I am busy trying to repair a IGBT H-Bridge inverter that forms part of an AC spot welder. The problem I am having is that it works perfectly with a resistive load, but when I connect the step down transformer to the output the output turns on again without a gate signal after turning off.
Here is the power circuit: The bit in blue is used for the current limit circuit. The resistor RL (circled in red) is the resistive load I referred to above. It seems to be there to ensure that the H-Bridge doesn't go open circuit as the transformer is pluggable in the field. The right half of the bridge circuit is identical to the left half. The IGBT's used are INFINEON FF300 range 300A 1200V 62mm modules. AH and AL form one module as do BH and BL.
And finally, here is the firing circuit which is repeated for each IGBT gate. This circuit is identical to other similar systems that I have seen in the industry with only the driver chip changing. (Currently Toshiba TLP250)
I have not shown the capacitor bank for the DC bus, but it does exist and consists of both electrolytic capacitors as well as RC circuit and is directly across the 320V DC bus that is supplying the H-Bridge. The firing circuit is microcontroller based.
I hope I have been specific enough in the information provided and would appreciate any assistance.
My suspicion is that the reverse EMF produced by the transformer when turning off is somehow causing the IGBT to turn on again, but I cannot prove this. I have also tried different gate resistors and external freewheel diodes over the IGBT Collector and Emitter.
Thank you in advance.