Battery Power - March/April 2013 - (Page 6)

Feature Circuit Protection Approach for High-Rate Discharge Li-Ion Battery Applications Ty Bowman, Global Battery Market Manager TE Circuit Protection, a Business Unit of TE Connectivity In 2010, a metal hybrid PPTC (MHP) technology was introduced to address the rapidly expanding market for high-ratedischarge lithium ion (Li-Ion) battery applications. The MHP arc-less contact technology results in circuit protection devices capable of providing 30 A+ hold currents at voltage ratings over 30 VDC. The MHP devices offer designers a cost-effective, space-saving alternative to conventional battery pack design solutions. This article describes how the latest MHP technology, introduced in 2012, builds upon the previous-generation MHP device family by adding Smart Activation functionality. PPTC device and no, or less, current remains on the contact, therefore preventing arcing between the contacts. When current shunts to the PPTC device, its resistance rapidly increases to a level much higher than the contact resistance and the PPTC device heats up. 3. After the contact opens, the PPTC device starts to heat up the bimetal and keeps it open until the overcurrent event ends or the power is turned off. New Approach to Battery Protection Due to advances in Li-Ion technology, smaller, lighter weight and higher-power Li-Ion batteries can now replace nickel cadmium or lead acid batteries previously used in high-rate-discharge battery applications. This trend has resulted in more high-power applications switching to Li-Ion battery technology. This, in turn, has created the need for more robust circuit protection solutions to help ensure battery safety in end-products. Currently, few protection solutions address high-rate-discharge Li-Ion battery applications, such as power tools, E-bikes, light electric vehicles (LEVs) and standby power applications. Furthermore, traditional circuit protection techniques tend to be large, complex and/or expensive. MHP technology addresses the design trends in the Li-Ion battery pack market by offering a cost-effective, space-saving circuit protection device. By connecting a bimetal protector in parallel with a polymeric positive temperature coefficient, or PPTC device, the MHP device provides resettable overcurrent protection while also utilizing the low resistance of the PPTC device to help prevent arcing in the bimetal protector at higher currents. Core Design Concept During normal operation of the MHP device, current passes through the bimetal contact due to its low contact resistance. 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 and helps prevent arcing between the contacts while also heating the bimetal, keeping it open and in a latched position. As shown in Figure 1, the activation steps of the MHP device include: 1. During normal operation, because contact resistance is very low, most of the current goes through the bimetal. 2. When the contact begins to open, contact resistance increases quickly. If the contact resistance is higher than the PPTC device’s resistance most of the current goes to the 6 Battery Power • March/April 2013 Figure 1. Activation steps for a standard MHP device. A PPTC device’s resistance is much lower than that of a ceramic PTC, which means that 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. Smart Activation The latest generation MHP technology, MHPSA devices, incorporate a third terminal as a signal line for over charge protection. This enables the device to take advantage of the advanced features of the IC that is monitoring various vital functions of the battery. If an abnormality is detected, Figure 2. Latest-generation MHP-SA the IC can send a signal device incorporates a third terminal via a low power switch for external activation. line to activate the MHPSA device and open the main line, as shown in Figure 2. The activation steps are: 1. The IC monitors the battery system for abnormalities in temperature, current and voltage.

Table of Contents for the Digital Edition of Battery Power - March/April 2013

Battery Power - March/April 2013
Editor's Choice
GTS and RGIS Launch Battery Test & Replace Service for Mobile Devices
Leyden Energy Raises $10 Million for Commercialization of Silicon Anode Mobile Pouch Cells and Automotive Start-Stop Batteries
Circuit Protection Approach for High-Rate Discharge Li-Ion Battery Application
Battery Power 2013 Conference Preview
Simplify Peripheral Connectivity and Extend Battery Life in Mobile Applications
Managing Lithium-Chemistry Batteries: It’s Mostly About Their Temperature
Lead Acid Batteries: A Proven Technology Marches Forward
New Products
Charging & Testing
Power Supplies & Conversion
ICs & Semiconductors
Industry New

Battery Power - March/April 2013