AN-6027 Datasheet, PDF (1/16 Pages) Fairchild Semiconductor – Design of Power Factor Correction Circuit

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Application Note AN-6027
Design of Power Factor Correction Circuit Using FAN7530
1. Introduction
The FAN7530 is an active power factor correction (PFC)
controller for the boost PFC application that operates in the
critical conduction mode (CRM). The critical conduction
mode boost power factor converter operates at the boundary
of continuous conduction mode and discontinuous conduc-
tion mode. The CRM PFC controllers are of two kinds: the
current-mode CRM PFC controller and the voltage-mode
CRM PFC controller. For the current mode, a boost switch is
turned on when the inductor current reaches zero and turned
off when the inductor current meets the desired current refer-
ence. In this case, the rectified AC line voltage should be
sensed to generate the current reference, as in the
FAN7527B; however, the sensing network can cause addi-
tional power loss. In the voltage mode, the switch turn-on is
the same as that of the current mode, but the switch turn-off
is determined by an internal ramp signal. The ramp signal is
compared with an error amplifier output and the switch turn-
on time is controlled to be constant, as shown in Figure 1. If
the turn-on time is constant, the peak inductor current is pro-
portional to the rectified AC line voltage, as shown in Figure
2. In this way, the input current waveform follows the wave-
form of the input voltage, thereby obtaining a good power
factor. The FAN7530 is a voltage-mode CRM PFC control-
ler. Because the voltage-mode CRM PFC controller does not
need the rectified AC line voltage information, it can save
the power loss of the sensing network.
L
D
VVO_OU_UTT
AACC
IN
TTuurrnn--OOnn
S
R
Q
TTuurrnn--Offff
OCP
R_SENSSEE
Ramp
FFeeedbacckk
OVP
Errrroor Amp
Disable
Figure 1. Voltage Mode CRM Boost PFC Circuit
Inductor
Current
MOSFET
Conduction
Diode
Conduction
Peak Inductor
Current
Average
Input
Current
Gating
Signal
Constant On-time & Variable Off-time
Figure 2. CRM Boost PFC Inductor Current Waveform
© 2006 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/11/07
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