Introduce the working principle of lithium battery protection circuit
The protection circuit is usually composed of control IC, MOs switch tube, fuse, resistor, capacitor and other components, as shown in Figure 2. Under normal circumstances, the control IC output signal controls the MOs switch to conduct, so that the cell and the external circuit are connected. When the cell voltage or loop current exceeds the specified value, it immediately controls the MOS tube to turn off to protect the cell. Safety.
1. Normal state
In the normal state, the CO and DO pins of N1 both output high voltage, and the two MOSFETs are in the on state. The lithium-ion battery can be charged and discharged freely. Because the on-resistance of the MOSFET is very small, usually less than 30 milliseconds ohms, so its on-resistance has little effect on the performance of the circuit. The current consumption of the protection circuit in this state is μA level, usually less than 7μA.
2. Overcharge protection
The charging method required for lithium-ion batteries is constant current/constant voltage. In the early stage of charging, it is charged with constant current. With the charging process, the voltage will rise to 4.2V (depending on the positive electrode material, some batteries require a constant voltage value of 4.1V). ), switch to constant voltage charging until the current becomes smaller and smaller.
During the charging process of the battery, if the charger circuit is out of 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 chemical Side reactions will intensify, resulting in battery damage or safety issues.
In a battery with a protection circuit, when the control IC detects that the battery voltage reaches 4.28V, its CO pin will change from high voltage to zero voltage, so that V2 will turn from on to off, thus cutting off the charging loop and charging The charger can no longer charge the lithium-ion battery, which is used for 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 still a delay time between when the control IC detects that the battery voltage exceeds 4.28V and sends the signal to turn off V2. The length of the delay time is determined by C3 and is usually set to about 1 second to prevent errors caused by interference. judge.
3. Over discharge protection
During the process of discharging the external load, the voltage of the lithium-ion battery 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 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 (this value is determined by the control IC, different ICs have different values), its DO pin will change from high voltage to zero voltage, so that V1 is driven by the lead. The on-turn is turned off, thereby cutting off the discharge circuit, so that the battery can no longer discharge the load, which is used for over-discharge protection. At this time, due to the existence of the body diode VD1 of V1, the charger can charge the battery through the diode.
Since the battery voltage can no longer be reduced 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 voltage of the lithium-ion battery is lower than 2.3V and sends the signal to turn off V1. The length of the delay time is determined by C3 and is usually set to about 100 milliseconds to prevent cause misjudgment.
4. Over current protection
Due to the chemical characteristics of lithium-ion batteries, lithium-ion battery manufacturers stipulate that the maximum discharge current cannot exceed 2C (C=battery capacity/hour). question.