Patent Publication Number: US-11658633-B2

Title: Impedance adjustment circuit, power conversion element, and power supply element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a 35 U.S.C. §§ 371 national phase conversion of International Application No. PCT/JP2019/016863, filed Apr. 19, 2019, which claims priority to Japanese Patent Application No. 2018-081478, filed Apr. 20, 2018, the contents of both of which are incorporated herein by reference. The PCT International Application was published in the Japanese language. 
     TECHNICAL FIELD 
     The present disclosure describes an impedance adjustment circuit, a power conversion element, and a power supply element. 
     BACKGROUND ART 
     Technologies for utilizing environmental energy that have not been used before are attracting attention. Technologies utilizing environmental energy are called energy harvesting. Non-Patent Literature 1 discloses a basic system relating to energy harvesting. The system disclosed in Non-Patent Literature 1 includes a micro energy transducer, a frequency converter, a control device, and an application unit. Environmental energy is converted into electric power by the micro energy transducer. This electric power is converted into a desired voltage and a desired frequency by the frequency converter. The application unit such as a sensor performs a desired operation by receiving electric power supplied from the frequency converter. The control device performs necessary control operations on such elements. Patent Literature 1 discloses a piezoelectric device for power generation. The technology of Patent Literature 1 focuses on the output resistance of the piezoelectric device for power generation. The technology of Patent Literature 1 adjusts the impedance for a load. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] Japanese Unexamined Patent Publication No. 2002-315362 
     Non-Patent Literature 
     [Non-Patent Literature 1] Chao Lu, Vijay Raghunathan, and Kaushik Roy, “Efficient Design of Micro-Scale Energy Harvesting Systems”, IEEE JOURNAL ON EMERGING AND SELECTED TOPICS IN CIRCUITS AND SYSTEMS, (United States of America), IEEE, September, 2011, No. 3, Vol. 1, pp. 254-266. 
     SUMMARY OF INVENTION 
     Technical Problem 
     A power generation element used for energy harvesting has a low output voltage. Thus, the electric power of a power generation element is supplied to a power conversion element. The power conversion element converts the electric power of a power generation element into a required form of electric power. The transmission of electric power from a power generation element to a power conversion element is influenced by a relation between the impedance of the power generation element and the impedance of the power conversion element. 
     As illustrated in Patent Literature 1, a power generation element has a high output resistance. Thus, when electric power is transmitted from a power generation element to a power conversion element, it is difficult to maintain an open voltage. In other words, a voltage supplied to the power conversion element becomes lower than an open voltage of the power generation element. The output voltage of the power generation element is low. As a result, there is a likelihood of a voltage supplied to the power conversion element being below an operating voltage of the power conversion element. Thus, there may be a case in which the power generation element is unable to supply desired electric power. 
     This, the present disclosure will describe an impedance adjustment circuit enabling transmission of electric power having high efficiency and a power conversion element and a power supply element capable of providing desired electric power. 
     Solution to Problem 
     One form of the present disclosure is an impedance adjustment circuit connected between a power generation circuit that converts external energy into electric power and outputs the electric power and a power conversion circuit that converts the electric power generated by the power generation circuit into a desired form. The impedance adjustment circuit includes a first circuit unit configured to have an input port connected to the power generation circuit and an output port connected to the power conversion circuit and a second circuit unit configured to have a connection point connected to the first circuit unit, a grounding point connected to a grounding electric potential, and a capacitor connected between the connection point and the grounding point. A magnitude of an output resistance included in the second circuit unit is smaller than a magnitude of an output resistance included in the power generation circuit. The capacitor is charged with the electric power output from the power generation circuit and outputs the charged electric power to the power conversion circuit. 
     The capacitor of the second circuit unit of the impedance adjustment circuit of the present disclosure is charged with electric power received from the power generation circuit through the input port of the first circuit unit. The capacitor transmits electric power to the power conversion circuit through the output port of the first circuit unit. 
     According to this configuration, in a form in which electric power is transmitted to the power conversion circuit, the power source of the power conversion circuit is capacitor rather than a power generation circuit. The output resistance present between the capacitor and the output port is smaller than the output resistance of the power generation circuit. As a result, a circuit configuration in which the impedance adjustment circuit is connected between the power generation circuit and the power conversion circuit can inhibit a voltage drop in the electric power transmitted to the power conversion circuit better than a circuit configuration in which the power generation circuit is directly connected to the power conversion circuit. Therefore, according to the impedance adjustment circuit, electric power can be transmitted with a high efficiency. 
     In another form of the present disclosure, a power conversion element connected to a power generation element including a power generation circuit that converts external energy into electric power and outputs the electric power is provided. The power conversion element includes a power conversion circuit configured to convert the electric power generated by the power generation circuit into a desired form and an impedance adjustment circuit configured to be connected between the power generation circuit and the power conversion circuit. The impedance adjustment circuit includes: a first circuit unit configured to have an input port connected to the power generation circuit and an output port connected to the power conversion circuit; and a second circuit unit configured to have a connection point connected to the first circuit unit, a grounding point connected to a grounding electric potential, and a capacitor connected between the connection point and the grounding point. A magnitude of output resistance included in the second circuit unit is smaller than a magnitude of output resistance included in the power generation circuit. The capacitor is charged with the electric power output from the power generation circuit and outputs the charged electric power to the power conversion circuit. 
     The power conversion element of this other form includes the impedance adjustment circuit described above. Therefore, the circuit configuration of the power conversion element can inhibit a voltage drop in the electric power transmitted to the power conversion circuit better than a circuit configuration in which the power generation circuit is directly connected to the power conversion circuit. Therefore, the power conversion element can transmit electric power with a high efficiency. 
     A power conversion element according to another form may further include a control unit configured to start and stop an operation of the power conversion circuit. The power conversion element may charge the capacitor by stopping the operation of the power conversion circuit. In addition, the power conversion element may discharge the capacitor by starting the operation of the power conversion circuit. According to this configuration, the configuration of the impedance adjustment circuit can be simplified. 
     In another form, the control unit may control starting of the operation and stopping of the operation of the power conversion circuit on the basis of a magnitude of a voltage supplied from the impedance adjustment circuit to the power conversion circuit. According to this configuration, desired electric power can be reliably acquired from the power conversion circuit. 
     In another form, the control unit may perform mutual switching between starting of the operation and stopping of the operation of the power conversion circuit every time a predetermined time elapses. According to this configuration, the impedance adjustment circuit can be controlled in a simplified manner. 
     In another form, the first circuit unit may have a first switch connected to the input port and a second switch connected to the first switch and the output port. The second circuit unit may have a connection point connected to the first switch and the second switch and a capacitor connected to the connection point and the grounding point. The power conversion element may further include a control unit configured to control the first switch and the second switch. The control unit may perform mutual switching between a charging operation of connecting the input port to the capacitor by controlling the first switch and disconnecting the output port from the capacitor by controlling the second switch and a discharging operation of disconnecting the input port from the capacitor by controlling the first switch and connecting the output port to the capacitor by controlling the second switch. According to this configuration, switching between the charging operation and the discharging operation can be reliably performed. 
     In another form, the control unit may control operations of the first switch and the second switch on the basis of a magnitude of a voltage supplied from the impedance adjustment circuit to the power conversion circuit. According to this configuration, desired electric power can be reliably obtained from the power conversion circuit. 
     In another form, the control unit may control operations of the first switch and the second switch every time a predetermined time elapses. According to this configuration, the impedance adjustment circuit can be controlled in a simplified manner. 
     In another form, the power conversion element further includes: a first power conversion circuit as the power conversion circuit described above; a second power conversion circuit other than the first power conversion circuit configured to convert the electric power generated by the power generation circuit into a desired form; and a control unit configured to control operations of the first power conversion circuit and the second power conversion circuit. The second power conversion circuit is disposed in parallel with the impedance adjustment circuit and the first power conversion circuit. An input impedance of the second power conversion circuit is closer to an output impedance of the power generation circuit than an input impedance of the first power conversion circuit. The control unit obtains electric power from the first power conversion circuit after obtaining electric power from the second power conversion circuit. 
     A power generation element according to a yet another form of the present disclosure includes: a power generation circuit configured to convert external energy into electric power and output the electric power; a power conversion circuit configured to convert the electric power generated by the power generation circuit into a desired form; and an impedance adjustment circuit connected between the power generation circuit and the power conversion circuit. The impedance adjustment circuit has: a first circuit unit configured to have an input port connected to the power generation circuit and an output port connected to the power conversion circuit; and a second circuit unit configured to have a connection point connected to the first circuit unit, a grounding point connected to a grounding electric potential, and a capacitor connected between the connection point and the grounding point. A magnitude of output resistance included in the second circuit unit is smaller than a magnitude of output resistance included in the power generation circuit. The capacitor is charged with the electric power output from the power generation circuit and outputs the charged electric power to the power conversion circuit. 
     The power generation element according to the yet another form includes the impedance adjustment circuit described above. The power generation element can inhibit a voltage drop in electric power transmitted to the power conversion circuit better than the configuration in which a power generation circuit is directly connected to a power conversion circuit. As a result, electric power is transmitted from the power generation circuit to the power conversion circuit with a high efficiency. Therefore, the power generation element can supply desired electric power. 
     Advantageous Effects of Invention 
     An impedance adjustment circuit of the present disclosure can efficiently transmit electric power from a power generation element to a power conversion element. A power conversion element and a power supply element of the present disclosure can provide desired electric power. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating the configuration of a sensor device using a power supply element according to an embodiment. 
         FIG.  2    is a diagram illustrating the configuration of a power supply element. 
         FIG.  3    is an example of a timing diagram of a power supply element. 
         FIG.  4    is a diagram illustrating the configuration of a power supply element of Modified Example 1. 
         FIG.  5    is a diagram illustrating the configuration of a power supply element of Modified Example 2. 
         FIG.  6    is a diagram illustrating the configuration of a power supply element of Modified Example 3. 
         FIG.  7    is a diagram illustrating the configuration of a power supply element of Modified Example 4. 
         FIG.  8    is a diagram illustrating the configuration of a power supply element of Modified Example 5. 
         FIG.  9    is a diagram illustrating the configuration of a power supply element of Modified Example 6. 
         FIG.  10    is an example of a timing diagram of the power supply element of Modified Example 6. 
         FIG.  11    is a diagram illustrating the configuration of a power supply element of Modified Example 7. 
       Part (a) of  FIG.  12    is a diagram illustrating the configuration of a power supply element of Examination Case 1. Part (b) of  FIG.  12    is a graph illustrating a result of Examination Case 1. 
       Part (a) of  FIG.  13    is a diagram illustrating the configuration of a power supply element of Examination Case 2. Part (b) of  FIG.  13    is a graph illustrating a result of Examination Case 2. 
         FIG.  14    is a graph illustrating a result of Examination Case 3. 
         FIG.  15    is a diagram illustrating the configuration of a sensor device for illustrating a background of Modified Example 8. 
         FIG.  16    is a diagram illustrating the configuration of a power supply element of Modified Example 8. 
       Part (a) of  FIG.  17   , Part (b) of  FIG.  17   , and Part (c) of  FIG.  17    are diagrams illustrating operations of the power supply element of Modified Example 8. 
       Part (a) of  FIG.  18   , Part (b) of  FIG.  18   , and Part (c) of  FIG.  18    are diagrams illustrating operations of the power supply element of Modified Example 8. 
         FIG.  19    is a diagram illustrating effects of the power supply element of Modified Example 8. 
       Part (a) of  FIG.  20    and Part (b) of  FIG.  20    are diagrams illustrating operations of the power supply element of Modified Example 8. 
         FIG.  21    is a diagram illustrating effects of the power supply element of Modified Example 8. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment for performing the present invention will be described in detail with reference to the attached drawings. In description of the drawings, the same reference signs will be assigned to the same elements, and duplicate description will be omitted. 
     As illustrated in  FIG.  1   , a power supply element  1  is used in a sensor device  2 . The sensor device  2  configures a so-called Internet of Things (IoT). 
     The sensor device  2  is connected to the Internet  300  through an antenna  3 . The sensor device  2  transmits collected data to another system such as a cloud  301  through the Internet  300 . The sensor device  2  receives various kinds of data such as a measuring program through the Internet  300 . 
     The sensor device  2  includes the power supply element  1  and a sensor element  4 . The sensor element  4  collects various kinds of data. In various kinds of data, for example, a temperature, a humidity, or a vibration frequency of a target object  302  is included. The sensor element  4  transmits data. In addition, the sensor element  4  receives data from the outside. For example, the sensor element  4  includes a sensor  4   a , a digital circuit  4   b , a memory circuit  4   c , and a communication circuit  4   d.    
     The power supply element  1  supplies electric power used for driving the sensor element  4 . The power supply element  1  supplies electric power that is necessary for operations of the sensor  4   a , the digital circuit  4   b , the memory circuit  4   c , and the communication circuit  4   d  configuring the sensor element  4 . The power supply element  1  is not a device that stores electric power in advance such as a so-called battery. The power supply element  1  obtains electric power, for example, by converting external energy such as vibration energy supplied from the target object  302 . Thus, when vibration energy is supplied from the target object  302 , the power supply element  1  continues to supply electric power to the sensor device  2 . The type of vibrations that become an energy source may be any type. For example, the vibrations that become an energy source may be sinusoidal vibrations. In addition, the vibrations that become an energy source may be random vibrations. The sensor device  2  installed in the target object  302  continues to supply electric power using vibration energy of the target object  302 . Therefore, in the sensor device  2 , a battery does not need to be replaced. 
     In addition, the target object  302  may include a target object  302   b  that is independent from target objects  302   a  and  302   a . The target object  302   a  may supply external energy to the power supply element  1 . The target object  302   b  may be measured by the sensor  4   a.    
     The power supply element  1  includes an energy conversion element  6  (a power generation element) and a power conversion element  7 . The energy conversion element  6  converts external energy supplied from the target object  302  into electric power. The power conversion element  7  converts electric power supplied from the energy conversion element  6  into electric power that can be used for driving the sensor element  4 . 
     As illustrated in  FIG.  2   , the energy conversion element  6  includes a power generation circuit  8 . The power generation circuit  8  includes outputs  8   a  and  8   b , a power generation unit  8 E, and an output resistor  8 R. The power generation unit  8 E is connected to the output resistor  8 R and a grounding electric potential GND. The output resistor  8 R is connected to the power generation unit  8 E and the output  8   a.    
     The power generation unit  8 E accepts energy supplied from the target object  302 . The power generation unit  8 E generates electric power using the accepted energy. The electric power is supplied to the output  8   a . For example, the power generation unit  8 E may use a piezoelectric phenomenon in which vibrations are converted into electric power. The power generation unit  8 E includes a vibrator such as a cantilever. A resonance frequency of the vibrator is adjusted to a vibration frequency of the target object  302 . The power generation unit  8 E generates AC power. A frequency of the AC power corresponds to a frequency at which the vibrator actually vibrates. In other words, the frequency of the AC power output from the power generation unit  8 E corresponds to the frequency of the vibrator. The frequency of the AC power output from the power generation unit  8 E corresponds to the vibration frequency of the target object  302 . For example, the frequency of the AC power is equal to or higher than several hundred hertz and is equal to or lower than several megahertz. 
     The power conversion element  7  is electrically connected to the energy conversion element  6 . The power conversion element  7  is electrically connected to the sensor element  4 . 
     The power conversion element  7  includes an impedance adjustment circuit (hereinafter, referred to as “adjustment circuit  9 ”), a power conversion circuit  11 , and a control unit  12 . The adjustment circuit  9  is connected to the energy conversion element  6  and the power conversion circuit  11 . The control unit  12  controls operations of the power conversion circuit  11  and the adjustment circuit  9 . 
     The adjustment circuit  9  is connected to the power generation circuit  8  and the power conversion circuit  11 . The adjustment circuit  9  adjusts the impedance of a circuit configuration that connects the power conversion circuit  11  to the power generation circuit  8 . 
     The adjustment circuit  9  includes an input  9   a  (an input port), an input  9   b , an output  9   c  (an output port), and an output  9   d . The input  9   a  is connected to the output  8   a  of the power generation circuit  8 . The input  9   b  is connected to the output  8   b  of the energy conversion element  6 . The outputs  9   c  and  9   d  are connected to the power conversion circuit  11 . 
     Between the input  9   a  and the output  9   c , a switch S 1  (a first switch) and a switch S 2  (a second switch) are disposed. The switch S 1  is connected to the input  9   a  and the switch S 2 . The switch S 2  is connected to the switch S 1  and the output  9   c . The input  9   a , the output  9   c , and the switches S 1  and S 2  configure a first circuit unit  13 . The switches S 1  and S 2  are connected to a capacitor C. A line L 1  connecting the switch S 1  to the switch S 2  has a connection point P 1 . A line L 2  connected to the capacitor C is connected to the connection point P 1 . 
     The input  9   b  is connected to the output  9   d . A line L 3  connecting the input  9   b  to the output  9   d  has a grounding point P 2 . The grounding point P 2  is connected to one end of the capacitor C. Thus, reference electric potential (GND) is supplied to the input  9   b , the output  9   d , and one end of the capacitor C. The connection point P 1 , the grounding point P 2 , and the capacitor C configure a second circuit unit  14 . 
     The capacitance of the capacitor C, for example, is set on the basis of a magnitude of an open voltage of the power generation circuit  8 , the electric power required by the power conversion circuit  11 , the electric power required by the sensor element  4 , and the like. For example, the capacitance of the capacitor C may be 10 μF. 
     The power conversion circuit  11  converts the electric power output from the adjustment circuit  9  into electric power according to the specifications of the sensor element  4 . More specifically, a voltage of the electric power output from the power conversion circuit  11  is higher than a voltage of the electric power output from the adjustment circuit  9 . In other words, the power conversion circuit  11  boosts the voltage. 
     The power conversion circuit  11  includes inputs  11   a  and  11   b  and outputs  11   c  and  11   d . The input  11   a  is connected to the output  9   c  of the adjustment circuit  9 . The input  11   b  is connected to the output  9   d  of the adjustment circuit  9 . The outputs  11   c  and  11   d  are connected to the sensor element  4 . 
     The power conversion circuit  11  includes a frequency modulating unit  16  and a transformation unit  17 . The frequency modulating unit  16  is connected to the inputs  11   a  and  11   b . The frequency modulating unit  16  receives electric power from the adjustment circuit  9 . The frequency modulating unit  16  generates a clock signal. The frequency modulating unit  16  superimposes a clock signal on an output voltage of the adjustment circuit  9 . The frequency modulating unit  16  may employ a ring oscillator. The ring oscillator, for example, includes a NAND circuit and an inverter circuit. A frequency of a clock signal generated by the frequency modulating unit  16  may be equal to or higher than several megahertz. The frequency of the clock signal may be equal to or higher than several gigahertz. 
     The frequency modulating unit  16  is further connected to the transformation unit  17 . The frequency modulating unit  16  supplies a voltage on which a clock signal is superimposed to the transformation unit  17 . 
     The transformation unit  17  is connected to the frequency modulating unit  16 . The transformation unit  17  receives a voltage on which a clock signal is superimposed. The transformation unit  17  boosts the voltage on which the clock signal is superimposed. For example, the transformation unit  17  may employ a charge pump. The charge pump includes a plurality of diodes and a plurality of capacitors. 
     The transformation unit  17  is connected to the outputs  11   c  and  11   d . The transformation unit  17  supplies electric power based on the boosted voltage to the outputs  11   c  and  11   d.    
     The control unit  12  generates control signals φ 1  and φ 2  used for the switches S 1  and S 2  of the adjustment circuit  9 . The control unit  12  is connected to the adjustment circuit  9 . The control unit  12  supplies the control signals φ 1  and φ 2  to the switches S 1  and S 2 . The control signal φ 1  is supplied to the switch S 1 . The control signal φ 2  is supplied to the switch S 2 . 
     Hereinafter, operations of the adjustment circuit  9  and the control unit  12  will be described in detail. 
     The control unit  12  supplies a control signal used for a charging operation to the adjustment circuit  9 . More specifically, the control unit  12  sets the control signal φ 1  to HIGH and sets the control signal φ 2  to LOW (see a period T 1  illustrated in  FIG.  3   ). The adjustment circuit  9  that has received the control signal φ 1  (H) closes the switch S 1 . In other words, the input  9   a  is connected to the capacitor C. As a result, the capacitor C is charged using electric power generated by the power generation circuit  8 . In addition, the adjustment circuit  9  that has received the control signal φ 2  (L) opens the switch S 2 . In other words, the output  9   c  is disconnected from the capacitor C. As a result, no electric power is supplied to the output  9   c . The power conversion circuit  11  is separated from the adjustment circuit  9 . In other words, the power conversion circuit  11  is separated from the power generation circuit  8 . Thus, in a charging operation, the power conversion circuit  11  does not output electric power. 
     After a predetermined time elapses from a timing at which a control signal for a charging operation is supplied, the control unit  12  supplies a control signal used for a discharging operation to the adjustment circuit  9 . More specifically, the control unit  12  sets the control signal φ 1  to LOW and sets the control signal φ 2  to HIGH (see a period T 2  illustrated in  FIG.  3   ). The adjustment circuit  9  that has received the control signal φ 1  (L) opens the switch S 1 . In other words, the input  9   a  is disconnected from the capacitor C. As a result, the power generation circuit  8  is separated from the adjustment circuit  9 . In other words, the power generation circuit  8  is separated from the power conversion circuit  11 . The adjustment circuit  9  that has received the control signal φ 2  (H) closes the switch S 2 . In other words, the output  9   c  is connected to the capacitor C. In other words, the adjustment circuit  9  is connected to the power conversion circuit  11 . As a result, a voltage is supplied from the capacitor C to the output  9   c . In this state, the impedance (the output resistance  8 R) of the power generation circuit  8  has no influence on the power conversion circuit  11 . Thus, the power conversion circuit  11  can receive a voltage that is close to an open voltage of the power generation circuit  8 . As a result, a voltage having a low influence (or no influence) of a voltage drop is supplied to the power conversion circuit  11 . In accordance with this voltage, the power conversion circuit  11  starts an operation. Then, the power conversion circuit  11  outputs a boosted voltage. 
     In the description presented above, the control unit  12  sets the length of the period T 1  of the charging operation and the length of the period T 2  of the discharging operation in advance. In other words, the periods T 1  and T 2  are fixed times. For example, the period T 1  of the charging operation is longer than the period T 2  of the discharging operation. The power conversion circuit  11  performs power conversion only during the period T 2  of the discharging operation. In other words, the power conversion circuit  11  supplies electric power to the output  11   c  only during the period T 2  of the discharging operation. 
     When a discharging operation is performed, the power conversion circuit  11  is separated from the power generation circuit  8 . Here, the adjustment circuit  9  includes switches S 1  and S 2  and a capacitor C. Thus, ideally, the adjustment circuit  9  may be regarded to have no output resistance. In an actual circuit configuration, an output resistance  14 R is present. However, the output resistance  14 R is extremely low. Thus, the output resistance  14 R may be regarded as being zero. In  FIG.  2   , the output resistance  14 R is illustrated. However, the adjustment circuit  9  does not have the output resistance  14 R as a resistor. For example, the output resistance  14 R illustrated in  FIG.  2    explicitly represents a resistance component of a line connecting the capacitor C to the switch S 2 . Generally, such a resistance component is ignored. This connection configuration is a state in which a power supply of which an impedance is zero is connected to the power conversion circuit  11 . Alternatively, the connection configuration may be a state in which a power supply of which an impedance is extremely low is connected to the power conversion circuit  11 . Thus, in accordance with a relation between the impedances of a power supply (the adjustment circuit  9 ) and a load (the power conversion circuit  11 ), the voltage of the power supply does not drop. As a result, electric power can be efficiently supplied from the power generation circuit  8  to the power conversion circuit  11 . 
     In other words, in order to operate the power conversion circuit  11 , a predetermined current needs to be generated by supplying a predetermined voltage required by an integrated circuit that becomes a load of the power conversion circuit  11  to the integrated circuit. When the power generation circuit  8  is directly connected to the power conversion circuit  11 , a voltage supplied to the power conversion circuit  11  is lowered in accordance with a high impedance (the output resistance  8 R) of the power generation circuit  8 . When this voltage drop is not present, a necessary output current may be able to be obtained. However, in accordance with the voltage drop, the output current markedly decreases. On the other hand, by using the adjustment circuit  9 , an ideal state in which an impedance is zero can be realized. By using the adjustment circuit  9 , electric power can be supplied to the power conversion circuit  11  in an ideal state. As a result, for example, a period (the period T 2  illustrated in  FIG.  3   ) in which the adjustment circuit  9  is connected to the power conversion circuit  11  is assumed to be set to  1 / 3  of a period (the period T 1  illustrated in  FIG.  3   ) in which the power generation circuit  8  is connected to the adjustment circuit  9 . Also in this case, ⅓ of an output current not influenced by the voltage drop can be supplied to the power conversion circuit  11 . 
     In other words, according to the adjustment circuit  9 , even in a case in which the open voltage of the power generation circuit  8  is low, power conversion can be performed. Therefore, even in a case in which energy input to the power generation circuit  8  is low, the energy can be collected as electric power. 
     [Operations and Effects] The power conversion element  7  and the power supply element  1  include the adjustment circuit  9 . The capacitor C of the adjustment circuit  9  is charged in accordance with electric power received from the power generation circuit  8  through the input  9   a  of the first circuit unit  13 . The capacitor C supplies electric power to the power conversion circuit  11  through the output  9   c  of the first circuit unit  13 . In the form in which electric power is supplied to the power conversion circuit  11 , a power source of the power conversion circuit  11  is not the power generation circuit  8  but is the capacitor C. The output resistance  14 R present between the capacitor C and the output  9   c  is smaller than the output resistance  8 R of the power generation circuit  8 . As a result, a circuit configuration in which the adjustment circuit  9  is connected between the power generation circuit  8  and the power conversion circuit  11  can inhibit a drop of a voltage supplied to the power conversion circuit  11  better than a circuit configuration in which the power generation circuit  8  is directly connected to the power conversion circuit  11 . Therefore, electric power can be transmitted with a high efficiency. 
     In a case in which an amount of external energy (vibrations, heat, or the like) input to the power generation circuit  8  is minute, an amount of electric power output from the power generation circuit  8  is low. For example, in a power generation circuit using a temperature difference, in a case in which a temperature difference caused in the power generation circuit is smaller than a predetermined temperature difference, the electric power required by the power supply element cannot be supplied. On the other hand, the power supply element  1  according to an embodiment can supply electric power from the power generation circuit  8  to the power conversion circuit  11  without causing a voltage drop. Thus, a threshold value of external energy that is necessary for supplying electric power from the power supply element  1  to the sensor element  4  can be lowered. 
     The first circuit unit  13  includes the switch S 1  connected to the input  9   a  and the switch S 2  connected to the switch S 1  and the output  9   c . The second circuit unit  14  includes the connection point P 1  and the capacitor C. The connection point P 1  is connected to the switch S 1  and the switch S 2 . The capacitor C is connected to the connection point P 1  and the grounding point P 2 . The power supply element  1  further includes the control unit  12  that controls the switches S 1  and S 2 . The control unit  12  performs switching between a charging operation and a discharging operation. In a charging operation, the input  9   a  is connected to the capacitor C by controlling the switch S 1 . In addition, in the charging operation, the output  9   c  is disconnected from the capacitor C by controlling the switch S 2 . In a discharging operation, the input  9   a  is disconnected from the capacitor C by controlling the switch S 1 . In addition, in the discharging operation, the output  9   c  is connected to the capacitor C by controlling the switch S 2 . According to this configuration, switching between the charging operation and the discharging operation can be reliably performed. 
     The control unit  12  controls operations of the switches S 1  and S 2  every time a predetermined time elapses. According to this configuration, the adjustment circuit  9  can be controlled in a simplified manner. 
     Although the embodiment of the present invention has been described, the present invention is not limited to the embodiment described above. 
     MODIFIED EXAMPLE 1 
     The control unit  12  has been described to regularly perform switching between a charging operation and a discharging operation every time a period set in advance elapses. A control unit  12 A of a power supply element  1 A of Modified Example 1 may perform control of switching between a charging operation and a discharging operation using a voltage supplied to the output  9   c  of the adjustment circuit  9 . As illustrated in  FIG.  4   , the power supply element  1 A of Modified Example 1 includes a line L 4  that is connected to the output  9   c  of the adjustment circuit  9  in addition to the configuration of the power supply element  1  according to the embodiment. A voltage is supplied to the output  9   c  in accordance with a discharging operation. This voltage is lowered in accordance with elapse of a time. The control unit  12 A monitors the voltage supplied to the output  9   c . When the voltage supplied to the output  9   c  decreases by a predetermined proportion from a voltage at the time of starting a discharging operation as a reference, the control unit  12 A switches from the discharging operation to a charging operation. For example, the voltage at the time of starting the discharging operation is set as 100%. When the voltage of the output  9   c  decreases to be 90% or less, the control unit  12 A may perform switching from the discharging operation to the charging operation. 
     MODIFIED EXAMPLE 2 
     As illustrated in  FIG.  5   , a power supply element  1 B of Modified Example 2 includes the power generation circuit  8 , an adjustment circuit  9 B, the power conversion circuit  11 , and a control unit  12 B. The adjustment circuit  9 B, the power conversion circuit  11 , and the control unit  12 B configure a power conversion element  7 B. The adjustment circuit  9 B of Modified Example 2 includes four switches S 3 , S 4 , S 5 , and S 6  and two capacitors C 1  and C 2 . This configuration is acquired by connecting the adjustment circuit  9  in parallel. 
     More specifically, the switch S 3  is connected to an input  9   a , the switch S 4 , and the capacitor C 1 . The switch S 4  is connected to the switch S 3 , the capacitor C 1 , and an output  9   c . The capacitor C 1  is connected to the switches S 3  and S 4  and the grounding electric potential GND. The switch S 5  is connected to the input  9   a , the switch S 6 , and the capacitor C 2 . The switch S 6  is connected to the switch S 5 , the capacitor C 2 , and the output  9   c . The capacitor C 2  is connected to the switches S 5  and S 6  and the grounding electric potential GND. The switches S 3  and S 6  are controlled in accordance with a control signal φ 1 . On the other hand, the switches S 4  and S 5  are controlled in accordance with a control signal φ 2 . The control unit  12 B performs switching between a charging operation and a discharging operation using a voltage V IN  supplied to the output  9   c  of the adjustment circuit  9 B. In addition, as in the embodiment, the control unit  12 B may regularly perform switching between the operations on the basis of a period set in advance. 
     When charging the capacitor C 1 , the adjustment circuit  9 B discharges the capacitor C 2 . For example, the control unit  12 B supplies a control signal φ 1  (H) and a control signal φ 2  (L) to the adjustment circuit  9 B. As a result, the capacitor C 1  is connected to the power generation circuit  8 . In addition, the capacitor C 1  is disconnected from the power conversion circuit  11 . On the other hand, the capacitor C 2  is disconnected from the power generation circuit  8 . In addition, the capacitor C 2  is connected to the power conversion circuit  11 . In other words, the capacitor C 1  is charged. On the other hand, the capacitor C 2  is discharged. For example, the control unit  12 B supplies a control signal φ 1  (L) and a control signal φ 2  (H) to the adjustment circuit  9 B. As a result, the capacitor C 1  is disconnected from the power generation circuit  8 . In addition, the capacitor C 1  is connected to the power conversion circuit  11 . On the other hand, the capacitor C 2  is connected to the power generation circuit  8 . In addition, the capacitor C 2  is disconnected from the power conversion circuit  11 . In other words, the capacitor C 1  is discharged. The capacitor C 2  is charged. 
     The adjustment circuit  9 B included in the power supply element  1 B of Modified Example 2 includes two capacitors C 1  and C 2 . As a result, a period in which electric power is supplied to the power conversion circuit  11  can be increased. In other words, a period (a voltage conversion period) in which a voltage is output from the power conversion circuit  11  can be increased. 
     MODIFIED EXAMPLE 3 
     As illustrated in  FIG.  6   , a power supply element  1 C of Modified Example 4 may further include an additional power conversion circuit  18  (a second power conversion circuit) in addition to the power generation circuit  8 , the adjustment circuit  9 , and the power conversion circuit  11  (a first power conversion circuit). The additional power conversion circuit  18  is connected to the outputs  8   a  and  8   b  of the power generation circuit  8  and the outputs  11   c  and  11   d  of the power conversion circuit  11 . The additional power conversion circuit  18  is disposed in parallel with the adjustment circuit  9  and the power conversion circuit  11 . The additional power conversion circuit  18  has a high input impedance of a degree not influenced by the output resistance  8 R of the power generation circuit  8 . Thus, the additional power conversion circuit  18  can be operated near an open voltage of the power generation circuit  8 . When the power generation circuit  8  starts to operate, the additional power conversion circuit  18  generates electric power having a predetermined voltage. Then, after the output voltage of the power generation circuit  8  reaches an output voltage at the time of a steady operation, the operation of the adjustment circuit  9  starts in accordance with the voltage. According to this configuration, a circuit area that is necessary for starting and steady operations of the adjustment circuit  9  and the power conversion circuit  11  can be decreased. In addition, the power efficiency of the power supply element  1 C can be raised. 
     MODIFIED EXAMPLE 4 
     As illustrated in  FIG.  7   , a power supply element  1 D of Modified Example 4 includes the power generation circuit  8 , an adjustment circuit  9 D, a power conversion circuit  11 D, and a control unit  12 D. The adjustment circuit  9 D includes a line L 5  and a capacitor C 3 . The line L 5  connects an input  9   a  and an output  9   c . The capacitor C 3  is connected to the line L 5  and grounding electric potential GND. The adjustment circuit  9 D of Modified Example 3 is acquired by removing the switches S 1  and S 2  from the adjustment circuit  9 . The control unit  12 D of Modified Example 3 is a constituent element of the power conversion circuit  11 D. The control unit  12 D controls the operation of the power conversion circuit  11 D. The control unit  12 D is a pulse generator (PG). The control unit  12 D controls starting and stopping of a transformation operation of the power conversion circuit  11 D. The power conversion circuit  11 D supplies a control signal to the frequency modulating unit  16 . The output  11   c  of the power conversion circuit  11 D may be connected to the capacitor C 4 . It is preferable that the capacitance of the capacitor C 4  be smaller than the capacitance of the capacitor C 3 . However, the magnitude relation between the capacitors C 3  and C 4  is not limited to the relation described above. For example, the capacitance of the capacitor C 4  may be equal to the capacitance of the capacitor C 3 . The capacitance of the capacitor C 4  may be larger than the capacitance of the capacitor C 3 . 
     Also according to this configuration, the control unit  12 D can perform switching between a charging operation and a discharging operation of the adjustment circuit  9 D. For example, the control unit  12 D may stop the operation of the power conversion circuit  11 D. The stopping of the operation of the power conversion circuit  11 D is stopping oscillation of the frequency modulating unit  16 . As a result, electric power supplied from the power generation circuit  8  is charged into the capacitor C 3  (a charging operation). In a state in which the operation of the power conversion circuit  11 D stops, the impedance of the power conversion circuit  11 D can be regarded to be almost infinity (a high impedance state). On the other hand, the control unit  12 D causes the operation of the power conversion circuit  11 D to be started. The starting of the operation of the power conversion circuit  11 D is starting the oscillation of the frequency modulating unit  16 . As a result, electric power is supplied from the capacitor C 3  to the power conversion circuit  11 D (a discharging operation). In a state in which the power conversion circuit  11 D operates, the impedance of the power conversion circuit  11 D can be regarded to be in a low impedance state. During a discharging operation, electric power output from the power generation circuit  8  is supplied to the power conversion circuit  11 . However, the operation of the power conversion circuit  11  is controlled by electric power supplied from the capacitor C 3 . According to this configuration, the operations of the switches S 1  and S 2  can be realized by the operation of the power conversion circuit  11 D. Thus, the switches S 1  and S 2  can be omitted. As a result, generation of parasitic resistance can be inhibited. In addition, a control circuit of the switches S 1  and S 2  can be also omitted. Therefore, the configuration of the power supply element  1 D including the adjustment circuit  9 D can be simplified. 
     MODIFIED EXAMPLE 5 
     As illustrated in  FIG.  8   , a power supply element  1 E of Modified Example 5 includes the power generation circuit  8 , the adjustment circuit  9 D, a power conversion circuit  11 E, a control unit  12 D, and the detection unit  20 . The detection unit  20  is connected to an output  11   c  of the power conversion circuit  11 E. The detection unit  20  monitors a voltage supplied to the output  11   c . When the voltage of the output  11   c  exceeds a threshold value, the detection unit  20  stops the operation of the frequency modulating unit  16 . When a voltage of the power conversion circuit  11 E is below a threshold value, the detection unit  20  starts the operation of the frequency modulating unit  16 . When the detection unit  20  stops the operation of the frequency modulating unit  16 , a type of control signal of the control unit  12 D may be any type. For example, even in a case in which the control unit  12  supplies a control signal used for starting the operation of the power conversion circuit  11 E (starting oscillation of the frequency modulating unit  16 ) to the frequency modulating unit  16 , when a control signal for stopping an operation is supplied from the detection unit  20 , the frequency modulating unit  16  stops the operation. According to this configuration, restrictions can be placed on an output from the power conversion circuit  11 E. 
     MODIFIED EXAMPLE 6 
     As illustrated in  FIG.  9   , a power supply element  1 F of Modified Example 6 includes the power generation circuit  8 , the adjustment circuit  9 D, a power conversion circuit  11 F, and a clock generating unit  21 . The power conversion circuit  11 F may be a switching regulator (SW). The switching regulator includes a coil  22 , a transistor  23 , and a diode  24 . In this circuit configuration, starting (see a period T 1  illustrated in  FIG.  10   ) and stopping (see a period T 2  illustrated in FIG.  10 ) of an operation of the power conversion circuit  11 F are controlled in accordance with a clock signal. The clock signal is supplied from the clock generating unit  21  to the diode  24 . Therefore, switching between a charging operation and a discharging operation of the adjustment circuit  9 D can be performed. 
     In other words, in the power supply element  1 F of Modified Example 6, the power conversion circuit is a first power conversion circuit, and a second power conversion circuit, which is different from the first power conversion circuit, converting electric power generated by a power generation circuit into a desired form and a control unit that controls operations of the first power conversion circuit and the second power conversion circuit are further included. The second power conversion circuit is disposed in parallel with an impedance adjustment circuit and the first power conversion circuit. An input impedance of the second power conversion circuit is closer to an output impedance of the power generation circuit than an input impedance of the first power conversion circuit. After obtaining electric power from the second power conversion circuit, the control unit obtains electric power from the first power conversion circuit. 
     In addition, the first power conversion circuit and the second power conversion circuit may be provided as mutually different circuits. In addition, by controlling operating conditions of one power conversion circuit, switching between a function of the first power conversion circuit and a function of the second power conversion circuit may be performed. 
     MODIFIED EXAMPLE 7 
     As illustrated in  FIG.  11   , a power supply element  1 G of Modified Example 7 includes the power generation circuit  8 A, the adjustment circuit  9 , a power conversion circuit  11 G, the additional power conversion circuit  18 , and a control unit  12 . The power generation circuit  8 A includes four outputs  8   c ,  8   d ,  8   e , and  8   f , two output resistors  8 Ra and  8 Rb, and two power generation units  8 Ea and  8 Eb. 
     The output resistor  8 Ra is connected to the output  8   c . The power generation unit  8 Ea is connected to the output resistor  8 Ra. The grounding electric potential (GND) is connected to the output  8   d . The output resistor  8 Rb is connected to the output  8   e . The power generation unit  8 Eb is connected to the output resistor  8 Rb. The grounding electric potential (GND) is connected to the output  8   f . The output resistor  8 Ra is larger than the output resistor  8 Rb. The outputs  8   e  and  8   f  are connected to the adjustment circuit  9 . The outputs  8   c  and  8   d  are connected to the additional power conversion circuit  18 . An output of the additional power conversion circuit  18  is connected to the adjustment circuit  9  and the power conversion circuit  11 . 
     In this circuit configuration, the additional power conversion circuit  18  autonomously starts a transformation operation. The additional power conversion circuit  18  supplies a voltage V ctrl  to the adjustment circuit  9  and the power conversion circuit  11 G The power conversion circuit  11 G includes a detection unit  25 . The detection unit  25  monitors the magnitude of the voltage V ctrl . In a case in which the voltage V ctrl  is determined to be higher than a threshold value, the detection unit  25  starts an operation of the power conversion circuit  11 G As a result, the power supply element  1 G can supply a desired electric potential. 
     [Examination Case 1] 
     In Examination Case 1, a theoretical limit of an open voltage of a power generation circuit has been checked. Part (a) of  FIG.  12    illustrates a model of a power supply element  100  according to Examination Case 1. The power supply element  100  of Examination Case 1 includes a power generation circuit  101  and a power conversion circuit  102 . The power generation circuit  101  includes a thermoelectric element (TEG). The power supply element  100  supplies an electric power P OUT  to a load  103 . The power generation circuit  101  has an output resistance R TEG . The power generation circuit  101  outputs an electric power P IN  represented using an open voltage V OC . The power conversion circuit  102  has a conversion efficiency η. The power conversion circuit  102  outputs the electric power P OUT . The load  103  requires an electric power represented using a current I PP  and a voltage V PP . 
     First, the operation of the power conversion circuit  102  is represented using Equation (1).
 
[Math 1]
 
 P   OUT   =η×P   IN   (1)
 
     A condition for efficiently performing transfer of electric power from the power generation circuit  101  to the power conversion circuit  102  is represented in Equation (2). In other words, the condition is to match a load resistance R CONV-LOAD  seen from the power generation circuit  101  to the output resistance R TEG  of the power generation circuit  101 .
 
[Math 2]
 
 R   CONV-LOAD   =R   TEG   (2)
 
     When the condition represented in Equation (2) is satisfied, 
     Equation (3) becomes valid.
 
[Math 3]
 
 P   IN =( V   OC /2) 2   /R   TEG   (3)
 
     A relation between the electric power P OUT  output by the power conversion circuit  102 , the open voltage V OC  of the power generation circuit  101 , and the output resistance R TEG  is represented using Equation (4).
 
[Math 4]
 
 P   OPUT =η×( V   OC /2) 2   /R   TEG   (4)
 
     A relation between the conversion efficiency η of the power conversion circuit  102  and the theoretical limit of the lower limit of the open voltage of the power generation circuit  101  has been checked using Equation (4). In Examination Case 1, the following conditions were set. 
     Voltage V PP  of the load  103 : 3.3 V 
     Current I PP  of the load  103 : 30 μA (Condition 1-1), 3 μA (Condition 1-2). 
     Output resistance R TEG  of the power generation circuit  101 :300Ω 
     Part (b) of  FIG.  12    represents a relation between the conversion efficiency η of the power conversion circuit  102  and the open voltage V OC  of the power generation circuit  8 . The horizontal axis represents the conversion efficiency η. The vertical axis represents the open voltage V OC . A graph G 1  is a result of Condition 1-1. A graph G 2  is a result of Condition 1-2. Here, the graphs G 1  and G 2  will be compared with each other. A difference between the graphs G 1  and G 2  exhibits an effect of enhancing the lower limit value V OC-MIN  of the open voltage V OC . According to the graphs G 1  and G 2 , it can be understood that the effect of enhancing the lower limit value V OC-MIN  of the open voltage V OC  is small in a case in which the conversion efficiency η is equal to or higher than 30%. A relation between the current I PP  of the power conversion circuit  102  and the lower limit value V OC_MIN  of the open voltage of the power generation circuit  8  is represented in Equation (5). Thus, it can be understood that the lower limit value V OC_MIN  of the open voltage strongly depends on the output condition of the power conversion circuit  102 .
 
[Math 5]
 
 V   OC_MIN   ˜I   PP   1/2   (5)
 
     [Examination Case 2] 
     In Examination Case 2, a relation between the conversion efficiency of the power conversion circuit  102  and the open voltage V OC  of the power generation circuit  8  in a case in which the adjustment circuit  104  is applied with the condition of the load  103  fixed was reviewed. Part (a) of  FIG.  13    represents a model of a power supply element  200  of Examination Case 2. The power supply element  200  includes a power generation circuit  101 , a power conversion circuit  102 , and an adjustment circuit  104 . In Examination Case 2, the condition of the load  103  was configured to be constant (the current I PP =30 uA, and the voltage V PP =3.3 V). In addition, the output resistance R TEG  of the power generation circuit  8  was configured to be 300Ω. A graph G 3  represents a relation between the conversion efficiency η and the open voltage V OC  in a case in which the adjustment circuit  104  is applied. A graph G 4  represents a relation between the conversion efficiency  11  and the open voltage V OC  in a case in which the adjustment circuit  104  is not applied. As illustrated in the graph G 3 , it can be understood that the minimum value V OC_MIN  of the open voltage of the power supply element  100  can halve by applying the adjustment circuit  104 . 
     [Examination Case 3] 
     A power supply element  1  including the adjustment circuit  9  has a high efficiency of transfer of electric power. As a result, the power supply element  1  including the adjustment circuit  9  can reduce the scale of the circuit used for obtaining desired electric power. In Examination Case 3, a circuit area of the power supply element  1  including the adjustment circuit  9  and a circuit area of a power supply element  200  (a comparative example) not including the adjustment circuit  9  were compared with each other. Part (a) of  FIG.  14    represents a relation between the output voltage V dd  and the circuit area A tot  of the power conversion circuit. Part (b) of  FIG.  14    represents a relation between the output voltage V dd  and the conversion efficiency η of the power conversion circuit. 
     Common conditions are as below. Output resistance R S  of the power generation circuit: 500Ω. 
     Output current I PP  from the power conversion circuit: 40 μA 
     The conversion efficiency η of the power conversion circuit of the power supply element (the comparative example) not including the adjustment circuit  9  is assumed to be 20%. As a result, the input current I dd  is 200 μA (I dd =I PP /η). When the output resistance is 500Ω, and the input current I dd  is 200 μA, a voltage drop is 0.1 V (Rs×I dd ). The output voltage V dd  of the power generation circuit is assumed to be 0.2 V. Then, an input voltage supplied to the power conversion circuit is 0.1 V. The graph represented in Part (a) of  FIG.  14    will be referred to. According to this graph, it can be understood that the circuit area of the power conversion circuit operating at 0.1 V is 2.5 mm 2 . The graph represented in Part (a) of  FIG.  14    is a result of a case in which the output current I PP  from the power conversion circuit is 10 μA. In other words, it can be understood that a circuit area that is four times the circuit area described above (2.5 mm 2 ×4) is necessary when the output current I PP  required for the power conversion circuit is 40 μA. 
     Next, the circuit area of the power supply element  1  including the adjustment circuit  9  was checked. The capacitance of the capacitor C of the adjustment circuit  9  was configured to be 10 μF. As an operating condition of the adjustment circuit  9 , the time of a charging operation was set to 3.5 msec. A time of a discharging operation was set to 1.0 msec. According to this operation, the output voltage from the capacitor C was set to 0.2 V±10 mV. In a case in which the adjustment circuit  9  is included, a voltage drop may not be taken into account. Thus, an input voltage supplied to the power conversion circuit  11  is 0.2 V. According to Part (a) of  FIG.  14   , it can be understood that the circuit area of the power conversion circuit  11  operating at 0.2 V was about 0.4 mm 2 . 
     According to Part (b) of  FIG.  14   , the conversion efficiency of the power conversion circuit  11  operating at 0.2 V was 30%. In other words, the output current I PP  of the power conversion circuit  11  was 60 μA. The current I PP  is output only for a predetermined period. Thus, an average value of the current I PP_AVG  was 13 μA (60 μA×1 msec/4.5 msec). It can be understood that a circuit area (about 1.2 mm 2 =0. 4 mm 2 ×3) that is about three times (40 μA/13 μA) the circuit area described above is necessary when the output current I PP  required for the power conversion circuit  11  is 40 μA. In other words, in the assumed conditions, it can be understood that the power supply element  1  including the adjustment circuit  9  can decrease the circuit area to be smaller than the circuit area of the power supply element not including the adjustment circuit  9 . 
     [Examination Case 4] 
     In Examination Case 4, a relation between the output resistance  8 R of the power generation circuit  8  and the circuit area of the power conversion circuit  11  was checked. In addition, a relation between the output resistance  8 R of the power generation circuit  8  and the conversion efficiency η was also checked. Furthermore, as a comparison target, a relation between the output resistance  8 R and the circuit area, the output resistance  8 R, and the conversion efficiency η were checked also for a case in which the adjustment circuit  9  is not included. The set conditions and results are illustrated in Table 1. In addition, a denotation inside parentheses in each of fields of the circuit area and the conversion efficiency η is a proportion when Condition 3-3 (or Condition 3-4) is set as a reference. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Voltage 
                 Output 
                   
                 Conversion 
               
               
                   
                 (V dd ) 
                 resistance 8 R 
                 Circuit area 
                 efficiency 
               
               
                   
                 [V] 
                 [Ω] 
                 [mm 2 ] 
                 [%] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Condition 3-1 
                 0.2 
                 500 
                 1.2 
                 (1/8) 
                 30 
                 (×1.5) 
               
               
                 Condition 3-2 
                 0.3 
                 1000 
                 0.3 
                 (1/30) 
                 45 
                 (×2.2) 
               
               
                 Condition 3-3 
                 0.2 
                 500 
                 10 
                 (1/1) 
                 20 
                 (×1) 
               
               
                 (comparative 
               
               
                 example) 
               
               
                 Condition 3-4 
                 0.3 
                 1000 
                 10 
                 (1/1) 
                 20 
                 (×1) 
               
               
                 (comparative 
               
               
                 example) 
               
               
                   
               
            
           
         
       
     
     In the power supply element (comparative example) not including the adjustment circuit  9 , the voltage drop was doubled (0.2 V). For example, in the power supply element (comparative example) not including the adjustment circuit  9 , when the voltage V dd  was 0.3 V, a voltage supplied to the power conversion circuit was 0.1 V (0.3 V−0.2 V). On the other hand, the power supply element  1  including the adjustment circuit  9  performs an intermittent operation. Thus, in the power supply element  1 , an average value of the output current decreases. As a result, in order to supplement the decrease in the average value of the output current, the circuit area of the power supply element  1  needs to be increased. However, in the power supply element  1 , a voltage drop may not be taken into account. Thus, a voltage of 0.3 V is supplied to the power conversion circuit  11  of the power supply element  1 . As a result, the current density of the output of the power supply element  1  is improved. The influence of the improvement of the current density is about one order of magnitude larger than the influence of the increase in the circuit area. As a result, the circuit area of the power supply element  1  can be configured to be 0.3 mm 2 . 
     The circuit areas in Condition 3-1 and Condition 3-2 will be focused on. The circuit area at a time when the output resistance  8 R is configured to be 500Ω was ⅛ of that of the comparative example (Condition 3-3). On the other hand, the circuit area at a time when the output resistance  8 R was configured to be 1000Ω was 1/30 of the circuit area of the comparative example (Condition 3-3). In other words, it can be understood that the circuit area can be decreased in a case in which the output resistance  8 R is high. As a result, it can be understood that, even when the output resistance  8 R of the power generation circuit  8  is increased, by configuring the open voltage to be high, a satisfactory design can be performed. For example, the number of energy harvesting elements is assumed to be a predetermined number. The number of energy harvesting elements connected in series is doubled. The number of energy harvesting elements connected in parallel is set to a half thereof. As a result, it can be understood that the open voltage can be doubled. In addition, it can be understood that the output resistance can be also doubled. As a result, a necessary circuit area can be decreased in size. Therefore, it can be understood that the degree of freedom of design of the power supply element  1  can be raised. 
     MODIFIED EXAMPLE 8 
       FIG.  15    illustrates the configuration of the sensor device  2  as an IoT terminal. The energy conversion element  6  including the power generation circuit  8  outputs electric power (P IN ) collected from environmental energy. A voltage (V IN ) output from the energy conversion element  6  is converted into a voltage (V OUT ) by the power conversion element  7  that is a power supply circuit. This voltage (V OUT ) is supplied to the sensor element  4  (load element) that is connected to the power conversion element  7 . The sensor element  4  includes a sensor  4   a  and a communication circuit  4   d  such as a wireless IC. 
     The operating power of such elements configuring the sensor element  4  is about 10 mW. For example, in a period in which the operation of the sensor element  4  is not required, the sensor element  4  is caused to be in an inactive state. According to such an operation, average electric power of the sensor element  4  can be configured to be about 10 μW. 
     Environmental energy sources include sun light, heat, vibrations, radio waves, and the like. In many cases, the open voltage (V OC ) of the energy conversion element  6  having these as energy sources is equal to or lower than 1 V. Thus, in a case in which the output of the energy conversion element  6  is used for driving the sensor element  4 , power transformation (boosting) is necessary. 
     In connection between the energy conversion element  6  outputting electric power and the power conversion element  7  receiving the electric power, it is preferable that electric power should be efficiently transferred from the energy conversion element  6  to the power conversion element  7 . Thus, it is preferable that the efficiency of transfer from the energy conversion element  6  to the power conversion element  7  should satisfy a desired value. An example of the desired value is a maximum value of the efficiency of transfer from the energy conversion element  6  to the power conversion element  7 . In order to obtain a maximum transfer efficiency, a connection configuration satisfying a so-called impedance matching condition is employed. The impedance matching condition, as a first condition, matches the input resistance of the power conversion element  7  to the output resistance (R ET ) of the energy conversion element  6 . In addition, as a second condition, the power conversion element  7  is controlled such that a voltage (V IN ) input to the power conversion element  7  is a half (V OC /2) of the open voltage (V OC ). 
     In addition, the second condition is not limited to the condition of being a half (V OC /2) of the open voltage (V OC ). As a condition in which the transfer efficiency of electric power becomes a maximum, various configurations may be employed in accordance with the type and the like of the energy conversion element  6 . The second condition may be appropriately set in accordance with the type and the like of the energy conversion element  6 . 
     In other words, there is a precondition of the power conversion element  7  being able to operate at the voltage (V OC /2). In more detail, there is a precondition of the power conversion element  7 , which has received the voltage (V OC /2), being able to output a voltage that can drive the sensor element  4 . However, the energy conversion element  6  that is a power generation element for IoT has a low open voltage (V OC ). In that case, in order to obtain a voltage (V OC /2) at which the sensor element  4  can operate from the energy conversion element  6 , the open voltage (V OC ) of the energy conversion element  6  needs to be high. 
     The magnitude of the open voltage (V OC ) corresponds to the magnitude of environmental energy. In other words, in order to obtain a high open voltage (V OC ), high environmental energy is necessary. In other words, the sensor device  2  cannot be operated without high environmental energy. 
     Thus, in Modified Example 8, a power supply element that enlarges the range of environmental energy that can drive a load element is provided. In other words, a circuit used for operating the sensor device  2  even in low environmental energy is provided. In other words, Modified Example 8 provides a power supply element that is a circuit system minimizing the open voltage (V OC ) of the energy conversion element  6  that can drive the sensor element  4 . In other words, Modified Example 8 provides a power supply element that is a circuit system minimizing input energy (environment energy) 0.5  for the energy conversion element  6  that can drive the sensor element  4 . 
     As illustrated in  FIG.  16   , a power supply element  1 H of Modified Example 8 includes the power generation circuit  8 , the power conversion element  7 H, and a control unit  12 H (control unit). 
     The power conversion element  7 H includes the adjustment circuit  9 D and a power conversion circuit  11 H. The power conversion circuit  11 H accepts a voltage (V IN ) from the adjustment circuit  9 D. The power conversion circuit  11 H outputs a voltage (V OUT ) acquired by boosting the voltage (V IN ). The power conversion circuit  11 H includes an oscillation circuit  16 H and a boosting circuit  17 H. 
     The oscillation circuit  16 H generates a clock signal (CLK) for the boosting circuit  17 H. The oscillation circuit  16 H supplies the clock signal (CLK) to the boosting circuit  17 H. The oscillation circuit  16 H generates a clock signal (CLK) on the basis of a control signal (V_ CONTROL ) supplied from the control unit  12 H. For example, the frequency of the clock signal (CLK) is in proportion to the magnitude of the voltage of the control signal (V_ CONTROL ). In a case in which the voltage of the control signal (V_ CONTROL ) is high, the frequency of the clock signal (CLK) is high. In a case in which the voltage of the control signal (V_ CONTROL ) is low, the frequency of the clock signal (CLK) is low. 
     The boosting circuit  17 H accepts a voltage (V IN ) and a clock signal (CLK). Then, the boosting circuit  17 H boosts the voltage (V IN ) in accordance with the clock signal (CLK). 
     The control unit  12 H outputs a control signal (V_ CONTROL ) used for the oscillation circuit  16 H. The control unit  12 H generates a control signal (V_ CONTROL ) on the basis of a voltage (V S ), a voltage (V IN ), a voltage (V OUT ), a target voltage (V IN_TARGET ), and a target voltage (V OUT_TARGET ). Thus, the control unit  12 H is connected to the output  8   a  of the power generation circuit  8 , thereby receiving the voltage (V S ). In addition, the control unit  12 H is connected to the output  9   c  of the adjustment circuit  9 D, thereby receiving the voltage (V IN ). Furthermore, the control unit  12 H is connected to the output  11   c  of the power conversion circuit  11 H, thereby receiving the voltage (V OUT ). The control unit  12 H receives the target voltage (V IN_TARGET ) and the target voltage (V OUT_TARGET ) from an input means not illustrated in the drawing. In addition, the control unit  12 H may generate a target voltage (V IN_TARGET ) on the basis of the voltage (V S ), the voltage (V IN ), and the voltage (V OUT ). 
     Hereinafter, the operation of the control unit  12 H will be described in detail. In the description,  FIGS.  17 ,  18 , and  19    will be referred to as is appropriate. 
     Graphs G 17   a , G 17   b , and G 17   c  of Part (a) of  FIG.  17   , Part (b) of  FIG.  17   , and Part (c) of  FIG.  17    represent relations between electric power and voltages received by the power conversion element  7 H. In each of the drawings, the horizontal axis represents a voltage (V) received by the power conversion element  7 H. The vertical axis represents electric power (P) received by the power conversion element  7 H. In addition, operating points Q 17   a , Q 17   b , and Q 17   c  of the power supply element  1 H are illustrated. 
     Part (a) of  FIG.  18   , Part (b) of  FIG.  18   , and Part (c) of  FIG.  18    represent relations between currents and voltages received by the power conversion element  7 H. In each of the diagrams, the horizontal axis represents the voltage (V) received by the power conversion element  7 H. The horizontal axis represents the current (I) received by the power conversion element  7 H. In addition, the voltage (V) received by the power conversion element  7 H may be regarded as a voltage (V) that is output by the energy conversion element  6 . Similarly, the current (I) received by the power conversion element  7 H may be regarded as a current (V) that is output by the energy conversion element  6 . Graphs G 18   a  and G 18   c  represent relations between output voltages and output currents of the energy conversion element  6 . 
     Graphs G 18   b , G 18   d , and G 18   e  represent relations between input voltages and input currents of the power conversion element  7 H. In a case in which electric power is supplied from the energy conversion element  6  to the power conversion element  7 H, the output current of the energy conversion element  6  and the input current of the power conversion element  7 H coincide with each other. Thus, for example, a point at which the graph G 18   a  and the graph G 18   b  intersect with each other represents an operating point of the power supply element  1 H. In other words, Q 18   a , Q 18   b , and Q 18   c  represent operating points of the power supply element  1 H 
     The connection between the energy conversion element  6  and the power conversion element  7 H satisfies the impedance matching condition as described above. Describing the impedance matching condition again, the first condition is matching the input resistance of the power conversion element  7 H to the output resistance (R ET ) of the energy conversion element  6 , and the second condition is performing control of the power conversion element  7 H such that the input voltage (V IN ) of the power conversion circuit  11 H is a voltage (V OC /2). 
     Now, a lowest output voltage that is required by the sensor element  4  for the power conversion element  7 H is assumed to be set. As a result, an input voltage for allowing the power conversion element  7 H to acquire this output voltage is determined. Then, an output voltage (V OC_MIN ) (a first voltage) of the energy conversion element  6  that is necessary for the power conversion element  7 H to obtain an input voltage is determined. 
     Part (a) of  FIG.  17    and Part (a) of  FIG.  18    will be referred to. A state in which sufficient environmental energy is supplied to the energy conversion element  6  will be assumed. At this time, the energy conversion element  6  outputs a voltage (V OC1 ). In order to satisfy the impedance matching condition, the operating voltage of the power conversion element  7 H needs to be a voltage (V OC1 /2) (a second voltage). This state is represented by the operating point Q 17   a  of Part (a) of  FIG.  17    and the operating point Q 18   a  represented by Part (a) of  FIG.  18   . When environmental energy that is sufficient for the energy conversion element  6  is provided, the voltage (V OC1 /2) is higher than a voltage (V OC_MIN ) to be output by the energy conversion element  6 . Thus, the voltage (V OC1 /2) can satisfy the voltage (V OC_MIN ) that is to be output by the energy conversion element  6 . 
     The voltage (V OC ) output by the energy conversion element  6  increases or decreases in accordance with the magnitude of environmental energy received by the energy conversion element  6 . For example, a case in which the environmental energy received by the energy conversion element  6  decreases will be assumed. In a case in which the environmental energy received by the energy conversion element  6  decreases, as illustrated in Part (b) of  FIG.  17    and Part (b) of  FIG.  18   , output characteristics (a graph Gl 7   b  and a graph G 18   c ) of the energy conversion element  6  change. 
     When the environmental energy received by the energy conversion element  6  decreases, the energy conversion element  6  outputs a voltage (V OC2 ). The voltage (V OC2 ) is lower than the voltage (V OC1 ). In this case, the power conversion element  7 H is operated such that the impedance matching condition is satisfied (see Part(b) of  FIG.  17    and Part (b) of  FIG.  18   ). In other words, as represented by the operating point Q 17   b  of Part (b) of  FIG.  17    and the operating point Q 18   b  of Part (b) of  FIG.  18   , the operating voltage of the power conversion element  7 H is set to the voltage (V OC2 /2). Then, a case in which the voltage (V OC2 /2) does not satisfy the voltage (V OC_MIN ) to be output by the energy conversion element  6  occurs. 
     In other words, the power conversion element  7 H is assumed to be driven such that it constantly satisfies the impedance matching condition without being in correspondence with an increase/decrease of the environmental energy received by the energy conversion element  6 . In that case, a case in which the voltage (V OC_MIN ) required for driving the sensor element  4  is not satisfied may occur. 
     Thus, the power supply element  1 H changes a drive condition of the power conversion element  7 H in accordance with an increase/decrease in the environmental energy received by the energy conversion element  6 . More specifically, in a case in which the environmental energy decreases, the operating voltage of the power conversion element  7 H is raised to be higher than the voltage (V OC /2). This state is represented by the operating point Q 17   c  of Part (c) of  FIG.  17    and the operating point Q 18   c  of Part (c) of  FIG.  18   . In other words, as illustrated in Part (c) of  FIG.  17    and Part (c) of  FIG.  18   , in a case in which the environmental energy decreases, a condition of satisfying a required voltage (V OC_MIN ) is prioritized over a condition of maximizing an efficiency of transferring electric power from the energy conversion element  6  to the power conversion element  7 H. As a result, as illustrated in the graph G 19   a  of  FIG.  19   , the range of environmental energy that can operate a circuit can be enlarged from a range B 1  to a range B 2 . In other words, a lowest value of the environmental energy that can operate the circuit can be set to a smaller value. 
     Hereinafter, first control and second control that are performed by the control unit  12 H will be described. 
     &lt;First Control of Control Unit&gt; 
     The first control is control in which the condition of satisfying impedance matching is prioritized. A state in which the first control is selected will be referred to as a standard state. The control unit  12 H assumes that an operation of satisfying the impedance matching condition (V OC /2=V IN ) is performed and compares the voltage (V OC /2) with the required voltage (V OC_MIN ). For example, the control unit  12 H may select the first control in a case in which the voltage (V OC /2) is equal to the required voltage (V OC_MIN ) (V OC /2=V OC_MIN ). In addition, for example, the control unit  12 H may select the first control in a case in which the voltage (V OC /2) is equal to or higher than the required voltage (V OC_MIN ) (V OC /2≥V OC_MIN ). Furthermore, for example, the control unit  12 H may select the first control in a case in which the voltage (V OC /2) is larger than a value acquired by multiplying the required voltage (V OC_MIN ) by a predetermined coefficient (a) (V OC /2&gt;a×V OC_MIN ). The predetermined coefficient (a), for example, may be 0.8. When the first control is determined to be performed, the control unit  12 H sets the target voltage (V IN_TARGET ) of the power conversion element  7 H to the voltage (V OC /2). Such a selection operation may be repeatedly performed at arbitrary timings during the operation of the power supply element  1 H. 
     When the first control is selected by the control unit  12 H, the oscillation circuit  16 H supplies a clock signal (CLK) illustrated in Part (a) of  FIG.  20    to the boosting circuit  17 H. The graphs G 21   b  and G 21   c  of Part (a) of  FIG.  20    illustrate an input voltage (V IN ) and an output voltage (V OUT ) of the power conversion circuit  11 H corresponding to the clock signal (CLK) represented in the graph G 21   a.    
     In addition, in the present disclosure, the input voltage (V IN ) for the power conversion circuit  11 H at the time of the standard state is set under a condition that the efficiency of transfer of electric power from the energy conversion element  6  to the power conversion circuit  11 H is a maximum. A setting of the input voltage (V IN ) for the power conversion circuit  11 H at the time of this standard state may be regulated from a viewpoint different from that of the condition described above. For example, the input voltage (V IN ) may be associated with an operating point at which electric power higher than the electric power that can be acquired at a minimum operating point is acquired when relatively compared with the electric power that can be acquired at the minimum operating point. This minimum operating point represents an operating point that is set near the open voltage (V OC ) of the energy conversion element  6 . 
     &lt;Second Control of Control Unit&gt; 
     The second control is a control in which a condition of satisfying a required voltage (V OC_MIN ) is prioritized. A state in which the second control is selected will be referred to as a minimum energy state. Similar to the first control, the control unit  12 H assumes that an operation satisfying the impedance matching condition (V OC /2=V IN ) is performed and compares the voltage (V OC /2) with the required voltage (V OC_MIN ). For example, the control unit  12 H may select the second control in a case in which the voltage (V OC /2) is lower than the required voltage (V OC_MIN ) (V OC /2&lt;V OC_MIN ). In addition, for example, the control unit  12 H may select the second control in a case in which the voltage (V OC /2) is smaller than a value acquired by multiplying the required voltage (V OC_MIN ) by a predetermined coefficient (a) (V OC /2&lt;a×V OC_MIN ). The predetermined coefficient (a), for example, may be 0.8. When the second control is determined to be performed, the control unit  12 H sets the target voltage (V IN_TARGET ) of the power conversion element  7 H to a value larger than the voltage (V OC /2). In addition, when the second control is determined to be performed, the control unit  12 H may set the target voltage (V IN_TARGET ) of the power conversion element  7 H to a value acquired by multiplying the voltage (V OC_MIN ) by a predetermined coefficient (b) (V IN_TARGET =b×V OC_MIN ). 
     In conclusion, when the second control is determined to be performed, the control unit  12 H may set the target voltage (V IN_TARGET ) to a value that is higher than the voltage (V OC /2) and is lower than the voltage (b×V OC_MIN ). Such a selection operation may be repeatedly performed at arbitrary timings during the operation of the power supply element  1 H. 
     When the second control is selected by the control unit  12 H, the oscillation circuit  16 H supplies a clock signal (CLK) illustrated in Part (b) of  FIG.  20    to the boosting circuit  17 H. The graphs G 2   le  and G 21   f  of Part (b) of  FIG.  20    illustrate an input voltage (V IN ) and an output voltage (V OUT ) of the power conversion circuit  11 H corresponding to the clock signal (CLK) represented in the graph G 21   d.    
     Here, the clock signal (CLK) (the graph G 21   d ) at the time of the second control has a frequency lower than the clock signal (CLK) (the graph G 21   a ) at the time of the first control. In a case in which the frequency of the clock signal (CLK) is high, the number of times of discharging of the capacitor C is large. Thus, an average input current and an average input power for the power conversion element  7 H become high. The average input voltage for the power conversion element  7 H becomes low. On the other hand, in a case in which the frequency of the clock signal (CLK) is low, the number of times of discharging of the capacitor C is small. Thus, an average input current and an average input power for the power conversion element  7 H become low. The average input voltage for the power conversion element  7 H becomes high. In other words, by relatively lowing the clock signal (CLK), the voltage (V IN ) input to the power conversion element  7 H can be raised. 
     Switching between the first control and the second control is performed in accordance with the frequency of the clock signal (CLK) of the oscillation circuit  16 H. Thus, in a case in which the control unit  12 H selects the first control, the control unit  12 H outputs a control signal (V_ CONTROL ) enabling the generation of the clock signal (CLK) represented in Part (a) of  FIG.  20   . Similarly, in a case in which the control unit  12 H selects the second control, the control unit  12 H outputs a control signal (V_ CONTROL ) enabling the generation of the clock signal (CLK) represented in Part (b) of  FIG.  20   . The frequency of the clock signal (CLK) corresponds to the magnitude of the control signal (V_ CONTROL ). For example, the voltage of the control signal (V_ CONTROL ) at the time of the second control is lower than the voltage of the control signal (V_ CONTROL ) at the time of the first control. 
     In other words, the frequency of the clock signal (CLK) may be regulated in accordance with a ratio between a discharging period (Ta) and a discharging period (Tb, Tc). In other words, a period (High) in which the voltage of the clock signal (CLK) is high is a discharging period, and a period (Low) in which the voltage of the clock signal (CLK) is low is a charging period. For example, a ratio of the discharging period (Ta) to the charging period (Tc) at the time of the second control is lower than a ratio of the discharging period (Ta) to the charging period (Tb) at the time of the first control. 
     In addition, the relation of the clock signal (CLK) may be regulated in accordance with a so-called duty ratio. For example, a duty ratio at the time of the first control is Ta/(Ta+Tb). A duty ratio at the time of the second control is Ta/(Ta+Tc). Thus, the duty ratio at the time of the second control is lower than the duty ratio at the time of the first control. 
     In other words, the power supply element  1 H of Modified Example 8 is a power supply element that is connected to a load element. 
     The power supply element includes: the impedance adjustment circuit including the power generation circuit that converts external energy into electric power and outputs the electric power, the power conversion circuit that converts the electric power generated by the power generation circuit into a desired form, the first circuit unit that includes an input port connected to the power generation circuit and an output port connected to the power conversion circuit, and the second circuit unit that includes a connection point connected to the first circuit unit, a grounding point connected to the grounding electric potential, and the capacitor connected between the connection point and the grounding point; and the control unit controlling the power conversion circuit. The control unit performs control of the operation of the power conversion circuit such that, when a second voltage is lower than a first voltage, the input voltage for the power conversion circuit is higher than the second voltage. The first voltage is an input voltage for the power conversion circuit for which the power conversion circuit can output a required voltage from the power conversion circuit for driving a load element. The second voltage is an input voltage for the power conversion circuit for which the efficiency of transfer of electric power from the power generation circuit to the power conversion circuit is a desired value. 
     The control unit may perform control of the operation of the power conversion circuit such that, when a first voltage is equal to or higher than a second voltage, the input voltage for the power conversion circuit is the second voltage. 
     The control unit may set a ratio of the discharging period of the capacitor to the charging period of the capacitor when the second voltage is lower than the first voltage to be lower than a ratio of the discharging period of the capacitor to the charging period of the capacitor when the first voltage is equal to or higher than the second voltage. 
     In a case in which necessary conditions represented in Equations (6) and (7) and  FIG.  21    are satisfied, the power supply element  1 H performing such an operation can raise the circuit operating point of the power conversion element  7 H to be higher than the voltage (V OC /2) and near the open voltage (V OC ). As a result, even in a state in which environmental energy is low, an IoT terminal can be operated. In other words, the power supply element  1 H is a circuit system that minimizes the open voltage (V OC ) of the energy conversion element  6  that can drive the sensor element  4 . In addition, the power supply element  1 H is a circuit system that minimizes input energy (environmental energy) 0.5  for the energy conversion element  6  that can drive the sensor element  4 . In addition, in the following equation, P out   ACT  is electric power that is input to the power conversion element  7 H. P out  is average power of P out   ACT . T ACT  is a period in which electric power (P out   ACT ) is transmitted. T CYC  is a period in which electric power (P out   ACT ) is transmitted. 
     
       
         
           
             
               
                 
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     OPERATION EXAMPLE 1 
     Case in which Environmental Energy Received by Energy Conversion Element  6  is Received 
     For example, the power supply element  1 H is assumed to be operating in the standard state. In this case, the control unit  12 H selects the first control. The environmental energy supplied to the energy conversion element  6  is assumed to decrease. As such a situation, there is a case in which a temperature difference decreases when the energy conversion element  6  is an element performing thermoelectric power generation as an example. In this case, the output voltage (V OC ) of the energy conversion element  6  decreases. Then, as a result of performing an operation of selecting control using the control unit  12 H, a condition for selecting the first control is not satisfied, and a condition for selecting the second control is satisfied. In other words, the standard state is switched to the lowest energy state. Thus, the control unit  12 H performs switching from the first control to the second control. According to such an operation, even in a case in which the environmental energy decreases, electric power can be continuously supplied to the sensor device  2 . 
     OPERATION EXAMPLE 2 
     Case in which Environmental Energy Received by Energy Conversion Element  6  Increase 
     For example, it is assumed that the power supply element  1 H is operating in the lowest energy state. In this case, the control unit  12 H selects the second control. Then, environmental energy supplied to the energy conversion element  6  is assumed to increase. In this case, the output voltage (V OC ) of the energy conversion element  6  increases as well. Then, as a result of performing a control selecting operation using the control unit  12 H, a condition for selecting the first control is satisfied. In other words, the lowest energy state is switched to the standard state. Thus, the control unit  12 H performs switching from the second control to the first control. According to such an operation, the efficiency of transfer of electric power from the energy conversion element  6  to the power conversion element  7 H can be maximized. 
     OPERATION EXAMPLE 3 
     The control unit  12 H may additionally control output of the power conversion element  7 H. For example, a case in which the operation of the power supply element  1 H is started will be considered. First, the control unit  12 H sets the target voltage (V IN_TARGET ) of the power conversion element  7 H. In a case in which the operation is started, the lowest energy state described above is assumed. Thus, the control unit  12 H sets a value higher than the voltage (V OC /2) and lower than the voltage (V OC_MIN ) as the target voltage (V IN_TARGET ). For example, the control unit  12 H may set a voltage (V OC_MIN)× 0.8 as the target voltage (V IN_TARGET ). When an operation is started, the output voltage (V OUT ) of the power conversion element  7 H is lower than the target voltage (V OUT_TARGET ) of the output. In this case, the frequency of the clock signal (CLK) supplied to the power conversion element  7 H is raised. Thus, the control unit  12 H raises the control signal (V_ CONTROL ). In accordance with this operation, the boosting circuit  17 H boosts an input voltage (V IN ) and outputs an output voltage (V OUT ). When the output voltage (V OUT ) rises, and the output voltage (V OUT ) becomes equal to the target voltage (V OUT_TARGET ) or higher than the target voltage (V OUT_TARGET ), the control unit  12 H stops the operation of the power conversion element  7 H. The power conversion element  7 H operates in accordance with reception of the clock signal (CLK). 
     Thus, the control unit  12 H outputs the control signal (V_ CONTROL ) that is 0 V. In addition, when the sensor element  4  connected to the power conversion element  7 H operates, and the output voltage (V OUT ) is lowered, the control unit  12 H starts the boosting operation again. In this way, the output voltage is controlled to be near the target voltage (V OUT_TARGET ). 
     In addition, the first control and the second control described above may be applied to the power supply devices of the embodiment and Modified Examples 1 to 7. In a case in which the first control and the second control are applied, in each of the power supply devices, a signal line connecting the output of the energy conversion circuit and the control unit is provided. In other words, by adding a configuration in which a voltage (V S ) is input to the control unit, operations to which the first control and the second control are applied can be performed. 
     REFERENCE SIGNS LIST 
       1 ,  1 A,  1 B,  1 C,  1 D,  1 E, IF,  1 G Power supply element 
       2  Sensor device 
       3  Antenna 
       4  Sensor element 
       6  Energy conversion element 
       7 ,  7 B Power conversion element 
       8 ,  8 A Power generation circuit 
       8 E,  8 Ea,  8 Eb Power generation unit 
       8 R,  8 Ra,  8 R Output resistance 
       9 ,  9 B,  9 D Adjustment circuit 
       11 ,  11 D,  11 E,  11 F,  11 G Power conversion circuit 
       12 ,  12 A,  12 B,  12 D Control unit 
       13  First circuit unit 
       14  Second circuit unit 
       14 R Output resistance 
       16  Frequency modulating unit 
       17  Transformation unit 
       18  Additional power conversion circuit 
       20  Detection unit 
       21  Clock generating unit 
       22  Coil 
       23  Transistor 
       24  Diode 
       25  Detection unit 
     C, C 1 , C 2 , C 3 , C 4  Capacitor 
     GND Grounding electric potential 
     S 1  Switch (first switch) 
     S 2  Switch (second switch) 
     S 3  to S 6  Switch 
     P 1  Connection point 
     P 2  Grounding point 
     φ 1 , φ 2  Control signal