Electronic – Air gap in switching power supply transformer

magneticspower electronicsswitch-mode-power-supplytransformer

An article I have read regarding switching power supplies (https://sound-au.com/articles/smps-primer.html) says that push-pull, half-bridge, and full-bridge switching supplies don't use an air gap in the transformer.

Now, flyback converters need an air gap since their unidirectional operation puts a DC component in the transformer's primary current.

But would it be advantageous to use an air gap in a high-power (say 24VDC out at 50 Amps) or very high power (340VDC out at 800 Amps for a high-power air ionizer) full-bridge converter's output transformer? It seems that it would be great to do so, since the transformer would be much more resistant towards saturation if the microcontroller ever fires the switching transistors on one side longer than the other, but what are the disadvantages?

Best Answer

Adding an air gap to the the flyback transformer increases the reluctance of the magnetic circuit. Doing this does a few things:

  • Decreases the effective permeability of the core
  • Decreasing permeability (also decreasing inductance) increases the amount of current that can be supplied to the transformer before saturating.

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These equations show how introducing a gap changes the effective permeability (z is the 'gap factor'). Lg is the gap length introduced and Le is the length of the path the magnetic field loops around (not including Lg). Here is a picture of how the gap affects the BH curve. You can see that you are able to apply a larger H field before the B field rolls off and the core is saturated.

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Since a larger primary side current can be used before saturation occurs, effectively more energy can be stored in the primary side magnetizing inductance.

Other topologies don't need an air gap like the flyback since they are transferring energy during both the on and off time of the switching. Energy is not stored in the magnetizing inductance in the same manner. The flyback is different since it stores energy when the primary side switch is on and transfers energy to the secondary size during the off time.

I would recommend you build a flyback (with safety precautions). You can wind your own transformer and file the core yourself if you're so inclined.

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