A new protection scheme for high-current discharge lithium-ion batteries

introduction

　　The rapidly growing market for high-current discharge lithium-ion batteries used in cordless power tools, electric bicycles, backup power supplies, and other fields has created a demand for high-current (maintaining current above 30A at 30V DC) circuit protection devices.

　　A new type of MHP protection device has emerged, which is composed of a bimetal protector and a PPTC (polymer positive temperature coefficient) device in parallel. This device can not only provide resettable overcurrent protection, but also use the low resistance characteristics of the PPTC device to suppress the arc generated by the bimetal protector when it operates under high current conditions.

　　1 Traditional solutions and MHP devices

　　High-current lithium-ion battery pack applications require stable and reliable circuit protection; however, the conventional circuit protection devices currently available are generally large, complex or expensive. Some circuit protection designs use a combination of ICs and MOSFETs or similar complex solutions; some consider using bimetallic protectors in DC applications with a working current of 30A+, but large contacts must be used to withstand such high currents, resulting in an overly large protection device. In addition, the number of operations must be limited because arcing between the contacts may damage the contacts.

　　In contrast, the new MHP integrated device developed by TE can replace or reduce the discharge FET and heat sink used in some complex IC/FET battery protection designs. Using MHP devices in high-rate discharge lithium-ion battery pack applications can reduce space occupation, save costs, and improve protection performance.

　　2 Working Principle

　　Under normal conditions, the current flows through the bimetallic strip due to its low resistance. Under abnormal conditions, such as when the rotor of a power tool is locked, a very high current will be generated in the circuit, causing the bimetallic contacts to open and their contact resistance to increase the new solution for resettable circuit protection for high-current discharge lithium-ion battery applications. 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. This integrated design meets the requirements of a resettable overcurrent protection device with arc suppression in high-current DC applications.

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

　　a During normal operation, most of the current will flow through the bimetal because the contact resistance is very low.

　　b The contacts begin to open and the contact resistance rises rapidly. When the contact resistance is higher than the PPTC device resistance, most of the current will be diverted to the PPTC device, and little or no current will flow through the contacts, thus preventing arcing between the contacts. When the current is diverted to the PPTC device, its resistance rises rapidly and reaches a level far higher than the contact resistance, causing the PPTC temperature to rise.

　　Once the contacts open, the PPTC device begins to heat the bimetal, keeping it open until the overcurrent condition disappears or the power is turned off. The resistance of the PPTC device is much lower than that of the ceramic PTC device, which means that even if the contacts are only opened a small amount, the contact resistance will only increase slightly, and the current will be shunted to the PPTC device, effectively preventing the contacts from arcing. In general, the resistance difference between ceramic PTC devices and polymer PTC devices is about 10 to the power of two (x10^2). Therefore, the higher resistance ceramic PTC devices are far less effective than polymer PTC devices in suppressing high current arc discharges.

　　Figure 2 is a circuit diagram showing a bimetallic protector connected in parallel with a PPTC device.

　　3 Advantages of combining bimetal and PPTC

　　Figures 3 and 4 show the current and voltage when only one bimetal protector is used. Figure 3 shows the typical opening of the bimetal protector at 24VDC/20A rated conditions, and it opens after 1.28 milliseconds. Figure 4 shows the performance of the bimetal protector at twice the rated voltage. A standard bimetal protector arcs under fault conditions, and the time from the beginning of the contact opening to the short circuit is 334 milliseconds.

　　Figure 5 shows the result of using a PPTC device and a bimetal protector in parallel - the current is cut off. The time from when the bimetal protector starts to operate to when the PPTC device is fully activated is 6.48 milliseconds. The time from when the protector starts to operate to when the current is cut off is 4.80 milliseconds (see the right side of Figure 5).

　　Combining the two images in Figure 5, we can see the smooth transition of current from the bimetallic protector to the PPTC device, with no fuse forming on the protector contacts, and we can also see how the PPTC device helps prevent arcing at the contacts.

　　4 MHP device advantages

　　The following sections describe the advantages that MHP devices offer over conventional circuit protection devices.

　　4.1 Small contacts and low resistance

　　A typical bimetallic protector usually has only one contact, so its withstand voltage capability 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-closed/double-opened" contact design, which greatly reduces the size of the device (see Figure 6).

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

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

　　b Heat is only generated at the contact points, allowing accurate thermal activation by thermal control devices;

　　c It allows MHP devices to be more compact than other circuit-breaking devices with comparable ratings.

　　Figure 6: Double make/double break contact design for a comprehensive MHP device For comparison purposes, Figure 7 shows a standard bimetallic contact.

　　As can be seen from Figure 7, the contact is only located at one position, so its voltage resistance is not as good as that of the MHP device.

　　4.2 Improve shock and vibration resistance

　　Figure 8 shows the specific design advantages of MHP devices, which enable MHP devices to provide longer service life, withstand greater vibration and shock, and can be used in harsh working environments of high current applications.

　　Typical power tools are often subjected to high levels of vibration and shock when in use.

　　To achieve such requirements, the contacts of MHP devices need to have sufficient contact pressure between them. Standard protection devices usually use strong springs to keep the moving contact arm in contact with the fixed contact. However, under conditions of large shock or vibration, the pressure generated by the spring (even a strong spring) is usually not enough to keep the contacts in contact.

　　Barb (ensuring stable contact under vibration and shock conditions) To address this issue, the MHP device focused its design on the bimetallic disc because it has enough strength to remain stable without hot contacts. In addition, we added a barb to the moving contact arm to increase the contact pressure provided by the bimetallic disc. 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 MHP device has been tested to 1000 shocks and 1500 drops without failure, and has also passed three 3000-gram shock tests.

　　4.3 Tripping cycle test

　　The resistance/temperature curve of the MHP device is shown in Figure 9. The opening and closing temperatures of the device can be customized by selecting bimetals with different opening and closing temperatures.

　　Figure 10 shows how the MHP30 device fared after being tested over 500 trip cycles.

Figure 10: Cycle life under DC36V/100A (rated) conditions

　　Figure 11 shows the vibration/shock resistance of the device resistor, which was tested for 1000 cycles at 1500 g vibration/shock. After applying shock or vibration forces to the device in the direction of contact opening, the device design always maintains contact, proving that the design can withstand large shock/vibration.

　　Figure 12 shows a test condition of "1500g shock/1000 cycles" with a device current load of 1A. The shock or vibration direction of this test is the same as Figure 11, that is, in the direction of contact opening. From Figure 12, we can see that the device did not experience power cut-off under the 1500g shock/vibration condition. Figure 13 shows a test condition of "3000g shock/3 cycles" with a device current load of 1A; the shock/vibration direction is the same as Figure 12. From Figure 13, it can be seen that the current was not cut off under this test condition either.

　　Drop test results:

　　1,500g x 1,000 cycles / no load, no resistance change

　　1,500gx1,000 cycles/1A load without current cutoff

　　3,000gx3 cycles/1A load without current cutoff

　　4.4 Reduce space occupation and save costs

　　Using MHP devices in cordless power tool battery packs can reduce space and save costs compared to conventional circuit protection devices (see Figures 14a and 14b). MHP devices can replace two more expensive P-channel FETs (for charge control only) with one less expensive N-channel FET. Another potential cost saving method is to move the IC to the system (tool) side of the application and use MHP devices to provide over-discharge protection/short-circuit protection in the battery pack to meet future regulations for lithium battery power tool applications.

　　5 MHP Device Specifications

　　Table 1 lists the specifications of the MHP30 device. The maximum rating of the MHP30 device is 36VDC/100A, and the trip time is 5 seconds at 100A (@25°C). The device has an operating current of 30A and an initial resistance of less than 2mohm, which is lower than the initial resistance of common bimetallic protectors (usually 3-4mohm).

Table 1: MHP30 reference values

　　The MHP30 has a trip time of 25 seconds +/- 5 seconds at 50A. This is just the right amount of time to protect the battery pack from over-discharge and overheating conditions, but not to inconvenience the power tool operator with frequent trips.

　　The trip time at 100A is the most critical parameter for protecting the battery pack in abnormal conditions, such as a stuck power tool rotor. In this case, the trip time should be no longer than 5 seconds, and the recovery time (the time required to re-energize the tool) should be no longer than 30 seconds—the optimum for both user convenience and preventing battery overheating.

　　Figure 15 shows the shape and size of the MHP device. The MHP device is rated for 30 A operation, while a commonly used bimetallic protector of the same size is rated for 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.

　　6 Conclusion

　　The compact, easy-to-install MHP30 device uses the low resistance of the PPTC device to suppress high current arc discharge and can provide 30A+ operating current under rated conditions exceeding 30VDC. The MHP device can provide resettable circuit protection under harsh conditions, providing battery pack designers and manufacturers with an effective way to optimize space, save costs, and meet future battery safety requirements.

　　The MHP device technology can be configured for a variety of applications, with devices currently under development for higher voltages (up to 400 VDC) and operating currents (60 A). Next design considerations include lithium battery pack protection for cordless power tools, e-bikes, e-scooters, light electric vehicles, backup power applications, and non-battery applications such as motor protection.