I am using two MOSFETs (IXFB210N30P3) at about 100kHz (with different duty cycles) to split current from a 100V rectifier into three separate coils. This is a very rough schematic of (one branch of) the circuit, where the PWM controller is putting 15V across the gate at 100kHz. This is a corrected repost of an earlier thread yesterday. I apologize for taking pictures of the scope, but I don't have a USB on me.


simulate this circuit – Schematic created using CircuitLab

Even at small voltages (~1-2V) from the DC source (V1), I am seeing odd voltage spikes at the drain of the MOSFET, of two different varieties. Without the capacitor/resistor snubber circuit (R2/C1), voltage builds at the drain to several times the applied voltage, and at even higher voltages, it spikes erratically and interferes with the gate voltage.

NOTE: cyan is gate, yellow is drain, purple is at the positive terminal of V1.

V1 at 1.1V: At 1.1VV1 at 1.5V:At 1.5V

However, with the capacitor and resistor snubber switch, up to a small voltage (around 1.2V), the voltage at the drain is flat but still nearly twice the applied voltage (around 2.2V), possibly because it is charging the capacitor, but this too breaks down if a larger voltage is applied. I've tried calculating the solution analytically, but none of the analytic solutions predict the amplified voltage. My solution in Mathematica would suggest a peak voltage of only 40 mV.

enter image description here

Here is a video of the voltage behavior (without the resistor/capacitor snubber) as the V1 voltage is increased. I have no idea what to attribute this to, and my various ferrite beads, diodes, and RC filters/snubber circuits haven't been able to stop it. I'd appreciate any advice/comments on how to fix this! Thanks.

  • \$\begingroup\$ Are you scope probes compensated? \$\endgroup\$ May 22 '17 at 20:44
  • \$\begingroup\$ What type of diode are you using? Have you decoupled the DC supply close to the load and the FET source? \$\endgroup\$ May 23 '17 at 2:47
  • \$\begingroup\$ @KevinWhite The diode is a STTH200W03TV1. I've noticed that removing the diode has basically no effect on the voltage, so perhaps it is simply too slow. I've tried running a capacitor from supply to the source, but it didn't do much to the voltage at the FET. \$\endgroup\$
    – JAustin
    May 23 '17 at 3:38
  • \$\begingroup\$ Add a bunch of ceramic disk caps etc. to decouple the power input as close to the FET as you can, maybe even solder to the same points as the FET. \$\endgroup\$
    – MadHatter
    May 23 '17 at 3:59

When the FET is turned off the current in the inductor has to go somewhere - the diode will start conducting but only after the forward recovery time. This can be up to 2400ns and a voltage of 2-3v.

The ringing on the purple trace in the last picture is due to inductance in the ground or power leads.

Have you followed my recommendation from your other question regarding a decoupling capacitor between the FET source and the positive end of the inductor?

You need to keep leads very short - with high currents and rapid switching it is important to keep inductances low.

  • \$\begingroup\$ Thanks! I have tried a decoupling capacitor, but maybe it was too large. I will try something smaller tomorrow, and I'll also shorten all the leads. I've been using long leads and clips to make switching components easier. I'm a little worried about the snubber circuit: I don't know how large the capacitor and resistor should be, and even at 1ohm, I'm dissipating a lot of power on a 100V source. Do you have any advice/thoughts on that? Is there a more efficient way to suppress the inductance of the coil (maybe a better diode)? \$\endgroup\$
    – JAustin
    May 23 '17 at 5:37
  • \$\begingroup\$ I'm also struggling to explain why everything seems to go crazy above a certain critical voltage (as in picture 2 and the video). Could it be that enough voltage is building up to short the MOSFET? \$\endgroup\$
    – JAustin
    May 23 '17 at 5:50
  • \$\begingroup\$ I already had decoupling capacitors on the PWM controller and the driver (which oddly hasn't eliminated the voltage spike at the gate, as in the yellow here or this video). The problem I'm having is that the inductor is a coil with a ~12 inch diameter, so it's almost impossible to put a capacitor close to it and the board. \$\endgroup\$
    – JAustin
    May 24 '17 at 22:13

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