Since Lithium-Ion (Li-ion) batteries have replaced nickel-metal hydride (NiMH) batteries, they are increasingly used in mobile phones, tablet computers, ultra-thin notebooks and other portable electronic products. The product has higher and higher battery capacity requirements, but at the same time requires the battery to be smaller and thinner, which makes the lithium battery have a higher capacity density.
Lithium Polymer (LiP) batteries can meet the thinner requirements, so their market share continues to increase, especially for smart phones, tablet computers and ultra-thin notebooks. However, lithium-ion batteries may be ignited and burned under certain abuse conditions, and even explosions, such as overcharge, short circuit, and high temperature, may cause heat and gas inside the battery, causing the battery to expand, rupture, and ignite. The safety of ion batteries has also attracted the attention of manufacturers, end users and management.
China and other countries have issued the country's own safety standards for batteries, such as China's GBT18287 and GB31241-2014, US and North America applicable UL1642 and UL2054, EU's EN60950 and EN62133, Japan PSE and Korea KC standards, etc. Standards have a positive role in regulating markets and protecting consumers.
Choose a suitable passive protection device to ensure lithium battery safety
In many cases, battery designers have some problems when choosing passive protection devices, such as whether to use passive protection devices? Which one to use? For lithium batteries, safety standards and performance requirements conflict with each other, sometimes it is difficult to get the best of both worlds. What should I do?
This article will explain how to choose the appropriate passive protection device fuse (Fuse) / Recoverable Polymer Positive Temperature Coefficient (PPTC) / MHP-TA (Metal Hybrid PPTC-Thermal AcTIvaTIon Device) The standard and the host charge and discharge performance requirements for lithium batteries. The safety standards in this paper take UL2054 as an example.
One-time blown fuse is more fragile
The fuses mentioned here are one-time blown fuses (fuse), which are typically included in lithium battery applications (Thermal Fuse) and slow-acting surface mount fuses (SMD Fuse) (Table 1).
The unrecoverable nature of the one-time blown fuses requires the designer to be extra cautious in the selection process to avoid damage to the battery due to mistakes in battery assembly, testing, and use.
Another main purpose of selecting a slow-break fuse is the Limited Power Source (LPS) requirement, as shown in Table 2. The surface-adhesive Fuse with a rated current of 5 amps (A) is usually selected. Because it can meet both LPS and battery charge and discharge requirements, the SMD Fuse cannot be restored, so the selected slow-break Fuse blow time must be better than that of the protection IC. The overcurrent action time is long, ensuring that the Fuse does not act and blow when the protection circuit is operating normally.
In order to meet this requirement, Fuse suppliers must have excellent quality control during production to ensure consistent performance. Since the LPS limits the Fuse maximum specification to 5A, it is difficult to meet the high application requirements for charge and discharge (Table 3).
PPTC performance / safety must be considered
PPTC is a positive temperature coefficient thermistor, which is widely used in lithium battery protection circuits. With the increasing capacity of lithium batteries and the increasing demand for battery currents in portable electronic products such as mobile phones, the factors that need to be considered when selecting PPTC are also increasing. The more you have to meet the performance requirements of the lithium battery, but also the safety requirements of the lithium battery (Figure 1).
Figure 1 When choosing PPTC, there are many factors to consider. To meet the performance of the product, it must meet the safety requirements of the lithium battery.
. Assembly method
PPTC is a temperature sensitive device, so it must be fully considered in the battery assembly method and production process, such as reflow soldering temperature setting and effective control, reflow times, board dispensing and curing methods (ultraviolet or high temperature baking) , low temperature injection molding parameter settings (pressure, temperature and time), and whether there is a manual welding process. The PPTC may increase the resistance due to pressure and high temperature in these processes, indirectly affecting the holding current and operating characteristics.
Surface-adhesive PPTC is mounted on a printed circuit board (PCB) through reflow soldering. The pad size, copper foil thickness, trace width, and heating of adjacent components all affect the thermal performance of the PPTC, which affects the holding current and action. characteristic.
The strip PPTC is connected to the lithium battery cell electrode and the nickel strip and the PCB board through resistance welding or laser welding. Therefore, the length, width and thickness of the nickel strip affect the heat dissipation performance, thereby affecting the PPTC holding current and the operating characteristics (Fig. 2).
Figure 2 Ribbon PPTC and lithium battery cell electrode and nickel tape connected to PCB board
. Safety requirements
When PPTC is used in a lithium battery, the requirements of LPS are different from those of Fuse. There are many factors to consider when selecting. In particular, it is necessary to meet the requirements of high current discharge at high temperature and overcharge in safety. It is also necessary to carry out an overcharge test in conjunction with the lithium battery cell selected for the specific project. Lithium battery cells of different manufacturers and different specifications have different temperature and safety characteristics in the overcharge test. The surface-adhesive PPTC is located far away from the battery core on the circuit board, so the temperature change of the battery core cannot be fully sensed, and it is also affected by the wiring and other components on the circuit board. Therefore, in order to meet the safety requirements, the surface adhesion is selected. The PFET should be slightly reduced in current specifications.
For strip PPTC, in the battery design and production process, it can not be subjected to external force and stress on the device; the connection with the battery cell, the degree of bonding and the size of the nickel strip will affect whether the battery can pass the safety test (Figure 3). .
Figure 3 Limitations of the power supply without overcurrent protection devices
MHP-TA small size / high temperature protection accuracy for high current demand
MHP-TA is a recoverable temperature protection device with small size and high temperature protection accuracy, so it is suitable for high current demand and is ideal for high-capacity lithium battery safety protection (Figure 4).
Figure 4 MHP-TA is small
MHP-TA is divided into two series of high current (15A) and low current (6A), which correspond to different operating temperatures, as shown in Table 4.
. Select component considerations
MHP-TA has high temperature protection accuracy, low on-resistance, and performance does not change with time and number of actions. Therefore, MHP is mostly used in lithium batteries with high temperature protection and performance requirements.
In terms of holding current, the low current specification MHP and PPTC have basically the same performance, but in the lithium battery core protection, the battery core with different temperature characteristics and safety performance can use different operating temperature MHP, thereby making full use of lithium battery charge and discharge performance. And provide more security protection. When the battery current is required to be high, for example, when the requirement is greater than 3.2A at 60 degrees, the PPTC is difficult to meet the requirements, and the MHP is a better choice, especially the high current series MHP, which can meet the requirements of large current and Effectively protects the lithium battery cell (Figure 5).
Figure 5 Different temperatures also affect the safety of the battery core
After selecting the MHP specification according to the maximum current requirement of the lithium battery, it is also required to pass the overcharge test, usually 6V/1C, 2C, or 9V, 12V, etc. The lower the MHP current and temperature specifications, the better the battery safety protection, but This is in contradiction with the battery current requirement. Therefore, it is necessary to balance the two requirements when selecting. However, in some cases, it is difficult to meet at the same time, it is necessary to communicate with the customer, and balance between the two requirements, such as reducing the battery at high temperatures. The current is required to meet the safety requirements.
During the design and assembly of the battery, the MHP-TA cannot be subjected to external forces, and the stress applied to the device or both terminals affects the resistance and temperature characteristics of the MHP. The connection to the cell, the degree of bonding, and the size of the nickel strip all affect whether the battery passes the safety test (Figure 6).
Figure 6 MHP-TA and battery cell connection, fit and nickel tape size will affect the safety test.
LPS requires a restricted power supply
Almost all mobile phone manufacturers require lithium batteries to meet the LPS requirements in safety regulations. MHP has the same requirements for LPS as PTC. Under normal circumstances, when the MHP is assembled into the battery, the heat dissipation condition is not as good as the external test, so the actual holding current characteristic of the MHP in the lithium battery will be slightly lower than the current value specified in the specification. Based on the charging characteristics of lithium batteries, MHPs with an operating temperature of 90 degrees can rarely be used for lithium battery safety protection. Therefore, except for the 90 degree MHP, all low current series MHP can basically satisfy LPS. Laptops and some tablet computers require higher discharge currents, high-current series MHPs can be used, and other design methods are used to satisfy LPS.
Understand the battery design method and choose the right battery core
In summary, the thermal fuse Thermal Fuse has been used less and less for lithium battery protection due to large volume and manual soldering; surface-adhesive PPTC is used in mobile phones due to its small size, low cost, and simple reflow soldering process. Battery protection is becoming more and more common, but it is difficult to meet the demand for high-current requirements for tablet computers and notebook computers. The protective effect of ribbon PPTC is good, so there are still some lithium batteries, especially square lithium which is more suitable for metal casings. Ion battery assembly; MHP-TA has high temperature protection accuracy, small on-resistance and stability, so it is widely used in high-end portable electronic products with high safety requirements, especially with high current and high voltage. Almost all of the required laptop batteries use this type of product.
The key to meeting the performance requirements of lithium batteries and passing the safety test is not only related to the selection of the protection device model and specifications, but also the specific battery core type, charge and discharge performance, safety characteristics, connection and installation method of the protection device, circuit board and battery assembly. The heat dissipation conditions are closely related. Therefore, in order to provide Fuse/PPTC/MHP-TA samples more accurately and quickly when developing new battery projects, communicate with battery designers to understand battery performance requirements, assembly methods, and safety. Regulatory requirements and detailed processes, etc., help to avoid project delays due to untested tests.
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