Case Temperature

Case temperature is the temperature of a semiconductor device/component’s case when it is operating in normal conditions. The Chemical Dictionary defines it as:

"In the measurement of thermal resistance, the temperature at a specified, accessible reference point on the package in which a semiconductor die is mounted"

Vishay Siliconix describes an application that benefits from the ability to measure case temperature relatively easily; it concerns a convenient method for estimating junction temperatures in power MOSFETs.

The thermal data provided on MOSFET datasheets is usually limited to thermal impedance between junction-to-lead and junction-to-ambient. While a number of sophisticated tools are available for more accurate thermal simulation based on customized conditions, quite often design engineers simply do not have resources or the time to utilize them. However, measuring the case top surface temperature is relatively easy, and a simple way of estimating the MOSFET junction temperature based on this top surface measurement would give designers a useful tool.

Vishay Siliconix provides this tool through their ThermaSim thermal simulation package, which helps designers speed time-to-market by allowing detailed thermal simulations to be performed before prototyping. The tool is the first online MOSFET simulation that uses structurally detailed power MOSFET models created using finite element analysis (FEA) techniques to increase the simulation's accuracy.

In an Application Note entitled "Estimating junction temperature by top surface temperature in power MOSFETS", the company shows how to study and exploit the relationship between top surface case temperature and junction temperature and use it with ThermaSim to provide an easy thermal simulation methodology.