Patent Publication Number: US-2020280216-A1

Title: Wireless Charging Method, Device, and Wireless Charging System

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application No. PCT/CN2018/096160, filed on Jul. 18, 2018, which claims priority to Chinese Patent Application No. 201711178938.2, filed on Nov. 21, 2017, both of which are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present disclosure relate to the field of electronic technologies, and in particular, to a wireless charging method, a device, and a wireless charging system. 
     BACKGROUND 
     Recent years have witnessed wider application of a wireless charging technology to electronic products. Compared with a conventional contact-type electrical energy transfer technology, a wireless electrical energy transfer technology is safer and more convenient because there is no cable connection between a power source and a load. There are mainly the following several manners of implementing wireless electrical energy transfer electromagnetic radiation, electromagnetic induction, electromagnetic resonance, and electric field coupling. Based on considerations of efficiency and safety, currently, wireless electrical energy transfer mostly uses electromagnetic induction and electromagnetic resonance for wireless charging. 
     Both electromagnetic induction and electromagnetic resonance wireless charging systems perform electrical energy transfer using electromagnetic induction between a coil in a transmitter and a coil in a receiver. During wireless charging, an oscillation circuit of the transmitter converts electrical energy into a high-frequency alternating current (AC) and supplies the high-frequency AC to a primary coil, the primary coil couples the electrical energy to a secondary coil of the receiver in proximity using a magnetic field that is generated from the high-frequency current, and the secondary coil receives the electrical energy, converts the electrical energy into a direct current (DC) using a converter circuit, and supplies the DC to a load for use. 
     However, during actual application, if there is a relatively large deviation between positions of a receiver (for example, a mobile phone) and a transmitter (for example, a wireless charging pad) during charging, efficiency in coupling between the primary coil and the secondary coil is reduced, and therefore charging efficiency of the receiver is reduced. In addition, due to a reduction in the coupling efficiency, the transmitter generates a stronger high-frequency current to generate a stronger magnetic field, and the stronger magnetic field and the low charging efficiency cause severe heating of the transmitter and the receiver. 
     SUMMARY 
     Embodiments of the present disclosure provide a wireless charging method, a device, and a wireless charging system in order to improve charging efficiency of a receiver. 
     According to a first aspect, an embodiment of this application provides a wireless charging system, where the wireless charging system includes a transmitter and a receiver, the transmitter includes a transmitter coil and a first series matching capacitor, the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, and the first oscillation circuit is configured to transfer power to the receiver, and the receiver includes a receiver coil and a second series matching capacitor, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and a self-inductance applied when the transmitter coil transfers the power to the receiver is L p , and a capacitance of the first series matching capacitor is C p , and a self-inductance applied when the receiver coil receives the power transferred by the first oscillation circuit is L s , and a capacitance of the second series matching capacitor is C s , where 
     
       
         
           
             
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
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                     p 
                   
                 
                 
                   
                     L 
                     s 
                   
                   * 
                   
                     C 
                     s 
                   
                 
               
               = 
               k 
             
             , 
           
         
       
     
     and k is a positive number that meets 0.8≤k≤1.2. Circuit parameters in the transmitter and the receiver meet 
     
       
         
           
             
               
                 
                   L 
                   p 
                 
                 * 
                 
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                   p 
                 
               
               
                 
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               . 
             
           
         
       
     
     The wireless charging system may be set to operate at a load-independent point by setting a voltage gain to a voltage gain at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, when a voltage gain between an output voltage of the receiver and an input voltage of the transmitter is a first voltage gain, the wireless charging system operates at a load-independent point, the load-independent point includes a first operating frequency and the first voltage gain, at the first operating frequency, the first voltage gain is independent of an output load of the receiver, and the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}. Regardless of a value of a coupling factor, a voltage gain at the load-independent point is invariably √{square root over (C p /C s )}, and the voltage gain is set to be within a range of 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )} or within a range of 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )} such that the wireless charging system operates near the load-independent point, and the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, k is 1, and X is √{square root over (C p /C s )}, or √{square root over (L s /L p )}. When k=1, L p *C p , C p −L s *C s =0. In this case, a voltage gain is regulated to √{square root over (C p /C s )} or √{square root over (L s /L p )}, and the wireless charging system operates at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, the transmitter further includes a first voltage regulation module, where the first voltage regulation module is connected to the first oscillation circuit in parallel, and the first voltage regulation module is configured to set the voltage gain between the output voltage of the receiver and the input voltage of the transmitter to the first voltage gain by regulating the input voltage of the transmitter. 
     In one embodiment, the receiver further includes a second voltage regulation module, where the second voltage regulation module is connected to the second oscillation circuit in parallel, and the second voltage regulation module is configured to set the voltage gain between the output voltage of the receiver and the input voltage of the transmitter to the first voltage gain by regulating the output voltage of the receiver. 
     According to a second aspect, an embodiment of this application provides a transmitter, where the transmitter includes a transmitter coil and a first series matching capacitor, and the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, the first oscillation circuit is configured to transfer power to a receiver, and a self-inductance applied when the transmitter coil transfers the power to the receiver is L p , and a capacitance of the first series matching capacitor is C p , where L p *C p =k*L s *C s , L s  is a self-inductance applied when a receiver coil in the receiver receives the power transferred by the first oscillation circuit, C s  is a capacitance of a second series matching capacitor in the receiver, and k is a positive number that meets 0.8≤k≤1.2, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit. Circuit parameters in the transmitter meet L p *C p =k*L s *C s . A wireless charging system may be set to operate at a load-independent point by setting a voltage gain to a voltage gain at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, when a voltage gain between an output voltage of the receiver and an input voltage of the transmitter is a first voltage gain, the wireless charging system operates at a load-independent point, the wireless charging system includes the transmitter and the receiver, the load-independent point includes a first operating frequency and the first voltage gain, at the first operating frequency, the first voltage gain is independent of an output load of the receiver, and the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}. Regardless of a value of a coupling factor, a voltage gain at the load-independent point is invariably √{square root over (C p /C s )}, and the voltage gain is set to be within a range of 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )} or within a range of 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )} such that the wireless charging system operates near the load-independent point, and the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, k is 1, and X is √{square root over (C p /C s )} or √{square root over (L s /L p )}. When k=1, L p *C p −L s *C s =0. In this case, a voltage gain is regulated to √{square root over (C p /C s )} or √{square root over (L s /L p )}, and the wireless charging system operates at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, the transmitter further includes a voltage regulation module, where the voltage regulation module is connected to the first oscillation circuit in parallel, and the voltage regulation module is configured to set the voltage gain between the output voltage of the receiver and the input voltage of the transmitter to the first voltage gain by regulating the input voltage of the transmitter. 
     According to a third aspect, an embodiment of this application provides a receiver, where the receiver includes a receiver coil and a second series matching capacitor, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive power transferred by a transmitter, and a self-inductance applied when the receiver coil receives the power transferred by the transmitter is L s , and a capacitance of the second series matching capacitor is C s , where 
     
       
         
           
             
               
                 
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                   s 
                 
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                   C 
                   s 
                 
               
               = 
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
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                     p 
                   
                 
                 k 
               
             
             , 
           
         
       
     
     L p  is a self-inductance applied when a transmitter coil in the transmitter transfers the power to the receiver, C p  is a capacitance of a first series matching capacitor in the transmitter, and k is a positive number that meets 0.8≤k≤1.2, the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, and the first oscillation circuit is configured to transfer the power to the second oscillation circuit. Circuit parameters in the receiver meet 
     
       
         
           
             
               
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                     p 
                   
                 
                 k 
               
               . 
             
           
         
       
     
     A wireless charging system may be set to operate at a load-independent point by setting a voltage gain to a voltage gain at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, when a voltage gain between an output voltage of the receiver and an input voltage of the transmitter is a first voltage gain, the wireless charging system operates at a load-independent point, the wireless charging system includes the transmitter and the receiver, the load-independent point includes a first operating frequency and the first voltage gain, at the first operating frequency, the first voltage gain is independent of an output load of the receiver, and the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}. Regardless of a value of a coupling factor, a voltage gain at the load-independent point is invariably √{square root over (C p /C s )}, and the voltage gain is set to be within a range of 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )} or within a range of 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )} such that the wireless charging system operates near the load-independent point, and the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, k is 1, and X is or √{square root over (C p /C s )} or √{square root over (L s /L p )}. When k=1, L p *C p −L s *C s =0. In this case, a voltage gain is regulated to √{square root over (C p /C s )} or √{square root over (L s /L p )}, and the wireless charging system operates at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, the receiver further includes a voltage regulation module, where the voltage regulation module is connected to the second oscillation circuit in parallel, and the voltage regulation module is configured to set the voltage gain between the output voltage of the receiver and the input voltage of the transmitter to the first voltage gain by regulating the output voltage of the receiver. 
     According to a fourth aspect, an embodiment of this application provides a charging method based on a wireless charging system, where the wireless charging system includes a transmitter and a receiver, the transmitter includes a transmitter coil and a first series matching capacitor, the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, and the first oscillation circuit is configured to transfer power to the receiver, and the receiver includes a receiver coil and a second series matching capacitor, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and a self-inductance applied when the transmitter coil transfers the power to the receiver is L p , and a capacitance of the first series matching capacitor is C p , a self-inductance applied when the receiver coil receives the power transferred by the first oscillation circuit is L s , and a capacitance of the second series matching capacitor is C s , where 
     
       
         
           
             
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 
                   
                     L 
                     s 
                   
                   * 
                   
                     C 
                     s 
                   
                 
               
               = 
               k 
             
             , 
           
         
       
     
     and k is a positive number that meets 0.8≤k≤1.2, and the method includes setting, by the transmitter, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to a first voltage gain, where the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}, when the voltage gain between the output voltage of the receiver and the input voltage of the transmitter is the first voltage gain, the wireless charging system operates at a load-independent point, the load-independent point includes a first operating frequency and the first voltage gain, and at the first operating frequency, the first voltage gain is independent of an output load of the receiver, and transferring, by the transmitter, the power to the receiver at the first voltage gain. Regardless of a value of a coupling factor, a voltage gain at the load-independent point is invariably √{square root over (C p /C s )}, and the voltage gain is set to be within a range of 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )} or within a range of 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )} such that the wireless charging system operates near the load-independent point, and the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, k is 1, and X is √{square root over (C p /C s )} or √{square root over (L s /L p )}. When k=1, L p *C p −L s *C s =0. In this case, a voltage gain is regulated to √{square root over (C p /C s )} or √{square root over (L s /L p )}, and the wireless charging system operates at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, the setting, by the transmitter, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to a first voltage gain includes receiving, by the transmitter, information sent by the receiver and indicating a first output voltage, where the first output voltage is an expected output voltage of the receiver, and setting, by the transmitter, the input voltage of the transmitter to a first input voltage based on the first output voltage and the first voltage gain. 
     In one embodiment, before the setting, by the transmitter, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to a first voltage gain, the method further includes receiving, by the transmitter, information sent by the receiver and indicating C s  and/or information sent by the receiver and indicating L s , where C s  and L s  are used by the transmitter to determine the first voltage gain, and/or receiving, by the transmitter, information sent by the receiver and indicating the first voltage gain. 
     According to a fifth aspect, an embodiment of this application provides a charging method based on a wireless charging system, where the wireless charging system includes a transmitter and a receiver, the transmitter includes a transmitter coil and a first series matching capacitor, the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, and the first oscillation circuit is configured to transfer power to the receiver, and the receiver includes a receiver coil and a second series matching capacitor, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and a self-inductance applied when the transmitter coil transfers the power to the receiver is L p , and a capacitance of the first series matching capacitor is C p , a self-inductance applied when the receiver coil receives the power transferred by the first oscillation circuit is L s , and a capacitance of the second series matching capacitor is C s , where 
     
       
         
           
             
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 
                   
                     L 
                     s 
                   
                   * 
                   
                     C 
                     s 
                   
                 
               
               = 
               k 
             
             , 
           
         
       
     
     and k is a positive number that meets 0.8≤k≤1.2, and the method includes sending, by the receiver to the transmitter, information that indicates a first output voltage, where the first output voltage is an expected output voltage of the receiver, and the first output voltage is used by the transmitter to set an input voltage of the transmitter to a first input voltage based on the first output voltage and a first voltage gain, where the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}, when a voltage gain between an output voltage of the receiver and the input voltage of the transmitter is the first voltage gain, the wireless charging system operates at a load-independent point, the load-independent point includes a first operating frequency and the first voltage gain, and at the first operating frequency, the first voltage gain is independent of an output load of the receiver, and receiving, by the receiver, the power transferred by the transmitter at the first voltage gain. Regardless of a value of a coupling factor, a voltage gain at the load-independent point is invariably √{square root over (C p /C s )}, and the voltage gain is set to be within a range of 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )} or within a range of 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )} such that the wireless charging system operates near the load-independent point, and the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     According to a sixth aspect, an embodiment of this application provides a charging method based on a wireless charging system, where the wireless charging system includes a transmitter and a receiver, the transmitter includes a transmitter coil and a first series matching capacitor, the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, and the first oscillation circuit is configured to transfer power to the receiver, and the receiver includes a receiver coil and a second series matching capacitor, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and a self-inductance applied when the transmitter coil transfers the power to the receiver is L p , and a capacitance of the first series matching capacitor is C p , a self-inductance applied when the receiver coil receives the power transferred by the first oscillation circuit is L s , and a capacitance of the second series matching capacitor is C s , where 
     
       
         
           
             
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 
                   
                     L 
                     s 
                   
                   * 
                   
                     C 
                     s 
                   
                 
               
               = 
               k 
             
             , 
           
         
       
     
     and k is a positive number that meets 0.8≤k≤1.2, and the method includes setting, by the receiver, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to a first voltage gain, where the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}, when the voltage gain between the output voltage of the receiver and the input voltage of the transmitter is the first voltage gain, the wireless charging system operates at a load-independent point, the load-independent point includes a first operating frequency and the first voltage gain, and at the first operating frequency, the first voltage gain is independent of an output load of the receiver, and receiving, by the receiver, the power transferred by the transmitter at the first voltage gain. Regardless of a value of a coupling factor, a voltage gain at the load-independent point is invariably √{square root over (C p /C s )}, and the voltage gain is set to be within a range of 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )} or within a range of 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )} such that the wireless charging system operates near the load-independent point, and the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, k is 1, and X is √{square root over (C p /C s )} or √{square root over (L s /L p )}. When k=1, L p *C p −L s *C s =0. In this case, a voltage gain is regulated to √{square root over (C p /C s )} or √{square root over (L s /L p )}, and the wireless charging system operates at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     In one embodiment, the setting, by the receiver, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to a first voltage gain includes receiving, by the receiver, information sent by the transmitter and indicating a first input voltage, where the first input voltage is the input voltage of the transmitter, and setting, by the receiver, the output voltage of the receiver to a first output voltage based on the first input voltage and the first voltage gain. 
     In one embodiment, before the setting, by the receiver, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to a first voltage gain, the method further includes receiving, by the receiver, information sent by the transmitter and indicating C p  and/or information sent by the transmitter and indicating L p , where C p  and L p  are used by the receiver to determine the first voltage gain, and/or receiving, by the receiver, information sent by the transmitter and indicating the first voltage gain. 
     According to a seventh aspect, an embodiment of this application provides a charging method based on a wireless charging system, where the wireless charging system includes a transmitter and a receiver, the transmitter includes a transmitter coil and a first series matching capacitor, the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, and the first oscillation circuit is configured to transfer power to the receiver, and the receiver includes a receiver coil and a second series matching capacitor, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and a self-inductance applied when the transmitter coil transfers the power to the receiver is L p , and a capacitance of the first series matching capacitor is C p , a self-inductance applied when the receiver coil receives the power transferred by the first oscillation circuit is L s , and a capacitance of the second series matching capacitor is C s , where 
     
       
         
           
             
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 
                   
                     L 
                     s 
                   
                   * 
                   
                     C 
                     s 
                   
                 
               
               = 
               k 
             
             , 
           
         
       
     
     and k is a positive number that meets 0.8≤k≤1.2, and the method includes sending, by the transmitter to the receiver, information that indicates a first input voltage, where the first input voltage is an input voltage of the transmitter, and the first input voltage is used by the receiver to set an output voltage of the receiver to a first output voltage based on the first input voltage and a first voltage gain, where the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}, when a voltage gain between the output voltage of the receiver and the input voltage of the transmitter is the first voltage gain, the wireless charging system operates at a load-independent point, the load-independent point includes a first operating frequency and the first voltage gain, and at the first operating frequency, the first voltage gain is independent of an output load of the receiver, and transferring, by the transmitter, the power to the receiver at the first voltage gain. Regardless of a value of a coupling factor, a voltage gain at the load-independent point is invariably √{square root over (C p /C s )}, and the voltage gain is set to be within a range of 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )} or within a range of 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )} such that the wireless charging system operates near the load-independent point, and the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     According to an eighth aspect, an embodiment of this application provides a charging method based on a wireless charging system, where the wireless charging system includes a transmitter and a receiver, the transmitter includes a transmitter coil and a first series matching capacitor, the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, and the first oscillation circuit is configured to transfer power to the receiver, the receiver includes a receiver coil and a second series matching capacitor, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and the method includes finding, by the transmitter from a first mapping table, a first load-independent point corresponding to a first coupling degree, or receiving, by the transmitter, information sent by the receiver and indicating a first load-independent point, where the first load-independent point is a first load-independent point corresponding to a first coupling degree and found by the receiver from a first mapping table, where the first load-independent point includes a first voltage gain, and the first coupling degree is a degree of coupling between the coil in the transmitter and the coil in the receiver, the first mapping table includes at least one coupling degree and a load-independent point corresponding to each of the at least one coupling degree, and the load-independent point is a combination of a voltage gain and an operating frequency, and at an operating frequency in a load-independent point corresponding to each coupling degree, a voltage gain in the load-independent point corresponding to the coupling degree is independent of an output load of the receiver, setting, by the transmitter, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to the first voltage gain, and transferring, by the transmitter, the power to the receiver at the first voltage gain. 
     The load-independent point is found from the first mapping table using the coupling degree. The wireless charging system may be set to operate at a load-independent point by setting a voltage gain to a voltage gain at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. Circuit parameters of the transmitter and the receiver do not need to be limited, and generality of the transmitter and the receiver is improved. 
     In one embodiment, the setting, by the transmitter, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to the first voltage gain includes receiving, by the transmitter, information sent by the receiver and indicating a first output voltage, where the first output voltage is an expected output voltage of the receiver, and setting, by the transmitter, the input voltage of the transmitter to a first input voltage based on the first output voltage and the first voltage gain. 
     In one embodiment, before the setting, by the transmitter, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to the first voltage gain, the method further includes receiving, by the transmitter, information sent by the receiver and indicating C s  and/or information sent by the receiver and indicating L s , where C s  and L s  are used by the transmitter to determine the first voltage gain, and/or receiving, by the transmitter, information sent by the receiver and indicating the first voltage gain. 
     According to a ninth aspect, an embodiment of this application provides a charging method based on a wireless charging system, where the wireless charging system includes a transmitter and a receiver, the transmitter includes a transmitter coil and a first series matching capacitor, the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, and the first oscillation circuit is configured to transfer power to the receiver, the receiver includes a receiver coil and a second series matching capacitor, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and the method includes sending, by the receiver to the transmitter, information that indicates a first output voltage, where the first output voltage is an expected output voltage of the receiver, and the first output voltage is used by the transmitter to set an input voltage of the transmitter to a first input voltage based on the first output voltage and a first voltage gain, where the first voltage gain is included in a first load-independent point corresponding to a first coupling degree and found by the transmitter from a first mapping table, or the first voltage gain is included in a first load-independent point corresponding to a first coupling degree and found by the receiver from a first mapping table, and the first load-independent point is added to information indicating the first load-independent point, and then sent by the receiver to the transmitter, where the first coupling degree is a degree of coupling between the transmitter coil in the transmitter and the receiver coil in the receiver, the first mapping table includes at least one coupling degree and a load-independent point corresponding to each of the at least one coupling degree, and the load-independent point is a combination of a voltage gain and an operating frequency, and at an operating frequency in a load-independent point corresponding to each coupling degree, a voltage gain in the load-independent point corresponding to the coupling degree is independent of an output load of the receiver, and receiving, by the receiver, the power transferred by the transmitter at the first voltage gain. 
     The load-independent point is found from the first mapping table using the coupling degree. The wireless charging system may be set to operate at a load-independent point by setting a voltage gain to a voltage gain at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. Circuit parameters of the transmitter and the receiver do not need to be limited, and generality of the transmitter and the receiver is improved. 
     According to a tenth aspect, an embodiment of this application provides a charging method based on a wireless charging system, where the wireless charging system includes a transmitter and a receiver, the transmitter includes a transmitter coil and a first series matching capacitor, the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, and the first oscillation circuit is configured to transfer power to the receiver, the receiver includes a receiver coil and a second series matching capacitor, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and the method includes finding, by the receiver from a first mapping table, a first load-independent point corresponding to a first coupling degree, or receiving, by the receiver, information sent by the transmitter and indicating a first load-independent point, where the first load-independent point is a first load-independent point corresponding to a first coupling degree and found by the transmitter from a first mapping table, where the first load-independent point includes a first voltage gain, and the first coupling degree is a degree of coupling between the transmitter coil in the transmitter and the receiver coil in the receiver, the first mapping table includes at least one coupling degree and a load-independent point corresponding to each of the at least one coupling degree, and the load-independent point is a combination of a voltage gain and an operating frequency, and at an operating frequency in a load-independent point corresponding to each coupling degree, a voltage gain in the load-independent point corresponding to the coupling degree is independent of an output load of the receiver, setting, by the receiver, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to the first voltage gain, and receiving, by the receiver, the power transferred by the transmitter at the first voltage gain. 
     The load-independent point is found from the first mapping table using the coupling degree. The wireless charging system may be set to operate at a load-independent point by setting a voltage gain to a voltage gain at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. Circuit parameters of the transmitter and the receiver do not need to be limited, and generality of the transmitter and the receiver is improved. 
     In one embodiment, the setting, by the receiver, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to the first voltage gain includes receiving, by the receiver, information sent by the transmitter and indicating a first input voltage, where the first input voltage is the input voltage of the transmitter, and setting, by the receiver, the output voltage of the receiver to a first output voltage based on the first input voltage and the first voltage gain. 
     In one embodiment, before the setting, by the receiver, a voltage gain between an output voltage of the receiver and an input voltage of the transmitter to the first voltage gain, the method further includes receiving, by the receiver, information sent by the transmitter and indicating C p  and/or information sent by the transmitter and indicating L p , where C p  and L p  are used by the receiver to determine the first voltage gain, and/or receiving, by the receiver, information sent by the transmitter and indicating the first voltage gain. 
     According to an eleventh aspect, an embodiment of this application provides a charging method based on a wireless charging system, where the wireless charging system includes a transmitter and a receiver, the transmitter includes a transmitter coil and a first series matching capacitor, the transmitter coil is connected to the first series matching capacitor in series to form a first oscillation circuit, and the first oscillation circuit is configured to transfer power to the receiver, the receiver includes a receiver coil and a second series matching capacitor, the receiver coil is connected to the second series matching capacitor in series to form a second oscillation circuit, and the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and the method includes sending, by the transmitter to the receiver, information that indicates a first input voltage, where the first input voltage is an input voltage of the transmitter, the first input voltage is used by the receiver to set an output voltage of the receiver to a first output voltage based on the first input voltage and a first voltage gain, the first voltage gain is included in a first load-independent point corresponding to a first coupling degree and found by the transmitter from a first mapping table, or the first voltage gain is included in a first load-independent point corresponding to a first coupling degree and found by the receiver from a first mapping table, and the first load-independent point is added to information indicating the first load-independent point, and then sent by the receiver to the transmitter, where the first coupling degree is a degree of coupling between the transmitter coil in the transmitter and the receiver coil in the receiver, the first mapping table includes at least one coupling degree and a load-independent point corresponding to each of the at least one coupling degree, and the load-independent point is a combination of a voltage gain and an operating frequency, and at an operating frequency in a load-independent point corresponding to each coupling degree, a voltage gain in the load-independent point corresponding to the coupling degree is independent of an output load of the receiver, and transferring, by the transmitter, the power to the receiver at the first voltage gain. 
     The load-independent point is found from the first mapping table using the coupling degree. The wireless charging system may be set to operate at a load-independent point by setting a voltage gain to a voltage gain at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. Circuit parameters of the transmitter and the receiver do not need to be limited, and generality of the transmitter and the receiver is improved. 
     According to a twelfth aspect, an embodiment of this application provides a transmitter, where the transmitter includes a module or a unit configured to perform the wireless charging method provided in the fourth aspect or any possible implementation of the fourth aspect. 
     According to a thirteenth aspect, an embodiment of this application provides a receiver, where the receiver includes a module or a unit configured to perform the wireless charging method provided in the fifth aspect. 
     According to a fourteenth aspect, an embodiment of this application provides a receiver, where the receiver includes a module or a unit configured to perform the wireless charging method provided in the sixth aspect or any possible implementation of the sixth aspect. 
     According to a fifteenth aspect, an embodiment of this application provides a transmitter, where the transmitter includes a module or a unit configured to perform the wireless charging method provided in the seventh aspect. 
     According to a sixteenth aspect, an embodiment of this application provides a transmitter, where the transmitter includes a module or a unit configured to perform the wireless charging method provided in the eighth aspect or any possible implementation of the eighth aspect. 
     According to a seventeenth aspect, an embodiment of this application provides a receiver, where the receiver includes a module or a unit configured to perform the wireless charging method provided in the ninth aspect or any possible implementation of the ninth aspect. 
     According to an eighteenth aspect, an embodiment of this application provides a receiver, where the receiver includes a module or a unit configured to perform the wireless charging method provided in the tenth aspect or any possible implementation of the tenth aspect. 
     According to a nineteenth aspect, an embodiment of this application provides a transmitter, where the transmitter includes a module or a unit configured to perform the wireless charging method provided in the eleventh aspect or any possible implementation of the eleventh aspect. 
     According to a twentieth aspect, an embodiment of this application provides a transmitter, including a processor, a memory, a transceiver, and a bus, where the processor, the transceiver, and the memory communicate with each other using the bus, the transceiver is configured to receive and send data, the memory is configured to store an instruction, and the processor is configured to invoke the instruction in the memory, to perform the wireless charging method provided in the fourth aspect or any possible implementation of the fourth aspect. 
     According to a twenty-first aspect, an embodiment of this application provides a receiver, including a processor, a memory, a transceiver, and a bus, where the processor, the transceiver, and the memory communicate with each other using the bus, the transceiver is configured to receive and send data, the memory is configured to store an instruction, and the processor is configured to invoke the instruction in the memory, to perform the wireless charging method provided in the fifth aspect or any possible implementation of the fifth aspect. 
     According to a twenty-second aspect, an embodiment of this application provides a receiver, including a processor, a memory, a transceiver, and a bus, where the processor, the transceiver, and the memory communicate with each other using the bus, the transceiver is configured to receive and send data, the memory is configured to store an instruction, and the processor is configured to invoke the instruction in the memory, to perform the wireless charging method provided in the sixth aspect or any possible implementation of the sixth aspect. 
     According to a twenty-third aspect, an embodiment of this application provides a transmitter, including a processor, a memory, a transceiver, and a bus, where the processor, the transceiver, and the memory communicate with each other using the bus, the transceiver is configured to receive and send data, the memory is configured to store an instruction, and the processor is configured to invoke the instruction in the memory, to perform the wireless charging method provided in the seventh aspect or any possible implementation of the seventh aspect. 
     According to a twenty-fourth aspect, an embodiment of this application provides a transmitter, including a processor, a memory, a transceiver, and a bus, where the processor, the transceiver, and the memory communicate with each other using the bus, the transceiver is configured to receive and send data, the memory is configured to store an instruction, and the processor is configured to invoke the instruction in the memory, to perform the wireless charging method provided in the eighth aspect or any possible implementation of the eighth aspect. 
     According to a twenty-fifth aspect, an embodiment of this application provides a receiver, including a processor, a memory, a transceiver, and a bus, where the processor, the transceiver, and the memory communicate with each other using the bus, the transceiver is configured to receive and send data, the memory is configured to store an instruction, and the processor is configured to invoke the instruction in the memory, to perform the wireless charging method provided in the ninth aspect or any possible implementation of the ninth aspect. 
     According to a twenty-sixth aspect, an embodiment of this application provides a receiver, including a processor, a memory, a transceiver, and a bus, where the processor, the transceiver, and the memory communicate with each other using the bus, the transceiver is configured to receive and send data, the memory is configured to store an instruction, and the processor is configured to invoke the instruction in the memory, to perform the wireless charging method provided in the tenth aspect or any possible implementation of the tenth aspect. 
     According to a twenty-seventh aspect, an embodiment of this application provides a transmitter, including a processor, a memory, a transceiver, and a bus, where the processor, the transceiver, and the memory communicate with each other using the bus, the transceiver is configured to receive and send data, the memory is configured to store an instruction, and the processor is configured to invoke the instruction in the memory, to perform the wireless charging method provided in the eleventh aspect or any possible implementation of the eleventh aspect. 
     According to a twenty-eighth aspect, an embodiment of this application provides a computer readable storage medium, where the storage medium includes an instruction, and when the instruction is run on a transmitter, the transmitter performs the wireless charging method provided in the fourth aspect or any possible implementation of the fourth aspect. 
     According to a twenty-ninth aspect, an embodiment of this application provides a computer readable storage medium, where the storage medium includes an instruction, and when the instruction is run on a receiver, the receiver performs the wireless charging method provided in the fifth aspect or any possible implementation of the fifth aspect. 
     According to a thirtieth aspect, an embodiment of this application provides a computer readable storage medium, where the storage medium includes an instruction, and when the instruction is run on a receiver, the receiver performs the wireless charging method provided in the sixth aspect or any possible implementation of the sixth aspect. 
     According to a thirty-first aspect, an embodiment of this application provides a computer readable storage medium, where the storage medium includes an instruction, and when the instruction is run on a transmitter, the transmitter performs the wireless charging method provided in the seventh aspect or any possible implementation of the seventh aspect. 
     According to a thirty-second aspect, an embodiment of this application provides a computer readable storage medium, where the storage medium includes an instruction, and when the instruction is run on a transmitter, the transmitter performs the wireless charging method provided in the eighth aspect or any possible implementation of the eighth aspect. 
     According to a thirty-third aspect, an embodiment of this application provides a computer readable storage medium, where the storage medium includes an instruction, and when the instruction is run on a receiver, the receiver performs the wireless charging method provided in the ninth aspect or any possible implementation of the ninth aspect. 
     According to a thirty-fourth aspect, an embodiment of this application provides a computer readable storage medium, where the storage medium includes an instruction, and when the instruction is run on a receiver, the receiver performs the wireless charging method provided in the tenth aspect or any possible implementation of the tenth aspect. 
     According to a thirty-fifth aspect, an embodiment of this application provides a computer readable storage medium, where the storage medium includes an instruction, and when the instruction is run on a transmitter, the transmitter performs the wireless charging method provided in the eleventh aspect or any possible implementation of the eleventh aspect. 
     According to a thirty-sixth aspect, an embodiment of this application provides a computer program, where the computer program includes an instruction, and when the instruction is run on a transmitter, the transmitter performs the wireless charging method provided in the fourth aspect or any possible implementation of the fourth aspect. 
     According to a thirty-seventh aspect, an embodiment of this application provides a computer program, where the computer program includes an instruction, and when the instruction is run on a receiver, the receiver performs the wireless charging method provided in the fifth aspect. 
     According to a thirty-eighth aspect, an embodiment of this application provides a computer program, where the computer program includes an instruction, and when the instruction is run on a receiver, the receiver performs the wireless charging method provided in the sixth aspect or any possible implementation of the sixth aspect. 
     According to a thirty-ninth aspect, an embodiment of this application provides a computer program, where the computer program includes an instruction, and when the instruction is run on a transmitter, the transmitter performs the wireless charging method provided in the seventh aspect or any possible implementation of the seventh aspect. 
     According to a fortieth aspect, an embodiment of this application provides a computer program, where the computer program includes an instruction, and when the instruction is run on a transmitter, the transmitter performs the wireless charging method provided in the eighth aspect or any possible implementation of the eighth aspect. 
     According to a forty-first aspect, an embodiment of this application provides a computer program, where the computer program includes an instruction, and when the instruction is run on a receiver, the receiver performs the wireless charging method provided in the ninth aspect. 
     According to a forty-second aspect, an embodiment of this application provides a computer program, where the computer program includes an instruction, and when the instruction is run on a receiver, the receiver performs the wireless charging method provided in the tenth aspect or any possible implementation of the tenth aspect. 
     According to a forty-third aspect, an embodiment of this application provides a computer program, where the computer program includes an instruction, and when the instruction is run on a transmitter, the transmitter performs the wireless charging method provided in the eleventh aspect or any possible implementation of the eleventh aspect. 
     According to a forty-fourth aspect, an embodiment of this application provides a chip product of a transmitter, to perform the method in the fourth aspect or any possible implementation of the fourth aspect. 
     According to a forty-fifth aspect, an embodiment of this application provides a chip product of a receiver, to perform the method in the fifth aspect or any possible implementation of the fifth aspect. 
     According to a forty-sixth aspect, an embodiment of this application provides a chip product of a receiver, to perform the method in the sixth aspect or any possible implementation of the sixth aspect. 
     According to a forty-seventh aspect, an embodiment of this application provides a chip product of a transmitter, to perform the method in the seventh aspect or any possible implementation of the seventh aspect. 
     According to a forty-eighth aspect, an embodiment of this application provides a chip product of a transmitter, to perform the method in the eighth aspect or any possible implementation of the eighth aspect. 
     According to a forty-ninth aspect, an embodiment of this application provides a chip product of a receiver, to perform the method in the ninth aspect or any possible implementation of the ninth aspect. 
     According to a fiftieth aspect, an embodiment of this application provides a chip product of a receiver, to perform the method in the tenth aspect or any possible implementation of the tenth aspect. 
     According to a fifty-first aspect, an embodiment of this application provides a chip product of a transmitter, to perform the method in the eleventh aspect or any possible implementation of the eleventh aspect. 
     In the embodiments of this application, the wireless charging system may be set to operate at a load-independent point by setting a voltage gain to a voltage gain at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module in the receiver can be reduced, and charging efficiency of the receiver can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       To describe the technical solutions in the embodiments of the present disclosure or in the background more clearly, the following describes the accompanying drawings required in the embodiments of the present disclosure or in the background. 
         FIG. 1  is a schematic architectural diagram of a wireless charging system to which an embodiment of this application relates. 
         FIG. 2  is a schematic diagram of a position deviation between a transmitter and a receiver according to an embodiment of this application. 
         FIG. 3  shows an equivalent circuit model of a wireless charging system according to an embodiment of this application. 
         FIG. 4  is a schematic diagram of a relationship between a voltage gain and an operating frequency according to an embodiment of this application. 
         FIG. 5  shows another equivalent circuit model of a wireless charging system according to an embodiment of this application. 
         FIG. 6  is a schematic diagram of another relationship between a voltage gain and an operating frequency according to an embodiment of this application. 
         FIG. 7  is a schematic diagram of still another relationship between a voltage gain and an operating frequency according to an embodiment of this application, 
         FIG. 8  is a schematic architectural diagram of another wireless charging system according to an embodiment of this application. 
         FIG. 9  is a schematic flowchart of a wireless charging method according to an embodiment of this application. 
         FIG. 10  is a schematic structural diagram of still another wireless charging system according to an embodiment of this application. 
         FIG. 11  is a schematic flowchart of another wireless charging method according to an embodiment of this application. 
         FIG. 12  is a schematic flowchart of still another wireless charging method according to an embodiment of this application. 
         FIG. 13  is a schematic flowchart of yet another wireless charging method according to an embodiment of this application. 
         FIG. 14  shows a test result of electrical energy conversion efficiency according to an embodiment of this application. 
         FIG. 15  shows another test result of electrical energy conversion efficiency according to an embodiment of this application. 
         FIG. 16  is a schematic diagram of a test of a voltage and a current according to an embodiment of this application. 
         FIG. 17  is a schematic structural diagram of a wireless charging system  100  according to an embodiment of this application. 
         FIG. 18  is a schematic structural diagram of another wireless charging system  100  according to an embodiment of this application. 
         FIG. 19  is a schematic structural diagram of still another wireless charging system  100  according to an embodiment of this application. 
         FIG. 20  is a schematic structural diagram of yet another wireless charging system  100  according to an embodiment of this application. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes the embodiments of the present disclosure with reference to the accompanying drawings in the embodiments of the present disclosure. Terms used in the embodiments of this application are only used to explain specific embodiments of this application, and are not intended to limit this application. 
     Embodiments of this application disclose a wireless charging method and a device in order to improve charging efficiency of a receiver. Detailed descriptions are separately provided below. 
       FIG. 1  is a schematic architectural diagram of a wireless charging system to which an embodiment of this application relates. As shown in  FIG. 1 , the wireless charging system  100  includes a transmitter  10  and a receiver  20 . The transmitter  10  can transfer power to the receiver  20 , to wirelessly charge the receiver  20 . 
     The receiver  20  may be mobile user equipment (UE), an access terminal, a subscriber unit, a subscriber station, a mobile station, a remote station, a remote terminal, a user terminal, or a user agent. The access terminal may be a cellular phone, a handheld device having a wireless communication function, a computing device or an in-vehicle device, a wearable device, a terminal in a fifth generation (5G) system or a terminal in a future evolved public land mobile network (PLMN), or the like. Specifically, the receiver  20  may be a mobile phone, a tablet computer, a computer having a wireless transceiver function, a virtual reality (VR) terminal device, an augmented reality (AR) terminal device, a wireless terminal in industrial control, a wireless terminal in self driving, a wireless terminal in remote medical surgery, a wireless terminal in a smart grid, a wireless terminal in transportation safety, a wireless terminal in a smart city, a wireless terminal in a smart home, or the like. Alternatively, the receiver  20  may be a wireless charging electric vehicle or white goods, for example, a no-tail television, a wireless charging soymilk maker, a wireless charging vacuum cleaning robot, or a multi-rotor drone. 
     A manner of data communication between the transmitter  10  and the receiver  20  may be wireless communication, and may be specifically in-band communication, BLUETOOTH communication, ZIGBEE communication, WI-FI communication, or the like. 
     The transmitter  10  may include a DC power source  101 , a DC/AC conversion module  102 , a series matching capacitor (whose capacitance is C p )  103 , a transmitter coil  104 , and a control module  105 . The receiver  20  may include a receiver coil  201 , a series matching capacitor (whose capacitance is C s )  202 , an AC/DC conversion module  203 , a voltage regulator module  204 , a output load  205 , a modulation module  206 , and a control module  207 . 
     Specifically, as shown in  FIG. 1 , the DC power source  101  is configured to supply charging power. The DC/AC conversion module  102  is connected to the DC power source  101 , and is configured to receive a DC that is output by the DC power source  101 , convert the received DC into an AC, and output the AC. The series matching capacitor (whose capacitance is C p )  103  and the transmitter coil  104  are connected to form an oscillation circuit. The oscillation circuit is connected to the DC/AC conversion module  102 , and is configured to receive the AC that is output by the DC/AC conversion module  102  and supply the AC to the transmitter coil  104 . Power of the transmitter coil  104  is transferred to the receiver coil  201  under coupling of the transmitter coil  104  and the receiver coil  201 . The control module  105  may be separately connected to the DC power source  101 , the DC/AC conversion module  102 , the series matching capacitor (whose capacitance is C p )  103 , and the transmitter coil  104 , and is configured to exchange a control parameter with each module, to implement control on each module. 
     The DC/AC conversion module  102  may be a full-bridge inverter circuit, or may be a half-bridge inverter circuit, or may be another inverter circuit that converts a DC into an AC. This is not limited in this embodiment of this application. 
     As shown in  FIG. 1 , the receiver coil  201  is connected to the series matching capacitor (whose capacitance is C s )  202 , to form an oscillation circuit on a side of the receiver  20 . The receiver coil  201  receives, through coil coupling, the power transferred by the transmitter coil  104 , and converts the power into an AC using the oscillation circuit. The AC/DC conversion module  203  is connected to the oscillation circuit, and is configured to receive the AC that is output by the oscillation circuit, and rectify the AC to obtain an output voltage Vrect. The voltage regulator module  204  is connected to the AC/DC conversion module  203 , and is configured to eliminate fluctuation in the output voltage Vrect from the AC/DC conversion module  203 , to output a stable voltage V 2 . The output load  205  is connected to the voltage regulator module  204 , and is configured to receive the supply voltage V 2  that is output by the voltage regulator module  204 . The modulation module  206  is configured to implement in-band communication with the transmitter  10 . 
     Specifically, as shown in  FIG. 1 , the modulation module  206  may use switched capacitor modulation and/or switched resistor modulation. The receiver  20  controls connection and disconnection of a switch  51  and/or a switch S 2 , to allow a capacitor C 1  and/or a resistor R 1  to be connected to a receiver circuit or not connected to a receiver circuit such that a voltage or a current in the receiver coil  201  in the receiver  20  is changed, and a voltage or a current in the transmitter  10  is changed. The transmitter  10  collects the voltage or the current and can obtain, through analysis after demodulation processing, a communication signal modulated by the receiver  20 . The control module  207  may be separately connected to the receiver coil  201 , the series matching capacitor (whose capacitance is C s )  202 , the AC/DC conversion module  203 , the voltage regulator module  204 , the output load  205 , and the modulation module  206 , and is configured to exchange a control parameter with each module, to implement control on each module. 
     The AC/DC conversion module  102  may be a diode full-bridge rectifier circuit, or may be a switching transistor synchronous rectifier circuit, or may be a half-bridge rectifier circuit, or may be another rectifier circuit that converts an AC into a DC. This is not limited in this embodiment of this application. 
     During in-band communication, connection of the modulation capacitor C 1  and connection of the modulation resistor R 1  may cause fluctuation in a load in a circuit of the receiver  20 . For example, if a load jump occurs in the receiver  20 , an operating frequency needs to be regulated to maintain a constant output voltage of the receiver according to the Wireless Power Consortium (WPC) standard. In this process, a control parameter for regulating the operating frequency needs to be transmitted through the in-band communication. During the in-band communication, the in-band communication may affect an output load of the receiver. For example, in a capacitor modulation mode, if the AC/DC conversion module  203  is a full-wave rectifier diode bridge, connection of the modulation capacitor C 1  is equivalent to a capacitance increase at two rectifier tubes of a rectifier bridge. When a polarity of a current flowing into the rectifier bridge changes, the modulation capacitor C 1  may be charged or discharged, which is equivalent to a load disturbance and may affect V 1 . 
     Connection and disconnection of the modulation capacitor and/or the modulation resistor cause V 1  to change at a relatively high amplitude. V 2  that is output by the voltage regulator module  204  is a stable DC voltage. A part of the voltage V 1  exceeding V 2  is consumed by the voltage regulator module  204 , and becomes power consumption of the voltage regulator module  204 , increasing an electrical energy loss of the receiver  20 . 
     When there is a deviation between positions at which the receiver  20  and the transmitter  10  are placed, the transmitter coil  104  and the receiver coil  201  do not directly face each other, and there is misalignment. A load jump may cause noticeable fluctuation in the output voltage V 1 . In addition, the in-band communication may cause a jump of the output voltage V 1 , the voltage regulator module  204  operates frequently, an electrical energy loss of the voltage regulator module  204  is increased, and charging efficiency is reduced. 
     Based on the schematic architectural diagram of the wireless charging system in  FIG. 1 , this application provides a wireless charging method such that output voltage fluctuation caused by a load jump can be reduced, and an output voltage jump caused by in-band communication can be reduced. Therefore, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of a receiver can be improved. 
     A main principle of this application may include enabling a wireless charging system to operate at a load-independent point through voltage regulation and operating frequency regulation such that a voltage gain is independent of a load impedance of a receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when a transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     For example, the wireless charging system operates at a load-independent point (   0 , X 0 ). In an embodiment, the voltage gain between the output voltage of the receiver and the input voltage of the transmitter is X 0 , the input voltage of the transmitter is V in0 , the output voltage of the receiver is V out0 , V out0 /V in0 =X 0 , and an operating frequency at which the transmitter transfers power to the receiver is    0 . When the output load of the transmitter jumps, because the wireless charging system operates at the load-independent point, an output load jump does not affect the voltage gain between the output voltage of the receiver and the input voltage of the transmitter, and the voltage gain is still X 0 . The input voltage V in0  of the transmitter does not change, and therefore the output voltage V out0  of the receiver does not change. Fluctuation of a voltage that is input into the voltage regulator module fluctuates very slightly before and after the output load jump. Therefore, the electrical energy loss of the voltage regulator module is reduced, and the charging efficiency of the receiver can be improved. 
     To help understand the embodiments of this application, some concepts or terms related to a load-independent point in the embodiments of this application are explained. 
     (1) Coupling Factor and Mutual Inductance 
     A coupling factor is used to represent closeness of coupling between the transmitter coil in the transmitter  10  and the receiver coil in the receiver  20 . A higher coupling factor indicates higher efficiency at which the transmitter coil in the transmitter  10  transfers power to the receiver coil in the receiver  20 . The coupling factor is related to the position deviation between the transmitter  10  and the receiver  20 . A larger position deviation between the transmitter  10  and the receiver  20  indicates lower closeness of the coupling between the transmitter coil in the transmitter  10  and the receiver coil in the receiver  20  and a smaller coupling factor. Therefore, during wireless charging, a smaller position deviation between the transmitter  10  and the receiver  20  leads to higher charging efficiency of the wireless charging system. That is, a coupling factor K is determined by the position deviation between the transmitter  10  and the receiver  20 . The position deviation herein is a deviation between positions of the transmitter coil in the transmitter  10  and the receiver coil in the receiver  20 . For details, refer to  FIG. 2 .  FIG. 2  is a schematic diagram of a position deviation between a transmitter and a receiver according to an embodiment of this application. As shown in  FIG. 2 , a position deviation s between the transmitter  10  and the receiver  20  may be understood as a deviation between positions of a center of the transmitter coil  104  in the transmitter  10  and a center of the receiver coil  201  in the receiver  20 , for example, may be a horizontal distance between the center of the transmitter coil  104  in the transmitter  10  and the center of the receiver coil  201  in the receiver  20 . If the transmitter  10  includes a plurality of transmitter coils, the position deviation s between the transmitter  10  and the receiver  20  may alternatively be understood as a deviation between positions of a center of a target transmitter coil in the transmitter  10  and the center of the receiver coil  201  in the receiver  20 . The target transmitter coil may be one or more transmitter coils in the transmitter  10  that are closest to the receiver coil  201  in the receiver  20 . 
     A mutual inductance is short for a coefficient of mutual induction, and may be used to represent magnitude of a phenomenon of mutual induction between the transmitter coil in the transmitter and the receiver coil in the receiver. A relationship between the coupling factor K and a mutual inductance M is 
     
       
         
           
             
               K 
               = 
               
                 M 
                 
                   
                     
                       L 
                       p 
                     
                     * 
                     
                       L 
                       s 
                     
                   
                 
               
             
             , 
           
         
       
     
     where L p  and L s  are respectively an equivalent inductance of the receiver and an equivalent inductance of the transmitter during power transfer. Magnitude of the mutual inductance can also reflect the position deviation between the transmitter and the receiver. 
     The coupling factor and the mutual inductance can both be used to indicate a degree of coupling between the transmitter coil in the transmitter  10  and the receiver coil in the receiver  20 . Certainly, a newly defined parameter may also be used to indicate the degree of coupling between the transmitter coil in the transmitter  10  and the receiver coil in the receiver  20 . The coupling factor K is used as an example for description in the embodiments of this application. It can be understood that in the embodiments of this application, the degree of coupling between the transmitter coil in the transmitter  10  and the receiver coil in the receiver  20  is not limited to being indicated by the coupling factor, but may be indicated by the mutual inductance or may be indicated by another newly defined parameter. 
     (2) Operating Frequency and Voltage Gain 
     An operating frequency is a frequency at which the transmitter transfers power to the receiver in the wireless charging system. Different power is transferred on a coupling coil at different operating frequencies. When the operating frequency is 
     
       
         
           
             
               1 
               
                 2 
                  
                 
                   ∏ 
                   
                     
                       
                         L 
                         p 
                       
                       * 
                       
                         L 
                         p 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
     power transferred on the coupling coil is maximum. 
     When output power or an output voltage of the receiver needs to be regulated, control information may be transmitted to the transmitter in a form of an error control information packet through in-band communication, the transmitter adjusts, based on the control information, an operating frequency at which the transmitter transfers power, thereby adjusting intensity of a magnetic field in the equivalent inductance L p  of the transmitter to achieve expected output power or an expected output voltage of the receiver. 
     A voltage gain is a ratio of an output voltage of a receiver to an input voltage of a transmitter. The output voltage is a voltage value that is obtained after rectification by an AC/DC conversion module in the receiver, and the input voltage is a voltage value that is obtained after conversion by a DC/AC conversion module in the transmitter. When an operating frequency of the wireless charging system changes, a voltage gain changes accordingly. 
     (3) Load-Independent Point 
     a. What is a Load-Independent Point? 
     The load-independent point is a point that is on a relationship curve between a voltage gain and an operating frequency and at which a change in an output load impedance of a receiver does not affect a voltage gain and an operating frequency of the wireless charging system. 
       FIG. 3  shows an equivalent circuit model of a wireless charging system according to an embodiment of this application. As shown in  FIG. 3 , the wireless charging system shown in  FIG. 1  may be equivalent to a magnetic coupling structure model of the wireless charging system shown in  FIG. 3 . An output voltage of a transmitter is U op , an output current of the transmitter is I op , and a frequency of the output voltage and the output current is f op . C p  is a series matching capacitance of the transmitter, L p  is a self-inductance applied when a transmitter coil in the transmitter transfers power to the receiver, and R p  is an input resistance of the transmitter. C s  is a series matching capacitance of the receiver, L s  is a self-inductance applied when a receiver coil in the receiver receives the power transferred by the transmitter, R s  is a resistance of the receiver, i s  is a current on the coil of the receiver, and Z L  is a load impedance. i L  and u L  are a load current and a load voltage, respectively. K is a coupling factor between the coil of the transmitter and the coil of the receiver. A series matching capacitor (whose capacitance is C p ) of the transmitter and the transmitter coil form a first oscillation circuit, a series matching capacitor (whose capacitance is C s ) of the receiver and the receiver coil form a second oscillation circuit, and the first oscillation circuit is configured to transfer power to the second oscillation circuit. The second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and transfer the power to an AC/DC conversion module. In the embodiments of this application, the series matching capacitor of the transmitter may be referred to as a first series matching capacitor, and the series matching capacitor of the receiver may be referred to as a second series matching capacitor. 
     When the coupling factor K is fixed, a curve of a relationship between a voltage gain X and an operating frequency   varies with an output load impedance of the receiver.  FIG. 4  is a schematic diagram of a relationship between a voltage gain and an operating frequency according to an embodiment of this application. As shown in  FIG. 4 , when the coupling factor K is a fixed value K 0 , a set of X−  curves corresponding to a load impedance Z L  whose values are Z 1 , Z 2 , and Z 3  are respectively drawn, where Z 1 , Z 2 , and Z 3  are different values. The set of X−  curves corresponding to Z 1 , Z 2 , and Z 3  intersect at a same point. At this intersection point, regardless of a value of the load impedance of the wireless charging system, an operating frequency is    0 , a voltage gain is X 0 , and (   0 , X 0 ) may be referred to as a load-independent point. The load-independent point is an inherent feature of the wireless charging system. At the load-independent point, if the operating frequency of the wireless charging system is regulated to    0 , the voltage gain is invariably X 0  regardless of a value of the output load impedance Z L  of the receiver. 
     The following provides specific calculation of the voltage gain X 0  and the operating frequency    0  of the load-independent point of the wireless charging system. 
     Because the wireless charging system has a fixed voltage gain at the load-independent point, and a transformer circuit has a constant voltage transformation ratio, if the voltage gain is analogized to the voltage transformation ratio, the wireless charging system that operates at the load-independent point can be equivalent to a transformer circuit model.  FIG. 5  shows another equivalent circuit model of a wireless charging system according to an embodiment of this application. 
     As shown in  FIG. 5 , in the equivalent transformer circuit model, L kp  is a transformer primary leakage inductance, L ks  is a transformer secondary leakage inductance, and L m  is a transformer primary excitation inductance. A transformer voltage transformation ratio may be understood as a voltage gain X, that is, Output voltage/Input voltage=X. It can be understood that the equivalent circuit model of the wireless charging system shown in  FIG. 3  can be equivalent to the transformer equivalent transformer circuit model shown in  FIG. 5  only when the wireless charging system operates at a load-independent point. 
     The following can be obtained based on the equivalent transformer circuit model 
         L   kp   =L   p   −L   m   (1)
 
         L   ks   =L   s   −L   m   *X   2   (2).
 
     In the equivalent transformer circuit model, it can be learned that C p  and L kp  form series resonance, and C s  and L ks  form series resonance. A primary side and a secondary side have a same series resonance frequency  , the series resonance frequency is an operating frequency of the wireless charging system, and the series resonance frequency   is calculated as follows 
     
       
         
           
             
               
                 
                   = 
                   
                     
                       1 
                       
                         
                           
                             ( 
                             
                               
                                 L 
                                 p 
                               
                               - 
                               
                                 L 
                                 m 
                               
                             
                             ) 
                           
                           * 
                           
                             C 
                             p 
                           
                         
                       
                     
                     = 
                     
                       1 
                       
                         
                           
                             ( 
                             
                               
                                 L 
                                 s 
                               
                               - 
                               
                                 
                                   L 
                                   m 
                                 
                                 * 
                                 
                                   X 
                                   2 
                                 
                               
                             
                             ) 
                           
                           * 
                           
                             C 
                             s 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The following can be obtained based on formula (3) 
       ( L   p   ,L   m )* C   p =( L   s   −L   m   *X   2 )* C   s   (4).
 
     The following can be obtained by sorting out formula (4) 
         L   p   *C   p   −L   s   *C   s   =L   m *( C   p   −C   s   *X   2 )  (5).
 
     Usually, after the wireless charging system has been designed, L p , C p , L s , and C s  are fixed values. Therefore, assuming that L p *C p −L s *C s =Const (briefly referred to as Const below), a voltage transformation ratio of the equivalent transformer circuit model can be obtained, that is, the voltage gain X of the wireless charging system is 
     
       
         
           
             
               
                 
                   
                     X 
                     = 
                     
                       
                         
                           
                             C 
                             p 
                           
                           - 
                           
                             Const 
                             
                               L 
                               m 
                             
                           
                         
                         
                           C 
                           s 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     where    0  and X 0  may be expressed as formula (3) and formula (6), respectively. 
     It can be learned that a load-independent point is related to a coupling factor. During actual wireless charging, when positions at which the transmitter and the receiver are placed are definite, the coupling factor is a fixed value. Therefore, when relative displacement of the transmitter and the receiver is constant, the load-independent point is also definite. The relationship between the coupling factor K and the mutual inductance M is 
     
       
         
           
             
               K 
               = 
               
                 M 
                 
                   
                     
                       L 
                       p 
                     
                     * 
                     
                       L 
                       s 
                     
                   
                 
               
             
             . 
           
         
       
     
     Therefore, a relationship between the mutual inductance M and the load-independent point is similar to that between the coupling factor and the load-independent point. 
     b. Two Load-Independent Points 
     Based on the foregoing properties of the load-independent point, a design idea of this application is described below. 
     It can be learned from formula (6) that the load-independent point is related to Const in addition to the coupling factor.  FIG. 6  is a schematic diagram of another relationship between a voltage gain and an operating frequency according to an embodiment of this application. As shown in  FIG. 6 , when Const&gt;0, and values of the coupling factor K are K 0 , K 1 , and K 2 , respectively, corresponding load-independent points are (   0 , X 0 ), (   1 , X 1 ), and (   2 , X 2 ), respectively. Position deviations, corresponding to the coupling factors K 0 , K 1 , and K 2 , between the transmitter and the receiver are s 0 , s 1 , and s 2 , respectively, where s 0 &gt;s 1 &gt;s 2 . It can be learned with reference to  FIG. 5  and formula (6) that when Const&gt;0, a voltage gain increases and an operating frequency decreases at the load-independent point as the position deviation increases. When Const&lt;0, both a voltage gain and an operating frequency decrease at the load-independent point as the position deviation increases. 
     When Const=0, the following is obtained by substituting Const=0 into formula (6) 
     
       
         
           
             
               
                 
                   
                     X 
                     = 
                     
                       
                         
                           C 
                           p 
                         
                         
                           C 
                           s 
                         
                       
                     
                   
                   . 
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     It can be learned from formula (7) that when Const=0, the voltage gain at the load-independent point is invariably √{square root over (C p /C s )}, and is no longer related to a coupling degree of the two coils.  FIG. 7  is a schematic diagram of still another relationship between a voltage gain and an operating frequency according to an embodiment of this application. When values of the coupling factor K are K 0 , K 1 , and K 2 , respectively, corresponding load-independent points are (   0 , X 0 ), (   1 , X 0 ), and (   2 , X 0 ), respectively, where X 0 =√{square root over (C p /C s )}. Position deviations, corresponding to the coupling factors K 0 , K 1 , and K 2 , between the transmitter and the receiver are s 0 , s 1 , and s 2 , respectively, where s 0 &gt;s 1 &gt;s 2 . It can be learned that regardless of a position deviation between the transmitter and the receiver, a voltage gain at the load-independent point is invariably X 0 =√{square root over (C p /C s )}. When Const=0, a voltage gain remains unchanged and an operating frequency gradually decreases at the load-independent point as the position deviation increases. In other words, when Const=0, the voltage gain at the load-independent point is unrelated to the position deviation between the two devices. Because Const=L p *C p −L s *C s =0, C p /C s =L s /L p , that is, the voltage gain at the load-independent point may also be expressed as √{square root over (L s /L p )}. 
     Based on the foregoing properties of Const=0 and Const≠0, wireless charging methods in which a wireless charging system operates at a load-independent point are designed, respectively. Descriptions are separately given below. 
     When Const=0, a process of adjusting a wireless charging system to operate at a load-independent point is as follows. Because the voltage gain at the load-independent point is unrelated to the position deviation between the two devices when Const=0, the voltage gain is always √{square root over (C p /C s )}. Based on the foregoing property, the voltage gain of the wireless charging system may be regulated to the voltage gain √{square root over (C p /C s )}, at the load-independent point by regulating an operating frequency at which the transmitter transfers power to the receiver. Correspondingly, the operating frequency at which the transmitter transfers the power to the receiver is    0 . 
     When Const≠0, a process of adjusting a wireless charging system to operate at a load-independent point is as follows. When Const≠0, a voltage gain X at a load-independent point and an operating frequency   change with a coupling factor. However, in formula (3) and formula (6), a primary excitation inductance L m  is related to a coupling factor, and L m  is unknown. Therefore, when Const≠0, for the wireless charging system, the load-independent point cannot be directly obtained through calculation. A mapping relationship between a coupling factor, Const, and a load-independent point may be pre-stored, to obtain a first mapping table. The wireless charging system may first determine the coupling factor between the transmitter and the receiver based on a position deviation between the transmitter and the receiver, and query the first mapping table, to obtain a load-independent point corresponding to the current coupling factor. In addition, the wireless charging system is adjusted to operate at this load-independent point. 
     c. How to Set a Load-Independent Point? 
     In the embodiments of this application, the wireless charging system needs to be set to operate at the load-independent point when transferring power. A voltage gain between an output voltage of the receiver and an input voltage of the transmitter may be set to be invariably a voltage gain X 0  at the load-independent point, and an operating frequency at which the transmitter transfers power to the receiver is the operating frequency    0  at the load-independent point. In a process of setting a gain to the voltage gain X 0  at the load-independent point, a voltage needs to be regulated. A voltage regulation module may be disposed in the transmitter, and is configured to regulate the input voltage of the transmitter to set the voltage gain to the voltage gain X 0  at the load-independent point. Alternatively, a voltage regulation module may be disposed in the receiver, and is configured to regulate the output voltage of the receiver to set the voltage gain to the voltage gain X 0  at the load-independent point. The operating frequency is regulated to the operating frequency    0  at the load-independent point through closed-loop frequency conversion. 
     In this embodiment of this application, the voltage regulation module may be disposed in the transmitter, or the voltage regulation module may be disposed in the receiver. When Const=0, several embodiments of the methods for regulating a wireless charging system to operate at a load-independent point are provided in the embodiments of this application based on the foregoing main principle and the foregoing two manners. 
     When Const=0, L p *C p −L s *C s =0 is required in the wireless charging system. Therefore, an embodiment of this application provides a wireless charging system. As shown in  FIG. 3 , in the wireless charging system, circuit parameters L p , C p , L s , and C s  of a transmitter and a receiver can allow the voltage gain at the load-independent point not to change with the coupling factor. During actual operation, a position deviation slightly affects L p  and L s . During circuit design, L p , C p , L s , and C s  may be designed to meet 
     
       
         
           
             
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 
                   
                     L 
                     s 
                   
                   * 
                   
                     C 
                     s 
                   
                 
               
               = 
               k 
             
             , 
           
         
       
     
     where k is a positive number that meets 0.8≤k≤1.2 in order to ensure that the wireless charging system operates near the load-independent point. It can be understood that in this embodiment of this application, the condition that k meets 0.8≤k≤1.2 is not limited. For example, it may also be 0.7≤k≤1.3. The range may be determined based on a design precision requirement of the wireless charging system. This is not limited in this application. 
     Specifically, for example, circuit design for the wireless charging system that includes the transmitter and the receiver renders k=1, that is, L p , C p , L s , and C s  in the wireless charging system meet L p *C p −L s *C s =0. For another example, circuit design for the wireless charging system that includes the transmitter and the receiver renders k=0.8, that is, L p , C p , L s , and C s  in the wireless charging system meet L p *C p −0.8*L s *C s =0. It should be noted that the foregoing examples are merely used to explain this embodiment of this application, and shall not be construed as a limitation. 
     (1) Implementation of Wireless Charging when Const=0 and the Voltage Regulation Module is Disposed in the Transmitter 
       FIG. 8  is a schematic architectural diagram of another wireless charging system according to an embodiment of this application. A transmitter  10  may include a DC power source  101 , a DC/AC conversion module  102 , a series matching capacitor (whose capacitance is C p )  103 , a transmitter coil  104 , and a control module  105 . A receiver  20  may include a receiver coil  201 , a series matching capacitor (whose capacitance is C s )  202 , an AC/DC conversion module  203 , a voltage regulator module  204 , a output load  205 , a modulation module  206 , and a control module  207 . For detailed descriptions of the foregoing modules, refer to the architecture of the wireless charging system described in  FIG. 1 . Details are not described herein again. 
     In the wireless charging system described in  FIG. 8 , a voltage regulation module  106  is further disposed in the transmitter, and is configured to regulate an input voltage of the transmitter, to set a voltage gain to a voltage gain X 0  at a load-independent point. Specifically, the voltage regulation module  106  is configured to receive a DC voltage that is output by the DC power source  101 , and receive a control signal that is output by the control module  105 , to regulate the DC voltage and set the voltage gain to the voltage gain X 0  at the load-independent point. 
     The voltage regulation module  106  may be a DC/DC output voltage regulation module or a voltage adapter circuit. The voltage regulation module  106  is a first voltage regulation module. 
     The receiver  20  further includes a battery management system (BMS)  208 . The receiver  20  is further configured to receive an expected output voltage V_out_target sent by the BMS. The expected output voltage has the following two purposes. First, the V_out_target is used by the receiver to regulate an output voltage to the expected output voltage. Second, the expected output voltage V_out_target is further used by the transmitter to determine the input voltage of the transmitter as a first input voltage Vin_set based on the expected output voltage and the voltage gain X 0  at the load-independent point, and regulate the input voltage to the first input voltage Vin_set using the voltage regulation module in the transmitter. Descriptions are separately given below. 
     First, regulation of the output voltage by the receiver to the expected output voltage V_out_target of the BMS is actually a closed-loop feedback regulation process. A specific regulation process is as follows. As shown in  FIG. 8 , the receiver uses V_out_target as a reference for the voltage regulator module  204 , and adds a preset increment to obtain a preset value of Vrect. The preset increment may be obtained through a table lookup, and is used to ensure normal operation of a related chip. The receiver obtains a current output voltage Vrect of a rectifier by detecting potential of a connection point of R 1  and R 2 , obtains an error voltage based on the preset value of Vrect and the detected current Vrect, and sends the error voltage to the transmitter through in-band communication. The transmitter can regulate Vrect by changing an operating frequency co of power transfer between the transmitter and the receiver. The receiver can finally regulate Vrect to the preset value of Vrect by repeatedly using the foregoing error voltage feedback manner. The preset value of Vrect obtained based on the expected output voltage is referred to as a first output voltage below. 
     Second, the expected output voltage V_out_target is used by the transmitter to determine the input voltage of the transmitter as a first input voltage Vin_set based on the expected output voltage and the voltage gain X 0  at the load-independent point, which is specifically based on the schematic structural diagram of the wireless charging system described in  FIG. 8 . 
     Based on the schematic structural diagram of the wireless charging system described in  FIG. 8 ,  FIG. 9  is a schematic flowchart of a wireless charging method according to an embodiment of this application. In the embodiment described in  FIG. 9 , a voltage gain X 0  at a load-independent point (   0 , X 0 ) of the wireless charging system is a first voltage gain. Regardless of a value of a coupling factor, the voltage gain X 0  at the load-independent point is the first voltage gain. The wireless charging method may include the following steps. 
     S 101 . A receiver sends, to a transmitter, information that indicates a first output voltage. The first output voltage is an expected output voltage of the receiver. 
     S 102 . The transmitter sets an input voltage of the transmitter to a first input voltage based on the first output voltage and a first voltage gain. 
     S 103 . The transmitter transfers power to the receiver at the first voltage gain. 
     Actually, when the transmitter transfers the power to the receiver at the first voltage gain, because the first voltage gain is the voltage gain at the load-independent point, an operating frequency at which the transmitter transfers the power to the receiver is a first operating frequency, and the input voltage of the transmitter is the first input voltage. 
     The expected output voltage of the receiver is sent by a BMS to a control module  207 . Actually, regulation of the input voltage of the transmitter to the first input voltage is also a closed-loop feedback regulation process. A specific process is as follows. The input voltage of the transmitter is gradually regulated based on the first input voltage. A change in the input voltage of the transmitter may cause an output voltage of the receiver to deviate from the first output voltage. A deviation of the output voltage of the receiver causes the receiver to start closed-loop feedback circuit regulation of the first output voltage again, to regulate the output voltage to the expected output voltage, causing the transmitter to regulate an operating frequency of power transfer. After the foregoing closed-loop feedback regulation process, the input voltage of the transmitter is finally regulated to the first input voltage, the voltage gain is the voltage gain X 0  at the load-independent point, and the operating frequency of the power transfer is exactly the operating frequency    0  at the load-independent point. 
     It should be noted that, in this embodiment of this application, the input voltage of the transmitter is an output voltage of a voltage regulation module  106  in the transmitter. The output voltage of the receiver is an output voltage Vrect of the AC/DC conversion module  203  in the receiver. In this embodiment of this application, the voltage gain X 0  at the load-independent point is the first voltage gain. 
     In this embodiment of this application, the voltage gain at the load-independent point of the wireless charging system is constant. To ensure that the wireless charging system operates at the load-independent point, a capacitance C p  of a series matching capacitor of the transmitter, a self-inductance L p  applied when a transmitter coil transfers power to the receiver, a capacitance C s  of a series matching capacitor of the receiver, and a self-inductance L s  applied when a receiver coil receives the power transferred by the transmitter need to be designed such that the foregoing parameters meet 
     
       
         
           
             
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 
                   
                     L 
                     s 
                   
                   * 
                   
                     C 
                     s 
                   
                 
               
               = 
               k 
             
             , 
           
         
       
     
     where k is a positive number that meets 0.8≤k≤1.2. 
     However, in an actual circuit, considering factors such as circuit losses, when the voltage gain at the load-independent point is set, the voltage gain may be increased or decreased by an offset. The first voltage gain X 0  may be a positive number that meets 0.8*√{square root over (L s /L p )}≤X 0 ≤1.2*√{square root over (L s /L p )}. For example, a voltage gain value at the load-independent point is set to √{square root over (C p /C s )} (1+20%) or √{square root over (C p /C s )} (1−20%). It can be understood that in this embodiment of this application, an offset specified for the voltage gain is not limited. For example, alternatively, the first voltage gain may be set to √{square root over (C p /C s )} (1+30%) or √{square root over (C p /C s )} (1−30%). The range may be determined based on a design precision requirement of the wireless charging system. This is not limited in this application. 
     Regardless of the value of the coupling factor, the voltage gain at the load-independent point is invariably √{square root over (C p /C s )}. The voltage gain is set to be within a range of 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )} or within a range of 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )} such that the wireless charging system operates near the load-independent point, and the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     The first output voltage may be the expected output voltage of the BMS in the receiver. During wireless charging, the expected output voltage of the BMS in the receiver may change. The expected output voltage of the BMS may be affected by factors such as a quantity of electricity of the receiver. For example, the voltage gain at the load-independent point is 1.1. If the receiver has a relatively small quantity of electricity (which may be lower than a threshold), the expected output voltage of the BMS in the receiver may be 9.9 volts (V). The receiver feeds back a difference between a current output voltage and a specified output voltage to the transmitter through a closed-loop circuit. The transmitter regulates an operating frequency when receiving the voltage difference. An output voltage of the voltage regulator module in the receiver is regulated to 9.9 V through the foregoing closed-loop regulation process. In addition, the receiver sends the expected output voltage 9.9 V of the BMS to the transmitter, and the transmitter determines, through calculation based on the expected output voltage 9.9 V of the BMS and the voltage gain of 1.1, that the input voltage of the transmitter is 9 V. Therefore, the transmitter gradually regulates the input voltage of the transmitter using the voltage regulation module, and a change in the input voltage of the transmitter causes the output voltage of the receiver to deviate from 9.9 V. When the receiver detects that the output voltage of the receiver deviates from 9.9 V, the receiver starts the closed-loop circuit to feed back the difference between the current output voltage and the specified output voltage to the transmitter, and the transmitter re-regulates the operating frequency based on the voltage difference. After the foregoing closed-loop circuit regulation is completed, the input voltage of the transmitter is 9 V, the output voltage of the receiver is 9.9 V, the voltage gain is 1.1, and the operating frequency is exactly the operating frequency at the load-independent point. 
     After the wireless charging continues for a period of time, if the receiver has a relatively large quantity of electricity (which may be higher than another threshold), the expected output voltage of the BMS may be 5.5 V. The current output voltage of the receiver is still 9.9 V, and then the receiver may repeat the foregoing closed-loop circuit regulation process. The input voltage of the transmitter is finally adjusted to 5 V, the output voltage of the receiver is regulated to 5.5 V, the voltage gain is 1.1, and the operating frequency is exactly the operating frequency at the load-independent point. 
     When the wireless charging system performs power transfer at the load-independent point, if the receiver moves, a position deviation between the receiver and the transmitter changes, and then a coupling factor between an inductor coil of the transmitter and an inductor coil of the receiver changes. As shown in  FIG. 7 , if the coupling factor applied before the receiver moves is K 0 , the wireless charging system operates at the load-independent point (   0 , X 0 ). If the coupling factor applied after the receiver moves is K 1 , the load-independent point of the wireless charging system becomes (   1 , X 0 ). After the receiver moves, as shown in  FIG. 7 , a  −X curve 1 becomes a  −X curve 2. Because the operating frequency is still    0 , a corresponding voltage gain increases, and the output voltage deviates from the expected output voltage of the BMS. The receiver may perform the closed-loop circuit regulation process. For a specific process, refer to detailed descriptions in step S 104 . Details are not described herein again. After the closed-loop circuit regulation is completed, the voltage gain is brought back to X 0 , the corresponding operating frequency is    1 , and the wireless charging system operates at the load-independent point (   1 , X 0 ) corresponding to the coupling factor K 1 . 
     It should be understood that the examples are merely used to explain this embodiment of this application, and shall not be construed as a limitation. 
     In this embodiment of this application, the first input voltage is obtained by dividing the first output voltage by the first voltage gain. The first voltage gain may be sent by the receiver to the transmitter. The receiver may obtain the first voltage gain through calculation based on C p  and C s  (or L s  and L p ) and send the first voltage gain to the transmitter. Alternatively, the receiver may preset a voltage gain and send the voltage gain to the transmitter. Alternatively, the first voltage gain may be obtained by the receiver through calculation based on C p  and C s  (or L s  and L p ), and the transmitter may receive in advance information sent by the receiver and indicating C s  or information sent by the receiver and indicating L s . C s  (L s ) and the first output voltage may be indicated by a same piece of information, or may be indicated by different pieces of information. This is not limited in this application. 
     In the foregoing description, the voltage regulation module is disposed in the transmitter, and the transmitter regulates the voltage gain to the voltage gain at the load-independent point by regulating the input voltage such that the wireless charging system finally operates at the load-independent point. Alternatively, the voltage regulation module may be disposed in the receiver, which is described in detail below. 
     The wireless charging system operates near the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     The voltage gain at the load-independent point is also unrelated to the coupling factor. In other words, during wireless charging, regardless of a position deviation between the transmitter and the receiver, the voltage gain at the load-independent point is invariably √{square root over (C p /C s )}. During a process of regulating the wireless charging system to operate at the load-independent point, it is only required to regulate the voltage gain between the output voltage of the receiver and the input voltage of the transmitter to be close to √{square root over (C p /C s )}, and the operating frequency at which the wireless charging system performs power transfer may be regulated to the operating frequency at the load-independent point under an effect of a closed-loop circuit such that the wireless charging system can operate at the load-independent point. It can be learned that regardless of the position deviation between the transmitter and the receiver, it is only required to regulate the voltage gain between the output voltage of the receiver and the input voltage of the transmitter to be close to √{square root over (C p /C s )}, and convenience for regulating the wireless charging system to operate at the load-independent point can be improved. 
     (2) Implementation of Wireless Charging when Const=0 and the Voltage Regulation Module is Disposed in the Receiver 
       FIG. 10  is a schematic structural diagram of still another wireless charging system according to an embodiment of this application. A transmitter  10  may include a DC power source  101 , a DC/AC conversion module  102 , a series matching capacitor (whose capacitance is Cp)  103 , a transmitter coil  104 , and a control module  105 . A receiver  20  may include a receiver coil  201 , a series matching capacitor (whose capacitance is C s )  202 , an AC/DC conversion module  203 , a voltage regulator module  204 , a output load  205 , a modulation module  206 , a control module  207 , and a BMS  208 . For detailed descriptions of the foregoing modules, refer to the architectures of the wireless charging systems described in  FIG. 1  and  FIG. 8 . Details are not described herein again. The series matching capacitor  103  is a first series matching capacitor, and the series matching capacitor (whose capacitance is C s )  202  is a second series matching capacitor. 
     In the wireless charging system described in  FIG. 10 , a voltage regulation module  209  is further disposed in the receiver, and is configured to regulate an output voltage of the receiver, to set a voltage gain to a voltage gain X 0  at a load-independent point. Specifically, the voltage regulation module  209  is configured to receive a voltage that is output by the AC/DC conversion module  203 , and receive a control signal that is output by the control module  207 , to regulate the voltage and set the voltage gain to the voltage gain X 0  at the load-independent point. The voltage regulation module  209  is a second voltage regulation module. 
     A process in which the receiver regulates the output voltage of the receiver to the first output voltage is also a closed-loop feedback regulation process. A specific process is as follows. The output voltage of the receiver is gradually regulated based on the first output voltage. The receiver obtains a voltage difference based on calculation of the first output voltage and the current output voltage of the voltage regulation module of the receiver, and sends the voltage difference to the transmitter through in-band communication. The transmitter regulates the operating frequency based on the voltage difference. After the foregoing closed-loop feedback regulation process, when the regulation is completed, the output voltage of the receiver is finally regulated to the first output voltage, the voltage gain is the voltage gain X 0  at the load-independent point, and the operating frequency of the power transfer is exactly the operating frequency    0  at the load-independent point. 
     Based on the schematic structural diagram of the wireless charging system described in  FIG. 10 ,  FIG. 11  is a schematic flowchart of another wireless charging method according to an embodiment of this application. In this embodiment described in  FIG. 11 , a voltage gain at a load-independent point (   0 , X 0 ) of the wireless charging system is a constant X 0 . Regardless of a value of a coupling factor, the voltage gain at the load-independent point is X 0 . The wireless charging method may include the following steps. 
     S 201 . A transmitter sends, to a receiver, information that indicates a first input voltage. The first input voltage is an input voltage of the transmitter. 
     S 202 . The receiver sets an output voltage of the receiver to a first output voltage based on the first input voltage and a first voltage gain. 
     S 203 . The transmitter transfers power to the receiver at the first voltage gain. 
     Actually, when the transmitter transfers the power to the receiver at the first voltage gain, because the first voltage gain is the voltage gain at the load-independent point, an operating frequency at which the transmitter transfers the power to the receiver is a first operating frequency, and the output voltage of the receiver is the first output voltage. 
     When the receiver regulates the output voltage to the first output voltage and the voltage gain is √{square root over (C p /C s )}, the transmitter regulates an operating frequency of power transfer to the operating frequency at the load-independent point through closed-loop feedback regulation. In this case, the wireless charging system operates at the load-independent point. 
     However, in an actual circuit, considering factors such as circuit losses, when the voltage gain at the load-independent point is set, the voltage gain may be increased or decreased by an offset. The first voltage gain X 0  may be a positive number that meets 0.8*√{square root over (L s /L p )}≤X 0 ≤1.2*√{square root over (L s /L p )}. For example, a voltage gain value at the load-independent point is set to √{square root over (C p /C s )} (1+20%) or √{square root over (C p /C s )}(1−20%). It can be understood that in this embodiment of this application, an offset specified for the voltage gain is not limited. For example, alternatively, the first voltage gain may be set to √{square root over (C p /C s )} (1+30%) or √{square root over (C p /C s )} (1−30%). The range may be determined based on a design precision requirement of the wireless charging system. This is not limited in this application. 
     Before step S 201 , the transmitter may receive information sent by the receiver and indicating the first output voltage, and determine the input voltage of the transmitter as the first input voltage based on the first output voltage. The transmitter may determine the first input voltage based on a pre-stored second mapping table. The second mapping table includes at least one output voltage and an input voltage of the transmitter corresponding to each of the at least one output voltage. When the input voltage of the transmitter is the input voltage of the transmitter corresponding to each output voltage, the wireless charging system is regulated to operate at the load-independent point, and the output voltage of the receiver is each output voltage. In this case, the first output voltage meets the expected output voltage of the BMS in the receiver. 
     During a charging process of the wireless charging system, if the expected output voltage of the BMS in the receiver varies in different charging phases, the transmitter is also required to include a voltage regulation module. For the voltage regulation module, refer to the voltage regulation module  106  in the transmitter in the embodiment described in  FIG. 8 . The voltage regulation module is configured to regulate the input voltage of the transmitter to the first input voltage after the transmitter determines the input voltage of the transmitter as the first input voltage based on the first output voltage. 
     When the wireless charging system performs power transfer at the load-independent point, if the receiver moves, a position deviation between the receiver and the transmitter changes, and then a coupling factor between an inductor coil of the transmitter and an inductor coil of the receiver changes. As shown in  FIG. 7 , if the coupling factor applied before the receiver moves is K 0 , the wireless charging system operates at the load-independent point (   0 , X 0 ). If the coupling factor applied after the receiver moves is K 1 , the load-independent point of the wireless charging system becomes (   1 , X 0 ). After the receiver moves, as shown in  FIG. 7 , a  −X curve 1 becomes a  −X curve 2. Because the operating frequency is still    0 , a corresponding voltage gain increases, and the output voltage deviates from the expected output voltage of the BMS. The receiver may perform the closed-loop circuit regulation process. For a specific process, refer to detailed descriptions in step S 104 . Details are not described herein again. After the closed-loop circuit regulation is completed, the voltage gain is brought back to X 0 , the corresponding operating frequency is and the wireless charging system operates at the load-independent point (   1 , X 0 ) corresponding to the coupling factor K 1 . 
     In this embodiment of this application, the first output voltage is obtained by multiplying the first input voltage by the first voltage gain. The first voltage gain may be sent by the transmitter to the receiver. The transmitter may obtain the first voltage gain through calculation based on C p  and C s  (or L s  and L p ) and send the first voltage gain to the receiver. Alternatively, the first voltage gain may be obtained by the transmitter through calculation based on C p  and C s  (or L s  and L p ). The receiver may receive in advance information sent by the transmitter and indicating C p  or information sent by the transmitter and indicating L p . C p  (L p ) and the first input voltage may be indicated by a same piece of information, or may be indicated by different pieces of information. This is not limited in this application. 
     The wireless charging system operates near the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. 
     The voltage gain at the load-independent point is also unrelated to the coupling factor. In other words, during wireless charging, regardless of a position deviation between the transmitter and the receiver, the voltage gain at the load-independent point is invariably √{square root over (C p /C s )}. During a process of regulating the wireless charging system to operate at the load-independent point, it is only required to regulate the voltage gain between the output voltage of the receiver and the input voltage of the transmitter to be close to √{square root over (C p /C s )}, and the operating frequency at which the wireless charging system performs power transfer may be regulated to the operating frequency at the load-independent point under an effect of a closed-loop circuit such that the wireless charging system can operate at the load-independent point. It can be learned that regardless of the position deviation between the transmitter and the receiver, it is only required to regulate the voltage gain between the output voltage of the receiver and the input voltage of the transmitter to be close to √{square root over (C p /C s )}, and convenience for regulating the wireless charging system to operate at the load-independent point can be improved. 
     During design of the wireless charging system, a capacitance and an inductance may be designed based on a desired voltage gain. During charging, the voltage gain of the wireless charging system is set to the voltage gain corresponding to the load-independent point, that is, the first voltage gain. The wireless charging system operates at the load-independent point. A change in an output resistance of the receiver does not affect the operating frequency of power transfer, and the change in the output resistance of the receiver does not cause a change in the voltage gain, either. Therefore, the output voltage of the receiver is constant, and an output voltage jump caused by an output resistance jump of the receiver is reduced, thereby reducing electrical energy consumed by the voltage regulator module and improving charging efficiency. Table 1 is an output voltage jump test result of a load jump at (   0 , X 0 ). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Test result of an output voltage jump caused by a load jump at (   0, X0) 
               
               
                 Resistance jump 
               
            
           
           
               
               
               
            
               
                   
                 Current jump  
                   
               
               
                   
                 0.3 amps 
                 Current jump 
               
               
                   
                 (A)→1.25 A  
                 1.25 A→0.3 A 
               
               
                 Voltage jump 
                 caused by a load 
                 caused by a  
               
               
                 Capacitance design 
                 jump 
                 load jump 
               
               
                   
               
               
                 Design values of C p  and C s   
                 0.65 V 
                 0.66 V 
               
               
                 meet X0 = {square root over (C p /C s )} 
                   
                   
               
               
                 A design value of C p  or C s   
                  4.8 V 
                  3.6 V 
               
               
                 deviates 50% 
               
               
                   
               
            
           
         
       
     
     In Table 1, when design values of C p  and C s  meet X 0 =√{square root over (C p /C s )}, the wireless charging system operates at the load-independent point, two jumps of a current of the receiver between 0.3 A and 1.25 A due to the output resistance jump in the receiver cause corresponding fluctuation of 0.65 V and 0.66 V in the output voltage of the receiver. When design values of C p  and C s  do not meet X 0 =√{square root over (C p /C s )}, that is, the wireless charging system does not operate at the load-independent point, two jumps of a current of the receiver between 0.3 A and 1.25 A due to the output resistance jump in the receiver cause corresponding fluctuation of 4.8 V and 3.6 V in the output voltage of the receiver. It can be learned that, the output voltage jump caused by the output resistance jump of the receiver can be reduced by setting the wireless charging system to operate at the load-independent point, thereby reducing electrical energy consumed by the voltage regulator module and improving charging efficiency. 
     In this embodiment of this application, actually, when Const≠0, the wireless charging system can also be designed to operate at the load-independent point. When the circuit parameters L p , C p , L s , and C s  of the transmitter and the receiver do not meet Const=0, the voltage gain at the load-independent point is no longer invariably √{square root over (C p /C s )}, and the voltage regulation module may be disposed in the transmitter, or the voltage regulation module may be disposed in the receiver. Several embodiments of the methods for regulating a wireless charging system to operate at a load-independent point are separately provided in the embodiments of this application based on the foregoing two manners. 
     (1) Implementation of wireless charging when Const≠0 and the voltage regulation module is disposed in the transmitter 
     Based on the schematic structural diagram of the wireless charging system described in  FIG. 8 ,  FIG. 12  is a schematic flowchart of still another wireless charging method according to an embodiment of this application. In this embodiment described in  FIG. 12 , a voltage gain X 0  at a load-independent point (   0 , X 0 ) of the wireless charging system is related to a coupling factor. The wireless charging method may include the following steps. 
     S 301 . A receiver finds, from a first mapping table, a first load-independent point corresponding to a first coupling degree, where the first load-independent point includes a first voltage gain. 
     S 302 . The receiver sends, to a transmitter, information that indicates the first load-independent point. 
     S 303 . The receiver sends, to the transmitter, information that indicates a first output voltage. 
     S 304 . The transmitter sets an input voltage of the transmitter to a first input voltage based on the first output voltage and a first voltage gain. 
     S 305 . The transmitter transfers power to the receiver at the first voltage gain. 
     Actually, when the transmitter transfers the power to the receiver at the first voltage gain, because the first voltage gain is the voltage gain at the load-independent point, an operating frequency at which the transmitter transfers the power to the receiver is a first operating frequency, and the input voltage of the transmitter is the first input voltage. 
     In this embodiment of this application, the receiver may pre-store the first mapping table, the first mapping table may include at least one coupling degree and a load-independent point corresponding to each of the at least one coupling degree, and the load-independent point is a combination of a voltage gain and an operating frequency. At an operating frequency in a load-independent point corresponding to each coupling degree, a voltage gain in the load-independent point corresponding to the coupling degree is independent of an output load of the receiver. The foregoing coupling degree may be a coupling factor or a mutual inductance. The coupling factor is described as an example below. 
     The first mapping table may have two forms depending on whether Const is known. Descriptions are separately given below. 
     (a) Const is known. 
     If C p  and L p  of the transmitter are known to the receiver, or C s  and L s  of the receiver are known to the transmitter, L p *C p −L s *C s =Const is known. For the known Const, different coupling factors may be preset to measure a load-independent point. For example, a position deviation is changed at fixed steps to test or calculate a corresponding coupling factor and a load-independent point corresponding to the coupling factor. For details, refer to Table 2. Table 2 is a schematic diagram of a first mapping table according to an embodiment of this application. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Example of a first mapping table (Const = Const 0) 
               
            
           
           
               
               
               
            
               
                 Position  
                 Coupling  
                 Load-independent  
               
               
                 deviation s 
                 factor K 
                 point (X,   ) 
               
               
                   
               
               
                 s0 
                 K0 
                 (X0,   0) 
               
               
                 s1 
                 K1 
                 (X1,   1) 
               
               
                 s2 
                 K2 
                 (X2,   2) 
               
               
                 . . .  
                 . . .  
                 . . .  
               
               
                 sn 
                 Kn 
                 (Xn,   m) 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, Const corresponding to the transmitter and the receiver is Const 0. In this case, a coupling factor Ku corresponding when the position deviation s is su (u=0, 1, 2, . . . , n), and a load-independent point (Xu,  ) may be tested and stored in advance, where n and u both are natural numbers. 
     The receiver may pre-store the foregoing first mapping table. When the transmitter transfers power to the receiver, the receiver finds a corresponding load-independent point based on a detected position deviation between the receiver and the transmitter, and then, sends the found load-independent point to the receiver. For example, if the receiver detects that a current position deviation between the receiver and the transmitter is s 2 , the receiver determines, through a table lookup based on the foregoing pre-stored first mapping table, that a coupling factor is K 2  and a load-independent point is (X 2 ,    2 ). 
     In addition, when the voltage gain and the input voltage of the transmitter are regulated, the operating frequency   is regulated using a closed-loop circuit. This is passive regulation, and there is no need to know a specific value of the operating frequency  . To reduce a stored data amount and save storage space, the receiver may alternatively pre-store only the voltage gain X at the load-independent point in the first mapping table. For details, refer to Table 3. Table 3 is a schematic diagram of another first mapping table according to an embodiment of this application. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Example of a first mapping table (Const = Const 0) 
               
            
           
           
               
               
               
            
               
                 Position  
                 Coupling  
                 Voltage gain X at a load- 
               
               
                 deviation s 
                 factor K 
                 independent point 
               
               
                   
               
               
                 s0 
                 K0 
                 X0 
               
               
                 s1 
                 K1 
                 X1 
               
               
                 s2 
                 K2 
                 X2 
               
               
                 . . .  
                 . . .  
                 . . .  
               
               
                 sn 
                 Kn 
                 Xn 
               
               
                   
               
            
           
         
       
     
     As shown in Table 3, Const corresponding to the transmitter and the receiver is Const 0. A coupling factor Ku corresponding when the position deviation s is su (u=0, 1, 2, . . . , n), and a voltage gain Xu at a load-independent point is tested and stored in advance, where n and u both are natural numbers. 
     The receiver may pre-store the foregoing first mapping table. When the transmitter transfers power to the receiver, the receiver finds a corresponding voltage gain at a load-independent point based on a detected position deviation between the receiver and the transmitter, and then, sends the found voltage gain at the load-independent point to the transmitter. For example, if the receiver detects that a current position deviation between the receiver and the transmitter is s 2 , the receiver determines, through a table lookup based on the foregoing pre-stored first mapping table, that a coupling factor is K 2  and a voltage gain at a load-independent point is X 2 . 
     After the receiver finds the voltage gain at the load-independent point, the receiver or the transmitter can alternatively calculate the operating frequency at the load-independent point based on formula (3) and formula (6). 
     (b) Const is unknown. 
     If the transmitter by which the receiver is charged is unknown, or the receiver charged by the transmitter is unknown, that is, C p  and L p  of the transmitter are unknown to the receiver, or C s  and L s  of the receiver are unknown to the transmitter, a value of Const may be changed at fixed steps to test different coupling factors under different Const and load-independent points corresponding to the coupling factors. Load-independent points corresponding to different Const and coupling factors K may be pre-stored. For details, refer to Table 4. Table 4 is an example of still another first mapping table according to an embodiment of this application. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Example of a first mapping table (Const is unknown) 
               
            
           
           
               
               
               
               
               
               
            
               
                 Load- 
                   
                   
                   
                   
                   
               
               
                 inde- 
                   
                   
                   
                   
                   
               
               
                 pendent 
                   
                   
                   
                   
                   
               
            
           
           
               
               
            
               
                 point 
                 Position deviation 
               
            
           
           
               
               
               
               
               
               
            
               
                 Const 
                 s0 
                 s1 
                 s2 
                 . . .  
                 sn 
               
               
                   
               
               
                 Const 0 
                 (X00,    00) 
                 (X01,    01) 
                 (X02,    02) 
                 . . .  
                 (X0n,    0n) 
               
               
                 Const 1 
                 (X10,    10) 
                 (X11,    11) 
                 (X12,    12) 
                 . . .  
                 (X1n,    1n) 
               
               
                 Const 2 
                 (X20,    20) 
                 (X21,    21) 
                 (X22,    22) 
                 . . .  
                 (X2n,    2n) 
               
               
                 . . .  
                 . . .  
                 . . .  
                 . . .  
                 . . .  
                 . . .  
               
               
                 Const m 
                 (Xm0,    n0) 
                 (Xm1,    n1) 
                 (Xm2,    m2) 
                 . . .  
                 (Xmn,    mn) 
               
               
                   
               
            
           
         
       
     
     As shown in Table 4, a load-independent point (Xvu,  vu) corresponding when the position deviation s is su (u=0, 1, 2, . . . , n), and Const is Const v (v=0, 1, 2, . . . , m) is tested and stored in advance, where m, n, u, and v all are natural numbers. 
     The receiver may pre-store the foregoing first mapping table. When the transmitter transfers power to the receiver, the receiver may first obtain L p  and C p  of the transmitter, and obtain Const through calculation. The receiver finds a corresponding load-independent point based on Const obtained through calculation and a detected position deviation between the receiver and the transmitter, and then, sends the found load-independent point to the transmitter. For example, if Const obtained by the receiver through calculation is Const 1, and the receiver detects that a current position deviation between the receiver and the transmitter is s 2 , the receiver determines, through a table lookup based on the foregoing pre-stored first mapping table, that the load-independent point is (X 12 ,    12 ). 
     Certainly, to save storage space, the receiver may alternatively pre-store only the voltage gain X at the load-independent point in the first mapping table. For details, refer to Table 5. Table 5 is a schematic diagram of yet another first mapping table according to an embodiment of this application. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Example of a first mapping table (Const is unknown) 
               
            
           
           
               
               
            
               
                 Gain X 
                 Position deviation s 
               
            
           
           
               
               
               
               
               
               
            
               
                 Const 
                 s0 
                 s1 
                 s2 
                 . . .  
                 sn 
               
               
                   
               
               
                 Const 0 
                 X00 
                 X01 
                 X02 
                 . . .  
                 X0n 
               
               
                 Const 1 
                 X10 
                 X11 
                 X12 
                 . . .  
                 X1n 
               
               
                 Const 2 
                 X20 
                 X21 
                 X22 
                 . . .  
                 X2n 
               
               
                 . . .  
                 . . .  
                 . . .  
                 . . .  
                 . . .  
                 . . .  
               
               
                 Const m 
                 Xm0 
                 Xm1 
                 Xm2 
                 . . .  
                 Xmn 
               
               
                   
               
            
           
         
       
     
     As shown in Table 5, a voltage gain Xvu at a load-independent point corresponding when the position deviation s is su (u=0, 1, 2, . . . , n) and Const is Const v (v=0, 1, 2, . . . , m) is tested and stored in advance, where m, n, u, and v all are natural numbers. 
     The receiver may pre-store the foregoing first mapping table. When the transmitter transfers power to the receiver, the receiver may first obtain L p  and C p  of the transmitter, and obtain Const through calculation. The receiver finds a corresponding voltage gain at a load-independent point based on Const obtained through calculation and a detected position deviation between the receiver and the transmitter, and then, sends the found voltage gain at the load-independent point to the transmitter. For example, if Const obtained by the receiver through calculation is Const 1, and the receiver detects that a current position deviation between the receiver and the transmitter is s 2 , the receiver determines, through a table lookup based on the foregoing pre-stored first mapping table, that the voltage gain at the load-independent point is X 12 . 
     Because the receiver is usually a mobile terminal and has very high data storage and processing capabilities, if the first mapping table is stored in the receiver, load on the transmitter for data processing and storage can be reduced, a table lookup speed can be accelerated, and a delay for regulating a load-independent point can be reduced. Certainly, the foregoing first mapping table may be alternatively stored in the transmitter. The transmitter can directly obtain the load-independent point through a table lookup, and the load-independent point does not need to be sent using the receiver. In-band communication may be reduced, and signaling overheads may be reduced. 
     When Const≠0 and the expected output voltage of the BMS in the receiver changes, neither of the voltage gain nor the operating frequency at the load-independent point changes. The receiver or the transmitter does not need to query the first mapping table again, and only needs to repeat steps S 303  to S 305 , to re-regulate the wireless charging system to operate at the load-independent point based on the changed first output voltage and the voltage gain of the load-independent point. 
     When Const≠0 and the wireless charging system operates at the load-independent point, if the position deviation between the transmitter and the receiver changes, for example, a charging position of the transmitter changes, the coupling factor changes, and the voltage gain and the operating frequency at the load-independent point of the wireless charging system also change. The load-independent point needs to be determined again, and the wireless charging system is regulated to operate at the load-independent point, that is, steps S 301  to S 305  are repeated based on a new coupling factor obtained after the position deviation changes. 
     However, in an actual circuit, considering factors such as circuit losses, after the voltage gain X 0  at the load-independent point corresponding to the coupling factor is found, when the voltage gain is set, the voltage gain may be set to the voltage gain X 0  at the load-independent point plus or minus an offset. For example, the voltage gain is set to the first voltage gain X 0 , and the first voltage gain X 0  may be a positive number that meets 0.8*√{square root over (L s /L p )}≤X 0 ≤1.2*√{square root over (L s /L p )}. For example, a voltage gain value at the load-independent point is set to √{square root over (C p /C s )} (1+20%) or √{square root over (C p /C s )} (1−20%). It can be understood that in this embodiment of this application, an offset specified for the voltage gain is not limited. For example, alternatively, the first voltage gain may be set to √{square root over (C p /C s )} (1+30%) or √{square root over (C p /C s )} (1−30%). The range may be determined based on a design precision requirement of the wireless charging system. This is not limited in this application. 
     The load-independent point is found from the first mapping table using the coupling degree. The wireless charging system may be set to operate at a load-independent point by setting a voltage gain to a voltage gain at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. Circuit parameters of the transmitter and the receiver do not need to be limited, and generality of the transmitter and the receiver is improved. 
     It can be learned that the first voltage gain at the load-independent point is determined in the foregoing manner of looking up the first mapping table, and the circuit parameters in the transmitter and the receiver do not necessarily need to meet 
     
       
         
           
             
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 
                   
                     L 
                     s 
                   
                   * 
                   
                     C 
                     s 
                   
                 
               
               = 
               k 
             
             , 
           
         
       
     
     where k is a positive number that meets 0.8≤k≤1.2. In this way, the wireless charging system can be set at the load-independent point. In other words, the transmitter or the receiver that has any circuit parameter can operate at the load-independent point. Therefore, the generality of the transmitter and the receiver can be improved. 
     In the foregoing description, the voltage regulation module is disposed in the transmitter, and the transmitter regulates the voltage gain to the voltage gain at the load-independent point by regulating the input voltage such that the wireless charging system finally operates at the load-independent point. Alternatively, the voltage regulation module may be disposed in the receiver, which is described in detail below. 
     (2) Implementation of Wireless Charging when Const≠0 and the Voltage Regulation Module is Disposed in the Transmitter 
     Based on the schematic structural diagram of the wireless charging system described in  FIG. 9 ,  FIG. 13  is a schematic flowchart of yet another wireless charging method according to an embodiment of this application. In this embodiment described in  FIG. 13 , a voltage gain X 0  at a load-independent point (   0 , X 0 ) of the wireless charging system is related to a coupling factor. The wireless charging method may include the following steps. 
     S 401 . A receiver finds, from a first mapping table, a first load-independent point corresponding to a first coupling degree, where the first load-independent point includes a first voltage gain. 
     S 402 . The receiver receives information sent by a transmitter and indicating a first input voltage. 
     S 403 . The receiver sets an output voltage of the receiver to a first output voltage based on the first input voltage and a first voltage gain. 
     S 404 . The transmitter transfers power to the receiver at the first voltage gain. 
     In this embodiment of this application, the receiver may pre-store the first mapping table, the first mapping table may include at least one coupling degree and a load-independent point corresponding to each of the at least one coupling degree, and the load-independent point is a combination of a voltage gain and an operating frequency. At an operating frequency in a load-independent point corresponding to each coupling degree, a voltage gain in the load-independent point corresponding to the coupling degree is independent of an output load of the receiver. The foregoing coupling degree may be a coupling factor or a mutual inductance. The mutual inductance is used as an example below. 
     For description of the first mapping table, refer to the embodiment described in FIG.  12 . Details are not described herein again. 
     A performing sequence of step S 401  and step S 402  is not limited. Before step S 401 , the transmitter may receive information sent by the receiver and indicating the first output voltage, and determine the input voltage of the transmitter as the first input voltage based on the first output voltage. The transmitter may determine the first input voltage based on a pre-stored second mapping table. The second mapping table includes at least one output voltage and an input voltage of the transmitter corresponding to each of the at least one output voltage. When the input voltage of the transmitter is the input voltage of the transmitter corresponding to each output voltage, the wireless charging system is regulated to operate at the load-independent point, and the output voltage of the receiver is each output voltage. In this case, the first output voltage meets the expected output voltage of the BMS in the receiver. 
     During a charging process of the wireless charging system, if the expected output voltage of the BMS in the receiver varies in different charging phases, the transmitter is also required to include a voltage regulation module. For the voltage regulation module, refer to the voltage regulation module  106  in the transmitter in the embodiment described in  FIG. 8 . The voltage regulation module is configured to regulate the input voltage of the transmitter to the first input voltage after the transmitter determines the input voltage of the transmitter as the first input voltage based on the first output voltage. 
     When the wireless charging system performs power transfer at the load-independent point, if the receiver moves, the coupling factor changes, and the voltage gain and the operating frequency at the load-independent point of the wireless charging system also change. The load-independent point needs to be determined again, and the wireless charging system is regulated to operate at the load-independent point, that is, steps S 401  to S 404  are repeated. 
     The load-independent point is found from the first mapping table using the coupling degree. The wireless charging system may be set to operate at a load-independent point by setting a voltage gain to a voltage gain at the load-independent point such that the voltage gain is independent of a load impedance of the receiver. Therefore, regardless of a value of an output load impedance of the receiver, both an operating frequency and a voltage gain that are used when the transmitter transfers power to the receiver are fixed values. In other words, the output load impedance of the receiver does not affect a voltage gain between an output voltage of the receiver and an input voltage of the transmitter. Therefore, the output voltage of the receiver is constant, an output voltage jump caused by a load jump can be reduced, an electrical energy loss of a voltage regulator module can be reduced, and charging efficiency of the receiver can be improved. Circuit parameters of the transmitter and the receiver do not need to be limited, and generality of the transmitter and the receiver is improved. 
     During charging of the wireless charging system, the wireless charging system is set at the load-independent point. At the load-independent point, a change in an output resistance of the receiver does not affect the operating frequency of power transfer, and the change in the output resistance of the receiver does not cause a change in the voltage gain either. Therefore, the output voltage of the receiver is constant, and an output voltage jump caused by an output resistance jump of the receiver is reduced, thereby reducing electrical energy consumed by the voltage regulator module and improving electrical energy conversion efficiency. 
       FIG. 14  shows a test result of electrical energy conversion efficiency according to an embodiment of this application. A test curve of electrical energy conversion efficiency shown in  FIG. 14  is a test result produced when a center of a transmitter coil in a transmitter directly faces a center of a receiver coil in a receiver, that is, when a position deviation is 0. The electrical energy conversion efficiency may be understood as a ratio of electrical energy that is output by the transmitter and electrical energy in the receiver that is used to supply power to a load. As shown in  FIG. 14 , for any load current, electrical energy conversion efficiency generated when a wireless charging system operates at a load-independent point is greater than electrical energy conversion efficiency generated when the wireless charging system deviates from the load-independent point. Therefore, when there is no position deviation between the transmitter and the receiver, the wireless charging system is set to operate at the load-independent point, and the electrical energy conversion efficiency of the wireless charging system can be improved. 
       FIG. 15  shows another test result of electrical energy conversion efficiency according to an embodiment of this application. A test curve of electrical energy conversion efficiency shown in  FIG. 15  is a test result produced when a center of a transmitter coil in a transmitter horizontally deviates from a center of a receiver coil in a receiver by 10 mm, that is, when a position deviation is 10 mm. As shown in  FIG. 15 , for any load current, electrical energy conversion efficiency generated when a wireless charging system operates at a load-independent point is greater than electrical energy conversion efficiency generated when the wireless charging system deviates from the load-independent point. Therefore, when there is a position deviation between the transmitter and the receiver, the wireless charging system is set to operate at the load-independent point, and the electrical energy conversion efficiency of the wireless charging system can be improved. 
     When the wireless charging system operates at the load-independent point, because the voltage gain does not change with a load change of the receiver, in this case, the wireless charging system is equivalent to a transformer. Therefore, a phase of an output voltage of a DC/AC conversion module in the transmitter is the same as a phase of an input voltage of an AC/DC conversion module in the receiver. In addition, the phase of the input voltage of the AC/DC conversion module in the receiver is also the same as a phase of an input current of the AC/DC conversion module in the receiver. If the AC/DC conversion module in the receiver is implemented using a rectifier diode circuit, when the phase of the input voltage of the AC/DC conversion module is the same as the phase of the input current of the AC/DC conversion module, a diode has a zero current turn-off property. The zero current turn-off property of the diode can reduce electromagnetic interference and improve rectifier efficiency. 
       FIG. 16  is a schematic diagram of a test of a voltage and a current according to an embodiment of this application. As shown in  FIG. 16 , an output voltage of a DC/AC conversion module in a transmitter is V 1 , an output current of the DC/AC conversion module in the transmitter is I 1 , an input voltage of an AC/DC conversion module in a receiver is V 2 , and an input current of the AC/DC conversion module in the receiver is I 2 , where phases of V 1  and V 2  are the same, and phases of V 2  and I 2  are the same. The AC/DC conversion module in the receiver uses a diode circuit for rectification, where the diode has a zero current turn-off property. The zero current turn-off property of the diode can reduce electromagnetic interference and improve rectifier efficiency. 
     The foregoing describes in detail the method in the embodiments of this application, and the following provides the transmitter and the receiver in the embodiments of this application. 
     Based on the system architecture in  FIG. 1 ,  FIG. 17  is a schematic structural diagram of a wireless charging system  100  according to an embodiment of this application. The wireless charging system  100  includes a transmitter  10  and a receiver  20 . 
     As shown in  FIG. 17 , the transmitter  10  includes a transmitter coil  104  and a first series matching capacitor  103 , and the transmitter coil  104  is connected to the first series matching capacitor  103  in series to form a first oscillation circuit  107 , the first oscillation circuit  107  is configured to transfer power to the receiver  20 , and a self-inductance applied when the transmitter coil  104  transfers the power to the receiver  20  is L p , and a capacitance of the first series matching capacitor  103  is C p , where L p *C p =k*L s *C s , is a self-inductance applied when a receiver coil  201  in the receiver  20  receives the power transferred by the first oscillation circuit, C s  is a capacitance of a second series matching capacitor  202  in the receiver  20 , and k is a positive number that meets 0.8≤k≤1.2, the receiver coil  201  is connected to the second series matching capacitor  202  in series to form a second oscillation circuit, the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and the transmitter  10  further includes a control unit  108  configured to set a voltage gain between an output voltage of the receiver  20  and an input voltage of the transmitter  10  to a first voltage gain, where the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}, when the voltage gain between the output voltage of the receiver and the input voltage of the transmitter is the first voltage gain, the wireless charging system operates at a load-independent point, the load-independent point includes a first operating frequency and the first voltage gain, and at the first operating frequency, the first voltage gain is independent of an output load of the receiver, and the control unit  108  is further configured to control the first oscillation circuit  107  to transfer the power to the receiver  20  at the first voltage gain. 
     As a possible implementation, k is 1, and X is √{square root over (C p /C s )} or √{square root over (L s /L p )}. 
     As a possible implementation, the transmitter  10  further includes a receiving unit  109 , and that a control unit  108  sets a voltage gain between an output voltage of the receiver  20  and an input voltage of the transmitter  10  to a first voltage gain includes the receiving unit  109  is configured to receive information sent by the receiver  20  and indicating a first output voltage, where the first output voltage is an expected output voltage of the receiver, and the control unit  108  is further configured to set the input voltage of the transmitter  10  to a first input voltage based on the first output voltage and the first voltage gain. 
     As a possible implementation, the transmitter  10  may include a voltage regulation module  106 . That the control unit  108  sets the input voltage of the transmitter  10  to a first input voltage based on the first output voltage and the first voltage gain includes the control unit  108  controls, based on the first output voltage and the first voltage gain, the voltage regulation module  106  to set the input voltage of the transmitter  10  to the first input voltage. 
     As a possible implementation, before the control unit  108  sets the voltage gain between the output voltage of the receiver  20  and the input voltage of the transmitter  10  to the first voltage gain, the receiving unit  109  is further configured to receive information sent by the receiver  20  and indicating C s  and/or information sent by the receiver  20  and indicating L s , where C s  and L s  are used by the transmitter  10  to determine the first voltage gain, and/or the receiving unit  109  is configured to receive information sent by the receiver  20  and indicating the first voltage gain. 
     As shown in  FIG. 17 , the receiver  20  includes the receiver coil  201  and the second series matching capacitor  202 , the receiver coil  201  is connected to the second series matching capacitor  202  in series to form a second oscillation circuit  210 , and the second oscillation circuit  210  is configured to receive power transferred by the transmitter  10 , a self-inductance applied when the receiver coil  201  receives the power transferred by the transmitter  10  is L s , and a capacitance of the second series matching capacitor  202  is C s , where 
     
       
         
           
             
               
                 
                   L 
                   s 
                 
                 * 
                 
                   C 
                   s 
                 
               
               = 
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 k 
               
             
             , 
           
         
       
     
     L p  is a self-inductance applied when the transmitter coil  104  in the transmitter  10  transfers the power to the receiver  20 , C p  is a capacitance of the first series matching capacitor  103  in the transmitter  10 , and k is a positive number that meets 0.8≤k≤1.2, the transmitter coil  104  is connected to the first series matching capacitor  103  in series to form the first oscillation circuit  107 , and the first oscillation circuit  107  is configured to transfer the power to the second oscillation circuit  210 . 
     The receiver  20  further includes a sending unit  212  and a control unit  211 , where the sending unit  212  is configured to send, to the transmitter  10 , the information that indicates the first output voltage, where the first output voltage is the expected output voltage of the receiver  20 , and the first output voltage is used by the transmitter  10  to set the input voltage of the transmitter  10  to the first input voltage based on the first output voltage and the first voltage gain, where the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}, when the voltage gain between the output voltage of the receiver  20  and the input voltage of the transmitter  10  is the first voltage gain, the wireless charging system  100  operates at a load-independent point, the load-independent point includes a first operating frequency and the first voltage gain, and at the first operating frequency, the first voltage gain is independent of an output load of the receiver  20 , and the control unit  211  is configured to control the second oscillation circuit  210  to receive the power transferred by the transmitter  10  at the first voltage gain. 
     In this embodiment of this application, the transmitter  10  may further include a DC power source  101  and a DC/AC conversion module  102 . The receiver  20  may include an AC/DC conversion module  203 , a voltage regulator module  204 , a output load  205 , a modulation module  206 , and a BMS  208 . For detailed descriptions of the foregoing modules, refer to the architectures of the wireless charging systems described in  FIG. 1  and  FIG. 8 . Details are not described herein again. 
     In this embodiment of this application, the control unit  108  may be implemented by the control module  105  in the embodiment described in  FIG. 8 . The control unit  211  may be implemented by the control module  207  in the embodiment described in  FIG. 8 . The control unit  108  may further have another function of the control module  105 . For details, refer to the embodiment described in  FIG. 8 . The control unit  211  may further have another function of the control module  207 . For details, refer to the embodiment described in  FIG. 8 . Functions of the control unit  108 , the receiving unit  109 , the control unit  211 , and the sending unit  212  may correspond to the corresponding descriptions of the wireless charging method embodiment shown in  FIG. 9 . Details are not described herein again. 
     Based on the wireless charging system  100  described in  FIG. 17 , in another possible embodiment, the control unit  108  is configured to find, from a first mapping table, a first load-independent point corresponding to a first coupling degree, or the receiving unit is configured to receive information sent by the receiver and indicating a first load-independent point, where the first load-independent point is a first load-independent point corresponding to a first coupling degree and found by the receiver from a first mapping table, where the first load-independent point includes a first voltage gain, and the first coupling degree is a degree of coupling between the coil in the transmitter and the coil in the receiver, the first mapping table includes at least one coupling degree and a load-independent point corresponding to each of the at least one coupling degree, and the load-independent point is a combination of a voltage gain and an operating frequency, and at an operating frequency in a load-independent point corresponding to each coupling degree, a voltage gain in the load-independent point corresponding to the coupling degree is independent of an output load of the receiver, the control unit  108  is configured to set a voltage gain between an output voltage of the receiver  20  and an input voltage of the transmitter  10  to the first voltage gain, and the control unit  108  is further configured to control the first oscillation circuit  107  to transfer the power to the receiver at the first voltage gain. 
     As a possible implementation, that a control unit  108  sets a voltage gain between an output voltage of the receiver  20  and an input voltage of the transmitter  10  to a first voltage gain includes the receiving unit  109  is configured to receive information sent by the receiver  20  and indicating a first output voltage, where the first output voltage is an expected output voltage of the receiver, and the transmitter  10  sets the input voltage of the transmitter to a first input voltage based on the first output voltage and the first voltage gain. 
     As a possible implementation, before the control unit  108  sets the voltage gain between the output voltage of the receiver  20  and the input voltage of the transmitter  10  to the first voltage gain, the receiving unit  109  is further configured to receive information sent by the receiver  20  and indicating C s  and/or information sent by the receiver  20  and indicating L s , where C s  and L s  are used by the transmitter  10  to determine the first voltage gain, and/or the transmitter  10  is configured to receive information sent by the receiver  20  and indicating the first voltage gain. 
     In the receiver  20 , the sending unit  212  is configured to send, to the transmitter  10 , information that indicates a first output voltage, where the first output voltage is an expected output voltage of the receiver  20 , and the first output voltage is used by the transmitter  10  to set an input voltage of the transmitter  10  to a first input voltage based on the first output voltage and the first voltage gain. The first voltage gain is included in a first load-independent point corresponding to a first coupling degree and found by the transmitter  10  from a first mapping table, or the first voltage gain is included in a first load-independent point corresponding to a first coupling degree and found by the receiver  20  from a first mapping table, and the first load-independent point is added to information indicating the first load-independent point, and then sent by the receiver  20  to the transmitter  10 . 
     The first coupling degree is a degree of coupling between the transmitter coil  104  in the transmitter  10  and the receiver coil  201  in the receiver  20 , the first mapping table includes at least one coupling degree and a load-independent point corresponding to each of the at least one coupling degree, and the load-independent point is a combination of a voltage gain and an operating frequency, and at an operating frequency in a load-independent point corresponding to each coupling degree, a voltage gain in the load-independent point corresponding to the coupling degree is independent of an output load of the receiver. 
     The control unit  211  is configured to control the second oscillation circuit to receive the power transferred by the transmitter  10  at the first voltage gain. 
     In this embodiment of this application, the control unit  108  may be implemented by the control module  105  in the embodiment described in  FIG. 8 . The control unit  211  may be implemented by the control module  207  in the embodiment described in  FIG. 8 . Alternatively, the control unit  108  may be a controller  30  in the embodiment described in  FIG. 19 . Alternatively, the control unit  211  may be a controller  40  in the embodiment described in  FIG. 19 . The control unit  108  may further have another function of the control module  105 . For details, refer to the embodiment described in  FIG. 8 . The control unit  211  may further have another function of the control module  207 . For details, refer to the embodiment described in  FIG. 8 . Functions of the control unit  108 , the receiving unit  109 , the control unit  211 , and the sending unit  212  may correspond to the corresponding descriptions of the wireless charging method embodiment shown in  FIG. 12 . Details are not described herein again. 
     Based on the system architecture in  FIG. 1 ,  FIG. 18  is a schematic structural diagram of another wireless charging system  100  according to an embodiment of this application. The wireless charging system  100  includes a transmitter  10  and a receiver  20 . 
     As shown in  FIG. 18 , the transmitter  10  includes a transmitter coil  104  and a first series matching capacitor  103 , and the transmitter coil  104  is connected to the first series matching capacitor  103  in series to form a first oscillation circuit  107 , the first oscillation circuit  107  is configured to transfer power to the receiver  20 , and a self-inductance applied when the transmitter coil  104  transfers the power to the receiver  20  is L p , and a capacitance of the first series matching capacitor  103  is C p , where L p *C p =k*L s *C s , L s  is a self-inductance applied when a receiver coil  201  in the receiver  20  receives the power transferred by the first oscillation circuit, C s  is a capacitance of a second series matching capacitor  202  in the receiver  20 , and k is a positive number that meets 0.8≤k≤1.2, the receiver coil  201  is connected to the second series matching capacitor  202  in series to form a second oscillation circuit, the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and the transmitter  10  further includes a sending unit  108  configured to send, to the receiver  20 , information that indicates a first input voltage, where the first input voltage is an input voltage of the transmitter  10 , and the first input voltage is used by the receiver to set an output voltage of the receiver to a first output voltage based on the first input voltage and a first voltage gain, where the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}, when the voltage gain between the output voltage of the receiver  20  and the input voltage of the transmitter  10  is the first voltage gain, the wireless charging system  100  operates at a load-independent point, the load-independent point includes a first operating frequency and the first voltage gain, and at the first operating frequency, the first voltage gain is independent of an output load of the receiver  20 , and a control unit  109  is configured to control the first oscillation circuit  107  to transfer the power to the receiver  20  at the first voltage gain. 
     As shown in  FIG. 18 , the receiver  20  includes the receiver coil  201  and the second series matching capacitor  202 , the receiver coil  201  is connected to the second series matching capacitor  202  in series to form a second oscillation circuit  210 , and the second oscillation circuit  210  is configured to receive power transferred by the transmitter  10 , a self-inductance applied when the receiver coil  201  receives the power transferred by the transmitter  10  is L s , and a capacitance of the second series matching capacitor  202  is C s , where 
     
       
         
           
             
               
                 
                   L 
                   s 
                 
                 * 
                 
                   C 
                   s 
                 
               
               = 
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 k 
               
             
             , 
           
         
       
     
     L p  is a self-inductance applied when the transmitter coil  104  in the transmitter  10  transfers the power to the receiver  20 , C p  is a capacitance of the first series matching capacitor  103  in the transmitter  10 , and k is a positive number that meets 0.8≤k≤1.2, the transmitter coil  104  is connected to the first series matching capacitor  103  in series to form the first oscillation circuit  107 , and the first oscillation circuit  107  is configured to transfer the power to the second oscillation circuit  210 . 
     The receiver  20  further includes a control unit  211 , where the control unit  211  is configured to set a voltage gain between an output voltage of the receiver  20  and an input voltage of the transmitter  10  to a first voltage gain, where the first voltage gain is X, and X is a positive number that meets 0.8*√{square root over (C p /C s )}≤X≤1.2*√{square root over (C p /C s )}, or X is a positive number that meets 0.8*√{square root over (L s /L p )}≤X≤1.2*√{square root over (L s /L p )}, when the voltage gain between the output voltage of the receiver  20  and the input voltage of the transmitter  10  is the first voltage gain, the wireless charging system  100  operates at a load-independent point, the load-independent point includes a first operating frequency and the first voltage gain, and at the first operating frequency, the first voltage gain is independent of an output load of the receiver  20 , and the control unit  211  is further configured to control the second oscillation circuit  210  to receive the power transferred by the transmitter  10  at the first voltage gain. 
     As a possible implementation, k is 1, and X is √{square root over (C p /C s )} or √{square root over (L s /L p )}. 
     As a possible implementation, the receiver  20  further includes a receiving unit  212 , and that the control unit  211  sets a voltage gain between an output voltage of the receiver  20  and an input voltage of the transmitter  10  to a first voltage gain includes the receiving unit  212  is configured to receive information sent by the transmitter  10  and indicating a first input voltage, where the first input voltage is the input voltage of the transmitter  10 , and the control unit  211  is further configured to set the output voltage of the receiver  20  to a first output voltage based on the first input voltage and the first voltage gain. 
     As a possible implementation, the receiver  20  may include a voltage regulation module  209 . That the control unit  211  sets the output voltage of the receiver  20  to a first output voltage based on the first input voltage and the first voltage gain includes the control unit  211  controls, based on the first input voltage and the first voltage gain, the voltage regulation module  209  to set the output voltage of the receiver  10  to the first output voltage. 
     As a possible implementation, before the control unit  211  sets the voltage gain between the output voltage of the receiver  20  and the input voltage of the transmitter  10  to the first voltage gain, the receiving unit  212  is further configured to receive information sent by the transmitter  10  and indicating C p  and/or information sent by the transmitter  10  and indicating L p , where C p  and L p  are used by the receiver  20  to determine the first voltage gain, and/or the receiving unit  212  is further configured to receive information sent by the transmitter  10  and indicating the first voltage gain. 
     In this embodiment of this application, the transmitter  10  may further include a DC power source  101  and a DC/AC conversion module  102 . The receiver  20  may further include an AC/DC conversion module  203 , a voltage regulator module  204 , a output load  205 , a modulation module  206 , and a BMS  208 . For detailed descriptions of the foregoing modules, refer to the architectures of the wireless charging systems described in  FIG. 1  and  FIG. 8 . Details are not described herein again. 
     In this embodiment of this application, the control unit  109  may be implemented by the control module  105  in the embodiment described in  FIG. 10 . The control unit  211  may be implemented by the control module  207  in the embodiment described in  FIG. 10 . The control unit  109  may be a controller  30  in the embodiment described in  FIG. 19 . The control unit  211  may be a controller  40  in the embodiment described in  FIG. 19 . The control unit  109  may further have another function of the control module  105 . For details, refer to the embodiment described in  FIG. 10 . The control unit  211  may further have another function of the control module  207 . For details, refer to the embodiment described in  FIG. 10 . Functions of the control unit  109 , the sending unit  108 , the control unit  211 , and the receiving unit  212  may correspond to the corresponding descriptions of the wireless charging method embodiment shown in  FIG. 11 . Details are not described herein again. 
     Based on the wireless charging system  100  described in  FIG. 18 , in another possible embodiment, the sending unit  108  is configured to send, to the receiver  20 , information that indicates a first input voltage, where the first input voltage is an input voltage of the transmitter  10 , and the first input voltage is used by the receiver  20  to set an output voltage of the receiver  20  to a first output voltage based on the first input voltage and the first voltage gain. The first voltage gain is included in a first load-independent point corresponding to a first coupling degree and found by the transmitter  10  from a first mapping table, or the first voltage gain is included in a first load-independent point corresponding to a first coupling degree and found by the receiver  20  from a first mapping table, and the first load-independent point is added to information indicating the first load-independent point, and then sent by the receiver to the transmitter  10 . 
     The first coupling degree is a degree of coupling between the transmitter coil  104  in the transmitter  10  and the receiver coil  201  in the receiver  20 , the first mapping table includes at least one coupling degree and a load-independent point corresponding to each of the at least one coupling degree, and the load-independent point is a combination of a voltage gain and an operating frequency, and at an operating frequency in a load-independent point corresponding to each coupling degree, a voltage gain in the load-independent point corresponding to the coupling degree is independent of an output load of the receiver, and the control unit  109  is further configured to control the first oscillation circuit  107  to transfer the power to the receiver  20  at the first voltage gain. 
     In the receiver  20 , the control unit  211  is configured to find, from a first mapping table, a first load-independent point corresponding to a first coupling degree, or the receiving unit  212  is configured to receive information sent by the transmitter  10  and indicating a first load-independent point, where the first load-independent point is a first load-independent point corresponding to a first coupling degree and found by the transmitter  10  from a first mapping table, where the first load-independent point includes a first voltage gain, and the first coupling degree is a degree of coupling between the transmitter coil  104  in the transmitter  10  and the receiver coil  201  in the receiver  20 , the first mapping table includes at least one coupling degree and a load-independent point corresponding to each of the at least one coupling degree, and the load-independent point is a combination of a voltage gain and an operating frequency, and at an operating frequency in a load-independent point corresponding to each coupling degree, a voltage gain in the load-independent point corresponding to the coupling degree is independent of an output load of the receiver, the control unit  211  is further configured to set a voltage gain between an output voltage of the receiver  20  and an input voltage of the transmitter  10  to the first voltage gain, and the control unit  211  is further configured to control the second oscillation circuit  210  to receive the power transferred by the transmitter  10  at the first voltage gain. 
     As a possible implementation, that the control unit  211  sets a voltage gain between an output voltage of the receiver  20  and an input voltage of the transmitter  10  to a first voltage gain includes the receiving unit  212  is further configured to receive information sent by the transmitter  10  and indicating a first input voltage, where the first input voltage is the input voltage of the transmitter  10 , and the control unit  211  is further configured to set the output voltage of the receiver to a first output voltage based on the first input voltage and the first voltage gain. 
     As a possible implementation, before the control unit  211  sets the voltage gain between the output voltage of the receiver  20  and the input voltage of the transmitter  10  to the first voltage gain, the receiving unit  212  is further configured to receive information sent by the transmitter  10  and indicating C p  and/or information sent by the transmitter  10  and indicating L p , where C p  and L p  are used by the receiver  20  to determine the first voltage gain, and/or receiving, by the receiver  20 , information sent by the transmitter  10  and indicating the first voltage gain. 
     In this embodiment of this application, the control unit  109  may be implemented by the control module  105  in the embodiment described in  FIG. 10 . The control unit  211  may be implemented by the control module  207  in the embodiment described in  FIG. 10 . The control unit  108  may further have another function of the control module  105 . For details, refer to the embodiment described in  FIG. 10 . The control unit  211  may further have another function of the control module  207 . For details, refer to the embodiment described in  FIG. 10 . Functions of the control unit  108 , the receiving unit  109 , the control unit  211 , and the sending unit  212  may correspond to the corresponding descriptions of the wireless charging method embodiment shown in  FIG. 13 . Details are not described herein again. 
     Based on the system architecture in  FIG. 1 ,  FIG. 19  is a schematic structural diagram of still another wireless charging system  100  according to an embodiment of this application. The wireless charging system  100  includes a transmitter  10  and a receiver  20 . 
     As shown in  FIG. 19 , the transmitter  10  includes a transmitter coil  104  and a first series matching capacitor  103 , and the transmitter coil  104  is connected to the first series matching capacitor  103  in series to form a first oscillation circuit  107 , the first oscillation circuit  107  is configured to transfer power to the receiver  20 , and a self-inductance applied when the transmitter coil  104  transfers the power to the receiver  20  is L p , and a capacitance of the first series matching capacitor  103  is C p , where L p *C p =k*L s *C s , L s  is a self-inductance applied when a receiver coil  201  in the receiver  20  receives the power transferred by the first oscillation circuit, C s  is a capacitance of a second series matching capacitor  202  in the receiver  20 , and k is a positive number that meets 0.8≤k≤1.2, the receiver coil  201  is connected to a second series matching capacitor  202  in series to form a second oscillation circuit, the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and the transmitter  10  further includes a voltage regulation module  106  configured to regulate an input voltage of the transmitter  10 . The transmitter  10  further includes a controller  30 , where the controller  30  includes one or more processors  301 , a memory  302 , and a communications interface  303 . These components may be connected using a bus  304  or in another manner. A bus connection is used as an example in  FIG. 19 . 
     The communications interface  303  may be configured for communication between the transmitter  10  and another communications device, for example, the receiver  20 . Specifically, the transmitter  10  and the receiver  20  may be the transmitter  10  and the receiver  20  shown in  FIG. 8  or  FIG. 17 . Specifically, the communications interface  303  may be an in-band communications interface in the wireless charging system. For detailed descriptions of in-band communication, refer to the embodiment described in  FIG. 1 . 
     Alternatively, the communications interface  303  may be an out-band interface, for example, a BLUETOOTH communications interface, a ZIGBEE communications interface, or a WI-FI communications interface, or may be extended to another communications interface. This is not limited in this embodiment of this application. 
     The memory  302  is coupled to the processor  301 , and is configured to store various software programs and/or a plurality of sets of instructions. The memory  302  may further store a data transmission program. The data transmission program may be used for communication with one or more additional devices, one or more transmitters, or one or more receivers. 
     In some embodiments of this application, the memory  302  may be configured to store a program for implementing, on a side of the transmitter  10 , the wireless charging method provided in one or more embodiments of this application. For implementation of the wireless charging method provided in one or more embodiments of this application, refer to the embodiments described in  FIG. 9  and  FIG. 12 . 
     The processor  301  may be configured to read and execute a computer readable instruction. Specifically, the processor  301  may be configured to invoke the program stored in the memory  302 , for example, the program for implementing, on the side of the transmitter  10 , the wireless charging method provided in one or more embodiments of this application, and execute the instruction included in the program. 
     It should be noted that the transmitter  10  shown in  FIG. 19  is merely one implementation in the embodiments of this application. During actual application, the transmitter  10  may further include more or fewer components, and this is not limited herein. 
     As shown in  FIG. 19 , the receiver  20  includes the receiver coil  201  and the second series matching capacitor  202 , the receiver coil  201  is connected to the second series matching capacitor  202  in series to form a second oscillation circuit  210 , and the second oscillation circuit  210  is configured to receive power transferred by the transmitter  10 , a self-inductance applied when the receiver coil  201  receives the power transferred by the transmitter  10  is L s , and a capacitance of the second series matching capacitor  202  is C s , where 
     
       
         
           
             
               
                 
                   L 
                   s 
                 
                 * 
                 
                   C 
                   s 
                 
               
               = 
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 k 
               
             
             , 
           
         
       
     
     L p  is a self-inductance applied when the transmitter coil  104  in the transmitter  10  transfers the power to the receiver  20 , C p  is a capacitance of the first series matching capacitor  103  in the transmitter  10 , and k is a positive number that meets 0.8≤k≤1.2, the transmitter coil  104  is connected to the first series matching capacitor  103  in series to form the first oscillation circuit  107 , and the first oscillation circuit  107  is configured to transfer the power to the second oscillation circuit  210 . 
     The receiver  20  further includes a controller  40 , where the controller  40  includes one or more processors  401 , a memory  402 , and a communications interface  403 . These components may be connected using a bus  404  or in another manner. A bus connection is used as an example in  FIG. 19 . 
     The communications interface  403  may be configured for communication between the receiver  20  and another communications device, for example, the transmitter  10 . Specifically, the transmitter  10  and the receiver  20  may be the transmitter  10  and the receiver  20  shown in  FIG. 8  or  FIG. 17 . Specifically, the communications interface  403  may be an in-band communications interface in the wireless charging system. For detailed descriptions of in-band communication, refer to the embodiment described in  FIG. 1 . 
     Alternatively, the communications interface  403  may be an out-band interface, for example, a BLUETOOTH communications interface, a ZIGBEE communications interface, or a WI-FI communications interface, or may be extended to another communications interface. This is not limited in this embodiment of this application. 
     The memory  402  is coupled to the processor  401 , and is configured to store various software programs and/or a plurality of sets of instructions. The memory  402  may further store a data transmission program. The data transmission program may be used for communication with one or more additional devices, one or more transmitters, or one or more receivers. In some embodiments of this application, the memory  402  may be configured to store a program for implementing, on a side of the receiver  20 , the wireless charging method provided in one or more embodiments of this application. For implementation of the wireless charging method provided in one or more embodiments of this application, refer to the embodiments described in  FIG. 9  and  FIG. 12 . 
     The processor  401  may be configured to read and execute a computer readable instruction. Specifically, the processor  401  may be configured to invoke the program stored in the memory  402 , for example, the program for implementing, on the side of the receiver  20 , the wireless charging method provided in one or more embodiments of this application, and execute the instruction included in the program. 
     It should be noted that the receiver  20  shown in  FIG. 19  is merely one implementation in the embodiments of this application. During actual application, the receiver  20  may further include more or fewer components, and this is not limited herein. 
     The receiver  20  may be a mobile phone, a tablet computer, a computer having a wireless transceiver function, a VR terminal device, or an AR terminal device. Alternatively, the receiver  20  may be a wireless charging electric vehicle, or white goods, for example, a no-tail television, a wireless charging soymilk maker, a wireless charging vacuum cleaning robot, or a multi-rotor drone. 
     Based on the system architecture in  FIG. 1 ,  FIG. 20  is a schematic structural diagram of yet another wireless charging system  100  according to an embodiment of this application. The wireless charging system  100  includes a transmitter  10  and a receiver  20 . 
     As shown in  FIG. 20 , the transmitter  10  includes a transmitter coil  104  and a first series matching capacitor  103 , and the transmitter coil  104  is connected to the first series matching capacitor  103  in series to form a first oscillation circuit  107 , the first oscillation circuit  107  is configured to transfer power to the receiver  20 , and a self-inductance applied when the transmitter coil  104  transfers the power to the receiver  20  is L p , and a capacitance of the first series matching capacitor  103  is C p , where L p *C p =k*L s *C s , L s  is a self-inductance applied when a receiver coil  201  in the receiver  20  receives the power transferred by the first oscillation circuit, C s  is a capacitance of a second series matching capacitor  202  in the receiver  20 , and k is a positive number that meets 0.8≤k≤1.2, the receiver coil  201  is connected to the second series matching capacitor  202  in series to form a second oscillation circuit, the second oscillation circuit is configured to receive the power transferred by the first oscillation circuit, and the transmitter  10  further includes a controller  50 , where the controller  50  includes one or more processors  501 , a memory  502 , and a communications interface  503 . These components may be connected using a bus  504  or in another manner. A bus connection is used as an example in  FIG. 20 . 
     The communications interface  503  may be configured for communication between the transmitter  10  and another communications device, for example, the receiver  20 . Specifically, the transmitter  10  and the receiver  20  may be the transmitter  10  and the receiver  20  shown in  FIG. 10  or  FIG. 18 . Specifically, the communications interface  503  may be an in-band communications interface in the wireless charging system. For detailed descriptions of in-band communication, refer to the embodiment described in  FIG. 1 . 
     Alternatively, the communications interface  503  may be an out-band interface, for example, a BLUETOOTH communications interface, a ZIGBEE communications interface, or a WI-FI communications interface, or may be extended to another communications interface. This is not limited in this embodiment of this application. 
     The memory  502  is coupled to the processor  501 , and is configured to store various software programs and/or a plurality of sets of instructions. The memory  502  may further store a data transmission program. The data transmission program may be used for communication with one or more additional devices, one or more transmitters, or one or more receivers. 
     In some embodiments of this application, the memory  502  may be configured to store a program for implementing, on a side of the transmitter  10 , the wireless charging method provided in one or more embodiments of this application. For implementation of the wireless charging method provided in one or more embodiments of this application, refer to the embodiments described in  FIG. 11  and  FIG. 13 . 
     The processor  501  may be configured to read and execute a computer readable instruction. Specifically, the processor  501  may be configured to invoke the program stored in the memory  502 , for example, the program for implementing, on the side of the transmitter  10 , the wireless charging method provided in one or more embodiments of this application, and execute the instruction included in the program. 
     It should be noted that the transmitter  10  shown in  FIG. 20  is merely one implementation in the embodiments of this application. During actual application, the transmitter  10  may further include more or fewer components, and this is not limited herein. 
     In this embodiment of this application, the transmitter  10  may further include a DC power source  101  and a DC/AC conversion module  102 . The receiver  20  may include an AC/DC conversion module  203 , a voltage regulator module  204 , a output load  205 , a modulation module  206 , and a BMS  208 . For detailed descriptions of the foregoing modules, refer to the architectures of the wireless charging systems described in  FIG. 1  and  FIG. 8 . Details are not described herein again. 
     As shown in  FIG. 20 , the receiver  20  includes the receiver coil  201  and the second series matching capacitor  202 , the receiver coil  201  is connected to the second series matching capacitor  202  in series to form a second oscillation circuit  210 , and the second oscillation circuit  210  is configured to receive power transferred by the transmitter  10 , a self-inductance applied when the receiver coil  201  receives the power transferred by the transmitter  10  is L s , and a capacitance of the second series matching capacitor  202  is C s , where 
     
       
         
           
             
               
                 
                   L 
                   s 
                 
                 * 
                 
                   C 
                   s 
                 
               
               = 
               
                 
                   
                     L 
                     p 
                   
                   * 
                   
                     C 
                     p 
                   
                 
                 k 
               
             
             , 
           
         
       
     
     L p  is a self-inductance applied when the transmitter coil  104  in the transmitter  10  transfers the power to the receiver  20 , C p  is a capacitance of the first series matching capacitor  103  in the transmitter  10 , and k is a positive number that meets 0.8≤k≤1.2, the transmitter coil  104  is connected to the first series matching capacitor  103  in series to form the first oscillation circuit  107 , the first oscillation circuit  107  is configured to transfer the power to the second oscillation circuit  210 , and the receiver  20  further includes a voltage regulation module  209  configured to regulate an output voltage of the receiver  20 . 
     The receiver  20  further includes a controller  60 , where the controller  60  includes one or more processors  601 , a memory  602 , and a communications interface  603 . These components may be connected using a bus  604  or in another manner. A bus connection is used as an example in  FIG. 20 . 
     The communications interface  603  may be configured for communication between the receiver  20  and another communications device, for example, the transmitter  10 . Specifically, the transmitter  10  and the receiver  20  may be the transmitter  10  and the receiver  20  shown in  FIG. 10  or  FIG. 18 . Specifically, the communications interface  603  may be an in-band communications interface in the wireless charging system. For detailed descriptions of in-band communication, refer to the embodiment described in  FIG. 1 . 
     Alternatively, the communications interface  603  may be an out-band interface, for example, a BLUETOOTH communications interface, a ZIGBEE communications interface, or a WI-FI communications interface, or may be extended to another communications interface. This is not limited in this embodiment of this application. 
     The memory  602  is coupled to the processor  601 , and is configured to store various software programs and/or a plurality of sets of instructions. The memory  602  may further store a data transmission program. The data transmission program may be used for communication with one or more additional devices, one or more transmitters, or one or more receivers. 
     In some embodiments of this application, the memory  602  may be configured to store a program for implementing, on a side of the receiver  20 , the wireless charging method provided in one or more embodiments of this application. For implementation of the wireless charging method provided in one or more embodiments of this application, refer to the embodiments described in  FIG. 11  and  FIG. 13 . 
     The processor  601  may be configured to read and execute a computer readable instruction. Specifically, the processor  601  may be configured to invoke the program stored in the memory  602 , for example, the program for implementing, on the side of the receiver  20 , the wireless charging method provided in one or more embodiments of this application, and execute the instruction included in the program. 
     It should be noted that the receiver  20  shown in  FIG. 20  is merely one implementation in the embodiments of this application. During actual application, the receiver  20  may further include more or fewer components, and this is not limited herein. 
     In this embodiment of this application, the transmitter  10  may further include a DC power source  101  and a DC/AC conversion module  102 . The receiver  20  may include an AC/DC conversion module  203 , a voltage regulator module  204 , a output load  205 , a modulation module  206 , and a BMS  208 . For detailed descriptions of the foregoing modules, refer to the architectures of the wireless charging systems described in  FIG. 1  and  FIG. 8 . Details are not described herein again. 
     It can be understood that when the embodiments of this application are applied to a transmitter chip, the transmitter chip implements functions of the transmitter in the foregoing method embodiment. The transmitter chip sends information to another module (for example, a radio frequency module or an antenna) of the transmitter, and the information is sent to the receiver using the other module of the transmitter. Alternatively, the transmitter chip may receive information from another module (for example, a radio frequency module or an antenna) of the transmitter, and the information is sent to the transmitter by the receiver. 
     It can be understood that when the embodiments of this application are applied to a receiver chip, the receiver chip implements functions of the receiver in the foregoing method embodiment. The receiver chip sends information to another module (for example, a radio frequency module or an antenna) of the receiver, and the information is sent to the transmitter using the other module of the receiver. Alternatively, the receiver chip may receive information from another module (for example, a radio frequency module or an antenna) of the receiver, and the information is sent to the transmitter by the receiver. 
     It can be understood that in this application, technical terms and technical solutions between different embodiments may be combined or referenced with each other based on internal logic of the different embodiments. The embodiments to which the technical terms and the technical solutions are applied are not limited in this application. A new embodiment may further be formed by mutually combining the technical solutions in the different embodiments. 
     It can be understood that the processor in the embodiments of this application may be a central processing unit (CPU), or may be another general-purpose processor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA) or another programmable logical device, a transistor logic device, a hardware component, or any combination thereof. The general-purpose processor may be a microprocessor or any conventional processor. 
     The method steps in the embodiments of this application may be implemented by hardware, or may be implemented by a processor executing a software instruction. The software instruction may include a corresponding software module. The software module may be located in a random access memory (RAM), a flash memory, a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), a register, a hard disk, a removable hard disk, a compact disc ROM (CD-ROM), or a storage medium of any other form known in the art. For example, a storage medium is coupled to a processor such that the processor can read information from the storage medium or write information into the storage medium. Certainly, the storage medium may be a component of the processor. The processor and the storage medium may be located in the ASIC. In addition, the ASIC may be located in a sending device or a receiving device. Certainly, the processor and the storage medium may exist in the sending device or the receiving device as discrete components. 
     All or some of the foregoing embodiments may be implemented using software, hardware, firmware, or any combination thereof. When software is used to implement the embodiments, the embodiments may be implemented completely or partially in a form of a computer program product. The computer program product includes one or more computer instructions. When the computer instructions are loaded and executed on a computer, the procedures or functions according to the embodiments of this application are all or partially generated. The computer may be a general-purpose computer, a dedicated computer, a computer network, or another programmable apparatus. The computer instructions may be stored in a computer readable storage medium or transmitted using the computer readable storage medium. The computer instructions may be transmitted from a website, computer, server, or data center to another website, computer, server, or data center in a wired (for example, a coaxial cable, an optical fiber, or a digital subscriber line (DSL)) or wireless (for example, infrared, radio, or microwave) manner. The computer readable storage medium may be any usable medium accessible by a computer, or a data storage device, such as a server or a data center, integrating one or more usable media. The usable medium may be a magnetic medium (for example, a floppy disk, a hard disk, or a magnetic tape), an optical medium (for example, a digital versatile disc (DVD)), a semiconductor medium (for example, a solid state disk (SSD)), or the like. 
     A person of ordinary skill in the art may understand that all or some of the procedures of the methods in the foregoing embodiments may be implemented by a computer program instructing related hardware. The program may be stored in the computer readable storage medium. When the program runs, the procedures of the foregoing method embodiments are performed. The foregoing storage medium includes any medium that can store program code, such as a ROM, a RAM, a magnetic disk, or an optical disc.