The MOSFET, a key component in charge/discharge protection circuits, also has a certain rate of short-circuit failure. If lithium battery production volume is not large, this effect will not be apparent. However, the demand for lithium batteries is enormous; in 2014 alone, global shipments of small lithium batteries reached 5.6 billion units. Given such a massive shipment volume, even a probability risk of 1 ppm translates to an average of 5,600 dangerous events per year. Therefore, in addition to the main protection circuit, a secondary protection layer is added to further reduce risk. The secondary protection components typically use only one element, including one-time-use fuses, PTCs, and thermal fuses, among others. Using a PTC eliminates the need for a fuse, and vice versa; the protection devices compete with each other, much like different alleles vying for the same location on a chromosome. However, because no single protection component is universally superior, a situation arises where multiple components coexist to meet various application requirements.
However, with the rapid popularization of smartphones and the increasing capacity of mobile phone batteries, the demand for fast charging has emerged. Currently, several standards such as OPPO VoOC, Qualcomm's QC2.0, and MediaTek's PumpExpressPlus have appeared. In the case of fast charging, the current will be very high in the first 30 minutes, generally reaching around 3A.
The high current surge during the first 30 minutes of fast charging, accompanied by heat generation and temperature rise, will change the competitive landscape of secondary protection components for lithium batteries. Instead, a cooperative model will emerge: PTC + fuse will form a protection combination.
First, PTC and fuse can complement each other for temperature and overcurrent protection. PTC has temperature protection, but due to its high temperature derating ratio, a larger current rating can be selected, resulting in relatively weaker overcurrent protection. Furthermore, PTC operates relatively slowly. Fuses, on the other hand, are not temperature-sensitive and do not provide temperature protection, but their temperature derating ratio is also very low, allowing for the selection of smaller current ratings. They offer relatively strong overcurrent protection and operate much faster.
Secondly, PTC + fuse will be a low-cost solution for passing UL2054. Under high-current charging conditions, relying on a single component is difficult to pass all UL2054 tests because each component has its advantages and disadvantages. First, the commonly used PTC. Because the charging current is very high, to ensure it doesn't trip under high temperature rise during fast charging, the selected specification must be at least 12066A/7A. Choosing such a high specification will make it difficult for lithium batteries to pass the UL2054 LPS test, as it's difficult to limit the current below 8A within 60 seconds. Second, the commonly used fuse. The biggest advantage is its insensitivity to temperature; a 5A specification can be selected. A fuse with a specification of ≤5A is extremely beneficial for lithium batteries to pass the UL2054 LPS test; however, because it is insensitive to temperature and lacks over-temperature protection, it is relatively difficult to pass the UL2054 6V/1C and 6V2C overcharge tests. Third, while three-terminal fuses can address over-temperature protection, their higher current rating (10A/12A) prevents them from passing the LPS test, and they are also very expensive. Fourth, some manufacturers use a dual-IC solution, which is effective but costly. Combining a PTC and a fuse allows for easy LPS and short-circuit tests using a temperature-insensitive 5A fuse, followed by a 12066A/7A PTC to pass overcharge tests such as 6V/1C and 6V2C. This overall solution is much cheaper.
Finally, the PTC+fuse protection scheme is safer than a single component. Combining the two components is equivalent to adding a second layer of protection on top of the existing secondary protection, further enhancing the safety of the lithium battery and significantly reducing the risk factor.
