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Topic: Mosfet body diode as flyback? (Read 3 times) previous topic - next topic

dc42

#15
Jul 24, 2012, 01:51 pm Last Edit: Jul 24, 2012, 03:00 pm by dc42 Reason: 1
For a very rough upper limit of the switching power dissipation of a logic level mosfet when switching an inductive load, I use the following formula:

P = 2VIQfR  [edit: corrected formula]

where V is the switched voltage, I is the switched current, Q is the total gate charge of the mosfet, f is the switching frequency and R is the gate series resistor (min. 100 ohms). For a resistive load, you can use 0.5 in place of 2. The actual switching power dissipation is likely to be somewhat less than this figure.
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scswift

Hm... Well, for
2 * 5v * 2A * (15C / 1000000000) * 400000hz / 180

I get 66mA, which @ 5V is 0.33W.

I chose 2A just cause that's the absolute maximum the connectors can handle.  I dunno if that result is right but it seems like a reasonable value. 

dc42

Sorry, the original formula I gave was incorrect, you need to multiply by R not divide. If it appears dimensionally incorrect, that's because you should really divide by the voltage that appears across the gate drive resistor when the gate is charging (I assumed this was 1V).

Did you really mean a switching frequency of 400kHz, or did you mean 400Hz? Arduino PWM is about 490Hz.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

scswift

Well, I meant 400khz, but that's because I assumed the Timer was /8.  It's /64 though and then halved for phase-correct PWM, so it's around 1000hz for the timers and 500hz for the PWM.  So I should have used 490hz I guess. 

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