Junction to Case Thermal Resistance

Junction to case thermal resistance is measured from the junction to the case. This is mostly specified for power semiconductor devices. This article will teach how to use it and when to use it.

Junction to Case Thermal Resistance Symbol and Units

Some datasheets use RthJC while others use RθJC. They are referring to the same parameter. The units used is either ‘C/W or K/W. They are interchangeable without any conversion. Below are portions of datasheets from ROHM, onsemi and Infineon showing how they define the junction to case thermal resistance.

When to Use the Junction to Case Thermal Resistance?

When the point of measurement is case temperature, then the thermal resistance to use is the junction to case. Another scenario is when a semiconductor device is designed to be mounted on a heat sink. A heat sink could be an external part like aluminum extrusion or a PCB copper pad and via combination.

How to Use Junction to Case Thermal Resistance?

It can be used to compute the power dissipation capability of a semiconductor or to know the maximum junction temperature.

Power dissipation = (Tjunction – Tcase) / RthJC

Tjunction max = Power dissipation x RthJC + Tcase

It can also be used to determine the case temperature of a semiconductor device.

Tcase = Tjunction / (Power dissipation x RthJC)

Example 1

Determine the maximum power dissipation capability of a ROHM RB078RSM10STF Schottky Barrier Diode with the given junction to case thermal resistance and junction temperature if the allowable maximum case temperature is 100’C.

Solution

Power dissipation capability = (Tjunction max – Tcase) / RthJC = (175’C – 100’C) / 3 ‘C/W = 25W

The diode can handle 25W of power dissipation, but this is only possible when the mounting pad is 50 X 50 X 1.6 mm FR4 board with a thickness of 35 microns. With a smaller footprint, the specified junction to case thermal resistance will increase.

Example 2

What is the junction temperature of a SiC MOSFET from Onsemi with a thermal resistance in below table. The allowed maximum case temperature is 100’C and the power dissipation is 20W.

Solution

Tjunction = Power dissipation x RthJC + Tcase = 20W x 0.35 ‘C/W + 100’C = 107’C

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