INTRODUCTION

Three different configurations that an L6561-based

flyback converter can assume have been identified.

They are illustrated in fig. 1.

Configurations a) and b) are basically conventional

flyback converters. The former works in TM (Transi-

tion Mode, i.e. on the boundary between continuous

and discontinuous inductor current mode), therefore

at a frequency depending on both input voltage and

output current. The latter works at a fixed frequency,

imposed by the synchronisation signal, and is there-

fore completely equivalent to a flyback converter

based on a standard PWM controller.

Configuration c), which most exploits the aptitude of

the L6561 for performing power factor correction,

works in TM too but quite differently: the input ca-

pacitance is so small that the input voltage is very close to a rectified sinusoid. Besides, the control loop

has a narrow bandwidth so as to be little sensitive to the twice mains frequency ripple appearing at the

output.

September 2003

®

AN1059

APPLICATION NOTE

DESIGN EQUATIONS OF HIGH-POWER-FACTOR

FLYBACK CONVERTERS BASED ON THE L6561

by Claudio Adragna

Despite specific for Power Factor Correction circuits using boost topology, the L6561 can be suc-

cessfully used to control flyback converters. Among the various configurations that an L6561-based

flyback converter can assume, the high-PF one is particularly interesting because of both its peculiar-

ity and the advantages it is able to offer. AC-DC adapters for mobile or office equipment, off-line bat-

tery chargers and low-power SMPS are the most noticeable examples of application that this configu-

ration can fit.

This paper describes the equations governing such a kind of flyback converter with the aim of provid-

ing a number of relationships useful to the system designer.

VCC

ZCD

GD

L6561

Vac

OPTO

+

TL431

Vout

DISABLE

Figure 1a. TM Flyback Configuration

VCC

ZCD

GD

L6561

Vac

OPTO

+

TL431

SYNCH

Vout

DISABLE

Figure 1b. Synchronised Flyback Configuration

VCC

ZCD

GD

L6561

Vac

INV

OPTO

+

TL431

Vout

COMP

DISABLE

MULT

Figure 1c. High-PF Flyback Configuration

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