Hard to say. Sometimes other parts are wounded and cause the
transistor on-times to be slightly different. Often a base
drive component, usually a resistor, changes value. Some
lousy designs just won't work without selected transistors
required to match the on-times. It only requires a small
imbalance for the push-pull transformer to accumulate enough
flux to eventually saturate. If the core is steel or powdered
iron which can be magnetized by the fault current of the first
failure sometimes (rarely) that causes problems too.

The other topologies we've discussed are more forgiving. BUT
if the switch is on for too long, or the voltage applied to the
winding too high, causing the flux density that the transformer
can sustain to be exceeded, it will saturate. The current will
then rise quite rapidly the sparks will fly.

It's just that they're not senstive to slight variations in the
on-time of the switch. They reset the transformer completely
during the dead time, so they don't accumulate any flux from
on-time imbalances.

An exception is current fed symmetrical topologies which are
just as senstive to imbalance. If the transformer saturates
the fault current is controlled by the inductor feeding the
converter and the current gradually increases. So the control
circuit can catch the fault before the transistors are turned
into lumps of glass.

Some topologies rely on saturation in order to function, like
the royer (and some forms of blocking oscillator supplies) where
saturation removes the positive feedback base drive and allows
the switch to turn off. Nasty things but sometimes useful for
low parts count, low-power, converters.

I'm sure you'll find Abe's book helpful. I still re-read it from
time to time.



I think I get it. Would this explain why in a push-pull topology after
the transistors have been replaced a couple of times the power supply
just keeps failing for no apparent reason?

Are you saying that in other topologies it doesnt saturate or that it
doesnt matter if it does?

Ordered Abe's book.

Jimmie