Secondary protection technology for fast-charging smartphones

Fast-charging smartphones and Lithium-ion batteries

The performance of smartphones improve every year, leading to an increase in the capacity of the Lithium-ion batteries installed in these devices. As battery capacity increases, the time required to recharge also increases. Therefore, some companies are now selling smartphones that can be recharged more rapidly by carrying a larger current than before.

Smartphones designed 5 to 6 years ago usually took 3 to 4 hours to fully charge, but some of the latest fast-charging smartphones can be fully charged in about 30 minutes. New technology for secondary protection of Lithium-ion batteries used in smartphones is required to cope with high-capacity and fast-charging batteries.

Fuse systems for secondary protection: SCP (Self Control Protector) and FET (Field Effect Transistor)

A solution often used for protection of Lithium-ion batteries in ordinary smartphones is to design a dual protection circuit that combines a protection IC and a field-effect transistor (FET). The FET is connected to an IC that detects voltage. If the IC detects a voltage that exceeds the standard amount, the current flowing to the charge/discharge circuit is cut off. In the case of the circuit shown on the left in the figure below, the voltage thresholds are slightly shifted, such as 4.4 V for FET-1 in the primary protection and 4.5 V for FET-2 in the secondary protection. This enables dual protection.

The dual FET method is widely used by major smartphone manufacturers. However, the FETs used in conventional dual protection systems have a resistance value of approximately 2mΩ. One way to reduce the resistance value significantly is to make the batteries larger. The lower the resistance value, the smaller the energy loss. Conversely, a higher resistance value means more heat generation (hotter), and so lower resistance is the best for suppressing heat generation.

As a protection method, it is possible to use two FETs. But since they operate the same way, there is a possibility that both FETs may not work when an incident outside of their capability occurs. To ensure protection for the battery’s safety and reliability, the primary and secondary protection circuits should operate using different mechanisms.

Therefore, in place of FETs, the low-resistance SCPs* developed by Dexerials are increasingly being adopted as a fuse element for secondary protection. Fast-charging smartphones need to minimize energy loss by lowering the resistance of circuit components as much as possible in order to reduce heat generated during charging. The fuse resistance of the SCP model SFJ-0422U is 0.9mΩ, the smallest among Dexerials’ SCPs for small devices, and less than half of that of general FETs used for secondary protection. This has led it to being adopted in many quick-charging smartphones.

*SCP is a fuse element used in the secondary protection of Lithium-ion batteries, which Sony Chemicals (now Dexerials) launched in 1994. It is now installed in many devices with Lithium-ion batteries.

The above figure shows the relationship between temperature and current of SCP mounted on a board. When the current is increased from 0.2A to 5A, the resistance of the SFJ-0422U remains almost unchanged at 0.9mΩ. Although the temperature rises due to Joule heat, the rise is gradual and stops at +11°C. (Note: temperatures are for reference only, as they are greatly affected by the surrounding environment.)

Smartphone manufacturers design their products so that they do not exceed specified temperatures in order to prevent the occurrence of low temperature burns and other health hazards even after prolonged use. Low-resistance SCPs help suppress temperature increases and power consumption.

The small size of SCP leads to its adoption in 200 million units

The diagram above shows how the SFJ-0412S, a standard product for small, low-resistance SCPs is integrated into a Lithium-ion battery of a smartphone. In smartphones, space is limited, and protection circuit modules must be miniaturized. In many cases, as shown in the figure, module boards are long and narrow. As such, the SFJ-0412S, with a mounting area width of 3 mm, has a great space-saving design. As of November 2020, more than 200 million units have already been shipped and are used in the Lithium-ion batteries of many smartphones. Dexerials also developed an even smaller product, the SFR-0412x (1.8 mm on the shortest side) to support the miniaturization of protection circuit modules.

SCP (SFJ-0422U) for fast-charging devices

Development of low-resistance SCPs, which are used in fast-charging smartphones, began several years ago with a customer request. Dexerials then developed SFJ-0422U, a low-resistance and small size SCP.

Currently, Dexerials is working on a challenging task to simultaneously achieve low fuse resistance to accommodate fast-charging miniaturized smartphones. Increasing the current path through cross-section of the fuse element can lower resistance, but conversely, the element becomes larger, making miniaturization impossible. Especially in cases where component height is limited, as in smartphones, the only way is to elongate them horizontally which results to a larger mounting area. On the other hand, miniaturization reduces the current path through cross-section, which leads to an increase in fuse resistance.

Below is the product information for the SCPs (SFJ-0412S and SFJ-0422U) that are now available.

In summary, the major difference between SCPs and FETs as secondary protection elements is their circuit breaking function. When a problem occurs in the battery of a smartphone, the protection IC detects an abnormal voltage and shuts down the circuit using FETs. However, this is only temporary; once the voltage returns to the normal range, charging and discharging become possible again. On the other hand, if an abnormal overvoltage or overcurrent problem occurs in the battery, the SCP will blow the fuse element and the circuit will be permanently shut off. Secondary protection elements are there to prevent damage in the case of an emergency, so it is safer to disable the battery as a precaution once a problem has occurred.

In fact, accidents involving Lithium-ion batteries are still being reported today. Dexerials will continue improving SCPs to enhance the safety of Lithium-ion batteries.