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Conduction Loss in Buck SMPS FET

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eem2am

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Hello,

I am trying to calculate conduction losses in the upper FET of a synchronous buck converter.

I thought conduction losses in an SMPS FET were I^2.R
-where I = I(rms) in the FET



-but equation (2) on page 1 of the following….


AN-6005
https://www.fairchildsemi.com/an/AN/AN-6005.pdf

.......states that it is I(OUT)^2 . R(DS)ON . Vout/Vin (2)


……surely equation (2) cannot be right ?
 

dick_freebird

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Your Vout/Vin is the duty cycle factor, which you need because
the FET is only dissipating power during that fraction of the time.
 

eem2am

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but the RMS value has already taken that into account.
...there is no need to take the duty cycle into account again.
 

FvM

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The calculation is assuming a constant Iout, commutated between two switches.
Sketch the waveform, perhaps you understand...
 

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