FAN6921ML Fairchild Semiconductor, FAN6921ML Datasheet - Page 15

• Manufacturer Part Number: FAN6921ML
• Description: The highly integrated FAN6921ML combines a Power Factor Correction (PFC) controller and a Quasi-Resonant PWM controller
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN6921ML • Rev. 1.0.2
Functional Description
PFC Stage
Multi-Vector Error Amplifier and THD Optimizer
For better dynamic performance, faster transient
response, and precise clamping on PFC output,
FAN6921ML uses a transconductance type amplifier
with proprietary innovative multi-vector error amplifier.
The schematic diagram of this amplifier is shown in
Figure 25. The PFC output voltage is detected from the
INV pin by an external resistor divider circuit that
consists of R
voltage reaches 6% over or under the reference voltage
of 2.5V, the multi-vector error amplifier adjusts its output
sink or source current to increase the loop response to
simplify the compensated circuit.
The feedback voltage signal on the INV pin is compared
with reference voltage 2.5V, which makes the error
amplifier source or sink current to charge or discharge
its output capacitor C
compared with the internally generated sawtooth
waveform to determine the on time of PFC gate.
Normally, with lower feedback loop bandwidth, the
variation of the PFC gate on time should be very small
and almost constant within one input AC cycle.
However, the power factor correction circuit operating at
light-load condition has a defect, zero crossing
distortion, that distorts input current and makes the
system’s Total Harmonic Distortion (THD) worse. To
improve the result of THD at light-load condition,
especially at high input voltage, an innovative THD
optimizer is inserted by sampling the voltage across the
current-sense resistor. This sampling voltage on
current-sense resistor is added into the sawtooth
waveform to modulate the on time of PFC gate, so it is
not constant on time within a half AC cycle. The method
of operation between THD optimizer and PWM is shown
in Figure 26. After THD optimizer processes, around the
valley of AC input voltage, the compensated on time
becomes wider than the original. The PFC on time,
which is around the peak voltage, is narrowed by the
THD optimizer. The timing sequences of the PFC MOS
and the shape of the inductor current are shown in
Figure 27. Figure 28 shows the difference between
calculated fixed on time and fixed on time with THD
optimizer during a half AC cycle.
Figure 25. Multi-Vector Error Amplifier
1
and R
2
. When PFC output variation
COMP
. The COMP voltage is
15
Figure 28. Calculated Waveforms of Fixed On Time
Figure 27. Operation Waveforms of Fixed On Time
R
S
with and without THD Optimizer During a Half
MOS
Figure 26. Multi-Vector Error Amplifier with
PFC
4
Filp-Flop
CSPFC
with and without THD Optimizer
RS
Optimizer
THD
THD Optimizer
AC Cycle
+
Generator
Sawtooth
V

COMP
+
Amplifier
Error
FAN6921
2.5V
INV
www.fairchildsemi.com
3
PFC V
R
R
1
2
O