Dr. Dark Current wrote ...

The part you posted is a relatively slow one. Have a look at the following part numbers:

HGTG20N60A4D (a relatively old transistor, but still fast even for today's standards and should be rugged)
(or other HGTG...A4D models)
FGH40N60SMD (a more modern part) (or other FGH...SMD models)
STGW30NC60WD (or other STGW...WD models)

Sigurthr wrote ...

The issue is that when using a secondary base current transformer the input to the schmitt triggers is pulled to ground through the DC resistance of the CT, so there will be NO feedback.

Dr. Dark Current wrote ...

Are the caps the power type (low dissipation factor)?

[img width=600]

Steve Conner wrote ...

I've always wondered why people don't bother to AC couple the feedback and DC bias the input approximately to the Schmitt trigger threshold. Seems like it would help the coil start up easier as oscillation would be sustained with a smaller signal from the CT.

Dr. Dark Current wrote ...

Unfortunately these caps are useless, they have a really huge dissipation factor.

Most coilers use polypropylene film / foil caps. Many manufacturers follow the markings - MKP for metallized film capacitor, MMKP for double metallized film and FKP for metal foil capacitor. FKP caps are most robust and most expensive. There are special kinds of HV RF power polypropylene caps - large cylinders. They are rare and very expensive usually.

Then there are another types of RF power caps which can be used: RF ceramic (values only up to few nF, several kV ratings, extremely low dissipation factor) and RF mica (higher capacitances). Unfortunately these caps are often quite expensive but almost indestructible.

Sigurthr wrote ...

I recommend WIMA or Cornell Dubilier brands for capacitors if you can afford them, they're tried and true tested to be reliable for TC use.

Required MMC voltage rating for DRSSTCs is a complex subject with no simple answer. Most suggest a rule of thumb something near 10x the DC bus voltage.

If you want to dig into the heart of the matter;

Each half cycle the inverter increases the voltage seen by the primary capacitor by 1/2 DC bus for a half bridge or by 1x DC bus for a full bridge*. This effect is lessened by the degree of loading the resonator puts on the inverter, which is determined by the loading of the streamer on the resonator (largely unknown and variable). If you design for a totally unloaded resonator you can determine a worst case scenario voltage expected to be seen across the primary capacitor if you know the number of cycles the inverter will switch per burst, which is determined by the burst length of the interrupter and the resonant frequency of the primary and the over current limiting circuitry (which should be set according to the maximum pulse Ids of the IGBTs).

*I think. I can't find my notes and there was some confusion in the discussion I had with several experienced members about whether the voltage increases by 1/2 or 1 per half cycle for half or full bridge. Logically my mind says it should be as I typed above: 1/2 Vbus per half cycle for half bridge.