Hi all,

Back again, and I have time today to get back to this.  First, I'll add=20
more detail...

Schematic...
http://bit.ly/1Q11zbg
The PIC controls the MOSFET, even during the "PWM" (current-controlled=20
hold) phase, using an interrupt at ~20kHz.  It checks the comparator to=20
see if the voltage across the sense resistor is above or below the 50mV=20
reference and toggles the MOSFET on/off accordingly.  In these, diode Df=20
was previously a ~30V 5W zener but got very hot, and I didn't have a=20
lower-voltage zener that could handle any decent power, so I swapped it=20
to a S2M 10A general-purpose rectifier.

Voltage across Rs (50mOhm), with different levels of detail...
http://bit.ly/1OtFNes
http://bit.ly/1OiCVmt
http://bit.ly/1T7FgA5

Vds...
http://bit.ly/1SmFymu

I've not seen any indication of ringing (though I see random spikes=20
occasionally that must be noise being picked up by the scope). However,=20
I have not considered the body diode causing the heat.

Cheers
-Neil.





On 11/25/2015 1:29 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.
>

--=20
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