Abstract:
A wireless power reception device and a wireless communication method thereby are provided. The wireless communication method by the wireless power reception device may comprise the steps of: receiving a wireless power signal from a wireless power transmission device; measuring the strength of the wireless power signal; modulating the amplitude of the wireless power signal according to the measured strength of the wireless power signal; and performing communication with the wireless power transmission device by using the signal having the amplitude modulated.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    Embodiments of the present invention relate to a wireless power receiving apparatus receiving wireless power from a wireless power transmitting apparatus and a wireless communication method thereby. 
         [0003]    Related Art 
         [0004]    In recent years, supply of portable electronic devices including a smart phone, a laptop, an MPEG-1 audio layer (MP3) player, a headset, and the like has been spread. However, since the portable electronic devices operate by consuming power stored in battery cells (e.g., a primary cell, a secondary cell, and the like), the battery cell needs to be charged or replaced in order to continuously operate the portable electronic devices. 
         [0005]    A method of charging the battery cell is generally divided into a contact type charging method of charging the battery cell by using a power supply line and a power supply terminal and a non-contact type charging method of charging the battery cell with wireless power induced by a magnetic field generated from a primary coil of a wireless power transmitting apparatus by using a wireless power receiving apparatus. However, in the contact type charging method, an instant discharge phenomenon occurs as different potential differences are generated at both terminals when a charger and a battery are coupled to or separated from each other and the power supply terminal is exposed to the outside, and as a result, fire may occur when foreign materials are accumulated in the power supply terminal and the battery is naturally discharged and the life-span and the performance of the battery deteriorate due to moisture. Accordingly, in recent years, in order to solve the problems, a research into the non-contact type charging method has been in active progress. 
         [0006]    As one of technologies associated with the non-contact type charging method, “Non-contact Charging System” of Korean Patent Registration No. 10-0971705 discloses that a wireless power signal is transmitted by determining measuring a delay time up to a time of receiving a response signal corresponding to a request signal from a time of outputting the request signal through a primary-side core unit and comparing the measured delay time with a reference stand-by time when a load change is sensed in the primary-side core unit of a non-contact power transmitting apparatus and thereafter, determining that a corresponding object is a foreign material when the measured time is shorter than the reference stand-by time and determining that the corresponding object is a normal non-contact power receiving apparatus when the measured time is longer than the reference stand-by time. 
         [0007]    In the magnetic induction type non-contact charging system, the wireless power receiving apparatus generally communicates with the wireless power transmitting apparatus by an amplitude-shift keying (ASK) modulation method. In detail, when an amplitude of the wireless power signal which the wireless power receiving apparatus receives from the wireless power transmitting apparatus is modulated, the modulated signal is induced to a transmitting coil of the wireless power transmitting apparatus. The wireless power transmitting apparatus performs communication by detecting the modulated signal induced to the transmitting coil. However, in the non-contact charging system, as the strength of the wireless power signal transmitted from the wireless power transmitting apparatus increases, distortion occurs in the modulated signal and this causes a communication error between the wireless power transmitting apparatus and the wireless power receiving apparatus. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a wireless power receiving apparatus which can smoothly communicate with a wireless power transmitting apparatus even when the strength of wireless power transmitted from the wireless power transmitting apparatus increases in a non-contact charging system. 
         [0009]    The present invention also provides a wireless communication method which enables a wireless power receiving apparatus and a wireless power transmitting apparatus to smoothly communicate with each other even when the strength of wireless power transmitted from the wireless power transmitting apparatus increases in a non-contact charging system. 
         [0010]    In an aspect, a wireless communication method by a wireless power receiving apparatus includes: receiving a wireless power signal from a wireless power transmitting apparatus; measuring the strength of the wireless power signal; modulating the amplitude of the wireless power signal according to the strength of the measured wireless power signal; and performing communication with the wireless power transmitting apparatus by using the signal having the amplitude modulated. 
         [0011]    The modulating of the amplitude of the wireless power signal may include selecting at least one of a plurality of modulators included in the wireless power receiving apparatus according to the measured strength of the wireless power signal. 
         [0012]    Each of the plurality of modulators may include at least one capacitor or resistor and includes at least one transistor. 
         [0013]    The at least one transistor may be a metal oxide silicon field effect transistor (MOSFET). 
         [0014]    The plurality of modulators may be configured to an alternating current (AC) terminal of the wireless power receiving apparatus and is connected to a controller of the wireless power receiving apparatus in parallel. 
         [0015]    At least one modulator of the plurality of modulators may be configured in a direct current (DC) terminal of the wireless power receiving apparatus. 
         [0016]    The modulating of the amplitude of the wireless power signal may include controlling a gate bias of the modulator included in the wireless power receiving apparatus according to the measured strength of the wireless power signal. 
         [0017]    The modulator may include a plurality of resistors and at least one transistor. 
         [0018]    In another aspect, a wireless power receiving apparatus includes: at least one secondary core receiving a wireless power signal transmitted from a wireless power transmitting apparatus; a detection circuit measuring the strength of the wireless power signal; a plurality of modulators modulating the amplitude of the wireless power signal; and a controller selecting at least one of the plurality of modulators based on the measured strength of the wireless power signal and controlling communication with the wireless power transmitting apparatus by using the signal having the amplitude modulated. 
         [0019]    In yet another aspect, a wireless power receiving apparatus includes: at least one secondary core receiving a wireless power signal transmitted from a wireless power transmitting apparatus; a detection circuit measuring the strength of the wireless power signal a modulator modulating the amplitude of the wireless power signal; and a controller controlling a gate bias of the modulator based on the measured strength of the wireless power signal and controlling communication with the wireless power transmitting apparatus by using the signal having the amplitude modulated by the modulator. 
         [0020]    Since a wireless power receiving apparatus modulates the amplitude of a wireless power signal according to the strength of the wireless power signal transmitted from a wireless power transmitting apparatus to prevent a modulated signal from being distorted, smooth wireless communication is available even when wireless power signals of various strength are transmitted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a diagram illustrating a non-contact charging system according to the present invention. 
           [0022]      FIG. 2  is a block diagram illustrating a wireless power transmitting apparatus included in the non-contact charging system. 
           [0023]      FIG. 3  is a circuit diagram illustrating a wireless power receiving apparatus included in the non-contact charging system. 
           [0024]      FIG. 4  is a block diagram illustrating a wireless power receiving apparatus according to an embodiment of the present invention. 
           [0025]      FIG. 5  is a circuit diagram illustrating the wireless power receiving apparatus according to the embodiment of the present invention. 
           [0026]      FIG. 6  is a block diagram illustrating a wireless power receiving apparatus according to another embodiment of the present invention. 
           [0027]      FIG. 7  is a circuit diagram illustrating the wireless power receiving apparatus according to another embodiment of the present invention. 
           [0028]      FIG. 8  is a flowchart illustrating a wireless communication method of a wireless power receiving apparatus according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0029]    The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. In addition, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
         [0030]    Throughout the specification, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
         [0031]    A term called “wireless power” used in the present specification means predetermined type of energy associated with an electric field, a magnetic field, an electromagnetic field, and the like transmitted from a transmitter to a receiver without using physical electromagnetic conductors. The wireless power may be called a power signal or a wireless power signal and mean an oscillating magnetic flux enclosed by a primary coil at a transmitting side and a secondary coil at a receiving side. Hereinafter, a wireless power receiving apparatus and a wireless communication method in a non-contact charging system for wirelessly charging devices including a mobile phone, a cordless phone, a smart phone, an MP3 player, a laptop, a headset, and the like will be described as an example. A fundamental principle of wireless power transmission includes both a magnetic induction coupling method and a magnetic resonance coupling (that is, resonance induction) method using frequencies less than 30 MHz. However, various frequencies including frequencies at which a license-exemption operation is permitted at comparative higher radiation levels, for example, less than 135 kHz (LF) or 13.56 MHz (HF) may be used. 
         [0032]      FIG. 1  is a diagram illustrating a non-contact charging system according to the present invention. 
         [0033]    Referring to  FIG. 1 , the non-contact charging system  100  includes a wireless power transmitting apparatus  110  and one or more wireless power receiving apparatus  150 - 1  to  150 - n  (herein, n is a natural number). 
         [0034]    The wireless power transmitting apparatus  110  includes a primary core. The primary coil may include at least one primary coil. The wireless power transmitting apparatus  110  may have a predetermined appropriate shape, but one preferred embodiment may be a flat platform having a power transmission surface. The respective wireless power receiving apparatuses  150 - 1  to  150 - n  are positioned on the platform or near the platform to receive wireless power from the wireless power transmitting apparatus  110 . 
         [0035]    The respective wireless power receiving apparatuses  150 - 1  to  150 - n  may be separated from the wireless power transmitting apparatus  110 . When the respective wireless power receiving apparatuses  150 - 1  to  150 - n  are positioned near the wireless power transmitting apparatus  110 , the respective wireless power receiving apparatuses  150 - 1  to  150 - n  include the secondary core coupled with an electromagnetic field generated by the primary core of the wireless power transmitting apparatus  110 . The secondary core may include one or more secondary coils. 
         [0036]    The wireless power transmitting apparatus  110  transmits power to the wireless power receiving apparatuses  150 - 1  to  150 - n  without a code contact. In this case, the primary core and the secondary core are magnetic induction coupled or magnetic resonance coupled to each other. The primary coil or the secondary coil may have predetermined appropriate shapes. As one example, the primary coil and the secondary coil may be copper wires wound around ferrite or an amorphous material. 
         [0037]    The wireless power receiving apparatuses  150 - 1  to  150 - n  are connected with external load (not illustrated, also referred to as an actual load of the wireless power receiving apparatus) to supply the power wirelessly received from the wireless power transmitting apparatus  110  to the external load. For example, each of the wireless power receiving apparatuses  150 - 1  to  150 - n  may transport the received power to an object which consumes or stores the power, such as a portable electric or electronic device or a rechargeable battery cell or battery. 
         [0038]      FIG. 2  is a block diagram illustrating a wireless power transmitting apparatus included in the non-contact charging system. Hereinafter, the wireless power transmitting apparatus will be described in more detail with reference to  FIG. 2 . 
         [0039]    The wireless power transmitting apparatus  200  may include a primary core  210 , an electric driving circuit  220 , a controller  230 , and a detection circuit  240 . 
         [0040]    The primary core  210  transmits a signal for detecting the wireless power receiving apparatus and a wireless power signal. 
         [0041]    The electric driving circuit  220  is connected to the primary core  210  to apply electric driving signals to the primary core so that the electromagnetic field is generated in the primary core  210 . 
         [0042]    The controller  230  is connected to the electric driving circuit  220  to generate a control signal  231  to control an alternating current (AC) signal required when the primary core  210  generates an induction magnetic field or causes magnetic resonance. The controller  230  may control an operation frequency, and voltage, current, and/or a duty cycle in the wireless power transmitting apparatus  200  according to a power control signal received from the wireless power receiving apparatus. 
         [0043]    The detection circuit  240  measures current that flows on the primary core  220 . The current measured by the detection circuit  240  may be alternating current (AC) or direct current (DC). As one example, the detection circuit  240  may be a current sensor or a voltage sensor. Alternatively, the detection circuit may be a transformer that lowers high current that flows on the primary core  210  to low current and uses the low current. 
         [0044]    The controller  230  may obtain information transmitted by the wireless power receiving apparatus by using a current or voltage value measured by the detection circuit  240 . The wireless power receiving apparatus may continuously or periodically transmit to the wireless power transmitting apparatus  200  a power control signal to request an increase of the power or a power control signal to request a decrease of the power until required power is satisfied by varying the load. When the wireless power transmitting apparatus  200  receives the power control signal to request the increase of the power from the wireless power receiving apparatus through the load variation, the wireless power transmitting apparatus  200  decreases the power controls to an appropriate strength by using the transformer or a voltage distributor using resistance, and the like and performs envelope detection by using a detector and thereafter, makes the power control signal pass through a low-pass filter to detect the signal form the wireless power receiving apparatus. In addition, the strength of the current which flows on the primary core  210  may be increases so as to transmit higher power as a response to the power control signal. In more detail, the controller  230  may adjust the control signal so as to apply an AC signal having a larger than a reference AC signal in order to make higher current to flow on the primary core  210 . On the contrary, when the controller  230  receives the power control signal to request the decrease of the power from the wireless power receiving apparatus, the controller  230  may adjust the control signal so as to an AC signal lower than the reference AC signal so that power lower than the current transmission power is transmitted. 
         [0045]      FIG. 3  is a circuit diagram illustrating a wireless power receiving apparatus included in the non-contact charging system. Hereinafter, a structure of a wireless power receiving apparatus will be described in more detail with reference to  FIG. 3 . 
         [0046]    A wireless power receiving apparatus  300  may include a secondary core  310 , a modulator  320 , a controller  330 , a rectifier  340 , and a regulator  350 . 
         [0047]    The secondary core  310  may be configured by at least one secondary coil. The secondary core  310  may receive a wireless power signal transmitted from the primary core of the wireless power transmitting apparatus. 
         [0048]    The modulator  320  may be configured by an AC terminal of the wireless power receiving apparatus  300  as illustrated in  FIG. 3  and modulate an amplitude of the wireless power signal received through the secondary core  310 . To this end, the modulator  320  may include a capacitor  321  and a transistor  322 . For example, the modulator  320  turns on/off the transistor  322  connected to the capacitor  321  to modulate the amplitude of the wireless power signal received through the secondary core  310 . The signal with the modulated amplitude may be induced to the primary core of the wireless power transmitting apparatus through the secondary core  310 . 
         [0049]    The controller  330  is to control an operation of the wireless power receiving apparatus  300 , and for example, may control power supplied to a load (not illustrated) connected to the wireless power receiving apparatus  300 . Further, the controller  330  may perform communication with the wireless power transmitting apparatus by controlling the modulator  320 . 
         [0050]    The rectifier  340  may rectify AC power received by the secondary core  310  to direct current (DC) power. The power rectified by the rectifier  340  may be supplied to a load which is connected, installed, or included to the wireless power receiving apparatus  300  by the regulator  350 . The rectifier  340  may be implemented by a half-bridge, a full-bridge, or the like as illustrated in  FIG. 3 . In  FIG. 3 , as an example, the rectifier  340  includes a plurality of diodes, but the diode of the rectifier  340  may be replaced with the transistor such as a field effect transistor (FET). 
         [0051]    The regulator  350  is configured by an output terminal of the rectifier  340  and may be regulated at low voltage so as to supply high voltage and/or irregular voltage output from the rectifier  340  as stable power. 
         [0052]    Meanwhile, although not illustrated in  FIG. 3 , the wireless power receiving apparatus  300  may include a detection circuit which measures strength of the wireless power signal transmitted from the wireless power transmitting apparatus by monitoring an output of the rectifier  340 . 
         [0053]    When the strength of the wireless power signal detected from the detection circuit is larger than or smaller than a predetermined control point, the controller  330  may transmit information on power control so that the wireless power signal with predetermined strength may be received by modulating the amplitude of the wireless power signal received from the wireless power receiving apparatus. Alternatively, when the strength of the wireless power signal detected from the detection circuit is beyond the predetermined range, the controller  330  may allow the strength of the wireless power signal to be maintained within the predetermined range by modulating the amplitude of the wireless power signal received from the wireless power receiving apparatus. In this case, as illustrated in  FIG. 3 , when the capacitor  321  is used in the modulator  320 , constant impedance for a change in frequency is maintained, and thus even though the frequency is changed, the modulator  320  may perform constant amplitude modulation. However, when a resistor instead of the capacitor  321  is used in the modulator  320 , a response time may be minimized according to the used resistor, but since the impedance is changed according to a frequency, when a full band is used, distortion according to a frequency may be caused. Further, even though the capacitor  321  is used in the modulator  320 , the size of the power received from the wireless power transmitting apparatus is larger than a predetermined value, the signal modulated by the modulator  320  may be distorted. The distortion of the modulated signal causes communication failure between the wireless power transmitting apparatus and the wireless power receiving apparatus  300 . 
         [0054]      FIG. 4  is a block diagram illustrating a wireless power receiving apparatus according to an embodiment of the present invention and  FIG. 5  is a circuit diagram illustrating the wireless power receiving apparatus according to the embodiment of the present invention. Hereinafter, the wireless power receiving apparatus will be described in detail with reference to  FIGS. 4 and 5 . 
         [0055]    First, referring to  FIG. 4 , a wireless power receiving apparatus  400  according to an embodiment of the present invention may include a secondary core  410 , a rectifier  420 , a regulator  430 , a detection circuit  440 , a controller  450 , and a plurality of modulators  460  and  470 . The wireless power receiving apparatus  400  is connected to an external load  480  to supply power wirelessly received from the wireless power transmitting apparatus to a load  480 . In  FIGS. 4 and 5 , as an example, it is illustrated that two modulators  460  and  470  are included in the wireless power receiving apparatus  400 , but the wireless power receiving apparatus  400  may include two or more modulators. 
         [0056]    The secondary core  410  may include at least one secondary coil receiving a wireless power signal transmitted from the wireless power transmitting apparatus. 
         [0057]    The rectifier  420  may rectify the wireless power signal received by the secondary core  410 . In detail, the rectifier  420  may convert an AC-waveform wireless power signal received by the secondary core  410  into DC power by using full-wave or half-wave rectification. 
         [0058]    The regulator  430  is configured by an output terminal of the rectifier  420  and may be regulated at low voltage so as to supply high voltage and/or irregular voltage output from the rectifier  420  as stable power. 
         [0059]    The detection circuit  440  is connected to an output terminal of the rectifier  420  to measure the strength of the wireless power signal transmitted from the wireless power transmitting apparatus by monitoring the DC voltage output from the rectifier  420 . 
         [0060]    The controller  450  selects any one or at least one of the plurality of modulators  460  and  470  based on the strength of the wireless power signal measured by the detection circuit  440  and may control communication with the wireless power transmitting apparatus by using a signal with the amplitude modulated by the selected modulator. 
         [0061]    The respective modulators  460  and  470  may modulate the amplitude of the wireless power signal transmitted from the wireless power transmitting apparatus. Herein, the respective modulators  460  and  470  may be used in the amplitude modulation for signals with different power. For example, the first modulator  460  may be used in the amplitude modulation of the corresponding signal when the strength of the wireless power signal received from the wireless power transmitting apparatus is a control point or more based on the value measured in the output terminal of the regulator  430 , and the second modulator  470  may be used in the amplitude modulation of the corresponding signal when the strength of the wireless power signal is less than the control point. Herein, the control point may be set to various values according to the strength of the wireless power signal. 
         [0062]    When three modulators are configured in the wireless power receiving apparatus  400 , any one of the three modulators may be used in the amplitude modulation when the wireless power signal is less than a first control point, another one of the three modulators may be used in the amplitude modulation when the wireless power signal is equal to or more than a second control point, and the other one of the three modulators may be used in the amplitude modulation when the wireless power signal is equal to or more than the first control point and less than the second control point. 
         [0063]    The controller  450  may generate charge control information, charge status information, full charge information, and the like by using the modulator selected from the plurality of modulators  460  and  470 . The charge control information may be transmitted to the wireless power transmitting apparatus through a control error packet. The charge status information may be transmitted to the wireless power transmitting apparatus through a charge status packet. The full charge information may be transmitted to the wireless power transmitting apparatus through an end power transfer packet. 
         [0064]    Hereinafter, a structure of the wireless power transmitting apparatus including two modulators  560  and  570  will be described in more detail with reference to  FIG. 5 . In  FIG. 5 , as an example, a case where a secondary core  510  includes one secondary coil is illustrated. 
         [0065]    Referring to  FIG. 5 , respective modulators  560  and  570  may be connected to the controller  550  in parallel and may include at least one capacitor and at least one transistor. In  FIG. 5 , as an example, a case where the respective modulators  560  and  570  include two capacitors and two transistors is illustrated. Herein, the transistor may be a field effect transistor (FET) or a metal oxide silicon field effect transistor (MOSFET). The respective modulators  560  and  570  may modulate the amplitude of the wireless power signal received through the secondary core  510  and/or the size of the amplitude according to a change amount of the load by turning on/off the transistors connected to the capacitor. 
         [0066]    In  FIG. 5 , the capacitor included in the respective modulators  560  and  570  may be replaced with a resistor. That is, the respective modulators  560  and  570  may include a plurality of resistors and a plurality of transistors. In this case, when the modulator modulates the amplitude of the wireless power signal, a time taken to store charges when using the capacitor is not required, and thus the response time of the wireless power receiving apparatus may be minimized. 
         [0067]    Meanwhile, in  FIG. 5 , a case where a plurality of modulators  560  and  570  is configured in an AC terminal of the wireless power receiving apparatus is illustrated, but the plurality of modulators  560  and  570  may be configured in a DC terminal (for example, between the rectifier  520  and the regulator  530 ) of the wireless power receiving apparatus and some of the plurality of modulators  560  and  570  may be configured in the AC terminal and the rest thereof may be configured in the DC terminal. 
         [0068]      FIG. 6  is a block diagram illustrating a wireless power receiving apparatus according to another embodiment of the present invention and  FIG. 7  is a circuit diagram illustrating the wireless power receiving apparatus according to another embodiment of the present invention. Hereinafter, a wireless power receiving apparatus according to another embodiment of the present invention will be described in detail with reference to  FIGS. 6 and 7 . 
         [0069]    First, referring to  FIG. 6 , a wireless power receiving apparatus  600  according to another embodiment of the present invention may include a secondary core  610 , a rectifier  620 , a regulator  630 , a detection circuit  640 , a controller  650 , and a modulator  660 . The wireless power receiving apparatus  600  according to the embodiment in  FIG. 6  may also supply power to an external load  670 . 
         [0070]    The secondary core  610  may include at least one secondary coil receiving a wireless power signal transmitted from the wireless power transmitting apparatus. 
         [0071]    The rectifier  620  may rectify the wireless power signal received by the secondary core  610 . In detail, the rectifier  620  may convert an AC-waveform wireless power signal received by the secondary core  610  into DC power by using wave rectification. 
         [0072]    The regulator  630  is configured by an output terminal of the rectifier  620  and may be regulated at low voltage so as to supply high voltage and/or irregular voltage output from the rectifier  620  as stable power. 
         [0073]    The detection circuit  640  is connected to an output terminal of the rectifier  620  to measure the strength of the wireless power signal transmitted from the wireless power transmitting apparatus by monitoring the DC voltage output from the rectifier  620 . 
         [0074]    The controller  650  may actively control a gate bias of the modulator  660  based on the strength of the wireless power signal measured in the detection circuit  640 . Herein, the gate bias may be defined as DC voltage or current applied between a gate and a source or substrate of a transistor (for example, a MOSFET) in the modulator  660 . For example, the controller may control a gate bias Vgs to be 3.3 V, 3 V, 2.5 V, and 2 V according to the strength of the wireless power signal. To this end, the controller  650  may include at least one capacitor, at least one transistor, and at least one resistor. 
         [0075]    The controller  650  controls the gate bias of the modulator  660  to generate charge control information, charge status information, full charge information, and the like and may transfer the generated information to the wireless power transmitting apparatus through the secondary core  610 . 
         [0076]    Hereinafter, a structure of the wireless power transmitting apparatus according to another embodiment of the present invention will be described in more detail with reference to  FIG. 7 . In  FIG. 7 , as an example, a case where a secondary core  710  includes one secondary coil is illustrated. 
         [0077]    Referring to  FIG. 7 , a modulator  760  may be connected to a controller  750  and for example, may include two capacitors, two transistors, and two resistors. Herein, the transistor may be a FET or MOSFET. 
         [0078]    In  FIG. 7 , the capacitor included in the modulator  760  may also be replaced with a resistor. That is, the modulator  760  may also include a plurality of resistors and a plurality of transistors. In this case, when the modulator  760  modulates the amplitude of the wireless power signal, a time taken to store the charges like when using the capacitor is not required. Accordingly, the response time of the wireless power receiving apparatus may be minimized. 
         [0079]    Meanwhile, in  FIG. 7 , the case where the modulator  760  is configured in the AC terminal of the wireless power receiving apparatus is illustrated, but the modulator  760  may be configured in the DC terminal (for example, between the rectifier  720  and the regulator  730 ) of the wireless power receiving apparatus. 
         [0080]      FIG. 8  is a flowchart illustrating a wireless communication method of a wireless power receiving apparatus according to an embodiment of the present invention. Hereinafter, a wireless communication method of a wireless power receiving apparatus according to the present invention will be described in more detail with reference to  FIG. 8 . 
         [0081]    The wireless power receiving apparatus according to the present invention receives a wireless power signal from a wireless power transmitting apparatus (S 810 ) and then may measure strength of the wireless power signal (S 820 ). In addition, according to strength of the wireless power signal measured for stable communication with the wireless power transmitting apparatus, a quantity of the amplitude modulation to be applied in communication with the wireless power transmitting apparatus may be determined (S 830 ). 
         [0082]    As an example, the wireless power receiving apparatus may select any one or at least one of a plurality of modulators included in the wireless power receiving apparatus according to the measured strength of the wireless power signal and modulate the amplitude of the wireless power signal by using the selected modulator. In this case, the plurality of modulators may include at least one capacitor or resistor and at least one transistor as illustrated in  FIG. 5 , respectively. Herein, the at least one transistor may be implemented by a FET, a MOSFET, or the like. Meanwhile, the plurality of modulators may be configured in an AC terminal or a DC terminal of the wireless power receiving apparatus. In addition, the plurality of modulators may be connected to the controller of the wireless power receiving apparatus in parallel. In  FIG. 5 , the capacitor included in each modulator may be replaced with a resistor in order to reduce a response time of the wireless power receiving apparatus. 
         [0083]    As another example, the wireless power receiving apparatus may actively control a gate bias of one modulator included in the wireless power receiving apparatus according to the measured strength of the wireless power signal. To this end, one modulator may include a plurality of resistors and at least one transistor as illustrated in  FIG. 7 . One modulator may be configured in an AC terminal or a DC terminal of the wireless power receiving apparatus and a capacitor included in one modulator may also be replaced with a resistor in order to reduce the response time of the wireless power receiving apparatus. 
         [0084]    The wireless power receiving apparatus may modulate a signal by using the determined amplitude modulation and then perform communication with the wireless power transmitting apparatus. Particularly, the signal with the modulated amplitude is induced to the primary core of the wireless power transmitting apparatus from the secondary core of the wireless power receiving apparatus, and the wireless power transmitting apparatus may control transmission power by detecting the signal induced in the primary core. 
         [0085]    The above description just illustrates the technical spirit of the present invention and various changes and modifications can be made by those skilled in the art to which the present invention pertains without departing from an essential characteristic of the present invention. Therefore, the embodiments disclosed in the present invention are used to not limit but describe the technical spirit of the present invention and the scope of the technical spirit of the present invention is not limited by the embodiments. The scope of the present invention should be interpreted by the appended claims and it should be analyzed that all technical spirit in the equivalent range thereto is intended to be embraced by the scope of the present invention.