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 satu-

rable 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.

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.

Vout

BIAS AND

CONTROL

MAG AMP

REGULATOR

Figure 2. Mag amp regulator block diagram.

3.3V

TL431

-

+

12 V

TO THE OUTPUT STAGE

OF THE 5V POWER RAIL

Figure 3. Mag amp regulator electrical schematic.

AN1007 APPLICATION NOTE

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