Patent Publication Number: US-2023147880-A1

Title: Wireless power transmission apparatus and wireless power supply system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of priority to International Patent Application No. PCT/JP2021/006439, filed Feb. 19, 2021, and to Japanese Patent Application No. 2020-116426, filed Jul. 6, 2020, the entire contents of each are incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a wireless power transmission apparatus that wirelessly transmits electric power and to a wireless power supply system that includes the wireless power transmission apparatus and a wireless power reception apparatus that wirelessly receives electric power. 
     Background Art 
     In Japanese Unexamined Patent Application Publication No. 2013-215065, a power transfer system that includes a power transmission apparatus including an AC converter that performs AC conversion of supplied AC power or DC power, a power-transmission-side resonance coil that wirelessly transmits AC power, and a power-transmission-side control device; and a power reception apparatus including a power-reception-side resonance coil, a rectifier, a DC converter, and a power-reception-side control device, is described. The power transfer system is described in which, after the power-reception-side control device is activated by receiving a control power supply voltage, the power-reception-side control device measures an output voltage of the rectifier and transmits the measured output voltage to the power-transmission-side control device, and the power-transmission-side control device controls, based on a measurement result of the output voltage of the rectifier, the AC converter in such a manner that the output voltage of the rectifier has an appropriate value as an input voltage of the DC converter. 
     In general, in a wireless power supply system, in order to adjust received power with respect to changes in a transfer distance and a power consumption at a load as a destination to which received power is supplied, control of electric power to be transmitted is required. In particular, in a system or an application handling high power, power management with a high-precision control function for power to be transmitted is required, in terms of temperature management with regard to heat generation caused by power loss and security in a circuit device. 
     Meanwhile, in the power reception apparatus, received power varies according to changes in the arrangement of the power transmission apparatus and the power reception apparatus and a transfer distance, and the power consumption at the load also varies. In the power transmission apparatus, to finely adjust electric power to be supplied to the power reception apparatus, being able to continuously adjust the strength of a high-frequency alternating magnetic field generated by a power transmission coil is required. However, continuously adjusting a high-frequency alternating magnetic field with excellent power efficiency is technically difficult. 
     In the power transmission apparatus, it is not easy to understand received power on the basis of voltage, current, and electric power in a circuit operation, and the received power and the strength of the high-frequency alternating magnetic field are not always correlated with each other. Thus, even if the strength of the high-frequency alternating magnetic field is continuously adjusted so that received power can be adjusted, voltage, current, or electric power of a circuit (power transmission circuit) inside the power transmission apparatus may become too high. 
     For example, even with the same strength of an alternating magnetic field generated by the power transmission coil, electromagnetic field energy generated by the power transmission coil is not always supplied efficiently. It is not easy to distinguish, with a simple configuration, between the case where the electromagnetic field energy is supplied efficiently and the case where the electromagnetic field energy is not supplied efficiently. There is no problem in the case where electric power of electromagnetic field energy generated by the power transmission coil is consumed in the power reception apparatus. However, if current flows only to a circuit in the power transmission apparatus without the electromagnetic field energy being supplied to the power reception apparatus, large power loss occurs in the power transmission circuit. As a result, voltage, current, and electric power in the power transmission circuit become too high, and this causes a problem of electrical stress and heat generation in the circuit. 
     SUMMARY 
     If only the strength of a high-frequency alternating magnetic field in the power transmission circuit is adjusted without detecting the efficiency with which electromagnetic field energy generated by the power transmission coil is supplied to the power reception apparatus, the voltage, current, electric power, and the like of the power transmission circuit become too high. Thus, the circuit may be destroyed or the reliability of circuit components may be significantly degraded. In contrast, if the voltage, current, and electric power of the power transmission circuit are controlled so that the circuit cannot be destroyed, the strength of the alternating magnetic field cannot be adjusted efficiently, and an excellent power supply efficiency cannot be achieved. 
     Thus, the present disclosure provides a wireless power transmission apparatus and a wireless power supply system with high security that are capable of continuously adjusting an alternating magnetic field with a simple circuit configuration and not causing excessive electrical stress or heat generation in a power transmission circuit without depending on changes in arrangement of and distance between the power transmission apparatus and a power reception apparatus. 
     A wireless power transmission apparatus according to an example of the present disclosure includes a power transmission resonance mechanism that includes a power transmission coil and a power transmission resonance capacitor; a power transmission circuit that performs switching in such a manner that a DC voltage or a DC current is intermittently provided to the power transmission resonance mechanism at a predetermined switching frequency; a voltage conversion circuit that performs voltage conversion on an input power supply; and an intermediate capacitor that is provided between the voltage conversion circuit and the power transmission circuit and shared between the voltage conversion circuit and the power transmission circuit. The wireless power transmission apparatus further includes an intermediate input current detection circuit that detects an intermediate input current input to the power transmission circuit from the voltage conversion circuit; and an electric power management circuit that, by setting an upper limit value for the intermediate input current and controlling an intermediate voltage, which is a voltage of the intermediate capacitor serving as an output voltage of the voltage conversion circuit, adjusts an amplitude of the DC voltage or the DC current intermittently provided to the power transmission resonance mechanism. The electric power management circuit controls strength of an alternating magnetic field at the switching frequency generated by the power transmission coil. 
     A wireless power supply system according to an example of the present disclosure includes a wireless power reception apparatus and a wireless power transmission apparatus. The wireless power reception apparatus includes a power reception coil. 
     The wireless power transmission apparatus includes a power transmission resonance mechanism that includes a power transmission coil magnetically coupled to the power reception coil, and a power transmission resonance capacitor, a power transmission circuit that performs switching in such a manner that a DC voltage or a DC current is intermittently provided to the power transmission resonance mechanism at a predetermined switching frequency, a voltage conversion circuit that performs voltage conversion on an input power supply, and an intermediate capacitor that is provided between the voltage conversion circuit and the power transmission circuit and shared between the voltage conversion circuit and the power transmission circuit. The wireless power transmission apparatus further includes an intermediate input current detection circuit that detects an intermediate input current input to the power transmission circuit from the voltage conversion circuit, and an electric power management circuit that, by setting an upper limit value for the intermediate input current and controlling an intermediate voltage, which is a voltage of the intermediate capacitor serving as an output voltage of the voltage conversion circuit, adjusts an amplitude of the DC voltage or the DC current intermittently provided to the power transmission resonance mechanism. The electric power management circuit controls strength of an alternating magnetic field at the switching frequency generated by the power transmission coil. 
     According to the present disclosure, a wireless power transmission apparatus and a wireless power supply system with high security that are capable of continuously adjusting an alternating magnetic field with a simple circuit configuration and not causing excessive electrical stress or heat generation in a wireless power transmission circuit without depending on changes in arrangement of and distance between the wireless power transmission apparatus and a wireless power reception apparatus, can be obtained. 
    
    
     
       BRIEF DESCRIPTON OF THE DRAWINGS 
         FIG.  1    is a block diagram illustrating a configuration of a wireless power supply system according to a first embodiment; 
         FIG.  2    is a circuit diagram of a wireless power supply system with a configuration different from that of the wireless power supply system illustrated in  FIG.  1   ; 
         FIG.  3    is a circuit diagram illustrating a configuration of a wireless power supply system according to a second embodiment; 
         FIG.  4    is a circuit diagram illustrating a configuration of a wireless power supply system according to a third embodiment; 
         FIG.  5    is a circuit diagram illustrating a configuration of a wireless power supply system according to a fourth embodiment; 
         FIG.  6    is a circuit diagram illustrating a configuration of a wireless power supply system according to a fifth embodiment; 
         FIG.  7    is a circuit diagram illustrating a configuration of a wireless power supply system according to a sixth embodiment; 
         FIG.  8    is a circuit diagram of a voltage conversion circuit provided in a wireless power transmission apparatus according to the seventh embodiment; 
         FIG.  9    is another circuit diagram of the voltage conversion circuit provided in the wireless power transmission apparatus according to the seventh embodiment; 
         FIG.  10    is a circuit diagram of a voltage conversion circuit provided in a wireless power transmission apparatus according to an eighth embodiment; 
         FIG.  11    is another circuit diagram of the voltage conversion circuit provided in the wireless power transmission apparatus according to the eighth embodiment; 
         FIG.  12    is a circuit diagram of an input power supply according to a ninth embodiment; and 
         FIG.  13    is a circuit diagram of an input power supply according to a tenth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       FIG.  1    is a block diagram illustrating a configuration of a wireless power supply system  301 A according to a first embodiment of the present disclosure. The wireless power supply system  301 A includes a wireless power transmission apparatus  101 , and a wireless power reception apparatus  201  including a wireless power reception coil Ls that is wirelessly and magnetically coupled to a wireless power transmission coil Lp provided in the wireless power transmission apparatus  101 . 
     The wireless power transmission apparatus  101  includes a power transmission resonance mechanism PR, a power transmission circuit  26 , a voltage conversion circuit  12 , an intermediate capacitor Ci, an intermediate input current detection circuit, and an electric power management circuit  39 . 
     The wireless power transmission coil Lp and a power transmission resonance capacitor Cr configure a resonance circuit. The power transmission resonance mechanism PR includes the wireless power transmission coil Lp and the power transmission resonance capacitor Cr. 
     The power transmission circuit  26  includes a switching circuit. The power transmission circuit  26  performs switching in such a manner that a DC voltage or a DC current is intermittently provided to the power transmission resonance mechanism PR at a predetermined switching frequency. 
     An input power supply Vi is connected to an input part of the voltage conversion circuit  12 . The voltage conversion circuit  12  converts the voltage of the input power supply Vi into a predetermined voltage and inputs the converted voltage to the power transmission circuit  26 . 
     The intermediate capacitor Ci is provided between the voltage conversion circuit  12  and the power transmission circuit  26  and is shared between the voltage conversion circuit  12  and the power transmission circuit  26 . Thus, the number of components can be reduced. Furthermore, current flowing into the intermediate capacitor Ci and current flowing out of the intermediate capacitor Ci are canceled out, and noise can thus be reduced. 
     The electric power management circuit  39  includes the above-mentioned intermediate input current detection circuit. The intermediate input current detection circuit detects an intermediate input current input to the power transmission circuit  26  from the voltage conversion circuit  12 . 
     By setting an upper limit value for the intermediate input current and controlling the output voltage of the voltage conversion circuit  12 , the electric power management circuit  39  adjusts the amplitude of a DC voltage or current intermittently provided to the power transmission resonance mechanism PR. The electric power management circuit  39  controls the voltage conversion circuit  12  in such a manner that, when the intermediate input current reaches the upper limit, the intermediate voltage serving as the output voltage of the voltage conversion circuit  12  decreases. 
     The wireless power reception apparatus  201  includes a power reception resonance mechanism SR, a rectifying and smoothing circuit  52 , a voltage stabilizing circuit  53 , and an electric power management circuit  59 . 
     A load Ro is connected to an output part of the voltage stabilizing circuit  53 . 
     The wireless power reception coil Ls and a power reception resonance capacitor Crs configure a resonance circuit. The power reception resonance mechanism SR includes the wireless power reception coil Ls and the power reception resonance capacitor Crs. 
     The rectifying and smoothing circuit  52  rectifies and smooths the output voltage of the power reception resonance mechanism SR. The voltage stabilizing circuit  53  stabilizes the output voltage of the rectifying and smoothing circuit  52  and outputs the stabilized output voltage to the load Ro. 
     The electric power management circuit  59  controls the rectifying and smoothing circuit  52  and includes received power request means for transmitting a received power request signal to the wireless power transmission apparatus  101  under resonance modulation control. 
     The electric power management circuit  39  on the wireless power transmission apparatus  101  side includes received power request signal receiving means for receiving the above-mentioned received power request signal under demodulation control for detecting the received power request signal. The electric power management circuit  39  controls the voltage conversion circuit  12  in accordance with the received power request signal. 
     A feedback system includes transmission of a received power request signal to the wireless power transmission apparatus  101  and adjustment of power to be transmitted according to the received power request signal. Thus, the wireless power reception apparatus  201  receives necessary electric power and supplies the received power to the load Ro. 
     Operation of the above-described wireless power supply system  301 A will be described below. When the strength of an alternating magnetic field generated by the wireless power transmission coil Lp increases, an intermediate input current flowing from the intermediate capacitor Ci to a subsequent stage increases. The electric power management circuit  39  detects the increase of the current, and reduces the output voltage of the voltage conversion circuit  12 . As a result, an intermediate voltage, which is the voltage of the intermediate capacitor Ci serving as the output voltage of the voltage conversion circuit  12 , decreases. 
     When the intermediate voltage decreases, a DC voltage or a DC current intermittently provided to the power transmission resonance mechanism PR, which includes the wireless power transmission coil Lp and the power transmission resonance capacitor Cr, decreases, and the amplitude voltage of a square wave decreases. Thus, a resonance current flowing to the wireless power transmission coil Lp decreases, and the strength of an alternating magnetic field decreases. The strength of the alternating magnetic field is feedback-controlled, as described above. 
     As described above, the intermediate input current detection circuit inside the electric power management circuit  39  detects the intermediate input current input to the power transmission circuit  26  from the voltage conversion circuit  12 , and the electric power management circuit  39  sets an upper limit value for the intermediate input current, so that the output voltage of the voltage conversion circuit  12  (voltage of the intermediate capacitor Ci) is controlled in such a manner that the intermediate input current does not exceed the upper limit value. By controlling the output voltage of the voltage conversion circuit  12 , the amplitude of the DC voltage or current intermittently provided to the power transmission resonance mechanism PR is adjusted. 
     In the case where the voltage of the intermediate capacitor Ci (intermediate input voltage) is constant, the intermediate input current is proportional to electric power handled by the power transmission circuit  26  and is substantially proportional to electric power supplied to the wireless power reception apparatus  201  from the wireless power transmission coil Lp. Thus, even if electromagnetic field energy generated by the wireless power transmission coil Lp is not always efficiently supplied to the wireless power reception apparatus  201 , electric power to be received by the wireless power reception apparatus  201  can be controlled to be equal to an electric power required by the wireless power reception apparatus  201 . 
     A state in which current flows only to a circuit inside the wireless power transmission apparatus  101  without electromagnetic field energy generated by the wireless power transmission coil Lp being supplied to the wireless power reception apparatus  201  represents a state in which reactive power (imaginary power when electric power is expressed by a complex number) is large. Meanwhile, active power (real power when electric power is expressed by a complex number), that is, power consumption, is determined by the product of “intermediate input voltage” and “intermediate input current”. In the case where electromagnetic field energy generated by the wireless power transmission coil Lp is supplied to the wireless power reception apparatus  201 , that is, only in the case where electric power is consumed at the wireless power reception apparatus  201 , the “intermediate input current” increases. 
      Next, another wireless power supply system according to the first embodiment will be described as an example. 
       FIG.  2    is a circuit diagram of a wireless power supply system  301 B with a configuration different from that of the wireless power supply system  301 A illustrated in  FIG.  1   . 
     The voltage conversion circuit  12  of the wireless power transmission apparatus  101  converts a DC input power supply voltage into a predetermined voltage. An MPU  30  is a digital control circuit that controls units of the wireless power transmission apparatus  101 . An input filter  21  removes a ripple component and a noise component. The intermediate capacitor Ci is provided between the voltage conversion circuit  12  and the input filter  21 . A current detection circuit  22  detects an intermediate input current, which is a current input in an inward direction from the voltage conversion circuit  12 . The current detection circuit  22  detects current flowing in a line so that a signal transmitted from the wireless power reception apparatus  201  can be detected. A demodulation circuit  23  demodulates a signal on the basis of a change in the current detected by the current detection circuit  22  and inputs the demodulated signal to the MPU  30 . A driver  25  performs switching of switching elements Q1 and Q2 in accordance with a control signal from the MPU  30 . 
     The power transmission circuit  26  includes the first switching element Q1, which is on a high side, and the second switching element Q2, which is on a low side, and turns on and off the switching elements Q1 and Q2 in accordance with a gate signal from the driver  25 . An EMI filter  27  reduces a noise component causing electromagnetic interference. A resonance adjusting circuit  28  includes a power transmission resonance capacitor. The resonance adjusting circuit  28  forms, together with the wireless power transmission coil Lp, a resonance circuit and adjusts the resonant frequency of the resonance circuit. 
      A voltage regulator circuit  31  stabilizes the power supply voltage with respect to the MPU  30 . An oscillator  32  provides a clock signal to the MPU  30 . 
     An overvoltage protection circuit  33  detects whether or not a voltage supplied to the power transmission circuit  26  is an overvoltage and inputs a result of the detection to the MPU  30 . A temperature detection circuit  34  detects whether or not temperatures of the switching elements Q1 and Q2 are in an overheated state and inputs a result of the detection to the MPU  30 . An overcurrent detection circuit  35  detects whether or not a current flowing to the power transmission circuit  26  is an overcurrent and inputs a result of the detection to the MPU  30 . An overpower detection circuit  36  detects, based on a voltage generated at the resonance adjusting circuit  28 , whether or not power to be transmitted is an overpower and inputs a result of the detection to the MPU  30 . 
     By providing a control signal to the voltage conversion circuit  12 , the MPU  30  adjusts the output voltage of the voltage conversion circuit  12 . Thus, protection against overpower supply can be achieved. Circuits between the input filter  21  and the power transmission circuit  26  configure the electric power management circuit  39 . 
     An MPU  50  inside the wireless power reception apparatus  201  controls units of the wireless power reception apparatus  201 . A power reception resonance adjusting circuit  51  includes a power reception resonance capacitor. The power reception resonance adjusting circuit  51  forms, together with the wireless power reception coil Ls, a resonance circuit and adjusts the resonant frequency of the resonance circuit. A rectifying and smoothing circuit  52  rectifies and smooths a voltage generated in the power reception resonance circuit, which includes the wireless power reception coil Ls and the power reception resonance adjusting circuit  51 , and inputs the rectified and smoothed voltage to the voltage stabilizing circuit  53 . A voltage stabilizing circuit converts the output voltage of the rectifying and smoothing circuit  52  into a specified voltage and supplies the converted voltage to the load Ro. 
     An overpower protection circuit  54  detects, based on a current flowing in the voltage stabilizing circuit  53 , whether or not electric power supplied to the load is an overpower, and inputs a result of the detection to the MPU  50 . An oscillator  55  provides a clock signal to the MPU  50 . A voltage regulator circuit  56  stabilizes a power supply voltage with respect to the MPU  50 . An overvoltage protection circuit  57  detects whether or not the output voltage of the rectifying and smoothing circuit  52  is an overvoltage and inputs a result of the detection to the MPU  50 . A modulation circuit  58  modulates the power reception resonance adjusting circuit  51 . By causing the modulation circuit  58  to modulate the power reception resonance adjusting circuit  51 , the MPU  50  transmits a predetermined signal to the wireless power transmission apparatus  101 . 
     A power reception voltage detection circuit  61  detects the output voltage of the rectifying and smoothing circuit  52  and inputs the detected output voltage to the MPU  50 . A temperature detection circuit  62  detects the temperature of the wireless power reception apparatus  201  and inputs the detected temperature to the MPU  50 . 
     By causing the modulation circuit  58  to modulate the power reception resonance adjusting circuit  51 , the wireless power reception apparatus  201  transmits a received power request signal to the wireless power transmission apparatus  101 . Furthermore, the wireless power transmission apparatus  101  detects a received power request signal under the demodulation control of the demodulation circuit  23 . 
     Operation of the above-described wireless power supply system  301 B will be described below. When the strength of an alternating magnetic field generated by the wireless power transmission coil Lp increases, the intermediate input current flowing from the intermediate capacitor Ci to a subsequent stage increases. When detecting the increase in the intermediate input current, the MPU  50  adjusts a detection value of the output voltage of the voltage conversion circuit  12  so that a potential returned to an output voltage feedback terminal is apparently increased. As a result, the width of pulses for driving a switching element in the voltage conversion circuit  12  is finely adjusted, and the voltage of the intermediate capacitor Ci (intermediate voltage) decreases. 
     When the intermediate voltage decreases, the DC voltage intermittently provided to the power transmission resonance mechanism PR, which includes the wireless power transmission coil Lp and the power transmission resonance capacitor Cr, decreases, and the amplitude voltage of a square wave decreases. Thus, the resonance current flowing in the wireless power transmission coil Lp decreases, and the strength of an alternating magnetic field decreases. The strength of the alternating magnetic field is feedback-controlled, as described above. 
     Furthermore, the MPU  30  controls the voltage conversion circuit  12  to reduce the intermediate voltage serving as the output voltage of the voltage conversion circuit  12  in such a manner that the temperature of the voltage conversion circuit  12  or the power transmission circuit  26  detected by the temperature detection circuit  34  does not exceed a predetermined upper limit value. 
     Furthermore, the MPU  30  detects, based on output of the overcurrent detection circuit  35  or the overpower detection circuit  36 , an abnormality of the voltage conversion circuit  12  or the power transmission circuit  26 . In the case where an abnormality of the voltage conversion circuit  12  or the power transmission circuit  26  is detected, the MPU  30  reduces the intermediate voltage serving as the output voltage of the voltage conversion circuit  12  to a value less or than equal to a predetermined value, and then stops switching of the power transmission circuit  26 . 
     As described above, the current detection circuit  22  detects the intermediate input current input to the power transmission circuit  26  from the voltage conversion circuit  12 , and the MPU  30  sets the upper limit value for the intermediate input current. Thus, the output voltage of the voltage conversion circuit  12  (voltage of the intermediate capacitor Ci) is controlled in such a manner that the intermediate input current does not exceed the upper limit value. By controlling the output voltage of the voltage conversion circuit  12 , the amplitude of the DC voltage or current intermittently provided to the power transmission resonance mechanism PR is adjusted. 
     In the case where the voltage of the intermediate capacitor Ci is constant, the intermediate input current is proportional to electric power handled by a power transmission circuit and is substantially proportional to electric power supplied to the wireless power reception apparatus  201  from the wireless power transmission coil Lp. Thus, even if electromagnetic field energy generated by the wireless power transmission coil Lp is not always efficiently supplied to the wireless power reception apparatus  201 , electric power to be received by the wireless power reception apparatus  201  can be controlled to be equal to an electric power required by the wireless power reception apparatus  201 . 
     Furthermore, the voltage conversion circuit  12  is controlled in such a manner that the current flowing in the voltage conversion circuit  12  or the power transmission circuit  26  or the temperature of the voltage conversion circuit  12  or the power transmission circuit  26  does not exceed a predetermined upper limit value. Thus, an abnormality such as an overpower can be avoided. 
     Furthermore, in the case where an abnormality of the voltage conversion circuit  12  or the power transmission circuit  26  is detected, the intermediate voltage serving as the output voltage of the voltage conversion circuit  12  is reduced to be less than or equal to the predetermined value. Thus, the operation of the power transmission circuit  26  stops, and the strength of an alternating magnetic field at a switching frequency can be fully controlled without depending on whether or not a desired power transmission is successfully performed. 
     Second Embodiment 
     In a second embodiment, a wireless power supply system that specifically represents a voltage conversion circuit, an intermediate input current detection circuit, a rectifying and smoothing circuit, and so on will be described as an example. 
       FIG.  3    is a circuit diagram illustrating a configuration of a wireless power supply system  302  according to the second embodiment. In this example, a circuit on a power transmission side performs a class-D converter operation, and a circuit on a power reception side performs a series resonance operation and a voltage-doubler rectifying operation. 
     The wireless power transmission apparatus  101  includes a first switch circuit S1 that equivalently includes a parallel connection circuit including the first switching element Q1, a diode Dds1, and a capacitor Cds1, a second switch circuit S2 that equivalently includes a parallel connection circuit including the second switching element Q2, a diode Dds2, and a capacitor Cds2, the wireless power transmission coil Lp, and the power transmission resonance capacitor Cr. The wireless power transmission coil Lp and the power transmission resonance capacitor Cr configure the power transmission resonance mechanism PR. 
     The wireless power transmission apparatus  101  further includes the voltage conversion circuit  12  including a switching element Q5, a diode D5, an inductor Li, and the intermediate capacitor Ci. By switching of the switching element Q5, the voltage conversion circuit  12  operates as a step-down converter. 
     The wireless power transmission apparatus  101  further includes a resistor element Ri and the electric power management circuit  39 . The electric power management circuit  39  controls the switching elements Q1, Q2, and Q5. In this example, the electric power management circuit  39  controls the switching element Q5 of the voltage conversion circuit  12 . Thus, the electric power management circuit  39  is part of the voltage conversion circuit  12 . The electric power management circuit  39  performs switching of the switching element Q5, so that the voltage conversion circuit  12  is caused to operate as a step-down DC-DC converter. The electric power management circuit  39  also controls the switch-ON duty ratio of the switching element Q5 in such a manner that the output voltage of the voltage conversion circuit  12  is maintained at a predetermined voltage. That is, the electric power management circuit  39  detects the output voltage and performs negative feedback control of the output voltage. 
     The switching elements Q1 and Q2 are alternately turned on and off in accordance with a signal from the electric power management circuit  39 . 
     The switching elements Q1 and Q2 are switching elements such as MOSFETs including a parasitic output capacitance or a parasitic diode. The switching elements Q1 and Q2 configure the switch circuits S1 and S2, respectively. The switch circuits S1 and S2 configure a power transmission circuit. 
     The electric power management circuit  39  performs switching of the first switching element Q1 and the second switching element Q2 at a predetermined operating frequency, so that the DC voltage is intermittently provided to the power transmission resonance mechanism PR and a resonance current is thus generated at the wireless power transmission coil Lp. Specifically, switching is performed at 13.56 MHz, which is used in NFC communication. 
     The wireless power reception apparatus  201  includes a third switch circuit S3 that equivalently includes a parallel connection circuit including the switching element Q3, a diode Dds3, and a capacitor Cds3, a fourth switch circuit S4 that equivalently includes a parallel connection circuit including the fourth switching element Q4, a diode Dds4, and a capacitor Cds4, the wireless power reception coil Ls, and the power reception resonance capacitor Crs. The wireless power reception coil Ls and the power reception resonance capacitor Crs configure the power reception resonance mechanism SR. 
     The wireless power reception apparatus  201  further includes a smoothing capacitor Co in a stage subsequent to the third switch circuit S3 and the fourth switch circuit S4. The smoothing capacitor Co, the third switch circuit S3, and the fourth switch circuit S4 configure a rectifying and smoothing circuit. 
     The third switch circuit S3 and the fourth switch circuit S4 rectify a voltage generated in the power reception resonance mechanism SR, which includes the wireless power reception coil Ls and the power reception resonance capacitor Crs, and the smoothing capacitor Co smooths the voltage. In this example, the wireless power reception coil Ls and the power reception resonance capacitor Crs configure a series resonance circuit. The wireless power transmission coil Lp and the wireless power reception coil Ls are magnetic-field coupled to each other. M in  FIG.  3    indicates coupling between the wireless power transmission coil Lp and the wireless power reception coil Ls. 
     Circuits in stages subsequent to the intermediate capacitor Ci configure a wireless power supply unit  120 . The intermediate capacitor Ci is part of the voltage conversion circuit  12  and is also part of the wireless power supply unit  120 . 
     The electric power management circuit  39  inside the wireless power transmission apparatus  101  detects, based on a step-down voltage of the resistor element Ri, a current (intermediate input current) input to the power transmission circuit from the voltage conversion circuit  12 , that is, a current supplied to the wireless power supply unit  120  from the voltage conversion circuit  12 . 
     The electric power management circuit  39  sets an upper limit value for the intermediate input current, so that the output voltage of the voltage conversion circuit  12  (voltage of the intermediate capacitor Ci) is controlled in such a manner that the intermediate input current does not exceed the upper limit value. The output voltage of the voltage conversion circuit  12  is controlled, so that the amplitude of a DC voltage or current intermittently provided to the power transmission resonance mechanism PR is adjusted. As a result, electric power to be received by the wireless power reception apparatus  201  is controlled to be equal to an electric power required by the wireless power reception apparatus  201 . Furthermore, electric power supplied to the power transmission circuit  26  is restricted, and breakdown of a circuit, an increase of electrical stress, and excessive heat generation are thus suppressed. 
     Furthermore, the electric power management circuit  39  may be configured to detect the voltage of the power transmission resonance capacitor Cr and control the voltage conversion circuit  12  to reduce the intermediate voltage in such a manner that the voltage of the power transmission resonance capacitor Cr does not exceed the upper limit value. Thus, the output voltage of the voltage conversion circuit  12  is controlled, and the amplitude of the DC voltage or current intermittently provided to the power transmission resonance mechanism PR is thus adjusted. As a result, electric power to be received by the wireless power reception apparatus  201  is controlled to be equal to an electric power required by the wireless power reception apparatus  201 . Furthermore, electric power supplied to the power transmission circuit  26  is restricted, and breakdown of a circuit, an increase of electrical stress, and excessive heat generation are thus suppressed. 
     Third Embodiment 
     In a third embodiment, a wireless power supply system that specifically represents a voltage conversion circuit, an intermediate input current detection circuit, a rectifying and smoothing circuit, and so on will be described as an example. 
       FIG.  4    is a circuit diagram illustrating a configuration of a wireless power supply system  303  according to the third embodiment. This example is different from the example described in the second embodiment especially in configurations of a power transmission resonance mechanism and a power reception resonance mechanism. 
     The wireless power transmission apparatus  101  includes the first switch circuit S1, the second switch circuit S2, and the power transmission resonance capacitor Cr. The wireless power transmission apparatus  101  also includes the voltage conversion circuit  12  including the switching element Q5, the diode D5, the inductor Li, and the intermediate capacitor Ci. The wireless power transmission apparatus  101  further includes the resistor element Ri and the electric power management circuit  39 . The electric power management circuit  39  controls the switching elements Q1, Q2, and Q5, The voltage conversion circuit  12  operates as a step-up converter. 
     The wireless power transmission coil Lp is not a coil whose both ends are supplied with electric power but is a helical coil (antenna) whose center is supplied with electric power. The wireless power transmission coil Lp includes the power transmission resonance capacitor Cr as a parasitic capacitance component. The wireless power transmission coil Lp and the power transmission resonance capacitor Cr configure the power transmission resonance mechanism PR. 
     The wireless power reception apparatus  201  includes the power reception resonance mechanism SR that includes the wireless power reception coil Ls and the power reception resonance capacitor Crs, and a rectifying and smoothing circuit. The rectifying and smoothing circuit includes the third switch circuit S3, the fourth switch circuit S4, and the smoothing capacitor Co. 
     The wireless power reception coil Ls is a helical coil (antenna) whose center is supplied with electric power. The wireless power reception coil Ls includes the power reception resonance capacitor Crs as a parasitic capacitance component. The wireless power reception coil Ls and the power reception resonance capacitor Crs configure the power reception resonance mechanism SR. 
     The other configurations are the same as those described above in the second embodiment. The electric power management circuit  39  inside the wireless power transmission apparatus  101  detects, based on the step-down voltage of the resistor element Ri, a current (intermediate input current) input to the power transmission circuit from the voltage conversion circuit, that is, a current supplied to the wireless power supply unit  120  from the voltage conversion circuit  12 . The electric power management circuit  39  sets an upper limit value for the intermediate input current, so that the output voltage of the voltage conversion circuit  12  (voltage of the intermediate capacitor Ci) is controlled in such a manner that the intermediate input current does not exceed the upper limit value. 
     Fourth Embodiment 
     In a fourth embodiment, a wireless power supply system that specifically represents a voltage conversion circuit, an intermediate input current detection circuit, a rectifying and smoothing circuit, and so on will be described as an example. 
       FIG.  5    is a circuit diagram illustrating a configuration of a wireless power supply system  304  according to the fourth embodiment. This example is different from the example described in the second embodiment especially in configurations of a rectifying and smoothing circuit, a power transmission resonance mechanism, and a power reception resonance mechanism. The wireless power transmission coil Lp and the wireless power reception coil Ls each have a loop shape. 
     In the wireless power supply system  304 , the wireless power transmission apparatus  101  is a circuit that performs a class-D converter operation, and the wireless power reception apparatus  201  is a circuit that performs a series resonance operation and a voltage-doubler rectifying operation. The configuration on the power transmission side is the same as that in the example illustrated in  FIG.  3   . The power reception resonance capacitor Crs connected to the wireless power reception coil Ls is provided on the power reception side. The wireless power reception coil Ls and the power reception resonance capacitor Crs configure a series resonance circuit. The rectifying and smoothing circuit includes the third switch circuit S3 including a diode D3 and a capacitor C3 and the fourth switch circuit S4 including a diode D4 and a capacitor C4. The other configurations are the same as those described above in the second embodiment. 
     The electric power management circuit  39  inside the wireless power transmission apparatus  101  detects, based on the step-down voltage of the resistor element Ri, a current (intermediate input current) input to the power transmission circuit from the voltage conversion circuit  12 , that is, a current supplied to the wireless power supply unit  120  from the voltage conversion circuit  12 . The electric power management circuit  39  sets an upper limit value for the intermediate input current, so that the output voltage of the voltage conversion circuit  12  (voltage of the intermediate capacitor Ci) is controlled in such a manner that the intermediate input current does not exceed the upper limit value. 
     Fifth Embodiment 
      In a fifth embodiment, a wireless power supply system that specifically represents a voltage conversion circuit, an intermediate input current detection circuit, a rectifying and smoothing circuit, and so on will be described as an example. 
       FIG.  6    is a circuit diagram illustrating a configuration of a wireless power supply system  305  according to the fifth embodiment. This example is different from the example described in the second embodiment especially in the configuration of a switching circuit. 
     In the wireless power supply system  305 , the wireless power transmission apparatus  101  performs a class-E converter operation, and the wireless power reception apparatus  201  performs a series resonance operation and a class-E rectifying operation. 
     The wireless power transmission apparatus  101  includes the first switch circuit S1 that equivalently includes a parallel connection circuit including the switching element Q1, the diode Dds1, and the capacitor Cds1, an inductor Lf, and the resonance capacitor Cr. The wireless power transmission coil Lp and the resonance capacitor Cr configure the power transmission resonance mechanism PR. 
     By switching of the first switching element Q1 at a predetermined operating frequency, the electric power management circuit  39  intermittently provides a DC voltage to a resonance circuit that includes the inductor Lf, the resonance capacitor Cr, and the wireless power transmission coil Lp, so that a resonance current is generated at the wireless power transmission coil Lp. 
     The wireless power reception apparatus  201  includes the power reception resonance mechanism SR that includes the wireless power reception coil Ls and the power reception resonance capacitor Crs, and a rectifying and smoothing circuit. The rectifying and smoothing circuit includes the third switch circuit S3 that equivalently includes a parallel connection circuit including the third switching element Q3, the diode Dds3, and the capacitor Cds3, an inductor Lfs, and the smoothing capacitor Co. 
     The third switch circuit S3 rectifies a voltage generated at a power reception resonance circuit that includes the wireless power reception coil Ls, the power reception resonance capacitor Crs, and the inductor Lfs, and the smoothing capacitor Co smooths the voltage. The other configurations are the same as those described above in the second embodiment. 
     The electric power management circuit  39  inside the wireless power transmission apparatus  101  detects, based on the step-down voltage of the resistor element Ri, a current (intermediate input current) input to the power transmission circuit from the voltage conversion circuit  12 , that is, a current supplied to the wireless power supply unit  120  from the voltage conversion circuit  12 . The electric power management circuit  39  sets an upper limit value for the intermediate input current, so that the output voltage of the voltage conversion circuit  12  (voltage of the intermediate capacitor Ci) is controlled in such a manner that the intermediate input current does not exceed the upper limit value. 
     Sixth Embodiment 
     In a sixth embodiment, a wireless power supply system that specifically represents a voltage conversion circuit, an intermediate input current detection circuit, a rectifying and smoothing circuit, and so on will be described as an example. 
       FIG.  7    is a circuit diagram illustrating a configuration of a wireless power supply system  306  according to the sixth embodiment. This example is different from the example described in the second embodiment in the configuration of an intermediate input current detection circuit. 
     In the wireless power supply system  306 , a transistor Tr1, resistor elements Ri, R1, R2, R3, R4, and R5, and capacitors C4 and C5 configure an intermediate input current detection circuit. In the intermediate input current detection circuit, the division ratio of a voltage-dividing circuit including the resistor elements R2, R3, and R4 and the transistor Tr1 varies according to the step-down voltage of the resistor element Ri, and the divided voltage is fed back to the electric power management circuit  39 . In this example, the detection resolution of the intermediate input current detection circuit increases in accordance with a gain by the transistor Tr1. 
     The resistor elements R4 and R5 and the capacitors C4 and C5 configure a filter of a feedback circuit. The pass band of the filter adjusts a frequency band in the feedback circuit for controlling switching operations of the switching elements Q1 and Q2. With the use of the filter, the output voltage of the voltage conversion circuit  12  (voltage of the intermediate capacitor Ci) can be controlled stably over a wide frequency band. 
     With the provision of the capacitor C6, the step-down voltage of the resistor element Ri can be compensated for, and the influence of a pulse current caused by switching operations of the switching elements Q1 and Q2 can be prevented from being exerted on the intermediate input current flowing in the resistor element Ri. That is, the capacitor C6 is capable of causing the intermediate input current flowing in the resistor element Ri to have a waveform close to the waveform of a DC current, not the waveform of a switching current. 
     The electric power management circuit  39  inside the wireless power transmission apparatus  101  detects, based on the step-down voltage of the resistor element Ri, a current (intermediate input current) input to the power transmission circuit from the voltage conversion circuit  12 , that is, a current supplied to the wireless power supply unit  120  from the voltage conversion circuit  12 . The electric power management circuit  39  sets an upper limit value for the intermediate input current, so that the output voltage of the voltage conversion circuit  12  (voltage of the intermediate capacitor Ci) is controlled in such a manner that the intermediate input current does not exceed the upper limit value. 
     Seventh Embodiment 
     In a seventh embodiment, a voltage conversion circuit with a configuration different from those of the voltage conversion circuits  12  described above will be described as an example. In this embodiment, a voltage conversion circuit that converts the voltage of an input power supply is configured as a DC-DC converter including a negative feedback control circuit that detects an output voltage and performs negative feedback control of the output voltage. The DC-DC converter is an isolated converter. 
       FIG.  8    is a circuit diagram of the voltage conversion circuit provided in a wireless power transmission apparatus according to the seventh embodiment. In  FIG.  8   , the switching element Q5 is connected between the input power supply Vi and a primary coil of a transformer T. A diode D6 and the intermediate capacitor Ci are connected to a secondary coil of the transformer T. The coupling polarity of the primary coil and the secondary coil of the transformer T is as illustrated in the drawing. A flyback converter is configured, as illustrated in  FIG.  8   . 
       FIG.  9    is another circuit diagram of the voltage conversion circuit provided in the wireless power transmission apparatus according to the seventh embodiment. In  FIG.  9   , the switching element Q5 is connected between the input power supply Vi and the primary coil of the transformer T. Furthermore, a parallel circuit including a capacitor C8 and a resistor element R8 and a series circuit including the capacitor C8 and the diode D8 are connected to the primary coil of the transformer T. Diodes D6 and D7, the inductor Li, and the intermediate capacitor Ci are connected to the secondary coil of the transformer T. The coupling polarity of the primary coil and the secondary coil of the transformer T is as illustrated in the drawing. A forward converter is configured, as illustrated in  FIG.  9   . 
     As described in this embodiment, with the use of an isolated DC-DC converter, electrical insulation between input and output can be obtained. Thus, effects such as prevention of electrical shock of a user and reduction of electromagnetic conduction noise can be achieved. In particular, a flyback converter needs only a small number of components and is thus suitable for size reduction. 
     Eighth Embodiment 
     In an eighth embodiment, a voltage conversion circuit with a configuration different from those of the voltage conversion circuits  12  described above will be described as an example. In this embodiment, a voltage conversion circuit that converts the voltage of an input power supply is configured as a series regulator including a negative feedback control circuit that detects an output voltage and performs negative feedback control of the output voltage. 
       FIG.  10    is a circuit diagram of the voltage conversion circuit provided in a wireless power transmission apparatus according to the eighth embodiment. In  FIG.  10   , the transistor Q6 is connected in series between the input power supply Vi and an output part. A series circuit including the resistor element R1 and a Zener diode ZD is provided between the collector of the transistor Q6 and a reference potential. The resistor element R2 is connected between the emitter of the transistor Q6 and the reference potential. The Zener diode ZD is connected to the base of the transistor Q6 in such a manner that the voltage of the Zener diode ZD is applied to the base of the transistor Q6. The intermediate capacitor Ci is connected to the output part of the voltage conversion circuit. 
       FIG.  11    is another circuit diagram of the voltage conversion circuit provided in the wireless power transmission apparatus according to the eighth embodiment. In  FIG.  11   , the transistor Q6 is connected in series between the input power supply Vi and the output part. A voltage-dividing circuit including the resistor elements R3 and R4 and a negative feedback circuit including a reference voltage circuit E and an error amplifier EA are arranged between the collector of the transistor Q6 and the reference potential. An output part of the error amplifier EA draws a base current of the transistor Q6. A capacitor C9 is connected to an input part of the voltage conversion circuit, and the intermediate capacitor Ci is connected to the output part. 
     As described above, with the use of a series regulator, voltage adjustment can be achieved with a small circuit scale. Furthermore, because a small number of components are required compared to the case of DC-DC converters, size reduction of a circuit can be achieved. 
     Ninth Embodiment 
     In a ninth embodiment, an input power supply with a configuration different from those of the input power supplies described above will be described as an example. In this embodiment, an input power supply to the voltage conversion circuit  12  is a DC voltage source. 
       FIG.  12    is a circuit diagram of the input power supply Vi according to the ninth embodiment. The input power supply Vi includes a diode bridge circuit DB that rectifies a commercial AC power supply, a capacitor C10, the transformer T, the switching element Q9, the diode D6, and a capacitor C11. 
     The switching element Q9 is connected between output of a rectifying and smoothing circuit including the diode bridge circuit DB and the capacitor C10 and the primary coil of the transformer T. A feedback path insulation element 2 is in an insulated state, and the output voltage of the voltage conversion circuit  12  is detected. A control IC 1 performs switching control of the switching element Q9 in such a manner that the output voltage of the input power supply Vi is equal to a predetermined voltage. 
     Tenth Embodiment 
     In a tenth embodiment, an input power supply with a configuration different from those of the input power supplies described above will be described as an example. In this embodiment, an input power supply to the voltage conversion circuit  12  is a DC current source. 
       FIG.  13    is a circuit diagram of the input power supply Vi according to the tenth embodiment. The input power supply Vi includes a capacitor C10, switching elements Q11, Q12, Q13, and Q14, the transformer T, diodes D9 and D10, an inductor L6, and a capacitor C11. 
     The above-mentioned switching elements Q11, Q12, Q13, and Q14, the transformer T, the diodes D9 and D10, the inductor L6, and the capacitor C11 configure a full-bridge DC-DC converter. A switching control circuit is connected to the switching elements Q11, Q12, Q13, and Q14, so that the switching control circuit controls the output current of the DC-DC converter to a constant value. With this configuration, the DC-DC converter operates as a DC current source. 
     Finally, the present disclosure is not limited to the embodiments described above. Modifications and changes can be made by those skilled in the art in an appropriate manner. The scope of the present disclosure is not illustrated by the embodiments described above but by the claims. Furthermore, the scope of the present disclosure covers modifications and changes made to embodiments that fall within the scope of the claims and their equivalents. 
     For example, a step-down converter, a step-up converter, and other various converters such as a step-up/step-down converter and an isolated converter that detect an output voltage, feed back a voltage signal, compare a feedback potential with a reference value, adjust the width of pulses for driving a switching element, and control the output voltage to a predetermined constant value, may be used for a voltage conversion circuit. 
     Furthermore, to detect the intermediate current, instead of a transistor illustrated in  FIG.  7   , a comparator or an operational amplifier may be used. 
     Furthermore, an electric power management circuit is not limited to an analog circuit or a digital circuit. The electric power management circuit may be configured to include both an analog circuit and a digital circuit.