In a push pull converter we often worry about volt second imbalance. But it seemed to me that when both the FETs are off the diodes in the transformer secondary demagnetizes the core. In that case ther is no flash x build up in the core. But there must be some catch with this conclusion as in all the resources explicit ways to mitigate this problem is discussed. Can anyone please explain this phenomenon?
It is more complicated than that!
The magnetization produced by the coils is perfectly determined by the current and the number of turns (NI). It has nothing to do with the volt-section you mention (but hey, wait a moment and see below).
And that will be correct for an air core transformer but not for a common magnetic core. Common magnetic cores have hysteresis. The figure is from
https://electronics.stackexchange.c...-with-hysteresis-doesnt-result-in-a-highly-no
You see that when the current becomes zero, there is still some magnetism left. Sooner or later, the diode will get the current close to zero but there will be considerable magnetism still left. To remove that, you need a current in the reverse direction. If you cycle the current in the positive quadrant only, the curve will be different. The residual magnetism will tend to accumulate over time.
About the volt-second: To pass current in an inductor, you need a voltage source. But an inductor develops a back emf and resists external voltage. When you apply a voltage pulse to an inductor, the current builds up linearly. But the core saturates after some time and the back emf falls and the pulse must be stopped before the core saturates and the current must be limited. During this linear period, the volt-sec is equal to the max current in the coil and this is value you need to use for all practical purposes.
The important lesson here is that you need to exercise the current in both directions in equal measure so that the magnetic field follows the curve. If you don't, you may reach in the saturation region of the curve.
I have omitted some details but hope this is clear for this purpose.