I am debugging a failed MOSFET (IRF6662), and in the process of doing so, am having difficulty understanding fully the thermal behaviour of the FETs. Two FETs used for DC switching both failed short independently (2 Ω short over S-D, and lower-than-usual ~1 kΩ resistances S-G and D-G). These failed when I turned the each FET on into a substantial load (34 V into 9 Ω an 1 mF of capacitance). These FETs are driven by a switch controller (LT4363) that has a soft-start feature, so the load voltage ramps up linearly over about 100 ms.
I believe what happened was thermal damage due to the higher-than-normal power generation into the FET when the switch controller held the FET in ohmic regions before it switched on fully. I estimate that an average power of 20 W was dissipated in that 100 ms period, concentrated mostly into the middle half of that time. Using a thermal resistance junction-to-ambient of 5°C/W (from Fig3, the transient impedance plot for a 100 ms pulse), I get a Tj temperature rise of 10°C, well within acceptable bounds. So, if I have understood this right, it doesn't look like a simple case of overheating. But, does this thermal calculation make sense in this case?
Another possibility is thermal instability. Looking at the SOA of the FET, and estimating the I-V curve across the 100 ms turn-on period, I get this:
It looks like I am venturing out-of-limits for a period of time, crossing not the max power or max voltage limits, but a "thermal instability limit". But it's unclear if my transient violation of this limit is dangerous, or if this violation is likely to cause a FET failure like I have seen. In general, could such a transient incursion over the thermal instability line of a FET cause it to become damaged like this?
Lastly, these (transient exceeding max junction temperature and thermal instability violation) are the only two thermal damage methods I can see being relevant. I'm not driving the gate anywhere near fast enough for dV/dt problems, nor am I approaching the avalanche breakdown voltage. Are there any other damage mechanisms that might give these same shorting symptoms?