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Gate drain capacitance value for switching loss calculation?

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grizedale

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Hello would you agree that the rquation for "CGD ave" on page 4 of this is nonsense....?........

**broken link removed**

If vds spec was 25V, and vds off was 600V, then Cgd would be smaller with the higher drain voltage , which is nonsense.


I am trying to calculate switching losses for a flyback , but i am close to concluding that an accurate calculation is simply not possible?
 

The junction part of Cdd varies with voltage (Cdb -> Qdb). The oxide part does
not, very much (Cdg, Qdg). I don't think there's an easy calculation there. But
the FET manufacturers do specify gate charge (Qgg) and maybe drain charge
(Qdd) at least at some operating point which you'd hope to be somewhat
application-relevant.

Any varactor reduces capacitance with more voltage. The charge, however,
will still go up.
 

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