Continue to Site

# MOSFET case temperature calculation

#### Techman_7

##### Full Member level 2
For a MOSFET, Rthja = 35, Rthjc = 0.6.
Then, Rthca = 35-0.6 = 34.4
Power dissipation, Pd = 4 W
Tj max as per datasheet = 175 deg C

If ambient temperature is 100 deg C, to calculate the Tj of a MOSFET can we assume Tc = 130 deg C.
So that Tj = Tc + Rthjc * Pd = 130 + 0.6*4 = 132.4 Deg C.

Here Tc is always above Tj so, its assumed to be 130 deg C (30 deg C more than ambient temp.) Because I dont know Tcase value right now.

Are the above calculations Right to calculate Tj approximately?

Reference: Tcase calculation link

Tj = Ta + Rja x Pd = 100 + 4 x 35 C/W = 240C

Regards, Dana.

Last edited:

Rthca is a design parameter in most applications, depending on heatsink, pcb layout, air flow.

Whats your opinion on Tcase calculation ??
To reinforce FVM comment :

Your "normal" goal is to back into a determination of Rthja (the complete thermal R path) for a design max of Tj.
Eg. to determine required heatsink and compound/thermal attach Thermal Rs for a given max Tj, max Pd, max
Ta, airt flow...... Your part has a fixed Rthjc (choice of case only option) so you design whats needed, thermal Rs, of
rest of interface, eg. heatsink and its attach thermals and air flow, convection and conduction, to achieve max Tj.

Regards, Dana.

Last edited: