Online Thermal Model Delivers Heat Sink Solutions

Aug 1, 2001 12:00 PM
By Dr. Ahmed Zaghlol, R-Theta Inc., Mississauga, Ontario, Canada

Today's power electronic system designers need quick and accurate heat sink solutions to ensure high reliability. The advent of the Internet and its potential for providing interactive design capability met this need. The result is R-Tools^®, a completely interactive online thermal design tool for heat sinks. You can run the simulation on any Internet browser capable of utilizing Java Applets.

The use of this analytically based design tool allows the user to perform the thermal design of the heat sink concurrent with the optimization of the electrical and manufacturing elements prior to any prototyping or testing. This thermal modeling tool is based on a set of analytical models for conduction heat transfer in the solid elements coupled with natural and forced convection heat transfer models in the cooling airflow. The basis of the conduction heat transfer model in the baseplate of the heat sink is the steady state solution of the Laplace equation for general rectangular geometry. The solution uses a general three-dimensional Fourier series solution, which satisfies the conduction equation in the base plate.

Forced convection air-cooled fins use an analytical model to predict the average heat transfer rate. The model used is a composite solution based on the limiting cases of fully developed and developing flow between parallel plates. Because of the analytically based R-Tools, you can achieve the solution within a few seconds, a very short time compared to the several hours required for a full CFD simulation. This provides a method for quickly and accurately testing various heat sink configurations. Use of analytically based design tools allows the user to perform the thermal design of the heat sink concurrent with the optimization of the electrical and manufacturing elements prior to any prototype or testing.

This approach reduces design time and improves the finished product's reliability. You can use R-Tools extensively for the thermal design and the selection of heat sinks for power conversion equipment. It can predict and display the temperature field on the heat sink's baseplate accurately and quickly. Hence, the user can locate the hot spots visually on the screen that will lead to an accurate prediction of the junction temperature. It predicts the total thermal resistance of the heat sinks by calculating the actual spreading resistance in the baseplate, which is typically different from the assumed coverage in most of the printed catalogues.

The program is also useful when you put more than one device on a heat sink, in particular when using high power devices. Users are able to select several heat sink sizes from a catalogue that fit the particular enclosure size. Then the software will predict the thermal profile of the MOSFETs mounted on the heat sink. The modeling software turns a difficult task into a simple one when users are limited to heat sinks that would fit the fixed enclosure dimensions required by the end user.

Experiments on heat sinks have found that the predicted temperature using an R-Tools thermal model of 78°C was very close to the measured value of the R-Theta heat sink baseplate. R-Tools design output contains detailed information about the physical, thermal, and hydraulic specifications of the designed heat sink. It also displays the temperature contours on the baseplate of the heat sink, as shown in Fig. 1.

You can find more information and the thermal modeling software at www.r-theta.com.

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