Active impedance control

Abstract

A wireless power transfer circuit has an impedance compression network to supply power to a load, and a controllable impedance in parallel in the compensation network, or a controllable capacitance in the compensation network to compensates for variability in the coupling with the another resonant inductive power transfer circuit and / or changes in the load supplied by the power converter. In some aspects a wireless power transfer circuit provides a current source output and comprises an impedance compression network.

Description

[0001] ACTIVE IMPEDANCE CONTROL

[0002] Field

[0003] This disclosure relates to wireless power transfer (WPT), also commonly referred to as inductive power transfer (IPT). The disclosure has particular relevance to active control of the impedance of an IPT system, or components thereof such as primary and secondary circuits, in order to regulate or improve power flow.

[0004] Background

[0005] IPT systems are well known, having a primary side and a secondary (also known as a pick-up) side that are loosely magnetically coupled. In uni-directional systems power flows in one direction, from the primary to the secondary. In bi-directional systems power may flow in either direction. This document should be interpreted as relating to uni- or bi-directional systems, so discussion of secondary circuits, apparatus or systems for example can equally apply to primary circuits, apparatus or systems.

[0006] The IPT systems and circuits that are the subject of this disclosure have many different applications in practice. One popular application is charging of electric vehicles (EVs). As the uptake of electric vehicles (EV’s) continues to grow around the world, effective charging techniques have become a major point of discussion. Traditionally, conductive charging techniques (wired charging) have been used, however these techniques suffer from reliability and safety issues at high power levels. To minimise safety concerns, minimal user input is preferred to initiate the charging process therefore there has been a shift towards wireless charging techniques. This trend is further highlighted by the introduction of standards such as SAE J2954, which provides specification guidelines on the implementation of wireless chargers for EV’s. SAE J2954 specifies wireless charging through inductive power transfer (IPT), which is currently the preferred technology for wireless power applications.

[0007] In a conventional IPT system, such as that shown in Figure 1 , the primary and / or secondary circuits have a power transfer coil, a compensation network which may comprise one or more elements or components, and a power converter which may be a half or full bridge. When either the magnetic coupling between the primary and secondary coils changes, or when the EV battery voltage changes, the controller(s) in the primary and / or the pick-up has to regulate the power flow by modulating the effective voltage applied across the input to the primary and / or pick-up compensation networks. Modulating the voltage across the input of a compensation network leads to a change in current if a fixed amount of power is delivered. So, for example, if the coupling increases by three times (typical for an EV charger), the voltage applied needs to be reduced by three times through modulation of switches, which leads to a three times increase in switch current. This not only increases the conduction losses of the switches, but also increases switching losses (due to modulation) and the cost of the devices required. These issues are not solely applicable to EV applications.

[0008] Constant output power is often desirable in inductive wireless transfer charging systems to ensure quality of power provided. This can be made difficult by a number of factors. For example, variation in distance of power transfer (for example because of the clearance between the load (e.g. a vehicle assembly) and the ground assembly. Variation in the distance causes the self-inductances of the coils and the magnetic coupling between them to change. In another example variations in the lateral alignment between the load and the ground assembly may also cause the self-inductances and their magnetic coupling to change. In another example as the load (for example a battery) is charged or discharged, the load voltage may increase or decrease respectively, resulting in a different impedance seen by the system.

[0009] In some applications the compensation network is designed to compensate for the non-linearity introduced by the power converter at an operating point, but there are still significant reactive impedance components, for example caused by changes in coupling or load in operation of the circuit, that are uncompensated, resulting in poor power factor and losses in the system.

[0010] Some systems have attempted to the above issue using an impedance compression network (abbreviated ZCN) and a tuneable matching network (abbreviated TMN). Figure 2 (annotated from SAE J2954) shows an example arrangement of a secondary with a compensation circuit, a TMN and a ZCN between the receiving coil and the load. The the ZCN is used between the output of the compensation circuit and the rectifier(s) and / or the load to passively regulate the power transfer when the load or output voltage changes (for example due to a battery charging from 280 V to 420 V). The TMN, placed in series with the ZCN input and the output of the compensation keeps the power constant under coupling variation. The ZCN, TMN, arrangement uses the controllable reactance of the TMN to regulate the power transfer. Controlling the impedance of the load branch helps the system to regulate the power flow without increasing the current stress of the power electronics employed.

[0011] WO2021130714A1 , incorporated herein by reference, shows an improvement to the combination of a ZCN and a TMN. As shown in Figure 3 the TMN comprises a power converter. This means that one or more switches or controllers are introduced to the TMN to enable the system to create an active impedance. This allows control of the impedance for power factor control, such as enabling the secondary to appear substantially resistive.

[0012] Unfortunately, impedance compression networks and tuneable matching networks can significantly increase the number of components, cost and size of a wireless power transfer system.

[0013] Object

[0014] It is an object of the present disclosure to provide an apparatus, system or method that overcomes one or more of the disadvantages of existing systems, or which at least provides a viable alternative.

[0015] Summary

[0016] This disclosure proposes a secondary or primary wireless power transfer circuit having a compensation network and controlled power converter which are configured to control the impedance of the circuit.

[0017] The circuits proposed may use the existing power converter and existing compensation networks to thus provide systems or methods that can be used to achieve a similar performance to those that employ ZCNs and TMNs. However, the operation of these circuits is distinctly different from known approaches to impedance control of wireless power transfer circuits.

[0018] In an aspect the disclosure may broadly be said to consist in a wireless power transfer circuit comprising: A power transfer coil, A compensation network, and An impedance compression network connected to the compensation network, wherein the compensation network comprises a controllable impedance in parallel with the power transfer coil.

[0019] In an aspect the disclosure may broadly be said to consist in a wireless power transfer circuit comprising: A power transfer coil, A compensation network, and An impedance compression network connected between the compensation network and a source or a load of the wireless power transfer circuit, wherein the compensation network comprises a controllable impedance in parallel with the power transfer coil.

[0020] Optionally the controllable impedance comprises a controllable reactance. Optionally the controllable impedance is a controllable capacitor. Optionally the controllable impedance is an inductor.

[0021] Optionally the controllable impedance is a switch-controlled capacitor.

[0022] Optionally the controllable impedance is connected to a fixed impedance. Optionally the base impedance is connected in series or parallel with the fixed impedance. Optionally the fixed impedance comprises a capacitor. Optionally the fixed capacitor is configured to set a controllable impedance range or limit.

[0023] Optionally the controllable impedance comprises one or more switches connected in parallel with an impedance.

[0024] Optionally comprising an impedance between the compensation network and the impedance compression network. Optionally wherein the impedance is one or more of a capacitance or an inductance. Optionally the impedance is a series impedance.

[0025] Optionally wherein the controllable impedance is connected between two branches and / or two legs of the impedance compression network.

[0026] Optionally comprising an impedance between the controllable impedance and the each of the input legs of the impedance compression network. Optionally wherein the impedance is one or more of a capacitance or an inductance.

[0027] Optionally wherein the controllable impedance is connected between the output legs power transfer coil.

[0028] Optionally wherein the impedance compression network is not controlled. The impedance compression network is optionally passive.

[0029] Optionally wherein the controllable impedance is between the power transfer coil and the impedance compression network.

[0030] Optionally wherein the controllable impedance is configured to regulate the output power of the impedance compression network. Optionally comprising a load connected to the output of the impedance compression network. Optionally comprising a rectifier connected to the output of the impedance compression network. Optionally comprising a rectifier connected between the output of the impedance compression network and the source or load. Optionally comprising rectifiers connected between the outputs of the impedance compression network and the source or load.

[0031] Optionally wherein the load is configured to vary in voltage level.

[0032] Optionally the wireless power transfer coil is a primary wireless power transfer coil or secondary wireless power transfer coil. Optionally the wireless power transfer circuit is a primary wireless power transfer circuit or secondary wireless power transfer circuit. Optionally the impedance compression network is between the compensation network and the load of the secondary wireless power transfer circuit. Optionally the impedance compression network is between the compensation network and the source of the primary wireless power transfer circuit.

[0033] Optionally the wireless power transfer coil is configured to receive or provide power from a second wireless power transfer coil comprising the same type of compensation network.

[0034] Optionally the compensation network comprises a first sub-network and a second sub-network, the first sub-network comprising the power transfer coil, the second sub-network comprising the controlled impedance, and wherein the reactance of the first sub-network is substantially cancelled by the combined reactance of the second sub-network and the impedance compression network.

[0035] Optionally wherein the controllable impedance is a first controllable impedance. Optionally comprising a second controllable impedance in series with the power transfer coil. Optionally wherein the second controllable impedance is a capacitance. Optionally wherein the second controllable impedance is connected between the power transfer coil and the first controllable impedance.

[0036] Optionally comprising a multi-phase wireless power transfer circuit comprising two or more wireless power transfer circuits as described herein.

[0037] Optionally the wireless power transfer circuit is a pick-up circuit and the reactance seen at the input comprises a reactance seen by a primary circuit. Optionally the wireless power transfer circuit is a primary circuit and the reactance seen at the input comprises a reactance seen by a power source connected to the primary circuit.

[0038] Optionally the controlled impedance is controlled by controlling a phase angle of one or more switches in the controlled impedance.

[0039] Optionally comprising a controller configured to determine one or more of a current or a voltage and control the controllable impedance.

[0040] Optionally wherein the current comprises a current in the compensation circuit. Optionally wherein the current is the current through a circuit leg comprising the controlled impedance.

[0041] Optionally wherein the voltage comprises a voltage in the compensation circuit. Optionally wherein the voltage is the voltage across a circuit leg comprising the controlled impedance.

[0042] Optionally the compensation network is a parallel compensation network. Optionally the compensation network is an LCL. Optionally the C is the controllable capacitor. Optionally the compensation network is and LXL or LXC network.

[0043] Optionally wherein the current or voltage comprises a current or voltage of the load. Optionally wherein comprising the load voltage.

[0044] In an aspect the disclosure may broadly be said to consist in a wireless power transfer circuit comprising: a power transfer coil, a compensation network, and an impedance compression network connected to the compensation network, wherein the compensation network comprises a controllable capacitance configured to control the input voltage to the impedance compression network.

[0045] In an aspect the disclosure may broadly be said to consist in a wireless power transfer circuit comprising: a power transfer coil, a compensation network, and an impedance compression network connected between the compensation network and a source or load of the wireless power transfer circuit, wherein the compensation network comprises a controllable capacitance configured to control the input voltage to the impedance compression network.

[0046] Optionally the controllable capacitance is a switch-controlled capacitor. Optionally the controllable capacitance is connected in series with a fixed capacitance. Optionally the controllable capacitance comprises one or more switches connected in parallel with an capacitance.

[0047] Optionally comprising an impedance between the compensation network and the impedance compression network. Optionally wherein the impedance is one or more of a capacitance or an inductance.

[0048] Optionally wherein the controllable capacitance is connected between the input legs of the impedance compression network. Optionally comprising an impedance between the controllable capacitance and the each of the input legs of the impedance compression network. Optionally wherein the impedance is one or more of a capacitance or an inductance.

[0049] Optionally wherein the controllable capacitance is connected between the output legs power transfer coil. Optionally wherein the controllable capacitance is connected in series with the power transfer coil.

[0050] Optionally wherein the impedance compression network is not controlled.

[0051] Optionally wherein the controllable capacitance e is connected between the power transfer coil and the impedance compression network. Optionally wherein the controllable capacitance is configured to regulate the output power of the impedance compression network.

[0052] Optionally comprising a load connected to the outlet of the impedance compression network. Optionally wherein the load is configured to vary in voltage level.

[0053] Optionally the wireless power transfer coil is a primary or secondary wireless power transfer coil. Optionally the wireless power transfer coil is configured to receive or provide power from a second wireless power transfer coil comprising the same type of compensation network.

[0054] Optionally the compensation network comprises a first sub-network and a second sub-network, the first sub-network comprising the power transfer coil, the second sub-network comprising the controlled impedance. Optionally wherein the reactance of the first sub-network is substantially cancelled by the combined reactance of the second sub-network and the impedance compression network. Optionally comprising a second controllable impedance in series with the power transfer coil.

[0055] Optionally wherein the second controllable impedance is a capacitance. Optionally wherein the second controllable impedance is connected between the power transfer coil and the controllable capacitance.

[0056] Optionally comprising a multi-phase wireless power transfer circuit comprising two or more wireless power transfer circuits as described.

[0057] Optionally the wireless power transfer circuit is a pick-up circuit and the reactance seen at the input comprises a reactance seen by a primary circuit.

[0058] Optionally the wireless power transfer circuit is a primary circuit and the reactance seen at the input comprises a reactance seen by a power source connected to the primary circuit.

[0059] Optionally the controlled impedance is controlled by controlling a phase angle two one or more switches in the controlled impedance.

[0060] Optionally comprising a controller configured to determine one or more of a current or a voltage and control the controllable capacitance.

[0061] Optionally wherein the current comprises a current in the compensation circuit. Optionally wherein the current is the current through a circuit leg comprising the controlled impedance.

[0062] Optionally wherein the voltage comprises a voltage in the compensation circuit. Optionally wherein the voltage is the voltage across a circuit leg comprising the controlled impedance.

[0063] Optionally the compensation network is a parallel compensation network. Optionally the compensation network is an LCL. Optionally the C is the controllable capacitor. Optionally the compensation network is and LXL or LXC network.

[0064] Optionally wherein the current or voltage comprises a current or voltage of the load. Optionally wherein comprising the load voltage.

[0065] In an aspect the disclosure may broadly be said to consist in a wireless power transfer circuit is provided comprising a compensation network comprising a first sub-network and a second subnetwork, the first sub-network comprising a power transfer coil, and the second sub-network comprising a controllable impedance in parallel with the power transfer coil; an impedance compression network connected to the compensation network, the impedance compression network being configured to supply a load, wherein the reactance of the first sub-network is substantially cancelled by the combined reactance of the controlled impedance of the second sub-network and the reactance of the impedance compression network.

[0066] In an aspect the disclosure may broadly be said to consist in a compensation network comprising a first sub-network and a second sub-network, the first sub-network comprising a power transfer coil, and the second sub-network comprising a controllable impedance in parallel with the power transfer coil; an impedance compression network connected between the compensation network and a source or load of the wireless power transfer circuit, wherein the reactance of the first sub-network is substantially cancelled by the combined reactance of the controlled impedance of the second sub-network and the reactance of the impedance compression network.

[0067] Optionally the the combined reactance is substantially cancelled for a range of voltage levels at the load or powers provided to the load.

[0068] Optionally the controllable impedance is configured to provide an impedance which, in combination with the compensation network and the impedance compression network, make an input impedance to the wireless power transfer circuit appear to be substantially resistive and / or minimally inductive.

[0069] Optionally comprising a controller configured to control the switching of the controllable impedance. Optionally the controller controls a phase angle of switches of the controllable impedance.

[0070] Optionally the controllable impedance is configured to control the real power provided to the load.

[0071] In an aspect the disclosure may broadly be said to consist in a method for controlling a wireless power transfer circuit comprising a compensation circuit comprising a controllable capacitance connected to an impedance compression network, the method comprising: switching the controllable capacitance to provide a reactance, which in combination with the compensation network and the impedance compression network substantially compensates for variations in reactance seen at an input to the wireless power transfer circuit. In an aspect the disclosure may broadly be said to consist in a method for controlling a wireless power transfer circuit comprising a compensation circuit comprising a controllable capacitance connected to an impedance compression network, the method comprising: switching the controllable capacitance to provide a reactance, which in combination with the compensation network and the impedance compression network substantially compensates for variations in voltage seen at an output of the wireless power transfer circuit.

[0072] In an aspect the disclosure may broadly be said to consist in a method for controlling a wireless power transfer circuit comprising a compensation circuit comprising a controllable capacitance and an impedance compression connected between the compensation circuit and a source or load of the wireless power transfer circuit network, the method comprising: switching the controllable impedance to provide a reactance, which in combination with the compensation network and the impedance compression network substantially compensates for variations in reactance seen at an input to the wireless power transfer circuit.

[0073] In an aspect the disclosure may broadly be said to consist in a wireless power transfer circuit comprising: a power transfer coil; a compensation circuit comprising a controllable impedance in parallel with the power transfer coil; an impedance compression network; and a controller configured to: switch the controllable impedance to provide a reactance, which in combination with the compensation network and the impedance compression network substantially compensates for variations in reactance seen at an input to the wireless power transfer circuit.

[0074] In an aspect the disclosure may broadly be said to consist in a wireless power transfer circuit comprising: a power transfer coil; a compensation circuit comprising a controllable capacitance; an impedance compression network; and a controller configured to: switch the controllable capacitance to provide a reactance, which in combination with the compensation network and the impedance compression network substantially compensates for variations in reactance seen at an input to the wireless power transfer circuit.

[0075] Optionally the controllable capacitance is in parallel with the power transfer coil. Optionally comprising a further fixed or controllable capacitance in series with the power transfer coil. Optionally the controllable capacitance is in series with the power transfer coil. In an aspect the disclosure may broadly be said to consist in a method for controlling a wireless power transfer circuit comprising a compensation circuit comprising a controllable impedance in parallel with an impedance compression network, the method comprising: switching the controllable impedance to provide a reactance, which in combination with the compensation network and the impedance compression network substantially compensates for variations in reactance seen at an input to the wireless power transfer circuit.

[0076] Optionally the wireless power transfer circuit is a tuned wireless power transfer circuit, and the method further comprises operating the tuned wireless power transfer circuit to receive or transfer power wirelessly at a frequency that is not the tuned frequency of the tuned wireless power transfer circuit. Optionally the controllable impedance is controlled to such that the tuned frequency of the wireless power transfer circuit is different to the tuned frequency of the wireless power transfer circuit it receives power from or provides power to.

[0077] Optionally the controllable impedance is switched to provide a controlled impedance to compensate for variations in reactance seen at an input to the wireless power transfer circuit. Optionally the variations in reactance occur while the wireless power transfer circuit is receiving power wirelessly from or transferring power wirelessly to another wireless power transfer circuit. Optionally variations in reactance at the input to the wireless power transfer circuit are caused by coupling variation. Optionally variations in reactance occur due to changing electrical characteristics of the load.

[0078] Optionally the controllable impedance is switched to provide a controlled impedance.

[0079] Optionally comprising switching the controllable impedance to minimise or cancel an impedance seen at one or more of the input and output of the wireless power transfer circuit.

[0080] Optionally the variations in reactance seen at one or more of the input or output of the wireless power transfer circuit are introduced by one or more of: receiving power wirelessly from, or transferring power wirelessly to, another wireless power transfer circuit; transferring power to a load supplied by the wireless power transfer circuit.

[0081] Optionally comprising monitoring an output voltage or current of the wireless power transfer circuit and switching the controllable impedance based, at least in part, on the output voltage or current. Optionally the controllable impedance is switched to control the resistance seen at the input to the wireless power transfer circuit.

[0082] Optionally comprising switching the controllable impedance such that the reactance seen at the input to the wireless power transfer circuit is minimally inductive.

[0083] Optionally comprising switching the controllable impedance such that the power transferred by the wireless power transfer circuit remains substantially constant over a two-fold change in voltage across a power transfer coil of the wireless power transfer circuit.

[0084] Optionally comprising controlling the controllable impedance such that the power factor at the input to the wireless power transfer circuit remains substantially constant over a two-fold change in voltage across a power transfer coil of the wireless power transfer circuit.

[0085] Optionally comprising controlling the controllable impedance such that the wireless power transfer circuit compensates for variations in the voltage output of the wireless power transfer circuit. Optionally the voltage output is dependent on a level of charge of a battery that is charged from the output. Optionally the battery is an electric vehicle battery. Optionally the battery is a battery in a personal electronic device. Optionally the battery is an aircraft battery.

[0086] In another aspect the disclosure may broadly be said to consist in a method comprising controlling a resonant inductive power pickup receiving power wirelessly from an inductive power primary, operating a controlled capacitance to substantially compensate for variability in one or more of: the coupling with the inductive power transfer primary, and changes in the load supplied by the pick-up.

[0087] Optionally the controlled capacitance is in parallel with the power transfer coil of the pick-up.

[0088] Optionally the controlled capacitance is on the primary. Optionally the controlled capacitance is in parallel with the power transfer coil of the primary.

[0089] Optionally controlling the controlled capacitance to substantially offset a variable reactance caused by one or more of the coupling with the inductive power transfer primary and the load supplied by the wireless power transfer primary. Optionally comprising controlling the controllable capacitance to correct a power factor between the inductive power transfer primary and the resonant inductive power pickup. Optionally the power factor is controlled towards, or substantially to, a unity power factor.

[0090] Optionally comprising controlling the controllable capacitance to concurrently regulate the power supplied to the load and control an impedance reflected, by the resonant inductive power pickup, onto the inductive power transfer primary.

[0091] Optionally comprising controlling the power supplied to the load to regulate the charge in a battery connected to the resonant inductive power pickup.

[0092] Optionally comprising controlling the controllable capacitance to substantially eliminate a reactive component of the reflected impedance.

[0093] In another aspect the disclosure may broadly be said to consist in a method comprising controlling a resonant inductive power transfer circuit transferring power wirelessly to another resonant inductive power transfer circuit, operating a controlled capacitance to condition power delivered to the other resonant inductive power transfer circuit that is connected to the resonant inductive power transfer circuit, and concurrently controlling the controlled capacitance to create a controlled reactance that substantially compensates for variability in the coupling with the other resonant inductive power transfer circuit and / or changes in the power supplied to the power transfer circuit.

[0094] Optionally comprising controlling the controlled capacitance to create a reactance that substantially offsets a reactance caused by the coupling with the other inductive power transfer circuit.

[0095] Optionally comprising controlling the controlled capacitance to substantially offset a reactance caused by changes in the load supplied by the wireless power transfer circuit.

[0096] In one aspect the disclosure may broadly be said to consist in an active impedance matching circuit for a wireless power transfer primary or secondary, the circuit comprising: an inductor-reactance-inductor (LXL) network or an inductor-reactance-capacitor (LXC) network, wherein the reactance is controllable, an impedance control network, and a controller to control the reactance to actively control the circuit impedance. The controller may control the control angle of the controllable capacitor.

[0097] The LXL network may comprise an LCL network, wherein the reactance is a capacitance.

[0098] In another aspect the disclosure may broadly be said to consist in a method of controlling power flow in a wireless power transfer primary or secondary, the method comprising: controlling the X component of an LXL network to provide a tuned matching network and thus control power flow.

[0099] Optionally the LXL network comprises an LCL network.

[0100] Optionally the method includes controlling one or more of the above to actively control the impedance and thus regulate power flow.

[0101] In another aspect the disclosure may broadly be said to consist in an IPT system comprising a primary and a secondary / pick-up, the secondary / pick-up comprising an impedance compression network; one or more of the primary and secondary comprising a controllable impedance.

[0102] In another aspect the disclosure may broadly be said to consist in an IPT system comprising a primary and a secondary each comprising a power transfer coil, the secondary comprising an impedance compression network connected between a compensation network and a load of the secondary; one or more of the primary and secondary comprising a controllable impedance.

[0103] Optionally the controllable impedance and impedance compression network are configured to adjust for load changes.

[0104] Optionally the primary and secondary comprise respective power transfer coils. Optionally the controllable impedance is in parallel with the respective power transfer coils. Optionally the controllable impedance comprises a controllable capacitance. Optionally the controllable capacitance comprises an SSC.

[0105] Optionally the secondary comprises any one or more of the features described above.

[0106] Optionally the primary and secondary have the same compensation network. Optionally the compensation network is an LCL. In another aspect the disclosure may broadly be said to consist in a wireless power transfer circuit comprising: A power transfer coil, A compensation network, and An impedance compression network connected between the compensation network and a source or load of the wireless power transfer circuit, wherein the compensation network comprises a controllable impedance in parallel with the power transfer coil.

[0107] In another aspect the disclosure may broadly be said to consist in a wireless power transfer circuit comprising: A power transfer coil, A compensation network, and An impedance compression network connected between the compensation network and a source or load of the wireless power transfer circuit, wherein the compensation network comprises a controllable impedance in parallel with the power transfer coil.

[0108] In another aspect the disclosure may broadly be said to consist in a wireless power transfer circuit comprising: A power transfer coil, A compensation network, A current source output, and An impedance compression network connected between the compensation network and the current source output.

[0109] Optionally the compensation network comprises a series compensated compensation network. Optionally comprising a rectifier between the compensation network and the current source output. Optionally the compensation network comprises a controllable impedance in parallel with the power transfer coil. Optionally the current source output comprises a DC inductor configured to supply the current source output.

[0110] Optionally wherein the rectifier is a passive rectifier, optionally a diode rectifier. Optionally wherein the impedance compression network is a full bridge compression network. Optionally wherein the compensation network comprises an LC compensation network. Optionally wherein the impedance compression network comprises an inductive branch and a capacitive branch. Optionally each branch comprises one or more legs. Optionally comprising one or more resistive legs.

[0111] In another aspect the disclosure may broadly be said to consist in a method of controlling the controllable impedance of any of the above aspects.

[0112] In another aspect the disclosure may broadly be said to consist in an IPT system comprising a primary and / or a secondary of any of the above aspects. In another aspect the disclosure may broadly be said to consist in an IPT system comprising a wireless power transfer primary comprising a primary compensation network and a wireless power transfer secondary comprising a secondary compensation network, the primary and secondary compensation networks configured to supply power the secondary as a current source, wherein the wireless power transfer secondary comprises an : impedance compression network connected between the compensation network and a current source output of the wireless power transfer secondary.

[0113] As used herein the term “and / or” means “and” or “or”, or both. As used herein “(s)” following a noun means the plural and / or singular forms of the noun. The term “comprising” as used in this specification means “consisting at least in part of’. When interpreting statements in this specification which include that term, the features, prefaced by that term in each statement, all need to be present, but other features can also be present. Related terms such as “comprise” and “comprised” are to be interpreted in the same manner. It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1 , 1.1 , 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1 .5 to 5.5 and 3.1 to 4.7). The entire disclosures of all applications, patents and publications, cited above and below, if any, are hereby incorporated by reference.

[0114] The disclosed subject matter also provides method or system which may broadly be said to consist in the parts, elements and features referred to or indicated in this specification, individually or collectively, in any or all combinations of two or more of those parts, elements or features. Where specific integers are mentioned in this specification which have known equivalents in the art to which the disclosure relates, such known equivalents are deemed to be incorporated in the specification.

[0115] Other aspects of the disclosure may become apparent from the following description which is given by way of example only and with reference to the accompanying drawings.

[0116] Drawing Description

[0117] Examples or embodiments are described below with reference to the drawings in which:

[0118] Figure 1 shows a schematic diagram of an IPT system; Figure 2 shows an IPT system pick-up having a tuneable matching network (TMN) and a impedance compression network (ZCN);

[0119] Figure 3 shows an IPT system pick-up having a controlled power converter with 2 switches;

[0120] Figure 4 shows an IPT system pick-up having a controlled capacitance in parallel with the power transfer coil;

[0121] Figure 5 shows an equivalent circuit for an AC switch controlled capacitance;

[0122] Figure 6 shows example waveforms for the controlled capacitance shown in Figures 4 and 5;

[0123] Figure 7 shows an IPT system pick-up having a two switches parallel to a capacitance and the power transfer coil;

[0124] Figure 8 shows a flow chart of the operation of a controller of the circuit of figure 4;

[0125] Figure 9 shows an IPT system pick-up with a controller to control the controllable capacitance;

[0126] Figure 10 shows a 3-phase IPT system with a variable impedance and impedance compression network on each phase;

[0127] Figure 11 shows an IPT primary with a variable impedance.

[0128] Figure 12 shows a circuit diagram of a power converter comprising a ZCN;

[0129] Figure 13 shows an equivalent circuit for an AC switch controlled capacitance with a series capacitance;

[0130] Figure 14 shows an IPT system pick-up having a current source output and a half-bridge ZCN.

[0131] Figure 15 shows an IPT system pick-up having a current source output and a half-bridge ZCN, with an LC compensation circuit.

[0132] Figure 16 shows an IPT system pick-up having a current source output and a full-bridge ZCN, with an LC compensation circuit and a boost capacitor. Figure 17 shows an IPT system pick-up having a current source output and a half-bridge ZCN, with a LC compensation circuit with a controllable parallel capacitance and a controllable boost capacitor.

[0133] Figure 18 shows an IPT system pick-up having a current source output and a half-bridge ZCN, with an LCL compensation circuit.

[0134] Detailed Description

[0135] This document discloses techniques that can be used to actively control the effective impedance of a WPT circuit, such as a primary or secondary power transfer circuit, or system in order to regulate power flow. These techniques include tuning the primary and / or pick-up compensation networks to facilitate active impedance matching. The tuning of the system may be independently controlled to be tuned or detuned. The controllable impedance allows the system to, for example, improve power factor, with a reduced number and / or type of circuit components. In some cases the controllable impedance will affect the tuning of the system. For example, the controllable impedance may be controlled to ensure a desired or required input voltage to the ZCN. If the coupling changes the controller will change the controllable impedance (and therefore the secondary tuning) to keep the voltage and / or current to the ZCN at the desired level. This ensures constant power transfer. However, this change will change the tuned frequency of the secondary relative to the primary because the frequency of the primary has not changed. Because of the control of the controllable impedance the power factor is also maintained. For example through cancellation between the compensation network including the controllable impedance and the ZCN.

[0136] The examples provided in this disclosure are largely directed to implementing a method and system for active impedance control implemented via a pick-up or secondary circuit of a wireless power transfer system. However, it will be understood by a person of ordinary skill in the art that the method or system may equally be performed on a primary circuit of a WPT I IPT system, or across both primary and secondary circuits.

[0137] As described above, with prior art circuits, power factor may be an issue. With the active impedance control of the present disclosure, power factor issues are addressed since impedance, specifically reactance, compensation allows a predominantly or solely resistive load to be seen looking into the circuit (either from a power source or load), so the power factor is essential unitary as the voltage and current appear, or are, in phase. A typical WPT / IPT system is shown in Figure 1 , generally referenced 1 , in which the primary circuit 2 has a power converter 3 connected between a DC source (or load) Vin and a compensation network 4 which includes a power transfer coil Lpt. Similarly, the pick-up or secondary circuit 5 has a compensation network 6 including a power transfer coil Lst, and a power converter 7 which is also connected to a load (or source) Vout. Figure 2 shows a pick-up with a compensation network 6, a tuneable matching network 18 and an impedance compression network with rectifier 19. As set out in wireless power standards the ZCN reduces battery voltage variation while the TMN in series with the ZCN keeps power constant under coupling variation. Figure 3 shows a secondary 10 which uses a controlled power converter 30, 32 as part of the ZCN. The controlled power converter 30, 32 is illustrated as a rectifier with one diode replaced with a controlled switch. This allows control of the circuit impedance. For example, the duty cycle of the power converter switches can be controlled to vary Lzc, Czc, RLac and RCac in order to regulate the power flow. The phase angle of the or each controlled power converter can be controlled to regulate the impedance of the impedance of the circuit as it appears at Vsr, i.e. as it appears to a primary circuit which is wirelessly supplying power. The secondary 10 has a compensation network 12 with a coil 14, parallel capacitance 16 and series impedance 18. The ZCN has two branches, with inductors 24 on both legs of the first branch and capacitors 26 on both legs of the second branch.

[0138] This disclosure presents a number of illustrative configurations for circuits that include compensation networks with a controlled impedance in connection with an impedance compression network. This structure can allow control of the circuit impedance with fewer components and / or complexity than prior art systems.

[0139] In Figure 4, a pick-up (or secondary) WPT circuit is shown, generally referenced 100. Vsr represents the voltage induced in power transfer coil Lst by a time varying magnetic field, generated from a primary WPT circuit 101 , for example. Lst, Csts,a, Cstp and Lsi,a form an LCL(lnductor, capacitor, inductor) topology compensation network. As shown in Figure 4 this structure is repeated on both sides of the power transfer coil Lst, with Csts,b and Lsi,b also forming part of the topology compensation network. However the capacitance may be provided by a single capacitor. In some cases the inductance is provided by a single inductor (i.e. leading to one leg of the impedance matching network). In some cases the capacitors Csts,a and Csts,b are balanced or substantially equal size. In some cases the inductances Lsi,a and Lsi, b are balanced or substantially equal size. In some cases Lsi, a and Lsi,b are replaced by impedances. For example, capacitors may be used instead. Changes may be made to the compensation network 112 to accommodate this change. These may be referred to as LXC networks, with X representing controllable impedance 200. Other compensation networks, with greater or fewer components may also be used where they have a controllable capacitance,

[0140] As shown in Figure 4 the branch 114 comprising the power transfer coil Lst is predominantly inductive although Csts,a and Csts,b are present. In some cases branch 114 may be entirely inductive. The leg or branch 116 of the parallel capacitance Cstp is predominantly or entirely capacitive, with the connection 118 between the Cstp and the impedance compression network being predominantly or entirely inductive. In other cases further inductors or capacitors may be present to alter the compensation work. The compensation network 112 has a first loop comprising first branch 114 and second branch 116. It has a second loop comprising second branch 116, third branch 118 and the components connected to that branch including the power converter 201 comprising an impedance compression network 130, 132. Branches 114 and 116 are provided in parallel with each other. The branches may form, or may combine to form subnetworks of the compensation network. A sub-network comprises one or more connected branches. For example, branch 114 may be a sub-network, or branches 116 and 118 may be a sub-network. In some cases, a first sub-network is formed by the branch comprising the power transfer coil (e.g. branch 114) with a second sub-network formed by the remaining compensation network. Branch 118 may be referred to, or considered as, a series impedance between the controllable impedance 200 and the ZCN 130, 132. Alternatively, the impedance of branch 118 may be varied from the expected resonance. This adjustment may improve performance of the ZCN 130, 132.

[0141] In some cases, the impedance compression network 130, 132 includes, or is connected to a power converter 140, 142. A power converter may include the ZCN, or the ZCN may be considered as a separate component (or separate power converter) preconfiguring the power supplied to a power converter. For example, the ZCN 130, 132 may supply an AC load. The power converter 140, 142 may be a controlled or passive power converter. A rectifier is a type of power converter, but other power converters may be used. Figure 4 shows diode rectifiers 140, 142 as example passive power converters or passive rectifiers. These act as respective power converters between the ZCNs 130, 132 and the load. A passive power converter (for example a passive rectifier) requires no controlled switching or input control signals. A controlled power converter (for example a controlled rectifier) has a switch or controller to alter the operation of the rectifier. Examples include a half or full bridge converters or rectifiers, as well as any other topologies wherein switches are used to modify a power converter forming part of the impedance compression network. In some cases, active components are used in the ZCN 130, 132 to configure the operation of the ZCN 130, 132. Although described here as a ZCN followed by a power converter it is possible to alternative describe the ZCN and power converter as a combined power converter. For example a combined power converter may comprise a ZCN and a power converter (such as a rectifier, or diode rectifier).

[0142] In the example shown a LCL type compensation network 112 is used. However alternative compensation networks may be used. For example additional components may be used to improve the resonance of the pick-up or primary and / or to improve tuning with a corresponding primary or pick-up. In some cases an LXC compensation network is used. In some cases an LXL compensation network is used. In some cases the associated primary compensation network type will be the same as the pick-up compensation network type. For example both may be LCL or LXL type networks. Or both may be parallel networks. Or the compensation networks may be LCL and parallel networks. In some cases the compensation networks of the primary and / or secondary converters are configured to supply a voltage source or a current source.

[0143] The compensation network 112 may be connected to a power converter 140, 142 such as a rectifier, which provides an output power, Vout, to a load. As shown in Figure 4 a passive power converter 201 , comprising a ZCN 130, 132 and rectifier 140, 142 connects the output of the LCL compensation network to two ZCNs 130 and 132 via Lzcl and Czc. In this example electrical energy from compensation network 112 is supplied to the load RLDC (including DC filter capacitor Cout) through both of a) a first output branch comprising zcl components and ZCN 130; and b) a second output branch comprising zee components and ZCN 132.

[0144] In each output branch a series inductor (Lzcl, a, Lzcl,b, Lzcc,a, and Lzcc,b) and capactor (Czcl,a, Czcl,b, Czcc,a, Czcc,b) provide an input impedance to the rectifiers 140, 142. However, these are not required on all the legs of each branch. The impedance compression network 130, 132 is therefore connected between the compensation network 112 and the output Vout. In some cases, the rectifier(s) 140, 142 (shown as two full bridge diode rectifiers in Figure 4) are not be considered part of the output or ZCN, meaning the ZCN is located between the compensation network 112 and the rectifier(s). Although all legs of the branches of the ZCN 130, 132 show inductances and capacitances the impedance is not limited to a combination. In some cases, legs may be purely inductive or purely capacitive. In some cases, selected legs may be simple connections (i.e. without specified components, or resistive) with the components on the other leg of the branch 130, 132. An explanatory ZCN 120 is shown Figure 12. It is shown as part of a power converter 125. The ZCN 120 is connected to a controlled resistance 124. This may represent a rectifier. The input 123 is split into two branches 121 , 122. Each branch has a single leg, but additional legs could be incorporated as in Figure 4. The impedances of the branches 121 , 122 are selected to be different. As shown in Figure 12 a first branch 121 may be inductive, with a second branch 122 being capacitive. The impedances (Xicn) may have the same magnitude, or the magnitude may be different on each branch. The deliberate mistuning can enable additional flexibility by allowing a choice to tune or mistune the coupling. The ZCN 120 is entirely passive. The outputs of branches 121 , 122 may be connected to outputs 124, such as the rectifiers shown in Figure 4, for example. Two rectifiers are used for a full-bridge ZCN while a single rectifier may be used for a half-bridge ZCN. The outputs or rectifiers 124 are preferably passive, because this reduces control requirements. The outputs 124 may be connected in parallel and applied to the load.

[0145] A controlled impedance, in this document, includes an impedance that can be controlled. It may be referred to as a controllable impedance, i.e., an impedance configured to be controlled or adjusted during operation of the system. One example is a switch controlled capacitor (SCC). A controllable impedance is configurable to present a different impedance based on the control signal provided. In a general example the control signal(s) may be provided to one or more switches on the controllable impedance. The switches then operate to short circuit or bypass the impedance for a time period, or portion of a waveform to alter the apparent impedance. Impedance is used to refer to one or more of a resistance or reactance. In most cases the impedance will be reactive, at least in part, comprising an inductive or capacitive value.

[0146] As shown in Figure 4 one example uses a controlled capacitor Cstp, 200, shown in branch 116. In some cases the controllable impedance or reactance may be referred to as a variable impedance or reactance. In some cases the controlled impedance may be connected to another impedance. For example the SCC may be in series with a capacitor. This may allow a minimum capacitance to be set and / or to allow more control over the capacitance range of interest (as the second capacitor provides a base impedance). For example the fixed capacitor may provide a base level of capacitance, while the controllable capacitor is able to increase this capacitance. Because the capacitance change may be across a smaller range, above the fixed capacitor the controllable capacitance can be focussed on the area of importance, greater control is achieved.

[0147] As shown in Figure 4 the controllable impedance 200 is part of the compensation circuit 112.

[0148] This means that controlling the impedance 200 modifies the running of the compensation circuit 112 or can be used to correct for changes in output voltage Vout while supplying constant power to the load. For example, consider a battery as a load. As the battery charges the voltage of the battery increases. This change would result in increasing load resistance and a drop in power supplied by the pick-up. However, it is desirable to continue to provide a set level of power, or at least a controlled level of power. With the circuit of Figure 4 the ZCN 130, 132 can passively regulate the power transfer, however this creates an impedance change in the circuit changing power transfer characteristics. By introducing the controllable impedance 200 the circuit of Figure 4 can adjust for this change in impedance and / or ensure a desired input voltage (power) level to the ZCN 130, 132. In comparison to a TMN this adjustment is achieved with fewer components. Comparing this configuration to the controlled TMN / ZCN of Figure 2, the number of variable impedances is reduced and switches of a control circuit of Figure 3 are similarly reduced.

[0149] Therefore, the described methods and systems propose a configuration of a controllable impedance 200 and a ZCN 130, 132 that significantly reduces the component cost of a method and system for delivery a set (e.g. constant) power level to a load while the load voltage changes. By using a controllable impedance 200 in the compensation network 112 (for example, as the capacitor in parallel with the secondary pad Lst) the two variable impedances of Figure 2 are eliminated. Figure 4 shows two inductors in the third branch 118 (Lsi,a, Lsi,b). These inductors may help to maintain high power factor because they provide further reactance controllable by the controllable impedance 200. As illustrated these inductors are not controlled directly, but rather their effect on the circuit characteristics is dependent on the controllable impedance 200. In some cases, further controllable impedances may be used, but these are typically not required and create additional control requirements and / or components. In some cases, capacitors may be used instead between the controllable impedance 200 and the ZCN 130, 132.

[0150] The controlled impedance 200 is shown, in Figure 4, between the branches, or input legs, of the ZCN 130, 132 or power converter 201 . The controlled impedance 200 is shown on connection or branch 116. As shown, it may be connected in parallel with the power transfer coil Lst and the ZCN 130, 132. A first end is connected to the input branch of a first ZCN 130, with the second end connected to the input branch of a second ZCN 132. This allows the controlled impedance to short circuit the legs of the ZCN (and therefore also the power converter 201). Although shown as passive rectifiers 140 (diode converters) the power converters could be active or controlled if required. The controlled impedance 200 is shown between the branches (in Figure 4 comprising two legs each) of the power transfer coil Lst. Is it separated from direct connection with the power transfer coil Lst by the capacitors Csts,a and Csts,b, although it may be directly connected or separated by alternative impedances dependent on the compensation circuit 112 used.

[0151] In some examples the switches of the controllable impedance 200 are used to short the branches of the ZCN 130, 132. This enables the system to control the input power into the ZCN 130, 132 and therefore balance the output power. In some cases the switches may be separate from (i.e., not directly connected to) the controllable impedance, but still configured to short the ZCN 130, 132. For example the switches may short the entire compensation network 112, or a branch, or portion, of the compensation network 112, which may include multiple components.

[0152] Figure 5 shows an example of a controllable impedance 200. The controllable impedance 200 is shown as a switch-controlled capacitor (SCC). This comprises a capacitor Cx in parallel with an AC switch. An AC switch may comprise two back-to-back switches. The switches may have body diodes to allow conduction when one switch is in the off state. The switches Sa, Sb are shown as MOSFETs, although alternative switches may be used. The switches are operated to control the amount of time the capacitor is bypassed, in effect providing a controlled capacitance value in the circuit. In alternative cases an inductor or other impedance or reactance could be similarly controlled.

[0153] Figure 13 shows a circuit diagram of a controlled impedance comprising a controlled capacitance Cx in series with fixed capacitance Cy. The fixed capacitance provides a minimum capacitance Cy (where both switches are closed) and increases the maximum capacitance (a series combination of Cy and Cx) when both switches are open. Alternative combinations may be used, including, for example a parallel combination.

[0154] There are known methods of controlling a switch-controlled capacitor. Typically they clamp the voltage across the capacitor to 0V for a period of time. The time period may be known as the clamping time tc (or 0c in the angular domain). Figure 6 shows example waveforms during control. Figure 8 and Figure 9 show an example method of control and a controller connected to the circuit respectively. In this case the duty cycle of the switches (shown as the portion of the waveform in which the gate drive voltages Sa, Sb are ON) are kept at fifty percent duty cycle (pi radians). When the switches are ON the capacitor is shorted. The switches are offset from one another and operate at the frequency of the circuit voltage and current. (Issc). The switches may be synchronized with the high-frequency current or voltage in the resonant circuit. The control signal for the switches may be determined by a current sensor detecting the current, a voltage sensor detecting the voltage or other sensor of an electrical characteristic of the circuit. The, for example current, measurement may then be filtered to determine the dominant or core frequency (for example a bandpass filter). A zero-crossing detector may then be used to determine switching points, or to send a signal to a controller. Alternative methods may be used. The method may be monitored by a controller 202. The controller 202 may receive readings from one or more sensors. For example, the controller 202 may receive a current measurement from a current sensor. The current may be indicative of a current flowing through the controlled impedance 200, through the leg 116 on which the controlled impedance is located, or another portion of the circuit. For example the controller may receive a voltage measurement from one or more voltage sensors. The voltage may be indicative a load voltage or a voltage across the controlled impedance.

[0155] As shown in Figure 6 in the positive half cycle of iscc Sb is turned ON and Cx is discharged until Vex is zero. After Cx is fully discharged, iscc flows through Sb and the antiparallel diode of Sa. When Sb is turned OFF, Cx is charged by the current iscc. For the negative half cycle of iscc, charge or discharge of Cx is controlled by Sa with similar procedure as positive half cycle. It can be found that Vex is always zero at turn-off points and thereby both Sa and Sb achieve ZVS. In an alternative method the switch-controlled capacitor can control the capacitance by regulating the duty cycles of Sa and Sb. The clamping time is shown as 0c. The equivalent capacitance (or control) is then controlled by adjusting the control angle (p (for example, TT / 2< (p< IT), or the angle, offset or time period, between the current zero crossings and the switching. In Figure 6 this is show as the time between a negative zero crossing and the switching off of Sa.

[0156] In one example we consider a fixed battery voltage. As coupling between the pick-up and an associated primary (and hence induced voltage) increases, power can be maintained. Table 1 shows example values for how the output power can be maintained with increasing induced voltage by reducing the clamping duration. The nominal output voltage is 350 Volts.

[0157] Table 1

[0158] The fourth column of Table 1 also shows that as the induced voltage changes, the power factor angle between vsr and ist remains small. This in turn keeps the power factor high and the VA ratings low. Though, the results show the PF angle to be negative / capacitive for the whole range, it is possible to get positive / inductive values depending on the mistuning used. The circuit characteristics or component values used for the present examples are shown in Table 2.

[0159] Table 2

[0160] The ZCN 130, 132 is insufficient to maintain constant power under coupling variation. For example if the coupling is increased, the input current to the ZCN 130, 132 will increase and the ZCN 130, 132 will simply maintain the new higher input current even if the load voltage (for example a battery voltage changes). In other words, to control power as coupling varies, the system must have a way of reducing the ZCN input current as coupling increases and decreasing ZCN input current as coupling decreases. The controllable impedance 200 is configured to short the ZCN input current for a portion of the switching period. This is controlled by changing the control angle of the controlled impedance, although other control methods are also possible.

[0161] In one example the voltage across parallel compensation capacitor Cst,p (controllable impedance 200) is monitored. The system is then configured to monitor the zero-crossing of the voltage and, after a specified or determined delay turn Sa on. Sb is operated in a complementary fashion. In this way the system maintains a 50 percent duty cycle, zero voltage switching, and ensures a correct input current to the impedance compression network (ZCN).

[0162] In a further example the coupling (e.g. the induced voltage) may stay substantially constant, but there may be variation in the load voltage. Table 3 shows that when the load voltage is varied the output power can remain constant. In fact, only a small amount of change is required to the clamping duration (controlled by the control angle of the controllable impedance). The output power (Column 3) remains substantially constant while the load voltage is varied from 280 V to 420 V (Column 1). This is based on a parallel TMN with substantially 350V induced voltage (Vst).

[0163] Table 3

[0164] Table 4 shows simulated results for load voltage and coupling variation. These are demonstrated by a series of operating positions or states A through G. For example, as the system moves from operating state A to operating state B the battery voltage increases while the induced voltage remains constant. In state B the power decreases. However, the system can react by changing the clamping duration to maintain the original power level. As shown in state C the original power level at A can be maintained by increasing the clamping duration to 0.25 us.

[0165] Table 4

[0166] In an example where coupling decreases (e.g., as the system moves from position A to D), supplied power (induced voltage) drops significantly. The system can be configured to maintain the power level by increasing the clamping duration. Power can be increased to the level at A by increasing the clamping duration to 1 ,83us in E. This adjusts to a greater than two-fold power change. In some example the system reacts to changes in power level by changing the clamping time (or control angle) of the switches of the controllable impedance. This corresponds to changing the apparent impedance of the controllable impedance. In some cases this action does not depend on the cause of the increase or decrease in output power. In some cases the clamping time is decreased / increased to counter the change and maintain constant power transfer. It is advantageous because there is no conflict or trade-off between correcting for load voltage variation and correcting for wireless power coupling variation. The system may be configured, for example by selecting an appropriate size of impedance 200 to handle large changes in load voltage and / or wireless power coupling between the primary and / or pick-up (secondary) The skilled person will be able to determine suitable component values. The compensation network values may be determined by a search algorithm. Simulation, such as use of PLECS™ (Piecewise Linear Electrical Circuit Simulation) may support or help the search algorithm. The search algorithm may determine the combined impedance of the secondary circuit and the effective series capacitor. The search algorithm may determine the total impedance of the inductors in series with the ZCN input. The search algorithm may determine a range of switched capacitor impedances required for the range of coupling factors that the system will operate over. The search algorithm may output one or more possible solutions. The solutions may be configured to meet one or more constraints. For example, based on power factor and magnitudes of system currents. In some cases, the search algorithm may make one or more assumptions. For example, that the ZCN constitutes a constant resistive load, or a range of resistive loads. In some cases estimated values for the capacitance and inductance of the ZCN may be obtained by considering the geometric mean value of the minimum and maximum impedance required. For example, if a known nominal resistance is required the geometric mean may be used to calculate suitable impedances to balance the system about that resistance.

[0167] In some cases the series impedances (series inductances Lsi,a and Lsi,b of Figure 4) can be determined from the search algorithm. However, in some cases the series impedances can be adjusted to adjust the apparent impedance of the ZCN 130, 132. The ZCN 130, 132 and / or rectifier 140, 142 often appears as a resistance in series with a capacitance. In many cases while the resistance is designed to be near constant with battery voltage, the size of the capacitance will vary. The capacitance occurs because the two rectifiers do not appear purely resistive. In some cases making series impedances (e.g., Lsi,a and Lsi,b of Figure 4) larger can offset the rectifier capacitance. In some cases, the series impedances are configured to substantially or completely cancel the capacitance (e.g. of the power converter or rectifier 140, 142) at least one, or one, load voltage. The capacitance may also be reduced for a range of battery voltages. The series impedances may be selected based on the predetermined range of battery voltages. Adjusting the series impedances may reduce the detuning effect of the ICN and improve power transfer.

[0168] In some examples the impedances of the ZCN 130, 132 may be altered to achieve the effect of making the output voltage resistive at a desired voltage, or for a desired voltage range. By changing the net inductance and net capacitance in the ZCN 130, 132 can be adjusted. Advantageously this may improve the balance of the currents into the two rectifiers 140, 142 (or other power converters), which can reduce the VA ratings of the inductors and capacitors in the ZCN. However, this may require careful selection of component values to maintain the desired operation of the ZCN 130, 132.

[0169] Figure 7 shows an alternative secondary circuit for receiving power from primary 301 . The secondary circuit combines a controllable impedance 200 with a power converter 201 comprising a ZCN and rectification. In this case the control switches 218 (shown as an AC switch) are illustrated separately from the impedance Cst,p, 200 in the compression network 216. In some examples they can be considered to be shorting the voltage between the branches of the ZCN 130, 132, rather than controlling the impedance 200. In Figure 7 the third branch 118 comprising the series inductors has been omitted. While the system may operate without these inductors it has been found that they improve performance. This is because they allow an increased power factor by allowing the system to better cancel reactive power. In a further example a half-bridge ZCN may be used with each side of the controllable impedance 200 connected to a branch of the ZCN with a single leg, and the branches of the ZCN connected to corresponding sides of a single rectifier.

[0170] In some cases the present system and method allows operation of the pick-up (secondary) or primary at either a tuned frequency or a mis-tuned frequency. This provides additional flexibility in controlling the circuit and wirelessly transferring power. In contrast the TMN / ZCN circuit must be mistuned to operate, reducing efficiency. In some cases alternative impedance compression networks are used between the controlled impedance and the load. The use of an alternative ZCN does not affect the overall action of the system but may require a change in the impedance used, location of the impedance used or control systems.

[0171] Figure 10 shows a three phase implementation of a pick-up. Single phase, dual phase, three phase or other multi-phase systems may also be configured. Three primary coils 301 are shown, each having potential coupling M to the three secondary coils are compensation networks 216. In each secondary a controllable impedance 200 is present to adjust the input voltage to the ZCNs 201 . The detail of the power converters 201 comprising ZCNs is not shown, but may be substantially the same as described previously. For example, each power converter may have ZCNs comprising one or more circuits (legs) of two branches (a capacitive and inductive branch) leading to rectifiers (optionally passive rectifiers such as diode rectifiers). The load is connected across each of the three phases to provide a suitable Vout. Although the system has been described with respect to a pick-up (e.g. a wireless power transfer pick-up, or a wireless power secondary) it is also applicable to a wireless power transfer primary. The use of the compensation networks takes advantage of resonance to improve power transfer. In particular in one or more examples where the input voltage to the primary may vary, the coupling to the secondary / pick-up may vary or where the required power transfer may vary. In some cases the system is used on connected primary and pick-up systems.

[0172] However it may be preferred to use a primary without an impedance matching network, as these are more difficult to construct in reverse (compared to the arrangement shown in Figure 4). In this case a primary may drive a pick-up with a controllable impedance and a ZCN. The primary may have a compensation circuit of the same type as the pick-up (e.g. a parallel or LCL compensation circuit). In some cases the variable impedance may be located on the primary. This means that the primary is able to control a portion of the impedance matching or input power control, with a second portion controlled by the ZCN on the pick-up.

[0173] Figure 11 shows a primary circuit with a power source 401 . A compensation network 312 is shown comprising an inductor 302 a power transfer coil 303 and a controllable impedance 300. The controllable impedance 300 may have the characteristics of the controllable impedance 200 described with respect to the pickup. The controllable impedance 300 may be controlled to provide the desired input voltage of the ZCN on a coupled secondary. This control may account for the coupling and / or power transmission between the primary and secondary. Placing the controllable impedance 300 on the primary may reduce control complexity on the secondary and / or reduce the size of the secondary, for example. In some cases, a controllable impedance 200, 300 is included on both the primary and secondary.

[0174] Figure 4 has shown the controllable capacitance 200 as in parallel with the power transfer coil Lst. In some cases a series controllable capacitance may be used. For example, this may replace Csts,a. In some cases the series controllable capacitance may be used as well as the parallel controllable impedance 200. In some cases the series controllable capacitance is used with a parallel fixed capacitance.

[0175] The previous examples showed a voltage source load, with the secondary having a series tuned compensation network. Figure 14 shows an example secondary with a parallel tuned (LC) compensation network 414 and a current source load. Series capacitor Cpc is used to boost the power, but is not required for operation. Cpc may be referred to as a partial series tuning capacitor, or as part of a partially series tuned compensation network. The load 402 may be identified by the presence of a DC inductor in series with the output voltage. The series tuned compensation network 414 has an inductor and capacitor in series. Other series tuning compensation networks such as LCL may also be used. The DC inductor (Ldc) may be referred to as an L-filer. In some cases, Lie and Cic are chosen to produce substantially a unity power factor. In some cases, the compensation network of the primary can affect the operation of the secondary. It may be understood that using a DC inductor on the output requires, or makes use of a current source output, or that the combination of the primary compensation network and secondary compensation network provide a current source input to the ZCN 430, 432. For example, a series tuned primary and secondary converters will provide a current source.

[0176] It has been found that the use of a ZCN 401 makes the current source output 402 more constant. Thus, a ZCN can be applied to a current source output 402 to achieve a similar effect to a voltage source output. Figure 14 shows a half-bridge example with the compensation network 414 connected to the ZCN 401 connected to rectifier 403. A first branch 430 of the ZCN 401 is shown as inductive, with the second branch 432 being capacitive. Each branch 430, 432 is shown as a single leg. In some cases, additional impedances may be present on the branches 430, 432. The branches 430, 432 are connected to opposite sides of a rectifier 403, shown as a diode bridge (i.e. a passive rectifier). The ZCN 401 is configured (e.g., through choice of suitable component values) to stabilize the output current even with variations from the input from the compensation network 414. This may not require a controllable impedance in the compensation network 414. Surprisingly the effect has been found to be as effective with the current source output as with a voltage source output.

[0177] Figure 15 shows a similar circuit to Figure 14 but with the compensation network 414 only having the resonant components, Lst and Cs. The series capacitor Cpc has been removed. The circuit operates similarly to Figure 14, with the ZCN 401 positioned between the compensation network 414 and the rectifier 403 to improve power transfer to the load Vb. The ZCN 401 maintains a more constant power transfer level with variation in, for example coupling. Again, in some cases the rectifier 403 (or other power converter) may not be present and the ZCN 401 may supply an AC load directly.

[0178] Figure 16 shows a current source output 402 system with full bridge rectification. In the full bridge arrangement, the compensation network 414 is the same as Figure 14, but there are now two legs for each of the ZCN 401 branches 430, 432. First legs of the branches 430, 432 (Z1 , Z2) extend to different rectifiers 403. Second legs of the branches 430, 432 extend to the other connections on the rectifiers 403. Although shown as one leg of each branch having an inductor or capacitor it will be understood that both branches may have some impedance, as shown in the voltage output of Figure 4. As with Figure 14 an LC compensation circuit 414 is shown, although other series compensation circuits may be used. Again, although series Cpc is shown it is not required.

[0179] Figure 17 shows an example of a current source output 402 system. A half-bridge system is shown with a ZCN 401 with inductive 430 and capacitive 432 branches. The compensation network 414 now has controllable impedance(s) 440, 441 . ZCN 401 is directly connected between the compensation network 441 and the rectifier 403. ZCN 401 is also between the compensation network 414 and the load 402, with the rectifier 403 positioned between them (although it may optionally be removed). Although both Cis 440 and Cs 441 are shown as controllable in some cases only one is controllable. In some cases only Cs is present. As with the example of Figure 4 the controllable impedances 441 allows the system to, for example, improve power factor, with a reduced number and / or type of circuit components. Controllable impedance 440, in series with the inductance may be used to control or boost the power output. In some cases, the controllable impedance 441 provides a required input characteristic (e.g. current or voltage) to the ZCN 401 . The control system is similar to the voltage source output described above, except in some cases current, or other electrical characteristic may be monitored instead of, or along with voltage, for example. Where no controllable impedance 441 is present the variation may be controlled for on the primary side.

[0180] Figure 17 illustrates a LC compensation network, with the inductor and capacitor arranged in parallel. However, Other compensation networks are possible. For example, Figure 18 shows an LCL compensation network 414. This comprises coil inductance Lst, parallel capacitance Cs and series inductance Lsi. ZCN 401 , rectifier 403 and output 402 (with series DC inductor Ldc to ensure a current output) are unchanged. Other compensation networks configured to provide a current source may also be used.

[0181] The system is not limited to a particular application. However, it may be particularly useful for EV charging. This is because an EV battery is typically a resistive load so it is helpful to improve power factor and, if wireless charging is used, there can be variation in coupling. Moreover the battery voltage may change during charging. For example by changes in the charging distance. The secondary may form the electric component of a charging pad. Various charging pad and / or coil structures may be used. In one example a doubled pad is chosen (DD WPT2 / Z2 pad) and the system is designed to transfer power at 7.3kW. The system may be configured for a target input voltage (Voc) of 115V with the ZCN allowing this to supply a battery voltage varying between 280V and 420V. Table 3 shows example values for a system:

[0182] In a system without the ZCN (i.e., without Impedance compression), power transfer varies as the variation of battery voltage. For example a 50% increase in power transfer when the battery voltage changes from 280 V to 420 V. When the ZCN is introduced the power transfer may remain substantially constant. For a ZCN with full-bridge rectifier, a suitable selection of ZCN components can make power transfer almost constant. The half-bridge rectifier, as shown, provides less constant power transfer but further reduces the number of components required.

[0183] Control for the various illustrative logical blocks, modules, routines, and algorithm steps described in connection with the examples disclosed herein can be implemented as electronic hardware (e.g., ASICs or FPGA devices), computer software that runs on computer hardware, or combinations of both. Moreover, the various illustrative logical blocks and modules described in connection with the examples disclosed herein can be implemented or performed by a machine, such as a processor device, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A processor device can be a microprocessor, but in the alternative, the processor device can be a controller, microcontroller, or state machine, combinations of the same, or the like. A processor device can include electrical circuitry configured to process computer-executable instructions. In another examples, a processor device includes an FPGA or other programmable device that performs logic operations without processing computer-executable instructions. A processor device can also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Although described herein primarily with respect to digital technology, a processor device may also include primarily analog components. For example, some or all of the rendering techniques described herein may be implemented in analog circuitry or mixed analog and digital circuitry. A computing environment can include any type of computer system, including, but not limited to, a computer system based on a microprocessor, a mainframe computer, a digital signal processor, a portable computing device, a device controller, or a computational engine within an appliance, to name a few.

[0184] The elements of a method, process, routine, or algorithm described in connection with the examples disclosed herein can be embodied directly in hardware, in a software module executed by a processor device, or in a combination of the two. A software module can reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of a non-transitory computer- readable storage medium. An exemplary storage medium can be coupled to the processor device such that the processor device can read information from, and write information to, the storage medium. In the alternative, the storage medium can be integral to the processor device. The processor device and the storage medium can reside in an ASIC. The ASIC can reside in a user terminal. In the alternative, the processor device and the storage medium can reside as discrete components in a user terminal.

[0185] Conditional language used herein, such as, among others, “can,” “could,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain examples include, while other examples do not include, certain features, elements or steps. Thus, such conditional language is not generally intended to imply that features, elements or steps are in any way required for one or more examples or that one or more examples necessarily include logic for deciding, with or without other input or prompting, whether these features, elements or steps are included or are to be performed in any particular example. The terms “comprising,” “including,” “having,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list.

[0186] Disjunctive language such as the phrase “at least one of X, Y, or Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to present that an item, term, etc., may be either X, Y, or Z, or any combination thereof (e.g., X, Y, or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain examples require at least one of X, at least one of Y, and at least one of Z to each be present. While the above detailed description has shown, described, and pointed out novel features as applied to various examples, it can be understood that various omissions, substitutions, and changes in the form and details of the devices or algorithms illustrated can be made without departing from the spirit of the disclosure. As can be recognized, certain examples described herein can be embodied within a form that does not provide all of the features and benefits set forth herein, as some features can be used or practiced separately from others. The scope of certain examples disclosed herein is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

[0187] Any routine descriptions, elements or blocks in the flow diagrams described herein and / or depicted in the attached figures should be understood as potentially representing modules, segments, or portions of code which include one or more executable instructions for implementing specific logical functions or elements in the routine. Alternate implementations are included within the scope of the examples described herein in which elements or functions may be deleted, or executed out of order from that shown or discussed, including substantially synchronously or in reverse order, depending on the functionality involved as would be understood by those skilled in the art.

[0188] It should be emphasized that many variations and modifications may be made to the abovedescribed examples, the elements of which are to be understood as being among other acceptable examples. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

Claims

Claims1 . A wireless power transfer circuit comprising:A power transfer coil,A compensation network, andAn impedance compression network connected between the compensation network and a source or a load of the wireless power transfer circuit, wherein the compensation network comprises a controllable impedance in parallel with the power transfer coil.

2. The wireless power transfer circuit as claimed in claim 1 wherein the controllable impedance comprises a controllable reactance.

3. The wireless power transfer circuit as claimed in claim 2 wherein the controllable impedance is a controllable capacitor.

4. The wireless power transfer circuit as claimed in any one of claims 1 to 3 wherein the controllable impedance is connected to a fixed impedance.

5. The wireless power transfer circuit as claimed in any one of claims 1 to 4 comprising a series impedance between the compensation network and the impedance compression network.

6. The wireless power transfer circuit as claimed in any one of claims 1 to 5 wherein the controllable impedance is connected between two branches of the impedance compression network.

7. The wireless power transfer circuit as claimed in any one of claims 1 to 6 wherein the impedance compression network is passive.

8. The wireless power transfer circuit as claimed in any one of claims 1 to 7 comprising a rectifier connected between the the output of the impedance compression network and the source or load.

9. The wireless power transfer circuit as claimed in any one of claims 1 to 8 wherein the wireless power transfer circuit is a secondary wireless power transfer circuit and theimpedance compression network is between the compensation network and the load of the secondary wireless power transfer circuit.

10. The wireless power transfer circuit as claimed in any one of claims 1 to 9 wherein the load is a current source load, optionally comprising a DC inductor in series with the load.11 . The wireless power transfer circuit as claimed in any one of claims 1 to 10 wherein the compensation network comprises a first sub-network and a second sub-network, the first sub-network comprising the power transfer coil, the second sub-network comprising the controlled impedance, and wherein the reactance of the first subnetwork is substantially cancelled by the combined reactance of the second subnetwork and the impedance compression network.

12. The wireless power transfer circuit as claimed in any one of claims 1 to 11 wherein the controlled impedance is controlled by controlling a phase angle of one or more switches in the controlled impedance.

13. The wireless power transfer circuit as claimed in any one of claims 1 to 12 comprising a controller configured to determine one or more of a current or a voltage and control the controllable impedance.

14. A wireless power transfer circuit comprising: a power transfer coil, a compensation network, and an impedance compression network connected between the compensation network and a source or load of the wireless power transfer circuit, wherein the compensation network comprises a controllable capacitance configured to control the input voltage to the impedance compression network.

15. A wireless power transfer circuit is provided comprising: a compensation network comprising a first sub-network and a second subnetwork, the first sub-network comprising a power transfer coil, and the second subnetwork comprising a controllable impedance in parallel with the power transfer coil; an impedance compression network connected between the compensation network and a source or load of the wireless power transfer circuit,wherein the reactance of the first sub-network is substantially cancelled by the combined reactance of the controlled impedance of the second sub-network and the reactance of the impedance compression network.

16. A method for controlling a wireless power transfer circuit comprising a compensation circuit comprising a controllable capacitance and an impedance compression connected between the compensation circuit and a source or load of the wireless power transfer circuit network, the method comprising: switching the controllable impedance to provide a reactance, which in combination with the compensation network and the impedance compression network substantially compensates for variations in reactance seen at an input to the wireless power transfer circuit.

17. An IPT system comprising a primary and a secondary each comprising a power transfer coil, the secondary comprising an impedance compression network connected between a compensation network and a load of the secondary; one or more of the primary and secondary comprising a controllable impedance.

18. The IPT system as claimed in claim 17 wherein the secondary is as claimed in any one or more of the preceding claims.

19. A wireless power transfer circuit comprising:A power transfer coil,A compensation network,A current source output, andAn impedance compression network connected between the compensation network and the current source output.

20. The wireless power transfer circuit as claimed in claim 19 wherein the compensation network comprises a series compensated compensation network.21 . The wireless power transfer circuit as claimed in claim 19 or claim 20 comprising a rectifier between the compensation network and the current source output.

22. The wireless power transfer circuit as claimed in any one of claims 19 to 21 wherein the compensation network comprises a controllable impedance in parallel with the power transfer coil.

23. The wireless power transfer circuit as claimed in any one of claims 19 to 22 wherein the current source output comprises a DC inductor configured to supply the current source output.

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