I went through the application notes of infineon and TI. In the datasheet on the MOSFET, there is a junction to case Thermal Resistance given.

Can we use these values to calculate the power dissipation in the MOSFET? Since I read that the Thermal resistances given in the datasheet are measured assuming maximum temperature?

Pd= (Tj(max)-Ta)/(Rthjc-Rthca)(°C/W)

Tjmax=maximum junction temperature Ta=Ambient temperature Rthjc=Junction to case thermal resistance Rthca=Case to ambient thermal resistance (Heat Sink assumed?)

It is also said the cooling method used will be forced-air cooling which is bulky and not very easy to achieve. How do I calculate the heat sink parameter required for the temperature rise I calculate? Do I assume the value of Rthca (Heat Sink thermal resistance)? Is it possible to make a calculated assumption for the Rthca value?

Is it possible to know what extreme test conditions were used to measure the thermal resistances in the datasheet? How do they find these thermal resistance values Rthjc?

How will I be able to calculate the power dissipation due to thermal resistance? Do we assume a Junction temperature of approximately 110 degrees?

Is 25 degrees a good assumption for Ambient temperature or we use some standard temperature according to the application (40 degrees)?

also, how do we use Rthja (Junction-Ambient Thermal Resistance) in our calculation depending on the cooling area given in the datasheet?


I have just focused on explaining certain things because, when you grasp them, your other questions will likely become irrelevant.

  • Junction to case thermal resistance tells you how warm the junction will get above the temperature of the case. If it has a value of 30 degC per watt and the device is dissipating 2 watts of power then the junction will rise 60 degC above the temperature of the case.

  • This figure is the main bottom line and, all we can hope to do is prevent the case from rising too high in temperature and causing the internal junction to exceed its stated limits. So, we use heatsinks attached to the case to stop the case getting very warm.

  • But there's another parallel path for heat to travel and that is directly to ambient - this is the Junction-Ambient Thermal Resistance and is usually a lot higher than the Junction to case thermal resistance. But, it's a parallel path and so every little helps.

  • Also be aware that the case and local ambient on the device will rise as heat is drawn from the device so you can't assume that the local ambient remains steady at the external temperature of any container for the device (I'm thinking a PCB enclosure here).

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