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help with my SSTC

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LAN_403

lightlinked

Tue Apr 21 2009, 09:39AM

Registered Member #2087 Joined: Tue Apr 21 2009, 08:32AM
Location:
Posts: 115

So I began building this SSTC a few months ago in metal shop and I got 10 inch sparks with an IGBT half bridge @ 120 volts, full wave, unfiltered, until they popped (I killed a handful of silicon) and I lost interest for a while. Iâ€™ve been tinkering with it again and I think I want to convert it to a DRSSTC.

But right now I have some issues with it. Iâ€™ve made a full bridge of MOSFETS but when I tried plugging it in the TVSâ€™ blew. I had it running ok on batteries ( around 30 volts) and the TVSâ€™ were just on one half of the bridge, kind of left there from when it used to be IGBTs. I replaced the TVSâ€™ when I put in the MOSFETS, but I did not put them on the new bridge half. Now I am kind of afraid to give it another full power run.

So here are a few points about it right now,
- open loop, tuned to 132 khz (brightest light from neon)
- antenna feedback was not as good as open loop
- it is interrupted
- there is a DC blocking cap on the primary, 4 x .68uf paralleled

Sec - 22â€ x 4.5â€ dia 30ga
Pri - 10 feet of copper ground shaped into a coil, 6 turns
Topload â€“ 14 ikea bowls tig welded
Full bridge of ST W18NB40 w/ diodes and 220 volt TVSâ€™
The base is 14 ga steel plasma cut and MIG welded together

So right now my issue is the TVSâ€™ exploding. Before that it was the half bridgeâ€™s going. When they went last time it kind of looked like the PLL locked (the sparks got bigger suddenly) and then the breaker tripped. But I donâ€™t think the PLL really works. I have it running ok right now on about 36 volts, no break out unless I pull a spark.

Any suggestions for why it keeps blowing up?
If I do go dual resonant, can I use multiple snap caps paralleled for the reservoir instead of a few gigantic ones?
Is it okay for me to be using two GDTs instead of one?
Iâ€™m thinking of running a light bulb in series to try it out on the mains again.
Thanks for your help! I read through pretty much every forum page here about SSTCs before I figured I should just ask.

MRacerxdl

Tue Apr 21 2009, 04:44PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Running SSTC's without breakpoint can easily blow your bridge up. That is because the current at primary increases so much and kill the mosfets/igbts...

About the TVS, how you wired it to the bridge? The TVS needs to be in anti-parallel with the IGBT/Mosfet/Power Supply, not parallel.

lightlinked

Tue Apr 21 2009, 06:16PM

Registered Member #2087 Joined: Tue Apr 21 2009, 08:32AM
Location:
Posts: 115

there is a breakout, it's pretty much the end of the secondary wire which is stripped and stuck to the top load with a magnet.
TVS' are unidirectional so i have one on each of the transistors

Renesis

Tue Apr 21 2009, 06:57PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

That is the meanest strike rail i've ever seen. Nice!

Mads Barnkob

Tue Apr 21 2009, 07:29PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I just realized your entire construction is metal.

Could it be your primary coils see's some of the construction as a single turn coil and it stresses it enough to kill it self? I am certainly not sure on this, so somebody else please input.

Really cool construction though, looks really badass :)

MRacerxdl

Tue Apr 21 2009, 10:27PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

If the TVS are unidirectional it needs to be in Antiparallel with the IGBT's like here:

(Sorry if I you say that is in that direction, I didnt understand)...

The metal construction (as MadsKaizer say) may be considered as a shorted turn on the coil circuit, for make sure is it, test it out of the metal case that you have built, if it works, that is the metal case, if not, is other thing...

lightlinked

Wed Apr 22 2009, 10:14AM

Registered Member #2087 Joined: Tue Apr 21 2009, 08:32AM
Location:
Posts: 115

i ran with 300 watts of incandescent in series and it seemed fine so i gave it another go at 120 volts. at about 50% duty cycle another TVS blew up. I put them on the bottom of the board this time and i can tell which one it was since it left explosm all over the glass it was over. it is one of the negative side FETs. TVS' are on the drain and source of each FET. I am now out of TVS'. should i order higher voltage TVS' next time? these were 220

so can i use an array of paralleled snap caps for a decoupling filter reservoir in a DRSSTC?

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