Full bridge much "weaker" than half bridge?

[nick]

Hi,

I have a half bridge of 2x IGBTs driven by a GDT.

I also have a full bridge of exactly the same IGBTs - everything else is identical except for the GDT.

The half bridge works well - big sparks, etc. However literally only swapping out the bridge and GDT, the sparks are puny and form only at higher voltage. 

Other than the GDT (I will investigate whether using the same core between the 2 bridges helps), has anyone seen similar issues / could something else be the problem?

Thanks

[Fumeaux]

try switching the connections of the gdt on the driver side. It sounds like a polarity issue

[nick]

I tried this and as I'm turning the knob on my variac, I see/hear an explosion one of the IGBTs and protection diodes literally had scorch marks on them. I was sure they were gonners, but having desoldered and checked them, they still worked - these are some hard ass components. I suspect the explosion was because my half-ass negative power connection was done with thin crocodile clips (connected to 3rd leg of that IGBT) and it accidentally shorted C-E.

I put that aside and connected back my half bridge. It worked great (as usual) for a short while and then an even bigger explosion happened. The inrush current limiting thermistor that I half-assed connected to my bus caps literally blew apart.

I interpreted these 2 events as a sign from god and went to bed. Will pick this project back up on the weekend.