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Registered Member #91
Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45
Hi,
Finally I picked up the build of my Mini SSTC (after Steve Ward's design) again. After I replaced the blown IRFP460 MOSFETs and two damaged zener diodes I managed to get some breakout with 230 V going into the bridge. For safety I put the primary of a MOT in series to limit the current in case things would go horribly wrong. The coil was drawing about 0,7 A and producing decent output (nothing spectacular, just what you expect from a first run I suppose).
Things were looking good so I removed the series MOT to see what would happen to the streamers. They got somewhat thicker, and the coil was drawing about 2,5 A. Then after a few seconds my MOSFETs exploded, taking with them two zeners (just like last time) and one gate resistor.
I replaced the blown components and I was able to run the coil with the MOT in series again. Increasing the coupling made the streamers somewhat longer (I increased it just a bit, I didn't search for maximum possible coupling yet).
My question however is if anyone knows why the FETs would promptly self-destruct after just a few seconds? Could my less-than-ideal coupling be the problem or should I look further? I didn't notice any significant heating of the FETs in my short runs with the MOT in series. Unfortunately, since everything is connected to the mains and I don't have an isolation transformer, I can't measure the signals going to the gates with my scope, but when I tested the circuit at just 35 V things were looking good.
I hope someone can give me some advice as I'm really excited to finally get this thing working reliably!
Registered Member #91
Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45
Sounds like an idea... I've got a coil lying around somewhere I suspect to be a current transformer, so I'll see if I can get some results with that, otherwise I have to make my own CT.
Registered Member #618
Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628
If your using an antenna another thing to watch out for is "faulty" feedback i guess you could call it, I can't remember what it all involves but it causes similar result, I've went threw maybe 36 460's, and about 12 260's, gonna try a full bridge with igbts next.
Registered Member #91
Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45
I tried to monitor the current going into the primary using a CT, but with the accuracy I got on low power tests it could be anywhere between 6 and 12 A or even further off.
Also, I think Ken is right that the feedback may be one of the causes the thing isn't working properly. I managed to get somewhat better streamers using a single loop of wire wrapped around the secondary and connected to the antenna via a 10k resistor, but the FETs would still heat too much. I also tried a CT on the RF ground wire, but so far this has only made matters worse (which means more blown FETs and diodes...).
Registered Member #618
Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628
The antenna method works great EXCEPT you need to find a good method to sheild it from the tc, Considering this was the same TC I made exactly to specs, but when I'd installed a toroid the arcs doubled in length, which is a good thng but ALSO made about 7" of arc FROM the antnna to the toroid, which immeaditly freaked me out, I killed the power, lowered the antenna since it was a telescoping one about 1" or so and then it immeaditly fried my fets due to driver switching issues, I've since used a CT which works alot better but using the wrong replaemet ets has since made me put mine on hold, plus an unattractive base IMo.
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Wrong coupling can cause MOSFET failure?
