No L DETECTION ( 31 ( 41, 42 ) IDC CIRCUIT CONTROLLER

Transcription

1 VDC w SECONDARY 4 VAC CONVERTER HAO WANATHI MOVIE PLANTA BANTAL ATT US B2 ( 12 ) United States Patent Sakai et al. ( 10 ) Patent No. : US 9, 948, 144 B2 ( 45 ) Date of Patent : Apr. 17, 2018 ( 54 ) POWER TRANSMISSION SYSTEM AND POWER TRANSMISSION DEVICE USED FOR POWER TRANSMISSION SYSTEM ( 71 ) Applicant : MURATA MANUFACTURING CO., LTD., Nagaokakyo Shi, Kyoto Fu ( JP ) ( 72 ) Inventors : Hironori Sakai, Nagaokakyo ( JP ) ; ( 56 ) References Cited U. S. PATENT DOCUMENTS 4, 549, 147 A * 10 / 1985 Kondo.... 5, 631, 611 A * 5 / 1997 Luu... Takanori Tsuchiya, Nagaokakyo ( JP ) ( Continued ) FOREIGN PATENT DOCUMENTS ( 73 ) Assignee : MURATA MANUFACTURING CO., LTD., Nagaokakyo Shi ( * ) Notice : Subject to any disclaimer, the term of this patent is extended or adjusted under 35 U. S. C. 154 ( b ) by 183 days. ( 21 ) Appl. No. : 14 / 481, 220 ( 22 ) Filed : Sep. 9, 2014 ( 65 ) Prior Publication Data US 2014 / A1 Dec. 25, 2014 Related U. S. Application Data ( 63 ) Continuation of application PCT / JP2013 / , filed on Feb. 26, ( 30 ) Foreign Application Priority Data, Kyoto Fu ( JP ) JE B2 11 / 2002 No. Mar. 26, 2012 ( JP ) ( 51 ) Int. Ci. H02J 17 / 00 ( ) HO2J 50 / 00 ( ) ( Continued ) ( 52 ) U. S. CI. CPC H02J 50 / 12 ( ) ; H02J ( ) ; H02J 17 / 00 ( ) ; H02J 50 / 05 ( ) ; H02M 7 / 537 ( ) ( 58 ) Field of Classification Search CPC HO2J 17 / 00 ( Continued ) A 9 / 2004 ( Continued ) GO5F 1 / / 207 P HO3H 7 / / OTHER PUBLICATIONS Written Opinion and International Search Report issued in PCT / JP2013 / , dated May 21, Primary Examiner Drew A Dunn Assistant Examiner Jerry D Robbins ( 74 ) Attorney, Agent, or Firm Arent Fox LLP ( 57 ) ABSTRACT A power transmission device wirelessly transmits power to a power receiving device rectifying a voltage received and supplying the voltage to a load circuit. The power transmis sion device includes a power transmitter that makes capaci tance or electromagnetic coupling to the power receiver and transmits electric power to the power receiver, a power supply unit that generates electric power to be transmitted to the power receiving device and supplies the electric power to the power transmitter, a load impedance detector that detects a load impedance of the power receiving device based on an electric signal detected in the power transmis sion device, and a controller that controls the electric power to be generated by the power supply unit such that an output voltage of the rectifying circuit of the power receiving device is equal to or lower than a predetermined voltage based on a detection result of the load impedance detector. 19 Claims, 10 Drawing Sheets Best 5 52AN IT DIRECT 7. CURRENT POWER SUPPLY 54! L. / DC DETECTION ( 31 ( 41, 42 ) VDC IDC NILA CONTROLLER 2005 DRIVE CONTROL L STEP RECTIFYING SWITCHING HBOOSTER DOWN 1 O wy = + LK UNIT DETECTION STORAGE ON / OFF VAC BATTERY DC / DC *

2 US 9, 948, 144 B2 Page 2 ( 51 ) Int. CI / A1 * HO2J H02J 50 / 05 ( / Al HO2J 7 / / A1 * HO2M 7 / 537 ( ) ( 58 ) Field of Classification Search USPC / 107 See application file for complete search history. ( 56 ) References Cited U. S. PATENT DOCUMENTS 8, 604, 873 B2 * 12 / 2013 Scott / A1 * 2012 / A / AL 2013 / A1 * 10 / 2011 Kozakai HO3F 1 / / / 2011 Kai et al. 1 / 2012 Chapman HO2M 3 / / 37 2 / 2012 Ichikawa HO2J 5 / / 97 5 / 2012 Takada et al. 11 / 2012 Ichikawa et al. 4 / 2013 Wheeland H04B 5 / / 713 FOREIGN PATENT DOCUMENTS HO3F 1 / / 104 JP A 12 / / A1 * 7 / 2009 Melde HO1P 5 04 JP / / 32 JP A / A1 * 11 / 2009 Toya HO2J WO WO 2011 / A / 152 WO WO A1 12 / / A1 * 12 / 2009 Kubono HO2J 5 / / 109 * cited by examiner

3 atent Apr. 17, 2018 Sheet 1 of 10 US 9, 948, 144 B2 FIG W WWW www

4 atent Apr. 17, 2018 Sheet 2 of 10 US 9, 948, 144 B2 FIG. 2 RL PS de E mm 200 I www w ww om fattot 36 frotto 5 re 100 TG se i oso

5 ? 1 w atent Apr. 17, 2018 Sheet 3 of 10 US 9, 948, 144 B2 3 FIG. Manaies 200 SECONDARY! BATTERY CONVERTER / DC ww www me N RECTIFYINGI STEP DOWN 40 ( 41, 42 ) ??????????? 12V?? DIRECT CURRENT POWER SUPPLY i 53 5V 30 ( 31, 32 ) DETECTION VDC / I 1 IDC 371 im w il DRIVE CONTROL wre we ww 111 BOOSTER 1 SWITCHING II CONTROLLER OFF 1 ON / 58 STORAGE UNIT DETECTION VAC 57 L VAC wwwwwwwwwwww

6 atent Apr. 17, 2018 Sheet 4 of 10 US 9, 948, 144 B2 FIG. 4 ww 56a ON / OFF O DRIVE CONTROL ww BOOSTER I h w 56b

7 U. S. Patent Apr. 17, 2018 Sheet 5 of 10 US 9, 948, 144 B2 FIG. 5 HIGH LOAD ( a ) IMPEDANCE ( 92 ) CHARGE TIME HIGH ( b ) IDC CHARGE TIME HIGH POWER RECEIVING MODULE OUTPUT VOLTAGE CHARGE TIME

8 U. S. Patent Apr. 17, 2018 Sheet 6 of 10 US 9, 948, 144 B2 FIG. 6 START S11 INITIAL SETTING 512 DETECT IDC 513 TSIDC EQUAL TO OR LOWERE JHAN PREDETERMINED VALUE2 S14 NO SET DUTY RATIO CORRESPONDING TO DETECTED IDC S15 CONTROL SWITCHING AT SET DUTY RATIO YES END

9 U. S. Patent Apr. 17, 2018 Sheet 7 of 10 US 9, 948, 144 B2 FIG. 7 IDC O ~ ~ ~ ~ DUTY RATIO mitt or tw FIG. 8 DUTY RATIO IDC

10 + U. S. Patent Apr. 17, 2018 Sheet 8 of 10 US 9, 948, 144 B2 FIG. 9 POWER RECEIVING MODULE OUTPUT 25 VOLTAGE 1 / FIG IDC POWER RECEIVING MODULE OUTPUT 25 VOLTAGE N N de LOAD IMPEDANCE 10000

11 U. S. Patent Apr. 17, 2018 Sheet 9 of 10 US 9, 948, 144 B2 FIG. 11 HIGH î DUTY RATIO FIG. 12 do IDC wyle HIGH HIGH POWER RECEIVING MODULE OUTPUT VOLTAGE IDC HIGH

12 U. S. Patent Apr. 17, 2018 Sheet 10 of 10 US 9, 948, 144 B2 FIG. 13 DEVICE 84 m 220 POWER CONTROL 83 RECTIFYING VOLTAGE REGULATOR SECONDARY BATTERY * FIG. 14 w ww ws 84 DEVICE 220 " POWER CONTROL 83 DECADES RESERVARAN RECTIFYING VOLTAGE REGULATOR CHARGING CONTROL SECONDARY BATTERY w power W

13 POWER TRANSMISSION SYSTEM AND POWER TRANSMISSION DEVICE USED FOR POWER TRANSMISSION SYSTEM CROSS REFERENCE TO RELATED APPLICATIONS US 9, 948, 144 B2 device. For this reason, the configurations of the power transmission device and the power receiving device become complicated. SUMMARY OF THE INVENTION An object of the present invention is to provide a power The present application is a continuation of PCT / JP2013 / transmission system that can supply appropriate electric filed Feb. 26, 2013, which claims priority to Japa power in accordance with a load state to a power receiving nese Patent Application No , filed Mar. 26, 10 device without providing communication circuits on a 2012, the entire contents of each of which are incorporated power transmission device and the power receiving device, herein by reference. and a power transmission device that is used for the power transmission system. FIELD OF THE INVENTION The present invention provides a power transmission 15 device that wirelessly transmits electric power to a power The present invention relates to a power transmission receiving device rectifying a voltage received by a power system that transmits electric power wirelessly and a power transmission device that is used for the power transmission receiver with a rectifying circuit and supplying the voltage system. to a load circuit. The power transmission device includes a 20 power transmitter that makes capacitance coupling or elec BACKGROUND OF THE INVENTION tromagnetic coupling to the power receiver of the power receiving device and transmits electric power to the power In recent years, for example, a wireless power transmis sion system that supplies electric power to a portable appa receiver, a power supply unit that generates electric power to be transmitted to the power receiving device and supplies ratus such as a smart phone and a laptop personal computer 25 the electric power to the power transmitter, a load impedance wirelessly has been put to practical use. As the wireless detector that detects a load impedance of the power receiv power transmission system, there is a wireless power trans ing device based on an electric signal detected in the power mission system disclosed in Patent Document 1, for transmission device, and a controller that controls the elec example. tric power to be generated by the power supply unit such that Patent Document 1 discloses an electromagnetic induc 30 an output voltage of the rectifying circuit of the power tion type wireless power transmission system. The electro receiving device is equal to or lower than a predetermined magnetic induction type wireless power transmission sys voltage based on a detection result of the load impedance tem includes a power transmission device and a power detector. receiving device. The power transmission device includes a The present invention provides a power transmission power transmission coil, the power receiving device 35 system that includes a power receiving device including a includes a power receiving coil, and electric power is power receiver and a load circuit, and the above mentioned transmitted between these coils by electromagnetic induc power transmission device. tion. The present invention provides a power transmission Patent Document 2 discloses an electric field coupling method in a power transmission device that wirelessly type wireless power transmission system. The electric field 40 transmits electric power to power receiving device recti coupling type wireless power transmission system includes fying a voltage received by a power receiver and supplying a power transmission device and a power receiving device. the voltage to a load circuit. The power transmission method The power transmission device includes a power transmis includes generating electric power to be transmitted to the sion electrode, the power receiving device includes a power power receiving device and supplying the electric power to receiving electrode, and electric power is transmitted 45 the power transmitter, detecting a load impedance of the between these electrodes by electrostatic induction. power receiving device based on an electric signal detected In the above mentioned systems, electric power that is in the power transmission device, and controlling electric transmitted from the power transmission device is required power to be supplied to the power transmitter such that an to be controlled to a value appropriate for the power receiv output voltage of a rectifying circuit of the power receiving ing device in consideration of an operation voltage, a 50 device is equal to or lower than a predetermined voltage withstand voltage, and the like of the power receiving based on a detection result of the load impedance. device. In the power transmission system as disclosed in According to the present invention, a load state of a power Patent Document 1, in order to control the electric power receiving device is detected in a power transmission device. that is transmitted to the value appropriate for the power The electric power to be supplied to the power receiving receiving device, wireless communication circuits are pro 55 device is controlled such that the output voltage of a vided on the power transmission device and the power rectifying circuit of the power receiving device is equal to or receiving device and transmit and receive a control signal to lower than the predetermined voltage based on a detection and from each other. result. Accordingly, appropriate electric power can be sup Patent Document 1 : Japanese Patent No plied to the power receiving device without providing com Patent Document 2 : International Publication Pamphlet 60 munication circuits on the power transmission device and No. WO2011 / the power receiving device. In the power transmission system as disclosed in Patent Document 1, in order to control the electric power that is BRIEF DESCRIPTION OF DRAWINGS transmitted to the value appropriate for a state of the power receiving device, the communication circuits for making 65 FIG. 1 is a perspective view showing a power transmis communication with each other are required to be provided sion device and a power receiving device according to a first on the power transmission device and the power receiving embodiment.

14 US 9, 948, 144 B2 FIG. 2 is an equivalent circuit diagram of a wireless between the power transmission device side passive elec power transmission system. trode 31 and the power transmission device side active FIG. 3 is a block configuration diagram of the power electrode 32. A capacitor CG is a parasitic capacitance that transmission device and the power receiving device. is generated between the power transmission device side FIG. 4 is a diagram illustrating the configuration of a 5 passive electrode 31 and the power transmission device side switching circuit. active electrode 32. The inductor LG and the capacitor CG FIG. 5 are graphs illustrating characteristics of an IDC, a constitute a power transmission device side resonance cir load impedance, an output voltage of a power receiving cuit. A step down circuit 45 constituted by a step down module with respect to charge passage time during charging transformer TL and an inductor LL is connected to and of a secondary battery. 10 interposed between the power receiving device side passive FIG. 6 is a flowchart relating to a control operation with electrode 41 and the power receiving device side active a control circuit. electrode 42 of the power receiving device 200. The step FIG. 7 is a view illustrating a specific example of a setting down circuit 45 steps down a voltage received by the power table of a duty ratio with respect to the IDC. receiving device side passive electrode 41 and the power FIG. 8 is a graph illustrating characteristics of the duty 15 receiving device side active electrode 42 and supplies the ratio with respect to the IDC, which corresponds to the table voltage to a load circuit. A capacitor CL is a parasitic in FIG 7. capacitance that is generated between the power receiving FIG. 9 is a graph illustrating characteristics of the output device side passive electrode 41 and the power receiving voltage of the power receiving module with respect to the device side active electrode 42. The inductor LL and the IDC, which corresponds to the table in FIG capacitor CL constitute a power receiving device side reso FIG. 10 is a graph illustrating the characteristics of the nance circuit. A load circuit RL is connected to the second output voltage of the power receiving module based on a ary side of the step down transformer TL. The load circuit relation with the load impedance. RL is constituted by a DC / DC converter and a secondary FIG. 11 is a graph illustrating characteristics of the duty battery. A capacitor Cm illustrates a state of the capacitance ratio with respect to the IDC. 25 coupling between the power transmission device side pas FIG. 12 is a graph illustrating characteristics of the output sive electrode 31 and the power receiving device side pas voltage of the power receiving module with respect to the sive electrode 41 and the capacitance coupling between the IDC. power transmission device side active electrode 32 and the FIG. 13 is a block configuration diagram illustrating power receiving device side active electrode 42. another example of a load circuit. 30 FIG. 3 is a block diagram illustrating the specific con FIG. 14 is a block configuration diagram illustrating still another example of the load circuit. 1. Configuration figurations of the power transmission device 100 and the power receiving device 200. Hereinafter, the respective specific configurations of the power transmission device 100 DETAILED DESCRIPTION OF THE and the power receiving device 200 will be described. PREFERRED EMBODIMENTS Power Transmission Device The power transmission device 100 includes a power First Embodiment transmitter 30, a power supply unit 54, a controller 52, a VDC / IDC detection circuit 53, and a VAC detection circuit Overall Configuration 40 The power transmitter 30 includes the power transmission FIG. 1 is a perspective view showing a power transmis device side passive electrode 31 and the power transmission sion device 100 and a power receiving device 200 consti device side active electrode 32. tuting an electric field coupling type wireless power trans The power supply unit 54 is a unit for supplying electric mission system in a first embodiment. power to the power transmitter 30 ( power transmission The power transmission device 100 includes a power 45 device side passive electrode 31 and power transmission transmission device side passive electrode 31 and a power device side active electrode 32 ) and includes a direct current transmission device side active electrode 32. The power ( DC ) power supply 51, a power converter 57, and a booster receiving device 200 includes a power receiving device side circuit 37. passive electrode 41 and a power receiving device side The DC power supply 51 is input with a DC voltage of DC active electrode V, generates a DC voltage of DC 5 V, and supplies it to The power receiving device 200 is placed on the power the controller 52. Furthermore, the DC power supply 51 transmission device 100 so that coupling capacitances are supplies the input DC voltage of DC 12 V to the VDC / IDC generated between the power transmission device side pas detection circuit 53 and the power converter 57. sive electrode 31 and the power receiving device side pas The VDC / IDC detection circuit 53 detects a voltage value sive electrode 41 and between the power transmission 55 VDC of the DC voltage that is supplied to a switching circuit device side active electrode 32 and the power receiving 56 from the DC power supply 51 and a current value IDC of device side active electrode 42. In this state, the power an electric current flowing through the switching circuit 56. transmission device 100 transmits electric power to the Furthermore, the VDC / DC detection circuit 53 outputs a power receiving device 200 by electric field coupling. VDC signal relating to the DC voltage value VDC and an FIG. 2 is an equivalent circuit diagram of the wireless 60 IDC signal relating to the DC current value IDC. power transmission system. In FIG. 2, a high frequency The power converter 57 includes the switching circuit 56 voltage generation circuit OSC of the power transmission and a drive control circuit 55. The power converter 57 device 100 generates a high frequency voltage of 100 khz corresponds to the high frequency voltage generation circuit to several tens MHz, for example. A booster circuit 37 OSC as illustrated in FIG. 2. formed by a booster transformer TG and an inductor LG 65 The switching circuit 56 switches the DC voltage of DC boosts the voltage that is generated by the high frequency 12 V supplied from the DC power supply 51 based on a voltage generation circuit OSC and applies the voltage driving signal from the drive control circuit 55 so as to

15 convert it to an alternating current ( AC ) voltage, and outputs US 9, 948, 144 B2 the power transmission device side passive electrode 31 and the AC voltage. FIG. 4 illustrates the specific configuration the power transmission device side active electrode 32, of the switching circuit 56. The switching circuit 56 includes respectively. a high side switch element 56a and a low side switch The step down circuit 45 steps down the voltage between element 566. The switching circuit 56 performs push pull 5 the power receiving device side passive electrode 41 and the operations by turning ON / OFF these switch elements 56a power receiving device side active electrode 42 and supplies and 56b so as to alternately drive the booster circuit 37. it to the load circuit 220 through the rectifying circuit 61. The drive control circuit 55 drives the switch elements The step down circuit 45 includes the step down trans 56a and 56b of the switching circuit 56 in accordance with former TL and the inductor LL ( see FIG. 2 ) and a primary a signal output from the controller winding of the step down transformer TL is connected to Returning back to FIG. 3, the booster circuit 37 boosts the and interposed between the power receiving device side AC voltage output from the switching circuit 56, and applies passive electrode 41 and the power receiving device side it between the power transmission device side passive elec active electrode 42 and a secondary winding thereof is trode 31 and the power transmission device side active 15 connected to the input of the load circuit 220. ice side, active 15 The rectifying circuit 61 rectifies AC power from the electrode 32. The booster circuit 37 includes the booster step down circuit 45 and outputs it to the load circuit 220. transformer TG and the inductor LG ( see FIG. 2 ). A sec The load circuit 220 includes a DC / DC converter 71 as a ondary winding of the booster transformer TG is connected voltage stabilization circuit and a secondary battery 72 to and interposed between the power transmission device connected to the secondary side of the DC / DC converter 71. side passive electrode 31 and the power transmission device 20 The DC / DC converter 71 converts the DC voltage output side active electrode 32 and a primary winding thereof is from the rectifying circuit 61 to a DC voltage having a connected to the output of the switching circuit 56. The voltage value appropriate for the secondary battery 72 and voltage after boosted by the booster circuit 37 is a voltage in outputs it to the secondary battery 72. The secondary battery a range of 100 V to 10 kv, for example. The voltage is 72 is charged with the voltage output from the DC / DC applied between the power transmission device side passive 25 converter 71. The secondary battery 72 has characteristics electrode 31 and the power transmission device side active that the impedance thereof is increased when it is closer to electrode 32, so that an electrostatic field is generated on a a full charged state. medium therearound. 2. Operations of Controller of Power Transmission Device The controller 52 is input with the VDC signal and the The power transmission device 100 in the embodiment IDC signal from the VDC / IDC detection circuit 53, a VAC 330 detects ( estimates ) a load impedance of the power receiving signal from the VAC detection circuit 58, and the like. device 200 using a physical amount in the power transmis sion device 100 and controls transmission power such that For example, the controller 52 determines whether or not the voltage and the current of the DC power of DC 12 V electric power can be supplied to the power receiving device output from the DC power supply 51 are within predeter 200 appropriately in accordance with a load state based on mined ranges based on the VDC signal and the IDC signal. 35 the detection result. The power transmission device 100 detects the load impedance of the power receiving device Then, when the voltage and the current of the DC power 200 using the physical amount in the power transmission of DC 12 V are within the predetermined ranges, the device 100 as described above. This eliminates necessity to controller 52 controls ON / OFF of the switch elements 56a obtain information relating to the load state from the power and 56b constituting the switching circuit 56 based on the 40 receiving device 200 so as to eliminate necessity to provide IDC signal. To be more specific, the controller 52 calculates communication circuits on the power transmission device a duty ratio of ON and OFF of the switch elements 56a and and the power receiving device. 56b constituting the switching circuit 56 based on the IDC In the embodiment, the value IDC of a current flowing signal, generates ON and OFF signals in the calculated duty through the switching circuit 56 is used as the physical ratio, and outputs them to the drive control circuit 55. Then, 45 amount in the power transmission device 100, which is used the drive control circuit 55 switches the switch elements 56a for detecting the load impedance of the power receiving and 56b constituting the switching circuit 56 in accordance device 200. Then, a reason why the current value IDC of the with the ON and OFF signals output from the controller 52. current flowing through the switching circuit 56 is used will The controller 52 also determines whether or not the be described with reference to FIG. 5. voltage between the power transmission device side passive 50 FIG. 5 ( a ) is a graph illustrating characteristics of the load electrode 31 and the power transmission device side active impedance when the secondary battery 72 in the load circuit electrode 32 is within a predetermined range based on the 220 is being charged. FIG. 5 ( b ) is a graph illustrating VAC signal from the VAC detection circuit 58, for example. characteristics of the DC current value IDC when the The controller 52 makes this determination, for example, for secondary battery 72 is being charged. FIG. 5 ( c ) is a graph determining whether or not the voltage is increased abnor 55 illustrating characteristics of an output voltage of the power mally. receiving module when the secondary battery 72 is being 1. 3 Power Receiving Device charged. As illustrated in FIG. 5 ( a ), as charging of the The power receiving device 200 includes a power receiv secondary battery 72 proceeds over charge time, the imped ing module 210 and a load circuit 220. The power receiving ance ( load impedance ) of the load circuit 220 is increased. module 210 includes a power receiver 40, the step down 60 The load impedance is obtained by combining the imped circuit 45, and a rectifying circuit 61. ance of the DC / DC converter 71 and the impedance of the The power receiver 40 includes the power receiving secondary battery 72. The secondary battery 72 has the device side passive electrode 41 and the power receiving characteristics that the impedance thereof is increased as the device side active electrode 42. The power receiving device charging thereof proceeds. As the charging of the secondary side passive electrode 41 and the power receiving device 65 battery 72 proceeds and the load impedance is increased, as side active electrode 42 receive electric power from the illustrated in FIG. 5 ( b ), the DC current value IDC is power transmission device by the capacitance coupling to decreased. In this manner, change in the load impedance in

16 US 9, 948, 144 B2 the power receiving device side and the DC current value approximately 21 V. For example, the duty ratio is set at 14 IDC in the power transmission device side have a correla stages in increments of 0. 1 A of the DC current value IDC tion. For this reason, in the embodiment, the current value in the setting table. The duty ratio is set such that the DC IDC of the current flowing through the switching circuit 56 voltage value VRO output from the rectifying circuit 61 of is used for detecting the load impedance of the power 5 the power receiving module 210 is substantially constant. receiving device 200. An output voltage ( VRO ) of the power FIG. 8 is a graph illustrating a relation of the duty ratio receiving module 210 is increased as the charging of the with the DC current value IDC on the setting table in FIG. secondary battery 72 proceeds, as illustrated in FIG. 5 ( c ). 7. As illustrated in FIG. 8, the duty ratio changes by 1 % in 2. 1 Control Flow a stepwise in increments of 0. 1 A following the change of the Control operations by the controller 52 in the power 10 DC current value IDC as indicated by a solid line A based transmission device 100 will be described with reference to on the setting table in FIG. 7. To be specific, the duty ratio FIG. 6. The case where the power receiving device 200 is is increased by 1 % in a stepwise every time the DC current placed on the power transmission device 100 and charging value IDC is increased by 0. 1 A. During the charging of the of the secondary battery 72 is performed is described as an secondary battery 72, the DC current value IDC is the largest example. 15 at the initial time of charging and is decreased toward the When the power receiving device 200 is placed on the full charged state. Accordingly, during the charging of the power transmission device 100, the controller 52 performs secondary battery 72, the duty ratio is decreased toward the initial setting for starting power transmission ( S11 ). In the full charged state. initial setting, a power transmission frequency, a duty ratio, FIG. 9 is a graph illustrating characteristics of an output a power transmission voltage, and the alike are set. 20 voltage of the power receiving module with respect to the The controller 52 is input with the IDC signal output from DC current value IDC on the setting table in FIG. 7. The the VDC / DC detection circuit 53 as a detection signal of the duty ratio is controlled based on the DC current value IDC load impedance of the power receiving device 200 ( S12 ). with reference to the setting table in FIG. 7. With this, as The IDC signal is a signal indicating the current value of the indicated by a solid line B even when the DC current value DC current flowing through the switching circuit IDC changes is lowered ), the output voltage of the power Then, the controller 52 determines whether or not the DC receiving module is controlled to be substantially constant at current value IDC indicated by the input IDC signal is equal approximately 22 V. to or lower than a predetermined value ( S13 ). When the DC FIG. 10 is a graph illustrating characteristics of the output current value IDC is equal to or lower than the predeter voltage of the power receiving module based on a relation mined value, the controller 52 finishes the control in the 30 with the load impedance. In the embodiment, as indicated by flowchart. The predetermined value is set to a value of the a solid line C, even when the load impedance is increased, DC current value IDC at which the secondary battery 72 can the output voltage of the power receiving module is sub be estimated to be in the full charged state. When the DC stantially constant at approximately 22 V. current value IDC is not equal to or lower than the prede Dashed dotted lines A ', B ', and C ' in FIG. 8, FIG. 9, and termined value, the controller 52 sets the duty ratio corre 35 FIG. 10, respectively, indicate characteristics in the existing sponding to the DC current value IDC ( S14 ). In the embodi technique. In the existing technique, as indicated by the ment, the duty ratio is set in accordance with the DC current dashed dotted line A ' in FIG. 8, the duty ratio is constant value IDC ( that is to say, load impedance ). With this, the regardless of the DC current value IDC. As indicated by the duty ratio in accordance with the state ( load impedance ) of dashed dotted line B ' in FIG. 9, when the DC current value the load circuit 220 of the power receiving device 200 can 40 IDC is decreased, the output voltage of the power receiving be set. The output power is controlled using the duty ratio, module is increased. As indicated by the dashed dotted line so that appropriate electric power can be supplied to the C ' in FIG. 10, when the load impedance is increased, the power receiving device 200. A specific setting method of the output voltage of the power receiving module is increased. duty ratio will be described later. In this example, the output voltage of the power receiving Subsequently, the controller 52 generates ON / OFF signals 45 module is increased by approximately 8 V from approxi at the set duty ratio and outputs them to the drive control mately 21 V to approximately 29 V. circuit 55 ( S15 ) and the process returns to step S12. The In the setting table as illustrated in FIG. 7, in a range drive control circuit 55 controls ON / OFF of the switch where the DC current value IDC is 0 to 1. 4 A, the duty ratio elements 56a and 56b of the switching circuit 56 based on is set at 14 stages in increments of 0. 1 A. However, the duty the ON / OFF signals. 50 ratio is not limited to being set in this manner. For example, 2. 2 Setting of Duty Ratio for Switching Circuit as indicated by a dashed line A " in FIG. 8, the duty ratio may As described above, in the embodiment, the duty ratio for be set at 2 stages in the range where the DC current value the switching circuit 56 is set in accordance with the DC IDC is 0 to 1. 4 A. In this example, the duty ratio is set to 37 % current value IDC ( that is, load impedance ). The duty ratio when the IDC is lower than 0. 5 A and is set to 50 % when the is set in advance such that an output voltage VRO of the 55 IDC is equal to or higher than 0. 5 A. Dashed lines B " and rectifying circuit 61 of the power receiving device 200 does C " in FIG. 9 and FIG. 10 indicate the characteristics when not exceed a constant voltage. Hereinafter, the setting of the the duty ratio is set at 2 stages in this manner. In this case, duty ratio for the DC current value IDC will be described as indicated by the dashed line B " in FIG. 9, the output In the embodiment, the duty ratio for the DC current value voltage of the power receiving module is also higher as a IDC is defined on a setting table. The controller 52 deter 60 whole and a fluctuation width thereof is larger than those in mines the duty ratio to the switch elements 56a and 56b of the case where the duty ratio is controlled at 14 stages as the switching circuit 56 based on the DC current value IDC described above. However, in comparison with the existing with reference to the setting table. FIG. 7 illustrates an case where the duty ratio is not controlled as indicated by the example of the setting table of the duty ratio for the DC dashed dotted line B ', the fluctuation width of the output current value IDC. The setting table is stored in a storage 65 voltage of the power receiving module is decreased and the unit 52 A in the controller 52. The setting table indicates maximum value thereof ( when the IDC is 0 A ) is also values when a rated voltage of the DC / DC converter 71 is lowered. Furthermore, as indicated by the dashed line C " in

17 US 9, 948, 144 B2 10 FIG. 10, the output voltage of the power receiving module to the power receiving device 200 without providing com is increased by approximately 5 V to be approximately 26 V munication circuits on the power transmission device 100 at the highest until the load impedance is increased to and the power receiving device 200. approximately At this time, the duty ratio is changed In addition, a sensor for detecting the state of the load to 37 %, so that the output voltage of the power receiving 5 circuit 220, or the like, is not also needed in the power module does not exceed the value thereafter. The duty ratio receiving device 200. Meanwhile, the power receiving is controlled at 2 stages without necessity to prepare the device 200 is a smart phone or a mobile phone, for example, table as illustrated in FIG. 7 or the like, thereby obtaining an and is required to be reduced in cost and size. According to effect that the configuration of switching control of the duty the embodiment, the power receiving device 200 is not ratio is simplified. This can be applied as long as the output 10 required to include the communication circuit with the voltage of the power receiving module is allowable for the power transmission device 100 and components such as the withstand voltage and the operation characteristics of the sensor for detecting the load state. This can reduce the DC / DC converter 71. substrate of the power receiving module 210 and the power In the above mentioned example, the duty ratio is con receiving device 200 in size and cost. trolled at 14 stages or 2 stages in accordance with the DC 15 In the power transmission device 100 in the embodiment, current value IDC. However, the setting manner of the duty the power supply unit 54 includes the DC power supply 51 ratio is not limited thereto. For example, the duty ratio may that supplies DC power at a predetermined voltage and the be set as illustrated in FIG. 11. FIG. 11 is a graph illustrating power converter 57 that converts the DC power supplied a setting table T of the duty ratio with respect to the IDC in from the DC power supply 51 to AC power and supplies it another example. In this example, as indicated by a solid line 20 to the power transmitter 30. The load impedance detector D in FIG. 11, the duty ratio is changed continuously ( with no detects the current value IDC of the DC current flowing from stage ) in proportion to the DC current value IDC. In this the DC power supply 51 to the power converter 57. The case, the duty ratio in accordance with the IDC can be controller 52 controls a power conversion mode in the power obtained by calculation using a function or the like instead converter 57 such that electric power appropriate for the of the setting table. FIG. 12 is a graph illustrating charac 25 load circuit 220 of the power receiving device 200 is teristics of the output voltage of the power receiving module supplied based on the DC current value IDC detected by the with respect to the IDC. The duty ratio is set such that the load impedance detector. DC voltage output from the rectifying circuit 61 of the With this configuration, the power conversion mode in the power receiving module 210 is substantially constant as power converter 57 is controlled such that electric power indicated by a solid line E in FIG. 12 even when the IDC is 30 appropriate for the load circuit 220 of the power receiving decreased. A dashed dotted line D ' in FIG. 11 and a dashed device 200 is supplied based on the current value IDC of the dotted line E ' in FIG. 12 indicate the above mentioned DC power which is supplied from the DC power supply 51 existing example. to the power converter 57. The value IDC of the DC current 3. Overview which is supplied from the DC power supply 51 to the power The power transmission device 100 in the embodiment is 35 converter 57 reflects the change in the impedance of the the power transmission device 100 that wirelessly transmits secondary battery 72 preferably as described above. Accord electric power to the power receiving device 200 rectifying ingly, even when the state of the load circuit 220 of the a voltage received by the power receiver 40 with the power receiving device 200 is changed, the electric power rectifying circuit 61 and supplying it to the load circuit 220. appropriate for the load circuit 220 can be supplied. The power transmission device 100 includes the power 40 In the power transmission device 100 in the embodiment, transmitter 30 that makes capacitance coupling or electro DC power is supplied to the load circuit 220 of the power magnetic coupling to the power receiver 40 of the power receiving device 200. The power converter 57 switches receiving device 200 and transmits electric power to the ON / OFF of the DC voltage that is supplied from the DC power receiver 40, the power supply unit 54 that generates power supply 51 periodically so as to generate AC power. electric power to be transmitted to the power receiving 45 The controller 52 controls the duty ratio of ON / OFF in the device 200 and supplies the electric power to the power power converter 57 based on the DC current value IDC transmitter 30, the load impedance detector that detects a detected by the load impedance detector. load impedance of the power receiving device 200 based on With this configuration, the duty ratio of ON / OFF in the an electric signal detected in the power transmission device power converter 57 is controlled based on the DC current 100, and the controller 52 that controls electric power to be 50 value IDC detected by the load impedance detector. This generated by the power supply unit 54 such that an output prevents an overvoltage from being applied to the load voltage of the rectifying circuit 61 of the power receiving circuit 220. In addition, control can be performed easily and device 200 is equal to or lower than a predetermined voltage reliably with the control of the duty ratio. based on a detection result of the load impedance detector. In the power transmission device 100 in the embodiment, The power transmission device 100 in the embodiment 55 the duty ratio for the detected current value IDC is deter detects ( estimates ) the load impedance of the power receiv mined based on the relation previously defined between the ing device 200 using the DC current value IDC ( electric current value IDC of the DC current and the duty ratio. signal ) detected in the power transmission device 100 and With this configuration, control of the duty ratio can be controls power to be supplied to the power receiving device performed easily with a simple configuration. 200 such that the output voltage of the rectifying circuit In the embodiment, the load circuit 220 includes the of the power receiving device 200 is equal to or lower than DC / DC converter 71 and the secondary battery 72 that is the predetermined voltage based on the detection result. connected to the secondary side of the DC / DC converter 71. With this, the output voltage can be controlled to be constant With this configuration, the voltage that is applied to the regardless of the coupling state between the power receiving DC / DC converter 71 and the secondary battery 72 can be set device and the power transmission device or the load state. 65 to be constant. This enables the power transmission device 100 to supply Furthermore, in the embodiment, the power receiver 40 appropriate electric power in accordance with the load state includes the power receiving device side active electrode 42

18 US 9, 948, 144 B2 12 ( power receiving side first electrode ) and the power receiv Other Embodiments ing device side passive electrode 41 ( power receiving side second electrode ), and the power transmitter 30 includes the In the embodiment, the invention is applied to the electric power transmission device side active electrode 32 ( power field coupling type power transmission system. However, transmission side first electrode ) that makes capacitance 5 the invention can be also applied to a magnetic field cou coupling to the power receiving device side active electrode 42 ( power receiving side first electrode ) and the power pling type power transmission system. transmission device side passive electrode 31 ( power trans Although the VDC / IDC detection circuit 53 detects the mission side second electrode ) that makes capacitance cou load impedance in the power receiving device in the pling to the power receiving device side passive electrode 10 embodiment, the VAC detection circuit 58 may detect the 41 ( power receiving side second electrode ). load impedance instead. The AC voltage value VAC that is With this configuration, the above mentioned various detected by the VAC detection circuit 58 indicates a voltage effects are obtained in the electric field coupling type power in accordance with the impedance of the secondary battery transmission. 72 same as the DC current value IDC that is detected by the In the power transmission device 100 in the embodiment, 15 VDC / IDC detector 53. Accordingly, effects that are the same the voltage of the power transmission device side active as those obtained in the case of using the VDC / DC detector electrode 32 ( power transmission side first electrode ) is higher than the voltage of the power transmission device 53 can be obtained. side passive electrode 31 ( power transmission side second Furthermore, in the embodiment, in control of the output electrode ) during power transmission. 20 power of the power transmission device 100, the duty ratio With this configuration, the voltages of the power trans of the respective switch elements of the switching circuit 56 mission device side active electrode 32 ( power transmis is controlled in accordance with the load impedance ( PWM sion side first electrode ) and the power receiving device side control ). The invention is not limited thereto. Alternatively, active electrode 42 ( power receiving side first electrode ) can in control of the output power of the power transmission be made high. This can increase power transmission effi 25 device 100, the magnitude of the DC voltage VDC that is ciency. input to the switching circuit 56 may be controlled in The power transmission device 100 in the embodiment accordance with the load impedance ( PAM control ). further includes the booster transformer TG of which second winding is connected to and interposed between the power Furthermore, although the secondary battery 72 as a load transmission device side active electrode 32 ( power trans 30 is connected to the secondary side of the DC / DC converter mission side first electrode ) and the power transmission 71 of the load circuit 220 in the embodiment, the invention device side passive electrode 31 ( power transmission side can be applied to the case where a component other than the second electrode ) and primary winding is connected to the secondary battery 72 is used. For example, a constant output of the power converter 57. voltage can be supplied to an electronic apparatus of which With this configuration, the voltages of the power trans 35 load impedance changes in accordance with the operation mission device side active electrode 32 ( power transmis state even when the load impedance changes. Therefore, the sion side first electrode ) and the power receiving device side operations of the electronic apparatus can be made stable active electrode 42 ( power receiving side first electrode ) can regardless of the variation of the load impedance. be made higher. This enables large electric power to be Although the load circuit 220 including the DC / DC transmitted with a low electric current so as to reduce power 40 converter 71 as the voltage stabilization circuit and the loss due to conductor loss in a power transmission path. This secondary battery 72 as the secondary side load of the can further increase power transmission efficiency. DC / DC converter 71 is explained in the embodiment, the Furthermore, in the embodiment, provided is the power transmission system including the power receiving device invention is not limited thereto. For example, as illustrated 200 having the power receiver 40 and the load circuit in FIG. 13, the invention can be applied to the case where a and the above mentioned power transmission device 100. load circuit 220 ' includes a voltage regulator 81 as a voltage stabilization circuit, a secondary battery 82 as a secondary With the power transmission system in the embodiment, side load of the voltage regulator 81, a device 84 operating the same effects as those described for the power transmis with electric power that is supplied from the secondary sion device 100 can be obtained. In addition, in the embodiment, provided is a transmission 50 battery 82 or the voltage regulator 81, and a power control method in the power transmission device 100 that wirelessly circuit 83 controlling supply of the electric power to the transmits electric power to the power receiving device 200 device 84. The secondary battery 82 incorporates a charge rectifying a voltage received by the power receiver 40 and control circuit. Also in this example, a constant voltage can be supplied even when the load impedance changes. There supplying the voltage to the load circuit 61. The transmis fore, the operations of the electronic apparatuses including sion method includes generating electric power to be trans mitted to the power receiving device 200 and supplying the the device 84 or the like can be made stable regardless of the electric power to the power transmitter 30, detecting a load variation of the load impedance. impedance of the power receiving device 200 based on an Although the secondary battery 82 is explained incorpo electric signal detected in the power transmission device rating the charge control circuit in the example of FIG. 13, 100, and controlling electric power to o be supplied to the 60the invention is not limited thereto. For example, like a load power transmitter 30 such that an output voltage of the circuit 220 " as illustrated in FIG. 14, a charge control circuit rectifying circuit 61 of the power receiving device 200 is 85 may be provided to be isolated between the voltage equal to or lower than a predetermined voltage based on a regulator 81 and a secondary battery 82 '. detection result of the load impedance. Furthermore, the invention can be applied to the case With the transmission method in the embodiment, the 65 where a voltage stabilization circuit of the load circuit of the same effects as those described for the power transmission power receiving device includes both of the DC / DC con device 100 can be obtained. verter and the voltage regulator.

19 US 9, 948, 144 B REFERENCE SIGNS LIST a controller configured to control the power supply unit to generate the electric power such that the voltage sup 30 POWER TRANSMITTER plied by the rectifying circuit to the load circuit is equal 31 POWER TRANSMISSION DEVICE SIDE PASSIVE to or less than a predetermined voltage based on a ELECTRODE ( POWER TRANSMISSION SIDE 5 signal output by the detection circuit, SECOND ELECTRODE ) wherein, when the detected current value exceeds a pre 32 POWER TRANSMISSION DEVICE SIDE ACTIVE determined value that is stored in memory of the ELECTRODE ( POWER TRANSMISSION SIDE controller and that relates to a charge state of a battery FIRST ELECTRODE ) of the load circuit, the controller sets a duty ratio of the 37 BOOSTER 10 AC power supplied by the power converter based on a 40 POWER RECEIVER relation previously defined between the current value 41 POWER RECEIVING DEVICE SIDE PASSIVE detected by the detection circuit and the duty ratio and ELECTRODE ( POWER RECEIVING SIDE SEC continuously changes the duty ratio of the AC power in OND ELECTRODE ) proportion to changes in the current value detected by 42 POWER RECEIVING DEVICE SIDE ACTIVE 15 the detection circuit, such that the rectifying circuit of ELECTRODE ( POWER RECEIVING SIDE FIRST the power receiver supplies the constant voltage to the ELECTRODE ) load circuit of the power receiving device, and 45 STEP DOWN wherein, when the current value is equal to or lower than 51 DIRECT CURRENT POWER SUPPLY the predetermined value relating to the charge state of 52 CONTROLLER 20 the battery, the controller stops setting the duty ratio of 52A STORAGE UNIT the AC power. 53 VDC / IDC DETECTION 2. The power transmission device according to claim 1, 54 POWER SUPPLY UNIT wherein the rectifying circuit supplies DC power to the 55 DRIVE CONTROL load circuit of the power receiving device, and 56 SWITCHING 25 wherein the power converter switches ON / OFF the DC 57 POWER CONVERTER voltage supplied by the DC power supply periodically 58 VAC DETECTION so as to generate an AC voltage. 61 RECTIFYING 3. The power transmission device according to claim 1, 71 DC / DC CONVERTER wherein the power receiver includes a power receiving 72, 82, 82 ' SECONDARY BATTERY 30 side first electrode and a power receiving side second 81 VOLTAGE REGULATOR electrode, and 83 POWER CONTROL wherein the power transmitter includes a power transmis 84 DEVICE sion side first electrode that generates capacitance cou 85 CHARGE CONTROL pling with the power receiving side first electrode and 100 POWER TRANSMISSION DEVICE 35 a power transmission side second electrode that gen 200 POWER RECEIVING DEVICE erates capacitance coupling with the power receiving 210 POWER RECEIVING MODULE side second electrode. 220, 220 ', 220 " LOAD 4. The power transmission device according to claim 3, CG CL CAPACITOR wherein a voltage of the power transmission side first elec LG, LL INDUCTOR 40 trode is greater than a voltage of the power transmission side OSC HIGH FREQUENCY VOLTAGE GENERATION second electrode during power transmission. 5. The power transmission device according to claim 4, RL LOAD wherein the power transmission device further includes a TG BOOSTER TRANSFORMER booster transformer with a secondary winding connected to TL STEP DOWN TRANSFORMER 45 and interposed between the power transmission side first electrode and the power transmission side second electrode The invention claimed is : and a primary winding connected to an output of the power 1. A power transmission device that wirelessly transmits converter. electric power to a power receiving device having a power 6. A power transmission system comprising : receiver with a rectifying circuit that supplies a constant 50 a power receiving device including a power receiver and voltage to a load circuit, the power transmission device a load circuit ; and comprising : a power transmission device including : a power supply unit that generates electric power and a power supply unit that generates electric power and includes : includes : a direct current ( DC ) power supply that supplies DC 55 a direct current ( DC ) power supply that supplies DC power, and power, and a power converter that switches a power converter that switches ON / OFF the DC power ON / OFF the DC power supplied by the DC power supplied by the DC power supply to generate an supply to generate an alternating current ( AC ) alternating current ( AC ) power and supplies the AC power and supplies the AC power to the power power to the power transmitter ; transmitter ; a power transmitter that transmits the AC power to the power receiver by capacitance coupling or electromag a power transmitter that transmits the electric power to the power receiver by capacitance coupling or elec netic coupling ; tromagnetic coupling ; a detection circuit configured to detect a current value of a detection circuit configured to detect a current value a current flowing from the power supply unit that 65 of a current flowing from the power supply unit that corresponds to a load impedance of the power receiving corresponds to a load impedance of the power device ; and receiving device ; and

20 US 9, 948, 144 B a controller configured to control the power supply unit supply and switching ON / OFF, by a power converter, to generate the electric power such that the voltage the DC power supplied by the DC power supply supplied to the load circuit is equal to or less than a periodically to generate an alternating current ( AC ) predetermined voltage based on a signal output by power ; the detection circuit, supplying the AC power to a power transmitter ; wherein, when the detected current value exceeds a detecting, by a detection circuit, a current value of a predetermined value that is stored in memory of the current flowing from the power supply unit that corre controller and that relates to a charge state of a sponds to a load impedance of the power receiving battery of the load circuit, the controller sets a duty device ; and ratio of the AC power supplied by the power con 10 controlling the power supply unit to generate the electric verter based on a relation previously defined between power such that the voltage supplied by the power the current value detected by the detection circuit receiver to the load circuit is equal to or less than a and the duty ratio and continuously changes the duty predetermined voltage based on the load impedance ; ratio of the AC power in proportion to changes in the and current value detected by the detection circuit, such 15 setting a duty ratio of the AC power supplied by the power that the power receiver supplies the voltage as a converter based on a relation previously defined constant voltage to the load circuit of the power between the current value detected by the detection receiving device, and circuit and the duty ratio, when the detected current wherein, when the current value is equal to or lower value exceeds a predetermined value relating to a than the predetermined value relating to the charge 20 charge state of a battery of the load circuit ; state of the battery, the controller stops setting the continuously changing the duty ratio of the AC power duty ratio of the AC power. supplied by the power converter in proportion to 7. The power transmission system according to claim 6, changes in the current value detected by the detection wherein the power receiving device further comprises a circuit, such that the power receiver supplies the con rectifying circuit that supplies DC power to the load circuit. 25 stant voltage to the load circuit of the power receiving 8. The power transmission system according to claim 6, device ; and wherein the power converter switches ON / OFF the DC ending the setting of the duty ratio of the AC power when voltage supplied by the DC power supply periodically the detected current value is equal to or lower than the so as to generate an AC voltage. predetermined value relating to the charge state of the 9. The power transmission system according to claim 6, 30 battery. wherein the load circuit includes a voltage stabilization 14. The power transmission device according to claim 1, circuit and the battery that is connected to a secondary side wherein the controller is configured to continually adjust the of the voltage stabilization circuit. duty ratio of the AC power based on the detected current 10. The power transmission system according to claim 6, value as long as the detected current value exceeds the wherein the power receiver includes a power receivino. 35 predetermined value. side first electrode and a power receiving side second 15. The power transmission device according to claim 14, electrode, and wherein the controller is configured to end power transmis sion to the power receiving device when the detected current sion side first electrode that generates capacitance cou value equal to or lower than the predetermined value The power transmission device according to claim 1, wherein the power transmitter includes a power transmis pling with the power receiving side first electrode and 40 a power transmission side second electrode that gen wherein the controller is configured to set and adjust the duty erates capacitance coupling with the power receiving ratio based on the current value detected by the detection side second electrode. circuit by calculation using a function. 11. The power transmission system according to claim 10, 17. The power transmission device according to claim 1, wherein a voltage of the power transmission side first elec 45 wherein the controller continuously changes the duty ratio of trode is greater than a voltage of the power transmission side the AC power smoothly and without stages, such that the second electrode during power transmission. power transmitter transmits the AC power to the power 12. The power transmission system according to claim 11, receiver causing the rectifying circuit to supply the constant wherein the power transmission device further includes a voltage to the load circuit of the power receiving device. booster transformer with a secondary winding connected to The power transmission system according to claim 6, and interposed between the power transmission side first wherein the controller continuously changes the duty ratio of electrode and the power transmission side second electrode the AC power smoothly and without stages, such that the and a primary winding connected to an output of the power power transmitter transmits the AC power to the power converter. receiver causing the power receiver to supply the constant 13. A power transmission method for wirelessly transmit 55 voltage to the load circuit of the power receiving device. ting electric power from a power transmission device to a 19. The power transmission method according to claim power receiving device having a power receiver that sup 13, further comprising continuously changing the duty ratio plies a constant voltage to a load circuit, the method com of the AC power smoothly and without stages, such that the prising : power receiver supplies the constant voltage to the load v 60 circuit of the power receiving device. generating electric power by a power supply unit by 60 cito supplying direct current ( DC ) power from a DC power * *