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

In contrast, moving to the right with the red high-theta system yields trouble. The system can't dissipate as much additional power as the junction produces, so the system will heat up some more, exacerbating the problem, and thermal runaway will be the result. Indeed, a simple-minded description of thermal runaway says that if the system slope is less than the slope of the device line, thermal runaway will occur.

If managing thermal runaway were always this simple, life would be easy. What could complicate matters is a device whose power/temperature behavior isn't a straight line. As shown in Fig. 2, you can have a perfectly stable operating point (where you're going to try to operate), yet at a temperature somewhat higher than your intended operation, the curves cross again, and there the slopes will necessarily have the opposite, unfavorable relationship. That is, if a perturbation is large enough, the system can move from the lower, stable point up to the higher, unstable point; once there, it keeps moving to the right and the system experiences complete thermal runaway.

Note that in Fig. 2, the device line has an increasing slope as the temperature goes up. Curves in the mathematical class known as power-law functions have this property, which turns out to describe many semiconductor devices over a useful range of current (hence, power) and temperature.

In this more interesting situation, is thermal runaway as simple as comparing the slopes? Not always. If the nonlinearity is modest, such as for the power MOSFET shown in Fig. 3, individual devices' lines are effectively straight over any realistic operating temperature range. You certainly have to pick a combination of system slope and T-intercept to give you an acceptable operating point. But all such lines will have a slope steeper than the device lines, so thermal runaway isn't going to happen.

On the other hand, the power Schottky device in Fig. 4 has strong temperature nonlinearity, and it is quite possible to build a cooling system that crosses any particular device line in two places, thus opening the door for thermal runaway.

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