I am going to use a nMOSFET (NTMFS5C670NL) which will a varied power dissipation. The current going from drain to source is determined by a control signal. The maximum power dissipation for the MOSFET is 1.29 W.

The junction-to-case thermal resistance Rjc is 2.4 ℃/W, and the junction-to-ambient thermal resistance Rja is 41 ℃/W.

The actual junction temperature formula is Tja = (Rja*Pd)+Ta. If the ambient temperature is 25 ℃, then Tja will be equal to 77.89.

Case temperature Tc will be (-Rjc*Pd) + Tj = 75 ℃.

These values are only valid if the MOSFET is surface mounted using a 650 mm^2, 2 Oz Cu pad. This is the only scenario the datasheet for the MOSFET gives.

Is there any way to give an educated guess on what the case temperature will be with a bigger or smaller Cu pad? I have failed to find any white papers that can give a decent estimation.


1 Answer 1


Yes, you can compute the \$R_{th}/cm^2\$ of that copper geometry in terms \$^\circ C/W ~per ~cm^2\$ in an open convection area then add Rth for the enclosure.

Then define the added subscript letter to abbreviate each interface or number for internal or external ambient layers.

  • \$\begingroup\$ So Rja will be 266.5 (℃/ W * cm2) in an open convection area? In other words the top and bottom layer? So if the Cu area is 9 cm^2, then Rja will be 29 ℃/W? It was my understanding that you could not do anything about the junction-to-case thermal resistance. Or is that thermal resistance affected by the inner layers in the PCB? \$\endgroup\$
    – Tungstein
    Nov 29, 2021 at 17:18
  • \$\begingroup\$ Yes it assumes you have the required amount of thermal vias. \$\endgroup\$ Nov 29, 2021 at 18:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.