TE Circuit Protection’s Metal Hybrid PPTC (MHP) Devices are designed for Resettable circuit protection for high-rate-discharge lithium-ion battery applications such as cordless power tools, electric vehicles (EV), E-bikes and back-up power supplies. The rapidly expanding market for high-rate-discharge lithium ion (Li-ion) batteries used in applications such as cordless power tools, e-bikes and back-up power supplies has created the need for cost-effective circuit protection devices capable of providing 30A+ hold currents at voltage ratings over 30VDC. To meet this need, a new hybrid device has been developed that connects a bimetal protector in parallel with a PPTC (polymeric positive temperature coefficient) device. The resulting Metal Hybrid PPTC (MHP) device helps provide resettable overcurrent protection while also utilizing the low resistance of the PPTC device to help prevent arcing in the bimetal protector at higher currents.

Design Concept of MHP Resettable Circuit Protection Devices

Resettable Circuit Protection for High-Rate-Discharge Lithium-Ion Battery PacksIn normal operation, current passes through the bimetal contact due to its low contact resistance. During an abnormal event, such as a power tool rotor lock, higher current is generated in the circuit causing the bimetal contact to open and its contact resistance to increase. At this point, the current shunts to the lower resistance PPTC device which helps prevent arcing between the contacts while also heating the bimetal, keeping it open and in a latched position. This integrated design addresses the need for compact, resettable overcurrent protection devices capable of arc suppression in high current DC power applications.

High-rate-discharge Li-ion battery pack applications require robust, reliable circuit protection; however, many of the conventional circuit protection techniques currently available tend to be large, complex or expensive. Some circuit protection design approaches might use a combination of ICs and MOSFETs or similar complex solutions. Other designs might consider a conventional bimetal protector in DC power applications requiring 30A+ hold current, but the contacts must be large enough to handle the high current, resulting in an overall device size that is much larger. Additionally, the number of switching cycles must be limited due to contact damage that may result from arcing between the contacts.

In comparison the new MHP hybrid device, developed by Tyco Electronics, can replace or help reduce the number of discharge FETs and accompanying heat sinks used in some complex IC/FET battery protection designs. The MHP device offers space-reduction, cost-reduction and protection enhancement benefits for high-rate-discharge Li-ion battery pack applications.