Abstract:
A wireless power transfer system includes a power transmitter configured to wirelessly transfer alternating-current power components of different frequencies simultaneously, the different frequencies including at least a first frequency and a second frequency, and a power receiver having a rectifier circuit configured to convert the alternating-current power components to a direct-current power component, wherein the first frequency is 0.5 MHz to 10 GHz, and the second frequency is 10 Hz to 300 kHz lower than the first frequency.

Description:
CROSS REFERENCE 
       [0001]    This application is a continuation application filed under 35 U.S.C. 111(a) and claims benefit under 35 U.S.C. 120 and 365(c) of PCT International Application No. PCT/JP2014/056176 filed on Mar. 10, 2014 and designating the United States, which International Application claims the priority of Japanese Patent Application No. 2013-050617 filed Mar. 13, 2013, both applications being incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a wireless power transfer system and a wireless power transmitter to transfer power to a wireless power receiver. 
       BACKGROUND ART 
       [0003]    In recent years studies have been actively made on wireless power transfer or wireless charging systems for charging electric vehicles, mobile devices, flat-panel TV sets, etc., without using solid wires or cables. Especially, wireless charging making use of electric field coupling (also called capacitive coupling) is attracting attention, which technique allows a battery of a consuming device to be charged without physical connection between the electrodes of a power transmitter and a power receiver. See, for example, Japanese Laid-open Patent Publication Nos. 2010-148287 and 2012-034447. Wireless power transfer using electromagnetic induction or magnetic resonance is also known, in addition to the electric field coupling scheme. Using a higher frequency band when performing wireless power transfer is advantageous because high-power transmission is achieved even if the coil turns or the permittivity of the medium existing between electrodes is small. 
         [0004]    In wireless power transfer, alternate current (AC) has to be converted to direct current (DC) at the power receiver side. At present, a satisfactory mechanism for rectifying a high frequency current has not been realized. The higher the frequency, the more energy to be lost, and the electric power efficiency in AC-to-DC conversion falls. For example, when using a diode as a rectifier, a portion of the high frequency current passes through without being rectified, depending on the parasitic capacitance (10 pF to 100 pF) generated in the depletion layer between the anode and the cathode of the diode. The alternating current (or power) having passed through the diode is consumed in a smoothing capacitor, and accordingly, the power transfer efficiency is degraded. It may be conceived to increase the high-frequency output level at the power transmitter side to achieve high power transfer. However, as the high-frequency output level increases, the loss in the rectifier also increases and the electric power efficiency cannot be improved. Besides, the capacitor may be destroyed due to the increased high-frequency output level. 
       SUMMARY 
       [0005]    There is a demand for a structure and a technique that can improve electric power efficiency in high-frequency wireless power transfer. 
         [0006]    It is an objective of the invention to provide a wireless power transfer system and a wireless power transmitter that can improve electric power efficiency using a simple structure. 
         [0007]    To achieve the objective, at least two alternating-current power components of different frequencies are transferred simultaneously, whereby the electric power efficiency in AC-to-DC conversion is improved at a power receiver side. 
         [0008]    In one aspect of the invention, a wireless power transfer system includes 
         [0009]    a power transmitter configured to wirelessly transfer alternating-current power components of a first frequency and a second frequency simultaneously, and 
         [0010]    a power receiver having a rectifier circuit configured to convert the alternating-current power components to a direct-current power component, 
         [0011]    wherein the first frequency is 0.5 MHz to 10 GHz, and the second frequency is 10 Hz to 300 kHz lower than the first frequency. 
         [0012]    In a preferred example, the alternating-current power components with the first frequency and the second frequency may be superimposed to produce a mixed wave in the power transmitter, and the mixed wave may be transferred wirelessly to the power receiver. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic block diagram of a wireless power transfer system according to the embodiment of the invention; 
           [0014]      FIG. 2  is a diagram illustrating an exemplified structure of a wireless power transfer system according to the embodiment of the invention; 
           [0015]      FIG. 3  is a chart illustrating an alternating-current power transfer efficiency of two mixed waves with the first and the second frequencies superimposed with each other, compared with that of a wave with the first frequency component only; 
           [0016]      FIG. 4  illustrates the first modification of the wireless power transfer system according to the embodiment, in which a resonant circuit is inserted in the power receiver; 
           [0017]      FIG. 5  illustrates the second modification of the wireless power transfer system according to the embodiment, which system is applied to an electromagnetic induction scheme and a magnetic resonance scheme; 
           [0018]      FIG. 6  illustrates the third modification of the wireless power transfer system of an electric field coupling scheme according to the embodiment, in which a parallel-resonant circuit is inserted in the power transmitter; and 
           [0019]      FIG. 7  illustrates the fourth modification of the wireless power transfer system according to the embodiment, in which parallel-resonant circuits are inserted in the power transmitter and the power receiver, respectively. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0020]    The embodiment of the invention is explained below with reference to the drawings. In the embodiment, a waveform generator  21  is used as an alternating-current (AC) power source to improve the efficiency for high-frequency electric power with a simple structure. 
         [0021]      FIG. 1  is a schematic block diagram of a wireless power transfer system  10  according to an embodiment of the invention. The wireless power transfer system  10  includes a wireless power transmitter (hereinafter referred to simply as “power transmitter”)  20  and a wireless power receiver (hereinafter referred to simply as “power receiver”)  30 . The wireless power transfer system  10  employs, for example, an electric field coupling scheme. The power transmitter  20  has power transmitting electrodes  29   a  and  29   b,  and the power receiver  30  has power receiving electrodes  31   a  and  31   b.  The power transmitting electrodes  29   a  and  29   b  and the power receiving electrodes  31   a  and  31   b  may be plate electrodes with a desired thickness formed of any suitable material such as a metal, a metal oxide, or carbon. The power transmitting electrodes  29   a  and  29   b  serve as contactless power transmission terminals. In operations, the power transmitting electrode  29   a  and the power receiving electrode  31   a  face each other, while the power transmitting electrode  29   b  and the power receiving electrode  31   b  face each other. Power is transferred from the power transmitter  20  to the power receiver  30  by means of electric field coupling between the facing electrode pairs. 
         [0022]    The power transmitter  20  may be a stationary apparatus, and the power receiver  30  may be a mobile object. When bringing the power receiving electrodes  31   a  and  31   b  of the power receiver  30  face to face with the power transmitting electrodes  29   a  and  29   b  of the power transmitter  20  via an arbitrary medium (such as the air, a resin sheet, a resin panel, etc.), wireless power transfer from the power transmitter  20  to the power receiver  30  is carried out. 
         [0023]    The power transmitter  20  also has a waveform generator  21  serving as an AC power source, a first coil L 21 , and a second coil L 22  in addition to the power transmitting electrodes  29   a  and  29   b.    
         [0024]    The power receiver  30  has a rectifier device D 31  and a load R 31 . The rectifier device D 31  may be a bridge rectifier circuit using diodes or a synchronous rectifier circuit using MOSFETs. 
         [0025]      FIG. 2  is a schematic diagram illustrating a wireless power transfer system  10 A according to an embodiment of the invention. The same elements or parts as those in  FIG. 1  are denoted by the same symbols and redundant explanation is omitted. The waveform generator  21 A has a first frequency power generator  22 , a second frequency power generator  25 , and a mixer  23 . 
         [0026]    The first frequency power generator  22  generates AC power at the first frequency, which frequency is 0.5 MHz to 10 GHz, preferably from 1 MHz to 30 MHz, and more preferably from 2 MHz to 15 MHz. If the first frequency is less than 0.5 MHz, it is unnecessary to produce a mixed wave because the efficiency of the rectifier device (e.g., the rectifier diode) is not so bad. If the first frequency is higher than 10 GHz, effective improvement in the electric power efficiency is not expected. 
         [0027]    The second frequency power generator  25  generates AC power at the second frequency, which frequency is lower than the first frequency by 10 Hz to 300 kHz, preferably by 20 Hz to 150 kHz, and more preferably by 40 Hz to 100 KHz. If the difference between the first frequency and the second frequency is less than 10 Hz, the electric power efficiency is significantly degraded during wireless power transfer via a dielectric medium. If the difference between the first and the second frequencies is greater than 300 kHz, the advantageous effect of the mixed wave is less likely to be seen. In the embodiment, the mixed wave is used to improve the electric power efficiency during the wireless power transfer process. 
         [0028]    The power levels of the first frequency AC power and the second frequency AC power are preferably similar to each other as much as possible. A certain degree of difference between the first frequency and the second frequency AC power levels may be acceptable. The advantageous effect of improving the electric power efficiency is still achieved unless the difference exceeds twice the power level of the lower one. 
         [0029]    The mixer  23  superimposes the first frequency AC power C generated at the first frequency power generator  22  and the second frequency AC power S generated at the second frequency power generator  25 , and outputs a mixed wave D. When the first frequency AC power C and the second frequency AC power S are input to the mixer  23 , an amplitude-modulated wave with a beat frequency corresponding to a frequency difference between the two AC power components is produced. The amplitude-modulated wave is output as the mixed wave D from the mixer  23 . The mixed wave D is also the output of the waveform generator  21 A. 
         [0030]    The electric power of the mixed wave D is supplied as an alternating current via the first coil L 21  and the second coil L 22 , to the power transmitting electrodes  29   a  and  29   b.    
         [0031]    The power receiver  30  has a coil L 31 , a rectifier device (such as a rectifier diode) D 31 , and a load R 31 , in addition to the electrodes  31   a  and  31   b.  By bringing the electrodes  31   a  and  31   b  face to face with the electrodes  29   a  and  29   b  of the power transmitter  20 , the electric field is coupled between the electrodes  29   a  and  31   a  and between the electrodes  29   b  and  31   b.  As a result, an alternating current is inducted in the power receiver  30 . 
         [0032]    The coil L 31  serves as an antireflection filter to prevent the power transferred to the power receiver  30  from being reflected to the power transmitter  20 . The antireflection filter is not limited to the coil L 31 . A capacitor may be used as the antireflection filter in place of the coil L 31 . 
         [0033]    The AC power having passed through the coil L 31  is rectified at a rectifier device (such as a rectifier diode) D 31 . In general, during rectification using a rectifier diode, the electric power efficiency is likely to degrade especially at a high frequency band. The configuration of the embodiment can improve the electric power efficiency by using a mixed wave even if a rectifier diode is used for rectification. 
         [0034]    The direct current rectified by the rectifier device D 31  is supplied to the load R 31 . 
         [0035]      FIG. 3  is a chart illustrating the advantageous effect of improvement of the electric power efficiency by the wireless power transfer system  10 A. The electric power efficiency achieved by supplying a mixed wave of a first frequency component and a second frequency component which is 20 kHz lower than the first frequency, as well as the electric power efficiency achieved by supplying a mixed wave of a first frequency component and a second frequency component which is 100 kHz lower than the first frequency, are plotted as a function of the first frequency, compared with that of a single frequency component (using only the first frequency). The electric power efficiency achieved by the first frequency of 1 MHz is set to the 100% efficiency, and then the first frequency is increased. Changes in the electric power efficiency according to the increase in the first frequency are illustrated. The electric power efficiency is estimated by measuring the received power level at each value of the first frequency and dividing the measured power level by the reference power level received at 1 MHz of the first frequency. As the first frequency is increased from 1 MHz to 2 MHz, each of the electric power efficiencies of the mixed wave is well maintained, compared with the first-frequency-only wave. The efficiency keeping effect of the mixed waves becomes more apparent above 2 MHz, and it becomes more conspicuous above 4 MHz. This effect is maintained even at higher frequency. 
         [0036]    By applying the waveform generator  21  of the embodiment to the power transmitter of a wireless power transfer system, degradation in the electric power efficiency can be prevented across a wide frequency band. 
         [0037]    In the foregoing embodiment, the AC power levels of the first and the second frequencies are adjusted so as to be equal to each other. This applies to other modifications of the invention unless otherwise noted. 
         [0038]      FIG. 4  is a schematic diagram of a wireless power transfer system  10 B, which system is illustrated as the first modification of the invention. The wireless power transfer system  10 B includes a power transmitter  20  and a power receiver  50 . The wireless power transfer system  10 B employs an electric field coupling scheme, as in the wireless power transfer system  10 A of  FIG. 2 . A parallel-resonant circuit  55  is inserted in the power receiver  50 . The parallel-resonant circuit  55  has a coil L 52  and a capacitor C 51  connected in parallel. 
         [0039]    In the power transmitter  20 , a waveform generator  21  is used as an AC power source. The waveform generator  21  may have the same structure as the waveform generator  21 A of  FIG. 2 . The mixed wave is supplied as an alternating current via the first coil L 21  and the second coil L 22  to the power transmitting electrodes  29   a  and  29   b.  Making use of electric field coupling between the power transmitting electrode  29   a  and the power receiving electrode  51   a,  and between the power transmitting electrode  29   b  and the power receiving electrode  51   b , AC power is transferred to the power receiver  50 . An alternating current is induced in the parallel-resonant circuit  55  due to electromagnetic induction between the coil L 51  and the coil L 52  of the parallel-resonant circuit  55 . With this configuration, even if the coupling capacitance between the facing electrodes changes, the frequency of the parallel-resonant circuit  55  is less affected by the change. Besides, by inserting the parallel-resonant circuit  55 , the resonance peak with respect to the power transferred to the power receiver  50  becomes sharp. Accordingly, the AC power transferred from the power transmitter  20  can be supplied efficiently to the rectifier device (such as a rectifier diode) D 51 . 
         [0040]      FIG. 5  is a schematic diagram of a wireless power transfer system  10 C, which system is illustrated as the second modification of the invention. The wireless power transfer system  10 C employs an electromagnetic induction scheme or a magnetic field resonance scheme, and it includes a power transmitter  60  and a power receiver  70 . 
         [0041]    The power transmitter  60  has a waveform generator  21 , a first coil L 61 , a second coil L 62 , and a third coil L 63 . The third coil L 63  serves as a contactless power transmission terminal. The AC power of the mixed wave produced by the waveform generator  21  is supplied through the first coil L 61  and the second coil L 62  to the third coil L 63 , and an alternating current flows in the third coil L 63 . The magnetic fluxes generated at the third coil L 63  penetrate through the coil L 71  of the power receiver  70 , and electromotive force is generated (electromagnetic induction). 
         [0042]    When the third coil L 63  of the power transmitter  60  and the coil L 71  of the power receiver  70  have the same resonant frequency, the magnetic field resonance scheme applies. In this case, the energy (that is, the oscillation of the magnetic field) of the mixed wave generated by the waveform generator  21  is transferred to the coil L 71  of the power receiver  70  by the magnetic field resonance. Employing the magnetic field resonance scheme is advantageous from the viewpoint of less degradation in the electric power efficiency with respect to positional offset between the power transmitter  60  and the power receiver  70 . 
         [0043]    The power receiver  70  has an LC resonant circuit  75 . The alternating current flowing via the coil L 72  in the LC resonant circuit  75  is rectified by a rectifier device (such as a rectifier diode) D 71 , and is consumed at the load R 71 . This configuration also allows the mixed wave alternating current to be rectified efficiently at the rectifier device D 71 . 
         [0044]      FIG. 6  is a schematic diagram of a wireless power transfer system  10 D, which system is illustrated as the third modification of the invention. The wireless power transfer system  10 D employs an electric field coupling scheme, and it includes a power transmitter  80  and a power receiver  90 . In the wireless power transfer system  10 D, the power transmitter  80  has an LC resonant circuit  87 , while the LC resonant circuit is omitted from the power receiver  90 . 
         [0045]    The power transmitter  80  has a waveform generator  21 , a coil L 81 , an LC resonant circuit  87 , and power transmitting electrodes  89   a  and  89   b.  The LC resonant circuit  87  has a coil L 82  and a capacitor C 81  connected in parallel. The AC power of the mixed wave generated by the waveform generator  21  is transferred via the coil L 81  to the LC resonant circuit  87 , and supplied as an alternating current to the power transmitting electrodes  89   a  and  89   b.  The AC power of the mixed wave is transferred to the power receiver  90  by means of the electric field coupling between the power transmitting electrode  89   a  and the power receiving electrode  91   a , and between the power transmitting electrode  89   b  and the power receiving electrode  91   b.  The AC power of the mixed wave supplied via the coil L 91  to the rectifier device (such as a rectifier diode) D 91  and rectified. The rectified current is consumed by the load R 91 . The coil L 91  serves as an antireflection filter; however, the invention is not limited to this example. A capacitor may be used as the antireflection filter. 
         [0046]    By transferring power or energy in the form of a mixed wave, loss in the rectifier device D 91  of the power receiver  90  can be reduced. By providing the LC resonant circuit  87  only in the power transmitter  80 , tuning is facilitated and the number of components used in the overall system is reduced. In addition, the power receiver  90  can be made compact. 
         [0047]      FIG. 7  is a schematic diagram of a wireless power transfer system  10 E, which system is illustrated as the fourth modification of the invention. The wireless power transfer system  10 E employs an electric field coupling scheme, and it includes a power transmitter  120  and a power receiver  130 . In the wireless power transfer system  10 E, each of the power transmitter  120  and the power receiver  130  has a parallel-resonant circuit. 
         [0048]    The power transmitter  120  has a waveform generator  21 , a coil L 21 , an LC resonant circuit  127 , and power transmitting electrodes  29   a  and  29   b.  The power receiver  130  has power receiving electrodes  131   a  and  131   b,  an LC resonant circuit  137 , a coil L 132 , a rectifier device (such as a rectifier diode) D 131 , and a load R 131 . 
         [0049]    The waveform generator  21  may have the same structure as that illustrated in  FIG. 2 . The transmitter-side LC resonant circuit  127  and the receiver-side LC resonant circuit  137  are coupled by the electric field between the electrodes  29   a  and  131   a  and between the electrodes  29   b  and  131   b.  The coupling capacities between the facing electrode pairs are independent from the LC resonant circuits  127  and  137 . Accordingly, when the coupling capacities are small compared with the capacitances C 121  and C 131  of the LC resonant circuits  127  and  137 , the frequencies of the LC resonant circuits  127  and  137  are less influenced by change in the coupling capacitance. 
         [0050]    The AC power of the mixed wave transferred to the power receiver  130  is supplied as an alternating current via the coil L 132  to the rectifier device D 131 , and rectified. The rectified current is supplied to the load R 131 . Using the mixed wave, degradation of the rectification efficiency can be reduced and the electric power efficiency is improved. 
         [0051]    As has been explained above, the wireless power transfer system of the invention is applicable to any of the electric field coupling scheme, the electromagnetic induction scheme, the magnetic field resonance scheme, and a radio wave charging scheme. The wireless power transfer system of the invention is also applicable regardless of whether a resonant circuit is used, or regardless of series resonance or parallel resonance. The technique of the embodiments can improve the electric power efficiency in wireless power transfer using an arbitrary scheme or configuration.