The principle of 3.7v lithium battery protection plate

1. Single-cell lithium battery protection circuit

A single lithium cell charge and discharge protection circuit more specific composition of embodiment, but the principle or less, in the following to a mobile phone used more as an example of a circuit for analysis, for reference.

The control chip of the circuit is DW01 (or 312F), and the MOS switch tube is 8205A. As shown in Figure 6, B+ and B- are the positive and negative poles of the battery, respectively; P+ and P- are the positive output of the protection board, respectively. Negative; T is the temperature resistance (NTC) port, which generally needs to cooperate with the CPU of the appliance to perform protection control.

DWO1 or 312F is a lithium battery protection chip, built-in high-accuracy voltage detection circuit and the delay time, the main parameters are as follows: overcharge detection voltage is 3V, overcharge releasing voltage of 4.05V; over discharge detection voltage of 2.5V The over-discharge voltage is 3.0V; the over-current detection voltage is 5V, and the short-circuit current detection voltage is 1.0V; DW01 allows the maximum output current of the battery to be 3.3A. The pin function of this chip is shown in Table 1.

(1) Normal work

The circuit of the protection board is shown in Figure 7. When the cell voltage is between 2.5V and 4.3V, both pins 1 and 3 of DW01 output a high level (equal to the supply voltage), and the voltage of the 2 pin is 0V. At this time, the two N-channel FETs Q1 and Q2 in the 8205A are all in the on state. Since the on-resistance of the 8205A is small, it is equivalent to the direct connection between the D and S poles. At this time, the negative pole of the cell and the P of the protection circuit The - terminal is equivalent to direct communication, and the protection circuit has a voltage output. The current loop is as follows: B+→P+→load. P-→8205A, 2, 3 feet → 8205 of 8205A → 8 feet of 8205A → 6, 7 feet of 8205A → B-.

[Hint] In this circuit, the 8205A internal FETs Q1 and Q2 can be equivalent to two switches. When the G pole voltage of Q1 or Q2 is greater than 1V, the switch is turned on, and the internal resistance between D and S is small ( Tens of milliohms), equivalent to the switch closure; when the G pole voltage is less than 0.7V, the switch tube is cut off, and the conduction internal resistance between the D and S poles is large (several megaohms), which is equivalent to the switch being disconnected.

(2) Overdischarge protection

When the battery is discharged through an external load, the voltage across the battery will gradually decrease, and the voltage inside the DW01 will be monitored in real time through the resistor R1. When the battery voltage drops to 2.3V (usually called over-discharge protection voltage) When DWO1 thinks that the cell is in an over-discharge state, its 1-pin voltage becomes 0, and Q1 is turned off in 8205A. At this time, the B- and - of the cell are disconnected, that is, the discharge circuit of the cell is cut off. The battery will stop discharging.

After entering the over-discharge protection state, the cell voltage will rise. If it can rise to the IC's threshold voltage (typically 3.1V, usually called over-discharge protection recovery voltage), DW0's 1 pin resumes output high level, within 8205A Q1 is turned on again.

(3) Battery charging

Regardless of whether the protection circuit enters the over-discharge state, as long as the charging voltage is applied between the P+ and P- terminals of the protection circuit, DW0 immediately detects the charging voltage through the B terminal, and immediately outputs a high level from the 3 pin, and the Q2 guide in the 8205A. Pass, that is, the P-pass of the B-protection circuit of the battery, the charger charges the battery, and its current loop is as follows: charger positive → p+ → B+ → B-, 8205 of 8205A → 8 pin of 8205A → 1 pin of 8205A → 2, 3 feet of 8205A → P- → negative charger.

(4) Overcharge protection

When charging, when the battery is normally charged by the charger, as the charging time increases, the voltage across the cell will gradually increase. When the cell voltage rises to 4.4V (commonly referred to as overcharge protection voltage), DW01 It will judge that the cell is in the overcharge state, and immediately reduce the voltage of pin 3 to 0V. Q2 in 8205A is cut off due to the low level of pin 4, at this time, the B pole of the cell and the P-end of the protection circuit The battery is disconnected and held, that is, the charging circuit of the battery cell is cut off, and charging is stopped.

When the P+ and P- terminals of the protection circuit are connected to the discharge load, although the Q2 is turned off, the positive direction of the diode inside is the same as the current direction of the discharge circuit, so the load can still be discharged. When the voltage across the battery is lower than 4.3V (commonly referred to as overcharge protection recovery voltage), DW01 will exit the overcharge protection state, 3 feet will re-output high level, Q2 will be on, ie the B-end of the battery core The P-terminal of the protection circuit is reconnected, and the battery can perform normal charging and discharging.

(5) Overcurrent protection

Since the internal resistance of the MOs switch tube is also saturated, when there is current flowing, the voltage drop occurs between the D and S poles of the MOs switch tube, and the protection control IC detects the voltage of the D and S poles of the MOs switch tube in real time. When the voltage rises to the IC protection threshold (generally 0.15V, called the discharge overcurrent detection voltage), the discharge protection actuator immediately outputs a low level, the discharge control MOs switch is turned off, and the discharge loop is disconnected.

In Figure 7, DW01 detects the voltage drop across the MOs switch in real time through a resistor R2 connected between the V- and VSS terminals. When the load current increases, the voltage drop on Q1 or Q2 also increases. When the voltage drop reaches 0.2V, DWO1 judges that the load current has reached the limit value, so its 1 pin voltage drops to 0V, and the inside of 8205A The discharge control tube Q1 is turned off, and the discharge circuit of the battery cell is cut off. Overcurrent protection is achieved.

(6) Over temperature protection

The T port on the protection board is an over temperature protection end and is connected to the CPU of the consumer. A common over-temperature protection circuit is simpler, that is, an NTC resistor (see R4 in Figure 7) is connected at the T terminal and the P- terminal, and the resistor is mounted close to the battery core. When the electrical appliance is in a high-power working state for a long time (such as when the mobile phone is in a call state for a long time), the temperature of the battery core will rise, and the resistance value of the NTC will gradually decrease. The resistance of the CPU of the electrical device detects the resistance value of the NTC, and when the resistance value decreases. When the CPU sets the threshold, the CPU immediately issues a shutdown command to stop the battery from supplying power, and only maintains a small standby current, thereby achieving the purpose of protecting the battery.

[Hint] When the protection board is in the protection state, you can short the B- and P- terminals to activate the protection board. At this time, the charge and discharge protection execution terminals (OC, OD) of the control chip will output a high level, let MOs The switch tube is turned on.