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):

LS turn on with load inductor of higher parasitic capacitance

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:

LS turn on with load ind of lower parasitic cap

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.

  • \$\begingroup\$ "The schematic is shown in figure (the values are not correct though)", what figure? \$\endgroup\$
    – Tyler
    Jan 3, 2018 at 13:54
  • 2
    \$\begingroup\$ 'rather' oscillatory? 'wasn't really' oscillatory? 'lower' capacitance? "The schematic is shown in figure (the values are not correct though)" - with such vagueness, how do you expect a good answer? Tell us what the correct values are, and show us the actual waveforms. Also a photo of the setup. \$\endgroup\$ Jan 3, 2018 at 20:10
  • \$\begingroup\$ Sorry, I am getting used to this forum.I have added the figures \$\endgroup\$
    – Autobot
    Jan 5, 2018 at 9:43
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    \$\begingroup\$ Where is the current probe? for double-pulse tests it is usual to put the could around the Source/Emitter of the UUT even if it is a tight squeeze. \$\endgroup\$
    – user16222
    Jan 5, 2018 at 9:59
  • 1
    \$\begingroup\$ What is the time unit of the plots? What are the different traces? \$\endgroup\$ Jan 23, 2018 at 15:56

1 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.


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..


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