Introduction

Desktop computer power supplies provide two or more low-voltage, high-current, isolated power rails,

typically a 5V rail and a 12V rail. More an more often, it also provides a 3.3V auxiliary rail with high-cur-

rent capability.

The power is generated by an off-line forward switching converter (inside the so-called "silver box") that

regulates only one power rail through an isolated feedback loop. The other power rails are usually post-

regulated to meet the specifications on the output voltage tolerance and regulation. A typical architecture

is shown in fig. 1.

Many power supply manufacturers use magnetic amplifiers (in short, "mag amps") to achieve secondary

post-regulation. Mag amps regulate the output voltage with a saturable core reactor that exhibits a

square B-H loop: when not saturated, the core material has a very high permeability and the reactor a

very high impedance, then it abruptly saturates thus the permeability drops to a very low value and so

does the impedance of the reactor.

By varying the magnetic DC flux inside the core, mag amps control the time needed for the core to satu-

rate under a given input voltage pulse. Therefore, the reactor acts basically as a delayed switch and per-

form PWM by modulating the leading-edge of the input voltage pulse applied to the output section.

October 2003

®

AN1007

APPLICATION NOTE

L6561 - BASED SWITCHER REPLACES MAG AMPS IN

SILVER BOXES

by Claudio Adragna

Mag amps (a contraction of "Magnetic Amplifier") are widely used in multi-output switching power

supplies to get auxiliary regulated power rails. However, they are expensive, bulky, and require a

high level of design expertise.

ST’s L6561, an 8-pin Transition Mode PFC (Power Factor Corrector) controller, is surprisingly suit-

able for implementing a switch-mode architecture as an alternative to mag amps. Much better per-

formance, a dramatic reduction of parts count, cost and design effort are the benefits of such an ap-

proach. Drawbacks? None. And once more the L6561 turns out to be a really versatile device.

DC Input

PWM CONTROLLER

& POWER SWITCH

RECTIFIER

MAG AMP

REGULATOR

RECTIFIER

+

FILTER

FILTER

RECTIFIER

+

FILTER

LINEAR

REGULATOR

(ISOLATED) FEEDBACK

MAIN

OUTPUT

AUXILIARY

DC OUTPUTS

Figure 1. Typical architecture of an SMPS for desktop computer ("silver box").

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