Application of MHP technology in lithium battery circuit protection

Advances in lithium-ion battery technology have enabled high-energy lithium-ion batteries with higher energy density and lighter weight, which can replace nickel-cadmium batteries and even lead-acid batteries used in applications such as power tools, electric bicycles, light electric vehicles and backup power supplies .

Lithium-ion technology is more environmentally friendly than nickel-cadmium or lead-acid technology. However, the challenge of adopting lithium-ion batteries in emerging markets is that system designers are placing more emphasis on battery safety requirements compared to nickel-cadmium or lead-acid technology.

A new approach to lithium-ion battery circuit protection now exists that addresses these market challenges by replacing traditional high-cost, space-consuming protection technologies. The new hybrid technology connects a bimetal protector and a PPTC (polymer positive temperature coefficient) device in parallel. The resulting bimetal-PPTC combination device (MHP) helps provide resettable overcurrent protection in high-rate discharge battery packs while preventing the arcing behavior of the bimetal protector at higher currents by utilizing the low impedance of the PPTC device and maintaining its open and latched state by heating the bimetal.

Traditional methods

Traditional circuit protection technologies in many high-energy discharge lithium-ion applications tend to be large, complex, or expensive. Some circuit protection designs may use a combination of ICs and MOSFETs, or similarly complex solutions. Other designs may consider that traditional bimetallic protectors in DC power applications require 30A+ holding currents, but the contact area must be large enough to handle this high current, which results in a larger device in overall size. In addition, the number of switching cycles must be limited because arcing between the contact surfaces may cause contact damage.

In contrast, Tyco Electronics' new MHP hybrid devices can replace or help reduce the number of discharge field-effect transistors and accompanying heat sinks used in some complex IC/FET battery protection designs. MHP devices offer space savings, cost reduction and enhanced protection for high-rate discharge lithium-ion battery pack applications. The first device, MHP30-36, has a maximum rating of 36VDC/100A and a holding current of 30A. MHP device technology can be configured for a variety of different applications, and devices with higher voltages (up to 400VDC) and holding currents (60A) are now being developed.

How it works

Under normal conditions, the current flows through the bimetallic strip due to its low resistance. Under abnormal conditions, such as when the power tool rotor is blocked, a very high current will be generated in the circuit, causing the bimetallic contacts to open and increase their contact resistance. At this time, the current will flow through the low-resistance PPTC. The current flowing through the PPTC not only suppresses the arc between the contacts, but also heats the bimetallic strip to keep it in the open state and locked position.

As shown in Figure 1, the activation steps of the MHP device include:

1. Under normal conditions, most of the current flows through the bimetallic strip due to its low resistance.

2. When the contacts are opened, the contact resistance increases rapidly. If the contact resistance is higher than the resistance of the PPTC device, most of the current will flow through the PPTC device, and no current or very little current will flow through the contacts, thus suppressing the generation of arcs between the contacts. When the current is diverted to the PPTC device, its resistance increases rapidly to be higher than the contact resistance, causing the PPTC to heat up.

3. Once the contacts open, the PPTC device begins to heat the bimetal and keeps it open until the overcurrent event ends or the power is turned off.

Figure 1: Activation steps of an MHP device.

The resistance of PPTC devices is much lower than that of ceramic PTC devices, which means that even if the contacts are only opened a small portion and the contact resistance only rises slightly, the current will be diverted to the PPTC device, effectively preventing the contacts from arcing. Generally speaking, at room temperature, the resistance difference between ceramic PTC devices and polymer PTC devices is about 10 to the power of two (x10^2). Therefore, when ceramic PTC devices with higher resistance are connected in parallel with bimetallic materials, they are far less effective than polymer MHP devices in suppressing high current arc discharges.

Figure 2 shows a circuit diagram of a bimetal protector connected in parallel with a PPTC device.

Figure 2: Circuit diagram of a bimetal protector in parallel with a PPTC device.

Smaller contacts, lower resistance

A typical bimetallic protector usually has only one contact, so its withstand voltage is not strong. For a single contact design, the contact size required for higher currents will also be large. To solve this problem, MHP devices use a "double close/double open" contact design, which greatly reduces the size of the device.

Dimensions (see Figure 3).

Figure 3: MHP hybrid devices use a “double make/double break” contact design.

This technology has the following advantages over commonly used bimetallic protectors:

1. Since the current path is extremely short, the resistance of the device is very low;

2. Heat is generated only at the contact point, so accurate thermal activation can be achieved without the use of thermal control.

3. It allows MHP devices to be more compact than other circuit breakers with comparable rated parameters.

In comparison, standard bimetallic contacts are generally not as voltage-resistant as MHP devices because the contacts are only in one location.

Shock and vibration resistance

A unique advantage of MHP devices is that they can provide longer service life, withstand greater vibration and shock, and can be used in the harsh environment of high current applications.

A typical power tool battery pack is often subjected to high vibration and shock during use. To meet such requirements, the contacts of the MHP device need to have sufficient contact pressure. Standard protection devices usually use strong springs to keep the moving contact arm in contact with the fixed contact. However, under high shock or vibration conditions, the pressure generated by the spring (even a strong spring) is usually not enough to keep the contacts in contact.

To address this issue, the MHP device design focuses on the bimetallic disk because it has enough strength to remain stable without a hot contact. In addition, a barb was added to the moving contact arm to increase the contact pressure provided by the bimetallic disk. The moving contact arm is secured by a latch on the other side of the device. Adding a barb to the contact point reduces the rotation of the moving arm and creates more downward pressure on both contacts. The first MHP device for electric battery applications has been tested to at least 500 1,500g drops without failure and has also passed three 3,000g tests.

The first in the planned MHP device family, the MHP30-36-T, has a maximum rating of 36VDC/100A and an operating time of less than 5 seconds at 100A (@25°C). The device has a holding current of 30A and an initial resistance of less than 2 milliohms, compared to 3 to 4 milliohms for typical bimetallic protectors.

Figure 4 shows the shape and size of the MHP30-36 device. It has a holding current of 30 A, while a commonly used bimetallic protector of the same size is rated at only 15 A. In addition, one side of the device is flattened to fit between the standard 18 mm diameter lithium battery cells in a battery pack.

Figure 4: Dimensions of the MHP30-36 device.

Conclusion

The rapid growth of the high-energy discharge lithium battery market is creating new requirements for battery circuit protection devices that can handle higher current and voltage ratings. The new MHP hybrid device provides battery pack designers with a new circuit protection approach that makes these designs more cost-effective. Compared with traditional methods, the hybrid device provides an enhanced protection to suppress arcing while also eliminating the need for multiple large discharge FETs and heat sinks used in previous IC plus MOSFET battery protection designs.

The MHP design can be configured for a variety of applications by using PPTCs in parallel and selecting PPTCs with different voltage ratings. The MHP device architecture can be configured for a variety of different applications, and devices for higher voltages (up to 400VDC) and operating currents (60A) are currently being developed.

The concept of adding a third terminal as a control signal line is under development, so that the MHP device can take advantage of the advanced features of the IC to monitor a variety of important battery operating conditions. If an abnormal condition is detected, the IC will send a signal through a low power switch line to activate the device and disconnect the main circuit. Such MHP devices with "smart activation" functions will provide more circuit protection control for large lithium-ion batteries and modules used in solar power systems and backup power applications.