Hi!
What make and model of P.S.W.I. is it?
The most likely problem is you've either got a fault in the gate–drivers that is either holding one MOSFET/IGBT partially on with a d.c. bias or there's some overload causing the drive waveforms to distort, either fault in this type of circuit will cause instant destruction of MOSFETs the moment power is applied.
You need to remove all the damaged MOSFETs/IGBTs (there will be several of them in parallel for each half of the output stage) and check individually all the small components associated with each gate circuit. As well as the gate circuits, there'll probably be fractional–ohm current sharing resistors in each MOSFET's source lead (right–hand lead) – any of these could have blown open–circuit.
Don't forget that catastrophic MOSFET/IGBT failure at power–on will almost certainly cause damage farther back in the drive circuits, so you may have to trace the gate–circuits back to the driver transistors or ICs.
Where IGBT output devices are in use, the driver–stages need to switch on and off in timed sequence such that a NEGATIVE gate–to–emitter voltage is provided during the turn–off phase, (this is not always provided with an auxiliary negative supply as multiple cross–connected driver transistors can do this – component – level circuit diagrams of IGBT inverters are virtually non–exsistent so I can't offer an example) – partial failure of drivers can delete the negative turn–off bias, which will also cause instant destruction of o/p devices!
The only way you'll trace this is by meticulous component–comparison checks on the smaller stages associated with all the IGBTs.
Nearly all PWM bridge–controller devices of reputable manufacture have reliable "dead–time" arrangements built into their o/p drivers to prevent cross–conduction of upper/lower sections of MOSFET output stages, so castrophic failure at switch–on is nearly always to an external fault degrading the drives to the gates in some way, or an overload.
If you can post as many clear pictures of your failed unit as you can, we can offer some more tips!
Chris Williams
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