Hello all,

I've got this strange issue on a Voltronic type inverter.

I've Replaced all IGBT's... quickly checked driver parts... seemed ok. Powered on... Boom ... 2 damaged IGBTS's again.

After checking again I found one zenner 5V6 damaged, replaced it and decided to use coulomb's method (2 separate power sources, 12V and 20V) to test the IGBT gate drivers.

All good, except the gate voltages for the 3 igbt's that are powered by same 20V power supply. In ON state i got 18V .... in OFF state I got only -1V instead of -5.6V.

Further checking .... in unpowered state, with DMM on ZD2 in diode mode - I only have there 0.3V forward voltage instead of 0.7V as on other zenners.

Also, when I check resistance of R10 (15K) - in one way (+...-) i get 15k.... when inverse the leads of DMM - I get around 8 kOhm.

Desoldered R10 - checked and is OK - 15K - but I also measure 1K on the pads where R10 was soldered.

Desoldered ACPL T350 - power pins 5 and 8 - same issue with that lower voltage drop on ZD2

desoldered TX7 - issue persists.

PCB a little bit burnt near IGBT legs - check resistance and is no leakage. (see last photo)

Any ideas what to check more?

Im afraid to run it power it ON having only -1 [V] on 3x IGBT's gate.

SOLVED. there was another bad zenner in parallel with the good one. on this particular model, the MPPT IGBT's are powered by the same power supply.