AN1686 datasheet(2/7 Pages) STMICROELECTRONICS

2 / 7 page

AN1686 APPLICATION NOTE

2/7

in case of anomalous power demand from the load (exceeding the peak power, e.g. due to a short circuit), con-

verter's shutdown must occur within 10

÷100 ms.

A power peak lasting 0.5 s is too long to be handled with just a reinforced output capacitor bank: as a matter of

fact, after few ms the converter's control loop has already reacted to maintain the output voltage regulated. Thus

a power demand lasting 0.5 s can be considered as a steady-state operating condition from the electrical point

of view. On the other hand the thermal time constants of the power components are such that the heat gener-

ated during a power peak of even a couple of seconds will not cause an excessive temperature rise: their ther-

mal

impedance is to be invoked rather than their thermal resistance.

From these considerations, it turns out that the converter can be thermally designed just considering the maxi-

mum continuous output power Poutx (and the related RMS currents) that it has to deliver and not the peak power

demand Poutpk. There are, however some points that one should consider for the electrical design:

1) the overcurrent limiting circuit must allow Poutpk to pass, hence its setpoint needs to be greater than the cor-

responding peak primary current Ippklim and, possibly, much greater than the peak primary current related

to Poutx;

2) the transformer and/or inductor must not saturate with this higher peak current;

3) there is a tradeoff between the input bulk capacitor size (which determines the minimum input DC voltage)

and the value of Ippklim: a low capacitance leads to a high input voltage ripple, then to a lower minimum input

DC voltage and to a higher Ippklim. Also the maximum duty cycle of the converter must account for the re-

sulting minimum DC input voltage.

4) As envisaged by the spec in table 1, designing the converter for a given power but allowing a much higher

level calls for a means to make the system work safely during abnormal operating conditions.

Except for the special requirement discussed so far, the specification is identical to that of the converter consid-

ered in [1], which will be then used as the starting point for the present design. In this context, only the modifi-

cations needed for fully complying with the spec of table 1 will be discussed. For reader's convenience figure 1

reproduces the electrical schematic of the original converter.

Additional circuitry will be needed for discriminating a peak power demand from a normal load condition and

detecting a short circuit as well as realizing the required double time-out.

Figure 1. 45W, wide-range mains AC-DC adapter: electrical schematic (original design)

88 to 264

Vac

BD1

DF04M

100 µF

400 V

56 k

Ω

R11

Ω

R14

1 k

Ω

R15

0.47

Ω

1/2 W

330 k

Ω

R9 6.8 k

Ω

3.3 nF

R13

8.2 k

Ω

STP7NB60

330 µF

25 V

GND

IC1

L5991

C12

4.7 nF

1kV

STTA106

R7 4.7

Ω

R5 47 k

Ω

47 µF

25 V

R10

Ω

DCC

R8 5.6 k

Ω

VREF

100 nF

56 nF

RCT

DC-LIM

SGND

2.2 M

Ω

100 pF

PGND

ISEN

OUT

VCC

DIS

COMP

VFB

Vout

ST-BY

R19

1.2 k

Ω

R17

4.3 k

Ω

R21

348

Ω

220 pF

C3 100 nF

C14

470 nF

R12

24 k

Ω

R16 100

Ω

R18

2.2 k

Ω

R20

5.6 k

Ω

D4 1N4148

BYW29-200

56 k

Ω

2.2 M

Ω

C10

330 µF

25 V

C11

330 µF

25 V

IC2

PC917

F1 T2A250V

1N4148

NTC1

R22

C17

C13

C15

100 nF

BZW06-154

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