Hi all,

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.

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 MOSFET=
=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.

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).

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?

Cheers,
-Neil.

--=20
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