> >
> > To get rid of the ringing you can slow down the switch on time with a
> > resistor in series with the gate. Too big resistor will mean too slow
> > switch on time which instead will produce heat in RDSOn while Vgs is
> > rising. Another option is to put a snubber (capacitor in series with a
> > resistor) in parallel with the transistor. The R and C should be
> > selected
> > based on the ringing frequency. The resistor may have to be a high
> > power
> > type (2+ Watts and could get hot in itself) and will contribute to
> > power
> > losses.
> I'm going to try a gate resistor.  I'm reading up on it now, but not=20
> seeing a way to calculate a fair starting value yet.

I imagined the schematics as a high side switch and seeing the scope images=
=20
doesn't indicate ringing in the sense I meant. So I don't think you need a=
=20
resistor (but - see below).

>=20
>=20
> > Ringing may be caused by inductive load and/or stray capacitances and
> > inductances in the switch circuit and long wire traces with impedance
> > mismatching between the driver and the transistor or between the micro
> > and
> > the driver. A 1 ns rise/fall time equals around 6 inches/150 mm in a
> > PCB
> > and if the traces are more than say 1/4 of that, they should be treated
> > as
> > critical and may have to be impedance matched on the receiver end.
> What do you mean by "A 1 ns rise/fall time equals around 6 inches/150 mm=
=20
> in a PCB" ?  I've got about 1.25" of trace length between my driver=20
> output and gate.  So you're suggesting that that would result in a much=20
> quicker rise/fall (at the gate), and hence be more prone to ringing?
>=20

I mean that 1 ns rise/fall time equals a distance on the PCB that is=20
approximately 6 inches / 150 mm. A voltage change propagates at about half =
the=20
speed of light on a typical PCB if I remember correctly. With trace lengths=
=20
longer than 1/4 of this length you may get problems with reflection if the=
=20
output impedance, transmission impedance (PCB trace) and input impedance ar=
e=20
different. This reflection can either add or subtract to the instant voltag=
e=20
level on the receiver side before the voltage has settled and can cause an=
=20
unwanted logic level transition in a digital input or even destroy it. The=
=20
total effect of this in this circuit could be that the turn on/off time for=
 the=20
transistor will be longer than you think which could cause higher switching=
=20
disspation before the minimum Rds on is reached.

If the rise/fall time is longer than 1 ns, the critical length also gets=20
longer.

Anyway, I don't think this will be a problem here since the current in the=
=20
inductor rises significantly slower than the rise time of the PIC and the=20
driver.

A small resistor in the gate could also help if you get a ringin effect on =
the=20
gate due to gate capacitance and stray board inductance.

/Ruben
--=20
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