Protection against thermal runaway

Jon Lawson

Thermal runaway is the overheating of a technical apparatus due to a self-reinforcing, heat-producing process. This usually causes the destruction of the apparatus and often leads to fire or explosion.

The causes of thermal penetration are manifold and often accidental. Of particular importance, however, are the ever-higher power density in electronic circuits and the trend towards miniaturization. An increasing number of functions are packaged in compact modules which then also have a correspondingly high power consumption.

And even slightly inflated currents in such power electronics with only a few watts of power loss can produce temperatures of approximately 200 °C. The possible consequences: damage or detachment of surrounding components, damage to the circuit board structure or, in the worst case, even the triggering of a fire.

In the case of a power MOSFET, for example, the drain-source transmission resistance increases in the connected state with increasing temperature, which causes an increasing power loss in the barrier layer. In the case of insufficient cooling - the high power density allows greetings - the heat dissipation in the form of heat can no longer be dissipated sufficiently, which further increases the transmission resistance. This process is rocking more and more and leads to the destruction of the component.

The excess currents occurring during the thermal penetration are too small for them to induce a conventional fuse. Thermal circuit breakers or PTCs would in principle be offered, but these are too complicated or completely unsuitable for the assembly of a SMD printed circuit board.

In the near future, SCHURTER is offering SMD thermal fuses with the lowest internal resistance for power electronics with the highest package density. They are reflow-solderable at 260 °C and are only activated at a later stage, so that they trigger at around 210° C. An area above the normal component temperatures, but still below the limit for error sequences. In this case, the tripping is effected with or without current flow, depending solely on the temperature.

The irreversible thermal fuses are resistant to mechanical shocks, vibrations, thermal shocks, temperature cycles and moisture. They are qualified according to AEC-Q200.