Parasitic capacitor current of an inductor

Question

I am doing a double pulse test on SiC MOSFETs from 100-800 V and current from 20-80 A. The schematic is shown below (the values are not correct though):

In my first round of tests, I was getting a rather oscillatory switch current for the lower switch (measured with a Rogowski coil of 30 MHz bandwidth):

When I replaced the inductor in circuit to an option with lower parasitic capacitance (had more distance between winding turns), the switch current curve improved considerably:

This made me believe that a part of those oscillations were because of parasitic capacitance of the inductor.

To confirm this, I wanted to check the inductor current difference in both the situations, so I probed with another Rogowski coil at the inductor with each inducor but IL wasn't really oscillatory and there was no difference in IL with either of the inductor. I repeated this with a current probe (50 MHz) but still no difference. The figure with load current is not attached. Is this a bandwidth issue? My rise and fall times are ~50 ns.

Answer 1

Yes your characteristic impedance is too low (L/C depenent and coax) ,
meaning the resonant frequency is too low,
meaning the interwinding capacitance is too high.

Choose a design where the L/C ratio with lower C meaning higher Zo and LC product is reduced to give higher fo, and L/R ratio gives lower Q.

suggestions.

This may depend more on your coaxial load or choice of dielectric as well as inter-winding space.

Air-gap clamshell, bifilar wound gives least L but higher C unless larger radius semi-rigid.

It is easy to measure Q and SRF. Do you know how by measuring current with sweep?

Try to make pulse look like my air coil current sense.

Note

area of -ve pulse matches +ve, but rise time limited by scope..