Jon,

I too am trying to do this except I already have a AC/DC Tig welder
with high frequency. Its an Lincoln Idealarc 300 and does not have
true squarewave. All I want to try is to generate a squarewave output
w/ variable frequency (20 - 300 Hz) So what I am going to do (or at
least try) is to take the DC side of it and run it through an H bridge
using 4 IGBT's. I will drive them similar to the circuit at this
site:

http://www3.telus.net/public/a5a26316/TIG_Welder.html

It might be along the lines of what you are trying to doing. I give
credit to Iggy, he too has invested a lot of time into this.