Voltage spike in MOSFET circuit

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: V1 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.

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.

• Are you scope probes compensated? May 22 '17 at 20:44
• What type of diode are you using? Have you decoupled the DC supply close to the load and the FET source? May 23 '17 at 2:47
• @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. May 23 '17 at 3:38
• 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. May 23 '17 at 3:59