AN1007 Datasheet, PDF (1/6 Pages) STMicroelectronics – L6561 - BASED SWITCHER REPLACES MAG AMPS IN SILVER BOXES

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.
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.
Figure 1. Typical architecture of an SMPS for desktop computer ("silver box").
(ISOLATED) FEEDBACK
RECTIFIER
FILTER
MAIN
OUTPUT
DC Input
PWM CONTROLLER
& POWER SWITCH
MAG AMP
REGULATOR
RECTIFIER
+
FILTER
RECTIFIER
+
FILTER
LINEAR
REGULATOR
AUXILIARY
DC OUTPUTS
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
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