Without going into details, the problems that arise in such regulators will be here summarized.

To ensure a safe operation also under short circuit conditions with maximum input voltage, the mag amp

core must be overdesigned (typically by 100%). This increases its size, weight and cost.

Furthermore, the switching characteristics of the magnetic core are not so good, which limits the opera-

tional frequency and cause a poor dynamic response of the regulator. This has a negative impact also

on the design of the main transformer of the supply and of the primary switch, which will require to be

overdesigned as well.

Both the reactor and the reset PNP exhibit considerable losses, thus the efficiency is not so good.

Another drawback is that the system loses regulation at light output current, thus causing the output volt-

age to drift high. Moreover, if the regulator starts up with a light load current, the output voltage will ex-

perience an overshoot which can be risky for the load. A minimum current consumption (that may be in

the hundred mA) must then be ensured in order to avoid these phenomena. This will require either an

even bigger mag amp core (further size, weight and cost increase) or a dummy load (efficiency will be

hurt) if the output current can go below the minimum value needed to ensure regulation.

Finally, it is not an easy task to tune a mag amp design so as to achieve a satisfactory behavior in the

whole and, unless the designer has a high level of expertise, it also takes a long time.

On the other hand, mag amp approach has been either the most competitive or the only practicable so-

lution for handling high load currents so far: it avoids the losses inherent in linear regulators and the

complexity of conventional switchers.

L6561-based post regulators

Figure 4 shows an L6561-based post-regulator for generating a 3.3 V power rail, with the same rating as

the one illustrated in fig. 3, running off the winding used for the 5V main output.

Compared to the schematic of fig. 3, the parts count is lower. Moreover, the bulkiest (as well as most ex-

pensive) components are replaced by smaller (and cheaper) ones. As a result, the Printed Circuit Board

area that accommodates the L6561-based regulator is very likely to be half the one needed for a simi-

larly rated mag amp-based regulator.

Ω, ac-

R1

820

Ω

R2

2.4 k

Ω

D1

Vcc

L6561

Q1

3.3 V

D3

1N4148

47 k

Ω

D2

T1

T2

T3

T1

61

2

3

8

5

4

7

Vsupply

MULT

COMP

INV

GND

ZCD

CS

GD

10 k

Ω

DISABLE

BC337

TO THE OUTPUT STAGE

OF THE 5V POWER RAIL

10 µF

Figure 4. L6561-based switcher for secondary regulation.

AN1007 APPLICATION NOTE

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