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[SOLVED] Kickback reduction without magnetic field reversal?

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DC177E

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Hello All,

I will be pumping ~250a (At 40v) through a 100 turn coil momentarily (hopefully ~5ms pulse length), controlled by a mosfet, and there will surely be a significant amount of inductive kickback that will probably kill said mosfet. I've seen the standard diode-across-inductor technique to remove this harmful kickback, but, to my understanding, this creates a magnetic field in the reverse of the one it was generating before (correct me if I'm wrong), which, since this is being used in a solenoid-type application to pull a magnet, would be unacceptable (it would push the magnet back away again).

So, basically, how could I remove kickback without reversing the magnetic field? Am I violating the laws of inductors by attempting to do so?

Any help would be appreciated.
 

FvM

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Your assumption is wrong. A freewheeling diode will simply allow the coil current to decay slowly to zero. Magnetic field H is proportional to current I (H = constant*n*I) and won't reverse. In some applications, the slow decay is unwanted, e.g. if you want to release a contactor or magnet fastly. In this case you'll allow a higher flyback voltage and respective higher dI/dt, e.g. by using a varistor or diode/Z-diode combination.
 

DC177E

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Ah, alright, sorry about that, don't know why I assumed the magnetic field would reverse.

Thank You!
 

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