Flashover

[alan sailer]

I am putting together a small/medium sized DRSSTC using SKM100 in a full bridge and a UD2.7C thru-hole
driver. The set-up is working to first order ie I am getting sparks and drawing current.

I was testing yesterday and I had a spark-over from the secondary to the primary. Didn't kill the IGBJTs
maybe because of the 44nF/3kV cap from buss negative to ground. This happened with a JAVATC coupling
of ~0.2. I do wonder if this hurt my secondary but the coil still coils.

The design I am using is closely based on OneKone's coil. To me the separation between the secondary coil
and the inner ring of the flat spiral primary has always seemed very tight. It is about 7-8mm.

It seems to me that with higher coupling factors the potential (ha) for flash-over gets greater.

Is there any guidance as to best practice? My past coils have all used an insulating lexan cylinder
with the primary wrapped in a rising spiral. This seems like a good idea but I have no experience
with DRSSTCs which involve higher voltages than my other coils.

My impression of DRSSTCs is that many people use the flat spiral and others use the inverted cone.
It seems like for all these designs an insulating lexan cylinder around the lower part of the secondary
would be a good idea.

Cheers.

[AstRii]

0.2 coupling and only 8mm distance between primary and secondary asks for trouble.

It seems to me that with higher coupling factors the potential (ha) for flash-over gets greater.

That's a fact, higher coupling causes higher voltage/turn on the secondary coil. Once a certain critical threshold is exceeded, the secondary will flashover and it may even strike your primary, especially if your primary is so close to the secondary.
The coupling factor for a DRSSTC is usually around ~0.15, I would suggest decreasing coupling by making the inner most diameter of the primary bigger, which will even solve the tight proximity to the secondary.

[alan sailer]

I forgot to update and end this topic. I did decide to both lower the coupling and remove the primary inside turn to increase
the primary/secondary spacing.

I'm tuning the coil and have had no more flash-overs. Up to 27 inch sparks and expect more.

Thanks for the input.

Cheers.