JP2003079137A - Constant current circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a constant current circuit that can be miniaturized and which has less heat generation. SOLUTION: This constant current circuit 10 is provided with a switching transistor Tr (FET) that, using as an input a DC voltage VIN obtained from a commercial power 'e' through an AC adapter 1, performs the switching of the DC voltage VIN; a filter circuit 3 that smoothes the output of the switching transistor Tr; a low-resistance element R1 for detecting a current inserted in series to a current path that supplies the output current 'i' coming out of the filter circuit 3 to a battery Bat; an amplifying circuit 5 that amplifies the voltage drop Vd of the low-resistance element R1; and a voltage-control circuit 7 that compares an output voltage Vo of the amplifying circuit 5 with a reference voltage Vref and controls the switching duty of a switching pulse wave P that is inputted to the gate G of the switching transistor Tr by the difference. The current control of the switching system enables the constant current circuit 10 to be miniaturized with proper efficiency and less heat generation, and makes it particularly well fit to the charging of a battery of a portable electronic device.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant current circuit, for example, a portable electronic device such as a mini disk player and a compact disk player, or a battery charger built in a battery charger. The present invention relates to a constant current circuit suitable for use in a constant current circuit. 2. Description of the Related Art To charge a battery (a nickel-metal hydride rechargeable battery or a nickel cadmium rechargeable battery) in a portable electronic device such as a portable mini disk player, a DC current of a predetermined value (500 mA for a nickel-metal hydride battery) is used. , About 300 mA for a nickel-cadmium rechargeable battery). Although there are various conventional constant current circuits for charging a battery, in principle, as shown in FIG.
A DC power supply (approximately 3 V to 10 V) obtained by passing a 00 V commercial power supply e through an AC (AC) adapter 1 (generally comprising a power transformer and a rectifying / smoothing circuit) is inserted into a series at an output as an input VIN. By performing feedback control of the output transistor Tr1 to apply negative feedback so that the voltage drop of the current detection resistor Rs becomes equal to the reference voltage VR, a constant output current of i = Vref / Rs is a load of the battery Bat. It is configured to flow. [0005] The constant current circuit 30 shown in FIG. 5 is a simple constant current circuit which applies the principle of the constant current circuit shown in FIG. 4 and uses the emitter-base voltage VEB of the transistor Tr 2 as a reference voltage. Conventionally, the type of the constant current circuit 30 has been widely used for charging a battery. [0005] In the above-described conventional constant current circuit 30 for charging a battery, a large loss is generated due to a potential difference between the collector side and the emitter side of the transistor Tr1 and heat is generated. Has been a major obstacle to incorporating circuits into small portable electronic devices and small chargers. That is, the above-described conventional constant current circuit 20 for charging a battery uses a relatively large-sized transistor or heat sink, and thus has a difficulty in downsizing. In the constant current circuit 30, the emitter-base voltage V of the transistor Tr2 is used as a reference voltage.
_{Since EB} is used, there is a disadvantage that when V _EB changes with temperature, the output current also changes at the same rate. In order to eliminate this disadvantage, it is necessary to provide a reference voltage separately and to use a differential amplifier input as a feedback amplifier. As a result, the circuit configuration becomes complicated, resulting in new cost and space problems. I will. The present invention has been made in view of the problems of the above-mentioned conventional constant current circuit for charging a battery, and has as its object to solve the heat generation problem and to save space by improving efficiency. In order to solve the above problems, the present invention provides a switching transistor Tr for switching a DC power supply VIN, a filter circuit 3 for smoothing the output of the switching transistor Tr, A low resistance element R1 inserted in series in a current path for supplying an output current i passing through the filter circuit 3;
Amplifying circuit 5 that amplifies the voltage drop Vd of the switching transistor Tr.
And a voltage control circuit 7 for controlling the switching duty of the switching pulse wave P inputted to the constant current circuits 10 and 20. An embodiment of a constant current circuit according to the present invention will be described in detail with reference to a circuit diagram of a constant current circuit for charging a battery. FIG. 1 is a circuit diagram of a constant current circuit according to the present invention, and FIG. 2 is a circuit diagram of another constant current circuit. FIG. 3 shows a DC-D suitable as a voltage control circuit of the constant current circuit of the present invention.
FIG. 2 is a block diagram of a C converter control IC (voltage control IC). First, the characteristic of the constant current circuit of the present invention is that the current control for keeping the output current (battery charging current) constant is performed by the switching control of the transistor. That is, the constant current circuit 10 shown in FIG.
A DC power supply VIN built in a battery charger (not shown) or a main body of a portable electronic device and obtained by passing a commercial power supply e through an AC adapter 1 is input, and the DC power supply VIN is input.
A switching transistor Tr for switching
A filter circuit 3 including a diode D, an inductor L, and a capacitor C for smoothing the output of the switching transistor Tr, and an output current i passing through the filter circuit 3 is supplied to a battery Bat. , A low-resistance element R1 for current detection inserted in series in a current path to be supplied to the amplifier, an amplifier circuit 5 for amplifying a voltage drop Vd of the low-resistance element R1, an output voltage Vo of the amplifier circuit 5, and a reference voltage Vref. And a voltage control circuit 7 for controlling the switching duty of the switching pulse wave P input to the gate G of the switching transistor Tr based on the difference between the two. The switching transistor Tr includes:
A large transistor as used in the conventional constant current circuit 30 is unnecessary, and a small field-effect transistor (F
ET) can be used. In addition, it has been found by the inventor's prototype that the switching-type current control secures an efficiency of 75% or more and reduces heat generation, thereby eliminating the need for a heat sink and enabling downsizing. It is desirable to use the low resistance element R1 having a resistance value as small as possible (1 Ω or less) from the viewpoint of improving efficiency and preventing heat generation. For example, a 0.1 Ω resistance element is used. When the output current (charging current) i = 500 mA,
Low resistance element R1 = 0.1Ω and voltage drop Vd = 0.05V
It becomes. The amplifying circuit 5 includes an operational amplifier 4, and includes a resistor R3 (= 1KΩ) and a resistor R4 (= 20KΩ).
Ω), the operational amplifier output voltage Vo becomes a voltage (1 V) equal to the reference voltage Vref in the voltage control circuit 7 when the desired output current i = 500 mA. The voltage control circuit 7 compares the output voltage Vo of the operational amplifier input to the feedback input terminal FB with an internal reference voltage Vref, and outputs a pulse at the switching output terminal EXT connected to the gate G of the switching transistor Tr. Control the duty of wave P (PW
M or PFM)
Can be diverted. The voltage control IC has an oscillation frequency of 300 as shown in the block circuit shown in FIG.
A DC-DC converter controller IC (Model No. XC6365) manufactured by Torex at a frequency of kHz is employed. The terminal CE is a high-active enable terminal having a threshold voltage of 0.35 V to 0.45 V. In the constant current circuit 10 of the present invention, when the oscillation stops for some reason, the input and output are short-circuited. To prevent this, a resistor R6 (= 560 KΩ) and a resistor R7 inserted in the DC power supply VIN
By applying a voltage divided by (= 100 KΩ) to the CE terminal, the switching transistor Tr is opened to return to a normal state when VIN decreases due to a short circuit. Next, the constant current circuit 20 shown in FIG.
The battery Bat. Is substantially the same as the constant current circuit 10 except that the low-resistance element R1 for current detection charges the battery Bat. Is inserted between the series and the ground. Although the amplifier circuit 5 'is simplified for cost reduction, the circuit operation is the same. However,
It goes without saying that the element constant is appropriately adjusted. In each of the above embodiments, the constant current circuit for charging the battery is used. However, the present invention can of course be used as a constant current circuit for other uses. Since the constant current circuit according to the present invention is configured as described above, it has excellent effects as described below. (1) 75% due to switching-type current control
The above efficiency is secured, and heat generation is small. (2) Since a small transistor such as an FET can be used and a heat sink is not required, miniaturization is possible. (3) It is particularly suitable as a constant current circuit for charging portable electronic devices. [0021]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a constant current circuit according to the present invention. FIG. 2 is a circuit diagram of another constant current circuit. FIG. 3 is a DC-DC converter control IC (voltage control IC) suitable as a voltage control circuit of the constant current circuit of the present invention.
FIG. FIG. 4 is a circuit diagram of a conventional constant current circuit in principle. FIG. 5 is an example of a simple constant current circuit used for conventional battery charging. [Description of Signs] 1 AC adapter 3 Filter circuit 4 Operational amplifier 5, 5 'amplifier circuit 7 Voltage control circuit 10, 20 Constant current circuit 30 Constant current circuit D Diode e Commercial power supply Tr Switching transistor (FET) Tr1 Transistors R2 to R7 Resistance R1 Low resistance element VIN DC power supply i Output current Bat. Battery Vd Voltage drop Vo Output voltage Vref Reference voltage

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Claims (1)

Claims: 1. A switching transistor for switching a DC power supply, a filter circuit for smoothing the output of the switching transistor, and a current path for supplying an output current through the filter circuit. A low-resistance element, an amplifying circuit for amplifying a voltage drop of the low-resistance element, and comparing an output voltage of the amplifying circuit with a reference voltage, and switching a switching pulse wave input to the gate of the switching transistor by a difference between the two. A constant current circuit, comprising: a voltage control circuit that controls a duty.