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.

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.

Ringing may be caused by inductive load and/or stray capacitances and=20
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.

/Ruben


On Wed, 25 Nov 2015 01:29:01 -0500, Neil <picdude3@narwani.org> wrote:
> Hi all,
>=20
> I got back to my solenoid/injector driver design this past weekend and=20
> built and coded it, and it works nicely, except that the MOSFET is=20
> getting very hot... though the calculations say it should run really cool=
..
>=20
> Quick overview -- PIC18F driving a TC4427A MOSFET driver (running off=20
> 12V), which drives this MOSFET ...=20
> http://www.nxp.com/documents/data_sheet/PSMN3R0-60PS.pdf
> It's ~3mOhm RdsOn at Vgs=3D10V.  There's a 0.05-ohm resistor on the MOSFE=
T=20
> source pin for current sensing.  And for the inductive snubber I was=20
> using a 30V 5W zener across the drain & source, but that was getting=20
> quite hot so for now I swapped it for a generic rectifier I had laying=20
> around (1.2Vf).  Much cooler and I notice the current drops off quite=20
> quickly still.
>=20
> it's currently setup to do 4.1A peak (approx 2.1ms), then 1.1A hold for=20
> up to ~22ms, before going into idle/off phase and then starting another=20
> cycle. The hold cycle is PWMed (I'm still calling it PWM, but in reality=
=20
> I'm current limiting by rapidly monitoring the output of a comparator=20
> (~10 khz) and toggling the MOSFET on/off accordingly).
>=20
> I'm sure I'm missing something here, but can't figure out what. Calcs=20
> say that if I run the peak hold cycle less frequently, I should get=20
> slightly less power consumption through the mosfet, but not significant=20
> enough to make a difference.  I'll run some tests tomorrow to see if I'm=
=20
> getting RdsOn in the range I expect, etc. but any thoughts on what may=20
> be causing this?
>=20
> Cheers,
> -Neil.

--=20
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