Abstract:
A wireless power reception method of a wireless power receiver for receiving power from a wireless power transmitter is disclosed. The wireless power receives a connection signal for identifying the wireless power receiver from the wireless power transmitter; transmitting a response signal in response to the connection signal to the wireless power transmitter; negotiating a power transmission condition with the wireless power transmitter; and receiving the power using resonance frequency band according to the negotiated power transmission condition. Furthermore, frequency band for exchanging information used for the power reception is different from the resonance frequency band. In addition, the negotiating of the power transmission condition comprises: receiving first state information of the wireless power transmitter from the identified wireless power transmitter, and transmitting second state information to the wireless power transmitter, the second state information being determined from the wireless power receiver based on the first state information of the wireless power transmitter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of co-pending U.S. patent application Ser. No. 14/616,367, filed Feb. 6, 2015 which is a divisional of co-pending U.S. patent application Ser. No. 13/673,163 filed Nov. 9, 2012, which claims the benefit of Korean Patent Application No. 10-2011-0117233, filed on Nov. 10, 2011, the entire contents of all of the above applications are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The embodiment relates to a wireless power transmitter, a wireless power receiver, a wireless power transmission method and a wireless power reception method. In more particular, the embodiment relates to a wireless power transmission for actively transmitting power according to power transmission state between a wireless power transmitter and a wireless power receiver. 
         [0004]    2. Discussion of the Related Art 
         [0005]    A wireless power transmission or a wireless energy transfer refers to a technology for wirelessly transferring electric energy to desired devices. In the  1800  &#39;s, an electric motor or a transformer employing the principle of electromagnetic induction has been extensively used and then a method for transmitting electric energy by irradiating electromagnetic waves, such as radio waves or lasers, has been suggested. Actually, electric toothbrushes or electric razors, which are frequently used in daily life, are charged based on the principle of electromagnetic induction. Until now, the long-distance transmission using the magnetic induction, the resonance and the short-wavelength radio frequency has been used as the wireless energy transfer scheme. 
         [0006]    However, according to the related art, power transmission environment between a transmission side and a reception side cannot be actively recognized upon the wireless power transmission between the transmission side and the reception side. Thus, a bidirectional communication for sharing state information between the transmission side and the reception side is necessary to achieve more effective and active power transmission. 
       SUMMARY OF THE INVENTION 
       [0007]    An embodiment provides a wireless power transmitter, a wireless power receiver, a wireless power transmission method and a wireless power reception method, capable of actively and effectively performing power transmission by transceiving information about the power transmission state between the wireless power transmitter and the wireless power receiver. 
         [0008]    According to one embodiment, there is provided a wireless power transmission method of a wireless power transmitter for transmitting power to a wireless power receiver. The wireless power transmission method includes transmitting a connection signal for identifying the wireless power receiver, identifying the wireless power receiver by receiving a response signal to the connection signal from the wireless power receiver, negotiating a power transmission condition with the identified wireless power receiver and transmitting the power to the identified wireless power receiver according to the negotiated power transmission condition. 
         [0009]    According to one embodiment, there is provided a wireless power reception method of a wireless power receiver for receiving power from a wireless power transmitter. The wireless power reception method includes receiving a connection signal for identifying the wireless power receiver, transmitting a response signal to the connection signal to the wireless power transmitter, negotiating a power transmission condition with the wireless transmitter and receiving the power from the wireless power transmitter according to the negotiated power transmission condition. 
         [0010]    According to one embodiment, there is provided a wireless power transmitter for transmitting power to a wireless power receiver, the wireless power transmitter including a transmitting unit to transmit a connection signal for identifying the wireless power receiver, an identifying unit to identify the wireless power receiver by receiving a response signal to the connection signal and a negotiating unit to negotiate a power transmission condition with the identified wireless power receiver, wherein the wireless power transmitter transmits the power to the identified wireless power receiver according to the negotiated power transmission condition. 
         [0011]    According to one embodiment, there is provided a wireless power receiver for receiving power from a wireless power transmitter, the wireless power receiver including a receiving unit to receive a connection signal for identifying the wireless power receiver, a transmitting unit to transmit a response signal to the connection signal to the wireless power transmitter and a negotiating unit to negotiate a power transmission condition with the wireless transmitter, wherein the wireless power receiver receives the power from the wireless power transmitter according to the negotiated power transmission condition. 
         [0012]    According to one embodiment, the power transmission can be actively and effectively achieved by using state information between the wireless power transmitter and the wireless power receiver. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a view showing a wireless power transmission system according to one embodiment; 
           [0014]      FIG. 2  is a circuit diagram showing an equivalent circuit of a transmission induction coil according to one embodiment; 
           [0015]      FIG. 3  is a circuit diagram showing an equivalent circuit of a power source and a transmitting unit according to one embodiment; 
           [0016]      FIG. 4  is a is a circuit diagram showing an equivalent circuit of a reception resonant coil, a reception induction coil, a smoothing circuit and a load according to one embodiment; 
           [0017]      FIG. 5  is a block diagram showing a wireless power transmitter and a wireless power receiver according to another embodiment; 
           [0018]      FIG. 6  is a flowchart showing a wireless power transmission method according to one embodiment; 
           [0019]      FIG. 7  is a ladder diagram for explaining a wireless power transmission method according to another embodiment; and 
           [0020]      FIG. 8  is a state transition diagram for wireless power transmission according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 1  is a view showing a wireless power transmission system according to one embodiment. 
         [0022]    Referring to  FIG. 1 , the wireless power transmission system may include a power supply apparatus  100 , a wireless power transmitter  200 , a wireless power receiver  300 , and a load  400 . 
         [0023]    According to one embodiment, the power supply apparatus  100  may be included in the wireless power transmitter  200 . 
         [0024]    The wireless power transmitter  200  may include a transmission induction coil  210  and a transmission resonant coil  220 . 
         [0025]    The wireless power receiver  300  may include a reception resonant coil  310 , a reception induction coil  320  and a rectifying unit  330 . 
         [0026]    Both terminals of the power supply apparatus  100  are connected to both terminals of the transmission induction coil  210 . 
         [0027]    The transmission resonant coil  220  may be spaced apart from the transmission induction coil  210  by a predetermined distance. 
         [0028]    The reception resonant coil  310  may be spaced apart from the reception induction coil  320  by a predetermined distance. 
         [0029]    Both terminals of the reception induction coil  320  are connected to both terminals of the rectifying unit  330 , and the load  400  is connected to both terminals of the rectifying unit  330 . According to one embodiment, the load  400  may be included in the wireless power receiver  300 . 
         [0030]    The power generated from the power supply apparatus  100  is transmitted to the wireless power transmitter  200 . The power received in the wireless power transmitter  200  is transmitted to the wireless power receiver  300 , that is, has the resonance frequency the same as that of the wireless power transmitter  200 , through a resonance phenomenon. 
         [0031]    Hereinafter, the power transmission process will be described in more detail. 
         [0032]    The power supply apparatus  100  generates AC power having a predetermined frequency and transmits the AC power to the wireless power transmitter  200 . 
         [0033]    The transmission induction coil  210  and the transmission resonant coil  220  are inductively coupled with each other. In other words, if AC current flows through the transmission induction coil  210  due to the power received from the power supply apparatus  100 , the AC current is induced to the transmission resonant coil  220  physically spaced apart from the transmission induction coil  210  due to the electromagnetic induction. 
         [0034]    Thereafter, the power received in the transmission resonant coil  220  is transmitted to the wireless power receiver  300 , which makes a resonance circuit with the wireless power transmitter  200 , using resonance. 
         [0035]    Power can be transmitted between two LC circuits, which are impedance-matched with each other, using resonance. The power transmitted using the resonance can be farther transmitted with higher efficiency when comparing with the power transmitted by the electromagnetic induction. 
         [0036]    The reception resonant coil  310  receives power from the transmission resonant coil  220  using the resonance. The AC current flows through the reception resonant coil  310  due to the received power. The power received in the reception resonant coil  310  is transmitted to the reception induction coil  320 , which is inductively coupled with the reception resonant coil  310 , due to the electromagnetic induction. The power received in the reception induction coil  320  is rectified by the rectifying unit  330  and transmitted to the load  400 . 
         [0037]    The transmission resonant coil  220  of the wireless power transmitter  200  may transmit power to the reception resonant coil  310  of the wireless power receiver  300  using a magnetic field. In detail, the transmission resonant coil  220  and the reception resonant coil  310  are resonance-coupled with each other so that the transmission resonant coil  220  and the reception resonant coil  310  operate at a resonance frequency. 
         [0038]    The resonance-coupling between the transmission resonant coil  220  and the reception resonant coil  310  can significantly improve the power transmission efficiency between the wireless power transmitter  200  and the wireless power receiver  300 . 
         [0039]    A quality factor and a coupling coefficient are important in the wireless power transmission. In other words, the power transmission efficiency can be gradually improved as the values of the quality factor and the coupling coefficient are increased. 
         [0040]    The quality factor may refer to an index of energy that may be stored in the vicinity of a wireless power transmitter or a wireless power receiver. 
         [0041]    The quality factor may vary according to the operating frequency w as well as a shape, a dimension and a material of a coil. The quality factor may be expressed as following equation, Q=w*L/R. In the above equation, L refers to the inductance of a coil and R refers to resistance corresponding to the quantity of power loss caused in the coil. 
         [0042]    The quality factor may have a value of 0 to infinity. The power transmission efficiency between the wireless power transmitter  200  and the wireless power receiver  300  can be improved as the value of the quality factor is increased. 
         [0043]    The coupling coefficient represents the degree of inductive magnetic coupling between a transmission coil and a reception coil, and has a value of 0 to 1. 
         [0044]    The coupling coefficient may vary according to the relative position and the distance between the transmission coil and the reception coil. 
         [0045]      FIG. 2  is a circuit diagram showing an equivalent circuit of the transmission induction coil  210  according to the one embodiment. 
         [0046]    As shown in  FIG. 2 , the transmission induction coil  210  may include an inductor L 1  and a capacitor C 1 , and a circuit having a desirable inductance and a desirable capacitance can be constructed by the inductor L 1  and the capacitor C 1 . 
         [0047]    The transmission induction coil  210  may be constructed as an equivalent circuit in which both terminals of the inductor L 1  are connected to both terminals of the capacitor C 1 . In other words, the transmission induction coil  210  may be constructed as an equivalent circuit in which the inductor L 1  is connected to the capacitor C 1  in parallel. 
         [0048]    The capacitor C 1  may include a variable capacitor, and impedance matching may be performed by adjusting the capacitance of the capacitor C 1 . The equivalent circuit of the transmission resonant coil  220 , the reception resonant coil  310  and the reception induction coil  320  may be the same as the equivalent circuit shown in  FIG. 2 . 
         [0049]      FIG. 3  is a circuit diagram showing an equivalent circuit of the power supply apparatus  100  and the wireless power transmitter  200  according to one embodiment. 
         [0050]    As shown in  FIG. 3 , the transmission induction coil  210  and the transmission resonant coil  220  may be constructed by using inductors L 1  and L 2  and capacitors C 1  and C 2  having predetermined inductances and capacitances, respectively. 
         [0051]      FIG. 4  is a circuit diagram showing an equivalent circuit of the wireless power receiver  300  according to one embodiment. 
         [0052]    As shown in  FIG. 4 , the reception resonant coil  310  and the reception induction coil  320  may be constructed by using inductors L 3  and L 4 , and capacitors C 3  and C 4  having predetermined inductances and capacitances, respectively. 
         [0053]    The rectifying unit  330  can transfer DC power to the load  400  by converting AC power received from the reception induction coil  320  into the DC power. 
         [0054]    In detail, the rectifying unit  330  may include a rectifier and a smoothing circuit. According to one embodiment, the rectifier may include a silicon rectifier and may be equivalent as a diode D 1  as shown in  FIG. 4 . 
         [0055]    The rectifier can convert AC power received from the reception induction coil  320  into the DC power. 
         [0056]    The smoothing circuit can output a smooth DC power by removing AC components included in the DC power converted by the rectifier. According to one embodiment, as shown in  FIG. 4 , the smoothing circuit may include a rectifying capacitor C 5 , but the embodiment is not limited thereto. 
         [0057]    The load  400  may be a predetermined rechargeable battery or a device requiring the DC power. For example, the load  400  may refer to a battery. 
         [0058]    The wireless power receiver  300  may be installed in an electronic device, such as a cellular phone, a laptop computer or a mouse, requiring the power. 
         [0059]    The wireless power transmitter  200  may interchange information with the wireless power receiver  300  through in-band communication or an out-of-band communication. 
         [0060]    The in-band communication refers to the communication for interchanging information between the wireless power transmitter  200  and the wireless power receiver  300  through a signal used in the wireless power transmission. The wireless power receiver  300  may further include a switch and may receive or may not receive power transmitted from the wireless power transmitter  200  through a switching operation of the switch. Accordingly, the wireless power transmitter  200  can recognize an on-signal or an off-signal of the switch included in the wireless power receiver  300  by detecting the quantity of power consumed in the wireless power transmitter  200 . 
         [0061]    In detail, the wireless power receiver  300  may change the power consumed in the wireless power transmitter  200  by adjusting the quantity of power absorbed in a resistor by using the resistor and the switch. The wireless power transmitter  200  may acquire the state information of the wireless power receiver  300  by detecting the variation of the power consumption. The switch may be connected to the resistor in series. According to one embodiment, the state information of the wireless power receiver  300  may include information about the present charge amount in the wireless power receiver  300  and the change of the charge amount. 
         [0062]    In more detail, if the switch is open, the power absorbed in the resistor becomes zero, and the power consumed in the wireless power transmitter  200  is reduced. 
         [0063]    If the switch is short-circuited, the power absorbed in the resistor becomes greater than zero, and the power consumed in the wireless power transmitter  200  is increased. If the wireless power receiver repeats the above operation, the wireless power transmitter  200  detects power consumed therein to make digital communication with the wireless power receiver  300 . 
         [0064]    The wireless power transmitter  200  receives the state information of the wireless power receiver  300  through the above operation so that the wireless power transmitter  200  can transmit appropriate power. 
         [0065]    To the contrary, the wireless power transmitter  200  may include a resistor and a switch to transmit the state information of the wireless power transmitter  200  to the wireless power receiver  300 . 
         [0066]    According to one embodiment, the state information of the wireless power transmitter  200  may include information about the maximum amount of power to be supplied from the wireless power transmitter  200 , the number of wireless power receivers  300  receiving the power from the wireless power transmitter  200  and the amount of available power of the wireless power transmitter  200 . 
         [0067]    Hereinafter, the out-of-band communication will be described. 
         [0068]    The out-of-band communication refers to the communication performed through a specific frequency band other than the resonance frequency band in order to exchange information necessary for the power transmission. The wireless power transmitter  200  and the wireless power receiver  300  can be equipped with out-of-band communication modules to exchange information necessary for the power transmission. The out-of-band communication module may be installed in the power supply apparatus. In one embodiment, the out-of-band communication module may use a short-distance communication technology, such as Bluetooth, Zigbee, WLAN or NFC, but the embodiment is not limited thereto. 
         [0069]    Hereinafter, a wireless power transmitter, a wireless power receiver, a wireless power transmission method, a wireless power reception method, an information transmission method and an information reception method according to another embodiment will be described with reference to  FIGS. 5 to 8  as well as  FIGS. 1 to 4 . 
         [0070]      FIG. 5  is a block diagram showing the wireless power transmitter and the wireless power receiver according to another embodiment. 
         [0071]    The wireless power transmitter  200  may include a transmitting unit  230 , an identifying unit  240 , a negotiating unit  250  and a control unit  260 . In addition, the wireless power transmitter  200  may further include the transmission induction coil  210  and the transmission resonant coil  220  shown in  FIG. 1 . 
         [0072]    The wireless power receiver  300  may include a receiving unit  350 , a response signal transmitting unit  360 , a negotiating unit  370  and a control unit  380 . In addition, the wireless power receiver  300  may further include the reception resonant coil  310 , the reception induction coil  320 , the rectifier circuit  330  and the load  400  shown in  FIG. 1 . 
         [0073]    The transmitting unit  230  transmits a connection signal to the receiving unit  350 . According to one embodiment, the connection signal refers to a power signal for detecting and identifying the wireless power receiver  300 . The transmitting unit  230  may periodically transmit the connection signal to detect the wireless power receiver  300 . 
         [0074]    The identifying unit  240  identifies the wireless power receiver  300  by receiving a response signal to the connection signal. If the identifying unit  240  does not receive the response signal to the connection signal, the transmission of the connection signal to the corresponding wireless power receiver  300  may be stopped. According to one embodiment, the identifying unit  240  may not receive the response signal when the connection signal is transmitted to a non-transmittable object, such as a key or a coin, other than a transmittable object. 
         [0075]    As the response signal to the connection signal is received in the identifying unit  240 , the negotiating unit  250  may negotiate the power transmission condition with the identified wireless power receiver  300 . According to one embodiment, the power transmission condition may refer to the factors required for power transmission or power transmission stop between the wireless power transmitter  200  and the wireless power receiver  300 . The power transmission may be achieved or not depending on the power transmission condition between the wireless power transmitter  200  and the wireless power receiver  300 . 
         [0076]    The control unit  260  controls the wireless power transmitter  200  such that the power can be transmitted to the identified wireless power receiver  300  according to the negotiated power transmission condition. 
         [0077]    The negotiating unit  250  transmits state information of the wireless power transmitter  200  to the identified wireless power receiver  300  to allow the identified wireless power receiver  300  to determine specific state information based on the state information of the wireless power transmitter  200  and receives the specific state information from the identified wireless power receiver  300 . 
         [0078]    According to one embodiment, the state information of the wireless power transmitter  200  may include information about the maximum amount of power to be supplied from the wireless power transmitter  200 , information about the number of wireless power receivers  300  receiving the power from the wireless power transmitter  200  and identification information of the wireless power transmitter  200 . 
         [0079]    According to one embodiment, the state information of the wireless power receiver  300  may include information about the present charge amount in the wireless power receiver  300  and the change of the charge amount. 
         [0080]    The negotiating unit  250  receives the state information of the identified wireless power receiver  300  to determine specific state information based on the state information of the wireless power receiver  300  and the control unit  260  controls the wireless power transmitter  200  to transmit the power corresponding to the specific state information to the identified wireless power receiver  300 . 
         [0081]    According to one embodiment, the negotiating unit  250  transmits the information about the available amount of power to the identified wireless power receiver  300  to allow the identified wireless power receiver  300  to determine the demanded amount of power within the range of the available amount of power. After that, if the wireless power receiver  300  determines the demanded amount of power, the negotiating unit  250  receives the information about the demanded amount of power. The control unit  260  transmits power corresponding to the demanded amount of power to the wireless power receiver  300 . 
         [0082]    Through the above procedure, the power transmission state between the wireless power transmitter  200  and the wireless power receiver  300  can be detected, so that the active power transmission can be performed. 
         [0083]    The wireless power receiver  300  may include the receiving unit  350 , the response signal transmitting unit  360 , the negotiating unit  370  and the control unit  380 . In addition, the wireless power receiver  300  may further include the reception resonant coil  310 , the reception induction coil  320 , the rectifier circuit  330  and the load  400  shown in  FIG. 1 . 
         [0084]    The receiving unit  350  may receive the connection signal from the wireless power transmitter  200 . 
         [0085]    The wireless power receiver  300  can transmit the response signal to the received connection signal to the wireless power transmitter  200 . 
         [0086]    The negotiating unit  370  can negotiate the power transmission condition with the wireless power transmitter  200  and the wireless power receiver  300  can receive the power from the wireless power transmitter  200  based on the negotiation result. 
         [0087]    According to one embodiment, the negotiating unit  370  receives the state information of the wireless power transmitter  200  to determine specific state information based on the state information of the wireless power transmitter  200  and can transmit the determined specific state information to the wireless power transmitter  200 . In addition, the wireless power receiver  300  can receive the power corresponding to the specific state information from the wireless power transmitter  200 . 
         [0088]    Through the above procedure, the power transmission state between the wireless power transmitter  200  and the wireless power receiver  300  can be detected, so that the active power transmission can be performed. 
         [0089]      FIG. 6  is a flowchart showing a wireless power transmission method according to one embodiment. 
         [0090]    Although a wireless power transmission using resonance is described below as an example of the wireless power transmission method according to one embodiment, a wireless power transmission using an electromagnetic induction is also possible. 
         [0091]    According to one embodiment, a bidirectional communication can be implemented through the configuration for changing the amount of current in the wireless power transmitter  200  and the wireless power receiver  300 . For instance, the bidirectional communication can be implemented through the in-band communication using a variable resistor and a switch or the out-of-band communication. The in-band communication and the out-of-band communication are the same as those described with reference to  FIG. 1 . 
         [0092]    Referring to  FIG. 6 , the wireless power transmitter  200  transmits the connection signal to the wireless power receiver  300  (S 101 ). According to one embodiment, the connection signal refers to a power signal for detecting and identifying the wireless power receiver  300 . The wireless power transmitter  200  may periodically transmit the connection signal to detect the wireless power receiver  300 . 
         [0093]    Upon receiving the connection signal, the wireless power receiver  300  transmits the response signal to the wireless power transmitter  200  (S 103 ). After that, the wireless power transmitter  200  receives the response signal to the connection signal to identify the wireless power receiver  300 . 
         [0094]    If the wireless power transmitter  200  does not receive the response signal to the connection signal, the transmission of the connection signal to the corresponding wireless power receiver  300  may be stopped. According to one embodiment, the wireless power transmitter  200  may not receive the response signal when the connection signal is transmitted to a non-transmittable object, such as a key or a coin, other than a transmittable object. 
         [0095]    If the wireless power transmitter  200  identifies the wireless power receiver  300 , the wireless power transmitter  200  negotiates the power transmission condition with the identified wireless power receiver  300  (S 105 ). 
         [0096]    If the negotiation between the wireless power transmitter  200  and the wireless power receiver  300  is succeeded (S 107 ), the wireless power transmitter  200  transmits the power to the wireless power receiver  300  based on the negotiated power transmission condition (S 109 ). 
         [0097]    Step (S 105 ) of negotiating the power transmission to the identified wireless power receiver  300  may include a step of transmitting the state information of the wireless power transmitter  200  to the identified wireless power receiver  300  to allow the identified wireless power receiver  300  to determine the specific state information based on the state information of the wireless power transmitter  200  and a step of receiving the specific state information from the identified wireless power receiver  300 . 
         [0098]    According to one embodiment, steps  105  to  109  may include the following process. The wireless power transmitter  200  transmits the information about the available amount of power to the identified wireless power receiver  300  to allow the identified wireless power receiver  300  to determine the demanded amount of power within the range of the available amount of power. After that, if the wireless power receiver  300  determines the demanded amount of power, the wireless power transmitter  200  receives the information about the demanded amount of power. Then, the wireless power transmitter  200  transmits the power corresponding to the demanded amount of power to the identified wireless power receiver  300 . 
         [0099]    According to one embodiment, steps  105  to  109  may include the following process. As an example, the wireless power transmitter  200  is assumed as it wirelessly transmits the power to a plurality of wireless power receivers  300 . 
         [0100]    Information about the priority of power transmission to the wireless power receivers  300  is transmitted from the wireless power transmitter  200  to the identified wireless power receiver  300  to allow the identified wireless power receiver  300  to determine whether it accepts the priority determined based on the information about the priority. Then, if the identified wireless power receiver  300  accepts the determined priority, the wireless power transmitter  200  transmits the power based on the priority. 
         [0101]    According to one embodiment, steps  105  to  109  may include the following process. As an example, it is assumed that a plurality of wireless power transmitters  200  are present. Each wireless power transmitter  200  transmits the identification information of each wireless power transmitter  200  to the identified wireless power receiver  300  to allow the identified wireless power receiver  300  to determine one of the wireless power transmitters  200 . Then, if the identified wireless power receiver  300  selects a specific wireless power transmitters  200 , the selected wireless power transmitters  200  receives a selection request signal from the identified wireless power receiver  300  and transmits the power to the identified wireless power receiver  300 . 
         [0102]    According to one embodiment, in the negotiation step, the wireless power transmitter  200  requests the state information of the wireless power receiver  300  and the state information of the wireless power receiver  300  is transmitted to the wireless power transmitter  200  in response to the request. In contrast, in the negotiation step, the wireless power receiver  300  may request the state information of the wireless power transmitter  200  and the state information of the wireless power transmitter  200  may be transmitted to the wireless power receiver  300  in response to the request. 
         [0103]    According to one embodiment, in the negotiation step, the power transmission is determined based on the state information of the wireless power transmitter  200  and the wireless power receiver  300 . 
         [0104]    According to one embodiment, the state information of the wireless power transmitter  200  may refer to information about the number of the wireless power receivers  300  that receive the power from one wireless power transmitter  200 . 
         [0105]    According to one embodiment, if the identification number is given for each wireless power receiver  300 , the state information of the wireless power transmitter  200  may refer to information about the given identification numbers of the wireless power receivers  300  that receive the power from a specific wireless power transmitter  200 . 
         [0106]    According to one embodiment, the state information of the wireless power transmitter  200  may refer to information about the amount of power supplied to the wireless power receiver  300 . 
         [0107]    According to one embodiment, when the power is transmitted to a plurality of wireless power receivers  300 , the state information of the wireless power transmitter  200  may refer to information about the priority of power transmission to the wireless power receivers  300 . 
         [0108]    According to one embodiment, when the power is transmitted to a plurality of wireless power receivers  300 , the state information of the wireless power transmitter  200  may refer to information about the differential ratio of the power transmission to the wireless power receivers  300 . For instance, when a specific wireless power transmitter  200  transmits the power to three wireless power receivers  300 , the specific wireless power transmitter  200  may transmit the power to the three wireless power receivers  300  at the differential ratio of 50%, 30% and 20%, respectively. 
         [0109]    According to one embodiment, the state information of the wireless power receiver  300  may refer to information about the present charge amount or the change of the charge amount in the wireless power receiver  300 . 
         [0110]    According to one embodiment, the state information of the wireless power receiver  300  may refer to information about the wireless power transmitter  200  that transmits the power to the wireless power receiver  300 . That is, the state information of the wireless power receiver  300  may signify the identification information of the wireless power receiver  300  that receives the power. 
         [0111]    If the negotiation between the wireless power transmitter  200  and the wireless power receiver  300  is succeeded (S 107 ), the wireless power transmitter  200  transmits the power to the identified wireless power receiver  300  based on the negotiated power transmission condition (S 109 ). 
         [0112]    If the state of the wireless power transmitter  200  is changed during the power transmission process (S 111 ), the process returns to negotiation step S 105  or the power transmission is stopped. 
         [0113]    According to one embodiment, the change of the state of the wireless power transmitter  200  may refer to information about the change of the amount of power to be supplied from the wireless power transmitter  200 . In this case, if the amount of power to be supplied from the wireless power transmitter  200  is reduced due to the external factors, the wireless power transmitter  200  stops the power transmission and the process enters negotiation step S 105  to perform the negotiation for adjusting the amount of power transmission between the wireless power transmitter  200  and the wireless power receiver  300 . Then, if the negotiation for adjusting the amount of power transmission is succeeded, the power transmission is performed. Otherwise, the process again enters negotiation step S 105 . If the power supplied to the wireless power transmitter  200  is shut off or the wireless power receiver  300  has been charged, the power transmission is finished. 
         [0114]    According to one embodiment, the change of the state of the wireless power transmitter  200  may refer to information about the change of the number of wireless power receivers  300  receiving the power from the wireless power transmitter  200 . 
         [0115]    If the state of the wireless power receiver  300  is changed during the power transmission process (S 113 ), the process returns to negotiation step S 105  or the power transmission is stopped. 
         [0116]    According to one embodiment, the change of the state of the wireless power receiver  300  may signify the detection of a new wireless power receiver  300  besides the wireless power receiver  300  that receives the power from the wireless power transmitter  200 . 
         [0117]      FIG. 7  is a ladder diagram for explaining a wireless power transmission method according to another embodiment. 
         [0118]    Referring to  FIG. 7 , the wireless power transmitter  200  requests the state information to each of wireless power transmitters  201 ,  202  and  203  (S 201 ). Although only three wireless power transmitters  201 ,  202  and  203  are illustrated in  FIG. 7 , the embodiment is not limited thereto. In addition, each of the wireless power transmitters  201 ,  202  and  203  may include the configuration of the wireless power transmitter  200  described with reference to  FIG. 5 . The state information of the wireless power transmitter  200  may include information about the maximum amount of power to be supplied from the wireless power transmitter  200 , the number of wireless power receivers  300  receiving the power from the wireless power transmitter  200  and the amount of available power of the wireless power transmitter  200 , but the embodiment is not limited thereto. 
         [0119]    According to one embodiment, the wireless power transmitter  200  may transceive information to/from other wireless power transmitters  201 ,  202  and  203  through the in-band communication or the out-of-band communication. 
         [0120]    Then, the wireless power transmitter  200  receives the state information of the wireless power transmitters  201 ,  202  and  203 , respectively, in response to the request for the state information (S 203 ). 
         [0121]    In addition, the wireless power transmitter  200  creates the state list based on the received state information of the wireless power transmitters  201 ,  202  and  203  (S 205 ). According to one embodiment, the state list may be created by collecting the state information of the wireless power transmitters  201 ,  202  and  203 . The state list will be described in more detail with reference to table 1. 
         [0000]    
       
         
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 State information of wireless power transmitter 
               
             
          
           
               
                 Wireless power 
                 Maximum  
                 Number of  
                 Available  
               
               
                 transmitter  
                 amount of power  
                 wireless power  
                 amount of 
               
               
                 (ID symbol) 
                 to be supplied 
                 receivers 
                 power 
               
               
                   
               
               
                 Wireless power 
                 100 W 
                 4 
                 20 W 
               
               
                 transmitter A 
                   
                   
                   
               
               
                 Wireless power 
                 200 W 
                 5 
                 15 W 
               
               
                 transmitter B 
                   
                   
                   
               
               
                 Wireless power 
                 300 W 
                 6 
                 30 W 
               
               
                 transmitter C 
               
               
                   
               
             
          
         
       
     
         [0122]    Table 1 is an example of the state list created by the wireless power transmitter  200  based on the state information received from three wireless power transmitters A, B and C. In addition, although not shown in the drawing, the wireless power transmitter  200  may create the state list by adding the state information of the wireless power transmitter  200  to the state list. 
         [0123]    Referring to table 1, the maximum amount of power to be supplied from the wireless power transmitter A is 100 W, and the wireless power transmitter A supplies the power to four wireless power receivers  300 . In addition, the available amount of power in the wireless power transmitter A is 20 W. The available amount of power refers to the amount of power to be supplied to another wireless power receiver  300  except for the four wireless power receivers  300 . In addition, the demanded amount of power of the four wireless power receivers  300  may not be needed to be constant, but may vary. 
         [0124]    The available amount of power in the wireless power transmitter may vary depending on the wireless power transmitters  201 ,  202  and  203 . 
         [0125]    The maximum amount of power of the wireless power transmitter B is 200 W and the wireless power transmitter B supplies the power to five wireless power receivers  300 . In addition, the available amount of power in the wireless power transmitter B is 15 W. Further, the demanded amount of power of the five wireless power receivers  300  may not be needed to be constant, but may vary. 
         [0126]    The maximum amount of power of the wireless power transmitter C is 300 W and the wireless power transmitter C supplies the power to six wireless power receivers  300 . In addition, the available amount of power in the wireless power transmitter C is 30 W. Further, the demanded amount of power of the six wireless power receivers  300  may not be needed to be constant, but may vary. 
         [0127]    After that, the wireless power transmitter  200  receives the request for transmission of the state list from the wireless power receiver  300  (S 207 ). According to one embodiment, the wireless power transmitter  200  and the wireless power receiver  300  may transceive information through the in-band communication or the out-of-band communication. 
         [0128]    Then, the wireless power transmitter  200  transmits the state list to the wireless power receiver  300  in response to the request for the transmission of the state list (S 209 ). 
         [0129]    Thereafter, the wireless power receiver  300  selects at least one of the wireless power transmitters  201 ,  202  and  203  based on the received state list to receive the power (S 211 ). That is, the wireless power receiver  300  may select at least one of the wireless power transmitters  201 ,  202  and  203  based on the state information of each wireless power transmitter, that is, the maximum of the power to be supplied, the number of the wireless power receivers that receive the power from each wireless power transmitter and the available amount of power in each wireless power transmitter. 
         [0130]    According to one embodiment, the wireless power receiver  300  may select the wireless power transmitter  203  having the highest available amount of power from among the wireless power transmitters  201 ,  202  and  203 . Such a selection signifies that the wireless power receiver  300  requires the power of 25 W so the wireless power receiver  300  selects the wireless power transmitter capable of supplying the power of 25 W. However, the embodiment is not limited to the above and the wireless power receiver  300  may select two wireless power transmitters  201  and  203 . 
         [0131]    According to one embodiment, the wireless power receiver  300  may select the wireless power transmitter A, which transmits the power to the wireless power receivers  300  of the fewest number, from among the wireless power transmitters  201 ,  202  and  203 . 
         [0132]    In this manner, the wireless power receiver  300  can select the wireless power transmitter suitable for the power transmission based on the state list. 
         [0133]    After that, the wireless power receiver  300  transmits information about the selection of at least one wireless power transmitter to the wireless power transmitter  200  (S 213 ). The selection information may include information about the wireless power transmitter selected by the wireless power receiver  300  from among the wireless power transmitters  201 ,  202  and  203 . In detail, if the wireless power transmitters  201 ,  202  and  203  have identification symbols A, B and C, respectively, the selection information may include information about the identification symbols. 
         [0134]    The wireless power transmitter  200  requests the power transmission to the wireless power transmitter  201  selected based on the received selection information (S 215 ). In  FIG. 7 , the wireless power transmitter  200  requests the power transmission to the wireless power transmitter  201 , but the embodiment is not limited thereto. 
         [0135]    The selected wireless power transmitter  201  transmits the power to the wireless power receiver  300  (S 217 ). According to one embodiment, the selected wireless power transmitter  201  may wirelessly transmit the power to the wireless power receiver  300  using electromagnetic induction or resonance. 
         [0136]    In this manner, according to the embodiment, the wireless power transmitter  200  and the wireless power receiver  300  may share the information through the bidirectional communication, so that the active power transmission can be achieved based on the power condition. Thus, the power transmission can be effectively performed between the wireless power transmitter  200  and the wireless power receiver  300 . 
         [0137]      FIG. 8  is a state transition diagram for wireless power transmission according to one embodiment. 
         [0138]    Hereinafter, the state transition diagram for the wireless power transmission according to one embodiment will be described with reference to  FIGS. 1 to 7 . 
         [0139]    First Mode (Selection Mode) 
         [0140]    At first, the wireless power transmission system including the wireless power transmitter  200  and the wireless power receiver  300  is in a first mode M 1 . According to one embodiment, the first mode M 1  may be the selection mode. In the first mode, the wireless power transmitter  200  may manage an interface region for removal and arrangement of objects. 
         [0141]    If the wireless power transmitter  200  detects at least one object, the wireless power transmitter  200  must try to detect the position of the at least one object. In particular, the wireless power transmitter  200  may try to distinguish the wireless power receiver  300  capable of receiving the power from other foreign objects (for instance, key or coin). 
         [0142]    In addition, the wireless power transmitter  200  must try to select the wireless power receiver  300  for power transmission. If the wireless power transmitter  200  has no sufficient information for accomplishing the above purposes at the early stage, the wireless power transmitter  200  sequentially and repeatedly performs the connection mode and the identification and setting mode, which will be described later, and then performs the selection mode. 
         [0143]    Second Mode (Connection Mode) 
         [0144]    According to one embodiment, the second mode M 2  may be the connection mode. In the second mode M 2 , the wireless power transmitter  200  may have been ready to make communication with the wireless power receiver  300 . 
         [0145]    If the wireless power transmitter  200  detects the wireless power receiver  300  in the first mode M 1 , the wireless power transmission system is transited to the second mode M 2  to transmit the power to the wireless power receiver  300 . That is, if the existence of the wireless power receiver  300  is recognized, the wireless power transmission system is transited to the second mode M 2  to transmit the connection signal to the wireless power receiver  300 . The connection signal is transmitted from the wireless power transmitter  200  to the wireless power receiver  300  to receive the state information of the wireless power receiver  300 . 
         [0146]    If the wireless power transmitter  200  does not receive the response signal to the connection signal from the wireless power receiver  300 , the transmission of the connection signal is stopped and the wireless power transmission system is transited to the first mode M 1 . If the wireless power transmitter  200  receives the response signal, which represents that the load (or battery)  400  has been sufficiently charged, from the wireless power receiver  300 , the wireless power transmitter  200  finishes the power transmission. 
         [0147]    If the wireless power transmitter  200  receives the response signal requesting the power transmission from the wireless power receiver  300  in the second mode M 2 , the wireless power transmitter  200  may transmit an additional connection signal to a new wireless power receiver  300 . The additional connection signal signifies a signal for detecting the new wireless power receiver  300 . 
         [0148]    If there is no response signal to the additional connection signal of the wireless power transmitter  200 , the operation for detecting the new wireless power receiver  300  is stopped. 
         [0149]    Third Mode (Identification and Setting Mode) 
         [0150]    If the wireless power transmitter  200  has received the response signal to the connection signal from the wireless power receiver  300  in the second mode M 2 , the wireless power transmission system is transited to the third mode M 3  for collecting identification information and state information of the wireless power receiver  300 . According to one embodiment, the third mode M 3  may be the identification and setting mode. The identification information of the wireless power receiver  300  may refer to the sort, type or model of the wireless power receiver  300 , but the embodiment is not limited thereto. The state information of the wireless power receiver  300  may signify the maximum amount of power (or maximum amount of charge) to be supplied to the load (or battery)  400  and the present amount of power remaining in the load (or battery)  400 , but the embodiment is not limited thereto. The wireless power transmitter  200  uses the identification information and state information of the wireless power receiver  300  to establish the power transmission condition with respect to the wireless power receiver  300 . The power transmission condition may refer to the group of conditions for specifying the power transmission from the wireless power transmitter  200  to the wireless power receiver  300 . 
         [0151]    If there is restriction for the power transmission condition in the third mode M 3 , the wireless power transmission system is transited to the first mode M 1 . According to one embodiment, the restriction for the power transmission condition may refer to the non-establishment of the power transmission condition. According to one embodiment, the restriction for the power transmission condition may signify a case in which an unexpected data packet is transmitted between the wireless power transmitter  200  and the wireless power receiver  300 . According to one embodiment, the restriction for the power transmission condition may refer to a situation, such as a time out. The time out may include a case in which the power transmission is stopped after a predetermined time has elapsed from the removal of the wireless power receiver  300  that receives the power. 
         [0152]    If the wireless power transmission system is transited to the first mode M 1  due to the restriction for the power transmission condition in the third mode M 3 , the wireless power transmitter  200  transmits the state information related to the wireless power transmitter  200  to the wireless power receiver  300  and then the wireless power transmission system is transited again to the third mode M 3 . According to one embodiment, the state information of the wireless power transmitter  200  may refer to information about the number of wireless power receivers  300  that receive the power from one wireless power transmitter  200 . 
         [0153]    According to one embodiment, if the identification number is dedicated for each wireless power receiver  300 , the state information of the wireless power transmitter  200  may refer to information about the dedicated identification numbers of the wireless power receivers  300  that receive the power from a specific wireless power transmitter  200 . 
         [0154]    According to one embodiment, the state information of the wireless power transmitter  200  may refer to information about the amount of power supplied to the wireless power receiver  300 . 
         [0155]    According to one embodiment, when the power is transmitted to a plurality of wireless power receivers  300 , the state information of the wireless power transmitter  200  may refer to information about the priority of power transmission to the wireless power receivers  300 . 
         [0156]    According to one embodiment, when the power is transmitted to a plurality of wireless power receivers  300 , the state information of the wireless power transmitter  200  may refer to information about the differential ratio of the power transmission to the wireless power receivers  300 . For instance, when a specific wireless power transmitter  200  transmits the power to three wireless power receivers  300 , the specific wireless power transmitter  200  may transmit the power to the three wireless power receivers  300  at the differential ratio of 50%, 30% and 20%, respectively. 
         [0157]    In the number of the wireless power receivers  300  exceeds a certain number in the third mode M 3 , the wireless power transmission system is transited again to the first mode M 1 . That is, when the specific wireless power transmitter  200  supplies the power to five wireless power receivers  300 , if a new wireless power receiver  300  is detected, the wireless power transmission system is transited to the first mode M 1  such that the power cannot be transmitted to the new wireless power receiver  300 . 
         [0158]    Fourth Mode (Power Transmission) 
         [0159]    If the power transmission condition is established in the third mode M 3 , the wireless power transmission system may be transited to the fourth mode M 4 . According to one embodiment, the fourth mode M 4  refers to the state in which the wireless power transmitter  200  supplies the power to the wireless power receiver  300 . According to one embodiment, the fourth mode M 4  refers to the state in which the wireless power transmitter  200  supplies the power to the wireless power receiver  300  while adjusting the current flowing through a coil in response to information received from the wireless power receiver  300 . 
         [0160]    If one of the power transmission conditions is not established in the fourth mode M 4 , the power transmission from the ireless power transmitter  200  to the wireless power receiver  300  is stopped and the wireless power transmission system is transited to the first mode M 1 . According to one embodiment, the restriction for the power transmission condition may refer to the non-establishment of the power transmission condition. According to one embodiment, the restriction for the power transmission condition may signify a case in which an unexpected data packet is transmitted between the wireless power transmitter  200  and the wireless power receiver  300 . According to one embodiment, the restriction for the power transmission condition may refer to a situation, such as a time out. The time out may include a case in which the power transmission is stopped after a predetermined time has elapsed from the removal of the wireless power receiver  300  that receives the power. 
         [0161]    If the wireless power transmitter  200  receives the response signal, which represents that the load (or battery)  400  has been sufficiently charged, from the wireless power receiver  300  in the fourth mode M 4 , the wireless power transmitter  200  finishes the power transmission and the wireless power transmission system is transited to the first mode M 1 . 
         [0162]    If the state of the wireless power receiver  300  is changed in the fourth mode M 4 , the wireless power transmission system may be transited to the third mode M 3 . 
         [0163]    According to one embodiment, the change of the state of the wireless power receiver  300  may signify the detection of a new wireless power receiver  300  during the power transmission from the wireless power transmitter  200  to the wireless power receiver  300 . In this case, the wireless power transmission system is transited from the fourth mode M 4  to the third mode M 3 . After that, the wireless power transmitter  200  works to collect the identification information and state information of the wireless power receiver  300  and the wireless power transmission system is transited to the fourth mode M 4  if the wireless power transmission condition has been established. In addition, the wireless power transmission system is transited to the first mode M 1  if there is restriction for the wireless power transmission condition. 
         [0164]    According to one embodiment, the change of the state of the wireless power receiver  300  may refer to the detection of at least one new wireless power receiver  300  due to the excessive number of the wireless power receivers  300  that receive the power from the wireless power transmitter  200 . In this case, the wireless power transmission system is transited from the fourth mode M 4  to the third mode M 3 . Then, the wireless power transmitter  200  works to collect the identification information and state information of the wireless power receiver  300  and the wireless power transmission system is transited to the first mode M 1  with respect to the excessive wireless power receivers  300 . 
         [0165]    Fifth Mode (Power Transmission and Bidirectional Communication) 
         [0166]    The wireless power transmission system may be transited from the fourth mode M 4  to the fifth mode M 5  to allow the wireless power transmitter  200  and the wireless power receiver  300  to share information. 
         [0167]    According to one embodiment, the fifth mode M 5  may be a power transmission and bidirectional communication mode. According to one embodiment, the power transmission is performed between the wireless power transmitter  200  and the wireless power receiver  300  and the wireless power transmitter  200  and the wireless power receiver  300  may share the information about the change of the state thereof in the fifth mode M 5 . 
         [0168]    According to one embodiment, the information about the change of the state of the wireless power transmitter  200  may refer to information about the change of the number of wireless power transmitters  200  and the change of the amount of power to be supplied from the wireless power transmitter  200 . 
         [0169]    According to one embodiment, the information about the change of the state of the wireless power transmitter  200  may refer to information about the change of the number of wireless power receivers  300  that receive the power from one wireless power transmitter  200 . 
         [0170]    According to one embodiment, if the identification number is dedicated for each wireless power receiver  300 , the information about the change of the state of the wireless power transmitter  200  may refer to information about the dedicated identification numbers of the wireless power receivers  300  that receive the power from a specific wireless power transmitter  200 . 
         [0171]    According to one embodiment, the information about the change of the state of the wireless power transmitter  200  may refer to information about the amount of power supplied to the wireless power receiver  300 . 
         [0172]    According to one embodiment, when the power is transmitted to a plurality of wireless power receivers  300 , the information about the change of the state of the wireless power transmitter  200  may refer to information about the priority of power transmission to the wireless power receivers  300 . 
         [0173]    According to one embodiment, when the power is transmitted to a plurality of wireless power receivers  300 , the information about the change of the state of the wireless power transmitter  200  may refer to information about the differential ratio of the power transmission to the wireless power receivers  300 . For instance, when a specific wireless power transmitter  200  transmits the power to three wireless power receivers  300 , the specific wireless power transmitter  200  may transmit the power to the three wireless power receivers  300  at the differential ratio of 50%, 30% and 20%, respectively. 
         [0174]    If there is restriction for the power transmission condition in the fifth mode M 5 , the wireless power transmission system is transited to the first mode M 1 . According to one embodiment, the restriction for the power transmission condition may refer to the non-establishment of the power transmission condition. According to one embodiment, the restriction for the power transmission condition may signify a case in which an unexpected data packet is transmitted between the wireless power transmitter  200  and the wireless power receiver  300 . According to one embodiment, the restriction for the power transmission condition may refer to a situation, such as a time out. The time out may include a case in which the power transmission is stopped after a predetermined time has elapsed from the removal of the wireless power receiver  300  that receives the power. 
         [0175]    If the wireless power transmission system is transited to the first mode M 1  due to the restriction for the power transmission condition in the fifth mode M 5 , the wireless power transmitter  200  transmits the state information related to the wireless power receivers  300  to the wireless power receiver  300  and then the wireless power transmission system is transited again to the fifth mode M 3 . 
         [0176]    If the wireless power transmitter  200  receives the response signal, which represents that the load (or battery)  400  has been sufficiently charged, from the wireless power receiver  300  in the fifth mode M 5 , the wireless power transmitter  200  finishes the power transmission and the wireless power transmissions system is transited to the first mode M 1 . 
         [0177]    If the state of the wireless power receiver  300  is changed in the fifth mode M 5 , the wireless power transmission system may be transited to the fourth mode M 4 . 
         [0178]    According to one embodiment, the change of the state of the wireless power receiver  300  may signify the detection of a new wireless power receiver  300  during the power transmission from the wireless power transmitter  200  to the wireless power receiver  300  through the bidirectional communication. 
         [0179]    In this case, the wireless power transmission system is transited from the fifth mode M 5  to the fourth mode M 4 . After that, the wireless power transmitter  200  works to collect the identification information and state information of a new wireless power receiver  300  and the wireless power transmission system is transited again to the fifth mode M 4  if the wireless power transmission condition has been established. In addition, the wireless power transmission system is transited to the first mode M 1  if there is restriction for the wireless power transmission condition. 
         [0180]    Although the embodiment has been described with reference to the wireless power transmission using the resonance, the wireless power transmission method, the wireless power reception method and the power control method using the bidirectional communication according to the embodiment may be applicable for other wireless power transmissions using the electromagnetic induction or the RF scheme. 
         [0181]    Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.