12v to 230v inverter

[p.larner]

I have a 600w mercury soft start inverter,its input was more or less a dead short,on investigation i found 2 banks of 4 fets, one of these was shorted between all legs,i replaced it but its still poorly,the 4 fets on the same bank/side of the pcb get hot in seconds,on further investigation all there gates have a squarewave drive but the waveform is riding on top of a 5v so the top of the peak is 12v the bottom of the squarewave is 5v,the fets are hard on as soon as power is supplied to the inverter,the pwm controler is a tl594 ic,any ideas what to check?.

[Jwillis]

One bank of MOSFETs will be the low side (12V) and the other will be the high side (230V in UK) . What often happens is if one of the MOSFETs is shorted  or a bank is shorted, that indicates that the inverter was over loaded. This usually destroys either the PWM controller and /or the the comparator.
A good way to destroy a high frequency inverters is using electric motors (like power tools) because of the extreme inrush current of the power tool. The inrush current can exceed the maximum power rating of the inverter instantly upon startup.