How lithium ions protect circuits

In recent years, more and more products such as PDAs, digital cameras, mobile phones, portable audio devices have adopted lithium ion batteries as the main power source. However, unlike nickel-cadmium, nickel-metal hydride and other batteries, lithium ion batteries must consider charging and discharging. Safety, to prevent character deterioration. At present, both square and cylindrical lithium-ion cores have adopted certain safety protection measures, such as explosion-proof holes, diaphragm and PTC. In order to further ensure the safety of the finished lithium ion battery in use, it is important to protect its overcharge, over-discharge, overcurrent and short circuit. Therefore, protection circuits are usually designed in the battery pack for the battery's charge and discharge state. Effective monitoring, and under certain conditions, the charge and discharge circuits are closed to prevent damage to the battery.

Under normal circumstances, the protective circuit of a lithium-ion battery is composed of a protective IC and two power MOSFETs, which protect the IC to monitor the battery voltage and switch to protect the battery with a plug-in power MOSFET when there is an overcharge and discharge state. The function of protecting IC is to overcharge protection, excessive discharge protection, and overcurrent / short circuit protection.

1, normal state

In the normal state of the circuit, MOSFET is in a state of conduction, and the battery can be charged and discharged freely. Since the MOSFET has a small conduction impedance, which is usually less than 30 mm, its conduction resistance has a small effect on the performance of the circuit. The current consumed by the protection circuit in this state is μA, usually less than 7μA.

2, overcharge protection

The charging method required for lithium-ion batteries is constant current / constant pressure. In the initial stage of charging, the constant current is charged. With the charging process, the voltage will rise to 4.2 V, and the constant pressure will be charged until the current becomes smaller and smaller.

When the battery is charged, if the charger circuit is out of control, the battery voltage will continue to charge after the battery voltage exceeds 4.2 V. At this time, the battery voltage will continue to rise. When the battery voltage is charged to more than 4.3 V, the chemical side of the battery will the reaction will intensify, It may cause battery damage or safety problems and need to terminate the charging state. At this point, the protection IC detects the battery voltage. When it reaches 4.28 V(assuming that the battery overcharge point is 4.28 V), the overcharge protection is activated, and the power MOSFET is switched from conduction to cut off, and then the charge is turned off. In addition, attention must also be paid to over-charging detection errors due to noise, so as not to be judged as overcharging protection. Therefore, the delay time needs to be set, usually set to about 1 second. And the delay time can not be shorter than the duration of the noise.

3, over discharge protection

In the process of external load discharge, the voltage of the battery will gradually decrease with the discharge process. In the case of excessive discharge, the electrolyte will degrade the battery characteristics due to decomposition. At this point, if the battery continues to discharge the load, it will cause permanent damage to the battery.

In the battery discharge process, when the control IC detects that the battery voltage is less than 2.3 V(this value is determined by the control IC, and different ICs have different values), excessive discharge protection will be activated, so that the power MOSFET will be switched from on to cut off. Discharge, In order to avoid excessive discharge of the battery, and keep the battery in a low-static current standby mode, the current at this time is only 0.1 μA. When the lithium ion battery is connected to the charger, and the voltage of the lithium ion battery is higher than the excessive discharge voltage, the excessive discharge protection function can be lifted. In addition, considering the pulse discharge situation, the over-discharge detection circuit has a delay time to avoid erroneous actions.

4, over current protection

To ensure safety, the discharge must be stopped immediately due to an overcurrent caused by unknown causes. That is, when the discharge current is too large, the protection IC will activate the overcurrent protection. At this time, the overcurrent detection uses the Rds(on) of the power MOSFET as the inductive impedance to monitor the voltage drop. If Gaoze stops discharging more than the specified overcurrent detection voltage, the formula is: V = I × Rds(on) × 2(V is the overcurrent detection voltage, I is the discharge current) the same, Overcurrent detection must also have a delay time to prevent accidental actions when there is a burst current inflow. This delay time is usually about 13 milliseconds. Usually after the overcurrent is generated, if the overcurrent factor can be removed(for example, immediately separated from the load), its normal state will be restored and normal charging and discharge actions can be performed again.

Short circuit protection

Short circuit caused by unknown reasons, to ensure safety, it must be stopped immediately. When the short-circuit situation occurs, the protection IC will activate the short-circuit protection. The delay time for short-circuit protection is extremely short, usually less than 7 microseconds. Its working principle is similar to that of overcurrent protection, but the method of judgment is different, and the protection delay time is also different.

At present, the company is conducting research and development of various power cores. In the long run, assembling finished batteries is also one of our goals. The protective circuit will be one of the important components we consider. With the wider application of lithium-ion batteries, the requirements for protective circuits have become more and more high. In the future, the protection IC will further improve the precision of the detection voltage, reduce the current consumption of the protection IC, and improve the prevention of erroneous actions. As the volume of the mobile phone becomes smaller and smaller, the requirements for the protection circuit volume of lithium-ion batteries have become smaller and smaller. In the past two years, there have been products that integrate the control IC and MOSFET into a protection IC, such as DIALOG's DA7112 series. Some manufacturers even encapsulate the entire protective circuit into a small size IC, such as MITSUMI's products.

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