Hi Neil The schematic should work OK provided that there is a good return path from the top of the coil/solenoid to ground - through a decoupling cap and power supply. When the MOSFET turns off, the current throught the inductor will case the voltae at the drain to increase to the breakdown voltage of the diode. It then continues through the diode and resistor to ground. Meantime the current through the coils will also cause the voltage at the top end of the coil to drop, Provided it is fed from the power supply that's ok, but the path needs to be short and fat. If the top end of the coild is decoupled to ground then this current can also be supplied from the current passing through Df and Rs, reducing the requirement on the power supply. This will only work if the decoupling is good for all frequencies involved. A single electrolytic at the feed point to the coil will be OK for the low frequency components (assuming low ESR) but will not be effective at the higher frequencies - you will need several lower value, low inductance, low esr, caps in parallel. say 100nF, 1nF sort of thing. Having Df where it is also presents a problem with the rectifier diode you are using. The voltage at the drain of the MOSFET will increase to the breakdown voltage of the diode (or the MOSFET, whichever is lower). the fact that the Zener was getting hot, indicates it was dissapating power. With a rectifier diode that energy is still being dissapated, possibly just somewhere else. Any high voltage will also capacitively couple to the gate and try and turn the MOSFET on. How successful it is depends on how good the driver circuit is but it could be slowing things down a bit and putting the dissapation into the MOSFET. The alternative position for the diode is directly across the coil. In this posistion, when the MOSFET turns off the voltage at the drain increase and the current loops back though the diode and back into the top of the coil. This does have the disadvantage that the coil current drops more slowly (as the energy is being dissipated by the coil windings mostly) so it drops out slower. May not be desirable. In this case you can add a zener in series with the catch diode and use it to dissipate the energy, shutting things down quicker. And the zener may need to be heatsunk. This presents problems if you need to monitor the current through Df as well as the MOSFET? Can you not just use the current value when the MOSFET is turned on, or is the LM339 used as a safety check that things have not blown? With the diode across the coil, the only problem would be if the diode goes s/c - is this an issue? The thing to remember is that energy is stored in the coil and that it will need to be directed to something that can handle it - zener, reistor or power supply/ battery. There are a number of clever ways to recover this energy, but they can get quite complex. Putting a rectifier diode across the coil may cause issues too as the reverse recovery is quite slow, you really need a fast diode for this. If RF noise is a problem, you'll need to add a snubber as well. This can be across either the coil or the MOSFET - assuming the top of the coil is properly decoupled. Richard On 27 November 2015 at 07:31, 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 MO= SFET >> > 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 fo= r >> > up to ~22ms, before going into idle/off phase and then starting anothe= r >> > cycle. The hold cycle is PWMed (I'm still calling it PWM, but in reali= ty >> > 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 significan= t >> > 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 >> > -- > 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 .