NCP1653A データシートの表示（PDF） - ON Semiconductor
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NCP1653A Datasheet PDF : 19 Pages
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NCP1653, NCP1653A
The input filter capacitor Cfilter and the front−ended EMI
filter absorbs the high−frequency component of inductor
current IL. It makes the input current Iin 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 IL.
From (eq.1) and (eq.2), the input impedance Zin is
formulated.
Zin
+
Vin
Iin
+
T
*
T
t1
Vout
IL−50
(eq.3)
Power factor is corrected when the input impedance Zin
in (eq.3) is constant or slowly varying in the 50 or 60 Hz
bandwidth.
Ich
Cramp
01
VM Vref
PFC Modulation
−
+
+
RQ
Vramp
S
clock
Vref
Vramp
VM
VM without
filtering
Clock
Latch Set
Latch Reset
Output
Inductor
Current
Figure 27. PFC Duty Modulation and Timing Diagram
The PFC duty modulation and timing diagram is shown
in Figure 27. The MOSFET on time t1 is generated by the
intersection of reference voltage Vref and ramp voltage
Vramp. A relationship in (eq.4) is obtained.
Vramp
+
VM
)
Icht1
Cramp
+
Vref
(eq.4)
The charging current Ich is specially designed as in
(eq.5). The multiplier voltage VM is therefore expressed in
terms of t1 in (eq.6).
Ich
+
Cramp
T
Vref
(eq.5)
VM
+
Vref
*
t1
Cramp
CrampVref
T
+
Vref
T
*
T
t1
(eq.6)
From (eq.3) and (eq.6), the input impedance Zin is
re−formulated in (eq.7).
Zin
+
VM
Vref
Vout
IL−50
(eq.7)
Because Vref and Vout are roughly constant versus time,
the multiplier voltage VM is designed to be proportional to
the IL−50 in order to have a constant Zin for PFC purpose.
It is illustrated in Figure 28.
V in
I in
time
IL
time
VM
time
Figure 28. Multiplier Voltage Timing Diagram
It can be seen in the timing diagram in Figure 27 that VM
originally consists of a switching frequency ripple coming
from the inductor current IL. The duty ratio can be
inaccurately generated due to this ripple. This modulation
is the so−called “peak current−mode”. Hence, an external
capacitor CM connected to the multiplier voltage VM pin
(Pin 5) is essential to bypass the high−frequency
component of VM. The modulation becomes the so−called
“average current−mode” with a better accuracy for PFC.
VM
5
IM
CM
RM
VM =
RM Ivac IS
2Icontrol
PFC Duty
Modulation
Figure 29. External Connection on the Multiplier
Voltage Pin
The multiplier voltage VM is generated according to
(eq.8).
VM
+
RM Ivac IS
2 Icontrol
(eq.8)
Input−voltage current Ivac is proportional to the RMS
input voltage Vac as described in (eq.9). The suffix ac
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