Quote:
Originally Posted by MazdaMatt
The voltage across those coils = L*dI/dt where L is the inductor rating. So if you switch a 1A charge off in 1uS, you are generating 1V for each uH of inductance. You have what, 150uH? So if you switch off 5A in 1us you generate 750 volts. There is also a "ringing" that occurs in fast switching.
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Matt and arnolde, I'm just being picky here, but it's not the 150uH inductors that cause the ringing. The current in those inductors do not change from full current to zero current. The 'flyback' diode allows the current to continue through the output loop, thus the current in those inductors do not change (much).
I guess there is a finite time in between the mosfet turning off and the diode becoming biased which I think can cause some voltage spikes. There's also a finite time for the diode to turn off, but that's another story.
On the other hand, the input current IS getting switched on and off from zero to full current. This is the main source of Drain/Source ringing - you don't want ANY inductance in the input loop (and since zero inductance is impossible, you'll at least want to minimize it). Minimize parasitic inductance by minimizing trace/wire lengths and keeping the caps/diode/mosfet close together.