JP2006296118A - Charger - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a charger applied to arbitrary external power supplies, and efficiently suppressing a temperature rise. <P>SOLUTION: The charger 2 has a switching voltage dropping converter comprising a switch element 5, a synchronous rectifier 3, an inductor 6 and a capacitor 7, a charge controlling element 8, a detection resistor 9 for regulating a constant current, and a control circuit 16, and is supplied with DC power from the external power supply 1. When a difference between voltages from the external power supply 1 and a battery 10 is sufficient to drive the charge controlling element 8, an output voltage from the voltage dropping converter is controlled so as to make a voltage applied across the charge controlling element 8 become the minimum voltage for driving the voltage across the charge controlling element 8 i. e. a voltage between a source and a drain. The charger is achieved, minimizes a power loss in the charge controlling element, and reduces the quantity of generated heat. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a charger that is provided in various battery-driven electronic devices and that is supplied with electric power from an external power source and charges the battery.

  In recent years, a charger provided in an electronic device such as a portable device having a battery is required to have a rapid and highly efficient charging function.

  Conventionally, as a charger, an apparatus having a circuit configuration as shown in FIG. 2 has been used. In FIG. 2, a charger 19 is supplied with electric power from an external power source 17 having a current limiting function, and includes a backflow prevention switch element 4 made of a P-channel MOSFET, a charge control element 8 made of a P-channel MOSFET, and a constant voltage. It comprises a current control detection resistor 9 and a control circuit 18. The control circuit 18 includes an external power supply detection circuit 11, a charge control element control circuit 13, and a charge detection circuit 14 that detects a charging current and a battery voltage.

  The direct current supplied from the external power source 17 charges the battery 10 via the backflow prevention switch element 4, the charge control element 8, and the constant current control detection resistor 9. The backflow prevention switch element 4 is controlled by the external power supply detection circuit 11. Specifically, the backflow prevention switch element 4 is turned on during the charging operation of the battery 10 by being supplied with electric power from the external power supply 17. Further, when there is no power supply from the external power supply 17, the battery is turned off so that the discharge current from the battery 10 does not flow backward. Therefore, in the following description of the charging operation of the charger 19, the backflow prevention switch element 4 is in the ON state.

  FIG. 4 shows the output characteristic X of the external power source 17 and the output characteristic Y of the charger 19. As shown in FIG. 4, the external power supply 17 has a constant voltage output function for outputting a predetermined voltage Va and a current limiting function for limiting the output current to a predetermined value Ia.

  Further, the charge control element 8 takes the following three states by controlling the impedance between the source and the drain by changing the gate voltage supplied from the charge control element control circuit 13. The first state is an on state. Further, in the second state, a voltage generated at both ends of the charging current flowing to the constant current control detection resistor 9 is detected by the charge detection circuit 14 and the voltage at the constant current control detection resistor 9 is detected. This is a constant current state in which the drop is controlled to be a constant value. The constant current controlled charging current flowing at this time is Ic. Further, the third state is a constant voltage state in which the voltage of the battery 10 is detected by the charge detection circuit 14 and controlled so as to stabilize the voltage of the battery 10 to a predetermined value Vc. In FIG. 4, the constant current state and the constant voltage state of the charger are shown, and the shaded portion indicates the power loss Z due to the charger.

  FIG. 5 is an operation waveform diagram showing the time change of the voltage Vb of the battery 10 and the charging current Ib to the battery 10 and the time change of the both-ends voltage Vy of the charge control element 8 due to the charging operation of the charger. The operation of the prior art charger shown in FIG. 2 will be described below with reference to the charging operation waveform diagram of FIG.

  First, the period T11 is a period in which the battery 10 is rapidly charged. In this period T11, the battery 10 is charged with the constant current Ia using the current limiting function of the external power supply 17. At this time, the charge control element 8 made of a P-channel MOSFET is in an on state. At this time, the battery voltage Vb rises with time. At this time, the voltage between the source and drain of the charge control element 8 is determined by the constant current Ia flowing through the battery 10 and the ON resistance of the MOS.

  Next, in the period T12, the charge control element 8 is controlled by the charge control element control circuit 13 based on the voltage drop at the constant current control detection resistor 9, and the charge control element 8 operates as a constant current control element. In this period, the battery 10 is charged with the constant current Ic. This period T12 is a section in which the battery voltage is not less than Vd and less than Vc. In this period T12, for the overvoltage protection of the battery 10, constant current charging is performed with the current Ic set smaller than the value of the constant current Ia in the period T11. Battery voltage Vb further increases. At this time, the voltage between the source and drain of the charge control element 8 is determined by the potential difference between the external power supply 17 and the battery voltage 10.

Finally, in the period T13, the charge detection circuit 14 detects the voltage of the battery 10 and transmits it to the charge control element control circuit 13, whereby the charge control element 8 is in a constant voltage state in which it operates as a constant voltage control element. It is. In this period T13, it is detected that the battery 10 voltage Vb is equal to Vc, and the battery voltage Vb is held at a constant voltage Vc. The charging current Ib gradually decreases. That is, in the period T12 and the period T13, the charger 19 operates as a series regulator.
Japanese Patent No. 3312422

  In the prior art charger as described above, when the charging control element 8 controls the charging current as shown in FIG. 5, the voltage across the charging control element 8, that is, the voltage between the source and drain can be driven. Since it is sufficiently larger than 0.2 to 0.3 V, a large power loss occurs, which may adversely affect peripheral devices due to an increase in surface temperature accompanying an increase in the amount of heat generated during charging. In order to suppress this, there is a problem that the charging current value cannot be set large and the charging time becomes long.

  Therefore, in order to shorten the charging time, at the initial stage of charging, the charging control element 8 is turned on and charging current control is not performed. The charging time was shortened. Therefore, there is a problem that an external power source other than a specific external power source cannot be used.

  An object of the present invention is to provide a charger capable of reducing a charging time by increasing a charging current while suppressing adverse effects on peripheral devices by reducing heat generation of a charging control element.

  Another object of the present invention is to provide a charger that can shorten the charging time without using an external power source having a current limiting function.

  Still another object of the present invention is to provide a highly efficient charger.

  In order to solve the above problems, a charger according to the present invention includes a DC voltage converter to which power is supplied from an external power source, a charge control element that supplies a charging current to a battery using the DC voltage converter as a power source, and charge control. The voltage detection circuit that detects the voltage across the element controls the output voltage of the DC voltage converter so that the voltage across the charge control element is the minimum voltage that can be driven, and a predetermined charging current flows through the battery. Has a function of driving the charge control element.

  According to this configuration, the output voltage of the DC voltage converter is controlled so that the voltage across the charge control element detected by the voltage detection circuit becomes the minimum voltage that can drive the charge control element. Loss can be minimized and heat generation can be minimized. Therefore, the charging current controlled by the charging control element can be increased. As a result, a relatively large charging current can be passed through the battery in a state controlled by the charging control element, and the charging time can be shortened. In addition, since a large charge current is controlled by the charge control element, it is not necessary to use an external power supply having a current limiting function.

  In the above charger, the DC voltage converter includes, for example, a switch element having one end connected to an external power source, a rectifier having one end connected to the other end of the switch element and the other end grounded, and other switch elements. The step-down converter comprises an inductor connected to one end and the other end serving as an output end of the DC voltage converter. This step-down converter controls the output voltage by changing the duty ratio of the on / off operation of the switch element by the switching control circuit.

  Here, the voltage detection circuit detects the voltage across the charge control element, and if the charge control element has a potential difference that can be driven, the switch control circuit and the synchronous rectifier are alternately turned on and off by the switching control circuit. The output voltage of the DC voltage converter is limited so that the control element can be driven to the minimum voltage.

  According to this configuration, a switching step-down converter is used, and the power loss in the charge control element is minimized, so that the efficiency is high.

  The charger of the present invention has an excellent effect of high efficiency and a small amount of heat generation by using a DC voltage conversion means, for example, a switching step-down converter, for controlling the voltage across the charging element for charge control. In addition, since the control circuit has a system that controls the voltage across the charging current control element and controls the charging current, it is possible to use an external power supply that does not have a voltage / current limiting function. It has the effect that charging is possible.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of a charger according to the invention will be described with reference to the accompanying drawings.

(Embodiment 1)
FIG. 1 is a circuit diagram showing a configuration of the charger according to the first embodiment of the present invention. In FIG. 1, the charger 2 is supplied with electric power from an external power source 1 that does not particularly require a current limiting function. The charger 2 includes a backflow prevention switch element 4 made of a P-channel MOSFET, a switch element 5 made of a P-channel MOSFET, a synchronous rectifier 3 made of an N-channel MOSFET, an inductor 6, a capacitor 7, and a charge control element. 8, a constant current control detection resistor 9, and a control circuit 16. The control circuit 16 includes an external power supply detection circuit 11, a voltage detection circuit 12, a charge control element control circuit 13, a charge detection circuit 14 that detects a charging current and a battery voltage, and a switching control circuit 15.

  The backflow prevention switch element 4 is controlled by the external power supply detection circuit 11 and is turned on during the charging operation to the battery 10 by being supplied with power from the external power supply 1, and when no power is supplied from the external power supply 1. Is turned off so that the discharge current from the battery 10 does not flow backward. Therefore, in the following description of the charging operation of the charger 2, the backflow prevention switch element 4 is always in the on state.

  The switch element 5 made of a P-channel MOSFET, the synchronous rectifier 3 made of an N-channel MOSFET, the inductor 6 and the capacitor 7 constitute a switching step-down converter. The step-down converter outputs DC power from the capacitor 7 via the inductor 6 when the switch element 5 and the synchronous rectifier 3 are alternately turned on and off. When the ON / OFF operation of the synchronous rectifier 3 and the switch element 5 is repeated periodically, the ratio of the ON time of the switch element 5 to one cycle (hereinafter, this ratio is described as a duty ratio) is adjusted. Thus, the output voltage of the step-down converter can be controlled.

  FIG. 3 is an operation waveform diagram showing the time change of the voltage Vb of the battery 10 and the charging current Ib to the battery 10 and the time change of the both-ends voltage Vx of the charge control element by the charging operation of the charger. Hereinafter, the charging operation of the charger 2 according to the first embodiment of the present invention will be described with reference to the operation waveform diagram shown in FIG.

  A period T1 in FIG. 3 is a period in which the battery charger 2 is in a constant current state and the battery 10 is charged with a constant current at the current value Ia. At this time, the charge control element 8 is controlled by the charge control element control circuit 13 based on the value of the charge current detected by the charge detection circuit 14 and operates at a constant current. The battery voltage Vb gradually increases. Further, the voltage detection circuit 12 in the control circuit 16 detects the voltage Vx across the charge control element 8, and the voltage Vx across the charge control element 8, that is, the minimum voltage (a constant value) that can drive the source-drain voltage. Thus, the switching control circuit 15 is controlled. As a result, power lost in the charge control element 8 can be minimized, and heat generation of the charge control element 8 can be minimized. In order to shorten the charging time, it is desirable to set the current value Ia in the period T1 large within the allowable range of the thermal burden on the charger 2. Further, by using a step-down converter having high-efficiency power conversion characteristics and minimizing the power loss in the charge control element 8, the current value Ia can be set large and the charging time can be shortened. This period T1 is a period from when charging proceeds until the voltage Vb of the battery 10 reaches the voltage value Vd, and when the battery voltage Vb reaches the voltage value Vd, the period T2 is entered.

  In the period T2, the charger 2 is in a constant voltage state, and the charge control element 8 is controlled by the charge control element control circuit 13 based on the value of the battery voltage detected by the charge detection circuit 14, and operates at a constant voltage. Thus, the battery voltage Vb is charged while being limited to the voltage value Vc. At this time, the charging current Ib gradually decreases. Further, the switching control circuit 15 controls the duty ratio so that the voltage across the charge control element 8, that is, the voltage between the source and the drain becomes the minimum drivable voltage (constant value) as in the above-described period T1. . Eventually, when the voltage of the battery 10 reaches a value close to the full charge voltage and reaches the full charge voltage completely, the charging is terminated.

  The charger according to the present invention uses a switching step-down converter, and controls the voltage across the charge control element, that is, the voltage between the source and the drain to be the minimum driveable voltage, so that the charging time is high efficiency. Is useful as a charger that can use an external power source that is short and does not have a current limiting function.

It is a circuit diagram which shows the structure of the charger of Embodiment 1 of this invention. It is a circuit diagram which shows the structure of the prior art of a charger. It is a wave form diagram of charging current Ib, charging voltage (battery voltage) Vb, and the both-ends voltage Vx of a charge control element which show operation | movement of the charger in Embodiment 1 of this invention. It is a characteristic view which shows the output characteristic of the external power supply and charger in a prior art. It is a wave form diagram of charge current Ib, charge voltage (battery voltage) Vb, and both-ends voltage Vy of a charge control element which show operation of the prior art of a charger.

Explanation of symbols

1 External power supply (no current limiting function)
2 Charger (charger of the present invention)
DESCRIPTION OF SYMBOLS 3 Synchronous rectifier 4 Switch element for backflow prevention 5 Switch element 6 Inductor 7 Capacitor 8 Charge control element 9 Detection resistor for constant current control 10 Battery 11 External power supply detection circuit 12 Voltage detection circuit 13 Charge control element control circuit 14 Charge detection circuit 15 Switching Control circuit 16 Control circuit (control circuit of the present invention)
17 External power supply (with current limiting function)
18 Control circuit (prior art control circuit)
19 Charger (prior art charger)

Claims (2)

A DC voltage converter to which power is supplied from an external power source;
A charge control element for supplying a charging current to the battery using the DC voltage converter as a power source;
The output control of the DC voltage converter is controlled so that the voltage across the charge control element is a minimum voltage that can drive the charge control element, and the charge control is performed so that a predetermined charge current flows through the battery. A charger comprising a control circuit for driving the element. The DC voltage converter includes a switch element having one end connected to the external power source, a rectifier having one end connected to the other end of the switch element and the other end grounded, and one end connected to the other end of the switch element. And the other end is composed of a step-down converter composed of an inductor serving as an output terminal of the DC voltage converter,
The charger according to claim 1, wherein the control circuit controls an output voltage of the DC voltage converter by changing a duty ratio of an on / off operation of the switch element.

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