Wireless Power Transmission

Abstract

The wireless power system includes a power receiver 105 that receives power from a power transmitter 101 having an output resonant circuit including a power transmit coil 103 and a capacitor 303. A driver 301 generates a drive signal for the output resonant circuit to generate an inductive power transfer signal. A frequency determiner 313 provides a load sensitivity reduced operating frequency for the drive signal, and a frequency controller 311 changes the operating frequency of the drive signal to the load sensitivity reduced operating frequency in response to receiving a load change message from the power receiver. A transmitter 309 sends a load change confirm message to the power receiver 105 to indicate the change in frequency. The frequency controller 311 changes the operating frequency from the load sensitivity reduced operating frequency to the load-dependent operating frequency in response to determining that the power receiver 105 has performed a load change.

Description

[Technical Field]

[0001] The present invention relates to wireless power transmission, and in particular, but not exclusively, to the operation of power transmitters and power receivers to support wireless transmission for typical high power devices such as kitchen appliances. [Background technology]

[0002] Most current electrical products require dedicated electrical contacts to receive power from an external power source. However, this tends to be impractical, requiring the user to physically insert a connector or otherwise establish physical electrical contact. Power requirements also typically vary widely, and currently most devices are provided with dedicated power sources, resulting in a typical user having a number of different power sources, each dedicated to a specific device. However, while the use of an internal battery can avoid the need for a wired connection to a power source during use, this only provides a partial solution, as the battery requires recharging (or replacement). Additionally, using a battery can substantially increase the weight and potential cost and size of the device.

[0003] To provide a significantly improved user experience, it has been proposed to use a wireless power source in which power is inductively transferred from a transmitter inductor in the power transmitting device to a receiver coil in the individual device.

[0004] Power transfer via magnetic induction is a well-known concept and is mostly applied to transformers with tight coupling between the primary transmitter inductor / coil and the secondary receiver coil. By separating the primary transmitter coil and secondary receiver coil between the two devices, wireless power transfer between them becomes possible based on the principle of a loosely coupled transformer.

[0005] Such a configuration allows for wireless power transmission to a device without the need for a wired or physical electrical connection. Indeed, a device can simply be placed adjacent to or on top of the transmitter coil for external recharging or powering. For example, the power transmitter can be configured to have a horizontal surface onto which a device can simply be placed to receive power.

[0006] Furthermore, such wireless power transmission configurations can be advantageously designed so that the power transmitter can be used with a range of power receiving devices. In particular, a wireless power transmission approach known as the Qi standard has been defined and is currently being further developed. This approach allows power transmitter devices that meet the Qi standard to be used with power receiver devices that meet the Qi standard, without the need for them to be from the same manufacturer or proprietary to each other. The Qi standard also includes several features that allow operation to be tailored to specific power receiving devices (e.g., depending on a specific power drain).

[0007] The Qi standard is developed by the Wireless Power Consortium, information about which can be found, for example, on its website http: / / www.wirelesspowerconsortium.com / index.html, where in particular the defined specifications can be found.

[0008] The Wireless Power Consortium has been developing the Ki standard (also known as the Cordless Kitchen standard), which builds on the Qi standard and aims to provide safe, reliable, and efficient wireless power transfer to kitchen appliances. Ki supports much higher power levels, up to 2.2KW.

[0009] Additionally, power receivers, such as many kitchen appliances and devices, may have several operating modes with very different power levels; for example, a device may include several different loads that are switched on or off. As a specific example, an air fryer appliance may switch its heating element on and off, resulting in typical load steps, e.g., between 50 and 1200 W. Such load changes may be repeated during the operation of the air fryer, for example, to maintain a reasonably constant temperature.

[0010] The system may also have a nonlinear load, for example, instead of a resistive component, the load may be, for example, a motor with significant reactance (e.g., a food processor appliance). Such a nonlinear load may result in a completely different response of the system, which may have a significant impact on the control system design.

[0011] Typically, systems use a control loop to compensate for load variations and ensure the correct operating point is reached. This control loop adapts the amount of power sent to the power receiver / device. The received power (or voltage or current) can be measured by the power receiver relative to a set power value, and an error signal can be determined. The power receiver can send this error signal to a control system in the power transmitter, which can adapt the power level to reduce the static error, ideally to zero. Summary of the Invention [Problem to be solved by the invention]

[0012] Such control loops can provide advantageous and appropriate operation in many scenarios. However, they cannot achieve ideal performance in all situations. For example, control loops inherently have dynamic performance that is a trade-off between response time on the one hand and accuracy and noise performance on the other. Practical control loops tend to have less than ideal transient response and tend not to provide ideal adaptation when significant load changes and steps occur.

[0013] Furthermore, the resulting system response and power transfer function can typically cause induced voltage changes and transients in the power receiver. Such transients can, in some cases, significantly fluctuate the output voltage when a load step occurs, which can potentially create an overvoltage condition, among other things. Such transients and voltage variations would be undesirable in many situations.

[0014] Therefore, improved operations and approaches for power transmission systems would be advantageous, particularly approaches that allow for increased flexibility, reduced cost, reduced complexity, improved backward compatibility, improved suitability for higher power level transmission, improved suitability to specific operating conditions, improved suitability to power receiver load variations, changes and / or steps, improved suitability to changing operating conditions, reduced voltage / current fluctuations, reduced overvoltage / current conditions, and / or improved performance. [Means for solving the problem]

[0015] Accordingly, the Invention seeks to preferably mitigate, reduce or eliminate one or more of the above mentioned disadvantages singly or in any combination.

[0016] According to one aspect of the present invention, there is provided a power transmitter for wirelessly supplying power to a power receiver via an inductive power transfer signal, the power transmitter comprising: an output resonant circuit having a transmitting coil and at least one capacitor; a driver configured to generate a drive signal for the output resonant circuit to produce the inductive power transfer signal; a frequency determiner configured to provide a load sensitivity reduced operating frequency for the drive signal; a receiver configured to receive a load change message from the power receiver, the load change message indicating an impending load change by the power receiver; a frequency controller configured to change the operating frequency of the drive signal to the load sensitivity reduced operating frequency in response to receiving the load change message; and a transmitter configured to send a load change confirmation message to the power receiver, the load change confirmation message indicating that the frequency controller has changed the operating frequency to the load sensitivity reduced operating frequency;

[0017] The present invention can provide improved power transfer in many embodiments. In many embodiments, it can provide improved load change performance and operation. In many scenarios, it can reduce the effects of load changes, especially large and / or fast load changes. In many scenarios, it can reduce overvoltage and / or overcurrent conditions associated with load changes and load steps. This approach allows for improved transient performance for load changes and load steps. This approach can reduce cost in many scenarios. It can, in many embodiments, facilitate implementation and / or provide improved backward compatibility.

[0018] In some embodiments, the load-sensitivity-reduced operating frequency is substantially the load-sensitivity minimum operating frequency. The minimum value can be a global minimum or a local minimum. The load-sensitivity-reduced operating frequency is a frequency that is less sensitive to load variations than any frequency within the operating frequency range. The operating frequency range is, for example, a power control loop operating frequency, which represents the range of operating frequencies that can be set by the power control loop. The sensitivity can be, for example, the sensitivity of power transfer parameters, such as rectified and smoothed inductive receiver coil voltage and / or current, power level, drive signal voltage / current / power / phase, and receiver load voltage, to variations / changes in the load of the power transfer signal / drive signal, and often variations / changes in the load of the power receiver.

[0019] In some embodiments, the load sensitivity reduction operating frequency can be a frequency at which the change in output voltage of the input resonant circuit of the power receiver as a function of change in load on the input resonant circuit is reduced relative to the range of power control operating frequencies (the range of frequencies that the drive signal can be controlled to have by the power control loop).

[0020] In some embodiments, the load sensitivity reduced operating frequency is the frequency at which the change in output voltage of the input resonant circuit of the power receiver as a function of change in load on the input resonant circuit has a local minimum, and in some cases may have a minimum value.

[0021] In some embodiments, the load sensitivity reduced operating frequency is the frequency at which the change in output voltage of the input resonant circuit of the power receiver as a function of change in load on the input resonant circuit has a local minimum, and in some cases may have a minimum value.

[0022] The above is believed to be particularly suitable for embodiments in which the input resonant circuit of the power receiver is a series resonant circuit. In such a circuit, the receiver coil of the input resonant circuit, the capacitor of the input resonant circuit and the load may be connected in series. In such a circuit, the currents flowing through the load, the receiver coil of the input resonant circuit and the capacitor of the input resonant circuit are the same.

[0023] In some embodiments, the load sensitivity reduction operating frequency is the frequency at which the change in output current of the input resonant circuit of the power receiver as a function of change in load on the input resonant circuit is reduced for the range of power control operating frequencies (the range of frequencies of the drive signal controlled by the power control loop).

[0024] In some embodiments, the load sensitivity reduced operating frequency is the frequency at which the change in output current of the input resonant circuit of the power receiver as a function of change in load on the input resonant circuit has a local minimum, possibly a minimum value.

[0025] In some embodiments, the load sensitivity reduced operating frequency is the frequency at which the change in output current of the input resonant circuit of the power receiver as a function of change in load on the input resonant circuit has a local minimum, possibly a minimum value.

[0026] The above may be particularly suitable for embodiments in which the input resonant circuit of the power receiver is a parallel resonant circuit. In such a circuit, the receiver coil of the input resonant circuit, the capacitor of the input resonant circuit and the load may be connected in parallel. In such a circuit, the voltages across the load, the receiver coil of the input resonant circuit and the capacitor of the input resonant circuit are the same.

[0027] In some embodiments, the load sensitivity reduction operating frequency can be the frequency of the output resonant frequency of the power transmitter and the combined resonant frequency of the input resonant circuit of the power receiver.

[0028] According to an optional feature of the invention, the power transmitter includes a power controller configured to implement a frequency power control loop by adapting an operating frequency in response to a power control error message received from the power receiver, the power controller being configured to suspend the frequency power control loop in response to receiving a load change message and to resume the frequency power control loop in response to determining that the power receiver has implemented a load change.

[0029] This approach allows for improved performance and / or operation and / or implementation for wireless power transfer systems. This feature allows, for example, for reduced transient impact from load steps.

[0030] According to an optional feature of the invention, the frequency controller is configured to determine that the power receiver has performed a load change in response to at least one of expiration of a timer, detecting a change in power extracted from the power transmission signal, detecting a change in the power level of the drive signal, and the receiver receiving a load change execution message from the power receiver.

[0031] This approach allows for improved performance and / or operation and / or implementation for wireless power transfer systems.

[0032] In accordance with an optional feature of the invention, the load sensitivity reduction operating frequency is a resonant frequency of a transfer function for power transfer.

[0033] This can provide efficient and / or reliable determination of an appropriate load sensitivity reduction operating frequency. A particular advantage of this approach is that in many embodiments, certain estimation processes or calculations can be power transmitter-based, not necessarily performed in the power receiver. This can reduce costs in many scenarios. It can also facilitate implementation and / or provide improved backward compatibility.

[0034] According to an optional feature of the invention, the frequency determiner is configured to determine the load sensitivity reduced operating frequency as a coupling resonant frequency for the output resonant circuit, the coupling resonant frequency being a resonant frequency for the output resonant circuit for a transmitter coil coupled to a receiver coil of a power transfer input resonant circuit of the power receiver.

[0035] This can provide particularly advantageous operation in many embodiments.

[0036] The coupled resonant frequency is the resonant frequency of the output resonant circuit when it is coupled to the receiver coil and the power receiver is in a power transfer position for power transfer.

[0037] In some embodiments, the power transfer input resonant circuit has a quality factor of 20, 50, 100 or even 500 or greater during the resonant measurement time interval.

[0038] The coupling resonant frequency for the output resonant circuit corresponds to a resonant frequency for the drive signal, and specifically can correspond to a local maximum (or in some cases a local minimum) of a characteristic of the drive signal for varying the frequency of the drive signal.

[0039] In some embodiments, the frequency determiner is configured to determine the load sensitivity reduced operating frequency as a coupled resonant frequency of the output resonant circuit (during the resonant measurement time interval), the coupled resonant frequency being a resonant frequency of the output resonant circuit of the transmitter coil coupled to a receiving coil of the power transfer input resonant circuit of the power receiver, the power transfer input resonant circuit having a quality factor of 5, 10, 15, 20, 50 or 100 or greater (during the resonant measurement time interval).

[0040] According to an optional feature of the invention, the frequency determiner is configured to control the drive signal to generate a drive signal having a variable frequency and determine a coupled resonant frequency that is dependent on at least one of a voltage of the drive signal, a current of the drive signal, and a phase difference between the voltage of the drive signal and the current of the drive signal.

[0041] This can provide a particularly advantageous approach, allowing for a very effective and practical determination of an appropriate load desensitization operating frequency.

[0042] According to an optional feature of the invention, the frequency determiner is configured to control the driver to perform a frequency sweep of the drive signal from a high frequency to a low frequency and determine the combined resonant frequency as the frequency at which the resonance criterion of the drive signal is first detected to be satisfied.

[0043] This, in many embodiments, provides improved detection of the coupling resonant frequency for the output resonant circuit and therefore improved detection of the load-sensitized operating frequency, thus resulting in improved power transfer operation.

[0044] In some embodiments, the frequency determiner can be configured to control the driver to perform a frequency sweep of the drive signal from a low frequency to a high frequency and determine the combined resonant frequency as the first frequency at which it is detected that the resonance criterion of the drive signal is met.

[0045] According to an optional feature of the invention, the frequency determiner is configured to initiate determination of the load sensitivity reduced operating frequency in response to detecting a change in the operating point of the power transfer.

[0046] This can provide particularly advantageous operation in many embodiments.

[0047] In accordance with an optional feature of the invention, the frequency determiner is configured to determine the load sensitivity reduced operating frequency as a predetermined function of at least one power transfer parameter.

[0048] This can provide particularly advantageous operation in many embodiments, allowing for facilitated operation and / or implementation in many scenarios.

[0049] According to an optional feature of the invention, the power controller is further configured to implement a non-frequency power control loop by adapting a parameter of the drive signal other than the operating frequency in response to a power control error message received from the power receiver when the drive signal is operated at the load sensitivity reduced operating frequency.

[0050] This allows for improved performance in many embodiments, allowing for an improved compromise between adaptability and load change transient performance.

[0051] According to another aspect of the present invention, there is provided a power receiver for a wireless power transfer system including a power transmitter that transfers power to a power receiver using an inductive power transfer signal, the power receiver having an input resonant circuit including at least one capacitor and a power receiving coil configured to inductively extract power from the power transfer signal to power a variable load; a determiner configured to determine that a load change of the variable load is imminent, and a transmitter configured to transmit a load change message to the power transmitter indicating that the load change is imminent; a receiver arranged to receive a load change confirmation message from the power transmitter indicating that the power transmitter has changed the operating frequency of the power transfer signal to a load sensitivity reduced operating frequency; and a load controller configured to initiate a load change in response to receiving the load change confirmation message.

[0052] In some embodiments, the transmitter of the power receiver can be configured to send a load change execution message to the power transmitter after a load change, the load change execution message indicating that a load change has been executed.

[0053] A load may be coupled to the output of the resonant circuit via a power path that includes rectification, and possibly smoothing, of the output signal of the power receiver input resonant circuit.

[0054] The comments provided regarding the power transmitter apply mutatis mutandis to the power receiver.

[0055] In many embodiments, the input resonant circuit can be a series resonant circuit. In many embodiments, the input resonant circuit can be a parallel resonant circuit.

[0056] According to another aspect of the present invention, there is provided a wireless power transmission system including the power transmitter and power receiver described above.

[0057] According to another aspect of the present invention, there is provided a method of operating a power transmitter that wirelessly supplies power to a power receiver via an inductive power transfer signal, the power transmitter having an output resonant circuit including a transmitting coil and at least one capacitor, the method comprising the steps of generating a drive signal for the output resonant circuit to produce the inductive power transfer signal, providing a load sensitivity reduced operating frequency for the drive signal, receiving a load change message from the power receiver indicating an impending load change by the power receiver, changing the operating frequency of the drive signal to the load sensitivity reduced operating frequency in response to receiving the load change message, transmitting a load change confirmation message to the power receiver indicating that the frequency controller has changed the operating frequency to the load sensitivity reduced operating frequency, and in response to determining that the power receiver has performed the load change, changing the operating frequency from the load sensitivity reduced operating frequency to the load dependent operating frequency.

[0058] According to another aspect of the present invention, there is provided a method of operating a power receiver of a wireless power transfer system including a power transmitter configured to transfer power to the power receiver using an inductive power transfer signal, the method comprising the steps of: inductively extracting power from the power transfer signal to power a variable load; determining that a load change of the variable load is imminent; transmitting a load change message to the power transmitter indicating that a load change is imminent; receiving a load change confirmation message from the power transmitter indicating that the power transmitter has changed the operating frequency of the power transfer signal to a load sensitivity reduced operating frequency; and initiating the load change in response to receiving the load change confirmation message.

[0059] These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. [Brief explanation of the drawings]

[0060] Embodiments of the present invention will now be described, by way of example only, with reference to the drawings in which: [Figure 1]FIG. 2 illustrates an example of elements of a power transfer system according to some embodiments of the present invention. [Figure 2] FIG. 2 is a diagram showing an example of an equivalent circuit of the power transmission system of FIG. 1. [Figure 3] FIG. 2 illustrates an example of elements of a power transmitter according to some embodiments of the present invention. [Figure 4] FIG. 1 illustrates an example of a half-bridge inverter for a power transmitter. [Figure 5] FIG. 1 illustrates an example of a full-bridge inverter for a power transmitter. [Figure 6] FIG. 2 illustrates an example of elements of a power receiver according to some embodiments of the present invention. [Figure 7] FIG. 10 is a diagram showing an example of a power transfer function. [Figure 8] FIG. 10 illustrates an example of power transfer coil current and power receiver output voltage as a function of frequency. [Figure 9] 2 is a diagram illustrating an example of a time frame for the wireless power transfer system of FIG. 1. DETAILED DESCRIPTION OF THE INVENTION

[0061] The following description focuses on embodiments of the invention applicable to wireless power transfer systems that utilize power transfer approaches such as those known from the Qi or Ki standards, however, it will be understood that the invention is not limited to this application and may be applied to many other wireless power transfer systems.

[0062] 1 illustrates an example of a power transfer system according to some embodiments of the present invention. The power transfer system includes a power transmitter 101 including (or coupled to) a transmitter coil / inductor 103. The system further includes a power receiver 105 including (or coupled to) a receiver coil / inductor 107.

[0063] The system provides an inductive electromagnetic power transmission signal capable of inductively transmitting power from a power transmitter 101 to a power receiver 105. Specifically, the power transmitter 101 generates an electromagnetic signal, which is propagated as magnetic flux by a transmitter coil or inductor 103. The power transmission signal may typically have a frequency between about 20 kHz and about 500 kHz, typically in the range of 95 kHz to 205 kHz for Qi-compatible systems, and typically in the range of 20 kHz to 80 kHz for Ki-compatible systems. The transmitter coil 103 and the receiver coil 107 are loosely coupled, such that the receiver coil 107 picks up (at least a portion of) the power transmission signal from the power transmitter 101. Thus, power is transmitted from the power transmitter 101 to the power receiver 105 via wireless inductive coupling from the transmitter coil 103 to the receiver coil 107. The term power transmission signal is primarily used to refer to the induction signal / magnetic field (magnetic flux signal) between the transmitter coil 103 and the receiving coil 107, but it will be understood that equivalently it can also be considered and used to refer to the electrical signal supplied to the transmitter coil 103 or picked up by the receiving coil 107.

[0064] In an embodiment, power receiver 105 is specifically a power receiver that receives power via receiver coil 107. However, in other embodiments, power receiver 105 may include a metallic element, such as a metallic heating element, where the power transmission signal directly induces eddy currents that result in direct heating of the element.

[0065] The system is configured to transmit substantial power levels, and specifically, in many embodiments, the power transmitter can support power levels of 500 mW, 1 W, 5 W, 50 W, 100 W, or greater than 500 W. For example, for Qi-enabled applications, power transmission can typically range from 1 to 5 W for low-power applications (baseline power profile), up to 15 W for Qi standard version 1.2, up to 100 W for high-power applications such as power tools, laptops, drones, robots, etc., and from greater than 100 W to greater than 2000 W for very high-power applications such as Qi kitchen applications.

[0066] The operation of power transmitter 101 and power receiver 105 is described below with particular reference to embodiments that generally conform to the Qi or Ki standard (except for the modifications and extensions described (or resulting from) herein) or are suitable for higher power kitchen specifications being developed by wireless power consortia. In particular, power transmitter 101 and power receiver 105 conform to or are substantially compatible with elements of Qi standard version 1.0, 1.1, 1.2, or 1.3 (except for the modifications and extensions described (or resulting from) herein).

[0067] Many wireless power transfer systems, particularly high-power systems such as Ki, utilize resonant power transfer in which the transmitter coil 103 is part of a resonant circuit, and typically the receiver coil 107 is also part of the resonant circuit. In many embodiments, the resonant circuit may be a series resonant circuit, such that the transmitter coil 103 and receiver coil 107 may be coupled in series with corresponding resonant capacitors. The use of a resonant circuit tends to provide more efficient power transfer.

[0068] In most power transmission systems, before power transmission begins, a communication channel is established between the power transmitter 101 and the power receiver 105. Once communication is set up and identification of the two devices is achieved, the power transmitter 101 can begin transmitting power to the power receiver 105.

[0069] FIG. 2 shows an example of an electrical equivalent diagram of the power transfer function of the power transmitter 101 and power receiver 105. Various power transmitters and receivers may be present in a given system, and these may have significantly different characteristics and parameters. For example, coil size, induction, and load may vary significantly. Thus, system parameters may in practice vary significantly between different devices, mechanical configurations, positioning, etc., as specifically shown in FIG. 2. In particular, the placement of the power receiver, and therefore the relative positions of the receiver coil 107 and transmitter coil 103, may substantially affect the coupling between the coils, i.e., the primary (transmitting) inductor Lp and the secondary (transmitting) inductor Ls, and therefore significantly change system behavior.

[0070] Additionally, power receivers can have several different modes in which they operate, e.g., several loads are switched on or off in different modes. For example, if the power receiver is an air fryer appliance, the heating element can be turned on and off. This can cause a very large load step, e.g., from 50 to 1200 W or vice versa. Furthermore, such load switching can be repeated during operation of the device to keep the temperature constant. The system may also include, for example, a nonlinear load rather than a resistive element, or the power receiver may drive a motor, such as the motor in a food processor, which results in a completely different response of the system, which has significant implications, especially for control system design.

[0071] Wireless power transfer systems typically use a power control loop to steer the system toward a suitable operating point. This power control loop varies the amount of power transmitted from the power transmitter to the power receiver. The received power (or voltage or current) can be measured, and an error signal can be generated depending on the set power value. The device sends this error signal, or sometimes the desired power set point, to a power control function in the power transmitter to reduce the static error, ideally to zero.

[0072] However, the performance and operation of the system varies greatly depending on the combination and placement of the existing power transmitters and receivers, and therefore the appropriate operating point also varies greatly, including the conditions at the start-up / initialization of the power transfer, and therefore the optimal initial operating point also varies greatly.

[0073] A key issue for power transmission systems is adaptability to load changes. In practice, power receivers, such as kitchen appliances, can change loads very dramatically and very quickly. Adapting to such load steps and changes tends to be very difficult due to the inherent delay and relatively slow response time of control loops based on the power receiver sending error messages to the power transmitter. A phenomenon that can occur in some scenarios is that an overvoltage condition can result in response to a load step. The induced voltage at the power receiver can exhibit a step-like increase until the system stabilizes at the new operating point. Such voltage transients can be undesirable for many devices and scenarios. Below, we describe an approach that enables improved operation, particularly in relation to load changes. This approach is illustrated with reference to Figure 1 and its power transmitter and power receiver.

[0074] FIG. 3 shows elements of the power transmitter 101 of FIG. 1 in more detail.

[0075] The power transmitter 101 includes a driver 301 capable of generating a drive signal that is supplied to a power transmitter coil 103, which in return generates an electromagnetic power transmission signal, thereby providing power transmission to a power receiver 105. The transmitter coil 103 is part of an output resonant circuit that includes the transmitter coil 103 and a capacitor 303. In this example, the output resonant circuit is a series resonant circuit, although it will be understood that in other embodiments the output resonant circuit can be a parallel resonant circuit. It will be understood that any suitable resonant circuit may be used, including one that includes multiple inductors and / or capacitors.

[0076] The driver 301 generates the current and voltage supplied to the output resonant circuit and thus to the transmitter coil 103. The driver 301 is typically a drive circuit in the form of an inverter that generates an AC signal from a DC voltage. The output of the driver 301 is typically a switch bridge that generates the drive signal by appropriate switching of the switches of the switch bridge. Figure 4 shows a half-bridge switch bridge / inverter. Switches S1 and S2 are controlled so that they are never closed simultaneously. Alternately, S1 is closed while S2 is open, and S2 is closed while S1 is open. The switches are opened and closed at a desired frequency, thereby generating an AC signal at the output. Typically, the inverter output is connected to the transmitter inductor via a resonant capacitor. Figure 5 shows a full-bridge switch bridge / inverter. Switches S1 and S2 are controlled so that they are never closed simultaneously. Switches S3 and S4 are controlled so that they are never closed simultaneously. Alternately, switches S1 and S4 are closed while S2 and S3 are open, and switches S2 and S3 are closed while S1 and S4 are open, thereby producing a square wave signal at the output. The switches are opened and closed at the desired frequency.

[0077] The power transmitter 101 further includes a power transmitter controller 305 configured to control the operation of the power transmitter 101 according to a desired operating principle. Specifically, the power transmitter 101 may include many of the functions necessary to perform power control according to the Qi or Ki standards.

[0078] The power transmitter controller 305 is configured, among other things, to control the generation of the drive signal by the driver 301 and can specifically control the power level of the drive signal by controlling the driver 301 to adapt parameters of the drive signal. The power level of the power transmission signal can be adapted in response to power control messages received from the power receiver 105 during the power transfer phase, as will be explained in more detail below.

[0079] It is well known that the use of a resonant circuit that includes the transmitter coil 103 provides more efficient power transfer in many scenarios. Furthermore, a power receiver that similarly uses a resonant circuit, i.e., where the receiver coil 107 is part of the resonant circuit, can result in resonant power transfer that offers a number of advantages, including highly efficient power transfer and ease of control of the power transfer, for example, by controlling the frequency of the drive signal.

[0080] FIG. 6 shows some exemplary elements of the power receiver 105.

[0081] The receiver coil 107 is coupled to the power receiver controller 601 via a capacitor 603, which together with the receiver coil 107 forms an input resonant circuit. Thus, the power transfer may be a resonant power transfer between the resonant circuits. While FIG. 6 and the following description focus on the input resonant circuit being a series resonant circuit, it will be understood that in other embodiments the input resonant circuit may be a parallel resonant circuit. It will be understood that any suitable resonant circuit including multiple inductors and / or capacitors may be used.

[0082] The power receiver controller 601 couples the receiver coil 107 to the load 605 via a switch 607, which is capable of, among other things, shorting the load 605. The switch 607 may also typically be configured to completely disconnect / isolate the load 605 from the power receiver. The power receiver controller 601 includes a power control path that converts the power extracted by the receiver coil 107 into a supply suitable for the load 605. In some embodiments, the power receiver controller 601 can provide a direct power path that simply connects the input resonant circuit to the switch 607 or the load 605; i.e., the power path of the power receiver controller 601 can be implemented by simply two wires. In other embodiments, the power path can include, for example, a rectifier and possibly a smoothing capacitor to provide a DC voltage. In still other embodiments, the power path can include more complex functions, such as, for example, a voltage control circuit, an impedance matching circuit, a current control circuit, etc.

[0083] The input resonant circuit can include a power receiver coil and one or more capacitors coupled in series. Further, the input resonant circuit load can be coupled in series with the power receiver coil and one or more capacitors. In some embodiments, the input resonant circuit can include a power receiver coil and one or more capacitors coupled in parallel. Further, the input resonant circuit load can be coupled in parallel with the power receiver coil and one or more capacitors.

[0084] The output signal of the input resonant circuit can be a signal applied / supplied to the input resonant circuit load. The output voltage of the input resonant circuit can be a voltage across the input resonant circuit load. The output current of the input resonant circuit can be a current supplied to the input resonant circuit load. The input resonant circuit load typically corresponds to the load 605 of the power receiver and the intervening power path (which may include, for example, rectification and smoothing). It may also include, for example, internal circuitry of the power receiver that is powered from the power transmission signal.

[0085] It will be understood that in a typical power path, the load voltage / signal corresponds directly to the input resonant circuit's output voltage / signal. For example, rectification and smoothing result in a load voltage that is the same as the input resonant circuit's output voltage (perhaps with some offset resulting from the power path conversion of an AC signal to a DC signal). Therefore, references to the input resonant circuit's output voltage / signal and to the load voltage are used interchangeably, based on the assumption that the power path maintains a direct relationship between them. Thus, characteristics of one voltage / signal are directly translated to the other voltage / signal, and operations or considerations based on one of these apply equally to the other voltage / signal (mutatis mutandis). The power receiver controller 601 can include various power receiver controller functions required to perform power transfer, particularly functions required to perform power transfer according to the Qi or Ki standards.

[0086] During operation, the switch 607 can be configured to repeatedly switch the load 605. For example, the load 605 can be a heating element that is continuously switched in and out to maintain a constant temperature. Such an approach can lead to very large load changes occurring at relatively frequent intervals, potentially resulting in an overvoltage condition. In this case, the power transmission system includes features to improve load change operation and mitigate the adverse effects and characteristics associated with load changes.

[0087] During operation, the system is configured to control the drive signal such that the power transmission signal achieves appropriate operating parameters / characteristics and the power transmission operates at an appropriate operating point. To do so, the power transmitter is configured to control parameters of the drive signal using a power control loop in which the power characteristics of the power transmission signal / drive signal are controlled in response to power control error messages received from the power receiver.

[0088] At regular, typically frequent intervals, the power receiver transmits a power control error message to the power transmitter. In some embodiments, a direct power setpoint change message indicating the desired absolute power level (rather than a relative error message) may be transmitted. The power receiver 105 may include functionality to support such a power control loop; for example, the power transmitter may continuously monitor the power or voltage of the load signal supplied to the load and detect whether this is above or below a desired value. At regular intervals, it may generate a power control error message requesting that the power level of the power transmission signal be increased or decreased, and it may transmit this power control error message to the power transmitter.

[0089] Upon receiving the power control error message from the power receiver, the power transmitter can determine how the drive signal parameters should be modified to increase or decrease the power level of the power transmission signal as required, and it can control and adapt the drive signal parameters accordingly.

[0090] Therefore, a power control loop is used to control the power characteristics of the power transmission signal to produce a desired operating point at the power receiver. The operation of the power transmission is therefore controlled by the power control loop, and its effective operation is important to the performance of the system. Therefore, initializing or adapting the power control loop to the operating conditions is important for optimal performance.

[0091] To support such operation, the power receiver and power transmitter are equipped with the capability for two-way communication.

[0092] The power transmitter comprises a first receiver 307 configured to receive messages from the power receiver and a first transmitter 309 configured to transmit messages to the power receiver. Similarly, the power receiver comprises a second receiver 609 configured to receive messages from the power transmitter and a second transmitter 611 configured to transmit messages to the power transmitter.

[0093] Various approaches for communicating between a power transmitter and a power receiver are known, and any suitable approach may be used according to the preferences and requirements of a particular embodiment.

[0094] For example, the first transmitter 309 may be configured to transmit data to the second receiver 609 by modulating the power transmission signal, and the first receiver 307 may receive data from the second transmitter 611 by detecting the load modulation of the power transmission signal. Similarly, the second receiver 609 may be configured, for example, to decode and demodulate data modulated onto the power transmission signal, and the second transmitter may be configured to transmit data to the power transmitter 101 by load modulating the power transmission signal. In other embodiments, other approaches may be used in one or both directions, such as, for example, a separate communication function, such as an NFC communication function.

[0095] The power transmitter 101 comprises a power controller 315 that implements at least some of the functionality of the described power control loop. The power controller 315 is coupled to the first receiver 307 and is supplied with the power control error message received from the power receiver 105. The power controller 315 is configured to adapt parameters of the power transmission signal to control the power level of the power transmission signal (this can typically be done via the power transmitter controller 305). Thus, in this example, the power transmitter 101 comprises a frequency controller 311 that is configured to control parameters of the power transmission signal that affect the power level of the power transmitted to the power receiver.

[0096] The power control loop is configured to dynamically vary a parameter of the power transmission signal in response to the error signal, or in some embodiments, can dynamically vary two (or possibly more) parameters of the drive signal / power transmission signal in response to the power control error signal.

[0097] The power control loop can adapt the power level by adapting the drive signal and, therefore, the drive / operating frequency of the power transmission signal. The use of resonant circuits at the power transmitter output and the power receiver input results in a frequency-dependent power transfer function (e.g., the power receiver load voltage Vload as a function of the inverter / drive signal voltage amplitude Vin), and the power transmission level is frequency-dependent. The power level of the power transmission is typically more efficient at resonance, and therefore the power level will often be higher at such frequencies. Typically, a frequency range below (or above) the resonant peak of the power transmission level is used during power transmission, and the frequency is adapted in response to a power control error message. In the case of an error message requesting an increase in the power level, the frequency is changed to approach the resonant frequency, resulting in an increased power level. Conversely, in the case of an error message requesting a reduced power level, the frequency is changed to be farther away from the resonant frequency, resulting in a reduced power level.

[0098] The power transmitter 101 includes a frequency controller 311 configured to control the operating frequency of the drive signal, and during power control operation, the power controller 315 can control the frequency controller 311 to adjust the operating frequency of the drive signal to increase and decrease the operating frequency in response to the power control error message when the system is operating in power control mode.

[0099] Additionally, the power controller 315, in some embodiments, can adapt other parameters of the drive signal that may affect the power level. In particular, in many embodiments, the power controller 315 can be configured to adapt the duty cycle of the drive signal. For example, to reduce the power level, the duty cycle can be reduced and the frequency can be moved away from the resonant peak. Similarly, to increase the power level, the duty cycle can be increased and the frequency can be moved closer to the resonant peak. In some embodiments, other parameters, such as the voltage or current level / amplitude of the drive signal, can be changed.

[0100] As the load on the power receiver changes, the power control loop can adapt the parameters of the drive signal accordingly to provide the desired operating point. However, power control loop changes tend to be inherently very slow, and power control loops tend to have slow response times / low-pass frequency responses. This is caused, for example, by the requirement for stable, noise-efficient operation, as well as inherent lags and delays, such as the time required to communicate power control error messages (and therefore the low update rate of the associated error messages). This results in a suboptimal transient response; in fact, for very fast, significant load changes, the initial response behaves as if the power control loop did not exist. As a result, rapid load changes, especially load steps, can result in step-like changes in the EMF voltage induced in the power receiver coil 107, which is undesirable. In particular, overvoltage conditions / transients can occur.

[0101] However, the power transmitter 101 and power receiver 105 are configured to perform specific load change operations that can mitigate and reduce such effects. In this approach, the power receiver and power transmitter are configured to implement a specific approach for improved load changes that is particularly suited to large and / or fast load changes that may occur in an air fryer when switching a heating element on or off, and that can be used in connection with load steps, for example.

[0102] In this approach, the power receiver can, for example, send a load change message to the power transmitter indicating that a large load step / change is to be performed. In response, the power transmitter can interrupt the power control loop and proceed to change frequency to a specific frequency that will remain substantially constant throughout the load step. As described in more detail below, this specific frequency is selected as a specific frequency that the inventors have recognized can provide improved load change / step characteristics for power transfer operation. The power transmitter can then send a load change acknowledgement message to the power receiver to indicate that the power transmitter is ready for the load step / change. In response, the power receiver can proceed to execute the load step / change. The power transmitter can then proceed to change its operating frequency from the specific frequency back to a variable frequency that can be adapted as appropriate for the new operating point / load. In particular, it can return the frequency to the normal frequency operating range and resume power control operation to control the parameters of the drive signal / power transmission signal.

[0103] Thus, the power transmitter can be configured, among other things, to suspend the power control loop and change to a specific load change frequency in response to receiving a load change message. Following the load change, the power transmitter can return the frequency to a frequency within the power control loop operating range and resume the power control loop. In some embodiments, the change to a dedicated load change frequency can be performed while still allowing some power control loop functions to operate. For example, during a load change operation, only the frequency variation power control loop function is suspended, while other frequency loop functions, such as a duty cycle power control loop, can remain active. In some embodiments, the frequency variation power control loop remains active during the load change. For example, if the frequency variation power control loop is so slow that the frequency impact is relatively small during the time it takes to execute the load change process, the frequency variation power control loop can be maintained throughout.

[0104] This approach can provide improved performance with load changes, especially load steps, and reduced transient conditions, such as overvoltage conditions, which occur in many scenarios.

[0105] More specifically, the load change process can begin with the power receiver controller 601 determining that a load change is imminent. In this example, the load of the power receiver relative to the power transmission signal is variable rather than a constant load. For example, switching by the switch 607 can result in a variable load that can result in a large load step, e.g., potentially exceeding 100 W or even a 1 KW load step. In some scenarios, the load can also have a static / permanent component in addition to the variable load component.

[0106] The power receiver controller 601 can determine that a load change is about to occur / approach according to different algorithms and criteria in each embodiment. Often, this determination can be based on receiving an indication of such a change from the load 605. In some cases, the power receiver controller 601 can be configured to determine various parameters / characteristics to evaluate whether a load change is about to occur. In many embodiments, the power receiver controller 601 can control the switch 607, so that when conditions are deemed appropriate for a load change, the power receiver controller 601 can hold off switching the load until a load change acknowledgement message is received, instead of directly executing the load change. Thus, in many embodiments, the power receiver controller 601 can determine or detect that it is appropriate to change the load / perform a load step / flip the switch 607. However, instead of directly executing the load change / switch, the power receiver controller 601 can initiate this particular load change process.

[0107] For example, the power receiver may be an air fryer and the load 605 may be the heating element of the air fryer. The power receiver controller 601 may continuously measure the temperature, and if this moves outside of an acceptable range, the power receiver controller 601 may determine that it is appropriate to switch the load (switching it in or out as needed). However, instead of simply changing / switching the load, the power receiver controller 601 may initiate the load changing approach described.

[0108] When the power receiver controller 601 determines that a load change / switch / step is necessary or appropriate, it can control the second transmitter 611 to send a load change message to the power transmitter. The load change message thus indicates to the power transmitter that a load change is imminent. It will be appreciated that many approaches to communicating such messages in a power transmission system can be used, and any suitable approach can be used.

[0109] The first receiver 307 can be configured to receive a load change message from the power receiver, which indicates this to the power controller 315, which in certain examples responsively suspends at least the frequency part of the power control operation and further controls the frequency controller 311 to switch to a particular load change frequency during the load change operation / process.

[0110] The frequency controller 311 is coupled to the frequency determiner 313, which is configured to determine and provide a load change frequency to the frequency controller 311. The load change frequency is selected to be the load sensitivity reduced operating frequency. The frequency controller 311 is then configured to change the drive signal to this load sensitivity reduced operating frequency in response to receiving the load change message. In a particular example, the frequency controller 311 is further configured to maintain this frequency until the power change process is completed / the load is changed.

[0111] The load sensitivity reduced operating frequency can be a frequency at which sensitivity to load variations is lower than any frequency within the frequency operating range. The load sensitivity reduced operating frequency can specifically be the input resonant circuit output voltage sensitivity reduced operating frequency. This frequency can be a frequency at which the sensitivity of the output of the power receiver's input resonant circuit to load variations, and therefore typically the sensitivity to the rectified and / or smoothed load voltage, is reduced, e.g., relative to the power transfer operating frequency, and specifically, in many embodiments, relative to all frequencies within the power control frequency operating range. In many embodiments, the load sensitivity reduced operating frequency is a frequency at which sensitivity to load variations is minimized (the load sensitivity as a function of frequency can be minimal or at a minimum). The load sensitivity can be a measure of the power receiver load voltage variation / input resonant circuit output voltage variation as a function of the value of the power receiver load. The power receiver load voltage can be the voltage of the load signal generated by rectifying and smoothing the input resonant circuit's output signal.

[0112] In a power receiver, the load voltage can be the voltage of a first signal generated by rectifying and smoothing (typically using an energy storage device such as a capacitor) a signal induced at the input resonant frequency. This signal can be supplied to the load of the power receiver. The load sensitivity reduction operating frequency can be the frequency at which the sensitivity of the load voltage to variations in the value of the first signal is minimal or at a minimum.

[0113] The load sensitivity reduced operating frequency is the operating frequency of the drive signal at which the sensitivity of the output voltage of the power receiver's input resonant circuit to load changes introduced by the power receiver is reduced (relative to the operating frequency before receiving the pre-load change / load change message and / or relative to frequencies within the operating range of the power control loop). This is particularly the case in embodiments where the input resonant circuit is a series resonant circuit. In some embodiments, the load sensitivity reduced operating frequency is the operating frequency of the drive signal at which the sensitivity of the output current of the power receiver's input resonant circuit to load changes introduced by the power receiver is reduced (relative to the operating frequency before receiving the pre-load change / load change message and / or relative to frequencies within the operating range of the power control loop). This is particularly the case in embodiments where the input resonant circuit is a parallel resonant circuit.

[0114] The load sensitivity reduced operating frequency is an operating frequency (of the drive signal / power transfer signal) that corresponds to / substantially equal to the combined resonant frequency of the output resonant circuit. The load sensitivity reduced operating frequency can be an operating frequency (of the drive signal / power transfer signal) that corresponds to / substantially equal to the combined resonant frequency of the output resonant circuit and input resonant circuit configuration.

[0115] After the frequency controller 311 is switched to the load sensitivity reduced operating frequency and the frequency power control loop is interrupted, the first transmitter 309 transmits a load change confirmation message to the power receiver. The load change confirmation message indicates to the power receiver that the power transmitter has changed the operating frequency of the drive signal / power transmission signal to the load change frequency / load sensitivity reduced operating frequency. This indicates to the power receiver that the power transmitter is ready for a load change, and the power receiver begins to perform a load change accordingly. Therefore, the first transmitter 309 can transmit a load change confirmation message to the power receiver, where the load change confirmation message can be received by the second receiver 609. This load change confirmation message indicates that the load processor has changed the operating frequency to the load sensitivity reduced operating frequency, and in response to this message, the power receiver controller 601 and the switch 607 can proceed to perform a load change accordingly, i.e., specifically, the switch 607 can be controlled to perform a switchover (to switch in or switch out the load 605).

[0116] The power transmitter can detect this load change (different approaches can be used, as described below), and in response, the frequency controller 311 changes its operating frequency from a load-sensitivity reduced operating frequency to a load-dependent operating frequency. The load-dependent operating frequency is a frequency that can change as a function of the loading of the power transmission signal by the power receiver, and specifically depends on the load of the power receiver. The load-dependent operating frequency is a variable frequency, and specifically depends on the operating characteristics of the power transmission, for example, depending on, among other things, the power level of the power transmission signal and / or the loading of the power transmission signal by the power receiver.

[0117] In particular, the load-dependent operating frequency can be a frequency controlled by the power control loop. Thus, the load-dependent operating frequency is under the control of the power control loop. It is a frequency that is variable in response to a power control error message received from the power receiver. Thus, after a load change occurs, the power transmitter can return operation to a frequency within the power transmission operating range and, in particular, can restart the power control loop.

[0118] The initial load-dependent operating frequency can be, for example, a frequency determined in response to an expected or estimated load on the power receiver. For example, a load change message can indicate a new load power level after the change, and the frequency controller 311 can estimate an appropriate operating frequency for this load. The frequency controller 311 can then return the drive signal to this frequency after the load change. The power control loop can then proceed to adjust the operating frequency in response to the received error message to fine-tune the frequency. In other embodiments, the initial load-dependent operating frequency can simply be a fixed, predetermined frequency, such as the mid-frequency of the operating band, from which an appropriate operating point / frequency can be reached using the power control loop (thus switching to a load-dependent frequency can be a two-step process, first switching to a predetermined frequency and then adapting to the load-dependent frequency, typically by allowing the power control operation to adapt the drive / operating frequency).

[0119] The frequency determiner 313 can be configured to provide a load-sensitivity-reduced operating frequency as a frequency at which sensitivity to load changes is lower than any frequency within the frequency operating range. The inventors recognized that the sensitivity of the power transmission signal and load voltage at the power receiver varies significantly as a function of frequency. They further recognized that in most practical applications, the minimum sensitivity is outside the frequency power control loop operating range because the characteristic required for power control (power level changes) is incompatible with the minimum sensitivity requirement. In fact, this characteristic tends to exist at resonant frequencies that tend to be unsuitable for power control because monotonic dependence on frequency typically does not apply in these cases (at least not for all loads). The inventors further recognized that improved load step / change operation can be achieved by a process of interrupting normal operation during a load change in order to change the operating frequency to a frequency that reduces or minimizes the impact of the load change.

[0120] Figure 7 shows an example of a power transfer function for power transfer between a power transmitter and a power receiver. The power transfer function specifically reflects the rectified and smoothed output load voltage (amplitude) of the input resonant circuit / power receiver coil induced voltage / EMF as a function of the frequency of the power transmission signal / drive signal. This transfer function can also be considered to indicate the power level of the power transmission / power transmission signal / drive signal. In fact, for a resistive load element, the power level and the load voltage have a monotonic relationship (and the logarithmic representations of power and voltage correspond).

[0121] As can be seen, for low frequencies, the load voltage and associated power level increase monotonically with frequency; similarly, for high frequencies, the voltage and associated power level decrease monotonically with increasing frequency. Thus, the operating frequency ranges are defined as ranges 701, 703 where the voltage / power level is a monotonic function of frequency. Such operating ranges are at high or low frequencies. Therefore, the power controller 315 can perform power control by varying the frequency within these ranges.

[0122] The inventors further recognized that the actual transfer curve (shape and value) may depend significantly on the load presented by the power receiver to the power transmission signal, but that this dependence varies across frequency, with some frequencies exhibiting a much higher dependence than others. They further recognized that this can be exploited by the described approach by the power transmitter switching to a load-reduced operating frequency before a load change and switching back to a load-dependent frequency after the load change.

[0123] Figure 7 shows the power transfer function for different loads. As can be seen, a more resistive load (higher power / lower load resistance) tends to result in a single-peak transfer function, while a less resistive load (lower power / higher load resistance) tends to result in a double-peak transfer function, reflecting the individual resonant behavior of the power transmitter's output resonant circuit and the power receiver's input resonant circuit. Furthermore, as shown in Figure 7, the transfer function is highly dependent on the load, but this sensitivity is substantially lower at some frequencies than at others. Notably, there exists a frequency at which this load sensitivity is minimized, to the point where the transfer function is practically independent of the load value.

[0124] The frequency determiner 313, in some embodiments, can be configured to determine the load sensitivity reduced operating frequency based on the transfer function. In some embodiments, the load sensitivity reduced operating frequency can be determined as the frequency at which the load sensitivity is minimum.

[0125] In some embodiments, the frequency determiner 313 can be configured to determine the load sensitivity reduction operating frequency as a predetermined frequency. Indeed, in some embodiments, the frequency determiner 313 can store a fixed / static / predetermined load sensitivity reduction operating frequency that can be retrieved and provided to the frequency controller 311. Such a predetermined load sensitivity reduction operating frequency can reflect average characteristics and properties of the power receiver, such as the average nominal characteristics, the nominal relative position of the power receiver, etc. For example, during design and / or manufacturing, various measurements can be performed using a nominal power receiver at a nominal position. The frequency at which a load step leads to a minimum voltage step at the output of the input resonant circuit and / or the power receiver load / receiver coil can be determined as the appropriate load sensitivity reduction operating frequency and stored in the power transmitter. In some cases, this operation can be performed for different types of power receivers, and a load sensitivity reduction operating frequency can be determined and stored for each of multiple types of power receivers. During initialization of the power transfer, the power receiver can indicate to the power transmitter what type it is, and the frequency determiner 313 can accordingly identify the corresponding stored reduced load sensitivity operating frequency and provide it to the frequency controller 311 when appropriate.

[0126] In some embodiments, the frequency determiner 313 can determine the load sensitivity reduced operating frequency as a predetermined function of one or more power transfer parameters, which can be, for example, coupling coefficients, power levels, relative positions of the power receivers, etc.

[0127] For example, during manufacturing, the load sensitivity reduced operating frequency can be determined while varying the range of each power transfer parameter. Based on the respective measurements, an equation can be derived or fitted to provide the load sensitivity reduced operating frequency as a function of one or more power transfer parameters. This equation / function can then be stored in the frequency determiner 313. During operation, the frequency determiner 313 can measure the required operating parameters and evaluate this function to determine the load sensitivity reduced operating frequency using the measured power transfer parameters. For example, a coupling coefficient can be determined, and a function dependent on the coupling coefficient can be evaluated to generate the load sensitivity reduced operating frequency.

[0128] In some embodiments, several different functions / formulas can be determined for different receiver types and the appropriate function can be selected for the current power receiver type.

[0129] In many embodiments, the frequency determiner 313 can be configured to perform measurements to dynamically determine the load sensitivity reduced operating frequency for the current power transmitter and power receiver setup, including determining the load sensitivity reduced operating frequency to reflect current operating conditions such as the current location of the power receiver.

[0130] Such a determination can be performed, inter alia, by measuring and considering a power transfer function, e.g., reflected by the power or input resonant circuit output voltage or load voltage as a function of the drive signal frequency / power transfer signal frequency. In particular, the inventors have recognized that the load sensitivity reduction operating frequency can be advantageously determined as the resonant frequency of such a transfer function. The resonant frequency can be determined, inter alia, for a low load of the resonant circuit, i.e., for a high resistive load of the receiver resonant circuit / a high Q of the resonant circuit. For example, FIG. 7 shows that the smallest variation in response to load variations occurs at the peak 705 of the high Q transfer function. These frequencies correspond to the resonant frequencies of the transfer function and correspond to the (combined) resonant frequencies of the output and input resonant frequencies.

[0131] In the described system, the frequency determiner 313 is provided with functionality for determining a load-sensitivity reduced operating frequency based on detecting / measuring one or more resonant frequencies for the output resonant circuit (and / or equivalently for the drive signal) when coupled to the power receiver, specifically the receiver coil 107 and the input resonant circuit. This corresponds to determining the load-sensitivity reduced operating frequency based on the resonant frequency of the power transfer function, specifically the peak of the power transfer function (for a higher Q factor). In particular, the load-sensitivity reduced operating frequency can be determined as the combined resonant frequency of the output resonant circuit (when coupled to the input resonant circuit).

[0132] The frequency determiner 313 may be configured to determine at least one combined operating resonant frequency for the output resonant circuit / drive signal / power transfer function. While the following primarily refers to the combined resonant frequency of the output resonant circuit, it will be understood that such references may be replaced by reference to the (combined) resonant frequency of the power transfer function, the drive signal, and / or the power transfer signal.

[0133] This determination can be based specifically on measurements during a resonance measurement time interval when the coupling resonance frequency is the resonance frequency of the output resonant circuit in the presence of the power receiver, i.e., when the transmitter coil 103 is coupled to the receiver coil 107 of the power receiver.

[0134] Thus, the coupled resonant frequency reflects the effective resonant frequency of the output resonant circuit when the transmitter coil 103 is coupled to the receiver coil 107. Due to the coupling of the two coils, the effective inductance of the transmitter coil 103 is different from the self-inductance of the transmitter coil 103 when it is not coupled to either receiver coil 107. Similarly, the effective inductance of the receiver coil 107 is different from the self-inductance of the receiver coil 107 when it is not coupled to either transmitter coil 103. As a result, in the absence of coupling, the effective resonance will be different from the self-resonance. Furthermore, due to the coupling of the two coils, and therefore the two resonant circuits, the drive signal will effectively experience two (different) resonant frequencies; i.e., due to the coupling, the output resonant circuit effectively has two resonant frequencies that are different from the self (uncoupled) resonant frequency of the output resonant circuit.

[0135] The frequency determiner 313 may proceed to determine the load sensitivity reduction operating frequency as (one of) the detected coupled resonant frequencies.

[0136] In some embodiments, the resonance detector 307 can be configured to detect the coupled resonant frequency during the resonance measurement time interval by varying the frequency of the drive signal. Specifically, the resonance detector 307 can perform a frequency sweep over a frequency range that may correspond to a frequency interval where the coupled resonant frequency is expected to exist or where the coupling coefficient is deemed high enough to provide acceptable power transfer. The resonance detector 307 can then monitor the drive signal and, for example, detect extrema in the current or voltage amplitude. For example, in the case of a series resonant circuit, the resonance detector 307 can control the driver 301 to vary the frequency over a range with a constant voltage amplitude. The current amplitude can then be measured for different frequencies to determine the coupled resonant frequency at which the maximum current amplitude is measured. Figure 8 shows how the power transmitter coil current / current IPTX of the drive signal corresponds to the voltage VPRX at the power receiver coil / input resonant circuit output.

[0137] As another example, the resonance detector 307 can detect when the phase difference between the current and voltage of the drive signal is zero (or close to zero), i.e., when the load presented by the output resonant circuit is purely resistive.

[0138] In some embodiments, the frequency sweep of the drive signal may be from a higher frequency to a lower frequency, and the coupling resonant frequency may be determined as the frequency at which the resonance criterion (e.g., an extremum in the current or voltage amplitude of the signal or a zero phase difference between the voltage and current) is first detected to be satisfied. Thus, for example, instead of detecting a global extremum, the first local extremum may be detected.

[0139] Such an approach allows for the detection of the two largest coupled resonant frequencies, which is advantageous in some embodiments because sensitivity to load variations tends to be lower around the higher coupled resonant frequencies than for the lower coupled resonant frequencies.

[0140] Detection of coupled resonance frequencies based on power transfer functions, particularly detection based on peak detection and / or frequency sweeping (e.g., detection of zero phase difference resonance points), can typically be advantageously performed for situations where the Q factor is high.

[0141] In many embodiments, the quality factor of the input resonant circuit may be 10 or greater, typically higher, during the resonant measurement operation / measurement time interval, so that detection of the coupled resonant frequency is reliable and reasonably accurate, and thus a reduced load sensitivity operating frequency reflecting low load sensitivity can be determined.

[0142] In some embodiments, a high Q during the resonance measurement time interval may be ensured by the Q value of the input resonant circuit always being greater than 10, i.e., by the power receiver being designed so that the input resonant circuit always has a quality factor greater than 10. However, this is typically at odds with the desire to provide adequate power to the load. For example, for a Ki system, loading of the input resonant circuit with typical power values ​​results in a Q factor that is typically less than 5, and often less than 2.

[0143] Thus, in many embodiments, the power receiver may be configured to switch its operating mode from a low quality factor mode for at least some time outside the resonant measurement time interval to a high quality factor mode having a Q-factor of at least 10 during the resonant measurement time interval. This allows for more accurate determination of an appropriate load sensitivity reduction operating frequency. Thus, the power receiver may be configured to switch from a power transfer mode, in which the quality factor is not constrained to be greater than 10 and may actually be substantially less than 10 to allow for efficient power transfer, to a measurement mode during the resonant measurement time interval in which the quality factor is 10 or greater.

[0144] This can be accomplished, for example, by switch 607. For example, if the power path directly couples switch 607 and load 605 to the input resonant circuit, the switch may normally couple load 605 to the input resonant circuit outside of the resonant measurement time interval. However, during the resonant measurement time interval, switch 607 can disconnect load 605. In the case of an input parallel resonant circuit, switch 607 can, for example, disconnect load 605 so that no current is drawn from the input resonant circuit. In contrast, in the case of an input series resonant circuit, switch 607 can disconnect the load by shorting load 605, thereby shorting the power transfer input resonant circuit during the resonant measurement time interval.

[0145] In some embodiments, such a change in quality factor can occur without the power receiver having any specific functionality. For example, in some embodiments, a load can essentially provide a short circuit during startup, which can essentially allow a resonant measurement time interval prior to power transfer with a high-Q input resonant circuit. For example, when the load is a motor, it can start up nearly like a short circuit. As another example, when a rectifier and a large output capacitor are present in the power path, this also behaves nearly like a short circuit when the capacitor is discharged.

[0146] In many embodiments, a resonance measurement time interval (or at least one resonance measurement time interval) may precede power transmission. Specifically, determination of the load sensitivity reduced operating frequency may be performed during initialization of the power transmission operation. Thus, prior to power transmission, the power transmitter may determine an appropriate load sensitivity reduced operating frequency for a particular power receiver and a particular placement of the power receiver. It may then store this load sensitivity reduced operating frequency and initiate power transmission ready to perform a load change using the determined load sensitivity reduced operating frequency.

[0147] Furthermore, during the coupling coefficient estimation before power transfer, the power receiver can enter a measurement mode for coupling resonant frequency detection. Specifically, the switch 607 can short-circuit the load to provide a high quality factor to the input resonant circuit. When the system enters the power transfer phase, the power receiver can be switched to a normal power transfer operation mode, and specifically, the short-circuit can be removed.

[0148] In some embodiments, the approach for determining the load sensitivity reduced operating frequency can alternatively or additionally be applied during the power transfer phase. In many embodiments in which the determination is performed during the power transfer phase, the system can be configured to operate in a time-slotted mode, with the measurement and coupling resonant frequency detection being performed during a measurement time interval. The resonant measurement time interval can specifically be performed during a measurement time interval of a recurring time frame that further includes at least one power transfer time interval in which power is transferred to the power receiver.

[0149] Thus, in such an embodiment, the system may utilize time division during the power transfer phase. In particular, the detection of the coupling resonant frequency and the power transfer may be performed, for example, in separate time intervals, thereby allowing interference therebetween to be substantially reduced.

[0150] In this example, the driver 301 and transmitter coil 103 are configured to generate an electromagnetic power transfer signal for transmitting power to the power receiver during a power transfer interval. Additionally, the drive signal can be used to detect the coupling resonance frequency during a measurement time interval. The power transmitter can employ a repetitive time frame for the drive signal during the power transfer phase, where the time frame includes at least one power transfer time interval and at least one resonance measurement time interval. An example of such a repetitive time frame is shown in FIG. 9 , where the power transfer time interval is denoted by PT and the measurement time interval (which may also be referred to as a detection time interval) is denoted by D. In this example, each time frame FRM includes only one resonance measurement time interval and one power transfer time interval, which (as well as the time frame itself) have the same duration in each frame. However, it will be understood that in other embodiments, other time intervals (e.g., communication intervals, etc.) may be included in a time frame, or multiple resonance measurement time intervals and / or power transfer time intervals may be included in each time frame. Furthermore, in some embodiments, the duration of each time interval (and indeed the time frame itself) may vary dynamically.

[0151] Therefore, in this approach, measurement and power transmission are separated in the time domain, thereby reducing mutual interference from power transmission to measurement. Therefore, variability and uncertainty arising from variations in operating conditions for power transmission can be isolated from measurement and estimation, resulting in a more reliable and accurate estimation process. Furthermore, it allows a drive signal to be generated (and optimized) for detection of the coupling resonance frequency. In particular, the resonance detector 307 can perform a frequency sweep and perform operations suitable for this detection.

[0152] Furthermore, it allows the power receiver to be specifically adapted to provide improved or optimal characteristics for detection. In particular, in many embodiments, the power receiver can be switched from a power operation mode during a power transfer time interval in which a load is coupled to the input resonant circuit (and therefore the quality factor of the input resonant circuit is low) to a measurement mode in which the quality factor of the input resonant circuit is ensured to be high, for example by switch 607 shorting the load.

[0153] Thus, a time-slotted approach can enable or facilitate the performance of coupling coefficient estimation during the power transfer phase.

[0154] The described approach, in many embodiments, provides improved operation, particularly improved load switching operation, which can reduce voltage or current fluctuations / steps that occur as a result of, for example, a load step that is too fast to be compensated for by the power control loop.

[0155] The inventors have recognized that the power transfer behavior of a coupled resonant circuit, including in particular the behavior of the input resonant circuit, is behavior that can reduce fluctuations in the output of the input resonant circuit in response to load changes. Specifically, the inventors have recognized that this operation and behavior is frequency dependent, and that frequency can be used to control the desired behavior. Furthermore, the inventors have recognized that by adapting the frequency to have different behaviors for different operations, improved overall performance can be achieved.

[0156] When the load of the power receiver changes, for example, when the load's resistance changes, the power transfer operating point changes, resulting in an operating point where the voltages and currents in the elements of the power receiver's input resonant circuit and the power transmitter's output resonant circuit change to reflect the changing load. This can further result in a change in the drive signal. In other words, the load typically imposes constraints on the power transfer path, such as a constant voltage amplitude, along with a drive signal constraint, resulting in a near-instantaneous change to the new operating point at the appropriate current and voltage in the circuit so that the constraints are met. The power control loop then adapts the drive signal, and therefore the operating point, so that the power transfer provides the desired power to the load. However, instantaneous changes can result in undesirable transients if the load change is too fast for the power control loop to compensate. This can, for example, cause overvoltage transients supplied to the load.

[0157] Thus, when the power receiving load changes rapidly and / or significantly, a new operating point for power transfer occurs. However, the inventors have recognized that the load sensitivity reduction frequency reduces this change (as described when the load reduction frequency is set to the coupling resonant frequency) by reducing, and in some cases substantially eliminating, the change in the load voltage / input resonant circuit output voltage.

[0158] For example, if a load step occurs with the operating frequency set to the coupling resonant frequency, the load voltage may remain substantially constant. However, when the load changes suddenly (i.e., the power drawn by the load changes suddenly), the current in the load also changes significantly and suddenly. For example, if the power changes from 20 W to 2000 W (e.g., a heating element is switched on), the load current changes 100 times more suddenly, while the voltage remains constant.

[0159] To accommodate this changed load, the power extracted from the transmitter source (mains power) will also change, and in fact the extracted power may change by a corresponding factor (i.e., a factor of 100 in the specific example). However, this change will not necessarily change only the current in the power receiving coil, but will typically result in a change in both the primary and secondary coil voltages and currents. This is due to a change in the total impedance present as seen by the output resonant circuit.

[0160] The entire power transfer system, including the resonant circuit, then adapts to a new operating point that matches the changed load. However, this can be done so that the output voltage of the input resonant circuit, and therefore the load voltage, remains substantially constant despite changes in load. This can be achieved by setting the resonant frequency to the coupled resonant frequency, which is the characteristic operating point of the loosely coupled resonant coil system.

[0161] More specifically, in the example shown where the input resonant circuit is a series resonant circuit, the output currents in the receiver coil 107 and capacitor 603 are the same and the output voltage is given as the combination / sum of the voltages in the receiver coil 107 and capacitor 603.

[0162] The elements in the output and input resonant circuits are all passive, and to accommodate different powers, they need to adapt the current / voltage (similar to resistors). Both inductors and capacitors behave roughly linearly, so that as the current increases, the voltage also increases. This is similar to resistors, but there is a phase difference between the voltage and the current.

[0163] The output voltage of the input resonant circuit is therefore the sum of two different voltages with a phase offset. At the load sensitivity reduction operating frequency or coupled resonant frequency, this operating point is such that the voltages compensate each other so that the resulting output voltage of the input resonant circuit does not change (or at least changes by a smaller amount).

[0164] In particular, at the load sensitivity reduction operating frequency, the phase difference between the coil voltage and the capacitor voltage is 180°, and therefore the imaginary components are almost cancelled, which is independent of the attached load (but frequency dependent). At a certain resonant frequency, the input and output resonant circuits can be thought of as acting like a voltage divider independent of the load.

[0165] At all other frequencies, the phase is not exactly 0° or 180°, so the load affects the resonant circuit. Therefore, in the output and input resonant circuits, the voltage at the input remains the same, and as the current load in the power receiver increases, the current increases. The voltage on the power receiver coil is higher because the current is also higher. The voltage on the capacitor also changes because its current is also higher, but the voltage is out of phase with the coil voltage. Therefore, on the primary side, the voltages are roughly balanced. A corresponding / complementary phenomenon occurs in the input resonant circuit. The entire resonant circuit combination essentially functions as a feedthrough, but the internal voltages change to accommodate changes in power in the passive process. Thus, the component voltages change, but compensate to keep the output voltage of the input resonant circuit substantially constant.

[0166] In particular, for both inductors and capacitors, the voltage increases as the current increases (i.e., by increasing the power drawn by the load). However, the voltages have opposite phases at the appropriate frequencies, so these changes can be (at least partially) canceled out.

[0167] Thus, the system can provide improved transient performance by controlling the frequency such that the voltage difference at the output of the input resonant circuit, and therefore typically at the load, is reduced.

[0168] When the output resonant circuit and / or the input resonant circuit are parallel resonant circuits, the voltage across the elements, not the current, is the same. The explanation provided above with reference to series resonant circuits is applicable with references to current and voltage interchanged. Thus, this approach can be used to provide load adaptation with a substantially constant current.

[0169] However, a frequency of minimum voltage sensitivity may also occur for a parallel resonant circuit. Such a point may occur between two frequencies of minimum current sensitivity. This minimum voltage sensitivity frequency may be identified, for example, by a testing process in which the load is varied and voltage transients are measured for different drive frequencies. Similarly, in a series resonant circuit embodiment, a minimum current sensitivity frequency may be identified and used as described.

[0170] The power transmitter may use different approaches to determine that the power receiver has performed a load change in different embodiments.

[0171] In some embodiments, this determination may be implicit in that the power receiver is expected to perform a load change within a given time interval, which is measured by a timer. Thus, when the timer expires, the frequency controller 311 may proceed to designate that a load change has occurred, change the drive signal frequency to an operating range / load-dependent frequency, and restart the power control loop. The timer may be started, for example, when the load change message is transmitted, and thus timer expiration indicates that a certain, e.g., predetermined, time interval has occurred since the transmission of the load change message occurred, which may be designated as a load change having occurred.

[0172] In other embodiments, the frequency controller 311 can be configured to measure the load on the power transmission signal and detect that a load change has been performed by the power receiver if a corresponding load change is detected on the power transmission signal. Thus, after transmitting a load change message, the frequency controller 311 can monitor the power level of the drive signal and, therefore, the load on the power transmission signal. When a change of an appropriate size is detected, the frequency controller 311 can designate that the indicated load change has occurred.

[0173] In some embodiments, the power receiver can be configured to transmit a load change execution message to the power transmitter after executing the load change, and the power transmitter can determine that a load change has occurred in response to receiving the load change execution message. Thus, upon receiving the load change message, the power transmitter can switch to a load sensitivity reduced operating frequency and suspend the frequency power control loop until it receives the load change execution message. Once this message is received, the power transmitter returns the drive signal frequency to within the operating range and resumes the frequency power control loop.

[0174] In some embodiments, the frequency determiner 313 can be configured to determine the load sensitivity reduced operating frequency once and then use it. In some embodiments, it can be determined, for example, as part of initializing a power transfer, and a new load sensitivity reduced operating frequency can be determined whenever a new power transfer is initiated. This frequency can, in some embodiments, be used throughout the power transfer operation, and a new load sensitivity reduced operating frequency may not be determined until the next power transfer operation is performed.

[0175] In some embodiments, the frequency determiner 313 can be configured to initiate a load-sensitivity reduced operating frequency determination in response to detecting a change in the operating point of the power transfer. In some embodiments, during the power transfer phase, the frequency determiner 313 can continuously monitor the operating point. It can measure one or more parameters, such as, for example, a coupling coefficient or coupling coefficient indicator, a foreign object loss level, or the relative position of the power receiver with respect to the power transmitter. The frequency determiner 313 can determine a load-sensitivity reduced operating frequency for a system operating at a given operating point. However, if the operating point changes beyond a given threshold, the frequency determiner 313 can detect this change and, in response, initiate a new determination of the load-sensitivity reduced operating frequency (e.g., using any of the techniques described above). Once the new load-sensitivity reduced operating frequency is determined, it can replace the previous load-sensitivity reduced operating frequency, and thus, the updated load-sensitivity reduced operating frequency appropriate for the new operating point is used.

[0176] As mentioned above, the power control loop may be a control loop that adapts only the frequency of the drive signal / power transmission signal. During the power change time interval, the power transmitter may hold the frequency constant at the load sensitivity reduced operating frequency as described and discontinue this frequency (adaptive / variable) power control loop.

[0177] However, in some embodiments, the power transmitter can be configured to operate a power control loop that adapts another parameter other than the drive signal frequency. For example, such a frequency-independent (adaptive / variable) power control loop can adapt the voltage amplitude of the drive signal, the current amplitude of the drive signal, and / or the duty cycle of the drive signal. In some embodiments, such a non-frequency power control loop can be operated during the power change time interval, specifically during the time that the frequency power control loop is suspended. In such a case, power control is still performed during the power change interval while simultaneously ensuring that the impact of the load change is reduced or minimized. Such an approach can provide improved overall load change operation.

[0178] The non-frequency power control loop may operate only when the frequency power control loop is interrupted, i.e., only during a load change interval. However, in other embodiments, the non-frequency power control loop may also operate outside of a load change interval, and in particular may operate continuously and / or simultaneously with the frequency power control loop.

[0179] It will be understood that, for clarity, the above description has described embodiments of the invention with reference to different functional circuits, units, and processors. However, it will be apparent that any suitable distribution of functionality between different functional circuits, units, or processors may be used without departing from the invention. For example, functionality shown to be performed by separate processors or controllers may be performed by the same processor or controller. References to specific functional units or circuits should therefore be seen only as references to suitable means for providing the described functionality, rather than to indicative of a strict logical or physical structure or organization.

[0180] In some embodiments, the load sensitivity reduced operating frequency can be determined as the coupled resonant frequency of the output resonant circuit. In such cases, all references to the load sensitivity reduced operating frequency can be replaced by references to the coupled resonant frequency. Thus, in such embodiments, a power transmitter for wirelessly supplying power to a power receiver via an inductive power transfer signal is provided, the power transmitter including: an output resonant circuit including a transmit coil and at least one capacitor; a driver configured to generate a drive signal for the output resonant circuit to produce the inductive power transfer signal; a frequency determiner configured to provide a coupled resonant frequency of the output resonant frequency; a receiver configured to receive a load change message from the power receiver indicating an impending load change by the power receiver; a frequency controller configured to change the operating frequency of the drive signal to the coupled resonant frequency in response to receiving the load change message; and a transmitter configured to send a load change confirmation message to the power receiver indicating that the frequency controller has changed the operating frequency to the coupled resonant frequency, wherein the frequency controller is further configured to change the operating frequency from the coupled resonant frequency in response to determining that the power receiver has performed a load change.

[0181] In some embodiments, the term "load sensitivity reduced operating frequency" can be replaced with the term "coupled resonant frequency."

[0182] The coupled resonant frequency can be the resonant frequency of the configuration of the output resonant circuit of the power transmitter and the input resonant circuit of the power receiver. The coupled resonant frequency can be the resonant frequency of the output resonant circuit of the power transmitter when it is (inductively / electromagnetically) coupled to the input resonant circuit of the power receiver.

[0183] The invention may be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and / or digital signal processors. The elements and components of embodiments of the invention may be physically, functionally, and logically implemented in any suitable way. Indeed, functionality may be implemented in a single unit, in multiple units, or as part of other functional units. Thus, the invention may be implemented in a single unit, or may be physically and functionally distributed between different units, circuits, and processors.

[0184] Although the present invention has been described in connection with several embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the appended claims. Furthermore, while certain features may appear to be described in connection with particular embodiments, those skilled in the art will recognize that various features of the described embodiments may be combined in accordance with the present invention. In the claims, the term "comprising" does not exclude the presence of other elements or steps.

[0185] Furthermore, although individually listed, a plurality of means, elements, circuits, or method steps may be implemented by, for example, a single circuit, unit, or processor. Furthermore, although individual features may be included in different claims, these may be advantageously combined in some cases, and their inclusion in different claims does not imply that a combination of features is not feasible and / or advantageous. Moreover, the inclusion of a feature in one category of claims does not imply a limitation to this category, but rather indicates that the feature is equally applicable to other claim categories, as appropriate. The inclusion of a feature in a dependent claim of one independent claim does not imply a limitation to this independent claim, but rather indicates that the feature is equally applicable to other independent claims, where appropriate. Furthermore, the order of features in the claims does not imply a particular order in which the features must operate, and in particular the order of individual steps in method claims does not imply that the steps must be performed in this order. Rather, steps may be performed in any suitable order. Furthermore, a reference to the singular does not exclude a plurality. Thus, references to "a," "an," "first," "second," etc. do not exclude a plurality. The terms "first" and "second" are used merely as labels (typically / commonly and as indicated by the context) and are not intended to have any limiting meaning or effect as such. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.

Claims

1. 1. A power transmitter for wirelessly supplying power to a power receiver via an inductive power transmission signal, comprising: an output resonant circuit including a transmitter coil and at least one capacitor; a driver configured to generate a drive signal for the output resonant circuit to generate the inductive power transfer signal; a frequency determiner configured to provide a load-reduced operating frequency for the drive signal; a receiver configured to receive a load change message from the power receiver indicating an impending load change by the power receiver; a frequency controller configured to change the operating frequency of the drive signal to the load sensitivity reduced operating frequency in response to receiving the load change message; a transmitter configured to transmit a load change confirmation message to the power receiver indicating that the frequency controller has changed the operating frequency to the load sensitivity reduced operating frequency; and the frequency controller is further configured to change the operating frequency from the load sensitivity reduced operating frequency to a load dependent operating frequency in response to determining that the power receiver has performed a load change. Power transmitter.

2. a power controller configured to implement a frequency power control loop by adapting the operating frequency in response to a power control error message received from the power receiver; 2. The power transmitter of claim 1, wherein the power controller is configured to suspend the frequency power control loop in response to receiving the power change message and to resume the frequency power control loop in response to the frequency controller determining that the power receiver has performed a load change.

3. The frequency controller: timer expiration, detecting a change in power extracted from the power transmission signal; a change in the power level of the drive signal; the receiver receiving a load change execution message from the power receiver; configured to determine that the power receiver has performed a load change in response to at least one of:

3. A power transmitter according to claim 1 or 2.

4. 4. The power transmitter of claim 1, wherein the load sensitivity reduced operating frequency is a resonant frequency of a power transfer function.

5. 5. The power transmitter of claim 4, wherein the frequency determiner is configured to determine the load sensitivity reduced operating frequency as a coupled resonant frequency of the output resonant circuit, the coupled resonant frequency being a resonant frequency of the output resonant circuit relative to the transmitter coil coupled to a receiver coil of a power transfer input resonant circuit of the power receiver.

6. 6. The power transmitter of claim 5, wherein the frequency determiner is configured to control the driver to generate the drive signals having different frequencies and to determine the coupled resonant frequency dependent on at least one of a voltage of the drive signal, a current of the drive signal, and a phase difference between the voltage of the drive signal and the current of the drive signal.

7. 7. The power transmitter of claim 5, wherein the frequency determiner is configured to control the driver to perform a frequency sweep of the drive signal from a high frequency to a low frequency and determine the coupled resonant frequency as the frequency at which a resonance criterion for the drive signal is first detected to be satisfied.

8. 8. A power transmitter according to any one of claims 5 to 7, wherein the frequency determiner is configured to initiate determination of the load sensitivity reduced operating frequency in response to detecting a change in the operating point of power transfer.

9. 9. A power transmitter according to any preceding claim, wherein the frequency determiner is configured to determine the load sensitivity reduced operating frequency as a predetermined function of at least one power transfer parameter.

10. 10. The power transmitter of claim 1, wherein the power controller is further configured to implement a non-frequency power control loop by adapting parameters of the drive signal other than the operating frequency in response to a power control error message received from the power receiver when the drive signal is operated at the load sensitivity reduced operating frequency.

11. 1. A power receiver for a wireless power transfer system including a power transmitter that transfers power to the power receiver using an inductive power transfer signal, an input resonant circuit including at least one capacitor and a power receiver coil configured to inductively extract power from the power transmission signal to power a variable load; a determiner configured to determine that a load change of the variable load is imminent; a transmitter configured to transmit a load change message to the power transmitter indicating that the load change is imminent; a receiver configured to receive a load change confirmation message from the power transmitter indicating that the power transmitter has changed the operating frequency of the power transmission signal to a load sensitivity reduced operating frequency; a load controller configured to initiate the load change in response to receiving the load change confirmation message; A power receiver having

12. A wireless power transmission system comprising a power transmitter according to any one of claims 1 to 10 and a power receiver according to claim 11.

13. 1. A method of operating a power transmitter that wirelessly supplies power to a power receiver via an inductive power transmission signal, the power transmitter having an output resonant circuit including a transmitter coil and at least one capacitor, the method comprising: generating a drive signal for the output resonant circuit to generate the inductive power transfer signal; providing a load-reduced operating frequency for the drive signal; receiving a load change message from the power receiver indicating an impending load change by the power receiver; changing the operating frequency of the drive signal to the load sensitivity reduced operating frequency in response to receiving the load change message; sending a load change confirmation message to the power receiver indicating that the frequency controller has changed the operating frequency to the load sensitivity reduced operating frequency; and changing the operating frequency from the load sensitivity reduced operating frequency to a load dependent operating frequency in response to determining that the power receiver has performed a load change.

14. 1. A method of operating a power receiver in a wireless power transfer system including a power transmitter configured to transfer power to the power receiver using an inductive power transfer signal, the method comprising: extracting power from the power transmission signal by induction to power a variable load; determining that a load change of the variable load is imminent; transmitting a load change message to the power transmitter indicating that the load change is imminent; receiving a load change confirmation message from the power transmitter indicating that the power transmitter has changed the operating frequency of the power transmission signal to a load sensitivity reduced operating frequency; and initiating the load change in response to receiving the load change confirmation message.

15. 15. A method of operating a wireless power transfer system including a power transmitter that transfers power via an inductive power transfer signal to a power receiver, wherein the power transmitter performs the method of claim 13 and the power receiver performs the method of claim 14.

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