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Old 09-13-2010, 12:59 PM   #3764 (permalink)
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Originally Posted by princeton View Post
Very interesting. Based on my testing with a small inductive load, I would have expected a lot higher voltage spikes. Maybe the low resistance of the motor is dampening the spikes? Would you be able to post the O-scope waveforms? Also, what type of motor are you using? I assume it is under a load if you're pulling 500 amps? .

See the post above for scope video. the motor is an impulse 9. I also set up a peak-voltage-measuring device (a diode and cap in series across Drain and Source, and voltmeter) and tested on the road at 500A. That's where the numbers that paul quoted came from.

The spike is low due to the physical arrangement of the parts and the modest turn-off time of the mosfet. We're driving the gates with 20ohm gate resistors. The slower you turn off the mosfets, dI/dt decreases, more time is available for the diodes to forward-conduct, and the voltage spike decreases. The downside is that switching losses go up and the mosfets run hotter. It's a fine balance of heat generation and voltage spikes.

The voltage spike is a function of current, (parasitic) inductance between the diode and mosfet connection, (parasitic) capacitance of the drain pin relative to source (make sure you isolate the back of your mosfet from something that could increase the parasitic capacitance, like a large heatsink), mosfet turn off time, and diode "turn on time".

One method of combating voltage spikes is using snubbers. In some applications, they sacrifice physical arrangement of the parts so they can fit everything in a smaller package. This results in large voltage spikes that can be mitigated with snubber circuits. I'm not sure if it'd be practical to use them in a high power application.

Search for RCD snubber circuit for details.
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