Small percentage of UCC28600 PWM controller chips are failing to restart during production temperature soak tests. The units are powered for 12-16 hours at high ambient temperature. The chips reach temperatures of 70-80oC. After soaking the circuits are restarted. It is at this stage that the chips are failing to restart.
It appears that the chip is drawing 600uA via Vdd pin 6 and the voltage on pin 6 is reaching 9.2V. From the datasheet, the start current should be less than 25uA up to 105C.
After cooling the units down, the unit works perfectly, until the above conditions occur again.
Localised heating of the power supply over short periods of time does not appear to cause the problem.
When the chip temperature is raised to 120-130C, the chip stops, but automatically restarts when the temperature falls slightly. During this region, the supply turns on and off and does not have the 15C hysteresis specified in the datasheet. This is a secondary problem.
This problem has just started occurring. (4 out of 200) We have manufactured approx 1000 units over the past year without this problem occurring.
