CN118783573A - Power Management Circuit - Google Patents

Abstract

The present invention provides a power management circuit, which includes: a first voltage terminal and a second voltage terminal that can be coupled to a charger or a load; a battery cell unit, which includes a first battery cell connection terminal and a second battery cell connection terminal, and the second battery cell connection terminal is conductively connected to the second voltage terminal; a conversion output circuit coupled to the first voltage terminal and the second voltage terminal, which includes a first switch tube, a second switch tube and an output inductor; a voltage conversion controller; when the discharge is overvoltage and the charger is not connected, the voltage conversion controller controls the first switch tube to be disconnected and the second switch tube to be turned on. In this way, the battery cell unit can avoid the risk caused by the parasitic body diode of the second switch when participating in the series application.

Description

Power Management Circuit

The application number of this invention is 202311673959.7, the application date is December 6, 2023, and the name of the invention is divisional application for power management circuit.

[Technical field]

The present invention relates to the field of battery management, and in particular to a power management circuit.

[Background technology]

Dry cells with a typical output voltage of 1.5V are widely used in consumer electronic products. With the emergence of energy storage cells such as lithium batteries and sodium batteries, product solutions to replace alkaline batteries have emerged, that is, using rechargeable cells, converting the cell voltage to alkaline battery voltage output through power management circuits, simulating dry cells, and replacing dry cells. This solution has the advantages of high power and increased cycle times.

FIG1 is a schematic diagram of the circuit structure of a power management circuit in the prior art. As shown in FIG1 , the power management circuit includes a first voltage terminal P+, a second voltage terminal P-, a battery cell Bat, a voltage conversion controller 110, a charging controller 120, a discharge overvoltage detection circuit 130, a charging detection circuit 140, a conversion output circuit 150, and a charging switch tube MP2. The conversion output circuit 150 includes a first switch tube MP1, a second switch tube MN1, an output inductor L, and an output capacitor C. The voltage conversion controller 110 is a DC-DC Buck voltage conversion controller.

When the charging detection circuit 140 detects that the voltage of the first voltage terminal P+ is higher than the voltage of the first cell terminal VB of the cell unit Bat, it is determined that a charger is connected between the first voltage terminal P+ and the second voltage terminal P-, and the charging controller 120 is controlled to work through the signal CHRG_ON, and the charging current of the cell unit is controlled through the charging switch tube MP2. At the same time, the voltage conversion controller 110 is controlled to stop working through the signal CHRG_ON, at which time the gate control signal P_DRV of the first switch tube MP1 is high, the first switch tube MP1 is disconnected, and the gate control signal N_DRV of the second switch tube MN1 is low, and the second switch tube MN1 is disconnected.

When the discharge overvoltage detection circuit 130 determines that the voltage of the first battery cell connection terminal VB is too low, the voltage conversion controller 110 is controlled to stop working through the output signal OD_STATE. At this time, the gate control signal P_DRV of the first switch tube MP1 is high, the first switch tube MP1 is disconnected, and the gate control signal N_DRV of the second switch tube MN1 is low, and the second switch tube MN1 is disconnected.

When the charging detection circuit 140 detects that the voltage of the first voltage terminal P+ is lower than the voltage of the first cell terminal VB of the cell unit Bat, it is determined that no charger is connected between the first voltage terminal P+ and the second voltage terminal P-, and the charging controller 120 is controlled to stop working through the signal CHRG_ON. At this time, the gate control signal CHG_DRV of the charging switch tube MP2 is at a high level, so that the charging switch tube MP2 is turned off. At the same time, the voltage conversion controller 110 is no longer prohibited from working through the signal CHRG_ON.

When the discharge overvoltage detection circuit 130 determines that the voltage at the first battery cell connection terminal VB is not in an excessively low state, the voltage conversion controller 110 is no longer prohibited from operating through the signal OD_STATE.

When the charging detection circuit 140 detects that the voltage of the first voltage terminal P+ is lower than the voltage of the first battery cell connection terminal VB and the discharge overvoltage detection circuit 130 determines that the VB voltage is not in a too low state, there is no signal to prohibit the voltage conversion controller 110 from working, and the voltage conversion controller 110 outputs control signals P_DRV and N_DRV to adjust the P+/P- output to have a constant voltage with load capacity.

When the cell voltage is too low, to ensure that the cell power does not continue to drop rapidly, the voltage conversion controller 110 will stop working, and to support charging from the P+/P- terminals, the first switch tube PM1 and the second switch tube NM1 will be disconnected or cut off. However, the method of cutting off the second switch tube NM1 will have the defect of safety risk when multiple simulated dry batteries are connected in series.

FIG2 is a schematic diagram of the circuit structure of the power management circuit in FIG1 when it is applied in series. When the power of the battery cell Bat1 decreases and triggers the discharge overvoltage protection, the voltage conversion controller 110 of the battery cell will turn off (disconnect) the second switch tube NM1. At this time, if the load LOAD of the multiple battery cells Bat1-Batn in series is still requesting the load current I_LOAD from the multiple battery cells Bat1-Batn in series, the load current will be in the parasitic body diode D1 of the second switch tube NM1 corresponding to the battery cell Bat1. The large current forward conduction of the parasitic body diode D1 will cause two risks. The first risk is: due to the conduction voltage of the parasitic body diode, the heat becomes larger; the second risk is: the forward bias conduction of the parasitic body diode may trigger the conduction of the parasitic triode, increase the power consumption of the battery cell Bat1 which is already at low power, and even trigger the latch effect. Both of the above risks may cause damage to the device or chip.

Therefore, it is necessary to propose a new technical solution to overcome the above problems.

[Summary of the invention]

The object of the present invention is to provide a power management circuit, which can prevent damage to battery cells when they are used in series.

According to one aspect of the present invention, a power management circuit is proposed, which includes: a first voltage terminal and a second voltage terminal that can be coupled to a charger or a load; a battery cell unit, which includes a first battery cell connection terminal and a second battery cell connection terminal, and the second battery cell connection terminal is conductively connected to the second voltage terminal; a conversion output circuit coupled to the first voltage terminal and the second voltage terminal, which includes a first switch tube, a second switch tube and an output inductor, the first battery cell connection terminal provides current to the output inductor via the first switch, and the second voltage terminal provides current to the output inductor via the second switch; a voltage conversion controller; when the discharge is overvoltage and the charger is not connected, the voltage conversion controller controls the first switch tube to disconnect and the second switch tube to turn on.

Compared with the prior art, in the present invention, when the discharge overvoltage signal is valid (indicating discharge overvoltage or the voltage of the battery cell unit is too low) and the charger is not connected, the voltage conversion controller stops working and controls the first switch tube to be disconnected and the second switch tube to be turned on; when the discharge overvoltage signal is valid and the charger is connected, the voltage conversion controller stops working and controls the first switch tube to be disconnected and the second switch tube to be disconnected; when the discharge overvoltage signal is invalid (indicating no discharge overvoltage or the voltage of the battery cell unit is not too low) and the charger is not connected, the voltage conversion controller works normally; when the discharge overvoltage signal is invalid and the charger is connected, the voltage conversion controller stops working and controls the first switch tube to be disconnected and the second switch tube to be disconnected, so that the battery cell unit can be prevented from being damaged when it is used in series.

【Brief Description of the Drawings】

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

FIG1 is a schematic diagram of a circuit structure of a power management circuit in the prior art;

FIG2 is a schematic diagram of a circuit structure when the power management circuit in FIG1 is applied in series;

FIG. 3 is a schematic diagram of the circuit structure of a power management circuit in one embodiment of the present invention.

[Specific implementation method]

In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

The term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments. Unless otherwise specified, the words "connected", "connected", "connected", and "coupled" used herein to indicate electrical connection all refer to direct or indirect electrical connection.

3 is a schematic diagram of the circuit structure of a power management circuit 300 in an embodiment of the present invention. The power management circuit includes a first voltage terminal P+, a second voltage terminal P-, a battery cell Bat, a voltage conversion controller 310, a charging controller 320, a discharge overvoltage detection circuit 330, a charging detection circuit 340, and a conversion output circuit 350.

The cell unit Bat includes a first cell connection terminal VB and a second cell connection terminal S. The conversion output circuit includes a first switch tube MP1, a second switch tube MN1, an output inductor L and an output capacitor C. The output capacitor C is coupled between the first voltage terminal P+ and the second voltage terminal P-, and the first switch tube MP1 is coupled between the first cell connection terminal VB and the intermediate node A. The output inductor L is coupled between the intermediate node A and the first voltage terminal P+. The second cell connection terminal S is coupled to the second voltage terminal P-, and the second switch tube MN1 is coupled between the intermediate node A and the second voltage terminal P-. The voltage conversion controller 310 is coupled between the first cell connection terminal VB and the second cell connection terminal S, and includes a first control output terminal, a second control output terminal, and a feedback input terminal. The first control output terminal is coupled to the control terminal of the first switch tube MP1, and the second control output terminal is coupled to the control terminal of the second switch tube MN1. The charging switch tube is coupled between the intermediate node A and the first cell connection terminal VB. A charging control output terminal of the charging controller 320 is coupled to a control terminal of the charging switch MP2 .

The discharge overvoltage detection circuit 330 is coupled between the first battery cell connection terminal VB and the second battery cell connection terminal S, and is used to detect whether the battery cell Bat is discharged with overvoltage. Specifically, the discharge overvoltage detection circuit 330 determines whether the battery cell Bat is discharged with overvoltage, that is, whether the voltage or power of the battery cell Bat is too low by comparing the voltage value of the first battery cell connection terminal VB with a predetermined voltage threshold. For example, when the voltage value of the first battery cell connection terminal VB is lower than the predetermined voltage threshold, it is considered that the battery cell Bat is discharged with overvoltage, and when the voltage value of the first battery cell connection terminal VB is higher than the predetermined voltage threshold, it is considered that the battery cell Bat is not discharged with overvoltage.

When the battery cell Bat discharges overvoltage, an effective discharge overvoltage signal OD_STATE is output to the voltage conversion controller 310 and the charging detection circuit 340. When the discharge overvoltage signal is valid and the charger is not connected, the voltage conversion controller 310 stops working and controls the first switch tube MP1 to be turned off and the second switch tube MN1 to be turned on by outputting control signals P_DRV and N_DRV. When the discharge overvoltage signal is valid and the charger is connected, the voltage conversion controller 310 stops working and controls the first switch tube MP1 to be turned off and the second switch tube MN1 to be turned off.

When the battery cell Bat is not over-discharged, an invalid discharge over-voltage signal OD_STATE is output to the voltage conversion controller 310 and the charging detection circuit 340. When the discharge over-voltage signal OD_STATE is invalid and the charger is not connected, the voltage conversion controller 310 works normally. At this time, the voltage conversion controller 310 controls the first switch tube MP1 and the second switch tube MN1 to be turned on alternately through the output control signals P_DRV and N_DRV to output a predetermined output voltage through the first voltage terminal P+ and the second voltage terminal P-. When the discharge over-voltage signal OD_STATE is invalid and a charger is connected, the voltage conversion controller 310 stops working and controls the first switch tube MP1 to be turned off and the second switch tube MN1 to be turned off.

The first voltage terminal P+ is coupled to the feedback input terminal of the voltage conversion controller 310. When the voltage conversion controller 310 is working normally, the voltage conversion controller 310 outputs control signals P_DRV and N_DRV according to the voltage of the first voltage terminal P+ to control the first switch tube MP1 and the second switch tube MN1 to be alternately turned on, so that the first voltage terminal and the second output terminal output the predetermined output voltage.

In a preferred embodiment, the second switch tube MN1 includes a plurality of second switch units connected in parallel. When the discharge overvoltage signal OD_STATE is valid and the charger is not connected, the voltage conversion controller 310 controls the first switch tube MP1 to be disconnected (i.e., cut off), and at least controls part of the second switch units to be turned on, and may also control all the second switch units to be turned on.

The charging detection circuit 340 is coupled between the first voltage terminal P+ and the first battery cell connection terminal VB, and determines whether a charger is connected between the first voltage terminal and the second voltage terminal. The control input terminal of the charging detection circuit 340 receives the discharge overvoltage signal OD_STATE from the discharge overvoltage detection circuit 330, and the output terminal of the charging detection circuit also outputs a charging enable signal CHRG_ON to the voltage conversion controller 310, and the charging enable signal CHRG_ON indicates whether a charger is connected between the first voltage terminal and the second voltage terminal.

In the present invention, the state of the discharge overvoltage signal OD_STATE is different, and the condition for the charging detection circuit 340 to determine whether the charger is connected is also different.

When the discharge overvoltage signal OD_STATE is invalid, the charging detection circuit 340 determines whether a charger is connected between the first voltage terminal and the second voltage terminal by comparing the voltage of the first voltage terminal P+ and the voltage of the first battery cell connection terminal VB. Specifically, when the voltage of the first voltage terminal P+ is higher than the voltage of the first battery cell connection terminal VB, it is considered that a charger is connected, and when the voltage of the first voltage terminal P+ is lower than the voltage of the first battery cell connection terminal VB, it is considered that no charger is connected. If it is determined that a charger is connected, a valid charging enable signal CHRG_ON is output to the charging controller 320 and the voltage conversion controller 310 to enable the charging controller 320 to work. At this time, the charging controller 320 controls the charging current of the battery cell Bat by controlling the charging switch MP2, and the voltage conversion controller 310 stops working, and controls the first switch tube MP1 to be disconnected and the second switch tube MN1 to be disconnected. If it is determined that the charger is not connected, an invalid charging enable signal CHRG_ON is output to the charging controller 320 and the voltage conversion controller 310 to stop the charging controller 320 from working. At this time, the charging controller 320 controls the charging switch tube to be disconnected, and the voltage conversion controller 310 works normally.

When the discharge overvoltage signal OD_STATE is valid, the charge detection circuit 340 determines whether the voltage of the first voltage terminal P+ is higher than a predetermined reference voltage Vref_chg_od.

If the voltage of the first voltage terminal P+ is higher than the predetermined reference voltage Vref_chg_od, a valid charging enable signal CHRG_ON is output to the charging controller 320 and the voltage conversion controller 310, so that the charging controller 320 works. At this time, the discharge overvoltage signal OD_STATE is valid and a charger is connected. The charging controller 320 controls the charging current of the battery cell Bat by controlling the charging switch tube MP2, and the voltage conversion controller 310 switches the second switch tube MN2 from on to off. Subsequently, the charging detection circuit 320 determines whether a charger Charger is connected between the first voltage terminal P+ and the second voltage terminal P- by comparing the voltage of the first voltage terminal P+ with the voltage of the first battery cell connection terminal VB.

If the voltage of the first voltage terminal P+ is lower than the predetermined reference voltage Vref_chg_od, an invalid charging enable signal CHRG_ON is output to the charging controller 320 and the voltage conversion controller 310, so that the charging controller 310 stops working. At this time, the discharge overvoltage signal OD_STATE is valid and the charger is not connected. The charging controller 310 controls the charging switch tube MP2 to be turned off, and at the same time, the voltage conversion controller 310 controls the first switch tube MP1 to be turned off and the second switch tube MN1 to be turned on.

In one embodiment, in an optional interval between 0V and the voltage pulse amplitude value generated by the current pulse of the charger short-circuit output flowing through the turned-on second switch tube MN1, an appropriate voltage is selected as the predetermined reference voltage Vref_chg_od in the discharge overvoltage protection state (the discharge overvoltage signal OD_STATE is valid).

In a preferred embodiment, the first switch tube and the charging switch tube are PMOS (P-channel Metal Oxide Semiconductor) transistors, and the second switch tube is NMOS (N-channel Metal Oxide Semiconductor) transistor.

The drain and source of the first switch tube MP1 are its first connection terminal and second connection terminal, and the gate is its control terminal. When the control terminal of the first switch tube MP1 is at a high level, the first switch tube MP1 is disconnected, and when the control terminal of the first switch tube MP1 is at a low level, the first switch tube MP1 is turned on. The source of the first switch tube MP1 is coupled to the first cell connection terminal, and its drain is coupled to the intermediate node A. The drain and source of the second switch tube MN1 are its first connection terminal and second connection terminal, and the gate is its control terminal. When the control terminal of the second switch tube MN1 is at a high level, the second switch tube MN1 is turned on, and when the control terminal of the second switch tube MN1 is at a low level, the second switch tube MN1 is turned off. The source of the second switch tube is coupled to the second voltage terminal, and its drain is coupled to the intermediate node A. The drain and source of the charging switch tube MP2 are its first connection terminal and second connection terminal, and its gate is its control terminal. When the control terminal of the charging switch tube MP2 is at a high level, the charging switch tube MP2 is disconnected, and when the control terminal of the charging switch tube MP2 is at a low level, the charging switch tube MP2 is turned on. The source of the charging switch tube is coupled to the middle node A, and the drain of the charging switch tube is coupled to the first battery cell connection terminal.

As shown in FIG. 3 , a load LOAD or a charger is coupled between the first voltage terminal P+ and the second voltage terminal P−.

In the present invention, when the discharge overvoltage signal is valid (discharge overvoltage protection state) and the charger is not connected, the voltage conversion controller stops working and controls the first switch tube MP1 to be disconnected and the second switch tube MN1 to be turned on, instead of directly turning off the second switch tube MN1 as in the prior art, so that when the battery cell is used in series, the risk caused by the parasitic body diode D1 of the second switch MN1 described in the prior art is eliminated, and damage to the device or chip is avoided. In other words, the battery cell Bat in the power management circuit 300 of the present invention can be used in series with other battery cells, and there will be no risk as in the prior art. In addition, when the discharge overvoltage signal is valid and the charger is connected, the voltage conversion controller stops working and controls the first switch tube to be disconnected and the second switch tube to be disconnected, so that the first voltage terminal and the second voltage terminal can be used for charging. Moreover, when the discharge overvoltage signal is valid, the condition for the charging detection circuit to determine whether the charger is connected becomes whether the voltage of the first voltage terminal is higher than the predetermined reference voltage, so that the hidden dangers of series application are eliminated while realizing the sharing of the load and charger ports in the battery management circuit.

In the present invention, words such as “connect”, “connected”, “connected”, “connected”, “coupled”, etc. indicating electrical connection, unless otherwise specified, indicate direct or indirect electrical connection.

It should be noted that any changes made by those skilled in the art to the specific embodiments of the present invention do not deviate from the scope of the claims of the present invention. Accordingly, the scope of the claims of the present invention is not limited to the above specific embodiments.

Claims (14)

1. A power management circuit, characterized in that it comprises: A first voltage terminal and a second voltage terminal capable of being coupled to a charger or a load; A battery cell unit, comprising a first battery cell connection end and a second battery cell connection end, wherein the second battery cell connection end is conductively connected to the second voltage end; A conversion output circuit coupled to the first voltage terminal and the second voltage terminal, comprising a first switch tube, a second switch tube and an output inductor, wherein the first battery cell connection terminal provides current to the output inductor via the first switch, and the second voltage terminal provides current to the output inductor via the second switch; Voltage conversion controller; When the discharge voltage is over-voltage and the charger is not connected, the voltage conversion controller controls the first switch tube to be turned off and the second switch tube to be turned on. 2. The power management circuit according to claim 1, characterized in that: The first switch tube is coupled between the first battery cell connection terminal and the intermediate node A, the output inductor is coupled between the intermediate node A and the first voltage terminal, the second battery cell connection terminal is coupled to the second voltage terminal, and the second switch tube is coupled between the intermediate node A and the second voltage terminal. The voltage conversion controller is coupled between the first cell connection terminal and the second cell connection terminal, a first control output terminal thereof is coupled to the control terminal of the first switch tube, and a second control output terminal thereof is coupled to the control terminal of the second switch tube. 3. The power management circuit according to claim 1, further comprising: A discharge overvoltage detection circuit coupled between the first battery cell connection terminal and the second battery cell connection terminal detects whether the battery cell unit is discharged overvoltage, and when the battery cell unit is discharged overvoltage, outputs a valid discharge overvoltage signal to the voltage conversion controller, and when the battery cell unit is not discharged overvoltage, outputs an invalid discharge overvoltage signal to the voltage conversion controller. When the discharge overvoltage signal is invalid and the charger is not connected, the voltage conversion controller controls the first switch tube and the second switch tube to be turned on alternately to output a predetermined output voltage through the first voltage terminal and the second voltage terminal. 4. The power management circuit according to claim 1, characterized in that: When a charger is connected, the voltage conversion controller controls the first switch tube to be turned off and the second switch tube to be turned off. 5. The power management circuit according to claim 3, characterized in that the discharge overvoltage detection circuit determines whether the battery cell unit is discharged overvoltage by comparing the voltage value of the first battery cell connection terminal with a predetermined voltage threshold. 6. The power management circuit according to claim 3, wherein the first voltage terminal is coupled to a feedback input terminal of the voltage conversion controller, When the voltage conversion controller works normally, the voltage conversion controller controls the first switch tube and the second switch tube to be turned on alternately according to the voltage of the first voltage terminal, so that the first voltage terminal and the second output terminal output the predetermined output voltage. 7. The power management circuit according to claim 1 is characterized in that the second switch tube includes multiple second switch units, and when the discharge overvoltage signal is valid and the charger is not connected, the voltage conversion controller stops working and controls the first switch tube to be disconnected and at least controls part of the second switch units to be turned on. 8. The power management circuit according to claim 3, further comprising: A charging detection circuit determines whether a charger is connected between the first voltage terminal and the second voltage terminal, wherein a control input terminal of the charging detection circuit receives a discharge overvoltage signal from the discharge overvoltage detection circuit, and an output terminal of the charging detection circuit outputs a charging enable signal, wherein the charging enable signal indicates whether a charger is connected between the first voltage terminal and the second voltage terminal. 9. The power management circuit according to claim 8, characterized in that: When the discharge overvoltage signal is invalid, the charging detection circuit determines whether a charger is connected between the first voltage terminal and the second voltage terminal by comparing the voltage of the first voltage terminal with the voltage of the first battery cell connection terminal, and outputs a valid charging enable signal if it is determined that the charger is connected, and outputs an invalid charging enable signal if it is determined that the charger is not connected; When the discharge overvoltage signal is valid, the charging detection circuit determines whether the voltage at the first voltage terminal is higher than a predetermined reference voltage. If the voltage at the first voltage terminal is higher than a predetermined reference voltage, a valid charging enable signal is output. If the voltage of the first voltage terminal is lower than a predetermined reference voltage, an invalid charging enable signal is output. 10. The power management circuit according to claim 9, characterized in that: When the charging detection circuit determines whether a charger is connected between the first voltage terminal and the second voltage terminal by comparing the voltage of the first voltage terminal with the voltage of the first battery cell connection terminal, When the voltage at the first voltage terminal is higher than the voltage at the first battery cell connection terminal, it is considered that a charger is connected. When the voltage at the first voltage terminal is lower than the voltage at the first battery cell connection terminal, it is considered that the charger is not connected. 11. The power management circuit according to claim 9, characterized in that: The predetermined reference voltage is a voltage between 0V and a voltage pulse amplitude value generated when a current pulse output by a short-circuit charger flows through the turned-on second switch tube. 12. The power management circuit according to claim 8, further comprising: Charging switch tube; A charging controller, whose charging control output terminal is coupled to the control terminal of the charging switch tube; When the charging enable signal is valid, the charging controller controls the charging current of the battery cell unit by controlling the charging switch tube. When the charging enable signal is invalid, the charging controller controls the charging switch tube to be disconnected. 13. The power management circuit according to claim 2, characterized in that: The first switch tube is a PMOS transistor, whose drain and source are its first connection terminal and second connection terminal, and its gate is its control terminal. When the control terminal of the first switch tube is at a high level, the first switch tube is disconnected, and when the control terminal of the first switch tube is at a low level, the first switch tube is turned on. The source of the first switch tube is coupled to the first battery cell connection terminal, and the drain is coupled to the intermediate node A. The second switch tube is an NMOS transistor, whose drain and source are its first connection end and second connection end, and its gate is its control end. When the control end of the second switch tube is at a high level, the second switch tube is turned on, and when the control end of the second switch tube is at a low level, the second switch tube is turned off. The source of the second switch tube is coupled to the second voltage end, and the drain is coupled to the intermediate node A. 14. The power management circuit according to claim 12, characterized in that: The charging switch tube is a PMOS transistor, whose drain and source are its first connection end and second connection end, and its gate is its control end. When the control end of the charging switch tube is at a high level, the charging switch tube is disconnected, and when the control end of the charging switch tube is at a low level, the charging switch tube is turned on.