I am using two MOSFETs (IXFB210N30P3) at about 100kHz (with different duty cycles) to split current from a 100-200V rectifier into three separate coils. This is a very rough schematic of (one branch of) the circuit (apologies for the diagram, I'm in no way an electrical engineer), where the box is actually a PWM controller putting 15V across the gate at 100kHz.

enter image description here

If the diagram isn't clear, here is a hand-drawn version with the details of the FET driver. The capacitor across the FET is supposed to be a snubber circuit, but I'm not sure if I need a smaller capacitor or a resistor in series to actually work. Either way, with even just a few volts across the coil (the 1 ohm, 6 mH component), I start seeing huge voltage spikes at the MOSFET gate

enter image description here

which I haven't been able to remove. I imagine this has something to do with the enormous inductance of the coil, but I've tried a number of filters and diodes with little success. I'm wondering if someone could explain this to me, and potentially recommend a solution. I have ferrite beads on just about every component, and a few low pass RC filters now, but the voltage spikes are still there. What value resistors and capacitors should I have on the snubber circuit across the FET? The diode and FET are quite large, so potentially they're just too slow for the high-frequency switching.

For reference, here is what the signal looks like for very small voltage across the coil, where yellow is voltage at the gate, cyan is the voltage at the drain, and purple is at the source.enter image description here

  • \$\begingroup\$ why did you photo your scope instead of print to file, that scope has usb... \$\endgroup\$
    – user16222
    May 21, 2017 at 16:47
  • 1
    \$\begingroup\$ Have try to add a flyback diode next to the FET? \$\endgroup\$
    – M.Ferru
    May 21, 2017 at 16:49
  • \$\begingroup\$ @JohnRB Because I'm an idiot and didn't think that through :| \$\endgroup\$
    – JAustin
    May 21, 2017 at 16:50
  • \$\begingroup\$ Is there actually a battery at the output? \$\endgroup\$
    – winny
    May 21, 2017 at 18:02
  • \$\begingroup\$ Battery? I think the thing which looks like a battery is the FET driver (its a controller and driver). This is a hand drawn schematic, but I couldn't find the right parts in the software I was using to draw the diagram. \$\endgroup\$
    – JAustin
    May 21, 2017 at 18:14

1 Answer 1


You have to be very careful about probing such circuits. It is easy to get large surge voltages in the ground system into the scope measurement point. I expect that what you are seeing on the scope is not really what is happening.

You will need to separate the current path for the gate drive from the path for the inductor. Use separate ground wires between each of the FET drivers and the FET it is driving. Keep them short, join the grounds at the FETs.

Make sure you use a very short ground lead on the scope connecting to the FET as directly as possible.

You don't show any decoupling of the 100v supply close to the inductor and back to the FET. When the FET stops conducting there will be a very rapid reduction in current from the battery, that can cause very large spikes in the inductance of the power leads. A capacitor from the source of the FET to the positive end of the inductor can reduce the effect of the lead inductance to the battery. Again keep the leads short.

You also show a P-channel FET running as a source follower? Is that true? I would expect to see an N-channel device with the source to ground.

  • \$\begingroup\$ I've updated the diagram, thanks. So to be clear, you're suggesting I separate the gate driver from the FET circuit entirely, grounding them separately to avoid interference/back EMF? \$\endgroup\$
    – JAustin
    May 21, 2017 at 17:09
  • \$\begingroup\$ Can you explain what you mean by "decoupling of the 100v supply close to the inductor and back to the FET"? I'm only showing part of the circuit, but the full circuit will have two FETs and three total coils connected in parallel to the 100V rectifier source, and I'm hoping to vary the duty cycle of the two FETs to split the current without using resistors. \$\endgroup\$
    – JAustin
    May 21, 2017 at 17:11
  • \$\begingroup\$ Thanks for your help, by the way. I have about three days worth of EE experience, so I really appreciate your advice. \$\endgroup\$
    – JAustin
    May 21, 2017 at 17:15
  • \$\begingroup\$ "I'm only showing part of the circuit, but the full circuit will have two FETs and three total coils connected in parallel to the 100V rectifier source" Please include that in your question or everyone here is just guessing by filling in the blanks or making assumptions. \$\endgroup\$
    – winny
    May 21, 2017 at 20:22

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