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mosfet speed versus power dissipation

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Simon:
I'm driving a power mosfet at 245 Hz, if has a combined rise and fall time of 184 ns and a channel resistance when full on of 6.6 mR, I'll be passing 2-3 amps through it, now I could easily work out the dissipation if it were just on, but how do I take into account the resistance during turn on and off ? clearly this will be of more impact than the actual current flow when fully on. is 1.6W allowance cutting it a bit fine ?

scrat:
Rise and fall times really depend on the gate charging current. If you know those times, you can quite easily approximate dissipation from Vds and Id at the switching instants (they rise/fall with a ramp).
You have to look at the gate charge curve on the datasheet for the time lengths.

I hope that a look here will help:http://www.btipnow.com/library/white_papers/MOSFET%20Power%20Losses%20Calculation%20Using%20the%20Data-Sheet%20Parameters.pdf

At your frequency and with a quite fast gate driver (high max current), maybe switching loss is low.
Another loss, not mentioned in the above document (but should be negligible at a low switching frequency and high output power) is the gate charge: at each on-off cycle the gate driver dissipates (Qg @Vgs final) * Vgs final of energy (then realted Pdiss = Ediss*fsw).

Simon:
Thanks scrat, I'll have a read,

The gate drive will not be that strong, I suppose I could do a test setup and and verify the dissipation buy observing the switch waveform with an oscilloscope to see how long it takes and calculating roughly the average resistance of the channel during the switch and use ohms law

tecman:
With 184 ns out of 245 Hz, you can determine the duty cycle of the rise-fall, which is quite small.  If you assume the highest possible dissipation (I x E), which is highly load dependent, you could come up with a max dissipation, multiplied by the duty cycle, you will have an approximate loss during switching.

Paul

Simon:
well my main unknown is the average resistance during the switching transition