On 11/25/2015 2:13 AM, rubenjonsson@bredband.net wrote:
> If it gets cooler when you ad an external diode it could mean that you
> have a lot of ringing when you switch the transistor and the peak
> voltage
> from that may be higher than the supply voltage so the body diode
> starts
> to conduct. This diode is not very good and will produce significant
> heat
> when conducting.

I haven't tried without an external flyback diode... with a ~30V 5W=20
zener, the zener got very hot and smoked.  But the MOSFET didn't get=20
very hot.  With a gen purpose 10A rectifier, the rectifier only got warm=20
but the MOSFET got very hot.  So this points to the internal body diode=20
conducting...?

> The ringing may also affect the gate voltage in such a way that the
> transistor is not switched on as sharply as you have calculated
> (capacitance between drain/source and gate).
>
> 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.


> 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?


> /Ruben
>

--=20
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