Neil, try to find out what is dissipating. Use a two channel scope and see both the drain current and the Vds during switching. Do math multiply between Id and Vds and see when you have a peak. Your MOS has already an internal diode, so either the diode either the transistor or both have large dissipation during switching. BTW, the Vds is not higher than 60V caused by the large coil inductance? Vasile On Wed, Nov 25, 2015 at 8:29 AM, Neil wrote: > Hi all, > > I got back to my solenoid/injector driver design this past weekend and > built and coded it, and it works nicely, except that the MOSFET is > getting very hot... though the calculations say it should run really cool= .. > > Quick overview -- PIC18F driving a TC4427A MOSFET driver (running off > 12V), which drives this MOSFET ... > 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 > source pin for current sensing. And for the inductive snubber I was > using a 30V 5W zener across the drain & source, but that was getting > quite hot so for now I swapped it for a generic rectifier I had laying > around (1.2Vf). Much cooler and I notice the current drops off quite > quickly still. > > it's currently setup to do 4.1A peak (approx 2.1ms), then 1.1A hold for > up to ~22ms, before going into idle/off phase and then starting another > cycle. The hold cycle is PWMed (I'm still calling it PWM, but in reality > I'm current limiting by rapidly monitoring the output of a comparator > (~10 khz) and toggling the MOSFET on/off accordingly). > > I'm sure I'm missing something here, but can't figure out what. Calcs > say that if I run the peak hold cycle less frequently, I should get > slightly less power consumption through the mosfet, but not significant > enough to make a difference. I'll run some tests tomorrow to see if I'm > getting RdsOn in the range I expect, etc. but any thoughts on what may > be causing this? > > Cheers, > -Neil. > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .