Identifying ‘hot spots’ before they cause thermal runaway
Despite all the necessary precautions, sticking to the proper operating conditions and guarding against external factors such as shock and vibration, in the real world components still fail. This is undesirable, but it’s even worse when the component fails in a way that damages other components in the system, or even worse, poses a safety hazard.
Thermal runaway is one such unsafe condition that can happen as a direct result of component failure; it can lead to overheating of the system and cause a lot of damage. It’s found in many types of semiconductor devices as well as passive components and batteries. For example, in a microprocessor or microcontroller without a sufficient heat sink, the heat generated by leakage currents in the device increases its temperature, which further increases the leakage current, which increases the temperature even more, leading to thermal runaway’s positive feedback loop which eventually breaks the device.
Outside of semiconductor devices, tantalum capacitors can experience thermal runaway; tiny manufacturing defects combined with voltage spikes can lead to an endothermic chemical reaction which is self-reinforcing, resulting in smoke, flames and the destruction of the capacitor.
A classic example of thermal runaway is found in power MOSFETs under certain conditions, caused by imperfections in the die attach process or degradation during the device’s lifetime, especially when used in harsh conditions such as automotive. It means that some of the many of cells in the device may take more current than others and run hotter. Since the MOSFET increases its RDSon with temperature, more power is dissipated in these cells, which increases the junction temperature, which increases the RDSon and leads to thermal runaway. This thermal runaway destroys the device and may lead to smoke and flames. If the temperature gets above around 180°C, this is enough to do serious damage to the PCB it’s mounted on.
One solution to the thermal runaway problem in power MOSFETs is to use a protective device that identifies when the temperature gets too hot and breaks the positive feedback loop by cutting the power supply. SCHURTER makes a reflowable thermal switch (RTS) device designed for exactly this purpose. The RTS is essentially a fuse which trips at 210°C (above the normal operating temperature for most electronics, but high enough to eliminate false-positives).
For the RTS device to work effectively, it has to be placed physically as close to the MOSFET as possible, which means it has to be surface mountable. This poses a challenge, as surface mount means reflow soldering at 260°C, which would trip the switch. SCHURTER has got around this by designing the device to require either manual or fully automated mechanical activation following assembly. Then, when in use, if temperatures above 210°C are experienced, the device trips and cuts off the power supplied to the FET to avoid a thermal runaway condition.
Another type of system where thermal runaway can have serious consequences, even resulting in explosion, is rechargeable batteries. Li-ion batteries are notorious for their thermal runaway condition, which can cause them to explode. This is caused by improper use conditions such as overcharging, which damage the cell structure causing a short, which allows current to flow between the positive and negative plates.
Bourns mini thermal cut off devices protect against thermal runaway in Li-ion battery packs
Bourns makes a range of thermal cut off devices (mini breakers) that provide protection from thermal runaway in battery packs for consumer devices such as notebook PCs and smartphones. These miniature circuit breakers are designed to open when abnormal temperatures are reached, but unlike the SCHURTER RTS, once the temperature reduces to a safe level, they will recover. Different parts in the range will trip at specified temperatures between 72 and 90°C, and they are compact enough to be mounted between a battery’s terminals.
Bourns’ mini breakers, such as the AA series, are hybrid devices, which include a bi-metal switch alongside a PTC (positive temperature coefficient) thermistor in parallel, in order to use the best properties of both technologies. The PTC enables latching operation, while the bi-metal switch provides higher operating current.
The structure of a Bourns mini breaker
Here’s how it works (see the picture below). Under normal circumstances, the bi-metal disc is curved downwards, which allows current to flow through the arm from contact to contact, in and out of the device. If it gets too hot, the shape of the disc changes so that it curves upwards, which deflects the arm and breaks the contact, shunting the current through the PTC thermistor. Heat in the thermistor keeps the bi-metal disc latched open until the system is back to normal temperatures, at which point the disc goes back to its original shape and normal operation is resumed.
Operation of the Bourns mini-breaker device
If you are working to identify hot spots in order to prevent thermal runaway in your system, whether it’s an automotive power system or a Li-ion battery pack design, there are plenty of solutions available that offer tried and tested protection. If you want to discuss your design in more detail, or you need samples, our technical specialists are on hand to help. Click the Ask an Expert button to get in touch.