JP2018033232A - Charger - Google Patents

Abstract

A charging device capable of continuing to charge a secondary battery even when the power supply capability of an input power supply fluctuates can be reduced in size and cost. A charging device of the present invention includes a DC-DC converter, a charging circuit, a power supply voltage detection circuit, and an output voltage setting circuit. The charging circuit is connected in series to a secondary battery. The charging current limiting resistor R8 that limits the charging current Iout of the secondary battery 20 is included. The charging current limiting resistor R8 charges the secondary battery 20 when the output voltage of the DC-DC converter 11 is the rated charging voltage V1. The current Iout has a resistance value at which the charging current becomes the maximum, and the charging current of the secondary battery 10 is lower than the maximum charging current on the condition that the input voltage Ve of the charging device 10 has decreased to the first threshold voltage Vth1 or less. The output voltage of the DC-DC converter 11 is set to the charging voltage V2. [Selection] Figure 1

Description

  The present invention relates to a charging device for a secondary battery such as a nickel metal hydride secondary battery.

  Secondary batteries such as nickel metal hydride secondary batteries can be used repeatedly by charging and are widely used in various fields. In such a charging device for charging a secondary battery, it is necessary to appropriately control a charging voltage and a charging current when charging the secondary battery. In order to complete the charging of the secondary battery in as short a time as possible, it is desirable to charge with a charging current as large as possible within a range where the secondary battery does not deteriorate.

  However, an input power source that supplies power to a charging device that charges a secondary battery often has a power supply capability that is not always constant. For example, when the input power supply supplies power to other devices and the load on the devices can fluctuate, the maximum power that can be supplied to the charging device can fluctuate. Therefore, for example, in a charging device, if the secondary battery is charged with a maximum charging current that can be charged in the shortest time, the power supply capacity of the input power supply is insufficient and the voltage of the input power supply decreases. In such a case, there is a possibility that the charging voltage of the minimum necessary voltage cannot be obtained and the charging of the secondary battery has to be interrupted.

  As an example of the prior art aiming to solve such a problem, when charging a secondary battery with the output voltage of a DC-DC converter, according to fluctuations in the voltage of a solar battery or the like as an input power supply A power supply device that adjusts output current characteristics (voltage droop start current) of a DC-DC converter is known (see, for example, Patent Document 1). As another conventional technique, when a decrease in input voltage is detected during charging of the secondary battery, an arithmetic process is performed to determine whether or not the secondary battery can be continuously charged with the current charging current. A charging device is known in which the setting of the operational amplifier circuit of the charging current control means is changed in accordance with the processing result to set the charging current to a low value (see, for example, Patent Document 2).

JP 2006-014526 A JP 2006-129619 A

  However, each of the above conventional techniques has a problem that it is difficult to reduce the size and cost of the charging device because the device configuration is large and the control procedure is complicated.

  In view of such circumstances, the present invention has been made, and the purpose thereof is to realize downsizing and cost reduction of a charging device that can continue to charge a secondary battery even if the power supply capability of the input power supply fluctuates. There is to do.

<First Aspect of the Present Invention>
According to a first aspect of the present invention, a DC-DC converter that converts DC power supplied from an input power source into constant-voltage DC power, and charging that charges a secondary battery with the DC voltage output by the DC-DC converter. A circuit, a power supply voltage detection circuit for detecting a voltage of the input power supply, an output voltage setting circuit for setting an output voltage of the DC-DC converter, and the charging circuit and the output voltage based on the voltage of the input power supply A control device that controls a setting circuit, and the charging circuit includes a charging current limiting resistor that is connected in series to the secondary battery and limits a charging current of the secondary battery, and the charging current limiting resistor is The DC-DC converter has a resistance value at which a charging current of the secondary battery becomes a maximum charging current when an output voltage of the DC-DC converter is a rated charging voltage, and the control device includes the DC-DC converter The output voltage of the secondary battery is set to the rated charging voltage and charging of the secondary battery is started, and the charging current of the secondary battery is set on the condition that the voltage of the input power source has dropped to a first threshold voltage or less. The charging device sets an output voltage of the DC-DC converter to a charging voltage lower than the rated charging voltage so as to be limited to a current lower than a maximum charging current.

  The charging of the secondary battery is started by setting the charging voltage to the rated charging voltage. Thereby, the secondary battery is charged with a constant current at the maximum charging current. At this time, it is preferable to gradually increase the charging voltage of the secondary battery to the rated charging voltage. Here, the maximum charging current is, for example, the maximum value of the charging current that can flow in a range in which the secondary battery does not deteriorate, and is a current that is large enough to charge the secondary battery in the shortest time. Therefore, if the power supply capacity of the input power supply is sufficient and the secondary battery can be charged with the maximum charging current until the secondary battery is fully charged, the secondary battery is fully charged in the shortest time. be able to.

  On the other hand, when the power supply capacity of the input power supply is insufficient and the voltage of the input power supply drops below the first threshold voltage, the charging current of the secondary battery is limited to a current lower than the maximum charging current. The charging voltage is set to a voltage lower than the charging voltage, and the secondary battery is subsequently charged. The charging voltage at this time is a voltage that is lower than the rated charging voltage and that is capable of charging the secondary battery with an optimal charging current until full charging. By reducing the charging voltage of the secondary battery to a charging voltage lower than the rated charging voltage, constant current charging of the secondary battery is performed at a current smaller than the maximum charging current, and as a result, the secondary battery is consumed for charging. The power that is used is reduced.

  In other words, when the power supply capacity of the input power supply is reduced, the power consumed by the secondary battery is reduced, so the charging time becomes longer, but a charging voltage higher than the necessary minimum voltage is maintained. Charging of the secondary battery can be continued. As a result, when the power supply capability of the input power supply is reduced, the possibility that the charging voltage of the secondary battery is lowered below a necessary minimum voltage and the charging of the secondary battery has to be interrupted can be reduced. Therefore, even if the power supply capability of the input power supply fluctuates, the secondary battery can be continuously charged.

  In the charging device according to the present invention, the charging voltage of the secondary battery is defined by the output voltage of the DC-DC converter, and the charging current of the secondary battery is defined by the current limiting resistor connected in series to the secondary battery. Is done. Therefore, the power consumed for charging the secondary battery is automatically increased or decreased when the output voltage of the DC-DC converter is variably set. That is, in the charging device according to the present invention, the output voltage of the DC-DC converter is variably set according to the decrease in the voltage of the input power supply so that the charging current of the secondary battery is limited to a current lower than the maximum charging current. Since the configuration is simple, a charging device that can continue to charge the secondary battery even if the power supply capability of the input power source varies can be configured in a small size and at low cost.

  As a result, according to the first aspect of the present invention, it is possible to achieve a reduction in the size and cost of the charging device that can continue to charge the secondary battery even if the power supply capability of the input power supply varies. It is done.

<Second Aspect of the Present Invention>
According to a second aspect of the present invention, in the first aspect of the present invention described above, the control device is higher than the first threshold voltage after the voltage of the input power supply drops below the first threshold voltage. A charging device that sets the output voltage of the DC-DC converter to the rated charging voltage on condition that the voltage of the input power source has risen above a second threshold voltage.
When the power supply capacity of the input power supply is insufficient and the voltage of the input power supply drops below the first threshold voltage, the rated charge voltage is set so that the charging current of the secondary battery is limited to a current lower than the maximum charging current. The charging voltage is set to a lower voltage, and the secondary battery is subsequently charged. When the power supply capability of the input power supply recovers during charging and the voltage of the input power supply rises above the second threshold voltage, the setting of the output voltage of the DC-DC converter is changed to the rated charge voltage. That is, when the power supply capability of the input power source is restored during charging, the secondary battery is thereafter charged with a constant current with the maximum charging current. Therefore, according to the second aspect of the present invention, the secondary battery can be charged by automatically and flexibly setting the charging current in accordance with fluctuations in the power supply capability of the input power supply.

<Third Aspect of the Present Invention>
According to a third aspect of the present invention, in the first or second aspect of the present invention described above, the output voltage setting circuit includes a voltage dividing circuit that divides an output voltage of the DC-DC converter, and the voltage dividing circuit. A voltage dividing ratio changing circuit for changing a voltage dividing ratio of the voltage dividing circuit, wherein the DC-DC converter controls an output voltage so that a voltage at a voltage dividing point of the voltage dividing circuit is maintained at a predetermined voltage. Device.

  According to the third aspect of the present invention, since the output voltage of the DC-DC converter can be variably set with a very simple circuit configuration, further miniaturization and cost reduction are realized in the charging device according to the present invention. can do.

  ADVANTAGE OF THE INVENTION According to this invention, size reduction and cost reduction of the charging device which can continue charge of a secondary battery even if the power supply capability of an input power supply changes are realizable.

1 is a circuit diagram illustrating a configuration of a charging device according to the present invention. The timing chart which illustrated an example of charge control in case there is sufficient margin in the power supply capability of an input power supply. The timing chart which illustrated an example of charge control in case the power supply capability of an input power supply is insufficient. 6 is a timing chart illustrating an example of charge control when the power supply capability of the input power source is restored during charging of the secondary battery. The circuit diagram which illustrated the principal part of the output voltage setting circuit. The circuit diagram which illustrated the principal part of the output voltage setting circuit.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
In addition, this invention is not specifically limited to the Example demonstrated below, It cannot be overemphasized that a various deformation | transformation is possible within the range of the invention described in the claim.

<Configuration of charging device 10>
The configuration of the charging apparatus 10 according to the present invention will be described with reference to FIG.
FIG. 1 is a circuit diagram illustrating the configuration of a charging device 10 according to the present invention.

  The charging device 10 includes a DC-DC converter 11, a charging circuit 12, a power supply voltage detection circuit 13, an output voltage setting circuit 14, and a charging control unit 15. The secondary battery 20 is a secondary battery such as a nickel hydride secondary battery.

  The DC-DC converter 11 is a constant voltage power source that converts DC power supplied from an input power source (not shown) connected to the input terminal IN and the ground terminal GND into constant voltage DC power. More specifically, the DC-DC converter 11 is a step-down converter that steps down the input voltage Ve of the charging device 10 and outputs a constant voltage. The DC-DC converter 11 is not particularly limited to a step-down converter, and may be a step-up converter or a step-up / step-down converter, for example.

  The charging circuit 12 is a circuit that charges the secondary battery 20 with a DC voltage output from the DC-DC converter 11, and includes a transistor Q1 and a charging current limiting resistor R8. The transistor Q1 is a semiconductor switch that opens and closes a charging path from the DC-DC converter 11 to the secondary battery 20, and is an NPN bipolar transistor in this embodiment. The transistor Q 1 has a collector connected to the output of the DC-DC converter 11 and an emitter connected to the positive electrode of the secondary battery 20. The base of the transistor Q1 is connected to the charge control unit 15, and is ON / OFF controlled by the charge control unit 15.

  The charging current limiting resistor R8 is a resistor that is connected in series to the secondary battery 20 and limits the charging current Iout of the secondary battery 20. In the charging current limiting resistor R8, when the output voltage of the DC-DC converter 11 is the rated charging voltage V1, the charging current Iout of the secondary battery 20 is the maximum charging current (first charging current I1 (FIGS. 2 to 4)). The resistance value is as follows.

  The power supply voltage detection circuit 13 is a circuit that detects the input voltage Ve of the charging device 10, and is a voltage dividing circuit that includes two resistors R1 and R2. The resistor R1 has one end connected to the input terminal IN and the other end connected to one end of the resistor R2. The other end of the resistor R2 is connected to the ground terminal GND. A connection point between the resistors R1 and R2 is connected to the charge control unit 15.

  The output voltage setting circuit 14 is a circuit for setting the output voltage of the DC-DC converter 11 and includes a voltage dividing circuit 141 and a voltage dividing ratio changing circuit 142.

  The voltage dividing circuit 141 is a circuit that divides the output voltage of the DC-DC converter 11, and includes two resistors R3 and R4. The resistor R3 has one end connected to the output of the DC-DC converter 11 and the other end connected to one end of the resistor R4. The other end of the resistor R4 is connected to the ground terminal GND. A connection point (voltage dividing point) between the resistor R3 and the resistor R4 is connected to a feedback control terminal of the DC-DC converter 11. The DC-DC converter 11 controls the output voltage so that the voltage at the connection point between the resistor R3 and the resistor R4 is maintained at a predetermined voltage.

  The voltage dividing ratio changing circuit 142 is a circuit that changes the voltage dividing ratio of the voltage dividing circuit 141, and includes three resistors R5 to R7 and a transistor Q2. The resistor R5 has one end connected to a connection point between the resistors R3 and R4, and the other end connected to the collector of the transistor Q2. The resistor R6 has one end connected to the output of the DC-DC converter 11 and the other end connected to one end of the resistor R7. The other end of the resistor R7 is connected to the charge control unit 15. An amplifier or a D / A converter may be connected between the resistor R7 and the charging control unit 15. The transistor Q2 is a semiconductor switch that opens and closes the parallel connection of the resistor R5 to the resistor R3 of the voltage dividing circuit 141, and is a PNP bipolar transistor in this embodiment. The transistor Q2 has a collector connected to the other end of the resistor R5, an emitter connected to the output of the DC-DC converter 11, and a base connected to a connection point between the resistor R6 and the resistor R7.

  The charge control unit 15 is a known microcomputer control device, and is a control device that executes charge control of the secondary battery 20. The charging control unit 15 may be a control circuit using an amplifier other than the microcomputer control device. The charging control unit 15 further controls the charging circuit 12 and the output voltage setting circuit 14 based on the input voltage Ve of the charging device 10. More specifically, the charging control unit 15 performs ON / OFF control of the transistor Q1 and base current control of the transistor Q2 based on the voltage at the connection point between the resistors R1 and R2.

<Operation of Charging Device 10>
The operation of the charging device 10 will be described with reference to FIGS.
FIG. 2 is a timing chart illustrating the operation of the charging apparatus 10 and illustrates an example of charging control when there is a sufficient margin in the power supply capability of the input power supply.

The charging control unit 15 sets the output voltage of the DC-DC converter 11 to the rated charging voltage V1 and starts charging the secondary battery 20 (timing T1). More specifically, the charge control unit 15 turns on the transistor Q1 with the charge control voltage _VA lowered. Then, since the base current I _Q2B of the transistor Q2 becomes the maximum, the operation starts with the charging voltage Vout and the charging current Iout being minimum. By gradually increasing the charging control voltage V _A , the base current I _Q2B of the transistor Q2 gradually decreases. As a result, the charging voltage Vout and the charging current Iout of the secondary battery 20 increase, and the input voltage Ve of the charging device 10 decreases.

  When there is a sufficient margin in the power supply capability of the input power supply, the charging voltage Vout of the secondary battery 20 reaches the rated charging voltage V1 before the input voltage Ve of the charging device 10 drops below the first threshold voltage Vth1. As a result, the charging current Iout of the secondary battery 20 rises to the first charging current I1 (timing T2). Therefore, the secondary battery 20 is charged with the first charging current I1, which is the maximum charging current. Here, the maximum charging current is, for example, the maximum value of the charging current Iout that can flow within a range in which the secondary battery 20 does not deteriorate, and is a current that is large enough to charge the secondary battery 20 in the shortest time. is there.

  While the secondary battery 20 is being charged, that is, while the transistor Q1 is ON, the charging control unit 15 determines the secondary battery 20 from the temperature of the secondary battery 20 detected by a temperature sensing element (not shown) such as a thermistor. Detect the state of charge. Then, the charging control unit 15 turns off the transistor Q1 when the charging state of the secondary battery 20 becomes fully charged, and ends the charging of the secondary battery 20 (timing T3). In this way, when there is a sufficient margin in the power supply capability of the input power supply, the secondary battery 20 can be charged with the first charging current I1 which is the maximum charging current. Can be fully charged.

  FIG. 3 is a timing chart illustrating the operation of the charging apparatus 10 and illustrates an example of charging control when the power supply capability of the input power supply is insufficient.

  The charging control unit 15 sets the output voltage of the DC-DC converter 11 to the rated charging voltage V1 and starts charging the secondary battery 20 (timing T11). As a result, the charging voltage Vout and the charging current Iout of the secondary battery 20 increase, and the input voltage Ve of the charging device 10 decreases. When the power supply capability of the input power supply is insufficient, the input voltage Ve of the charging device 10 is reduced to the first threshold voltage Vth1 or less before the charging voltage Vout of the secondary battery 20 rises to the rated charging voltage V1. Decrease (timing T12).

  The first threshold voltage Vth1 detects a state in which the power supply capability of the input power supply is insufficient when the secondary battery 20 is charged at a constant current with the rated charging voltage V1 and the maximum charging current (first charging current I1). Is for. Therefore, the first threshold voltage Vth1 is set to a voltage equal to or higher than the lower limit of the input voltage of the DC-DC converter 11 that can output the rated charging voltage V1, for example. The first threshold voltage Vth1 may be set, for example, as a potential difference ΔV between the rated value and detected value of the input voltage Ve of the charging device 10.

When the input voltage Ve of the charging device 10 drops below the first threshold voltage Vth1, the charging control unit 15 stops the charging control voltage _VA and returns the applied voltage to the previous state (timing) T12). As a result, the current flowing through the resistor R4 of the voltage dividing circuit 141 is fixed. By doing so, the voltage dividing ratio of the voltage dividing circuit 141 is changed. Due to the change of the voltage dividing ratio, the voltage at the voltage dividing point of the voltage dividing circuit 141 (the connection point between the resistors R3 and R4) rises, so that the output voltage of the DC-DC converter 11 decreases, and as a result, the secondary battery 20 The output voltage of the DC-DC converter 11 is automatically reduced to the charging voltage V2 so that the charging current is limited to a current lower than the maximum charging current (first charging current I1). That is, the charging control unit 15 is configured such that the charging current of the secondary battery 20 is lower than the maximum charging current (first charging current I1) on condition that the input voltage Ve of the charging device 10 has decreased to the first threshold voltage Vth1 or less. The output voltage of the DC-DC converter 11 is set to a charging voltage V2 lower than the rated charging voltage V1 so as to be limited to (second charging current I2). This charging voltage V2 is a voltage lower than the rated charging voltage V1, and is a voltage that can charge the secondary battery 20 to full charge with the maximum current of the input power supply capability. The voltage is set to the same voltage as the minimum required charging voltage of the secondary battery 20 or higher.

  By reducing the output voltage of the DC-DC converter 11 to the charging voltage V2 so that the charging current of the secondary battery 20 is limited to a current lower than the maximum charging current (first charging current I1), the secondary battery 20 Charging current Iout decreases to the second charging current I2, whereby the charging voltage Vout of the secondary battery 20 decreases to the charging voltage V2 (timing T13). That is, when the power supply capability of the input power supply is insufficient and the input voltage Ve of the charging device 10 decreases to the first threshold voltage Vth1 or less, the charging current of the secondary battery 20 is the maximum charging current (first charging current I1). The secondary battery 20 is continuously charged at a charging voltage V2 lower than the rated charging voltage V1 so as to be limited to a lower current (second charging current I2). Then, by reducing the charging voltage Vout of the secondary battery 20 to the charging voltage V2, the constant current charging of the secondary battery 20 is performed with the second charging current I2 smaller than the maximum charging current, and as a result, the secondary battery The power consumed by charging 20 is reduced. Then, when the charging state of the secondary battery 20 becomes fully charged, the charging control unit 15 turns off the transistors Q1 and Q2 and ends the charging of the secondary battery 20 (timing T14).

  Thus, when the power supply capability of the input power supply is reduced, the power consumed by charging the secondary battery 20 is reduced, so that the charging time is increased by that amount, but the maximum power supply capability of the input power supply is increased. The charging of the secondary battery 20 can be continued while maintaining the charging current (second charging current I2). As a result, when the power supply capability of the input power supply is reduced, the possibility that the charging voltage Vout of the secondary battery 20 drops below a necessary minimum voltage and the charging of the secondary battery 20 has to be interrupted is reduced. Therefore, charging of the secondary battery 20 can be continued even if the power supply capability of the input power supply fluctuates.

  In the charging device 10 according to the present invention, the charging voltage Vout of the secondary battery 20 is defined by the output voltage of the DC-DC converter 11, and the charging current Iout of the secondary battery 20 is connected in series to the secondary battery 20. Defined by the charging current limiting resistor R8. Therefore, the power consumed for charging the secondary battery 20 is automatically increased or decreased when the output voltage of the DC-DC converter 11 is variably set. That is, the charging device 10 according to the present invention outputs the output of the DC-DC converter 11 in accordance with a decrease in the input voltage Ve of the charging device 10 so that the charging current of the secondary battery 10 is limited to a current lower than the maximum charging current. Since it is a simple configuration in which the voltage is variably set, the configuration can be reduced in size and cost. Therefore, according to the present invention, it is possible to reduce the size and cost of the charging device 10 that can continue charging the secondary battery 20 even if the power supply capability of the input power supply varies.

  FIG. 4 is a timing chart illustrating the operation of the charging device 10, and illustrates an example of charging control when the power supply capability of the input power source is restored during the charging of the secondary battery 20.

  The charging control unit 15 sets the output voltage of the DC-DC converter 11 to the rated charging voltage V1 and starts charging the secondary battery 20 (timing T21). As a result, the charging voltage Vout and the charging current Iout of the secondary battery 20 increase, and the input voltage Ve of the charging device 10 decreases. When the power supply capability of the input power supply is insufficient, the input voltage Ve of the charging device 10 is reduced to the first threshold voltage Vth1 or less before the charging voltage Vout of the secondary battery 20 rises to the rated charging voltage V1. Decrease (timing T22).

When the input voltage Ve of the charging device 10 drops below the first threshold voltage Vth1, the charging control unit 15 stops the charging control voltage _VA and returns the applied voltage to the previous state (timing) T22). As a result, the output voltage of the DC-DC converter 11 decreases to the charging voltage V2. Then, the output voltage of the DC-DC converter 11 is reduced to the charging voltage V2 so that the charging current of the secondary battery 10 is limited to a current lower than the maximum charging current, whereby the charging voltage Vout of the secondary battery 20 is charged. The voltage V2 is lowered to the maximum, and the charging current Iout of the secondary battery 20 can continue to flow with the maximum supply capacity of the input power supply (timing T23).

The charging control unit 15 is configured when the power supply capability of the input power supply is recovered before the charging state of the secondary battery 20 becomes fully charged, and the input voltage Ve of the charging device 10 rises to the second threshold voltage Vth2 or more. Starts to increase the charging control voltage _VA at that time (timing T24). As a result, the resistor R5 is not connected in parallel to the resistor R3 of the voltage dividing circuit 141, so that the voltage dividing ratio of the voltage dividing circuit 141 is changed to the original voltage dividing ratio. Due to the change of the voltage dividing ratio, the voltage at the voltage dividing point of the voltage dividing circuit 141 (the connection point between the resistors R3 and R4) is lowered, so that the output voltage of the DC-DC converter 11 rises, and as a result, the DC-DC converter. 11 output voltage rises to the rated charge voltage V1. That is, after the input voltage Ve of the charging device 10 decreases to the first threshold voltage Vth1 or lower, the charging control unit 15 increases the input voltage Ve of the charging device 10 to the second threshold voltage Vth2 or higher that is higher than the first threshold voltage Vth1. As a condition, the output voltage of the DC-DC converter 11 is set to the rated charging voltage V1.

  The second threshold voltage Vth2 indicates that the power supply capability of the input power source has been restored to such an extent that the secondary battery 20 can be charged with a constant current at the rated charging voltage V1 and the maximum charging current (first charging current I1). It is for detection. Therefore, the second threshold voltage Vth2 is set to a voltage at least equal to or higher than the first threshold voltage Vth1, and is preferably set to a voltage higher than the first threshold voltage Vth1. The second threshold voltage Vth2 may be set as a potential difference ΔV between the rated value and the detected value of the input voltage Ve of the charging device 10, for example.

  As the output voltage of the DC-DC converter 11 increases to the rated charging voltage V1, the charging voltage Vout and the charging current Iout of the secondary battery 20 increase and the input voltage Ve of the charging device 10 decreases. . Then, before the input voltage Ve of the charging device 10 drops below the first threshold voltage Vth1, the charging voltage Vout of the secondary battery 20 rises to the rated charging voltage V1, thereby the charging current Iout of the secondary battery 20 becomes the first It rises to 1 charging current I1 (timing T25). Therefore, the secondary battery 20 is charged with the first charging current I1, which is the maximum charging current, after the power supply capability of the input power supply is restored. Then, when the charging state of the secondary battery 20 is fully charged, the charging control unit 15 turns off the transistor Q1 and ends the charging of the secondary battery 20 (timing T26).

  Even when the power supply capability of the input power source is recovered during charging even if the power supply capability of the input power source is insufficient and the secondary battery 20 is charged with the charging voltage V2. The setting of the output voltage of the DC-DC converter 11 is changed from the charging voltage V2 to the rated charging voltage V1. That is, when the power supply capability of the input power source is restored during charging, the secondary battery 20 is then charged with a constant current with the maximum charging current (first charging current I1). Accordingly, the secondary battery 20 can be charged by setting the charging current Iout flexibly and accurately in accordance with the fluctuation of the power supply capability of the input power supply.

  5 and 6 are circuit diagrams illustrating the main part of the output voltage setting circuit 14. FIG. 5 shows a state in which the transistor Q2 is OFF, and FIG. 6 shows a state in which the transistor Q2 is ON. It is illustrated.

The current i3 is a current that flows through the resistor R3. The current i4 is a current that flows through the resistor R4. The current i5 is a current that flows through the resistor R5. The voltage Vfe is a voltage at the connection point between the resistor R3 and the resistor R4. As described above, the DC-DC converter 11 controls the output voltage so that the voltage Vfe is maintained at a constant voltage. The output voltage of the DC-DC converter 11 when charging the secondary battery 20, that is, the charging voltage Vout is expressed by the following formula (1).
Vout = R3 × i3 + Vfe (1)
In a state where the transistor Q2 is OFF (FIG. 5), i3 = i4. The voltage Vfe is expressed by the following equation (2).
Vfe = R4 × i4 (2)
On the other hand, in the state where the transistor Q2 is ON (FIG. 6), the charging current of the secondary battery 20 is constant current control. Since i4 = i3 + i5, the voltage Vfe is expressed by the following equation (3).
Vfe = R4 × (i3 + i5) (3)
DC-DC converter 11, for controlling the output voltage so that the voltage Vfe is maintained at a constant voltage, the current i5 relative to the base current I _Q2B of the transistor Q2 increases to increase, whereby the current i3 decreases To go. As the current i3 decreases, the charging voltage Vout decreases accordingly.

DESCRIPTION OF SYMBOLS 10 Charging apparatus 11 DC-DC converter 12 Charging circuit 13 Power supply voltage detection circuit 14 Output voltage setting circuit 15 Charge control part 20 Secondary battery 141 Voltage dividing circuit 142 Voltage dividing ratio change circuit R8 Charging current limiting resistance

Claims (3)

A DC-DC converter that converts DC power supplied from an input power source into constant voltage DC power;
A charging circuit for charging the secondary battery with a DC voltage output from the DC-DC converter;
A power supply voltage detection circuit for detecting a voltage of the input power supply;
An output voltage setting circuit for setting an output voltage of the DC-DC converter;
A control device for controlling the charging circuit and the output voltage setting circuit based on the voltage of the input power supply,
The charging circuit includes a charging current limiting resistor connected in series to the secondary battery and limiting a charging current of the secondary battery, and the charging current limiting resistor has a rated charge when an output voltage of the DC-DC converter is rated. Having a resistance value at which the charging current of the secondary battery is the maximum charging current when the voltage is,
The control device sets the output voltage of the DC-DC converter to the rated charging voltage and starts charging the secondary battery, on the condition that the voltage of the input power source has dropped below a first threshold voltage. The charging device sets the output voltage of the DC-DC converter to a charging voltage lower than the rated charging voltage so that the charging current of the secondary battery is limited to a current lower than the maximum charging current.   2. The charging device according to claim 1, wherein after the voltage of the input power supply has decreased to be equal to or lower than the first threshold voltage, the control device is configured to have the input power supply not lower than a second threshold voltage higher than the first threshold voltage. A charging device that sets the output voltage of the DC-DC converter to the rated charging voltage on condition that the voltage has increased. 3. The charging device according to claim 1, wherein the output voltage setting circuit includes a voltage dividing circuit that divides an output voltage of the DC-DC converter, and a voltage dividing ratio changing circuit that changes a voltage dividing ratio of the voltage dividing circuit. , Including
The DC-DC converter is a charging device that controls an output voltage so that a voltage at a voltage dividing point of the voltage dividing circuit is maintained at a predetermined voltage.

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