Newsgroups: comp.robotics
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From: cs911225@ariel.yorku.ca (KEN E WILLMOTT)
Subject: MOSFET clamping diode
Message-ID: <1993Sep10.140648.2958@newshub.ariel.yorku.ca>
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Organization: York University, Toronto, Canada
Date: Fri, 10 Sep 1993 14:06:48 GMT
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>> Now, the neat part is that a lot of MOSFETs have these diodes
>> built in, so you needn't bother adding them externally.
>>
>>         -Ken Willmott

>But you may need the freewheeling diode for PWM because the Non-repetitive
>clamping energy is a lot higher than the Repetitive clamping energy.

By this, you mean (an) external diode, not the internal one?

>I was stumped on why did my power Mosfet FRY when it had its own clamping
>diodes. For a philips BUK101-50GL its 625 mJ  to 25 mJ respectively.
>Supposedly this Power Mosfet could handle 100A peak and 26 A continuous.
>And I was only putting 2 amps MAX through at 10 KHZ!

>Regards
>cliff

I am aware of the issue of heat dissipation in this context.
When the diodes clamp, there is some power generated due to
the junction drop. I didn't mention it, however, because I
assumed that the original circuit under discussion was driving
a low current load - that assumption may have been unwarranted.

You haven't said whether your MOSFET was in a bridge circuit,
or just a driver in a switching power supply or whatever.
Are you sure you had it properly heatsinked? Suppose the
diode were conducting the full 2A. Then its power dissipation
would be  2.0A * 0.7V = 1.4W. Doesn't seem like enough,
somehow.

I'll have to review my MOSFET book to see what they have to
say about the effects of high frequency in this regard,
meanwhile, I wonder if you could clarify.

The freewheeling diode is not just for protection, but is
an important part of the current cycle of many switching
power supplies. In those cases, the diode carries about
as much current as the MOSFET, on average. But, it seems
to me that the bridge circuit is a different ball game.

	-Ken Willmott

