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AN1376 APPLICATION NOTE

14 SHORT CIRCUIT OF THE OUTPUT RECTIFIERS

A frequent problem in a power supply is relevant to the protection of the SMPS itself: thus

sometimes 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 output electrolytic capacitor

or a diode. Besides, in case of a rectifier shorted the equivalent circuit changes and the energy

is delivered even during the on time, like in forward mode.

To insure reliable operation of the design, even this fault condition has been simulated for each

rectifier. Thanks to the controller functionality, the SMPS can withstand this failure, working in

burst mode as visible in the pictures,

Figure 23. RECTIFIERS SHORT: @FULL LOAD - 220 VAC

In case of an output diode short, the current sensing voltage exceeds a second protection lev-

el, then the controller stops the operation, so avoiding the destruction of the components at

primary side. The controller remains in off-state until the voltage across the Vcc pin decreases

below the UVLO threshold. Then it try to restart and it will switch off again until the secondary

short is removed. This provides for the hic-cup working mode, preventing the circuit destruc-

tion. The operating frequency inside the burst is the internal timer one (~2.5 Khz).

7V

CH1:

DRAIN VOLTAGE

CH1:

DRAIN VOLTAGE

CH2:

CH2:

12V

35V

CH1:

DRAIN VOLTAGE

CH1:

DRAIN VOLTAGE

CH2:

CH2:

3.3V