Help debug my mosfet inverter circuit


Hi all,

I’m hoping someone can shed some light on the below circuit; I’ve got a in inverter welder with an open circuit DC output of 72V, I’m trying to use a teensy or some other micro to output a square wave and drive IGBTs in parallel to convert it to a square wave to give me a 140A square wave.

I’ve managed to get as far as the below in LTSpice, but the lower end of the “arc” resistor (blue trace) won’t drop to gnd potential. What I think I’m doing is using the P2 PMOS to ground that end of the resistor but it doesn’t seem to be working, even though the gate (red trace) looks as I expect. It looks like the gate wire isn’t quite dropping to gnd; could this be the cause, and if so, why? It’s got a pulldown BJT which is working okay for the other FETs.

I did play about with swapping the drain & source of the M4 PMOS over, which fixed it temporarily but I don’t know why and i’ve tried doing it again to no avail so I think i just fixed it by accident and don’t know how!!

Can anyone give me a hint or point me at what concept I’m missing?
Any help much appreciated :slight_smile:


Firstly, draw your schematic as a proper “H-bridge” circuit. And use proper models for the transistors, select ones that have appropriate Vds, and lowest Rds(on) specifications.

I would also consider using an H-bridge driver IC to drive the transistors with the correct gate voltages and drive currents.