Electronic – Parasitic Capacitor current of an inductor

current measurementinductormosfetparasitic-capacitance

LS turn on with load ind of lower parasitic capLS turn on with load inductor of higher parasitic capacitanceI am doing a double pulse test on SiC MOSFETs from 100-800 V and current from 20-80 A. The schematic is shown in figure (the values are not correct though).
In my first round of tests (Figure 2), I was getting rather oscillatory switch current for lower switch (measured with 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 (Figure 1). 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 (50MHz) but still no difference. The figure with load current is not attached.
Is this a bandwidth issue? My ride time and fall time are ~50 ns


Best Answer

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.


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. enter image description here


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