[SMPS] Ferrite core transformer design

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Joined Sep 30, 2018

I'm trying for the first time to design a ferrite core transformer for an SMPS. I 'm actually trying to select the size of the core. I tried two differents methods in order to compute the size of the core. I actually don't know what is the better method... At the end, I hoped the results from the two differents methods would be the same. It is not the case...

First method is extracted from a book "Switching power supply design 3rd Edition" written by I Pressman ...
By knowing the flux density swing, the operating transformer frequency and the output power necessary and the kind of the ferrite (geometry, EE core for example), thanks to a chart we are able to determine what should be the size of the core for a certain topology.

2nd method is thanks to the area product
By knowing the flux density swing, the operating frequency, the output power and a certain factor K which is dependant on the topology, you can estimate what should be the size of the core... In the following expression, the geometry parameter of the ferrite doesn't take into account.


Actually what i want to do is compute the size of a planar transformer and i don't know if the previous formula would apply for this kind of transformer.

I would like to know what is the problem if i surestimate the size of a core ? I will increase the hysteresis losses ... But I prefer to do this than saturate the core... Could you please help me to select the core by telling me other methods to compute the size ?

Thanks a lot and have a nice day !