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Old 01-21-2009, 01:29 PM   #267 (permalink)
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A motor is essentially a massive inductor, and with PWM current is sent to the motor in Pulses, on and off, but with an inductor current always wants to flow, so during the time the MOSFETs are off, it needs somewhere to flow. A FreeWheel Diode "absorbs" the current during the time the MOSFETs are off, and these diodes need to be big, something like 1200 volts and at least 600 AMPs continuous.

Not having a FreeWheel diode is probably a reason why Paul is busting MOSFETs.

I have been developing my own controller for my EV, but had a FreeWheel diode failure and I rocketed down the driveway full reverse when the whole system blew. My diode failed and took everything else with it.

I would suggest reading up on PWM, freewheeling, smoothing capacitors and circuit theory before throwing a homemade controller into a car. While it may work on the bench, without proper caps, diodes, heatsinks, and current limiting it will probably fail in the worst way possible.

Also, I have a little side note about MOSFETs. I don't care what they are labeled as, always current limit them to HALF their rated value, and no MOSFET should go over 50 AMPs per module because that is the max the physical casing can take before it explodes, with a heatsink. Without a heat sink, 2 AMPs would probably kill it.

Just my $0.02 from me majoring in Computer Engineering.
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