Vasile, What do you mean by "anti-parallel"? But I'm avoiding putting it across=20 the load as the expectation is that the positive side of the load (+12V)=20 will not be in the actual circuit. Cheers, -Neil On 11/26/2015 1:31 PM, embedded systems wrote: > Schematic looks good. > I prefer a fast rectifying diode in anti-parallel on the load versus your > zener on the MOSFET. > Maybe you should try. > At turn on you have 4x nominal current on the load. > Say the value of this current. > > Vasile > > On Thu, Nov 26, 2015 at 6:59 PM, Neil wrote: > >> Hi all, >> >> Back again, and I have time today to get back to this. First, I'll add >> more detail... >> >> Schematic... >> http://bit.ly/1Q11zbg >> The PIC controls the MOSFET, even during the "PWM" (current-controlled >> hold) phase, using an interrupt at ~20kHz. It checks the comparator to >> see if the voltage across the sense resistor is above or below the 50mV >> reference and toggles the MOSFET on/off accordingly. In these, diode Df >> was previously a ~30V 5W zener but got very hot, and I didn't have a >> lower-voltage zener that could handle any decent power, so I swapped it >> 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 >> occasionally that must be noise being picked up by the scope). However, >> 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 MOS= FET >>> 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 realit= y >>> 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 .