Ty Bowman, global battery market manager for TE Circuit Protection, a business unit of TE Connectivity, takes a closer look at the technological developments for high rate discharge Li-ion battery applications
Metal hybrid PPTC (MHP) technology was introduced around two years ago for high rate discharge lithium-ion (Li-ion) battery applications. The technology offered designers a cost effective, space saving alternative to conventional battery pack design solutions as its arc-less contact technology results in circuit protection devices capable of providing 30A+ hold currents at voltage ratings over 30V DC.
Technological advances have enabled these batteries to replace nickel cadmium or lead acid batteries in high rate discharge battery applications, providing robust circuit protection solutions for safety in end products such as power tools, e-bikes and light electric vehicles (LEVs). These are applications where standby power applications and traditional circuit protection techniques can be large, complex and/or expensive.
Providing protection
MHP technology brings a cost effective, space saving circuit protection device – by connecting a bimetal protector in parallel with a polymeric positive temperature coefficient (PPTC) device, it provides resettable overcurrent protection while utilising the low resistance of the PPTC device to prevent arcing in the bimetal protector at higher currents.
Due to its low contact resistance, during normal operation of the MHP device, current passes through the bimetal contact. When an abnormal event occurs, 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 to help prevent arcing between the contacts while also heating the bimetal, keeping it open and in a latched position.
A PPTC device’s resistance is much lower than that of a ceramic PTC. Even when the contact opens just a small amount, the contact resistance increases only slightly and the current can be shunted to the PPTC device to help prevent arcing on the contacts. Typically, the resistance difference at room temperature between ceramic and polymer PTC devices is in the range of two decades (x10^2), so when higher resistance ceramic PTC devices are combined in parallel with a bimetal they are less effective than MHP devices at suppressing arcs at higher currents.
The latest generation MHP technology, MHP-SA devices, incorporate a third terminal as a signal line for overcharge protection, to take advantage of the advanced features of the IC that is monitoring various vital functions of the battery. If an abnormality is detected, the IC sends a signal via low power switch line to activate the MHP-SA device and open the main line.
Figure 1 (below) shows an example of the overcharge protection concept and activation steps for the model MHP-SA50-400-M5 (50A, 400V DC) device that has a hold current of 50A on the main line. In this example, the IC monitors individual cell voltage, the FET turns on during an abnormal voltage event, the heater (PPTC) activates and heats the bimetal, and the contact opens and cuts the current to the main line.
Main line current (1-2): Under normal operation it passes 50A. Rated at 400V DC.
Switch line current (2-3): Turns on with short circuit current. Rated at 60V DC.
Application possibilities
By selecting PPTCs with different voltage ratings, the MHP-SA device can be configured for a broad range of applications. Device design capabilities with voltage ratings up to 400V DC and hold current ratings up to 50A are currently possible, and higher ratings up to 800V DC are in development.
The benefits of the MHP-SA device include external activation via signal line to utilise the battery monitoring IC to detect over voltage, over temperature, etc, it is smaller in size and has a thinner form factor, it has arc suppression PPTC design, and low power switch line can be used to open the main line (e.g. a lower cost FET can be used).
Incorporating a third terminal in the latest MHP-SA devices provides a signal line to utilise the smart battery monitoring available in today’s electronics, and gives more control over the circuit protection in large Li-ion battery packs and modules for energy storage systems. The MHP device provides a rugged, resettable circuit protection solution giving battery pack designers and manufacturers a way to optimise space, reduce cost and enhance safety.
TE Circuit Protection
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