Originally posted by Aaron
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The only mechanisms by which current can pass through or around the MOSFET D-S, are by parasitic capacitances, channel capacitance, body diode capacitance, and by avalanche breakdown of the body diode. In order to break down that body diode in the reverse direction, it's going to need to see at least 1000V from D-S. Now this is not even going to happen if the flyback diode is present across the coil. The Drain to Source voltage is always positive in this circuit, which means the body diode is never forward biased. So this only leaves the capacitances in and around the MOSFET, and they do allow spikes to reach ground potential and hence a spikey reverse current, so in that sense there is AC current in this ciruit. The vast majority of the current however (about 99%) is pulsed-DC current.
Aaron's circuit probably exhibits much more reverse spiking due to the higher inductance as compared to the resistor used in the RA circuit.
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