Thermal runaway

This article is about Thermal runaway. For other uses, see Runaway.

Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature leading to a destructive result. It is a kind of positive feedback.

Chemical engineering

In chemical engineering, thermal runaway is a process by which an exothermic reaction goes out of control, often resulting in an explosion.

Thermal runaway occurs when the reaction rate increases due to an increase in temperature, causing a further increase in temperature and hence a further increase in the reaction rate. Exothermic side reactions and combustion begins at higher temperatures, accelerating the thermal runaway. It has contributed to industrial chemical accidents, most notably the 1984 explosion of a Union Carbide plant in Bhopal, India that produced methyl isocyanate. Thermal runaway is also a concern in hydrocracking, an oil refinery process.

Thermal runaway is most often caused by failure of the reactor vessel's cooling system. Failure of the mixer can result in localized heating, which initiates thermal runaway. Wrongful component installation is also a common cause. Many chemical production facilities are designed with high-volume emergency venting to limit the extent of injury and property damage when such accidents occur.

Electronics

Bipolar transistors

Bipolar transistors (notably germanium based bipolar transistors) increase significantly in current gain as they increase in temperature. Depending on the design of the circuit, this increase in current gain can increase the current flowing through the transistor and with it the power dissipation. This causes a further increase in current gain. This is frequently seen in a push-pull stage of a class AB amplifier. If the transistors are biased to have minimal crossover distortion at room temperature, and the biasing is not made temperature-dependent, as the temperature rises, both transistors will be increasingly turned on, causing current and power to further increase, eventually destroying one or both devices.

If multiple bipolar transistors are connected in parallel (which is typical in high current applications) one device will enter thermal runaway first, taking the current which originally was distributed across all the devices and exacerbating the problem. This effect is called current hogging. Eventually one of two things will happen, either the circuit will stabilize or the transistor in thermal runaway will be destroyed by the heat.

Power MOSFETs

Power MOSFETs display increase of the on-resistance with temperature. Power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature, in a positive feedback loop. (However, the increase of on-resistance with temperature helps balance current across multiple MOSFETs connected in parallel and current hogging does not occur). If the transistor produces more heat than the heatsink can dissipate, the thermal runaway happens and destroys the transistor. This problem can be alleviated to a degree by lowering the thermal resistance between the transistor die and the heatsink. See also Thermal Design Power.

• Java applet demo of MOSFET thermal runaway

Microwave heating

Microwaves are used for heating of various materials in cooking and various industrial processes. The rate of heating of the material depends on the energy absorption, which depends on the dielectric constant of the material. The dependence of dielectric constant on temperature varies for different materials; some materials display significant increase with increasing temperature. This behavior, when the material gets exposed to microwaves, leads to selective local overheating, as the warmer areas are better able to accept further energy than the colder areas - potentially dangerous especially for thermal insulators, where the heat exchange between the hot spots and the rest of the material is slow. These materials are called thermal runaway materials. This phenomenon occurs in some ceramics.

Batteries

When handled improperly, some rechargeable batteries can experience thermal runaway, resulting in overheating. Sealed cells will sometimes explode. Especially prone to thermal runaway are lithium-ion batteries. A report of an exploding cellphone occasionally appears in newspapers. Laptop batteries from Dell were recalled because of fire and explosions: http://www.theinquirer.net/default.aspx?article=32550

Electronics and tropical environments

Thermal runaway in electronic equipment is not usually a design factor for most electronics manufacturers and engineers, but it can cause trouble and malfunctions in tropical countries. It is seen in satellite TV receivers, portable computers or industrial programmable controllers. When the air temperature that surrounds the equipment is over a threshold level, the temperature starts a heat run, a thermal runaway that makes the electronics fail, and the electronics then need to be cooled down and reset to work properly again. This problem is caused by the high frequency chopped power supplies, where MOSFET transistors are used.

Digital electronics

The leakage currents of transistors increase with temperature. In rare instances, this may lead to thermal runaway in digital electronics. This is not a common problem, since leakage currents make up a small portion of overall power consumption, so the increase in power is fairly modest -- for an Athlon 64, the power dissipation increases by about 10% for every 30 degrees Celsius. For a device with a TDP of 100W, for thermal runaway to occur, the heat sink would have to have a thermal resistivity of over 3 C/W (about 6 times worse than a stock Athlon 64 heat sink). A heat sink with a thermal resistance of over 0.5-1C/W would result in the destruction of a 100W device even without thermal runaway effects.

This problem is predicted to be more common in the future. As devices become smaller, static power dissipation contributes to an increasing portion of overall CPU power consumption. Leakage currents increase by a factor of close to 100 over the temperature range of a CPU. As a result, it is likely that this thermal runaway will be a common problem with future CPUs .

External links

• Safetycenter.navy.mil: Thermal runaway
• Thermal runaway of batteries in an airplane