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sparky

Mon Mar 26 2007, 06:53PM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

20' discharge ...hum mm- sounds like it would be quite possible.

I'd adjust your top load for sure, and maybe add another turn of copper tube?? I wonder what a Tesla magnifier system would do on your monster power supply!! ....anyways this makes me really want to build one just for the hell of it!

I just found a brand new Powerex CM1200HA-24J ... I wonder if these kind are okay for a Tesla Coil ???

and even one DIM2400ESM17A00 .... I'm not sure if these are very useful??

Tell me... why are you running 2 IGBTs together?? ...maybe a stupid question but I'm a little new to this kind of coil system.

Marko

Mon Mar 26 2007, 09:20PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

As fas as I see, he's running a most basic fullbridge with a beefy GDT powered directly from mains.

In a way, it's more reliable than isolated driver since there's practically no way for both IGBT's in bridge to turn on simultaneously and for long.

I just found a brand new Powerex CM1200HA-24J ... I wonder if these kind are okay for a Tesla Coil ???
and even one DIM2400ESM17A00 .... I'm not sure if these are very useful??

Well.. if you can find a fullbridge of them, surely! Althoguh they are slow and huge overkill considering those IGBT's in this monster actually have a lower current rating.

sparky

Tue Mar 27 2007, 01:37AM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

I wonder what current/volt rating those IGBT are that Aaron is using??
1200V 600A? Humm... curious.

Steve Ward

Tue Mar 27 2007, 04:51AM

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

Steve, believe it or not all the problems I was having at the thon were because of a half dead UCC part.

Here's one for ya. When my coil failed at DCs thon, and the gate driver card was "down"... guess what the problem was? The negative supply regulation zener! Took me so long to figure that one out as it seems so unlikely. So that, along with the obviously blown IGBT. So i will have to agree, your driver is probably a little easier to de-bug.

hvguy

Tue Mar 27 2007, 05:49AM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

Well the weather didnâ€™t just hold up for tonight, it was perfect; practically no wind, about 65 and 50% humidity. So given these ideal circumstances I expected to push the 20â€™ mark with relative ease. Everything was set up as the night before. I did a few test runs and nailed down the tuning. Things were looking good with a few ~10sec runs easily reaching out to 18â€™ at about 70% input power. I set up the camera and DSO to capture some primary current waveforms, ran the variac stack up to about 50% thenâ€¦ This happen:

Shit! I hope I didnâ€™t kill an $1100 transistor (not that I paid that for it, but stillâ€¦)! I checked the 60A ultra-fast semiconductor fuse on my PS and it was blown so I shut everything down and went to take a look. This is what I found:

After a few intense moments checking the IGBTs I found they were fine; looks like the TVSs, which all had holes blown out the sides, did their job. Everything else is tested and functional except the other TVS strings. They too showed signs of imminent failure (one string measured 1180ohms, both ways).

Ok, so why did 20 (1.5KE200 2 parallel strings of 5 in series on each IGBT) TVSs fail so catastrophically after only maybe 2min total run time? Upon closer inspection the solder had begun to melt off the undetonated strings on the other brick. This tends toward a fairly obvious answer of over-dissipation. This does not sound unreasonable as the total delay time at the switching transitions, because of these huge, slow, IGBTs it almost 1uS. While I did not get a scope shot of the primary current I did manage to read the screen before the failure. My guess was right on at nearly 2KA Pk on the primary, possibly less (lower Vin) when it failed; I was not watching the scope then.

So if TVSs can not handle the spikes whatâ€™s next? Well, I will be bench testing the system without the secondary in circuit to get an idea of the voltage spikes I am dealing with. Given the scale of things I would not be surprised if they were 2X the rail. All this with only 2â€ between the caps and the IGBT... I really think the delay in these things is causing this problem. The thing is I canâ€™t fix the delay, its inherent to these IGBTs, so now I have to figure out how to deal with it. Any Ideas?

Steve, I hate problems like that, they just suck time like a black hole and itâ€™s never that satisfying in the end. Mine is probably easier to trouble shoot but itâ€™s still a Tesla coil; Iâ€™d swear it is impossible to build one of these things with out something failingâ€¦

sparky

Tue Mar 27 2007, 07:54AM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

I'm wondering why all the complexity in your circuit...excuse me for being naive I've never built one of these systems before, but I can't help but notice the circuit controls, variacs, diode this diode that.

Why not boost the voltage using a rewired transformer and rectify it to a higher voltage - say 800-900VDC. Maybe this would be less of a problem a give a bigger bang size- cut down on the amount of silicon used between the IGBT and the TC primary???

Steve Conner

Tue Mar 27 2007, 10:44AM

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

Hi Aron,

The scale of this thing is really awesome!

Even if you "only" ever got 18' you could still be pretty proud of it. Here are a few things I thought of:

1) Most even halfway recent IGBTs have avalanche capability. That is to say, the IGBT dies themselves have TVS-like behaviour. Even if they only have a very modest avalanche rating, I bet they can still take more abuse than those tiny TVS you burnt out. These IGBTs typically hard-switch hundreds of amps in industrial service, with maybe a snubber capacitor, but certainly no bolt-on TVS.

So the system may work fine with no TVS at all (but check the IGBT datasheet, or call the IGBT manufacturer, if you want to be sure) If you really want to use TVS, a series-parallel "MMC" array of them with forced air or oil cooling may be a good option.

2) As for coping with the turn-off delays of large IGBTs, my solution to that was the PLL driver, which can anticipate the current zero crossing and send the turn-off signal in advance. This was pretty much the only reason I spent so much effort on PLLs, because I thought they would scale to the biggest coils. Unfortunately I've been too busy with other stuff to try it out on IGBT bricks yet, it could be a total flop

Sparky: The bigger you make a solid-state coil, the more expensive the damage when it blows up. So, for bigger coils, it's worth spending more effort on protection circuitry. The power circuitry on Aron's coil is already as simple as a SSTC can be.

sparky

Tue Mar 27 2007, 04:27PM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

I was thinking along the same lines too... having protection circuits is a bonus and I'd hate to see an expensive IGBT fry!!

I am really amazed by the sheer output of Arons Tesla Coil. I'd love to build one - though I must admit I've never really been one for Solid State devices --- guess I should buy a book on it. I'd would really like to learn more about these kinds of Tesla Coils...

Dr. Drone

Tue Mar 27 2007, 05:34PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

hvguy

Tue Mar 27 2007, 07:46PM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

Steve C., you make a good point. I had not considered that as an option so I called Powerex and spoke to an applications engineer and, believe it or not, the entire CM series has NO avalanche capability! He said if you exceed Vce a few times the part will most likely fail. In light of this new information I may have to consider an active clamp. This would allow me to dissipate the spikes in something more conveniently heat sunk than a DO-201 diode or even recover the spike energy. Of course, pending the bench testing, this may be over kill. How about the good-ol RCD snubber and a big fan?

Also, you had mentioned a PLL. I experimented with this back in 98 when I was working with the SSTCs. I do not recall having much luck with it. Ultimately I did get a CD4046 based circuit working but it was such a hassle to adjust and so much more complicated than the feedback scheme of the time that I abandon the idea. Perhaps I should reconsiderâ€¦ Is your design as reliable as my current driver? I have a small DRSSTC that has logged more than 200hrs with this set up so I am a little leery to change to something new.

Sparky, these parts are rated 1000A 1700V. You can google the PN for a data sheet. Yes, I would much rather nuke $40 of TVSs than one IGBT.

Chris, thanks! One thing I learned from the 100 or so coils I have built is that a larger top=less primary and ground hits. If you look at the e-field shape of a coil with a wide top it becomes fairly obvious why this is. Further more if you place a break out point up and at an angle off this wide toroid the arcs have a tendency to reach a bit further before grounding. So, no, for now I will leave it at this height. If ground strikes become a problem at higher power levels I may reconsider. Far as the thon goes...I don't know. It depends on whether time/money permit.

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