Working principle of lithium ion battery protection circuit
1. Normal stateUnder normal conditions, the CO and do pins of N1 output high voltage, and both MOSFETs are in on state. Lithium ion battery can charge and discharge freely. Because the on impedance of MOSFET is very small, usually less than 30 milliohm, its on resistance has little effect on the performance of the circuit. In this state, the current consumption of the protection circuit is μ a, usually less than 7μa.
2. Overcharge protection
The charging mode of lithium-ion battery is constant current / constant voltage. In the initial stage of charging, constant current charging is adopted. With the charging process, the voltage will rise to 4.2V (some batteries require a constant voltage value of 4.1V according to the different cathode materials) and change to constant voltage charging until the current becomes smaller and smaller.
In the process of battery charging, if the charger circuit is out of control, the battery voltage will continue to be charged at 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 chemical side reaction of the battery will be intensified, which will lead to battery damage or safety problems.
In the battery with protection circuit, when the control IC detects that the battery voltage reaches 4.28v, the co pin of the battery will change from high voltage to zero voltage, which will turn V2 from on to off, thus cutting off the charging circuit, so that the charger can no longer charge the lithium-ion battery, playing the overcharge protection purpose. At this time, due to the existence of VD2, the battery can discharge the external load through the diode. There is a delay time between the control IC detects that the battery voltage exceeds 4.28v and sends off V2 signal. The delay time is determined by C3 and is usually set at about 1 second to prevent misjudgment caused by interference.
3. Over discharge protection
In the process of discharging external load, the voltage of lithium-ion battery will gradually decrease with the discharge process. When the battery voltage drops to 2.5V, its capacity has been fully discharged. At this time, if the battery continues to discharge the load, it will cause permanent damage to the battery.
In the process of battery discharge, when the control IC detects that the battery voltage is lower than 2.3V (the value is determined by the control IC, different IC has different values), its do pin will change from high voltage to zero voltage, making V1 from on to off, thus cutting off the discharge circuit, so that the battery can no longer discharge the load and play the role of over discharge protection. At this time, due to the existence of VD1 body diode, the charger can charge the battery through the diode.
Since the battery voltage can not be reduced in the over discharge protection state, the current consumption of the protection circuit is required to be very small. At this time, the control IC will enter the low power consumption state, and the power consumption of the whole protection circuit will be less than 0.1 μ a. There is also a time delay between the control IC detecting the lithium-ion battery voltage lower than 2.3V and sending off V1 signal. The delay time is determined by C3, which is usually set at about 100ms to prevent misjudgment caused by interference.
4. Over current protection
Due to the chemical characteristics of lithium-ion battery, the manufacturer of lithium-ion battery has stipulated that the maximum discharge current should not exceed 2C (C = battery capacity / hour). When the battery discharge current exceeds 2c, it will lead to permanent damage or safety problems of the battery.
During the normal discharge of the battery to the load, when the discharge current passes through two MOSFETs in series, a voltage will appear at both ends of the MOSFET due to the on impedance of the MOSFET. The voltage value u = I * RDS * 2, RDS is the conduction impedance of a single MOSFET. The v-pin on the control IC is used to detect the voltage value. If the load is abnormal for some reason and the loop current increases, when the loop current is large enough to make u > 0.1V (the value is determined by the control IC, different IC has different values), its do pin will change from high voltage to zero voltage, making V1 change from on to off, thus cutting off the discharge circuit and making the circuit The medium current is zero, which is used for over-current protection.
There is also a time delay between the occurrence of over-current detected by the control IC and the sending of the turn off V1 signal. The length of the delay time is determined by C3, usually about 13 MS, to prevent misjudgment caused by interference.
5. Short circuit protection
In the process of discharging the load of lithium-ion battery, if the loop current is large enough to make u > 0.9V (the value is determined by the control IC, and different IC has 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, making V1 turn from on to off, so as to cut off the discharge circuit and play a short-circuit protection purpose. The delay time of short circuit protection is very 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.
