ST AN1007 Application note

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 suit­able for implementing a switch-mode architecture as an alternative to mag amps. Much better per­formance, a dramatic reduction of part s count, cost and design effort are the benefit s of such an ap­proach. Drawbacks? None. And once more the L6561 turns out to be a really versatile device.

Introduction

Desktop computer power supplies provide two or more low-voltage, high-current, isolated power rails,
typically a 5V rail and a 12V r ail. More an more of ten, it also provides a 3.3V auxiliary rail with high-cur­rent capability.

The power is generated by an off -line forward switching c onverter (inside the s o-called "silver box") that
regulates only one power rail t hrough an isolated feedback loop. The other power rails are usually post­regulated to meet the specifications on the output voltage tolerance and regulation. A typical architecture
is shown in fig. 1.

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

(ISOLATED) FEEDBACK

MAIN

OUTPUT

AUXILIARY

DC OUTPUTS

DC Input

PWM CONTROLLER

& POWER SWITCH

RECTIFIER

MAG AMP

REGULATOR

MAIN TRANSFORMER

RECTIFIER

+

FIL TER

FIL TER

RECTIFIER

+

FIL TER

LINEAR

REGULATOR

Many power supply manufacturers us e m agnetic am plifiers (in short, "mag amps") to achiev e s econdary
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 satu­rate under a given input voltage pulse. Therefore, the reactor acts basically as a delayed switch and per­form PWM by modulating the leading-edge of the input voltage pulse applied to the output section.

October 2003

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AN1007 APPLICATION NOTE

Mag amps have a number of dr awbacks: they are expensive, bulky, slow, inefficient and, last but not
least, their design is not easy.

In this paper a switch-mode architecture is proposed as an alternative to mag amps. Basically, the satu­rable core reactor is replaced by a power MOSFET. The control circuitry is based on a well-known 8-pin
controller IC, in this context used in a not conventional way: the PFC controller L6561.

The benefits of this architecture, which takes advantage of the current mode control offered by the
L6561, will be detailed in this paper and can be summarized as follows: much better performance at a
much lower cost.

Mag amps as secondary post-regulators

Figure 2 shows a block diagram of a mag amp regulator. It looks simple but there are several aspects of
the design that must be accounted for in order to ensure a proper operation.

Figure 2. Mag amp regulator block diagram.

MAG AM P

REGULATOR

Vout

MAIN

TRANSFORM ER

BIAS AND
CONTROL

In figure 3, a real example of a mag amp regulator for a 3.3V power rail is shown.
The TL431 acts as a reference voltage/error amplifier. The PNP transistor, driven by the amplified and

frequency-compensated error signal, acts as a controlled current generator that resets the mag amp
core through the auxiliary winding wound on it. The sense resistor Rsense is part of the current limiting
circuit, which includes also the overcurrent comparator with open collector output. The comparator,
when triggered, saturates the reset PNP transistor, thus delaying as much as possible the positive
pulses delivered downstream the reactor and limiting the overcurrent at the output.

Figure 3. Mag amp regulator electrical schematic.

TO THE OUTPUT STA GE
OF THE 5V POWER RAIL

R

sense

MAIN

TRANSFORMER

12 V

+

-

TL431

3.3V

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