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PDF AN1007 Data sheet ( Hoja de datos )
| Número de pieza | AN1007 | |
| Descripción | Switcher Replaces MAG Amps in Silver Boxes | |
| Fabricantes | ST Microelectronics | |
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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.
March 1998
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1 page  
AN1007 APPLICATION NOTE
(±1% @ Tj = 25°C) voltage reference.
VMUL voltage is connected to the output voltage, in this way providing foldback current limiting. This
function provides a short circuit current lower then the normal load current, thus greatly reducing the
stress on the power components in case of failure.
The diode D3 avoids latchoff, that is prevents the output current from dropping to zero during short cir-
cuits and staying at zero even when the short circuit is removed. Furthermore, at start-up, there is a soft-
start action because of the limitation of the current inflow.
The regulator will exhibit a better efficiency over its mag amp counterpart: the dissipation on the MOS-
FET is quite limited (about 1W with the parts suggested in Tab.1), while the mag amp dissipates on the
copper and the core of the reactor, as well as some hundreds mW on the reset PNP transistor.
The "peak" current mode control performed by the circuit offers many more benefits: excellent load regu-
lation from very low output current up to the maximum load, fast response to step-load changes, good
line regulation due to the inherent input voltage feedforward and one-pole transfer function of the output
stage, which simplifies frequency compensation.
Additionally, since the L6561 can be disabled by grounding the ZCD pin, the entire regulator can be
either enabled or shut down with a logic signal. Besides, being the L6561 on the secondary side, the in-
terface needed to feed the enable/disable signal will be extremely simple, like the one shown in the
schematic of fig. 4.
This disable feature achieves a real zero-power shutdown. In fact, when the regulator is disabled, Q1 is
kept off and the secondary winding is an open circuit . The only power consumption is a couple of mA
from the supply pin of the L6561.
This functionality turns out to be extremely useful in those systems were power management is required,
as stated in Intel’s Power Supply ’98 specifications, for example.
Conclusions
The replacement of a mag amp-based post regulator with an L6561-based one for the generation of a
given auxiliary power rail allows to save numerous external parts among which the bulky and expensive
mag amp core. Such a regulator will benefit a dramatic cost reduction from the lower number and cost of
the parts used, from the shrinkage of the PCB area needed to accommodate the circuit and from a cut of
the design time.
In spite of that, the result is a much more performant system which takes advantage of all the benefits
inherent in switching regulation with current mode control as well as of the functionalities available from
the versatile L6561.
REFERENCE
[1] "L6561, Enhanced Transition Mode Power Factor Corrector" (AN966)
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| Páginas | Total 6 Páginas | |
| PDF Descargar | [ Datasheet AN1007.PDF ] | |
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