Solid state stuff is inherently weak in tough di/dt service. Special snubber networks are demanded and often these are just stop gap. Most spot welders use transofmrers to step down the high voltage and never apply it directly to the work. Still anti kickback scheme must be used. Solid state junctions are all just fusible links waiting to bite th' dust. They all have a glass jaw and can't take real punches. Somehow engineers manage to make modern welders just about as big as the old tube based spot welders. The solid state switches are small, but there gaurdian components fill up the cabinet to virtual tube level volumes.
My spot welder uses 2 paralleled xenon tyratrons with zero protection circuitry. having a nice big gas gap in the switch prevents device melt down. At the worst, you'l just blast tiny bits of metal off the plates and or cathode in arc reversals. They then fall harmlessly into the bottom of the tube. They don't self heal, they just don't get injured in the first place. Ignitrons are used extensively in kiloamp spot welders, though small ones exist for light duty units.
If I were stuck with designing a solid state spot welder I would study some schematics of successful units already out there. (you might be winding some special inductors and buying some expensive snubber caps.)
Richard Hull
Created on Monday, February 19, 2001 4:46 PM EDT by Richard Hull