SG6902 Fairchild Semiconductor, SG6902 Datasheet - Page 17

• Manufacturer Part Number: SG6902
• Description: Green-Mode PFC / Flyback-PWM Controller
• Manufacturer: Fairchild Semiconductor

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Green-Mode PFC / Flyback-PWM Controller
Multi-vector Error Amplifier
The voltage-loop error amplifier is transconductance,
which has high output impedance (> 90kΩ). A capacitor
C
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 6 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
SG6902 shuts off immediately to prevent extra-high
voltage on the output capacitor.
FIG.6 Multi-vector Error Amplifier
PFC Over-Voltage Protection (OVP)
When
over-voltage threshold, the SG6902 inhibits the PFC
switching signal. This protection also prevents the PFC
power converter from operating abnormally while the
FBPFC pin is open.
© System General Corp.
Version 1.3.1 (IAO33.0022.B3)
EA
(1µ ~ 10µF) connected from VEA to ground provides
the
OVP
feedback
voltage
exceeds
A
is opened,
the
- 17 -
Cycle-by-Cycle Current Limiting
SG6902 provides cycle-by-cycle current limiting for both
PFC and PWM stages. Figure 7 shows the peak current
limit for the PFC stage. The PFC gate drive is terminated
once the voltage on the ISENSE pin goes below V
The voltage of V
relationship between V
The amplitude of the constant current, I
the internal current reference, I
following equation:
The peak current of the I
Flyback PWM and Slope Compensation
As shown in Figure 8, peak-current-mode control is
utilized for flyback PWM. The SG6902 inserts a
synchronized 0.5V ramp at the beginning of each
switching cycle. This built-in slope compensation ensures
stable operation for continuous-current-mode operation.
When the IPWM voltage, across the sense resistor,
reaches the threshold voltage, 0.65V or 0.7V selected by
RANGE, the OPWM is turned off after a small
propagation delay, T
introduces
T
and L
transformer. Since the propagation delay is nearly
constant, higher V
I
I
P
S_peak
PD-PWM
=
2
×
p
=
I
•V
T
FIG.7 V
is the magnetized inductance of the flyback
(I
=
P
PFC
2
×
/L
×
R
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additional
1.2V
R
P
p
RMS
, where V
R
- )
S
RMS
PFC
I
0.2V
Controlled Current Limiting
determines the voltage of V
results in a larger additional current
---------------------
PK
PD-PWM
S
and V
------------------
is given by (V
PFC
current,
. This propagation delay
is the output voltage of PFC
Product Specification
RMS
T
is shown in Figure 7.
, according to the
P
proportional
, is determined by
RMS
September, 2007
<1.05V):
SG6902
PK
PK
(8)
(9)
. The
.
to