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Finished SSTC but having problems

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Danielle

Wed May 30 2007, 06:25PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

I just finished my second SSTC but while I was raising the voltage both my IGBTs blew and my 12 A fuse on the variac blew. The wave forms are good so that's not the problem. I know these were the conditions at failure.

Half bridge of IGBTs
HGTG30N60C3D

Voltage divider

Voltage full wave rectified failed at 37v

2 turns on primary secondary fres= 270KHz
4.5"*9" dimensions of secondary, 30 gage wire.

Have a large filter cap and a film cap across +\- on IGBTs.

2 3.3 uf 250v ceramic film caps as divider.
circuit is protected by zeners, TVS and diodes
GDT 1:1:1 commercial because my 1:2:2 didn't have as clean wave forms.
I had a fluorescent tube 2 ft from the coil and it was glowing brightly in full daylight.
1\4 " spark off breakout point

no toroid.

please help I really don't want to blow more IGBTs I have 24 left and blown 30 on all my coils.
I think its a tuning issue but where should I start?

Thanks for the help,

Daniel.
edit: wouldn't adding more turns help me limit current helping this problem or is it involving coupling too?

Steve Ward

Wed May 30 2007, 08:18PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

2 turn primary might be a bit small. Ive gone down to this few turns, and it didnt help performance (but increased the current draw a lot).

Id also suspect your gate drive. Its possible you have a shoot-through condition where 2 IGBTs conduct at the same time during the switching transitions. You can detect this by scoping the +/- supply rails right at the IGBTs. If you see really large dips in the supply voltage, this can be an indication.

Its all in the details with this stuff, and ive just scratched the surface.

Danielle

Wed May 30 2007, 08:21PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

I was thinking that shoot through was possible, so I am planning to build a new self resonant driver soon but for now..... Ill get to it. and my DRSSTC. For this SSTC how many windings on the primary should I start with?, and also what should the maximum sparks I should expect. I ran this similarly at full power and got only a 6" flame. BTW I am running it non interrupted.

thedatastream

Thu May 31 2007, 06:39AM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Is 270kHz a bit too fast for IGBTs? Running at these frequencies with a 50% duty cycle, I would have thought you would expect to see some shoot through caused by the slow turn off of the devices...

Steve Conner

Thu May 31 2007, 11:59AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

270kHz uninterrupted may be too fast. My DRSSTC runs at 220kHz, but I used the fastest IGBTs available, and it's interrupted with a 1 to 2% duty cycle. The dual resonant topology also lowers the switching losses compared to a regular SSTC. I estimated the switching losses in my setup a while back, and they were over 1kW IIRC, far too high for uninterrupted operation.

Devices can die for a lot of reasons, far too many to list them all here, but other posters have raised some good points that you should check out.

Danielle

Thu May 31 2007, 03:24PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

Thanks for the help I guess I should build a new coil for my DRSSTC so I can run it much more efficiently, do you have any recommendations for fast IGBTs? I Saw some IGBTs that can run at 900Khz I should get those for something like this. Whats nice is that its an acrylic tube and really nicly wound is there something als I could do with it I really don't want to use FETs they caint supply enough power.

Brett Miller

Thu May 31 2007, 05:08PM

Registered Member #593 Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50

Daniel,

I wouldn't run 100% duty cycle (CW) with IGBT's at all if I were you. You might be able to get away with this with some hyper fast IGBT's and a 60Khz huge secondary...but not something I would want to attempt. I get 14 inch sparks on my SSTC (Steve Ward type MiniSSTC ant feedback) at 8% duty...30 prf with 3ms on-time. For those sparks I used a 31 awg 12.5" x 4.5" resonator (Fo == 170Khz) and a 11.75 x 2.75 toroid. With this same setup (IRFP260N FETS by the way) I get an 8" CW flame with no interrupter and the fets stay ice cold, as long as I don't start drawing flaming power arcs...in which case they do warm up.

I've thought about trying IGBT's in this coil by winding a slower secondary and upgrading the gate drive, but then why? It would be time, money and effort that I could put into my DRSSTC anyway so what's the point. If you want a good rock solid SSTC, I'd use a half bridge of IRFP260's and make the good old self resonant. I've never had a single failure, and I've really abused this thing. I've vaporized several sets of jumpers that were too thin to be good primary wires...I've tried all sorts of secondary sizes and Fo speeds with it, and not a single failure.

-Brett

Danielle

Thu May 31 2007, 09:33PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

Thanks for the help and I guess Ill work on my DRSSTC Ill get this running with fets and packaged it away for demos and do some serious work with the DRSSTC.

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