Push Pull Converter Transformer Saturation Rating

Question

How do you determine the required saturation rating for a transformer to be used in a push pull converter? Can a transformer in this topology pass more than the saturation rating since it is not storing the energy as it would in a flyback type converter?

Some background: I am building a push pull converter that will take 24V at up to 1A peak and convert it to 15V. I found a transformer (VPH5-1200-R) with a RMS current rating of 1.7A but a saturation current rating of only 0.14A. Will this transformer saturate out before it reaches the 1A through the primary?

Answer 1

The saturation rating refers to the algebraic total current, net curent, through the transformer. Current in the secondary adds, preserving sign, to current in the primary.

For convenience, assume 1:1 windings. For a transformer with a different ratio 1:N, just multiply the appropraite winding current by N.

When you apply the input voltage waveform with no output load, so zero secondary current, the primary current should stay below the satauration limit, if you have designed it with enough primary turns. Any given core will have a volts per turn at a given frequency, and you need enough turns to support the input voltage.

When you connect an output load, a secondary current will flow, which increases the primary current, such that the net current stays pretty much as before (exactly the same as before in a transformer with no primary resistance). The secondary current cancels out the primary current increase.

As the input voltage is the same, the net current through the coils is the same, as it's changes in this net current that creates the back emf that balances the primary input voltage.

In a transformer with primary resistance, the extra current through the primary creates an IR voltage drop, which subtracts from the input voltage, so the net current falls slightly under load, as it is supporting a slightly smaller input voltage across the primary inductance.

Answer 2

You need to do some learning here. Transformer saturation is almost 100% unrelated to load currents. If you apply a voltage to the primary, with the secondary unloaded, it will (largely) saturate no more or no less than under full load conditions.

I know this sounds unintuitive but any current in the primary that is due to secondary load current produces a magnetic field that is totally cancelled by the magnetic field produced by secondary current.

If this were not the case then Faraday's law of induction would be violated.