Patent Publication Number: US-2022224161-A1

Title: Wireless charging device, a receiver device, and a method of operating the same

Description:
BACKGROUND 
     Embodiments of the present specification relate generally to wireless power transfer systems, and more particularly to a wireless charging device, and a receiver device in wireless power transfer systems. 
     In general, power transfer system includes a charging devices widely used to transfer power from a power source to one or more receiver devices, such as mobile devices, biomedical devices, and portable consumer devices. Typically, the power transfer systems are contact based power transfer systems or wireless power transfer systems. In certain applications, where instantaneous or continuous power transfer is required but interconnecting wires are inconvenient, the wireless power transfer systems are desirable. 
     In wireless power transfer systems, a charging device is used to convert an input power to a transferrable power which is then wirelessly transmitted to charge one or more batteries in the receiver devices. Each receiver device is compatible with one wireless frequency standard. Currently available frequency standards include a Wireless Power Consortium (WPC) with Qi standard and an Air fuel Alliance (AFA) standard. The Qi standard is defined in a frequency range from 100 kHz to 400 kHz, while the AFA standard is defined in a frequency range from 6 MHz to 8 MHz, for example. Further, design of any new wireless receiver devices will entail use of one of these two frequency standards. 
     A conventional charging device transmits the input power at only one frequency standard. Hence, for each frequency standard, separate charging devices are employed to transmit electric power to the corresponding receiver device. However, using separate charging devices for each frequency standard substantially increases set-up costs and maintenance costs of the wireless power transfer systems. Moreover, the conventional charging device may continuously transmit the power even if the receiver device is not present. Such transmission of power results in power loss and affects efficiency of the charging device. 
     BRIEF DESCRIPTION 
     Briefly in accordance with one aspect of the present specification, a wireless charging device is presented. The wireless charging device includes a driver unit configured to generate one of a first alternating current (AC) voltage signal having a first frequency and a second alternating current (AC) voltage signal having a second frequency. Also, the wireless charging device includes a transmitting unit coupled to the driver unit, wherein the transmitting unit includes a first coil and a first capacitor coupled to each other and configured to transmit the first AC voltage signal having the first frequency. Further, the transmitting unit includes a second coil and a second capacitor coupled to each other and configured to transmit the second AC voltage signal having the second frequency. In addition, the wireless charging device includes a control unit coupled to the transmitting unit and the driver unit, wherein the control unit is configured to detect a first receiver device operating at the first frequency based on a change in a first voltage with reference to a first threshold value, at a first junction between the first coil and the first capacitor, and detect a second receiver device operating at the second frequency based on a change in a second voltage with reference to a second threshold value, at a second junction between the second coil and the second capacitor. 
     In accordance with another embodiment of the present specification, a receiver device is presented. The receiver device includes a receiving unit comprising a receiver coil and a receiver capacitor coupled to each other and configured to receive one of a first alternating current (AC) voltage signal having a first frequency and a second alternating current (AC) voltage signal having a second frequency from a wireless charging device. Also, the receiver device includes a rectifier unit coupled to the receiving unit and configured to convert one of the first alternating current (AC) voltage signal having the first frequency and the second alternating current (AC) voltage signal having the second frequency to a DC load signal. Further, the receiver device includes a receiver confirmation unit coupled to the rectifier unit and configured to receive the DC load signal and in response change an impedance across the receiving unit for a predefined time period to confirm a presence of the receiver device to the wireless charging device. 
     In accordance with another embodiment of the present specification, a method for detecting a receiver device is presented. The method includes generating, by a driver unit, one of a first alternating current (AC) voltage signal having a first frequency and a second alternating current (AC) voltage signal having a second frequency. Also, the method includes transmitting, by a first coil and a first capacitor in a transmitting unit, the first AC voltage signal having the first frequency, if the first AC voltage signal is generated. Further, the method includes transmitting, by a second coil and a second capacitor in the transmitting unit, the second AC voltage signal having the second frequency, if the second AC voltage signal is generated. In addition, the method includes detecting, by a control unit, the first receiver device based on a change in a first voltage at a first junction between the first coil and the first capacitor. Furthermore, the method includes detecting, by the control unit, the second receiver device based on a change in a second voltage at a second junction between the second coil and the second capacitor. 
     In accordance with another embodiment of the present specification, a wireless power transfer system is presented. The wireless power transfer system includes a wireless charging device including a driver unit configured to generate one of a first alternating current (AC) voltage signal having a first frequency and a second alternating current (AC) voltage signal having a second frequency. Also, the wireless charging device includes a transmitting unit coupled to the driver unit and configured to transmit the first AC voltage signal having the first frequency and the second AC voltage signal having the second frequency. Furthermore, the wireless charging device includes a control unit coupled to the transmitting unit and the driver unit, wherein the control unit is configured to detect a first receiver device operating at the first frequency based on a change in a first voltage with reference to a first threshold value, at a first junction in the transmitting unit and a second receiver device operating at the second frequency based on a change in a second voltage with reference to a second threshold value, at a second junction in the transmitting unit. In addition, the wireless power transfer system includes the receiver device configured to be coupled to the wireless charging device, wherein the receiver device includes a receiving unit configured to receive one of the first AC voltage signal having the first frequency and the second AC voltage signal having the second frequency from the wireless charging device. Also, the receiver device includes a rectifier unit coupled to the receiving unit and configured to convert one of the first AC voltage signal having the first frequency and the second AC voltage signal having the second frequency to a DC load signal. Further, the receiver device includes a receiver confirmation unit coupled to the rectifier unit and configured to receive the DC load signal and in response change an impedance across the receiving unit for a predefined time period to confirm a presence of the receiver device to the wireless charging device. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a block diagram of a wireless power transfer system in accordance with certain embodiments of the present invention; 
         FIG. 2  is a diagrammatical representation of a wireless power transfer system in accordance with certain embodiments of the present invention; 
         FIG. 3  is a graphical representation of a first AC voltage signal and a second AC voltage signal in accordance with certain embodiments of the present invention; and 
         FIG. 4  is a flow chart illustrating a method for detecting a receiver device in in accordance with certain embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As will be described in detail hereinafter, various embodiments of a system and method for charging one or more wireless receiver devices are disclosed. In particular, embodiments of the system and the method disclosed herein disclose detecting the receiver device prior to continuously transmitting electric power to the receiver device. Further, the embodiments of the system and the method disclosed herein disclose confirming a presence of the receiver device at regular intervals while transmitting the electric power to the receiver device. 
     Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this specification belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The use of terms “including,” “comprising” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “connected” and “coupled” are not restricted to physical or mechanical connections or couplings and can include electrical connections or couplings, whether direct or indirect. Furthermore, terms “circuit” and “circuitry” and “control unit” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together to provide the described function. In addition, the term operationally coupled as used herein includes wired coupling, wireless coupling, electrical coupling, magnetic coupling, radio communication, software based communication, or combinations thereof. 
       FIG. 1  is a block diagram of a wireless power transfer system  100  in accordance with embodiments of the present invention. The wireless power transfer system  100  is used to transmit electric power. 
     In the illustrated embodiment, the wireless power transfer system  100  includes a universal wireless charging device  102  that is wirelessly coupled to a first receiver device  104  and a second receiver device  106 . It may be noted that the terms “universal wireless charging device,” “wireless charging device,” and “charging pad” may be used interchangeably in the following description and may be denoted by same reference numeral. 
     Further, the first and second receiver devices  104 ,  106  may be compatible with one of the wireless frequency standards. For example, one of the first and second receiver devices  104 ,  106  may be compatible with an Air Fuel Alliance (AFA) standard that is defined in a frequency range from 6 MHz to 8 MHz. Similarly, another of the first and second receiver devices may be compatible with the Wireless Power Consortium (WPC) with Qi standard that is defined in a frequency range from 100 kHz to 400 kHz. For ease of explanation, the first receiver device  104  is considered to be compatible with a first frequency standard, such as the AFA standard. Similarly, the second receiver device  106  is considered to be compatible with a second frequency standard, such as the Qi standard. Although the first and second receiver devices  104 ,  106  are described as adhering to one of the two currently available frequency standards, it may be noted that the receiver devices  104 ,  106  may adhere to other frequency standards and are not limited to the frequency standards mentioned herein. Although the wireless power transfer system  100  is depicted as including two receiver devices  104 ,  106 , use of any number of receiver devices that are compatible with one or more frequency standards is envisioned. 
     The first and second receiver devices  104 ,  106  may be mobile devices, biomedical devices, portable consumer devices, and the like. The mobile devices may be cell phones, laptops, and the like. 
     As previously noted, in conventional power transfer systems, a charging device is operated at only one frequency standard to transmit electric power to receiver devices. Hence, a separate charging device having a separate converter and a separate coil for each frequency standard are employed to transmit electric power to the corresponding receiver device. Use of separate charging devices for each frequency standard substantially increases costs associated with the set-up and maintenance of the conventional power transfer systems. Also, use of separate charging devices to charge the receiver devices having different frequency standards is inconvenient for a user. Moreover, the charging device may continuously generate electromagnetic field to transmit electric power even if the receiver device is not present, resulting in power loss and reduction in overall efficiency. 
     The wireless charging device  102  is configured to charge the first and second receiver devices  104 ,  106  only if the receiver devices  104 ,  106  are present at a proximate location. Further, the wireless charging device  102  is configured to charge both the first and second receiver devices  104 ,  106  having different frequency standards, thereby obviating the need for separate charging devices for charging receiver devices that adhere to different frequency standards. In particular, the wireless charging device  102  includes a control unit  112  configured to detect the first and second receiver devices  104 ,  106 . 
     In addition to the control unit  112 , the wireless charging device  102  includes a power source  108 , a driver unit  110 , and a transmitting unit  114 . The driver unit  110  is coupled to the power source  108 , the control unit  112 , and the transmitting unit  114 . The power source  108  is used to supply input power having a DC voltage  120  to the driver unit  110 . In one embodiment, the input power may be in a range from about 1 Watt to about 200 Watts. In one embodiment, the power source  108  may be a part of the wireless charging device  102 . In another embodiment, the power source  108  may be positioned external to the wireless charging device  102 . It may be noted that the input power having the DC voltage  120  may also be alternatively referred to as a “DC voltage signal” in the following description. 
     The control unit  112  is configured to generate a first control signal  122  and a second control signal  124 . In particular, prior to detecting the first and second receiver devices  104 ,  106 , the control unit  112  is configured to repeatedly and alternately generate the first control signal  122  and the second control signal  124 . In one embodiment, the first control signal  122  is generated for a first time period and the second control signal  124  is generated for a second time period after a predetermined time interval from the first time period. In one example, the first and second time periods are in a range from about 5 milliseconds to about 100 milliseconds. The predetermined time interval is in a range from about 5 milliseconds to about 500 milliseconds. It may be noted that the first and second control signals  122 ,  124  may be generated in any desired pattern, and is not limited to the pattern of “repeatedly and alternately” generating the first and second control signals  122 ,  124  as mentioned above. Further, if the first receiver device  104  is detected, the control unit  112  continuously generates the first control signal  122  until the presence of the first receiver device  104  is detected. Similarly, if the second receiver device  106  is detected, the control unit  112  continuously generates the second control signal  124  until the presence of the second receiver device  106  is detected. 
     If both the first and second receiver devices  104 ,  106  are detected, the control unit  112  alternately and repeatedly communicates the first control signal  122  and the second control signal  124  to transmit electric power to both the first and second receiver devices  104 ,  106 . It may be noted that the first and second control signals  122 ,  124  may be communicated in any desired pattern, and is not limited to the pattern of “repeatedly and alternately” communicating the first and second control signals  122 ,  124  as mentioned above. For ease of understanding of the specification, the embodiment of detecting one of the first and second receiver devices  104 ,  106  and communicating the first control signal  122  or the second control signal  124  is discussed in the following description. 
     Further, the driver unit  110  is configured to receive the DC voltage signal  120  from the power source  108 . Additionally, the driver unit  110  is configured to receive the first control signal  122  or the second control signal  124  from the control unit  112 . The driver unit  110  is configured to transform the DC voltage signal  120  based on the first control signal  122  or the second control signal  124  received from the control unit  112 . By way of example, if the first control signal  122  is received from the control unit  112 , the driver unit  110  is configured to convert the DC voltage signal  120  to a first AC voltage signal  126  having a first frequency. It may be noted that the first frequency may be one of the frequencies corresponding to a frequency band associated with the first frequency standard. In one embodiment, the first frequency may be about 6.8 MHz. In a similar manner, if the second control signal  124  is received from the control unit  112 , the driver unit  110  is configured to convert the DC voltage signal  120  to a second AC voltage signal  128  having a second frequency. It may be noted that the second frequency may be one of the frequencies corresponding to a frequency band associated with the second frequency standard. In one embodiment, the second frequency may be about 200 kHz. Furthermore, in one embodiment, magnitudes of the first and second AC voltage signals  126 ,  128  may be in a range from about 5 Volts to about 50 Volts. It may be noted that the first AC voltage signal  126  may be referred as a “high frequency signal” and the second AC voltage signal  128  may be referred as a “low frequency signal.” Further, the driver unit  110  is configured to transmit the first AC voltage signal  126  or the second AC voltage signal  128  to the transmitting unit  114 . 
     The transmitting unit  114  is configured to wirelessly transmit the first AC voltage signal  126  or the second AC voltage signal  128  to the first and second receiver devices  104 ,  106 . It may be noted that the electric power for charging the first and second receiver devices  104 ,  106 , is transmitted in the form of the first AC voltage signal  126  or the second AC voltage signal  128  to the first and second receiver devices  104 ,  106 . As depicted in  FIG. 1 , the transmitting unit  114  includes a first coil  130 , a first capacitor  132 , a second coil  134 , and a second capacitor  136  that are coupled to the driver unit  110 . It may be noted that in other embodiments, the transmitting unit  114  may include a plurality of first coils  130  and a plurality of second coils  1348 . Further, the first coil  130  and the first capacitor  132  are tuned to the first frequency or proximate to the first frequency to transmit the first AC voltage signal  126  having the first frequency. In a similar manner, the second coil  134  and the second capacitor  136  are tuned to the second frequency or proximate to the second frequency to transmit the second AC voltage signal  128  having the second frequency. 
     The first receiver device  104  includes a first receiving unit  140 , a first rectifier unit  142 , a first load  144 , and a first receiver confirmation unit  146 . Similarly, the second receiver device  106  includes a second receiving unit  148 , a second rectifier unit  150 , a second load  152 , and a second receiver confirmation unit  154 . Also, as previously noted, the first and second receiver devices  104 ,  106  are configured to receive electric power in the form of the first AC voltage signal  126  or the second AC voltage signal  128  to charge the first load  144  or the second load  152 . The first and second loads  144  or  152  may be batteries. Alternatively, the first and second loads  144 ,  152  may be passive loads like resistive loads or other kinds of active loads. 
     Further, the first receiving unit  140  includes a first receiver coil  156  and a first receiver capacitor  158  that are tuned to the first frequency to receive the first AC voltage signal  126  having the first frequency from the wireless charging device  102 . In particular, the first receiver coil  156  is wirelessly coupled to the first coil  130  in the wireless charging device  102 . If the driver unit  110  generates the first AC voltage signal  126  having the first frequency, the first coil  130  is excited to transmit the first AC voltage signal  126  having the first frequency to the first receiver coil  156  in the first receiver device  104 . In one embodiment, the first coil  130  generates a magnetic field that is corresponding to the first AC voltage signal  126 . Further, the generated magnetic field is received by the first receiver coil  156  in the first receiver device  104  in the form of the first AC voltage signal  126 . The first rectifier unit  142  in the first receiver device  104  is configured to receive the first AC voltage signal  126  from the first receiver coil  156  and convert the first AC voltage signal  126  to a first DC load signal  160 . The first DC load signal  160  is used to charge the first load  144 , such as a battery in the first receiver device  104 . 
     In one exemplary embodiment, the first receiver confirmation unit  146  is configured to change an impedance across the first receiving unit  140  at regular time intervals to confirm a presence of the first receiver device  104  to the wireless charging device  102 . More specifically, the first receiver confirmation unit  146  receives the first DC load signal  160  from the first rectifier unit  142 . In one embodiment, the first DC load signal  160  is used to activate or switch ON the first receiver confirmation unit  146 . In response to receiving the first DC load signal  160 , the first receiver confirmation unit  146  changes the impedance across the first receiving unit  140 . The change in impedance may cause a first voltage at the first coil  130  to change accordingly. The first voltage may be referred to as a magnitude or a peak voltage of the first AC voltage signal  126  at the first coil  130 . Further, the control unit  112  monitors the change in the first voltage and detects the first receiver device  104 , if a change in the first voltage at the first coil  130  is greater than a first threshold value. Upon detecting the first receiver device  104 , the control unit  112  continuously generates the first control signal  122  to drive the driver unit  110  to continuously transmit electric power in the form of the first AC voltage signal  126  to the first receiver device  104 . Further, the driver unit  110  transmits the first AC voltage signal  126  until the presence of the first receiver device  104  is detected. The aspect of detecting the first receiver device  104  and transmitting the electric power to the first receiver device  104  is described in greater detail with reference to  FIG. 2 . 
     In a similar manner, the second receiving unit  148  includes a second receiver coil  162  and a second receiver capacitor  164  that are tuned to the second frequency to receive the second AC voltage signal  128  having the second frequency from the wireless charging device  102 . In particular, the second receiver coil  162  is wirelessly coupled to the second coil  134  in the wireless charging device  102 . If the driver unit  110  generates the second AC voltage signal  128  having the second frequency, the second coil  134  is excited to transmit the second AC voltage signal  128  having the second frequency to the second receiver coil  162  in the second receiver device  106 . In one embodiment, the second coil  134  generates a magnetic field that is corresponding to the second AC voltage signal  128 . Further, the generated magnetic field is received by the second receiver coil  162  in the second receiver device  106  in the form of the second AC voltage signal  128 . The second rectifier unit  150  in the second receiver device  106  is configured to receive the second AC voltage signal  128  from the second receiver coil  162  and convert the second AC voltage signal  128  to a second DC load signal  166 . The second DC load signal  166  is used to charge the second load  152 , such as a battery in the second receiver device  106 . 
     In one exemplary embodiment, the second receiver confirmation unit  154  is configured to change an impedance across the second receiving unit  148  at regular time intervals to confirm a presence of the second receiver device  106  to the wireless charging device  102 . More specifically, the second receiver confirmation unit  154  receives the second DC load signal  166  from the second rectifier unit  150 . In one embodiment, the second DC load signal  166  is used to activate or switch ON the second receiver confirmation unit  154 . In response to receiving the second DC load signal  166 , the second receiver confirmation unit  154  changes the impedance across the second receiving unit  148 . The change in the impedance causes a second voltage at the second coil  134  to change accordingly. The second voltage may be to referred as a magnitude or a peak voltage of the second AC voltage signal  128  at the second coil  134 . Further, the control unit  112  monitors the change in the second voltage and detects the second receiver device  106 , if the change in the second voltage at the second coil  134  is greater than a second threshold value. Upon detecting the second receiver device  106 , the control unit  112  continuously generates the second control signal  124  to drive the driver unit  110  to continuously transmit electric power in the form of the second AC voltage signal  128  to the second receiver device  106 . Further, the driver unit  110  transmits the second AC voltage signal  128  until the presence of the second receiver device  106  is detected. The aspect of confirming the presence of the second receiver device  106  and transmitting the electric power to the second receiver device  106  is described in greater detail with reference to  FIG. 2 . 
     In accordance with the exemplary embodiment, the wireless power transfer system  100  allows transmission of the electric power continuously to the first and second receiver devices  104 ,  106  only after detecting the presence of the first and second receiver devices  104 ,  106 . As a result, power loss in the wireless power transfer system  100  is reduced and efficiency of the wireless charging device  102  is enhanced. Further, the exemplary wireless charging device  102  allows wireless transmission/transfer of electric power to the first and second receiver devices  104 ,  106  that are compatible with one or more frequency standards. Accordingly, use of a single wireless charging device enables charging of a plurality of receiver devices that conform to different frequency standards, thereby obviating the need of a plurality of charging devices. 
     Referring to  FIG. 2 , a schematic representation of the wireless power transfer system  100  in accordance with certain embodiments of the present invention is depicted. The wireless power transfer system  100  includes the wireless charging device  102  that is capable of magnetically coupling to a receiver device  202  for performing wireless power transmission to the receiver device  202 . The receiver device  202  may be similar to the first receiver device  104  or the second receiver device  106  shown in the embodiment of  FIG. 1 . 
     As discussed earlier, the wireless charging device  102  includes the power source  108 , the driver unit  110 , the transmitting unit  114 , and the control unit  112 . The driver unit  110  includes a first leg of switches  206  and a second leg of switches  208  that form a bridge circuit between first terminals  210  and second terminals  212  of the driver unit  110 . The first leg of switches  206  and the second leg of switches  208  are activated or deactivated based on the first and second control signals  122 ,  124  received from the control unit  112  to generate the first AC voltage signal  126  or the second AC voltage signal  128 . 
     The transmitting unit  114  includes the first coil  130  and the first capacitor  132  that are coupled in series to each other and electrically coupled to the second terminals  212  of the driver unit  110 . In addition, the transmitting unit  114  includes the second coil  134  and the second capacitor  136  that are coupled in series to each other and electrically coupled to the second terminals  212  of the driver unit  110 . 
     The control unit  112  includes a first peak detecting unit  214 , a first comparator  216 , a second peak detecting unit  218 , a second comparator  220 , and a detector  222 . The first peak detecting unit  214  is electrically coupled to a first junction  224  between the first coil  130  and the first capacitor  132  in the transmitting unit  114 . The first peak detecting unit  214  includes a diode  226 , a capacitor  228 , and a resistor network  230 , configured to measure the first voltage at the first junction  224  in the transmitting unit  114 . Further, the first comparator  216  is electrically coupled to the first peak detecting unit  214  and configured to receive the measured first voltage from the first peak detecting unit  214 . The first comparator  216  includes resistors  232 , a reference voltage source  234 , and an opamp-comparator unit  236 , configured to determine whether a change in the first voltage at the first junction  224  is greater than a first threshold value. In one embodiment, a voltage of the reference voltage source  234  may be set to the first threshold value. Further, the opamp-comparator unit  236  is configured to compare the first voltage received from the first peak detecting unit  214  with the first threshold value of the reference voltage source  234  to determine whether the change in the first voltage is greater than the first threshold value. 
     In a similar manner, the second peak detecting unit  218  is electrically coupled to a second junction  240  between the second coil  134  and the second capacitor  136 . The second peak detecting unit  218  includes a diode  242 , a capacitor  244 , and a resistor network  246 , configured to measure a second voltage at the second junction  240  in the transmitting unit  114 . Further, the second comparator  220  is electrically coupled to the second peak detecting unit  218  and configured to receive the measured second voltage from the second peak detecting unit  218 . The second comparator  220  includes resistors  248 , a reference voltage source  250 , and an opamp-comparator unit  252 , configured to determine whether the change in the second voltage at the second junction  240  is greater than a second threshold value. In one embodiment, a voltage of the reference voltage source may be set to the second threshold value. Further, the opamp-comparator unit  252  is configured to compare the second voltage received from the second peak detecting unit  218  with the second threshold value of the reference voltage source  250  to determine whether the change in the second voltage is greater than the second threshold value. 
     Further, the detector  222  is electrically coupled to the first comparator  216  and the second comparator  220  and configured to generate the first control signal  122  or the second control signal  124 . More specifically, if the change in the first voltage is greater than the first threshold value, the first comparator  216  sends a signal to the detector  222  to generate the first control signal  122 . Similarly, if the change in the second voltage is greater than the second threshold value, the second comparator  220  sends a signal to the detector  222  to generate the second control signal  124 . The detector  222  communicates the first control signal  122  or the second control signal  124  to the driver unit  110  to convert the DC voltage signal  120  to the first AC voltage signal  126  or the second AC voltage signal  128 . 
     As depicted in  FIG. 2 , the receiver device  202  includes a receiving unit  256 , a rectifier unit  258 , a load  260 , and a receiver confirmation unit  262 . The receiving unit  256  includes a receiver coil  264  and a receiver capacitor  266  that are coupled to each other and configured to receive the first AC voltage signal  126  or the second AC voltage signal  128  from the wireless charging device  102 . More specifically, if the receiver coil  264  and the receiver capacitor  266  are tuned to the first frequency, the receiving unit  256  receives the first AC voltage signal  126  having the first frequency from the first coil  130  in the wireless charging device  102 . Similarly, if the receiver coil  264  and the receiver capacitor  266  are tuned to the second frequency, the receiving unit  256  receives the second AC voltage signal  128  having the second frequency from the second coil  134  in the wireless charging device  102 . Moreover, if the receiver coil  264  and the receiver capacitor  266  are tuned to one frequency i.e., the first frequency, the receiver coil  264 , and the receiver capacitor  266  generates a high impedance to the AC voltage signal having other frequency i.e., the second frequency. As a result, flow of current at the other frequency i.e., the second frequency, is restricted in the receiver device  202 . 
     Further, the rectifier unit  258  is electrically coupled to the receiving unit  256  and configured to receive the first AC voltage signal  126  or the second AC voltage signal  128  from the receiving unit  256 . The rectifier unit  258  includes diodes  268  configured to convert the first AC voltage signal  126  or the second AC voltage signal  128  to the DC load signal  270 . Further, the DC load signal  270  is transmitted to the load  260 , such as a battery in the receiver device  202 . 
     In the exemplary embodiment, the receiver confirmation unit  262  includes a pulse generator  272  and a switching sub-unit  274  that are electrically coupled to each other. The pulse generator  272  is coupled to the rectifier unit  258  via a diode  290  and a capacitor  292  and is configured to receive the DC load signal  270 . In response to receiving the DC load signal  270 , the pulse generator  272  generates one or more signal pulses  276 . In one example, the pulse generator  272  may be a timer that is configured to transmit signal pulses  276  at regular intervals when activated or switched ON. The DC load signal  270  is used to activate or switch ON the pulse generator  272 . 
     Further, the switching sub-unit  274  is coupled in parallel to the rectifier unit  258  and configured to receive the signal pulses  276  from the pulse generator  272 . The switching sub-unit  274  includes an impedance element (Z)  278  and a switch  280  that are coupled in series to each other. In one embodiment, the impedance element (Z)  278  may include a resistor, a capacitor, or a combination thereof. In one embodiment, the switch  280  may be an electronic switch, such as a metal-oxide-semiconductor field-effect transistor (MOSFET). The switch  280  is closed when the signal pulses  276  are received from the pulse generator  272 . Further, when the switch  280  is closed, the impedance element (Z)  278  is coupled across the rectifier unit  258  to change the impedance across the receiving unit  256 . Further, the switch  280  is opened when the signal pulses  276  are not received from the pulse generator  272 . In one embodiment, the switch  280  is closed for a predefined time period after every selected time period. In one example, the predefined time period is in a range from about 0.1 millisecond to about 20 milliseconds. The selected time period between two consecutive predefined time periods may be in a range from about 0.1 second to about 5 seconds. 
     During operation, the control unit  112  repeatedly and alternately generates the first control signal  122  and the second control signal  124 . In particular, prior to detecting the receiver device  202 , the control unit  112  generates the first control signal  122  for a first time period and the second control signal  124  for a second time period. In one embodiment, the first and second time periods are in a range from about 5 milliseconds to about 100 milliseconds. In one embodiment, the second control signal  124  is generated after the predetermined time interval from the first time period. It may be noted that this predetermined time interval is also referred to as a third predetermined time interval in the below description. In one embodiment, the third predetermined time interval is in a range from about 5 milliseconds to about 500 milliseconds. Also, generation of one set of first and second AC voltage signals  126 ,  128  and another consecutive set of first and second AC voltage signals  126 ,  128  are separated by a fourth predetermined time interval. In one embodiment, the fourth predetermined time interval is in a range from about 500 milliseconds to about 10 seconds. 
     The driver unit  110  is configured to receive the DC voltage signal  120  from the power source  108 . Additionally, the driver unit  110  is configured to receive the first control signal  122  or the second control signal  124  from the control unit  112 . The first control signal  122  is generated for the first time period. Hence, the driver unit  110  converts the DC voltage signal  120  to the first AC voltage signal  126  having the first frequency and transmits the first AC voltage signal  126  for the first time period via the first coil  130  in the transmitting unit  114 . If the first receiver  104  is not detected, the second control signal  124  is generated for the second time period. The driver unit  110  converts the DC voltage signal  120  to the second AC voltage signal  128  having the second frequency and transmits the second AC voltage signal  128  for the second time period via the second coil  134  in the transmitting unit  114 . Further, the control unit  112  monitors the change in the first voltage at the first junction  224  and the change in the second voltage at the second junction  240  of the transmitting unit  114 . 
     In one embodiment, if the receiver device  202  is compatible with the first frequency standard, the receiver coil  264  in the receiver device  202  receives the first AC voltage signal  126  having the first frequency from the first coil  130  in the wireless charging device  102 . Further, the rectifier unit  258  converts the first AC voltage signal  126  to the DC load signal  270  and transmits the DC load signal  270  to the load  260  in the receiver device  202 . Also, the rectifier unit  258  transmits the DC load signal  270  to the receiver confirmation unit  262  to activate or switch ON the pulse generator  272  in the receiver confirmation unit  262 . Further, the pulse generator  272  transmits one signal pulse  276  to the switching sub-unit  274  to close the switch  280  in the switching sub-unit  274  for a predefined time period. More specifically, the signal pulse  276  is transmitted to the switching sub-unit  274  to communicate to the wireless charging device  102  that the receiver device  202  is present and the receiver device  202  is capable of receiving the first AC voltage signal  126  from the wireless charging device  102 . More signal pulses  276  are transmitted to the switch  280  for a predefined time period after every selected time period. In one example, the predefined time period is in a range from about 0.1 milliseconds to about 20 milliseconds. The selected time period between two consecutive predefined time periods may be in a range from about 0.1 seconds to about 5 seconds. 
     Further, when the switch  280  is closed for the predefined time period, the impedance across the receiver coil  264  is changed to below a predefined value. Consequently, the first voltage at the first junction  224  between the first coil  130  and the first capacitor  132  is increased. More specifically, when the switch  280  is closed for the predefined time period, the change in the first voltage is greater than the first threshold value for the predefined time period. 
     The control unit  112  monitors the change in the first voltage at the first junction  224 . Further, the control unit  112  detects the receiver device  202  if the change in the first voltage at the first junction  224  is greater than the first threshold value. More specifically, the control unit  112  determines that the receiver device  202  is positioned within a predetermined distance from the wireless charging device  102 . In one embodiment, the predetermined distance is in a range from about 2 mm to about 10 mm. In some instances, a foreign object may be positioned proximate to the wireless charging device  102 . As a result, the change in the first voltage at the first junction  224  may increase to greater than the first threshold value. Such a change in the first voltage to greater than the first threshold value may occur for more than the predefined time period. The control unit  112  verifies a duration of the change in the first voltage at the first junction  224  to distinguish the receiver device  202  from the foreign object. If the change in the first voltage to greater than the first threshold value occurs for only the predefined time period, the control unit  112  confirms that the receiver device  202  is present. Otherwise, the control unit  112  determines that the foreign object is present and stops transmitting the electric power in the form of the first AC voltage signal  126  or the second AC voltage signal  128 . In one embodiment, the control unit  112  is configured to control the driver unit  110  to stop transmitting the first AC voltage signal  126  or the second AC voltage signal  128 . It may be noted that the foreign object may be any electrically conductive element/component other than the receiver device  202 . In one example, the foreign object may include a metal sheet, a coin, or the like. 
     Upon detecting the receiver device  202 , the control unit  112  generates the first control signal  122  continuously so that the driver unit  110  continuously transmits the first AC voltage signal  126  as the electric power to the receiver device  202 . At the receiver device  202 , the rectifier unit  258  continues to convert the first AC voltage signal  126  to the DC load signal  270  which is further transmitted to the load  260  in the receiver device  202 . Further, the rectifier unit  258  continuously transmits the DC load signal  270  to the receiver confirmation unit  262  until the first AC voltage signal  126  is received from the wireless charging device  102 . In one embodiment, the rectifier unit  258  continuously transmits the DC load signal  270  to the receiver confirmation unit  262  to communicate the presence of the receiver device  202  to the wireless charging device  102 . More specifically, the pulse generator  272  is in active state until the DC load signal  270  is received from the rectifier unit  258 . The pulse generator  272  repeatedly transmits the signal pulses  276  to the switch  280  after every selected time period to close the switch  280  for the predefined time period. As a result, the change in the impedance across the receiving unit  256  is less than the predefined value, which in-turn increases the first voltage at the first junction  224  after every selected time period for only the predefined time period. 
     Further, the control unit  112  continuously monitors the change in the first voltage at the first junction  224  after detecting the receiver device  202 . In particular, the control unit  112  monitors the change in the first voltage at the first junction  224  for at least once in a first predetermined time interval from the previous detection or confirmation instance of the receiver device  202 . For example, a duration of the first predetermined time interval may be greater than a duration of the selected time period. In one example, the first predetermined time interval is in a range from about 500 milliseconds to about 5000 milliseconds. The control unit  112  confirms the presence of the receiver device  202  until the change in the first voltage is greater than the first threshold value for at least once in the first predetermined interval. Further, the control unit  112  drives the driver unit  110  to continuously transmit the first AC voltage signal  126  to the receiver device  202  until the presence of the receiver device  202  is detected. If the change in the first voltage is not greater than the first threshold value for at least once in the first predetermined interval, the control unit  112  confirms that the receiver device  202  is not and controls the driver unit  110  to stop transmitting the first AC voltage signal  126 . 
     In another embodiment, if the receiver device  202  is compatible with the second frequency standard, the receiver coil  264  in the receiver device  202  receives the second AC voltage signal  128  having the second frequency from the second coil  134  in the wireless charging device  102 . Further, the rectifier unit  258  converts the second AC voltage signal  128  to the DC load signal  270 . The rectifier unit  258  transmits the DC load signal  270  to the load  260  and the receiver confirmation unit  262  in the receiver device  202 . 
     As discussed above, the receiver confirmation unit  262  changes the impedance across the receiving unit  256  for the predefined time period after every selected time period. The change in the impedance across the receiving unit  256  changes the second voltage at the second junction  240  in the transmitting unit  114 . Further, the control unit  112  detects the receiver device  202  if the change in the second voltage at the second junction  240  is greater than the second threshold value for only the predefined time period. 
     Upon detecting the receiver device  202 , the control unit  112  generates the second control signal  124  continuously so that the driver unit  110  continuously transmits the second AC voltage signal  128  to the receiver device  202 . At the receiver device  202 , the rectifier unit  258  continuously transmits the DC load signal  270  to the receiver confirmation unit  262  until the second AC voltage signal  128  is received from the wireless charging device  102 . Further, the pulse generator  272  is in active state until the DC load signal  270  is received from the rectifier unit  258 . The pulse generator  272  repeatedly transmits the signal pulses  276  to the switch  280  after every selected time period to close the switch  280  for the predefined time period. As a result, the change in the impedance across the receiving unit  256  is less than the predefined value, which in-turn increases the second voltage at the second junction  240  after every selected time period for only the predefined time period. 
     Further, the control unit  112  repeatedly monitors the change in the second voltage at the second junction  240  after detecting the receiver device  202 . In particular, the control unit  112  monitors the change in the second voltage at the second junction  240  for at least once in a second predetermined time interval from the previous detection or confirmation instance of the receiver device  202 . The second predetermined time interval may be in a range from about 500 milliseconds to about 5000 milliseconds. The control unit  112  confirms a presence of the receiver device  202  until the change in the second voltage is greater than the second threshold value for at least once in the second predetermined interval. The control unit  112  drives the driver unit  110  to continuously transmit the second AC voltage signal  128  to the receiver device  202 . If the change in the second voltage is not greater than the second threshold value for at least once in the second predetermined interval, the control unit  112  confirms that the receiver device  202  is not present. As a result, the control unit  112  controls the driver unit  110  to stop transmitting the second AC voltage signal  128 . 
     In one embodiment, the control unit  112  is configured to detect misalignment of the receiver device  202  with reference to the wireless charging device  102  based on the first voltage and/or the second voltage in the wireless charging device  102 . More specifically, the control unit  112  monitors the change in the first voltage and the second voltage in the transmitting unit  114  of the wireless charging device  102 . If the change in the first voltage or the change in the second voltage is less than a predefined misalignment value, the control unit  112  confirms that the receiver device  202  is misaligned with reference to the wireless charging device  102 . 
     Referring to  FIG. 3 , a graphical representation  300  of a first AC voltage signal  126  and a second AC voltage signal  128  in accordance with certain embodiments of the present invention is depicted. Reference numeral  302  represents a scenario of transmitting the first and second AC voltage signals  126 ,  128  prior to detecting a receiver device. In particular, the first and second AC voltage signals  126 ,  128  are repeatedly and alternately transmitted from the wireless charging device. The first AC voltage signal  126  is transmitted for a first time period  304  and the second AC voltage signal  128  is transmitted for a second time period  306 . The second time period  306  is after a third predetermined time interval  308  from the first time period  304 . The reference numeral  310  represents a fourth predetermined time interval between generation of one set of first and second AC voltage signals  126 ,  128  and another set of first and second AC voltage signals  126 ,  128 . 
     Further, reference numeral  312  represents a scenario of transmitting the first AC voltage signal  126  after detecting the receiver device that is compatible with the first frequency standard. The first AC voltage signal  126  is continuously transmitted to the receiver device until the presence of the receiver device is detected. More specifically, after transmitting the first AC voltage signal  126  for the first time period  304 , the control unit monitors a change in the first voltage at the first junction  224  in the transmitting unit. If the change in the first voltage is greater than the first threshold value, within the first time period  304 , the control unit confirms that the receiver device is present. Further, the control unit repeatedly monitors the change in the first voltage at the first junction after detecting the receiver device. Further, the control unit confirms the presence of the receiver device until the change in the first voltage is greater than the first threshold value at least once in a first predetermined interval  314  from the previous confirmation instance of the receiver device  202 . If the receiver device is not present, the change in the first voltage is not increased to greater than the first threshold value for more than the first predetermined interval  314  from the previous confirmation of the receiver device. The control unit controls the driver unit to stop continuous transmission of the first AC voltage signal  126 . 
     Further, reference numeral  324  represents a scenario of transmitting the second AC voltage signal  128  after detecting the receiver device that is compatible with the second frequency standard. The second AC voltage signal  128  is continuously transmitted to the receiver device until the presence of the receiver device is detected. More specifically, after transmitting the second AC voltage signal  128  for the second time period  306 , the control unit monitors a change in the second voltage at the second junction in the transmitting unit. If the change in the second voltage is greater than the second threshold value, within the second time period  306 , the control unit confirms that the receiver device is present. Further, the control unit repeatedly monitors the change in the second voltage at the second junction after detecting the receiver device. Further, the control unit confirms the presence of the receiver device until the change in the second voltage is greater than the second threshold value at least once in a second predetermined interval  316  from the previous detection or confirmation instance of the receiver device. If the receiver device  202  is not present, the change in the second voltage is not increased to greater than the second threshold value for more than the second predetermined interval  316  from the previous confirmation instance of the receiver device. As a result, the control unit controls the driver unit to stop continuous transmission of the second AC voltage signal  128 . 
       FIG. 4  is a flow chart illustrating a method  400  for detecting a receiver device in accordance with certain embodiments of the present invention. At step  402 , one of a first alternating current (AC) voltage signal having a first frequency and a second alternating current (AC) voltage signal having a second frequency is generated by a driver unit. In particular, if a first control signal is received from the control unit, the driver unit converts a DC voltage signal to the first AC voltage signal having the first frequency. Similarly, if a second control signal is received from the control unit, the driver unit converts the DC voltage signal to the second AC voltage signal having the second frequency. 
     Subsequently, at step  404 , the first AC voltage signal having the first frequency is transmitted by a first coil and a first capacitor in a transmitting unit. More specifically, the first coil and the first capacitor in the transmitting unit are tuned to the first frequency. Further, when the driver unit converts the DC voltage signal to the first AC voltage signal having the first frequency, the first coil is excited to generate a magnetic field that is associated with the first AC voltage signal having the first frequency. The generated magnetic field enables to wirelessly transmit the first AC voltage signal to the receiver device that is compatible with the first frequency standard. 
     In addition, at step  406 , the second AC voltage signal having the second frequency is transmitted by a second coil and a second capacitor in a transmitting unit. More specifically, the second coil and the second capacitor in the transmitting unit are tuned to the second frequency. Further, when the driver unit converts the DC voltage signal to the second AC voltage signal having the second frequency, the second coil is excited to generate a magnetic field that is associated with the second AC voltage signal having the second frequency. The generated magnetic field enables to wirelessly transmit the second AC voltage signal to the receiver device that is compatible with the first frequency standard. 
     Further, at step  408 , the first receiver device is detected by the control unit based on a change in the first voltage at the first junction between the first coil and the first capacitor in the transmitting unit. In particular, when the first receiver device receives the first AC voltage signal, the first receiver confirmation unit in the first receiver device changes the impedance across the first receiving unit. The change in impedance causes a first voltage at the first coil to change accordingly. Further, the control unit monitors the first voltage and detects the first receiver device, if the change in the first voltage at the first coil is greater than a first threshold value. Upon detecting the first receiver device, the control unit continuously generates the first control signal to drive the driver unit to continuously transmit the electric power in the form of the first AC voltage signal to the first receiver device. Further, the driver unit transmits the first AC voltage signal until the presence of the first receiver device is detected. 
     Further, at step  410 , the second receiver device is detected by the control unit based on a change in a second voltage at a second junction between the second coil and the second capacitor in the transmitting unit. In particular, when the second receiver device receives the second AC voltage signal, the second receiver confirmation unit in the second receiver device changes the impedance across the second receiving unit. The change in impedance causes a second voltage at the second coil to change accordingly. Further, the control unit monitors the second voltage and detects the second receiver device, if the change in the second voltage at the second coil is greater than a second threshold value. Upon detecting the second receiver device, the control unit continuously generates the second control signal to drive the driver unit to continuously transmit the electric power in the form of the second AC voltage signal to the second receiver device. The driver unit transmits the second AC voltage signal until the presence of the second receiver device is detected. It may be noted that the steps  408  and  410  are interchangeable. Also, it may be noted that any instance one of the steps  408 ,  410  is performed. For example, if the step  408  is performed then the step  410  is not performed. 
     The various embodiments of the exemplary system and method described hereinabove discloses transmitting voltage signals having different frequencies, thereby enabling charging of wireless receiver devices operating at different frequency standards. In addition, the exemplary system and method described hereinabove discloses transmitting the electric power only if the receiver device is detected. As a result, power loss of the wireless power transfer system is reduced and efficiency is enhanced. Also, by reducing the power loss, efficiency of the wireless charging device may be substantially improved. 
     While only certain features of the present disclosure have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the present specification.