Fairchild ML4824IP2 Power factor correction and pwm controller combo Datasheet

December 2000
ML4824
Power Factor Correction and PWM Controller Combo
GENERAL DESCRIPTION
FEATURES
The ML4824 is a controller for power factor corrected,
switched mode power supplies. Power Factor Correction
(PFC) allows the use of smaller, lower cost bulk capacitors,
reduces power line loading and stress on the switching
FETs, and results in a power supply that fully complies
with IEC1000-2-3 specification. The ML4824 includes
circuits for the implementation of a leading edge, average
current, “boost” type power factor correction and a trailing
edge, pulse width modulator (PWM).
■
Internally synchronized PFC and PWM in one IC
■
Low total harmonic distortion
■
Reduces ripple current in the storage capacitor between
the PFC and PWM sections
■
Average current, continuous boost leading edge PFC
■
Fast transconductance error amp for voltage loop
■
High efficiency trailing edge PWM can be configured
for current mode or voltage mode operation
■
Average line voltage compensation with brownout
control
■
PFC overvoltage comparator eliminates output
“runaway” due to load removal
■
Current fed gain modulator for improved noise immunity
■
Overvoltage protection, UVLO, and soft start
The device is available in two versions; the ML4824-1
(fPWM = fPFC) and the ML4824-2 (fPWM = 2 x fPFC).
Doubling the switching frequency of the PWM allows the
user to design with smaller output components while
maintaining the best operating frequency for the PFC. An
over-voltage comparator shuts down the PFC section in the
event of a sudden decrease in load. The PFC section also
includes peak current limiting and input voltage brownout protection. The PWM section can be operated in
current or voltage mode at up to 250kHz and includes a
duty cycle limit to prevent transformer saturation.
BLOCK DIAGRAM
16
VFB
POWER FACTOR CORRECTOR
+
2.5V
13.5V
+
IEA
3.5kΩ
2.7V
+
+
–
2
–1V
GAIN
MODULATOR
VRMS
4
S
Q
R
Q
S
Q
R
Q
S
Q
R
Q
VREF
14
+
–
PFC OUT
3.5kΩ
ISENSE
7.5V
REFERENCE
–
–
IAC
VCC
VCCZ
OVP
VEA
–
15
13
1
IEAO
VEAO
PFC ILIMIT
12
3
RAMP 1
OSCILLATOR
7
(-2 VERSION ONLY)
RAMP 2
x2
DUTY CYCLE
LIMIT
8
8V
VDC
6
1.25V
+
VCC
SS
–
–
50µA
5
+
VFB
–
2.5V
+
VIN OK
1V
8V
–
+
PWM OUT
11
DC ILIMIT
DC ILIMIT
9
PULSE WIDTH MODULATOR
VCCZ
UVLO
REV. 1.01 12/7/2000
ML4824
PIN CONFIGURATION
ML4824
16-Pin PDIP (P16)
16-Pin Wide SOIC (S16W)
IEAO
1
16
VEAO
IAC
2
15
VFB
ISENSE
3
14
VREF
VRMS
4
13
VCC
SS
5
12
PFC OUT
VDC
6
11
PWM OUT
RAMP 1
7
10
GND
RAMP 2
8
9
DC ILIMIT
TOP VIEW
PIN DESCRIPTION
PIN
NAME
FUNCTION
1
IEAO
PFC transconductance current error
amplifier output
2
IAC
PFC gain control reference input
3
ISENSE
Current sense input to the PFC current
limit comparator
4
VRMS
5
NAME
FUNCTION
9
DC ILIMIT
PWM current limit comparator input
10
GND
Ground
11
PWM OUT PWM driver output
12
PFC OUT
PFC driver output
Input for PFC RMS line voltage
compensation
13
VCC
Positive supply (connected to an
internal shunt regulator)
SS
Connection point for the PWM soft start
capacitor
14
VREF
Buffered output for the internal 7.5V
reference
6
VDC
PWM voltage feedback input
15
VFB
PFC transconductance voltage error
amplifier input
7
RAMP 1
Oscillator timing node; timing set
by RTCT
16
VEAO
PFC transconductance voltage error
amplifier output
8
2
RAMP 2
PIN
When in current mode, this pin
functions as as the current sense input;
when in voltage mode, it is the PWM
input from PFC output (feed forward
ramp).
REV. 1.01 12/7/2000
ML4824
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings are those values beyond which
the device could be permanently damaged. Absolute
maximum ratings are stress ratings only and functional
device operation is not implied.
VCC Shunt Regulator Current .................................. 55mA
ISENSE Voltage .................................................. –3V to 5V
Voltage on Any Other Pin ... GND – 0.3V to VCCZ + 0.3V
I REF ............................................................................................ 20mA
IAC Input Current .................................................... 10mA
Peak PFC OUT Current, Source or Sink ................ 500mA
Peak PWM OUT Current, Source or Sink .............. 500mA
PFC OUT, PWM OUT Energy Per Cycle .................. 1.5µJ
Junction Temperature .............................................. 150°C
Storage Temperature Range ..................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec) ..................... 260°C
Thermal Resistance (θJA)
Plastic DIP ....................................................... 80°C/W
Plastic SOIC ................................................... 105°C/W
OPERATING CONDITIONS
Temperature Range
ML4824CX ................................................. 0°C to 70°C
ML4824IX .............................................. –40°C to 85°C
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, ICC = 25mA, RT = 52.3kΩ, CT = 470pF, TA = Operating Temperature Range (Note 1)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
7
V
VOLTAGE ERROR AMPLIFIER
Transconductance
0
VNON INV = VINV, VEAO = 3.75V
Feedback Reference Voltage
Input Bias Current
50
85
120
µ
2.46
2.53
2.60
V
-0.3
–1.0
µA
Note 2
Output High Voltage
Ω
Input Voltage Range
6.0
Output Low Voltage
6.7
0.6
V
1.0
V
Source Current
∆VIN = ±0.5V, VOUT = 6V
–40
–80
µA
Sink Current
∆VIN = ±0.5V, VOUT = 1.5V
40
80
µA
60
75
dB
60
75
dB
Open Loop Gain
Power Supply Rejection Ratio
VCCZ - 3V < VCC < VCCZ - 0.5V
CURRENT ERROR AMPLIFIER
Transconductance
–1.5
VNON INV = VINV, VEAO = 3.75V
Input Offset Voltage
V
130
195
310
µ
0
8
15
mV
–0.5
–1.0
µA
Input Bias Current
Output High Voltage
2
Ω
Input Voltage Range
6.0
Output Low Voltage
6.7
0.6
V
1.0
V
Source Current
∆VIN = ±0.5V, VOUT = 6V
–40
–90
µA
Sink Current
∆VIN = ±0.5V, VOUT = 1.5V
40
90
µA
60
75
dB
60
75
dB
Open Loop Gain
Power Supply Rejection Ratio
REV. 1.01 12/7/2000
VCCZ - 3V < VCC < VCCZ - 0.5V
3
ML4824
ELECTRICAL CHARACTERISTICS (Continued)
SMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Threshold Voltage
2.6
2.7
2.8
V
Hysteresis
80
115
150
mV
Threshold Voltage
–0.8
–1.0
–1.15
V
∆(PFC ILIMIT VTH - Gain Modulator Output)
100
190
OVP COMPARATOR
PFC ILIMIT COMPARATOR
Delay to Output
mV
150
300
ns
1.02
1.07
V
Input Bias Current
±0.3
±1
µA
Delay to Output
150
300
ns
DC ILIMIT COMPARATOR
Threshold Voltage
0.97
VIN OK COMPARATOR
Threshold Voltage
2.4
2.5
2.6
V
Hysteresis
0.8
1.0
1.2
V
IAC = 100µA, VRMS = VFB = 0V
0.36
0.55
0.66
IAC = 50µA, VRMS = 1.2V, VFB = 0V
1.20
1.80
2.24
IAC = 50µA, VRMS = 1.8V, VFB = 0V
0.55
0.80
1.01
IAC = 100µA, VRMS = 3.3V, VFB = 0V
0.14
0.20
0.26
GAIN MODULATOR
Gain (Note 3)
Bandwidth
IAC = 100µA
Output Voltage
IAC = 250µA, VRMS = 1.15V,
VFB = 0V
10
MHz
0.74
0.82
0.90
V
71
76
81
kHz
OSCILLATOR
Initial Accuracy
TA = 25°C
Voltage Stability
VCCZ - 3V < VCC < VCCZ - 0.5V
Temperature Stability
Total Variation
Line, Temp
1
%
2
%
68
Ramp Valley to Peak Voltage
84
2.5
kHz
V
Dead Time
PFC Only
270
370
470
ns
CT Discharge Current
VRAMP 2 = 0V, VRAMP 1 = 2.5V
4.5
7.5
9.5
mA
Output Voltage
TA = 25°C, I(VREF) = 1mA
7.4
7.5
7.6
V
Line Regulation
VCCZ - 3V < VCC < VCCZ - 0.5V
2
10
mV
Load Regulation
1mA < I(VREF) < 20mA
2
15
mV
REFERENCE
Temperature Stability
4
0.4
Total Variation
Line, Load, Temp
Long Term Stability
TJ = 125°C, 1000 Hours
7.35
5
%
7.65
V
25
mV
REV. 1.01 12/7/2000
ML4824
ELECTRICAL CHARACTERISTICS (Continued)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
0
%
PFC
Minimum Duty Cycle
VIEAO > 4.0V
Maximum Duty Cycle
VIEAO < 1.2V
Output Low Voltage
IOUT = -20mA
0.4
0.8
V
IOUT = -100mA
0.8
2.0
V
IOUT = 10mA, VCC = 8V
0.7
1.5
V
Output High Voltage
90
95
%
IOUT = 20mA
10
10.5
V
IOUT = 100mA
9.5
10
V
50
ns
Rise/Fall Time
CL = 1000pF
Duty Cycle Range
ML4824-1
0-44
0-47
0-50
%
ML4824-2
0-37
0-40
0-45
%
IOUT = -20mA
0.4
0.8
V
IOUT = -100mA
0.8
2.0
V
IOUT = 10mA, VCC = 8V
0.7
1.5
V
PWM
Output Low Voltage
Output High Voltage
Rise/Fall Time
IOUT = 20mA
10
10.5
V
IOUT = 100mA
9.5
10
V
50
ns
CL = 1000pF
SUPPLY
Shunt Regulator Voltage (VCCZ)
12.8
13.5
14.2
V
±100
±300
mV
14.6
V
VCCZ Load Regulation
25mA < ICC < 55mA
VCCZ Total Variation
Load, Temp
Start-up Current
VCC = 11.8V, CL = 0
0.7
1.0
mA
Operating Current
VCC < VCCZ - 0.5V, CL = 0
16
19
mA
12.4
Undervoltage Lockout Threshold
12
13
14
V
Undervoltage Lockout Hysteresis
2.7
3.0
3.3
V
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst-case test conditions.
Note 2: Includes all bias currents to other circuits connected to the VFB pin.
Note 3: Gain = K x 5.3V; K = (IGAINMOD - IOFFSET) x IAC x (VEAO - 1.5V)-1.
REV. 1.01 12/7/2000
5
ML4824
250
200
200
Ω
TRANSCONDUCTANCE (µ )
250
Ω
TRANSCONDUCTANCE (µ )
TYPICAL PERFORMANCE CHARACTERISTICS
150
100
50
0
1
0
2
4
3
150
100
50
0
–500
5
0
500
IEA INPUT VOLTAGE (mV)
VFB (V)
Voltage Error Amplifier (VEA) Transconductance (gm)
Current Error Amplifier (IEA) Transconductance (gm)
VARIABLE GAIN BLOCK CONSTANT - K
400
300
200
100
0
0
1
2
3
4
5
VRMS (mV)
Gain Modulator Transfer Characteristic (K)
16
1
IEAO
VEAO
VFB
15
VEA
–
OVP
+
IEA
3.5kΩ
+
2.5V
+
+
IAC
VRMS
ISENSE
–
–1V
+
–
2
4
2.7V
–
GAIN
MODULATOR
–
S
Q
R
Q
S
Q
R
Q
PFC OUT
3.5kΩ
PFC ILIMIT
12
3
RAMP 1
7
OSCILLATOR
Figure 1. PFC Section Block Diagram.
6
REV. 1.01 12/7/2000
ML4824
FUNCTIONAL DESCRIPTION
The ML4824 consists of an average current controlled,
continuous boost Power Factor Corrector (PFC) front end
and a synchronized Pulse Width Modulator (PWM) back
end. The PWM can be used in either current or voltage
mode. In voltage mode, feedforward from the PFC output
buss can be used to improve the PWM’s line regulation. In
either mode, the PWM stage uses conventional trailingedge duty cycle modulation, while the PFC uses leadingedge modulation. This patented leading/trailing edge
modulation technique results in a higher useable PFC error
amplifier bandwidth, and can significantly reduce the size
of the PFC DC buss capacitor.
The synchronization of the PWM with the PFC simplifies
the PWM compensation due to the controlled ripple on
the PFC output capacitor (the PWM input capacitor). The
PWM section of the ML4824-1 runs at the same frequency
as the PFC. The PWM section of the ML4824-2 runs at
twice the frequency of the PFC, which allows the use of
smaller PWM output magnetics and filter capacitors while
holding down the losses in the PFC stage power
components.
In addition to power factor correction, a number of
protection features have been built into the ML4824. These
include soft-start, PFC over-voltage protection, peak
current limiting, brown-out protection, duty cycle limit,
and under-voltage lockout.
POWER FACTOR CORRECTION
Power factor correction makes a non-linear load look like
a resistive load to the AC line. For a resistor, the current
drawn from the line is in phase with and proportional to
the line voltage, so the power factor is unity (one). A
common class of non-linear load is the input of most
power supplies, which use a bridge rectifier and capacitive
input filter fed from the line. The peak-charging effect
which occurs on the input filter capacitor in these supplies
causes brief high-amplitude pulses of current to flow from
the power line, rather than a sinusoidal current in phase
with the line voltage. Such supplies present a power factor
to the line of less than one (i.e. they cause significant
current harmonics of the power line frequency to appear
at their input). If the input current drawn by such a supply
(or any other non-linear load) can be made to follow the
input voltage in instantaneous amplitude, it will appear
resistive to the AC line and a unity power factor will be
achieved.
To hold the input current draw of a device drawing power
from the AC line in phase with and proportional to the
input voltage, a way must be found to prevent that device
from loading the line except in proportion to the
instantaneous line voltage. The PFC section of the ML4824
uses a boost-mode DC-DC converter to accomplish this.
The input to the converter is the full wave rectified AC line
voltage. No bulk filtering is applied following the bridge
rectifier, so the input voltage to the boost converter ranges
(at twice line frequency) from zero volts to the peak value
of the AC input and back to zero. By forcing the boost
REV. 1.01 12/7/2000
converter to meet two simultaneous conditions, it is
possible to ensure that the current which the converter
draws from the power line agrees with the instantaneous
line voltage. One of these conditions is that the output
voltage of the boost converter must be set higher than the
peak value of the line voltage. A commonly used value is
385VDC, to allow for a high line of 270VACrms. The other
condition is that the current which the converter is
allowed to draw from the line at any given instant must be
proportional to the line voltage. The first of these
requirements is satisfied by establishing a suitable voltage
control loop for the converter, which in turn drives a
current error amplifier and switching output driver. The
second requirement is met by using the rectified AC line
voltage to modulate the output of the voltage control loop.
Such modulation causes the current error amplifier to
command a power stage current which varies directly with
the input voltage. In order to prevent ripple which will
necessarily appear at the output of the boost circuit
(typically about 10VAC on a 385V DC level) from
introducing distortion back through the voltage error
amplifier, the bandwidth of the voltage loop is deliberately
kept low. A final refinement is to adjust the overall gain of
the PFC such to be proportional to 1/VIN2, which linearizes
the transfer function of the system as the AC input voltage
varies.
Since the boost converter topology in the ML4824 PFC is
of the current-averaging type, no slope compensation is
required.
PFC SECTION
Gain Modulator
Figure 1 shows a block diagram of the PFC section of the
ML4824. The gain modulator is the heart of the PFC, as it
is this circuit block which controls the response of the
current loop to line voltage waveform and frequency, rms
line voltage, and PFC output voltage. There are three
inputs to the gain modulator. These are:
1) A current representing the instantaneous input voltage
(amplitude and waveshape) to the PFC. The rectified AC
input sine wave is converted to a proportional current
via a resistor and is then fed into the gain modulator at
IAC. Sampling current in this way minimizes ground
noise, as is required in high power switching power
conversion environments. The gain modulator responds
linearly to this current.
2) A voltage proportional to the long-term rms AC line
voltage, derived from the rectified line voltage after
scaling and filtering. This signal is presented to the gain
modulator at VRMS. The gain modulator’s output is
inversely proportional to VRMS2 (except at unusually
low values of VRMS where special gain contouring takes
over, to limit power dissipation of the circuit
components under heavy brownout conditions). The
relationship between VRMS and gain is called K, and is
illustrated in the Typical Performance Characteristics.
7
ML4824
FUNCTIONAL DESCRIPTION (Continued)
VREF
3) The output of the voltage error amplifier, VEAO. The
gain modulator responds linearly to variations in this
voltage.
The output of the gain modulator is a current signal, in the
form of a full wave rectified sinusoid at twice the line
frequency. This current is applied to the virtual-ground
(negative) input of the current error amplifier. In this way
the gain modulator forms the reference for the current
error loop, and ultimately controls the instantaneous
current draw of the PFC from the power line. The general
form for the output of the gain modulator is:
IGAINMOD =
IAC × VEAO
× 1V
VRMS2
PFC
OUTPUT
16
VFB
15
VEA
–
IEA
+
2.5V
+
+
–
IAC
(1)
More exactly, the output current of the gain modulator is
given by:
IGAINMOD = K × ( VEAO − 15
. V) × I AC
where K is in units of V-1.
1
IEAO
VEAO
–
2
VRMS
4
GAIN
MODULATOR
ISENSE
3
Figure 2. Compensation Network Connections for the
Voltage and Current Error Amplifiers
Note that the output current of the gain modulator is
limited to ≅ 200µA.
Current Error Amplifier
Overvoltage Protection
The current error amplifier’s output controls the PFC duty
cycle to keep the average current through the boost
inductor a linear function of the line voltage. At the
inverting input to the current error amplifier, the output
current of the gain modulator is summed with a current
which results from a negative voltage being impressed
upon the ISENSE pin (current into ISENSE ≅ VSENSE/3.5kΩ).
The negative voltage on ISENSE represents the sum of all
currents flowing in the PFC circuit, and is typically derived
from a current sense resistor in series with the negative
terminal of the input bridge rectifier. In higher power
applications, two current transformers are sometimes used,
one to monitor the ID of the boost MOSFET(s) and one to
monitor the IF of the boost diode. As stated above, the
inverting input of the current error amplifier is a virtual
ground. Given this fact, and the arrangement of the duty
cycle modulator polarities internal to the PFC, an increase
in positive current from the gain modulator will cause the
output stage to increase its duty cycle until the voltage on
ISENSE is adequately negative to cancel this increased
current. Similarly, if the gain modulator’s output decreases,
the output duty cycle will decrease, to achieve a less
negative voltage on the ISENSE pin.
The OVP comparator serves to protect the power circuit
from being subjected to excessive voltages if the load
should suddenly change. A resistor divider from the high
voltage DC output of the PFC is fed to VFB. When the
voltage on VFB exceeds 2.7V, the PFC output driver is shut
down. The PWM section will continue to operate. The
OVP comparator has 125mV of hysteresis, and the PFC
will not restart until the voltage at VFB drops below 2.58V.
The VFB should be set at a level where the active and
passive external power components and the ML4824 are
within their safe operating voltages, but not so low as to
interfere with the boost voltage regulation loop.
Cycle-By-Cycle Current Limiter
The ISENSE pin, as well as being a part of the current
feedback loop, is a direct input to the cycle-by-cycle
current limiter for the PFC section. Should the input
voltage at this pin ever be more negative than -1V, the
output of the PFC will be disabled until the protection flipflop is reset by the clock pulse at the start of the next PFC
power cycle.
8
Error Amplifier Compensation
The PWM loading of the PFC can be modeled as a
negative resistor; an increase in input voltage to the PWM
causes a decrease in the input current. This response
dictates the proper compensation of the two
transconductance error amplifiers. Figure 2 shows the
types of compensation networks most commonly used for
the voltage and current error amplifiers, along with their
respective return points. The current loop compensation is
returned to VREF to produce a soft-start characteristic on
the PFC: as the reference voltage comes up from zero
volts, it creates a differentiated voltage on IEAO which
prevents the PFC from immediately demanding a full duty
cycle on its boost converter.
There are two major concerns when compensating the
voltage loop error amplifier; stability and transient
response. Optimizing interaction between transient
response and stability requires that the error amplifier’s
REV. 1.01 12/7/2000
ML4824
FUNCTIONAL DESCRIPTION (Continued)
open-loop crossover frequency should be 1/2 that of the
line frequency, or 23Hz for a 47Hz line (lowest
anticipated international power frequency). The gain vs.
input voltage of the ML4824’s voltage error amplifier has a
specially shaped nonlinearity such that under steady-state
operating conditions the transconductance of the error
amplifier is at a local minimum. Rapid perturbations in
line or load conditions will cause the input to the voltage
error amplifier (VFB) to deviate from its 2.5V (nominal)
value. If this happens, the transconductance of the voltage
error amplifier will increase significantly, as shown in the
Typical Performance Characteristics. This raises the gainbandwidth product of the voltage loop, resulting in a
much more rapid voltage loop response to such
perturbations than would occur with a conventional linear
gain characteristic.
The current amplifier compensation is similar to that of
the voltage error amplifier with the exception of the
choice of crossover frequency. The crossover frequency of
the current amplifier should be at least 10 times that of
the voltage amplifier, to prevent interaction with the
voltage loop. It should also be limited to less than 1/6th
that of the switching frequency, e.g. 16.7kHz for a
100kHz switching frequency.
There is a modest degree of gain contouring applied to the
transfer characteristic of the current error amplifier, to
increase its speed of response to current-loop
perturbations. However, the boost inductor will usually be
the dominant factor in overall current loop response.
Therefore, this contouring is significantly less marked than
that of the voltage error amplifier. This is illustrated in the
Typical Performance Characteristics.
For more information on compensating the current and
voltage control loops, see Application Notes 33 and 34.
Application Note 16 also contains valuable information for
the design of this class of PFC.
Oscillator (RAMP 1)
The oscillator frequency is determined by the values of RT
and CT, which determine the ramp and off-time of the
oscillator output clock:
fOSC =
1
t RAMP + t DEADTIME
(2)
The deadtime of the oscillator is derived from the
following equation:
FG
H
t RAMP = C T × R T × In
IJ
VREF − 125
.
VREF − 375
.
at VREF = 7.5V:
K
(3)
The deadtime of the oscillator may be determined using:
t DEADTIME =
25
. V
× C T = 490 × C T
51
. mA
(4)
The deadtime is so small (tRAMP >> tDEADTIME) that the
operating frequency can typically be approximated by:
fOSC =
1
t RAMP
(5)
EXAMPLE:
For the application circuit shown in the data sheet, with
the oscillator running at:
fOSC = 100kHz =
1
t RAMP
t RAMP = C T × R T × 0.51 = 1 × 10 −5
Solving for RT x CT yields 2 x 10-4. Selecting standard
components values, CT = 470pF, and RT = 41.2kΩ.
The deadtime of the oscillator adds to the Maximum PWM
Duty Cycle (it is an input to the Duty Cycle Limiter). With
zero oscillator deadtime, the Maximum PWM Duty Cycle
is typically 45%. In many applications, care should be
taken that CT not be made so large as to extend the
Maximum Duty Cycle beyond 50%. This can be
accomplished by using a stable 470pF capacitor for CT.
PWM SECTION
Pulse Width Modulator
The PWM section of the ML4824 is straightforward, but
there are several points which should be noted. Foremost
among these is its inherent synchronization to the PFC
section of the device, from which it also derives its basic
timing (at the PFC frequency in the ML4824-1, and at
twice the PFC frequency in the ML4824-2). The PWM is
capable of current-mode or voltage mode operation. In
current-mode applications, the PWM ramp (RAMP 2) is
usually derived directly from a current sensing resistor or
current transformer in the primary of the output stage, and
is thereby representative of the current flowing in the
converter’s output stage. DC ILIMIT, which provides cycleby-cycle current limiting, is typically connected to RAMP
2 in such applications. For voltage-mode operation or
certain specialized applications, RAMP 2 can be
connected to a separate RC timing network to generate a
voltage ramp against which VDC will be compared. Under
these conditions, the use of voltage feedforward from the
PFC buss can assist in line regulation accuracy and
response. As in current mode operation, the DC ILIMIT
input is used for output stage overcurrent protection.
t RAMP = C T × R T × 0.51
REV. 1.01 12/7/2000
9
ML4824
FUNCTIONAL DESCRIPTION (Continued)
No voltage error amplifier is included in the PWM stage
of the ML4824, as this function is generally performed on
the output side of the PWM’s isolation boundary. To
facilitate the design of optocoupler feedback circuitry, an
offset has been built into the PWM’s RAMP 2 input which
allows VDC to command a zero percent duty cycle for
input voltages below 1.25V.
Solving for the minimum value of CSS:
CSS = 5ms ×
50µA
= 200nF
125
. V
The DC ILIMIT pin is a direct input to the cycle-by-cycle
current limiter for the PWM section. Should the input
voltage at this pin ever exceed 1V, the output of the PWM
will be disabled until the output flip-flop is reset by the
clock pulse at the start of the next PWM power cycle.
Caution should be exercised when using this minimum
soft start capacitance value because premature charging
of the SS capacitor and activation of the PWM section
can result if VFB is in the hysteresis band of the VIN OK
comparator at start-up. The magnitude of VFB at start-up is
related both to line voltage and nominal PFC output
voltage. Typically, a 1.0µF soft start capacitor will allow
time for VFB and PFC out to reach their nominal values
prior to activation of the PWM section at line voltages
between 90Vrms and 265Vrms.
VIN OK Comparator
Generating VCC
The VIN OK comparator monitors the DC output of the PFC
and inhibits the PWM if this voltage on VFB is less than its
nominal 2.5V. Once this voltage reaches 2.5V, which
corresponds to the PFC output capacitor being charged to
its rated boost voltage, the soft-start begins.
The ML4824 is a current-fed part. It has an internal shunt
voltage regulator, which is designed to regulate the voltage
internal to the part at 13.5V. This allows a low power
dissipation while at the same time delivering 10V of gate
drive at the PWM OUT and PFC OUT outputs. It is
important to limit the current through the part to avoid
overheating or destroying it. This can be easily done with a
single resistor in series with the Vcc pin, returned to a bias
supply of typically 18V to 20V. The resistor’s value must be
chosen to meet the operating current requirement of the
ML4824 itself (19mA max) plus the current required by the
two gate driver outputs.
PWM Current Limit
PWM Control (RAMP 2)
When the PWM section is used in current mode, RAMP 2
is generally used as the sampling point for a voltage
representing the current in the primary of the PWM’s
output transformer, derived either by a current sensing
resistor or a current transformer. In voltage mode, it is the
input for a ramp voltage generated by a second set of
timing components (RRAMP2, CRAMP2), which will have a
minimum value of zero volts and should have a peak
value of approximately 5V. In voltage mode operation,
feedforward from the PFC output buss is an excellent way
to derive the timing ramp for the PWM stage.
EXAMPLE:
With a VBIAS of 20V, a VCC limit of 14.6V (max) and the
ML4824 driving a total gate charge of 110nC at 100kHz
(e.g., 1 IRF840 MOSFET and 2 IRF830 MOSFETs), the gate
driver current required is:
IGATEDRIVE = 100kHz × 100nC = 11mA
(7)
20V − 14.6V
= 180Ω
19mA + 11mA
(8)
Soft Start
Start-up of the PWM is controlled by the selection of the
external capacitor at SS. A current source of 50µA supplies
the charging current for the capacitor, and start-up of the
PWM begins at 1.25V. Start-up delay can be programmed
by the following equation:
CSS = t DELAY
50µA
×
. V
125
To check the maximum dissipation in the ML4824, find
the current at the minimum VCC (12.4V):
ICC =
(6)
where CSS is the required soft start capacitance, and
tDELAY is the desired start-up delay.
It is important that the time constant of the PWM soft-start
allow the PFC time to generate sufficient output power for
the PWM section. The PWM start-up delay should be at
least 5ms.
10
RBIAS =
20V − 12.4V
= 42.2mA
180Ω
(9)
The maximum allowable ICC is 55mA, so this is an
acceptable design.
The ML4824 should be locally bypassed with a 10nF and
a 1µF ceramic capacitor. In most applications, an
electrolytic capacitor of between 100µF and 330µF is also
required across the part, both for filtering and as part of
the start-up bootstrap circuitry.
REV. 1.01 12/7/2000
ML4824
In the case of leading edge modulation, the switch is
turned OFF right at the leading edge of the system clock.
When the modulating ramp reaches the level of the error
amplifier output voltage, the switch will be turned ON.
The effective duty-cycle of the leading edge modulation
is determined during the OFF time of the switch. Figure 5
shows a leading edge control scheme.
VBIAS
RBIAS
VCC
ML4824
10nF
CERAMIC
1µF
CERAMIC
One of the advantages of this control teccnique is that it
requires only one system clock. Switch 1 (SW1) turns off
and switch 2 (SW2) turns on at the same instant to
minimize the momentary “no-load” period, thus lowering
ripple voltage generated by the switching action. With
such synchronized switching, the ripple voltage of the first
stage is reduced. Calculation and evaluation have shown
that the 120Hz component of the PFC’s output ripple
voltage can be reduced by as much as 30% using this
method.
GND
Figure 3. External Component Connections to VCC
LEADING/TRAILING MODULATION
Conventional Pulse Width Modulation (PWM) techniques
employ trailing edge modulation in which the switch will
turn on right after the trailing edge of the system clock. The
error amplifier output voltage is then compared with the
modulating ramp. When the modulating ramp reaches the
level of the error amplifier output voltage, the switch will
be turned OFF. When the switch is ON, the inductor
current will ramp up. The effective duty cycle of the
trailing edge modulation is determined during the ON
time of the switch. Figure 4 shows a typical trailing edge
control scheme.
SW2
L1
+
I2
I1
TYPICAL APPLICATIONS
Figure 6 is the application circuit for a complete 100W
power factor corrected power supply, designed using the
methods and general topology detailed in Application
Note 33.
I3
I4
VIN
RL
SW1
DC
C1
RAMP
VEAO
REF
U3
+
–EA
TIME
DFF
RAMP
OSC
U4
CLK
+
–
U1
R
Q
D U2
Q
CLK
VSW1
TIME
Figure 4. Typical Trailing Edge Control Scheme.
REV. 1.01 12/7/2000
11
ML4824
SW2
L1
+
I2
I1
I3
I4
VIN
RL
SW1
DC
C1
RAMP
VEAO
REF
U3
+
–EA
RAMP
OSC
U4
CLK
VEAO
+
–
CMP
U1
TIME
DFF
R
Q
D U2
Q
CLK
VSW1
TIME
Figure 5. Typical Leading Edge Control Scheme.
12
REV. 1.01 12/7/2000
ML4824
AC INPUT
85 TO 265VAC
F1
3.15A
C1
470nF
L1
3.1mH
D1
8A, 600V
Q1
IRF840
C4
10nF
R2A
357kΩ
BR1
4A, 600V
Q2
R17 IRF830
33Ω
C5
100µF
C25
100nF
T1
R1A
499kΩ
R21
22Ω
R2B
357kΩ
D13
1A, 50V
C2
470nF
D6
600V
R1B
499kΩ
C30
330µF
C21
1800µF
C20
1µF
D3
50V
C12
10µF
R14
33Ω
C7
220pF
C22
4.7µF
Q3
IRF830
R23
1.5kΩ
2
3
4
5
C19
1µF
6
7
8
VEAO
IEAO
VFB
IAC
ISENSE
VREF
VRMS
VCC
PFC OUT
SS
PWM OUT
VDC
RAMP 1
GND
RAMP 2
DC ILIMIT
R6
41.2kΩ
R18
220Ω
R26
10kΩ
R22
8.66kΩ
C23
100nF
R25
2.26kΩ
MOC
8102
R7B
178kΩ
TL431
16
15
14
13
C15
10nF
12
C16
1µF
C13
100nF
C14
1µF
R8
2.37kΩ
C31
1nF
R11
750kΩ
C9
8.2nF
C8
82nF
11
10
D8
1A, 20V
9
ML4824
C18
470pF
R20
1.1Ω
R19
220Ω
C6
1nF
R12
27kΩ
12VDC
RTN
R7A
178kΩ
R4
13kΩ
C24
1µF
R24
1.2kΩ
R3
75kΩ
1
R5
300mΩ
1W
L2
D11
MBR2545CT 33µH
T2
R15
3Ω
R28
180Ω
D12
1A, 50V
D7
15V
R30
4.7kΩ
R27
39kΩ
C3
470nF
D5
600V
R10
6.2kΩ
C17
220pF
D10
1A, 20V
L1:
L2:
T1:
T2:
Premier Magnetics #TSD-734
33µH, 10A DC
Premier Magnetics #TSD-736
Premier Magnetics #TSD-735
Premier Magnetics: (714) 362-4211
C11
10nF
Figure 6. 100W Power Factor Corrected Power Supply, Designed Using Micro Linear Application Note 33.
REV. 1.01 12/7/2000
13
ML4824
PHYSICAL DIMENSIONS inches (millimeters)
Package: P16
16-Pin PDIP
0.740 - 0.760
(18.79 - 19.31)
16
0.240 - 0.260 0.295 - 0.325
(6.09 - 6.61) (7.49 - 8.26)
PIN 1 ID
0.02 MIN
(0.50 MIN)
(4 PLACES)
1
0.055 - 0.065
(1.40 - 1.65)
0.015 MIN
(0.38 MIN)
0.170 MAX
(4.32 MAX)
0.125 MIN
(3.18 MIN)
14
0.100 BSC
(2.54 BSC)
0.016 - 0.022
(0.40 - 0.56)
SEATING PLANE
0º - 15º
0.008 - 0.012
(0.20 - 0.31)
REV. 1.01 12/7/2000
ML4824
PHYSICAL DIMENSIONS inches (millimeters)
Package: S16W
16-Pin Wide SOIC
0.400 - 0.414
(10.16 - 10.52)
16
0.291 - 0.301 0.398 - 0.412
(7.39 - 7.65) (10.11 - 10.47)
PIN 1 ID
1
0.024 - 0.034
(0.61 - 0.86)
(4 PLACES)
0.050 BSC
(1.27 BSC)
0.095 - 0.107
(2.41 - 2.72)
0º - 8º
0.090 - 0.094
(2.28 - 2.39)
REV. 1.01 12/7/2000
0.012 - 0.020
(0.30 - 0.51)
SEATING PLANE
0.005 - 0.013
(0.13 - 0.33)
0.022 - 0.042
(0.56 - 1.07)
0.009 - 0.013
(0.22 - 0.33)
15
ML4824
ORDERING INFORMATION
PART NUMBER
PWM FREQUENCY
TEMPERATURE RANGE
ML4824CP-1
ML4824CP-2
ML4824CS-1
ML4824CS-2
1 x PFC
2 x PFC
1 x PFC
2 x PFC
0°C
0°C
0°C
0°C
ML4824IP-1
ML4824IP-2
ML4824IS-1
ML4824IS-2
1 x PFC
2 x PFC
1 x PFC
2 x PFC
–40°C
–40°C
–40°C
–40°C
to
to
to
to
70°C
70°C
70°C
70°C
to
to
to
to
85°C
85°C
85°C
85°C
PACKAGE
16-Pin PDIP (P16)
16-Pin PDIP (P16)
16-Pin Wide SOIC (S16W)
16-Pin Wide SOIC (S16W)
16-Pin PDIP (P16)
16-Pin PDIP (P16)
16-Pin Wide SOIC (S16W)
16-Pin Wide SOIC (S16W)
DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO
ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME
ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN;
NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES
OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body,
or (b) support or sustain life, and (c) whose failure to
perform when properly used in accordance with
instructions for use provided in the labeling, can be
reasonably expected to result in a significant injury of the
user.
www.fairchildsemi.com
16
2. A critical component in any component of a life support
device or system whose failure to perform can be
reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
© 2000 Fairchild Semiconductor Corporation
REV. 1.01 12/7/2000
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