I'm considering this project Completed, at least for the time being. I'm incredibly proud of what I've created -- every design goal was achieved, and it looks even better than I'd dared to hope.

However, this is far, far from perfect. If I were to continue this project, I have a real hit-list of what could be improved.

Overall System

The biggest issue is that controlling drive intensity by adjusting mosfet feedback bias voltage is janky and unstable. A proper system would leave a bias voltage set with a trimmer, and use high-side limiting on the primary input current for primary drive control. I realized this kinda late in the design process, and wanted to reach "done" rather than do another round of prototyping.

Also, having a breakout board for the USB-PD trigger module is pretty inelegant. Having a trigger module on-board would be better, but when I started this project I was having trouble finding a suitable IC with decent availability in the US. Several folks have since suggested good candidate ICs. 

MOSFET and driving

A straightforward error is that the gate-drive resistors have been undersized. The design shown in the video uses two 10-ohm 1206-size resistors in parallel (per mosfet), and they get incineratingly hot if there's no ionization/plasma to load the circuit. For the published release version, I increased those to a pair of 1-watt 2010-size resistors which should hopefully fare a bit better.

I probably ought to be using an asymmetric resistor / diode pair on the gate pins. Almost wish I'd investigated that sooner since the footprints are already there, but, hey, like I said I'm declaring this project Done.  

More generally: I'm convinced my mosfets are running hotter than they really ought to be.

I still don't feel like I have a sufficiently deep understanding of mosfet switching behavior, especially when driven from a sine-wave rather than a proper square wave. Using two parallel mosfets for the final version was a callous attempt at reducing heating. In prototypes, heat was observed as being heavily dependent on how much current I was drawing and that made me hope that parallel would help, but I also should have known that I was looking at switching losses rather than anything Rds(ON) related. I did some quick math before going parallel which made it seem promising, but like I said there's aspects of mosfet switching that I'm still learning. 

The most-correct version of this circuit would probably use an actual mosfet driver IC, with something like a PLL as part of a feedback loop. Would be lovely to have both a nice square-wave drive and finely adjustable phase offset.

Using something like a GaN mosfet might also perform a lot better!

Arc Start

For the first start when the glass vessel is room-temperature cold, button-press arc-start reliability isn't as good as I'd like -- sometimes it does take several tries to get the toroid running.

This particular arc start method is too expensive. If I were designing for even small-run production, it'd get a major overhaul.

Both arc-start transformers and the GDT are all what I'd call "kinda-specialty components" which isn't ideal even if their BoM cost is modest. More seriously though, the flyback controller chip LT3420 is both expensive ($6.75/ea at qnty 10) and has limited availability. The good news is that the fancy chip is probably completely unnecessary. I initially thought I might be using the advanced functionality like keep-charged and the "done" signal, but the system I ended up with would probably be just fine with a fixed-duty-cycle flyback charger.

There's a lot of different ways of getting pushbutton arc start though. I know one person who reports success using a cheap aliexpress "arc ignitor" module with one leg of the output tied to ground and the other pressed up against the globe. Another promising idea which was suggested to me would be to have an entire autotransformer-style secondary winding driven by the primary inductor, which would offer the benefit of automatic start!

I'll continue to edit this post as I think of more improvements, and as improvements are suggested :)

EDIT 2024-09-19

Matthew Sylvester assembled this project from the PCB, and let me know that they experienced arcing underneath the xenon flash trigger transformer. I've increased the clearances, and released the changes as v1.03

Double-checking against IPC 2221, the clearances still aren't "enough" but it's the best I could do without a redesign of the entire arc start section.