ringing with mosfets problems

I'd like to know how they fit all that smoke in there in the first place?

I think its why mosfets have all those holes in them, the holes are the storing space for all the smoke I think......

Chris

Chris,

I can help. There are several things going on here. First, The upper fets ( high side ) should have no traces of copper under them. The lower fets can be coupled to ground but never the upper ones. Second, their is a current mirror , during switching, that tries to turn both the upper and lower fets on at the same time. To eliminate this, add a 5 times larger resistor for turn on than the turn off. to do this you will need to ad a diode, schottky,

1 amp or so in series with the lower value resistor that is in parallel with the larger resistor. I know this seems like excessive but this will work every time and eliminates the trickey layout problems. I can furnish photos of wave forms.

Ray King

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Ray,

The best I have found is a schottky diode on each mosfet to turn them on fast, a 22R on the low side, and 56R on the high side (at least I think it was that way around).

I think actually now the problem is just ground noise, I think when the mosfets are switching they are just spiking the ground and that spike is being feed though the driver stage as false pulses. I think there are a lot of problems to be addressed, the ringing being the first one I am trying to cure as its the worst.

cheers, chris

Chris, The issue of the current mirror, if two fets are in series the turn on gate current is reflected into the other fets gate. So the turn off current should exceed the turn on current ( by 5 x )to prevent current shoot through.

Ray

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Ray,

Is 5x in general or you experience of it ?

I will be building a new circuit up tomorrow, so will try out your ideas then. I do agree that the highside mosfet effects the turn on of the low side, I think this is where half the problems start. curing that should help a great deal. Chrsi