KR100539216B1 - Battery charge circuit - Google Patents

Abstract

The present invention relates to a battery charging circuit, comprising: a primary circuit for inputting commercial AC voltage, a transformer for stepping down the output of the primary circuit, and a rectifying voltage output from the transformer to output a DC voltage output voltage. The secondary side circuit includes a secondary side circuit, and when the battery is connected to the secondary side circuit to charge the battery, when the voltage of the battery is lower than the first voltage, the secondary side circuit outputs a second constant current, and the voltage of the battery is increased. When the voltage is higher than the first voltage, the secondary-side circuit outputs less current than the second constant current, and when the voltage of the battery increases and reaches the second voltage, the secondary-side circuit is smaller than the second constant current. By outputting a constant current, the battery charging time is accelerated.

Description

Battery charge circuit

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to electrical circuits, and more particularly, to a battery charging circuit for charging a lithium ion battery.

In the case of portable products such as notebook computers (PCs), how long they can be used continuously has become an important factor in evaluating their performance. Therefore, a lot of developments and investments are being made to improve the performance of the battery as well as the performance of the product, and portable products using the battery such as lithium ions that can store more energy even with the same weight of electricity are mainly focused on expensive products. It is gradually increasing. Compared to conventional Nicad batteries or NiMH batteries, lithium ion batteries have more energy density at the same weight. The charging method of such a lithium ion battery is different from that of a nickel-card battery or a nickel-hydrogen battery. The charging method of lithium ion battery is Ni-cad battery. In order to charge nickel-metal hydride batteries, constant current and constant voltage charging methods are basically required. More specifically, in the case of a lithium ion battery, the battery is initially charged with a constant current, and when the battery voltage is above a certain level, the battery is charged with a constant voltage thereafter. If the voltage of the lithium ion battery increases above a certain voltage, it is necessary to control to maintain a constant voltage, which may cause a dangerous situation leading to the explosion of the battery.

When the lithium ion battery is continuously charged with a constant current until the voltage of the lithium ion battery reaches the maximum voltage, the lithium ion battery is controlled only in the constant voltage mode in which the voltage is kept constant from the moment when the voltage of the lithium ion battery reaches the maximum voltage. The maximum output power of the output stage to which the ion storage battery is connected becomes relatively large. If the maximum output power is increased by charging the lithium ion battery only by the constant current and the constant voltage method as described above, there are many problems such as difficulty in thermal problems and increase in size in terms of the system using the lithium ion battery. . When the products of the conventional charging adapters are more compact and have a confined space, problems such as a decrease in reliability due to thermal characteristics and an increase in system size will be very important problems. The maximum voltage of a Li-ion battery is a rated voltage defined by the battery manufacturer, which is determined by a constant voltage, depending on how many battery cells are connected in series. Therefore, in order to lower the maximum output power, it is possible to reduce the output power by lowering the constant current value in the constant current mode, which causes a longer charging time of the battery. Therefore, in the case of a product such as a small sealed lithium-ion charging adapter, it is charged with a large constant current up to a predetermined voltage lower than the maximum voltage of the lithium-ion battery in order to satisfy the temperature reliability and to shorten the charging time of the battery. To the maximum voltage of the lithium-ion battery by implementing a circuit to charge in a constant power mode that maintains a constant power it is possible to achieve faster charging and thermal stability.

An object of the present invention is to provide a battery charging circuit that outputs a constant power above a predetermined output voltage.

The present invention to achieve the above technical problem,

A secondary side circuit for inputting a commercial AC voltage, a transformer for stepping down the output of the primary side circuit, and a secondary side circuit for rectifying the voltage output from the transformer to output a DC level output voltage; When the battery is charged by connecting a battery to the battery, the secondary circuit outputs a second constant current when the voltage of the battery is lower than the first voltage, and when the voltage of the battery is higher than the first voltage, the secondary side. The circuit outputs less current than the second constant current, and when the voltage of the battery increases and reaches the second voltage, the secondary circuit outputs a first constant current less than the second constant current. Provide a circuit.

Preferably, the secondary circuit has one end connected to the secondary side of the transformer and a sensing resistor for generating an output voltage of the secondary circuit from the other end and a first voltage for sensing and amplifying a voltage generated in the sensing resistor. A first transistor connected to a base of an operational amplifier and an output terminal of the first operational amplifier, the first transistor being turned on when the output voltage is higher than a voltage applied to one end of the sense resistor, and a Zener diode connected to the other end of the sense resistor (Zener Diode), a second operational amplifier which is deactivated when the zener diode is turned on and is activated when the zener diode is turned off, a base is connected to an output terminal of the second operational amplifier, A second transistor, the first and second transistors that turn on when the output voltage reaches a predetermined level and flow current generated at the other end of the sensing resistor; A control resistor connected between the output terminals of the resistors and ground, first and second resistors connected in series to the other end of the sense resistor, a first comparator comparing the voltage generated by the control resistor with a reference voltage, and the first and second resistors. And a second comparator for comparing the voltage generated between the second resistor and the reference voltage, wherein the first and second transistors are both PNP transistors.

Preferably, the secondary circuit includes a sensing resistor for generating an output voltage of the secondary circuit from one end connected to the secondary side of the transformer, and sensing and amplifying a voltage generated at the sensing resistor. An amplifier, a first transistor connected to a base of an output terminal of the first operational amplifier and turned on when the output voltage is higher than a voltage applied to one end of the sense resistor, a current mirror connected to the other end of the sense resistor, and the current Zener diode connected to the output terminal of the mirror, a control resistor connected between the connection point and the ground connected to the output terminal of the current mirror and the output terminal of the first transistor, the first and second resistors connected in series to the other end of the sense resistor, the control resistor A first comparator for comparing the reference voltage and the voltage generated in the circuit, and a foot between the first and second resistors. And voltage and a second comparator for comparing the reference voltage.

Preferably, the current mirror is a second transistor connected between the other end of the sense resistor and the output end of the first transistor, and the other end of the sense resistor and the zener diode and is based on the base of the second transistor. Has a third transistor to which the second and third transistors are both PNP transistors.

By the battery charging circuit of the present invention, the charging time of the battery becomes faster and is thermally stable.

In order to fully understand the present invention, the operational advantages of the present invention, and the objects achieved by the practice of the present invention, reference should be made to the accompanying drawings which illustrate preferred embodiments of the present invention and the contents described in the accompanying drawings.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Like reference numerals in the drawings denote like elements.

1 is a circuit diagram of a battery charging circuit according to a first embodiment of the present invention. Referring to FIG. 1, the battery charging circuit according to the first embodiment of the present invention includes a primary side circuit 111, a transformer 115, and a secondary side circuit. The secondary circuit includes the rectifier circuit 121, the sense resistor R _sense1 , the resistors R4 to _R12 , the capacitors C11 to C15, the zener diode ZD1, the PNP transistors Q1 and Q2, and an operational amplifier. 151, 152 and comparators 161, 162. A commercial AC voltage input to the primary side circuit 11, for example 110 volts or 220 volts, is stepped down through the transformer 115, and the stepped down voltage is converted into a DC voltage through the rectifier circuit 121. The output voltage Vo1 output from the secondary side circuit is supplied to the storage battery 125, for example, a lithium ion storage battery. The secondary circuit supplies a constant current to the storage battery 125 when the secondary circuit operates in the constant current mode, and supplies a constant voltage to the storage battery 125 when the secondary circuit operates in the constant voltage mode. When the circuit operates in the constant power mode, constant power is supplied to the storage battery 125.

The current flowing through the resistor R6 and the PNP transistor Q2 is as follows.

I _R6 = (Vo1-V _ZD1 ) / R6

I _Q2 = I _R6 × ((β-1) / β)

Here, β = Ie / I _b , which is β >> 1. therefore,

I _Q2 ≒ I _R6

Is equivalent to And I _Q1 = I _R4 . The voltage V _R10 generated in the resistor R10 is as follows.

V _R10 = (I _Q1 + I _Q2 ) × R10

The output characteristics of the battery charging circuit shown in FIG. 1 are as shown in FIG. 2. That is, when the voltage of the battery 125 is lower than the first voltage V1, much current is supplied from the battery charging circuit to the battery 125, such as the second current A2, and the voltage of the battery 125 is second. When the voltage is increased to V2, a current as low as the first current A1 is supplied from the battery charging circuit to the battery 125, and the voltage of the battery 125 is higher than the first voltage V1 and the second voltage V2. When lower, more current than the first current A1 and less than the second current A2 is supplied from the battery charging circuit to the storage battery 125.

As described above, since the constant current supplied from the battery charging circuit to the battery 125 is elastically adjusted according to the size of the charging voltage of the battery 125, not only the charging speed of the battery 125 is fast but also the battery is thermally stable.

FIG. 3 is an equivalent circuit diagram of a secondary circuit when the battery charging circuit shown in FIG. 1 operates in the constant current mode. The reason why the circuit shown in FIG. 1 is represented by the equivalent circuit shown in FIG. 3 will be described. When the voltage of the battery 125 is lower than the control voltage V _ZD1 of the zener diode ZD1, the battery charging circuit illustrated in FIG. 1 operates in the constant current mode as shown in FIG. 3. When the voltage of the storage battery 125 is lower than the zener voltage of the zener diode ZD1, the zener diode ZD1 is open. Then, the collector current i _Q2 of the transistor (Q2 in FIG. 1) becomes zero (0). In addition, since the voltage V _R12 of the inverting input terminal (-) of the comparator 154 is lower than the voltage Vref of the non-inverting input terminal (+), the transistor (not shown) inside the comparator 154 is opened. . As a result, when the output voltage Vo1 is lower than the zener voltage V _ZD1 of the zener diode ZD1, the voltage is equalized as shown in FIG. 6. When operating in the constant current mode of FIG. 6, the constant current Io1 supplied to the storage battery 125 is determined as follows.

Since the output of the comparator 153 is fed back to the inverting input terminal (-) by the capacitor C12, the voltage of the inverting input terminal (-) of the comparator 153 is a reference voltage input to the non-inverting terminal (+) of the comparator 153. The primary circuit 111 of the transformer 115 is controlled through the photo coupler 131 so as to have a voltage equal to (Vref). Therefore, in the normal state, the voltage V _R10 generated across the resistor R10 is equal to the reference voltage Vref. Substituting this in Equation 4 expands,

Vref = I _Q1 × R10 = I _R4 × R10, V _R4 = I _R4 × R4 = (Vref × R4) / R10

In addition, in the steady state, the voltages of the inverting input terminal (-) and the non-inverting input terminal (+) of the operational amplifier 151 are the same. At this time, if the input bias current of the operational amplifier 151 is zero, the voltage V _R4 at both ends of the resistor _R4 is expressed by Equation 6 below.

V _R4 = V _sense1 + V _R5

Here, the sense voltage V _sense1 is the sense resistor R _sense1 . The voltage is generated across both ends, and the voltage V _R5 is a voltage generated across the resistor R5. If the input bias current of the operational amplifier 151 is zero,

V _R4 = V _sense1 = Io1 × R _sense1

Therefore, the output current Io1 is

Io1 = V _R4 / R _sense1 = (Vref × R4) / (R10 × R _sense1 )

It becomes

FIG. 4 is an equivalent circuit diagram of a secondary circuit when the battery charging circuit shown in FIG. 1 operates in the constant voltage mode. The reason why the circuit shown in FIG. 1 is represented by the equivalent circuit shown in FIG. 4 will be described. When the voltage of the battery 125 reaches the second voltage (V2 shown in FIG. 2), the battery charging circuit shown in FIG. 1 operates in the constant voltage mode as shown in FIG. 4.

When the battery 125 is charged and the voltage of the battery 125 gradually increases to reach the second voltage V2, the comparator 162 remaining in the open collector state until then becomes normal operation, and thus the The internal output transistor (not shown) is open. When the output terminal of the comparator 161 is in the open collector state, it can be easily seen that the operation of the operational amplifier 151 and the operational amplifier 152 has no influence on the control. Thus, the battery charging circuit shown in FIG. 1 operates in the constant voltage mode as shown in FIG. 4. When the battery charging circuit shown in FIG. 1 is in the constant voltage mode, the voltage at the inverting input terminal (-) of the comparator 162 maintains the primary circuit of the transformer 115 through the photo coupler opt1 to maintain the reference voltage Vref. To control. Therefore, the output voltage Vo1 is maintained at a constant constant voltage as shown in Equation 9 below.

Vo1 = {Vref × (R11 + R12)} / R12

FIG. 5 is an equivalent circuit diagram of a secondary circuit when the battery charging circuit shown in FIG. 1 operates in the constant power mode. If the voltage of the battery 125 is higher than the first voltage (V1 shown in FIG. 2) and lower than the second voltage (V2 shown in FIG. 2), the battery charging circuit shown in FIG. It will operate in constant power mode. The operation of the circuit shown in FIG. 5 will be described.

When the voltage of the storage battery 125 starts to increase than the zener voltage V _ZD1 , the voltage V _R7 across the resistor _R7 also increases, which is expressed by Equation 10.

V _R7 = Vo1-V _ZD1

At this time, since the voltage of the non-inverting input terminal (+) of the comparator 162 operating as a shunt regulator and the voltage of the inverting input terminal (-) are the same, the voltage across the resistor (R7) (V _R7 ) The voltage V _R6 across the resistor R6 is the same. Therefore, the current I _R6 flowing through the resistor R6 is defined by the following Equation 11 and increases to a larger value as the battery 125 voltage Vo1 becomes larger than the zener voltage V _ZD1 .

I _R6 = (Vo1-V _ZD1 ) / R6 = I _Q2

As can be seen in FIG. 5, when the voltage of the battery 125 gradually increases and becomes larger than the zener voltage V _ZD1 , the collector current I _Q2 of the transistor Q2 gradually increases, whereas the collector current I of the transistor Q1 is increased. It can be seen that _Q1 ) gradually decreases because the current I _R10 flowing through the resistor _R10 has a constant value. That is, since the current I _R10 flowing through the resistor R10 is a constant constant, the collector current I _Q1 of the transistor Q1 decreases as the collector current I _{Q2 of the} transistor _Q2 increases. The decrease in the collector current I _O1 of the transistor Q1 by the increase amount of the collector current I _Q2 of the transistor _Q2 results in a voltage drop across the resistor R4. At this time, since the voltage across the resistor R4 and the voltage across the sense resistor R _sense1 are the same, the output current Io1 supplied to the battery 125 is also linearly reduced. In FIG. 5, it can be seen that the decreasing slope of the output current Io according to the increase of the output voltage Vo1 varies according to the change of the resistance R6 value. Therefore, in order for the voltage of the battery 125 to operate in the constant power mode in the interval between the first voltage V1 and the second voltage V2, a value of the resistor R6 satisfying the condition (V2 × A1 = V1 × A2) is selected. Should be.

I _Q1 = I _R10 -I _Q2 , I _R10 = V _R10 / R10, I _Q2 = (Vo1-V _ZD1 ) / R6

I _Q1 = (V _R10 / R10)-(Vo1-V _ZD1 ) / R6

In addition, since Io1 = V _R4 / R _sense , V _R4 = I _Q1 × R4 = {(V _R10 / R10)-(Vo1-V _ZD1 ) / R6} × R4, the output current Io1 is

Io1 = [{(V _R10 / R10)-(Vo1-V _ZD1 ) / R6} × R4] / R _sense1

= [{(Vref / R10)-(Vo1-V _ZD1) / R6} × R4] / R _sense1

It becomes

In Equation 14, the value of the resistor R6 for operating in the constant power mode between the first voltage V1 and the second voltage V2 is substituted with the first current A1 of FIG. 2 in the output current Io1. It is obtained by substituting the second voltage V2 for the output voltage Vo1. At this time, it should be preceded by first defining the value of the resistor R4 in a state in which the reference voltage Vref, the resistor R10 and the sense resistor R _sense1 are arbitrarily designated, which is expressed in Equation 8 by the output current Io1. Can be obtained by substituting a second current (A2 in FIG.

6 is a circuit diagram of a battery charging circuit according to a second embodiment of the present invention. Referring to FIG. 6, the battery charging circuit according to the second embodiment of the present invention includes a primary side circuit 611, a transformer 615, and a secondary side circuit. The secondary circuit includes the rectifier circuit 621, the sense resistor R _sense2 , the resistors R14 to R22, the capacitors C21 to C25, the zener diode ZD2, the PNP transistors Q11, Q12 and Q13, An operational amplifier 651 and comparators 661 and 662. A battery 625, for example, a lithium ion battery, is connected and charged at both ends of the capacitor C21.

A commercial AC voltage input to the primary side circuit 611, for example 110 volts or 220 volts, is stepped down through the transformer 615, and the stepped down voltage is converted into a direct current voltage through the rectifier circuit 621. The output voltage Vo2 output from the secondary side circuit is supplied to a storage battery 625, for example, a lithium ion storage battery. The secondary circuit supplies a constant current to the storage battery 625 when the secondary circuit operates in the constant current mode, and supplies a constant voltage to the storage battery 625 when the secondary circuit operates in the constant voltage mode. When the circuit operates in the constant power mode, constant power is supplied to the storage battery 625.

The collector currents I _Q11 , I _Q12 and I _{Q13 of the} transistors Q11, Q12 and Q13 and the voltage V _R20 generated across the resistor _R20 are as follows.

I _Q13 ≒ (Vo2-V _ZD2 -0.6) / R16

I _Q12 = I _Q13

I _Q11 ≒ I _R14 = (Io2 × R _sense2 ) / R14

V _R20 = (I _Q11 + I _Q12 ) × R20

Here, it is assumed that the input bias current of the operational amplifier 651 and the base current of the PNP transistors Q11, Q12, and Q13 are negligibly small.

Output characteristics of the battery charging circuit shown in FIG. 6 are as shown in FIG. 7. That is, when the voltage of the battery 625 is lower than the first voltage V3, a large amount of current is supplied from the battery charging circuit to the battery 625, such as the second current A4, and the voltage of the battery 625 is second. When the voltage is increased to V4, a current as low as the first current A3 is supplied from the battery charging circuit to the battery 625, and the voltage of the battery 625 is higher than the first voltage V3 and the second voltage V4. When lower, more current than the first current A3 and less than the second current A4 is supplied from the battery charging circuit to the storage battery 625.

As described above, since the constant current supplied from the battery charging circuit to the battery 625 is elastically adjusted according to the size of the charging voltage of the battery 625, not only the charging speed of the battery 625 is increased but also the battery is thermally stable.

8 and 9 are equivalent circuit diagrams of a secondary circuit when the battery charging circuit illustrated in FIG. 6 operates in the constant current mode and the constant voltage mode, respectively. Since the circuits shown in FIGS. 8 and 9 have the same configuration and operation as the circuits shown in FIGS. 3 and 4, respectively, redundant description will be omitted.

FIG. 10 is an equivalent circuit diagram of a secondary circuit when the battery charging circuit illustrated in FIG. 6 operates in the constant power mode. If the voltage of the storage battery 625 is higher than the first voltage (V3 shown in FIG. 7) and lower than the second voltage (V4 shown in FIG. 6), the battery charging circuit shown in FIG. 6 may be configured as shown in FIG. 10. It will operate in constant power mode. The operation of the circuit shown in FIG. 10 will be described.

When the voltage of the battery 625 starts to be greater than the predetermined voltage (0.6 + V _ZD1 ), the zener diode ZD2 is turned on, and from this time, the voltage V _R16 across the resistor _R16 is also proportional to the increase in the output voltage Vo2. It is increased by the equation (19).

V _R16 = Vo2-(0.6 + V _ZD2)

At this time, the collector current I _Q12 of the transistor _Q12 has the same current value as the collector current I _Q13 of the transistor Q13 due to the characteristics of the current mirror, and is output as shown in Equation 20 below. As the voltage is greater than (0.6 + V _ZD2 ), it will increase to a larger value.

I _Q12 = I _Q13 = (Vo2-0.6-V _ZD2 ) / R16

As can be seen in FIG. 10, when the voltage of the battery 625 gradually increases and becomes larger than a predetermined voltage (0.6 + V _ZD2 ), the collector current I _Q12 of the transistor Q12 gradually increases, whereas the collector current of the transistor Q11 is increased. It can be seen that (I _Q11 ) gradually decreases because the current I _R20 flowing through the resistor _R20 has a constant value as the voltage across the resistor R20 is maintained at the same voltage as the reference voltage Vref. to be. That is, since the current I _R20 is a constant constant at (I _R20 = I _Q11 + I _Q12 ), the current I _Q11 decreases as the current I _Q12 increases.

Reduction of current I _Q11 by the increase amount of current I _Q12 results in a voltage drop across resistor R14. In this case, As previously mentioned, the resistance (R14) the voltage across (V _R14) and the sense resistor (R _sense2) output current (Io2) supplied to the battery 625, the sense voltage (V _semse2) at both ends is the same, also linearly decreases Is done. In FIG. 10, it can be seen that the slope of the decrease in the output current Io2 varies with the increase in the output voltage Vo2 according to the change in the value of the resistor R16. Therefore, in order for the voltage of the storage battery 625 to operate in the constant power mode in the period between the first voltage V3 and the second voltage V4, the value of the resistor R16 satisfying the condition (V4 ㅧ A3 = V3 ㅧ A4) is selected. Should be.

Here, I _Q11 = I _R20 -I _Q12 , I _R20 = V _R20 / R20, I _Q12 = (Vo2-0.6-V _ZD2 ) / R16. therefore,

I _Q11 = (V _R20 / R20)-(Vo2-0.6-V _ZD2 ) / R16

In addition, since Io2 = V _R14 / R _sense2 , V _R14 = I _Q11 × R14 = {(V _R20 / _R20 )-(Vo2-0.6-V _ZD2 ) / R16} × R14, the output current Io2 is

Io2 = [{(V _R20 / R20)-(Vo2-0.6-V _ZD2 ) / R16} × R14] / R _sense2

= [{(Vref / R20)-(Vo2-0.6-V _ZD2) / R16} × R14] / R _sense2

It becomes

In Equation 22, the value of the resistor R16 for operating in the constant power mode between the first voltage V3 and the second voltage V4 is substituted with the first current A3 of FIG. 7 in the output current Io2. It is obtained by substituting the second voltage V4 for the output voltage Vo2. At this time, it should be preceded by first defining the value of the resistor R14 in a state in which the reference voltage Vref, the resistor R20 and the sense resistor R _sense2 are arbitrarily designated. Can be obtained by substituting a second current (A4 in FIG.

As such, the circuit shown in FIG. 6 has a simpler circuit configuration than the circuit shown in FIG. 1, thereby reducing the manufacturing cost.

The present invention is not limited to the above embodiments, and it is apparent that many modifications are possible by those skilled in the art within the technical spirit of the present invention.

As described above, according to the present invention, since the constant current supplied from the battery charging circuit to the battery is elastically adjusted according to the size of the charging voltage of the battery, not only the charging speed of the battery is increased but also the battery is thermally stable.

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

2 is a view showing output characteristics of the battery charging circuit shown in FIG. 1;

3 is an equivalent circuit diagram of a secondary circuit when the battery charging circuit shown in FIG. 1 operates in the constant current mode.

FIG. 4 is an equivalent circuit diagram of a secondary circuit when the battery charging circuit shown in FIG. 1 operates in the constant voltage mode. FIG.

FIG. 5 is an equivalent circuit diagram of a secondary circuit when the battery charging circuit shown in FIG. 1 operates in the constant power mode. FIG.

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

7 is a view showing output characteristics of the battery charging circuit shown in FIG. 6;

FIG. 8 is an equivalent circuit diagram of a secondary circuit when the battery charging circuit shown in FIG. 6 operates in the constant current mode. FIG.

Fig. 9 is an equivalent circuit diagram of a secondary circuit when the battery charging circuit shown in Fig. 6 operates in the constant voltage mode.

FIG. 10 is an equivalent circuit diagram of a secondary circuit when the battery charging circuit shown in FIG. 6 operates in the constant power mode. FIG.

Claims (5)

A primary circuit for inputting commercial AC voltage; A transformer for stepping down the output of the primary circuit; And One end is connected to the secondary side of the transformer and a sense resistor for generating the output voltage of the secondary circuit from the other end, a first operational amplifier for sensing and amplifying the voltage generated in the sense resistor, and the first operational amplifier of the A base is connected to an output terminal and the first transistor is turned on when the output voltage is higher than the voltage applied to one end of the sense resistor, a zener diode connected to the other end of the sense resistor, and is connected to the control diode, and the zener diode is turned on. A second operational amplifier that is deactivated when the zener diode is turned off and a base is connected to an output terminal of the second operational amplifier, and is turned on when the output voltage reaches a predetermined level to generate a current generated at the other end of the sensing resistor. Flowing a second transistor and the output terminals of the first and second transistors. A control resistor connected therebetween, first and second resistors connected in series with the other end of the sense resistor, a first comparator comparing the voltage generated by the control resistor with a reference voltage, and the first and second resistors. And a secondary side circuit having a second comparator for comparing the voltage generated in the circuit with the reference voltage, When the battery is connected to the secondary circuit and charged, the secondary circuit outputs a second constant current when the voltage of the battery is lower than the first voltage, and the voltage of the battery is higher than the first voltage and the second voltage. A lower battery than the second constant current and a higher current than the first constant current, and a battery charging circuit for outputting the first constant current when the voltage of the battery is higher than the second voltage; The battery charging circuit of claim 1, wherein the first and second transistors are both PNP transistors. A primary circuit for inputting commercial AC voltage; A transformer for stepping down the output of the primary circuit; And One end is connected to the secondary side of the transformer and from the other end a sense resistor for generating the output voltage of the secondary circuit, an operational amplifier for sensing and amplifying the voltage generated in the sense resistor, and an output terminal of the first operational amplifier A first transistor turned on when a base is connected and the output voltage is higher than a voltage applied to one end of the sense resistor, a current mirror connected to the other end of the sense resistor, a zener diode connected to an output end of the current mirror, and the current A control resistor connected between a connection point between the output terminal of the mirror and the output terminals of the first transistor and ground, first and second resistors connected in series with the other end of the sensing resistor, and a voltage and a reference voltage generated in the control resistor. Comparing the reference voltage with a voltage generated between the first comparator and the first and second resistors Claim and a secondary circuit having a second comparator, When the battery is connected to the secondary circuit and charged, the secondary circuit outputs a second constant current when the voltage of the battery is lower than the first voltage, and the voltage of the battery is higher than the first voltage and the second voltage. A lower battery than the second constant current and a higher current than the first constant current, and a battery charging circuit for outputting the first constant current when the voltage of the battery is higher than the second voltage; The method of claim 3, wherein the current mirror is A second transistor connected between the other end of the sense resistor and the output end of the first transistor; And A third transistor connected between the other end of the sense resistor and the zener diode and having a base connected to a base of the second transistor; 5. The battery charging circuit of claim 4, wherein the second and third transistors are all PNP transistors.