LR921 battery.Detailed introduction to the principle of mobile phone battery circuit

The working mechanism of lithium-ion batteries is: when the battery is charged, lithium in the positive electrode material forms ions and is dissolved and embedded in the modified graphite layer of the negative electrode; when the battery is discharged, lithium ions are deintercalated from the graphite layer and backfilled to the positive electrode through the isolation film. In the layered structure of lithium cobalt oxide. As charging and discharging proceed, lithium ions continue to flow from the positive and negative electrodes.

The working mechanism of lithium-ion batteries is: when the battery is charged, lithium in the positive electrode material forms ions and is dissolved and embedded in the modified graphite layer of the negative electrode; when the battery is discharged, lithium ions are deintercalated from the graphite layer and backfilled to the positive electrode through the isolation film. In the layered structure of lithium cobalt oxide. As charging and discharging proceed, lithium ions are constantly embedded and extracted from the positive and negative electrodes, so some people call it a rocking chair battery. The rated voltage of a lithium-ion battery cell is 3.6V, the charging limit voltage is 4.2V, and the discharge limit voltage is 2.5V. .

The charging process of lithium-ion batteries is divided into two steps: first, constant current charging, whose current is constant and the voltage continues to increase. When the voltage reaches 4.2V, it automatically switches to constant voltage charging. During constant voltage charging, the voltage is constant and the current It becomes smaller and smaller until the charging current is less than the preset value. Therefore, when someone uses direct charging to charge the mobile phone battery, the battery level is obviously full, but it still shows that it is charging. In fact, the voltage at this time has reached 4.2V, so the battery is displayed as full. At that time, the constant voltage charging process is being carried out. Then some people may ask, why is constant voltage charging required? Isn’t it enough to directly charge to 4.2V with constant current? In fact, it is easy to explain. , because each battery has a certain internal resistance. When charging to 4.2V with a constant current, this 4.2V is not actually the actual voltage of the battery, but the voltage of the battery plus the voltage consumed by the internal resistance of the battery. And, if the current is very large, the voltage consumed on the internal resistance will also be very large, so the actual battery voltage may be much smaller than 4.2V, so a constant voltage charging process must be used to slowly reduce the charging current. In this way, the actual voltage of the battery is very close to 4.2V.

Working principle diagram of a switch-type mobile phone charger

This charger uses an RCC switching power supply, which is an oscillation suppression converter, which is somewhat different from a pWM switching power supply. The pWM switching power supply consists of an independent sampling error amplifier and a DC amplifier to form a pulse width modulation system; while the RCC switching power supply only consists of a voltage regulator to form a level switch, and the control process is an oscillation state and a suppression state. Since the switching tube in the pWM switching power supply is always on and off periodically, the system control only changes the pulse width of each cycle, while the control process of the RCC switching power supply does not change linearly and continuously. It has only two states: when the switching power supply When the output voltage exceeds the rated value, the pulse controller outputs a low level and the switch tube is turned off; when the output voltage of the switching power supply is lower than the rated value, the pulse controller outputs a high level and the switch tube is turned on. When the load current decreases, the discharge time of the filter capacitor is extended, the output voltage will not decrease quickly, and the switch tube is in a cut-off state. The switch tube will not turn on again until the output voltage drops below the rated value. The cut-off time of the switch tube depends on the load current. The on/off of the switch tube is controlled by the level switch sampling the output voltage. Therefore, this power supply is also called aperiodic switching power supply.

The 220V mains power is bridge-rectified by VD1~VD4 and forms a DC voltage of about 300V on the collector of V2. The intermittent oscillator is composed of V2 and switching transformer. After starting up, 300V DC voltage is added to the collector of V2 through the primary of the transformer. At the same time, this voltage also supplies a bias voltage to the base of V2 through the starting resistor R2. Due to the use of positive feedback, V2Ic rises rapidly and becomes saturated. During the cut-off period of V2, the induced voltage in the secondary winding of the switching transformer turns VD7 on and outputs a DC voltage of about 9V to the load. The induced pulses appearing in the feedback winding of the switching transformer are rectified by VD5 and filtered by C1, and then a DC voltage appears that is proportional to the number of oscillation pulses. If this voltage exceeds the voltage stabilization value of the voltage regulator tube VD17, VD17 will be turned on, and the negative polarity rectified voltage will be added to the base of V2, causing it to quickly cut off. The cut-off time of V2 is inversely proportional to its output voltage. The turn-on/cut-off of VD17 is directly affected by the grid voltage and load. The lower the grid voltage or the larger the load current, the shorter the conduction time of VD17 and the longer the conduction time of V2. On the contrary, the higher the grid voltage or the smaller the load current, the higher the rectified voltage of VD5 and the longer the conduction time of VD17. The longer it is, the shorter the conduction time of V2 is. V1 is an overcurrent protection tube, and R5 is the sampling resistor of V2Ie. When V2Ie is too large, the voltage drop on R5 causes V1 to turn on and V2 to turn off, which can effectively eliminate the inrush current at the moment of power-on and also compensate for the control function of VD17. VD17 uses voltage sampling to control the oscillation time of V2, while V1 uses current sampling to control the oscillation time of V2.

If you are charging nickel-cadmium or nickel-metal hydride batteries, these batteries have a certain memory effect and need to be discharged from time to time. SW1 is a charging switch for nickel-cadmium, nickel-metal hydride and lithium-ion batteries. SW1 and precision reference power supply SL431 provide two different precision reference sources for the operational amplifier LM324⑨, which are switched by SW1. When charging nickel-cadmium and nickel-hydrogen batteries, the reference voltage of pin LM324 is about 0.09V (no load); when charging lithium-ion batteries, the reference voltage of pin LM324 is about 0.08V (no load). The design is dictated by the chemistry unique to these two types of batteries. When SW2 is pressed, the base of V5 instantly reaches a low level and is turned on. The residual voltage on the rechargeable battery is discharged on R17 through the ec pole of V5, and the discharge indicator VD14 lights up at the same time. After pressing SW2, it will be released immediately. At this time, the residual voltage on the rechargeable battery is divided by R16 and R13. After filtering, C9 supplies a high level to the base of V4, and V4 is turned on, which is equivalent to shorting SW2. As the discharge time prolongs, the residual voltage on the rechargeable battery becomes lower and lower. When the voltage on the base of V4 cannot maintain its continued conduction, V4 is cut off, the discharge is terminated, and the charger immediately enters the charging state.

Since there is no memory effect in lithium batteries, when the battery is lower than 3V, it cannot be turned on. Its residual voltage is divided by resistors R40 and R41 to obtain 2.53V, which is sent to the non-inverting terminals ③, ⑤ and ⑩ of the operational amplifier. It is always 2.66V under load, so pin ⑧ outputs low level, V3 is turned on, and the +9V voltage charges the rechargeable battery through V3ec pole and VD8. When IC1d is used as capacitor C6, the {14} pin outputs a pulse signal. Since IC1⑧ pin is low level, VD12 is in a flashing state to indicate that the battery is charging, and the corresponding capacity is 20%. As the charging time increases, the voltage on the rechargeable battery gradually increases. When the divided voltage value of R40 and R41 is approximately equal to 2.58V, that is, IC1③ pin equals 2.58V, IC1② pin is divided by a resistor to obtain 2.57V, and its ① pin outputs a high level (because during charging, IC1⑨ The pin voltage is always 2.66V, V6 is on; on the contrary, when no load is present, IC1⑨ pin is 0.08V, V6 is off), VD10 and VD11 light up, and the corresponding indicated capacities are 40% and 60%. When the divided voltage value of R40 and R41 rises to 2.63V, that is, the ⑤ pin of IC1 is equal to 2.63V, and its ⑥ pin is divided by a resistor to obtain 2.63V. The ⑦ pin outputs high level, and VD9 lights up, corresponding to the charging capacity. is 80%. Only when the voltage of pin ⑩ of IC1 is ≥ 2.66V, pin ⑧ outputs high level, VD13 lights up, and the corresponding charging capacity is 100%. Even when VD13 is on, VD12 is still flashing, which means that the battery has not yet reached full saturation. Only when the voltage at pin 8 of IC1 is 6.5V, VD12 gradually goes out, indicating that the battery is fully charged to saturation.

VD16 serves as overcharge and overcurrent protection in the circuit, and VD8 serves as reverse protection to prevent the battery from being discharged in reverse after the charger is powered off.