Patent Publication Number: US-10790701-B2

Title: Wireless power transmitter, wireless power receiver, and control methods thereof

Description:
PRIORITY 
     This application claims priority under 35 U.S.C. § 119(a) to Korean Patent Application Serial Nos. 10-2013-0004350, 10-2013-0033917, and 10-2013-0053452, which were filed in the Korean Intellectual Property Office on Jan. 15, 2013, Mar. 28, 2013, and May 10, 2013, respectively, the entire disclosure of each of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to a wireless power transmitter and a wireless power receiver, and control methods thereof, and more particularly, to a wireless power transmitter, a wireless power receiver, and method of communication therebetween. 
     2. Description of the Related Art 
     Mobile terminals, such as a mobile phone, a Personal Digital Assistant (PDA), etc., are powered by rechargeable batteries. Commonly, the battery of the mobile terminal is charged through supplied electrical energy using a separate charging apparatus. For example, a separate contact terminal electrically connects the charging apparatus and the battery to each other. 
     However, because the contact terminal typically protrudes outward, the contact terminal is often contaminated by foreign substances or damaged due to moisture, which inhibits proper charging. 
     Wireless charging (or a non-contact charging) has been developed in an effort to address the above-mentioned problems. 
     Wireless charging uses wireless power transmission and reception. For example, wireless charging is used in a system in which a battery can be automatically charged, when the battery is laid on a charging pad, without having to physically connect the mobile phone or battery to a separate charging connector. 
     The wireless charging typically utilizes an electromagnetic induction scheme using coils, a resonance scheme using a resonance, or a Radio Frequency (RF)/microwave radiation scheme that converts electrical energy to microwaves and then transmits the microwaves. 
     Power transmission through electromagnetic induction transmits power between a first coil and a second coil. More specifically, when a magnet approaches the first coil, an induced current is generated. A transmission side generates a magnetic field using the induced current and a reception side generates energy through an induced current according to changes in the magnetic field. This phenomenon is referred to as magnetic induction, and the power transmission method using magnetic induction has a high energy transmission efficiency. 
     Power transmission through the resonance scheme is based on a coupled mode theory and may charge a battery of a device that is separated from a charging device by several meters. More specifically, an electromagnetic wave is resonated, which includes electrical energy instead of resonating sounds. The resonated electrical energy is directly transferred to a device having a corresponding resonance frequency. Accordingly, electrical energy that is not used is reabsorbed into an electromagnetic field, instead of being spread in the air. As a result, the electrical energy in the resonance scheme does not affect surrounding machines or people, unlike other electromagnetic waves. 
     A wireless power transmitter and a wireless power receiver may communicate using various schemes, for example, a Zig-Bee scheme or a Bluetooth low energy scheme. By an out-band scheme such as the Zig-Bee scheme or the Bluetooth low energy scheme, an available distance of communication increases. Accordingly, even when the wireless power transmitter and the wireless power receiver are located a relatively far distance from each other, the wireless power transmitter and the wireless power receiver may still perform the communication. That is, the wireless power transmitter may perform communication with the wireless power receiver even though the wireless power transmitter is located farther than a distance for which wireless power generally cannot be transmitted. 
       FIG. 1  is a diagram illustrating a concept of cross-connection. 
     Referring to  FIG. 1 , a first wireless power receiver RX1 is located near a first wireless power transmitter TX1, and a second wireless power receiver RX2 is located near a second wireless power transmitter TX2. The first wireless power transmitter TX1 transmits power to the first wireless power receiver RX1 and the second wireless power transmitter TX2 transmits power to the second wireless power receiver RX2. Accordingly, the first wireless power transmitter TX1 communicates with the first wireless power receiver RX1 and the second wireless power transmitter TX2 communicates with the second wireless power receiver RX2. 
     However, if the first wireless power receiver RX1 is moved away from the first wireless power transmitter TX1, the first wireless power receiver RX1 may enter a wireless power network controlled by the second wireless power transmitter TX2. Similarly, if the second wireless power receiver RX2 is moved away from the second wireless power transmitter TX2, the second wireless power receiver RX2 may enter a wireless power network controlled by the first wireless power transmitter TX1. This commonly called a cross-connection. 
     During the cross-connection, a problem may occur when the first wireless power transmitter TX1 transmits power requested by the second wireless power receiver RX2, not power requested by the first wireless power receiver RX1. For example, when a capacity of the second wireless power receiver RX2 is greater than that of the first wireless power receiver RX1, over capacity power may be applied to the first wireless power receiver RX1, which causes a problem. 
     Further, when the capacity of the second wireless power receiver RX2 is smaller than that of the first wireless power receiver RX1, a problem may occur in which the first wireless power receiver RX1 receives less power than its actual charging capacity. 
     SUMMARY 
     Accordingly, the present invention is designed to address at least the above-described problems and/or disadvantages and to provide at least the advantages described below. 
     An aspect of the present invention is to address problems associated with a cross-connection. 
     Another aspect of the present invention is to provide a wireless power transmitter and method for excluding a wireless power receiver that is cross connected. 
     In accordance with an aspect of the present invention, a method is provided for controlling a wireless power transmitter to transmit charging power to a wireless power receiver. The method includes transmitting, to the wireless power receiver, a control signal including first time information and load change information; detecting a load change of the wireless power receiver during a period of time corresponding to the first time information; and determining that the wireless power receiver is authorized for charging, if the detected load change of the wireless power receiver corresponds to the load change information included in the control signal. 
     In accordance with another aspect of the present invention, a wireless power transmitter is provided for transmitting charging power to a wireless power receiver. The wireless power transmitter includes a communication unit configured to transmit a control signal including first time information and load change information to the wireless power receiver; a controller configured to detect a load change of the wireless power receiver during a period of time corresponding to the first time information, and to determine that the wireless power receiver is authorized for charging, if the detected load change of the wireless power receiver corresponds to the load change information included in the control signal; and a power transmitting unit configured to apply the charging power to the wireless power receiver authorized for charging. 
     In accordance with another aspect of the present invention, a method is provided for controlling a wireless power receiver to receive charging power from a wireless power transmitter. The control method includes receiving, from the wireless power transmitter, a control signal including first time information and load change information; changing a load state according to the load change information, during a period of time corresponding to the first time information; and returning the load state back to a previous state before the changing, after a lapse of the period of time corresponding to the first time information. 
     In accordance with another aspect of the present invention, a wireless power receiver is provided for receiving charging power from a wireless power receiver. The wireless power receiver includes a communication unit configured to receive a control signal including first time information and load change information from the wireless power transmitter; a charging unit configured to charge the wireless power receiver at the charging power received from the wireless power transmitter; a load switch configured to switch a connection state of the charging unit to be in an on or off state; and a controller configured to controlling the load switch to be changed into the on state during a period of time corresponding to the first time information, based on the load change information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram illustrating a concept of cross-connection; 
         FIG. 2  illustrates a wireless charging system operation according to an embodiment of the present invention; 
         FIG. 3A  illustrates a wireless power transmitter and a wireless power receiver according to an embodiment of the present invention; 
         FIG. 3B  illustrates a wireless power receiver according to an embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating a method of controlling a wireless power transmitter according to an embodiment of the present invention; 
         FIG. 5  is a flowchart illustrating a method of controlling a wireless power transmitter according to an embodiment of the present invention; 
         FIG. 6  is a signal flow diagram illustrating a charging process of a wireless power transmitter and a wireless power receiver according to an embodiment of the present invention; 
         FIG. 7A  illustrates a cross-connection scenario; 
         FIG. 7B  is a signal flow diagram illustrating a charging process according to an embodiment of the present invention; 
         FIG. 8A  illustrates a cross-connection scenario; 
         FIG. 8B  is a signal flow diagram illustrating a charging process according to an embodiment of the present invention; and 
         FIG. 9  is a signal flow diagram illustrating signaling between a wireless power transmitter and a wireless power receiver, according to an embodiment of the present disclosure. 
     
    
    
     Throughout the drawings, like reference numerals will be understood to refer to like parts, components, and structures. 
     DETAILED DESCRIPTION 
     Various embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the following description, specific details such as detailed configuration and components are merely provided to assist the overall understanding of these embodiments of the present invention. Therefore, it should be apparent to those skilled in the art that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the present invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
       FIG. 2  illustrates a wireless charging system operation according to an embodiment of the present invention. 
     Referring to  FIG. 2 , a wireless charging system includes a wireless power transmitter  100  and a plurality of wireless power receivers, e.g., wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n.    
     The wireless power transmitter  100  may wirelessly transmit power 1-1, 1-2, and 1-n to the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - 3 , respectively. More specifically, the wireless power transmitter  100  may wirelessly transmit the power 1-1, 1-2, and 1-n to an authenticated wireless power receiver. 
     The wireless power transmitter  100  configures an electrical connection with the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n . For example, the wireless power transmitter  100  transmits wireless power in an electromagnetic wave type to the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n.    
     The wireless power transmitter  100  performs bidirectional communication with the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n . Here, the wireless power transmitter  100  and the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n  process, transmit, and/or receive packets 2-1, 2-2, and 2-n including predetermined frames. The frames will be described below in more detail. For example, the wireless power receiver may be implemented by a mobile communication terminal, a PDA, a Personal Media Player (PMP), a smart phone, etc. 
     The wireless power transmitter  100  wirelessly provides power to a plurality of wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n . For example, the wireless power transmitter  100  transmits power to the plurality of wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n  through a resonant scheme. Using the resonant scheme, distances between the wireless power transmitter  100  and the plurality of wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n  should be no more than approximately 30 m. However, when the wireless power transmitter  100  uses an electromagnetic induction scheme, the distances between the wireless power transmitter  100  and the plurality of wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n  should be no more than approximately 10 cm. 
     The wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n  receive wireless power from the wireless power transmitter  100  and charge batteries therein using the received power. Further, each of the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n  may transmit, to the wireless power transmitter  100 , a signal requesting wireless power transmission, information used for wireless power reception, state information of the wireless power receiver, and/or control information of the wireless power transmitter  100 . 
     Further, each of the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n  transmit a message indicating its respective charging state to the wireless power transmitter  100 . 
     The wireless power transmitter  100  may include a display, which displays a state of each of the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n  based on the messages received from each of the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n . Further, the wireless power transmitter  100  may also display charging times associated with the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n . For example, the wireless power transmitter  100  may display an approximate remaining charging time for each of the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n.    
     The wireless power transmitter  100  may transmit a control signal for disabling a wireless charging function to each of the wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n . The wireless power receivers  110 - 1 ,  110 - 2 , and  110 - n  having received a disable control signal of the wireless charging function from the wireless power transmitter  100  disable the wireless charging function. 
       FIG. 3A  illustrates a wireless power transmitter and a wireless power receiver according to an embodiment of the present invention. 
     Referring to  FIG. 3A , a wireless power transmitter  200  includes a power transmitter  211 , a controller  212 , and a communication unit  213 . Further, a wireless power receiver  250  includes a power receiver  251 , a controller  252 , and a communication unit  253 . 
     The power transmitter  211  provides power required by the wireless power transmitter  200  and wirelessly provides the power to the wireless power receiver  250 . For example, when the power transmitter  211  receives power in an Alternating Current (AC) waveform type, e.g., from an electrical outlet, it may directly supply power in the AC waveform type. However, when the power transmitter  211  receives power in a Direct Current (DC) waveform type, it first converts the received DC waveform type power, to finally supply the power in the AC waveform type. 
     For example, the power transmitter  211  may be implemented in the form of a battery included in the wireless power transmitter  200 , i.e., it may be part of a battery, or may be implemented in the form of a power reception interface, as a component of the wireless power transmitter  200 , to receive power from a battery or other source, such as an electrical outlet. It should be easily understood by those ordinarily skilled in the art that structure of the power transmitter  211  has no limitation as long as the power transmitter  21  is capable of providing AC waveform type power. 
     Further, the power transmitter  211  provides the AC waveform in an electromagnetic wave to the wireless power receiver  250 . The power transmitter  211  may include a loop coil for transmitting and receiving an electromagnetic wave. When the power transmitter  211  is implemented by the loop coil, inductance L of the loop coil may change. The power transmitter  211  is not limited to the description above and may be embodied differently, as long as the power transmitter  211  is capable of transmitting and receiving the electromagnetic wave. 
     The controller  212  controls the overall operation of the wireless power transmitter  200 , e.g., by using an algorithm, a program, or an application read from a storage unit (not shown). The controller  212  may be implemented in a form of a CPU, a microprocessor, or a mini computer. 
     The communication unit  213  communicates with the wireless power receiver  250 . For example, the communication unit  213  may communicate with the communication unit  253  of the wireless power receiver  250  using Near Field Communication (NFC), ZigBee communication, infrared communication, visible ray communication, etc. Herein, it is assumed that the communication unit  213  communicates using ZigBee communication of IEEE802.15.4, and uses a Carrier Sense Multiple Access with a Collision Avoidance (CSMA/CA) algorithm, although the present invention is not limited thereto. 
     The communication unit  213  transmits a signal including information about the wireless power transmitter  200 . For example, the communication unit  213  may unicast, multicast, or broadcast the signal. 
     Table 1 shows a frame data structure of a signal, i.e., a notice signal, transmitted from the wireless power transmitter  200  according to an embodiment of the present invention. Herein, the wireless power transmitter  200  periodically transmits the notice signal. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                 RX to 
                   
                   
               
               
                   
                   
                   
                 Net- 
                 Report 
               
               
                 Frame 
                 Protocol 
                 Sequence 
                 work 
                 (schedule 
                   
                 Number 
               
               
                 type 
                 version 
                 number 
                 ID 
                 mask) 
                 Reserved 
                 of Rx 
               
               
                   
               
             
            
               
                 Notice 
                 4 bit 
                 1 Byte 
                 1 Byte 
                 1 Byte 
                 5 bit 
                 3 bit 
               
               
                   
               
            
           
         
       
     
     In Table 1, the frame type field indicates a type of the frame, i.e., a notice signal frame. The protocol version field indicates a type of protocol and is allocated, for example, 4 bits. The sequence number field indicates a sequential order of the frame and is allocated, for example, 1 byte. For example, the sequence number may increase by one for each signal transmission/reception step. 
     The network IDentifier (ID) field indicates a network ID of the wireless power transmitter  200  and is allocated, for example, 1 byte. An Rx to Report (schedule mask) field indicates wireless power receivers for providing a report to the wireless power transmitter  200  and is allocated, for example, 1 byte. 
     Table 2 shows the Rx to Report (schedule mask) field according to an embodiment of the present invention. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Rx to Report (schedule mask) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Rx1 
                 Rx2 
                 Rx3 
                 Rx4 
                 Rx5 
                 Rx6 
                 Rx7 
                 Rx8 
               
               
                   
               
               
                 1 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
               
               
                   
               
            
           
         
       
     
     In Table 2, Rx1 to Rx8 correspond to first to eighth wireless power receivers, respectively. The Rx to Report (schedule mask) field is implemented such that the wireless power receiver having a schedule mask number of 1 provides a report. 
     Referring back to Table 1, the reserved field is reserved for being used in the future and is allocated, for example, 5 bytes. The number of Rx field indicates the number of wireless power receivers located near the wireless power transmitter  200  and is allocated, for example, 3 bits. 
     A signal having the frame type as shown in Table 1 may be allocated to Wireless Power Transmission (WPT) of a data structure in an IEEE802.15.4 form. 
     Table 3 shows a data structure of IEEE802.15.4. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
             
            
               
                 Preamble 
                 SFD 
                 Frame Length 
                 WPT 
                 CRC16 
               
               
                   
               
            
           
         
       
     
     As shown in Table 3, the data structure of IEEE802.15.4 includes a Preamble field, a Start Frame Delimiter (SFD) field, a Frame Length field, a WPT field, and a Cyclic Redundancy Check (CRC)  16  field. For example, the data structure shown in Table 1 may be included in the WPT field shown in Table 3. 
     The communication unit  213  receives power information from the wireless power receiver  250 . For example, the power information may include at least one of a capacity, a residual quantity of the battery, the number of times of charging, a usage quantity, a battery capacity, and a battery ratio of the wireless power receiver  250 . Further, the communication unit  213  transmits a charging function control signal for controlling a charging function of the wireless power receiver  250 . The charging function control signal is a control signal controls the power receiver  251  of the particular wireless power receiver  250  to enable or disable the charging function of the particular wireless power receiver  250 . 
     The communication unit  213  may receive signals from another wireless power transmitter (not shown) as well as the wireless power receiver  250 . For example, the communication unit  213  may receive the notice signal of Table 1 from another wireless power transmitter. 
     Although  FIG. 3A  illustrates the power transmitter  211  and the communication unit  213  as different hardware elements, the power transmitter  211  and the communication unit  213  may alternatively be implemented as a single hardware structure. 
     The wireless power transmitter  200  and the wireless power receiver  250  transmit and receive various types of signals. Accordingly, the wireless power receiver  250  may join a wireless power network controlled by the wireless power transmitter  200  and the charging process through the wireless power transmission and reception may be performed. 
       FIG. 3B  illustrates a wireless power receiver according to an embodiment of the present invention. Specifically,  FIG. 3B  illustrates a more detailed description of the wireless power receiver  250  illustrated in  FIG. 3A . 
     Referring to  FIG. 3B , the wireless power receiver  250  includes the power receiver  251 , the controller  252 , the communication unit  253 , a rectifier  254 , a DC/DC converter  255 , a switching unit  256 , and a charging unit  257 . 
     Descriptions of the power receiver  251 , the controller  252 , and the communication unit  253  have been provided above with reference to  FIG. 3A . Therefore, a repetitive description of these elements will be omitted here. 
     Referring to  FIG. 3B , the rectifier  254  rectifies wireless power received from the power receiver  251  to DC power and may be implemented, for example, as a bridge diode type. The DC/DC converter  255  converts the rectified power to a preset gain. For example, the DC/DC converter  255  may convert the rectified power such that a voltage at an output terminal  259  becomes 5V. A minimum value and a maximum value of a voltage applied to a front end  258  of the DC/DC converter  255  may be preset, and the aforementioned information may be recorded in an input voltage MIN field and an input voltage MAX field of a request join signal, which will be described below. A rated voltage applied to the rear end  259  of the DC/DC converter  255  and a rated current flowing to the rear end  259  may be included in an output voltage field and a an output current field of the request join signal. 
     The switching unit  256  connects the DC/DC converter  255  with the charging unit  257 . The switching unit  256  maintains an on/off state according to a control of the controller  252 . 
     The charging unit  257  stores the converted power received from the DC/DC converter  255  when the switch unit  256  is in the on state. 
     The communication unit  253  receives the command signal for starting charging, and the control unit  252  controls the switch unit  256  to maintain an on state at the predetermined time based on the received command signal. 
       FIG. 4  illustrating a method for controlling a wireless power transmitter according to an embodiment of the present invention. 
     Referring to  FIG. 4 , the wireless power transmitter receives a wireless power transmitter search signal (hereinafter, referred to as a search signal) from the wireless power receiver in step S 401 . For example, the search signal has a data structure as shown in Table 4 below. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Frame 
                 Protocol 
                 Sequence 
                 Company 
                 Product 
                   
                   
               
               
                 Type 
                 Version 
                 Number 
                 ID 
                 ID 
                 Impedance 
                 Class 
               
               
                   
               
             
            
               
                 Search 
                 4 bit 
                 1 Byte 
                 1 Byte 
                 4 Byte 
                 4 bit 
                 4 bit 
               
               
                   
               
            
           
         
       
     
     In Table 4, the frame type field indicates a type of frame, i.e., a search frame. The protocol version field indicates a type of protocol of a communication scheme and is allocated, for example, 4 bits. The sequence number field indicates a sequential order of the corresponding signal and is allocated, for example, 1 byte. For example, the sequence number may increase by one for each signal transmission/reception step. That is, when the sequence number of the notice signal of Table 1 is 1, the sequence number of the search signal of Table 4 may be 2. 
     The company ID field indicates manufacturer information of the wireless power receiver and is allocated, for example, 1 byte. The product ID field indicates product information of the wireless power receiver and includes, for example, serial number information on the wireless power receiver. The product ID field is allocated, for example, 4 bytes. The impedance field indicates impedance information of the wireless power receiver and is allocated, for example, 4 bits. The class field indicates rated power information of the wireless power receiver and is allocated, for example, 4 bits. 
     In step S 403 , the wireless power transmitter detects whether there is a load change. When it is determined that there is the load change i.e., when the wireless power receiver having transmitted the search signal is disposed on the wireless power transmitter, the wireless power transmitter performs a process for joining the corresponding wireless power receiver to the wireless power network in step S 405 . 
     However, when no load change is detected in step S 403 , i.e., when the wireless power receiver having transmitted the search signal is not disposed on the wireless power transmitter, the wireless power transmitter excludes the corresponding wireless power receiver from the wireless power network in step S 407 . 
     Basically, when a wireless power receiver is disposed on the wireless power transmitter, a load or impedance at one point of the wireless power transmitter will changed. However, when the wireless power receiver is disposed on another wireless power transmitter, the load or impedance at the one point of the wireless power transmitter does not change. Accordingly, after receiving a search signal, the wireless power transmitter determines whether the wireless power receiver is disposed on the wireless power transmitter or another wireless power transmitter through the detection of the load change. 
       FIG. 5  is a flowchart illustrating a method or controlling a wireless power transmitter according to an embodiment of the present invention. 
     Referring to  FIG. 5 , the wireless power transmitter receives, for example, a search signal having a data structure as shown in Table 1 from the wireless power receiver in step S 501 . 
     In step S 503 , the wireless power transmitter lets the corresponding wireless power receiver join a wireless power network controlled by the wireless power transmitter and applies charging power to the joined wireless power receiver. 
     In step S 505 , the wireless power transmitter transmits a load switch on control command for controlling the wireless power receiver to control the load switch to switch to the on state. For example, the load switch may be connected to a charging unit, as illustrated in  FIG. 3B . 
     In step S 507 , the wireless power transmitter monitors whether there is a load change. When the load switch is controlled to be in the on state, the load is connected to the wireless power receiver, and the load value at one point of the wireless power transmitter may be changed. 
     When the wireless power transmitter detects the load change in step S 509 , the wireless power transmitter identifies that the wireless power receiver is disposed on the wireless power transmitter, and continues to charge the wireless power receiver in step S 511 . However, when the wireless power receiver is disposed on a different wireless power transmitter, i.e., the wireless power transmitter does not detect the load change in step S 509 , the wireless power transmitter identifies that the wireless power receiver is disposed on another wireless power transmitter and stops charging the wireless power receiver in step S 513 . 
     For example, the wireless power transmitter may exclude the wireless power receiver from the wireless power network. Alternatively, the wireless power transmitter may transmit a network exclusion message to the wireless power receiver, and then the wireless power receiver may be excluded from the wireless power network based on the network exclusion message. 
     For example, when the wireless power transmitter is also supplying charging power to another wireless power receiver, the wireless power transmitter will only decrease the charging power for the wireless power receiver to be excluded, while maintaining the charging power for the other wireless power receiver. 
       FIG. 6  is a signal flow diagram illustrating a charging process of a wireless power transmitter and a wireless power receiver according to an embodiment of the present invention. 
     Referring to  FIG. 6 , a first wireless power transmitter  601  and a second wireless power transmitter  602  are available for supplying power, and a wireless power receiver  603  is disposed on the first wireless power transmitter  601 . Further, the wireless power receiver  603  is located at a communicable distance from both the first wireless power transmitter  601  and the second wireless power transmitter  602 . Further, both the first wireless power transmitter  601  and the second wireless power transmitter  602  can detect a load change based on the location of the wireless power receiver  603 . 
     The first wireless power transmitter  601  periodically or aperiodically applies detection power  611  and  614  for detecting the wireless power receiver  603 . The second wireless power transmitter  602  periodically or aperiodically applies detection power  612  and  615  for detecting the wireless power receiver  603 . The detection power is power applied for detecting the wireless power receiver  603  by the first wireless power transmitter  601  or the second wireless power transmitter  602 . 
     As described above, when the wireless power receiver  603  is disposed on one of the wireless power transmitters, a load or impedance at one point of the corresponding wireless power transmitter changes. The first wireless power transmitter  601  or the second wireless power transmitter  602  then detects the load change at the one point based on detection power, while applying the corresponding detection power. 
     In step  613 , a user disposes the first wireless power receiver  603  on the first wireless power transmitter  601 . 
     The first wireless power transmitter  601  detects the load change during a process of applying detection power  614 . Thereafter, the first wireless power transmitter  601  stops applying the detection power  614  and applies driving power  616 . 
     The second wireless power transmitter  602  also detects the load change during a process of applying the detection power  615 . Thereafter, the second wireless power transmitter  602  stops applying the detection power  615  and applies driving power  617 . Here, the driving power may have a power quantity for driving a controller or a Micro Control Unit (MCU) of the wireless power receiver  603  or a power quantity for driving the controller or the MCU, and operating a communication module. 
     In step  619 , the wireless power receiver  603  transmits a search signal, e.g., as shown in Table 1, based on the applied driving power  616  or  617 . For example, the wireless power receiver transmits the search signal based on a multicast or a broadcast technique. Accordingly, both the first wireless power transmitter  601  and the second wireless power transmitter  602  receive the search signal in steps  619  and  621 , respectively. 
     In step  620 , the first wireless power transmitter  601  transmits a wireless power transmitter search response signal to the wireless power receiver  603 , based on the received search signal. Similarly, in step  622 , the second wireless power transmitter  602  also transmits a wireless power transmitter search response signal to the wireless power receiver  603  based on the received search signal. For example, the wireless power transmitter search response signal has a data structure as shown in Table 5 below and is referred to as a response search signal hereinafter. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Frame Type 
                 Reserved 
                 Sequence Number 
                 Network ID 
               
               
                   
               
             
            
               
                 Response Search 
                 4 bit 
                 1 Byte 
                 1 Byte 
               
               
                   
               
            
           
         
       
     
     In table 5, the frame type field of Table 5 indicates a type of the frame, i.e., a response search signal frame. The reserved field is reserved for future use and is allocated, for example, 4 bits. The sequence number field indicates a sequential order of the corresponding signal and is allocated, for example, 1 byte. For example, the sequence number may increase by one for each signal transmission/reception step. 
     The network ID field indicates a network ID of the wireless power transmitter and is allocated, for example, 1 byte. 
     In step  623 , the wireless power receiver  603  determines the wireless power transmitter to perform the joining from the first wireless power transmitter  601  and the second wireless power transmitter  602  by comparing Received Signal Strength Indicators (RSSIs) or energy levels of the received response search signals. For example, the wireless power receiver  603  may determine the second wireless power transmitter  602  as the wireless power transmitter to perform the joining. 
     In step  624 , the wireless power receiver  603  transmits a join request signal to the second wireless power transmitter  602 . The join request signal may also be referred to as a communication request signal, because the join request signal is the signal for setting up communication between the wireless power receiver  603  and the second wireless power transmitter  602 . The join request signal is referred to as a request join signal hereinafter, and may have a data structure as shown in Table 6 below. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Input 
                 Input 
                 Typical 
                 Typical 
               
               
                 Frame 
                   
                 Sequence 
                 Network 
                 Product 
                 Voltage 
                 Voltage 
                 Output 
                 Output 
               
               
                 Type 
                 Reserved 
                 Number 
                 ID 
                 ID 
                 MIN 
                 MAX 
                 Voltage 
                 Current 
               
               
                   
               
             
            
               
                 Request 
                 4 bit 
                 1 Byte 
                 1 Byte 
                 4 Byte 
                 1 Byte 
                 1 Byte 
                 1 Byte 
                 1 Byte 
               
               
                 join 
               
               
                   
               
            
           
         
       
     
     In Table 6, the frame type field indicates a type of the frame of the signal, i.e., a request join frame. The reserved field is reserved for future use and is allocated, for example, 4 bits. The sequence number field indicates a sequential order of the corresponding signal and is allocated, for example, 1 byte. For example, the sequence number may increase by one for each signal transmission/reception step. 
     The network ID field indicates a network ID of the wireless power transmitter and is allocated, for example, 1 byte. The product ID field indicates product information of the wireless power receiver and includes, for example, serial number information of the wireless power receiver. The input voltage MIN field indicates a minimum voltage value applied to a front end of a DC/DC inverter (not shown) of the wireless power receiver and is allocated, for example, 1 byte. The input voltage MAX field indicates a maximum voltage value applied to the front end of the DC/DC inverter (not shown) of the wireless power receiver and is allocated, for example, 1 byte. The typical output voltage field indicates a rated voltage value applied to a rear end of the DC/DC inverter (not shown) of the wireless power receiver and is allocated, for example, 1 byte. The typical output current field indicates a rated current value flowing to the rear end of the DC/DC inverter (not shown) of the wireless power receiver and is allocated, for example, 1 byte. 
     The second wireless power transmitter  602  may determine whether to establish communication with the wireless power receiver based on the request join signal. First, the second wireless power transmitter  602  may determine whether to establish communication based on a signal strength of the request join signals, e.g., an RSSI value. If a received RSSI value of the request join signal is greater than a predetermined threshold, the second wireless power transmitter  602  may determine to establish communication. However, if a received RSSI value of the request join signal is not greater than the predetermined threshold, the second wireless power transmitter  602  may determine not to establish communication. 
     Alternatively, the second wireless power transmitter  602  may determine whether to establish communication by checking an ID of the request join signal. Although not shown in Table 6, the request join signal may further include an ID of the wireless power receiver. The second wireless power transmitter  602  may check an ID of the wireless power receiver and determine whether the ID is allowed for wireless power transmission. If the ID is allowed for the wireless power transmission, the second wireless power transmitter  602  may determine to establish communication with the wireless power receiver. 
     The second wireless power transmitter  602  transmits a join response signal (hereinafter, referred to as a response join signal) corresponding to the received request join signal in step  625 . 
     For example, the response join signal has a data structure as shown in Table 7. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                   
                   
                 Sequence 
                 Network 
                   
                 Session 
               
               
                 Frame Type 
                 Reserved 
                 Number 
                 ID 
                 Permission 
                 ID 
               
               
                   
               
             
            
               
                 Response 
                 4 bit 
                 1 Byte 
                 1 Byte 
                 4 bit 
                 4 bit 
               
               
                 join 
               
               
                   
               
            
           
         
       
     
     In Table 7, the frame type field indicates a type of the frame, i.e., that the corresponding frames is in a response join signal. The reserved field is reserved for future use and is allocated, for example, 4 bits. The sequence number field indicates a sequential order of the corresponding signal and is allocated, for example, 1 byte. For example, the sequence number may increase by one for each signal transmission/reception step. 
     The network ID field indicates a network ID of the wireless power transmitter and is allocated, for example, 1 byte. The permission field indicates whether the wireless power receiver joins a wireless power network and is allocated, for example, 4 bits. For example, when the permission field indicates 1, the wireless power receiver is allowed to join the wireless power network, and when the permission field indicates 0, the wireless power receiver is not allowed to join the wireless power network. 
     The session ID field indicates a session ID assigned to the wireless power receiver by the wireless power transmitter for controlling the wireless power network. The session ID is allocated, for example, 4 bits. 
     The second wireless power transmitter  602  determines whether to transmit charging power to the wireless power receiver  603  and transmit a result thereof to the wireless power receiver  603  by using the response join signal. Here, it is assumed that the second wireless power transmitter  602  determines to apply the charging power to the wireless power receiver  603 . 
     In step  626 , the wireless power receiver  603  transmits an Acknowledgement (Ack) signal to the second wireless power transmitter  602 . In step  627 , the second wireless power transmitter  602  transmits a command signal for instructing a charging initiation to the wireless power receiver  603 . 
     For example, the command signal has a data structure as shown in Table 8. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 8 
               
               
                   
               
               
                 Frame 
                   
                 Sequence 
                   
                 Command 
                   
               
               
                 Type 
                 Session ID 
                 number 
                 Network ID 
                 Type 
                 Variable 
               
               
                   
               
             
            
               
                 Command 
                 4 bit 
                 1 Byte 
                 1 Byte 
                 4 bit 
                 4 bit 
               
               
                   
               
            
           
         
       
     
     In Table 8, the frame type field indicates a type of frame, i.e., indicates that the corresponding frame is a command signal frame. The session field indicates a session ID assigned to each of the wireless power receivers by the wireless power transmitter for controlling the wireless power network. The session ID field is allocated, for example, 4 bits. The sequence number field indicates a sequential order of the corresponding signal and is allocated, for example, 1 byte. For example, the sequence number may increase by one for each a signal transmission/reception step. 
     The network ID field indicates a network ID of the wireless power transmitter and is allocated, for example, 1 byte. The command type field indicates a type of command and is allocated, for example, 4 bits. Further, the variable field supplements the command type field and is allocated, for example, 4 bits. 
     The command type field and the variable field may be used to indicate various commands as shown below in Table 9. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 9 
               
               
                   
                   
               
               
                   
                 Command Type 
                 Variable 
               
               
                   
                   
               
             
            
               
                   
                 Charge start 
                 Reserved 
               
               
                   
                 Charge finish 
                 Reserved 
               
               
                   
                 Request Report 
                 CTL level 
               
               
                   
                 Reset 
                 Reset type 
               
               
                   
                 Channel Scan 
                 Reserved 
               
               
                   
                 change channel 
                 Channel 
               
               
                   
                 load switch on 
                 Reserved 
               
               
                   
                   
               
            
           
         
       
     
     In Table 9, a charge start command instructs the wireless power receiver to initiate charging, a charge finish command instructs the wireless power receiver to end the charging, a request report command instructs the wireless power receiver to transmit a report signal, a reset command instructs the wireless power receiver to reset, a channel scan command instructs the wireless power receiver to search for a channel, a channel change command instructs the wireless power receiver to change a communication channel, and a load switch on command instructs the wireless power receiver to control a load switch thereof to be in an on state, e.g., immediately, or after or at a preset time. 
     The above-listed commands may be set independently or simultaneously. For example, the command signal may simultaneously instruct to initiate the charging and instruct to control the load switch to be in the on state. 
     In step  627 , the second wireless power transmitter  602  initiates the charging of the wireless power receiver  603  by instructing to control the load switch to be in the on state. In step  628 , the second wireless power transmitter  602  increases a power quantity to charging power  629  from the driving power  617 . In step  630 , the second wireless power transmitter  602  monitors whether there is a load change within a preset time period. 
     In step  631 , the wireless power receiver  603  initiates the charging and controls the load switch to be in the on state, based on the commands received from the second wireless power transmitter  602 . In step  632 , the wireless power receiver  603  transmits a report signal to the second wireless power transmitter  602 . 
     For example, the report signal has a data structure as shown in Table 10. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 10 
               
               
                   
               
               
                 Frame 
                 Session 
                 Sequence 
                 Network 
                 Input 
                 Output 
                 Output 
                   
               
               
                 Type 
                 ID 
                 number 
                 ID 
                 Voltage 
                 Voltage 
                 Current 
                 Reserved 
               
               
                   
               
             
            
               
                 Report 
                 4 bit 
                 1 Byte 
                 1 Byte 
                 1 Byte 
                 1 Byte 
                 1 Byte 
                 1 Byte 
               
               
                   
               
            
           
         
       
     
     In Table 10, the frame type field indicates a type of frame, i.e., indicates that the corresponding frame is a report signal frame. The session field indicates a session ID assigned to each of the wireless power receivers by the wireless power transmitter for controlling the wireless power network. The session ID field is allocated, for example, 4 bits. The sequence number field indicates a sequential order of the corresponding signal and is allocated, for example, 1 byte. For example, the sequence number may increase by one for each signal transmission/reception step. 
     The network ID field indicates a network ID of the wireless power transmitter and is allocated, for example, 1 byte. The input voltage field indicates a voltage value applied to a front end of a DC/DC inverter (not shown) of the wireless power receiver and is allocated, for example, 1 byte. The output voltage field indicates a voltage value applied to a rear end of the DC/DC inverter (not shown) of the wireless power receiver and is allocated, for example, 1 byte. The output current field indicates a rated current value flowing to the rear end of the DC/DC inverter (not shown) of the wireless power receiver and is allocated, for example, 1 byte. 
     As described above, the wireless power receiver  603  may not actually be disposed on the second wireless power transmitter  602 , i.e., the wireless power receiver  603  may actually be disposed on the first wireless power transmitter  601 , and as a result, the second wireless power transmitter  602  may not detect the load change within a preset time (Tloadon), in step  633 . Accordingly, the second wireless power transmitter  602  excludes the wireless power receiver  603  from the wireless power network controlled by the second wireless power transmitter  602 . That is, the wireless power transmitter  602  may decide not to communicate with the wireless power receiver  603 , and then returns to a load change detection state. 
     However, when another wireless power receiver, instead of the wireless power receiver  603 , joins the wireless power network controlled by the second wireless power transmitter  602 , the second wireless power transmitter  602  only stops applying the charging power to the wireless power receiver  603  and continues to charge the another wireless power receiver, without returning to the load change detection state. In  FIG. 6 , it is assumed that the second wireless power transmitter  602  returns to the load change detection state. 
     Accordingly, the first wireless power transmitter  601  and the second wireless power transmitter  602  apply detection power  634  and  635 , respectively. 
     The wireless power receiver  603  is continuously disposed on the first wireless power transmitter  601 . Accordingly, the first wireless power transmitter  601  and the second wireless power transmitter  602  apply driving power  636  and  637 , respectively. In step  638 , the wireless power receiver  603  is driven based on the driving power  636  and  637 . The wireless power receiver  603  transmits a search signal to the first wireless power transmitter  601  and the second wireless power transmitter  602  in steps  639  and  640 , respectively. 
     In step  641 , the first wireless power transmitter  601  transmits a response search signal to the wireless power receiver  603  in response to the search signal. Because the second wireless power transmitter  602  has excluded the wireless power receiver  603  from the wireless power network controlled by the second wireless power transmitter  602 , the search signal from the wireless power receiver  603  may be ignored for a preset period (tignore). For example, the second wireless power transmitter  602  may exclude the wireless power receiver  603  from the wireless power network by storing an ID or a serial number of the wireless power receiver  603  and ignoring the search signal transmitted from the corresponding wireless power receiver  603 . 
     In step  642 , the wireless power receiver  603  forms a communication with the first wireless power transmitter  601  according to the received response search signal. 
     In step  643 , the wireless power receiver  603  transmits a request join signal to the first wireless power transmitter  601 , and in step  644 , the first wireless power transmitter  601  transmits a response join signal to the wireless power receiver  603 . In step  645 , the wireless power receiver  603  transmits an Ack signal to the first wireless power transmitter  601 . 
     In step  646 , the first wireless power transmitter  601  initiates the charging and control the on state of the load switch at a particular point in time by using a command signal. In step  647 , the first wireless power transmitter  601  increases applied power to charging power  652  from driving power  637 . 
     In step  648 , the first wireless power transmitter  601  monitors for a load change. 
     Both the first wireless power transmitter  601  and the wireless power receiver  603  may use a command signal or an Ack signal as a synchronization signal for calculating the predetermined time Tloadon. For example, a point in time of receiving the command signal or the Ack signal may be used as a point in time to start calculating the predetermined time Tloadon. 
     In step  649 , the wireless power receiver  603  initiates the charging and also controls the load switch to be in the on state after a preset time (Tloadon). 
     In step  650 , the wireless power receiver  603  transmits the report signal to the first wireless power transmitter  601 . 
     In step  651 , the first wireless power transmitter  601  detects a load change due to an on state control of the load switch, after a preset time (Tloadon). Accordingly, the first wireless power transmitter  601  determines that the wireless power receiver  603  is disposed on the first wireless power transmitter  601  and continues to charge. The first wireless power transmitter  601  may set up a tolerance for the predetermined time Tloadon. The first power transmitter  601  may continue to charge even though the first power transmitter  601  detects a load change earlier than the predetermined time Tloadon or later than the predetermined time Tloadon. 
       FIG. 7A  illustrates a cross-connection scenario. 
     Referring to  FIG. 7A , a first wireless power receiver  703  is disposed on a first wireless power transmitter  701 , and a second wireless power receiver  704  is disposed on a second wireless power transmitter  702 . However, the first wireless power transmitter  701  is communicating with the second wireless power receiver  704 , and the second wireless power transmitter  702  is communicating with the first wireless power receiver  703 . 
       FIG. 7B  is a signal flow diagram illustrating a charging process according to an embodiment of the present invention. Specifically,  FIG. 7B  illustrates a procedure for addressing the problems created by the scenario illustrated in  FIG. 7A . 
     In  FIG. 7B , steps  711  to  745  are the same as steps  611  to  645  as illustrated in  FIG. 6 , and thus repetitive descriptions of these steps will be omitted here. 
     Referring to  FIG. 7B , in step  746 , the first wireless power transmitter  701  instructs the wireless power receiver  703  to initiate charging and controls an on state of the load switch at a particular time point by using a command signal. In step  747 , the first wireless power transmitter  701  monitors for a load change, after a preset time (tloadon). 
     In step  748 , the wireless power receiver  703  controls the load switch to be in the on state after the preset time (tloadon), and in step  749 , the wireless power receiver  703  transmits a report signal to the first wireless power transmitter  701 . 
     In step  750 , the first wireless power transmitter  701  continues applying charging power  752 , after gradually increasing charging power in step  751 . 
       FIG. 8A  illustrates a cross-connection scenario. 
     Referring to  FIG. 8A , a first wireless power receiver  803  and a third wireless power receiver  805  are disposed on a first wireless power transmitter  801 , and a second wireless power receiver  804  is disposed on a second wireless power transmitter  802 . However, the first wireless power transmitter  801  communicates with the second wireless power receiver  804 , and the second wireless power transmitter  802  communicates with the first wireless power receiver  803  and the third wireless power receiver  805 . For example, the third wireless power receiver  805  was disposed on the first wireless power transmitter  801 , after the first wireless power receiver  803 . 
       FIG. 8B  is a signal flow diagram illustrating a charging process according to an embodiment of the present invention. Specifically,  FIG. 8B  illustrates transmission and reception between the wireless power transmitter and the wireless power receivers in the scenario illustrates in  FIG. 8A . 
     Referring to  FIG. 8B , the first wireless power transmitter  801  periodically or aperiodically applies detection power  811  and  814  for detecting the first wireless power receiver  803 . The second wireless power transmitter  802  periodically or aperiodically applies detection power  812  and  815  for detecting the first wireless power receiver  803 . The detection power is power applied for detecting the first wireless power receiver  803  by the first wireless power transmitter  801  or the second wireless power transmitter  802 . As described above, when the first wireless power receiver  803  is disposed on one of the wireless power transmitters, a load or impedance at one point of the first wireless power transmitter and the second wireless power transmitter may be changed. The first wireless power transmitter  801  or the second wireless power transmitter  802  detects a load change at one point based on detection power while applying the corresponding detection power. In  FIG. 8B , the user disposes the first wireless power receiver  803  on the first wireless power transmitter  801  in step  813 . 
     The first wireless power transmitter  801  detects the load change during a process of applying the detection power  814 . The first wireless power transmitter  801  stops applying the detection power  814  and applies driving power  816 . The second wireless power transmitter  802  detects the load change during a process of applying the detection power  815 . The second wireless power transmitter  802  stops applying the detection power  815  and applies driving power  817 . 
     In step  818 , the first wireless power receiver  803  transmits a search signal as shown in Table 1 based on the applied driving power  816  or  817 . For example, the first wireless power receiver  803  may transmit the search signal based on a multicast or a broadcast technique. Accordingly, both the first wireless power transmitter  801  and the second wireless power transmitter  802  receive the search signal in steps  818  and  820 . 
     In step  821 , the first wireless power transmitter  801  transmits a wireless power transmitter search response signal to the first wireless power receiver  803 , based on the received search signal. In step  819 , the second wireless power transmitter  802  also transmits the wireless power transmitter search response signal to the first wireless power receiver  803 , based on the received search signal. 
     In step  822 , the first wireless power receiver  803  determines the first wireless power transmitter  801  as a wireless power transmitter to perform joining, based on an RSSI or an energy level of the received response search signal. The second wireless power transmitter transmits detecting power  823 . 
     In step  824 , the first wireless power receiver  803  transmits a request join signal to the first wireless power transmitter  801 . In step  825 , the first wireless power transmitter  801  transmits a response join signal to the first wireless power receiver  803 , and in step  826 , the first wireless power receiver  803  transmits an Ack signal to the first wireless power transmitter  801 . 
     In step  827 , the first wireless power transmitter  801  transmits a notice signal to the first wireless power receiver  801 , and in step  828 , initiates the charging and controls an on state of the load switch at a particular point in time by using a command signal. 
     The first wireless power transmitter  801  monitors for a load change after a preset time (tloadon), and in step  829 , increases a power quantity applied to charging power, when the load change due to an on state  830  of the load switch of the first wireless power receiver  803  is detected. 
     In step  831 , the wireless power receiver  803  transmits a report signal to the first wireless power transmitter  801 . The first wireless power transmitter  801  maintains applying charging power  832  after gradually increasing the charging power  829 . 
     The second wireless power transmitter  802  may periodically apply detection power  834  and  835 . 
     In step  836 , the third wireless power receiver  805  is disposed on the first wireless power transmitter  801  between applying of the detection power  834  and applying of the detection power  835 . In step  838 , the second wireless power transmitter  802  applies driving power  837  and the third wireless power receiver  805  turns on. 
     In step  8339 , the third wireless power receiver  805  transmits a search signal to the second wireless power transmitter  802 , and in step  840 , the second wireless power transmitter  802  transmits a response search signal to the third wireless power receiver  805 . 
     In step  841 , the third wireless power receiver  805  transmits the search signal to the first wireless power transmitter  801 , and in step  842 , the first wireless power transmitter  801  transmits the response search signal to the third wireless power receiver  805 . 
     In step  843 , the third wireless power receiver  805  determines the first wireless power transmitter  801  as a wireless power transmitter to perform joining by comparing RSSIs or energy levels of the response search signals received from the first wireless power transmitter  801  and the second wireless power transmitter  802 . 
     In step  844 , the third wireless power receiver  805  transmits a request join signal to the first wireless power transmitter  801 , and in step  846 , the first wireless power transmitter  801  transmits a response join signal to the third wireless power receiver  805 . 
     In step  847 , the third wireless power receiver  805  transmits an Ack signal to the first wireless power transmitter  801 , and the second wireless power transmitter  802  periodically applies detection power  845  and  848 . 
     In step  849 , the first wireless power transmitter  801  defines a new period by transmitting a notice signal to the first wireless power transmitter  803 . In step  850 , the notice signal transmitted from the first wireless power transmitter  801  is also received by the third wireless power receiver  805 . 
     In step  850 , the first wireless power transmitter  801  transmits a report signal instructing the first wireless power receiver  803  to report a charging state. In response to the report signal, in step  852 , the first wireless power receiver  803  transmits a report signal including information such as a charging state, impedance information, remaining charging amount, etc. 
     In step  853 , the first wireless power transmitter  801  initiates the charging and controls an on state of the load switch at a particular point in time by using a command signal. 
     In step  854 , the first wireless power transmitter  801  monitors a load change, and detects the load change by a load switch on after a preset time  855 . 
     In step  856 , the third wireless power receiver  803  transmits the report signal. 
     In step  857 , the first wireless power transmitter  801  having detected the load change maintains charging power, which has been gradually increased at step  858 . 
     As described above, when two or more wireless power receivers are disposed, preventing cross-connection is possible. 
       FIG. 9  is a signal flow diagram illustrating signaling between a wireless power transmitter and a wireless power receiver, according to an embodiment of the present disclosure. 
     Referring to  FIG. 9 , a wireless power transmitter  901  transmits a load change command signal to a wireless power receiver  902  in step S 911 . The load change command signal may be a signal for the wireless power receiver  902  to change a load during a first period of time (Tset1) and to control a load switch to be in the off state during a second period of time (Tset2). Alternatively, the load change command signal may be a signal to request a load change during the first period of time Tset1. For example, the load change may be a change of the load switch from off state to on state. For example, changing the load during the first period of time may be to change the load switch from off state to on state and remain the load switch in the on state for the first period of time Tset1. 
     Alternatively, changing the load during the first period of time may be changing the load according to a predetermined pattern during the first period of time Tset1. 
     In step S 912 , the wireless power receiver  902  changes the load during the predetermined first period of time Tset1, based on the received load change command signal. For example, the wireless power receiver  902  may change the load switch from off state to on state and remain the load switch to be in the on state during the first period of time Tset1. Alternatively, the wireless power receiver  902  may change the load according to a predetermined pattern during the first period of time Tset1. 
     The wireless power receiver  902  transmits dynamic signals to the wireless power transmitter  901  at predetermined intervals in steps S 913 , S 914 , S 917 , and S 918 . 
     The wireless power receiver  902  stops changing the load after the lapse of the first period of time Tset1 in step S 915 . For example, the wireless power receiver  902  may control the load switch having been in the on state during the first period of time Tset1 to be in off state. Alternatively, the wireless power receiver  902  may stop changing the load according to the predetermined pattern. 
     When the load change command signal includes a command to control the load switch to be in off state during the second period of time Tset2, the wireless power receiver  902  changes the load switch to be in on state after remaining the load switch in off state during the second period of time Tset2 in step S 916 . However, if the load change command signal includes only a command to change the load during the first period of time Tset1, the aforementioned step of controlling the load switch to be in off state during the second period of time Tset2 may be omitted. 
     The wireless power transmitter  901  may detect a load change of the wireless power receiver  902 . The wireless power transmitter  901  may compare information of the transmitted load change command signal to the detected load change, and determine from the comparison whether the wireless power receiver is cross-connected. For example, the wireless power transmitter  901  may detect a load change during the first period of time Tset1 and detect the load switch being off during the second period of time Tset2. That is, if it is determined that the load change detected by the wireless power transmitter  901  corresponds to the information of the load change command signal, the wireless power transmitter  901  determines that the wireless power receiver  902  is a wireless power receiver for charging, which is not cross-connected. However, if the load change detected by the wireless power transmitter  901  does not correspond to the information of the load change command signal, the wireless power transmitter  901  determines that the wireless power receiver  902  is a cross-connected wireless power receiver. 
     If the load change command signal indicates only a load change during the first period of time Tset1, the wireless power transmitter  901  may determine that the wireless power receiver is a wireless power receiver for charging, which is not cross-connected, upon detection of a load change during the first period of time Tset1. However, if the load change detected by the wireless power transmitter  901  does not correspond to the information of the load change command signal, the wireless power transmitter  901  may determine that the wireless power receiver  902  is a cross-connected wireless power receiver. 
     In accordance with another embodiment of the present invention, the wireless power receiver  902  may send a control signal (for example, a load change signal) including the first period of time Tset1 and/or second period of time Tset2 to the wireless power transmitter  901 . The control signal indicates that the wireless power receiver  902  changes a load during the first period of time Tset1 and changes a load switch into off state during the second period of time Tset2. Alternatively, the control signal may indicate that the wireless power receiver  902  changes a load during the first period of time Tset1. For example, the load change may be a change of the load switch from off state to on state. Further, changing the load during the first period of time may be to change the load switch from off state to on state and remain the load switch in the on state for the first period of time Tset1. Alternatively, changing the load during the first period of time may change the load according to a predetermined pattern during the first period of time Tset1. 
     The wireless power receiver  902  may change the load during the predetermined first period of time Tset1 based on the control signal. For example, the wireless power receiver  902  may change the load switch from off state to on state and remain the load switch to be in the on state during the first period of time Tset1. Alternatively, the wireless power receiver  902  may change the load according to a predetermined pattern during the first period of time Tset1. 
     The wireless power receiver  902  may also stop changing the load after the lapse of the first period of time Tset1. For example, the wireless power receiver  902  may control the load switch having been in the on state during the first period of time Tset1 to be in off state. Alternatively, the wireless power receiver  902  may stop changing the load according to the predetermined pattern. 
     If the load change signal includes information to control the load switch to be in off state during the second period of time Tset2, the wireless power receiver  902  may change the load switch to be in on state after remaining the load switch in off state during the second period of time Tset2. However, if the load change signal includes only a command to change the load during the first period of time Tset1, the aforementioned step of controlling the load switch to be in off state during the second period of time Tset2 may be omitted. 
     The wireless power transmitter  901  may receive the control signal from the wireless power receiver  902  and detect a load change of the wireless power receiver  902 . 
     The wireless power transmitter  901  may compare information of the received control signal (e.g., load change signal) to the detected load change, and determine from the comparison whether the wireless power receiver is cross-connected. For example, the wireless power transmitter  901  may detect a load change during the first period of time Tset1 and detect the load switch being off during the second period of time Tset2. In other words, if it is determined that the load change detected by the wireless power transmitter  901  matches to the information of the control signal, the wireless power transmitter  901  may determine that the wireless power receiver  902  is a wireless power receiver for charging, which is not cross-connected. On the other hand, if it is determined that the load change detected by the wireless power transmitter  901  does not match to the information of the load change signal, the wireless power transmitter  901  may determine that the wireless power receiver  902  is a cross-connected wireless power receiver. 
     If the load change signal indicates only a load change during the first period of time Tset1, the wireless power transmitter  901  may determine that the wireless power receiver is a wireless power receiver for charging, which is not cross-connected, upon detection of a load change during the first period of time Tset1. On the other hand, if it is determined that the load change detected by the wireless power transmitter  901  does not match to the information of the load change signal, the wireless power transmitter  901  may determine that the wireless power receiver  902  is a cross-connected wireless power receiver. 
     In accordance with an embodiment of the present invention, the information about a load change, first period of time, second period of time, etc., transmitted by the wireless power transmitter  901  or the wireless power receiver  902 , may be included as static parameters or dynamic parameters in any signal of many signals sent during a registration procedure or a charging procedure about the wireless power receiver  902 . 
     According to the above-described various embodiments of the present invention, it is possible to address many of the problems associated with a wireless power receiver located on a wireless power transmitter connected to another wireless power transmitter and receiving charging power. 
     While the present invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.