The MOSFET, a key component in the charge/discharge protection circuit, also has a certain rate of short-circuit failure. If the production volume of lithium batteries is not large, this effect will not be apparent. However, the demand for lithium batteries is enormous; in the first half of 2017 alone, global shipments of small lithium batteries reached 10 billion units. Given such a massive shipment volume, even a probability risk of 1 ppm translates to an average of 1000 dangerous events per year. Therefore, in addition to the main protection circuit, a secondary protection layer is added to further reduce the risk.
Mainstream solutions:
In secondary protection schemes, the commonly used protection schemes currently include the following:
A standard fuse.
Various components, including PTC temperature-controlled fuses.
Three-terminal fuse (SCP).
Dual IC solution
Highly integrated four-in-one solution
These five options are generally chosen for secondary protection.
Solution Introduction
Using a PTC eliminates the need for a three-terminal fuse (SCP), and vice versa. These protective devices are in a competitive relationship, much like different alleles vying for the same location on a chromosome. However, because no single protective element is universally superior, a variety of elements coexist to meet diverse application needs.
However, with the rapid popularization of smartphones, the demand for fast charging has emerged, and the charging current of mobile phones is getting larger and larger. Currently, several standards such as OPPO's VoOC standard, Qualcomm's QC4.0 standard, and MTK's P 3.0 standard have appeared. In the case of fast charging, the current will be very high in the first 30 minutes, reaching a maximum of about 6A.
The high current surge during the first 30 minutes of fast charging will bring heat and temperature rise, changing the competitive landscape of secondary protection components for lithium batteries. With increasingly faster charging currents, simple overcurrent protection and PTC can no longer meet the protection requirements of fast charging. Instead, a cooperative model will emerge: primary protection MOS + secondary protection SCP (commonly known as the SCP solution), and primary protection MOS + secondary MOS (commonly known as the dual MOS solution).
MOS and PTC can complement each other for temperature protection and overcurrent protection. PTC has temperature protection, but because of its high temperature derating ratio, it requires a larger size and has relatively weaker overcurrent protection capability. In addition, PTC has a slower operating speed and a higher overall pack impedance.
MOS + fuse cannot meet overvoltage requirements, but the temperature derating ratio is also very low, so a smaller current rating can be selected. It has relatively strong overcurrent protection capability, but no overvoltage function.
A MOS + secondary MOS (commonly known as a dual MOS solution) can meet overvoltage requirements, but the impedance of the MOS will increase with the rise in temperature, and the MOSFET also has a certain rate of short-circuit failure, making the design relatively complicated. Moreover, since the MOS can be reused, there is a certain risk from a safety perspective.
The MOS + secondary protection SCP (commonly known as the SCP solution) can meet the protection requirements of overvoltage and overcurrent, and it also has a very low temperature derating ratio, low impedance, low power loss, is relatively simple to design, and has a much faster response speed, making it a relatively safe protection mechanism.
Authentication Implementation
The aforementioned secondary protection solutions are all used in the market, and each application has its own characteristics. However, in terms of certification, it is difficult to pass all the tests of UL2054 by relying on a single component, because each component has some advantages and disadvantages.
First, the commonly used PTC. Because the charging current is very large, in order to ensure that it does not activate under the condition of high temperature rise during fast charging, the selected specification will inevitably be 1206 6A/7A. Choosing such a large specification will make it difficult for lithium batteries to pass the UL2054 LPS test, because it is difficult to limit the current below 8A within 60 seconds.
Second, commonly used fuses. The advantage is that they are not sensitive to temperature, and a 5A specification can be selected. A ≤5A specification fuse is very helpful for lithium batteries to pass the UL2054 LPS test; however, because they are not sensitive to temperature, they do not have over-temperature and over-voltage protection functions, so it is relatively difficult to pass the UL2054 6V/1C and 6V2C overcharge test items.
The latter two options are easier to get certified by UL, have more complete functions, and are safer in terms of protection mechanisms. The only drawback is that the cost is relatively high. In terms of performance, they add another layer of protection to the safety of lithium batteries, further reducing the risk factor significantly.
