NCP1603 ON Semiconductor, NCP1603 Datasheet - Page 29

• Manufacturer Part Number: NCP1603
• Description: PFC/PWM Combo Controller
• Manufacturer: ON Semiconductor

PFC Toggling
the PFC−on or PFC−off can be very large. When the
NCP1603 circuit is operating at some conditions between
PFC on and off boundary, the duty ratio variation can lead
to unwanted on/off toggling in the PFC stage. A current
feedforward resistor R
between V
prevent the toggling. The value of R
current sense feedback resistor R
effect when V
(or PFC is on), the R
pin voltage and it allows the feedback voltage V
shift slightly but provide a dramatic duty cycle reduction
in Figure 77. It slight movement of the feedback voltage
can reduce the change to reach the PFC stage on/off
threshold. Hence, the current feedforward resistor can help
to improve the toggling.
The variation of the duty ratio in the PWM stage between
Figure 76. Feedforward Resistor R
R
aux
FF
aux
and CS2 pin (pins 1 and 3) in Figure 76 to
CS2
= 0 (or PFC is off). When V
FF
V
aux
FF
creates a positive offset on the CS2
is hence recommended to added
S2
R
FF
S2
and plays very little
is much larger than
FF
aux
V
Added
bulk
is available
FB2
to only
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NCP1603
29
V
V
Out 2
V
Figure 77. Timing Diagram of PWM Stage When R
FB2
CS2
3
aux
High duty when PFC is off.
is Added
Low duty when PFC is on.
FF