Dynamic capacitance tuning for wireless power transfer

Abstract

The invention relates to dynamic capacitance tuning for wireless power transfer. A method of operating a wireless power transmitter may be performed by a control circuit of the wireless power transmitter, the wireless power transmitter has a wireless power transmitting coil, an inverter receiving an input voltage and generating an AC output voltage driving the wireless power transmitting coil, and at least one adjustable capacitor coupled to the wireless power transmitting coil to form a series tuned circuit with the wireless power transmitting coil, the method may include, in response to an increased power demand from the wireless power receiver, determining whether the input voltage is at a maximum value, and if so, decreasing the at least one adjustable capacitance to increase a gain; or in response to a reduced power demand from the wireless power receiver, determining whether the input voltage is at a minimum value, and if so, increasing the at least one adjustable capacitance to reduce the gain.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 585,279, entitled “Dynamic Capacitance Tuning For Wireless Power Transfer,” filed on September 26, 2023, U.S. Patent Application No. 18 / 529,564, entitled “Dynamic Capacitance Tuning For Wireless Power Transfer,” filed on December 5, 2023, and U.S. Patent Application No. 18 / 613,196, entitled “Dynamic Capacitance Tuning For Wireless Power Transfer,” filed on March 22, 2024, each of which is incorporated herein by reference in its entirety for all purposes. Background Art

[0003] Wireless power transfer is becoming increasingly popular in a wide variety of electronic devices. For example, many electronic devices such as smart phones, tablet computers, smart watches, wireless headphones, styluses, etc. may employ wireless power transfer to facilitate charging of batteries within the device. In some applications, higher levels of wireless power transfer may be desired, for example to provide faster charging. Such higher power transfer levels may benefit from techniques that tune various system parameters to improve operating efficiency, voltage regulation, etc. Summary of the invention

[0004] A wireless power transmitter may include: a wireless power transmission coil; an inverter that receives an input voltage and generates an AC output voltage that drives the wireless power transmission coil; at least one adjustable capacitor that is coupled to the wireless power transmission coil to form a series tuned circuit with the wireless power transmission coil; and a control circuit that operates the inverter and the at least one adjustable capacitor to deliver wireless power to a wireless power receiver, wherein the control circuit responds to a power demand from the wireless power receiver by: determining whether the input voltage is at a maximum value, and if so, reducing the at least one adjustable capacitor to increase gain; or determining whether the input voltage is at a minimum value, and if so, increasing the at least one adjustable capacitor to reduce gain.

[0005] The at least one adjustable capacitance may include one or more capacitors that can be selectively coupled to the wireless power transmission coil through one or more switches controlled by the control circuit. The at least one adjustable capacitance may include at least one capacitor coupled to the wireless power transmission coil and at least two capacitors that can be selectively coupled to the wireless power transmission coil through one or more switches controlled by the control circuit. Determining whether the input voltage is at a maximum value, and if so, reducing the at least one adjustable capacitance to increase the gain may also include at least temporarily adjusting the input voltage before reducing the at least one adjustable capacitance. At least temporarily adjusting the input voltage before reducing the at least one adjustable capacitance may be responsive to the operating power level of the wireless power transmitter. Determining whether the input voltage is at a minimum value, and if so, increasing the at least one adjustable capacitance to reduce the gain may also include at least temporarily adjusting the input voltage before increasing the at least one adjustable capacitance. At least temporarily adjusting the input voltage before increasing the at least one adjustable capacitance may be responsive to the operating power level of the wireless power transmitter.

[0006] The wireless power transmitter may also include a control circuit that increases the at least one adjustable capacitance in response to one or more hard switching events to mitigate the hard switching events. The at least one adjustable capacitance may include: one or more capacitors that can be selectively coupled between the inverter output and the wireless power transmission coil through one or more first switches controlled by the control circuit; and one or more capacitors that can be selectively coupled between the inverter output and the ground through one or more second switches controlled by the control circuit. The one or more capacitors that can be selectively coupled between the inverter output and the wireless power transmission coil may include at least one capacitor coupled between the inverter output and the wireless power transmission coil and at least two capacitors that can be selectively coupled between the inverter output and the wireless power transmission coil through the one or more first switches. The one or more capacitors that can be selectively coupled between the inverter output and the ground may include at least one capacitor coupled between the inverter output and the ground and multiple capacitors that can be selectively coupled between the inverter output and the ground through the one or more second switches. The wireless power transmitter may further include a control circuit that, in response to an absence of one or more hard switching events, reduces the at least one adjustable capacitance to mitigate the hard switching events.

[0007] A method of operating a wireless power transmitter may be performed by a control circuit of the wireless power transmitter, the wireless power transmitter having a wireless power transmission coil, an inverter that receives an input voltage and generates an AC output voltage that drives the wireless power transmission coil, and at least one adjustable capacitor coupled to the wireless power transmission coil to form a series tuned circuit with the wireless power transmission coil, and the method may include: determining whether the input voltage is at a maximum value in response to an increased power demand from a wireless power receiver, and if so, reducing the at least one adjustable capacitor to increase gain; or determining whether the input voltage is at a minimum value in response to a decreased power demand from the wireless power receiver, and if so, increasing the at least one adjustable capacitor to reduce gain.

[0008] Determining whether the input voltage is at a maximum value, and if so, reducing the at least one adjustable capacitance to increase the gain may also include at least temporarily adjusting the input voltage before reducing the at least one adjustable capacitance. Determining whether the input voltage is at a minimum value, and if so, increasing the at least one adjustable capacitance to decrease the gain may also include at least temporarily adjusting the input voltage before increasing the at least one adjustable capacitance.

[0009] The method may further include increasing the at least one adjustable capacitance to mitigate the hard switching events in response to one or more hard switching events, and decreasing the at least one adjustable capacitance to mitigate the hard switching events in response to the absence of one or more hard switching events.

[0010] A wireless power receiver may include: a wireless power receiving coil; a rectifier that receives an AC voltage induced in the wireless power receiving coil by a wireless power transmitter and generates a DC output voltage therefrom; at least one adjustable capacitor that is coupled to the wireless power receiving coil to form a series tuned circuit with the wireless power receiving coil; and a control circuit that operates the rectifier and the at least one adjustable capacitor to deliver power to a load coupled to the DC output voltage of the rectifier. The control circuit may respond to power demand by determining whether a DC input voltage of an inverter of the wireless power transmitter is at a maximum value, and if so, reducing the at least one adjustable capacitor to increase gain; or determining whether the DC input voltage of the inverter of the wireless power transmitter is at a minimum value, and if so, increasing the at least one adjustable capacitor to reduce gain. The DC input voltage may be determined by the control circuit in response to one or more messages received from the wireless power transmitter.

[0011] The at least one adjustable capacitance may include one or more capacitors that can be selectively coupled to the wireless power receiving coil through one or more switches controlled by the control circuit. The at least one adjustable capacitance may include at least one capacitor coupled between the wireless power receiving coil and the rectifier and at least one capacitor that can be selectively coupled between the wireless power receiving coil and the rectifier through one or more switches controlled by the control circuit.

[0012] Determining whether the DC input voltage of the inverter is at a maximum value, and if so, reducing the at least one adjustable capacitor to increase gain may also include, in response to rectifier power, at least temporarily reducing the rectifier power before reducing the at least one adjustable capacitor. Determining whether the DC input voltage of the inverter is at a minimum value, and if so, increasing the at least one adjustable capacitor to decrease gain may also include, in response to rectifier power, at least temporarily reducing the rectifier power before increasing the at least one adjustable capacitor.

[0013] A method of operating a wireless power receiver may be performed by a control circuit of the wireless power receiver, the wireless power receiver having a wireless power receiving coil, a rectifier that receives an AC voltage induced in the wireless power receiving coil by a wireless power transmitter and generates a DC output voltage therefrom, and at least one adjustable capacitor coupled to the wireless power receiving coil to form a series tuned circuit with the wireless power receiving coil, and the method may include: determining whether a DC input voltage of an inverter of the wireless power transmitter is at a maximum value in response to an increased power demand, and if so, reducing the at least one adjustable capacitor to increase gain; or determining whether the DC input voltage of the inverter of the wireless power transmitter is at a minimum value in response to a decreased power demand, and if so, increasing the at least one adjustable capacitor to reduce gain; wherein the DC input voltage is determined by the control circuit in response to one or more messages received from the wireless power transmitter.

[0014] Determining whether the DC input voltage of the inverter is at a maximum value, and if so, reducing the at least one adjustable capacitor to increase gain may also include, in response to rectifier power, at least temporarily reducing the rectifier power before reducing the at least one adjustable capacitor. Determining whether the DC input voltage of the inverter is at a minimum value, and if so, increasing the at least one adjustable capacitor to decrease gain may also include, in response to rectifier power, at least temporarily reducing the rectifier power before increasing the at least one adjustable capacitor. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A simplified block diagram of a wireless power transfer system is illustrated.

[0016] FIG. 2A to FIG. 2E A schematic diagram of a wireless power transfer system is illustrated.

[0017] FIG. 3A to FIG. 3C A flow chart illustrating a technique for capacitive tuning of a wireless power transmitter is shown.

[0018] Figure 4 A flow chart illustrating a wireless power receiver capacitance tuning technique. DETAILED DESCRIPTION

[0019] In the following description, for the purpose of explanation, many specific details are set forth in order to provide a thorough understanding of the disclosed concepts. As part of this description, some of the drawings in the drawings of the present disclosure represent structures and devices in block diagram form for simplicity. For the sake of clarity, not all features of actual implementations are described in the present disclosure. In addition, the language used in the present disclosure is selected for readability and instructional purposes and has not been selected to describe or limit the disclosed subject matter. Instead, the attached claims are intended for this purpose.

[0020] Various embodiments of the disclosed concepts are illustrated by way of example, and are not limited to individual drawings, in which similar reference numerals indicate similar elements. For simplicity and clear illustration, reference numerals are repeated in different drawings to indicate corresponding or similar elements, where appropriate. In addition, many specific details are shown in order to provide a full understanding of the specific implementation described herein. In other cases, methods, procedures, and components are not described in detail to avoid blurring the related functions described. Mentioning "one", "a kind of" or "another" embodiment in this disclosure is not necessarily the same or different embodiments, and this means at least one. A given drawing may be used to illustrate more than one embodiment or more than one type of features of the present disclosure, and all elements in the drawings may not be required for a given embodiment or type. When provided in a given drawing, the reference numeral refers to the same element throughout several drawings, but it may not be repeated in each drawing. Unless otherwise specified, the drawings are not drawn to scale, and the proportions of certain components may be enlarged to better illustrate the details and features of the present disclosure.

[0021] Figure 1A simplified block diagram of a wireless power transmission system 100 is illustrated. The wireless power transmission system includes a power transmitter (PTx) 110 that transmits power wirelessly to a power receiver (PRx) 120, such as via an inductive coupling 130. The power transmitter 110 can receive input power converted into an AC voltage having a specific voltage and frequency characteristic by an inverter 114. The inverter 114 can be controlled by a controller / communication module 116, which operates as further described below. In various embodiments, the inverter controller and the communication module can be implemented in a common system (such as a system based on a microprocessor, a microcontroller, etc.). In other embodiments, the inverter controller can be implemented by a separate controller module and a communication module with a communication device therebetween. The inverter 114 can be constructed using any suitable circuit topology (e.g., full bridge, half bridge, etc.) and can be implemented using any suitable semiconductor switch device technology (e.g., MOSFET, IGBT, etc. manufactured using silicon, silicon carbide, or gallium nitride devices).

[0022] The inverter 114 can deliver the generated AC voltage to the transmitter coil 112. In addition to allowing magnetic coupling to the wireless coil of the receiver, Figure 1 The illustrated transmitter coil block 112 may include tuning circuits (such as additional inductors and capacitors) that facilitate operation of the transmitter under different conditions (such as different degrees of magnetic coupling to the receiver, different operating frequencies, etc.). The wireless coil itself can be constructed in a variety of different ways. In some embodiments, the wireless coil can be formed as a winding of wire around a suitable bobbin. In other embodiments, the wireless coil can be formed as a trace on a printed circuit board. Other arrangements are also possible and can be used in conjunction with the various embodiments described herein. The wireless transmitter coil may also include a magnetically permeable material (e.g., ferrite) core that is configured to affect the flux pattern of the coil in a manner suitable for a particular application. The teachings herein may be applied in conjunction with any of a variety of transmitter coil arrangements suitable for a given application.

[0023] The PTx controller / communication module 116 may monitor the transmitter coil and use information derived therefrom to control the inverter 114 to suit a given situation. For example, the controller / communication module may be configured to cause the inverter 114 to operate at a given frequency or output voltage depending on the particular application. In some embodiments, the controller / communication module may be configured to receive information from the PRx device and control the inverter 114 accordingly. This information may be received via the power transmission coil (i.e., in-band communication) or may be received via a separate communication channel (not shown, i.e., out-of-band communication). For in-band communication, the controller / communication module 116 may detect and decode signals (such as voltage, frequency, or load changes) applied by the PRx on the magnetic link to receive information, and may instruct the inverter to modulate the delivered power by manipulating various parameters of the generated voltage (such as voltage, frequency, etc.) to transmit information to the PRx. In some embodiments, the controller / communication module may be configured to employ frequency shift keying (FSK) communication to convey data to the PRx, in which the frequency of the inverter signal is modulated. The controller / communication module 116 may be configured to detect amplitude shift keying (ASK) communications or load modulation based communications from the PRx. In either case, the controller / communication module 126 may be configured to vary the current drawn on the receiver side to manipulate the waveform seen on the Tx coil, thereby delivering information from the PRx to the PTx. For out-of-band communications, additional modules may be provided that allow communication between the PTx and PRx, such as WiFi, Bluetooth, or other radio links, or any other suitable communication channel.

[0024] As described above, the controller / communication module 116 can be a single module, for example, disposed on a single integrated circuit, or can be constructed from a plurality of modules / devices disposed on different integrated circuits or a combination of integrated circuits and discrete circuits having analog components and digital components. The teachings herein are not limited to any particular arrangement of controller / communication circuits.

[0025] The PTx device 110 may optionally include other systems and components, such as a separate communication module 118. In some embodiments, the communication module 118 may communicate with a corresponding module in the PRx via a power transmission coil. In other embodiments, the communication module 118 may communicate with a corresponding module using a separate physical channel 138.

[0026] As mentioned above, the wireless power transmission system also includes a wireless power receiver (PRx) 120. The wireless power receiver may include a receiver coil 122 that may be magnetically coupled 130 to the transmitter coil 112. As discussed above, the transmitter coil 112, Figure 1The illustrated receiver coil block 122 may include tuning circuits (such as additional inductors and capacitors) that facilitate operation of the transmitter under different conditions (such as different degrees of magnetic coupling to the receiver, different operating frequencies, etc.). The wireless coil itself can be constructed in a variety of different ways. In some embodiments, the wireless coil can be formed as a winding of wire around a suitable bobbin. In other embodiments, the wireless coil can be formed as a trace on a printed circuit board. Other arrangements are also possible and can be used in conjunction with the various embodiments described herein. The wireless receiver coil may also include a magnetically permeable material (e.g., ferrite) core that is configured to affect the flux pattern of the coil in a manner suitable for a particular application. The teachings herein may be applied in conjunction with any of a variety of receiver coil arrangements suitable for a given application.

[0027] The receiver coil 122 outputs an AC voltage induced therein by magnetic induction via the transmitter coil 112. The output AC voltage may be provided to a rectifier 124, which provides DC output power to one or more loads associated with the PRx device. The rectifier 124 may be controlled by a controller / communication module 126, which operates as further described below. In various embodiments, the rectifier controller and the communication module may be implemented in a common system (such as a system based on a microprocessor, a microcontroller, etc.). In other embodiments, the rectifier controller may be implemented by a separate controller module and a communication module with a communication device therebetween. The rectifier 124 may be constructed using any suitable circuit topology (e.g., full bridge, half bridge, etc.) and may be implemented using any suitable semiconductor switching device technology (e.g., MOSFETs, IGBTs, etc. manufactured using silicon, silicon carbide, or gallium nitride devices).

[0028] The PRx controller / communication module 126 may monitor the receiver coil and use the information derived therefrom to control the rectifier 124 to suit a given situation. For example, the controller / communication module may be configured to cause the rectifier 124 to provide a given output voltage depending on the particular application. In some embodiments, the controller / communication module may be configured to transmit information to the PTx device to effectively control the power delivered to the receiver. This information may be transmitted via the power transmission coil (i.e., in-band communication) or may be transmitted via a separate communication channel (not shown, i.e., out-of-band communication). For in-band communication, the controller / communication module 126 may, for example, modulate the load current or other electrical parameters of the received power to transmit the information to the PTx. In some embodiments, the controller / communication module 126 may be configured to detect and decode signals (such as voltage, frequency, or load changes) imposed on the magnetic link by the PTx to receive information from the PTx. In some embodiments, the controller / communication module 126 may be configured to receive frequency shift keying (FSK) communications in which the frequency of the inverter signal has been modulated to convey data to the PRx. The controller / communication module 126 may be configured to generate amplitude shift keying (ASK) communications or load modulation based communications from the PRx. In either case, the controller / communication module 126 may be configured to vary the current drawn on the receiver side to manipulate the waveform seen on the Tx coil, thereby delivering information from the PRx to the PTx. For out-of-band communications, additional modules may be provided that allow communication between the PTx and PRx, such as WiFi, Bluetooth, or other radio links, or any other suitable communication channel.

[0029] As described above, the controller / communication module 126 can be a single module, for example, disposed on a single integrated circuit, or can be constructed from multiple modules / devices disposed on different integrated circuits or a combination of integrated circuits and discrete circuits having analog components and digital components. The teachings herein are not limited to any particular arrangement of controller / communication circuits. The PRx device 120 may optionally include other systems and components, such as a communication ("communication") module 128. In some embodiments, the communication module 128 can communicate with a corresponding module in the PTx via a power transmission coil. In other embodiments, the communication module 128 can communicate with a corresponding module or tag using a separate physical channel 138.

[0030] Many variations and enhancements of the wireless power transmission system 100 described above are possible, and the following teachings apply to any of such variations and enhancements.

[0031] FIG. 2A to FIG. 2E A schematic diagram of a wireless power transfer system is illustrated. Figure 2AA general schematic diagram 200 of an exemplary wireless power transmission system includes a wireless power transmitter (PTx) 210 and a wireless power receiver 220, similar to the above description of the wireless power transmission system. Figure 1 Those described with similar reference numbers (e.g., PTx 110 and PRx 120). The wireless power transmitter 210 may include an inverter 114 that may drive the transmitter coil 112. The inverter 114 is depicted as a full-bridge inverter, although other inverter configurations may be used depending on application requirements. The inverter 114 may receive an input voltage Vin from an input voltage source represented by a capacitor Cin, which may include any suitable DC power source, such as a battery, a DC-DC converter, an AC-DC converter, etc. The inverter 114 may drive the transmitter coil 112 via a series tuned network 231, which will be described below with respect to Figure 2B Describe in more detail. Figure 2A Also depicted is an optional slew rate tuning circuit 233, which will be described below. Figure 2D Describe in more detail. Figure 2B Also depicted is a parasitic element 239 which may represent the effects of various wireless charger system implementations, such as capacitance associated with the wires / cords, etc. The illustrated representation is only one example, and other representations may be more appropriate depending on the exact physical configuration of a particular implementation.

[0032] Figure 2A A wireless power receiver 220 is also depicted. The wireless power receiver 220 may include a rectifier that may be driven by the receiver coil 122, which may have a current induced therein by the transmitter coil 112. The rectifier 124 is depicted as a full bridge rectifier, although other rectifier configurations may be used depending on the application requirements. The rectifier 124 may generate an output voltage Vrect / Vout from the AC voltage present across the receiver coil 122. The rectifier 124 may be driven via a series tuned circuit 235, which will be described below with respect to Figure 2C Describe in more detail. Figure 2A The following also describes Figure 2E The in-band communications circuit 237 is described in more detail.

[0033] Figure 2B Describe the above reference Figure 2ASchematic diagram of a wireless power transmitter series tuning circuit 231 is introduced. The transmitter series tuning circuit may include capacitance, such as capacitors Ctx1, Ctx2, and Ctx3, and the inductance of the transmitter coil 112. Depending on various parameters, such as the amount of power being transmitted, the operating voltage, the degree of coupling between PTx and PRx, etc., it may be desirable to tune the series tuning circuit to improve the overall operation. For example, as described in more detail below, it may be desirable to adjust the series tuning circuit to change the gain of the wireless power transmission system, thereby allowing adaptation to various input voltages, power levels, etc.

[0034] The series tuned circuit 231 may include a base capacitance of a capacitor Ctx1 that may always be present in the series tuned circuit, and selectable capacitance from capacitors Ctx2 and Ctx3. These capacitors may be selectively connected in parallel with capacitor Ctx1 to increase the effective capacitance by closing switches Qctx2 and / or Qctx3, respectively. Correspondingly, if one or more of switches Qctx2 and Qctx3 are closed, thereby connecting the corresponding capacitor to the series tuned circuit, opening such switches may reduce the capacitance of the series tuned circuit. By selecting different values ​​for the corresponding capacitors, a range of capacitances may be provided. Switches Qctx2 and Qctx3 may be selected by the above description of Figure 1 Such control may be implemented as described below with respect to the wireless power transmitter controller / communication circuit 116. FIG. 3A to FIG. 3C One or more of the control techniques described. Figure 2B The configurations are exemplary only, and other configurations including additional selective capacitors, all selectable capacitors, series combinations of capacitors (to selectively reduce capacitance), etc. may all be implemented as appropriate for a given application.

[0035] Figure 2C Describe the above reference Figure 2A Schematic diagram of a wireless power receiver series tuning circuit 235 is introduced. The receiver series tuning circuit may include capacitance, such as capacitors Ctx1 and Ctx2, and the inductance of the receiver coil 122. Depending on various parameters, such as the amount of power being transferred, the operating voltage, the degree of coupling between PTx and PRx, etc., it may be desirable to tune the series tuning circuit to improve overall operation. For example, as described in more detail below, it may be desirable to adjust the series tuning circuit to change the gain of the wireless power transfer system, thereby allowing adaptation to various input voltages, power levels, etc.

[0036] The series tuned circuit 235 may include a base capacitance of capacitor Crx1, which may always be present in the series tuned circuit, and a selectable capacitance from capacitor Crx2. This capacitor may be selectively connected in parallel with capacitor Ctx1 to increase the effective capacitance by closing switch Qcrx2. Correspondingly, if switch Qcrx2 is closed, thereby connecting the corresponding capacitor to the series tuned circuit, opening this switch may reduce the capacitance of the series tuned circuit. By selecting different values ​​for the corresponding capacitors, a range of capacitances may be provided. Switch Qcrx2 may be selected by the above description of Figure 1 The wireless power receiver controller / communication circuit 126 described herein can be controlled. Such control can implement control techniques such as those described below with respect to Figure 4 Describe the control technique. Figure 2C The configurations are exemplary only, and other configurations including additional selective capacitors, all selectable capacitors, series combinations of capacitors (to selectively reduce capacitance), etc. may all be implemented as appropriate for a given application.

[0037] Figure 2D Illustrated above about Figure 2A An optional slew rate tuning circuit 233 is introduced. To improve operating efficiency, it may be preferred to operate the inverter 114 with zero voltage switching (ZVS), which may reduce switching losses associated with inverter operation. Depending on various parameters, such as operating voltage, operating power level, etc., ZVS may not be possible for a given inverter circuit configuration, including but not limited to the total capacitance coupled to the inverter. To allow ZVS over a wider operating range, a slew rate tuning circuit 233 may be provided. More specifically, if one or more hard switching events are detected (i.e., lack of ZVS), the capacitance may be reduced. Alternatively, if no hard switching events are detected within a given time period (potentially corresponding to a greater than necessary ZVS margin), the capacitance may be increased, which may increase the voltage slew rate of the inverter.

[0038] The slew rate tuning circuit may include the base capacitance of capacitor Csw1, which may always be present, and optional capacitance from capacitors Csw2, Csw3, and Csw4. These capacitors may be selectively connected in parallel with capacitor Csw1 to increase the effective capacitance by closing switches Qcsw2, Qcsw3, and Qcsw4. Correspondingly, if one or more of switches Qcrx2 are closed, thereby connecting the corresponding capacitor into the series tuned circuit, opening this switch may reduce the capacitance of the series tuned circuit. A corresponding slew rate capacitor and associated switch may be connected to each inverter branch, such as Figure 2A By selecting different values ​​for the corresponding capacitors, a range of capacitances can be provided. Switches Qcsw2, Qcsw3 and Qcsw4 can be configured as described above with respect to Figure 1 The wireless power transmitter controller / communication circuit 116 described above may be controlled by the wireless power transmitter controller / communication circuit 116 described above. Such control may implement various control techniques depending on the specific implementation. In some embodiments, the slew rate capacitance may be tuned in conjunction with a change in the series tuning capacitance, as described in more detail below. Figure 2D The configurations are exemplary only, and other configurations including additional selective capacitors, all selectable capacitors, series combinations of capacitors (to selectively reduce capacitance), etc. may all be implemented as appropriate for a given application.

[0039] Figure 2E Illustrated above about Figure 2A In-band communication circuit 237 is described. In-band communication circuit 237 is only one example of a variety of in-band communication circuit types associated with corresponding in-band communication methods. In many embodiments, various forms of load modulation may be employed. Such load modulation may be used to implement replicated on-off keying ("ASK") communication, where the connection or disconnection of an additional load element may be used to change the received power, which may be detected by a corresponding change in the power transmitted by the transmitter. As an example, in-band communication circuit 237 includes switches Qcc1 and Qcc2 that can selectively couple capacitors Ccomm1 and Ccomm2 to / from respective branches of the (full-bridge) rectifier 124, in Figure 2E 224a and 224b. This configuration is just one example, and many other configurations of this circuit can be implemented.

[0040] FIG. 3A to FIG. 3C The flowchart of the wireless power transmitter capacitor tuning technique is illustrated. PTx input voltage Vin( Figure 2A ) can be varied to adjust the output power of the inverter, and thereby adjust the power delivered to the wireless power receiver. Increasing the input voltage Vin can increase the power delivered, and vice versa. Thus, the wireless power transmitter can adjust the power delivered by changing the input voltage Vin (e.g., by modifying the control signal provided to the DC-DC converter that supplies the input voltage). Depending on the output voltage provided as Vin and the power level requested by the wireless power receiver, the system may end up operating in an area where it becomes difficult or impossible to adjust the output voltage to the desired level. This can be overcome in at least some cases by adjusting the series tuning capacitor, such as using the above reference FIG. 2A to FIG. 2B Describe the circuit.

[0041] Figure 3AFlowchart 300a illustrates an advanced wireless power transmitter capacitance tuning technique. Beginning at block 341, a transmitter controller (e.g., controller / communication circuit 116) may determine whether the input voltage Vin has reached a maximum value (i.e., a high threshold). For example, this may occur in response to a request from a wireless power receiver to increase power when the wireless power transmitter is already operating at its maximum input voltage. If so, the controller may reduce Ctx to increase the gain of the wireless power transfer system (block 342) while continuing to monitor the input voltage. For example, the input voltage may be increased by opening switches Qctx2, Qctx3 ( Figure 2B ) to disengage one or more of the selectable capacitors Ctx2, Ctx3 coupled to the transmitter coil to achieve a reduction in Ctx. If in block 341, the transmitter controller determines that the input voltage is not at a maximum / high threshold, the controller may determine in block 343 whether the input voltage has reached a minimum value (i.e., a low threshold). This may occur, for example, in response to a reduction in the transmitted wireless power or a request to reduce the transmitted wireless power. If so, the controller may increase Ctx to reduce the gain of the wireless power transfer system (block 344) while continuing to monitor the input voltage. For example, the controller may close switches Qctx2, Qctx3 ( Figure 2B ) to couple one or more of the selectable capacitors Ctx2, Ctx3 to the transmitter coil to achieve the increase in Ctx. Thus, the transmitter controller can adjust or regulate the input voltage and the series tuning capacitance in response to the power demand from the wireless power receiver.

[0042] As described above, different capacitance values ​​can be achieved by different combinations of capacitors Ctxn. In addition, different numbers of resonant capacitors Ctxn can be provided, and each capacitor can have a different capacitance value (or the same capacitance value). Therefore, the system can be designed to provide any range of capacitance values ​​in response to the input voltage.

[0043] Figure 3BFlowchart 300b illustrates a modified wireless power transmitter capacitance tuning technique with optional additional steps that may be advantageous under various conditions, such as relatively high transmitted power levels. As previously described, at block 341, a transmitter controller (e.g., controller / communication circuitry 116) may determine whether the input voltage Vin has reached a maximum value (i.e., a high threshold), for example, in response to an increased power demand from a wireless power receiver. If so, the controller may first adjust (e.g., reduce) Vin (block 345a) before making any changes to the series tuning capacitance, possibly at least temporarily reducing the transmitted power level. Adjusting the input voltage before changing the input capacitance may prevent an overvoltage event from occurring at the wireless power receiver due to a change in transmitter-side capacitance. Subsequently, the controller may reduce Ctx to increase the gain of the wireless power transmission system (block 342) while continuing to monitor the input voltage. As described above, the input voltage may be increased by opening switches Qctx2, Qctx3 ( Figure 2B ) to disconnect one or more of the selectable capacitors Ctx2, Ctx3 coupled to the transmitter coil to achieve the reduction of Ctx. For example, if the wireless power receiver requests a higher power level, the transmitted power level can be increased again to a relatively high value.

[0044] Alternatively, if in box 341, the transmitter controller determines that the input voltage is not at a maximum / high threshold, the controller may determine in box 343 whether the input voltage has reached a minimum value (i.e., a low threshold), for example, in response to a reduced power demand of the wireless power receiver. If so, the controller may (optionally) first adjust Vin (box 345b) before making any changes to the series tuning capacitance, possibly at least temporarily reducing the transmitted power level. As described above, adjusting the input voltage (and the associated transmitted power level) before changing the input capacitance may prevent an overvoltage event from occurring at the wireless power receiver due to changes in transmitter-side capacitance. The controller may then increase Ctx to reduce the gain of the wireless power transfer system (box 344) while continuing to monitor the input voltage. For example, the control may be performed by closing switches Qctx2, Qctx3 ( Figure 2B ) to couple one or more of the selectable capacitors Ctx2, Ctx3 to the transmitter coil to achieve the increase in Ctx. For example, if the wireless power receiver requests a higher power level, the transmitted power level may be increased again to a relatively high value.

[0045] The input voltage regulation described above can be achieved in various ways. For example, in at least some embodiments, voltage regulation can be achieved by changing a feedback control signal provided to a DC-DC converter or other regulator that provides Vin. In other embodiments, such as those in which the input voltage is provided by a USB-PD power adapter, appropriate USB-PD control signals can be provided to the adapter to change the voltage provided.

[0046] As described above, certain operating scenarios (e.g., higher power levels) may require that the input voltage be regulated before a transmitter-side capacitance change is made, while other operating scenarios (e.g., lower transmitted power levels) may not necessarily benefit from such operation. Therefore, the above-described techniques may be modified to selectively provide input voltage regulation before the capacitor is switched in response to the transmitted power level. As an example, the input voltage regulation function may be enabled in response to the power level being above a first threshold power level and disabled in response to the power level being below a second threshold power level. The first and second threshold power levels may be the same value or different values, thereby providing a degree of hysteresis in the operation.

[0047] Figure 3C Flowchart 300c illustrating a further modified wireless power transmitter capacitance tuning technique with optional additional steps that may be useful for addressing inverter hard switching (i.e., non-ZVS) events that may result in increased losses and reduced operating efficiency. Beginning at block 346, the control circuit may determine whether a hard switching condition exists. This may be the occurrence of a single hard switching event or, more preferably, in at least some cases, multiple hard switching events and / or multiple consecutive hard switching events are detected over a period of time. Such hard switching events may indicate an opportunity to improve operating efficiency by increasing the capacitance coupled to the inverter, which may include adjustment of the series tuning capacitance Ctx and / or adjustment of the optional slew rate tuning capacitance, as described above with respect to Figure 3C described.

[0048] In any case, if a hard switching condition is detected, the controller may increase the series tuning capacitance (block 347) or adjust both the slew rate capacitance Csw and the series tuning capacitance Ctx to provide an overall increase in capacitance. For example, the adjustment may include increasing the slew rate capacitance Csw and decreasing the series tuning capacitance Ctx while still providing an overall capacitance increase. Additionally (and possibly optionally), the controller may adjust the input voltage Vin (block 345c) before changing the capacitance to prevent receiver side overvoltage events (as described above). Figure 3CNot shown is a process for reducing capacitance if the conditions that caused the previous hard switch to trigger the capacitance no longer exist. For example, such a change can be made by detecting that no hard switching events have been detected for a period of time. The system can then reduce the capacitance (by adjusting the series tuning capacitance Ctx and / or the slew rate capacitance Csw). If the reduction triggers a hard switching event, the capacitance adjustment may be reversed (i.e., the capacitance is increased again). Alternatively, if the reduction does not trigger a hard switching condition, the capacitance reduction can be maintained.

[0049] Otherwise, if the hard switch condition does not trigger capacitance adjustment in block 346, the technique depicted in flowchart 300c may be similar to that described above with reference to Figure 3A and Figure 3B As previously described, at box 341, the transmitter controller (e.g., the controller / communication circuit 116) may determine whether the input voltage Vin has reached a maximum value (i.e., a high threshold), for example, in response to an increased power demand from the wireless power receiver. If so, the controller may (optionally, for example, in response to the transmitted power level) first adjust (reduce because it is already at a maximum voltage) Vin (box 345a) and also at least temporarily reduce the transmitted power level before making any changes to the series tuning capacitance. Adjusting (e.g., reducing) the input voltage before changing the input capacitance may prevent an overvoltage event from occurring at the wireless power receiver due to a change in the capacitance on the transmitter side. Subsequently, the controller may reduce Ctx to increase the gain of the wireless power transmission system (box 342) while continuing to monitor the input voltage. As described above, the input voltage may be increased by opening switches Qctx2, Qctx3 ( Figure 2B ) to disconnect one or more of the selectable capacitors Ctx2, Ctx3 coupled to the transmitter coil to achieve the reduction of Ctx. For example, if the wireless power receiver requests a higher power level, the transmitted power level can be increased again to a relatively high value.

[0050] Alternatively, if in box 341, the transmitter controller determines that the input voltage is not at a maximum / high threshold, the controller may determine in box 343 whether the input voltage has reached a minimum value (i.e., a low threshold), for example, in response to a reduced power demand from the wireless power receiver. If so, the controller may (optionally, for example, in response to the transmitted power level) first adjust (increase since it is already at a minimum voltage) Vin (box 345b) and may also temporarily reduce the transmitted power level before making any changes to the series tuning capacitance. As described above, adjusting (increasing) the input voltage (and the associated transmitted power level) before changing the input capacitance may prevent an overvoltage event from occurring at the wireless power receiver due to a change in the transmitter-side capacitance. Subsequently, the controller may increase Ctx to reduce the gain of the wireless power transmission system (box 344) while continuing to monitor the input voltage. For example, the controller may adjust (increase) the input voltage (and the associated transmitted power level) by closing switches Qctx2, Qctx3 ( Figure 2B ) to couple one or more of the selectable capacitors Ctx2, Ctx3 to the transmitter coil to achieve the increase in Ctx. For example, if the wireless power receiver requests a higher power level, the transmitted power level may be increased again to a relatively high value.

[0051] Various techniques for adjusting the wireless power transmitter series tuning capacitance in response to input voltage and load conditions to achieve improved voltage regulation are described above. Additionally or alternatively, series tuning capacitance adjustment may also be performed on the wireless power receiver side to achieve a similar effect. Figure 4 A flow chart of a wireless power receiver capacitance tuning technique 400 is illustrated, in which a receiver-side resonant gain associated with a receiver-side resonant circuit including a receiver coil and a series tuning capacitor Crx is tuned based on a power level and a transmitter-side input voltage. Starting from block 451, a receiver controller (e.g., a receiver controller / communication circuit 126) may determine whether increased power is needed or desired. If so, in block 452, the receiver controller may transmit an increased power request to a wireless power transmitter (PTx). If PTx can transmit an increased power level, it may do so, for example, by increasing its input voltage Vin. Otherwise, if PTx cannot transmit an increased power level, it may communicate this to the wireless power receiver, for example, by transmitting a NAK (not acknowledged) message in response to a request for increased power from the receiver. The NAK message (as well as previous messages from the receiver requesting increased power) may conform to one or more standard protocols, such as the Qi wireless charging standard promulgated by the Wireless Power Alliance, or may be any other format suitable for communication between PTx and PRx, such as a proprietary protocol.

[0052] If no NAK message is received (block 453), the PTx will increase power, and the process may return to block 451. Otherwise, if a NAK message is received (block 453), the receiver controller may determine whether the PTx cannot deliver the increased power because it is already at its maximum input voltage Vin. For example, a NAK message received from the PTx may include an indication of why the increased power is not available (e.g., because the PTx is already at its maximum input voltage), or may be followed by a subsequent message indicating such information. If the NAK message indicates a reason why the increased power cannot be provided other than that it is already at its maximum input voltage, the receiver may optionally engage in other compensation or negotiation with the PTx (block 457) to attempt to increase the received power, after which control returns to block 451. Such other compensation or negotiation is beyond the scope of the present application and is therefore not discussed in detail.

[0053] If a NAK message is received (block 453) and the reason for the NAK is that PTx has reached its maximum input voltage Vin (block 454), the receiver controller may reduce the receiver side series tuning capacitance Crx to increase gain and thus increase the received power (block 456). Before doing so, the receiver controller may optionally reduce its power level to prevent overvoltage (block 455). This is similar to the transmitter side process described above. In certain operating scenarios (e.g., relatively high power levels), changing the series tuning capacitance to increase gain and thus increase the received power may cause an overvoltage event on the receiver side. To mitigate such situations, the power may be temporarily reduced before the capacitance change is made. Then, after the capacitance change, the power level may be subsequently increased. The capacitance change on the receiver side may be performed in a manner similar to that on the transmitter side, by disconnecting the switch Qcrx2 to reduce the capacitance, and one or more additional receiver side resonant capacitors Crx2 may be optionally decoupled from the receiver coil 122, thereby increasing gain and thus increasing the power received in block 456. Control may then return to block 451.

[0054] If in block 451, the receiver controller determines that an increase in the wireless power transmitter is not needed or expected, the receiver controller may determine whether it is at a low power level (block 458). If not, control may return to block 451. Otherwise, if the receiver controller determines that the receiver is operating at a low power level (block 458), the receiver controller may increase the receiver series tuning capacitance back to the nominal value. The increase in capacitance may be achieved by closing switch Qcrx2 to connect the resonant capacitor in parallel with capacitor Crx1, thereby increasing the capacitance and reducing the gain of the system. As with transmitter-side regulation, the power levels associated with increasing or decreasing capacitance may be associated with one or more threshold power levels, which may be the same value or different values ​​to allow for hysteresis in the transition.

[0055] All of the above operations involve various voltage levels and thresholds, power levels and thresholds, etc. The teachings herein can be applied to various systems operating at different voltage levels, different power levels, etc. For example, in some embodiments, whether by manipulating the control signal of the DC-DC converter or otherwise, the input voltage can be controlled within a range between about 16V and 20V. Such a voltage range can but need not correspond to a USB-PD power supply that provides a 20V input voltage to such a DC-DC converter. However, operations within other voltage ranges corresponding to other USB-PD voltage levels may also be appropriate. For example, a range between about 10V and 15V or between 12V and 15V input voltage can be used with a 15V USB-PD supply. Alternatively, if a buck-boost converter is used to provide Vin from a power supply, the upper limit of the voltage range supplied may be higher than the voltage supplied to such a DC-DC converter. Similarly, with respect to power thresholds, a power threshold of 15W can be used as a dividing line between a low power solution and a high power solution. Operation above this threshold can be used to selectively enable or disable functions such as input voltage reduction before initiating a capacitance change on the PTx or PRx side. However, 15W is only one example of such a threshold, and the threshold can be 10W, 12W, 16W, 18W, 20W, 22W, 25W, 28W, 30W, 32W, 35W, 38W, 40W, 45W, 50W, or any other suitable value. If a certain degree of hysteresis is required, an additional power threshold can be used to indicate a return to a low power scheme. Such a threshold can be 9W, although any value less than the high power threshold can be used, such as 12W, 10W, 7.5W, 5W, etc. Unless otherwise specified herein or in the appended claims, any of the above values ​​can be used; however, for at least some applications, it may be advantageous to use certain specific thresholds.

[0056] Various features and embodiments of dynamic capacitance tuning in wireless power transmission systems are described above. Such arrangements can be used in a variety of applications, but can be particularly advantageous when used in conjunction with electronic devices (such as mobile phones, tablet computers, laptops or notebook computers) and accessories (such as wireless headsets, styluses, etc.). In addition, although many specific features and various embodiments have been described, it should be understood that various features and embodiments may be arranged and combined in various combinations in specific implementations unless otherwise specified as mutually exclusive. Therefore, the various embodiments described above are provided by way of example only and should not be construed as limiting the scope of the present disclosure. Various modifications and changes may be made to the principles and embodiments herein without departing from the scope of the present disclosure and without departing from the scope of the claims.

[0057] The foregoing describes an exemplary embodiment of a wireless power transmission system capable of transmitting certain information between a PTx and a PRx in the system. The present disclosure contemplates that this information transfer improves the ability of devices to provide wireless power signals to each other in an efficient manner to facilitate battery charging, such as by sharing the power handling capabilities of the devices with each other. Entities implementing the present technology should be careful to ensure compliance with established privacy policies and / or privacy practices to the extent that any sensitive information is used in a particular implementation. In particular, such entities will be expected to implement and consistently apply privacy practices that are generally recognized as meeting or exceeding industry or government requirements for maintaining user privacy. Implementers should inform users where personally identifiable information is expected to be transmitted in wireless power transmission systems, and allow users to "opt-in" or "opt-out" participation. For example, if a power transmitter is configured to poll for sensitive information from a power receiver, such information may be presented to the user when the user places the device onto the power transmitter.

[0058] Once the data is no longer needed, risks can be minimized by limiting data collection and data deletion. In addition, and when applicable, data de-identification can be used to protect the privacy of users. For example, a device identifier may be partially masked to convey the power characteristics of a device without uniquely identifying the device. De-identification can be facilitated by removing identifiers, controlling the amount or specificity of stored data (e.g., collecting location data at a city level rather than an address level), controlling how data is stored (e.g., aggregating data across users), and / or other methods such as differential privacy when appropriate. Robust encryption can also be used to reduce the likelihood that communications between inductively coupled devices can be spoofed.

Claims

1. A wireless power transmitter, comprising: Wireless power transmission coil; an inverter receiving an input voltage and generating an AC output voltage that drives the wireless power transmission coil; at least one adjustable capacitor coupled to the wireless power transmission coil to form a series tuned circuit with the wireless power transmission coil; as well as a control circuit that operates the inverter and the at least one adjustable capacitor to deliver wireless power to a wireless power receiver, wherein the control circuit responds to a power demand from the wireless power receiver by: determining whether the input voltage is at a maximum value, and if so, reducing the at least one adjustable capacitance to increase gain; or A determination is made as to whether the input voltage is at a minimum value, and if so, the at least one adjustable capacitance is increased to reduce the gain.

2. The wireless power transmitter of claim 1 , wherein the at least one adjustable capacitance comprises one or more capacitors, the one or more capacitors being selectively coupled to the wireless power transmitting coil via one or more switches controlled by the control circuit.

3. The wireless power transmitter according to claim 2, wherein the at least one adjustable capacitance comprises at least one capacitor coupled to the wireless power transmitting coil and at least two capacitors that can be selectively coupled to the wireless power transmitting coil by one or more switches controlled by the control circuit.

4. The wireless power transmitter of claim 1 , wherein determining whether the input voltage is at a maximum value, and if so, reducing the at least one adjustable capacitance to increase gain further comprises at least temporarily adjusting the input voltage before reducing the at least one adjustable capacitance.

5. The wireless power transmitter of claim 4, wherein at least temporarily adjusting the input voltage before reducing the at least one adjustable capacitance is performed by the control circuit in response to an operating power level of the wireless power transmitter.

6. The wireless power transmitter of claim 1 , wherein determining whether the input voltage is at a minimum value, and if so, increasing the at least one adjustable capacitance to reduce gain further comprises at least temporarily adjusting the input voltage before increasing the at least one adjustable capacitance.

7. The wireless power transmitter of claim 6, wherein the input voltage is at least temporarily adjusted in response to an operating power level of the wireless power transmitter before increasing the at least one adjustable capacitance. 8 . The wireless power transmitter of claim 1 , wherein the control circuit increases the at least one adjustable capacitance in response to one or more hard switching events to mitigate the hard switching events.

9. The wireless power transmitter of claim 8, wherein the at least one adjustable capacitor comprises: one or more capacitors selectively coupleable between an inverter output and the wireless power transmission coil via one or more first switches controlled by the control circuit; as well as One or more capacitors are selectively coupleable between the inverter output and ground via one or more second switches controlled by the control circuit.

10. The wireless power transmitter according to claim 9, wherein: The one or more capacitors selectively coupleable between the inverter output and the wireless power transmission coil include at least one capacitor coupled between the inverter output and the wireless power transmission coil and at least two capacitors selectively coupleable between the inverter output and the wireless power transmission coil via the one or more first switches; and The one or more capacitors selectively coupleable between the inverter output and ground include at least one capacitor coupled between the inverter output and ground and a plurality of capacitors selectively coupleable between the inverter output and ground via the one or more second switches. 11 . The wireless power transmitter of claim 8 , wherein the control circuitry reduces the at least one adjustable capacitance to mitigate the hard switching events in response to an absence of one or more hard switching events.

12. A method of operating a wireless power transmitter, the wireless power transmitter having a wireless power transmission coil, an inverter that receives an input voltage and generates an AC output voltage that drives the wireless power transmission coil, and at least one adjustable capacitor coupled to the wireless power transmission coil to form a series tuning circuit with the wireless power transmission coil, the method being performed by a control circuit of the wireless power transmitter, and comprising: In response to an increased power demand from the wireless power receiver, determining whether the input voltage is at a maximum value, and if so, reducing the at least one adjustable capacitance to increase gain; or In response to a reduced power demand from the wireless power receiver, determining whether the input voltage is at a minimum value, and if so, increasing the at least one adjustable capacitance to reduce gain.

13. The method of claim 12, wherein determining whether the input voltage is at a maximum value, and if so, reducing the at least one adjustable capacitance to increase gain further comprises at least temporarily adjusting the input voltage before reducing the at least one adjustable capacitance.

14. The method of claim 12, wherein determining whether the input voltage is at a minimum value, and if so, increasing the at least one adjustable capacitance to reduce gain further comprises at least temporarily adjusting the input voltage before increasing the at least one adjustable capacitance.

15. The method according to claim 12, further comprising: The at least one adjustable capacitance is increased in response to one or more hard switching events to mitigate the hard switching events.

16. The method according to claim 15, further comprising: The at least one adjustable capacitance is reduced in response to an absence of one or more hard switching events to mitigate the hard switching events.

17. A wireless power receiver, comprising: Wireless power receiving coil; a rectifier that receives an AC voltage induced in the wireless power receiving coil by a wireless power transmitter and generates a DC output voltage therefrom; at least one adjustable capacitor, the at least one adjustable capacitor coupled to the wireless power receiving coil to form a series tuned circuit with the wireless power receiving coil; as well as a control circuit that operates the rectifier and the at least one adjustable capacitor to deliver power to a load coupled to the DC output voltage of the rectifier, wherein the control circuit responds to the power demand by: determining whether a DC input voltage of an inverter of the wireless power transmitter is at a maximum value, and if so, reducing the at least one adjustable capacitance to increase gain; or determining whether the DC input voltage of the inverter of the wireless power transmitter is at a minimum value, and if so, increasing the at least one adjustable capacitor to reduce gain; Wherein the DC input voltage is determined by the control circuit in response to one or more messages received from the wireless power transmitter.

18. The wireless power receiver of claim 17, wherein the at least one adjustable capacitance comprises one or more capacitors, the one or more capacitors being selectively coupleable to the wireless power receiving coil via one or more switches controlled by the control circuit.

19. The wireless power receiver of claim 18, wherein the at least one adjustable capacitance comprises at least one capacitor coupled between the wireless power receiving coil and the rectifier via one or more switches controlled by the control circuit.

20. The wireless power receiver of claim 17, wherein determining whether the DC input voltage of the inverter is at a maximum value, and if so, reducing the at least one adjustable capacitance to increase gain further comprises: In response to the rectifier power, the rectifier power is at least temporarily reduced before reducing the at least one adjustable capacitance.

21. The wireless power receiver of claim 17, wherein determining whether the DC input voltage of the inverter is at a minimum value, and if so, increasing the at least one adjustable capacitor to reduce gain further comprises: In response to the rectifier power, the rectifier power is at least temporarily reduced before the at least one adjustable capacitance is increased.

22. A method of operating a wireless power receiver having a wireless power receiving coil, a rectifier that receives an AC voltage induced in the wireless power receiving coil by a wireless power transmitter and generates a DC output voltage therefrom, and at least one adjustable capacitor coupled to the wireless power receiving coil to form a series tuned circuit with the wireless power receiving coil, the method being performed by a control circuit of the wireless power receiver and comprising: determining, in response to an increased power demand, whether a DC input voltage of an inverter of the wireless power transmitter is at a maximum, and if so, reducing the at least one adjustable capacitance to increase gain; or determining, in response to a reduced power demand, whether the DC input voltage of the inverter of the wireless power transmitter is at a minimum value, and if so, increasing the at least one adjustable capacitance to reduce gain; Wherein the DC input voltage is determined by the control circuit in response to one or more messages received from the wireless power transmitter.

23. The method of claim 22, wherein determining whether the DC input voltage of the inverter is at a maximum value, and if so, reducing the at least one adjustable capacitance to increase gain further comprises: In response to the rectifier power, the rectifier power is at least temporarily reduced before reducing the at least one adjustable capacitance.

24. The method of claim 22, wherein determining whether the DC input voltage of the inverter is at a minimum value, and if so, increasing the at least one adjustable capacitor to reduce gain further comprises: In response to the rectifier power, the rectifier power is at least temporarily reduced before the at least one adjustable capacitance is increased.

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