Patent Publication Number: US-2023134591-A1

Title: Wireless Power Transfer System and Control Method Thereof

Description:
PRIORITY CLAIM 
     This disclosure claims the benefit of and priority to Chinese Patent Application No. 202111273455.7, filed on Oct. 29, 2021, which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to the technical field of electronic circuits, and, in particular embodiments, to a wireless power transfer system and control method thereof. 
     BACKGROUND 
     As technologies further advance, wireless power transfer has emerged as an efficient and convenient mechanism for powering or charging battery based mobile devices such as mobile phones, tablet PCs, digital cameras, MP3 players and/or the like. A wireless power transfer system typically comprises a primary side transmitter and a secondary side receiver. The primary side transmitter is magnetically coupled to the secondary side receiver. The magnetic coupling may be implemented as a loosely coupled transformer having a primary side coil formed in the primary side transmitter and a secondary side coil formed in the secondary side receiver. 
     The primary side transmitter may comprise a power conversion unit such as a primary side of a power converter. The power conversion unit is coupled to a power source and is capable of converting electrical power to wireless power signals. The secondary side receiver is able to receive the wireless power signals through the loosely coupled transformer and convert the received wireless power signals to electrical power suitable for a load. 
     Power loss of the wireless power transfer system is an important parameter. For example, a foreign object detection (FOD) can be performed using the calculated power loss. Additional circuits are needed in conventional wireless power transfer system to detect parameters such as an input current and a coil current to realize power loss detection and foreign object detection. 
     It would be desirable to have a simple and reliable control mechanism to reduce cost of power loss detection. 
     SUMMARY 
     These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present disclosure which provide a wireless power transfer system and control method thereof. 
     In accordance with an embodiment, a wireless power transfer system is provided, comprising: a transmitter coil configured to be magnetically coupled to a receiver coil; a power conversion device coupled to the transmitter coil; and a controller including an one-half cycle detection block configured to set up a current sensing time instant corresponding to one half of switching cycle of the power converter device, and wherein at the current sensing time instant, a current flowing through the transmitter coil is detected and compared with a predetermined threshold to perform foreign object detection, so as to determine whether a foreign object is coupled to the wireless power transfer system. 
     In accordance with another embodiment, a control method of a wireless power transfer system is provided, comprising: finding a time instant corresponding to one half of the switching cycle in the switching cycle of power converter of the wireless power transfer system; at the time instant, comparing the current flowing through the transmitter coil with the predetermined threshold to perform foreign object detection, so as to determine whether a foreign object is coupled to the wireless power transfer system. 
     In accordance with yet another embodiment, a controller is provided, including an one-half cycle detection block configured to set up a current sensing time instant corresponding to one half of the switching cycle of the power conversion device, and wherein at the current sensing time instant, the current flowing through the transmitter coil can be detected and compared with a predetermined threshold to perform foreign object detection, so as to determine whether a foreign object is coupled to the wireless power transfer system. 
     The foregoing has outlined rather broadly the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the claims of the disclosure. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the disclosure as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    illustrates a block diagram of a wireless power transfer system in accordance with various embodiments of the present disclosure; 
         FIG.  2    illustrates a schematic diagram of the wireless power transfer system shown in  FIG.  1    in accordance with various embodiments of the present disclosure; 
         FIG.  3    illustrates various waveforms of the wireless power transfer system shown in  FIG.  1    in accordance with various embodiments of the present disclosure; 
         FIG.  4    illustrates a block diagram of a controller in accordance with various embodiments of the present disclosure; 
         FIG.  5    illustrates a schematic diagram of a comparison and interruption block in accordance with various embodiments of the present disclosure; and 
         FIG.  6    illustrates a flow chart of a method for controlling the wireless power transfer system shown in  FIG.  1    in accordance with various embodiments of the present disclosure. 
     
    
    
     Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the disclosure, and do not limit the scope of the disclosure. The present disclosure will be described with respect to preferred embodiments in a specific context, namely a foreign object detection device for a wireless power transfer system. The invention may also be applied, however, to a variety of power systems. Hereinafter, various embodiments will be explained in detail with reference to the accompanying drawings. 
       FIG.  1    illustrates a block diagram of a wireless power transfer system in accordance with various embodiments of the present disclosure. The wireless power transfer system  100  comprises a power stage  104  and a wireless power transfer device  101  connected in cascade between an input power source  102  and a load  114 . The wireless power transfer device  101  includes a transmitter  110  and a receiver  120 . As shown in  FIG.  1   , the transmitter  110  comprises a transmitter circuit  107  and a transmitter coil L 1  connected in cascade. The input of the transmitter circuit  107  is coupled to an output of the power stage  104 . The receiver  120  comprises a receiver coil L 2  and a rectifier  112  connected in cascade. The output of the rectifier  112  is coupled to the load  114 . 
     The transmitter  110  is magnetically coupled to the receiver  120  through a magnetic field when the receiver  120  is placed near the transmitter  110 . A loosely coupled transformer  115  is formed by the transmitter coil L 1 , which is part of the transmitter  110 , and the receiver coil L 2 , which is part of the receiver  120 . As a result, power may be transferred from the transmitter  110  to the receiver  120 . 
     In some embodiments, the transmitter  110  may be inside a charging pad. The transmitter coil is placed underneath the top surface of the charging pad. The receiver  120  may be embedded in a mobile phone. When the mobile phone is place near the charging pad, a magnetic coupling may be established between the transmitter coil and the receiver coil. In other words, the transmitter coil and the receiver coil may form a loosely coupled transformer through which a power transfer occurs between the transmitter  110  and the receiver  120 . The strength of coupling between the transmitter coil L 1  and the receiver coil L 2  is quantified by the coupling coefficient k. In some embodiments, k is in a range from about 0.05 to about 0.9. 
     In some embodiments, after the magnetic coupling has been established between the transmitter coil L 1  and the receiver coil L 2 , the transmitter  110  and the receiver  120  may form a power system through which power is wirelessly transferred from the input power source  102  to the load  114 . 
     The input power source  102  may be a power adapter converting a utility line voltage to a direct-current (dc) voltage. Alternatively, the input power source  102  may be a renewable power source such as a solar panel array. Furthermore, the input power source  102  may be an energy storage device such as rechargeable batteries, fuel cells and/or the like. 
     The load  114  represents the power consumed by the mobile device (e.g., a mobile phone) coupled to the receiver  120 . Alternatively, the load  114  may refer to a rechargeable battery and/or batteries connected in series/parallel, and coupled to the output of the receiver  120 . 
     The transmitter circuit  107  may comprise primary side switches of a full-bridge power converter according to some embodiments. The full-bridge is also known as an H-bridge. Alternatively, the transmitter circuit  107  may comprise the primary side switches of other converters such as a half-bridge converter, a push-pull converter and the like. The detailed configuration of the transmitter circuit  107  will be described below with respect to  FIG.  2   . 
     It should be noted that the converters described above are merely examples. One having ordinary skill in the art will recognize other suitable power converters such as class E topology based power converters (e.g., a class E amplifier), may alternatively be used. 
     The transmitter circuit  107  may further comprise a resonant capacitor. The resonant capacitor and the magnetic inductance of the transmitter coil may form a resonant tank. Depending on design needs and different applications, the resonant tank may further include a resonant inductor. In some embodiments, the resonant inductor may be implemented as an external inductor. In alternative embodiments, the resonant inductor may be implemented as a connection wire. 
     The receiver  120  comprises the receiver coil L 2  magnetically coupled to the transmitter coil L 1  after the receiver  120  is placed near the transmitter  110 . As a result, power may be transferred to the receiver coil and further delivered to the load  114  through the rectifier  112 . The receiver  120  may comprise a secondary resonant capacitor. 
     The rectifier  112  converts an alternating polarity waveform received from the output of the receiver coil L 2  to a single polarity waveform. In some embodiments, the rectifier  112  is implemented as a synchronous rectifier including four switches. In alternative embodiments, the rectifier  112  comprises a full-wave diode bridge and an output capacitor. 
     Furthermore, the synchronous rectifier may be formed by any controllable devices such as metal oxide semiconductor field effect transistor (MOSFET) devices, bipolar junction transistor (BJT) devices, super junction transistor (SJT) devices, insulated gate bipolar transistor (IGBT) devices, gallium nitride (GaN) based power devices and/or the like. The detailed structure of the rectifier  112  will be discussed below with respect to  FIG.  2   . 
     The power stage  104  is coupled between the input power source  102  and the input of the wireless power transfer device  101 . Depending design needs and different applications, the power stage  104  may comprise many different configurations. In some embodiments, the power stage  104  may be a non-isolated power converter such as a buck converter. In some embodiments, the power stage  104  may be implemented as a linear regulator. In some embodiments, the power stage  104  may be an isolated power converter such as a forward converter. 
     The implementation of the power stage  104  described above is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. Furthermore, depending on different applications and design needs, the power stage  104  may be an optional element of the wireless power transfer system  100 . In other words, the input power source  102  may be connected to the transmitter circuit  107  directly. 
       FIG.  2    illustrates a schematic diagram of the wireless power transfer system shown in  FIG.  1    in accordance with various embodiments of the present disclosure. The wireless power transfer device  101  comprises a transmitter circuit  107 , a resonant capacitor Cp, a loosely coupled transformer  115 , a resonant capacitor Cs and a rectifier  112  connected in cascade. The loosely coupled transformer  115  is formed by the transmitter coil L 1  and the receiver coil L 2 . The transmitter circuit  107  is implemented as a full-bridge as shown in  FIG.  2   . Throughout the description, the full-bridge shown in  FIG.  2    may be alternatively referred to as a power converter or a full-bridge power converter. 
     The full-bridge  107  includes four switching elements, namely S 1 , S 2 , S 3  and S 4 . As shown in  FIG.  2   , the switching elements S 1  and S 2  are connected in series between an input voltage bus VIN and ground. The input voltage bus VIN is connected to the output of the power stage  104  shown in  FIG.  1   . Likewise, the switching elements S 3  and S 4  are connected in series between the input voltage bus VIN and ground. The common node (SW 1 ) of the switching elements S 1  and S 2  is coupled to a first input terminal of the transmitter coil L 1 . The common node (SW 2 ) of the switching elements S 3  and S 4  is coupled to a second input terminal of the transmitter coil L 1  through the resonant capacitor Cp. The voltage between SW 1  and SW 2  is denoted as VSW. The waveform of VSW will be illustrated below with respect to  FIG.  4   . 
     According to some embodiments, the switching elements S 1 , S 2 , S 3  and S 4  are implemented as MOSFET or MOSFETs connected in parallel, any combinations thereof and/or the like. According to alternative embodiments, the switching elements (e.g., switch S 1 ) may be an insulated gate bipolar transistor (IGBT) device. Alternatively, the primary switches can be any controllable switches such as integrated gate commutated thyristor (IGCT) devices, gate turn-off thyristor (GTO) devices, silicon controlled rectifier (SCR) devices, junction gate field-effect transistor (JFET) devices, MOS controlled thyristor (MCT) devices, gallium nitride (GaN) based power devices and/or the like. 
     It should be noted that while the example throughout the description is based upon a full-bridge converter (e.g., full-bridge  107  shown in  FIG.  2   ), the implementation of the transmitter circuit  107  shown in  FIG.  2    may have many variations, alternatives, and modifications. For example, half-bridge converters, push-pull converters, class E based power converters (e.g., a class E amplifier) may be alternatively employed. Furthermore, an inductor-inductor-capacitor (LLC) resonant converter may be formed when the transmitter coil L 1  is tightly coupled with the receiver coil L 2  in some applications. 
     In sum, the full-bridge  107  illustrated herein is limited solely for the purpose of clearly illustrating the inventive aspects of the various embodiments. The present invention is not limited to any particular power topology. 
     It should further be noted that while  FIG.  2    illustrates four switches S 1 -S 4 , various embodiments of the present disclosure may include other variations, modifications and alternatives. For example, a separate capacitor may be connected in parallel with each switch of the full-bridge  107 . Such a separate capacitor helps to better control the timing of the resonant process of the full-bridge  107 . 
     The outputs of the receiver coil L 2  are coupled to the load RL through the resonant capacitor Cs, the rectifier  112  and a capacitor Co. The rectifier converts an alternating polarity waveform received from the outputs of the receiver coil L 2  to a single polarity waveform. The capacitor Co is employed to attenuate noise and provide a steady output voltage. The resonant capacitor Cs helps to achieve soft switching for the wireless power transfer system. 
     In some embodiments, the rectifier  112  is implemented as a synchronous rectifier. The rectifier  112  includes four switching elements, namely S 5 , S 6 , S 7  and S 8 . As shown in  FIG.  2   , the switching elements S 5  and S 6  are connected in series between the output terminal of the rectifier  112  and ground. Likewise, the switching elements S 7  and S 8  are connected in series between the output terminal of the rectifier  112  and ground. As shown in  FIG.  2   , the common node of the switching elements S 5  and S 6  is coupled to a first terminal of the receiver coil L 2 . The common node of the switching elements S 7  and S 8  is coupled to a second terminal of the receiver coil L 2  through the resonant capacitor Cs. 
     According to some embodiments, the switching elements S 5 , S 6 , S 7  and S 8  are implemented as MOSFET or MOSFETs connected in parallel, any combinations thereof and/or the like. 
     Referring back to  FIG.  1   , in the related art, foreign object detection is required during wireless power transfer. There are many ways to detect foreign objects, such as detecting capacitance, resistance, power loss, etc. The power loss detection method is to determine whether there is a foreign object around the wireless power transfer system by detecting whether the power loss deltaP between the transmitter  110  and the receiver  120  is less than a threshold. Specifically, deltaP=Pt-Pr, wherein Pt is the transmit power of the transmitter  110 , and Pr is the receive power of the receiver  120 . When using the power loss detection method to detect foreign objects, it is necessary to precisely detect Pt and Pr, so as to accurately determine whether there is a foreign object around the wireless power transfer system. However, it is difficult to precisely detect Pt and Pr. 
     Therefore, the present application improves the power loss detection method, which simplifies the calculation of power loss. Specifically, the power loss equation deltaP=Pt-Pr=[Pi-Pt (loss) ]-[Po-Pr (loss) ] is used, wherein Pt is the transmit power of the transmitter  110 , Pr is the receive power of the receiver  120 , Pi is the input power of the system, Pt (loss)  is the power loss at the input terminal, Po is the output power of the system, and Pr (loss)  is the power loss at the output terminal. This application provides a simple method for calculating Pt (loss) , which can take into account the simplicity and accuracy of the power loss calculation at the input terminal. If Pi, Po and Pr (loss)  are known, Pt (loss)  can be easily calculated to obtain the power loss deltaP, so as to determine whether the power loss is less than the threshold and whether a foreign object is around the wireless power transfer system. The relationship between the power loss of the transmitter and the current flowing through the transmitter coil is described below with respect to  FIG.  3   . 
       FIG.  3    illustrates various waveforms of the wireless power transfer system shown in  FIG.  1    in accordance with various embodiments of the present disclosure. The horizontal axis of  FIG.  3    represents the time interval. The first waveform  402  represents VSW (the voltage between the two switch nodes SW 1  and SW 2  shown in  FIG.  2   ). The second waveform  404  represents a fundamental frequency waveform of the first waveform  402 . The third waveform  406  represents the current flowing through the transmitter coil. 
     As shown in  FIG.  3   , the current flowing through the transmitter coil lags behind the voltage (a fundamental frequency waveform of the first waveform  402 ), the phase difference between voltage and current is expressed as ϕ . 
     Transmitter power loss includes coil loss, power switch loss and resonant capacitor loss. The above three losses are equivalent to the AC equivalent resistance of the transmitter coil multiplied by the current. Assuming the AC equivalent resistance is ACR, the transmitter power loss is ACR*Icoil rms ^2, wherein Icoil rms  is the rms value of the transmitter coil current. 
     Define the current flowing through the transmitter coil as equation (1): 
     
       
         
           
             I 
             c 
             o 
             i 
             l 
             
               θ 
             
             = 
             
               2 
             
             ∗ 
             I 
             c 
             o 
             i 
             
               l 
               
                 r 
                 m 
                 s 
               
             
             s 
             i 
             n 
             
               
                 θ 
                 − 
                 φ 
               
             
             = 
             
               α 
               1 
             
             s 
             i 
             n 
             
               θ 
             
             + 
             
               α 
               2 
             
             c 
             o 
             s 
             
               θ 
             
           
         
       
     
     The transmitter coil current can be divided into two parts, including an active power and a reactive power, which can be expressed as equation (2): 
     
       
         
           
             I 
             c 
             o 
             i 
             
               l 
               
                 r 
                 m 
                 s 
               
             
             
                 
               2 
             
             = 
             I 
             c 
             o 
             i 
             l 
             
               α 
               
                 r 
                 m 
                 s 
               
             
             
                 
               2 
             
             + 
             I 
             c 
             o 
             i 
             l 
             
               r 
               
                 r 
                 m 
                 s 
               
             
             
                 
               2 
             
           
         
       
     
      wherein, Icoila rms   2  represents the active power part, and Icoilr rms   2  represents the reactive power part. 
     The active power part of the transmitter coil current can be expressed as equation (3): 
     
       
         
           
             I 
             c 
             o 
             i 
             l 
             
               α 
               
                 r 
                 m 
                 s 
               
             
             = 
             
               
                 P 
                 i 
                 n 
               
               
                 
                   V 
                   
                     r 
                     m 
                     s 
                   
                 
               
             
             = 
             
               π 
               
                 2 
                 
                   2 
                 
               
             
             ∗ 
             I 
             I 
             N 
           
         
       
     
      wherein, IIN represents the input current of the transmitter coil. 
     The current at time instant π of the transmitter coil current can be expressed as equation (4): 
     
       
         
           
             I 
             c 
             o 
             i 
             l 
             
               π 
             
             = 
             
               2 
             
             ∗ 
             I 
             c 
             o 
             i 
             
               l 
               
                 r 
                 m 
                 s 
               
             
             s 
             i 
             n 
             
               
                 π 
                 − 
                 φ 
               
             
             = 
             
               2 
             
             ∗ 
             I 
             c 
             o 
             i 
             
               l 
               
                 r 
                 m 
                 s 
               
             
             s 
             i 
             n 
             
               φ 
             
           
         
       
     
     Then, the reactive power part of the transmitter coil current can be expressed as equation (5): 
     
       
         
           
             I 
             c 
             o 
             i 
             l 
             
               r 
               
                 r 
                 m 
                 s 
               
             
             = 
             
               
                 P 
                 i 
                 
                   n 
                   r 
                 
               
               
                 
                   V 
                   
                     r 
                     m 
                     s 
                   
                 
               
             
             = 
             
               
                 
                   V 
                   
                     r 
                     m 
                     s 
                   
                 
                 ∗ 
                 I 
                 c 
                 o 
                 i 
                 
                   l 
                   
                     r 
                     m 
                     s 
                   
                 
                 s 
                 i 
                 n 
                 
                   φ 
                 
               
               
                 
                   V 
                   
                     r 
                     m 
                     s 
                   
                 
               
             
             = 
             
               
                 I 
                 c 
                 o 
                 i 
                 l 
                 
                   π 
                 
               
               
                 
                   2 
                 
               
             
           
         
       
     
     Therefore, the transmitter coil current can be expressed as equation (6): 
     
       
         
           
             I 
             c 
             o 
             i 
             
               l 
               
                 r 
                 m 
                 s 
               
             
             
                 
               2 
             
             = 
             
               
                 
                   π 
                   2 
                 
                 ∗ 
                 I 
                 I 
                 
                   N 
                   2 
                 
               
               8 
             
             + 
             
               
                 I 
                 c 
                 o 
                 i 
                 l 
                 
                   
                     
                       π 
                     
                   
                   2 
                 
               
               2 
             
           
         
       
     
     In the related art, a complicated equation needs to be used to calculate the power loss between the transmitter  110  and the receiver  120  (for example, the power loss is calculated by using the voltage), so as to further perform foreign object detection. 
     The present disclosure provides an improved detection circuit, which can easily obtain the power loss of the input terminal, so as to calculate the power loss deltaP between the transmitter  110  and the receiver  120 , achieving the purpose of foreign object detection. 
     In operation, in each switching cycle, finding the time instant corresponding to the one half of the switching cycle. At this time instant, the current flowing through the transmitter coil is detected. Based on the detected current, it can be calculated and determined accordingly whether the power loss of the transmitter coil is greater than the threshold, and the purpose of foreign object detection can be achieved based on the determined result. 
     In operation, the detected current is compared with a predetermined threshold. In some embodiments, the threshold of a present cycle is set based on the threshold of a previous cycle. If the detected current is greater than the predetermined threshold, there is a foreign object around, and a protection mechanism for foreign object detection is activated. The detailed implementation of the detection circuit provided by the present disclosure will be described below with reference to  FIGS.  4 - 6   . 
       FIG.  4    illustrates a block diagram of a controller in accordance with various embodiments of the present disclosure. The controller comprises a microcontroller unit (MCU)  502 , a π detection block  504 , a comparison and interruption block  506  and a current sense block  508 . This controller can realize the function of foreign object detection. 
     The MCU  502  sets up an integer equivalent to one half of the switching cycle of the power converter. The MCU  502  feeds this integer to the π detection block  504 . The input terminal of the current sense block  508  is directly or indirectly connected to the transmitter coil, so as to sense the current flowing through the transmitter coil. 
     In every switching cycle of the power converter, the π detection block  504  detects a falling edge of one PWM signal applied to the full-bridge, and sends the falling edge to the comparison and interruption block  506 . The comparison and interruption block  506  receives the falling edge of the PWM signal and detects the current (Icoil) flowing through the transmitter coil at the time instant equivalent to one half of the switching cycle. The detected current (current flowing through the transmitter coil) is fed into the comparison and interruption block  506 . The detected current is compared with the predetermined threshold. If a foreign object exists nearby, the comparison and interruption block  506  generates a corresponding interrupt signal and feed this interrupt signal to the MCU  502 . The MCU  502  applies a corresponding control mechanism to the wireless power transfer system to control the wireless power transfer system not to enter the power transfer mode, or to stop performing power transfer. For example, MCU  502  applies the corresponding control mechanism to a power converter, so that the power converter stops performing power conversion. 
       FIG.  5    illustrates a schematic diagram of the comparison and interruption block in accordance with various embodiments of the present disclosure. The comparison and interruption block  506  comprises a first comparator  601  and a first interrupt generator  602 . As shown in  FIG.  5   , the inverting input of the first comparator  601  is coupled to a predetermined threshold. The predetermined threshold is used to determine whether foreign objects are present around the wireless power transfer system. The non-inverting input of the first comparator  601  is coupled to the current (Icoil) flowing through the transmitter coil. 
     It should be noted that the comparison and interruption block  506  discussed herein is provided for illustrative purposes only, and is provided only as an example of the functionality that may be included in the comparison and interruption block  506 . One of ordinary of skill in the art would realize that the comparison and interruption block  506  may be implemented in many different ways, and it may include other function blocks. 
       FIG.  6    illustrates a flow chart of a method for controlling the wireless power transfer system shown in  FIG.  1    in accordance with various embodiments of the present disclosure. This flowchart shown in  FIG.  6    is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, various steps illustrated in  FIG.  6    may be added, removed, replaced, rearranged and repeated. 
     A wireless power transfer system comprises a transmitter and a receiver. The transmitter comprises a power converter and a transmitter coil. The receiver comprises a receiver coil and a rectifier. The transmitter coil is magnetically coupled to the receiver coil. 
     In operation, the power loss of the wireless power transfer system increases when there are foreign objects around the wireless power transfer system. 
     A controller is employed to detect the current flowing through the transmitter coil at a time instant corresponding to one half of a switching cycle of the power converter, the detected current of this time instant can determine whether a foreign object is around the wireless power transfer system. When the detected current is greater than the predetermined threshold, it is determined that a foreign object is around the wireless power transfer system. 
     At step  802 , in a switching cycle of the power converter of the wireless power transfer system, finding a time instant corresponding to one half of the switching cycle. For instance, in the switching cycle of power converter, the falling edge of a gate drive signal of the power converter is used as the time instant corresponding to one half of the switching cycle. 
     At step  804 , at the time instant corresponding to one half of the switching cycle, the current flowing through the transmitter coil of the wireless power transfer system is detected. 
     At step  806 , the detected current flowing through the transmitter coil is compared with the predetermined threshold to perform foreign object detection, so as to determine whether a foreign object is coupled to the wireless power transfer system. For example, the predetermined threshold of present cycle is updated based on the predetermined threshold of previous cycle. 
     At step  808 , a control mechanism is applied to the wireless power transfer system when it is determined that a foreign object is coupled to the wireless power transfer system. In particular, when it is determined that a foreign object is coupled to the wireless power transfer system, the interrupt signal is generated and sent to the power converter device, in order to control the wireless power transfer system not to enter power transfer mode, or to stop performing power transfer. 
     In other embodiments, according to a comparison result of a comparison between the current flowing through the transmitter coil and the predetermined threshold, the wireless power transfer system is controlled not to enter the power transfer mode, or to stop performing power transfer. 
     Although embodiments of the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.