Power
Figure 2: Secondary protection devices in Li-ion battery application. (Source: Littelfuse)
battery management system (BMS), including fuel-gauge ICs and charge/discharge control MOSFETs. These can be regarded as primary safety features that prevent dangerous conditions from occurring. With built-in circuitry such as voltage regulation to ensure that the voltage applied to the battery remains within safe limits, as well as current-limiting circuitry and fuses, the BMS can prevent excessive charging voltage, short circuits due to manufacturing defects, overtemperature, excessive discharge current, and discharging to a dangerously low level. The BMS circuitry focuses on preventing known threats from occurring. However, also needed are secondary protection devices that respond to protect the cells against dangerous conditions when they occur. These are needed at the cell level, battery pack level, and charging port level (Figure 1). At the cell level, overcurrent and overcharging protection can be implemented using a battery mini breaker, like the Littlefuse MHP-TAT18 or a PPTC device, such as the PPTC LSP. These can be added as a ‘fail-safe’ to protect battery-management ICs and fuel gauges. To protect the communications interface from overcurrent and ESD events, the compact Littelfuse zeptoSMDC PPTC can be combined with a TVS diode array. At the battery pack level, devices such
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as the Littelfuse ITV battery protectors are used to protect against overcurrent and overcharging. These three-terminal surface- mountable devices contain an embedded fuse element that cuts off the circuit in the event of an overcurrent condition. The ITV protectors also integrate a heater directly under the fuse element to blow the fuse if the fuel-gauge circuit or if a charge/discharge MOSFET malfunctions.
Port protection
With USB-C connection standard to many Li-ion charging systems, port protection is crucial to the overall device safety. Here, eFuses can interrupt current as effectively as a conventional fuse and reset when the exceptional condition has passed. They provide protection against threats such as overload, short circuits, input voltage surges, and excessive inrush current. Reverse- current protection can also be implemented, sometimes requiring an additional external MOSFET. Protection using an eFuse offers performance advantages, including fast response in the event of dangerous operating conditions as well as real-time diagnostics that can be used to infer information about the condition of the battery and the load. Also applicable to port protection is temperature monitoring using devices such as
Littelfuse setP temperature indicators. These are designed to protect systems rated at 100W or higher and meet the specific requirements of the USB Type-C interface. They can be used with USB-C plugs and chargers with captive Type-C cables. Littelfuse setP temperature indicators can replace a combination of a current limiting switch and PPTC device while adding no I2R loss to the system. A complete protection solution for Li-ion batteries and associated circuitry, including external connections such as a USB Type-C charging port, should also include protection against electrical surges and electrostatic discharge (ESD). A TVS diode array, such as the Littelfuse SP1006 series, leverages silicon avalanche technology to protect against high-energy surges and meets applicable IEC 61000 specifications. ESD protection meets IEC 61000-4-2 up to ±30kV contact, ±30kV air and there is protection against electrical fast transients up to 40A, meeting IEC 61000-4-4, and protection against lightning strikes meeting IEC 61000-4-5.
Figure 2 shows how protection devices are implemented to provide cell, battery pack, and port protection.
Conclusion
Li-ion batteries are the portable power of choice for popular applications such
as cordless power tools and e-mobility. To take advantage of their long-lasting performance, safety precautions must be built into the battery pack and any external ports to prevent the cells from overheating that can lead to thermal runaway, potentially resulting in fire or explosion. Overheating can result from excessive charging voltage, excessively fast or extensive discharging, short circuits or excessive load current.
A combination of protection devices is needed, operating at various levels from protection of individual cells to protecting the battery pack and BMS circuitry as well as external ports. These can include familiar devices such as eFuses, PPTCs and TVS diodes, as well as battery protectors that combine overcurrent and overvoltage protection and temperature indicators that combine the roles of a current limiter and PPTC into a single device. By adhering to safety standards, implementing appropriate protection devices, and following recommended charging and usage guidelines, the risks associated with Li-ion batteries can be effectively managed, ensuring their safe operation.
www.mouser.com Components in Electronics June 2024 15
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