What are the protection circuits and working principles of lithium batteries?

　　With the development of science and technology, the volume of portable devices is getting smaller and smaller. With this trend, the requirements for the volume of the protection circuit of lithium-ion batteries are getting smaller and smaller. This article will elaborate on the protection circuit of single-cell lithium-ion batteries. The working principle is similar to the protection principle of multi-cell series lithium-ion batteries.

　　The circuit has the functions of overcharge protection, overdischarge protection, overcurrent protection and short circuit protection. Its working principle is analyzed as follows:

　　1 Normal state

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

　　2 Overcharge protection

　　Lithium-ion battery is a kind of rechargeable battery. The required charging method is constant current/constant voltage. In the initial stage of charging, it is constant current charging. With the charging process, the voltage will rise to 4.2V (depending on the positive electrode material, some The battery requires a constant voltage value of 4.1V), and switch to constant voltage charging until the current becomes smaller and smaller. When the battery is being charged, if the charger circuit loses control, the battery voltage will continue to be charged with constant current after the battery voltage exceeds 4.2V. At this time, the battery voltage will continue to rise. When the battery voltage is charged to more than 4.3V, the battery's chemistry Side reactions will intensify, causing battery damage or safety issues.

　　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, different ICs have different values), its "CO" pin will change from high voltage to zero voltage. Turn V2 from on to off, thereby cutting off the charging circuit, so that the charger can no longer charge the battery, which plays a role of overcharge protection. At this time, due to the existence of the body diode VD2 of V2, the battery can discharge the external load through the diode.

　　There is a delay time between when the control IC detects that the battery voltage exceeds 4.28V and when the V2 signal is turned off. The length of the delay time is determined by C3 and is usually set to about 1 second to avoid errors caused by interference. judge.

　　3 Over discharge protection

　　When the battery discharges to an 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. At this time, if the battery continues to discharge the load, it will cause battery damage. Permanent damage.

　　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, different ICs have different values), its "DO" pin will change from high voltage to zero voltage, making V1 Turning from on to off, which cuts off the discharge circuit, so that the battery can no longer discharge the load, playing a role of over-discharge protection. At this time, due to the existence of the body diode VD1 of V1, the charger can charge the battery through this diode.

　　Because the battery voltage cannot be lowered any more in the over-discharge protection state, the current consumption of the protection circuit is required to be extremely small. 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 the V1 signal is turned off. The length of the delay time is determined by C3 and is usually set to about 100 milliseconds to avoid errors caused by interference. judge.

　　4 Overcurrent protection

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

　　When the battery discharges the load normally, when the discharge current passes through two MOSFETs connected in series, due to the on-resistance of the MOSFETs, a voltage will be generated at both ends of the MOSFET. The voltage value U=I*RDS*2, RDS is a single MOSFET on-resistance, control the "V-" pin on the IC to detect the voltage value. If the load is abnormal for some reason, the loop current will increase. When the loop current is so large that U>0.1V (the value is determined by When the control IC decides that different ICs have different values), its "DO" pin will change from high voltage to zero voltage, and turn V1 from on to off, thereby cutting off the discharge loop and making the current in the loop zero. Play the role of over-current protection.

　　There is also a delay time between when the control IC detects the occurrence of overcurrent and when the V1 signal is turned off. 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 overcurrent 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 overcurrent protection of the same control IC The smaller the value.

　　5 short circuit protection

　　When the battery is discharging the load, if the loop current is so large that U>0.9V (this value is determined by the control IC, different ICs have different values), the control IC will judge that the load is short-circuited, and its "DO" pin will be It quickly changes from high voltage to zero voltage, so that V1 is turned from on to off, thereby cutting off the discharge circuit and playing a role of short-circuit protection. 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 working principle of the single-cell lithium-ion battery protection circuit is explained in detail above. The protection principle of the multi-cell series-connected lithium-ion battery is similar. In the actual battery protection circuit, there are many other types of control ICs, such as the S-8241 series of Seiko, the MM3061 series of MITSUMI, the FS312 and FS313 series of Taiwan Fujing, the AAT8632 series of Taiwan Analog Technology, etc. The working principle is the same, but the specific parameters are different. In order to save the peripheral circuit, some control ICs have the filter capacitor and the delay capacitor built into the chip, and the peripheral circuit can be very few, such as the S-8241 series of Seiko. In addition to the control IC, there is also an important component in the circuit, which is the MOSFET, which acts as a switch in the circuit. Because it is directly connected in series between the battery and the external load, its on-resistance affects the performance of the battery. Influence, when the selected MOSFET is better, its on-resistance is small, the internal resistance of the battery pack is small, the load capacity is also strong, and it consumes less electric energy when discharging.

　　With the development of science and technology, the volume of portable devices is getting smaller and smaller, and with this trend, the requirements for the volume of the protection circuit of lithium-ion batteries are getting smaller and smaller. In the past two years, there has been a control IC and MOSFET. Integrated into a protection IC product, such as DIALOG's DA7112 series, some manufacturers even encapsulate the entire protection circuit into a small-size IC, such as MITSUMI's products.