Principle and Analysis of Lithium Ion Battery Protection Circuit

LC03-3.3 SOP8 TVS Static Protection 3.3V

Principle Analysis of Lithium Ion Battery Protection Circuit

With the advancement of technology and social development, portable devices such as mobile phones, notebook computers, MP3 players, PDAs, handheld game consoles, and digital video cameras have become more and more popular. Many of these products are powered by lithium-ion batteries. The characteristics of lithium-ion batteries are different from those of other rechargeable batteries. They usually have a circuit board inside. Many people don't understand the function of this circuit. This paper will explain the characteristics of lithium-ion batteries and the working principle of their protection circuits.

Lithium batteries are divided into primary batteries and secondary batteries. At present, some portable electronic products with low power consumption mainly use non-rechargeable primary lithium batteries, while in laptops, mobile phones, PDAs, digital cameras, etc. Larger electronic products use rechargeable secondary batteries, ie lithium-ion batteries.

Compared with nickel-cadmium and nickel-hydrogen batteries, lithium-ion batteries have the following advantages:

1. The voltage is high. The voltage of a single-cell lithium-ion battery can reach 3.6V, which is much higher than the 1.2V voltage of nickel-cadmium and nickel-hydrogen batteries.
2. The capacity density is large, and its capacity density is 1.5-2.5 times that of nickel-hydrogen battery or nickel-cadmium battery.
3. The charge retention ability is strong (ie, self-discharge is small), and its capacity loss is small after being placed for a long time.
4, long life, normal use of its cycle life can reach more than 500 times.
5, no memory effect, do not have to empty the remaining power before charging, easy to use.

Due to the chemical characteristics of lithium-ion batteries, during normal use, the internal chemical reaction of electrical energy and chemical energy is mutually converted, but under certain conditions, such as overcharging, overdischarging and overcurrent, the battery will be caused. Chemical side reactions occur internally, which will seriously affect the performance and service life of the battery, and may generate a large amount of gas, causing the internal pressure of the battery to rapidly increase and explode, resulting in safety problems. Therefore, all lithium ion batteries are required. A protection circuit for effectively monitoring the charging and discharging states of the battery, and turning off the charging and discharging circuits under certain conditions to prevent damage to the battery.

The figure below shows a typical lithium-ion battery protection circuit schematic.

As shown in the figure above, the protection loop consists of two MOSFETs (V1, V2) and a control IC (N1) plus some RC components. The control IC is responsible for monitoring the battery voltage and loop current, and controlling the gates of the two MOSFETs. The MOSFET acts as a switch in the circuit to control the conduction and shutdown of the charging and discharging circuits, respectively, and C3 is a time delay capacitor. With overcharge protection, over-discharge protection, over-current protection and short-circuit protection, its 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. 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 batteries require a constant current/constant voltage. In the initial stage of charging, they are charged at a constant current. As the charging process, the voltage rises to 4.2V (depending on the cathode material, some batteries require a constant voltage of 4.1V). ), switch 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, different ICs have different values), the "CO" pin will change from high voltage to 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 V2's own body diode VD2, 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 when the V2 signal is turned off. The delay time is determined by C3, usually set to about 1 second to avoid the error caused by 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 the 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 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 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-ion 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 increases. 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 shutdown V1 signal is issued. The length of the delay is determined by C3, usually about 13 milliseconds, to avoid misjudgment due to 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 discharging to the load, if the loop current is so large that U>0.9V (this value is determined by the control IC and different ICs have different values), the control IC judges 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 Co., Ltd. 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 the specific parameters are different. Some control ICs save the peripheral circuits, and the filter capacitors and delay capacitors are implemented inside the chip. The peripheral circuits 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, and 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.