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Calculating switching losses of a mosfet when switching energy is not given

Cecemel

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Hi,

The title pretty much says it all. I’m not sure what factors these losses are based on. I saw somewhere you need to multiply the sum of the rise and fall time by the drain-source voltage and current, but I don’t think the losses are this high.

Thanks
 

KlausST

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Hi,

It depends what values you use

P(t) = V(t) × I(t)

But P(avg) is not V(avg) × I(avg)
And P(RMS) is not V(RMS) × I(RMS)

Klaus
 

Cecemel

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Thanks for your reply.

I don’t really understand what you’re saying. Could you please elaborate or maybe give an example for this mosfet at say 500V and 10A drain-source driven by a 1kHz square wave?
 

KlausST

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Hi,

Let's say you have a Mosfet with 50% duty cycle, average drain current of 10A and average drain-source voltage of 10V.
Then the power dissipation may be from about zero (let's say 0.1W) up to 100W.
It depends on at which time which voltage and which current is.
It's not (only) a question of the used Mosfet.

Klaus
 

Easy peasy

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typically in a circuit with an inductive load, the current must rise to full value before the volts across the device turning on can fall - so turn on losses are Vbus x 0.5 Iload x time to turn ON x freq = losses, assuming the current ramps up linearly. For resistive load losses = 0.5 times prev example as we assume the volts fall linearly at turn on.

Very similar for turn off losses for turning off an inductive load the volts go up quickly & stay high as the current goes from full to zero -> same formula losses = 0.5 x Vbus x Iload x time to turn off x freq.

ditto for resistive load turn off as turn on, for losses, as the V & I cross over linearly, 0.25 x V x I x Toff time x freq.

hope this helps
 

Cecemel

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Thanks for your replies.
I guess what it comes down to is that it's quite hard to calculate switching losses as they depend on a lot of factors, and that the calculations from my first post are about twice as high as a typical worst case scenario because they assume max voltage and current throughout the whole switching process instead of a (still not accurate) linear increase or decrease.

I've been playing with LTspice for a bit and I think those calculations are the most accurate I'm going to get, given a good model.

LTspice mosfet losses.png
 

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