Growth in the wearable electronics market increases with new types of useful devices every year. The lithium-ion or lithium polymer batteries that designers typically specify for wearable applications have advanced with higher energy density technologies that offer improved charge capabilities. Higher density batteries also make sense to meet the inherent space constraints in wearable designs.

A shortcoming of lithium-ion batteries is that they are susceptible to short circuit and overcurrent threats during the energy transfer process. Designers need to be aware that a short circuit can trigger the battery to overheat, leading to a condition known as a thermal runaway. Not only is this a safety issue causing discomfort to the person wearing the device, but it can reflect poorly on the manufacturer, resulting in large numbers of product returns or even recalls.

This issue was documented last year by the U.S. Consumer Product Safety Commission. In the Commission’s Status Report on High Energy Density Batteries Project, it found that more than 25,000 overheating or fire incidents in some 400 types of lithium battery-powered consumer products had occurred over a five-year period.

Protection against overtemperature threats

The basic function of the lithium-ion cell is to transform chemical energy into electricity. An individual lithium-ion cell is comprised of an intercalating (i.e. inserting between layers in a structure) lithium compound cathode, a carbon based (typically graphite) anode, as well as a liquated or gel-type electrolyte with lithium salts through which ions travel, and a polymer separator to act as an internal insulator to the electrons. The use of the two intercalation electrodes has led to the lithium-ion batteries being called “rocking-chair” batteries as ions shuttle back and forth between the electrodes and through the electrolyte in a lithiation/delithiation process1. The separator plays a critical role in cell safety by ensuring there is no physical contact between the cathode and anode.

While separators have evolved from simple single-layer sheets to multilayer sheets with shutdown features, they alone cannot ensure complete cell safety. The lithium-ion cell is constructed with materials that are flammable and degradable where mechanical and electrical shock can lead to a thermal runaway. The lithium-ion cell materials that are stable at lower temperatures start to breakdown when temperatures exceed 130˚C. If a cell starts to enter thermal runaway, the results can be catastrophic.

Thermal runaway in a lithium-ion cell is a highly exothermic, self-propagating process that results in the venting of toxic and highly flammable gases and the release of significant energy in the form of heat greater than 1000˚C.

The requirement for protection circuits to maintain the voltage and current within safe limits is one of the primary limitations of a lithium-ion battery. To prevent thermal events from occurring within the wearable device's specifications, overcurrent and overtemperature protection has become a critical safety requirement that also contributes to longer battery life.

Circuit protection, however, is often overlooked during the design stage in many applications. In deciding whether to add protection, design engineers typically evaluated the trade-offs of additional cost and increased parasitic loading that have a tendency to affect data rates and signal integrity in I/O interfaces. In next-generation designs, engineers have more to contend with. They must take into consideration today’s ever-shrinking sub-micron semiconductor technologies coupled with the effects of electrostatic discharge (ESD) transients. There is also a whole new range of basic safety concerns in the growing wearable device market such as the potential of a fire from faulty charging units. It is easy to see why adding circuit protection in these designs has become the preferred practice.

Resettable fuses meet protection needs

Polymer positive temperature coefficient (PPTC) thermistors or resettable fuses are common overcurrent protection devices used in consumer applications such as personal computers, game consoles, smartphones, tablets and now, wearables. Demand for PPTC thermistors has grown due to their ultra-low impedance “on” resistance values, extremely small form factor, and enhanced reliability.

PPTC resettable fuses are made from conductive filled polymer. In normal operation, the conductive particles in the polymer form a continuous path, which allows current to flow through the device without interruption. Typical base resistance of the device may range from a few milliohms to a few ohms. When there is an overload condition, the polymer heats up internally from I²R heating. When the polymer heats up to approximately 90°C - 160°C, its molecular structure changes from semi-crystalline to amorphous. This causes a macroscopic expansion, which breaks the conductive paths. When the conductive paths are broken there is a large increase in resistance - typically several orders of magnitude. At this point, the device is in the “tripped state.” Upon cooling, the polymer reforms to its semi-crystallized state and the conductive pathways are reestablished.

In batteries, the lower the resistance, the less restriction it has to operate at its fullest. High resistance causes the battery to heat up and the voltage to drop under load that can lead to a shorter operational life. A resettable fuse can be integrated into the design to keep parasitic resistances or impedances to a minimum to enable lower voltage and longer device operating times.

Now available in a small 0402 package size, a new generation of resettable fuses delivers the required performance from higher hold currents (Ihold), higher voltages (Vmax) and post trip resistance values for enhanced resistance stability. Leveraging new low resistance conductive materials, resettable fuses can provide post-trip resistance values as low as 0.5 ohms and initial resistance values as low as 0.04 ohms. Such low resistance values help boost the current carrying capability of wearable applications. These features also help enable longer battery life and faster charging in today’s smaller lithium-ion battery-based devices where a compact, low-profile resettable protection solution makes sense for mobile space-constrained wearable applications.

 Multifuse device (Image courtesy of Bourns)

Bourns designed its Multifuse Model MF-ASML/X series to keep parasitic resistances or impedances to a minimum, thereby contributing to lower battery voltage and longer operating times. This small form factor 0402-size resettable PPTC fusing device acts as a secondary overcurrent and overtemperature protection device to help prevent thermal events from occurring within the wearable device’s specifications. 

Protecting the battery and circuitry

Many users of wearable devices have migrated to faster battery charging cables that use the new USB-C connector. The connector features 24 pins in a smaller form factor than previous USB designs, yet it is capable of delivering up to 100 W of power. While the USB-C provides charging benefits, the downside of this combination of increased power and extremely tight pin spacing is a greater possibility of thermal runaway events. These faults can generate a tremendous amount of heat that can harm not only the charging cable and connector, but also the devices they charge or even the people using them.

For consumer wearable applications, it is possible to implement a small form factor (0402-size) PPTC resettable fuse into both the connector head of the USB charging cable and the PCB in the wearable device itself. The result of implementing a dual approach is that it protects the wearable device battery unit during a charging cycle, and will also protect the circuitry during discharging, and powering of the device during usage if a short circuit or overcurrent/overtemperature event occurs.

In addition, designers can protect USB Type-C connectors and other charging cables by adding a polymeric thermal cutoff (P-TCO) device. The Bourns P-TCO model series is an example of an optimized resettable thermal sensor for overtemperature and overcurrent protection. These devices deliver ultra-low resistance, and are designed for thermal cutoff temperatures from 75°C to 100°C with a 12 V maximum operating voltage rating and a 50 A maximum operating current rating.

Including circuit protection such as a PPTC resettable fuse into the cable connector of device protects against overcurrent and overtemperature threats to avoid safety issues and assist in improving positive perceptions that can ultimately spur wearable market growth. (Image courtesy of Bourns)

This allows designers to eliminate the damaging effects of an unspecified charging event, transient, or overtemperature condition of the battery within the device’s specifications, thereby, resulting in a safer end product for the consumer.

Adding reliability and safety

The need for small, reliable protection and the value it brings to consumer wearable products is increasingly more evident. Companies that produce wearable devices have learned that a field failure may not only signal possible safety-related design issues and large numbers of returns, but also trigger negative social media and viral news that can severely hurt the reputation of the manufacturer’s brand and image, which can eventually affect future product sales. Including circuit protection in the design provides proven return on investment (ROI) and risk-mitigation benefits. A small form factor PPTC resettable fuse such as the Bourns MF-ASML/X series helps wearable device manufacturers meet their small space, automated assembly and protection needs. Wearable device users are also able to enjoy significant total cost of ownership savings from a more reliable, high-quality product that is designed to meet their operational expectations and improve their total experience.


  1. This is a reference to describe the terms Lithiation and Delithiation: Lithiation and Delithiation Processes in Lithium–Sulfur Batteries from Ab Initio Molecular Dynamics Simulations, Claire Arneson, Zachary D. Wawrzyniakowski, Jack T. Postlewaite, Ying Ma, J. Phys. Chem. C2018122168769-8779, Publication Date: April 9, 2018
Lee Bourns is the Product Manager of the Multifuse Polymer PTC Resettable Fuse and SingleFuse SMD Fuse product portfolios at Bourns, Inc.