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[TCML] DRSSTC ring-up

Hello all,

I'm still tussling with my DRSSTC:
In the list archives and the internet there are very few pictures showing the ring-up of
a DRSSTC. Sometimes the amplitude rises linearly, sometimes it goes up exponentially. My
DRSSTC showes linear ring-up, but with a strong sinusoidal modulation, if the natural
frequencies of primary and secondary coil are identical. As this modulation extends the
time-to-breakout, I have tried to avoid it by altering the coupling between the coils -
but without avail. The only possibility to get a smoother envelope was to tune the
primary coil lower.
My circuit:
half-bridge with TO-247-IGBTs - primary coil - MMC - GND
ON-time of burst 100 µs, OFF-time of burst 8 ms
peak current limited to 75 A to keep the IGBTs alive
natural frequency of primary and secondary coil: 299 kHz
after coupling the lower pole is 264 kHz and the upper pole is 313 kHz
MMC for natural frequency is 75 nF and for tuning low (see above) it is 94 nF
inductance of primary coil is constant
rail voltage is +57 Vdc at the begin of current limitation
no streamers, only strong corona from breakout pin (distance to grounded rod 2")

What's wrong with my system? Any advice is appreciated very much!


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