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[TCML] New SISG failure mode

    I was going over the rest of my SISG to see how  many replacement SIDACs 
to order *this* time (note to self: buy stock in  Littlefuse). Previously 
damaged sections either had both the IGBT and all the  SIDACs blown, or recently 
just the SIDACs. So I had figured it was the SIDACs  dying first that took out 
their IGBT.
    But this time I found a damaged section  that had good SIDACs, but a 
blown IGBT! Even more significantly, the  gate was still at a high resistance to 
the body, and I observed its  capacitance charging up from the digital 
multimeter. But the emitter and  collector were dead shorted! 
    My guess is that this IGBT suffered from secondary  breakdown. Does that 
make it an IGSS now? (Insulated Gate Silicon Shunt).  All other components in 
that module appeared and measured perfectly fine,  including the TVS's. Could 
this be the result of excessive pulsed current or  overheating?
    (Note to self: buy stock in International  Rectifier...)
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities

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