Battery Power - March/April 2013 - (Page 6)
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
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
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.
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
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
Charging & Testing
Power Supplies & Conversion
ICs & Semiconductors
Battery Power - March/April 2013