Its a small tesla coil with some inductors and caps added to the welding ci rcuit. A low pass LC or just an L capable of carrying the weld current keep s the HF out of the welder and/or rectifiers. A spark gap or special mica capacitor is used in older systems to couple the HF to one electrode, mode rn systems may have the Tesla secondary carry the weld current.
In the old days it was HF on or off. Now they time/gate the RF burst for v arious purposes by modulating the power to the Tesla primary.
Basically anywhere from 50 to 300 KHz, very high peak voltage, beefy tesla. coil.
How do I know, well, Big Xenon Arc Lamps need very similar starters and sim ilar protection schemes for the DC power supply.. I spent a few days design ing suitable LC low pass filter and shunt capacitors to keep the HF into t he lamp and out of the PSUs output FETS. Then it was off to design a decent spark gap. Spark gaps do a better job then diodes at coupling the HV into the anode lead..
For example, series injection transformers for starting krypton lamps in l asers and xenon lamps in Medical IPL systems often have secondaries that ca n handle 100 or more amps, so the technology is out there...
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Steve