AN-6026 Datasheet, PDF(1/23 Page) Fairchild Semiconductor – Design of Power Factor Correction Circuit

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AN-6026 Datasheet, PDF (1/23 Pages) Fairchild Semiconductor – Design of Power Factor Correction Circuit

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Application Note AN-6026

Design of Power Factor Correction Circuit Using FAN7529

1. Introduction

The FAN7529 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 FAN7529 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.

VVOOUUTT

AACC

IIN

TTuurrnn--OOnn

TTuurrnn--Offff

OCCPP

RSEENSSEE

RRaamp

FFeeeed b ackk

OVPP

EErroor Amp

DDis a ble

Figure 1. Voltage Mode CRM Boost PFC Circuit

Induc tor

Current

Diode

C onduc tion

MOSFET

C onduc tion

Peak

Inductor

C urrent

Average

Input

C urrent

Gating

Signal

Constant On- time & Variable Off- time

Figure 2. CRM Boost PFC Inductor Current Waveform

Rev. 1.0.4 â¢ 4/25/08

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