ringing with mosfets problems

Hi all,

I've converted a H-bridge from a P-N type design to using all N-chan
mosfets. While the conversion works its left me with a problem. When you
turn the rail voltage up from 1-40V the mosfet gates get really bad
overshoot and ringing on high and low edges. The mosfets actually start to
turn on, then have a large overshoot spike which then turns negative and
turns the mosfet off, then it goes high again. Its overheating the mosfets
really bad because of these false turn ons.

I managed to filter out the problem with the P-N H-bridge, though I'm having
real problems with the N-chan design :-(

I tuned up the H-bridge so the input waves look perfect using a fast diode
and a 33R resistor on the mosfet gates, though it wasn't until I actually
ran power though the mosfets that the squarewave input looked more like
spaghetti.

I've read countless articles on driving the mosfets, bascially now all my
copper traces are that short that they is almot nothing there, the whole
design is about credit card size, one solid ground plain on the back of the
board, no sharp T edges, all traces are rounded. I've done everything no
matter how small to keep it as good as possible, and while I've got the P-N
bridge sussed, the N-chan version seems a whole new ball game.

I tried adding a small snubber circuit but the mosfets just drew top amps
from the PSU and smoked out. I already had to do 13 PCB revisions to get the
P-N bridge to work right and that seems easy now compaird to this new set of
problems. Its also becomming very tiresome to get something so simple to
work right :-(

Reading up on all the pro'c & Con's to driving mosfets in H-bridge style, or
any mosfet in general, you could pretty much conclude that its going to be
impossible to build a perfect design, the slightest little thing of even
making one small copper trace 5mm longer is enough to totally trash the
whole design. Hey you can even build your own PCB from a manufactures
gerbers and build it up and have it not work right, when your up against
impossible problems like that it makes you wonder...

Chris


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