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SRF of a transformer

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Full Member level 3
Nov 8, 2014
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Hi guys,

Quick question about the SRF of a transformer. How does the SRF specification effect the design of a power supply. For instance if a transformer is designed to switch at 100KHz and the SRF of the transformer is 150KHz how would you know the following:

1) Whether the SRF is too close to the switching frequency.
2) If it is too close, what effects would you be likely to see.
3) What are the main external aspects within a design that can affect the SNF of a transfomer, i.e input ripple caps (if there are any aspects that do)
4) Does the core contribute/effect the SRF?

Many thanks in advance

What you are likely to see are high switching losses and poor overall efficiency.

One solution is to re think your magnetics design.
Too many turns on one winding, often a single high voltage secondary, and poor winding technique are usually the cause of a low self resonant frequency.

You need to reduce the ac voltage gradients between adjacent turns, and hence the inter turn capacitance by careful design.
One trick is to break up a high voltage secondary into several individual sections, each with its own rectifier and filter capacitor, and series connect the dc outputs.

Perhaps one complete full layer per high voltage section.
Several single full layers placed one top of the other can have a high dc voltage between layers, but zero ac voltage between individual turns to the layer above, and the layer below.

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