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'[EE]: AUVic Motor Control (Why do we add dissipate'
2002\09\20@195706 by Donovan Parks

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Hello Madhu and fellow PICsters,

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
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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2002\09\21@053810 by Peter L. Peres

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During turn on and turn off the power dissipation is significantly higher
than during conduction. These losses must be added to the dissipation
during conduction. This leads to interesting results as you see. One way
to down the switching losses is to decrease the frequency.

Peter

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2002\09\21@082025 by Olin Lathrop

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> 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
> be dissipating 80W per cycle.  Please explain.  Thanks.

I didn't look at the equations you copied, but your logic is absolutely
correct.  Anything that comes up with nonsensical units like "80W per cycle"
should be discarded without further waste of brain cycles.


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2002\09\23@100156 by Madhu Annapragada

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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}

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