Thermal fuse with that certain extra

In modern highly compacted electronic devices, safety in operation is increasingly threatened by excessively high temperatures. As a rule, thermal fuses are used to counter this risk. But only in combination with a shunt resistor is it possible to provide a fail-proof protection.

In order to achieve ever greater power densities, modern electronic devices need to accommodate more and more components and functions within a very tight space. As a result, new products – small, but offering extremely high-performance – are being produced, but which nevertheless bring with them the risk of thermal runaway. Thermal runaway means overheating within a technical device caused by heat-producing and self-energising processes. With a power semi-conductor such as a MOSFET, in connected status, the drain-source forward resistance increases as temperature increases, resulting in an increasing power dissipation in the barrier layer. If the cooling system is insufficient, this power dissipation, which is released in the form of heat, cannot be fully discharged thus raising the forward resistance and the process very quickly becomes increasingly unstable. That can result in the component or the device being destroyed and in the worst situation, cause fires or explosions.

Adding a current measurement sensor

When designing components and circuits, engineers need to consider the temperature dependency of the component’s electrical resistance. This current measurement property can be a specific benefit. Because an abnormal temperature protection only triggers in heat conditions, it is often a good idea to combine a thermal fuse with a current measurement sensor in the form of a shunt, so that current flow can be sufficiently taken into account. This combination means that the thermal fuse can identify a creeping temperature increase while the shunt will identify a rapid onset malfunction (excess current).

A shunt is a low-ohm resistor with as low a temperature dependency as possible, which enables accurate measurements to be made of the current flow inside the component without being influenced by the ambient temperature. The small voltage present at this resistor is measured in order to calculate the current intensity. The lower the voltage, the less the circuit is influenced by the measuring device. A controller processes the measurements taken and if the current is too high, it separates the circuit. The permitted current intensity depends on the system and can be configured accordingly.

Clever combination for failsafe protection

To prevent thermal runaway, in its RTS (Reflow Thermal Switch) with shunt Schurter is supplying a better failsafe protection. It is not a replacement for the conventional over-current protection but rather an extension to the safety chain to provide functionality that safety fuses have not been able to provide previously. The RTS is located as close as possible to the component it is protecting. As soon as the ambient temperature of the power semi-conductor exceeds a certain threshold, the thermal fuse separates the components from the circuit. If thermal runaway is detected galvanic separation occurs. Because it is enhanced with the addition of a shunt measuring resistor it is also possible to measure the extent of the current flow and correct it with control electronics.

Adding an over-current fuse to the RTS combines three functions into a single reflow soldered SMD component inside the space-saving RTS housing with its small 6.6mm x 8.8mm footprint: abnormal temperature protection, over-current protection and current intensity measurement. This way costs are reduced while operational safety is greatly increased.

Standard parameters and customized variants

The RTS from Schurter copes with operating currents up to 130A and rated voltages up to 60VDC. The standard version has a measuring sensor resistance of 500μΩ and a tripping temperature of 210°C. The current design of the standard RTS with shunt is designed especially for the 12V power electronics sector within the automotive industry and is particularly suitable for cooling fans, ABS, servo steering, PTC heaters, HVAC, glowplugs and diesel heaters. But it can also be deployed in industrial applications, particularly in fields such as battery protection, power adaptors, control-gear lighting, H-bridge circuits and motor drives.

Besides the standard products, customized designs with adapted parameters (e.g. resistance level or tripping temperature) are also available to provide effective protection for individual needs.

A small component like this RTS from Schurter can prevent heat damage; in the event of a fault, it does not need to be replaced like a safety fuse, and can offer much more.

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