SymTech Laboratories wrote:At ~310Hz (18,600 RPM) and 50% DC, the temperature rise over a thirty minute period was approximately 10.0°C. At 85% DC, the temperature rise was about 14.5°C. The measured temperature rise is consistent with the DPAK part's junction-to-ambient thermal resistance and total power dissipation at that frequency and duty cycle.
A quick note about the test conditions: the readings were taken from bare parts on a paper substrate (no copper, no heatsinks). The VND5N07 was covered to minimize air flow over it.
So from these test, I don't see how any normal operating conditions could ever be problematic, at least from a heat generation point of view. From your description, you have use an absolute worst case scenario in terms of dissipating the heat from the VND5N07.
I assume it would be possible to find other injectors that would generate more flyback energy or more steady state current but even if it were to double the generated heat (which would be unlikely) it would still not be enough to create an issue.
So there is a need to find some abnormal conditions that could create the observed failures. Or it is a manufacturing or QA issue and some substandard parts are getting through. Or assembly is damaging the part somehow. Or am I missing something?