First SSTC build - some questions

[davekni]

As expected.  It is nice when the world starts to make sense.

I've learned quite a bit from this long thread, especially with all your detailed scope measurements.  I hadn't realized how significant induced sine-wave primary current was compared to the triangle component of a primary alone for normal SSTC designs.  (My only personal SSTC experience is unconventional.  H-Bridge through ferrite-core transformer generates +-4kV square wave fed to bottom of secondary.  No primary winding.)

BTW, way back you'd asked about my PFC.  No, I haven't documented it.  No one would want to copy.  As typical for me, most of the analog circuitry is implemented with discrete transistors rather than ICs.

[zytra]

Thanks, I'm a bit surprised but happy you were able to learn some!

Everything makes a lot more sense now; I'll continue experimenting with primary turns (coupling/inductance) I have a feeling this part of the system has room for improvement. I'll convert this into a double resonance shortly as well, I have almost everything built just to put it all together.

[zytra]

I'll post some stuff soon in a thread about the DR that I am putting together reusing this full bridge initially.

Meanwhile, I wanted to do an experiment with this bridge and driver. I didn't want to create a new thread for this especially as it's directly related to the stuff built in this thread. Basically I turned a multi-chamber secondary structure out of PTFE and got around to wind it today. I initially hooked it up to one of those ZVS boards with underwhelming results. It's a possibility that my primary didn't have enough turn (I went with 2x 2.5 turns - the driver is designed for a center tap primary). Anyway, I thought I'd give a shot on my SSTC driver/bridge. So I disconnected the 7414 and feedback, and taped my signal generator to the input pin of the driver chip. Results were quite a bit better than with the ZVS driver. And since I still had the oscilloscope hooked up the same way it was for the SSTC I immediately saw the IGBT were hard switching. With very little input power my temperature probe already showed they were heating more than they do with the SSTC at much great input power. I tried adding the DC blocking cap again thinking it might move the phase a bit, and it did but nothing significant. First screenshot is without the 4.7uF and the second is with it.

Anyway I stopped there and started doing some research on the topic and found that Steve Ward actually did something very similar: https://www.stevehv.4hv.org/FBD.htm
He doesn't talk about hard/soft switching but other than for a half bridge and a couple of caps it's basically the same concept. He does mention he was able to pull 450W which is probably over 10 times what I was able to pull safely today. IGBT's may not be the best for this, but I was comfortably switching at 150 kHz which is well in the capabilities of those IGBT's. With this soft switching, this project could work. The transformer itself (well the secondary) is a success from what I can tell. I have at least over 3000 turns, everything submerged in mineral oil.

[davekni]

I'd suggest sweeping frequency at low voltage to find the transformer's secondary resonant frequency.  Even with a segmented bobbin, 3000 turns may have enough intra-winding capacitance to resonate well below your 50kHz drive frequency.

More information on your transformer would help too.  Roughly what is the cross-sectional area and path length through the core?  What is the total gap length between core halves?  Or, is it not gapped?  (That would explain it not working well with ZVS oscillators.)

[zytra]

The transformer is fully custom, I'll post pictures later tonight. It's made of 2x "C" shape ferrite bolted together. The secondary is roughly 26mm ID x 50 mm long (that includes 15 separations that are about 2mm thick). The core has an OD of 20mm, meaning 3mm of insulating PTFE. The segmented secondary was turned on a piece of PTFE. The core is not gapped.

For the primary I have 2x 2.5 turns, which is a bit on the small side which could explain the lack of success with the ZVS driver.

After running the transformer on the SSTC driver, I decided to make my own ZVS driver. And since I ended up using roughly the same components, results were the same outside of the capacitor that was film but with high ESR, it ended up heating a lot so I replaced it with a better quality capacitor with significantly lower ESR but also slightly higher capacitance. Results were immediately better, and after playing with the few caps I had available, found that 4.7 uF was giving me the best results. Note: I did go back to the commercial ZVS and put the same 4.7uF cap across the far ends taps of the primary and results were much better, but as good as mine oddly - perhaps due to the other caps still being there. I say "caps" because that driver used 2 for some reason. The ZVS circuit I am familiar with use 1 cap across the far ends of the primary, and this one uses two. I looked at the PCB and it looks like they use 2x ~0.330F but I am not sure how it is all wired together.

I proceeded with assembly of my 200mm plasma ball, flushed it with argon at 1atm and results were great. Voltage was probably a bit high because I can't run a vacuum on that cheap plastic globe (that has a flat bottom - I tried by the way, and it's not a good idea, haha). I used a grounded striker to test contact and it would definitely shock you as opposed to normal plasma ball.

The goal is to make a bigger one (glass, round, under vacuum) so I don't think I will dial down on the voltage yet, just to make sure I have enough for the larger one down the road.


I did experience a set back with a failure of the secondary. The last chamber, last few turns (hard to tell exactly) arc'ed through the PTFE to the ferrite. Not through the thickness which was 3mm but through the last separation which was only 2mm in my design for some reason.

I've made revisions to the design and will turn it this week, increased the thicknesses separating the windings from the ferrite to a total of 4mm. I had to reduce the thickness of chamber to chamber separation to compensate, but I think this will be fine, as I don't expect any arc to get through 1mm PTFE on two adjacent chamber. If it took 16 chambers to go through 2mm or so on the HV side where the potential difference is maximum, I think I should be fine with 1mm between 2 adjacent chamber.

Another setback today after rewinding a spare V1 core. I wound this one with only 2 differences: I didn't fill the last chamber (HV side), and I tried to compensate by adding 1mm more thickness on all of the other chambers. That one failed with an arc within one chamber, along the surface of the PTFE which darkened, but no penetration. I think what happened is that my layering wasn't great on this way, I kinda rushed it last night, the added layers didn't help as it increased the voltage difference within one chamber.


With all tests I found that I was lacking a direct voltage measurement on the HV side. I actually was able to use a HV probe on my early tests (that didn't work well) and measured 8kV. After tweaking the capacitor on my own ZVS I was able to get intense plasma at ambient air and discharges over 2" long, so there is no doubt that my 10kV probe wouldn't be able to work there.

I think I'll copy this in a new thread in the appropriate section because it's pretty interesting too, and some my findings could be of use to someone else.

[davekni]

It would be interesting to see waveforms for this - primary voltage and secondary with just antenna pickup (reasonable phase, just no amplitude calibration).  ZVS drivers usually aren't used with ungapped inductors.  Inductance is high and saturation current low.  I wonder if your system is running at a frequency determined by leakage inductance and the primary and secondary (intrawinding) capacitance.

Besides the HV end failure, the common tricky problem with segmented bobbins is the wire transition from the top of one segment to the bottom of the next.  Unless there are slots or other accommodations in the separation walls, that wire to the bottom ends up adjacent every layer (including the top) of the new segment as it is wound.  That leaves a full segment's voltage across two thicknesses of enamel insulation.

BTW, the two 0.33uF (1200Vdc, 630Vac) caps in the commercial ZVS are almost certainly in parallel.  They are standard Chinese induction cooktop capacitors, the ones I use for my DRSSTC MMC.  Some ZVS units have 6 or 8 in parallel.

[zytra]

Thanks Dave, I split the thread here and replied there as well: https://highvoltageforum.net/index.php?topic=1391.msg10494#msg10494