Introduction

The front-end stage of conventional off-line converters, typically made up of a full wave rectifier bridge

with a capacitor filter, gets an unregulated DC bus from the AC mains. The filter capacitor must be large

enough to have a relatively low ripple superimposed on the DC level. This means that the instantaneous

line voltage is below the voltage on the capacitor most of the time, thus the rectifiers conduct only for a

small portion of each line half-cycle. The current drawn from the mains is then a series of narrow pulses

whose amplitude is 5-10 times higher than the resulting DC value.

Lots of drawbacks result from that: much higher peak and RMS current drawn from the line, distortion of

the AC line voltage, overcurrents in the neutral line of the three-phase systems and, after all, a poor utili-

sation of the power system’s energy capability.

This can be measured in terms of either harmonic contents, as norms EN61000-3-2 envisage, or Power

Factor (PF), intended as the ratio between the real power (the one transferred to the output) and the ap-

parent power (RMS line voltage times RMS line current) drawn from the mains, which is more immedi-

ate. A traditional input stage with capacitive filter has a low PF (0.5-0.7) and high harmonic contents.

March 2003

®

AN966

APPLICATION NOTE

L6561, ENHANCED TRANSITION MODE

POWER FACTOR CORRECTOR

by Claudio Adragna

The TM (Transition Mode) technique is widely used for Power Factor Correction in low power appli-

cations, such as lamp ballasts or low-end monitors. The L6561 is the latest ST’s proposal for this

market and the emerging ones that are supposed to require a low-cost Power Factor Correction.

Based on a well-established architecture, the L6561 offers excellent performance that enlarges its

field of application considerably.

MULTIPLIER

OVER-VOLTAGE

DETECTION

VOLTAGE

REGULATOR

UVLO

INTERNAL

SUPPLY 7V

+

-

2.5V

R1

R2

R

S

Q

+-

DRIVER

STARTER

+

-

ZERO CURRENT

DETECTOR

DISABLE

2.1V

1.6V

8

1

23

4

ZCD

INV

COMP

MULT

CS

GD

7

5

GND

6

D97IN547D

20V

40K

5pF

Figure 1. Internal Block Diagram of the L6561.

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