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Problem Calculating Power in Flyback Converters

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alisarhangpour

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I'm going to design a 25W Flyback SMPS, and I had a look at Abraham Pressman's book: "Switching Power Supply Design 2nd. Ed.".

The problem is: according to the relations given in the book, when I calculate the RMS current of primary and secondary windings of the transformer, I find out that Irms(pri.)*VDC(nominal) or Irms(sec.)*VDC(out) is twice the intended power.

And when I checked the example given in the book, I found the same: Power is twice the intended figure. Am I making any mistakes simply multiplying Irms by VDC? or is there anything I'm not taking into account? look at example 4.3.2.7 in attachment.
 

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No answer? well, I answer it myself!
It's simply because not all over a period the voltage across primary is VDC. Actually only when the switch is ON, Voltage across primary is VDC. so the RMS voltage across primary is simply less than VDC, and multiplying Irms(pri.) by the RMS voltage mentioned so far, will give the correct result.

The same holds for secondary, I have to take into account that during a period, rectifier diode is OFF for a fraction of one period, So RMS value of voltage across secondary is different from Vout, as well.

I was making a stupid mistake by misunderstanding the situation.
 
alisarhangpour said:
No answer? well, I answer it myself!
It's simply because not all over a period the voltage across primary is VDC. Actually only when the switch is ON, Voltage across primary is VDC. so the RMS voltage across primary is simply less than VDC, and multiplying Irms(pri.) by the RMS voltage mentioned so far, will give the correct result.

The same holds for secondary, I have to take into account that during a period, rectifier diode is OFF for a fraction of one period, So RMS value of voltage across secondary is different from Vout, as well.

I was making a stupid mistake by misunderstanding the situation.

Good for you following up with what you have discovered!
 
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