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Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I finally did it. After being on the forum for over 2.5 years it's pretty embarrassing not to have made one yet, but better late than never.
Fres: 525 kHz Winding dimensions: 11cm diameter, 20.5 cm tall
PLL courtesy of Steve Conner. Jugding by the output waveform when driving a resistive load the poor driver does ok. 10 ohms looked pretty square, so I assume the driver is good enough for the task. The internal diodes of the IRFP450s are not disabled as of yet. I know IRFP450s have terribly slow internal diodes, so I'll disable them soon. The coil is still in the lash-up phase and in need of tuning. So far I've achieved 15cm ground arcs, so 3/4 of the secondary length. I haven't been able to take it to full power yet as the fets heat too much.
There is much work to do on it still, but for now I'm focusing on tuning. Is there any point in reducing the coupling? It's at max right now.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Nice work
Uzzors wrote ...
Is there any point in reducing the coupling? It's at max right now.
I think you actually have too low coupling.. spread the turns a bit, you'll notice you can then add more turns to maintain the same output power. Here the higher coupling the better, make it as high as possible without flash over.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Awesome to see someone finally build a driver with discrete parts. I tried building a driver out of IRFZ44-based halfbridge completely discrete but I didn't get but to light a flourescent lamp near the secondary.
I don't know whether you should use external diodes or not. They are expensive and may not be needed at all.
I admit IRFP450's are difficult to drive, but you should be able to do with very high speed using larger signal transistors like 2N2222+2N2907 and higher drive voltage, at speed comparable to UCC's.
BD's are very slow in comparison. You could try to use class AB instead of class B in order to prevent transistors from going into cutoff - that might give you somewhat faster transitions.
With smaller transistors I didn't see much improvement from class AB, though.
Regarding coupling, I second that you should increase it as much as possible - flashover is what usually limits it.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I think I have it tuned now, the sparks are bigger and it can run from the full 230V at least. I forgot to mention I was using a SS variac, which is why the sparks were more branch like before. Without it I get the normal sword-streamers. I'm satisfied with the output, nice 20 cm streamers, but IMO the fets heat too much. What is normal for an offline halfwave SSTC? My fets have an 8 cm fan keeping them cool, but after just 1 minute the heatsink is 60-70 degrees? I'm no good at estimating temp, but they're hot enough to only be held for a few seconds at a time before becoming uncomfortable. How cool do yours stay?
I measured the primary current with a 33 turn CT combined with 33 load resistor. This gives a 1A = 1V ratio, so with the scope set to 10A div in the scopeshot. Looks like 18A to me, which explains the IRFP450s sweating like they are. 65W of dissipation per device, so 130W on the puny heatsink! I've tried a few different primary inductances, but all they did was lower the primary current to 10A, while halving the streamer length. Better coupling needed as well maybe? Probably, but I can't get plastic pipe wide enough and long enough, so it'll have to stay as is.
I think I'll just put up with the massive heating current and enjoy longer streamers for shorter lengths of time The GDT and gate drivers get warm too anyway. (btw, I gave up and switched to UCC's, the BD1XX'es are just too slow at these frequencies.)
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hey uzzors- nice sparks!!
From your pictures it seems to me you still have poor coupling.. Add some turns and wind the primary closer to the resonator and make it longer (vertically), I recommend doing this until you start getting flash overs- then back up a little
I don't know why your fets heat up, I used little IRF740's and they got a bit hot after a minute, but on small heatsinks and without a fan. Though they were making just ~17cm arcs. I was using the "Mini SSTC" driver, maybe you could give it a try.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Thanks. It's probably a coupling matter like you say. More turns and winding the primary higher up would probably help. The problem is the secondary is wound on 110mm diameter secondary, which is the largest pipe size available. I found some sort of converter piece which I'm using now, but the length is limited. So changing the coupling is impossible atm. Before I was just using some plastic sheets, but they arced over rather easily. How much primary current are you running through your SSTC, jmartis?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I measured the primary current with a 33 turn CT combined with 33 load resistor. This gives a 1A = 1V ratio, so with the scope set to 10A div in the scopeshot. Looks like 18A to me, which explains the IRFP450s sweating like they are. 65W of dissipation per device, so 130W on the puny heatsink! I've tried a few different primary inductances, but all they did was lower the primary current to 10A, while halving the streamer length. Better coupling needed as well maybe? Probably, but I can't get plastic pipe wide enough and long enough, so it'll have to stay as is.
I guess you now notice the problem!
I wonder how are 14amp mosfets living t all with that kind of current
You should definitely increase coupling, as much as possible. At some point it's going to flash over though :(. You can try moving the primary somewhat up, but it's a gamble. Making a wider primary former could help somewhat, and best of all would be an inverse conical primary (although at much larger angle) like those used with SGTC's.
You could hardly get better, unless if you used a ferrite transformer and base feed to the coil. That's an intristic limitation for the SSTC's.
Ata point you might really want beefier switches, fullbridge and so on. 20cm spark after all isn't too bad for two IRFP450's.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
The RMS current is only 12.7A so they're playing within the survivable range.
I've put a CT on it and after some fiddling I've gotten the same output as with antenna feedback. The resonance point seems less defined with a CT. Also I had to reverse the phasing. Both worked, but one had a lower fres, with a little less output, while the opposite phasing had the same output as the antenna feedback and higher fres. The higher fres was higher than the antenna fres. Seems odd to me, but CTs use current feedback while antennas use voltage, so it must be caused by the current/voltage phase shift.
Pulling ground arcs works fine, the benefit of PLL I guess. Beefier switches are what I really want as lower on resistance could double run times, to the point where the UCC's and GDT are the limiting factor. However this coil will be mostly used as an HV supply for my x-ray setup so the run times won't be long at all.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
I looked at this when you first posted, but I had to come back to it and say well done!!! I really like the PLL driver, it's just so elegant. Really nice sparks Uzzors, they're as long as the coil, right!?! And the fact that you didn't use Mosfet drivers is an extra nice touch. Good job!
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Uzzors' First SSTC
The GDT and gate drivers get warm too anyway. (btw, I gave up and switched to UCC's, the BD1XX'es are just too slow at these frequencies.)
