Re: ringing with mosfets problems

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




"exxos" <exxos@xxxxxxxxxx> wrote in message
news:42d8e45f$0$6466$cc9e4d1f@xxxxxxxxxxxxxxxxxxxxxxxxxxx
> 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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