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AN1523 APPLICATION NOTE

Short circuit of the output rectifiers

Another frequent problem in a power supply is rele-

vant to the protection of the SMPS itself: thus some-

times it is easy to find circuits with a good protection

capability against shorts of the load but which are not

able to survive in case of a very hard short like an out-

put electrolytic capacitor or a diode. Besides, in case

of a rectifier shorted, the equivalent circuit of the ba-

sic converter changes: in fact, due to the missing

(shorted) rectifier the energy stored is delivered even

during the on time, like in forward mode with reverse

polarity of the trafo. To insure reliable operation of

the circuit, even this fault condition has been simulat-

ed (figure 25) shorting each rectifier, then has been

proven that the circuit can withstand this failure with-

out any performance degradation. The circuit in fact

works in hic-cup mode and then it restarts correctly to

deliver the output voltages if the short is removed.

This exceeds the requirements of the VDE and IEC

safety rules, and ensures a considerable time saving

during the qualification phase of the SMPS, avoiding

failures during the qualification tests, retrofit and new

testing, sometimes with a short time available to

solve the issue.

Figure 25. SHORT ON +5V RECTIFIER

Vin = 220 Vrms - 50 Hz @FULL LOAD

CH1:

CH2:

Switch On and Turn Off In Short Circuit Condition

The following pictures show the SMPS behaviour during the start-up phase with an output voltage shorted. As

clearly visible the circuit starts correctly then it works in hiccup mode protecting itself. The start-up phase is

clean in all conditions, without showing any dangerous transition for the SMPS circuitry.

Figure 26. SWITCH ON WITH +5V SHORTED

Figure 27. SWITCH ON WITH +5V SHORTED

Vin = 88 Vrms - 50 Hz @FULL LOAD

Vin = 265 Vrms - 50 Hz @STAND-BY

CH1:

CH1:

CH2:

CH2:

CH3:

+5Vout

CH3:

+5Vout