Overtemperature protection for power semiconductors

SCHURTER (S) PTE LTD
Wednesday, 17 March, 2021


Overtemperature protection for power semiconductors

SCHURTER reintroduces its RTS (Reflowable Thermal Switch) — a particularly compact overtemperature protection device using advanced SMD technology, suited for the highest demands of protection in power semiconductors.

Need for protection

Electrical and electronic equipment cannot always count on constant voltage, constant current or constant working temperatures. Various influences lead to fluctuations, which must be secured by suitable means. Thermal runaway, for example, refers to the overheating of a power semiconductor due to a self-reinforcing, heat-producing process. The reasons for this are ever-increasing power density and the trend towards miniaturisation of electronic circuits.

Inflated currents in power electronics with little power dissipation can lead to elevated temperatures of approximately 200°C. The possible consequences are serious: damage or detachment of surrounding components, damage to the printed circuit board structure or, in the worst case, triggering a fire.

With a power semiconductor (eg, MOSFET), the drain-source transmission resistance increases with rising temperatures when connected, resulting in an increasing loss of power in the barrier layer. If the elements are not sufficiently cooled — the high power density permits cooling — the power loss output in the form of heat can no longer be sufficiently dissipated, which also increases the transmission resistance. This process escalates and ultimately leads to destruction of the component.

The RTS

SCHURTER’s RTS, which stands for ‘Reflowable Thermal Switch’, is a patented surface mount fuse that was developed to protect highly integrated power semiconductors like MOSFETs, ICs, IGBTs, Triacs, SCRs, etc, from overheating. It protects these devices not through a logic circuit but rather through basic laws of physics, providing physical protection in cases where all software-based security measures have failed. In case of thermal runaway, the RTS reliably interrupts the circuit precisely at the well-defined temperature.

Before mechanical activation, the RTS can be mounted using conventional reflow soldering techniques with temperature profiles up to 260°C. There is no danger that the fuse will blow during the soldering process as its temperature sensitivity will be activated in the next step.

Mechanical activation

After having the RTS reflow soldered on a PCB, mechanical activation arms the RTS for tripping at 210°C. This activation can be done manually or through fully automated means. In contrast to electrical activation, the RTS activation status is immediately visible and clear to the installer. Furthermore, the third additional contact required for electrical activation is thereby eliminated.

Compact dimensions, high load capacity

The new type of overtemperature protection shines thanks to small dimensions and high load capacity. Operating currents up to 100 A at rated voltages of up to 60 VDC can be handled by the RTS at just 6.6 x 8.8 mm in size. Customer-specific variants are available with an integrated shunt or an additional overcurrent fuse. These integrated functions result in less space consumption on the printed circuit board.

Applications

The RTS meets the requirements of AEC-Q200 and MIL-STD and can be deployed wherever power transistors are used — such as in the automotive sector, which covers cooling fans, ABS power steering, PTC heaters, HVAC, glow plugs and diesel fuel heaters. It is also suited to the industrial sector, for battery protection, power supplies, lighting ballasts, H-bridge circuits and motor drivers.

For more information on thermal protection, visit https://www.schurter.com/Thermal-Protection and our previous article at https://www.electronicsonline.net.au/content/protection/sponsored/the-final-authority-41463451.

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