The principle and functional requirements of lithium battery protection are explained in detail here, which has certain guiding significance for the design of lithium battery protection! Lithium ion battery protection circuit includes overcharge protection, overcurrent/short circuit protection and overdischarge protection, requiring high precision overcharge protection Features such as low power consumption, high withstand voltage, and zero-volt rechargeability. This text introduces the principle, new function and characteristic requirement of these three kinds of protection circuits in detail.

　　In recent years, more and more products such as PDAs, digital cameras, mobile phones, portable audio devices and Bluetooth devices have adopted lithium batteries as their main power source. Lithium batteries have the advantages of small size, high energy density, no memory effect, high cycle life, high-voltage batteries and low self-discharge rate. Unlike nickel-cadmium and nickel-hydrogen batteries, lithium batteries must be safe during charging and discharging. To prevent deterioration of characteristics. The overcharge, overdischarge, overcurrent and short circuit protection of lithium batteries is very important, so protection circuits are usually designed in the battery pack to protect the lithium batteries.　　 Due to the high energy density of lithium-ion batteries, it is difficult to ensure battery safety. In the overcharged state, the battery will have excess energy after the temperature rises, so the electrolyte will decompose to produce gas, which may cause spontaneous combustion or rupture due to the increase in internal pressure; on the contrary, in the over-discharged state, the electrolyte will decompose and cause the battery characteristics and The durability is deteriorated, thus reducing the number of rechargeable times.

　　The protection circuit of lithium-ion battery is to ensure the safety in such overcharge and discharge state and prevent the deterioration of characteristics. The protection circuit of the lithium ion battery is composed of a protection IC and two power MOSFETs. The protection IC monitors the battery voltage and switches to an external power MOSFET to protect the battery when it is overcharged and discharged. The function of the protection IC is excessive. Charge protection, over discharge protection and over current/short circuit protection.

　　One, overcharge protection

　　The principle of overcharge protection IC is that when an external charger is charging a lithium battery, in order to prevent the internal pressure from rising due to temperature rise, it is necessary to terminate the charging state. At this time, the protection IC needs to detect the battery voltage, and when it reaches 4.25V (assuming the battery overcharge point is 4.25V), it activates the overcharge protection, turns the power MOSFET from on to off, and then stops charging.

　　In addition, you must also pay attention to the malfunction of overcharge detection caused by noise, so as not to be judged as overcharge protection. Therefore, the delay time needs to be set, and the delay time cannot be shorter than the duration of the noise.

　　2. Over discharge protection

　　In the case of over-discharge, the electrolyte will decompose and cause the deterioration of battery characteristics and reduce the number of recharges. The use of lithium battery protection IC can avoid over-discharge and realize the battery protection function.

　　The principle of over-discharge protection IC: In order to prevent the over-discharge of the lithium battery, suppose that the lithium battery is connected to the load. When the voltage of the lithium battery is lower than its over-discharge voltage detection point (assumed to be 2.3V), the over-discharge protection will be activated to enable the power MOSFET Switch from on to off and cut off the discharge to avoid over-discharge of the battery and keep the battery in a low quiescent current standby mode. The current at this time is only 0.1μA.

　　When the lithium battery is connected to the charger, and the voltage of the lithium battery is higher than the over-discharge voltage at this time, the over-discharge protection function can be released. In addition, considering the pulse discharge, the over-discharge detection circuit is equipped with a delay time to avoid malfunction.

　　Three, over current and short circuit current

　　Overcurrent or short-circuit is caused by unknown reasons (when discharging or the positive and negative electrodes are touched by metal objects). To ensure safety, the discharge must be stopped immediately.

　　The principle of the overcurrent protection IC is that when the discharge current is too large or a short circuit occurs, the protection IC will activate the over (short circuit) current protection. At this time, the detection of overcurrent is to use the Rds(on) of the power MOSFET as an inductive impedance to monitor When the voltage drops, if the overcurrent detection voltage is higher than the predetermined overcurrent detection voltage, the discharge will be stopped. The calculation formula is:

　　V-=I×Rds(on)×2 (V- is the overcurrent detection voltage, I is the discharge current)

　　Assuming V-=0.2V, Rds(on)=25mΩ, the protection current is I=4A

　　Similarly, over-current detection must also be equipped with a delay time to prevent malfunction when sudden current flows in.

　　Normally after the overcurrent is generated, if the overcurrent factor can be removed (for example, immediately separated from the load), the normal state will be restored, and normal charging and discharging operations can be performed.

　　Fourth, the new function of lithium battery protection IC

　　In addition to the above-mentioned lithium battery protection IC functions, the following new functions are also worthy of attention:

　　1. Overcurrent protection during charging

　　When the charger is connected for charging, an overcurrent occurs suddenly (such as the charger is damaged), the circuit will immediately perform overcurrent detection, at this time, Cout will change from high to low, and the power MOSFET will be switched from on to off, realizing the protection function.　　V-=I×Rds(on)×2

　　(I is the charging current; Vdet4, the overcurrent detection voltage, Vdet4 is -0.1V) 　　2. Locking mode when overcharging

　　Generally, the protection IC will pass a delay time during overcharge protection, and then cut off the power MOSFET to achieve the purpose of protection. When the lithium battery voltage drops to the release point (overcharge hysteresis voltage), it will recover. Will continue to charge→protect→discharge→charge→discharge. The safety problem of this state will not be effectively solved. The lithium battery will always repeat the action of charging→discharging→charging→discharging, and the gate of the power MOSFET will be in the alternating state of high and low voltage repeatedly, which may cause the MOSFET to become hot. , It will also reduce battery life, so the lock mode is very important. If the lithium battery protection circuit has a lock mode when the overcharge protection is detected, the MOSFET will not become hot, and the safety is relatively improved.

　　After the overcharge protection, as long as the charger is connected to the battery pack, it will enter the overcharge lock mode at this time. At this time, even if the voltage of the lithium battery drops, there will be no recharging. Remove the charger and connect the load to restore the charging and discharging state.

　　3. Reduce the size of protection circuit components

　　The delay capacitor for overcharge and short circuit protection is integrated into the protection IC to reduce the size of the protection circuit components.

　　5. Requirements for protection IC performance

　　1. High precision of overcharge protection

　　When the lithium ion battery is overcharged, in order to prevent the internal pressure from rising due to temperature rise, the charging state must be cut off. The protection IC will detect the battery voltage, and when overcharge is detected, the power MOSFET for overcharge detection will cut off and stop charging. At this time, attention should be paid to the high-precision detection voltage of overcharge. When the battery is charged, the user is very concerned about charging the battery to a full state. At the same time, safety issues must be taken into account. Therefore, it is necessary to achieve the allowable voltage When the charging state is cut off. To meet these two conditions at the same time, there must be a high-precision detector. The current precision of the detector is 25mV, and this precision needs to be further improved.

　　2. Reduce the power consumption of the protection IC

　　As the use time increases, the voltage of the charged lithium-ion battery will gradually decrease, and finally fall below the standard value of the specification, at which time it needs to be recharged. If you continue to use it without charging, it may cause the battery to fail to continue to use due to over-discharge. In order to prevent over-discharge, the protection IC must detect the battery voltage. Once it reaches the over-discharge detection voltage or lower, the power MOSFET on the discharge side must be cut off to cut off the discharge. But at this time, the battery itself still has natural discharge and current consumption of the protection IC, so it is necessary to minimize the current consumption of the protection IC.

　　3. Over current/short circuit protection needs to have low detection voltage and high precision requirements

　　The discharge must be stopped immediately when a short circuit is caused by an unknown reason. The overcurrent detection uses the Rds(on) of the power MOSFET as the inductive impedance to monitor the voltage drop. If the voltage at this time is higher than the overcurrent detection voltage, the discharge will stop. In order to make the Rds(on) of the power MOSFET effective in the charging current and discharging current, the impedance value needs to be as low as possible. At present, the impedance is about 20mΩ～30mΩ, so the overcurrent detection voltage can be lower.

　　4. High voltage resistance

　　When the battery pack is connected to the charger, high voltage will be generated instantaneously, so the protection IC should meet the requirements of high voltage resistance.

　　5. Low battery power consumption

　　In the protection state, the quiescent current consumption must be less than 0.1μA.

　　6. Zero volt rechargeable

　　Some batteries may have a voltage as low as 0V due to excessive storage or abnormalities during storage. Therefore, the protection IC needs to be charged at 0V.

　　VI. Prospects for the protection of IC development

　　As mentioned above, the future protection IC will further improve the precision of the detection voltage, reduce the current consumption of the protection IC, and improve the malfunction prevention function. At the same time, the high withstand voltage of the charger connection terminal is also the focus of research and development. In terms of packaging, SOT23-6 has gradually shifted to SON6 packaging. In the future, there will be CSP packaging and even COB products to meet the current emphasis on thin, thin and short.

　　In terms of function, the protection IC does not need to integrate all functions. A single protection IC can be developed according to different lithium battery materials, such as only overcharge protection or overdischarge protection, which can greatly reduce the cost and size.

　　Of course, the single crystal of functional components is the unchanging goal. For example, mobile phone manufacturers are currently moving towards a two-chip chipset composed of peripheral circuits such as protection ICs, charging circuits, power management ICs, and logic ICs. However, the open-circuit impedance of the power MOSFET is currently required. It is difficult to integrate with other ICs. Even if a single chip is made with special technology, the cost will be too high. Therefore, it will take some time to protect the single crystal of IC.