You are right about suspecting the 80W figure. Ideally, you would want to
figure out the worst case load conditions; how much time in switching, how
much in conduction and how much in the off state. Then you just get the
weighted average as you mentioned for sizing your heatsinks and you should
be okay.
Madhu
> I am confused to why we add dissipated power instead of taking a weighted
> average. For example, if a device is periodically dissipating 5W for 1ms
> followed by 0W for 1ms shouldn't the average dissipated power be 2.5W
> (5W*(1ms/2ms) + 0W*(1ms/2ms)) and the peak dissipated power be 5W. For
the
> calculations below relating to a MOSFET in a motor controller, shouldn't
the
> total losses per cycle of the PWM signal be a waited average and not the
> sum? If at no time the MOSFET is dissipating 80W I fail to see how it
can
{Quote hidden}> be dissipating 80W per cycle. Please explain. Thanks.
>
> Regards,
> Donovan Parks
> AUVic Team Leader
>
>
>
> > (7) Turn on losses = [0.55 * Vb * (Iload + Irrm)^2 * f * 2E-9] / Di/Dt
=
> > 8.475 Watts
> > (8) Turn off losses = Vb^2 * ILoad E-9 * f / (dv/dt) = 6.667 Watts
> > (9) Conduction Losses = RdsonTj * ILoad^2 = 64 Watts.
> > (10) Total Losses are approximately 80 Watts per cycle of your PWM
> > waveform.
>
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