CN207218340U - A switch type constant current and constant voltage lithium battery charger - Google Patents

Abstract

A kind of switching mode constant current constant voltage lithium battery charger is the utility model is related to, its structure is sequentially connected in series by current rectifying and wave filtering circuit, high frequency transformer and low-voltage direct rectification circuit, composition charging major loop；By voltage sampling circuit, amplifying circuit connects compared with first, and by current sampling circuit, amplifying circuit connects compared with second, and two-way is total to reversed feedback regulating networks, composition voltage x current sampling feedback loop again；It is sequentially connected in series by feedback regulation network, photoelectric isolating circuit, error amplifying circuit, pwm control circuit and high frequency power on-off circuit, composition charging voltage and current adjustment control loop.The utility model charger first charges under constant current mode to battery, after the magnitude of voltage of setting is reached, circuit automatically switches to constant-voltage charge pattern, and the charge mode for stopping such a constant-current constant-voltage charging of charged state is automatically switched to after full of voltage.The utility model efficiency high, do not generate heat, without radiator fan, noiseless, the full level of battery can be improved.

Description

A switch type constant current and constant voltage lithium battery charger

technical field

The utility model relates to a lithium battery charging device, in particular to a switch-type constant current and constant voltage lithium battery charger.

Background technique

In recent years, with the rapid development of lithium battery technology and the advancement of industrialization, lithium battery electric bicycles have been more and more widely popularized and applied, bringing great convenience to people's lives. Lithium battery charger is the supporting equipment for electric bicycle charging, which plays a decisive role in maintaining the capacity of the battery and the service life of the battery.

At present, most of the cheap lithium battery chargers on the market are mostly single constant voltage charging mode, without constant current and constant voltage charging function; and the few constant current chargers, in fact, cannot maintain the constant voltage when the load status changes. The stability of the charging current will cause irreversible capacity loss to the lithium battery itself. At the same time, linear chargers also have problems such as high heat generation, fan cooling is required during charging, low efficiency and high noise.

Utility model content

The purpose of the utility model is to provide a switch-type constant current and constant voltage lithium battery charger to solve the problems of the existing chargers such as unstable constant current charging current, large calorific value, low efficiency, and large noise.

The utility model is achieved in this way: a switch type constant current constant voltage lithium battery charger, comprising:

The rectifying and filtering circuit is connected with the high-frequency transformer, and is used for rectifying and filtering the AC mains power supply;

The high-frequency transformer is connected with the rectification and filtering circuit and the low-voltage DC rectification circuit respectively, and is used to convert the rectified and filtered 300V DC voltage into a low-voltage AC signal with the amplitude required for charging through PWM mode at high frequency;

The low-voltage DC rectification circuit is respectively connected to the positive terminal of the high-frequency transformer, the voltage sampling circuit and the charging output terminal, and is used for DC rectifying the low-voltage AC signal output by the high-frequency transformer and then sending it to the charging output terminal;

The current sampling circuit is respectively connected with the negative terminal of the charging output terminal and the comparison amplifier circuit, and is used for sampling the charging current;

The voltage sampling circuit is respectively connected with the low-voltage direct current rectification circuit and the comparison amplifier circuit, and is used for sampling the charging voltage;

The comparison and amplification circuit is respectively connected with the current sampling circuit, the voltage sampling circuit and the feedback regulation network, and is used to compare and amplify the sampled current signal and voltage signal respectively and send them to the feedback regulation network;

The feedback regulation network is respectively connected with the comparison amplifier circuit and the photoelectric isolation circuit, and is used to send out a feedback regulation signal for pulse width modulation according to the change of the sampled current signal and/or voltage signal;

The photoelectric isolation circuit is respectively connected with the feedback adjustment network and the error amplifier circuit, which is used to isolate the strong current circuit and the weak current circuit, and at the same time complete the transmission of the feedback adjustment signal;

The error amplifier circuit is respectively connected with the photoelectric isolation circuit and the PWM control circuit, and is used to amplify the feedback adjustment signal for pulse width modulation;

The PWM control circuit is connected to the error amplifier circuit and the high-frequency power switch circuit respectively, and is used to adjust the on-time and off-time of the high-frequency power switch according to the feedback adjustment signal; and

The high-frequency power switching circuit is connected with the PWM control circuit and the high-frequency transformer respectively, and is used for constant current and constant voltage control of the charging circuit.

The utility model charger also includes:

The charging display circuit is connected with the comparison amplifier circuit for displaying the charging status. The charging display circuit is set to indicate the charging state, specifically, the red light-emitting diode is turned on in the charging state, and the red light-emitting diode is turned off and the green light-emitting diode is turned on when it is fully charged. This way the display is intuitive and the charging status is clear at a glance.

An electronic fuse is connected to the connection line between the rectification filter circuit and the high frequency transformer. Electronic fuses are resettable electronic overcurrent protectors. The electronic fuse is used to realize over-current protection, so that when the rectified and filtered 300V DC voltage signal passes through the electronic fuse, if the charging current is too high, the electronic fuse will present a high-resistance state, so that the charging current in the charging circuit will decrease rapidly. When the current drops to a safe range, the electronic fuse returns to a low-resistance state, so that circuit protection can be achieved without frequent replacement of fuses in the charger, and the work is simple and effective.

The utility model comprises a rectifying and filtering circuit of 220V/50Hz AC electric signal, which is used for generating DC voltage. The DC voltage signal provides a DC charging voltage for the lithium battery after passing through a high-frequency transformer and a low-voltage DC rectifier circuit. Design the current sampling circuit and voltage sampling circuit in the battery charging circuit. The sampling signal is input to the feedback regulation network after being processed by the comparison amplifier. Since the circuit involves high voltage and low voltage parts, the feedback regulation signal is input to the error amplifier circuit through the photoelectric isolation circuit. , and then control the PWM control circuit to generate a suitable pulse width to adjust the on and off time of the high-frequency power switch, and finally realize the charging control of constant current and constant voltage through the high-frequency transformer.

The charging process of the utility model charger is to charge the battery in the constant current mode first. When the charging voltage of the lithium battery reaches the set voltage value, the charging circuit automatically switches to the constant voltage charging mode. After reaching the full voltage, It will automatically switch to the state of stopping charging. This is a constant-current constant-voltage charging mode that is different from traditional chargers, and the charging power source works under the control mode of high-frequency switches. This charging method is characterized by high efficiency, no heat, no need for cooling fans, and no It is safer and more reasonable than the traditional constant voltage charging method.

The charger of the utility model adopts a pure analog circuit design, and has a constant current and constant voltage charging mode at the same time without the need for a single-chip computer and program design. After the constant current charging is completed, it enters the constant voltage charging mode, which is a more reasonable and effective charging mode. Compared with the charging method that only stops charging after the constant current is charged to the required voltage of the battery, the fullness of the battery can be improved.

The high-voltage circuit and the low-voltage circuit in the charger of the utility model are electrically isolated, so that the charger is more safe and reliable during use. On the premise of maintaining the low cost of the charger, the charging process of the battery is optimized, the operation is simplified, and the current stability during the constant current charging process is ensured.

Description of drawings

Fig. 1 is the circuit block diagram of the utility model charger.

Fig. 2 is a schematic circuit diagram of the utility model charger.

Detailed ways

As shown in Figure 1, the charger of the utility model is sequentially connected in series by a rectification filter circuit, an electronic fuse, a high-frequency transformer and a low-voltage DC rectification circuit to form a charging main circuit; the current sampling circuit and the voltage sampling circuit are connected with a comparison amplifier circuit, The comparison amplifier circuit is connected with the feedback adjustment network to form a voltage and current sampling feedback loop. The feedback adjustment network, photoelectric isolation circuit, error amplifier circuit, PWM control circuit and high-frequency power switch circuit are sequentially connected in series to form a charging voltage and current adjustment control loop. , it can be realized to charge the battery in the constant current mode first, when the set voltage value is reached, the circuit automatically switches to the constant voltage charging working mode, and automatically switches to the stop charging state after the voltage is fully charged, forming a new The working mode of constant current and constant voltage charging.

Figure 2 shows an embodiment of a charger for charging a 48V/10AH lithium battery.

In Figure 2, a rectifier bridge is composed of diodes D ₂ , D _{3 ,} D ₆ , and D ₇ , and the input terminal of the rectifier bridge is connected to power terminal P ₁ , which is used to connect to 220V/50Hz AC mains; rectification The output end of the bridge is connected to the digital ground through the filter capacitor _C2 , thus forming a rectification filter circuit. The output end of the rectification filter circuit is connected to the high-frequency transformer T ₁ through the electronic fuse. The primary side of the high-frequency transformer _T1 has two sets of reverse-wound primary coils, one end of the first set of primary coils is connected to the output end of the rectifier bridge through the electronic fuse _F2 , and the other end of the first set of primary coils is divided into two circuits , one way is connected to the high-frequency power switching circuit, the other way is connected to the output terminal of the rectifier bridge through the diode _D1 and the capacitor _C1 , and a resistor _R1 is connected in parallel at both ends of the capacitor _C1 ; one end of the second group of primary coils is connected in series Resistor R ₄ , diode D ₈ and resistor R ₄ are connected to the output terminal of the rectifier bridge. There is a lead wire at the connection node between diode D ₈ and resistor R ₄ , and the digital ground is connected after the capacitor C _4. The second group of primary coils The other end is connected to the digital ground. One end of the secondary coil of the high-frequency transformer _T1 is connected to the positive end of the charging terminal _P2 through the Zener diode _D4 and the diode _D5 , and the other end of the secondary coil of the high-frequency transformer _T1 is grounded; in the high-frequency transformer T The two ends of the secondary coil of ₁ are connected in parallel with a capacitor C ₃ , and the node between the Zener diode D ₄ and the diode D ₅ is connected with the integrated chip U ₁ and the peripheral resistor R ₃ , capacitors C ₅ , C ₆ , C ₇ is connected to the three-terminal voltage regulator integrated circuit. The voltage stabilizing diode D ₄ , the diode D ₅ , the capacitor C ₃ and the three-terminal voltage stabilizing integrated circuit form a low-voltage DC rectifying circuit. The rectification and filtering circuit, electronic fuse, high-frequency transformer and low-voltage DC rectification circuit are sequentially connected in series to form the charging main circuit.

The negative terminal of the charging terminal _P2 is divided into two paths, one path is grounded through the sampling resistor _R6 , and the other path is connected to the comparison amplifier circuit through the sampling resistor _R5 . Sampling resistors R ₅ and R ₆ form a current sampling circuit. The positive end of the charging terminal _P2 is divided into three circuits after passing through the resistor R _28. The first circuit is connected to the control electrode of the controllable precision voltage regulator TL431, the second circuit is grounded after passing through the resistor R ₃₀ and the adjustable resistor R ₃₁ , and the third circuit After passing through capacitor _C17 , it is grounded. The resistor R ₂₈ , the resistor R ₃₀ and the adjustable resistor R ₃₁ form a voltage sampling circuit.

The comparative amplifying circuit includes operational amplifier U _2A , operational amplifier U _2B , peripheral components of operational amplifier, voltage reference circuit and controllable precision voltage regulator TL431. The voltage reference circuit is composed of resistors R ₇ , R ₁₁ , R ₁₅ and reference voltage source LM336. In the voltage reference circuit, resistors R ₇ , R ₁₁ , and R ₁₅ are connected in series to form a voltage divider circuit. The reference voltage source LM336 is connected between the voltage divider circuit and the ground wire. The control pole of the reference voltage source LM336 is connected to the negative pole. A capacitor C ₉ is connected between the positive and negative poles of the reference voltage source LM336. The sampling resistor _R5 branch in the current sampling circuit is connected to the non-inverting input terminal of the operational amplifier U _2A , and the inverting input terminal of the operational amplifier U _2A is connected to the output terminal of the voltage reference circuit; the voltage reference circuit provides voltage for the comparison amplifier circuit reference signal. The output terminal of the operational amplifier _U2A is divided into three routes, the first route is connected to the inverting input terminal of the operational amplifier _U2B , the second route is connected to the base of the triode _Q5 in the charging display circuit through the resistor _R20 , and the third route is connected to the diode D ₁₃ and adjustable resistor R ₂₆ are connected to the control pole of the controllable precision voltage regulator TL431. The non-inverting input terminal of the operational amplifier _U2B is connected with the output terminal of the voltage reference circuit, and the output terminal of the operational amplifier _U2B is connected to the base of the triode _Q2 in the charging display circuit through the resistor _R14 . The emitter of the transistor _Q5 in the charging display circuit is connected to the red light-emitting diode _D12 , and the emitter of the transistor _Q2 in the charging display circuit is connected to the green light-emitting diode _D10 . Transistor Q ₂ , Q ₅ together with red diode D ₁₂ and green light-emitting diode D ₁₀ form a charging display circuit.

The operational amplifier U _2A and the controllable precision voltage regulator TL431 also constitute a feedback regulation network, which is used to send out a feedback regulation signal for pulse width modulation according to the change of the sampled current signal and/or voltage signal. The output terminal of the operational amplifier U _2A is connected to the control pole of the controllable precision voltage regulator TL431 through the diode D ₁₃ and the adjustable resistor R ₂₆ ; the positive end of the charging terminal P ₂ is connected to the controllable precision voltage regulator TL431 through the resistor R ₂₈ The control pole; the positive pole of the controllable precision voltage regulator TL431 is grounded, and the negative pole is connected to the input terminal of the photoelectric isolation circuit.

The photoelectric coupler U5 and the peripheral capacitor C ₁₆ , resistors R ₂₄ and R ₂₉ form a photoelectric isolation circuit, which completes the photoelectric isolation of strong and weak currents for the feedback adjustment signal.

The pulse width modulation chip U ₄ of the core device of the PWM control circuit. The pulse width modulation chip U ₄ uses UC3842 integrated chip, its pin 1 is connected to the emitter of the transistor Q ₁ , and the collector of the transistor Q ₁ is connected to the digital ground; its 8 pins are connected to four external circuits, and the first circuit is connected to the transistor Q ₁ through the resistor R ₈ The base of the transistor _Q4 , the second road is connected to the collector of the transistor Q4, the third road is connected to the output terminal of the photocoupler _U5 , and the fourth road is connected to the digital ground through the capacitor _C13 ; its 4 pins are connected to the base of the transistor _Q4 , The emitter of the transistor Q ₄ is connected to the digital ground through the capacitor C ₁₄ , the resistor R ₂₃ and the capacitor C ₁₅ ; its 3-pin is connected to the digital ground through the capacitor C ₁₅ ; the 5-pin of the pulse width modulation chip U ₄ is connected to the digital ground; its 7-pin is connected to the digital ground through Capacitor _C12 is connected to the digital ground; its pin 6 is the output terminal, which is connected to the gate of thyristor _Q3 through resistor _R18 , so as to control the on-off time of the high-frequency high-power drive switch—thyristor Q3 through pulse width modulation.

The error amplification circuit is the internal circuit of the pulse width modulation chip U ₄ (UC3842), which is used to amplify the feedback regulation signal for pulse width modulation.

The high-frequency power switching circuit is composed of thyristor _Q3 and peripheral components. The gate of thyristor _Q3 is connected to the output terminal of the PWM control circuit through resistor _R18 . The source of thyristor _Q3 is divided into two circuits, one of which is connected to digital ground through resistor _R21 . , the other path is connected to the digital ground through the resistor _R22 and the capacitor C15, the drain of the thyristor _Q3 is connected to one end of the first group of primary coils of the high-frequency transformer _T1 , and the source and drain of the thyristor _Q3 are connected with Zener diode. Thyristor Q ₃ is a MOSFET for realizing high frequency power switching function.

The charger of the utility model adopts a reference voltage source LM336 and a controllable precision voltage stabilizer TL431 to provide a voltage reference signal, so that the circuit design is simple, and the control of the current and voltage is very precise. The adjustable resistor R ₂₆ and the adjustable resistor R ₃₁ in the circuit can be set to change the constant current value and the charging voltage value, so that it can be flexibly applied to the lithium battery charging work of electric bicycles of different specifications.

The utility model adopts pure analog circuit design, combines low-cost electronic fuse, special pulse width modulation (PWM) chip, voltage and current sampling circuit, feedback network and high-frequency power switch, etc., and realizes the scientific and reasonable charging process of lithium battery.

The working process of the charger of the utility model is: a rectification and filter circuit composed of a rectifier bridge (D ₂ , D ₃ , D ₆ , D ₇ ) and a filter capacitor C ₂ connected to its output terminal, which is used to convert the 220V AC market The electrical signal is converted into a DC pulsating signal output of about 300V. The DC pulsating signal is input to one end of the primary coil of the 20KHz-50KHz high-frequency transformer _T1 after passing through the electronic fuse _F2 with a withstand voltage of 300V, and then applied to the charging terminal _P2 after being processed by a low-voltage DC rectifier circuit. Outputs a DC charging signal to the lithium battery connected to the charging terminal _P2 . The rectifier diode _D5 adopts a fast recovery Schottky diode, which has a small forward voltage drop and high efficiency. The other end of the primary coil of the high-frequency transformer _T1 is connected to the drain of the thyristor _Q3 , and the pulse width modulation chip _U4 controls the on or off working state of the thyristor _Q3 to control the secondary of the high-frequency transformer _T1. Energy coupled to the stage coil. The Zener diode D ₄ connected to the secondary coil of the high-frequency transformer T ₁ can also ensure the direction of the charging current.

The charging current I flowing through the charging load and the current sampling resistor _R6 forms a voltage drop on the sampling resistor _R6 , and this voltage is the sampling signal of the charging current. The signal is input to the non-inverting input terminal of the operational amplifier _U2A through the sampling resistor _R5 , compared with the reference potential of the inverting input terminal of the operational amplifier _U2A , and then amplified in the same phase. If the potential of the non-inverting input terminal is higher than the reference potential of the inverting terminal, the operational amplifier U _2A outputs high level. This high level signal turns on the transistor Q ₅ and lights up the red LED D ₁₂ , indicating that the charger is charging. At the same time, the high-level signal is simultaneously input to the control electrode of the controllable precision voltage regulator TL431 in the comparison amplifier circuit, so that the potential of the control electrode reaches or exceeds 2.5V, and the conduction degree of the controllable precision voltage regulator TL431 is increased. Drive and light the LED inside the optocoupler U ₅ , make the phototransistor at the output end of the optocoupler U ₅ turn on, the current on the resistor R ₉ increases, and the potential rises, so that the V _FB pin on the pulse width modulation chip U ₄ As the potential rises, the pulse width of the output PWM signal becomes narrower, reducing the conduction time of the thyristor _Q3 , thereby reducing the output voltage of the power supply, and reducing the output current of the charger, thereby realizing negative feedback control.

Similarly, when the charging current is less than the designed constant current value, the voltage on the sampling resistor _R6 is lower than the reference voltage value of the inverting input terminal of the op amp U2A, the op amp U2A outputs a low level, and the controllable precision voltage regulator TL431 tends to stop , the phototransistor at the output end of the photocoupler _U5 also tends to be cut off, the current flowing through the resistor _R29 decreases, and the potential of the upper end of the resistor _R29 decreases, so that the potential of the V _FB pin on the pulse width modulation chip _U4 decreases, and the output The pulse width of the PWM signal is widened, so that the output current can be adjusted incrementally.

Through the above dynamic adjustment, the charging current I flowing through the load is finally maintained at a stable set value to realize constant current charging.

As the constant current charging progresses, the voltage on the load lithium battery gradually increases. When the battery voltage rises to 54V, the voltage divider circuit composed of resistor _R28 and resistor _R30 connected in series starts to work, and the charger enters the constant voltage charging mode. The constant voltage charging voltage is set to 115% of the battery nominal voltage value, so the constant voltage charging voltage value is set to 55.2V. As shown in Figure 2, the output terminal voltage of the charging circuit is input to the control pole of the controllable precision voltage regulator TL431 after being divided by the resistor R ₂₈ and the resistor R ₃₀ , by adjusting the adjustable resistor R ₂₆ and the adjustable resistor R ₃₁ , adjust the voltage divider value so that when the power supply output is 55.2V, the resistor divider voltage output is 2.5V, so that when the power supply voltage is greater than 55.2V, the controllable precision voltage regulator TL431 is turned on, and the feedback is the pulse width modulation chip U ₄ As the voltage signal rises, the pulse width of the PWM pulse signal narrows, thereby reducing the output voltage and forming a negative feedback process.

Similarly, when the output voltage is lower than 55.2V, the pulse width of the PWM signal is widened, and finally the output voltage is stabilized at the designed 55.2V, thereby realizing constant voltage charging.

During the constant current charging process, the triode _Q5 is turned on, and the red light-emitting diode _D12 lights up, indicating that it is charging. During constant voltage charging, the battery is almost fully charged, and the charging current gradually decreases, which is lower than the previously set constant current charging current value. Therefore, the constant current control circuit gradually fails. When the current in the constant current control circuit is much smaller than the design value, the operational amplifier U _2A outputs a low level, the operational amplifier U _2B outputs a high level, the triode _Q5 is cut off, the triode _Q2 is turned on, the red light-emitting diode D ₁₂ is off, and the green LED is off. The light-emitting diode D ₁₀ lights up, indicating that the battery is fully charged.

Claims (5)

1. a kind of switching mode constant current constant voltage lithium battery charger, it is characterized in that, including：

Current rectifying and wave filtering circuit, connect with high frequency transformer, for carrying out rectification and filtering to AC mains；

High frequency transformer, connect respectively with current rectifying and wave filtering circuit and low-voltage direct rectification circuit, for by after rectifying and wave-filtering Low-voltage ac signal of the 300V DC voltages by PWM mode high frequency conversion for amplitude needed for charging；

Low-voltage direct rectification circuit, respectively with high frequency transformer, voltage sampling circuit and charge output end positive terminal connect, use Charging output end is sent after the low-voltage ac signal for exporting high frequency transformer carries out DC rectifier；

Current sampling circuit, respectively with charge output end negative pole end and compared with amplifying circuit connect, for entering to charging current Row sampling；

Voltage sampling circuit, respectively with low-voltage direct rectification circuit and compared with amplifying circuit connect, for being carried out to charging voltage Sampling；

Compare amplifying circuit, connect respectively with current sampling circuit, voltage sampling circuit and feedback regulation network, for sampling Current signal, voltage signal feedback regulation network is sent after being compared and amplify respectively；

Feedback regulation network, amplifying circuit and photoelectric isolating circuit connect compared with respectively, for the current signal according to sampling And/or the change of voltage signal sends the feedback adjustment signal for carrying out pulsewidth modulation；

Photoelectric isolating circuit, connect respectively with feedback regulation network and error amplifying circuit, for isolating forceful electric power circuit and light current Circuit, while complete the transmission of feedback adjustment signal；

Error amplifying circuit, connect respectively with photoelectric isolating circuit and pwm control circuit, the anti-of pulsewidth modulation is carried out for amplifying Present Regulate signal；

Pwm control circuit, connect respectively with error amplifying circuit and high frequency power on-off circuit, for according to feedback adjustment signal Adjust conducting and the deadline of high frequency power switch；And

High frequency power on-off circuit, connects with pwm control circuit and high frequency transformer respectively, for carrying out the constant current of charging circuit Isobarically Control.

2. switching mode constant current constant voltage lithium battery charger according to claim 1, it is characterized in that, also include：

Charging display circuit, amplifying circuit is connected compared with, for showing charged state.

3. switching mode constant current constant voltage lithium battery charger according to claim 1 or 2, it is characterized in that, filtered in the rectification Electrical fuse is connected on connecting line between wave circuit and the high frequency transformer.

4. switching mode constant current constant voltage lithium battery charger according to claim 3, it is characterized in that, the relatively amplifying circuit Including amplifier U_2A, amplifier U_2B, operational amplifier peripheral cell, voltage reference circuit and controllable accurate source of stable pressure TL431； Voltage reference circuit is by resistance R₇、R₁₁、R₁₅Composition is connected with reference voltage source LM336；In voltage reference circuit, resistance R₇、 R₁₁、R₁₅Bleeder circuit is composed in series, reference voltage source LM336 is connected between bleeder circuit and ground wire, reference voltage source LM336 control pole connects with negative pole, and electric capacity C is connected between reference voltage source LM336 positive and negative electrode₉；Current sampling circuit In sampling resistor R₅Branch road is connected to amplifier U_2AIn-phase input end, amplifier U_2AThe voltage reference that is connected to of inverting input The output end of circuit；Voltage reference circuit provides voltage reference signal to compare amplifying circuit；Amplifier U_2AOutput end point three tunnels, The first via meets amplifier U_2BInverting input, the second tunnel is through resistance R₂₀Meet the triode Q in charging display circuit₅Base stage, Three tunnels are through diode D₁₃With adjustable resistance R₂₆Connect controllable accurate source of stable pressure TL431 control pole；Amplifier U_2BIn-phase input end Connect with the output end of voltage reference circuit, amplifier U_2BOutput end through resistance R₁₄Meet the triode Q in charging display circuit₂'s Base stage.

5. switching mode constant current constant voltage lithium battery charger according to claim 4, it is characterized in that, the feedback regulation network Including amplifier U_2AWith controllable accurate source of stable pressure TL431；Amplifier U_2AOutput end through diode D₁₃With adjustable resistance R₂₆Being connected to can Control precision voltage regulator TL431 control pole；Charging terminal P₂In positive terminal through resistance R₂₈Connect controllable accurate source of stable pressure TL431's Control pole；Controllable accurate source of stable pressure TL431 plus earth, negative pole connect the input of photoelectric isolating circuit.