SG6932 Fairchild Semiconductor, SG6932 Datasheet - Page 14

• Manufacturer Part Number: SG6932
• Description: The highly integrated SG6932 is designed for power supplies with boost PFC and forward PWM
• Manufacturer: Fairchild Semiconductor

© 2007 Fairchild Semiconductor Corporation
SG6932 • Rev. 1.1.3
PFC Operation
The purpose of a boost active power factor corrector
(PFC) is to shape the input current of a power supply.
The input current waveform and phase follow that of the
input voltage. Average-current-mode control is utilized
for continuous-current-mode operation for the PFC
booster. With the innovative multi-vector control for
voltage loop and switching charge multiplier-divider for
current reference, excellent input power factor is
achieved with good noise immunity and transient
response. Figure 27 shows the control loop for the
average-current-mode control circuit.
The current source output from the switching charge
multiplier-divider can be expressed as:
I
of I
sources. R
pull HIGH the operating point of the IMP and IPFC pins
when the voltage across R
to ground.
Through the differential amplification of the signal
across R
output of IEA is compared with an internal sawtooth
and the pulsewidth for PFC is determined. Through the
average-current-mode control loop, the input current I
is proportional to I
According to Equation 4, the minimum value of R
maximum of R
not exceed the specified maximum value.
There are different considerations in determining the
value of the sense resistor R
be small enough to reduce power consumption, but
large enough to maintain the resolution. A current
transformer (CT) may be used to improve the efficiency
of high-power converters.
To achieve good power factor, the voltage for V
V
Equation 3. Good RC filtering for V
bandwidth (lower than the line frequency) for voltage
loop are suggested for better input current shaping.
MP
EA
, the current output from IMP pin, is the summation
MO
should be kept as DC as possible, according to
and I
Figure 27. Control Loop of PFC Stage
S
, better noise immunity is achieved. The
MR1
2
and R
I
MO
. I
S
can be determined because I
I
MR1
MO
=
MO
K
3
×
×
and I
:
R
are also identical and are used to
I
AC
2
V
RMS
=
×
I
MR2
S
V
EA
S
×
2
R
goes negative with respect
are identical, fixed-current
S
S
(
μ
. The value of R
A)
RMS
and narrow
MO
S
RMS
should
should
2
and
and
(3)
(4)
S
14
The transconductance error amplifier has output
impedance R
10μF) connected to ground (as shown in Figure 28).
This establishes a dominant pole f
The average total input power can be expressed as:
From Equation 6, V
amplifier, actually controls the total input power and the
power delivered to the load.
Multi-vector Error Amplifier
The voltage-loop error amplifier is transconductance,
which has high output impedance (> 90kΩ). A capacitor
C
provides a dominant pole for the voltage loop. Although
the PFC stage has a low bandwidth voltage loop for
better input power factor, the innovative multi-vector
error amplifier provides a fast transient response to
clamp the overshoot and undershoot of the PFC output
voltage.
Figure 28 shows the block diagram of the multi-vector
error amplifier. When the variation of the feedback
voltage exceeds ±5% of the reference voltage, the
transconductance error amplifier adjusts its output
impedance to increase the loop response. If R
opened, SG6932 shuts off immediately to prevent
extra-high voltage on the output capacitor.
EA
R
R
A
B
C
(1μF ~ 10μF) connected from VEA to ground
EA
Figure 28. Multi-Vector Error Amplifier
FBPFC
V
EA
P
∝
∝
∝
IN
O
V
V
V
RMS
RMS
RMS
=
2.85V
3.15V
f
(>90kΩ) and a capacitor C
1
V
=
IN
×
×
×
(
2
3V
rms
I
π
I
MO
R
AC
EA
V
V
×
AC
IN
V
RMS
, the output of the voltage error
R
)
RMS
×
×
O
1
V
×
I
IN
+
EA
-
×
2
V
2
(
EA
C
rms
+
-
EA
∝
)
V
EA
1
for the voltage loop:
SG69XX
K
www.fairchildsemi.com
I
AC
V
EA
RMS
x
V
EA
2
(1μF ~
A
(5)
(6)
is