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# Coupled output inductor design

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Hello everybody! I have a question about coupled output inductor design. Reading this paper from unitrode, i
undestood the physics behind it, but not completely... In the example a forward converter is given with 2
outputs, one is 5 volts 20 amp ( 100 watts ) and the other 15 volts 5 amp ( 80 watts ). For the calculation of the inductance the turns ratio of the transformer secondaries is defined ( as the turn ration on the inductor need to be the same ), then the 15 volt is normalized to 5 volt with a 15 amp and then combined with the other one and herein on treated as a sigle output of 5 volts 35 amp ( 180 watts ). Then the inductance needed is calculated as 7µH, then  it's beeing told that since the first output is only 20 amp you have to reduce the area of the conductor and  then wind the other output wich will have three times more turns because of the turns ratio. Now the question is for what inductance do i design the windings? I mean do I have to split the inductance between the outputs in such a way that it gives me a 3:1 turns ratio and select the core for a 7µH inductance? Or I make a 7µH winding for the 5 volt output then wind the second 15 volt winding with 3 times more turns and the calculate the resulting inductance then add it to the 7µH from the first output and select the core for the total inductance?
Sorry if i am not making any sense but i am asking your help after i have read all the unitrode documents, 3 other separate documents and a thesis about this topic and the info in my head looks like it was in a blender.
Thank you in advance!

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I don't understand the question.

Generally, you don't worry about the inductance of the secondary coils. Wind the primary for the desired inductance, then wind the secondaries based on the turns ratio, rather than the inductance.

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Thank you very much for your answer. My, lenghty, question was: for wich inductance do i select the size of the core? For the 7µH resulted from the calculation?

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• 6 months later...

In a multi output forward converter, calculate the inductance needed for the "higher" power output. Then, for the coupled filter inductor, the turns ratio between the transformer secondaries has to match the inductors turns ratio.

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