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# Question about a secondary of the transformer in the push-pull topology

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#### mbmsv

##### Newbie level 3
Hello experts,

In a classic push-pull topology a secondary supplies a pair of out-of-phase square waves. Two diodes are then used to rectify the output voltage. In some applications it is useful to also generate equal negative voltage. This is done by adding two more reverse biased diodes and another output inductor and a capacitor as shown for example in the last example circuit in the LT1683 datasheet. My question is how do I take this additional output power into account when designing the transformer. Is it equivalent to saying that the output current should be double or do I actually get the second output for free (somehow hard to believe )???

Thanks.

My question is how do I take this additional output power into account when designing the transformer. Is it equivalent to saying that the output current should be double or do I actually get the second output for free (somehow hard to believe )???

The question suggests, that you actually know how to calculate the transformer for only positive output voltage. somehow hard to believe

Seriously speaking, the sum of instantaneous output currents, multiplied with the windings ratio, is transformed (or "reflected") to the input. There's no principle difference between single or multiple outputs.

In addition, each winding should be dimensioned for the respective RMS current. Skin effect needs to be considered for high frequency transformers.

RMS calculation shows, that some copper is saved if a winding carries current during full cycle instead of half cycle.

The question suggests, that you actually know how to calculate the transformer for only positive output voltage. somehow hard to believe

Well, that's not something that I do on the regular basis, and all of the examples and software such as Poweresim use a secondary to generate single polarity voltage. So, if I wanted to use Poweresim I need to somehow tweak the input data to force it to take into account the fact that I will be using the same secondary to produce negative voltage as well.

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