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
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.
AS gigawatts has pointed out you might have to consider re-drawing the circuit to make it look like a H-bridge circuit. It took me a long time to figure that out before reading his comment.
Also as for the reason why your simulation is messed up might be because you circuit is missing a pull down resistor on the gate pin of mosfet. Whenyou trigger the mosfet the gate capacitor (inside the mosfet) will be charged and when you remove this trigger signal you must provide a path for this capacitor to discharge through a pull down resistor. Else it will partiality go low just like the blue wave form in your simulation graph
