WO2015016427A1

WO2015016427A1 - Secondary battery charging circuit using linear regulator

Info

Publication number: WO2015016427A1
Application number: PCT/KR2013/009893
Authority: WO
Inventors: 박시홍; 김준식; 진기웅
Original assignee: 단국대학교 산학협력단
Priority date: 2013-08-01
Filing date: 2013-11-04
Publication date: 2015-02-05
Also published as: KR20150016442A; KR101509323B1

Abstract

A method for charging a secondary battery is disclosed. A constant voltage mode is used when charging with low current and a constant current mode using a switching-operation is used when charging with high current. A linear regulator is used for the low current charging operation in the constant voltage mode, and switching is repeatedly performed according to a PWM operation for the high current charging operation in the constant current mode. Therefore, a voltage ripple applied to a cell when charging is reduced.

Description

Secondary Battery Charging Circuit Using Linear Regulator

The present invention relates to a secondary battery charging circuit, and more particularly, to a secondary battery charging circuit using a switching method and a linear regulator method and a charging method for driving the same.

The secondary battery has a positive electrode and a negative electrode, and lithium ions are reversibly transferred between the two electrodes. Secondary batteries have many advantages, such as high energy density, high operating voltage and excellent retention and life characteristics.

The charging operation of the secondary battery is to apply electrons to the negative electrode, and is usually performed by applying a DC component controlled by a constant voltage to the electrode and supplying electrons due to the current caused by the difference between the applied voltage and the battery internal voltage. Applied voltage is defined per cell (denoted C), which is the nominal unit of the battery. Normally, the lithium ion battery and lithium polymer battery for mobile phones should be applied with a strictly limited constant voltage of 4.2V per cell (rated 3.6V). When 4.5 V or more is applied per unit cell, the electrolyte is decomposed to generate gas, leakage occurs, and there is a risk of explosion. In addition, in an over-discharge state of 1.5 V or less per unit cell, the current collector of the negative electrode is dissolved in the electrolyte, and the battery performance is lowered. Therefore, the secondary battery is provided with a protection circuit in order to set a voltage range for stable charging and discharging.

The protection circuit has a function to stop charging current above 4.35V, discharge current stop below 2.3V, and discharge current stop at output terminal short circuit. The charging current is performed in the range of 0.1C to 1.5C. In the case of a lithium ion battery of 550mAh (1C = 550mAh), charging is performed at a current of 600 mA to 700 mA. When the charging current is increased for rapid charging, the temperature rises. This affects the charge / discharge cycles (usually 300 times), resulting in a problem of reduced battery life. The charging circuit should be configured for stable charging and discharging operation as soon as possible without adversely affecting the life and performance of the secondary battery.

In addition, a constant current method is used at the start of charging of the secondary battery, and the charging current is applied at a constant size. In addition, when the terminal voltage rises to a certain level, the constant voltage circuit is driven to apply a constant voltage to the electrodes of the secondary battery. When the charging operation proceeds and the voltage level between the electrodes exceeds the level of the voltage to which the constant voltage circuit is applied, the secondary battery becomes overcharged, causes defects, or degrades life and stability.

In order to perform the charging operation of the secondary battery, a charging circuit, a regulator, and a switch are provided. The charging circuit receives power from the outside to charge the cell, and the regulator forms a power supply voltage applied from the outside to a constant DC level or sets the output voltage of the charging circuit to a specific voltage level. The switch is used to select a constant current method or a constant voltage method.

In particular, in the charging circuit of the secondary battery, a pulse-frequency modulation control method is mainly used during low current driving, which is a constant voltage method. In this method, a large current is supplied through the inductor during the turn-on period of switching, and the charging is repeated while the turn-off operation is repeated. In the low current configuration, a large amount of current ripple occurs, and an output voltage also appears ripple. In most cases, the ripple component at the output voltage of a secondary battery is strongly related to the life of the battery.

Disclosure of Invention The present invention has been made to solve the above-described problems, and provides a charging circuit having a simple structure including a linear regulator and minimizing current and voltage ripple when charging at low current.

The present invention for solving the above problems, the linear regulator unit for operating in a constant voltage mode; PWM operation unit for operating in the constant current mode; And a mode selection unit for selectively receiving output signals of the linear regulator unit and the PWM operation unit to perform a charging operation in the constant voltage mode or the constant current mode.

The object of the present invention, the linear regulator unit for operating in the constant voltage mode is supplied with a low current at the time of the charging operation; A PWM operation unit for operating in the constant current mode through PWM control according to the increase in the terminal voltage of the cell in the constant voltage mode; A mode selection unit for selectively receiving output signals of the linear regulator unit and the PWM operation unit to perform a charging operation in the constant voltage mode or the constant current mode; And a sensing unit for sensing an output of the mode selection unit and controlling an operation of the PWM operation unit.

According to the present invention, the low current charging method of the secondary battery charging circuit is a constant voltage mode using a linear regulator. The use of the linear regulator makes the structure simpler than the conventional PFM control method, and can reduce the ripple of the output voltage in the low current charging method, thereby extending system stability and battery life.

1 is a circuit diagram of a rechargeable battery charging circuit according to a first embodiment of the present invention.

2 is a circuit diagram of a rechargeable battery charging circuit according to a second embodiment of the present invention.

3 is a circuit diagram of a rechargeable battery charging circuit according to a third embodiment of the present invention.

4 is a circuit diagram illustrating an example of a voltage sensing unit of a sensing unit.

5 is an image showing a comparison of the waveform of the charging current and voltage of the conventional PFM method and the method using the linear regulator of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In describing the embodiments of the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

Referring to FIG. 1, the charging circuit of the present embodiment includes a linear regulator unit 100, a PWM operation unit 200, a first mode selector 300, and a sensing unit 400.

The linear regulator 100 receives an input charging voltage VCHG and performs a linear regulation operation on the charging voltage VCHG. The linearly regulated voltage is applied to the first mode selector 300. The linear regulator unit 100 includes a current source 102, an error amplifier 101, a power transistor QP and a feedback unit 103.

The PWM operation unit 200 receives the voltage sensed by the feedback unit 103 of the linear regulator, and is activated when a voltage equal to or higher than a predetermined reference level is applied. The output signal of the PWM operation unit 200 is applied to the first mode selection unit 300.

The first mode selector 300 receives an output signal of the linear regulator unit 100 and an output signal of the PWM operation unit 200. Transistors QNM and QNS operate complementarily by the reception of signals. The output signal of the first mode selector 300 is applied to the sensing unit 400.

The sensing unit 400 has a sensing resistor Rs and a voltage sensing unit 401. The current flowing through the sensing resistor Rs is represented by the voltage difference Vs, and the voltage difference Vs displayed at both ends of the sensing resistor Rs is sensed by the voltage sensing unit 401. According to the exemplary embodiment, the voltage difference Vs sensed by the sensing unit 400 may be output at a voltage level of a specific type, and the output voltage level may be used for the activation operation of the PWM operation unit 200.

Therefore, the sensing voltage sensed by the feedback unit 103 or the output of the sensing unit 400 may be applied to the PWM operation unit 200.

First, when the charging operation for the secondary battery C is started, the charging circuit operates in the constant voltage mode. That is, the terminal voltage of the secondary battery C is kept very low. The feedback unit 103 of the linear regulator has two resistors R1 and R2 and senses the voltage applied to the secondary battery C according to the distribution ratio of the resistor. The feedback voltage, which is the sensed voltage, is applied to the error amplifier 101. The feedback voltage of the feedback unit 103 of the linear regulator unit 100 is low and becomes less than the reference voltage VREF applied to the positive input terminal of the error amplifier 101.

Therefore, the error amplifier 101 outputs a high level signal, and the power transistor QP is turned off. Therefore, the linear regulator applies the charging voltage VCHG to the gate terminal of the transistor QNS of the first mode selector 300. The transistor QNS is turned on by the charging voltage applied to the gate terminal of the transistor QNS. Therefore, the input voltage VIN having a constant level is applied to the sensing unit 400 by the turned-on transistor QNS. The input voltage VIN applied to the sensing unit 400 is applied to the secondary battery C, and the charging operation of the constant voltage mode is performed in the secondary battery C.

In addition, due to the feedback voltage of the feedback unit 103 of the low level, the PWM operation unit 200 is stopped or inactivated. Thus, in the constant voltage mode, the transistor QNM remains off. In addition, since the amount of current charged in the secondary battery C at the beginning of the charging operation has a small value, the voltage difference Vs of the sensing resistor Rs sensed by the sensing unit 400 maintains a low value. If the element that determines the operation of the PWM operation unit 200 is an output signal of the sensing unit 400, the PWM operation unit 200 is deactivated due to the voltage difference Vs of the sensing resistor Rs having a low level.

As the charging operation in the constant voltage mode proceeds, the amount of current flowing into the secondary battery C increases. In addition, the voltage appearing at the electrodes of the secondary battery C also increases. Therefore, the feedback voltage sensed by the feedback unit 103 of the linear regulator unit 100 also increases. In particular, when the feedback voltage sensed by the feedback unit 103 is greater than or equal to the reference voltage VREF, the error amplifier 101 outputs a low level, and the power transistor QP is turned on. Accordingly, the linear regulator applies a low level signal to the gate terminal of the transistor QNS of the first mode selector 300, and the transistor QNS is turned off.

At the same time, the feedback voltage sensed by the feedback unit 103 activates the PWM operation unit 200. In the PWM operation unit 200, a specific reference level is preset, and when a voltage exceeding the set reference level is generated in the feedback unit 103, the PWM operation unit 200 is activated to form a PWM signal. Therefore, the transistor QNM of the first mode selector 300 repeats the on / off operation. Thus, driving in the constant current mode is started.

As the charging operation in the constant current mode proceeds, a periodic signal of the transistor QNM generates an induced electromotive force in the inductor L, and a high level current is applied to the sensing resistor Rs. Therefore, the voltage difference Vs of the sensing resistor Rs maintains a high value, and the secondary battery C is charged due to the high level current. At the same time, the voltage of the secondary battery C rises.

When the charging operation for the secondary battery C is performed so that a voltage of a certain level or more appears on the electrode of the secondary battery C, charging in the constant current mode is terminated, and charging operation in the constant voltage mode is performed again.

That is, while the terminal voltage of the secondary battery C is maintained at a very high value, the secondary battery C

The current supplied decreases. Accordingly, the feedback voltage of the feedback unit 103 of the linear regulator unit 100 is lowered, and becomes less than the reference voltage VREF applied to the positive input terminal of the error amplifier 101. Therefore, the error amplifier 101 outputs a high level signal, and the power transistor QP is turned off. Therefore, the linear regulator applies the charging voltage VCHG to the gate terminal of the transistor QNS of the first mode selector 300. The transistor QNS is turned on by the charging voltage applied to the gate terminal of the transistor QNS. Therefore, the input voltage VIN having a constant level is applied to the sensing unit 400 by the turned-on transistor QNS. The input voltage VIN applied to the sensing unit 400 is applied to the secondary battery C, and the charging operation of the constant voltage mode is performed in the secondary battery C.

In addition, due to the sensed voltage of the low-level feedback unit 103, the PWM operation unit 200 again stops or deactivates the operation. Thus, in the constant voltage mode, the transistor QNM remains off.

Second Embodiment

Referring to FIG. 2, the charging circuit of the present embodiment includes a linear regulator unit 100, a PWM operation unit 200, a second mode selection unit 310, and a sensing unit 400.

The linear regulator unit 100 has the same configuration as that of FIG. 1. Accordingly, the linear regulator unit 100 includes a current source 102, an error amplifier 101, a power transistor QP and a feedback unit 103.

The PWM operation unit 200 receives the feedback voltage sensed by the feedback unit 103 of the linear regulator, and is activated when a voltage equal to or higher than a predetermined reference level is applied. The output signal of the PWM operation unit 200 is applied to the second mode selection unit 310.

The second mode selector 310 receives an output signal of the linear regulator unit 100 and an output signal of the PWM operation unit 200. The first switch 301 may select the output of the linear regulator or the output of the PWM operation unit 200. In addition, the second switch 302 may bypass the inductor L and the sensing resistor Rs through an on / off operation. The output signal of the second mode selector 310 is selectively applied to the sensing unit 400.

Therefore, the feedback voltage sensed by the feedback unit 103 or the output of the sensing unit 400 may be applied to the PWM operation unit 200.

First, when the charging operation for the secondary battery C is started, the charging circuit operates in the constant voltage mode. In the constant voltage mode, the first switch 301 is connected to the linear regulator unit 100 and the second switch 302 is turned on. In addition, the terminal voltage of the secondary battery C is kept very low. The feedback voltage of the feedback unit 103 of the linear regulator unit 100 is low and becomes less than the reference voltage VREF applied to the positive input terminal of the error amplifier 101. Therefore, the error amplifier 101 outputs a high level signal, and the power transistor QP is turned off. Accordingly, the linear regulator applies the charging voltage VCHG to the gate of the transistor QNM through the first switch 301 of the second mode selector 310. The transistor QNM is turned on by a specific level of charging voltage applied to the gate terminal of the transistor QNM, and the input voltage VIN having a constant level does not pass through the inductor L and the sensing unit 400 by the turned-on transistor QNM, Is applied to the switch 302. The input voltage VIN applied to the second switch 302 is applied to the secondary battery C, and the charging operation in the constant voltage mode is performed in the secondary battery C.

In addition, due to the feedback voltage of the feedback unit 103 of the low level, the PWM operation unit 200 is stopped or inactivated.

As the charging operation in the constant voltage mode proceeds, the amount of current flowing into the secondary battery C increases. In addition, the voltage appearing at the electrodes of the secondary battery C also increases. Therefore, the feedback voltage sensed by the feedback unit 103 of the linear regulator unit 100 also increases. In particular, when the feedback voltage sensed by the feedback unit 103 is greater than or equal to the reference voltage VREF, the error amplifier 101 outputs a low level, the power transistor QP is turned on, and the first switch 301 selects the second mode. An electrical connection between the unit 310 and the PWM signal generator is achieved.

At the same time, the feedback voltage sensed by the feedback unit 103 activates the PWM operation unit 200. In the PWM operation unit 200, a specific reference level is preset, and when a voltage exceeding the set reference level is generated in the feedback unit 103, the PWM operation unit 200 is activated to form a PWM signal. Therefore, the transistor QNM of the second mode selector 310 repeats the on / off operation. Thus, driving in the constant current mode is started, and the second switch 302 is opened.

That is, while the terminal voltage of the secondary battery C is maintained at a very high value, the current supplied to the secondary battery C decreases. Accordingly, the feedback voltage of the feedback unit 103 of the linear regulator unit 100 is lowered, and becomes less than the reference voltage VREF applied to the positive input terminal of the error amplifier 101. Therefore, the error amplifier 101 outputs a high level signal, and the power transistor QP is turned off. Accordingly, the linear regulator applies the charging voltage VCHG to the gate terminal of the transistor QNM of the second mode selector 310.

The transistor QNM is turned on by a specific level of charging voltage applied to the gate terminal of the transistor QNM, and the input voltage VIN having a constant level does not pass through the inductor L and the sensing unit 400 by the turned-on transistor QNM, Is applied to the switch 302. Since the current also has a constant level due to the input voltage VIN having a constant level, no induced electromotive force is generated by the inductor L. The input voltage VIN applied to the second switch 302 is applied to the secondary battery C, and the charging operation in the constant voltage mode is performed in the secondary battery C.

Third Embodiment

Referring to FIG. 3, the charging circuit of the present embodiment includes a linear regulator unit 100, a PWM operation unit 200, a third mode selection unit 320, and a sensing unit 400.

The configuration and operation of the linear regulator unit 100 is the same as described with reference to FIGS. 1 and 2.

However, in the present embodiment, the configuration of the third mode selector 320 is different from those of FIGS. 1 and 2. Therefore, the charging circuit of the present embodiment will be described centering on the third mode selector 320 having a different configuration.

The third mode selector 320 of the present embodiment has a third switch 303. The third switch 303 may select the output of the linear regulator or the output of the PWM operation unit 200. Accordingly, the third mode selector 320 may selectively receive the output of the linear regulator and the output of the PWM operation unit 200.

First, when the charging operation for the secondary battery C is started, the charging circuit operates in the constant voltage mode. In the constant voltage mode, the third switch 303 is turned on. In addition, the terminal voltage of the secondary battery C is kept very low. The feedback voltage of the feedback unit 103 of the linear regulator unit 100 is low and becomes less than the reference voltage VREF applied to the positive input terminal of the error amplifier 101. Therefore, the error amplifier 101 outputs a high level signal, and the power transistor QP is turned off. Accordingly, the linear regulator applies the charging voltage VCHG to the gate of the transistor QNM through the third switch 303 of the third mode selector 320.

The transistor QNM is turned on by a specific level of charging voltage applied to the gate terminal of the transistor QNM, and the input voltage VIN having a constant level is applied to the inductor L by the turned-on transistor QNM. In addition, since the current also has a constant level due to the input voltage VIN having a constant level, no induced electromotive force is generated by the inductor L. The input voltage VIN applied to the sensing unit 400 is applied to the secondary battery C, and the charging operation of the constant voltage mode is performed in the secondary battery C.

As the charging operation in the constant voltage mode proceeds, the amount of current flowing into the secondary battery C increases. In addition, the voltage appearing at the electrodes of the secondary battery C also increases. Therefore, the feedback voltage sensed by the feedback unit 103 of the linear regulator unit 100 also increases. In particular, when the feedback voltage sensed by the feedback unit 103 is greater than or equal to the reference voltage VREF, the error amplifier 101 outputs a low level, the power transistor QP is turned on, and the third switch 303 selects the third mode. It is turned on between the unit 320 and the PWM signal generator.

At the same time, the feedback voltage sensed by the feedback unit 103 activates the PWM operation unit 200. In the PWM operation unit 200, a specific reference level is preset, and when a voltage exceeding the set reference level is generated in the feedback unit 103, the PWM operation unit 200 is activated to form a PWM signal. Therefore, the transistor QNM of the third mode selector 320 repeats the on / off operation. Thus, driving in the constant current mode is started.

As the charging operation in the constant current mode proceeds, a periodic signal of the transistor QNM generates an induced electromotive force in the inductor L, and a high level current is applied to the sensing resistor Rs. Therefore, the voltage difference Vs of the sensing resistor Rs maintains a high value, and the secondary battery C is charged due to the high level current. At the same time, the terminal voltage of the secondary battery C rises.

When the charging operation for the secondary battery C is completed, the charging circuit operates again in the constant voltage mode. That is, while the terminal voltage of the secondary battery C is maintained at a very high value, the current supplied to the secondary battery C decreases. Accordingly, the feedback voltage of the feedback unit 103 of the linear regulator unit 100 is lowered, and becomes less than the reference voltage VREF applied to the positive input terminal of the error amplifier 101. Therefore, the error amplifier 101 outputs a high level signal, and the power transistor QP is turned off. Accordingly, the linear regulator applies the charging voltage VCHG to the gate terminal of the transistor QNM of the third mode selector 320.

Transistor QNM is turned on by a specific level of charging voltage applied to the gate terminal of QNM, and input voltage VIN having a constant level is applied to inductor L by turned-on transistor QNM. In addition, since the current also has a constant level due to the input voltage VIN having a constant level, no induced electromotive force is generated by the inductor L.

The input voltage VIN applied to the sensing unit 400 is applied to the secondary battery C, and the charging operation of the constant voltage mode is performed in the secondary battery C.

In addition, the PWM operation unit 200 is stopped or inactivated due to the sensed voltage of the feedback unit 103 at a low level.

The voltage detector 401 of FIG. 4 operates as the voltage detector 401 disclosed in the first to third embodiments.

Referring to FIG. 4, the voltage detector 401 has a configuration of a subtractor using the OP amplifier 402.

The first input voltage VIN1 and the second input voltage VIN2 represent voltages at both ends of the sensing resistor Rs. Therefore, the voltage difference Vs across the sensing resistor Rs is VIN1-VIN2. When the input voltage is a low level value, the signal input to the feedback unit 103 of the linear regulator also has a low level value, so that the charging method is in the constant voltage mode. On the other hand, when the input voltage is a high level value, since the signal input to the feedback unit 103 of the linear regulator also has a high level value, the charging method is a constant current mode.

The reference voltage VREF2 is a specific reference voltage applied to the positive input terminal of the OP amplifier 402. When the input voltage is less than the reference voltage VREF2, the output voltage output from the sensing unit 400 through the operation of the OP amplifier 402. Has a high level value. On the other hand, when the input voltage is greater than or equal to the reference voltage VREF2, the output voltage has a low level. Accordingly, signal control applied to the PWM operation unit 200 and the switch may be performed through the value of the output voltage of the voltage sensing unit 401.

Therefore, the output Vout of the voltage detector 401 may be represented by Equation 1 below.

Equation 1

Referring to FIG. 5, in the PFM method, when the level of the charging current is varied, ripple occurs in the output voltage. However, in the present invention, even when the level of the charging current is varied, the ripple of the output voltage is greatly reduced. This is because a mode change is not performed when switching from a low current state to a high current state in the PFM method, so that a sudden change in the amount of current in the charging circuit causes a ripple in the charging voltage.

On the other hand, in the present invention, since it operates in the constant voltage mode while being charged with low current, the ripple of the voltage may be blocked at source.

Therefore, according to the present invention, the ripple of the charging voltage due to the change of the charging current amount is significantly reduced. In addition, when the linear regulator unit 100 is driven, the voltage charged in the secondary battery C may be kept constant to prevent overcharging.

In the foregoing detailed description of the present invention, specific embodiments have been described, but various modifications may be made without departing from the scope of the present invention.

It will be obvious to those with ordinary knowledge in the field.

Claims

A linear regulator unit for operating in a constant voltage mode;

PWM operation unit for operating in the constant current mode; And

And a mode selection unit for selectively receiving output signals of the linear regulator unit and the PWM operation unit to perform a charging operation in the constant voltage mode or the constant current mode.
The linear regulator unit of claim 1,

A feedback unit for sensing a voltage charging the cell and forming a feedback voltage according to a resistance distribution ratio;

An error amplifier for receiving a feedback voltage and a reference voltage of the feedback unit and amplifying a difference between the feedback voltage and the reference voltage; And

And a power transistor for receiving the output of the error amplifier and performing an on / off operation.
The power regulator of claim 2, wherein the power transistor is turned off in the constant voltage mode, the linear regulator part outputs a charging voltage, and the power transistor is turned on in the constant current mode, and the linear regulator outputs a low level. Secondary battery charging circuit.
The method of claim 3, wherein the mode selection unit,

A first transistor for receiving an output signal of the PWM operation unit; And

And a second transistor for receiving the output of the linear regulator unit.
The secondary battery charging circuit of claim 4, wherein the second transistor is turned on in the constant voltage mode to transmit an input voltage to a cell.
The secondary battery charging circuit of claim 4, wherein in the constant current mode, the first transistor is repeatedly turned on to transmit an input voltage to a cell.
The method of claim 3, wherein the mode selection unit,

A first switch for selecting an output of the linear regulator unit or an output of the PWM operation unit; And

And a third transistor for performing an on / off operation according to a signal transmitted through the first switch.
The method of claim 7, wherein the first switch transmits the output of the linear regulator part to the third transistor in the constant voltage mode, and transmits the output of the PWM operation part to the third transistor in the constant current mode. Secondary battery charging circuit.
10. The method of claim 7, wherein the mode selector further comprises a second switch for selectively receiving an input voltage transmitted through the third transistor and bypassing an inductor connected to the third transistor. Primary battery charging circuit.
The secondary battery charging circuit of claim 1, wherein the secondary battery charging circuit further comprises a sensing unit configured to sense a current for charging the cell by the mode selection unit.
A linear regulator unit for operating in a constant voltage mode to which a low current is supplied at the time of the charging operation;

A PWM operation unit for operating in the constant current mode through PWM control according to the increase in the terminal voltage of the cell according to the constant voltage mode;

A mode selection unit for selectively receiving output signals of the linear regulator unit and the PWM operation unit to perform a charging operation in the constant voltage mode or the constant current mode; And

And a sensing unit for sensing an output of the mode selection unit and controlling an operation of the PWM operation unit.
The method of claim 11, wherein the mode selection unit,

A first transistor for receiving an output signal of the PWM operation unit; And

And a second transistor for receiving the output of the linear regulator unit.
The method of claim 11, wherein the mode selection unit,

A first switch for selecting an output of the linear regulator unit or an output of the PWM operation unit; And

And a third transistor for performing an on / off operation according to a signal transmitted through the first switch.
The method of claim 13, wherein the first switch transmits the output of the linear regulator part to the third transistor in the constant voltage mode, and delivers the output of the PWM operation part to the third transistor in the constant current mode. Secondary battery charging circuit.