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

So, given that nominal system cooling capacity equals nominal device power dissipation, a somewhat more accurate statement of the thermal runaway problem is that a small perturbation in the power output of the device is more than can be dissipated by the system. For example, maybe the device and system are balanced at 9 W, but suddenly the device surges to 10 W. This will cause the device to heat up, or in semiconductor jargon, the internal junction temperature (T_J) of the device increases. The question is, can the system handle it?

A Stable Operating Point

To figure out whether a stable operating point is possible, we must discuss the relative change in the device versus the system. In Fig. 1, the straight, green device line represents the power output of a hypothetical device as a function of its temperature. (We'll talk about real device lines later, but this simple example helps illustrate the basic concept.) Fig. 1 also illustrates two possible linear system lines that describe how much power the system might be able to dissipate as a function of junction temperature.

As depicted in Fig. 1, the reality is that most systems are able to dissipate more heat as the driving temperature increases (and a straight-line model is often quite reasonable), but the difference between the two systems shown is in their theta (θ) values. (Broadly speaking, theta is a measure of the cooling capacity of the system, usually expressed in degrees Celsius per watt. This means that, with the chosen axes, theta is actually the inverse of the slope.) The red system line is a high-theta system with a relatively large temperature rise per watt; the blue one is a low-theta system with a relatively small temperature rise per watt.

If we define the operating point of the system as the place where device power dissipation equals system cooling capacity, both system lines intersect the device line at the same point. The critical distinction is that, as you move away from the operating point, one system illustrates stable behavior and the other unstable behavior.

Consider the separation between the device line and a given system line: It is evident that when the system line is steeper than the device line (for example, the blue low-theta), a perturbation in device power (or temperature) to the right results in a temporary situation in which the system can successfully remove the excess power, cooling the device back down to the original operating point.

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© 2007 Penton Media, Inc.