So I found a load of IGBTs going cheap on ebay (fairchild FGPF4633)

And then it turned out that I didn't really look at the data sheet properly.

I bought 40 of these, (to put ten in parallel per leg of the H-bridge)
However I really only looked at the maximum pulse current handling which is 300A of 0.01Seconds, and the low saturation voltage (VceSat is 1.55V @ Ic = 70A)
Which is good - running the transistors in linear modes of operation would have meant running them really hot!

The welding transformer draws 240V and runs from a 16 Amp supply.
The open circuit voltage is 48, (so the 330V max Vce voltage was well selected).

There is no specific current handling listed in the data sheet but there is a maximum power listed.

The welding transformer draws 240V and runs from a 16A supply.
That's a draw of around 4800 Watts.

I've decided to de-rate everything by a lot, but even so, that's still expecting that (if the transformer were perfect) that each IGBT should handle at least 480W of power.

Power handling is listed for the transistor as 30w @ 25 degrees

Temperature parameters are listed as:
4.1 c/w junction case resistance (Rjc)
and 62.5 c/w dissipation into air. (Rca)
Maximum operating temperature of 150degrees (Tm)
I'll assume ambient temperature is 25Degrees (Ta)

This gives me (working without a heat sink)
Pmax = (Tm - Ta) / (Rjc + Rca)
125 / 66.6 = 1.8W
Sadly much much less than the 480W I require!

If I add a heat sink (and insulator)
Rinsulation = 1 (Ri)
RHeatsink-Air = 4 (Rha) - what Wikipedia lists as a "standard" T0-220 heat sink

Then I get
Pmax = (Tm - Ta / (Rjc + Ri + Rha)
Pmax = 125 / 4.1 + 1 + 4 = 13.7 W (still a lot less than the 480 I need!)

in fact.
even if I said that ambient temperature was -55 Degrees Centigrade.

150 - -55 = 255 and that I had an ideal conductor of heat with a resistance of 0, case to heat sink, and a resistance of 0 heat sink to air.

255/ 4.1 (junction to case) = ~62W and is still less than the 480W I'd need to only use ten of these.

It doesn't really matter how bigger heat sink I use, these transistors will cook if I try to switch anything more than a few milli amps in a standard room temperature with a open circuit welder voltage of 48V (~ 24v actual welding droop voltage)

These were at least cheap - and I'm sure I'll find a use for them! -just not in this project.

If anyone can see anything wrong with my working above then let me know before I consign these to a parts draw!