Abstract:
Provided is a maximum power extraction devices including: a battery; a voltage control unit adjusting a size of a first power outputted from the battery according to a resistor selected from a plurality of resistors, and generating a compare signal according to a size difference between an operating voltage adjusting the size of the first power depending on the selected resistor and a reference voltage; a switching unit connected between the battery and a load and adjusting a size of the operating voltage according to a size difference of the compare signal in response to first and second switching control signals; a switching control unit generating the first and second switching control signals to allow a size between the operating voltage according to the compare signal and the reference voltage to be within an error range; and a maximum power control unit measuring the number of first operations obtained by counting the occurrence number of the first or second switching control signals for a predetermined time, when the compare signal is within the error range.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 of Korean Patent Application No. 10-2012-0151064, filed on Dec. 21, 2012, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention disclosed herein relates to a maximum power extraction device, and more particularly, to a maximum power extraction device extracting maximum power by using the number of operations representing the size of current. 
     Among renewable energy sources, a solar energy resource generates the largest amount of power. Accordingly, various solar energy developments have been made until now. Especially, various developments on a solar battery collecting solar energy and converting it into electrical energy have been made. 
     In relation to a solar battery, the amount of energy varies according to the intensity of solar light or the angle of light. The intensity of solar light, i.e., a condition given from the outside, cannot be artificially changed. Also, the angle of solar light may be adjusted by changing the direction of a solar battery, but changing the direction requires high power consumption. 
     Moreover, an output voltage determining the size of a power generated from a solar battery may be easily adjusted through a power controlling device. That is, an output power may be adjusted by adjusting an output voltage. Accordingly, in order to extract the maximum power from a solar battery, it is necessary to adjust an output voltage. 
     There is a method of controlling voltage and current by using a DC-DC converter in order to extract the maximum power from a solar battery. However, a device including a voltmeter and an ammeter to extract the maximum power becomes more complex as various digital signals are generated. 
     SUMMARY OF THE INVENTION 
     The present invention provides a maximum power extracting device extracting the maximum power without a complex configuration such as an ammeter and a voltmeter. 
     Embodiments of the present invention provide maximum power extraction devices including: a battery; a voltage control unit adjusting a size of a first power outputted from the battery according to a resistor selected from a plurality of resistors, and generating a compare signal according to a size difference between an operating voltage adjusting the size of the first power depending on the selected resistor and a reference voltage; a switching unit connected between the battery and a load and adjusting a size of the operating voltage according to a size difference of the compare signal in response to first and second switching control signals; a switching control unit generating the first and second switching control signals to allow a size between the operating voltage according to the compare signal and the reference voltage to be within an error range; and a maximum power control unit measuring the number of first operations obtained by counting the occurrence number of the first or second switching control signals for a predetermined time, when the compare signal is within the error range, wherein the maximum power control unit compares the number of the first operations with the number of second operations obtained by counting a size of a maximum power in the load according to the internally stored operating voltage, and then, generates a select signal for changing a selection on the plurality of resistors on the basis of a comparison result to adjust the size of the first power. 
     In some embodiments, the battery may receive solar energy and may convert the received solar energy into electrical energy. 
     In other embodiments, when the number of the first operations is less than the number of the second operations, the select signal may be generated to select a resistor for lowering the operating voltage from among the plurality of resistors, and when the number of the first operations is greater than the number of the second operations, the select signal may be generated to select a resistor for raising the operating voltage from among the plurality of resistors, so as to adjust the size of the first power. 
     In still other embodiments, the maximum power control unit may include: a counter unit storing the number of the first operations obtained by counting the occurrence number of the first or second switching control signals; a data unit storing the number of the second operations obtained by counting a size of a maximum power in the load according to the operating voltage; a counter comparator comparing the number of the first operations with the number of the second operations; and a resistor selection unit generating the select signal to adjust the size of the first power on the basis of a comparison result from the counter comparator. 
     In even other embodiments, the voltage control unit may include: the plurality of resistors; a multiplexer selecting one of the plurality of resistors in response to the select signal; and a comparator comparing a size difference between an output signal for a size of an operating voltage of the selected resistor and a signal of the reference voltage and delivering the compare signal to the switching control unit on the basis of a comparison result. 
     In yet other embodiments, the size of the first power may be adjusted through a voltage distribution using the plurality of resistors. 
     In further embodiments, the switching unit may convert the size of the first power into a size of a second power through a DC-DC conversion and may deliver the converted second power to the load. 
     In other embodiments of the present invention, maximum power extraction devices include: a battery; a voltage control unit adjusting a size of a first power outputted from the battery according to a resistor selected from a plurality of resistors, and generating a compare signal according to a size difference between an operating voltage adjusting the size of the first power depending on the selected resistor and a reference voltage; a switching unit connected between the battery and a load and adjusting a size of the operating voltage according to a size difference of the compare signal in response to first and second switching control signals; a switching control unit generating the first and second switching control signals to allow a size between the operating voltage according to the compare signal and the reference voltage to be within an error range; and a maximum power control unit storing a size of a first power resulting from a product of the number of operations obtained by counting the occurrence number of the first or second switching control signals for a predetermined time and the operating voltage, when the compare signal is within the error range, wherein the maximum power control unit compares the size of the first power with a size of a second power obtained by the most recent multiplication, and generates a select signal to change a selection on the plurality of resistors on the basis of a comparison result. 
     In some embodiments, the maximum power control unit may include: a counter unit storing the number of the first operations obtained by counting the occurrence number of the first or second switching control signals; an arithmetic unit outputting the size of the first power by multiplying the operating voltage by the number of the operations; a data unit storing the size of the second power obtained by the most recent multiplication; a first comparator receiving the sizes of the first and second powers and compare the received sizes; and a resistor selection unit generating the select signal for adjusting the size of the first power on the basis of a comparison result from the first comparator. 
     In other embodiments, the voltage control unit may include: the plurality of resistors; a multiplexer selecting one of the plurality of resistors in response to the select signal; and a comparator comparing a size difference between an output signal for a size of an operating voltage of the selected resistor and a signal of the reference voltage and delivering the compare signal to the switching control unit on the basis of a comparison result. 
     In still other embodiments, the size of the first power may be adjusted through a voltage distribution using the plurality of resistors. 
     In even other embodiments, the switching unit may convert the size of the first power into a size of a second power through a DC-DC conversion and may deliver the converted second power to the load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present invention and, together with the description, serve to explain principles of the present invention. In the drawings: 
         FIG. 1  is a block diagram of a maximum power extraction device according to an embodiment of the present invention; 
         FIG. 2  is a current-voltage graph of an operating voltage change according to an embodiment of the present invention; 
         FIG. 3  is a view illustrating the number of operations according to each operating voltage of  FIG. 2 ; 
         FIG. 4  is a flowchart illustrating operations of the maximum power extraction device of  FIG. 1 ; 
         FIG. 5  is a block diagram of a maximum power extraction device according to another embodiment of the present invention; and 
         FIG. 6  is a flowchart illustrating operations of the maximum power extraction device of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. 
       FIG. 1  is a block diagram of a maximum power extraction device according to an embodiment of the present invention. Referring to  FIG. 1 , the maximum power extraction device  100  includes a solar battery  110 , a voltage control unit  120 , a switching unit  130 , a switching control unit  140 , and a maximum power control unit  150 , and a load  160 . 
     The solar battery  110  receives solar energy from the sun. Also, the solar battery  110  converts the received solar energy into the form of electrical energy. In this way, the solar battery  110  converts the received solar energy into the form of electrical energy as a first power, and then, delivers the converted first power into the switching unit  130 . 
     Here, the size of solar energy is a factor that cannot be artificially changed. However, although there is a method of adjusting the size of solar energy by adjusting the angle of light, this requires high power consumption. Accordingly, as a method of adjusting the size of the output power of the solar battery  110 , a method of adjusting the size of the output power through the output voltage of the solar battery  110  is mainly used. 
     The voltage control unit  120  includes a plurality of resistors R 1  to Rn, a multiplexer  121 , a reference voltage generator  122 , and a comparator  123 . The voltage control unit  120  may adjust the size of the first power generated from the solar battery  110  by using an operating voltage Vn. Here, the operating voltage Vn refers to an output voltage adjusting the size of a power to be generated from the solar battery  110 . That is, the size of an output power generated from the solar battery  110  may be changed through an output voltage. In this way, the voltage control unit  120  may maximize the size of the first power generated from the solar battery  110  by using the operating voltage Vn. 
     The multiplexer  121  selects one of the plurality of resistors R 1  to Rn in response to a select signal applied from the maximum power control unit  150 . Also, the multiplexer  121  delivers the size signal of the operating voltage Vn according to the selected resistor to the comparator  123 . 
     The reference voltage generator  122  generates a predetermined size of a reference voltage Vref, which is not affected from an external voltage, and then, delivers the reference voltage Vref to the comparator  123 . 
     The comparator  123  receives the size signal of the operating voltage for the resistor selected from the multiplexer  121  among the plurality of resistors R 1  to Rn. Also, the comparator  123  receives the reference voltage Vref from the reference voltage generator  122  and compares it with the operating voltage Vn. The comparator  123  compares a size difference between the operating voltage Vn and the reference voltage Vref, and then, according to a comparison result, generates a compare signal to deliver it to the switching control unit  140 . Additionally, in comparing the operating voltage Vn and the reference voltage Vref, in addition to the comparator  123 , an error amplifier may be used. 
     The switching unit  130  receives the first power generated from the solar battery  110 , and converts the received first power into DC-DC as a second power. That is, the switching unit  130  receives the operating voltage Vn and generates a driving voltage to supply it to the load  160 . The switching unit  130  includes an NMOS transistor M 1 , a PMOS transistor M 2 , and an inductor L. 
     The switching control unit  140  receives a compare signal from the comparator  123 . Then, the switching control unit  140  controls the switching unit  130  to allow a size difference between the operating voltage Vn and the reference voltage Vref to be equal in response to the received compare signal. Here, the switching control unit  140  adjusts a size difference of the compare signal by controlling the switching unit  130 , instead of adjusting a size difference of the compare signal by changing the selection on a resistor. Also, when a size difference of the compare signal is within a predetermined error range, a counter unit  1515  counts an operation of the switching control unit  140  for a predetermined time so as to measure a maximum power. Also, since the size of a power outputted from the solar battery  100  changes continuously, it is assumed that the size of power is identical when a size difference of the compare signal is within a predetermined error range. 
     The switching control unit  140  generates first and second control signals S 1  and S 2  to control a DC-DC conversion operation of the switching unit  130 . Also, the NMOS and PMOS transistors M 1  and M 2  serve as switches. The switching control unit  140  generates a first control signal S 1  to control the NMOS transistor M 1  and generates a second control signal to control the PMOS transistor M 2 . The switching control unit  140  controls the turn-on and turn-off operations of the NMOS and PMOS transistors M 1  and M 2  by using the first and second control signals S 1  and S 2 . 
     In more detail, when the NMOS transistor M 1  is turned on in response to the first control signal S 1 , the PMOS transistor M 2  may be turned off in response to the second control signal S 2 . At this point, induced voltage is formed in the inductor L. Also, when the NMOS transistor M 1  is turned off in response to the first control signal S 1 , the PMOS transistor M 2  may be turned on in response to the second control signal S 2 . At this point, the induced voltage formed in the inductor L is delivered to the load  160 . In this way, the NMOS and PMOS transistors M 1  and M 2  operate complementary to each other. 
     Thus, the switching control unit  140  controls a DC-DC conversion operation to allow a size difference between the reference voltage Vref and the operating voltage Vn to be identical in response to a compare signal according to the selected resistor. Also, when a size difference of the compare signal is within a predetermined error range, the counter unit  151  counts an operation of the switching control unit  140  for a predetermined time. 
     The maximum power control unit  150  includes the counter unit  151 , a data unit  152 , a counter comparator  153 , and a resistor selection unit  154 . The counter unit  151  counts the repeating turn-on and turn-off operations of the NMOS and PMOS transistors M 1  and M 2  in response to the first and second control signals S 1  and S 2 . That is, the counter unit  151  counts the operations of the NMOS and PMOS transistors M 1  and M 2 , which are controlled by the switching control unit  140 , for a predetermined time when a compare signal according to the selected resistor among the plurality of resistors R 1  to Rn is within a predetermined error range. The counted operations are digitized and stored as the number of operations. Here, the number of operations refers to a total number of iterations of the process that the NMOS or PMOS transistor M 1  or M 2  is turned on and then turned off once. 
     Then, the counter unit  151  delivers a counter signal Cn for the stored number of operations to the counter comparator  153 . Additionally, each time the size of the operating voltage Vn according to the selected resistor from among the plurality of resistors R 1  to Rn is changed, information on a previous count is initialized. 
     The data unit  152  stores the number of operations of the maximum power of each operating voltage Vn according to the selected resistor from among the plurality of resistors R 1  to Rn. In this way, since the number of operations of the maximum power of each operating voltage Vn is stored in the data unit  152 , it is determined whether a currently selected operating voltage Vn is the maximum power. Additionally, the data unit  152  receives information on the operating voltage Vn selected from the resistor selection unit  154 . In this way, the data unit  152  delivers a maximum counter signal Dn for the maximum power of the selected operating voltage Vn to the counter comparator  153 . 
     The counter comparator  153  compares the counter signal Cn received from the counter unit  151  and the maximum counter signal Dn received from the data unit  152 . Also, the counter comparator  153  delivers a signal for adjusting the size of an operating voltage Vn to the resistor selection unit  154  in response to the compared value. 
     Also, the counter comparator  153  delivers a signal for adjusting the size of an operating voltage Vn to the resistor selection unit  154  in response to the compared value. Then, the resistor selection unit  154  delivers the generated select signal SEL to the multiplexer  121 . Accordingly, the multiplexer  121  selects one of the plurality of resistors R 1  to Rn in response to the received select signal SEL. Additionally, the resistor selection unit  154  delivers information on an operating voltage Vn changed in response to the select signal SEL to the data unit  152 . That is, one resistor is selected from the plurality of resistors R 1  to Rn in response to the select signal SEL, so that the size of an operating voltage Vn is adjusted. 
     Also, the resistor selection unit  154  delivers information on a change in operating voltage Vn according to the selected resistor to the data unit  152 . Accordingly, the data unit  152  delivers the number of operations of a maximum power for an operating voltage Vn according to the selected resistor to the counter comparator  153 . 
     In this way, the maximum power extraction device  100  adjusts the size of a power by using the operating voltage Vn, instead of using an arithmetic unit having a complex configuration, in order to maximize the first power outputted from the solar battery  110 . The maximum power extraction device  100  maintains the maximum size of a power more simply by adjusting the size of a power with an operating voltage Vn. 
       FIG. 2  is a current-voltage graph of an operating voltage change according to an embodiment of the present invention. Referring to  FIG. 2 , it may be seen that the position of a power is changed according to the size of an operation voltage Vn. It is assumed that first to third operating voltages V 1 , V 2 , and V 3  shown in  FIG. 2  are selected and determined by three resistors from among the plurality of resistors R 1  to Rn. Then, first to third currents I 1 , I 2 , and I 3  according to the first to third operating voltages V 1 , V 2 , and V 3  are delivered to the switching unit  130  of  FIG. 1 . 
     In more detail, the size of a current for each operating voltage according to the selected resistor may be recognized through the number of operations measured by the counter unit  151  of  FIG. 1 . That is, as a current according to each operating voltage is applied to the inductor L of  FIG. 1 , forming an induced voltage and discharging it to a load are regarded as one operation. Accordingly, the large number of operations means a large size of current. Therefore, the number of operations is the largest in the first current I 1  having the largest size and the number of operations is the smallest in the third current I 3  having the smallest size. 
     In this way, the size of a power outputted from the solar battery  110  may be changed in response to the size of the operating voltage Vn and current In. That is, the size of an operating voltage Vn may be adjusted by the size of a current. 
     For example, it is assumed that the second power P 0  is the point at which the maximum power is extracted as shown in  FIG. 2 . When the voltage control unit  120  of  FIG. 1  selects a resistor for the size of the third operating voltage V 3 , the maximum power control unit  150  of  FIG. 1  recognizes the number of operations in response to the size of the third current I 3 . Then, since the number of operations of the third current I 3  is less than the number of operations of the second current I 2 , the maximum power control unit  150  delivers a select signal SEL to the voltage control unit  120  to lower the size of an operating voltage Vn. Accordingly, the size of the third operating voltage V 3  is lowered and the size of the third current I 3  is increased, so that the number of operations is increased. That is, the third power P 3  approaches the second power P 0 . 
     On the contrary, when the voltage control unit  120  of  FIG. 1  selects a resistor for the size of the first operating voltage V 1 , the maximum power control unit  150  of  FIG. 1  recognizes the number of operations in response to the size of the first current I 1 . Then, since the number of operations of the first current I 1  is greater than the number of operations of the second current I 2 , the maximum power control unit  150  delivers a select signal SEL to the voltage control unit  120  to increase the size of an operating voltage Vn. Accordingly, the size of the third operating voltage V 3  is increased and the size of the third current I 3  is decreased, so that the number of operations is reduced. That is, the first power P 1  approaches the second power P 0 . 
       FIG. 3  is a view illustrating the number of operations according to each operating voltage of  FIG. 2 . Referring to  FIG. 3 , an induced voltage is formed in the inductor L during a section T 1  in response to the turn-on of the NMOS transistor M 1 , and the induced voltage formed in the inductor L is supplied to the load  160  of  FIG. 1  during a section T 2  in response to the turn-on of the PMOS transistor M 2 . It is regarded that the number of operations in the section T 1  or T 2  is measured once. 
     Therefore, the number of operations is the largest in the first current I 1  having the largest size and the number of operations is the smallest in the third current I 3  having the smallest size. 
     In this way, the maximum power extraction device  100  adjusts the operating voltage Vn according to the number of operations representing the size of current, so that a point having the maximum power may be found. 
       FIG. 4  is a flowchart illustrating operations of the maximum power extraction device of  FIG. 1 . Referring to  FIG. 4 , the voltage control unit  120  of  FIG. 1  determines an operating voltage Vn for adjusting the size of the first power supplied from the solar battery  110  in operation S 110 . In more detail, the voltage control unit  120  of  FIG. 1  selects a resistor with which the operating voltage Vn has the maximum value from among the plurality of resistors R 1  to Rn. Here, the first resistor R 1  is determined as a resistor with which the operating voltage Vn has the maximum value. 
     In operation S 120 , the counter unit  151  of  FIG. 1  performs an initializing operation on previous information to measure the number of operations of the NMOS and PMOS transistors M 1  and M 2  according to the selected operating voltage Vn. In more detail, the counter unit  151  determines the number of operations of the NMOS and PMOS transistors M 1  and M 2  according to the number of operations that generate the first and second control signals S 1  and S 2  of the switching control unit  140 . Here, the number of operations measured by the counter unit  151  is measured when a size difference of compare signal, i.e., the operating voltage Vn and the reference voltage Vref, become identical. That is, when a size difference of the compare signal is within a predetermined error range, the counter  151  measures the number of operations for a predetermined time. 
     In operation S 130 , the counter unit  151  measures the number of operations of the operating voltage Vn according to the selected resistor from among the plurality of resistors R 1  to Rn in response to the first and second control signals S 1  and S 2 . 
     In operation S 140 , the data unit  152  of  FIG. 1  determines the number N 2  of operations having the maximum power of the operating voltage Vn according to the selected resistor from among the plurality of resistors R 1  to Rn. The data unit  152  stores the number of operations of the maximum power for each operating voltage Vn. 
     In operation S 150 , the counter comparator  153  performs a comparison operation to determine whether the number N 1  of operations of the operating voltage Vn selected from the voltage control unit  120  is greater than the number N 2  of operations of the operating voltage Vn having the maximum power. The resistor selection unit  154  of  FIG. 1  generates a select signal SEL for adjusting the size of the operating voltage Vn according to the comparison result. 
     In operation S 160 , it shows when the number N 1  of operations of the operating voltage Vn selected from the voltage control unit  120  is greater than the number N 2  of operations of the operating voltage Vn having the maximum power. Then, the resistor selection unit  154  generates the select signal SEL for increasing the size of the operating voltage Vn, and then, delivers it to the multiplexer  121 . The multiplexer  121  selects a resistor for increasing the size of the operating voltage Vn from among the plurality of resistors R 1  to Rn in response to the received select signal SEL. 
     In operation S 170 , it shows when the number N 1  of operations of the operating voltage Vn selected from the voltage control unit  120  is less than the number N 2  of operations of the operating voltage Vn having the maximum power. Then, the resistor selection unit  154  generates the select signal SEL for decreasing the size of the operating voltage Vn, and then, delivers it to the multiplexer  121 . The multiplexer  121  selects a resistor for decreasing the size of the operating voltage Vn from among the plurality of resistors R 1  to Rn in response to the received select signal SEL. 
     In operation S 180 , the multiplexer  121  selects a resistor for changing the size of the operating voltage Vn from among the plurality of resistors R 1  to Rn in response to the select signal SEL generated from operation S 160  or S 170 . Then, the maximum power extraction device  100  performs operations S 110  to S 170  repeatedly in order to maximize the size of a power delivered to the load  160 . In this way, the load  160  maintains the maximum power according to the iterative processes of operation S 110  to S 170 . Then, when the iterative processes for changing the operating voltage Vn are not performed any more, the maximum power extraction device  100  is terminated. 
       FIG. 5  is a block diagram illustrating a maximum power extraction device according to another embodiment of the present invention. Referring to  FIG. 5 , the maximum power extraction device  200  includes a solar battery  210 , a voltage control unit  220 , a switching unit  230 , a switching control unit  240 , and a maximum power control unit  250 , and a load  260 . The maximum power extraction device  200  of  FIG. 5  has the same configuration as the maximum power extraction device  100  of  FIG. 1 , except for the maximum power control unit  250 . Accordingly, the maximum power control unit  250  will be described in more detail. 
     The maximum power control unit  250  includes a counter unit  251 , an arithmetic unit  252 , a data unit  253 , a comparator  254 , and a resistor selection unit  245 . 
     The counter unit  251  counts the repeating turn-on and turn-off operations of the NMOS and PMOS transistors M 1  and M 2  in response to the first and second control signals S 1  and S 2 . The counter unit  251  determines the number of operations of the NMOS and PMOS transistors M 1  and M 2  in response to the first and second control signals S 1  and S 2  generated from the switching control unit  240 . 
     Then, the number of operations measured by the counter unit  251  means the size of current. That is, the large number of operations means that a large size of current, and the small number of operations means a small size of current. This is because that forming an induced voltage according to a current applied to the inductor L and discharging it to a load according to operations of the NMOS and PMOS transistors M 1  and M 2  are regarded as one operation. Then, the counter unit  251  delivers information on the measured number of operations to the arithmetic unit  252 . 
     The arithmetic unit  252  calculates the product of the size of a current according to the number of operations received from the counter unit  251  and the operating voltage Vn so as to measure the size of the first power. The calculating unit receives information on the operating voltage Vn through the resistor selection unit  255 . Then, the arithmetic unit  252  delivers information on the measured size of the first power to the data unit  253  and the comparator  254 . 
     The data unit  253  receives the size of the first power measured by the arithmetic unit  252  and delivers the previously-stored size of the second power to the comparator  254 . Then, in relation to an initial operation of the maximum power extraction device  200 , the data unit  253  delivers initial power information having a set arbitrary value to the comparator  254 . Here, the arbitrary value means a power value designated by a user. 
     The comparator  254  receives the size of the first power from the arithmetic unit  252  and also the size of the second power from the data unit  253 , and then, compares them in terms of a size. Then, the comparator  254  delivers a comparison result to the resistor selection unit  255 . 
     The resistor selection unit  255  generates a select signal SEL for changing the size of the operating voltage Vn in response to the comparison result received from the comparator  254 . Then, the resistor selection unit  244  delivers the generated select signal SEL to the multiplexer  221 . Accordingly, the multiplexer  221  selects one of the plurality of resistors R 1  to Rn in response to the received select signal SEL. Additionally, the resistor selection unit  255  delivers information on the operating voltage Vn in response to the select signal SEL to the arithmetic unit  252 . 
     In this way, the maximum power extraction device  200  repeatedly performs an operation for comparing a currently-measured first power value and a previously-measured second power value through the counter unit  251  and the arithmetic unit  252 . Therefore, the maximum power extraction device  100  maintains the size of the second power delivered to the load  160 . 
       FIG. 6  is a flowchart illustrating operations of the maximum power extraction device of  FIG. 5 . Referring to  FIG. 6 , operations S 210  to S 230  are identical to operations S 110  to S 130  of the maximum power extraction device  100  of  FIG. 4 . Therefore, description will be made from operation S 240 . 
     In operation S 240 , the product of the size of a current received from the counter  251  through the arithmetic unit  252  and the size of an operating voltage Vn generated from the voltage control unit  220  is calculated. Then, the arithmetic unit  252  delivers the calculated size of the first power P 1  to the data unit  253  and the comparator  254 . 
     In operation S 250 , the size of the second power P 0  on previous information is delivered to the comparator  254 , and the size of the first power P 1  received from the arithmetic unit  243  is stored in the comparator  254 . 
     In operation S 260 , the comparator  2  compares the size of the first power P 1  and the size of the second power P 0 . In more detail, when the size of the first power P 1  is greater than the size of the second power P 0 , a select signal SEL is generated to select a resistor of the next stage on the basis of a recently-selected resistor. Then, the resistor selection unit  255  delivers the selected select signal SEL to the multiplexer  221 . 
     On the contrary, when the size of the first power P 1  is less than the size of the second power P 0 , a select signal SEL is generated to select a resistor of the previous stage on the basis of a recently-selected resistor. Then, the resistor selection unit  255  delivers the selected select signal SEL to the multiplexer  221 . 
     In operation S 270 , the case that the size of the first power P 1  is greater than the size of the second power P 0  is shown. The multiplexer  221  selects a resistor of the next stage on the basis of a recently-selected resistor in response to the received select signal SEL. 
     In operation S 280 , the case that the size of the first power P 1  is less than the size of the second power P 0  is shown. The multiplexer  221  selects a resistor of a previous stage on the basis of a recently-selected resistor in response to the received select signal SEL. 
     In operation S 290 , the size of the operating voltage Vn is changed according to the resistor selected from operation S 270  or S 280 . Then, the maximum power extraction device  200  performs operations S 210  to S 280  in order to maximize the size of a power delivered to the load  260 . In this way, the load  160  maintains the maximum power according to the iterative processes of operation S 210  to S 280 . Then, when the iterative processes for changing the operating voltage Vn are not performed any more, the maximum power extraction device  200  is terminated. 
     In this way, a maximum power extraction device according to an embodiment of the present invention does not include a complex arithmetic unit in order to obtain maximum power. That is, the maximum power extraction device may measure a maximum power by using the number of operations representing the size of current. In this way, since the maximum power extraction device has a simple configuration, it may be effective in terms of cost and production. 
     According to an embodiment of the present invention, since a configuration such as a voltmeter and an ammeter is not required, a maximum power extraction device is manufactured through simple processes. 
     The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments, which fall within the true spirit and scope of the present invention. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.