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Old 05-30-2011, 05:07 AM   #4812 (permalink)
jyanof
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Quote:
Originally Posted by MPaulHolmes View Post
I've got a way to get rid of the isolation pads now...

Each mosfet will be bolted to a 1/2" x 1.5" copper bus bar which will also act as the heat spreader. That heat spreader will be bolted directly to a finned heatsink. A fan will suck air across the heatsink fins. The fins will be inside the controller, since they will be connected to M-.
Cool! Just an experience of mine...

I tried something similar when I was developing the powerboard for my charger. I had the mosfet and diode on different heatsinks so they could be attached with minimal thermal resistance, but I had awful results. The circuit made an audible noise and a scope of Vds looked really weird. I don't really remember, but either it blew up or I didn't feel comfortable pushing it past just a couple of amps.

I think the problem was that the back of the mosfets are connected to the drain pin. This node of a low side buck converter circuit sees a very high dV/dt during switching, so maybe the electrically attached heatsink added a significant amount of stray capacitance which caused bad things to happen. I ended up spending a few extra bucks for a package with an isolated back. Another option may have been going to a high side switch where the drain pin (and heatsink) is now at the positive rail voltage and the source pin sees the high dV/dt.

Of course, I was switching rectified 240 (so like 350VDC), and at a faster frequency and maybe even with harder gate driving. So, it may not be a problem for the motor controller, but it might be something to look out for when testing starts.
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