AN1335

AN1335
Phase-Shifted Full-Bridge (PSFB) Quarter Brick DC/DC Converter
Reference Design Using a dsPIC® DSC
Author:
Ramesh Kankanala
Microchip Technology Inc.
ABSTRACT
This application note provides the digital implementation of a telecom input 36 VDC-76 VDC to output 12 VDC,
200W Quarter Brick DC/DC Brick Converter using the
Phase-Shifted Full-Bridge (PSFB) topology. This topology combines the advantages of Pulse-Width
Modulation (PWM) control and resonant conversion.
The dsPIC33F “GS” family series of Digital Signal Controllers (DSCs) was introduced by Microchip Technology Inc., to digitally control Switched Mode Power
Converters. The dsPIC33F “GS” family of devices consists of an architecture that combines the dedicated
Digital Signal Processor (DSP) and a microcontroller.
These devices support all of the prominent power conversion technologies that are used today in the power
supply industry.
In addition, the dsPIC33F “GS” family of devices controls the closed loop feedback, circuit protection, fault
management and reporting, soft start, and output voltage sequencing. A DSC-based Switched Mode Power
Supply (SMPS) design offers reduced component
count, high reliability and flexibility to have modular
construction to reuse the designs. Selection of peripherals such as the PWM module, Analog-to-Digital Converter (ADC), Analog Comparator, Oscillator and
communication ports are critical to design a good
power supply. MATLAB® based simulation results are
compared to the actual test results and are discussed
in subsequent sections.
INTRODUCTION
Recently, Intermediate Bus Converters (IBCs) have
become popular in the telecom power supply industry.
Most telecom and data communication systems contain ASIC, FPGAs and integrated high-end processors.
These systems require higher currents at multiple lowlevel voltages with tight load regulations. Traditionally,
bulk power supplies deliver different load voltages. In
the conventional Distributed Power Architecture (DPA),
the front-end AC/DC power supply generates 24V/48V
and an individual isolated Brick Converter supports the
required low system voltages. These systems become
inefficient and costly where very low voltages are
© 2010 Microchip Technology Inc.
required. In the Intermediate Bus Architecture (IBA),
the IBC generates 12V/5V. Further, these voltages are
stepped down to the required load voltages by Point of
Loads (PoLs).
In IBA, the high-density power converters, IBC and
PoLs are near to the load points, which bring considerable financial gains with the improved performance.
Because these converters are at the load points, PCB
design will be simpler with reduction in losses.
Electromagnetic Interference (EMI) is also considerably reduced due to minimum routing length of high
current tracks. Due to the position of these converters,
the transient response is good and the system performance is improved. Modern systems require voltage
sequencing, load sharing between the converters,
external communication and data logging.
Conventional Switched Mode Power Supplies are
designed with Analog PWM control to achieve the
required regulated outputs, and an additional microcontroller performs the data communication and load
sequencing. To maximize the advantages of IBC, the
converter must be designed with reduced component
count, higher efficiency, and density with lower cost.
These requirements can be achieved by integrating the
PWM controller, communication and load sharing with
the single intelligent controller. The dsPIC33F “GS” family series of DSCs have combined these design features
in a single chip that is suitable for the bus converters.
Some of the topics covered in this application note
include:
• DC/DC power module basics
• Topology selection for the Quarter Brick DC/DC
Converter
• DSC placement choices and mode of control
• Hardware design for the isolated PSFB Quarter
Brick DC/DC Converter
• Planar magnetics design
• Digital PSFB Quarter Brick DC/DC Converter
design
• Digital control system design
• Digitally controlled load sharing
• MATLAB modeling
• Digital nonlinear control techniques
• Circuit schematics and laboratory test results
• Test demonstration
DS01335A-page 1
AN1335
DISTRIBUTED POWER ARCHITECTURE (DPA)
Isolation Barrier
FIGURE 1:
Load
DC/DC Brick
Converter
3.3 VDC
Load
Load
AC/DC Power
Supply
24V/48V Bus
DC/DC Brick
Converter
2.5 VDC
Load
Load
DC/DC Brick
Converter
1.8 VDC
Load
INTERMEDIATE BUS ARCHITECTURE (IBA)
Isolation Barrier
FIGURE 2:
AC/DC Power
Supply
24V/48V Bus
Intermediate
Bus Converter
(IBC)
1.3 VDC
PoL
Load
1.8 VDC
PoL
12V/5V Bus
Load
1.5 VDC
Load
PoL
1.2 VDC
PoL
Load
1.0 VDC
PoL
Load
0.8 VDC
PoL
Load
PoL = Point of Load
DS01335A-page 2
© 2010 Microchip Technology Inc.
AN1335
QUARTER BRICK CONVERTER
Remote ON/OFF Control
The Distributed-Power Open Standards Alliance
(DOSA) defines the specifications for the single output
pin Quarter Brick DC/DC Converter. These specifications are applicable to all Quarter Bricks (unregulated,
semi-regulated and fully regulated) for an output
current range up to 50A.
Remote ON/OFF control is used to enable or disable
the DC/DC converter through an external control signal. The most common method to enable or disable the
converter is from the primary side (input side). Because
the controller exists in the secondary side of the isolated barrier, an isolation circuit must be used to transfer the signal from the primary side to the secondary
side. This can be achieved using the opto-isolator,
which is illustrated in Figure 3.
The AC/DC converter output is 48V in the IBA. This
voltage is further stepped down to an intermediate voltage of 12V by an isolated IBC. This voltage is further
stepped down to the required low voltage using PoL.
DOSA Quarter Brick DC/DC converters are offered in
through-hole configurations only.
FIGURE 3:
REMOTE ON/OFF
Remote ON/OFF – (I/P)
Some advantages of the Quarter Brick Converter are:
•
•
•
•
•
Improved dynamic response
Highest packaging density
Improved converter efficiency
Isolation near the load end
Output voltage ripple below the required limit
+3.3V ANA
R
U
Remote ON/OFF
Signal to DSC
C
1
DC/DC POWER MODULES BASICS
R
Before discussing the design aspects of the Quarter
Brick Converter, the following requirements should be
understood:
•
•
•
•
•
•
•
•
Input Capacitance
Output Capacitance
Remote ON/OFF Control
Ripple and Noise
Remote Sense
Forced Air Cooling
Overvoltage
Overcurrent
Input Capacitance
For DC/DC converters with tight output regulation
requirements, it is recommended to use an electrolytic
capacitor of 1 µF/W output power at the input to the
Quarter Brick Converter. In the Quarter Brick Converter
designs, these capacitors are external to the converter.
Output Capacitance
To meet the dynamic current requirements and the
output voltage regulations at the load end, additional
electrolytic capacitors must be added. As a design
guideline, in Quarter Brick Converter designs, 100 µF/A
to 200 µF/A of output current can be added and an effective lower Equivalent Series Resistance (ESR) can be
achieved by using a number of capacitors in parallel.
© 2010 Microchip Technology Inc.
C
GND
DIG_GND
Ripple and Noise
The output of a rectifier consists of a DC component
and an AC component. The AC component, also
known as ripple, is undesirable and causes pulsations
in the rectifier output. Ripple is an artifact of the power
converter switching and filtering action, and has a frequency of some integral multiple of the power converter
operating switching frequency.
Noise occurs at multiples of the power converter
switching frequency, and is caused by a quick charge
and discharge of the small parasitic capacitances in the
power converter operations. Noise amplitude depends
highly on load impedance, filter components and the
measurement techniques.
Remote Sense
Remote sense can be used to compensate voltage
drop in the set voltage when long traces/wires are used
to connect the load. In applications where remote sensing is not required, the sense pins can be connected to
the respective output pins.
DS01335A-page 3
AN1335
Forced Air Cooling
To remove heat from the high density board mount
power supplies, forced air cooling is applied using a
fan.
Forced air cooling greatly reduces the required PCB
size and heat sink. However, installation of a fan
consumes additional power, causes acoustic noise and
also the maintenance requirements are significant.
In forced air cooling SMPS applications, reliability of
the converter highly depends on the fan. A temperature
sensing device is used to monitor the temperature and
shuts down the converter when the Quarter Brick
Converter
exceeds
the
maximum operating
temperature.
Overvoltage
Overvoltage protection is required to protect the load
circuit from excessive rated voltage because of a
malfunction from the converter’s internal circuit. This
protection can be implemented by Latch mode or
Cycle-by-Cycle mode. In Latch mode, the circuit will be
in the OFF condition on the occurrence of overvoltage
fault until the input voltage is cycled. The system
automatically recovers in the Cycle-by-Cycle mode. If
faults still exist in the system, the system is turned OFF
and this cycle is repeated.
Overcurrent
Overcurrent protection prevents damaging the
converter from short circuit or overload conditions. In
Hiccup mode, the converter will be OFF when an
overcurrent or short circuit occurs, and will recover in
the specified time period. If the converter still sees the
fault, it will turn OFF the converter again and this cycle
repeats. In the Latch mode, the circuit is recovered only
after recycling the input power.
higher switching losses while the switch turns ON or
OFF, which results in a reduction in the efficiency of the
converter.
Soft switching techniques are used to reduce the
switching losses of the PWM converter by controlling
the ON/OFF switching of the power devices. Soft
switching can be done using the Zero Voltage
Switching (ZVS) and Zero Current Switching (ZCS)
techniques. These soft switching techniques have
some design complexity and in turn, produce higher
efficiency at high-power levels.
Non-Isolated Forward Mode Buck
Converter
If the required output voltage is always less than the
specified input voltage, the Buck Converter can be
selected from the following three basic topologies:
Buck, Boost and Buck Boost.
The Buck topology can be implemented in the isolated
and non-isolated versions. As per the bus converter
specification requirement, isolated converter design is
selected for this application. In the Forward mode Buck
Converter, energy is transferred from the primary side
to the secondary side when the primary side switch is
turned ON. The output voltage can be controlled by
varying the duty cycle with respect to the input voltage
and load current. This is done with the feedback loop
from the output that controls the duty cycle of the
converter to maintain the regulated output.
FIGURE 4:
NON-ISOLATED FORWARD
MODE BUCK CONVERTER
Q1
L1
VOUT+
VIN+
D1
C1
TOPOLOGY SELECTION
VOUT-
VIN -
The bus converter specifications are standardized, and
are used or assembled as one of the components in the
final system. The user must consider the end-system
characteristics such as reliability, efficiency, foot prints
and cost. There is no universally accepted topology for
the bus converters. However, the following sections
describe a few topologies that are commonly used for
DC/DC converter applications with their pros and cons.
A fundamental distinction among the PWM switching
topologies is hard switching and soft switching/
resonant topologies. Typically, high frequency
switching power converters reduce the size and weight
of the converter by using small magnetics and filters.
This in turn increases the power density of the
converter. However, high frequency switching causes
DS01335A-page 4
Isolated Forward Converter
In the Forward Converter, the energy from the input to
the output is transferred when the switch Q1 is ON.
During this time, diode D1 is forward biased and diode
D2 is reverse biased. The power flow is from D1 and L1
to output. During the switch Q1 OFF time, the
transformer (T1) primary voltages reverse its polarity
due to change in primary current. This also forces the
secondary of T1 to reverse polarity. Now, the secondary
diode, D2 is forward biased and freewheels the energy
stored in the inductor during switch Q1 ON time. This
simple topology can be used for power levels of 100W.
Some of the commonly used variations in Forward
© 2010 Microchip Technology Inc.
AN1335
Converter topologies are active Reset Forward
Converter, Two Transistor Forward or Double-ended
Forward Converter.
FIGURE 7:
HALF-BRIDGE CONVERTER
VIN+
D1
FIGURE 5:
ISOLATED FORWARD
CONVERTER
D1
VIN+
L1
T1
L1
VOUT+
Q3
T1
VOUT+
C1
Q4
VOUTD2
D2
C1
VIN-
Q1
VOUT-
VIN-
Push-Pull Converter
The Push-Pull Converter is a two transistor topology
that uses a tapped primary on the converter
transformer T1. The switches Q1 and Q2 conduct their
respective duty cycles and the current in the primary
changes, resulting in a bipolar secondary current
waveform. This converter is preferred in low input
voltage applications because the voltage stress is twice
the input voltage due to the tapped primary
transformer.
FIGURE 6:
PUSH-PULL CONVERTER
Full-Bridge Converter
The Full-Bridge Converter is configured using the four
switches: Q1, Q2, Q3 and Q4. The diagonal switches
Q1, Q4 and Q2, Q3 are switched ON simultaneously.
This provides full input voltage (VIN) across the primary
winding of the transformer. During each half cycle of
the converter, the diagonal switches Q1, Q4 and Q2,
Q3 are turned ON, and the polarity of the transformer
reverses in each half cycle. In the Full-Bridge
Converter, at a given power compared to the HalfBridge Converter, the switch current and primary
current will be half. This makes the Full-Bridge
Converter suitable for high-power levels.
FIGURE 8:
FULL-BRIDGE CONVERTER
VIN +
Q1
D1
T1
-
VIN
D1
L1
VOUT +
Q1
Q3
T1
Q2
Q4
+
L1
VOUT +
C1
C1
VOUT D2
VOUT D2
VIN -
Q2
Half-Bridge Converter
Half-Bridge converters are also known as two switch
converters. Half the input voltage level is generated by
the two input capacitors, C1 and C2. The transformer
primary is switched alternatively between VIN+ and
input return VIN- such that the transformer primary sees
only half the input voltage (VIN/2). The input switches,
Q1 and Q2, measure the maximum input voltage, VIN
compared to 2 * VIN in the Push-Pull Converter. This
allows the Half-Bridge Converter to use higher power
levels.
© 2010 Microchip Technology Inc.
However, the diagonal switches are hard switched
resulting in high turn ON and turn OFF switching
losses. These losses increase with frequency, which in
turn limits the frequency of the operation. To overcome
these losses, the PSFB converter is introduced. In this
topology, the switch turns ON after discharging the
voltage across the switch. This eliminates the turn ON
switching losses.
DS01335A-page 5
AN1335
FIGURE 9:
ZERO VOLTAGE
SWITCHING (ZVS)
FIGURE 10:
VDS(t)
FULL-BRIDGE CONVERTER
WITH SYNCHRONOUS
RECTIFICATIONS
PWMH
PWMH
VDS
ID(t)
Q3
Q1
Q6
PWML
ID
TX
TXVPRI
ZVS
t
PWM
PWML
PWML
t
Q4
Q2
PWMH
Q5
Synchronous Rectification
In synchronous rectification, the secondary diodes, D1
and D2 are replaced with MOSFETs. This yields lower
rectification losses because a MOSFET will have
minimum DC losses compared to the Schottky
rectifiers. The forward DC losses of a Schottky rectifier
diode will be forward voltage drop multiplied by the
forward current. The power dissipation by a conducting
MOSFET will be RDS(ON) multiplied by the square of
the forward current. The loss comparison will be
significant at considerably higher current >15A and
lower output voltages.
This configuration involves complexity and cost to an
extent because a gate drive circuit is required to control
the synchronous MOSFET. The efficiency of this
configuration can be further increased by designing the
complex gate drive signals, which are discussed in the
section “Digital Nonlinear Implementations”.
Many topologies are available and one of them can be
chosen depending on the given power level, efficiency
of the converter, input voltage variations, output voltage
levels, availability of the components, cost, reliability of
the design, and good performance characteristics.
With the discussed advantages for the topologies and
efficiency considerations, the PSFB topology was
selected for the Quarter Brick DC/DC Converter
design. The operation, design and performance of this
topology is discussed in following sections.
TABLE 1:
TOPOLOGY COMPARISON
Topology
No. of Switches in the
Primary
Stress Level of Primary
Switches
Power Levels (Typical)
Forward converter
2
VIN
100W
Push-Pull converter
2
2 * VIN
150W
Half-Bridge converter
2
VIN
Full-Bridge converter
4
VIN
PSFB converter
4
VIN
DS01335A-page 6
200W
~ 200W
~ 200W
© 2010 Microchip Technology Inc.
AN1335
PRIMARY SIDE CONTROL VS.
SECONDARY SIDE CONTROL
After selecting the topologies based on the merits for
the given application, the next challenge faced by
designers is to position the controller either on the
primary or secondary side. The power converter
demands
the
galvanic
isolation
between
primary (input) and secondary (output load) due to
safety reasons. There should not be any direct
conductive path between the primary and secondary.
Isolation is required when signals are crossing from the
primary to the secondary and vice versa. The power
path isolation will be given by the high frequency
transformers. Gate drive signals can be routed through
optocouplers or gate drive transformers.
FIGURE 11:
In the primary side controllers, the output feedback
signal is transferred from the secondary to the primary
using the optocouplers. These devices have limited
bandwidth, poor accuracy, and tend to degrade over
time and temperature.
Again, the transfer of signals from the primary to the
secondary or the secondary to the primary is
dependant on the features demanded by the
application. Figure 11, Figure 12 and Table 2 show the
comparison between the primary side controller and
the secondary side controller. The secondary side
controller is selected in this application.
SECONDARY SIDE CONTROL
VOUT +
VIN +
36V-76V
12V/17A
Sync Rectifier
PSFB MOSFET
200W
VIN -
VOUT Driver
Current TX
Drive TX
Driver
Drive TX
Driver
dsPIC®
DSC
Communication
Remote Control
OPTO
VINOV
VINUV
3.3V Reg
NCP 1031
© 2010 Microchip Technology Inc.
Auxiliary TX
3.8V
12V
To Driver’s VCC
DS01335A-page 7
AN1335
TABLE 2:
PRIMARY SIDE CONTROL VS. SECONDARY SIDE CONTROL
Primary Side dsPIC® DSC Control
Secondary Side dsPIC® DSC Control
Isolated feedback is required to regulate the output. A
linear optocoupler can be used to achieve the
regulation, which requires an auxiliary supply and an
amplifier in the secondary.
Isolated feedback is not required because the controller is
on the secondary.
Remote ON/OFF signal isolation is not required.
Remote ON/OFF signal isolation is required.
Isolation is required for communication signals.
Isolation is not required for communication signals.
Load sharing signal is transferred from the secondary to Load sharing isolation is not required because the
the primary.
controller is in the secondary.
Overvoltage protection signal is transferred from the
secondary to the primary.
Isolation for overvoltage is not required because the
controller is in the secondary.
Frequency synchronization signal is transferred from the Isolation for frequency synchronization is not required
secondary to the primary.
because the controller is in the secondary.
Input undervoltage and overvoltage can be measured
without isolation.
Isolation is required. However, in this application, the input
undervoltage or overvoltage protection is provided by the
NCP 1031 auxiliary converter controller.
Gate drive design for the primary side switches is
simple.
Gate drive is transferred from the secondary to the primary
either by using driver transformers or opto isolators.
FIGURE 12:
PRIMARY SIDE CONTROL
VOUT+
VIN+
36V-76V
12V/17A
Sync Rectifier
PSFB MOSFET
200W
VIN-
VOUT Current TX
Drive TX
Drive TX
Driver1
Driver2
dsPIC®
DSC
I/P UV
O/P OV
To Driver’s ICs
Remote ON/OFF
Drive TX
Linear
OPTO
Driver3
LM358
OPTO
Communication
+12V
Reg 3.3V
3.8V
Isolated 12V for Driver3
VIN+
NCP1031
Auxiliary
TX
VIN-
DS01335A-page 8
© 2010 Microchip Technology Inc.
AN1335
VOLTAGE MODE CONTROL (VMC) VS.
CURRENT MODE CONTROL (CMC)
FIGURE 14:
The preference to implement VMC or CMC as the
feedback control method is based on applicationspecific requirements. In VMC, change in load current
will have effect on the output voltage before the
feedback loop reacts and performs a duty cycle
correction. In CMC, change in load current is sensed
directly and corrects the loop before the outer voltage
loop reacts.
This cause and then react process in the VMC is slower
to respond than in the CMC for highly varying load
transients.
The fundamental difference between VMC and CMC is
that CMC requires accurate and high grade current
sensing. In VMC, output voltage regulation is
independent of the load current. Therefore, relatively
low grade current sensing is enough for overload
protection. This saves significant circuit complexity and
power losses.
TABLE 3:
VMC AND CMC DIFFERENCES
VMC
CMC
Single feedback loop.
Dual feedback loop.
Provides good noise
margin.
Poor noise immunity.
Slope compensation
required, instability at
more than 50% duty
cycles.
Poor dynamic response.
Good dynamic response.
VOLTAGE MODE
CONTROLLER (VMC)
ID
L
IS
S
PWM1H
+
Comp
-
Ramp
Generator
EA
+
IO
D
C
Ref
L
ID
IS
IO
D
S
PWM1H
C
+
Comp
-
EA
+
Ref
HARDWARE DESIGN FOR THE
ISOLATED QUARTER BRICK DC/DC
CONVERTER
The average Current mode control PSFB topology with
secondary side controller was selected for this design.
The digital Quarter Brick DC/DC Converter design is
discussed in the following sections.
High switching frequency and high voltage stress on
the primary side transistors produce switching losses.
PSFB transformer isolated buck converter attains zero
voltage transition (ZVT) without increasing the
MOSFET’s peak voltage stress.
Slope compensation not
required.
IL
IL
Phase-Shifted Full-Bridge (PSFB)
Converter Design
Current measurement not Current measurement
required for feedback.
required.
FIGURE 13:
CURRENT MODE
CONTROLLER (CMC)
In Figure 15, MOSFET (Q1-Q4), body diodes (D1-D4)
leakage
output
capacitance
(COSS1-COSS4)
inductance of the transformer are illustrated. Leakage
inductance causes the full-bridge switching network to
drive an effective inductive load, and results in ZVT on
the primary side switching devices.
The output voltage is controlled through a phase shift
between the two half-bridge. Both halves of the bridge
switch network operate with a 50% duty cycle and the
phase difference between the half-bridge switch
networks is controlled. A maximum duty cycle of 50%
ensures that the gate drive transformer and gate drive
circuit design will be simple.
The ZVT is load related and at some minimum load, the
ZVT will be lost. Linear output voltage control can be
achieved by controlling the phase shift between the
right leg and left leg of the bridge configuration.
In ZVT, the switches are turned ON when the voltage
seen by the switches are zero, resulting in no switch
ON losses. Phase shift control of a Full-Bridge
Converter can provide ZVT in the primary side which
results in lower primary side switching losses and lower
EMI losses.
© 2010 Microchip Technology Inc.
DS01335A-page 9
AN1335
Operation of the PSFB converter and detailed primary
side waveforms with different time intervals are
illustrated in Figure 15.
FIGURE 15:
PSFB CONVERTER WITH FULL WAVE SYNCHRONOUS RECTIFICATIONOPERATIONAL WAVEFORMS
Q3
Q1
TX
Q6
LL
IPRI
L0
VPRI
C0
Q4
Q2
V0
Q5
Q1
Q2
Q3
Q4
VPRI
IPRI
t
t0
DS01335A-page 10
t1
t2 t3 t4
© 2010 Microchip Technology Inc.
AN1335
• Initial Conditions t0: Q1 = ON; Q4 = ON;
Q2 = OFF; Q3 = OFF
• Time interval t3 to t4: Q3 = ON; Q2 = ON;
Q1 = OFF; Q4 = OFF;
The PSFB converter operation is described with the
power transfer from primary side to secondary side with
the conduction of diagonal switches, Q1 and Q4. The
primary side current (IPRI) was conducting through the
switches, Q4 and Q1, but in this period, the full input
voltage VIN is across the primary side of the
transformer TX and VIN/N is across the secondary of
the transformer. The slope of the current is determined
by VIN, magnetizing inductance and the output
inductance.
In this time interval, both the diagonal switches Q3 and
Q2 are ON and input voltage VIN is applied across the
primary of transformer. The rate of rise of the current is
determined by the input voltage VIN, magnetizing
inductance and the output inductance. However, the
current flows at negative value as opposed to zero.
Now, the current flowing through the primary switches
is the magnetizing current along with the reflected
secondary current into the primary.
• Time interval t0 to t1: Q1 =ON; Q4 = OFF;
Q2 =OFF; Q3 = OFF
Switch Q4 is turned OFF and Switch Q1 remains ON,
the primary current continues to flow taking the Q4
switch output capacitor C4. This charges the
capacitor C4 to VIN from 0V, at the same time the
capacitor C3 of Switch Q3 is discharged because its
source voltage rises to input voltage VIN. This transition
puts Q3 with no drain to source voltage prior to turn ON
and ZVS can be observed. Therefore there will not be
any turn ON switching losses. During this transition
period, the primary voltage of the transformer
decreases from VIN to zero, and the primary no longer
supplies power to the output. Simultaneously, the
energy stored in the output inductor starts supplying
the decaying primary power.
• Time interval t1 to t2: Q1 = ON; Q3 = ON;
Q4 = OFF; Q2 =OFF; D3 = ON
The input voltage, the transformer turns ratio and
output voltage determine the exact diagonal switch ON
time. After the switch-on time period of the diagonal
switches, Q3 is turned OFF at t4. One switching cycle
is completed when the switch Q3 is turned OFF and the
resonant transition to switch Q4 starts.
In the PSFB converter, the left leg transition requires
more time than the right leg transition to complete. The
maximum transition time occurs for the left leg at
minimum load current and maximum input voltage,
while minimum transition time occurs for the right leg at
maximum load current and minimum input voltage.
To achieve ZVT for all the switches, the leakage
inductor must store sufficient energy to charge and
discharge the output capacitance of the switches in the
allocated time. The energy stored in the inductor must
be greater than the capacitive energy required for the
transition.
After Q3 output capacitance is charged to full input
voltage VIN, the primary current free wheels through
switch Q1 and body diode D3 of switch Q3. The current
remains constant until the next transition occurs. Q3
can be turned ON any time after t1 and the current
shares between the body diode D3 and the switch Q3
channel.
HARDWARE DESIGN AND
SELECTION OF COMPONENTS
• Time interval t2 to t3: Q3 = ON; Q1 = OFF;
Q4 = OFF; Q2 =OFF;
Specifications
At time t2, Q1 is turned OFF, the primary current
continues to flow through the body diode, D1 of the
switch Q1. The direction of the current flow increases
the switch Q1 source to drain voltage, and voltage
across the switch Q2 decreases from high to lower
voltage. During this transition, the primary current
decays to zero. ZVS of the left leg switches depending
on the energy stored in the resonant inductor,
conduction losses in the primary switches and the
losses in the transformer winding. Because the left leg
transition depends on leakage energy stored in the
transformer, it may require an external series inductor
if the stored leakage energy is not enough for ZVS.
When Q2 is then turned ON in the next interval, voltage
VIN is applied across the primary in the reverse
direction.
© 2010 Microchip Technology Inc.
Selection of components for a quarter brick converter
design is critical to achieve high efficiency and high
density.
•
•
•
•
•
•
•
•
•
•
•
Input voltage: VIN = 36 VDC-76 VDC
Output voltage: VO = 12V
Rated output current: IORATED = 17A
Maximum output current: IO= 20A
Output power: PO = 200W
Estimated efficiency: 95%
Switching frequency of the converter:
FSW = 150 kHz
Switching period of the converter:
TP = 1/150 kHz = 6.66 µs
Chosen duty cycle: D = 43.4%
Full duty cycle: DMAX = 2 * 43.4% = 86.8%
Input power pin = 214.75W
DS01335A-page 11
AN1335
EQUATION 1:
EQUATION 4:
TURN ON TIME
Conduction losses of the MOSFET at 48V:
43.4
TurnOnTime = 6.66μs × ---------- = 2.89μs
100
P COND = I
2
SRMS
× R DS ( ON ) HOT = 0.171W
where:
PSFB MOSFET Selection
ISRMS = Switch rms current
EQUATION 2:
Conduction losses of all the four PSFB
MOSFETs = 0.687W
Input line current at 36V
P IN
I AVE = ------------------ = 5.96A
V INMIN
EQUATION 5:
Maximum Line Current at 36V
I MAX
1
P SW = --- × V IN × I SRMS × T F × F SW = 0.05W
2
I AVE
5.96
= -------------- = ------------- = 6.87A
D MAX
0.868
where:
TF = Fall time of the MOSFET = 5.7ns
Line rms current at 36V
Switching losses of all the four PSFB
MOSFETs = 0.21W
I RMS = I MAX × D = 6.40A
Switch rms current at 36V
I SRMS
In the ZVT, MOSFETs have only turn OFF switching
losses.
= I MAX × D
---- = 4.53A
2
EQUATION 6:
Because the maximum input voltage is 76 VDC, select
a MOSFET voltage rating that is higher than 76V and
the current rating higher than IMAX at 36 VDC.
RDS(ON) HOT can be calculated either from the graphs
provided in the data sheet or by using the empirical
formula shown in Equation 3.
= 0.126W
RDS(ON) HOT = 0.02625E
where:
RDS(ON) at 25 = 0.015E
Maximum junction temperature, TMAX = 125oC
Ambient temperature, TAMB = 25oC
DS01335A-page 12
where:
For all the four PSFB MOSFETs = 0.504W
Bias voltage to the gate drive, VDD = 12V
MOSFET total gate charge QG = 70 ns
RDS(ON) EMPIRICAL
FORMULA
RDS(ON) HOT =
RDS(ON) @ 25 * [1+0.0075*(TMAX-TAMB)
MOSFET GATE CHARGE
LOSS
MOSFETGateCh arg eLosses = Q G × F SW × V DD
The device selected is Renesas HAT2173 (LFPAK),
and has VDS 100V, ID 25A, RDS(ON) 0.015E.
EQUATION 3:
SWITCHING LOSSES OF
MOSFET
Synchronous MOSFET Selection
]
The ability of the MOSFET channel to conduct current
in the reverse direction makes it possible to use a
MOSFET where a fast diode or Schottky diode is used.
In the fast diodes, junction contact potential limits to
reduce the forward voltage drop of diodes. Schottky
diodes will have reduced junction potential compared
to the fast diode. In the MOSFETs, the conduction
losses will be RDS(ON) * I2RMS. The on-resistance can be
decreased by using parallel MOSFETs; this will reduce
the losses further significantly.
© 2010 Microchip Technology Inc.
AN1335
When full wave center tapped winding is used in the
transformer secondary side, the MOSFET voltage
stress is twice the output voltage, as shown in
Equation 7.
EQUATION 7:
MOSFET VOLTAGE STRESS
MOSFETVoltageStress = 2 × ( V O + V FET + V DROP )
The maximum transition cannot exceed one-fourth of
the resonant period to gain the ZVT.
EQUATION 10:
TTRANSMAX
MaximumTransitionTime, T
TRANSM AX
π
= --- ⋅ ( L R ⋅ C R )
2
= 2 × ( 12 + 0.6 + 0.2 )
where:
The capacitive energy required to complete the
transition, ECR is shown in Equation 11.
= 25.6V
Secondary MOSFET Drop, VFET = 0.6V
Total Trace Drops, VDROP = 0.2V
This is the minimum voltage stress, seen by the
MOSFET when the lower input voltage is 36V. For the
maximum input voltage of 76V, the stress is as shown
in Equation 8.
EQUATION 8:
25.6
MOSFET Voltage Stress @ 76V = 76 × ---------- = 54.04V
36
EQUATION 11:
1
2
E CR = --- × C R × ( V INMAX )
2
where:
VIN MAX = maximum input voltage
The energy stored in the resonant inductor LR must be
greater than the energy required to charge and
discharge the COSS of the MOSFET and transformer
capacitance CTX of the leg transition within the
maximum transition time.
The device selected is Renesas HAT2173 (LFPAK).
The energy stored in the resonant inductor (LR), is as
shown in Equation 12.
Transformer Design
EQUATION 12:
DESIGN CONSIDERATIONS FOR RESONANT
TANK CIRCUIT ELEMENTS
Design of resonant tank is critical to achieve ZVT.
Resonant capacitor (CR) and resonant Inductor (LR)
forms resonant tank. A factor of 4/3 is multiplied to the
Output Capacitance of MOSFET (COSS) to
accommodate the increase in capacitance with
voltage, and a factor of two is also multiplied because
two output capacitances (COSS) will come in parallel in
each resonant transition.
1
2
E LR = --- × L R × I PRI
2
The slope of the primary current during transition is as
shown in Equation 13.
EQUATION 13:
I PRI
VP
------ = ----------------LR
T TRANS
EQUATION 9:
4
TotalResonantCapaci tan ce, C R = --- × 2 × C OSS + C TX
3
8--= × C OSS + C TX
3
= 1.387nF
where:
COSS = Output capacitance of the MOSFET
= 5.20E-10F
CTX = Transformer capacitance (neglected)
ENERGY STORED IN THE
RESONANT INDUCTOR (LR)
NS
MaximumPrimaryCurrent, I PRI = ------- × I O
NP
= 6.8A
where:
IO = Output current
NP = TX primary turns = 5
NS = TX secondary turns = 2
VP = Input voltage = 32.5V
FSW = Converter switching frequency
ResonantTankFrequency, F R
1
= ------------------------------2π L R × C R
where:
TP = Switching period = 1/FSW
Resonant transition estimated TTRANS = 0.15 * TP
LR = Transformer leakage inductance + Additional
leakage inductance
© 2010 Microchip Technology Inc.
DS01335A-page 13
AN1335
EQUATION 14:
TTRANS
LR
T TRANS = 2 × I PRI × -----VP
Two transitions per period. Hence, multiplied with 2.
EQUATION 15:
LR
T TRANS V P
L R = ------------------ × ---------2
I PRI
L R = 2.3μH
The energy stored in the inductor, ELR must be greater
than the capacitive energy, ECR, which is required for
the transition to occur within the allocated transition
time.
EQUATION 16:
2
1
2
1
E LR > E CR = --- × L R × I PRIMIN > --- × C R × V INMAX
2
2
2
( V IN )
I PRIMIN > C R × ---------------- = 0.88A
LR
The magnetic cross section area must be large to
minimize the number of turns that are required for the
given application. Ensure that the core covers the
winding that is laid on the PCB. Such design types
reduce the EMI, heat dissipation and allow small height
cores. Copper losses can be reduced by selecting the
round center leg core because this reduces the length
of turns.
The Planar Magnetics design procedure is the same as
that of the wire wound magnetics design:
1.
2.
3.
4.
5.
6.
7.
8.
Select the optimum core cross-section.
Select the optimum core window height.
Iterate turns versus duty cycle.
Iterate the core loss.
Iterate the copper loss (Cu).
Evaluate the thermal methods.
Estimate the temperature rise.
What is the cost trade-off versus the number of
layers.
9. Does the mechanical design fit the envelope
and pad layout?
10. Fit within core window height.
11. Is the size sufficient for power loss and thermal
solution?
Magnetics Design
Full-Bridge Planar Transformer Design
Magnetics design also plays a crucial role in achieving
high efficiency and density. In the Quarter Brick DC/DC
Converter design, planar magnetics are used to gain
high efficiency and density.
The two considerations for secondary rectifications are
Full
Wave
Center
Tapped (FWCT)
rectifier
configuration and Full Wave Current Doubler rectifier
configurations. It is observed that the FWCT rectifier
makes optimum use of board space and efficiency
goals. Preliminary testing has validated this
conclusion.
DESIGN OF PLANAR MAGNETICS
Planar magnetics are becoming popular in the high
density power supply designs where the winding height
is the thickness of the PCB. Planar magnetics design
can be constructed stand-alone with a stacked layer
design or as a small multi-layer PCB or integrated into
a multi-layer board of the power supply.
The advantages of planar magnetics are:
•
•
•
•
•
•
Low leakage inductance
Very low profile
Excellent repeatability of performance
Economical assembly
Mechanical integrity
Superior thermal characteristics
Planar E cores offer excellent thermal resistance.
Under normal operating conditions, it is less than 50%
as compared to the conventional wire wound
magnetics with the same effective core volume, VE.
This is caused by the improved surface to the volume
ratio. This results in better cooling capability and can
handle higher power densities, while the temperature is
within the acceptable limits.
DS01335A-page 14
A further optimization goal is to offer a broad operating
frequency from 125 kHz to 200 kHz to provide wide
latitude for customers to optimize efficiency.
The input voltage range is 36 VDC-76 VDC nominal with
extended VINMIN OF 32.5 VDC.
Analysis of the transformer design begins with the
given input parameters:
• VIN = 36V
• Frequency = 150 kHz
• TP = 6.667 x 10-6
The intended output voltage was meant to supply a
typical bus voltage for distributed power applications
and the output voltage. VO = 12.00V and the maximum
output load current, IO = 25A
No substitute exists for the necessary work to perform
calculations sufficient to evaluate a particular core size,
turns, and core and copper losses. These must be
iterated for each design. One of the design
considerations is to maximize the duty cycle, but the
limitation of resolution offered by integer turns will
quickly lead to the turn ratio of NP = 5 and NS = 2.
© 2010 Microchip Technology Inc.
AN1335
In the design of the magnetics, users must select the
minimum number of turns. There is a cost or penalty to
placing real-world turns on a magnetic structure such
as, resistance, voltage drop and power loss. Therefore,
use the least number of integer turns possible.
FIGURE 16:
PLANAR TRANSFORMER
21.00 mm
(0.83”)
14.90 mm
(0.59”)
Thereafter, a reasonable assessment for turn ratio,
duty cycle, peak flux density, and core loss can be done
until a satisfactory point is reached for the designer.
The duty cycle (more than each half-period) to produce
the desired output is as follows:
Champs Technologies
MCHP-045-V31-1
• TON = 2.89 µs
• D = TON/TP = 0.434
16.90”
(0.67 mm)
Over a full period, the duty cycle is 86.8% at a VIN of
36 VDC.
5.90 mm
(0.23”)
• Secondary MOSFET drop, VFET SEC = 0.1V
• Total trace drops, VDROP = 0.2V
• Primary MOSFET drop, VFET PRI = 0.6V
9.80 mm (0.39”)
In this design, the following regulation drops are used:
EQUATION 17:
N
V
O
= ( V IN – V FETPRI ) × -------S – V FETSEC – V DROP × 2D
N
P
= 12.03V
The iteration method is followed again to select the
core size from the available cores.
The selected core has the following magnetic
parameters:
• AC = 0.45 cm2
• LE = 3.09 cm
• VE = 1.57 cm3
This core shape is a tooled core and is available from
the Champs Technologies. In general, a power material
in the frequency range of interest must be considered.
Materials such as 2M, 3H from Nicera™, the PC95
from TDK™, or the 3C96, 3C95 from Ferroxcube™ are
the most recommended options. The peak-to-peak and
rms flux densities arising from this core choice are
shown in Equation 18.
EQUATION 18:
8
( V IN × t ON ) × 10
B PKPK = ------------------------------------------NP × AC
3
B PKPK = 4.624 × 10 Gauss
t ON
B RMS =
2----×
TP
∫
O
8 2
( V IN × t ON ) × 10
------------------------------------------- dT
2 × NP × AC
3
B RMS = 2.153 × 10 Gauss
© 2010 Microchip Technology Inc.
DS01335A-page 15
AN1335
The power loss density is calculated using the
parameters shown in Table 4.
TABLE 4:
Material
3C92
3C96
3F35
Note:
FIT PARAMETERS TO CALCULATE THE POWER LOSS DENSITY
f (kHz)
Cm
x
y
Ct1
Ct0
20-100
26.500
1.19
2.65
2.68E-04
5.43E-02
3.75
100-200
0.349
1.59
2.67
1.51E-04
3.05E-02
2.55
200-400
1.19E-04
2.24
2.66
2.08E-04
4.37E-02
3.29
20-100
5.120
1.34
2.66
5.48E-04
1.10E-01
6.56
100-200
8.27E-02
1.72
2.80
1.83E-04
3.66E-02
2.83
200-400
9.17E-05
2.22
2.46
2.33E-04
4.72E-02
3.39
400-1000
1.23E-08
2.95
2.94
1.38E-04
2.41E-02
2.03
Source – New ER Cores for Planar Converters, Ferroxcube™ Publication 939828800911, Sept. 2002.
Core loss density can be approximated by the formula
shown in Equation 19. The core constants are made
available by Ferroxcube™. In this design:
•
•
•
•
•
•
•
•
•
Ct2
Temp = 50oC
Frequency = 150000 Hz
B = BRMS * 10-4 = 0.2153 Tesla
x =1.72
y = 2.80
Ct2 = 1.83 * 10-4
Ct1 = 3.66 * 10-2
Ct0 = 2.83
Cm = 8.27 * 10-2
EQUATION 19:
SECONDARY RMS
CURRENT
I SEC = I O × D
I SEC = 16.47A
Primary rms current is calculated as shown in
Equation 21:
EQUATION 21:
PRIMARY RMS CURRENT
NS
I PRI = I O × 2D × ------NP
CORE LOSS DENSITY
Core Loss Density Pcore
2
x
y
C m × Freq × B × ⎛ C t0 – C t1 × Temp + C t2 × Temp ⎞
⎝
⎠
------------------------------------------------------------------------------------------------------------------------------------------=
1000
3
P = 1.307 × 10 mW/Cm3
CoreLoss = P × V E × 10
EQUATION 20:
–3
CoreLoss = 2.052W
I PRI = 9.317A
The DCR values are computed from the CAD
drawings:
• Secondary DCR: SecDCR = 0.0023E
• Primary DCR: PriDCR = 0.025E
Secondary copper loss is multiplied by two because it
is a center tapped winding.
EQUATION 22:
Sec_Loss = 2 * I2SEC * Sec_DCR = 1.248W
One of the benefits of using planar construction is the
opportunity to utilize 2 oz., 3 oz., and 4 oz. copper
weight, which results in very thin copper. The impact is
that skin depth and proximity loss factors are usually
considerably reduced versus using wire wound magnetic structures. The copper losses are calculated
using DC Resistance (DCR).
Pri_Loss = I2PRI* Pri_DCR = 2.17W
Total_Loss = Sec_Loss + Pri_Loss + Core Loss
Total_Loss = 5.466W
The secondary rms current in each half of the center
tapped winding is shown in Equation 20.
DS01335A-page 16
© 2010 Microchip Technology Inc.
AN1335
The stacking of the main transformer
arrangement is shown in Table 5.
TABLE 5:
Planar Output Inductor Design
layers
The output inductor serves the following functions:
• Stores the energy during the OFF period to keep
the output current flowing continuously to the
load.
• Smooths out and average the output voltage
ripple to an acceptable level.
STACKING LAYERS FOR
PLANAR TRANSFORMER
Layers
Cu Weight
(Oz.)
Winding
Primary
Sec1
Layer 1
Sec2
Sec2
2
Layer 2
2
Layer 3
Sec1
2
Layer 4
2
Layer 5
Primary
4
Layer 6
4
Layer 7
Sec1
2
Layer 8
2
Layer 9
Primary
2
Layer 10
3
Layer 11
Sec2
3
Layer 12
Layer 13
2
Primary
4
Layer 14
4
Layer 15
Sec1
2
Layer 16
2
Layer 17
Sec2
2
Layer 18
Turns
2
5
FIGURE 17:
Q1
2
2
FULL-BRIDGE CONVERTER WITH CENTER TAPPED FULL WAVE SYNCHRONOUS
RECTIFIER
Q3
TX
Q6
LL
VPK2
IPRI
L0
VPRI
C0
Q2
Q4
© 2010 Microchip Technology Inc.
V0
Q5
DS01335A-page 17
AN1335
FIGURE 18:
PLANAR OUTPUT
INDUCTOR
• Switch turn ON time, TON = 2.89 μs
• Total Switching period, TP = 6.667μs
• Duty cycle, D = TON/TP = 0.434
18.35 mm
(0.72”)
Over a full period the duty cycle is 86.8% at
VINMIN 36 VDC.
12.00 mm
(0.47”)
The duty cycle (more than each half-period) to produce
the desired output is as follows:
Champs Technologies
MCHP1825-V31-1
EQUATION 23:
V
O
= (V
IN
–V
FETPRI
N
S
) × ------- – V
–V
× 2D
FETSEC
DROP
N
P
15.0 mm
(0.59”)
V O = 12.03V
5.40 mm
(0.21”)
9.80 mm (0.39”)
In the case of output inductor, consider the choice of
inductance value at the maximum off time. This occurs
in PWM regulated DC-DC converters at the maximum
input voltage, VIN MAX = 76V, and the feedback loop
adjusts the switch ON time accordingly.
TONMIN = 1.415 µs
The duty cycle is as follows:
D_MIN = TONMIN/TP = 1.3689 µs
The peak voltage at the transformer secondary is as
shown in Equation 24.
EQUATION 24:
NS
V PK2 = ( V INMAX – V FETPRI ) × ------- – V FETSEC – V DROP
NP
V PK2 = 28.26V
Maximum output load current, IO = 25A. A ripple current of 25% of the total output current is considered in
this design.
EQUATION 25:
I MIN = I O × 0.25 = 6.5A
EQUATION 26:
L OMIN
OUTPUT INDUCTANCE
(LOMIN)
( V PK2 – V O ) × T ONMIN
= --------------------------------------------------------- = 3.54μH
I MIN
This core is also a tooled core as the main
transformer, TX1. It is available from Champs
Technologies as PN MCHP1825-V31-1. Materials such
as 7H from Nicera™, the PC95 from TDK™, or the
3C94, 3C92 from Ferroxcube™ are the recommended
choices.
•
•
•
•
Core cross section, AC = 0.4 cm2
Core path length, LCORE = 3.09 cm
Rated output current: IRATED = 17A
Defined saturation current: ISAT = 20A
The process of inductor design involves iterating the
number of turns possible and solving for a core air gap.
The air gap is checked for operating the flux below
maximum rated flux in the core material at the two
operating current values that is rated current and
saturation current.
In this design, if the 18 layers are available, these
layers can be split into balanced integer turns. This is a
practical method and the number of turns Nt = 6.
In this design, a fringing flux factor assumption of 15%
is done that is FFF = 1.15.
The iterative process begins by calculating the air gap
equation. The air gap is calculated using Equation 27.
EQUATION 27:
2
In this design, the core window height and its adequacy
in terms of accommodating the 18 layer PCB stack is to
be assessed since the windings/turns for the inductor
are also embedded.
DS01335A-page 18
–8
⎛ 0.4 × π × Nt × A C × 10 ⎞
L GAP = ⎜ --------------------------------------------------------------⎟ × FFF
L OMIN
⎝
⎠
= 0.058cm
L GAP
L GAPIN = ------------- = 0.023inch
2.54
© 2010 Microchip Technology Inc.
AN1335
EQUATION 28:
OPERATING FLUX DENSITY
AT DEFINED SATURATION
CURRENT
0.4 × π × Nt × I SAT
B DC = ---------------------------------------------L GAP
3
B DC = 2.598 × 10 Gauss
EQUATION 29:
OPERATING FLUX DENSITY
AT RATED CURRENT
0.4 × π × Nt × I RATED
B RATED = ----------------------------------------------------L GAP
3
B RATED = 2.208 × 10 Gauss
The BDC and BRATED values are conservative
compared to the commercially rated devices. Typical
BMAX values are 3000 Gauss at 100ºC.
The required AL value is calculated, as shown in
Equation 30.
EQUATION 30:
AL VALUE
9
L OMIN × 10
- = 98.32mH
A L = -----------------------------2
Nt
This is helpful for instructing the core manufacturer for
gapping instructions. The inductor traces are designed
using a CAD package and are integrated into the PCB
layout package. The CAD package facilitates the
calculation of trace resistance for each layer. The
calculated DCR values DCRRATED = 3.5 * 10-3E.
Copper loss is computed at the DC values of rated and
saturation-defined currents, as shown in Equation 31.
EQUATION 31:
2
Cu LossSAT = ( I SAT ) × DCR RATED
Cu LossSAT = 1.4W
EQUATION 32:
2
Cu LossRATED = ( I RATED ) × DCR RATED
Cu LossRATED = 1.012W
One of the design goals is to make it universal for other
lower and higher power implementations of the digital
converter and to keep the overall efficiency high. It fits
comfortably with its footprint in the PCB. However, we
consider that a smaller core and footprint optimization
is quite possible.
© 2010 Microchip Technology Inc.
Planar Drive Transformer Design
To drive each leg (high side and low side) of the gates,
the high side/low side driver, or low side driver with
isolated drive transformer is required. A minimum of
500 VDC isolation is required in the drive transformer
from the high side to low side winding. Because the
gate drive is derived from secondary side controller,
primary to secondary 2500 VDC isolation is required.
The following critical parameters must be controlled
while designing the gate drive transformer:
• Leakage inductance
• Winding capacitance
A high leakage inductance and capacitance causes an
undesirable gate signal in the secondary, such as
phase shift, timing error, overshoot and noise. Winding
capacitance results when the design has a higher
number of turns. Leakage inductance results when the
turns are not laid uniformly. Because planar magnetics
are used in this application, these parameters may not
be a problem. Since the absolute number of turns
required is low and the primary and secondary side
high/low drive windings can be interleaved to minimize
leakage without increasing the overall capacitance.
Typical gate drive transformers are designed with
ferrite cores to reduce cost and to operate them at high
frequencies. Ferrite is a special material that comprises
high electrical resistivity and can be magnetized quickly
with minor hysteresis losses. Because of its high
resistance, eddy currents are also minimal at high
frequency.
Selection of Core Materials and Core
Selection of core material depends on the frequency of
the operation. 3F3 from Ferroxcube™ is one of the best
options for the operating frequencies below 500 kHz.
The power loss levels of gate drive transformers is
usually not a problem and thus Ferroxcube RM4/ILP is
selected. The magnetic parameters of Ferroxcube
RM4/ILP are as follows:
•
•
•
•
AC = 0.113 cm2
Lm = 1.73 cm
AL =1200 nH
µEFF = 1140
One of the primary goals of the design is to embed all
the magnetics as part of the overall PCB design of the
main power stage. A small size core geometry is
selected, that has sufficient window height to
accommodate the overall PCB thickness and also
gives reasonable window width to accommodate the
PCB trace width that comprises the turns. The
resulting “footprint” or core cut-out required of the RM4/
ILP was found to be acceptable.
DS01335A-page 19
AN1335
We will iterate the primary turns to arrive at a suitable
peak flux density and magnetizing current using the
formula shown in Equation 33.
The peak and RMS flux densities can be pushed
higher. However, a reasonably low value of magnetizing current has been maintained such that the driver is
not loaded much.
EQUATION 33:
EQUATION 37:
8
NP
( V IN × T ON ) × 10
= --------------------------------------------B PKPK × A C
CALCULATION OF
MAGNETIZING
INDUCTANCE
2
In the application, VIN = 12V as set by the bias supply.
The operating frequency for main power processing is
selected as 150 kHz. The result is the gate drive
transformer operates at the same frequency.
The duty cycle is also determined by the power stage.
The basic input parameters, TP and TON are set.
Iterating for primary turns, NP = 10.
The peak-to-peak flux density can be achieved as
shown in Equation 34:
LA L = N P × A L × 10
–9
–4
LA L = 1.2 × 10 Henry
Or,
2
–8
0.4 × π × μ EFF × ( N P ) × A C × 10
L M = --------------------------------------------------------------------------------------Lm
–5
L M = 9.57 × 10 Henry
Conversely, the inductance minimum will be
between ~70 µH.
L MIN = 0.75L M Henry
EQUATION 34:
PEAK-TO-PEAK FLUX
DENSITY
–5
L MIN = 6.699 × 10 Henry
8
( V IN × T ON ) × 10
B PKPK = --------------------------------------------NP × AC
The magnetizing current is thus reasonable for this
application, and is shown in Equation 38:
3
B PKPK = 3.069 × 10 Gauss
EQUATION 38:
The peak flux density is shown in Equation 35, which
yields a volt-µs rating of (VIN * TON) = 37.7. This is well
below the typical saturation curves for 3F3 of 3000
Gauss at 85ºC operational ambient temperature.
However, potential saturation is not a design concern.
EQUATION 35:
PEAK FLUX DENSITY
8
( V IN × T ON ) × 10
B PK = --------------------------------------------2 × NP × AC
3
B RMS =
O
Farad
EQUATION 39:
F R = 2.3MHz
8 2
( V IN × T ON ) × 10
--------------------------------------------2 × NP × AC
∫
– 12
1
F R = ------------------------------------------------2 × π × ( LM × CD )
RMS FLUX DENSITY
T ON
Assuming the worst case, the distributed capacitance
is shown as follows:
Any ringing on the gate drive waveforms due to the
transformer will possess a frequency of 2.3 MHz.
The RMS flux density is calculated as shown in
Equation 36.
2----×
TP
di = 0.362A
C D = 50 × 10
B PK = 1.534 × 10 Gauss
EQUATION 36:
V IN × T ON
di = ------------------------LM
× DT
3
B RMS = 1.363 × 10 Gauss
results in mW of core loss.
DS01335A-page 20
© 2010 Microchip Technology Inc.
AN1335
In this design, the selection of track width or trace width
was fairly conservative. Given the RM4/ILP core
window width of 2.03 mm (80 mils), and an allowable
PCB width accommodated inside this core
of 1.63 mm (64 mils), and a further conservative
assumption of trace-to-trace clearance of 0.3 mm (12
mils), we can either place 2T/layer of 0.39 mm (15 mil)
width or 3T/layer of 0.18 mm (7 mils) width. If 4 oz.
copper was used per layer the 0.18mm trace width
would result in too much “under-etch” in the fabrication.
We had ~14 layers dictated by the power stage and the
resulting PCB thickness of 3.5-3.8 mm could be easily
accommodated by the RM4/ILP core window height.
Hence, it is easier to select 2T/layer. This selection also
allowed three opportunities for an interleave to occur
between the primary and each secondary drive
winding. A choice of 3T/layer may have resulted in an
imbalance and with less opportunity for interleave.
EQUATION 42:
–6
T ON = 2.89 × 10 Sec
TP
T OFF = ------ – T ON
2
–7
T OFF = 4.433 × 10 Sec
The core used on this part = E5.3/2.7/2-3C96
The core parameters are as follows:
• LM = 1.25 cm
• AC = 0.0263 cm2
• VE = 0.0333 cm3
The nominal current sense termination resistor
value: RB = 10.0E.
EQUATION 43:
Current Sense Transformer Design
I MAX
V PKSECY = ------------ × R B
NS
The current sense transformer selected is a
conventional stand-alone magnetic device. The
decision was made earlier to have a 1:100 current
transformation ratio. Therefore, it is difficult to
implement this device as an embedded structure.
Therefore, the rating is 0.1 V/amp.
We repeat some aspects of the TX1 main transformer
design such as switching frequency.
EQUATION 44:
8
EQUATION 40:
3
F SW = 150 × 10 HZ
1
T p = ---------F SW
–6
T P = 6.667 × 10 Sec
The transformation ratio, NC = NS/NP = 100
Maximum rated current, IMAX = 10A
Therefore, secondary RMS current is computed as
shown in Equation 41:
EQUATION 41:
V PKSECY = 1V
SECONDARY RMS
CURRENT
I RMSPRIM
I RMSSECY = -----------------------NC
I RMSSECY = 0.093A
© 2010 Microchip Technology Inc.
( V PKSECY × T ON ) × 10
B PK = ----------------------------------------------------------NS × AC
B PK = 109.886Gauss
It is considered that the peak flux density is very low
and it is fine. Usually, the current to voltage gain is this
low in most switched mode converters. The current
ramp signal at the current sense (CS) input for most
analog controllers is <1V so always select a low value
termination resistor. In this case, the voltage gain is
conditioned with differential op amps prior to sending it
to the input ADC of the dsPIC® DSC.
It is helpful to know that higher current to voltage gains
are possible simply by selecting higher value
termination resistors. The only limitation will be a
ceiling imposed by the saturation of the ferrite core.
The volt-µs rating of the CH-1005 Champs
Technologies is 58V-µs. In this design, if a termination
impedance of 100Ω is selected, a 10V signal amplitude
is gained. The current transformer reproduces the
current wave shape until it is not saturated, that is as
long as it is performing as a transformer. In this design,
a maximum ON time of 5.8 µs can be permitted.
DS01335A-page 21
AN1335
EQUATION 46:
The rated maximum flux is shown in Equation 45.
RMS FLUX DENSITY
t ON
EQUATION 45:
–6
B RMS =
8
( 58 × 10 ) × 10
B RATED = ------------------------------------------NS × AC
2----×
TP
∫
O
8 2
( V PKSECY × T ON ) × 10
----------------------------------------------------------- dTp
2 × NS × AC
B RMS = 51.159Gauss
3
B RATED = 2.205 × 10 Gauss
The BPK is rated as 2200 Gauss peak for 100ºC
operation unipolar excursion. The RMS flux density is
calculated as shown in Equation 46.
EQUATION 47:
x
2
y
C m × F × ( B RMS ) × ( C t0 – Ct1 × Temp + C t2 × Temp )
CoreLossDensity, P = -----------------------------------------------------------------------------------------------------------------------------------1000
P = 0.048 mW/cm3
where setting up core loss coefficients:
Cm = 8.27 * 10-2
x = 1.72
y = 2.80
Temp = 30
Ct1 = 3.66 * 10-2
Ct2 = 1.83 *10-4
Ct0 = 2.83
F = 1.5 * 105
CoreLoss = P × V E × 10
–3
–6
CoreLoss = 1.592 × 10 W
Core loss is about zero or negligible.
Secondary SecDCR = 6.6E.
DS01335A-page 22
© 2010 Microchip Technology Inc.
AN1335
FIGURE 19:
CURRENT TRANSFORMER
2
3.70 mm
(0.146'')
Secloss = ( I RMSSECY ) × SecDCR
2
Priloss = ( I RMSPRIM ) × PriDCR
1.85 mm
(0.073'')
1
8
= 0.057W
100T
7
1T
3
= 0.173W
5.3 mm
(0.21'')
EQUATION 48:
7
+
8
TotalLoss = Secloss + Priloss + Coreloss
= 0.23W
where:
4.9 mm
(0.19'')
Secloss = Secondary copper losses
Priloss = Primary copper losses
0.25 mm
(0.010'')
PriDCR = 0.002E
SecDCR = 6.6E
Total loss for this device at maximum ratings is less
than 1/4W.
Calculate the inductance value for the selected 3C96
material.
7.80 mm
(0.31'')
5.3 mm
(0.21'')
EQUATION 49:
2
L AL = ( N S ) × A L × 10
L AL = 3 × 10
–3
–9
~ 3mH
0.006
where:
Ns = 100
0.15
6.8 mm
(0.27'')
AL = 300 nH
EQUATION 50:
L MIN = L AL × 0.75
= 2.25mH
Minimum secondary inductance = 2.25 mH.
X L = 2 × π × f × L MIN
3
= 2.12 × 10 E
Effective termination impedance is as shown in
Equation 51:
Planar Auxiliary Power Supply
Transformer Design
The digital DC/DC converter requires auxiliary power
supply. The dsPIC DSC requires 3.3V and the gate
drivers require 12V.
The dsPIC DSC must have power supplied to it prior to
start-up of the power converter. The scheme to
accomplish this is to utilize an analog converter for
start-up and also for continuous operation. This avoids
possible glitches or uncontrolled operation events
during abnormal operation or unanticipated transient
conditions. The analog controller requires a boot strap
supply once it has gone through soft-start.
EQUATION 51:
X L × Rb
X EFF = ------------------X L + Rb
X EFF = 9.953E
Deviation from ideal is < 0.1%.
© 2010 Microchip Technology Inc.
DS01335A-page 23
AN1335
The dsPIC DSC requires 3.3V. A linear regulator is
inserted prior to 3.3V so that the headroom required at
one output is 4V. The 3.3V output voltage before
regulator V01 = 4V.
•
•
•
•
•
•
EQUATION 57:
2 × I SCDC
PeakSecondaryCurrent, ISCPK = -----------------------DS
Load current, I3.3V = 0.3A
12V output voltage before regulator, V02 = 12V
Load current, I12V = 0.4A
Total output power = 6W
Consider an overall efficiency of 80%
Input power = 7.5W
2---------------× 0.1
= 3.33A
0.6
D
------S × I SCPK
3
= 1.489A
SecondaryRMSCurrent, I SRMS =
Consider minimum input voltage, VINMIN = 32V. The
converter is designed to operate at a maximum duty
cycle, D = 40%. The nominal operating frequency, FSW
of the IC is 250 kHz.
where:
Short circuit current: ISCDC = 1A
Secondary duty cycle: DS = 0.6
The turns ratio for 12V and 3.3V output is shown in
Equation 58.
EQUATION 52:
3
F SW = 250 × 10 Hz
EQUATION 58:
N P [ V IN – ( I PPK × R DS ( ON ) ) ] × D
------- ≥ -------------------------------------------------------------------------NS
( V OUT + V fD1 ) × ( 0.8 – D )
Total period, TP = 4 µs
On period, TON = 1.6 µs
NP
---------- = 2.60
N S12
EQUATION 53:
InputPower
AverageCurrent, IAVE = -----------------------------------------------------------MinimumInputVoltage
NP
------------- = 7.828
N S 3.3
7.5W
= ------------ = 0.234A
32V
where:
Voltage drop on the diode, VFD1 = 0.7V
RDS(ON) = 4E
Peak current of a Discontinuous mode Flyback
Converter, IPPK is shown in Equation 54.
A quick check of the available standard core structures
indicates that there was a distinct possibility to use a
standard size RM-4 core.
EQUATION 54:
I AVE
I PPK = 2 × ----------- = 1.17A
D
Primary rms current, IRMSPRIM is shown in Equation 55:
An important feature of this core for this design is, it
consists of a core window with nominal 4.3 mm that
clears the 4.0 mm PCB thickness. The overall height of
this core is 7.8 mm so it is <10 mm height of the DC/DC
Converter mechanical height.
The RM-4 core parameters are:
EQUATION 55:
T ON
- = 0.427A
I RMSPRI = I PPK × -------------3 × TP
•
•
•
•
AE = 0.145 cm2
ICORE = 1.73 cm
µ = 2000
VE = 0.25 cm3
EQUATION 56:
V INMIN × D
PrimaryInduc tan ceL P = ----------------------------F SW × I PPK
32 × 0.4
= ---------------------------------- = 43.6μH
250000 × 1.17
DS01335A-page 24
© 2010 Microchip Technology Inc.
AN1335
FIGURE 20:
AUXILIARY PLANAR
TRANSFORMER
EQUATION 61:
9
L P × 10
A L = --------------------2( N PRI )
15.51 mm
(0.61'')
2
1
5
A L = 164.063nH
6
The flux density is calculated as shown in Equation 62.
11.51 mm
(0.45'')
EQUATION 62:
7
4
0.4 × π × N PRI × I PPK
B PK = ----------------------------------------------------L GAP
8
3
3
B PK = 1.959 × 10 Gauss
Inner Layer Top Points
1
*
BPK is lesser than BSAT limitation of 3000 Gauss
at 85ºC. The required maximum output power for DCM
operation, factoring in efficiency is shown in
Equation 63.
5
*
6T
16T
6
2
3
*
7
*
6T
EQUATION 63:
2T
4
1
2
P O = --- × L P × ( I PPK ) × F SW
2
8
The footprint (length x width) of the device is not greater
than that of a stand-alone magnetic device. The
footprint shown above has been further reduced in the
final implementation and the entire bias converter has
been implemented as part of the embedded design.
P O = 7.46W
The peak AC flux density is calculated as shown in
Equation 64:
EQUATION 64:
EQUATION 59:
8
( V IN × T ON ) × 10
B PKAC = --------------------------------------------N PRI × A E
8
V INMIN × T ON × 10
N PRI = -------------------------------------------------- = 16T
B MAX × A E
3
B PKAC = 2.48 × 10 Gauss
where:
BMAX = 2200Gauss
The required center post air gap based on the formula
is shown in Equation 60:
The RMS flux density is calculated as shown in
Equation 65.
EQUATION 65:
EQUATION 60:
2
L GAP
T ON
–8
0.4 × π × ( N PRI ) × A E × 10
= ------------------------------------------------------------------------- × FFF
LP
B RMS =
1----×
TP
∫
O
8 2
( V IN × T ON ) × 10
-------------------------------------------- dTp
2 × N PRI × A E
B RMS = 771.454Gauss
L GAP = 0.012cm
L GAP
L GAPIN = ------------2.54
–3
L GAPIN = 4.591 × 10 in
The core loss equation parameters are used for
Ferroxcube “3C92” material at 40ºC rise in
temperature.
The AL value is calculated as shown in Equation 61.
© 2010 Microchip Technology Inc.
DS01335A-page 25
AN1335
EQUATION 66:
A calculated core loss value of 76 mW is acceptable
and a good reason to use ferrite for the core material.
B RMS
B = --------------10000
1
f = -----TP
The CAD package is used in the PCB trace design to
calculate the trace DCR for the primary and secondary
DC resistance.
• DCRSEC = 0.023E
• DCRPRI = 0.088E
5
f = 2.5 × 10 Hz
°
EQUATION 68:
Temp = 40 C
The operating coefficients are:
CM = 9.17 x 10-5
EQUATION 67:
2
opperLoss = ⎛ I
2
× DCR PRI⎞ + ⎛ I RMSSEC × DCR SEC⎞
⎝ RMSPRI
⎠ ⎝
⎠
CORE LOSS DENSITY
FORMULA
CopperLoss = 0.067W
2
x
y ( C t0 – C t1 ) × Temp + C t2 × Temp
P = C M × f × B × ------------------------------------------------------------------------------------1000
P = 303.063 mW/cm3
The overall loss is shown in Equation 69.
where:
EQUATION 69:
Ct2 = 2.33 * 10-4
Ct1 = 4.72 * 10-2
TotalLoss = CuLoss + CoreLoss
Ct0 = 3.39
TotalLoss = 0.142
x = 2.22
y = 2.46
CoreLoss = P × V OL × 10
CoreLoss = 0.076W
–3
The only efficiency penalty in using a digital controller
is the bias supply converted efficiency of 80%. All
converters will share approximately the same FET
driver loss.
The only further penalty is the footprint or space
occupied by the bias supply within the available outline
package of the converter itself. The main advantage as
discussed at the outset is that the controller is “always
on”, that is, it supplies power in a controlled fashion and
rides out abnormalities and transients that might at the
least require a hiccup start-up for an analog controller.
DS01335A-page 26
© 2010 Microchip Technology Inc.
AN1335
DESIGNING A DIGITAL QUARTER
BRICK CONVERTER
The Quarter Brick DC/DC Converter was designed
using the dsPIC33FJ16GS502. The design analysis is
described in the following sections.
FIGURE 21:
Sensing
Element
REAL WORLD SIGNAL
CHAIN: DIGITAL POWER
SUPPLY
Scaling
Filter
What is a Digitally Controlled Power
Supply?
A digital power supply can be broadly divided into
power control and power management. Power control
is relatively a new trend when compared to power
management.
Power management is data communication,
monitoring, data logging, power supply protection, and
sequencing of the outputs. This is not real time
because the switching frequencies of the converters
are higher than the power management functions.
Power control is defined as the flow of power in the
converter and it is controlled from one PWM cycle to
another PWM cycle. Power control is performed with
both the DSCs and analog controllers without much
variation in the design.
Advantages of DSC
In modern SMPS applications, power conversion is
only part of the total system solution. In addition, many
other requirements and features are required to make
the system more reliable. These features can be
realized using a DSC and are as follows:
• Improved level of portability to other converter
topologies
• Adaptive and predictive control mechanism to
achieve high efficiency and improved dynamic
response
• Software implementation of the protections to
reduce the component count
• Improved scalability
• Active load balancing in the parallel connected
systems
• Improved overall system reliability and stability
• System performance monitoring capability
• Real time algorithms for the regulation of power
converters
• Less susceptibility to parameter variations from
thermal effects and aging
ADC
CMP
Load
Power Converter
Analog Hardware
DSC Core
PWM
Digital Signal Controller (DSC)
DIGITAL PHASE-SHIFTED FULLBRIDGE (PSFB) DESIGN
In the digital power supply design, the power train is
same as the analog power converter design. The
difference exists in the way it is controlled in the digital
domain. The analog signals such as voltage and
current are digitized by using the ADC, and fed to the
DSC. These feedback signals are processed with the
digital compensator and modulate the PWM gate drive
to get the desired control on the output.
Few critical peripherals that are used in digital power
supply are listed below:
• PWM generator
• ADC
• Analog comparator
PWM Generator
The PWM generator must have the ability to generate
high operating frequencies with good resolution,
dynamically control PWM parameters such as duty
cycle, period, and phase, and to synchronously control
all PWMs, fault handling capability, and CPU load
staggering to execute multiple control loops.
The PWM resolution determines the smallest
correction to be done on the PWM time base.
EQUATION 70:
PWMClockFrequency
PWMResolution = ---------------------------------------------------------------DesiredPWMFrequency
EQUATION 71:
PWMClockFrequency
BitResolution = log 2 ⎛ ----------------------------------------------------------------⎞
⎝ DesiredPWMFrequency⎠
© 2010 Microchip Technology Inc.
DS01335A-page 27
AN1335
EXAMPLE 1:
EQUATION 72:
FullScaleVoltageADCResolution = ---------------------------------------------n
2
where:
PWM Clock Frequency = 60 MHz
Desired PWM Frequency = 500 kHz
PWM Resolution = 120 = One part in 120
n = Number of bits in the ADC
Bit Resolution = log2 (120) ~ 7 bits
EXAMPLE 3:
EXAMPLE 2:
Example A:
PWM Clock Frequency = 1000 MHz
ADC full voltage = 3.3V
Desired PWM Frequency = 500 kHz
PWM Resolution = 2000 = One part in 2000
Bit Resolution = log2 (2000) ~ 11 bits
A resolution of 11 bits indicates that the user can
have 2048 different steps from zero to full power of the
converter. This gives finer granularity in control of the
duty cycle when compared to the seven bits resolution
where only 128 steps are available for control.
Analog-to-Digital Converter (ADC)
All the real world feedback signals are continuous
signals, and should be digitized to process in the DSC.
A built-in ADC performs this process. ADC requires a
voltage signal that is to be provided as an input. The
input signals are scaled down to the ADC reference
voltage. These voltages are typically 3.3V and 5V.
FIGURE 22:
ANALOG-TO-DIGITAL
CONVERTER (ADC)
AN1
Inputs
MUX
SAR
Core
Data
Format
ADC
Result
Buffers
ANx
Sample and Hold Circuit
In digital SMPS applications, higher bit resolutions and
higher speed are the two characteristics that determine
the ADC selection.
The ADC resolution indicates the number of discrete
values it can produce over the range of analog values,
hence the resolution is expressed in bits.
DS01335A-page 28
CALCULATING THE ADC
RESOLUTION
Number of bits in an ADC = 10
Therefore, ADC resolution = 3.22mV
Another parameter to be considered is the sample and
conversion time (time taken by ADC to sample an
analog signal and to deliver the equivalent digital
value). Usually, the conversion time is specified in
million samples per second (Msps). For example, if the
conversion time is specified as 2 Msps, the ADC can
convert two million samples in one second. Hence, the
sample and conversion time is 0.5 µs.
The conversion speed plays an important role to
replicate the sampled signal. As per Nyquist criterion,
the sampling frequency must be greater than twice the
bandwidth of the input signal (Nyquist frequency). As a
guideline in SMPS applications, sampling of the analog
signal at a frequency greater than 10x of the signal
bandwidth is required to maintain fidelity.
Analog Comparator
Most of the DSCs consists of an analog comparator as
a built-in peripheral which enhances the performance
of SMPS applications. Analog comparator can be used
in cycle-by-cycle control method to improve the
response time of the converter and also in the fault
protection applications.
ADC and PWM Resolution in SMPS
Applications
Usually, analog controllers provide fine resolution to
position the output voltage. The output voltage can be
adjusted to any arbitrary value, and is only limited by
loop gain and noise levels. However, a DSC consists of
a finite set of discrete levels, because the quantizing
elements, ADC and PWM generator exist in the digital
control loop. Therefore, the quantization of ADC and
PWM generator is critical to both static and dynamic
performance of switched mode power supplies.
© 2010 Microchip Technology Inc.
AN1335
The ADC resolution must be lower than the permitted
output voltage variation to achieve the specified output
voltage regulation. The required ADC resolution is
shown in Equation 73.
EQUATION 73:
⎛ V MAX A ⁄ D V o ⎞
N A ⁄ D = Int log 2 ⎜ ----------------------- × ----------⎟
ΔV o⎠
⎝ V REF
where:
VMAX A/D = ADC full range voltage in this
application
VREF = Reference voltage
The digital PWM produces an integer number of duty
values (it produces a discrete set of output voltage
values). If the desired output value does not belong to
any of these discrete values, the feedback controller
switches among two or more discrete values of the duty
ratio. In digital control system, this is called as limit
cycle and it is not desirable.
Limit cycling can be avoided by selecting the change in
output voltage caused by one LSB change in the duty
ratio has to be smaller than the analog equivalent of the
LSB of ADC. For a buck type forward regulator, NPWM
is shown in Equation 74.
EQUATION 74:
NPWM > = NA/D + log2
NA/D = Number of bits in ADC
VO = Signal to be measured (output voltage)
ΔVO = Allowed output voltage variation
Int [ ] = Denotes taking the upper rounded integer
EXAMPLE 4:
ADC Resolution
Vref
VMAX A/D * D
where:
NPWM = Number of bits in a PWM controller
D = Duty ratio
To generalize, NPWM must be minimum of one bit
more than NA /D.
VMAX A/D = 3.3V
Note:
VO = 12V
Δ VO = 1% of 12V = 120 mV
VREF = 2.6V which is 80% of the ADC full range
voltage
To have a stable output, that is without
limit cycling, the down stream quantizer of
the ADC should have higher resolution.
NA/D = 7, (therefore, a 7-bit ADC can be used)
ADC resolution can also be expressed as follows:
ADC LSB << (VREF/VO) * ΔVO
TABLE 6:
SWITCHING FREQUENCIES OF THE CONVERTER
Signal Name
Description
Type of Signal
dsPIC® DSC
Resource
Frequency of
Operation
PWM1H,PWM1L
Left Leg Gate Drive
PWM Output
PWM1H,PWM1L
150 kHz
PWM2H,PWM2L
Right Leg Gate Drive
PWM Output
PWM2H,PWM2L
150 kHz
Synchronous Rectifier Gate Drive
PWM Output
PWM3H,PWM3L
150 kHz
—
—
75 kHz
PWM3H,PWM3L
—
Control Loop Frequency
© 2010 Microchip Technology Inc.
DS01335A-page 29
AN1335
TABLE 7:
Pin
DSC PERIPHERALS MAPPED
TO PSFB CONVERTER
Peripheral
Description
1
AN2
Load share
2
AN3
Temp
3
CMP2C
Output overvoltage
4
RP10
TX secondary voltage
5
VSS
Ground
6
CMP4A
TX overcurrent
7
RP2
EXT SYNCI1
8
PGD2
Programming
9
PGC2
Programming
10
VDD
Bias supply +ve
11
RB8
COM1
12
RB15
COM2
13
RB5
Remote ON/OFF
14
SCL1
COM4
15
SDA1
COM3
16
VSS
Ground
17
VDDcore
VDD core
18
PWM3H
Sync gate drive
19
PWM3L
Sync gate drive
20
PWM2H
PSFB gate drive
21
PWM2L
PSFB gate drive
22
PWM1H
PSFB gate drive
23
PWM1L
PSFB gate drive
24
AVSS
Ground
25
AVDD
Bias supply +ve
26
MCLR
Master clear
27
AN0
TX current
28
AN1
12V output
DS01335A-page 30
© 2010 Microchip Technology Inc.
AN1335
FIGURE 23:
dsPIC® DSC RESOURCES FOR THE QUARTER BRICK CONVERTER
Full-Bridge Converter
+
Output Voltage
36 VDC – 76 VDC
Synchronous Rectifier
+
12 VDC /17A
CT
-
-
Drive TX
Drive IC
Drive IC
Drive TX
Drive IC
PWM1 PWM2 AN1
Remote ON/OFF
Opto
Isolator
AN0
dsPIC33FJ16GS502
RB5
RB8
PWM3
RB15
SCL1 SDA1
RP2
AN3
AN2
External Communication
External Over
Sync Temp
© 2010 Microchip Technology Inc.
Load
Share
DS01335A-page 31
AN1335
DIGITAL CONTROL SYSTEM DESIGN
Digital Average Current Mode Control
Technique
Digital control system design is a process of selecting
the difference equation or Z-domain transfer function
for the controller to achieve good closed loop response.
Parameters such as settling time, output overshoot,
rise time, control loop frequency and bandwidth must
be considered to achieve acceptable performance.
Digital current mode control is a new approach for
improving the dynamic performance of high frequency
switched mode PWM converters, and is used in this
design. In this method, DSC performs the entire control
strategy in software. The current mode control (CMC)
strategy consists of two control loops. The inner current
loop subtracts a scaled version of the inductor current
from the current reference. The current error is further
processed with the PID or PI compensator and the
result is appropriately converted into duty or phase.
Any dynamic changes in the output load current directly
modifies the duty or phase of the converter. The outer
loop subtracts the scaled output voltage from a
reference and the error is processed using the PID or
PI compensator. The output of the voltage loop
compensator provides the current reference for the
inner loop. Current and voltage compensators allow
tuning of the inner and outer loops to ensure converter
stability and to achieve the desired transient response.
The denominator polynomial of transfer function
provides the roots of the equation. These roots are the
poles of the transfer function. This equation is called
the characteristic equation.
The nature of roots of the characteristic equation
provides an indication of the time response. The
system stability can be determined by finding the roots
of the characteristic equation and its location. The
system is considered to be stable if the roots of the
characteristic equation are located in left half of
the ‘S’ plane. This causes the output response due to
bounded input to decrease to zero as the time
approaches infinity.
In the quarter brick converter design, the controller is
designed in the continuous time domain and then
converted to an equivalent digital controller. This
approach is called digital re-design approach or digital
design through emulation.
FIGURE 24:
AVERAGE CURRENT MODE CONTROL
DCR Compensation
Voltage Loop Compensation
VO*
+
Current Loop Compensation
IL *
VERROR
-
VO
+
+
PI Control
P Control
-
Phase/Duty
VO
Plant
+
IL
VO Decouple
Compensation
ITX
ADC
Sensor
ADC
Sensor
Digital Signal Controller (DSC)
DS01335A-page 32
© 2010 Microchip Technology Inc.
AN1335
Deriving the Characteristic Equation for
the Current Mode Control (CMC)
Let us take a simple buck converter to derive the
characteristic equation.
FIGURE 25:
BUCK CONVERTER
IL
VL
D * VIN
LM
DCR
VX
Buck Inductor
ESR
VIN
IC
Output
Capacitor
IO
VO
Load
C
Based on Figure 25, and applying Kirchhoff's laws
results in the expressions and equations shown in
Equation 75.
EQUATION 75:
(A)
IC = IL – IO
(B)
V O = D × V IN – V L
(C)
VL
VL
VL
VL
I L = ------ = ------------ = ---------- = -----XL
2πfL
JωL
sL
(D)
VO
IC
IC
Ic
= I C × X c = ------------- = ----------- = -----2πfC
JωC
sC
The current compensator proportional gain is denoted
as RA, and it has a dimension of resistance. The value
of RA can be determined from the system characteristic
equation. Higher value of RA implies higher current
loop bandwidth. With the current mode control, the ‘D’
term performance in the voltage PID can be achieved.
EQUATION 76:
VX = VO + VL
The current reference (IL*) is generated using the outer
voltage loop.
[IL* = (VO* - VO) * G] (because current loop performs the
function of differential gain in the voltage loop, the outer
voltage loop will have only proportional and integral
gain).
From the physical capacitor system, IC = IL - IO. In the
equation, IO is made as constant and analyzed the
relation between VO and VO*. Therefore, IL = SCVO.
EQUATION 77:
(F)
KI
( I L )∗ = ( V O∗ – V O ) × ⎛ K P + -----⎞
⎝
s⎠
KI
( Ra + sL )
I L × ------------------------ = [ ( V O∗ ) – V O ] × K P + ⎛ -----⎞
⎝
s⎠
Ra
The Equation 77 is rearranged to find VO*/VO and is
shown in Equation 78.
EQUATION 78:
KI
K P × R A + ⎛⎝ -----⎞⎠
V O∗
S
---------- = ---------------------------------------------------------------------------------------------------------VO
KI
2
s LC + ( sC × R A ) + ( K P × R A ) + ⎛ -----⎞ × R A
⎝ s⎠
V X = V O + sLI L = R A × [ ( I L∗ ) – I L ] + [ V X – sLI L ]
(E)
[ R A × ( I L∗ ) ]
I L = -----------------------------R A + sL
© 2010 Microchip Technology Inc.
DS01335A-page 33
AN1335
The denominator [s2LC + sCRa + KPRa + (KI/s)Ra]
denotes the characteristic equation. The denominator
should have three roots known as three poles or three
bandwidths, f1 > f2 > f3 (units of Hz) of the controller.
These roots correspond to current loop bandwidth (f1),
proportional voltage loop bandwidth (f2) and integral
voltage loop bandwidth (f3). These roots should be
selected based on the system specifications. f1, f2 and
f3 should be separated with a factor minimum of three
between them. This ensures that any parameter
variation (L and C) due to manufacturing tolerance or
inductor saturation will not affect the stability of the
system.
EQUATION 81:
The f3 determines the settling time (TS), that is the
output voltage of the converter takes to settle
within 98% of VO* for a step change in load. Ts should
be selected less than the specification settling time.
Substituting the actual design parameters used in the
PSFB converter to have the KP, KI, RA gains.
TS = 4/2πf3
The f2 determines the ability of the controller to track
changes in VO*. If VO* varies, VO can track VO*
variations up to a frequency f2 Hz.
The f1 exists only to make the system non-oscillatory or
resonant at frequencies greater than f2.
The gains KP, KI and RA can be determined once f1, f2
and f3 are selected. The characteristic equation:
s3LC + s2CRa + s KP Ra + KI Ra = 0 is a cubic
equation.
Because ‘s’ is -2πf1(ω1), -2πf2 (ω2) and -2πf3 (ω3),
which are the roots of the characteristic equation and
should make the equation equal to zero after
substituting for ‘s’. The three unknown coefficients KP,
KI and RA can be obtained by solving the following
three equations shown in Equation 79:
EQUATION 79:
2
3
2
3
ω 1 CR A + ω 1 K P R A + K I R A = – ω 1 LC
ω 2 CR A + ω 2 K P R A + K I R A = – ω 2 LC
2
3
ω 3 CR A + ω 3 K P R A + K I R A = – ω 3 LC
Y1
–1
Y = Y2 = A × B
Y3
Y1 = C RA and RA = Y1/C
Y2 = KP RA and KP = Y2/RA
Y3 = KI RA and KI = Y3/RA
Finding the Gains
• Transformer turns ratio = 5:2
• Primary input voltage, VIN = 76V
• Nominal primary input voltage, VNOM = 48V
The maximum primary input current is selected as
9.75A and is reflected to the secondary because the
controller exists on the secondary side of the isolation
barrier.
The base value of the current INBASE is 24.38A and the
base value of the voltage VNBASE is 14.2V. All the
voltage and current quantities are referenced with the
base values INBASE and VNBASE.
Transformer secondary voltage is:
• VINS = VIN/turns ratio = 30.4V
• Output inductor L = 3.4e-6 Henry
• DC resistance of the inductor and tracks is
considered as DCR = 0.05E
• Output capacitance, C = 4576e-6F (4400 µF
external to converter)
• Equivalent series resistance of the capacitor,
ESR = 0.0012E
• Switching frequency of the converter,
FSW = 150000 Hz
• Control loop frequency TS is 1/2 of the FSW that is:
1
T S = ----------------F SW ⁄ 2
This can be solved by using the matrix method shown
in Equation 80.
• Integral voltage BW, f3 = -1000 * 2 * π
• Proportional voltage BW, f2 = -2000 * 2 * π
• Proportional current loop BW, f1 = -4000 * 2 * π
EQUATION 80:
The characteristic equation is solved using the above
three bandwidths.
ω1
2
ω1 1
ω2
2
ω2
ω3
2
ω3
3
– ω 1 LC
CRa
3
1 × K P R A = – ω 2 LC
KI RA
3
1
– ω 3 LC
• RA = 0.1495
• KP = 57.5037
• KI = 2.0646e + 005
The matrix shown in Equation 80 is made equivalent to
A * Y = B for simplicity purpose.
DS01335A-page 34
© 2010 Microchip Technology Inc.
AN1335
FIGURE 26:
CONTROL LOOP COMPENSATOR DESIGN BLOCK DIAGRAM
Inner Current Loop Compensator
Outer Voltage Loop Compensator
IL * DCR
+
VERROR
+
IREF (IL*)
IERROR
+
Compensator
VO*
VO
Scaling
The gains calculated previously are based on real units
(volts, amps, and so on). The dsPIC DSC consists of a
fixed point processor and the values in the processor
comprise linear relationship with the actual physical
quantities they represent.
The gains calculated are in real units, and cannot be
directly applied to these scaled values (representation
of physical quantities). Therefore, for the consistency
these gains must be scaled.
The scaling feedback section and the prescalar section
provide general concepts of scaling.
The basic idea behind scaling is the quantities that are
to be added or subtracted should have the same scale.
Scaling does not affect the structure of the control
system block diagram. Scaling only affects the software
representation of various quantities used in the
software.
Scaling Feedback
To properly scale the PID gains, it is imperative to
understand the feedback gain calculation. The
feedback can be represented in various formats.
Fractional format (Q15) is a very convenient
representation.
Fractional format allows easy migration of code from
one design to another with different ratings where most
of the changes that exist only in the coefficients and are
defined in the header file.
To use the available 16 bits in the processor, the Q15
format is most convenient as it allows signed
operations and full utilization of the available bits
(maximum resolution). Other formats can also be used,
but resolution is lost in the process. Q15 allows using
the fractional multiply MAC and MPY operation of the
dsPIC DSC effectively.
The feedback signal (typically voltage or current) is
usually from a 10-bit ADC. Based on the potential
divider or amplifier in the feedback circuitry, actual
voltage and current is scaled.
© 2010 Microchip Technology Inc.
Compensator
VX
VL
Phase/Duty
+
IL
+
VO
Typically, the feedback 10-bit value (0 -1023) is brought
to ±32767 range by multiplying with 32. This format is
also known as Q15 format: Q15(m) where -1<m<1 and
is defined as (int) (m * 32767).
These formulae will have some error as 215 = 32768 is
required, but due to finite resolution of 15 bits, only
±32767 is used. From a control perspective, for most
systems these hardly introduce any significant error. In
this format, +32767 correspond to +3.3V and 0
corresponds to 0V.
Prescalar
As most physical quantities are represented as Q15
format for easy multiplication with gains, the gains must
also be represented in fractional format. If the value of
gain (G * VNBASE/INBASE) is between -1 and +1, it can
be easily represented as fractional format.
Multiplication can then be performed using fractional
multiply functions such as MAC or using builtin_mul
functions and shifting appropriately. For example,
z = (__builtin_mulss(x,y) >> 15) results in
z = Q15(fx,fy), where all x, y, and z are in Q15 format
(fx and fy are the fractions that are represented by x
and y).
In many cases, the gain terms are greater than unity.
Because 16-bit fixed point is a limitation, a prescalar
may be used to bring the gain term within the ± range.
In this application, voltage loop proportional gain KP
value is higher than one. Therefore, it is normalized
using the defined current, voltage base values with the
pre scalar 32. For simplifying the calculations, the
voltage integral gain (KI) is also scaled with 32, that
means if a prescalar is used for P term in a control
block, it must also be used for the ‘I’ and ‘D’ term in the
control block since all the terms are added together.
To prevent the number overflows, PID output and ‘I’
output must be saturated to ±32767.
The saturation limits for the PID output must be set at
1/32 of the original ±32767 to account for the prescalar.
Therefore, saturation limits are set at ±1023. Finally,
after saturation, the output must be post scaled by five
to bring it to proper scale again.
DS01335A-page 35
AN1335
Gain Scaling
LOAD SHARING
The voltage compensator input is in voltage
dimensions and the output is in current dimensions, the
voltage loop coefficients dimensions will be in mho
(Siemens).
In the traditional analog controller, regulation of the
converter is achieved by a simple PWM controller, and
load sharing of the converter is achieved by an additional load sharing controller/equivalent amplifier circuit. Recently, high end systems are calling for logging
of converter parameters, which requires a microcontroller to communicate to the external world. Therefore,
each converter needs a PWM controller, a load sharing
controller, and a Microcontroller to meet the desired
specifications.
New value voltage loop proportional gain KP after normalizing and scaling will be (KP * VNBASE)/(INBASE *
prescalar) that is 1.04.
New value voltage loop integral gain, KI after
normalizing and scaling will be KI * TS * VNBASE/
(INBASE * prescalar) = 0.0501.
The current compensator input is in current dimensions
and the output is in voltage dimensions, the current
loop coefficients dimensions will be in Ω.
New value current loop Integral gain, RA after
normalizing is [(RA/VINS) * INBASE] = 0.1495.
A few more contributors for the Phase/Duty control, are
voltage decouple term and DCR compensation term.
These are discussed below.
Because at steady state (VL = 0), the average output of
switching action will be equal to VO. A contribution of
VO can be applied towards VX (the desired voltage at
primary of the transformer).
VO information is available in the software, so the
voltage decouple term can be easily calculated. This
will improve the dynamic performance and make the
design of control system easier. PI output performs
only small changes to correct for load and line
variations and most of the variation in PHASE/DUTY is
contributed by VO.
The voltage decouple term after scaling will be
VNBASE/VINS.
The other parameters that need to be addressed are
the resistance drops in the traces and magnetic winding resistance drop which may cause the current loop
to function less than ideal. The dimension of gain of the
current loop is in ohms. The physical resistance may
interfere with the control action. If this resistance is
known and measured during the design stage, then this
resistance drop in the software can be compensated.
The DC resistance compensation term after scaling will
be (DCR/VINS) * INBASE.
The input quantity should be in fractional format (this
must be ensured in code). Then, the output current
quantity will automatically be in the correct fractional
quantity. This essentially solves the objective of scaling.
The same logic applies to any control block.
In the recent past, cost of the DSCs has reduced drastically and are highly attractive for use by power supply
designers in their applications. Digital controllers are
immune to component variations and have the ability to
execute sophisticated nonlinear control algorithms,
which are not common or unknown in analog controlled
power systems.
Apart from closing the control loop digitally, the DSC
can perform fault management and communicate with
the external applications which is becoming more and
more significant in server applications. Digitally controlled power systems also offer advantages where
very high precision, flexibility and intelligence are
required.
For the overcurrent protection or short circuit protection
of the converters, load current or load equivalent current will be measured and the same will be used for the
load sharing between the converters. Therefore, an
additional circuitry/additional controller is not required
in the case of a digitally controlled power supply compared to its analog counterpart for load sharing. This
reduces overall cost as the component count is lower
and easier to implement by adding a few lines of code
to the stand-alone converter design.
Digital Load Sharing Implementation
Basic operation of the analog and digital load sharing
concept is the same; however, implementation is completely different. In the digital implementation, the ADC
will sample the continuous signals of output voltage
and output current. The sampling frequency of the
output voltage and output current signal is user configurable. The PID compensator design calculations are
performed in the Interrupt Service Routine (ISR) and
are updated based on the control loop frequency.
By considering the input and output units and scale of
each block to be implemented in software, the proper
scaled values can be arrived.
DS01335A-page 36
© 2010 Microchip Technology Inc.
AN1335
In the dual load sharing implementation, for additional
current, error information is added and this combined
data will be given to the PWM module to generate
appropriate phase/duty cycle. The PID compensator
design will be same as the standalone individual
converter. The load sharing compensator depends on
the expected dynamic performance and this depends
on the bandwidth of the current feedback. The current
loop compensator forces the steady state error, (δIL)
between individual converter currents IL1, IL2 and
average current (IAVE) to zero.
load sharing will be done with the single controller and
this results in fewer components, less complexity and
increased reliability. Poor noise immunity is a
disadvantage of this design.
Load sharing loop
be 2πfL = 0.0021
gain,
IKP
will
Load sharing loop integral gain, IKI will be 2πf5
IKI = 0.3356, where f5 (25 Hz) is the zero of the PI.
New value voltage loop proportional gain, IKI after
normalizing and scaling w ill be as shown below:
Typically, temperature is a criteria for stress on the
components and the junction temperature bandwidth is
around 5 ms (about 30 Hz). Therefore, it is sufficient to
use ~500 Hz bandwidth current data and the current
share loop can have a bandwidth of ~100 Hz. Here, the
DSC allows output voltage regulation by designing the
voltage/current loop compensator and load current
sharing by load current loop compensator design.
Effectively, both the output voltage regulation and the
FIGURE 27:
proportional
IKP * INBASE/VNBASE * prescaler2 * 1.25 = 0.0734
New value voltage loop proportional gain, IKI after
normalizing and scaling will be as follows:
IKI * INBASE/VNBASE * prescaler2 * TSLOADSHARE =
0.0092
In this application, the load sharing sampling time
(TS LOADSHARE) is selected as 1 kHz.
SINGLE WIRE LOAD SHARE COMPENSATOR DESIGN BLOCK DIAGRAM
Outer Voltage Loop Compensator
(IL * DCR)
Inner Current Loop Compensator
VERROR
+
VO*
+
IREF(IL*)
+
IERROR
Compensator
VX
Phase/Duty
+
-
VO
+
VL
Compensator
+
IL1
VO
Load Share Loop Compensator
Load Share
IL1
Converter 1
δ IL
Compensator
IAVEBLOCK
(IL1 + IL2)/2
Load Share Loop Compensator
Converter 2
δ IL
Compensator
IL2
Load Share
Outer Voltage Loop Compensator
Inner Current Loop Compensator
+
I REF(I L*)
V ERROR
+
Compensator
VO *
(IL * DCR)
IL2
-
+
-
+
IERROR
Compensator
VL
Vx
+
Phase/Duty
+
VO
VO
© 2010 Microchip Technology Inc.
DS01335A-page 37
AN1335
MATLAB MODELING
The disturbance rejection plot is defined as: I(S)/VO(S).
The .m file is used to generate the coefficients that are
used in the MATLAB model (.mdl). This file also
generates the scaled values to be used in the software.
The generated values are in fractional format. In
software, the coefficients must be represented as
Q15(x), where ‘x’ is a fractional value.
For more detailed calculations, refer to the MATLAB
(.m) file in the PSFB_MATLAB file. For the MATLAB
Simulink block diagram, refer to the MATLAB (.mdl)
file.
The transfer function IO(S)/VO(S) (with VO*(S) = 0) is
called as dynamic stiffness or disturbance rejection.
This plot explains us for a unit amplitude distortion in
VO, the amount of load needed as a function of
frequency. The system needs to be as robust as
possible so that the output does not change under load.
The higher this absolute figure of merit, the
stiffer (better) the power supply output will be. The
minimum is 35 db in this application, which will
correlate to 56A (20logI = 35 dB) at approximately 1300
Hz of load producing 1.0V ripple on the output voltage.
The following Bode plots (Figure 29 through Figure 31)
are generated from the MATLAB (.m) file. Each plot is
used to describe the behavior of the system.
FIGURE 28:
MATLAB® DIGITAL IMPLEMENTATION FOR THE PSFB CONVERTER
(FROM MATLAB FILE)
Quantizer2
Quantizer1
VO
IL
12
VO *
Phifactor
VO*
VIN
Control System
Zero-Order
Hold2
Zero-Order
Hold1
Phifactor
VIN_PHIFACTOR
LC Voltage 1
IL1
VIN
IL1
VO1
VO2
ZVT Modulation
L1
13.6
iLoad
C
VIN
Scope1
Pulse Generator
DS01335A-page 38
© 2010 Microchip Technology Inc.
AN1335
FIGURE 29:
DISTURBANCE REJECTION PLOT
© 2010 Microchip Technology Inc.
DS01335A-page 39
AN1335
The loop gain voltage plot illustrated in Figure 30 is
used to calculate the phase and gain margin. In the
plot, the phase margin (difference between 180º and
the phase angle where the gain curve crosses 0 db) is
50º. To prevent the system from being conditionally
unstable, it is imperative that the gain plot drops below
0 db when the phase reaches 180º.
FIGURE 30:
DS01335A-page 40
The blue curve is for the analog implementation and
the green curve is for the digital implementation.
It is generally recommended to have a phase margin of
at least 40º to allow for parameter variations. The gain
margin is the difference between gain curve at 0 db and
where the phase curve hits 180º. The gain margin
(where the green line on the phase plot reaches 180º)
is -20 db.
LOOP GAIN PLOT
© 2010 Microchip Technology Inc.
AN1335
Figure 31 illustrates the closed loop Bode plot. The
point where the gain crosses -3 db or -45º in phase is
usually denoted as the bandwidth. In this system, the
bandwidth
of
the
voltage
loop
is
approximately 2700 Hz (17000 rad/s), which is closely
matched by the Bode plot.
FIGURE 31:
CLOSED LOOP PLOT
© 2010 Microchip Technology Inc.
DS01335A-page 41
AN1335
SOFTWARE IMPLEMENTATION
The Quarter Brick DC/DC Converter is controlled using
the dsPIC33FJ16GS502 device. This device controls
the power flow in the converter, fault protection, soft
start, remote ON/OFF functionality, external
communication, adaptive control for the synchronous
MOSFET’s and single wire load sharing.
Note:
For more information on this device, refer
to the “dsPIC33FJ06GS101/X02 and
dsPIC33FJ16GSX02/X04 Data Sheet”
(DS70318).
For information on the peripherals, refer to
Section
43. “High-Speed
PWM”
(DS70323), Section 44. “High-Speed 10Bit Analog-to-Digital Converter (ADC)”
(DS70321), and Section 45. “HighSpeed Analog Comparator” (DS70296)
in
the
“dsPIC33F/PIC24H
Family
Reference Manual”.
These documents are available from the
MIcrochip website (www.microchip.com).
Init_CMC.c
Functions present in this file are:
init_PSFBDrive ()
Configure the primary MOSFET’s PWM module.
init_SYNCRECTDrive ()
Configure the synchronous MOSFET’s PWM
module.
init_ADC()
Configure the ADC module.
InitRemoteON_OFF()
Configure the System state for remote ON/OFF
functionality.
init_Timer1()
Configure Timer1.
Variables_CMC.c
Description of Software Functional
Blocks
Declarations and Initialization of all the global
variables.
The source files and header files describe the functions
used in the software.
Compensator_CMC.c
Source Files
DigitalCompensator(void)
Main_CMC.c
Function to execute the voltage PI compensator
and current P compensator.
LoadshareCompensator(void)
Functions present in this file are:
main()
Configures the operating frequency of the
device.
Function to execute
compensator.
the
load
share
PI
delay.s
Configures the auxiliary clock module.
Calls functions for configuring GPIO, ADC and
PWM modules.
_Delay to get ms delay.
_Delay_Us to get µs delay.
Checks for fault status.
ADCP1Interrupt()
Read values of currents and voltages.
Check for any fault condition.
If fault does not exist, execute the control loop.
If fault exists, disable PWM outputs.
INT1Interrupt()
Remote ON/OFF functionality.
T1Interrupt()
Averaging the PID output.
Over current limit selection.
Over temperature fault.
DS01335A-page 42
© 2010 Microchip Technology Inc.
AN1335
Header Files
dsp.h
Define_CMC.h
Standard library file for all DSP related operations.
This file has all the global function prototype definitions
and global parameter definitions.
delay.h
This is the file where all the modifications must be done
based on the requirements of hardware components,
power level, control loop bandwidth and other
parameters. They are given below for reference.
Presentable delay definition in ms and µs.
Variables_CMC.h
Supporting file for Variables_CMC.c and contains all
the external global definitions.
FIGURE 32:
SOFTWARE FLOW CONTROL CMC WITH LOAD SHARING
Wait for A/D
Interrupt
Initialization
Soft Start
Reset
Voltage PI
Compensator
VO
IREF
Phase +
ΔPhase
PWM
Phase
Load Sharing
PI
Compensator
ΔPhase
ISHARE
© 2010 Microchip Technology Inc.
Current P
Compensator
IPSFB
IPSFB
DS01335A-page 43
AN1335
Digital Nonlinear Implementations
DSCs allow implementing customized configurations to
gain performance improvements of the SMPS.
Adaptive Control to Improve the
Efficiency
Achieving ultra high efficiency specifications in power
supply designs require unique configuration of PWM.
This can be achieved by using external hardware or
with software in digital controllers. In the PSFB
converter, the software is designed to get the efficiency
benefit at higher specified input voltages.
Most of the DC/DC converters (part of AC/DC
converter/Brick DC/DC converter) are designed using
the isolation transformer for user safety and is also
imposed by regulatory bodies. These power supplies
are designed primary with push-pull, half-bridge, fullbridge and PSFB, in the secondary with synchronous
MOSFET configurations to gain high efficiency.
To avoid cross conduction, there will be a defined dead
band and during this period neither of the synchronous
MOSFET’s conduct so, the current will take the path of
MOSFET body diode. These MOSFET body diodes
has high forward drop compared to the RDS(ON) of the
MOSFET, that is, VF * I >> IRMS2 * RDS(ON). Therefore,
the losses are higher and the efficiency is less.
DS01335A-page 44
© 2010 Microchip Technology Inc.
AN1335
FIGURE 33:
FULL-BRIDGE CONVERTER WITH CONVENTIONAL SYNCHRONOUS MOSFET
GATE DRIVES
Q1
Q3
Q6
TX
LO
TXVPRI
Q2
Q4
CO
Q5
Q1 / Q5
Q2 /Q 6
Q3
Q4
VPRI
t
During this period, there will be circulating currents in the Primary side MOSFET’s.
These circulating currents are prominent at higher input voltages
© 2010 Microchip Technology Inc.
DS01335A-page 45
AN1335
These problems can be overcome by unique
configuration of PWM gate drive of the synchronous
MOSFETs.
To control the output voltage of the converter with
variation of input voltage, the duty cycle/phase is
controlled. At high input voltages, the energy transfer
from primary side to secondary side will be in small
portions of the total period (zero states will exist). Due
to the presence of inductors in secondary side of the
converter, current continues to flow through the
transformer coils through the MOSFET’s channel or
through MOSFET body diodes. Due to reflection of
current from secondary to primary, there will be a
circulating current during the zero states in the primary
and particularly this will be predominant at higher input
voltage than the nominal input voltages of the input
voltage range.
Losses occurring during zero state of the primary side
of the transformer can be avoided by overlapping the
PWM gate drive of the synchronous MOSFETs. This
method solves the problems which cause losses during
zero states of the transformer.
DS01335A-page 46
© 2010 Microchip Technology Inc.
AN1335
FIGURE 34:
FULL-BRIDGE CONVERTER WITH OVERLAP OF SYNCHRONOUS MOSFET GATE
DRIVES
Q1
Q3
Q6
TX
L
LO
TXVPRI
Q4
Q2
CO
Q5
Q1
Q2
Q3
Q4
VPRI
Q5
Q6
t
Zero
States
© 2010 Microchip Technology Inc.
DS01335A-page 47
AN1335
MOSFET body diode conduction in the primary side of
the transformer is stopped so there are no reflected
currents from the secondary side. The secondary side
coils conduct in a way that there are no circulating
currents in the primary side, effectively cancellation of
currents. If a center tapped configuration is used in the
secondary side of the transformer, the two coils cancel
the flux and no flux is linked to the primary side
because of the cancellation of currents. In case of
“synchronous current doubler configuration” in the
secondary side, both the synchronous MOSFETS are
ON and the current does not pass in secondary side
coil of the transformer, and therefore there is no
reflected current in the primary side of the converter.
This drastically reduces the circulating current losses in
primary side body diodes of the MOSFETs.
• In the case of center tapped transformer
secondary configuration, instead of one
synchronous MOSFET and one coil of the center
tapped transformer, two synchronous MOSFETS
and two transformer coils conduct simultaneously.
Therefore, the secondary current will have only
half the effective resistance, and the losses are
reduced by half compared to when only one
synchronous MOSFET is ON.
• In the conventional switching methodology,
intentional dead time is introduced between the
two synchronous MOSFETS and typically this
may be 10% of switching period based on the
designs. During this dead time, the high
secondary current flows through the high forward
drop body MOSFET and cause losses. By
configuring the overlap of the PWM gate drive of
the synchronous MOSFET, the high secondary
currents flow through the channel of the MOSFET.
In this instance there will be only RDS(ON) losses
that are very less compared to the losses incurred
by the MOSFET body diodes in the dead time.
Overcurrent Protection Implementation
A current transformer is located in the primary side of
the converter and the output of the current transformer
also varies with the line conditions. To have the specific
current limit across the line voltages, the compensator
final output is averaged over a period of 10 ms. The
compensator final output provides the line voltage
variation data. This data is used as a modifier to
change the current limit setting.
DS01335A-page 48
PRINTED CIRCUIT BOARD (PCB)
In the Quarter Brick DC/DC Converter design, an 18layer PCB is used to achieve the standard quarter
brick dimensions. The PCB tracks routing is a challenging task in the quarter brick converter design. The
PCB layers are described in Table 8.
TABLE 8:
PCB
Layer
Stacking of PCB Layers
PCB Layer Description
1
Top layer traces, magnetic winding and
component assembly.
2
Analog GND, magnetics and primary,
and secondary side Cu pours.
3
4
5
6
Analog GND, +3.3V, magnetics and
primary, and secondary side Cu pours.
7
Analog GND, gate drive traces,
magnetics and primary, and secondary
side Cu pours.
8
Analog GND, magnetics and primary,
and secondary side Cu pours.
9
10
11
Analog GND, DIG GND, magnetics and
primary, and secondary side Cu pours.
12
Analog GND, DIG GND, gate drive
traces, magnetics and primary, and
secondary side Cu pours.
13
Analog GND, DIG GND, magnetics and
primary, and secondary side Cu pours.
14
Analog GND, DIG GND, gate drive
traces, magnetics and primary, and
secondary side Cu pours.
15
Analog GND, DIG GND, magnetics and
primary, and secondary side Cu pours.
16
Analog GND, DIG GND, magnetics and
primary, and secondary side Cu pours.
17
Digital GND and signal traces, magnetics
and primary, and secondary side Cu
pours.
18
Bottom layer traces, magnetic winding
and component assembly.
© 2010 Microchip Technology Inc.
AN1335
LABORATORY TEST RESULTS AND
CIRCUIT SCHEMATICS
The Laboratory test results provide an overview of the
quarter brick PSFB electrical specifications as well as
the scope plots from initial test results. The test results
are illustrated in Figure 35 to Figure 65.
FIGURE 35:
OUTPUT VOLTAGE RIPPLE: 8.5A AT 75V
FIGURE 36:
OUTPUT VOLTAGE RIPPLE: 17A AT 75V
© 2010 Microchip Technology Inc.
DS01335A-page 49
AN1335
FIGURE 37:
OUTPUT VOLTAGE RIPPLE: 0A AT 75V
FIGURE 38:
OUTPUT VOLTAGE RIPPLE: 8.5A AT 48V
DS01335A-page 50
© 2010 Microchip Technology Inc.
AN1335
FIGURE 39:
OUTPUT VOLTAGE RIPPLE: 17A AT 48V
FIGURE 40:
OUTPUT VOLTAGE RIPPLE: 0A AT 75V
© 2010 Microchip Technology Inc.
DS01335A-page 51
AN1335
FIGURE 41:
OUTPUT VOLTAGE RIPPLE: 8.5A AT 36V
FIGURE 42:
OUTPUT VOLTAGE RIPPLE: 17A AT 36V
DS01335A-page 52
© 2010 Microchip Technology Inc.
AN1335
FIGURE 43:
OUTPUT VOLTAGE TRANSIENT: 4.25A, 12.75A AT 48V
FIGURE 44:
OUTPUT VOLTAGE TRANSIENT: 4.25A, 12.75A AT 75V
© 2010 Microchip Technology Inc.
DS01335A-page 53
AN1335
FIGURE 45:
OUTPUT VOLTAGE TRANSIENT: 4.25A, 12.75A AT 36V
FIGURE 46:
START-UP TIME: 8.5A AT 53V
DS01335A-page 54
© 2010 Microchip Technology Inc.
AN1335
FIGURE 47:
OUTPUT VOLTAGE RAMP UP TIME: 17A AT 53V
FIGURE 48:
OUTPUT VOLTAGE RIPPLE: 8.5A AT 53V
© 2010 Microchip Technology Inc.
DS01335A-page 55
AN1335
FIGURE 49:
OUTPUT VOLTAGE OVERSHOOT: 8.5A AT 53V
FIGURE 50:
REMOTE ON/OFF, OUTPUT VOLTAGE RISE TIME: 17A AT 53V
DS01335A-page 56
© 2010 Microchip Technology Inc.
AN1335
FIGURE 51:
REMOTE ON/OFF, OUTPUT VOLTAGE FALL TIME: 17A AT 53V
FIGURE 52:
REMOTE ON/OFF, OUTPUT VOLTAGE FALL TIME: 0A AT 53V
© 2010 Microchip Technology Inc.
DS01335A-page 57
AN1335
FIGURE 53:
PRIMARY TX AND SYNCHRONOUS FET GATE WAVEFORMS: 48V/8.5A
FIGURE 54:
PRIMARY TX AND SYNCHRONOUS FET GATE WAVEFORMS: 48V/17A
DS01335A-page 58
© 2010 Microchip Technology Inc.
AN1335
FIGURE 55:
PRIMARY TX AND SYNCHRONOUS FET GATE WAVEFORMS: 48V/0A
FIGURE 56:
PRIMARY TX AND SYNCHRONOUS FET GATE WAVEFORMS: 76V/8.5A
© 2010 Microchip Technology Inc.
DS01335A-page 59
AN1335
FIGURE 57:
PRIMARY TX AND SYNCHRONOUS FET GATE WAVEFORMS: 76V/17A
FIGURE 58:
PRIMARY TX AND SYNCHRONOUS FET GATE WAVEFORMS: 76V/0A
DS01335A-page 60
© 2010 Microchip Technology Inc.
AN1335
FIGURE 59:
PRIMARY TX AND SYNCHRONOUS FET GATE WAVEFORMS: 36V/8.5A
FIGURE 60:
PRIMARY TX AND SYNCHRONOUS FET GATE WAVEFORMS: 36V/17A
© 2010 Microchip Technology Inc.
DS01335A-page 61
AN1335
FIGURE 61:
PRIMARY TX AND SYNCHRONOUS FET GATE WAVEFORMS: 36V/0A
FIGURE 62:
SYNCHRONOUS MOSFET GATE AND DRAIN WAVEFORM: 48V/17A
DS01335A-page 62
© 2010 Microchip Technology Inc.
AN1335
FIGURE 63:
SYNCHRONOUS MOSFET GATE AND DRAIN WAVEFORM: 76V/17A
FIGURE 64:
PRIMARY MOSFET GATE AND DRAIN WAVEFORM: 48V/17A
© 2010 Microchip Technology Inc.
DS01335A-page 63
AN1335
FIGURE 65:
DS01335A-page 64
PRIMARY MOSFET GATE AND DRAIN WAVEFORM: 76V/17A
© 2010 Microchip Technology Inc.
AN1335
FIGURE 66:
LOOP GAIN PLOT: 36V AND 12V/9A
Phase Margin: 61.430
Gain Margin:-7.53 dB
Crossover frequency: 2.17 kHz
© 2010 Microchip Technology Inc.
DS01335A-page 65
AN1335
FIGURE 67:
LOOP GAIN PLOT: 48V AND 12V/9A
Phase Margin: 59.800
Gain Margin:-6.508 dB
Crossover frequency: 2.67 kHz
DS01335A-page 66
© 2010 Microchip Technology Inc.
AN1335
FIGURE 68:
LOOP GAIN PLOT: 76V AND 12V/9A
Phase Margin: 53.080
Gain Margin:-3.60 dB
Crossover frequency: 3.57 kHz
© 2010 Microchip Technology Inc.
DS01335A-page 67
AN1335
CONCLUSION
REFERENCES
This application note presents the design of a PSFB
Quarter Brick DC/DC Converter through the average
current mode control using a Microchip dsPIC “GS”
family Digital Signal Controller (DSC). Various
nonlinear techniques implemented in this design
explore the benefits of DSCs in Switched Mode Power
Converter applications.
The following resources are available from Microchip
Technology Inc., and describe the use of dsPIC DSC
devices for power conversion applications:
Microchip has various resources to assist you in
developing this integrated application. For more details
on the PSFB Quarter Brick DC/DC Converter
Reference Design using a dsPIC DSC, please contact
your local Microchip sales office.
DS01335A-page 68
• “dsPIC33FJ06GS101/X02 and
dsPIC33FJ16GSX02/X04 Data Sheet” (DS70318)
• Dedicated Switch Mode Power Supply (SMPS)
Web site: http://www.microchip.com/SMPS
In addition, the following resource was used in the
development of this application note:
“Design and Implementation of a Digital PWM
Controller for a High-Frequency Switching DC-DC
Power Converter”. Aleksandar Prodic, Dragan
Maksimovic and Robert W. Erickson
© 2010 Microchip Technology Inc.
AN1335
APPENDIX A:
SOURCE CODE
Software License Agreement
The software supplied herewith by Microchip Technology Incorporated (the “Company”) is intended and supplied to you, the
Company’s customer, for use solely and exclusively with products manufactured by the Company.
The software is owned by the Company and/or its supplier, and is protected under applicable copyright laws. All rights are reserved.
Any use in violation of the foregoing restrictions may subject the user to criminal sanctions under applicable laws, as well as to civil
liability for the breach of the terms and conditions of this license.
THIS SOFTWARE IS PROVIDED IN AN “AS IS” CONDITION. NO WARRANTIES, WHETHER EXPRESS, IMPLIED OR STATUTORY, INCLUDING, BUT NOT LIMITED TO, IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE APPLY TO THIS SOFTWARE. THE COMPANY SHALL NOT, IN ANY CIRCUMSTANCES, BE LIABLE FOR
SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES, FOR ANY REASON WHATSOEVER.
All of the software covered in this application note is
available as a single WinZip archive file. This archive
can be downloaded from the Microchip corporate Web
site at:
www.microchip.com
© 2010 Microchip Technology Inc.
DS01335A-page 69
FIGURE B-1:
PSFB QUARTER BRICK DC/DC CONVERTER BOARD LAYOUT AND SCHEMATICS
PSFB Quarter Brick DC/DC Converter Board Layout (Bottom View)
Output Inductor (L2)
Main Transformer (TX1)
DT1
VO +ve
VIN +ve
U5
Q2
Q5
Remote ON/OFF
© 2010 Microchip Technology Inc.
VO -ve
VIN -ve
Q6
3.3V Regulator (U8)
Auxiliary Transformer (TX3)
U7
Auxiliary Controller (U9)
DT2
dsPIC33FJ16GS502 (U1)
Note:
This view lists a few key components. Refer to the Bottom Silk drawing in Figure B-2, which lists all board components.
AN1335
DS01335A-page 70
APPENDIX B:
© 2010 Microchip Technology Inc.
FIGURE B-2:
PSFB Quarter Brick DC/DC Converter Board Layout (Bottom Silk)
C47
C10
DT1
Q1
L2
C11
TX1
R5
C9
R1
C1
D1
R47
C40
DS01335A-page 71
C24
D15
D16
R59
C37
R61
DT2
AN1335
RS7
C23
C27
TX3
C41
R40
C25
C36
R43
C20
U9
C3
R62
R80
C14
R6
C31
D18
R58
L4
U1
R55
C32
L3
U8
R54
R76
C26
R48
C30
R77
C46
C13
U7
R74
C22
R60
C39
C28
C35
R81
C48
R46
C21
D2
R33
R41
U5
R35
C19
R73
Q5
R2
R44
C29
Q2
Q6
C7
PSFB Quarter Brick DC/DC Converter Board Layout (Top View)
Q3
TX1
Q14
Q13
AN1335
DS01335A-page 72
FIGURE B-3:
L2
DT1
Q4
J4
TX2
U4
© 2010 Microchip Technology Inc.
U6
DT2
Note:
TX3
U3
U2
This view lists a few key components. Refer to the Top Silk drawing in Figure B-4, which lists all board components.
J1
© 2010 Microchip Technology Inc.
FIGURE B-4:
PSFB Quarter Brick DC/DC Converter Board Layout (Top Silk)
C4
M
DT1
Q3
C5
L2
C6
C8
R3
R11
R50
7
6
C18
R75
C42
C44
D7
R52
R39
R12
TX2
D8
R8
D4
R13
R4
15
14
R10
J4
R51
Q13
Q14
R49
Q4
D3
C2
R7
TX1
R28
C15
C12
R18
R15
R14
R20
R19
R16
R78
R79
C43
R69
R64
U6
R65
R66
DS01335A-page 73
J1
C38
R63
R68
AN1335
D14
R17
C45
C16
TX3
C33
R56
R22
U2
R21
C34
DT2
D17
R23
R53
C17
U4
U3
PSFB Quarter Brick DC/DC Converter Board Dimensions
AN1335
DS01335A-page 74
FIGURE B-5:
VO+
VIN+
ON/OFF
VIN-
VO-
© 2010 Microchip Technology Inc.
© 2010 Microchip Technology Inc.
FIGURE B-6:
PSFB Quarter Brick DC/DC Converter Schematic (Sheet 1 of 4)
+12V
R23
10
U2
1
2
3
4
PWM1H
PWM1L
R75
10K
NC
IN/A
GND
IN/B
8
NC1 7
OUT/A 6
VDD 5
OUT/B
+12V
C42
.1 µF
R28
10
DT1
1
5
4
MCP1404-SO8
R74
10K
GATE1
U3
1
2
3
4
PWM2H
6
7
S1
PWM2L
NC
IN/A
GND
IN/B
R77
10K
C16
2.2 µF
8
8
7
6
5
NC1
OUT/A
VDD
OUT/B
DT2
1
5
4
6
7
MCP1404-SO8
GATE3
S3
C17
2.2 µF
R76
10K
GATE2
C44
.1 µF
8
GATE4
ANA_GND
ANA_GND
SECY side components. Require 2250 VDC isolation.
SECY side components. Require 2250 VDC isolation.
+12V
R39
10
1
2
3
4
PWM3H
PWM3L
R79
10K
NC
IN/A
GND
IN/B
8
7
6
5
NC1
OUT/A
VDD
OUT/B
GATE5
TEMP
U5
5
NC1
NC
VSS
4
VDD
VOUT
MCP9700-SC70
GATE6
MCP1404-SO8
+3.3V_ANA
C19
2.2 µF
ANA_GND
C18
2.2 µF
ANA_GND
ANA_GND
AN1335
DS01335A-page 75
R78
10K
U4
1
2
3
PSFB Quarter Brick DC/DC Converter Schematic (Sheet 2 of 4)
Pin 1
AN1335
4.7
S1
R3
GATE1
5
4.7
GATE6
R11
HAT2173H
10K
4
CT
5
5
3
2
1
R7
10K
Pin 5
9
4.7
VO+
GATE3
S3
C2
L2a
11
C4
3.5 µH
22 µF
C6
C7
C8
22 µF
22 µF
22 µF
C5
22 5F
C9
22 µF
C10
C11
22 µF
22 µF
R2
R4
4.7
8
7
5
VAux
eSMP
D18
TX2
Pin 4
L2b
C45
47 µF
R13
10K
3
2
1
GATE4
4
HAT2173H
ANA_GND
R12
5
5
Q6
DIG_GND
4.7
GATE5
4
3
2
1
Pin 3
R73
0.0
1
3
10K
INPUT VOLTAGEGATE2
VO-
1
2
3
R8
1
2
3
R6
10K
4.7
D4 BAT54-XV
1
2
Q4
HAT2173H
4
D2 BAT54-XZ
2
1
Q2
HAT2173H
5
2.2 µF
4
2.2 µF
R10
Q13
L1
C1
HAT2173H
7
4
1
TX1
3
2
1
R1
1
BAT54-XV
1
2
3
R5
10K
BAT54-XV
D3
2
1
2
3
1
Q3
HAT2173H
4
D1
2
5
Q1
HAT2173H
5
Q5
INPUT VOLTAGE+
4
DS01335A-page 76
FIGURE B-7:
Q14
HAT2173H
R80
Pin 2
470 pF
C46
150K
CT
REMOTE ON/OFF-I/P
D7
BAS40
3
3
R81 4.99K
D8
BAS40
+3.3V_ANA
1
8
R69
LOAD SHARE
47
C43
.1 µF
U6B
MCP6022-TSSOP
+
7
-
5
R63 620
ANA_GND
R15 100
R68
2K
C38
470 pF
R16 620
8
3
R14
5.6
ANA_GND
2.2 µF
1
2
100
6
4
R66
2
R18
2K
C12
470 pF
+
2
+3.3V_ANA
U6A
MCP6022-TSSOP
R20 47
1
TX CURRENT
C14
-
.1 µF
© 2010 Microchip Technology Inc.
4
ANA_GND
R21
4.7K
TX OVER CURRENT
ANA_GND
ANA_GND
ANA_GND
R64
R65
2K
620
ANA_GND
ANA_GND
R17 620
C15
470 pF
R22
4.7K
R19
2K
ANA_GND
ANA_GND
ANA_GND
ANA_GND
DIG_GND
VSecy
C13
PSFB Quarter Brick DC/DC Converter Schematic (Sheet 3 of 4)
R33 10
VO+
+3.3V_DIG
REMOTE+
MCLR
R48 4.7K
R44
5.23K
1
2.2uF
C22 2.2
+3.3V_ANA
2
OUTPUT FEEDBACK
R46
1.5K
C21
2200pF
2200
7.5K
OUTPUT OVERVOLTAGE
C20
TX OVER CURRENT
EXTSYNCI1
1
2
3
4
5
6
7
AN2
AN3
CMP2C
RP10
VSS
CMP4a
RP2
470pF
8
9
10
11
12
13
14
dsPIC33FJ16GS502-QFN28
DIG_GND
VSS
PGD
5
PGC
N/C
ICSP_6_HDR
PWM2L
PWM2H
PWM3L
PWM3H
VDDCORE
VSS
SDA1
21
20
19
18
17
16
15
PWM2L
PWM2H
PWM3L
PWM3H
C23
2.2 µF
PGD2
PGC2
DIG_GND
VDD
6
GND
LOAD SHARE
VSecy
OUTPUT OVERVOLTAGE
TEMP
R41
AN1
AN0
MCLR
AVdd
AVss
PWM1L
PWM1H
U1
VO+
PGC2
28
27
26
25
24
23
22
R35 10
DIG_GND
PWM1L
PWM1H
MCLR
TX CURRENT
OUTPUT FEEDBACK
J1
MCLR
29
VO-
1K
4
PGD2
REMOTE-
R43
3
C26
.1 µF
ANA_GND
PGD2
PGC2
Vdd
RB8
RB15
RB5
SCL1
© 2010 Microchip Technology Inc.
FIGURE B-8:
DIG_GND
COM3
COM4
REMOTE ON/OFF
COM2
COM1
+3.3V_DIG
REMOTE ON/OFF-I/P
C24
2.2 µF
R62
DIG_GND
+3.3V_ANA
1.6K
4
1
+3.3V_DIG
REMOTE ON/OFF
3
2
U7
2801
R49
R47
C25
470 pF
6.8K
DIG_GND
R50
4.99K
COM3
DIG_GND
COM4
J4.1
R51 220
DS01335A-page 77
R52
COM1
14
15
COM2
EXTSYNCI1
12
13
DIG_GND
10
11
8
9
6
7
220
LOAD SHARE
REMOTE+
REMOTE-
AN1335
PRI side components.
Require 2250 VDC Isolation.
4.99K
PSFB Quarter Brick DC/DC Converter Schematic (Sheet 4 of 4)
AN1335
DS01335A-page 78
FIGURE B-9:
VAux
+12V
D15
2
TX3
5
eSMP
L4
C41
XPL2010
C30
47 µF
47 µF
INPUT VOLTAGE +
C33
2200 pF
6
7
4
8
R56
1K
1 MB
2
U8
2
D17
MURA110
eSMP
C3
15uF
6
VIN
VEN
47
2
4
7
D16
1
R57
+3.3V_DIG
ANA_GND
D14
GND
GND1
R53
1
3
VOUT
VOUT-SEN
BYPASS
LP3961-LLP6
10000 pF
1
+3.3V_ANA
L3
1
3
5
XPL2010
C40
C29
C39 15 µF
15uF
BAT54-XV
C31
DIG_GND
R54
10000 pF
R58 10
R59 4.7K
R60 1.6K
45.3K
8
7
6
5
VDRAIN VSS
VCC
CT
UV
FB
OV
COMP
1
2
3
4
+3.3V_ANA
C35
NCP1031-SO8
U9
C32
10000 pF
R55
C36 680pF
C34
34K
2.2 µF
C27
680 pF
C37
10000 pF
R40 0.0
R61
10K
DIG_GND
INPUT VOLTAGE -
2.2uF
ANA_GND
ANA_GND
C28
15 µF
© 2010 Microchip Technology Inc.
AN1335
TABLE B-1:
PSFB Quarter Brick DC/DC Converter Pin Out Details
Pin Number
Pin Designation
Function
1
VIN+
Input Voltage Plus
2
Remote
ON/OFF
Remote ON/OFF
3
VIN-
Input Voltage Minus
4
V0-
Output Voltage Minus
5
V0+
Output Voltage Plus
J4-6
Remote+
Remote Sense Plus
J4-7
Remote-
Remote Sense Minus
J4-8
Load Share
J4-9
NC
J4-10
COM 4
J4-11
COM 3
J4-12
EXTSYNCI 1
J4-13
DIG_GND
Single Wire Load Share
Not Connected
Serial Clock Input/Output
Serial Data Input/Output
External Synchronization Signal
Digital Ground
J4-14
COM 1
PORTB - 8
J4-15
COM 2
PORTB - 15
J1-1
MCLR
Master Clear
J1-2
+3.3V
J1-3
DIG_GND
J1-4
PGD2
Data I/O Pin for Programming/Debugging
J1-5
PGC2
Clock Input Pin for Programming/Debugging
© 2010 Microchip Technology Inc.
Supply
Digital Ground
DS01335A-page 79
BASE BOARD SCHEMATIC
P1
5
4
1
N/C
VO-
5
TP7
TP8
White
White
Fan circuitry
U2
7
1
8
C17
0.1 µF
L1
D2
50SQ100
ON/OFF
3
FB
GND
R4
139K
5
SGND
C15
68 µF
LX
6
R3
1M
BST
VIN
VD
220 µH
C16
15 µF
4
2
MAX5035
J6
1
2
Fan Header
3
C18
0.1 µF
4
© 2010 Microchip Technology Inc.
J3
Auxiliary Fan Input
Input Power Selection for Fan
S1
1
1
2
J7
3
1
3
4
5
2
2
4
C14
22 µF
Black
TP2
White
C13
22 µF
2
J2
GND
REMOTE-
COM4
COM3
COM2
C12
100 µF
1
20
Black
14
COM1
11
TP3
VI-
13
3
C11
47 µF
3
ON/OFF
C9
2200 µF
2
9
10
+3.3V
MCLR
6
7
8
TP6
C8
2200 µF
REMOTE+
2
D1
3.3V
TP1
4
DC-DC
LOADSHARE
TP5
VO+
19
C7
5
4
PGC
Orange
C6
17
C5
Red
18
C4
EXTSYNCI1
C3
16
C2
1
3
J1
P2
2
2
C1
180 µF/100V
12
1
2.2 µF/100V
C2-C7
VI+
PGD
U1
1
GND
Red
TP4
15
FIGURE C-1:
BASE BOARD SCHEMATIC AND LAYOUT
AN1335
DS01335A-page 80
APPENDIX C:
6
J4
MCLR
J5
1
1
2
VDD
2
3
VSS
3
4
PGD
4
PGC
N.C.
RJ-11
5
6
PMBUS
TP9
White
© 2010 Microchip Technology Inc.
FIGURE C-2:
BASE BOARD LAYOUT (TOP VIEW)
R1
C7
C8 C5
C4 C3 C2
R5
D1
C10
C12
C14
C18
D3
U5
D5
C17
R3 C19
R4
AN1335
DS01335A-page 81
AN1335
BASE BOARD LAYOUT (BOTTOM VIEW)
DS01335A-page 82
FIGURE C-3:
© 2010 Microchip Technology Inc.
AN1335
C.1
Efficiency Improvement Proposals
The following proposals can be implemented to
improve the efficiency of the converter.
1.
2.
3.
4.
5.
Improving the rise and fall times of the
MOSFETs.
Investigating the feasibility of using a single gate
drive transformer in the Full-Bridge reference
design.
Investigating the feasibility of using high-side
and low-side drivers.
Using 3+3 synchronous MOSFETs in the
secondary rectifications.
Investigating the feasibility of using fractional
turns in the main transformer.
In the present design, some of the layers were made
using 2 oz. copper. As an improvement, these layers
could be made using 4 oz. copper.
© 2010 Microchip Technology Inc.
DS01335A-page 83
AN1335
APPENDIX D:
PSFB QUARTER
BRICK DC/DC
REFERENCE DESIGN
DEMONSTRATION
This appendix guides the user through the evaluation
process to test the Quarter Brick DC/DC Converter.
The Phase-Shifted Full-Bridge Quarter Brick DC/DC
Converter Reference Design is a 200W output isolated
converter with 36V-76V DC input and produces
12V DC output voltage.
D.1
Tests Performed on the Quarter
Brick DC/DC Converter
• Input characteristics
- Input undervoltage/overvoltage
- No load power
- Input power when remote ON/OFF is active
• Output characteristics
- Line regulation
- Load regulation
- Output voltage ramp-up time
- Start-up time
- Remote ON/ OFF start-up time
- Remote ON/OFF shutdown fall time
- Output overcurrent threshold
- Output voltage ripple and noise
- Load transient response
• Efficiency of the converter
FIGURE D-1:
D.2
Test Equipment Required
• DC source 30 VDC-100 VDC @ 8A (programmable
DC power supply, 62012P-600-8 from Chroma or
equivalent)
• DC electronic load (DC electronic load 6314/
63103 from Chroma or equivalent)
• Digital multimeters (six and one-half digit multimeter,
34401A from Agilent or equivalent)
• Oscilloscope (mixed-signal oscilloscope,
MSO7054A from Agilent or equivalent)
• Differential probe (high-voltage differential probe,
P5200 from Tektronix or equivalent)
D.3
Test Setup Description
The Quarter Brick DC/DC Converter is assembled on the
base board for evaluation purposes. The location of the
Quarter Brick DC/DC Converter and its associated components used for testing are illustrated in Figure D-1.
QUARTER BRICK DC/DC CONVERTER CONNECTED TO THE BASE BOARD IN THE
REFERENCE DESIGN ENCLOSURE
Quarter Brick DC/DC Converter
DS01335A-page 84
© 2010 Microchip Technology Inc.
AN1335
FIGURE D-2:
FRONT VIEW OF THE QUARTER BRICK DC/DC CONVERTER REFERENCE DESIGN
Note: The check mark on the front of the enclosure identifies the reference design model.
FB = Full-Bridge Quarter Brick DC/DC Converter (to be discussed in a future application note)
PSFB = Phase-Shifted Full-Bridge DC/DC Converter
Use the following procedure to connect the DC load
and source.
1.
Connect the DC source +ve terminal and -ve terminals to the + and – input terminals (INPUT 3676V) of the connector, as illustrated in Figure D-3.
FIGURE D-3:
LEFT SIDE VIEW OF THE
QUARTER BRICK DC/DC
CONVERTER REFERENCE
DESIGN
© 2010 Microchip Technology Inc.
2.
Connect the DC load +ve terminal and –ve terminals to the + and – output terminals (OUTPUT
12V) of the converter, as illustrated in Figure D-4.
Note:
The PROGRAM/DEBUG socket is used to
program the converter with software.
FIGURE D-4:
RIGHT SIDE VIEW OF THE
QUARTER BRICK DC/DC
CONVERTER REFERENCE
DESIGN
DS01335A-page 85
AN1335
Use the following procedure to prepare the reference
design for testing.
1.
Connect the DMM +ve terminal and –ve terminals to the +ve and –ve terminals of the input
current measurement resistor, as illustrated in
Figure D-5. The current measurement resistor
used to measure the input current is 10 mE. For
example, if the measured voltage across the
resistor is 60 mV, the input current will be 6A.
FIGURE D-5:
2.
INPUT CURRENT MEASUREMENT
Connect the DMM +ve terminal and –ve terminals to the +ve and –ve terminals of the output
current measurement resistor, as illustrated in
Figure D-6. The current measurement resistor
used to measure the output current is 5 mE. For
example, if the measured voltage across the
resistor is 85 mV, then the output current will be
17A.
FIGURE D-6:
DS01335A-page 86
OUTPUT CURRENT MEASUREMENT
© 2010 Microchip Technology Inc.
AN1335
3.
Connect the DMM for input voltage measurement,
as illustrated in Figure D-7.
FIGURE D-7:
4.
INPUT VOLTAGE MEASUREMENT
Connect the DMM for output voltage
measurement, as illustrated in Figure D-8.
FIGURE D-8:
OUTPUT VOLTAGE MEASUREMENT
© 2010 Microchip Technology Inc.
DS01335A-page 87
AN1335
5.
Connect the oscilloscope probe for output
voltage (in DC coupling) and ripple and noise (In
AC coupling) measurement, as illustrated in
Figure D-9.
FIGURE D-9:
6.
OUTPUT VOLTAGE MEASUREMENT
Connect the oscilloscope probe for remote ON/
OFF testing, as illustrated in Figure D-10.
FIGURE D-10: CONNECTING THE OSCILLOSCOPE PROBE FOR REMOTE ON/OFF TESTING
Remote ON/OFF Pin
Note:
Differential probe must be used to monitor
the remote ON/OFF signal.
DS01335A-page 88
© 2010 Microchip Technology Inc.
AN1335
7.
Connect the oscilloscope probe for start-up
time, as illustrated in the Figure D-11.
FIGURE D-11: CONNECTING THE OSCILLOSCOPE PROBE FOR START-UP TIME
Note:
Differential probe must be used to monitor
the input voltage.
Instructions to connect two quarter brick
converters for parallel operation
1.
2.
3.
For N+1 system operations connect COMM2-3
(load share) of converter 1 to converter2
COMM2-3(load share).
Connect a common DC source to the
Converter1 and Converter2 input terminals as
illustrated in Figure D-3.
Connect a Common DC electronic load to the
Converter1 and Converter2 output terminal as
illustrated in Figure D-4.
© 2010 Microchip Technology Inc.
DS01335A-page 89
AN1335
D.4
Forced Air Cooling
a)
The Quarter Brick DC/DC Converter is designed to work
with forced air cooling, which is provided by the fans
illustrated in Figure D-2. Ensure that the fans are circulating air into the enclosure after providing the DC input
supply at the + and – input terminals (INPUT 36-76V) of
the connector, as illustrated in Figure D-3.
D.5
b)
Powering Up the Quarter Brick
DC/DC Converter
c)
Before powering up the converter, ensure that polarity
of the input source and DC load are connected as per
the guidelines described in the section “Test Setup
Description”.
Typically, the unit may enter into the regulation range at around 35 VDC, undervoltage
lockout at approximately 33.5 VDC, and
overvoltage lockout at approximately 81
VDC.
Use the following procedure to power up the reference
design.
1.
2.
3.
Turn the DC source ON and measure the input
voltage with DMM, as illustrated in Figure D-7.
This voltage should be in the range of 36 VDC76 VDC. Check to see that the fans are circulating air into the enclosure.
Ensure that the connected DC load is in the
range of 0A-17A. The output load current measurement resistor provides a value in the range
of 0 mV-85 mV when measuring with DMM, as
illustrated in Figure D-6.
Ensure that the output voltage read by DMM
(see Figure D-8) is in the range of 11.88 VDC to
12.12 VDC.
D.6
Test Procedure
The following two sections provide detailed procedures
for each test.
D.6.1
1.
INPUT CHARACTERISTICS
Input undervoltage/overvoltage.
The Quarter Brick DC/DC Converter is rated to
operate with regulation between the input voltage ranges 36 VDC-76 VDC. The converter features input undervoltage and overvoltage
protection. This feature will not allow the converter to start-up unless the input voltage
exceeds the turn-on voltage threshold and shuts
down the converter when the input voltage
exceeds the overvoltage threshold.
DS01335A-page 90
Set the DC load at 8.5A and increment the
input voltage from 33 VDC (read the input
voltage with DMM illustrated in Figure D-7)
to the voltage where output voltage is in the
regulation range of 11.88 VDC to 12.12 VDC.
Read the output voltage with DMM illustrated in Figure D-8.
Start decrementing the input voltage and
observe at what input voltage the converter
shuts OFF. This input voltage point will be
the input undervoltage threshold.
Start incrementing the voltage from 76 VDC
input and observe at what input voltage
converter shuts OFF. This input voltage
point will be the input overvoltage threshold.
2.
No load power.
a) Set the input voltage at 53 VDC and disconnect or turn OFF the load from the converter
and record the input power.
This value will be the product of input voltage and input current measured using the
DMM illustrated in Figure D-5 and
Figure D-7.
3.
Input power when remote ON/OFF is active.
Remote ON/OFF will be used to turn OFF the
converter by applying a 3.3 VDC signal on the
pin illustrated in Figure D-10. A high signal (3.3
VDC) will turn OFF the converter and there is no
output. When a high signal is sensed by the
dsPIC DSC, all of the PWM generators are shutdown. When the dsPIC DSC detects a low
remote ON/OFF signal, the converter will be
turned ON.
a)
b)
Turn ON the converter with 53 VDC input at
8.5A output load. Connect an oscilloscope
voltage probe to measure the output voltage and a differential voltage probe to measure the external 3.3 VDC supply, as
illustrated in Figure D-10.
Turn ON the external 3.3 VDC supply and
the system will shut down (there will be no
voltage at the output of the converter).
Record the input voltage and input current
to calculate the input power.
© 2010 Microchip Technology Inc.
AN1335
D.6.2
1.
OUTPUT CHARACTERISTICS
7.
The output overcurrent limit will protect the unit
from excessive loading than the rated load current. Increment the output load beyond the rated
17A, the converter enters into Hiccup mode for
a few milliseconds. If overcurrent persists, the
converter enters into Latch mode.
Line regulation.
Change the input DC voltage from 36 VDC to 76
VDC to the converter and record the output voltage. The output voltage deviation should be in
the range of 11.88 VDC to 12.12 VDC.
2.
Load regulation.
Change the output load from 0A to 17A at various input voltages in the range of 36 VDC to 76
VDC and record the output voltage variations.
The output voltage deviation should be in the
range of 11.88 VDC to 12.12 VDC.
3.
Set the input voltage at various points in the
specified range 36 VDC to 76 VDC and increment
the load at the output insteps. To monitor the
output voltage, connect the voltage probe, as
illustrated in Figure D-9.
8.
Output voltage ramp-up time.
Start-up time.
This is the time when the input voltage applied to
the converter (in the range of 36 VDC-76 VDC)
when the output voltage reaches 90% of the rated
12V output voltage. Connect the voltage differential probe at the input voltage terminals and the
voltage probe at the output to the oscilloscope, as
illustrated in Figure D-11.
5.
9.
Output voltage ripple and noise.
Measure the AC component on the output voltage
of the converter by connecting the oscilloscope
output voltage probe, as illustrated in Figure D-9.
Read the output voltage by configuring the oscilloscope in the AC couple mode. The output ripple is
measured in terms of peak-to-peak voltage.
Remote ON/OFF start-up time.
Remote ON/OFF will be used to disable/enable the
converter by applying or removing a 3.3 VDC signal
on the Remote ON/OFF pin, as illustrated in
Figure D-10. Applying 3.3 VDC on the remote ON/
OFF pin turns the converter OFF. Remote ON/OFF
start-up time is the time duration from when the
remote ON/OFF is disabled, to when the output
voltage rises to 90% of the rated output voltage.
6.
Load transient response.
Observe the variation on the DC output voltage
while step changing the output load from 25% to
the 75% of the rated output load 17A. The parameters to be measured are peak-to-peak output
voltage variation and load transient recovery
time. Configure the oscilloscope in AC couple
mode and connect the oscilloscope output voltage probe as illustrated in Figure D-9 to measure
the peak-to-peak output voltage variation and
load transient recovery time.
Turn ON the converter with the specified input
voltage in the range of 36 VDC to 76 VDC and
observe the DC output voltage raise time.
Ramp-up time is the time taken to reach output
voltage from 10% to 90% of the rated output
voltage. Ramp-up time can be measured by
connecting the oscilloscope voltage probe, as
illustrated in Figure D-9.
4.
Output overcurrent threshold.
Remote ON/OFF shut down fall time.
Removing the 3.3 VDC signal on the remote ON/
OFF pin, turns the converter ON. The remote ON/
OFF fall time is the time duration from when the
remote ON/OFF signal is enabled, to when the
output voltage falls to 10% of the rated output
voltage.
D.7
Efficiency is the ratio of output power to the input
power:
Efficiency (%) = Output Power / Input Power * 100
= [(Output voltage * Output current) / (Input Voltage
* Input Current)] * 100
Use the following procedure to measure the efficiency
of the converter.
1.
2.
3.
4.
© 2010 Microchip Technology Inc.
Efficiency of the Quarter Brick DC/
DC Converter
Connect the DMM +ve terminal and –ve terminals
to the +ve and –ve of the input current
measurement resistor, as illustrated in Figure D-5.
Connect the DMM +ve terminal and –ve terminals to the +ve and –ve of the output current measurement resistor, as illustrated in Figure D-6.
Connect the DMM for input voltage
measurement, as illustrated in Figure D-7.
Connect the DMM for output voltage
measurement, as illustrated in Figure D-8.
DS01335A-page 91
AN1335
D.8
COMM 1 and COMM 2
Connectivity
The COMM 1 and COMM 2 signal connectors, pin termination, and functionality are described in Table D-1.
The pin sequence is illustrated in Figure D-12.
TABLE D-1:
Pin
PIN, PERIPHERAL AND FUNCTIONALITY TABLE
Peripheral
COMM 1 - 1
RB8
Functionality
Remappable I/O
COMM 1 - 2
—
COMM 1 - 3
VSS
COMM 1 - 4
SDA1
Synchronous serial data input/output for I2C1.
COMM 1 - 5
SCL1
Synchronous serial clock input/output for I2C1.
COMM 1 - 6
RB15
Remappable I/O.
COMM 2 - 1
—
COMM 2 - 2
—
COMM 2 - 3
AN2
COMM 2 - 4
RP2/SYNCI1
No connect.
DIG_GND
Remote Sense -ve.
Remote Sense +ve.
Load share.
External synchronization signal to PWM master time base.
FIGURE D-12: COMM 1 AND COMM 2 SIGNAL CONNECTORS
Pin 6
Pin 1
Pin 4
Pin 1
Note 1: For N+1 system operations connect
COMM2-3 (load share) of Converter 1 to
Converter2 COMM2-3(load share).
2: Connect a common DC source to the
Converter1
and Converter2
input
terminals as illustrated in Figure D-3.
3: Connect a Common DC electronic load to
the Converter1 and Converter2 output
terminal as illustrated in Figure D-4.
DS01335A-page 92
© 2010 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is provided only for your convenience
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Trademarks
The Microchip name and logo, the Microchip logo, dsPIC,
KEELOQ, KEELOQ logo, MPLAB, PIC, PICmicro, PICSTART,
PIC32 logo, rfPIC and UNI/O are registered trademarks of
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trademarks of Microchip Technology Incorporated in the
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SQTP is a service mark of Microchip Technology Incorporated
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All other trademarks mentioned herein are property of their
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© 2010, Microchip Technology Incorporated, Printed in the
U.S.A., All Rights Reserved.
Printed on recycled paper.
ISBN: 978-1-60932-409-4
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© 2010 Microchip Technology Inc.
DS01335A-page 93
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DS01335A-page 94
© 2010 Microchip Technology Inc.
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