This is a question which was originally asked by my power electronics professor but left unanswered.
Suppose you are given the following system (kindly borrowed from this question)
- The inductance L is so big that the load current is practically constant (remove Vx and think of the load as an ideal current source).
- The diodes are ideal.
- The voltage source is a secondary side of an ideal transformer (no leakage reactance no resistance and no magnetizing current).
As you can see, the secondary current is a square wave. How can a square wave (i.e. a constant current on a half period basis) cause a primary current in the transformer to flow?
My answer is the following:
We can see the secondary current as a Fourier series, that is as an infinite sum of sinewaves. This means that, at least on the transformer part of the circuit which is linear, we can apply the superposition principle and therefore obtain a lot of AC circuits to be solved and then combined. It's the fundamental frequency and the harmonics which are causing primary current to flow. If it were pure DC it wouldn't work.
In reality of course, the transformer has a leakage reactance which will smooth out the edges of the square wave, providing a (perhaps more evident) time varying current on both its sides.