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 parts count, cost and design effort are the benefits 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 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 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
			RECTIFIER	FILTER	MAIN
					OUTPUT
		TRANSFORMER
DC Input	PWM CONTROLLER
	& POWER SWITCH		MAG AMP	RECTIFIER
				+
			REGULATOR
				FILTER
		MAIN			AUXILIARY
					DC OUTPUTS
			RECTIFIER
				LINEAR
			+
				REGULATOR
			FILTER

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 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.

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AN1007 APPLICATION NOTE

Mag amps have a number of drawbacks: 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.

MAIN TRANSFORMER

MAG AMP

REGULATOR

Vout

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 STAGE
OF THE 5V POWER RAIL
Rsense	3.3V
MAIN TRANSFORMER	TL431
12 V
+
-

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