Wireless power transfer

Abstract

A wireless power system comprises a power receiver (105) that receives power from a power transmitter (101), the power transmitter comprising an output resonant circuit, the output resonant circuit comprising a transmitter 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 reduced load-sensitive operating frequency for the drive signal, and a frequency controller (311) changes the operating frequency of the drive signal to the reduced load-sensitive operating frequency in response to receiving a load change message from the power receiver. The transmitter (309) sends a load change acknowledgement message to the power receiver (105) to indicate a change in frequency. In response to determining that the power receiver (105) has performed the load change, the frequency controller (311) changes the operating frequency from a reduced load-sensitive operating frequency to a load-dependent operating frequency.

Description

Technical Field

[0001] The present invention relates to wireless power transfer and particularly, but not exclusively, to the operation of a power transmitter and a power receiver supporting wireless transfer for typically high power devices such as kitchen appliances. Background Art

[0002] Most electrical products today require dedicated electrical contacts to be powered from an external power source. However, this is often unrealistic and requires the user to physically plug in a connector or otherwise establish physical electrical contact. Typically, power requirements also vary significantly, and most current devices are provided with their own dedicated power source, resulting in a typical user having a large number of different power sources, each dedicated to a specific device. Although the use of internal batteries can avoid the need for wired connections to a power source during use, this only provides a partial solution because the battery will need to be recharged (or replaced). The use of batteries may also significantly increase the weight of the device and potential cost and size.

[0003] In order to provide a significantly improved user experience, the use of wireless power has been proposed, wherein power is inductively transferred from a transmitter inductor in a power transmitter device to a receiver coil in an individual device.

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

[0005] Such an arrangement allows wireless power transfer to the device without requiring any wired or physical electrical connection to be made. In fact, it can simply allow the device to be placed near or on top of the transmitter coil for external recharging or powering. For example, the power transmitter device can be arranged with a horizontal surface on which the device can simply be placed for powering.

[0006] In addition, such a wireless power transmission arrangement can be advantageously designed to enable a power transmitter device to be used with a range of power receiver devices. Specifically, a wireless power transmission method known as the Qi specification has been defined and is currently being further developed. This method allows a power transmitter device that meets the Qi specification to be used with a power receiver device that also meets the Qi specification without the devices having to be from the same manufacturer or having to be dedicated to each other. The Qi standard also includes a function for allowing operation to be adapted to a specific power receiver device (e.g., depending on a specific power consumption).

[0007] The Qi specification was developed by the Wireless Power Consortium and more information can be found, for example, on their website:

[0008] http: / / www.wirelesspowerconsortium.com / index.html, where, in particular, the defined specification documents can be found.

[0009] The Wireless Power Consortium continues to develop the Ki specification (also known as the Cordless Kitchen Specification) based on the Qi specification, which aims to provide safe, reliable and efficient wireless power transfer to kitchen appliances. Ki supports much higher power levels up to 2.2KW.

[0010] Furthermore, power receivers such as many kitchen appliances and devices may have several operating modes which may have very different power levels, e.g. an appliance may include several different loads which may be turned on or off. As a specific example, an air fryer appliance may turn a heating element on and off resulting in typical load steps of, e.g., between 50 and 1200 W. Such load changes may be repeated, e.g., during operation of an air fryer in order to maintain a reasonably constant temperature.

[0011] The system may also have nonlinear loads, for example, instead of resistive components, the load may be, for example, a motor with significant reactance (e.g., a food processor appliance). Such nonlinear loads may result in a completely different response of the system and may have a significant impact on the control system design.

[0012] Typically, the system uses a control loop to compensate for load variations and ensure that the correct operating point is reached. This control loop will adjust the amount of power sent to the power receiver / device. The received power (or voltage or current) can be measured by the power receiver in conjunction with the set point power value, and an error signal can be determined. The power receiver can send this error signal to the control system in the power transmitter, which can adjust the power level to ideally reduce the static error to zero.

[0013] While such a control loop may provide advantageous and suitable operation in many scenarios, ideal performance may not be achieved in all cases. For example, a control loop will inherently have dynamic performance, which is a trade-off between response time on the one hand and accuracy and noise performance on the other hand. Practical control loops will tend to have non-ideal transient responses and tend not to provide ideal regulation when significant load changes and steps occur.

[0014] Furthermore, the resulting system response and power transfer function may generally result in voltage changes and transients induced at the power receiver. In some cases, such transients may result in the output voltage changing significantly when a load step occurs, and this may specifically result in a potential overvoltage condition. In many cases, such transients and voltage changes may be undesirable.

[0015] Accordingly, improved operations and methods for power transmission systems would be advantageous, and in particular, allowing increased flexibility, reduced cost, reduced complexity, improved backward compatibility, improved suitability for higher power level transmission, improved adaptation to specific operating conditions, improved adaptation to power receiver load variations, changes and / or steps, improved adaptation to varying operating conditions, reduced voltage / current variations, reduced overvoltage / current conditions and / or improved performance would be advantageous. Summary of the invention

[0016] Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

[0017] According to one aspect of the present invention, there is provided a power transmitter for wirelessly providing power to a power receiver via an inductive power transfer signal; the power transmitter comprising: an output resonant circuit comprising a transmitter coil and at least one capacitor; a driver arranged to generate a drive signal for the output resonant circuit to generate the inductive power transfer signal; a frequency determiner arranged to provide a reduced load-sensitive operating frequency for the drive signal, a receiver arranged to receive a load change message from the power receiver, the load change message indicating that a load change by the power receiver is imminent; a frequency controller arranged to change the operating frequency of the drive signal to the reduced load-sensitive operating frequency in response to receiving the load change message; a transmitter arranged 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 reduced load-sensitive operating frequency; wherein the frequency controller is further arranged to change the operating frequency from the reduced load-sensitive operating frequency to a load-dependent operating frequency in response to determining that the power receiver has performed the load change.

[0018] In many embodiments, the present invention can provide improved power delivery. In many embodiments, it can provide improved load change performance and operation. In many scenarios, it can reduce the impact of load changes, and particularly the impact of large and / or rapid load changes. In many scenarios, it can reduce overvoltage and / or overcurrent conditions associated with load changes and load steps. The method can allow for improved transient performance of load changes and load steps. The method can reduce costs in many scenarios. In many embodiments, it can facilitate implementation and / or provide improved backward compatibility.

[0019] In some embodiments, the reduced load sensitivity operating frequency may be a substantially minimum load sensitivity operating frequency. The minimum may be a global or local minimum. The reduced load sensitivity operating frequency may be a frequency that is less sensitive to load changes than any frequency belonging to an operating frequency range. The operating frequency range may be a power control loop operating frequency, e.g. representing a range of operating frequencies that may be set by the power control loop. The sensitivity may be the sensitivity of a power transfer parameter (such as, e.g., rectified and smoothed inductive power receiver coil voltage and / or current, power level, drive signal voltage / current / power / phase, power receiver load voltage, etc.) to changes / changes in the load of the power transfer signal / drive signal and often to changes / changes in the load of the power receiver.

[0020] In some embodiments, the reduced load sensitivity operating frequency may be a frequency for which a change in the output voltage of the input resonant circuit of the power receiver as a function of a change in the loading of the input resonant circuit is reduced relative to a range of power control operating frequencies (a range of frequencies that the drive signal may be controlled to have by the power control loop).

[0021] In some embodiments, the reduced load sensitivity operating frequency may be a frequency for which a change in the output voltage of the input resonant circuit of the power receiver as a function of a change in loading of the input resonant circuit has a local minimum and in some cases a global minimum.

[0022] In some embodiments, the reduced load sensitivity operating frequency may be a frequency for which a change in the output voltage of the input resonant circuit of the power receiver as a function of a change in loading of the input resonant circuit has a local minimum and in some cases a global minimum.

[0023] The above may be particularly applicable to 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 coupled in series. In such a circuit, the current through the load, the receiver coil of the input resonant circuit and the capacitor of the input resonant circuit is the same.

[0024] In some embodiments, the reduced load sensitivity operating frequency may be a frequency for which a change in the output current of the input resonant circuit of the power receiver as a function of a change in the loading of the input resonant circuit is reduced relative to a range of power control operating frequencies (a range of frequencies over which the drive signal may be controlled to be controlled by the power control loop).

[0025] In some embodiments, the reduced load sensitivity operating frequency may be a frequency for which a change in the output current of the input resonant circuit of the power receiver as a function of a change in loading of the input resonant circuit has a local minimum and in some cases a global minimum.

[0026] In some embodiments, the reduced load sensitivity operating frequency may be a frequency for which a change in the output current of the input resonant circuit of the power receiver as a function of a change in loading of the input resonant circuit has a local minimum and in some cases a global minimum.

[0027] The above may be particularly applicable to 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 coupled 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.

[0028] In some embodiments, the reduced load sensitivity operating frequency may be a frequency of an output resonant frequency of the power transmitter and a coupled resonant frequency of an input resonant circuit of the power receiver.

[0029] According to an optional feature of the invention, the power transmitter comprises a power controller arranged to implement a frequency power control loop by adjusting the operating frequency in response to a power control error message received from the power receiver, and wherein the power controller is arranged to suspend the frequency power control loop in response to receiving a load change message, and to restart the frequency power control loop in response to the frequency controller determining that the power receiver has performed a load change.

[0030] The method may allow improved performance and / or operation and / or implementation of a wireless power transfer system. The features may, for example, allow reduction of transient effects from load steps.

[0031] According to an optional feature of the invention, the frequency controller is arranged to determine that the power receiver has performed a load change in response to at least one of the following: expiration of a timer; detection of a change in power extracted from the power transfer signal; detection of a change in the power level of the drive signal; and receipt by the receiver of a load change performed message from the power receiver.

[0032] The method may allow for improved performance and / or operation and / or implementation of wireless power transfer systems.

[0033] According to an optional feature of the invention, the reduced load sensitivity operating frequency is a resonant frequency of a transfer function for the power transfer.

[0034] This may provide an efficient and / or reliable determination of a suitable reduced load sensitivity operating frequency. A particular advantage of the method is that in many embodiments it may be power transmitter based without having to perform specific estimation processes or calculations at the power receiver. This may reduce costs in many scenarios. It may facilitate implementation and / or provide improved backward compatibility.

[0035] According to an optional feature of the invention, the frequency determiner is arranged to determine the reduced load sensitivity operating frequency as a coupling resonant frequency for an output resonant circuit, the coupling resonant frequency being a resonant frequency of the output resonant circuit for the transmitter coil, the transmitter coil being coupled to a receiver coil of a power transfer input resonant circuit of the power receiver.

[0036] This may provide particularly advantageous operation in many embodiments.

[0037] The coupling resonant frequency may be the resonant frequency of the output resonant circuit when this is coupled to the receiver coil and when the power receiver is in the power transfer position for power transfer.

[0038] In some embodiments, the power transfer input resonant circuit has a quality factor of no less than 20, 50, 100, or even 500 during the resonance measurement time interval.

[0039] The coupled resonant frequency for the output resonant circuit corresponds to the resonant frequency for the drive signal and may specifically correspond to a local maximum (or possibly a local minimum) of a property of the drive signal for a varying frequency of the drive signal.

[0040] In some embodiments, the frequency determiner is arranged to determine the reduced load sensitivity operating frequency as a coupled resonant frequency of the output resonant circuit (during the resonance measurement time interval), the coupled resonant frequency being the resonant frequency of the output resonant circuit of a transmitter coil coupled to a receiver coil of a power transfer input resonant circuit of the power receiver, the power transfer input resonant circuit having a quality factor of not less than 5, 10, 15, 20, 50 or even 100 (during the resonance measurement time interval).

[0041] According to an optional feature of the invention, the frequency determiner is arranged to control the driver to generate the drive signal with a varying frequency, and to determine the coupling resonant frequency based 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.

[0042] This may provide a particularly advantageous approach and may enable efficient and practical determination of a suitable reduced load sensitivity operating frequency.

[0043] According to an optional feature of the invention, the frequency determiner is arranged to control the driver to perform a frequency sweep of the drive signal from a higher frequency towards a lower frequency and to determine the coupling resonant frequency as a first detected frequency satisfying a resonance criterion for the drive signal.

[0044] In many embodiments, this may provide improved detection of the coupled resonant frequency of the output resonant circuit and therefore provide a reduced load sensitivity operating frequency, thereby enabling improved power transfer operation.

[0045] In some embodiments, the frequency determiner may be arranged to control the driver to perform a frequency sweep of the drive signal from a lower frequency towards a higher frequency and to determine the coupling resonant frequency as a first detected frequency satisfying a resonance criterion for the drive signal.

[0046] According to an optional feature of the invention, the frequency determiner is arranged to initiate determination of the reduced load sensitivity operating frequency in response to detecting a change in the operating point of the power transfer.

[0047] This may provide particularly advantageous operation in many embodiments.

[0048] According to an optional feature of the invention, the frequency determiner is arranged to determine the reduced load sensitivity operating frequency as a predetermined function of at least one power transfer parameter.

[0049] This can provide particularly advantageous operation in many embodiments. It can allow facilitating operation and / or implementation in many scenarios.

[0050] According to an optional feature of the present invention, the power controller is further arranged to implement a non-frequency power control loop by adjusting 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 operates at the reduced load sensitivity operating frequency.

[0051] This may allow improved performance in many embodiments and may allow an improved tradeoff between adaptability and load change transient performance.

[0052] According to another aspect of the present invention, a power receiver for a wireless power transmission system is provided, the wireless power transmission system comprising a power transmitter, the power transmitter transmitting power to the power receiver using an inductive power transmission signal, the power receiver comprising: an input resonant circuit comprising at least one capacitor and a power receiver coil, the power receiver coil being arranged to extract power from the power transmission signal by induction to provide power to a variable load; a determiner, arranged to determine that a load change of the variable load is about to occur; a transmitter, arranged to send a load change message to the power transmitter, the load change message indicating that the load change is about to occur; a receiver, arranged to receive a load change confirmation message from the power transmitter, the load change confirmation message indicating that the power transmitter has changed the operating frequency of the power transmission signal to a reduced load sensitivity operating frequency; and a load controller, arranged to initiate the load change in response to receiving the load change confirmation message.

[0053] In some embodiments, the transmitter of the power receiver may be arranged to send a load change performed message to the power transmitter after the load change. The load change performed message indicates that the load change has been performed.

[0054] The load may be coupled to the output of the resonant circuit via a power path including rectification and possibly smoothing of the output signal of the input resonant circuit of the power receiver.

[0055] The comments provided with respect to the power transmitter also apply mutatis mutandis to the power receiver.

[0056] In many embodiments, the input resonant circuit may be a series resonant circuit. In many embodiments, the input resonant circuit may be a parallel resonant circuit.

[0057] According to another aspect of the present invention, a wireless power transmission system is provided, comprising the power transmitter and the power receiver as described above.

[0058] According to another aspect of the present invention, there is provided an operating method for a power transmitter, the power transmitter wirelessly providing power to a power receiver via an inductive power transfer signal; the power transmitter comprises: an output resonant circuit comprising a transmitter coil and at least one capacitor; and the method comprises: generating a drive signal for the output resonant circuit to generate the inductive power transfer signal; providing a reduced load-sensitive operating frequency for the drive signal, receiving a load change message from the power receiver, the load change message indicating that a load change through the power receiver is imminent; changing the operating frequency of the drive signal to the reduced load-sensitive operating frequency in response to receiving the load change message; sending 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 reduced load-sensitive operating frequency; and changing the operating frequency from the reduced load-sensitive operating frequency to a load-dependent operating frequency in response to determining that the power receiver has performed the load change.

[0059] According to another aspect of the present invention, a method for operating a power receiver for a wireless power transmission system, the wireless power transmission system including a power transmitter, the power transmitter being arranged to transmit power to the power receiver using an inductive power transmission signal, the method comprising: extracting power from the power transmission signal by induction to provide power to a variable load; determining that a load change for the variable load is imminent; sending a load change message to the power transmitter, the load change message indicating that the load change is imminent; receiving a load change confirmation message from the power transmitter, the load change confirmation message indicating that the power transmitter has changed the operating frequency of the power transmission signal to a reduced load-sensitivity operating frequency; and initiating the load change in response to receiving the load change confirmation message.

[0060] These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0062] Figure 1 illustrates examples of elements of a power delivery system according to some embodiments of the present invention;

[0063] Figure 2 The diagram shows the Figure 1 An example of an equivalent circuit of a power transmission system;

[0064] Figure 3 illustrates an example of elements of a power transmitter according to some embodiments of the present invention;

[0065] Figure 4 An example of a half-bridge inverter for a power transmitter is illustrated;

[0066] Figure 5 An example of a full-bridge inverter for a power transmitter is illustrated;

[0067] Figure 6 illustrates examples of elements of a power receiver according to some embodiments of the present invention;

[0068] Figure 7 An example of a power transfer function is illustrated;

[0069] Figure 8 illustrates examples of power transmitter coil current and power receiver output voltage as a function of frequency;

[0070] Fig. 9 Picture shows Figure 1 Example of a time frame for a wireless power transfer system. DETAILED DESCRIPTION

[0071] The following description focuses on an embodiment of the invention applicable to a wireless power transfer system utilizing a power transfer method such as known from the Qi specification or the Ki specification. However, it should be understood that the invention is not limited to this application but may be applied to many other wireless power transfer systems.

[0072] Figure 1 An example of a power transmission system according to some embodiments of the present invention is illustrated. The power transmission system includes a power transmitter 101, which includes (or is coupled to) a transmitter coil / inductor 103. The system also includes a power receiver 105, which includes (or is coupled to) a receiver coil / inductor 107.

[0073] The system provides an inductive electromagnetic power transfer signal that can inductively transfer power from the power transmitter 101 to the power receiver 105. In particular, the power transmitter 101 generates an electromagnetic signal that is propagated as magnetic flux by the transmitter coil or inductor 103. The power transfer signal can typically have a frequency between about 20 kHz and about 500 kHz, and the frequency is often in the range of 95 kHz to 205 kHz for Qi-compatible systems, or in the range of 20 kHz to 80 kHz for Ki-compatible systems. The transmitter coil 103 is loosely coupled with the power receiving coil 107, and therefore, the power receiving coil 107 picks up (at least part of) the power transfer signal from the power transmitter 101. Therefore, power is transferred from the power transmitter 101 to the power receiver 105 via wireless inductive coupling from the transmitter coil 103 to the power receiving coil 107. The term power transfer signal is primarily used to refer to the induced signal / magnetic field (magnetic flux signal) between the transmitter coil 103 and the power receiving coil 107, but it should be understood that, by equivalents, it can also be considered and used as a reference to the electrical signal provided to the transmitter coil 103 or picked up by the power receiving coil 107.

[0074] In this example, the power receiver 105 is specifically a power receiver that receives power via a receiver coil 107. However, in other embodiments, the power receiver 105 may include a metallic element, such as a metallic heating element, in which case the power transmission signal directly induces eddy currents resulting in direct heating of the element.

[0075] The system is arranged to transfer substantial power levels, and in particular the power transmitter may in many embodiments support power levels in excess of 500 mW, 1 W, 5 W, 50 W, 100 W or 500 W. For example, for Qi corresponding applications, the power transfer may be typically in the 1-5 W power range for low power applications (baseline power profile), up to 15 W for Qi specification version 1.2, in the range up to 100 W for higher power applications (such as power tools, laptops, drones, robots, etc.), and in excess of 100 W and reaching over 2000 W for very high power applications (such as for example for Ki kitchen applications).

[0076] Hereinafter, the operation of the power transmitter 101 and the power receiver 105 will be described with specific reference to embodiments generally in accordance with the Qi or Ki specifications (except for modifications and enhancements described herein (or as a result thereof)) or embodiments adapted for the higher power kitchen specifications developed by the Wireless Power Alliance. Specifically, the power transmitter 101 and the power receiver 105 may comply with or be substantially compatible with elements of Qi specification versions 1.0, 1.1, 1.2, or 1.3 (except for modifications and enhancements described herein (or as a result thereof)).

[0077] Many wireless power transfer systems, and in particular high power systems such as Ki, utilize resonant power transfer, where the transmitter coil 103 is part of a resonant circuit, and typically the receiver coil 107 is also part of a resonant circuit. In many embodiments, the resonant circuit may be a series resonant circuit, and thus the transmitter coil 103 and the 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.

[0078] In most power transfer systems, before initiating power transfer, a communication channel is established between the power transmitter 101 and the power receiver 105. When communication has been established and recognition of the two devices has been achieved, the power transmitter 101 can begin sending power to the power receiver 105.

[0079] An example of an electrical equivalent diagram of the power transfer function of the power transmitter 101 and the power receiver 105 is shown in Figure 2 A wide range of power transmitters and power receivers may exist in a given system, and these power transmitters and power receivers may have substantially different properties and parameters. For example, coil size, inductance, and load may vary significantly. Thus, Figure 2 The system parameters specifically represented in may in practice vary significantly between different devices, mechanical configurations, positioning, etc. In particular, the placement of the power receiver and therefore the relative position of the receiver coil 107 and the transmitter coil 103 substantially affects the coupling between the coils (i.e., the primary (power transmitter side) inductor Lp and the secondary (power transmitter side) inductor Ls), and thus may significantly change the system behavior.

[0080] Furthermore, power receiving devices may have several different modes in which they operate, such as, for example, where several loads are turned on or off in different modes. For example, for a power receiver that is an air fryer appliance, a heating element may be turned on and off. This may, for example, result in a very significant load step from, say, 50 W to 1200 W, and vice versa. Furthermore, such load switching may be repeated during operation of the device to keep the temperature constant. The system may also contain nonlinear loads, for example, instead of resistive components, the power receiver may drive a motor, such as a motor of a food processor. This results in a completely different response of the system, and this has a great impact on the control system design in particular.

[0081] Typically, wireless power transfer systems employ a power control loop in order to steer the system to the appropriate operating point. This power control loop varies the amount of power sent 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 along with the set point power value. The appliance sends this error signal, or possibly the desired power set point, to the power control function in the power transmitter to ideally reduce the static error to zero.

[0082] However, since system performance and operation vary widely depending on the existing power transmitter and power receiver combination and placement, the appropriate operating point also varies widely. This includes the conditions at start-up / initialization of power transfer, and therefore the optimal initial operating point also varies widely.

[0083] A significant problem with power transmission systems is adaptability to load changes. In practice, a power receiver such as a kitchen appliance may change load very drastically and very quickly. Adapting to such load steps and changes is often very difficult due to the inherent hysteresis and relatively slow response time of the control loop based on the power receiver sending error messages to the power transmitter. A phenomenon that may occur in some scenarios is that an overvoltage condition may arise in response to a load step. The induced voltage at the power receiver may exhibit a step-change increase until the system stabilizes at the new operating point. Such voltage transients may be undesirable for many devices and scenarios, and in the following, a method will be described that may in particular allow for improved operation in conjunction with load changes. Reference will be made to Figure 1 The method is described with reference to a power transmitter and a power receiver thereof.

[0084] Figure 3 A more detailed illustration Figure 1 Elements of the power transmitter 101.

[0085] The power transmitter 101 includes a driver 301 that can generate a drive signal that is fed to the transmitter coil 103, which in turn generates an electromagnetic power transfer signal to provide power transfer to the 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, but it should be understood that in other embodiments, the output resonant circuit can be a parallel resonant circuit. It should be understood that any suitable resonant circuit can be used, including a resonant circuit including multiple inductors and / or capacitors.

[0086] The driver 301 generates a current and a voltage which is fed to the output resonant circuit and hence to the transmitter coil 103. The driver 301 is typically a drive circuit in the form of an inverter which generates an alternating signal from a DC voltage. The output of the driver 301 is typically a switching bridge which generates the drive signal by appropriate switching of the switches of the switching bridge. Figure 4 A half-bridge switching bridge / inverter is shown. Switches S1 and S2 are controlled so that they are never closed at the same time. Alternatingly, 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, generating an alternating signal at the output. Typically, the output of the inverter is connected to the transmitter inductor via a resonant capacitor. Figure 5 A full-bridge switching bridge / inverter is shown. Switches S1 and S2 are controlled so that they are never closed at the same time. Switches S3 and S4 are controlled so that they are never closed at the same time. Alternatingly, switches S1 and S4 are closed while S2 and S3 are open, and then S2 and S3 are closed while S1 and S4 are open, producing a square wave signal at the output. The switches are opened and closed at the desired frequency.

[0087] The power transmitter 101 further comprises a power transmitter controller 305, which is arranged to control the operation of the power transmitter 101 according to a desired operating principle. In particular, the power transmitter 101 may comprise a number of functions required to perform power control according to the Qi specification or the Ki specification.

[0088] The power transmitter controller 305 is specifically arranged to control the generation of the drive signal by the driver 301, and it can specifically control the power level of the drive signal by controlling the driver 301 to adjust the parameters of the drive signal. The power level of the power transfer signal can be adjusted in response to a power control message received from the power receiver 105 during the power transfer phase, as will be described in more detail later.

[0089] As is well known, the use of a resonant circuit including the transmitter coil 103 provides more efficient power transfer in many scenarios. Furthermore, having a power receiver that also employs a resonant circuit (i.e., where the receiver coil 107 is part of the resonant circuit) can result in resonant power transfer, which provides many advantages, including efficient power transfer and easy control of the power transfer, such as by controlling the frequency of the drive signal.

[0090] Figure 6 Some exemplary elements of the power receiver 105 are illustrated.

[0091] 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 resonant circuits. Figure 6 The following description will focus on the input resonant circuit being a series resonant circuit, but it should be understood that in other embodiments, the input resonant circuit can be a parallel resonant circuit. It should be understood that any suitable resonant circuit can be used, including resonant circuits including multiple inductors and / or capacitors.

[0092] The power receiver controller 601 couples the receiver coil 107 to the load 605 via a switch 607, which may specifically be capable of short-circuiting and / or short-circuiting the load 605. The switch 607 may also be generally arranged to completely decouple / disconnect 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 suitable power supply for the load 605. In some embodiments, the power receiver controller 601 may 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 may be simply implemented by two wires. In other embodiments, the power path may include, for example, a rectifier and possible smoothing capacitors to provide a DC voltage. In yet other embodiments, the power path may include more complex functions, such as a voltage control circuit, an impedance matching circuit, a current control circuit, etc.

[0093] The input resonant circuit may include a power receiver coil and one or more capacitors coupled in series. In addition, the input resonant circuit load may be coupled in series with the power receiver coil and the one or more capacitors. In some embodiments, the input resonant circuit may include a power receiver coil and one or more capacitors coupled in parallel. In addition, the input resonant circuit load may be coupled in parallel with the power receiver coil and the one or more capacitors.

[0094] The output signal of the input resonant circuit may be a signal applied / provided to the input resonant circuit load. The output voltage of the input resonant circuit may be a voltage across the input resonant circuit load. The output current of the input resonant circuit may be a current provided to the input resonant circuit load. The input resonant circuit load typically corresponds to the load 605 of the power receiver and the intermediate power path (which may, for example, include rectification and smoothing). It may also include, for example, internal circuits of the power receiver powered by the power transfer signal.

[0095] It will be appreciated that for a typical power path, the load voltage / signal corresponds directly to the output voltage / signal of the input resonant circuit. For example, rectification and smoothing will result in a load voltage that is the same as the output voltage of the input resonant circuit (possibly with some offset caused by the power path conversion of the AC signal to the DC signal). Therefore, references to the output voltage / signal of the input resonant circuit and references to the load voltage are used interchangeably, based on the assumption that the power path maintains a direct relationship between them. Therefore, the properties of one voltage / signal are directly converted to another, and operations or considerations based on one of these (mutatis mutandis) are equally applicable to the other. The power receiver controller 601 may include various power receiver controller functions required to perform power transfer, particularly functions required to perform power transfer according to the Qi or Ki specifications.

[0096] During operation, the switch 607 may be arranged to repeatedly switch the load 605 on and off. For example, the load 605 may be a heating element that is continuously switched on and off to maintain a constant temperature. Such an approach may result in very significant load changes occurring at relatively frequent intervals, which may result in overvoltage conditions. In the present case, the power transmission system includes functionality for improving load change operations and mitigating adverse effects and properties associated with load changes.

[0097] In operation, the system is arranged to control the drive signal so that the power transfer signal reaches suitable operating parameters / properties and so that the power transfer operates at a suitable operating point. To do so, the power transmitter is arranged to control the parameters of the drive signal using a power control loop, wherein the power properties of the power transfer signal / drive signal are controlled in response to a power control error message received from the power receiver.

[0098] Periodically and typically frequently, the power receiver sends power control error messages to the power transmitter. In some embodiments, direct power set point change messages indicating the desired absolute power level (rather than relative error messages) may be sent. The power receiver 105 includes functionality for supporting such a power control loop, for example, the power transmitter may continuously monitor the power or voltage of the load signal provided to the load and detect whether this is higher or lower than the desired value. It may generate a power control error message at regular intervals that requests an increase or decrease in the power level of the power transfer signal, and it may send the power control error message to the power transmitter.

[0099] When 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 transfer signal as requested. It can then control and adjust the drive signal parameters accordingly.

[0100] A power control loop is accordingly employed which controls the power properties of the power transfer signal to produce a desired operating point at the power receiver. The operation of the power transfer is therefore controlled by the power control loop, and its efficient operation is critical to the performance of the system. Initializing or adapting the power control loop to the operating conditions is therefore critical to optimal performance.

[0101] To support such operation, the power receiver and the power transmitter include functionality for bidirectional communication.

[0102] The power transmitter comprises a first receiver 307 arranged to receive a message from the power receiver and a first transmitter 309 arranged to send a message to the power receiver. Similarly, the power receiver comprises a second receiver 609 arranged to receive a message from the power receiver and a second transmitter 611 arranged to send a message to the power receiver.

[0103] Various methods for communicating between a power transmitter and a power receiver are known, and any suitable method may be used depending on the preferences and requirements of an individual embodiment.

[0104] For example, the first transmitter 309 may be arranged to send data to the second receiver 609 by modulating the power transfer signal, and the first receiver 307 may receive data from the second transmitter 611 by detecting the load modulation of the power transfer signal. Similarly, the second receiver 609 may be arranged to decode and demodulate the data modulated onto the power transfer signal, and the second transmitter may be arranged to send data to the power transmitter 101 by modulating the load of the power transfer signal. In other embodiments, other methods may be used, for example, a separate communication function may be employed in one or both directions, such as an NFC communication function.

[0105] The power transmitter 101 comprises a power controller 315 implementing 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 fed with a power control error message received from the power receiver 105. The power controller 315 is arranged to adjust a parameter of the power transfer signal to control the power level of the power transfer signal (this may typically be done via the power transmitter controller 305). Thus, in this example, the power transmitter 101 comprises a frequency controller 311, which is arranged to control a parameter of the power transfer signal that affects the power level of the power transferred to the power receiver.

[0106] The power control loop is arranged to dynamically change a parameter of the power transfer signal in response to the error signal, or in some embodiments it may dynamically change two (or possibly even more) parameters of the drive signal / power transfer signal in response to the power control error signal.

[0107] The power control loop can adjust the power level by adjusting the drive / operating frequency of the drive signal (and therefore the drive / operating frequency of the power transfer signal). A resonant circuit is used in the power transmitter output and the power receiver input to implement a power transfer function (e.g., a power receiver load voltage Vload as a function of the inverter / drive signal voltage amplitude Vin), which is frequency-dependent and the power transfer level depends on the frequency. The power level of the power transfer is generally more efficient at resonance, and therefore the power level can often be higher at such a frequency. Typically, a frequency range below (or above) the resonant peak of the power transfer level is used during power transfer, where the frequency is adjusted in response to a power control error message. For an error message requesting an increased power level, the frequency is changed to be closer to the resonant frequency, resulting in an increased power level. Conversely, for an error message requesting a reduced power level, the frequency is changed to be further away from the resonant frequency, resulting in a reduced power level.

[0108] The power transmitter 101 includes a frequency controller 311, which is arranged to control the operating frequency of the drive signal. During power control operation, when the system operates in the power control mode, the power controller 315 can therefore control the frequency controller 311 to adjust the operating frequency of the drive signal in response to the power control error message to increase and decrease the operating frequency.

[0109] In addition, in some embodiments, the power controller 315 may be able to adjust other parameters of the drive signal that may affect the power level. Specifically, in many embodiments, the power controller 315 may also be arranged to adjust the duty cycle of the drive signal. For example, to reduce the power level, the duty cycle may be reduced and the frequency may be moved further away from the resonant peak. Similarly, to increase the power level, the duty cycle may be increased and the frequency may be moved closer to the resonant peak. In some embodiments, other parameters may be changed, such as the voltage or current level / amplitude of the drive signal.

[0110] When the power receiver load changes, the power control loop may adjust the parameters of the drive signal accordingly to provide the desired operating point. However, power control loop changes tend to be inherently quite slow, and the power control loop tends to have a low response time / low-pass frequency response. This may be due, for example, to the requirement for stable and noise-efficient operation, and also caused by inherent hysteresis and delays such as the time required to communicate power control error messages (and the low update rate of the error messages associated therewith). This may result in a suboptimal transient response, and indeed for very rapid and significant load changes, the initial response may be as if the power control loop does not exist. As a result, rapid load changes and in particular load steps may result in a step change in the EMF voltage induced in the power receiver coil 107, which may be undesirable. In particular, an overvoltage condition / transient may occur.

[0111] However, the power transmitter 101 and the power receiver 105 are arranged to perform specific load change operations that can mitigate and reduce such effects. In this method, the power receiver and the power transmitter are arranged to implement a specific method for improved load change that may be particularly suitable for large and / or rapid load changes and may be used, for example, in conjunction with a load step, such as may occur in an air fryer when this turns a thermal element on or off.

[0112] In this method, the power receiver may send a load change message to the power transmitter indicating, for example, that a large load step / change is to be performed. In response, the power transmitter may proceed to suspend the power control loop and change the frequency to a specific frequency that remains substantially constant throughout the load step. As will be described in more detail below, the specific frequency is selected as a specific frequency that the inventors have realized can provide improved load change / step properties for the power transfer operation. The power transmitter may then send a load change confirmation message to the power receiver to indicate that the power transmitter is ready for a load step / change. In response, the power receiver may proceed to perform the load step / change. Subsequently, the power transmitter may proceed to change the operating frequency from the specific frequency back to a variable frequency that can adapt to the new operating point / load. Specifically, it may change the frequency back to the normal frequency operating range and restart the power control operation to control the parameters of the drive signal / power transfer signal.

[0113] Therefore, the power transmitter can be specifically arranged to suspend the power control loop and change to a specific load change frequency in response to receiving a load change message. After the load change, the power transmitter can change the frequency back to a frequency within the power control loop operating range and restart the power control loop. In some embodiments, changes to the dedicated load change frequency can be performed while still allowing some power control loop functions to operate. For example, only the frequency change power control loop function can be suspended, while other frequency loop functions (such as the duty cycle power control loop) can remain active throughout the load change operation. In some embodiments, the frequency change power control loop can even remain active during the load change. For example, if the frequency power control loop is so slow that the impact on the frequency during the time it takes to perform the load change process will be relatively small, the frequency power control loop can always be maintained.

[0114] The method may provide improved load change, and in particular improved load step, performance while reducing transients and, for example, overvoltage conditions that occur in many scenarios.

[0115] In more detail, the load change process may begin when the power receiver controller 601 determines that a load change of the load is imminent. In this example, the load of the power receiver on the power transfer signal is not a constant load, but is variable. For example, switching by the switch 607 may result in a variable load that may result in a large load step, such as a load step potentially exceeding 100W or even 1KW. In some scenarios, the load may have a static / permanent component in addition to a variable load component.

[0116] In different embodiments, the power receiver controller 601 may determine that a load change is pending / imminent according to different algorithms and criteria. In many cases, the determination may be based on an indication of such a change received from the load 605. In some cases, the power receiver controller 601 may be arranged to determine various parameters / characteristics and evaluate whether a load change is about to occur. In many embodiments, the power receiver controller 601 may control the switch 607, and thus when the condition is deemed suitable for a load change, the power receiver controller 601 may suppress the switching of the load instead of directly performing a load change until a load change confirmation message is received. Therefore, in many embodiments, the power receiver controller 601 may determine or detect that it will be appropriate to change the load / perform a load step / switching the switch 607. However, instead of directly performing a load change / switching, the power receiver controller 601 may initiate a specific load change process.

[0117] For example, the power receiver may be an air fryer and the load 605 may be a heating element of the air fryer. The power receiver controller 601 may continuously measure the temperature and when this moves outside of an acceptable range, the power receiver controller 601 may determine that it is appropriate to switch the load (on or off as appropriate). However, instead of just changing the load / switch, the power receiver controller 601 initiates the load change method described.

[0118] When the power receiver controller 601 determines that a load change / switch / step is required or appropriate, it may control the second transmitter 611 to send a load change message to the power transmitter. Thus, the load change message indicates to the power transmitter that a load change is imminent. It should be understood that many methods of communicating such messages in a power transmission system may be used, and any suitable method may be used.

[0119] The first receiver 307 may be arranged to receive a load change message from the power receiver. The first receiver 307 indicates this to the power controller 315, which in response in a specific example suspends at least the frequency portion of the power control operation and also controls the frequency controller 311 to switch to a specific load change frequency for the duration of the load change operation / process.

[0120] The frequency controller 311 is coupled to a frequency determiner 313, which is arranged to determine a load change frequency and provide the load change frequency to the frequency controller 311. The load change frequency is selected as a reduced load sensitivity operating frequency. The frequency controller 311 is then arranged to change the drive signal to the reduced load sensitivity operating frequency in response to receiving the load change message. In a specific example, the frequency controller 311 is also arranged to maintain the frequency until the power change process has been completed / the load has changed.

[0121] The reduced load sensitivity operating frequency may be a frequency that is less sensitive to load variations than any frequency within the frequency operating range. The reduced load sensitivity operating frequency may specifically be a reduced input resonant circuit output voltage sensitivity operating frequency. The frequency may be a frequency for which the sensitivity to load variations of the output of the input resonant circuit of the power receiver (and thus the rectified and / or smoothed load voltage is generally reduced) is reduced, for example 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 reduced load sensitivity operating frequency may be a frequency at which the sensitivity to load variations is minimized (which may be a local or global minimum of the load sensitivity as a function of frequency). The load sensitivity may be an indication of a 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 may be the voltage of a load signal generated by rectification and smoothing of the output signal of the input resonant circuit.

[0122] At the power receiver, the load voltage may be the voltage of the first signal generated by rectifying and smoothing (typically using an energy storage such as a capacitor) the signal induced in the input resonant frequency. The signal may be provided to the load of the power receiver. The reduced load sensitivity operating frequency may be the frequency at which the sensitivity of the load voltage to changes in the value of the load of the first signal is at a (local or global) minimum.

[0123] The reduced load sensitivity operating frequency may be an operating frequency of the drive signal for which the output voltage of the input resonant circuit of the power receiver is less sensitive to load changes introduced by the power receiver (relative to the operating frequency before the load change / receiving the load change message and / or a frequency within the operating range of the power control loop). This may be particularly true for embodiments where the input resonant circuit is a series resonant circuit. In some embodiments, the reduced load sensitivity operating frequency may be an operating frequency of the drive signal for which the output current of the input resonant circuit of the power receiver is less sensitive to load changes introduced by the power receiver (relative to the operating frequency before the load change / receiving the load change message and / or a frequency within the operating range of the power control loop). This may be particularly true for embodiments where the input resonant circuit is a parallel resonant circuit.

[0124] The reduced load sensitivity operating frequency may be an operating frequency (of the drive signal / power transfer signal) corresponding to / substantially equal to the coupled resonant frequency of the output resonant circuit. The reduced load sensitivity operating frequency may be an operating frequency (of the drive signal / power transfer signal) corresponding to / substantially equal to the coupled resonant frequency of the output resonant circuit and the input resonant circuit arrangement.

[0125] After the frequency controller 311 has switched to the reduced load sensitivity operating frequency and the frequency power control loop has been suspended, the first transmitter 309 sends 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 / reduced load sensitivity operating frequency. This indicates to the power receiver that the power transmitter is in a state where it is ready for load change, and therefore the power receiver proceeds to perform the load change. Therefore, the first transmitter 309 can send a load change confirmation message to the power receiver, which can be received by the second receiver 609. The load change confirmation message indicates that the load processor has changed the operating frequency to the reduced load sensitivity operating frequency, and in response to the message, the power receiver controller 601 and the switch 607 can proceed to perform the load change accordingly, that is, specifically, the switch 607 can be controlled to perform switching (to connect or disconnect the load 605).

[0126] The power transmitter may detect this load change (different methods may be used as will be described later) and in response, the frequency controller 311 changes the operating frequency from the reduced load-sensitive operating frequency to a load-dependent operating frequency. The load-dependent operating frequency is a frequency that may vary depending on the load of the power transfer signal through the power receiver, and in particular, it may depend on the load of the power receiver. The load-dependent operating frequency is a variable frequency, and in particular, it is a frequency that may depend on operating characteristics of the power transfer, such as in particular may depend on the power level of the power transfer signal and / or the load of the power transfer signal through the power receiver.

[0127] In particular, the load-dependent operating frequency may be a frequency controlled by a power control loop. Thus, the load-dependent operating frequency may be under the control of the power control loop. It may be a frequency that is variable in response to a power control error message received from the power receiver. Thus, after a load change has occurred, the power transmitter may switch operation back to a frequency within the power transfer operating range, and it may in particular restart the power control loop.

[0128] The initial load-dependent operating frequency may, for example, be an operating frequency determined in response to an expected or estimated value of the load of the power receiver. For example, the load change message may indicate a new load power level after the change, and the frequency controller 311 may estimate a suitable operating frequency for the load. The frequency controller 311 may then return the drive signal to that frequency after the load change. The power control loop may then adjust the operating frequency in response to the received error message in order to fine-tune the frequency. In other embodiments, the initial load-dependent operating frequency may simply be a fixed predetermined frequency, such as a mid-frequency of the operating frequency band, and from that frequency, the power control loop may be used to reach an appropriate operating point / frequency (thus, switching to a load-dependent frequency may be a two-stage process of first switching to a predetermined frequency and then adapting to the load-dependent frequency, typically by allowing the power control operation to adapt to the drive / operating frequency).

[0129] The frequency determiner 313 may be arranged to provide a reduced load sensitivity operating frequency as a frequency that is less sensitive to load changes than any frequency within the frequency operating range. The inventors have recognized that the sensitivity of the power transfer signal and the load voltage at the power receiver may vary significantly as a function of frequency. They have also recognized that the lowest sensitivity is in most practical applications outside the frequency power control loop operating range because the properties required for power control (changes in power levels) are incompatible with the requirement for lowest sensitivity. In practice, this property tends to exist at resonant frequencies that tend to be unsuitable for power control because the monotonic dependence on frequency is generally not for these cases (at least not for all loads). The inventors have also recognized that improved load step / change operation can be achieved by interrupting normal operation during load changes to change the operating frequency to a frequency that reduces or minimizes the impact of load changes.

[0130] Figure 7 An example of a power transfer function for power transfer between a power transmitter and a power receiver is illustrated. 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 transfer signal / drive signal. The transfer function can also be considered to indicate the power level of the power transfer / power transfer signal / drive signal. In fact, for resistive load components, the power level and load voltage have a monotonic relationship (and the logarithmic representation of power and voltage will correspond).

[0131] As can be seen, for low frequencies, the load voltage and associated power level increase monotonically with frequency, and similarly, for high frequencies, the voltage and associated power level may decrease monotonically with increasing frequency. Thus, an operating frequency range may be defined as a range in which the voltage / power level is a monotonic function with respect to frequency 701, 703. Such an operating range may be at higher or lower frequencies. Thus, the power controller 315 may perform power control by varying the frequency within these ranges.

[0132] The inventors have further recognized that while the exact transfer curve (shape and value) may depend heavily on the load presented by the power receiver to the power transfer signal, the dependence varies over frequency, and some frequencies exhibit much higher dependence than others. They have further recognized that this can be exploited by the described method where the power transmitter switches to a reduced load-sensitive operating frequency before a load change and returns to a load-dependent frequency after the load change.

[0133] Figure 7The power transfer function for different loads is illustrated. As can be seen, higher resistive loads (higher power / lower load resistance value) tend to result in a single peak transfer function, while lower resistive loads (lower power / higher load resistance value) tend to result in a dual peak transfer function reflecting the individual resonant behavior of the power transmitter output resonant circuit and the power receiver input resonant circuit. In addition, as Figure 7 As shown, the transfer function depends strongly on the load, but this sensitivity is significantly lower at certain frequencies than at others. In particular, there are frequencies at which this load sensitivity is minimized, and even a point is reached at which the transfer function is almost independent of the load value.

[0134] In some embodiments, the frequency determiner 313 may be arranged to determine the reduced load sensitivity operating frequency based on the transfer function.In some embodiments, the reduced load sensitivity operating frequency may be determined as a minimum load sensitivity frequency.

[0135] In some embodiments, the frequency determiner 313 may be arranged to determine the reduced load sensitivity operating frequency as a predetermined frequency. Indeed, in some embodiments, the frequency determiner 313 may store a fixed / static / predetermined reduced load sensitivity operating frequency that may be called up and provided to the frequency controller 311. Such a predetermined reduced load sensitivity operating frequency may reflect average characteristics and properties, such as average nominal properties of the power receiver, nominal relative position of the power receiver, etc. For example, during design and / or manufacturing, various measurements may be made using a nominal power receiver at a nominal position. The frequency at which a load step results in a minimum voltage step at the output of the input resonant circuit and / or the power receiver load / receiver coil may be determined as a suitable reduced load sensitivity operating frequency and stored in (one or more) power transmitters. In some cases, operations may be performed for different types of power receivers, and a reduced load sensitivity operating frequency may be determined and stored for each of a plurality of types of power receivers. During initialization of power transmission, the power receiver may indicate to the power transmitter which type it is, and the frequency determiner 313 may identify the corresponding stored reduced load sensitivity operating frequency in response and provide it to the frequency controller 311 when appropriate.

[0136] In some embodiments, the frequency determiner 313 may determine the reduced load sensitivity operating frequency as a predetermined function of one or more power transfer parameters. The power transfer parameters may be, for example, coupling factor, power level, relative position of the power receiver, etc.

[0137] For example, during manufacturing, a reduced load sensitivity operating frequency may be determined while varying ranges of different power transfer parameters. Based on the different measurements, an equation may be derived or fitted to provide a reduced load sensitivity operating frequency as a function of one or more power transfer parameters. The equation / function may then be stored at the frequency determiner 313. During operation, the frequency determiner 313 may measure the required operating parameters, and may then evaluate a function to determine a reduced load sensitivity operating frequency using the measured power transfer parameters. For example, a coupling factor may be determined, and a function dependent on the coupling factor may be evaluated to generate a reduced load sensitivity operating frequency.

[0138] In some embodiments, a plurality of different functions / equations may be determined for different types of power receivers, and an appropriate function may be selected for the current power receiver type.

[0139] In many embodiments, the frequency determiner 313 may be arranged to perform measurements to dynamically determine a reduced load sensitivity operating frequency for the current power transmitter and power receiver settings, including determining the reduced load sensitivity operating frequency to reflect current operating conditions, such as a current positioning of the power receiver.

[0140] Such a determination may in particular be performed by measuring and taking into account a power transfer function, such as for example 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 reduced load sensitivity operating frequency may advantageously be determined as the resonant frequency of such a transfer function. The resonant frequency may in particular be determined for low loading of the resonant circuit. That is, for a high resistive load of the receiver resonant circuit / a high Q of the resonant circuit(s). For example, Figure 7 It is shown that the lowest variability with respect to load changes occurs at the peak of the transfer function for high Q 705. These frequencies correspond to the resonant frequencies of the transfer function and to the (coupled) resonant frequencies of the output and input resonant frequencies.

[0141] In the described system, the frequency determiner 313 may include functionality for determining a reduced load sensitivity operating frequency based on detection / measurement of one or more resonant frequencies for the output resonant circuit (and / or equivalently for the drive signal) when the output resonant circuit is coupled to the power receiver and specifically coupled to the receiver coil 107 and the input resonant circuit. This may correspond to determining the reduced load sensitivity operating frequency based on the resonant frequency of the power transfer function and specifically based on the peak value of the power transfer function (for higher Q values). In particular, the reduced load sensitivity operating frequency may be determined as the coupled resonant frequency of the output resonant circuit (when coupled to the input resonant circuit).

[0142] The frequency determiner 313 may be arranged to determine at least one coupled operating resonant frequency of the output resonant circuit / drive signal / power transfer function. In the following, reference will mainly be made to the coupled resonant frequency of the output resonant circuit, but it will be appreciated that such reference may be replaced by a reference to the (coupled) resonant frequency of the power transfer function, the drive signal and / or the power transfer signal.

[0143] The determination may specifically be based on measurements during a resonance measurement time interval, wherein the coupled resonance frequency is the resonance frequency of the output resonance circuit in the presence of the power receiver, ie when the transmitter coil 103 is coupled to the receiver coil 107 of the power receiver.

[0144] When the transmitter coil 103 is coupled to the receiver coil 107, the coupled resonant frequency accordingly reflects the effective resonant frequency of the output resonant circuit. 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 any 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 any transmitter coil 103. As a result, the effective resonance will be different from the self-resonance when there is no coupling. In addition, 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 will effectively have two resonant frequencies, and these resonant frequencies are different from the self (uncoupled) resonant frequency of the output resonant circuit.

[0145] The frequency determiner 313 may proceed to determine the reduced load sensitivity operating frequency as (one of) the detected coupling resonant frequencies.

[0146] In some embodiments, the resonance detector 307 may be arranged to detect the coupled resonant frequency during the resonance measurement time interval by changing the frequency of the drive signal, and in particular, it may perform a frequency sweep within a frequency range, which may correspond to a frequency interval in which the coupled resonant frequency is expected to be, or within which the coupling factor may be considered to be high enough to provide acceptable power transfer. The resonance detector 307 may then monitor the drive signal, and it may detect, for example, extreme values ​​of current or voltage amplitude. For example, for a series resonant circuit, the resonance detector 307 may control the driver 301 to change the frequency within a certain range and at a fixed voltage amplitude. It may then measure the current amplitude for different frequencies and determine the coupled resonant frequency for which the highest current amplitude is determined. Figure 8 Figure 2 shows the power transmitter coil current / drive signal I PTX How does the current correspond to the voltage V at the output of the power receiver coil / input resonant circuit?PRX .

[0147] As another example, the resonance detector 307 may detect when the phase difference between the current and voltage of the drive signal is zero (or close to zero), ie, when the loading through the output resonant circuit is purely resistive.

[0148] In some embodiments, the frequency sweep of the drive signal may be from a higher frequency toward a lower frequency, and the coupled resonant frequency may be determined as the first detected frequency that satisfies a resonance criterion (e.g., an extreme value of the current or voltage amplitude of the signal or a zero phase difference between the voltage and current). Thus, instead of detecting a global extreme value, for example, a first local extreme value may be detected.

[0149] Such an approach may allow detection of the highest coupled resonant frequency of the two. This may be advantageous in some embodiments because sensitivity to load variations tends to be lower frequency errors around higher coupled resonant frequencies than lower coupled resonant frequencies.

[0150] Detection of one or more coupled resonant frequencies based on a power transfer function and in particular based on peak detection and / or frequency sweeping (eg, detecting a zero phase difference resonance point) may generally be advantageously performed for high Q-factor conditions.

[0151] In many embodiments, the quality factor of the input resonant circuit during the resonant measurement operation / measurement time interval is not less than 10, and generally it may be higher. This may enable reliable and fairly accurate detection of the coupled resonant frequency(s), and therefore a determined reduced load sensitivity operating frequency reflecting low load sensitivity.

[0152] In some embodiments, a high Q during the resonance measurement time interval may be ensured by the Q value of the input resonant circuit being always above 10, i.e. by the power receiver being designed such that the input resonant circuit always has a quality factor above 10. However, this is often in contrast to the desire to provide appropriate power to the load. For example, for a Ki system, loading of the input resonant circuit by typical power values ​​results in a Q value that is typically less than 5, and often less than 2.

[0153] In many embodiments, the power receiver may accordingly be arranged to switch the operating mode from a lower quality factor mode to a higher quality factor mode having a Q value of at least 10 during the resonance measurement time interval for at least some of the time outside of the resonance measurement time interval. This may allow a more accurate determination of a suitable reduced load sensitivity operating frequency. Thus, the power receiver may be arranged to switch from a power transfer mode to a measurement mode during the resonance measurement time interval with a quality factor of not less than 10, during the power transfer mode the quality factor is not constrained to be above 10, and during the power transfer mode the quality factor may in fact be substantially less than 10 in order to allow efficient power transfer.

[0154] This can be achieved, 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 can couple a normal load 605 to the input resonant circuit outside of the resonant measurement time interval(s). However, during the resonant measurement time interval(s), switch 607 can decouple load 605. For an input parallel resonant circuit, switch 607 can, for example, disconnect load 605 so that no current is drawn from the input resonant circuit. Conversely, for an input series resonant circuit, switch 607 can decouple the load by shorting load 605, thereby shorting the power transfer input resonant circuit during the resonant measurement time interval.

[0155] In some embodiments, such a change in the quality factor may occur without the power receiver including specific functionality. For example, in some embodiments, the load may inherently provide a substantially short circuit during startup, which may allow a resonant measurement time interval prior to power transfer with an inherently high Q input resonant circuit. For example, when the load is a motor, it may start up almost as a short circuit. As another example, when there is a rectifier and a large output capacitor in the power path, this will also behave almost like a short circuit as the capacitor discharges.

[0156] In many embodiments, the resonance measurement time interval (or at least one resonance measurement time interval) may precede the power transfer. Specifically, the determination of the reduced load sensitivity operating frequency may be performed during initialization of the power transfer operation. Thus, before the power transfer, the power transmitter may determine a suitable reduced load sensitivity operating frequency for a particular power receiver and a particular positioning of the power receiver. It may then store the reduced load sensitivity operating frequency and begin preparing to perform a load-changed power transfer using the determined reduced load sensitivity operating frequency.

[0157] During the pre-power transfer coupling factor estimation, the power receiver can enter a measurement mode for coupled resonant frequency detection. Specifically, switch 607 can short-circuit the load to provide a high quality factor for the input resonant circuit. When the system enters the power transfer phase, the power receiver can switch back to the normal power transfer operation mode, and specifically, the short circuit can be removed.

[0158] In some embodiments, the method for determining the reduced load sensitivity operating frequency may alternatively or additionally be applied during a power transfer phase. In many embodiments in which the determination is performed during the power transfer phase, the system may be arranged to operate in a time slot mode, wherein the measurements and the coupled resonant frequency detection are performed during a measurement time interval. The resonant measurement time interval may specifically be performed during such a measurement time interval of a repeating time frame, the repeating time frame also comprising at least one power transfer time interval during which power is transferred to the power receiver.

[0159] In such an embodiment, the system may thus utilize time division during the power transfer phase. Specifically, the detection of the coupling resonance frequency and the power transfer may be performed, for example, in separate time intervals, allowing interference between these to be significantly reduced.

[0160] In this example, the driver 301 and the transmitter coil 103 are arranged to generate an electromagnetic power transfer signal during the power transfer interval for transferring power to the power receiver. In addition, the drive signal can be used to detect the coupling resonant frequency during the measurement time interval. The power transmitter can use a repeating time frame for the drive signal during the power transfer phase, wherein the time frame includes at least one power transfer time interval and at least one resonance measurement time interval. An example of such a repeating time frame is shown in FIG. Fig. 9 , where the power transfer time interval is indicated by PT and the measurement time interval is indicated by D (the time interval may also be referred to as the detection time interval). In this example, each time frame FRM includes only one resonance measurement time interval and one power transfer time interval, and these (and the time frame itself) have the same duration in each frame. However, it should be understood that in other embodiments, other time intervals may also be included in the time frame (e.g., communication intervals), or multiple resonance measurement time intervals and / or power transfer time intervals may be included in each time frame. In addition, in some embodiments, the duration of different time intervals (and indeed the time frame itself) may vary dynamically.

[0161] In this method, measurement and power transfer are thus separated in the time domain, resulting in reduced cross-interference from power transfer to measurement. Thus, variability and uncertainty caused by changes in the operating conditions of the power transfer can be isolated from the measurement and estimation, thereby achieving a more reliable and accurate estimation process. In addition, it allows the generation (and optimization) of the drive signal to detect the coupled resonant frequency. Specifically, it allows the resonance detector 307 to perform a frequency sweep and perform appropriate operations for this detection.

[0162] Furthermore, it allows the power receiver to be specifically adapted to provide improved or optimal properties for detection. Specifically, in many embodiments, the power receiver can switch from a power operation mode during a power transfer time interval, during which the 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 short-circuiting the load by switch 607.

[0163] Thus, the time slot approach may allow or facilitate performing coupling factor estimation during the power transfer phase.

[0164] In many embodiments, the described methods can provide improved operation, and in particular improved load switching operation. For example, it can reduce voltage changes / steps or current changes / steps that occur due to load steps that are too fast to be compensated by the power control loop.

[0165] The inventors have recognized that for a coupled resonant circuit, the power transfer behavior (particularly including the behavior of the input resonant circuit) is such that changes at the output of the input resonant circuit that occur in response to load changes can be reduced. In particular, they have recognized that the operation and behavior are frequency dependent, and that frequency can be used to control the desired behavior. Furthermore, they have recognized that adapting the frequency to have different behaviors for different operations can achieve improved overall performance.

[0166] When the load of the power receiver changes, such as, for example, when the resistance of the load changes, the power transfer operating point changes to produce an operating point where the voltage and current in the components of the input resonant circuit of the power receiver and the output resonant circuit of the power transmitter change to reflect the changed load. This may further result in a change in the drive signal. In other words, the load imposes constraints on the power transfer path, which, together with the constraints on the drive signal (such as a fixed voltage amplitude), usually results in an almost instantaneous change to a new operating point, where the appropriate current and voltage of the circuit are such that the constraints are met. Subsequently, the power control loop adjusts the drive signal, and therefore the operating point, so that the power transfer provides the desired power to the load. However, if the load changes too fast for the power control loop to compensate, the instantaneous change may result in undesirable transients. This may, for example, result in an overvoltage transient being provided to the load.

[0167] Thus, when the power receiver load changes rapidly and / or substantially, a new operating point for power transfer is created. However, the inventors have recognized that, for a reduced load sensitivity frequency, such a change may occur with a reduced (and in some cases substantially no (such as described when the reduced load frequency is set to the coupling resonant frequency)) change in the load voltage / output voltage of the input resonant circuit.

[0168] For example, in the case of a load step occurring with the operating frequency set to the coupled resonant frequency, the load voltage may remain substantially constant. However, as the load changes suddenly (i.e., the power drawn by the load changes suddenly), the current of the load also changes significantly and suddenly. For example, if the power changes from, for example, 20 W to 2000 W (e.g., the heating element is turned on), the load current changes suddenly by a factor of 100, and the voltage remains constant.

[0169] In order to adapt to this changed load, the power extracted from the transmitter source (mains) will also change, and in fact the power extracted may change by a corresponding factor (i.e. by a factor of 100 in the specific example). However, this change does not necessarily result in a change in the current of only the power receiver coil, but generally both the coil voltage and current will change on both the primary and secondary sides. This is due to the change in the total impedance present and visible from the output resonant circuit.

[0170] The entire power transmission system including the resonant circuit will accordingly adapt to a new operating point matching the changed load. However, this may occur such that the output voltage of the input resonant circuit and therefore the load voltage is substantially constant despite the load changes. This can be achieved by setting the resonant frequency to the coupling resonant frequency, which is a characteristic operating point of the loosely coupled resonant coil system.

[0171] In more detail, for the example of the figure where the input resonant circuit is a series resonant circuit, the output current is the same as the current in the receiver coil 107 and capacitor 603, and the output voltage is given as the combination / sum of the voltages across the receiver coil 107 and capacitor 603.

[0172] The components of the output resonant circuit and the input resonant circuit are all passive components, and in order to adapt these components to different powers, the current / voltage needs to be adjusted (similar to resistors). Both inductors and capacitors behave more or less linearly, so that when the current increases, the voltage also increases. This is similar to a resistor, but with a phase difference between voltage and current.

[0173] Therefore, the output voltage of the input resonant circuit is the sum of two different voltages with a phase offset. At the reduced load sensitivity operating frequency or coupled resonant frequency, this operating point can be 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).

[0174] Specifically, at reduced load sensitivity operating frequencies, the phase difference between the coil voltage and the capacitor voltage can be 180° degrees, so the imaginary components can mostly cancel, and this is independent of the attached load (but frequency dependent). At a specific resonant frequency, the input resonant circuit and the output resonant circuit can be considered to act like a load-independent voltage divider. At all other frequencies, the load will have an effect on the resonant circuit because the phase is not exactly 0 or 180°.

[0175] Thus, for both the output resonant circuit and the input resonant circuit, the voltage at the input remains the same, and the current increases as the current load at the power receiver increases. The voltage on the power receiver coil is higher because the current is also higher. The voltage on the capacitor also changes because the current is also higher, but the voltage is in antiphase with the coil voltage. Thus, on the primary side, the voltages are more or less balanced. A corresponding / complementary phenomenon occurs at the input resonant circuit. The total resonant circuit combination essentially acts as a feedthrough, but the internal voltage changes to accommodate the power changes in the passive process. Thus, the component voltages change but compensate, so that the output voltage of the input resonant circuit is substantially constant.

[0176] Specifically, for both the inductor and the capacitor, the voltage increases for an increased current (ie, by increasing the power drawn by the load). However, the voltages have opposite phases at the appropriate frequency, and so these changes may (at least partially) cancel out.

[0177] The system may therefore provide improved transient performance by controlling the frequency such that the voltage difference at the output of the input resonant circuit, and therefore typically the voltage difference at the load, is reduced.

[0178] For the output resonant circuit and / or the input resonant circuit being a parallel resonant circuit, the voltage across the components is the same, rather than the current. The description provided above with reference to the series resonant circuit applies to the exchange of the reference current and voltage. Thus, the method can be used to provide load regulation with a substantially constant current.

[0179] However, a minimum voltage sensitivity frequency may also occur for a parallel resonant circuit. Such a point may occur between two minimum current sensitivity frequencies. The minimum voltage sensitivity frequency may be identified, for example, by a test process of varying the load and measuring the voltage transients for different drive frequencies. Similarly, for the series resonant circuit embodiment, the minimum current sensitivity frequency may be identified and used as described above.

[0180] In different embodiments, the power transmitter may use different methods to determine that the power receiver has performed a load change.

[0181] In some embodiments, this determination may be implicit, as the power receiver will be expected to perform a load change within a given time interval, and this time interval may be measured by a timer. Thus, when the timer expires, the frequency controller 311 may proceed to specify that a load change has occurred, and it may proceed to change the drive signal frequency to an operating range / load-dependent frequency and restart the power control loop. The timer may, for example, be started when a load change message is sent, and expiration of the timer may therefore indicate a certain (e.g., predetermined) time interval since the sending of the load change message has occurred, and this may result in the load change being specified as having occurred.

[0182] In other embodiments, the frequency controller 311 may be arranged to measure the load of the power transfer signal and detect that the power receiver has performed a load change if a (corresponding) load change is detected for the power transfer signal. Thus, after sending a load change message, the frequency controller 311 may monitor the power level of the drive signal and thus the loading of the power transfer signal. When a change of suitable size is detected, the frequency controller 311 may specify that the indicated load change has occurred.

[0183] In some embodiments, the power receiver may be arranged to send a load change performed message to the power transmitter after the power transmitter has performed the load change, and the power transmitter may then determine that the load change has occurred in response to receiving the load change performed message. Thus, the power transmitter may switch to a reduced load sensitivity operating frequency upon receiving the load change message and suspend the frequency power control loop until it receives the load change performed message. Upon receiving the message, the power transmitter returns the drive signal frequency to within the operating range and restarts the frequency power control loop.

[0184] In some embodiments, the frequency determiner 313 may be arranged to determine the reduced load sensitivity operating frequency once and then subsequently use this operating frequency. In some embodiments, it may for example be determined as part of initialization of a power transfer and each time a new power transfer is started, a new reduced load sensitivity operating frequency may be determined. In some embodiments, this frequency may be used throughout the power transfer operation and a new reduced load sensitivity operating frequency may not be determined until the next power transfer operation is performed.

[0185] In some embodiments, the frequency determiner 313 may be arranged to initiate determination of a reduced load sensitivity operating frequency in response to detecting a change in the operating point of the power transfer. In some embodiments, the frequency determiner 313 may continuously monitor the operating point during the power transfer phase. It may measure one or more parameters, such as a coupling factor or coupling factor indicator, a foreign object loss level, a relative position of the power receiver relative to the power transmitter, etc. The frequency determiner 313 may determine a reduced load sensitivity operating frequency for the system operating at a given operating point. However, if the operating point changes by more than a given threshold, the frequency determiner 313 may detect the change and, in response, initiate a new determination of a reduced load sensitivity operating frequency (e.g., using any of the previously described methods). Once a new reduced load sensitivity operating frequency has been determined, this may replace the previously reduced load sensitivity operating frequency, and thus an updated reduced load sensitivity operating frequency appropriate for the new operating point is then used.

[0186] As previously described, the power control loop may be a control loop that only adjusts the frequency of the drive signal / power transfer signal. During the power change time interval, the power transmitter may maintain the frequency constant at a reduced load sensitivity operating frequency and suspend the frequency (adjusting / variable) power control loop as described.

[0187] However, in some embodiments, the power transmitter may be arranged to operate a power control loop that adjusts another parameter other than the drive signal frequency. For example, such a non-frequency (adjustable / variable) power control loop may adjust the drive signal voltage amplitude, the drive signal current amplitude and / or the drive signal duty cycle. In some embodiments, such a non-frequency power control loop may operate during a power change time interval, and may specifically operate during the time when the frequency power control loop is suspended. In such a case, power control is still performed during the power change interval while ensuring that the impact of the load change is reduced or even minimized. Such a method may provide an improved overall load change operation.

[0188] The non-frequency power control loop may be a power control loop that operates only when the frequency power control loop is suspended (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.

[0189] It should be understood that, for the sake of clarity, the above description has described embodiments of the present invention with reference to different functional circuits, units and processors. However, it will be apparent that any suitable distribution of functions between different functional circuits, units or processors may be used without departing from the present invention. For example, functions illustrated as being performed by a separate processor or controller may be performed by the same processor or controller. Therefore, references to specific functional units or circuits are only considered as references to suitable modules for providing the described functions, and do not indicate a strict logical or physical structure or organization.

[0190] In some embodiments, the reduced load sensitivity operating frequency may be determined as a coupled resonant frequency of the output resonant circuit. In such a case, all references to the reduced load sensitivity operating frequency may be replaced by references to the coupled resonant frequency. Thus, in such an embodiment, there may be provided a power transmitter for wirelessly providing power to a power receiver via an inductive power transfer signal; the power transmitter comprising: an output resonant circuit comprising a transmitter coil and at least one capacitor; a driver arranged to generate a drive signal for the output resonant circuit to generate the inductive power transfer signal; a frequency determiner arranged to provide a coupled resonant frequency of the drive signal, a receiver arranged to receive a load change message from the power receiver, the load change message indicating that a load change by the power receiver is imminent; a frequency controller arranged to change the operating frequency of the drive signal to the coupled resonant frequency in response to receiving the load change message; a transmitter arranged 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 coupled resonant frequency; wherein the frequency controller is further arranged to change the operating frequency from the coupled resonant frequency to a load-dependent operating frequency in response to determining that the power receiver has performed the load change.

[0191] In some embodiments, the term "reduced load sensitivity operating frequency" may be replaced by the term "coupled resonant frequency".

[0192] The coupling resonant frequency may be the resonant frequency of the arrangement of the output resonant circuit of the power transmitter and the input resonant circuit of the power receiver. The coupling resonant frequency may be the resonant frequency of the output resonant circuit of the power transmitter when (inductively / electromagnetically) coupled to the input resonant circuit of the power receiver.

[0193] The present invention can be implemented in any suitable form, including hardware, software, firmware or any combination of these. The present invention can be optionally implemented at least in part as computer software running on one or more data processors and / or digital signal processors. The elements and components of an embodiment of the present invention can be implemented physically, functionally and logically in any suitable manner. In fact, the function can be implemented in a single unit, in multiple units or as part of other functional units. In this way, the present invention can be implemented in a single unit, or can be physically and functionally distributed between different units, circuits and processors.

[0194] Although the present invention has been described in conjunction with some embodiments, it is not intended to limit the present invention to the specific forms set forth herein. On the contrary, the scope of the present invention is limited only by the appended claims. In addition, although it may appear that features have been described in conjunction with specific embodiments, those skilled in the art will recognize that the various features of the described embodiments may be combined according to the present invention. In the claims, the term "comprising" does not exclude the presence of other elements or steps.

[0195] In addition, although listed individually, multiple modules, elements, circuits or method steps can be implemented by, for example, a single circuit, unit or processor. In addition, although individual features can be included in different claims, these can be advantageously combined, and inclusion in different claims does not mean that the combination of features is not feasible and / or disadvantageous. The inclusion of features in a class of claims does not mean a restriction on the class, but indicates that the feature is equally applicable to other claim classes when appropriate. Including features in a dependent claim of an independent claim does not mean a restriction on the independent claim, but indicates that the feature is equally applicable to other independent claims when applicable. In addition, the order of features in the claims does not mean any specific order in which the features must work, and in particular, the order of individual steps in the method claims does not mean that the steps must be performed in that order. Instead, the steps can be performed in any suitable order. In addition, singular references do not exclude multiples. Therefore, references to "one", "one", "first", "second", etc. do not exclude multiples. The terms "first", "second" (usually / generally and otherwise indicated by the context) are used only as labels, and are therefore not intended to have any restrictive meaning or effect. 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. A power transmitter (101) for wirelessly providing power to a power receiver (105) via an inductive power transfer signal; the power transmitter (101) comprising: an output resonant circuit comprising a transmitter coil (103) and at least one capacitor (303); a driver (301) arranged to generate a drive signal for the output resonant circuit to generate the inductive power transfer signal; a frequency determiner (313) arranged to provide a reduced load sensitivity operating frequency for the drive signal, a receiver (307) arranged to receive a load change message from the power receiver (105), the load change message indicating that a load change by the power receiver (105) is imminent; a frequency controller (311) arranged to change the operating frequency of the drive signal to the reduced load sensitivity operating frequency in response to receiving the load change message; a transmitter (309) arranged to send a load change confirmation message to the power receiver (105), the load change confirmation message indicating that the frequency controller (311) has changed the operating frequency to the reduced load sensitivity operating frequency; Wherein the frequency controller (311) is further arranged to change the operating frequency from the reduced load-sensitive operating frequency to a load-dependent operating frequency in response to determining that the power receiver (105) has performed the load change.

2. The power transmitter of claim 1, comprising a power controller (315) arranged to implement a frequency power control loop by adjusting the operating frequency in response to a power control error message received from the power receiver (105), and wherein, The power controller (315) is arranged to suspend the frequency power control loop in response to receiving the load change message and to restart the frequency power control loop in response to the frequency controller (311) determining that the power receiver (105) has performed the load change.

3. A power transmitter according to any preceding claim, wherein: The frequency controller (311) is arranged to determine that the power receiver has performed the load change in response to at least one of the following: The timer expires; detecting a change in power extracted from the power transfer signal; detecting a change in the power level of the drive signal; as well as The receiver (307) receives a load change performed message from the power receiver (105).

4. A power transmitter according to any preceding claim, wherein: The reduced load sensitivity operating frequency is a resonant frequency of a transfer function for the power transfer.

5. The power transmitter according to claim 4, wherein: The frequency determiner (313) is arranged to determine the reduced load sensitivity operating frequency as a coupling resonant frequency for the output resonant circuit, the coupling resonant frequency being a resonant frequency of the output resonant circuit for the transmitter coil (103), the transmitter coil being coupled to a receiver coil (107) of a power transfer input resonant circuit of the power receiver (105).

6. The power transmitter according to claim 5, wherein: The frequency determiner (313) is arranged to control the driver (301) to generate the drive signal with a varying frequency, and to determine the coupling resonant frequency based 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. The power transmitter according to claim 5 or 6, wherein: The frequency determiner (313) is arranged to control the driver (301) to perform a frequency sweep of the drive signal from a higher frequency towards a lower frequency, and to determine the coupling resonance frequency as a first detection frequency that meets a resonance criterion for the drive signal.

8. The power transmitter according to any one of claims 5 to 7, wherein: The frequency determiner (313) is arranged to initiate determination of the reduced load sensitivity operating frequency in response to detecting a change in the operating point of the power transfer.

9. A power transmitter according to any preceding claim, wherein: The frequency determiner (313) is arranged to determine the reduced load sensitivity operating frequency as a predetermined function of at least one power transfer parameter.

10. A power transmitter according to any preceding claim, wherein: The power controller (315) is further arranged to implement a non-frequency power control loop by adjusting 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 operates at the reduced load sensitivity operating frequency.

11. A power receiver for a wireless power transmission system, the wireless power transmission system comprising a power transmitter (101), the power transmitter transmitting power to the power receiver (105) using an inductive power transmission signal, the power receiver (105) comprising: an input resonant circuit comprising at least one capacitor and a power receiver coil (107) arranged to extract power from the power transfer signal by induction to provide power to a variable load (605, 607); a determiner (601) arranged to determine that a load change for the variable load is imminent; a transmitter (611) arranged to send a load change message to the power transmitter (101), the load change message indicating that the load change is imminent; a receiver (609) arranged to receive a load change confirmation message from the power transmitter (101), the load change confirmation message indicating that the power transmitter (101) has changed the operating frequency of the power transfer signal to a reduced load sensitivity operating frequency; A load controller (601) is arranged to initiate said load change in response to receiving said load change confirmation message.

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

13. A method of operating a power transmitter (101) for wirelessly providing power to a power receiver (105) via an inductive power transfer signal; the power transmitter (101) comprising: an output resonant circuit comprising a transmitter coil (103) and at least one capacitor (303); And the method comprises: generating a drive signal for the output resonant circuit to generate the inductive power transfer signal; providing a reduced load sensitivity operating frequency for the drive signal, receiving a load change message from the power receiver (105), the load change message indicating that a load change by the power receiver (105) is imminent; changing the operating frequency of the drive signal to the reduced load sensitivity operating frequency in response to receiving the load change message; sending a load change confirmation message to the power receiver (105), the load change confirmation message indicating that the frequency controller (311) has changed the operating frequency to the reduced load sensitivity operating frequency; and The operating frequency is changed from the reduced load sensitivity operating frequency to a load dependent operating frequency in response to determining that the power receiver (105) has performed the load change.

14. A method for operating a power receiver for a wireless power transfer system, the wireless power transfer system comprising a power transmitter (101), the power transmitter being arranged to transfer power to the power receiver (105) using an inductive power transfer signal, the method comprising: extracting power from the power transfer signal by induction to provide power to a variable load (605, 607); determining that a load change for the variable load is imminent; sending a load change message to the power transmitter (101), the load change message indicating that the load change is imminent; receiving a load change confirmation message from the power transmitter (101), the load change confirmation message indicating that the power transmitter (101) has changed the operating frequency of the power transfer signal to a reduced load sensitivity operating frequency; and The load change is initiated in response to receiving the load change confirmation message.

15. An operating method for a wireless power transmission system, the wireless power transmission system comprising a power transmitter (101), the power transmitter transmitting power to a power receiver (105) to induce a power transmission signal, the method comprising: The power transmitter performs the method according to claim 13 and the power receiver (105) performs the method according to claim 14.

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