Junction Temperature (TJ)
The temperature not of the outer body of a power semiconductor but of the silicon die inside it. In power MOSFETs the absolute limit is typically 150–175 °C, but in a reliable design 70–80% of that value is targeted. It is estimated by adding the product of the lost power and the thermal resistance to the ambient temperature.
The temperature that determines how long a driver will last is not the body you touch with your hand but the junction you cannot see. While the body is at 60 °C the junction can be at 110 °C; the difference between them arises entirely from thermal resistance.
The estimation formula is simple: the junction temperature is the sum of the ambient temperature and the product of the lost power and the total thermal resistance. Inside a 45 °C cabinet, 6 W of loss and 5 °C/W total resistance give a 75 °C junction; the same board reaches 90 °C in a 60 °C ambient.
- Feedback effect: The channel resistance of a MOSFET rises with temperature, the increased resistance produces more loss, and the loss raises the temperature further. If you keep the margin narrow this cycle speeds up.
- Life: In semiconductors every noticeable increase in operating temperature shortens the expected life; running at the limit does not postpone the failure, it brings it forward.
Practical rule: the junction temperature you calculate should stay well below the limit, and the body temperature you measure should be at a level that does not burn your hand.
The context in which this term is used: DC Motor Driver Selection Guide