AN1059 Datasheet, PDF (1/20 Pages) STMicroelectronics – DESIGN EQUATIONS OF HIGH-POWER-FACTOR

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.
INTRODUCTION
Figure 1a. TM Flyback Configuration
Three different configurations that an L6561-based
flyback converter can assume have been identified.
Vac
Vout
They are illustrated in fig. 1.
CBULK
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
DISABLE
at a frequency depending on both input voltage and
output current. The latter works at a fixed frequency,
ZCD VCC
imposed by the synchronisation signal, and is there-
L6561
fore completely equivalent to a flyback converter
GD
OPTO
based on a standard PWM controller.
+
TL431
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.
Figure 1b. Synchronised Flyback Configuration Figure 1c. High-PF Flyback Configuration
Vac
SYNCH
DISABLE
ZCD VCC
L6561
GD
CBULK
Vout
Vac
OPTO
+
TL431
DISABLE
ZCD MULT VCC
L6561
COMP INV ^GD
Vout
CIN
OPTO
+
TL431
(BW<100 Hz)
September 2003
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