AN4121 Datasheet, PDF (1/22 Pages) Fairchild Semiconductor – Design of Power Factor Correction Circuit

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Application Note AN4121
Design of Power Factor Correction Circuit Using
FAN7527B
1. Introduction
The FAN7527B is an active power factor correction(PFC)
controller for boost PFC application which operates in the
critical conduction mode. It turns on MOSFET when the
inductor current reaches zero and turns off MOSFET when
the inductor current meets the desired input current reference
voltage as shown in Fig. 1. In this way, the input current
waveform follows that of the input voltage, therefore a good
power factor is obtained.
1-1. Internal Block Diagram
It contains following blocks.
• Error amplifier (E/A)
• Zero current detection (Idet)
• Switch current sensing (CS)
• Input voltage sensing (MULT)
• Switch drive (OUT)
.
Inductor Peak Current
Inductor Current
Inductor Average Current
Gating
Signal
Figure 1. Inductor Current Waveform
Vcc 8
2.5V Ref
Vcc
+
UVLO
−
Internal
Bias
Idet 5
CS 4
MULT 3
1¹²1^V.5V ^9V 8.5V
Drive
Output
7 OUT
Timer R
+
S
6⁷.^.5²V^V
−
2V 1.5V
Zero Current
Q
Detector
40k
+
8pF
−
Current Sense
Comparator
1.8V
Vm1
0 ~ 3.8V
Vmo
Multiplier
K=
Vmo
Vm 1 ⋅ (Vm 2 − Vref )
Static OVP
Vref
R
Veao(L)=2.25V
+
2.25V
−
0.25V
−+
+
Vref
Vm2 Vref~Vref+2.5V
OVP
Current
Detector
Isovp=30uA
Idovp=40uA
−
Vea(-)
Error Amp
1 INV
6
2
GND
EA_OUT
Figure 2. Block diagram of the FAN7527B
©2002 Fairchild Semiconductor Corporation
Rev. 1.0.1