23 Years' Battery Customization

Portable lithium battery protection circuit

Jun 03, 2019   Pageview:569

In recent years, a growing number of products, including pdas, digital cameras, mobile phones, portable audio equipment and bluetooth devices, have used lithium batteries as their main power source. Lithium battery has the advantages of small size, high energy density, no memory effect, high cycle life, high voltage battery and low self-discharge rate, etc. Different from nickel-cadmium and nickel-hydride batteries, lithium battery must consider the safety during charging and discharging to prevent the deterioration of characteristics. Protection against overcharge, overdischarge, overcurrent and short circuit of lithium batteries is very important, so protective lines are usually designed in the battery pack to protect lithium batteries.

Due to the high energy density of lithium ion batteries, it is difficult to ensure the safety of batteries. In the overcharge state, the energy will be excessive after the battery temperature rises, and then the electrolyte will decompose and produce gas, and the internal pressure will rise and generate the risk of spontaneous combustion or rupture; On the other hand, in the case of excessive discharge, the electrolyte will degrade the battery characteristics and durability due to decomposition, thus reducing the number of charges.

The protection circuit of lithium ion battery is to ensure the safety of such overcharge and discharge state and prevent the deterioration of characteristics. The protective circuit of lithium ion battery is composed of protective IC and two power MOSFET, in which the protective IC monitors the battery voltage, switches to the power MOSFET hanging outside to protect the battery when there is overcharge and discharge state, and the protective IC functions include overcharge protection, overdischarge protection and over current/short circuit protection.

I. overcharging protection

The principle of overcharging protection IC is as follows: when the external charger charges the lithium battery, in order to prevent the rise of internal pressure caused by temperature rise, the charging state shall be terminated. At this point, the protection IC needs to detect the battery voltage, and when it reaches 4.25v (assuming the battery overcharge point is 4.25v), it will activate the overcharge protection and turn the power MOSFET from on to off, thus cutting off the charging.

In addition, it is also necessary to pay attention to the noise generated by the overcharge detection error action, 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.

Overdischarge protection

In the case of excessive discharge, the electrolyte will degrade the battery characteristics due to decomposition and reduce the charging times. Lithium battery protection IC can avoid excessive discharge and realize battery protection function.

Excessive discharge protection IC principle: in order to prevent the excessive discharge of the lithium battery, assuming that the lithium battery connected to the load, when the lithium battery voltage is less than its excessive discharge voltage testing point (2.3 V) is assumed to activate the excessive discharge protection, make the power MOSFET by open to cut off by the discharge, in order to avoid excessive battery discharge phenomenon, and keep the battery in low static current standby mode, the current is only 0.1 mu A.

When the lithium battery is connected to the charger, and the lithium battery voltage is higher than the overdischarge voltage, the overdischarge protection function can be removed. In addition, considering the situation of pulse discharge, delay time is set in the over-discharge detection circuit to avoid misoperation.

Overcurrent and short-circuit current

The discharge must be stopped immediately in order to ensure the safety of over current or short circuit caused by unknown reasons (when discharging or the positive and negative poles are mistakenly touched by metal objects).

For over current protection IC principle, when the discharge current is too large, or short circuit occurs, protection IC will activate a short circuit current protection, over current at this time of testing is to power MOSFET Rds (on) as inductive impedance to monitor its voltage drop, if is higher than set by the over-current detection voltage is stop discharge, computation formula is:

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

Suppose V - = 0.2 V, Rds (on) = 25 m Ω, protect the current size of I = 4 a

Similarly, over current detection must have a delay time in case there is a sudden influx of current to produce the wrong action.

Usually after the over-current is generated, if the over-current factor can be removed (for example, it will be separated from the load immediately), its normal state will be restored, and the normal charging and discharging action can be carried out again.

Iv. New functions of lithium battery protection IC

In addition to the above 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 and overcurrent is generated suddenly (such as the charger is damaged), the circuit immediately detects the overcurrent. At this time, the Cout will be changed from high to low, and the power MOSFET will be changed from on to off to realize the protection function.

V minus lambda is I times Rds of on times 2

(I is the charging current; Vdet4, over current detection voltage, Vdet4 is -0.1v)

2. Lock mode during overcharging

Normally, the protection IC will go through a period of delay time in the overcharge protection, and then will cut off the power MOSFET to achieve the purpose of protection, when the lithium battery voltage has been dropped to the release point (overcharge lagging voltage) will recover, at this time will continue to charge protection discharge charge discharge. The safety problem in this state cannot be effectively solved. The lithium battery will keep repeating the action of charging, discharging, charging and discharging, and the gate of power MOSFET will be in the alternating state of high and low voltage repeatedly, which may make the MOSFET hot and reduce the battery life. Therefore, the locking mode is very important. The MOSFET will not get hot and will be relatively safer if the li-ion protection circuit has a locking mode when overcharging protection is detected.

After the overcharge protection, as long as the charger is connected to the battery pack, the overcharge lock mode will be entered. At this time, even if the voltage of lithium battery drops, there will be no recharging. The state of charging and discharging can be restored by removing the charger and connecting the load.

3. Reduce the size of protection circuit components

Integrate overcharge and short circuit protection delay capacitors into the protection IC to reduce the size of the protection circuit components.

Five, the protection of IC performance requirements

1. High precision of overcharge protection

When the lithium ion battery has overcharging state, in order to prevent the rise of internal pressure caused by temperature rise, the charging state must be cut off. The protective IC will detect the battery voltage, and when overcharging is detected, the power MOSFET detects for overcharging will cause it to be cut off and cut off for charging. At this time, it should be noted that the detection voltage of overcharging is of high precision. When the battery is charged, it is a matter of great concern to users to charge the battery to a full state, while taking safety into consideration. Therefore, it is necessary to cut off the charging state when the allowable voltage is reached. In order to meet the two conditions at the same time, a detector with high precision is required. Currently, the precision of the detector is 25mV, and the precision will be further improved.

2. Reduce the power consumption of protection IC

With the increase of the service time, the voltage of the charged lithium ion battery will gradually decrease and finally drop below the standard value. At this time, it needs to be recharged. If the battery continues to be used without charging, the battery may not be able to continue to be used due to excessive discharge. In order to prevent excessive discharge, the protective IC must test the battery voltage. Once the voltage below the detection voltage of excessive discharge is reached, the power MOSFET on the discharging side must be cut off to stop the discharge. However, at this time, the battery itself still has natural discharge and consumption current of protection IC, so it is necessary to minimize the consumption current of protection IC.

3. Over current/short circuit protection requires low detection voltage and high precision

Discharge must be stopped immediately if short circuit is caused by unknown reasons. The detection of overcurrent is based on the power MOSFET's Rds (on) as the induced impedance to monitor the voltage drop. At this time, if the voltage is higher than the detection voltage of overcurrent, the discharge will be stopped. In order to make the power MOSFET Rds (on) valid at the time of charging current and discharge current application, need to make the low impedance value as far as possible, at present the impedance about 20 m Ω ~ 30 m Ω, this current voltage can be lower.

4. High voltage resistance

High voltage is generated instantly when the battery pack is connected to the charger, so the protection IC should meet the requirement of high voltage resistance.

5. Low battery power consumption

The static power consumption must be 0.1 A smaller in the protected state.

Zero volt rechargeable

In the process of storage, some batteries may be put for too long or abnormal reasons to lower the voltage to 0V, so the protection IC needs to be in 0V can also achieve charging.

Vi. Prospect of IC development

As mentioned above, in the future, protection IC will further improve the precision of detection voltage, reduce the power consumption of protection IC and improve the function of preventing misoperation, etc. At the same time, the high voltage resistance of charger connecting terminal is also the focus of research and development. In terms of packaging, sot23-6 has been gradually transferred to SON6 packaging, and there will be CSP packaging in the future, and even COB products to meet the current emphasis on thin and light requirements.

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

, of course, functional components of single crystal is the same goal, such as mobile phone manufacturers are currently facing will protect IC, charging circuit and power management IC, and other peripheral circuits and logic IC constitute a dual chip chipset, but at the moment, to reduce the open circuit impedance power MOSFET, hard integrate with other IC, even in a particular technology made into single chip, I'm afraid the cost will be high. Therefore, the protection of IC single crystal will take some time to solve.

The page contains the contents of the machine translation.

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