(Note the added cap)
A cap for one, or better yet a pi filter (Google it), but neither is a particularly good solution when your inverter wants to switch a square output!
But clearly, since the inverter has a low impedance output, it is going to shunt the HF across it anyway (remember the HF start has its own current loop, "starting" at its transformer, going through the torch, work and welder supply transformer before returning). That just leaves the voltage produced between transistor source/emitter and whatever the circuit ground is defined as. If you ground the control circuitry to the inverter transistors, which makes sense, then there's no problem, it all follows the hash with respect to whatever higher plane you're observing form (earth ground?).
Now, if you require the circuits be earthed, and you can't earth the output (which as I recall, your output is floating, and all the better to leave it that way, minus a TVS or so to keep things within limits), you might go with isolation from transistors to control. Optoisolators could do this. Since you require variable frequency (down to ~0Hz), coupling transformers are no good. 10us is fast enough switching time IIRC, so even a relatively shoddy opto would still be suitable here.
Tim
-- Deep Fryer: a very philosophical monk. Website: