JP2012060697A - Constant current circuit for charge, and charging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low cost constant current circuit for charge where temperature dependence of a charging current is small, and a charging apparatus provided with an overcharge preventing function using the constant current circuit for charge.SOLUTION: There are provided: a first transistor and second transistor having the same polarity where a base and an emitter are connected with each other, and a collector and the base are connected with each other; a diode of which one end is connected to the base of the first transistor in the forward direction; a first resistance element connected in series between the other end of the diode and the emitter of the first transistor; and a second resistance element of which one end is connected to a node between the collector of the first transistor and the base of the second transistor. A temperature change of a voltage between the base and emitter of the first transistor is canceled by a temperature change of the diode.

Description

  The present invention relates to a charging constant current circuit used for charging a secondary battery, and a charging device for rapidly charging a secondary battery with a constant current using the charging constant current circuit.

  As a method of charging secondary batteries such as lithium ion batteries and nickel metal hydride batteries in a short period of time, when the voltage of the battery to be charged is low, charging is progressed with a constant current (CC), and the voltage of the battery to be charged is regulated. A CC / CV system that charges at a constant voltage (CV) when the value is exceeded is well known. FIG. 6 shows the functional configuration of a typical CC / CV charging device 600.

  The charging device 600 includes a current control circuit 60, a voltage control circuit 61, a selection circuit 62 that switches between current control and voltage control, a pass transistor 63, a backflow prevention diode 64, and a current detection resistor 65. The selection circuit 62 functions the current control circuit 60 in the initial stage of charging to charge the charged secondary battery 3 with a constant current, and functions the voltage control circuit 61 in the latter stage of charging to make the charged secondary battery constant voltage. Switch to charge. The CC / CV charging device 600 is extremely useful for rapid charging of the secondary battery 3 to be charged, but has a complicated circuit configuration and is expensive.

  Patent Document 1 discloses a CC / CV charging device that performs current control and voltage control in one control system. Although this charging device has a simpler configuration than the charging device 600, the charging device is expensive and difficult to adopt for an extremely low-priced charging device.

  As the simplest charging circuit for the secondary battery, a series circuit of a backflow prevention diode and a current limiting resistor is used. This charging circuit consisting of only two elements is the cheapest. However, if the current limiting resistor is made small for rapid charging, a very large current flows at the beginning of charging, the current limiting resistor generates heat, and the life of the secondary battery is impaired. On the other hand, if the current limiting resistance is increased, charging takes a long time.

  Therefore, it is also useful to use a constant current circuit that always charges with a constant current regardless of the voltage of the secondary battery to be charged. FIG. 7 shows a conventional constant current circuit 700 for charging.

  The charging constant current circuit 700 includes a first transistor 71, a second transistor 72, a first resistance element 73, and a second resistance element 74. The base of the first transistor 71 is connected to the emitter of the second transistor 72, the collector of the first transistor 71 is connected to the base of the second transistor 72, and the first resistance element 73 is connected to the base of the first transistor 71. And one end of the second resistance element 74 is connected to a connection point between the collector of the first transistor 71 and the base of the second transistor 72.

  The positive voltage of the charging power supply 4 is supplied to the emitter of the first transistor 71, the negative voltage of the charging power supply 4 is supplied to the other end of the second resistance element 74, and the secondary secondary charge is supplied to the collector of the second transistor 72. The positive voltage of the battery 3 is supplied to the other end of the second resistance element 74 and the negative voltage of the charged secondary battery 3 is supplied.

When the voltage between the terminals of the first resistance element 73 turns on the first transistor 71, the second transistor 72 is turned off, so that the charging constant current circuit 700 has the charging current I _CH given by the equation (1). The secondary battery 3 to be charged is charged. As apparent from the equation (1), the charging current I _CH does not depend on the voltage of the charging power source 4.

Here, V _BE is the base-emitter voltage of the first transistor 71, and R ₇₃ is the resistance value of the first resistance element 73.

JP 2008-31426 A JP 2009-131143 A

The conventional constant current circuit for charging 700 has a simple circuit configuration and is low in cost. However, since V _BE changes depending on the temperature, the charging current value changes depending on the ambient temperature. In addition, when the charging device is configured only by the charging constant current circuit 700, the charging current continues to flow even after the charged secondary battery is fully charged, and the secondary battery is overcharged. When the secondary battery to be charged is a lithium ion battery, the battery may burst in an overcharged state.

  The present invention has been made in view of such a problem, and includes a low-cost constant current circuit for charging with low temperature dependency of the charging current, and an overcharge prevention function using the constant current circuit for charging. An object of the present invention is to provide a charging device.

  The constant current circuit for charging according to the present invention includes a first transistor and a second transistor having the same polarity, a diode, and first and second resistance elements. The base of the first transistor and the emitter of the second transistor, the collector of the first transistor and the base of the second transistor are respectively connected, and the diode and the first resistance element are the base and emitter of the first transistor, respectively. The diode is connected in the direction in which the polarity of the base of the first transistor coincides with the polarity of the first transistor, and the second resistance element has one end of the collector of the first transistor and the base of the second transistor. Connected to the connection point.

  Further, the charging device of the present invention includes the above-described charging constant current circuit of the present invention, and a comparator that connects one input terminal to the secondary battery to be charged and connects the other input terminal to the reference voltage source, The output terminal of the comparator is connected to the other end of the second resistance element.

The constant current circuit for charging according to the present invention works in the direction in which the temperature change of the forward drop voltage of the diode cancels the temperature change of the base-emitter voltage V _BE of the first transistor. Therefore, the temperature change of the charging current can be reduced. Further, the charging device of the present invention can prevent the secondary battery from being overcharged by using a charging constant current circuit whose charging current has a small temperature change.

The figure which shows the circuit structural example of the constant current circuit 1 for charging of this invention. The figure which shows the circuit structural example of the constant current circuit 1 'for charging which made the transistor of the constant current circuit 1 for charging reverse polarity. The figure which shows the circuit structural example of the charging device 100 of this invention. The figure which shows the circuit structural example of the charging device 200 of this invention. The figure which shows the circuit structural example of charging device 200 'which made the transistor of the charging device 200 reverse polarity. The figure which shows the circuit structure of the conventional charging device 600. FIG. The figure which shows the circuit structure of the conventional constant current circuit 700 for charge.

  Embodiments of the present invention will be described below with reference to the drawings. The same reference numerals are given to the same components in a plurality of drawings, and the description will not be repeated.

[Constant current circuit for charging]
FIG. 1 shows a circuit configuration example of a charging constant current circuit 1 of the present invention. The charging constant current circuit 1 is different from the conventional charging constant current circuit 700 only in that a diode 1c is provided. It is the same as the charging constant current circuit 700 in that it operates as a constant current circuit.

  The diode 1c is connected in series with the first resistance element 1d between the base and the emitter of the first transistor 1b. The direction of the diode 1c is connected so that the polarity of the base feature of the first transistor 1b matches the polarity. The direction in which the base feature and the polarity match will be described later.

By connecting the diode 1c, the temperature change of the charging current _ICH can be reduced. The charging current I _CH can be expressed by equation (2).

Here, R _1d is the resistance value of the first resistance element 1d, V _BE is the base-emitter voltage of the first transistor, and V _D is the forward voltage drop of the diode 1c. From equation (2), it is necessary that V _BE > V _D. From this condition, a Schottky diode with a low forward voltage drop is suitable for the diode 1c.

The temperature change of the charging current I _CH can be expressed by equation (3).

The temperature coefficient of V _BE is about −2 mV / ° C., and the temperature coefficient of the Schottky diode is about −1.2 mV / ° C. Accordingly, the temperature change of the charging current I _CH by connecting the diode 1c, the diode 1c is reduced to approximately 40% of the conventional charging constant current circuit is not connected.

  Although the first and second transistors in FIG. 1 are shown as examples using PNP transistors, this type of circuit can be replaced with an NPN transistor having a reverse polarity. FIG. 2 shows a circuit configuration example of a charging constant current circuit 1 ′ using an NPN transistor. Further, even if the positions of the first resistance element 1d and the diode 1c are replaced, the operation of the charging constant current circuits 1 and 1 'remains unchanged.

  That is, the constant current circuit for charging according to the present invention includes the bases of the first transistors 1b and 1b ', the emitters of the second transistors 1a and 1a', the collectors of the first transistors 1b and 1b ', and the second transistor 1a. , 1a 'are connected to each other, the diode 1c and the first resistance element 1d are arranged in series between the bases and emitters of the first transistors 1b, 1b', and the diode 1c is connected to the first transistors 1b, 1b, The second resistance element 1e is connected in the direction in which the polarity of the base of 1b 'coincides with the polarity of the base, and one end of the second resistance element 1e is connected to the collectors of the first transistors 1b and 1b' Connected to the connection point. The charging power supply 4 is connected between the emitters of the first transistors 1b and 1b 'and the other end of the second resistor element 1e, and the emitters of the second transistors 1a and 1a' and the second resistor element. The charged secondary battery 3 is connected between the other end of 1e.

Here, the direction in which the polarity of the base of the diode 1c matches the polarity is that the cathode (N) of the diode 1c is directed to the base of the first transistor 1d when the first transistor 1d is formed of a PNP transistor. is there. Further, when the first transistor 1d is composed of an NPN transistor, the anode (P) of the diode 1c is directed to the base of the first transistor 1d.
[Charging device]
FIG. 3 shows a circuit configuration example of the charging device 100 of the present invention. The charging device 100 includes the charging constant current circuit 1 and the overcharge detection circuit 2 described above. The overcharge detection circuit 2 includes a comparator 2a and a reference voltage source 2b.

  The non-inverting input terminal (+) of the comparator 2a is connected to the positive electrode of the charged secondary battery 3, and the inverting input terminal (−) is connected to the reference voltage source 2b. The output terminal of the comparator 2a is connected to the other end of the second resistance element 1e.

In the initial state of charging, the voltage V ₃ of the secondary battery 3 to be charged is lower than the reference voltage V _ref , so that the output voltage of the comparator 2 a is the negative voltage of the charging power source 4. Therefore, the charging constant current circuit 1 generates the charging current I _CH to charge the charged secondary battery 3.

When the secondary battery 3 to be charged continues to be charged, its voltage V ₃ rises, and when it exceeds the reference voltage V _ref , the output voltage of the comparator 2 a is inverted and changes to a positive voltage of the charging power source 4. Then, the second transistor 1a is cut off, and the supply of the charging current _ICH is stopped. By setting the reference voltage V _ref to the overcharge prevention voltage of the charged secondary battery 3, overcharge of the charged secondary battery 3 can be prevented.

Although the example in which the positive electrode of the secondary battery 3 to be charged is connected to the non-inverting input terminal (+) of the comparator 2a and the reference voltage source 2b is connected to the inverting input terminal (−) has been described, the non-inverting input terminal (+) Even if the negative electrode of the reference voltage source 2b and the negative electrode of the secondary battery 3 to be charged are connected to the inverting input terminal (−), the comparator 2a performs the same operation. That is, the charged secondary battery 3 may be connected to one input terminal of the comparator 2a, and the reference voltage source 2b may be connected to the other input terminal.
[Modification of charging device]
FIG. 4 shows a circuit configuration example of a charging device 200 obtained by modifying the charging device 100. In the charging apparatus 200, the second transistor 1a is replaced with a PMOS transistor 20, and the overcharge detection circuit 2 is replaced with a reset IC 30. Further, a light emitting diode 22 is added for the purpose of displaying the state of charge.

  The body diode 20a connected in parallel between the source and drain of the PMOS transistor 20 is a parasitic diode provided in the structure of the MOS transistor. The current path of the body diode 20a is blocked by the diode 1c. Further, no current flows through the gate of the PMOS transistor 20. Therefore, the charging device 200 can improve the power efficiency as compared with the charging device 100, and the circuit design is facilitated.

  The reset IC 30 is a general-purpose IC that is also called a voltage detector IC, and is extremely small and inexpensive. Replacing the overcharge detection circuit 2 of the charging device 100 with a reset IC 30 makes it possible to make the charging device 200 smaller and less expensive than configuring the overcharge detection circuit 2 with discrete components.

The reset IC 30 has a size of 1.2 mm ^W × 1.6 mm ^L × 0.5 mm ^H , for example, according to a package specification called SNT-4A. The operation of the charging apparatus 200 will be described with a specific example.

It is assumed that the reference voltage V _ref of the reference voltage source 30b of the reset IC 30 is 4.3V, and the output stage of the reset IC 30 is configured by an open drain NMOS transistor 30c. The output voltage of the charging power source 4 is a 5.5V DC power source, and the charged secondary battery 3 is a nominal 3.7V / 480mAh lithium polymer battery. The full charge equivalent voltage of this lithium polymer battery is, for example, 4.3V. As the diode 1c of the charging constant current circuit 1, a Schottky diode having a forward voltage drop of 0.4V when 250 mA is energized is used. The first resistance element 1d, which is a current limiting resistor, is set to 1Ω, for example.

In the initial state of charging, the voltage of the secondary battery 3 to be charged is lower than the reference voltage V _ref (4.3V), so the output voltage of the comparator 30a is the positive voltage of the charging power source 4. Therefore, the NMOS transistor 30c constituting the output stage of the reset IC 30 is in the ON state, and the PMOS transistor 20 is also in the ON state. At this time, the charging the secondary battery 3 is charged at a charge current I _CH of about 200mA. Further, the light emitting diode 22 is turned on because current is supplied from the charging power supply 4 via the resistor 21, and displays that charging is in progress.

As the charging proceeds, the voltage of the secondary battery 3 to be charged increases and becomes higher than the reference voltage V _ref (4.3V). Then, the output voltage of the comparator 30a changes to the negative voltage of the charging power supply 4. When the output voltage of the comparator 30a changes to the negative voltage of the charging power supply 4, the NMOS transistor 30c is turned off. When the NMOS transistor 30c is turned off, the voltage at the other end of the second resistance element 1e changes to a voltage close to the positive voltage of the charging power supply 4, so that the PMOS transistor 20 is turned off and the charging current I _CH is Blocked. At the same time, the light emitting diode 22 is turned off because no current is supplied.

  Note that the charging device 200 can be formed of a transistor having a reverse polarity, as described in the charging constant current circuit 1. FIG. 5 shows a circuit configuration example of a charging device 200 ′ configured using reverse polarity transistors. The operation is the same as that of the charging device 200 except that the polarity is reversed.

In addition, although the example which combined the PMOS transistor 20 and the reset IC30 was demonstrated, you may replace the 2nd transistor of the charging device 100 (FIG. 3) with a MOS transistor.
Moreover, although the example which combined the light emitting diode 22 and the reset IC30 was demonstrated, you may make it connect the cathode of the light emitting diode 22 to the output terminal of the comparator 2a of the charging device 100 (FIG. 3). In other words, the charging display can be performed by providing the third resistance element 21 connected in series between the charging power supply 4 and the output terminal of the comparator and the light emitting diode 22 connected in the forward direction. is there.

  As described above, the charging constant current circuit 1 of the present invention can reduce the temperature dependence of the charging current and reduce the cost. Moreover, the charging apparatus 100 of this invention can prevent the overcharge of a secondary battery using the constant current circuit for charging with the small temperature change of the charging current. Moreover, the charging device 200 of the present invention using a general-purpose reset IC can be a smaller and lower cost charging device.

Claims (7)

A first transistor and a second transistor of the same polarity;
A diode,
First and second resistance elements,
The base of the first transistor and the emitter of the second transistor, the collector of the first transistor and the base of the second transistor are connected, respectively.
The diode and the first resistance element are arranged in series between a base and an emitter of the first transistor, and the diode is connected in a direction in which the feature and polarity of the base of the first transistor coincide with each other.
The second resistance element is a charging constant current circuit having one end connected to a connection point between the collector of the first transistor and the base of the second transistor. A first transistor and a second transistor of the same polarity;
A diode,
First and second resistance elements,
The base of the first transistor and the emitter of the second transistor, the collector of the first transistor and the base of the second transistor are connected, respectively.
The diode and the first resistance element are arranged in series between a base and an emitter of the first transistor, and the diode is connected in a direction in which the feature and polarity of the base of the first transistor coincide with each other.
The second resistive element is a charging constant current circuit having one end connected to a connection point between the collector of the first transistor and the base of the second transistor,
A charging power source is connected between the emitter of the first transistor and the other end of the second resistance element,
A charging constant current circuit, wherein a charged secondary battery is connected between the emitter of the second transistor and the other end of the second resistance element. In the constant current circuit for charging according to claim 1 or 2,
A constant current circuit for charging, wherein the diode is a Schottky diode. The constant current circuit for charging according to any one of claims 1 to 3,
The constant current circuit for charging, wherein the second transistor is a MOS transistor. A constant current circuit for charging according to any one of claims 1 to 4,
A comparator that connects one input terminal to the charged secondary battery and connects the other input terminal to a reference voltage source;
The charging device, wherein an output terminal of the comparator is connected to the other end of the second resistance element. The charging device according to claim 5,
A third resistance element connected in series between the charging power source and the output terminal of the comparator, and a light emitting diode connected in the forward direction,
A charging device which performs charging display with the light emitting diode. The charging device according to claim 5 or 6,
The charging device, wherein the comparator and the reference voltage source are constituted by a general-purpose reset IC.

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