Working principle of lithium battery protection circuit

The circuit has overcharge protection, over-discharge protection, overcurrent protection and short-circuit protection functions. Its working principle is analyzed as follows:

1 Normal state

In normal state, both the "CO" and "DO" pins of N1 in the circuit output high voltage, and both MOSFETs are in the on state. The battery can charge and discharge freely. Since the on-resistance of MOSFET is very small, usually less than 30 milliohms, its on-resistance has little effect on the performance of the circuit. In this state, the current consumption of the protection circuit is μA level, usually less than 7μA.

2 Overcharge protection

As a kind of rechargeable battery, lithium-ion batteries require constant current/constant voltage charging. In the initial stage of charging, constant current charging is used. As the charging process progresses, the voltage will rise to 4.2V (depending on the positive electrode material, some batteries require a constant voltage value of 4.1V), and then switch to constant voltage charging until the current becomes smaller and smaller. During the battery charging process, if the charger circuit loses control, the battery voltage will exceed 4.2V and continue to charge at a constant current. At this time, the battery voltage will continue to rise. When the battery voltage is charged to more than 4.3V, the chemical side reactions of the battery will intensify, causing battery damage or safety problems.

In a battery with a protection circuit, when the control IC detects that the battery voltage reaches 4.28V (this value is determined by the control IC, and different ICs have different values), its "CO" pin will change from high voltage to zero voltage, causing V2 to turn from on to off, thereby cutting off the charging circuit, making the charger unable to charge the battery, and playing an overcharge protection role. At this time, due to the presence of V2's own body diode VD2, the battery can discharge to the external load through the diode.

There is a delay time between the control IC detecting that the battery voltage exceeds 4.28V and sending a signal to shut down V2. The length of the delay time is determined by C3 and is usually set to about 1 second to avoid misjudgment due to interference.

3 Over-discharge protection

During the discharge process of the battery to the external load, its voltage will gradually decrease with the discharge process. When the battery voltage drops to 2.5V, its capacity has been completely discharged. If the battery continues to discharge to the load at this time, it will cause permanent damage to the battery.

During the battery discharge process, when the control IC detects that the battery voltage is lower than 2.3V (this value is determined by the control IC, and different ICs have different values), its "DO" pin will change from high voltage to zero voltage, causing V1 to turn from on to off, thereby cutting off the discharge circuit, making it impossible for the battery to discharge to the load, and playing an over-discharge protection role. At this time, due to the presence of V1's own body diode VD1, the charger can charge the battery through this diode.

Since the battery voltage cannot be reduced in the over-discharge protection state, the protection circuit is required to consume very little current. At this time, the control IC will enter a low power consumption state, and the power consumption of the entire protection circuit will be less than 0.1μA.

There is also a delay time between when the control IC detects that the battery voltage is lower than 2.3V and when it sends out the signal to shut down V1. The length of this delay time is determined by C3 and is usually set to about 100 milliseconds to avoid misjudgment due to interference.

4 Overcurrent protection

Due to the chemical characteristics of lithium batteries, battery manufacturers stipulate that the maximum discharge current cannot exceed 2C (C = battery capacity/hour). When the battery discharges at a current exceeding 2C, it will cause permanent damage to the battery or safety problems.

When the battery is discharging normally to the load, when the discharge current passes through the two MOSFETs in series, a voltage will be generated at both ends due to the on-resistance of the MOSFET. The voltage value is U=I*RDS*2, RDS is the on-resistance of a single MOSFET. The "V-" pin on the control IC detects the voltage value. If the load is abnormal for some reason, the loop current increases. When the loop current is large enough to make U>0.1V (this value is determined by the control IC, and different ICs have different values), its "DO" pin will change from high voltage to zero voltage, causing V1 to turn from on to off, thereby cutting off the discharge circuit and making the current in the circuit zero, which plays an overcurrent protection role.

There is also a delay time between the control IC detecting the occurrence of overcurrent and sending the V1 shutdown signal. The length of the delay time is determined by C3, usually about 13 milliseconds, to avoid misjudgment due to interference.

In the above control process, it can be seen that the over-current detection value depends not only on the control value of the control IC, but also on the on-resistance of the MOSFET. When the on-resistance of the MOSFET is larger, the over-current protection value is smaller for the same control IC.

5 Short-circuit protection

When the battery is discharging to the load, if the loop current is large enough to make U>0.9V (this value is determined by the control IC, and different ICs have different values), the control IC will judge that the load is short-circuited, and its "DO" pin will quickly change from high voltage to zero voltage, causing V1 to turn from on to off, thereby cutting off the discharge circuit and playing a short-circuit protection role. The delay time of short-circuit protection is extremely short, usually less than 7 microseconds. Its working principle is similar to that of over-current protection, but the judgment method is different and the protection delay time is also different.

The above describes in detail the working principle of the single-cell lithium-ion battery protection circuit. The protection principle of multiple-cell lithium-ion batteries in series is similar and will not be repeated here. The control IC used in the above circuit is the R5421 series of Ricoh, Japan. In the actual battery protection circuit, there are many other types of control ICs, such as Seiko's S-8241 series, MITSUMI's MM3061 series, Taiwan Fujing's FS312 and FS313 series, Taiwan Analog Technology's AAT8632 series, etc. Their working principles are similar, but they differ in specific parameters. In order to save peripheral circuits, some control ICs have built-in filter capacitors and delay capacitors inside the chip, so the peripheral circuits can be very few, such as Seiko's S-8241 series. In addition to the control IC, there is another important component in the circuit, which is MOSFET, which acts as a switch in the circuit. Since it is directly connected in series between the battery and the external load, its on-resistance affects the performance of the battery. When the MOSFET is selected, its on-resistance is very small, the internal resistance of the battery pack is small, the load capacity is strong, and the power consumed during discharge is also small.

With the development of technology, the size of portable devices is getting smaller and smaller. With this trend, the requirements for the size of lithium-ion battery protection circuits are also getting smaller and smaller. In the past two years, products that integrate control ICs and MOSFETs into a protection IC have appeared, such as DIALOG's DA7112 series. Some manufacturers even package the entire protection circuit into a small-sized IC, such as MITSUMI's products.