Patent Publication Number: US-2016226298-A1

Title: Power receiver, power source, and wireless power transfer system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application and is based upon PCT/JP2013/082391, filed on Dec. 2, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate to a power receiver, a power source and a wireless power transfer system. 
     BACKGROUND 
     Recently, wireless power transfer techniques for supplying power or charging a secondary battery have attracted attention. Research and development have been conducted on wireless power transfer systems (wireless power supply systems) wirelessly transferring power to, e.g., various electronic devices such as mobile terminals and notebook computers, electrical household appliances or power infrastructure equipment. 
     Incidentally, a magnetic field coupling method of employing coils to both of a power source and a power receiver has been generally used for a wireless power transfer method of wirelessly transferring power over several Watts class in a distance of several cm to several ten cm. The power transfer method using the magnetic field may be known as an electromagnetic induction method, and a magnetic field resonance method which is proposed by the United States MIT (Massachusetts Institute of Technology) in recent years. 
     Further, the electromagnetic induction method may be known as a Qi standard formulated by WPC (Wireless Power Consortium). Further, the magnetic field resonance method may be known as a WiPower formulated by A4WP (Alliance for Wireless Power). 
     Therefore, the magnetic field coupling method of employing coils to the power source and the power receiver may be realized for small-sized electronic devices of several Watts class in a commercialization phase, and may be standardized for home appliances using a target power of over 100 Watts. Further, a wireless power transfer technology of transferring power over several kilo-Watts for an electric vehicle has been researched and developed in the center of automobile manufacturers. 
     In these wireless power transfer systems, for the purpose of safety of heat generation or a condition optimization in efficiency, power may be transferred from a power source to a power receiver by using any communications technique between the power source and the power receiver in general. 
     Specifically, in the above described Qi standard, for example, in-band method for modulating an energy transfer waveform by controlling a connection of on/off at the power receiver side is adopted. Further, in the WiPower standard, for example, out-of-band scheme to both the power source and the power receiver is equipped with communication devices such as Bluetooth (registered trademark) to exchange information in both directions. 
     In this specification, a wireless power transferring (wireless power transfer) using magnetic field resonance will be explained as an example of the embodiments, however, the present embodiments described later in detail are not limited to apply the magnetic field resonance, but may be applied to a wireless power transfer using electric field resonance, or the like. 
     As described above, the wireless power transfer technique has been applied to electronic devices and home appliances, for example, including a portable terminal, in addition to an electric vehicle or the like. Further, a wireless power transfer technique has been sought for small sensors and small devices of several ten milli-Watts class. The small sensors and the small devices may include various types of sensors embedded in a wall and medical devices mounted in the human body or the like. 
     Conventionally, for the sensors embedded in the wall and the medical devices mounted in the body or the like, a battery may be replaced at fixed intervals, and therefore, by taking into consideration the use of facilities and the burden on the human body, benefits of using a wireless power transfer technique may be very large. 
     Specifically, the sensors embedded in the wall and the medical devices mounted in the body or the like include a size restriction of a power receiver, and therefore, if it is possible to remove a communication circuit unit from the power receiver, a size or a consumption power of the receiver may be reduced. 
     However, when power is transferred to a plurality of power receivers at the same time, charging characteristics of secondary batteries in the plurality of power receivers may be different from each other, for example, by power receiving characteristics of respective power receivers. Therefore, if removing the communication circuit unit from the power receiver, it may be difficult to suitably charge the secondary battery of the respective power receivers. These problems are not only caused by the power transfer using magnetic field resonance or electric field resonance, but also by a power transfer using an electromagnetic induction or an electric field induction. 
     Conventionally, various wireless power transfer techniques for wirelessly supplying power have been proposed. 
     Patent Document 1: Japanese Laid-open Patent Publication No. 2013-090483 
     Patent Document 2: Japanese Laid-open Patent Publication No. 2000-287375 
     Patent Document 3: Japanese Laid-open Patent Publication No. 2006-006948 
     Non-Patent Document 1: Andre KURS et al., “Wireless Power Transfer via Strongly Coupled Magnetic Resonances,” SCIENCE Vol. 317, pp. 83-86, Jul. 6, 2007 
     Non-Patent Document 2: Aristeidis KARALIS et al., “Efficient wireless non-radiative mid-range energy transfer,” Cornel University Library, arXiv:physics/0611063v2 [physics.optics], (Journal-ref: Annals of Physics, vol. 323, No. 1, pp. 34-48, January 2008) 
     SUMMARY 
     According to an aspect of the embodiments, there is provided a power receiver that includes a power receiver coil, a secondary battery, a load resistor, a switch, and a power receiver controller. The power receiver coil is configured to wirelessly receive power from a power source, and the secondary battery is configured to be charged by power from the power receiver coil. 
     The switch is configured to selectively connect the power receiver coil to the secondary battery or the load resistor, and the power receiver controller is configured to control the switch in accordance with a power supply state of the power source based on the power receiver coil, and a charged state of the secondary battery. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram schematically depicting an example of a wireless power transfer system; 
         FIG. 2A  is a diagram (1) for illustrating a modified example of a transmission coil in the wireless power transfer system of  FIG. 1 ; 
         FIG. 2B  is a diagram (2) for illustrating a modified example of the transmission coil in the wireless power transfer system of  FIG. 1 ; 
         FIG. 2C  is a diagram (3) for illustrating a modified example of the transmission coil in the wireless power transfer system of  FIG. 1 ; 
         FIG. 3  is a block diagram schematically depicting the wireless power transfer system and an example of a power receiver; 
         FIG. 4A  is a diagram (1) for illustrating a first embodiment of a wireless power transfer system; 
         FIG. 4B  is a diagram (2) for illustrating the first embodiment of the wireless power transfer system; 
         FIG. 4C  is a diagram (3) for illustrating the first embodiment of the wireless power transfer system; 
         FIG. 5  is a diagram for explaining an impedance characteristic of a resonance coil of a power source; 
         FIG. 6  is a block diagram illustrating an example of a power receiver controller in the first embodiment of the wireless power transfer system; 
         FIG. 7  is a diagram illustrating signal waveforms of the power source and the power receiver in the first embodiment of the wireless power transfer system; 
         FIG. 8  is a flowchart for explaining an example of a power transfer process in the first embodiment of the wireless power transfer system; 
         FIG. 9  is a block diagram illustrating an example of a power receiver controller in a second embodiment of a wireless power transfer system; 
         FIG. 10  is a diagram illustrating signal waveforms of a power source and a power receiver in the second embodiment of the wireless power transfer system; 
         FIG. 11  is a flowchart for explaining an example of a power transfer process in the second embodiment of the wireless power transfer system; and 
         FIG. 12  is a diagram for explaining a power receiver of a third embodiment of a wireless power transfer system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First, before describing embodiments of a power receiver, a power source and a wireless power transfer system in detail, examples of wireless power transfer systems (wireless power supply systems) will be described with reference to  FIG. 1  to  FIG. 3 . 
       FIG. 1  is a block diagram schematically depicting an example of a wireless power transfer system including one power source and one power receiver. In  FIG. 1 , reference sign  1  denotes a power source (primary side: power source side), and reference sign  2  denotes a power receiver (secondary side: power receiver side). 
     As depicted in  FIG. 1 , power source (power source device)  1  includes a wireless power transfer unit (power source coil)  11 , a high frequency power supply unit  12 , a power transfer control unit  13 , and a power source communication circuit unit  14 . In addition, power receiver (power receiver device)  2  includes a wireless power receiver unit (power receiver coil)  21 , a power receiver circuit unit (rectifier unit)  22 , a power receiver control unit  23 , and a power receiver communication circuit unit  24 . 
     The wireless power transfer unit  11  includes a power supply coil  11   b  and a power source resonance coil  11   a , and the wireless power receiver unit  21  includes a power receiver resonance coil  21   a  and a power extraction coil  21   b.    
     As depicted in  FIG. 1 , the power source  1  and the power receiver  2  perform energy (electric power) transmission from the power source  1  to the power receiver  2  by magnetic field resonance (electric field resonance) between the power source resonance coil  11   a  and the power receiver resonance coil  21   a . Power transfer from the power source resonance coil  11   a  to the c may be performed not only by magnetic field resonance but also electric field resonance or the like. However, the following description will be given mainly by way of example of magnetic field resonance. 
     The power source  1  and the power receiver  2  communicate with each other (near field communication) by the power source communication circuit unit  14  and the power receiver communication circuit unit  24 . Note that, a distance of power transfer by the power source resonance coil  11   a  of power source  1  and the power receiver resonance coil  21   a  of power receiver  2  is set to be shorter than a distance of communication by the power source communication circuit unit  14  of power source  1  and the power receiver communication circuit unit  24  of power receiver  2 . 
     In addition, power transfer by the power source resonance coil  11   a  and the power receiver resonance coil  21   a  is performed by a system (out-band communication system) independent from communication by the power source communication circuit unit  14  and the power receiver communication circuit unit  24 . 
     Specifically, the power transfer performed by the power source resonance coil  11   a  and the power receiver resonance coil  21   a  uses, for example, a frequency band of 6.78 MHz, whereas communication performed by the power source communication circuit unit  14  and the power receiver communication circuit unit  24  uses, for example, a frequency band of 2.4 GHz. 
     The communication performed by the power source communication circuit unit  14  and the power receiver communication circuit unit  24  may use, for example, a DSSS wireless LAN system based on IEEE 802.11b or Bluetooth (registered trademark). 
     The above described wireless power transfer system performs power transfer using magnetic field resonance or electric field resonance by the power source resonance coil  11   a  of the power source  1  and the power receiver resonance coil  21   a  of the power receiver  2 , for example, in a near field at a distance of about a wavelength of a frequency used. Accordingly, the range of power transfer (power transfer range) PR varies with the frequency used for power transfer. 
     The high frequency power supply unit  12  supplies power to the power supply coil  11   b , and the power supply coil  11   b  supplies power to the power source resonance coil  11   a  arranged very close to the power supply coil  11   b  by using electromagnetic induction. The power source resonance coil  11   a  transfers power to the power receiver resonance coil  21   a  (the power receiver  2 ) at a resonance frequency that causes magnetic field resonance between the resonance coils  11   a  and  21   a.    
     The power receiver resonance coil  21   a  supplies power to the power extraction coil  21   b  arranged very close to the power receiver resonance coil  21   a , by using electromagnetic induction. The power extraction coil  21   b  is connected to the power receiver circuit unit  22  to extract a predetermined amount of power. The power extracted from the power receiver circuit unit  22  is used, for example, for charging a secondary battery  25 , as a power supply output to the circuit (load) of power receiver  2 , or the like. 
     Note that, the high frequency power supply unit  12  of power source  1  is controlled by the power transfer control unit  13 , and the power receiver circuit unit  22  of power receiver  2  is controlled by the power receiver control unit  23 . Then, the power transfer control unit  13  and the power receiver control unit  23  are connected via the power source communication circuit unit  14  and the power receiver communication circuit unit  24 , and adapted to perform various controls so that power transfer from power source  1  to power receiver  2  may be performed in an optimum state. 
       FIG. 2A  to  FIG. 2C  are diagrams for illustrating modified examples of a transmission coil in the wireless power transfer system of  FIG. 1 . Note that,  FIG. 2A  and  FIG. 2B  depict exemplary three-coil structures, and  FIG. 2C  depicts an exemplary two-coil structure. 
     Specifically, in the wireless power transfer system depicted in  FIG. 1 , the wireless power transfer unit  11  includes the first coil  11   b  and the second coil  11   a , and the wireless power receiver unit  21  includes the third coil  21   a  and the power extraction coil  21   b.    
     On the other hand, in the example of  FIG. 2A , the wireless power receiver unit (power receiver coil)  21  is set as a single coil (power receiver resonance coil: LC resonator)  21   a , and in the example of  FIG. 2B , the wireless power transfer unit (power source coil)  11  is set as a single coil (power source resonance coil: LC resonator)  11   a.    
     Further, in the example of  FIG. 2C , the wireless power receiver unit (power receiver coil)  21  is set as a single power receiver resonance coil  21   a  and the wireless power transfer unit (power source coil)  11  is set as a single power source resonance coil  11   a . Note that,  FIG. 2A  to  FIG. 2C  are merely examples and, obviously, various modifications may be made. 
       FIG. 3  is a block diagram schematically depicting the wireless power transfer system and an example of a power receiver, and illustrates the case wherein a power source resonance coil  11   a  (power source coil  11 : power source  1 ) transfers power to a plurality of power receivers  2 . 
     In  FIG. 3 , a power receiver  2  is, for example, a medical device mounted on a body  3 , specifically, a cardiac pacemaker inserted into a heart  3 . Note that, the power receivers  2  may be temperature sensors embedded in a wall ( 3 ) or various types of sensors or micro devices that are sprayed on soil or the like. 
     As depicted in  FIG. 3 , the power receiver  2  includes a wireless power receiver unit (power receiver coil)  21 , a power receiver circuit unit  22 , a power receiver control unit  23 , a power receiver communication circuit unit  24 , a secondary battery  25 , and a device unit  26 . 
     The wireless power receiver unit  21  includes a power receiver resonance coil  21   a  and a power extraction coil  21   b , and the power receiver circuit unit  22  includes a rectifier circuit  22   a  and a DC/DC converter  22   b . The device unit  26  includes a driver  26   a  and a device  26   b.    
     Note that, the power receiver  2  depicted in  FIG. 3  is, for example, equivalent to the power receiver described with reference to above described  FIG. 1 . In  FIG. 3 , the power receiver circuit unit  22  depicted in  FIG. 1  is divided and illustrated as the rectifier circuit  22   a  and the DC/DC converter  22   b.    
     Further, in the power receiver  2  depicted in  FIG. 3 , the device unit  26  (driver  26   a  and device  26   b ) applied with output voltage of the secondary battery  25  is illustrated, but the power receiver  2  of  FIG. 3  is substantially equivalent to that of  FIG. 1 . Note that, the power receiver communication circuit unit  24  of the power receiver  2  depicted in  FIG. 3  is used to communicate with the power source communication circuit unit  14  of the power source  1  depicted in  FIG. 1 . 
     Incidentally, for example, by applying a wireless power transfer technique to medical devices attached to a human body or various types of sensors embedded in a wall, a burden on the human body may be reduced and a facility of operation may be improved. In the medical devices attached to the human body or the like, a constraint on size of the power receiver is large, and a consumption power is preferable as small as possible, by considering power supply frequency. 
     Therefore, after enabling wireless power transfer, if the communication circuit unit of the power receiver is simplified or removed, it is possible to reduce the size and the consumption power of the power receiver. This reduction of the consumption power may realize to suppress the size of the secondary battery, and is more preferable. 
     Note that, for example, even in the power transfer system for transmitting data from the sensors embedded in the wall to a host device by using wireless communication circuits, if removing the power receiver communication circuit unit used for a power receiving operation from the power receiver and retaining a wireless communication circuit for only transmitting data, the benefits of reducing the size and the consumption power may be expected. 
     Incidentally, for example, when simultaneously transferring power to a plurality of power receivers, secondary batteries of some power receivers may be reached to fully charged states and the other may be reached to fully charged states others are not reached to the fully charged states, in accordance with characteristic differences of respective power receivers. 
     Note that, the differences of charging characteristics of the secondary batteries are not only caused by manufacturing differences, but also caused by, for example, differences in the relative positional relationship and power transfer efficiency (power supply efficiency) between the power source and the power receiver, individual consumption powers of respective power receivers, and various kinds of matters. 
     Therefore, if no communication function is provided, it is difficult to detect the state of the power receivers and preferably control power transfer conditions by the power source. This problem is not limited to power transfer systems using magnetic field resonance or electric field resonance, but the problem may be caused in, for example, power transfer systems using electromagnetic induction and electric field induction. 
     Note that, the present embodiments to be described later are preferable to apply wireless power transfer system wherein the number of power receivers and positions of the power receivers with respect to the power source are substantially fixed, however, the present embodiments are not limited to apply such a wireless power transfer system. 
     Further, the power receiver to be applied the present embodiments is not limited to sensors embedded in a wall and medical devices mounted in a body or the like, and is not also limited to a device driven by a secondary battery. 
     Hereinafter, embodiments of a power receiver, a power source and a wireless power transfer system will be described in detail with reference to the accompanying drawings. In each of the embodiments to be described below, a power source coil (wireless power transfer unit)  11  and a receiver coil (wireless power receiver unit)  21  may be applied, for example, the various configurations described with reference to  FIG. 2A  to  FIG. 2C . 
       FIG. 4A  to  FIG. 4C  are diagrams for illustrating a first embodiment of a wireless power transfer system. In  FIG. 4A  to  FIG. 4C , references  1  denotes a power source,  2 - 1  to  2 -N denotes power receivers, and all of the power receivers  2 - 1  to  2 -N include the same configurations. 
     In the following descriptions, for simplicity, a first power receiver  2 - 1  and a N-th power receiver  2 -N are mainly explained. Specifically,  FIG. 4A  illustrates the case that the first power receiver  2 - 1  and the N-th power receiver  2 -N (for example, all of the power receivers  2 - 1  to  2 -N) are normal states and in a test power transfer (mode). 
     Further,  FIG. 4B  illustrates the case that the first power receiver  2 - 1  and the N-th power receiver  2 -N (for example, all of the power receivers  2 - 1  to  2 -N) are charging states. In addition,  FIG. 4C  illustrates the case that the first power receiver  2 - 1  is fully charged state and the N-th power receiver  2 -N is a charging state, for example, one power receiver  2 - 1  is fully charged, and other power receivers  2 - 2  to  2 -N are not fully charged and continued in charging states. 
     The power source  1  includes a wireless power transfer unit (power source coil)  11  including a power supply coil  11   b  and a power source resonance coil  11   a , an amplifier  15 , a matching circuit  16 , and a power source control unit (including memory)  17 . 
     In each power receiver  2  (each of the power receivers  2 - 1  to  2 -N), a switch  28  selectively connects an output (received voltage Vr) of a rectifier circuit  22   a  to a dummy load resistor (load resistance)  29  or a DC/DC converter  22   b  in accordance with a switch control signal Ss from a power receiver controller  27 . 
     The power receiver controller  27  receives the received voltage Vr from the rectifier circuit  22   a  and an output (charging power: power signal) Pc of the DC/DC converter  22   b , and controls the switch  28  by the switch control signal Ss. 
     Specifically, in the power receiver  2  ( 2 - 1  to  2 -N) of the wireless power transfer system of the first embodiment, a three terminal switch  28  is provided between the rectifier circuit  22   a  and the DC/DC converter  22   b.    
     Based on switching operation of the switch  28 , in a normal state (normally used state, and an initial state of the test power transfer), an output (received voltage Vr) of the rectifier circuit  22   a  is connected to the dummy load resistor  29 , and in a full power transfer for charging a secondary battery  25 , the output of the rectifier circuit  22   a  is connected to the DC/DC converter  22   b.    
     Further, in the case that the secondary battery  25  is fully charged, the output of the rectifier circuit  22   a  is opened or connected to a high impedance resistor by the switch  28 . Specifically, the switch  28  is controlled by the switch control signal Ss from the power receiver controller  27 , and switching operations of the switch  28  may be performed suitable for respective operation phases. 
     Note that, the power receiver controller  27  monitors the charging power Pc of the receiving voltage Vr and the DC/DC converter  22   b  from the rectifier circuit  22   a , to perform control by outputting a switch control signal Ss to the switch  28  ringing are. Incidentally, monitoring of the charging power Pc of the DC/DC converter  22   b  is actually may be monitoring the current so that a constant voltage output. 
     The power receiver controller  27 , for example, may be used in combination with those used for the operation control of the sensors and devices provided in the power receiver  2 , also, the dummy load resistor  29  is a known value (e.g., a secondary battery  25  in may be set to the impedance) during charging. However, for example, in a test transmission, to be suitable for the state detection of the power receiver  2 , it is preferably avoided to set an extremely high value and low value. 
     The power source control unit  17 , receives the waveform of the power source coil  11  (power supply coil  11   b ), and controls the output of the amplifier  15  by the amplifier control signal Sa, to control the switching of the matching circuit  16  by the matching control signals Sm. 
     That is, the power source control unit  17  monitors the voltage and current waveforms of the power supply coil  11   b , and by referring to the information stored in the input impedance characteristic and the internal memory which is obtained from the result, the power receiving to be charged. It has a function of determining the number of devices  2  and the like. Hereinafter, it will be described in detail with reference to  FIG. 4A  to  FIG. 4C . 
     As depicted in  FIG. 4A , when the first power receiver  2 - 1  and the N-th power receiver  2 -N are normally used, in each power receiver  2  ( 2 - 1 ,  2 -N), the switch  29  is connected to the dummy load resistor  29 , and the connection between the switch  29  and the DC/DC converter  22   b  is cut-off. 
     Incidentally, the normal use state, the secondary battery  25  in the power receiver  2  is, for example, powered to the apparatus  26  described with reference to  FIG. 3  (driver  26   a  and device  26   b ), predetermined processes are performed . The switch  29  in the power receiver  2  is the same connection state in the normal state and the initial state of the test power transfer. 
     On the other hand, the power source  1  is stopped when the respective power receivers  2  are normally used states, and when starting the charging, a full power transfer (mode) is established after starting a test power transfer mode, and power transfer (power supply) to the power receiver  2  may be performed. 
     That is, as depicted in  FIG. 4A , in the power source  1 , the power source control unit  17  controls the amplifier  15  by an amplifier control signal Sa so as to supply power for a test power transfer to the power source coil  11  (power supply coil  11   b ) via a matching circuit  16 . 
     The power from the power source coil  11  (power source resonance coil  11   a ) is output to the power receiver coil  21  (power receiver resonance coil  21   a ) of the N power receivers  2 - 1  to  2 -N. Note that, in the initial state of the test power transfer mode, the switch  29  of the power receiver  2  ( 2 - 1 ,  2 -N) is connected to the dummy load resistor  29 , and thus the received voltage Vr from the rectifier circuit  22   a  is applied to the dummy load resistor  29 . 
     In the power source  1 , when starting a test power transfer in the test power transfer mode, waveforms of the power source coil  11  (power supply coil  11   b ) are detected, and verified an impedance with the memory. Specifically, in the power source  1 , the power source control unit  17  receives a coil waveform signal Fc from the power supply coil  11   b , recognizes the number or the like of the power receivers  2 - 1  to  2 -N by referring to an internal memory, establishes a full power transfer mode, and starts a full power transfer by a predetermined power. 
     Note that, the dummy load resistor  29  is, for example, set to a value such as corresponding to the impedance of the secondary battery  25 . Further, an input impedance characteristics (amplitude or phase difference in the voltages, currents, etc.) obtained by waveforms (voltage waveforms or current waveforms) of the power supply coil  11   b  may be detected and referring to characteristics stored in the internal memory. 
     In each power receiver  2 , when the power receiver controller  27  counts a predetermined fixed time from detecting the received voltage Vr output from the rectifier circuit  22   a  or when the received voltage Vr exceeds a predetermined threshold voltage, the connection of the switch  29  is changed from the dummy load resistor  29  to the DC/DC converter  22   b.    
     Specifically, as depicted in  FIG. 4B , in each power receiver  2 , the connection of the switch  29  is switched from the dummy load resistor  29  to the DC/DC converter  22   b , so that the received voltage Vr from the rectifier circuit  22   a  is applied to the DC/DC converter  22   b.    
     Therefore, the power transfer to the power receiver  2 , that is, the charging of the secondary battery  25  of the power receiver  2  is advanced, for example, the first power receiver  2 - 1  becomes fully charged, and the N-th power receiver  2 -N is not fully charged and the charging state is continued. 
     Specifically, as depicted in  FIG. 4C , for example, in the first power receiver  2 - 1  wherein the power receiver controller  27  detects the fully charged state by the charging power Pc, and the power receiver controller  27  controls the switch  28  to an open state (any of the dummy load resistor  29  and the DC/DC converter  22   b  is not connected). 
     Note that, for example, in the N-th power receiver  2 -N, if the power receiver controller  27  judges that the fully charged state is not established by the charging power Pc, the switch  29  is controlled to connect the received voltage Vr from the rectifier circuit  22   a  to the DC/DC converter  22   b  so as to maintain the charging. 
     On the other hand, in the power source  1 , the power source control unit  17  refers to the memory upon detecting a change in the waveforms by the coil waveform signal Fc from the power supply coil  11   b , for example, to estimate the number of power receivers according to the charging target, and controls the amplifier control signal Sa and the matching control signal Sm. 
     Specifically, the power source control unit  17  controls an output of the amplifier  15  suitable for the estimated number of the power receivers and the switching of the matching circuit  16 . Note that, the power source  1  stops an charging operation, or a power transfer operation, for example, if the number of estimated power receivers is equal to or less than a predetermined number or if the charging operation elapses a predetermined time. 
     Therefore, in each power receiver  2 , for example, if the received voltage Vr from the rectifier circuit  22   a  is no longer detected, it is determined that the power transfer (charging) is completed, so that the switch  28  is connected to the dummy load resistor  29  and returned to a normal operation state. 
     As described above, in the present embodiment, before performing the full power transfer, a test power transfer wherein the dummy load resistor  29  for calibration including an impedance equivalent to the secondary battery  25  is connected is performed. 
     The characteristics required from the waveform of the test transmission time of the power source coil  11  (power supply coil  11   b ) (the input impedance, etc.) is detected, and matching the characteristics of the recorded power receiver in the memory. Then, for example, after a test transmission has passed a predetermined time, I do the transmission are switched to connect the secondary battery  25  of the switch  29  of the power receiver  2  (DC/DC converter  22   b ). 
     When the secondary battery is fully charged during normal power transmission, in the power receiver  2  ( 2 - 1 ), the charging path is opened by controlling the switch  28 . In this case, the power source  1 , the waveform of the power source coil  11  (power supply coil  11   b ) is changed according to the input impedance changes. Therefore, the number of secondary batteries  25  to be charged object (the number of powered devices for charging), is estimated by referring to the memory, and it is possible to appropriately perform the adjustments and switching of the transmission power and the matching circuit. 
     As will be described later with reference to  FIG. 12 , instead of opening the charging path by controlling the switch  28 , the power receiver which is fully charged may be determined by connecting to a high-impedance resistance of which resistance value is previously known. 
     Further, in the application of the present embodiment, a wireless power transfer system may be preferable wherein the number of the power receivers  2  ( 2 - 1  to  2 - n ) and the locations thereof with respect to the power source  1  are fixed. Nevertheless, the application of the present embodiment is not limited to such a wireless power transfer system including predetermined number and locations of the power receivers. 
     According to the wireless power transfer system of the first embodiment, the power receiving communication circuit used for the power receiving operation is removed, and therefore, simplification of the wireless power transfer system, as well as reducing the size and consumption power of the power receiver may be possible. Note that this effect is exerted as well as in other embodiments. 
       FIG. 5  is a diagram for explaining an impedance characteristic of a resonance coil of a power source, wherein the power source resonance coil  11   a  (power source coil  11 , power supply coil  11   b ) and the power receiver resonance coil  21   a  (power receiver coil  21 , power external coil  21   b ) are illustrated as equivalent circuits. Note that, an input impedance Zin is defined at an input port of the power source coil  11 . 
     The references C 1 , L 1 , R 1  and I 1  denote equivalent values of capacitance, inductance, resistance and current of the power source resonance coil  11   a , and references C 2 , L 2 , R 2  and I 2  denote equivalent values of capacitance, inductance, resistance and current of the power receiver resonance coil  21   a . Further, references R L  denotes a value of a load in the power receiver  2 , Vin denotes an input voltage of a power source resonance coil  11   a , and M denotes a transfer efficiency between the power source resonance coil  11   a  and the power receiver resonance coil  21   a.    
     Note that, circuit equations with respect to the power source resonance coil  11   a  and the power receiver resonance coil  21   a  (power source and receiver coils) may be described below. 
       ( R   1   +jωL   1 +1 /ωC   1 )× I   1   +jωMI   2   =V in
 
         jωMI   1 +( R   2   +jωL   2 +1 /jωC   2   +R   L )× I   2 =0
 
     In the above simultaneous equations, I 1  may be obtained, and thus input impedance Zin may be obtained by the following equation. 
         Z in= V in/ I   1 =( R   1   +jωL   1 +1 /ωC   1 )× I   1 +(ω M ) 2 /( R   2   +jωL   2 +1 /jωC   2   +R   L )
 
     Therefore, in the power source  1 , the impedance Zin may be calculated by measuring the input voltage Vin and the current I 1 , for example, by comparing (referring to) contents of the memory provided in the power source control unit  17 , it is possible to recognize various kinds of information in the power receiver  2 . 
     Specifically, an right-hand side of the equation Zin includes the load R L  of the power receiver  2  side, and thus Zin may be varied when a value of the load R L  is changed. Note that, when the number of the power receiving coils  21  (power receiver resonance coils  21   a : power sources) is increased, various kinds of information in the plurality of power receivers  2 - 1  to  2 -N may be recognized from the impedances Zin. 
       FIG. 6  is a block diagram illustrating an example of a power receiver controller in the first embodiment of the wireless power transfer system. As depicted in  FIG. 6 , the power receiver controller  27  of the first embodiment includes a microcontroller  271 , a digital input/output unit (DIO)  272 , an analog-to-digital converter (ADC)  273 , a memory  274  and a timer  275 . 
     The ADC  273  receives a received voltage (analog value) Vr from a rectifier circuit  22   a  and a charging power (analog value) Pc from a DC/DC converter  22   b , converts to a digital value, and outputs the digital value to the microcontroller  271 . 
     The microcontroller  271  receives information from the memory  274  and the timer  275 , and performs various controls. Note that, the DIO  272  receives a signal from the microcontroller  271 , and outputs a switch control signal Ss to the switch  28 . 
       FIG. 7  is a diagram illustrating signal waveforms of the power source and the power receiver in the first embodiment of the wireless power transfer system. Note that references Fc denotes a waveform of the power source coil  11  (power supply coil  11   b ) of the power source  1 , and Vr denotes an output voltage (received voltage) of the rectifier circuit  22   a , and Ss denotes the switch control signal output from the power receiver controller  27  (microcontroller  271 ). 
     In the wireless power transfer system of the first embodiment, the microcontroller  271  of the power receiver controller  27  monitors the received voltage Vr input through the ADC  273 , and switches to a full power transfer after starting a test power transfer by a predetermined time (X seconds). 
     For example, when the microcontroller  271  detects that a voltage level of the received voltage Vr becomes to V 0 , it is recognized that the test power transfer is started from time T 0 , and a time measurement is started by using the timer  275 . Further, if the time measured by the timer  275  exceeds a predetermined X seconds, the microcontroller  271  controls the switch  28  by the switch control signal Ss. 
     Specifically, applying of the received voltage Vr from the rectifier  22   a  is switched from the dummy load resistor  29  to the DC/DC converter  22   b , and charging of the secondary battery  25  (full power transfer) is performed. Note that, when the full power transfer is started, the voltage level of the received voltage Vr is changed from V 0  of the test power transfer to a predetermined charging voltage V 1 . 
     Further, the microcontroller  271  monitors the charging power Pc and, for example, when detecting that the secondary battery  25  is fully charged, the microcontroller  271  controls the switch  28  by the switch control signal Ss so as to apply the received voltage Vr to the dummy load resistor  29 . 
     Alternatively, the microcontroller  271  monitors the received voltage Vr and, for example, when detecting that the power source  1  stops power transfer, the microcontroller  271  controls the switch  28  by the switch control signal Ss so as to apply the received voltage Vr to the dummy load resistor  29 . 
       FIG. 8  is a flowchart for explaining an example of a power transfer process in the first embodiment of the wireless power transfer system. Note that, all of the power receivers  2 - 1  to  2 -N perform the same processes, and therefore, in the following description, explanations will be focused on the power receiver  2 - 1 . 
     First, the power receiver  2 - 1  ( 2 - 1  to  2 -N) is used in a normal state, that is, in the case of consuming electric power stored in the secondary battery  25 , the switch  28  connects the battery  25  to the dummy load resistor  29  (step ST 20 ). 
     When a wireless power transfer process (charging process) is started, the power source  1  performs a test power transfer in a test power transfer mode so as to check whether or not power receivers  2 - 1  to  2 -N are located in a predetermined area and in a predetermined number, before performing a full power transfer in a full power transfer mode (charging mode). 
     Specifically, in the test transfer mode, the power source  1  sets an output of the amplifier  15  and the matching circuit  16  to those of the test transfer mode in accordance with an instruction from the power source control unit  17  (step ST 10 ), and starts the test power transfer (step ST 11 ). 
     In this case, as described above, the power receiver  2 - 1  may perform a test power transfer of transferring a relatively small power between the power source  1 , since the switch  28  connects to the dummy load resistor  29  in an initial state of the test power transfer. 
     Further, the power receiver  2 - 1  detects the received voltage Vr by using the power receiver controller  27  (step ST 21 : YES), and judges whether or not a predetermined received voltage (for example, voltage level V 0  depicted in  FIG. 7 ) is maintained during to pass predetermined X seconds (step ST 22 ). 
     In the power receiver  2 - 1 , when a predetermined received voltage is detected (step ST 21 : YES), and when this state is maintained over X seconds (step ST 22 : YES), the switch  28  connects to a DC/DC converter  22   b  (secondary battery  25 ). 
     In the power source  1 , for example, the power source control unit  17  detects an impedance of the power supply coil  11   b  (step ST 12 ), and confirms the power receivers  2 - 1  to  2 -N by the detected impedance characteristics with reference to the memory (step ST 13 ). 
     Specifically, in the power source  1  side, by monitoring the waveforms of the power supply coil  11   b  (wireless power transfer unit  11 ) with reference to memory information, and judging whether or not a predetermined number of power receivers are located in a predetermined area, the test power transfer may be stopped (step ST 14 ). 
     Note that, in the test power transfer, relative small power transfer may be performed, however, a prescribed large electric power (V 2 /RL) determined by a set output voltage (V) and a load (RL) may be transferred in the case of performing a full power transfer for transferring electric power via the DC/DC converter  22   b  to the secondary battery  25 . 
     Further, even if the secondary battery  25  was already fully charged, it is easily performed to judge the number and the locations of the power receivers by monitoring the coil waveforms with reference to the memory information in the power source  1  side. 
     Note that, in the test power transfer mode, the power source  1  may be changed to the full power transfer mode when a predetermined time (X seconds) elapses. Further, in the power receiver  2 - 1  ( 2 - 1  to  2 -N) the switch  28  is switched to the secondary battery  25  (DC/DC converter  22   b ) when a state of detecting a predetermined received voltage is maintained over X seconds. 
     Next, in the full power transfer mode, the power source  1  performs the setting of the amplifier  15  and the matching circuit  16  based on the results obtained in the test power transfer mode and starts the power transfer at a predetermined electric power (step ST 15 ). Note that, the power source control unit  17  performs continuously the monitoring of the coil waveforms (waveforms of the power supply coil  11   b ). 
     On the other hand, in the power receiver  2 - 1 , the switch  28  is switched to connect to the DC/DC converter  22   b  (secondary battery  25 ). Then, the power receiver controller  27  performs the monitoring of an output power (charging power) Pc of the DC/DC converter  22   b  (step ST 24 ), and the monitoring of an output voltage (received voltage) Vr of the rectifier circuit  22   a  (step ST 25 ). 
     If a fully charged state is detected by monitoring the charging power Pc (step ST 25 : YES), the switch  28  is opened (step ST 26 ), specifically, the switch  28  is controlled so as not to connect to any of the DC/DC converter  22   b  and the dummy load resistor  29 . After that, the monitoring of the received voltage Vr may be performed (the step ST 27 ). 
     If a power transfer stop is detected by monitoring the received voltage Vr (step ST 24 , ST 27 : YES), the switch  28  is controlled to connect to the dummy load resistor  29  (step ST 20 ), and is returned to the initial state. 
     Therefore, the power receiver controller  27  detects that the output power Pc of the DC/DC converter  22   b  becomes lower than a predetermined value, for example, if it the secondary battery  25  is turned around the fully charged state, the switch is opened (or connected to high impedance). 
     Note that, the power receiver controller  27 , which detects a completion of the charged state of the secondary battery  25  by the output power Pc, the completion of the charged state is not limited to the state of full charged, and a predetermined charging rate (e.g., 80% of full charged state) may be detected to control the switch  28 . These processes are performed independently in each of the power receiver  2 - 1  to  2 -N. 
     Therefore, for example, a power receiver wherein a secondary battery  25  is full charged state and the switch is opened, and a power receiver wherein the switch  28  is controlled to connect to the DC/DC converter  22   b  and the charging of power is maintained may be both included. 
     In this way, in the power receivers  2 - 1  to  2 -N, when the switch  28  is opened (or connected to high impedance), in the power source  1  side, changes in the waveforms of the coil (power supply coil  11   b ) may be appeared by changing the impedance (step ST 17 : YES). 
     The power source control unit  17  estimates a full charged power receiver (or a charging target power receiver continuously to be charged) with reference to the changed waveforms and the memory information (step ST 18 ). Further, the power source control unit  17  outputs a control signal for controlling the switching of an output of the amplifier  15  and the matching circuit  16  in accordance with estimated results, and changes the setting of the full power transfer (step ST 19 ). 
     Note that, in the power receivers  2 - 1  to  2 -N wherein the switch  28  is opened by the full charged state (or high impedance connection), the received voltage Vr from the rectifier circuit  22   a  is continuously monitored, and when the power transfer stop is detected, a power transfer stop process may be performed (step ST 16 : YES). 
     That is, in the power transfer stop process, the power source  1  is, for example, interrupts the output of the amplifier  15  based on a stop instruction from an operator. Specifically, when the power transfer is stopped, the received voltage Vr from the rectifier circuit  22   a  of the power receiver  2 - 1  to  2 -N is, for example, dropped to near zero volts (0V). 
     The power receiver controller  27  of the power receiver  2 - 1  to  2 -N judges that the power transfer is stopped, when detecting a decrease of the received voltage Vr, the switch  28  is controlled to connect to the dummy load resistor, and is returned to the initial state (test power transfer mode). 
     Note that, with respect to the received voltage Vr from the rectifier circuit  22   a , even though voltage values of a charging state (which connects to the DC/DC converter  22   b ) and a full charged state (which is opened or connected to a high impedance) are different, however, the received voltage Vr is commonly dropped to near 0V when the power transfer is stopped. Therefore, as described above, by setting the threshold to an appropriate value, no problems may be caused, even in the same operation flow. 
     Consequently, in the case of existing no communication means between the power source  1  and the power receivers  2 - 1  to  2 -N, it may be possible to realize a stable wireless power transfer therebetween. 
       FIG. 9  is a block diagram illustrating an example of a power receiver controller in a second embodiment of a wireless power transfer system, and  FIG. 10  is a diagram illustrating signal waveforms of a power source and a power receiver in the second embodiment of the wireless power transfer system. 
     As apparently depicted from a comparison of  FIG. 9  and above described  FIG. 6 , a power receiver controller  27  of the second embodiment includes a comparator  276  in place of the timer  275  of the first embodiment. In the wireless power transfer system of the second embodiment, the microcontroller  271  of the power receiver controller  27  monitors the received voltage Vr input through the ADC  273 , and compares the received voltage Vr by the comparator  276 . 
     Specifically, when detecting the voltage level of the received voltage Vr is changed from a voltage V 0  for the test power transfer to a voltage V 1  for the full power transfer which exceeds a predetermined threshold voltage V 2  by using the comparator  276 , the switch  28  is controlled by the switch control signal Ss. Note that, the other processes are the same as those of the first embodiment, and the explanations thereof will be omitted. 
       FIG. 11  is a flowchart for explaining an example of a power transfer process in the second embodiment of the wireless power transfer system. Note that, as apparently depicted from a comparison of  FIG. 11  and above described  FIG. 8 , processes of the power source  1  are commonly performed in the first and second embodiments. Further, in the wireless power transfer system of the second embodiment, processes of the power receivers  2 - 1  to  2 -N are the same as the processes of the first embodiment except that the process of step ST 22  of the first embodiment is changed to a process of step ST 32 . 
     Specifically, in the power receiver  2 - 1 , the power receiver controller  27  detects the received voltage Vr (step ST 21 : YES), and determines whether or not a voltage level of the received voltage Vr exceeds a predetermined threshold voltage (for example, a threshold voltage V 2  depicted in  FIG. 10 ) (step ST 32 ). 
     That is, in the power receiver  2 - 1 , it is judged that the received voltage Vr exceeds the threshold voltage (V 2 ) (step ST 32 : YES), the switch  28  connects to the DC/DC converter  22   b  (secondary battery  25 ). The other processes are the same as those described with reference to  FIG. 8 , and the explanations thereof will be omitted. 
     As described above, in the wireless power transfer system of the second embodiment, the microcontroller  271  prepares the full power transfer by controlling the switch  28  to connect from the dummy load resistor  29  to the DC/DC converter  22   b , when the voltage level of the received voltage exceeds the predetermined threshold voltage (V 2 ). 
       FIG. 12  is a diagram for explaining a power receiver of a third embodiment of a wireless power transfer system. As depicted in  FIG. 12 , in the wireless power transfer system of the third embodiment, a switch  28  of a power receiver  2  ( 2 - 1  to  2 -N) selects from a dummy load resistor  29 , a DC/DC converter  22   b  and a high-impedance resistor  30 , and connects the selected one to an output of a rectifier circuit  22   a.    
     Specifically, in the first embodiment, as explained with reference to  FIG. 4A  to  FIG. 4C , for example, when the secondary battery  25  is fully charged, the switch  28  is controlled to an open state (any of the dummy load resistor  29  and the DC/DC converter  22   b  are not connected). 
     In contrast, in the third embodiment, for example, when the secondary battery  25  is fully charged, the switch  28  is controlled to apply the received voltage Vr from the rectifier circuit  22   a  to the high-impedance resistor  30 . 
     For example, by setting the high-impedance resistors  30  in respective power receivers to different values, the power source  1  may identify a power receiver that has been fully charged. 
     Note that, the high impedance resistor  30  of each of the power receivers  2 - 1  to  2 -N may be set to a value which enables to identify a waveform of the power source coil  11  (power supply coil  11   b ) in the power source  1 , when applying the received voltage Vr to the high impedance resistors  30 . Specifically, resistance values of the high impedance resistors (power receiver identification resistors)  30  may be set, for example, as a resistance value of a first power receiver  2 - 1  is set to 1000Ω, a resistance value of a second power receiver  2 - 2  is set to 2000Ω, . . . . 
     In the above descriptions, a power transfer (transmission) using magnetic field resonance is mainly described, but the present embodiment is possible to apply a power transfer using electric field resonance or a power transfer using electromagnetic induction or electric field induced. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.