My QCW DRSSTC, small questions.

[davekni]

Okay, Here's my new setup for the IGBTs. I have them on 1x1x1/8 copper blocks, held down with 3d-printed clamps, pushing down on the die. Is this good?

Looks good.  My only concern would be that printed clamp plastic might soften at temperature or perhaps relax some even if not heated.  It is important to maintain force during operation.  Perhaps these clamps will do fine.  Or perhaps a metal spring could be added somewhere to provide compliance for any plastic creep.  Or change to a bent metal piece.

[Lucasww]

The heat is a fair concern, which I somehow didn't realize. Fortunately, I don't think it will be much of an issue. It's printed from PETG which shouldn't soften until above 80C. When running previously it was barely heating at all without any air circulation, I'm going to have a fan running over the heatsink now so I doubt it will even reach 40C. I'll keep my eye on it, though.

[Lucasww]

Okay, Heres an update.
I spent some time making a nice little enclosure for everything and put it together. Overall the results are good and the IGBTs don't get hot.




However, I'm having trouble getting arcs any longer than before. I decided to try making a new secondary with thicker wire, inspired by Loneoceans' QCW 1.5. the new coil with the same topload has an unloaded frequency around 370kHz with upper pole around 450kHz. As expected the arcs were a bit straighter, although unfortunately not as big. I messed around with coupling and toploads a bit and was able to get good results placing another topload on top of it, lowering the effect of spark loading.



Of course I want to use a single topload, and I don't really want to make a larger topload, Although I will if I need to. I've seen Steve Ward using a small MMC running through the secondary coil to add to the topload capacitance (here https://www.flickr.com/photos/kickermagnet/7383788136/)
I have a bunch of 82p 3kV C0G ceramic caps originally purchased for HFSSTC stuff and I'm wondering if it would be possible to run like 30 of them in series in parallel with the secondary coil. That should have a similar effect as adding the second topload did.

[davekni]

I have a bunch of 82p 3kV C0G ceramic caps originally purchased for HFSSTC stuff and I'm wondering if it would be possible to run like 30 of them in series in parallel with the secondary coil. That should have a similar effect as adding the second topload did.

Interesting idea.  Expect it will work.  My QCW top measurements indicate ~30kV RMS, so ~44kV peak.  30 x 3kV would be plenty safe for that.  Would be only 2.7pF total addition, however.  Perhaps two or three such strings in parallel would make a more useful difference.

[Lucasww]

Okay, I think i'm going to try that. Maybe 2x20 series would be better. Also, I've noticed the top of my secondary coil and underside of the topload get warm pretty quick. I'm guessing this is because of inductive heating? is this a problem?

[davekni]

I'm guessing this is because of inductive heating? is this a problem?

I'd also presume inductive heating.  If it gets hot enough to melt coil form or loosen winding or whatever, then it is obviously a problem.  Otherwise it is just a source of power loss that could otherwise go to increasing arc power.  Many TC designs include a gap between top end of secondary winding and top load to minimize eddy current losses (to provide a return path for secondary magnetic field).  My SSTC has a layer of plate ferrites between secondary top and top load.  That is not a normal solution.

[Lucasww]

It's been a little while and I think i'm done with this for now, so I figured a final update on this project would be good.
I had a friend print a form for a slightly larger topload, and added an extra 3pF with a C0G MMC through the secondary (27 series 82pF 3kV) and the results are very nice. Peak primary current is generally around 160A (OCD set to 180A), unloaded secondary resonant frequency is around 340kHz and the primary is tuned to around 360kHz if i remember correctly.

sloppily made secondary MMC, potted in epoxy

Initial testing with the secondary MMC


I had some unexpected issues with the primary MMC arcing across its terminals, which in hindsight were way too close together.

A good amount of kapton tape seemed to fix it.


So here's the final (for now) product. BTW, I haven't had any issues with the 3D printed IGBT clamps, they still provide a strong force on the IGBTs after a few months.

~70 inch (1.8m) ceiling strikes