APPLICATION NOTE AN008

SYNC POWER CORP.
APPLICATION NOTE AN008
SP6019 SIMPLIFIES IMPLEMENTATION OF
SYNCHRONOUS RECTIFIER IN HARD SWITCH
CONVERTER
JUL
2009
V1.2
Synchronous Rectifier Products
AN008-EN
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SYNC POWER CORP.
Table of Contents
Page
1.
Abstract ...................................................................................................... 3
2.
Operation Principle.................................................................................... 4
3.
Pin Configuration....................................................................................... 4
4.
Block Diagram............................................................................................ 5
5.
Operational Details .................................................................................... 6
5.1 Input Voltage ........................................................................................ 6
5.2 Synchronizing Voltage ...................................................................... 6
5.3 Converter Load Determination ........................................................... 6
5.4 Prediction ............................................................................................. 7
5.5 Dynamic Load Adjustment.................................................................. 7
6.
Application Circuit (Demo Board) ............................................................ 8
6.1 Demo Board Schematic ...................................................................... 8
6.2 Demo Board PCB Layout .................................................................... 8
6.3 Demo Board Part list ......................................................................... 9
6.4 Key Layout Issue ................................................................................. 9
7.
Double Forward Application Schematic .................................................. 10
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1. Abstract：
The application details of SP6019 for controlling SR MOSFET to operate like a Rectifier
as shown in Figure 1 is demonstrated in this application note.
Figure 1. Application Circuit
Lm1
Vout
+12Vdc
Rt
Lp
Ls2
SP6019
Timing
SYNC
Pred
Vdd
VR
MOSG-C
Adj
GND
SR
Mosfet Rs1
Rg
Rs2
C2
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2. Operation Principles：
SP6019 is an intelligent Synchronous Rectifier Control IC, it controls the ON and OFF sequence
of a SR MOSFET (Low Ron) to make the active switch act diode like, to achieve high rectification
efficiency.
The SP6019 takes the SR MOSFET drain voltage (Vds) as the synchronize signal to control
SP6019 output, which drives the SR MOSFET gate (Vgs). When the Vds of SR MOSFET is high,
its Vgs is kept low, and when the Vds is low, SR MOSFET Vgs is kept high. To avoid cross
conduction, a predictive dead time is built in between Vgs and Vds to ensure that Vgs is turn
OFF prior Vds going high.
SP6019 also has a built in circuit that senses the dV/dT of the SR MOSFET Vds, which allows it
to operate in the Discontinuous Mode (DCM).
3. Pin Configuration:
Figure 2. IC Configuration
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Table 1. Pin Configuration
Pin
Symbol
1
Timing
2
Pred
3
VR
Internal Reference Voltage.
4
Adj
Dynamic Response Sensitivity Adjustment.
5
GND
6
Description
Output (Vgs) Enable Adjustment
(for SR MOSFET Activation Based on Load Current)
SR MOSFET Vgs and Vds Dead Time Adjustment.
Ground
MOSG-C Output for Driving SR MOSFET Gate。
7
Vdd
8
SYNC
IC Supply Voltage
Input for Synchronous Signal。
4. Internal Block Diagram：
Figure 3. Internal Diagram
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5. Operational Details：
5.1 Supply Voltage and Under Voltage Lock Out (UVLO)：
The suggested supply voltage for SP6019 is between 10.5 and 16 volts; Maximum
DC voltage is 17V；Decoupling Capacitor is required and should be proportional to the Ciss
of the SR MOSFET.
Under Voltage Lock Out (UVLO) for the supply voltage is 10.5V (UVLO_on=10.5V)
5.2 Obtaining the Synchronous Signal：
SP6019 takes SR MOSFET Drain voltage (Vds) as it’s synchronous signal via Pin
8, the Sync pin. The Sypc Pin (Pin 8) is clamped at 5V internally, so a resistor divider from
Vds is used to obtain the synchronous signal.
5.3 Explanation of Load Current Based SR MOSFET Activation：
There are 2 comparators inside the SP6019 each with a trigger voltage of 3.9V and
0.9V respectively. When SR MOSFET Vds falls through these two trigger voltages at a
predetermined time (adjustable at the Timing Pin), the SR MOSFET Vgs is enabled.
Otherwise, the SR MOSFET gate is not enabled. This is to avoid false turn ON of the SR
MOSFET during DCM mode as shown in Figure 4：
Figure 4. SR MOSFET Activation
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5.4 Prediction Control：
SP6019 uses prediction technology to control the Dead Time between SR
MOSFET Vgs and Vds. The prediction circuit uses previous cycle‘s timing information for
the predictive turn OFF of the Vgs in the current cycle, thus creating a Dead Time for the SR
MOSFET as shown in Figure 5.
Figure 5. Predication Sequence Waveforms
5.5 Dynamic Response Circuit：
When SP6019 detects a change in PWM ON Time between 2 cycles that is greater
than 600ns, (PWM on_time；T2-T1 >600ns), SP6019 would reduce the output (Pin 6) to 1us
minimum ON time for the protection of SR MOSFET as shown in Figure 6：
Figure 6. Dynamic Response Adjustment
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6. Demo Board Schematic：
6.1 Demo Board：
Figure 7.
6.2 Demo Board Layout (top、bottom side)
Figure 8. Top side layout
Bottom side layout
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6.3 Demo Board Part list：
Table 2. Part list
P/N
TYPE
FUNCTION
VR、R1
50K、10K
R2
5.1K
Provide Q1 Base Bias
R3、R4
TBD
Resistor divider providing signal for the SYNC pin from SR
MOSFET Vds, values To Be Determined, Initial values
R3=10K、R4=5.1K。
R5
10K
Pull hi resister for sync signal
R6
5.1
Clamping current resister。
Potentiometer, exchange for fixed resistor after design is
completed.
SR Mosfet Gate protective resistor
Rg
C1
104pF
Noise Filter
C2
68pF
for dead time adjustment。C2=68p Æ dead time set =600~700ns
C3
0.22uF
C4
5~100pF
C5
0.1uf
C6
5pF~33pF
Capacitor for Internal Reference Voltage ( Typ.= +5Vdc )
Dynamic Response Adjustment
Decoupling Capacitor。
Noise Filter。
10uF、10uF Decoupling Capacitor for 78L12
C7、C8
D1
1N4148
Q1、Q2
2907/2222A
DZ
5.1V
MOSFET Gate Protection Diode
External Driver Enhancer
Zener Diode for initial start up supply voltage
6.4 Key Layout Issue：
•
All ground lines from SP6019 and its auxiliary components should be connected
together to Pin 5 before connected to SR MOSFET source.
•
PIN 8 SYNC synchronous signal is from SR MOSFET DRAIN through a resistor
divider R3、R4。
•
SR MOSFET and SP6019 should be as close as possible in layout.
•
SP6019 should be far from the transformer in PCB Layout to avoid possible
interferences.
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7. 7.Application and Adjustments：
7.1 Connection the Demo Board with Your Converter:
First, replace the free-wheel diode with SR MOSFET, then connect the SP6019
Demo Board as shown in Figure 9：
Figure 9. SP6019 Demo Board connections：
Ls
Co
+12V
Lp
Ls
SP6019 +12V
Demo
Board
SR
Mosfet
Co
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7.2 Adjusting Demo Board：
z
Determine R3(to limit current)、R4(divide voltage) as follows：
Ex：if secondary side voltage is 30~60V、On Duty(max)=40%、VR pin=5V
SYNC pin input max current 3mA。
R3 (min) =
R3 (min) =
R4 (min) =
R4 (min) =
※
z
Vds(max) X On_time (max) - Set voltage
Limit Current SYNC pin
60V X 40% - 5V
3mA
= 6.33K
Set voltage SYNC pin X R3
Vds(min) - Set voltageSYNC pin
5V X R3
30V - 5V
= 1.267k
Initial values can be set at R3=10K、R4=5.1K。
The RT on Demo Board is a potentiometer. The smaller the RT, the easier to
activate SR MOSFET as shown in Figure 10.
Figure 10.
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z
Put the converter in light load, and adjust RT on the Demo Board to the max.
SP6019 PIN 6 should have no output and the SR MOSFET is not activated, as
shown in Figure 10.
z
Increase the converter load to about 1A, then adjust RT until SP6019 PIN 6
has output as shown in Figure 11.
Figure 11.
z
Note the resistance of the Potentiometer, and replace the Potentiometer with a
fix resistor. Thus, complete the SR adjustment.
z
Avoid making RT too small which would cause over trigger of SR MOSFET.
z
Scope waveforms for a 12V/7A converter is shown below：
Io=0.3A
Io=7A
CH2-SR MOSFET Vds、CH3-SR MOSFET Vgs
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7.3 Double Forward Application Schematic：
( double feedback control loop )
Figure 12.
7.3 Flyback topology Application Schematic：
low side
+12V
Figure 13.
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7.4 Flyback topology Application Schematic：
high side
Figure 14.
SP6019
Demo
Board
G D S
T1
V1
SR Mosfet
C3
Ls1
Lp
7.6 Forward topology Application Schematic：
Figure 15.
T1
Lm1
V1
+12Vdc
Lp
Ls2
SP6019 +12V
Demo
Board
C3
SR
Mosfet
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