What is lithium battery protection board schematic?

What is lithium battery protection board schematic?

The lithium battery protection board is mainly composed of maintenance IC (overvoltage maintenance) and MOS tube (overcurrent maintenance), and used to protect safety of lithium battery cells. Lithium batteries have large discharge current, low internal resistance, long life, and no recall effect. Lithium-ion batteries are prohibited from overcharging, over-discharging, and short-circuit during operation, or it cause fire or blasting. Therefore, when use rechargeable lithium batteries they will have a maintenance board to maintain safety of battery.

1, voltage protection capability

Overcharge protection board: It is necessary for the protection board to have the ability to prevent the battery voltage from exceeding the preset value. Over-discharge maintenance is required: The protection board must have the ability to prevent the battery voltage from being at a preset value.

2, current capability

(overcurrent protection current, short circuit protection)

As a safety protection device for lithium batteries, the protection board should work reliably in the normal operating current scale of the equipment, and can operate quickly when the battery is accidentally short-circuited or over-current, so that the battery core can be protected.

3.7v lithium battery protection board schematic

3, on resistance

Definition: The on-resistance of the MOS transistor when the charging current is 500 mA.

Due to the high operating frequency of the communication equipment, the data transmission requires a low bit error rate, and the rising and falling edges of the pulse train are steep, so the current output capability and voltage stability of the battery are required to be high, and thus the MOS tube switch of the protection board is turned on. When the resistance is small, the single-cell protection board is usually "70mΩ. If it is too large, the communication equipment will not work properly, such as the phone suddenly disconnects during the call, the phone is not connected, the noise and so on.

4, self-consumption current

Definition: The IC operating voltage is 3.6V. Under no-load conditions, the operating current flowing through the protection IC is generally extremely small.

The self-consumption current of the protection board directly affects the standby time of the battery, and the self-consumption current of the regular protection board is usually less than 10 microamperes.

5, mechanical function, temperature adaptability, antistatic ability

It is necessary for the protection board to pass the sensation of the national standard rules and impact test; the protection board can work safely at 40 to 85 degrees and can withstand the ±15KV non-touch ESD static test.

The protection function of the lithium battery is usually completed by the protection circuit board and the PTC. The protection board is composed of electronic components. It can accurately monitor the voltage of the battery core and the current of the charge and discharge circuit in an environment of -40 ° C to +85 ° C, and control it in time. The current circuit is switched on and off; the main function of the PTC is to protect it in a high temperature environment to prevent the battery from burning or exploding.

[Hint] PTC is the abbreviation of English PosiTIvetemperature coefficient, which means positive temperature coefficient resistance (the higher the temperature, the larger the resistance). The component can be protected from overcurrent, that is, to prevent high-temperature discharge of the battery and unsafe large current charge and discharge. The PTC device is made of a polymer material polymer, which is made through a strict process and consists of a polymer tree vinegar matrix and conductive particles distributed therein. Under normal conditions, the conductive particles form a conductive path in the tree vinegar, and the device exhibits a low impedance; when an overcurrent phenomenon occurs in the circuit, the heat generated by the large current flowing through the PTC causes the volume of the polymer tree vine to expand. The connection between the conductive particles is cut off to provide overcurrent protection to the circuit. When the fault is solved, the component can automatically return to the initial state to ensure the normal operation of the circuit.

First, the charge and discharge requirements of lithium batteries

1. Lithium battery charging

The maximum charge termination voltage of a single-cell lithium battery is 4.2V, and it cannot be overcharged. Otherwise, the battery will be scrapped due to too much lithium ion loss from the positive electrode. When charging the lithium battery, a dedicated constant current and constant voltage charger should be used. After constant current charging, the voltage across the lithium battery is 4.2V, then it is switched to the constant voltage charging mode; when the constant voltage charging current is reduced to 100mA, the charging should be stopped.

The charging current (mA) can be 0.1 to 1.5 times the battery capacity, for example, a 1350 mAh lithium battery, and the charging current can be controlled between 135 mA and 2025 mA. The normal charging current can be selected to be about 0.5 times the battery capacity, and the charging time is about 2 to 3 hours.

2. Discharge of lithium battery

Due to the internal structure of the lithium battery, lithium ions cannot all move to the positive electrode during discharge, and a part of lithium ions must be retained at the negative electrode to ensure that lithium ions can be smoothly inserted into the channel during the next charging. Otherwise, battery life will be shortened. In order to ensure that some lithium ions remain in the graphite layer after discharge, it is necessary to strictly limit the minimum voltage at the end of discharge, that is, the lithium battery cannot be over-discharged. The discharge termination voltage of a single-cell lithium battery is usually 3.0V, and the minimum cannot be lower than 2.5V. The length of battery discharge is related to the battery capacity and discharge current. Battery discharge time (hours) = battery capacity / discharge current, and lithium battery discharge current (mA) should not exceed 3 times the battery capacity, for example: 1000mAh lithium battery, the discharge current should be strictly controlled within 3A, otherwise it will the battery is damaged.

Second, the composition of the protection circuit

The protection circuit is usually composed of control IC, MOs switch tube, blown fuse, resistor, capacitor and other components, as shown in Figure 2. Under normal circumstances, the control IC output signal controls the turn of the MOs switch to make the cell and the external circuit conduct. When the cell voltage or loop current exceeds the specified value, it immediately controls the MOS transistor to turn off to protect the cell. Safety.

The control IC has a built-in high-precision voltage detection circuit and a multi-stage current detection circuit. Among them, the voltage detecting circuit detects the charging voltage, and once it reaches its set threshold (usually 3.9V to 4.4V), it immediately enters the overcharge protection state; the second is to detect the discharge voltage once it reaches its set threshold. (usually 2.0V ~ 3.0V), immediately enter the over-discharge protection state.

In this circuit, the MOS switch tube is mostly in the form of a thin TSSOP-8 or SOT23 -6 package, and its appearance is shown in Figure 3. Some of these MOS switch tubes contain an N-channel field effect transistor, such as the FDMC7680. The 1st to 3rd pins are S poles, the 4th leg is G pole, and the 5th to 8th feet are D poles. The internal structure is shown in Figure 4. Some contain two N-channel FETs, such as FDW9926A, 8205A, etc. The pin function is related to the package form, as shown in Figure 5.

In addition, NTC and ID signal forming elements are also mounted in some lithium battery protection circuits. NTC is the abbreviation of English NegaTIvetemperature coefficient, which means negative temperature coefficient resistance. The component mainly plays the role of overheat protection in the circuit, that is, when the temperature of the battery itself or its surrounding environment rises, the resistance value of the NTC component is lowered, and the electric device or the charging device is used to react in time. If the temperature exceeds a certain value, the system enters. Protected state, stop charging and discharging. ID is the abbreviation of IdenTIficaTIon, which means identification. There are two types of information recognition components: memory, often beast line interface memory, storage battery type, production date, etc. Second, identification resistance, both Both can be used to trace the product and limit the application.

Third, the protection circuit working principle analysis

The normal output voltage of a single-cell lithium battery is about 3.7V, which can be used directly as a power source for mobile phones, MP3/MP4 and some small-screen tablets. For appliances that require higher voltages, such as mobile DVD/EVD or large-screen tablets, multi-cell lithium batteries can be used in series to obtain the required voltage. For example, a tablet with 11.1V power supply is equipped with a battery pack. Three lithium batteries connected in series.

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