Lithium battery protection circuit works

The circuit has overcharge protection, overdischarge protection, overcurrent protection and short circuit protection. The working principle is as follows:

1 normal state

In the normal state, the "CO" and "DO" pins of N1 output high voltage in the circuit, both MOSFETs are in conduction state, and the battery can be freely charged and discharged. Since the on-resistance of the MOSFET is small, it is usually smaller than 30 milliohms, 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

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 charged with constant current. With the charging process, the voltage will rise to 4.2V (depending on the cathode material, some) The battery requires a constant voltage of 4.1V) and is switched to constant voltage charging until the current is getting smaller and smaller. When the battery is being charged, if the charger circuit loses control, the battery voltage will exceed 4.2V and continue constant current charging. At this time, the battery voltage will continue to rise. When the battery voltage is charged to over 4.3V, the battery chemistry Side effects will increase, 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 and different ICs have different values), the "CO" pin will change from a high voltage to a zero voltage. Turning V2 from on to off, thus cutting off the charging circuit, so that the charger can no longer charge the battery, which acts as an overcharge protection. At this time, due to the presence of the body diode VD2 that is provided by the 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, usually set to about 1 second to avoid the error caused by the interference. judgment.

3 over discharge protection

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

During 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 be converted from high voltage to zero voltage, making V1 Turning from conduction to shutdown, the discharge circuit is cut off, so that the battery can no longer discharge the load, which acts as an over-discharge protection. At this time, due to the presence of the V1 body diode VD1, the charger can charge the battery through the diode.

Since the battery voltage can no longer be lowered under 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 state, and the entire protection circuit consumes less than 0.1 μA.

There is also a delay time between when the control IC detects that the battery voltage is lower than 2.3V and the signal that turns off the V1. The length of the delay time is determined by C3, usually set to about 100 milliseconds to avoid errors caused by interference. judgment.

4 over current protection

Due to the chemical characteristics of lithium batteries, battery manufacturers stipulate that their discharge current should not exceed 2C (C = battery capacity / hour). When the battery exceeds 2C current discharge, it will cause permanent damage or safety problems.

During the normal discharge of the battery, the discharge current is passed through two MOSFETs in series. Due to the on-resistance of the MOSFET, a voltage is generated across the MOSFET. The voltage value U=I*RDS*2, RDS is a single MOSFET on-resistance, the "V-" pin on the control IC detects the voltage value. If the load is abnormal for some reason, the loop current is increased. When the loop current is so large that U"0.1V (this value is When the control IC determines that different ICs have different values, the "DO" pin will be converted from a high voltage to a zero voltage, causing V1 to turn from on to off, thereby cutting off the discharge loop and causing zero current in the loop. It acts as an overcurrent protection.

There is also a delay between when the control IC detects an overcurrent and when the V1 signal is turned off. The length of the delay is determined by C3, usually about 13 milliseconds, to avoid misjudgment caused by interference.

In the above control process, 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 MOSFET on-resistance is larger, the over-current protection is applied to the same control IC. The smaller the value.

5 short circuit protection

When the battery is discharged to 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 It quickly turns from a high voltage to a zero voltage, turning V1 from on to off, thereby cutting off the discharge loop and providing short-circuit protection. The short-circuit protection has a very short delay time, usually less than 7 microseconds. Its working principle is similar to overcurrent protection, except that 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 described in detail above. The protection principle of the multi-cell series lithium-ion battery is similar. Therefore, the control IC used in the above circuit is the R5421 series of Ricoh Corporation of Japan. There are many other types of control ICs in the actual battery protection circuit, such as Seiko's S-8241 series, Japan's MITSUMI's MM3061 series, Taiwan's Fujing's FS312 and FS313 series, Taiwan's analog technology's AAT8632 series, etc. The working principle is similar, but there are differences in specific parameters. Some control ICs have implemented the filter capacitor and the delay capacitor inside the chip in order to save the peripheral circuit. The peripheral circuit can be few, such as Seiko's S-8241 series. In addition to the control IC, there is another important component in the circuit, the 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 has a performance on the battery. The effect is that when the selected MOSFET is better, its on-resistance is small, the internal resistance of the battery pack is small, the load carrying capacity is also strong, and the power consumed during discharge is also small.

With the development of technology, the volume of portable devices is getting smaller and smaller. With this trend, the requirements for the protection circuit volume of lithium-ion batteries are getting smaller and smaller. In the past two years, control ICs and MOSFETs have appeared. A synthetic IC product, such as DIALOG's DA7112 series, some manufacturers even package the entire protection circuit into a small-sized IC, such as MITSUMI's products.

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