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AN1376 APPLICATION NOTE

11 SHORT-CIRCUIT TESTS @ FULL LOAD

The short circuit tests have been done in two phases, both making the test shorting by a power

switch the output electrolytic capacitor or making the short by the active load option. This gives

an idea about the circuit behaviour with a hard short (at very low impedance) or with a "soft"

short that could happen on the STB main board, having slightly higher impedance. All the tests

have been done at maximum and minimum input voltage. For all conditions the drain voltage

is always below the BVDSS, while the mean value of the output current has a value close to

the nominal one, then preventing component melting for excessive dissipation. The auto-re-

start is correct at short removal in all conditions.

Figure 16. 7V OUTPUT: SHORT C3

As clearly indicated by the waveforms, the circuit start to work in hic-cup mode, so maintaing the mean

value of the current at levels supported by the component rating. Because the working time and the dead

time are imposed by the charging and discharging time of the auxiliary capacitor C11, it is proportional

to the input mains voltage.

Figure 17. 7V OUTPUT: SHORT BY ACTIVE LOAD

CH1:

DRAIN VOLTAGE

CH1:

DRAIN VOLTAGE

CH2:

CH2:

CH4:

CH4:

CH1:

DRAIN VOLTAGE

CH1:

DRAIN VOLTAGE

CH2:

CH2:

CH4:

CH4: