Operating Principle Of Lithium-ion Battery Protection Circuit

- Jun 05, 2020-

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


1. Normal state

The "CO" and "DO" pins of N1 in the circuit output high voltage, and both MOSFETs are in the on state, and the battery can be freely charged and discharged. Because the on-resistance of the MOSFET is small, it is usually less than 30 mΩ, 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 7 μA.


2. Overcharge protection

Lithium ion battery is a kind of rechargeable battery. The required charging method is CC/CV. At the beginning of charging, is CC charging. With the charging process, the voltage will rise to 4.2V (depending on the positive electrode material, some requires a constant voltage value of 4.1V), which is converted to CV 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 charge after the voltage exceeds 4.2V. At this time, the battery voltage will continue to rise. When the battery voltage is charged to exceed 4.3V, the battery chemistry The side reactions will intensify, causing damage to the battery 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, V2 is turned from on to off, which cuts off the charging circuit, so that the charger can no longer charge the battery, and serves as an overcharge protection. At this time, due to the presence of the VD2 body diode VD2, the battery can discharge external loads through the diode.


Before the control IC detects that the battery voltage exceeds 4.28V to send off the V2 signal, there is a delay time, the length of the delay time is determined by C3, usually set to about 1s to prevent errors caused by interference judgment.


3. Over-discharge protection

During the discharge 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 fully discharged. At this time, if the battery continues to discharge the load, it will cause the battery 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 From on to off, the discharge circuit is cut off, so that the battery can no longer discharge the load, and it is used for over-discharge protection. At this time, due to the presence of the VD1 body diode VD1, the charger can charge the battery through the diode.


Since the battery voltage cannot be lowered 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 the time 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 this delay time is determined by C3, and is usually set to about 100ms to prevent errors caused by interference judgment.


4. Overcurrent protection

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


During the normal discharge of the battery to the load, when the discharge current passes through the two MOSFETs in series, a voltage will appear at both ends due to the on-resistance of the MOSFET. The voltage value U=I*RDS*2, RDS is a single MOSFET on-resistance, the "V-" pin on the control IC detects this voltage value. If the load is abnormal for some reason, the loop current increases. When the loop current is so large that U>0.1V (this value is When the control IC decides that different ICs have different values), the "DO" pin will change from high voltage to zero voltage, making V1 turn from on to off, thereby cutting off the discharge circuit and making the current in the circuit zero for over-current protection purposes.


There is also a delay time between the time when the control IC detects the occurrence of overcurrent and when the V1 signal is turned off. The length of this delay time is determined by C3, which is usually about 13ms to prevent 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 greater, the value of overcurrent protection is smaller.


5. Short circuit protection

When the battery discharges 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 determines that the load is short-circuited, and its "DO" pin will be Quickly change from high voltage to zero voltage, making V1 turn from on to off, thereby cutting off the discharge circuit and serving as short circuit protection. The delay time of short-circuit protection is extremely short, usually less than 7ms. Its working principle is similar to overcurrent protection, but the judgment method is different, and the protection delay time is also different.