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NCP1653, NCP1653A
Zin + in +
t1 CrampVref T * t1
VM + Vref *
+ Vref
VM Vout
Zin +
Vref IL ? 50
The input filter capacitor C filter and the front ? ended EMI
filter absorbs the high ? frequency component of inductor
current I L . It makes the input current I in a low ? frequency
signal only of the inductor current.
Iin + IL ? 50 (eq.2)
The suffix 50 means it is with a 50 or 60 Hz bandwidth
of the original I L .
From (eq.1) and (eq.2), the input impedance Z in is
formulated.
V T * t1 Vout (eq.3)
Iin T IL ? 50
Power factor is corrected when the input impedance Z in
in (eq.3) is constant or slowly varying in the 50 or 60 Hz
bandwidth.
(eq.6)
Cramp T T
From (eq.3) and (eq.6), the input impedance Z in is
re ? formulated in (eq.7).
(eq.7)
Because V ref and V out are roughly constant versus time,
the multiplier voltage V M is designed to be proportional to
the I L ? 50 in order to have a constant Z in for PFC purpose.
It is illustrated in Figure 28.
V in
C ramp
I ch
0
1
V M
+
V re f
PFC Modulation
?
+
V ramp
R
S
Q
IL
I in
time
time
clock
VM
time
V ref
Figure 28. Multiplier Voltage Timing Diagram
V ramp
V M
V M without
filtering
Clock
Latch Set
Latch Reset
It can be seen in the timing diagram in Figure 27 that V M
originally consists of a switching frequency ripple coming
from the inductor current I L . The duty ratio can be
inaccurately generated due to this ripple. This modulation
is the so ? called “peak current ? mode”. Hence, an external
capacitor C M connected to the multiplier voltage V M pin
(Pin 5) is essential to bypass the high ? frequency
component of V M . The modulation becomes the so ? called
“average current ? mode” with a better accuracy for PFC.
Output
Inductor
I M
V M
5
V M =
R M I vac I S
2I control
Current
Figure 27. PFC Duty Modulation and Timing Diagram
The PFC duty modulation and timing diagram is shown
C M
R M
PFC Duty
Modulation
Vramp + VM ) ch 1 + Vref
I t
VM +
2 Icontrol
in Figure 27. The MOSFET on time t 1 is generated by the
intersection of reference voltage V ref and ramp voltage
V ramp . A relationship in (eq.4) is obtained.
(eq.4)
Cramp
The charging current I ch is specially designed as in
(eq.5). The multiplier voltage V M is therefore expressed in
terms of t 1 in (eq.6).
Figure 29. External Connection on the Multiplier
Voltage Pin
The multiplier voltage V M is generated according to
(eq.8).
RM Ivac IS
(eq.8)
Ich +
Cramp Vref
T
(eq.5)
Input ? voltage current I vac is proportional to the RMS
input voltage V ac as described in (eq.9). The suffix ac
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