Notebook lithium battery is how to protect the circuit and function

The ion battery protection circuit includes overcharge protection, overcurrent/short-circuit protection and over discharge protection, requiring overcharge protection, high precision, low power consumption, high voltage and 0 volt charging characteristics. The principle, new function and characteristic requirements of these three kinds of protection circuits are introduced in detail in this paper.

Lithium battery has small size, high energy density, no memory effect, high cycle life, the advantages of high voltage battery and low self-discharge rate are different from Ni-Cd and Ni-MH battery, lithium battery must consider the safety of charging and discharging in order to prevent the property from deteriorating. It is important to overcharge, over discharge, overcurrent and short-circuit protection for lithium batteries, So a protective line is usually designed in the battery pack to protect the lithium battery.

Because of the high energy density of lithium-ion batteries, it is difficult to ensure the safety of batteries. In the excessive charging state, the energy will be excess after the battery temperature rises, so the electrolyte decomposes and produces the gas, because of the risk of spontaneous combustion or rupture due to the increase of internal pressure; Conversely, in the excessive discharge state, the electrolyte causes the battery characteristic and the durability deterioration, So as to reduce the number of rechargeable.

The protection circuit of lithium-ion battery is to ensure the safety of such overcharge and discharge state, and to prevent the degradation of properties. Lithium-ion battery protection circuit is composed of protection IC and two power MOSFET, which protects the IC monitor battery voltage, When there is excessive charge and discharge state switch to plug the Power MOSFET to protect the battery, the function of protection IC has over charge protection, over discharge protection and overcurrent/short-circuit protection.

Excess charge protection

The principle of excessive charge protection IC is: when the external charger to charge lithium battery, in order to prevent the temperature rise caused by the increase in pressure, the need to terminate the charging state. At this point, the protection IC needs to detect battery voltage, when reaching 4.25V (assuming the battery over filling point for 4.25V) that is activated over charge protection, Switch the Power MOS from open to shut off, and then stop charging.

In addition, it is necessary to pay attention to the excessive charging of noise caused by the detection of misoperation, so as to avoid the determination of overcharge protection. Therefore, it is necessary to set the delay time, and the delay time can not be shorter than the noise duration.

Over discharge protection

In the case of excessive discharge, the electrolyte is degraded by decomposition, and the charge times are reduced. Using Lithium battery protection IC can avoid excessive discharge phenomenon and realize battery protection function.

The principle of over discharge protection IC: In order to prevent the excessive discharge state of lithium battery, assuming that the lithium battery is connected with the load, when the lithium battery voltage is lower than its excess discharge voltage detection point (2.3V) will activate over discharge protection, so that the power MOSFET from open to shut off and cut-off discharge, in order to avoid the battery over discharge phenomenon occurs, And keep the battery in a low static current standby mode, at this time the current is only 0.1uA.

When the lithium battery is connected to the charger, and the lithium battery voltage is higher than the excess discharge voltage, the excess discharge protection can be discharged. In addition, considering the pulse discharge, the overvoltage detection circuit has the delay time to avoid the misoperation.

Over current and short circuit current

In order to ensure safety, the discharge must be stopped immediately for unexplained reasons, such as electric current or short circuit, caused by a false contact between the metal and the positive or negative.

Overcurrent protection IC principle, when the discharge current is too large or short-circuit situation occurs, protection IC will be activated (short-circuit) current protection, at this time overcurrent detection is the Power MOSFET Rds (on) as the induction impedance to monitor the voltage drop, if more than the fixed current detection voltage is higher then stop discharging, The formula is: V-=ixrds (on) X2 (V for overcurrent detection voltage, I for discharge current). Assuming V-=0.2v,rds (on) =25mω, the protection current is the size of i=4a.

In the same way, overcurrent detection must be provided with a delay time in case of a sudden current inflow and a false action.

Usually after the overcurrent occurs, if you can remove the overcurrent factors (such as immediately with the load off), will return to their normal state, you can carry out normal charging and discharging action.

New functions of lithium battery protection IC

In addition to the above lithium-ion battery protection IC features, the following new features are also worth paying attention to:

1. Over current protection when charging

When the connection charger to charge the sudden occurrence of electricity (such as the charger damage), the circuit immediately to detect the current, at this time cout will be converted from high to low, power MOSFET from open to shut off, to achieve protection functions.

V (Vdet4 overcurrent detection voltage, VDET4 for -0.1v) =i (charging current) xrds (on) x2

2. Locking mode when over charging

Usually protection IC will pass a period of delay when overcharge protection, then power MOSFET will be turned off for protection purposes, when the lithium battery voltage down to the release point (overcharge hysteresis voltage) will be restored, This will continue to charge-protect-discharge-charge-discharge. This state of security problems will not be effectively resolved, lithium batteries will always repeat the charging-discharge-charging-discharge action, power MOSFET grid will be repeatedly in the high and low voltage alternating state, which may make the MOSFET hot, It also lowers battery life, so locking mode is important. If the lithium-ion protection circuit has a locking mode when it detects an overcharge protection, the MOSFET will not become hot and the security is relatively much improved.

After the overcharge protection, as long as the charger is connected to the battery pack, this will enter the overcharge locking mode. At this point, even if the lithium battery voltage drop will not occur in the case of recharging, the charger removed and connected to the load can be restored charge and discharge status.

3. Reduce Protection Circuit assembly size

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

Requirements for the protection of IC performance

1. High precision of overcharge protection

When a lithium-ion battery has an excess charge state, in order to prevent the increase of the internal pressure caused by temperature rise, shall be closed to the charging state. The protection IC will detect the battery voltage, when excessive charge is detected, then the power MOSFET for excessive charge detection is turned off and the charge is off. Attention should be paid to the high precision of the detection voltage of overcharge, When the battery is charged, the battery charge to the full state of the user is very concerned about the problem, at the same time, it is necessary to take into account the safety problem, so it is required to reach the charging state when the allowable voltage is reached. To meet these two conditions at the same time, must have a high-precision detector, the current detector precision of 25mV, this precision will need to be further improved.

2. Reduce the power consumption of the protection IC

With the increase in usage time, the voltage of the lithium-ion battery that has been charged will gradually decrease to below the standard value, and it will need to be recharged again. If you continue to use without recharging, the battery may be unable to continue to use because of excessive discharge. In order to prevent excessive discharge, the protection IC must detect the battery voltage, Once the excess discharge detection voltage is reached, the power MOSFET on one side of the discharge is shut off and the discharge is closed. But at this time the battery itself still has a natural discharge and protection IC consumption current exists, so need to protect the IC consumption of the current to the lowest degree.

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

The discharge must be stopped immediately for unexplained short circuit. Overcurrent detection is based on the Power MOSFET RDS (ON) as the inductive impedance to monitor the voltage drop, at this time if the voltage than the current detection voltage is higher when the discharge is stopped. To make the Power MOSFET rds (ON) In the charging current and discharge current effective application, need to make this impedance value as low as possible, the current impedance is about 20mω~30mω, so that the overvoltage detection voltage can be lower.

4. High Voltage Resistance

Battery pack and charger when connected to the moment there will be high pressure, so the protection IC should meet the requirements of high pressure.

5. Low battery power consumption

When protecting state, its static power consumption flow must be 0.1uA.

6.0 Volt Rechargeable

Some batteries in the process may be too long or abnormal due to the reason that the voltage is low to 0V, so the protection IC need to be in 0V can also realize charging.

Development prospect of Protection IC

As mentioned earlier, the future protection IC will further improve the accuracy of the detection voltage, reduce the power consumption of the IC protection and improve the prevention of misoperation, and the Charger connection Terminal High Pressure is also the focus of research and development.

In the packaging, has been gradually from sot23-6 to SON6 package, there will be CSP package, and even cob products to meet the current emphasis on the thin short requirements.

In the functional aspect, the protection IC does not need to integrate all functions, may develop the single protection IC according to the different lithium battery material, if only has the overcharge protection or the protection function, this can greatly reduce the cost and the size.

Of course, the functional component of the single crystal is the same goal, such as the current mobile phone manufacturers will protect the IC, charging circuit and power management IC and other peripheral circuits and logic IC to form a dual-chip chipset, but at present to enable power MOSFET open circuit impedance, it is difficult to integrate with other IC, even with special technology made of single chip , I'm afraid the cost will be too high. Therefore, the single crystal of the protection IC will take some time to solve.