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Author Topic: Mosfet body diode as flyback?  (Read 3529 times)
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Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.
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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.
« Last Edit: July 24, 2012, 08:00:06 am by dc42 » Logged

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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. 
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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.
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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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