Beyond the Data Sheet: Demystifying Thermal Runaway

Nov 1, 2007 12:00 PM
By Roger Stout, Senior Research Scientist, ON Semiconductor, Technology Development, Advanced Packaging, Phoenix

Thermal runaway in a semiconductor device is a mysterious and scary phenomenon. A device can cross a magic threshold and burn up in an instant — something you'll never see coming. What makes thermal runaway so interesting and important to understand is that it can happen at a much lower temperature than the normal maximum junction temperature rating of a device.

In other words, if you inadvertently designed your system too close to the runaway temperature, even though it may seem to be operating safely away from the maximum-rated junction temperature, there is no safe operating margin. Once runaway begins, no temperature is out of reach.

By exploring a specific mathematical model of a semiconductor device and applying it, we can make some quantitative statements about thermal runaway and attain a better understanding of this condition. This will allow designers to more accurately predict the types of operating margins in their designs with respect to thermal runaway, while using data supplied from manufacturers' data sheets.

Briefly, thermal runaway is what can happen in an electronics system when a particular device starts to generate too much heat. Over time, the excess heat increases, raising the device temperature, which, in a circular fashion, increases the device power dissipation, and so on, until destruction of the device (if not more) results. Two conditions must converge to cause thermal runaway, or at least to make its possibility a design issue. The first condition is that the thermal system surrounding the device be unable to dissipate as much heat as the device produces. The second is that the device power versus temperature characteristic be significantly nonlinear.

The first condition is fairly obvious: If the device produces 10 W and the system can only dissipate 9 W, bad things should be expected. Clearly, your initial design goal is a thermal system capable of removing at least as much power as you expect the device to dissipate. If you can't match device and system characteristics at this point, you don't have a viable design.

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© 2007 Penton Media, Inc.