JP2006158103A - Charging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simultaneously perform preferable charge of a secondary cell and power supply to a load using only a simple circuit constitution and required minimum power supply capability. <P>SOLUTION: This charger consists of a power supply device 1, and a charging portion 2 constituted of a charger 20, a current detecting resistor 21, and a voltage detector 22 to charge the secondary cell 3 and supply output voltage Vo to the load 4. The voltage detector 22 detects the load 4 and a voltage Vo of the secondary cell 3, and outputs a first voltage detecting signal V1 and a second voltage detecting signal V2 to the charger 20. The first voltage detecting signal V1 indicates a compared result between the output voltage Vo and a first prescribed voltage Vo1, and a second voltage detecting signal V2 indicates a compared result between the output voltage Vo and a second prescribed voltage Vo2. In this constitution, the first prescribed voltage Vo1 is set at a cell voltage at the time of full charge of the secondary cell 3, the second voltage detecting signal Vo2 is set to be lower than the first prescribed voltage Vo1 and higher than a driving lower limit voltage of the load 4. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a charger that is supplied with power from a power supply means such as an AC adapter to supply a power supply voltage to a load and charges a secondary battery such as a lithium ion battery.

  Generally, an electronic device such as a portable device includes a charger that uses a secondary battery such as a lithium ion battery as a power source and charges the secondary battery. Such a charger converts the input power supply voltage from a power supply means such as an AC adapter to charge the secondary battery and supplies the power supply voltage to the load. As such a conventional charger, there is a charger described in Patent Document 1, for example.

  FIG. 5 is a configuration diagram of the charger described in Patent Document 1. In FIG.

  In FIG. 5, reference numeral 1 denotes a power supply means such as an AC adapter, and 5 denotes a charging unit, which is composed of a constant current means 50, a charging current detection resistor 51, and a quick charge end detection means 52 to charge the secondary battery 3. In addition, the power supply voltage is supplied to the load 4. The constant current converting means 50 converts the input power supply voltage from the power supply means 1 into power and outputs a desired direct current. The charging current detection resistor 51 is connected in series to the secondary battery 3 to detect the charging current, and outputs a detection signal to the constant current converting means 50. Since the load current does not flow through the charging current detection resistor 51 but only the charging current flows, the constant current converting means 50 controls the output so that the charging current is constant, and the secondary battery 3 is rapidly charged. The quick charge end detection means 52 detects the charge voltage of the secondary battery 3 and outputs a detection signal to the constant current means 50 when the charge voltage reaches a predetermined value. As a result, the constant current means 50 reduces the charging current. As described above, the charging current to the secondary battery 3 can be kept constant regardless of the fluctuation of the load 4.

  Patent Document 2 discloses a charger that includes a control circuit that outputs a control signal for controlling driving of a load, and means for controlling a charging current to the secondary battery based on the control signal.

  6 is a configuration diagram of a charger in which the charging control method of Patent Document 2 is applied to the charger shown in FIG.

  In FIG. 6, reference numeral 61 denotes a control circuit that outputs a control signal Vc for controlling driving of the load 4, and this control signal Vc is input to the constant current converting means 60. The constant current means 60 adjusts the charging current to the secondary battery 3 according to the control signal Vc. Specifically, the charging current to the secondary battery 3 is reduced as the power consumption at the load 4 is increased.

With this configuration, the secondary battery 3 is not charged at a constant current regardless of the load 4 as in the charging unit 6, but the power consumed by the load 4 is subtracted from the power that can be supplied by the power supply means 1 and the charging unit 6. The charged electric power is applied to charge the secondary battery 3. Compared with the charging unit 5 of FIG. 5, if the load 4 is heavy, rapid charging with a constant current cannot be performed, but the maximum supply power of the power supply means 1 and the charging unit 6 can be small.
Japanese Utility Model Publication No. 2-122540 JP 2001-275272 A

  However, in the conventional configuration of Patent Document 1, the power supply means and the charging unit are required to be capable of supplying the sum of the constant current charging power to the secondary battery and the maximum power consumption at the load. The For this reason, the power supply means and the charging unit are increased in size and cost. You may prioritize the miniaturization of the charging unit from the advantage that it can be charged quickly with a constant current regardless of load fluctuations, but even in that case the constant current charging power in the charging unit is, of course, the load. The ability to supply the maximum power consumption is required.

  If the maximum power consumption at the load is greater than the constant current charging power and the charging unit does not have the ability to supply the maximum power consumption at the load, the charging unit is supplied to the load even though it is operating There is a problem that the secondary battery is discharged in order to compensate for the power shortage.

  Further, according to the charger of Patent Document 2, the secondary battery can be charged simultaneously with the driving of the load without increasing the size of the power supply means and the charging unit. However, a control circuit and a control signal for knowing the driving state of the load are necessary, and there is a problem that the circuit becomes complicated.

  In addition, in the two conventional chargers, a charging current detection resistor and a secondary battery are connected in series, and a load is connected in parallel with the series circuit in order to make the charging current constant regardless of the fluctuation of the load. It becomes the composition which is done.

  In such a configuration, when there is no power supply from the power supply means and the load is driven by the discharge current from the secondary battery, the discharge current, that is, the load current flows through the charging current detection resistor. There is also a problem that occurs.

  On the other hand, if the load current does not flow through the charging current detection resistor 71 when there is no power supply from the power supply unit 1 as in the circuit configuration of the charging unit 7 shown in FIG. When the load 4 is driven during power supply, the sum of the charging current and the load current for the secondary battery 3 flows through the charging current detection resistor 71. If the load 4 is not driven, it can be charged with a constant current set by the constant current converting means 70. However, if the load 4 is driven and the load current is larger than the charging current value at the time of constant current charging, the secondary battery 3 has a problem of being discharged.

  The present invention solves the above-described conventional problems, and preferably performs simultaneous charging of a secondary battery and power supply to a load while having only a simple circuit configuration and a necessary minimum power supply capability. An object of the present invention is to provide a battery charger that can be used.

  To achieve the above object, the present invention provides power supply means for supplying an input DC voltage, charging means for converting the input DC voltage to an output DC voltage, voltage detection means for detecting the output DC voltage, A charger for supplying the output DC voltage to a load, wherein the voltage detector is a fully charged secondary battery. A charge voltage or a voltage in the vicinity of the full charge voltage is set as the first predetermined voltage, a second predetermined voltage lower than the first predetermined voltage and higher than the drive lower limit voltage of the load is set, and the output DC voltage is set The first predetermined voltage is compared and amplified to output a first voltage detection signal, a comparison result between the output DC voltage and the second predetermined voltage is output as a second voltage detection signal, and the charging means Is the first voltage When an output signal, the second voltage detection signal, and a current detection signal from the current detection means are input, and the output DC voltage is less than or equal to the second predetermined voltage, an input / output short-circuit state is established, and the output DC voltage is When the output is controlled so that the current flowing through the current detection means is constant when the output voltage is higher than the second predetermined voltage and lower than the first predetermined voltage, and the output DC voltage reaches the second predetermined voltage The output is controlled so that the output DC voltage becomes the second predetermined voltage.

  Here, the current detection means may form a series circuit only with the secondary battery, or may form a series circuit with the load.

  The voltage detection means may include a hysteresis comparator as means for comparing the output DC voltage with the second predetermined voltage.

  Further, the voltage detecting means may be configured to detect a decrease in the output DC voltage and output a second voltage detection signal instead of comparing the output DC voltage with the second predetermined voltage.

  Furthermore, the input / output short-circuit state of the charging unit may be configured by providing a through switch between the input and output of the charging unit, or may be configured to cancel the constant current control by the current detection signal. Good.

  With the above configuration, the present invention allows the charging of the secondary battery by detecting the voltage drop of the secondary battery during constant current charging and releasing the charger from the constant current control and switching to the input / output short-circuit state. It is possible to simultaneously supply power to the load while securing a current.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a configuration diagram of a charger according to a first embodiment of the present invention.

  In FIG. 1, reference numeral 1 denotes a power supply means such as an AC adapter, and 2 denotes a charging unit, which comprises a charging means 20, a current detection resistor 21, and a voltage detection means 22, and charges the secondary battery 3 to the load 4. An output voltage Vo is supplied. The charging means 20 is constituted by, for example, a switching type step-down converter, and converts the input power supply voltage from the power supply means 1 to output a desired direct current.

  The current detection resistor 21 has a resistance value Rs, is connected in series to the secondary battery 3, detects the charging current Ic, and outputs a current detection signal Vc = Ic × Rs to the charging unit 20. The voltage detection means 22 detects the voltage Vo of the load 4 and the secondary battery 3, and outputs the first voltage detection signal V1 and the second voltage detection signal V2 to the charging means 20. The first voltage detection signal V1 indicates a comparison result between the output voltage Vo and the first predetermined voltage Vo1, and the second voltage detection signal V2 indicates a comparison result between the output voltage Vo and the second predetermined voltage Vo2. Here, the first predetermined voltage Vo1 is set to the battery voltage when the secondary battery 3 is fully charged, and the second predetermined voltage Vo2 is lower than the first predetermined voltage Vo1 and lower than the drive lower limit voltage of the load 4. Set to high voltage.

  In the first embodiment, the power supply means 1 outputs a predetermined DC power supply voltage Vi, and the output current is made constant when the output current reaches the maximum value Imax by the overcurrent protection function. The maximum output current Imax of the power supply means 1 is assumed to be larger than the maximum current Iomax consumed by the load 4.

  The charging unit 20 of the charging unit 2 is supplied with the current detection signal Vc, the first voltage detection signal V1, and the second voltage detection signal V2. When the second voltage detection signal V2 indicates that the output voltage Vo is lower than the second predetermined voltage Vo2, the charging unit 20 directly sets the output of the power supply unit 1 by short-circuiting the input and output. Perform output short-circuit mode operation.

  When the first voltage detection signal V1 and the second voltage detection signal V2 indicate that the output voltage Vo is higher than the second predetermined voltage Vo2 and lower than the first predetermined voltage Vo1, the charging unit 20 Performs a constant current mode operation for controlling the output so that the current detection signal becomes a constant value Vs. The charging current at this time is Ic = Vs / Rs.

  When the output voltage Vo reaches the first predetermined voltage Vo1, the charging unit 20 performs a constant voltage mode operation for controlling the output so that the output voltage Vo is stabilized at the first predetermined voltage Vo1. In the constant current mode operation and the constant voltage mode operation for controlling the output, the charging unit 20 becomes constant current when the output current reaches the maximum value Icm by the overcurrent protection function.

  The operation of the charger in the first embodiment configured as described above will be described with reference to FIGS.

  FIG. 2A shows changes with time of the output voltage Vo and the charging current Ic when the load 4 is not driven, that is, when the output of the charging unit 2 is all consumed for charging the secondary battery 3. In the initial charging stage (1), the output voltage Vo is lower than the second predetermined voltage Vo2, and the charging means 2 is in short-circuit mode operation. For this reason, the charging current Ic is rapidly charged with the maximum current Imax of the power supply means 1. When the output voltage Vo rises and becomes higher than the second predetermined voltage Vo2 in (2), the charging unit 2 enters the constant current mode operation and charges the secondary battery 3 with constant current at Ic = Vs / Rs. Further, in (3), when the output voltage Vo rises and reaches the first predetermined voltage Vo1, the charging means 2 enters the constant voltage mode operation, and the charging means 20 stabilizes the output voltage Vo to the first predetermined voltage Vo1. Control. For this reason, the charging current Ic decreases with time.

  Next, with reference to FIG. 2B, the operation when the load 4 is driven during the constant current mode operation and the load current Io increases to reach the maximum current Iomax will be described. In the constant current mode operation, the load current does not flow through the current detection resistor 21, but only the charging current flows. Therefore, in the time region (1) where the load current Io is small, the secondary battery 3 is charged with a constant current at Ic = Vs / Rs. At this time, the output current of the charging means 20 is the sum of the charging current Ic = Vs / Rs and the load current Io. When this output current becomes the maximum output current Icm of the charging means 20, as shown in (2), the charging current Ic decreases as the load current Io increases.

  When the load current Io further increases and exceeds the maximum output current Icm of the charging means 20, as shown in (3), the charging current Ic becomes negative and the secondary battery 3 starts to be discharged. The voltage Vo also decreases.

  When the lowered output voltage Vo becomes lower than the second predetermined voltage Vo2, as shown in (4), the charging unit 2 enters a short-circuit mode operation, and the maximum current Imax from the power supply unit 1 is supplied. Therefore, the secondary battery 3 is charged again, and the charging current is a value obtained by subtracting the load current Io from the maximum current Imax from the power supply means 1 (Imax−Io).

  Even when the load current reaches the maximum value Iomax, since Imax> Iomax, the secondary battery 3 is secured with a charging current. When the secondary battery 3 is charged, the output voltage Vo rises again and becomes higher than the second predetermined voltage Vo2, and the charging means 20 enters the constant current mode operation. At this time, the load current Io is the maximum output of the charging means 20. If it is larger than the current Icm, the secondary battery 3 is discharged and the output voltage Vo decreases. That is, when the load current Io is larger than the maximum output current Icm of the charging means 20, the secondary battery 3 repeats charging / discharging, and the output voltage Vo rises and falls near the second predetermined voltage Vo2.

  FIG. 3 is a configuration diagram illustrating an example of a circuit configuration of the charging unit 2 illustrated in FIG. 1.

  In FIG. 3, the voltage detection means 22 includes a reference voltage source 220, a resistor 221, a resistor 222, a resistor 223 that divides the output voltage Vo, and a reference voltage Vr, a resistor 221, and a resistor 222 that are output from the reference voltage source 220. An error amplifier 224 that compares and amplifies the connection point voltage, and a comparator 225 that compares the reference voltage Vr and the connection point voltage of the resistor 222 and the resistor 223 are included. The error amplifier 224 is the first voltage detection signal V1. And the comparator 225 outputs the second voltage detection signal V2.

Here, when the resistance values of the resistor 221, the resistor 222, and the resistor 223 are R1, R2, and R3, respectively, the first predetermined voltage Vo1 is expressed by the following equation (Equation 1).
(Equation 1)
Vo1 = (1 + R2 / R1 + R3 / R1) × Vr
When the output voltage Vo is going to be higher than the first predetermined voltage Vo1, the first voltage detection signal V1 is lowered, and when the output voltage Vo is going to be lower than the first predetermined voltage Vo1, the first voltage detection is performed. Signal V1 rises.

The second predetermined voltage Vo2 is expressed by the following equation (Equation 2).
(Equation 2)
Vo2 = {1 + R3 / (R1 + R2)} × Vr
When the output voltage Vo is higher than the second predetermined voltage Vo2, the second voltage detection signal V2 is “H”, and when the output voltage Vo is lower than the second predetermined voltage Vo2, the second voltage detection signal V2 is “L”.

  In FIG. 3, the charging unit 20 is connected between a switching step-down converter including a P-channel FET 200, a diode 201, an inductor 202, and a capacitor 203, and an input / output of the charging unit 20, and is turned on / off by a second voltage detection signal. It comprises a P-channel FET 204 and a control drive circuit 205 that outputs a drive signal Vg for driving the FET 200.

  Further, the control drive circuit 205 selects the lower one of the error amplifier 206 that compares and amplifies the current detection signal Vc with the reference voltage Vs, and the error signal Ve that is the output of the error amplifier 206 and the first voltage detection signal V1. A selection circuit 207 that outputs, a triangular wave oscillator 208 that outputs a triangular wave signal Vt, a PWM circuit 209 that outputs a drive signal Vg that is a comparison result between the output of the selection circuit 207 and the triangular wave signal Vt, and a voltage across the FET 200 are detected. From the overcurrent protection circuit 210 that detects the current flowing through the FET 200 from the voltage drop at the time of ON and adjusts the pulse width of the drive signal Vg so that the output current of the charging unit 20 is limited to the maximum output current Icm. Composed.

  The error signal Ve is lowered when the current detection signal Vc is to be higher than the reference voltage Vs, and is increased when the current detection signal Vc is to be lower than the reference voltage Vs. The drive signal Vg increases the output power of the charging means 20 by extending the “L” period when the output of the selection circuit 207 rises and extending the ON period of the FET 200. Conversely, the drive signal Vg reduces the output power of the charging means 20 by narrowing the “L” period and shortening the ON period of the FET 200 when the output of the selection circuit 207 decreases.

  Therefore, when the selection circuit 207 selects and outputs the error signal Ve, the charging unit 20 adjusts the pulse width of the drive signal Vg so that the current detection signal Vc becomes the reference voltage Vs, and the constant current mode. Operate. On the other hand, when the selection circuit 207 selects and outputs the first voltage detection signal V1, the charging unit 20 sets the pulse width of the drive signal Vg so that the output voltage Vo becomes the first predetermined voltage Vo1. Adjust and operate in constant voltage mode. Further, the FET 204 to which the second voltage detection signal V2 is applied to the gate is turned off when the output voltage Vo is higher than the second predetermined voltage Vo2, but when the output voltage Vo becomes lower than the second predetermined voltage Vo2. In order to short-circuit between the input and output of the charging means 20, the charging means 20 operates in a short-circuit mode.

  As described above, according to the charger of the first embodiment, since the power supply to the load and the charging of the secondary battery are performed simultaneously, the power supply means has a power supply capability larger than the maximum power consumption at the load. The charging means only needs to have a current supply capacity equal to or higher than the charging current at the time of constant current charging to the secondary battery. If the load current is small, the charging current to the secondary battery can be kept constant regardless of the load variation. In addition, the charging operation is maintained even when the load current increases, and even when the load current exceeds the current supply capability of the charging means, the secondary battery is repeatedly charged and discharged, so that a predetermined voltage higher than the load drive lower limit voltage is loaded. Can be supplied to.

(Second Embodiment)
FIG. 4 is a configuration diagram of a charger according to the second embodiment of the present invention. Reference numeral 1 denotes a power supply means such as an AC adapter, and 2A denotes a charging unit, which is composed of a charging means 23, a current detection resistor 24, and a voltage detection means 25. Supply. The charging means 23 is composed of, for example, a switching type step-down converter, and converts the input power supply voltage from the power supply means 1 to output a desired direct current.

  The current detection resistor 24 has a resistance value Rs, is connected in series to the charging unit 23, detects the output current Io, and outputs a current detection signal Vc = Io × Rs to the charging unit 23. The voltage detection means 25 detects the voltage Vo of the load 4 and the secondary battery 3 and outputs the first voltage detection signal V1 and the second voltage detection signal V2 to the charging means 23. The first voltage detection signal V1 indicates a comparison result between the output voltage Vo and the first predetermined voltage Vo1, and the second voltage detection signal V2 indicates a comparison result between the output voltage Vo and the second predetermined voltage Vo2.

  Here, the first predetermined voltage Vo1 is set to the battery voltage when the secondary battery 3 is fully charged, and the second predetermined voltage Vo2 is lower than the first predetermined voltage Vo1 and lower than the drive lower limit voltage of the load 4. Set to high voltage.

  In the second embodiment, the power supply means 1 outputs a predetermined DC power supply voltage Vi, and the output current is constant when the output current reaches the maximum value Imax by the overcurrent protection function. The maximum output current Imax of the power supply means 1 is assumed to be larger than the maximum current Iomax consumed by the load 4.

  The charging unit 23 of the charging unit 2A receives the current detection signal Vc, the first voltage detection signal V1, and the second voltage detection signal V2. When the second voltage detection signal V2 indicates that the output voltage Vo is lower than the second predetermined voltage Vo2, the charging unit 23 directly outputs the output of the power supply unit 1 with the input and output shorted. Performs short-circuit mode operation.

  When the first voltage detection signal V1 and the second voltage detection signal V2 indicate that the output voltage Vo is higher than the second predetermined voltage Vo2 and lower than the first predetermined voltage Vo1, the charging unit 20 Performs a constant current mode operation for controlling the output so that the current detection signal becomes a constant value Vs.

  The output current of the charging means 23 at this time is Ic = Vs / Rs. When the output voltage Vo reaches the first predetermined voltage Vo1, the charging unit 23 performs a constant voltage mode operation for controlling the output so that the output voltage Vo is stabilized at the first predetermined voltage Vo1.

  The operation of the charging unit 2A in the second embodiment configured as described above differs from the charging unit 2 of the first embodiment shown in FIG. 1 in that the maximum output current of the charging means 23 is in the constant current mode. The point is the output current during operation. The operation when the load 4 is not driven, that is, when the output of the charging unit 2A is all spent for charging the secondary battery 3, is the same as that of the charging unit 2 in the first embodiment.

  Next, in the second embodiment, an operation when the load 4 is driven during the constant current mode operation to increase the load current Io and reach the maximum current Iomax will be described.

  In the constant current mode operation, a current that is the sum of the charging current Ic and the load current Io flows through the current detection resistor 24. Therefore, the charging current Ic decreases as the load current Io increases. When the load current Io exceeds the constant current output value (Vs / Rs) of the charging means 23, the charging current Ic becomes negative, the secondary battery 3 is discharged, and the output voltage Vo also decreases. When the reduced output voltage Vo becomes lower than the second predetermined voltage Vo2, the charging unit 2 enters the short-circuit mode operation, and the maximum current Imax from the power supply unit 1 is supplied.

  For this reason, the secondary battery 3 is charged again, and the charging current is a value obtained by subtracting the load current Io from the maximum current Imax from the power supply means 1 (Imax−Io). Even when the load current reaches the maximum value Iomax, since Imax> Iomax, the secondary battery 3 is ensured with a charging current.

  When the secondary battery 3 is charged, the output voltage Vo rises again and becomes higher than the second predetermined voltage Vo2, and the charging unit 23 enters the constant current mode operation. At this time, the load current Io is the maximum of the charging unit 23. If it is larger than the output current (Vs / Rs), the secondary battery 3 is discharged and the output voltage Vo decreases. That is, when the load current Io is larger than the maximum output current (Vs / Rs) of the charging means 23, the secondary battery 3 repeats charging / discharging, and the output voltage Vo rises and falls near the second predetermined voltage Vo2.

  As described above, according to the charging unit 2A of the second embodiment, since the power supply to the load 4 and the charging of the secondary battery 3 are performed at the same time, the power supply means 1 is based on the maximum power consumption at the load 4. There is a large power supply capability, and the charging unit 23 only needs to have a current supply capability during constant current charging of the secondary battery. Even if the load current flows, the charging operation is maintained. Even if the load current exceeds the current supply capability of the charging means 23, the secondary battery 3 is repeatedly charged and discharged, so that a predetermined voltage equal to or higher than the load drive lower limit voltage is applied to the load. Can be supplied.

  In the charging unit 20 of the charging unit 2 of the first embodiment and the charging unit 23 of the charging unit 2A of the second embodiment, the short-circuit mode is operated according to the second voltage detection signal. The short-circuit mode operation is not limited to that using a through switch as shown in FIG. For example, the overcurrent protection circuit or the constant current mode may be disabled. The object of the present invention can be achieved by canceling or relaxing the output current limitation by the charging means and effectively using the power limiting function of the power supply means.

  Further, in the voltage detection means 22 of the charging unit 2 of the first embodiment or the voltage detection means 25 of the charging unit 2A of the second embodiment, the output voltage Vo is compared with the second predetermined voltage Vo2. By using a hysteresis comparator as means for outputting the detection signal 2, it is possible to prevent the secondary battery 3 from repeatedly charging and discharging when the load current exceeds the current supply capability of the charging means. . In order to prevent the power supply means 1 from being removed in an operation state where the secondary battery 3 is not sufficiently charged as described above, a display for displaying the charge state of the secondary battery 3 may be provided.

  Further, although the output voltage Vo and the second predetermined voltage Vo2 are compared in order to output the second detection signal, the present invention is not limited to this method. What is necessary is just to detect a decrease in the output voltage due to the negative overcurrent exceeding the current supply capability of the charging means 20 and 23.

  The present invention is useful when applied to an electronic device that uses a secondary battery such as a lithium ion battery as a power source and has a charging function for the secondary battery.

The block diagram of the charger which is the 1st Embodiment of this invention The wave form diagram showing the time-dependent change of the charging current and output voltage of the charger in 1st Embodiment Circuit configuration diagram of the charger in the first embodiment The block diagram of the charger which is the 2nd Embodiment of this invention Configuration diagram of conventional charger Configuration diagram of conventional charger Configuration diagram of conventional charger

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Power supply means 2, 2A Charging part 3 Secondary battery 4 Load 20, 23 Charging means 21, 24 Current detection resistors 22, 25 Voltage detection means

Claims (7)

Power supply means for supplying an input DC voltage, charging means for converting the input DC voltage to an output DC voltage, voltage detection means for detecting the output DC voltage, and a secondary battery to which the output DC voltage is applied A charger for supplying the output DC voltage to a load comprising current detection means connected in series;
The voltage detecting means sets a full charge voltage of the secondary battery or a voltage in the vicinity of the full charge voltage as a first predetermined voltage, and is a second lower than the first predetermined voltage and higher than the drive lower limit voltage of the load The output DC voltage is compared and amplified with the first predetermined voltage to output a first voltage detection signal, and the comparison result between the output DC voltage and the second predetermined voltage 2 as a voltage detection signal
The charging means receives the first voltage detection signal, the second voltage detection signal, and the current detection signal from the current detection means,
When the output DC voltage is equal to or lower than the second predetermined voltage, an input / output short-circuit state is established,
When the output DC voltage is higher than the second predetermined voltage and lower than the first predetermined voltage, the output is controlled so that the current flowing through the current detection means is constant,
When the output DC voltage reaches the second predetermined voltage, the output is controlled so that the output DC voltage becomes the second predetermined voltage.   The charger according to claim 1, wherein the current detection unit forms a series circuit only with the secondary battery.   The charger according to claim 1, wherein the current detection unit forms a series circuit with the load.   2. The charger according to claim 1, wherein the voltage detecting means includes a hysteresis comparator as means for comparing the output DC voltage with the second predetermined voltage.   The voltage detection means detects a decrease in the output DC voltage and outputs a second voltage detection signal instead of comparing the output DC voltage with the second predetermined voltage. 1. The charger according to 1.   The charger according to claim 1, wherein a switch is provided between the input and output of the charging means to short-circuit the switch in the input / output short-circuit state of the charging means.   2. The charger according to claim 1, wherein control by a current detection signal from the current detection unit is canceled in an input / output short-circuit state of the charging unit.

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