FIELD OF THE INVENTION

The present invention relates to an inductive power transfer transmitter and an inductive power transfer system and method.

BACKGROUND TO THE INVENTION

Inductive power transfer is the transmission of electrical energy without a physical or wired connection. A typical inductive power transfer system comprises an inductive power transfer transmitter, or primary device, which generates an electromagnetic field, and which is used to transfer electric power across space to an inductive power transfer receiver, or secondary device, which extracts power from the field and supplies this to a load. A typical inductive power transfer transmitter is driven by an electric power source, and comprises an inverter, a primary tuning network, and a primary coil. A typical inductive power transfer receiver comprises a secondary coil, a secondary tuning network, a rectifier and a load.

The system uses a magnetic field to transfer electrical power wirelessly from the primary coil to a secondary coil. The magnetic field is created by inputting an AC current at the primary coil. The secondary coil, when placed in the magnetic field produced by the primary coil, generates an AC voltage across its terminals for driving or powering a connected load(s). Inductive power transmission can eliminate the use of wires and batteries, thus increasing the mobility, convenience, and safety of electronic devices.

In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art. SUMMARY OF THE INVENTION

It is an object of at least some preferred embodiments of the invention to provide a wireless power transmitter and system, and/or to at least provide the public with a useful alternative.

In one aspect the present invention may be said to comprise an inductive power transfer transmitter comprising : a plurality of parallel sub-circuits acting as current sources, each comprising : one or more inverters, and a tuning network configured to provide a current source output from the inverter output, wherein the parallel sub-circuit outputs are coupled so their respective output currents are added, and a transmitter coil sub-circuit coupled to the outputs of the parallel sub-circuits, wherein the inverter output of each parallel sub-circuit can be coupled or decoupled from the transmitter coil sub-circuit.

Optionally the inverter output can be coupled or decoupled from the transmitter coil sub-circuit by operating one or more switches.

Optionally the inverter output can be coupled or decoupled from the transmitter coil sub-circuit by operating switches in the inverter.

Optionally the inverter output can be coupled or decoupled from the transmitter coil sub-circuit by operating one or more switches between a respective inverter and tuning circuit.

Optionally the inverter output can be coupled or decoupled from the transmitter coil sub-circuit by operating one or more switches between a respective tuning circuit and the transmitter coil cub-circuit.

Optionally the transmitter coil sub-circuit comprises a series tuned transmitter coil and capacitor.

Optionally the transmitter coil sub-circuit comprises a tuned transmitter coil and capacitor.

Optionally the transmitter coil sub-circuit comprises a tuned transmitter coil and at least one variable capacitor and/or at least one variable conductor. Optionally coupling or decoupling the inverter output of each parallel sub-circuit increases or decrease current provided to the transmitter coil sub-circuit.

Optionally the current provided to the transmitter coil sub-circuit can be adjusted by altering the phase of the current output from one or more of the inverters.

Optionally the inductive power transfer transmitter further comprises a circuit to sense the k coefficient between the transmitter coil and a receiver, and a controller to switch one or more inverters on or off and/or to alter the phase of the current output from one or more the inverters to provide current to the transmitter coil sub-circuit that takes into account changes in the k coefficient and/or a load.

Optionally the power factor and/or impedance and/or other characteristics of the remaining parallel sub-circuits are not altered when an inverter and/or parallel circuit are disconnected.

In another aspect the present invention may be said to comprise an inductive power transfer transmitter for providing desired power to a receiver comprising a plurality of inverters coupled to a transmitter coil via a plurality of tuning circuits and a controller monitoring the K coefficient between the transmitter coil and the receiver, wherein the controller is configured to switch inverters on and/or off to control the power provided to the transmitter coil so that the desired power is received at the receiver.

The embodiments here might optionally also comprise one or more of the following.

• A controller is configured to take as input the required power or current at the power transmission coil. Optionally the controller is configured to determine which of the plurality of parallel sub-circuits are required to be on or off based on the required power or current at the power transmission coil. Optionally the controller is configured to output one or more control signals, each relating to one of the plurality of parallel sub-circuits. The plurality of primary power circuits are configured to be controlled or turned on/off in order to provide a substantially constant current or power output at the power transmission coil/antenna.

• The current or power output at the power transmission coil/antenna can be controlled by turning on or off different combinations of primary power circuits, and/or controlling the phase shift of each primary power circuit.

• The primary power circuits are controlled in order to compensate for a change in the K coefficient or the mutual inductance at the power transmission coil/antenna.

• A change in the 'k' coefficient at the power transmission coil/antenna is caused by a change in the alignment with a secondary coil or wireless receiver.

• Each primary power circuit is configured to provide an output current. The current at the primary coil is a sum of all output currents from the plurality of primary power circuits. The phase shift of each inverter can be controlled in order to vary the output current of the primary power circuit.

• The or each inverter is configured to be controlled or turned on or off through controlling one or more inverter switches. The inverter switches are configured to short circuit the output of the inverter.

• The tuning or compensation network further comprises one or more DC blocking capacitors, configured to reduce a DC common mode current which may form as a result of switching off one or more inverters. The tuning or compensation network further comprises one or more common mode chokes configured to minimise or suppress AC common mode current in the wireless power transmitter.

Definitions, terms and phrases

In this specification, "high power application" means an application (of the inductive power transfer system) with a high-power rating. This high-power rating could be around lOkW or more for example. In this specification, "low power application" means an application (of the inductive power transfer system) with a low power rating. This low power rating could be around lOkW or less for example.

In this specification, reference to "wireless charging of electric vehicles" relates to wireless charging of electric vehicle at big enough scale suitable for industrial/commercial use. This is different to home domestic wireless charging of electric vehicles, which may or may not have different design considerations to the inductive power transfer system described in this specification.

In this specification, reference to "inductive power transfer" can refer to "wireless charging" and "real-time wireless power transfer". Any reference to wireless charging, inductive charging or similar can also where technically feasible as understood by those skilled in the art can also apply to real-time wireless power transfer.

The term "capacitor" is a well understood term in the art. However, in this specification, a "capacitor" may also refer to any component that has a capacitive reactance. A "capacitor" may also refer to any combination of components (which may or may not include any capacitors) arranged such that the net reactance of the combination of components is capacitive, and can therefore be remodelled into a capacitor.

The term "inductor" is a well understood term in the art. However, in this specification, an "inductor" may also refer to any component that has an inductive reactance. An "inductor" may also refer to any combination of components (which may or may not include any inductors) arranged such that the net reactance of the combination of components is inductive, and can therefore be remodelled into an inductor.

Although the inductive power transfer system described below can be used in a variety of applications, it should be noted the inductive power transfer system (including the described embodiments and the sub-circuits that make up the inductive power transfer system) have been designed to have a high power application, including for example, wireless charging of electric vehicles in an industrial/commercial setting (as opposed to a domestic setting). Such a design should be differentiated from inductive power transfer systems that only have a lower power application (such as wireless charging of electronic devices such like mobile phones for example), even if the electronic circuitry may appear similar on a circuit diagram. Inductive power transfer systems that have a high-power application have particular design considerations that are not applicable when designing an inductive power transfer system with a low power application. For example, an inductive power transfer system that has a high-power application will have significantly greater problems with heat dissipation and power efficiency, which at least in some cases cannot be adequately addressed using techniques known to a person skilled in the art. Some of these particular design considerations (and their respective solutions) will be discussed later on in the detailed description.

The term 'comprising' as used in this specification and indicative claims "consisting at least in part of". When interpreting each statement in this specification and indicative independent claims that includes the term 'comprising', features other than that or those prefaced by the term may also be present. Related terms such as 'comprise' and 'comprises' 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) and, therefore, all sub-ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will be described by way of example only and with reference to the drawings, in which :

Figure 1 is a block diagram showing a circuit representation of an inductive power transfer system according to an embodiment of the invention;

Figure 2 is a block diagram showing a circuit representation of an inductive power transfer transmitter according to an embodiment of the invention;

Figure 3 is a block diagram showing a circuit representation of an inductive power transfer transmitter according to an embodiment of the invention;

Figures 4a and 4b are circuit diagrams of inverters used in an inductive power transfer transmitter according to embodiments of the invention;

Figures 5a, 5b and 5c are circuit and block representations showing configurations for switching an inverter off in an inductive power transfer transmitter according to embodiments of the invention;

Figures 6a to 6g are circuit diagrams of parallel tuning networks used in an inductive power transfer transmitter according to different embodiments of the invention;

Figure 7 is a circuit and block diagram showing a representation of a LCL tuned primary network and an inductive power transfer transmitter;

Figures 8a to 8e and 9a to 9d are an equivalent circuit diagrams of Figures 6a to 6g, demonstrating their impedance.

Figure 10a is a block diagram showing a circuit representation of an inductive power transfer transmitter according to an embodiment of the invention;

Figure 10b is a block diagram showing a circuit representation of an inductive power transfer transmitter according to an embodiment of the invention;

Figures Ila, 11b and 11c are circuit diagrams of shared tuning sub-circuits used in an inductive power transfer transmitter according to embodiments of the invention;

Figure 12 is a graphical representation of an example embodiment of an inductive power transfer system showing the change in coupling coefficient k as the distance between a transmitter and receiver is changed according to an embodiment of the invention;

Figure 13 is a graphical representation of an example embodiment of an inductive power transfer system showing the change in mutual inductance as the distance between a transmitter and receiver is changed according to an embodiment of the invention;

Figure 14 is a graphical representation of an example embodiment of an inductive power transfer transmitter showing the change in current provided at the power transmission coil as the magnetic coupling between a transmitter and receiver is changed according to an embodiment of the invention;

Figure 15 shows different graphical representation of an example embodiment of an inductive power transfer transmitter according to an embodiment of the invention; and

Figure 16 is a flow diagram showing an example embodiment of a control process performed by a controller of an inductive power transfer transmitter according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

1. Overview

In inductive power transfer systems, power is transferred from a transmitter to a receiver. The receiver then provides the power to the device being powered by inductive power transfer.

There is a nominal desired power required to be transferred in such systems. If there is misalignment between the receiver and the transmitter (e.g. receiver is too closer to far away from the transmitter and/or the receiver is not aligned correctly with the transmitter) then the nominal desired power being provided by the transmitter might not actually be transferred to and received by the receiver. As such, it is desirable to control the power at the transmitter to compensate for misalignment of the receiver with the transmitter. Embodiments described herein enable control of the power at the transmitter through turning on/off inverters that form part of the power transfer system on the transmitter side.

2. Background to inductive power transfer systems

Embodiments described relate to an inductive power transfer transmitter 2 for use in an inductive power transfer system. An example inductive power transfer system is shown in Figure 1. The system 1 comprises an inductive power transfer transmitter 2 (also termed "primary device", "transmitter circuit", "transmitter side", "transmitter module"), which generates an electromagnetic field which is used to transfer electric power across space to an inductive power transfer receiver 3 (also termed "secondary device", "receiver circuit", "receiver side", "receiver module") which extracts power from the electromagnetic field and supplies this to a load.

With reference to the example system shown in Figure 1, the inductive power transfer transmitter 2 is driven by an electric power source 4, and comprises at least one inverter 5, a primary tuning network 6, and a primary coil 7. The inductive power transfer receiver 3 comprises a secondary coil 11, a secondary tuning network 10, a rectifier 9 and a load or loads 8.

The example system 1 uses a magnetic field generated at the primary coil 7 to transfer electrical power wirelessly from the primary coil 7 to the secondary coil 11. The magnetic field is created by inputting an AC current at the primary coil 7. The secondary coil 11, when placed in the magnetic field produced by the primary coil, generates an AC voltage across its terminals for driving or powering a connected load(s) 8.

2.1 Power transfer and alignment

Inductive power transfer systems such as that of which the present invention is intended to form a part of, allow electrical energy to be transferred without physical contact. An inductive power transfer system should be able to operate to produce a constant output for reception at the load or loads. For example, this could be in order to achieve a good user experience for applications such as wireless charging of electric vehicles.

In the present embodiments, as will be described in more detail, this involves controlling the power the primary device provides from the primary coil in response to changes in or at the secondary device. For example, such changes could relate to a certain amount of misalignment between the primary coil and the secondary coil, and/or variations of the load or loads connected to the secondary device.

Inductive power transfer systems transfer power optimally when the primary and secondary coils are properly aligned. Alignment is such that the secondary coil is within the strongest portion of the electromagnetic field produced by the primary coil. Any misalignment between the primary and secondary coils could lead to less than optimal power transfer from the primary device to the secondary device.

Misalignment can refer to the distance between the transmitter (primary) and receiver (secondary) not being optimal and/or the orientation between the transmitter and receiver not being correct. For example, misalignment between the primary and secondary coils may occur, for example, because an electric vehicle may have a different ground clearance depending on its loading or it may be challenging to park such that the secondary coil, located in the electric vehicle, is well aligned with the primary coil each time the electric vehicle is to be charged.

Misalignment between the primary and secondary coils can cause the magnetic coupling factor (k coefficient), between primary and secondary coils to vary. A change in the coupling coefficient k can subsequently cause the power at the secondary device to change. For example, the power at the secondary coil and output increases with increasing values of magnetic coupling, for example, as the primary and secondary coils are better aligned.

In other examples, using different types of tuning, the power at the secondary coil may decrease as the coupling coefficient k (also termed" magnetic coupling factor") increases. Typically, the magnetic coupling factor (k coefficient) can change by a factor of 2 or more. For example, the k coefficient may change from for example a value of 0.15 at the furthest distance between the primary and secondary coils, to a value of 0.3 at the closest position between the primary and secondary coils, in the context of a wireless electric vehicle charging application. Alternatively, the secondary device may be varied in a way which does not relate to a misalignment of the coils, but which causes more or less power to be required by or drawn by the secondary device. In one such example situation, the load or loads connected to the secondary device may be changed, which causes the amount of power drawn by the load or loads to also change. As such, the secondary device may require more or less power to be drawn from the primary device in order to provide sufficient power to the load or loads in response to this change.

In an inductive power transfer system, the output power provided by the secondary device preferably is kept constant, in order to maintain power to the load or loads. Power control is therefore desired in order to maintain constant power at the output of the secondary device. Typically, this power control is implemented on both the primary device and the secondary device, for example to compensate for changes in the coupling coefficient k.

Power control at the primary device typically requires providing current to the primary coil such that the power provided by the primary coil varies in correlation with the changes in the system. For example, power control at the primary device may take the form of controlling the current provided to the primary coil in relation to change in the coupling coefficient k with the secondary coil, such that the product of these two variables i.e. the power received by the secondary device from the primary device, stays largely unchanged.

For example, if the secondary coil moves further away from the primary coil, the coupling coefficient k is decreased, and as such, the primary device provides more current to the primary coil in order to produce more power. This increase in power is in line with the change in coupling coefficient k. As such, the secondary device does not see the effect of the change in coupling coefficient k, as the increased power from the primary coil offsets the change in coupling coefficient k.

In a typical inductive power transfer transmitter, the current at the power transmission coil can be varied by changing the output voltage of the inverter. The ratio between inverter output voltage and transmission coil current varies depending on the primary tuning topology. For example, for an LCL tuned primary, the ratio is the characteristic impedance of the LCL network. For other types of tuning network, the ratio may be different and more complicated to derive. However, practically there is a lower limit for the output voltage from the inverter. This is because for the same power level, reducing the output voltage from the inverter leads to a higher inverter output current and potentially higher switching currents, which not only require expensive inverter switches but also can generate excessive amount of heat. This conventional solution increases system cost and reduces system reliability. This problem becomes more severe at higher power levels.

3. General overview of embodiments for power compensating for misalignment

Present embodiments relate to an inductive power transfer transmitter, and a control method for use within an inductive power transfer system such as that outlined in the example embodiment described in relation to Figure 1.

As mentioned above, the present embodiments provide an inductive power transfer transmitter or primary device which is able to control the power at the primary coil in response to changes at or in the inductive power transfer receiver, which might be due to change in k due to misalignment and/or other load changes. The embodiments assist in maintaining constant power (or otherwise control power) even with change in k due to misalignment and/or other load changes. The embodiments provide a circuit and control method to vary the primary coil current safely and efficiently such that changes in the coupling coefficient k due to misalignment, or other changes in the secondary device, can be compensated on the primary side with low inverter output current.

To do this, the present embodiments provide a circuit that provides current sources rather than voltage sources. Current sources can be summed, and selective controlled to control the total current provided. This total current can be controlled to compensate for changes in k coupling coefficient, so that the power actually provided to a load is the desired power. Also described are control methods to vary the current at the power transmission coil of an inductive power transfer transmitter such that large changes in the coupling coefficient k (due to for example a misalignment between the primary and secondary coils), or power requirement at the load or loads connected to the inductive power transfer receiver can be compensated for in the inductive power transfer transmitter by varying the output current provided by the inverter(s) by selectively switching in/out inverters of the circuit and/or using phase control or inverter output current. In the present embodiments, the current at the power transmission coil is varied such that any change in current acts to compensate for changes in the coupling coefficient k or loading requirements of an inductive power transfer receiver being supplied with power. As a result, the product of these two variables, the power received by the inductive power transfer receiver, stays largely unchanged - that is, the desired power is still provided despite the change in the k coefficient. Because of this, an inductive power transfer receiver which is being supplied with power from the inductive power transfer transmitter, receives a substantially constant level of power. As such, the inductive power transfer receiver does not see the effects of the changes in the coupling coefficient k or loading at the output.

With reference to Figure 2, a general embodiment is shown. The inductive power transfer transmitter 20 comprises an arrangement of a plurality of parallel subcircuits 22 and a shared tuning sub-circuit (also termed "transmitter coil subcircuit") 30. The shared tuning sub-circuit 30 is provided in a series arrangement with the plurality of parallel sub-circuits 22, which are provided in a parallel arrangement. The series arrangement is formed by connecting a tuning capacitor Cpi (32) in series with the primary coil Lpt (34). The shared network 30 is not limited to series tuned topology. Other tuning networks such as parallel or LCL may also be suitable - see for example Figure 10a as described below. Other options are possible also. It is also possible to have at least one variable capacitor and/or at least one variable inductor in the shared network 30 for controlling inverter power factor and/or adjusting current/power sent to the transmitting coil 34. The variable components may be connected in series or in parallel with transmitting coil 34.

The plurality of parallel sub-circuits 22 each provide a current source. Each comprises one or more inputs for connection to a power source 24, one or more inverters 26 each configured to invert a direct current (DC) voltage source to alternating current (AC) voltage output, and a tuning network 28 configured to convert the inverter voltage output into a current source. This means that instead of a voltage output being provided to the shared sub-circuit 30 from each inverter, a summation of current source outputs from each parallel sub-circuit 22 are provided to the shared sub-circuit. The tuning circuit may also optionally compensate and/or minimise the reactive power of the inverter(s) 26 and provide a substantially real output/impedance. Another function of the network 28 is to ensure reactive power is minimised when one or more inverters are switched off. In one option, the tuning networks 28 are tuning networks, but that is not essential. The tuning networks 28 will be described in detail later.

The shared tuning sub-circuit 30 comprises at least one capacitor 32, and a power transmission coil 34 configured to generate a field for wireless transmission of power. The parallel arrangement of the inverters 26 and tuning network 28 in the parallel sub-circuits 22 provides a power into series tuned network at the power transmission coil 34.

As a result of the parallel arrangement of the parallel sub-circuits 22 in relation to the shared tuning sub-circuit 30, the current at the power transmission coil 34 (for a series tuned transmission coil) is a sum of the output currents of the plurality of parallel sub-circuits 22. If the tuning network of 30 contains a capacitor in parallel with transmission coil 34 (for example, parallel or LCL tuned), the current at the transmission coil 34 is a portion of the sum of the output currents of the plurality of parallel sub-circuits 22. This is because part of the current flows through the parallel capacitor. The shared tuning sub-circuit 30 behaves as a current source (when all tuning circuits 28 are LCL tuned), and as such, all the output currents from the parallel sub-circuits 22 add up at the shared tuning sub-circuit 30.

Using the parallel arrangement, the current at the power transmission coil 34 can be controlled in large steps by turning on or off different combinations of the inverters 26 in the plurality of parallel sub-circuits 28 (regardless of the tuning topology of the shared sub-circuit 30.) . The inverters can be turned on and off using a number of different techniques, which will be discussed further below. In some embodiments, a common mode choke may be required to suppress AC common mode current when an inverter is turned off.

The phase shift of each inverter 26 can also be controlled in order to vary the current at the power transmission coil 34. The phase shift of each inverter can be adjusted with a very fine resolution between the large steps. As such, by controlling the phase shift of an inverter as well as turning on or off individual inverters in the plurality of parallel sub-circuits, the current at the power transmission coil 34 can be controlled in a continuous manner in response to any changes in the system affecting the power drawn by the power transmission coil 34. It is not trivial simply to turn off/on inverters as per the present embodiments. The arrangement of the present embodiments allows for a switching off current of between 25A to 15A lower than a conventional solution, which enables a significant reduction in switching losses and conduction loses for the inductive power transfer transmitter. In a conventional inductive power transfer system, a high switching off current typically occurs in all inverters in the transmitter or primary. In typical inductive power transfer systems, the high switching current occurs because of a much smaller inverter output voltage. In the present invention, as will be discussed further, an inverter, when turned off, has minimal or no switching losses because its outputs are shorted and not switching.

In one possible general embodiment, the coupling coefficient k can be monitored by way of a sensor and a controller 36. When the controller determines a change in k coefficient that requires some compensation to maintain the desired power level being received at the receiver, the controller turns on/off one or more inverters to provide the required power level at the transmitter that results in the desired power level being received.

4. Possible embodiments

Each of the components of the inductive power transfer transmitter of possible embodiments will now be discussed in more detail.

4.1 Parallel sub-circuits

With reference to Figures 2 and 3, the present invention relates to a circuit arrangement wherein a shared tuning sub-circuit 30 is provided in a series arrangement with a plurality of parallel sub-circuits 22, which are provided in a parallel arrangement. This forms an inductive power transfer transmitter.

The plurality of parallel sub-circuits 22 comprise one or more inputs for connection to a power source 24, one or more inverters 26 each configured to invert a direct current (DC) voltage source to alternating current (AC), and a tuning network 28 configured to compensate and/or minimise the reactive power of the inverter(s) 26 and provide a substantially real output/impedance.

As a result of the parallel arrangement of the parallel sub-circuits 22 in relation to the shared tuning sub-circuit 30, the current at the power transmission coil 34 is a sum of the output currents of the plurality of parallel sub-circuits 22. With other forms of tuning topology in shared tuning sub-circuit 30, the current at the power transmission coil 34 can be a portion of or higher than a sum of the output currents of the plurality of parallel sub-circuits 22.) When all tuning circuits 28 are LCL tuned, all the output currents from the parallel sub-circuits 22 add up at the shared tuning sub-circuit 30.

Each parallel sub-circuit is configured to provide an output current, which is provided to the shared tuning sub-circuit. In a preferred embodiment, each parallel sub-circuit is configured to provide an output current of the same or substantially similar maximum magnitude when turned on, however it will be appreciated that each parallel sub-circuit may be configured to provide an output current having a different maximum magnitude. In a preferred embodiment, each parallel sub-circuit is configured to provide an output current of the same or substantially similar minimum magnitude when turned on, however it will be appreciated that each parallel sub-circuit may be configured to provide an output current having a different minimum magnitude. The magnitude of the output current between the maximum and minimum magnitudes for each inverter is configured to be controlled by varying the phase shift or the voltages of the power sources connecting to the the one or more inverters of the parallel sub-circuit. In these embodiments, the output current provided by each parallel sub-circuit is configured to be in phase with that provided by the other one or more parallel sub-circuits which are turned on or are providing current to the shared tuning sub-circuit, however it will be appreciated that in alternative embodiments the output current provided by one or more of the parallel sub-circuits may be out of phase.

Each of the components comprising each of the parallel sub-circuits will now be discussed.

4.2 Connections for power source

In this embodiment, each of the parallel sub-circuits 22 comprises one or more inputs for connection to a power source 24. The power source which is intended for connection may be DC voltage source, though in a preferred embodiment is a single DC voltage source. Once connected using the one or more inputs, the power source is configured to provide power to the parallel sub-circuit. The power source may have a variable current and/or voltage, but in a preferred embodiment the voltage and current provided by a connected power source are fixed. In an embodiment, the power sources configured to be connected to each of the parallel sub-circuits might have a shared grounding. Furthermore, the power sources may be provided from a single mains source with shared ground. In such embodiments, it is envisaged that a number of power factor correction (PFC) devices, corresponding with the number of parallel sub-circuits, can be provided which split the mains into multiple DC voltage sources. Each of these DC voltage sources is then provided or connected to the connection for input to each parallel sub-circuit as the power source. These could be for example between the voltages of 300-1000V, or more preferably between 500-800V.

In a further embodiment, each power source may be configured to be turned off in order to prevent the corresponding parallel sub-circuit from providing an output current to the power transmission coil, rather than through short-circuiting the inverter of the corresponding parallel sub-circuit (which will be discussed in the proceeding section).

4.3 Inverters

Still referring to Figures 2 and 3, each of the parallel sub-circuits 22 comprises one or more inverters 26. Each inverter 26 is configured to invert an input direct current (DC) voltage source to an alternating current (AC) voltage as an output. The input direct current voltage source is provided through the one or more inputs for connection to a power source 24, to which the inverter 26 is electrically connected. The output alternating current source is provided to a tuning network 28 which is electrically connected to the output of the one or more inverters. In a preferred embodiment the parallel sub-circuit 22 comprises one input for connection to a power source 24 electrically connected to one inverter 26. In alternative embodiments, the parallel sub-circuit may comprise one input for connection to a power source electrically connected to two or more inverters, or two or more inputs for connection to a power source electrically connected to two or more inverters.

Referring to Figure 4b, in which a preferred embodiment of an inverter 26b is shown. In this embodiment, the or each inverter of each parallel sub-circuit is a full bridge inverter 26b. The full bridge inverter 26b is connected to an input for connection to a power source 24, which is configured to provide a DC power source. The full bridge inverter 26b inverts the DC source and provides an AC voltage at its output. Referring now to Figure 4a, in which an alternative embodiment of an inverter 26a is shown. In this embodiment, the or each inverter of each parallel sub-circuit is a half bridge inverter 26a. The half bridge or h-bridge inverter 26a is connected to an input for connection to a power source 24, which is configured to provide a DC power source. The half bridge or h-bridge inverter 26a inverts the DC source and provides an AC voltage at its output. Alternatively, in embodiments where each parallel sub-circuit has two or more inverters, one or more may take a different form, for example one may be a full bridge inverter and one a half bridge.

It is envisaged that any form of inverter which is configured to invert a direct current (DC) voltage source to an alternating current (AC) could be used in the present invention, and the form of the inverter is not intended to be limiting. The type of inverter may vary depending on the application the inductive power transfer transmitter is intended to be used in. For example, a h-bridge inverter as shown in Figure 4b may be more suitable in applications requiring higher power such as those relating to electric vehicle charging.

In operation, each of the parallel sub-circuits inverter(s) can be driven synchronously such that their outputs are of substantially the same magnitude and are substantially in phase. Each of the parallel sub-circuits inverter(s) can also be driven with different phase shift such that their output voltages are different in magnitude but are still substantially in phase. Each of the parallel sub-circuits inverter(s) can also be driven asynchronously such that their output voltages are different in both magnitude and phase. However, a large phase difference may result in circulating reactive power between inverters, leading to unnecessary losses in the inverters, which is undesirable.

Because of the parallel arrangement provided by the present invention, the inverters of each parallel sub-circuit are able to be controlled in response to changing power output required at power transmission coil (e.g. due to changing K and/or loads). The current output of each inverter is able to be controlled with precision between a minimum magnitude and a maximum magnitude through controlling the phase shift of the inverter. The inverter(s) of each parallel subcircuit is also able to be switched on or off in order to provide its current output at the shared tuning sub-circuit. In this way, the current and therefore the power at the power transmission coil is able to be controlled in a similar way to a gear box in a car. 4.4 Tuning network

Referring back now to Figures 2 and 3, each of the parallel sub-circuits 22 comprises a tuning network 28. The tuning network 28 of each parallel sub-circuit 22 is configured to provide a current source output from the respective inverter. Each can also compensate and/or minimise the reactive power of the inverter(s) 26.

LCL is a preferred network for 28 due to its characteristics, although this is not essential - other types could be used. More specifically, when both 30 and 28 (LCL) are tuned well, each inverter sees a largely real impedance, which helps to minimise inverter losses. Another feature of the LCL network is when an inverter is turned off by shorting its outputs, the LCL network ensures that:

1) all remaining inverters still see largely real impedance, and

2) short-circuit current is not too large for the short-circuited inverter.

In addition, the LCL circuit suppresses harmonics current also by having a large impedance in blocks 1 and 2 (see Figure 6a) at harmonics frequency and a smaller reactance in block 3.

Referring now to Figure 6a, a preferred embodiment of a parallel tuning network structure 28 is shown. This embodiment is an LCL type tuning network. In this embodiment, the LCL type tuning network has an impedance of block A in addition to the impedance of block X-A, the result of which is approximately equal to a positive X, where X is a positive floating number. Also, the impedance of block B in addition to the impedance of block X-B is approximately equal to positive X. The impedance of block -X is approximately -X.

Figures 6b to 6g show several practical implementations of an LCL type network. The output current of the LCL tuning network is equal to the output voltage of the inverter connected as input, divided by X. In these examples, X is the characteristics impedance of the LCL type tuning network. It should however be noted that in alternative embodiments, other types of tuning networks that satisfy the impedance requirements described above in relation to X and would be suitable as parallel tuning networks within one or more parallel sub-circuits may be used. The proposed topology with an LCL type parallel tuning network can suppress higher order harmonics current provided to the power transmission coil. With reference to Figure 6a, the LCL type tuning network works by having a larger impedance at higher harmonic frequencies for example in the path containing blocks B and X-B and the power transmission coil. This aspect of the LCL type tuning network is especially relevant for high power applications such as for example electric vehicle charging. Also, the LCL suppresses harmonics current also by having a large impedance in blocks 1 and 2 at harmonics frequency and a smaller reactance in block 3.

In Figure 6 (and by extension, Figures 6a to 6d), one or both of components 4 and 5 (Cb, Lb) can also be variable. Making them variable does not alter the current source characteristics.

By way of example, a typical LCL type parallel tuning network is described briefly in a typical wireless power transfer system for context. Figure 7 shows a traditional LCL tuned primary network, where Lpi and C_Lpi are tuned inductively with a positive combined reactance of X, Cpt is tuned with a negative impedance of X and Lpt and Cpi are also tuned inductively with a positive X. The LCL network 28 is configured to compensate and/or minimise the reactive power of the inverter 26 when the secondary circuit is configured to reflect a substantially resistive load back onto the primary coil Lpt such that the inverter drives a substantially real load, which optimizes/minimizes the inverter output current and inverter losses . In alternative embodiments, other types of tuning networks that offer good power factor for inverter are envisaged as being able to be used.

The tuning circuits described herein assist with turn off and turn on of inverters.

When an inverter 26 is turned on, the inverter and the respective tuning network 28 cause more current to flow into the shared sub-circuit 30. For example, an LCL tuning network behaves as a current source see Figure 8e. A parallel connection of multiple LCL tuned networks is equivalent to connecting multiple current sources in parallel, and the sum of all currents flow into the shared sub-circuit 30. Figure 8e illustrates such a circuit, where three parallel inverters and tuned circuits feed currents into the shared sub-circuit 30. When turned off, the tuning circuit behaves as an open-circuit, which in effect removes this parallel sub-circuit from the remaining circuits. This means the power factor or characteristics (including current source characteristics) of the remaining inverters are not affected by this turn-off action. In addition, the tuning network when the inverter is turned off outputs no current so the current in the shared subcircuit is reduced.

The reason for why an LCL circuit behaves as a current source is explained below. The general form of an LCL tuned network is shown in Figure 8a. Figure 8b shows a simplified version of it by combining impedances of blocks 1 and 2 of Figure 8a into block 1 of Figure 8b, and combining impedances of blocks 4 and 5 of Figure 8a into block 3 of Figure 8b. According to Norton's theorem, we can transform a series connection of Vinv and block 1 of Figure 8a into a parallel connection of a current source and block 1 as shown in Figure 8c, where the magnitude of the current source is Vinv/X. A parallel connection of blocks 1 and 2 (enclosed by the dashed box in 8c) forms an open-circuit that has an infinitely large impedance. Its equivalent impedance can be derived as equation 1:

Therefore, blocks 1 and 2 can be removed, leading to the current source in series with block 3 as shown in 8d. Value of block 3 does not affect the magnitude of the current source, and its purpose is to ensure the power factor of Vinv is close to 1. Therefore, if we turn off an inverter by short-circuiting its output terminals, Vinv becomes zero and the magnitude of the current source also becomes zero. Therefore, this inverter and its tuning network no longer contributes current into the shared sub-circuit 30. Turning off an inverter using other methods described in the draft essentially achieves the same outcome; the decoupled parallel branch stops contributing current to the shared sub-circuit.

The arrangement described can also provide a power factor of 1. Figure 9a shows the general form of the LCL network, and Figure 9b shows a simplified version of the LCL tuned network by combining impedances of blocks 1 and 2 of Figure 9a into block 1 of Figure 9b, and combining impedances of blocks 4 and 5 of Figure 9a into block 3 of Figure 9b. Here a positive impedance of X indicates an inductive element while a negative impedance of X indicates a capacitive element. In Figure 9c, an impedance Z is connected between terminals Bl and B2. Z represents the impedance of the shared sub-circuit.

Mathematics derivation shows the input impedance Zc (across terminals Al and A2) is equation 2: z _c ^{2 c} = — z

Zc represents the inverter impedance when it is turned on.

If Z is largely resistive, Zc is also largely resistive. Because Z is on the denominator, Zc becomes inductive if Z is capacitive, and Zc becomes capacitive if Z is inductive.

As can be seen, the inverter impedance not only depends on the LCL tuning, but also depends on the equivalent impedance Z of the shared sub-circuit. The reflected impedance of the secondary circuit forms part of Z. Although it is possible to design the reflected impedance to be largely real at full power (to make Zc largely real at full power), the reflected impedance does change towards either more inductive or capacitive (depending on the secondary topology) during a charging cycle as power level changes. This means if the system is tuned at full power (so the inverter power factor is largely real under this operating condition), the LCL network cannot ensure the inverter power factor stays close to 1 for all other operating conditions.

The invention can also work with the situation when the impedance for an inverter is not too real, as the user can deal with the power factor issue. If we turn off an inverter using the first method shown in Figure 5a, terminals Al and A2 are short- circuited by the inverter switches, as shown in Figure 9d. Because it is a mirror image of Figure 9c, the impedance across terminals Bl and B2 can also be calculated using the equation above with Z set to approximately 0, which means Zd is infinitely large and can therefore be considered open-circuit. This effectively removes the LCL network from the rest of the circuit. Therefore the parallel subcircuit allows an inverter to be turned off without affecting the current source characteristics of the remaining parallel sub-circuits.

Turning off one inverter using the methods described in Figure 5b and 5c achieves a similar result. In Figure 5b, SI is turned on instead of inverter switches to short- circuit terminals Al and A2 of the inverter. In Figure 5c, SI and S2 are turned off to remove the LCL network, which is similar to an open-circuit condition creating by the first method.

Having an open circuit/real load removes the impedance from the other inverters. As described earlier, turning off an inverter by shorting its outputs, as illustrated in Figure 9d, makes the impedance Zd infinitely large; this is equivalent to disconnecting LCL tuned network and its inverter from the rest of the system.

4.5 Shared tuning sub-circuit

With reference to Figures 2 and 3, the present invention relates to a circuit arrangement wherein a shared tuning sub-circuit 30 is provided in a series arrangement with a plurality of parallel sub-circuits 22, which are provided in a parallel arrangement. This forms an inductive power transfer transmitter. The series tuned network is shared amongst all parallel inverters and tuning networks. Note, as later described, parallel, LCL or other topologies could be used.

The shared tuning sub-circuit 30 comprises at least one capacitor 32, and a power transmission coil 34 configured to generate a field for wireless transmission of power. The parallel arrangement of the inverters 26 and tuning network 28 in the parallel sub-circuits 22 provides current into a series (or other shared) tuned network at the power transmission coil 34.

In the embodiments shown, the capacitor(s) 32 and the inductance of the power transmission coil 34 form a series tuned sub-circuit or network which acts to tune the current received from the sum of operating parallel sub-circuits. The total reactance of the capacitor(s) 32 and the power transmission coil 34 equals to substantially zero at the operating frequency of the inductive power transfer transmitter 20 when in operation. In some embodiments, the value of the inductance of the power transmission coil 34 can change slightly due to misalignment and value of the capacitance of the capacitor(s) 32 can change slightly due to temperature, capacitor aging and capacitor voltage.

4.6 Capacitor(s) in the shared tuning sub-circuit

In the embodiments shown in Figures 2 and 3, the capacitor(s) and the inductance of the power transmission coil form the series tuned sub-circuit or network which acts to tune the current received from the sum of operating parallel sub-circuits. One or more capacitor(s) are provided in series arrangement with the power transmission coil. The total reactance of the capacitor(s) and the power transmission coil equals to substantially zero at the operating frequency of the inductive power transfer transmitter when in operation.

Figures 10a, 10b and Ila show an alternative embodiment of a shared tuning network 50. In this embodiment there is a capacitor 52 provided in parallel to the power transmission coil 54, and therefore in parallel to the inductance of the power transmission coil, forming a parallel tuned network. In further embodiments, as shown in Figures lib and 11c, at least one partial series tuning capacitor 56 is provided in series with the power transmission coil 54 in the shared tuning subcircuit 50, along with the capacitor 52 provided in parallel to the power transmission coil 54.

The features of the shared tuning sub-circuit of these alternative embodiments are shown in Figures 10a, 10b and 11a to 11c is similar to the original embodiments shown in previous embodiments.

Figures 2, 3, 10a, 10b, Ila, l ib and 11c are just some examples of LC tuning forms. The shared tuning sub-circuit can have other multiple L and C combined tuning forms, also, as will be appreciated by those skilled in the art.

4.7 Power transmission coil

The power transmission coil is provided in the shared tuning sub-circuit alongside the one or more capacitors as described. The power transmission coil is configured to generate a field for wireless transmission of power to an inductive power transfer receiver, as will be appreciated. The current at the power transmission coil is a sum or part of all output currents from the plurality of parallel sub-circuits, depending on the shared tuning topology. Therefore, the power provided by the power transmission coil is able to be controlled by controlling the current provided by each of the plurality of parallel sub-circuits.

If the output voltages of the plurality of parallel sub-circuits are kept substantially constant, the current at the power transmission coil can be controlled in large steps by turning on different combinations of parallel sub-circuits. For example, if N number of parallel sub-circuits comprising an inverter and LCL tuning network are connected in parallel, and the output voltage of each parallel sub-circuit is fixed at voltage Vb, the primary coil current is (N-M)*Vb/X for a series tuned sub-circuit, where M is the number of parallel sub-circuits that are turned off or short circuited.

Therefore, by controlling N-M, the primary coil current can be varied in steps.

Between these steps, the phase shift of each operating inverter can be used to vary the current at the power transmission coil so that the power at the coil can change in a continuous manner. Typically, phase shift of each inverter can be adjusted with a very fine resolution.

4.8 Controller and sensor to determine K coefficient

A controller is provided 36 in combination with a sensor circuit 50 to determine k coefficient, and control switching o the inverters accordingly

5. Control of inductive power transmitter

As discussed previously, it is desirable to couple and/or decouple inverters from the transmitter coil based on the K coefficient to keep the received power constant/at a desired level.

There are several ways this can be achieved based on the disclosed embodiment, or with small variations to the disclosed embodiment. This has been enabled because the tuning circuits 28 in effect create a current source from each inverter. This enables the current to be controlled e.g. through turning off/on inverters and the total summed current provided to the shared circuit. Also, more fine tuning can be achieved with controlling phasing of the output from inverters as will be described below.

Control aspects might further include a controller and sensor to monitor the k coefficient and then operate the inverters accordingly. Alternatively, the inverters might be operated manually.

5.1 Power control through turning inverters on/off and/or controlling phasing

In this embodiment, each inverter is also able to be turned on, where it provides an output current, or off, where no output current is provided. In an embodiment, the or each inverter is configured to be turned on or off through controlling one or more inverter switches. The inverter switches are configured to short circuit the output of the inverter. An inverter of a parallel sub-circuit can be switched out or turned off to stop its contribution to the power transmission coil current using a number of different configurations and methods. Three different example configurations and methods are shown in Figures 5a, 5b and 5c, and will be explained in more detail below. These configurations and methods are not intended to be limiting, and it will be appreciated that an inverter in the inductive power transfer transmitter can be turned on and off using one or more of these configurations and methods, or another known configuration and method. Figures 5a, 5b, 5c show a series tuned circuit, but they could instead be parallel, LCL or other tuned circuit also.

A first example configuration and method is shown in Figure 5a. In this example embodiment, the inverter 26 is a full bridge inverter, and is turned off by shortcircuiting its output terminals A and B using either the top two or bottom two inverter switches. More specifically, this can be performed by turning on Sna and Snc and turning off Snb and Snd, or by turning on Snb and Snd and turning off Sna and Snc.

In some embodiments, when the inverter is turned off in the way described in relation to Figure 5a, a DC common mode current can flow between the inverters of the plurality of parallel sub-circuits. In these embodiments, one or more DC blocking capacitors can be used to control or limit this DC common mode current. In such embodiments, the one or more DC blocking capacitors can be added to the tuning network of the parallel sub-circuit. For example, in the example embodiments of the tuning networks 28 shown in Figures 6b to 6g, a DC blocking capacitor 42 is provided in the network in order to control any DC common mode current which may form. The DC blocking capacitors have infinitely large impedance when operated at DC.

In further embodiments, an AC common mode current can also flow between parallel inverters modules when one inverter is turned off in the way described in relation to Figure 5a. In such embodiments, a common mode choke can be used to suppress this AC common mode current.

Figure 6f shows one example implementation of a common mode choke 44 within the parallel tuning network 28. In this example embodiment, the first set of inductors are realized using the differential mode inductance of the common mode choke 44. The common mode choke 44 presents a large AC impedance at the operating frequency of the inductive power transfer transmitter in the path of the AC common mode current.

Figure 6g shows another example implementation of a common mode choke 44 within the parallel tuning network 28. In this example embodiment, the second set of inductors are realized using the differential mode inductance of the common mode choke 44. It will be appreciated that other implementations that cooperate common mode chokes are also possible.

It should be noted that Figures 6a to 6g show example embodiments of parallel tuning networks 28 and may not necessarily show all different implementations of the tuning network possible. Regardless of the tuning network used, in these embodiments as described in relation to Figure 5a, DC blocking capacitors and common mode chokes in the tuning network may be required in order to achieve the best common mode suppression when one or more inverters are turned off as described in relation to Figure 5a.

By turning off the inverter in this manner, the LCL circuit becomes an open circuit, and the load becomes real. The high impedance is removed so doesn't affect the other sub-circuits or current provided by them.

A second example configuration and method for turning off or on one or more of the inverters is shown in Figure 5b. In this example embodiment, an AC switch 40a is added to the output of an inverter between points A and B. Examples of AC switches include solid-state relays and two series connected electronic switches. An inverter can be turned off by turning off inverter switches Sna, Snb, Snc and Snd as previously described in relation to the first example configuration and method, and additionally, or alternatively by tuning on the AC switch 40a. The example embodiments of parallel tuning networks 28 as described in Figures 6a to 6g in relation to the first example configuration and method are applicable for this second configuration and method for turning off and on an inverter also.

By turning off the inverter in this manner, the LCL circuit becomes an open circuit, and the load becomes real. The high impedance is removed so doesn't affect the other sub-circuits or current provided by them. A third example configuration and method for turning off or on one or more of the inverters is shown in Figure 5c. In this example embodiment, an inverter can be turned off by turning off inverter switches Sna, Snb, Snc and Snd as previously described in relation to the first example configuration and method, and additionally, or alternatively by turning off AC switches 40b or 40c, or both switches 40b and 40c. The example embodiments of parallel tuning networks 28 as described in Figures 6a to 6g in relation to the first example configuration and method are applicable for this third configuration and method for turning off and on an inverter also.

The three example embodiments for configurations and methods for turning on and off inverter in a parallel sub-circuit described above and in relation to Figures 5a to 5c do not introduce reactive loads to the other parallel sub-circuits inverters that are on or otherwise operational, which optimize the total output current from the parallel sub-circuits received at the power transmission coil. In some embodiments, when used with an inductive power transfer receiver, after an inverter has been switched off, there may still be some current circulating in an inverter due to reflected impedance from the inductive power transfer receiver.

In the above example embodiments of the present invention described in relation to Figures 5a to 5c, an inverter, when turned off, has minimal or no switching losses because its outputs are shorted and not switching. As such, in combination with the parallel arrangement of the present invention, this allows for a switching off current of between 25A to 15A lower than a conventional solution, which enables a significant reduction in switching losses for the inductive power transfer transmitter. In a conventional inductive power transfer system, a high switching off current typically occurs in all inverters in the transmitter or primary. In typical inductive power transfer systems, the high switching current occurs because of a much smaller inverter output voltage.

5.2 Power control using phase shift

In a preferred embodiment, each inverter is able to provide an output current between a maximum magnitude and a minimum magnitude. The magnitude of the output current between the maximum and minimum magnitudes is controlled by varying the phase shift of the one or more inverters of the parallel sub-circuit. As such the phase shift of each parallel sub-circuit can be controlled in order to vary the output current of the primary power circuit. 5.3 Controller and K monitoring

As shown in Figures 3 and 10b, in some embodiments, the inductive power transfer transmitter further comprises a controller 36. In these embodiments, the controller 36 is operably connected to each of the plurality of parallel sub-circuits 22 and is configured to control each of the parallel sub-circuits 22 separately. The controller may be operably connected to the inverter 26 of each parallel sub-circuit 22 and may be configured to control each of the inverters 26 separately.

In some embodiments, the controller 36 may further be operably connected to the shared tuning sub-circuit, or more specifically the power transmission coil, and/or may have a sensor provided in the shared tuning sub-circuit. In these embodiments, the controller 36 takes as input one or more measurements or readings relating to the shared tuning sub-circuit and/or power transmission coil.

In some embodiments, the controller may alternatively, or additionally, have one or more sensors external to the inductive power transfer transmitter 20. For example, the controller 36 may be operatively connected to one or more sensors which measure or estimate the magnetic coupling factor (k coefficient) between the inductive power transfer transmitter and an inductive power transfer receiver when in use. For example, the magnetic coupling factor (k coefficient) could be measured by the controller by providing a small current and seeing what pickup is obtained by the controller or connected sensor.

The controller may additionally, or alternatively be operatively connected to one or more sensors in or at the inductive power transfer receiver or secondary device and may provide readings or measurements relating to one or more parameters of the inductive power transfer receiver. For example, the controller may operatively connect to a sensor which takes readings of the loading requirements at the inductive power transfer receiver. In these embodiments, the controller is configured to take readings from the sensor(s) as input and use these for processing. Alternatively, a communications module may be provided in the inductive power transfer receiver which is operable to communicate any relevant readings or measurements related to the loading requirements of the receiver to the controller of the transmitter. Figure 16 shows an example flow diagram of the control process performed by the controller. In this example, the controller is configured at step 100 to receive as input data from any source which allows it to determine the power/current required to be provided to the power transmission coil in order for an inductive power transfer receiver which is in use with the inductive power transfer transmitter of the present invention to receive a constant amount of power from the inductive power transfer transmitter.

Once the controller has received relevant data as input at step 100, as outlined above, the controller is then configured to firstly determine the power requirements at the power transmission coil based on the received input at step 102. The power determination performed by the controller calculates the power required at the power transmission coil in order for the inductive power transfer receiver to receive a constant level of power. The controller is configured to calculate the requisite power at the power transmission coil based on one or more of the input measurements. An example embodiment of this determination and possible readings is described in the section below.

Once the controller has determined the level of power required at the power transmission coil at step 102, it then proceeds to determine a configuration or arrangement of parallel sub-circuits required in order to achieve determined level of power required at the power transmission coil at step 104. Because of the parallel arrangement provided by the present invention, the inverters of each parallel subcircuit are able to be controlled to meet the power output required at power transmission coil, as determined by the controller at step 102 based on the input received. The current output of each inverter is able to be controlled with precision between a minimum magnitude and a maximum magnitude through controlling the phase shift of the inverter. The inverter(s) of each parallel sub-circuit is also able to be switched on or off in order to provide its current output at the shared tuning sub-circuit. In this way, the current and therefore the power at the power transmission coil is able to be controlled in a similar way to a gear box in a car.

Based on the received input(s) and the determined level of power required at the power transmission coil, the controller is then configured to determine which of the plurality of parallel sub-circuits should be turned on and which should be turned off, and of those that should be on, what phase shift is required for each parallel sub- circuit. This determination is so that the sum of the parallel sub-circuits outputs is equal or substantially equal to the required power determined at step 102.

The determinations of steps 102 and 104 may be performed by the controller in real time, or may alternatively be performed on a periodic basis, for example every 30 seconds. Alternatively, this determination may be performed when the inductive power transfer transmitter starts, before it provides any power to the power transmission coil and therefore any power to the power transfer receiver. Data may be received as input by the controller in accordance with the rate of determination or at another rate, for example, may be received by the controller continuously, periodically, and/or upon start up.

The controller is then configured to provide as output one or more control signals which are operable to control the amount of current provided by each of the plurality of parallel sub-circuits at step 106. For example, the controller may provide as output one control signal to each of the plurality of parallel sub-circuits based on the power determination performed. Alternatively, the controller may provide as output one or more control signals to each of the inverters in the parallel sub-circuits directly. In either of these embodiments, the controller may be configured to output a first control signal to a parallel sub-circuit and/or corresponding inverter which signals to the parallel sub-circuit and/or inverter that it should be turned on or off. The controller may further be configured to output a second control signal to a parallel sub-circuit and/or corresponding inverter which signals to a parallel sub-circuit and/or inverter which is on or operational, what phase shift it should operate at. In alternative embodiments, the controller may provide as output a single control signal to each parallel sub-circuit and/or inverter both whether it should be on or off, and also signal if on, what phase shift it should operate at.

The controller is configured to control each of the parallel sub-circuits separately in order to ensure the power received by the inductive power receiver or secondary device is constant. The controller is configured to provide a control signal for or otherwise control a parallel sub-circuit or inductor within a parallel sub-circuit to turn off or turn on, in a method such as previously described. Therefore, the controller is configured to control whether the parallel sub-circuit provides current to the shared tuning sub-circuit and the power transmission coil. The controller is also configured to provide a control signal for or otherwise control a parallel sub-circuit or inductor within a parallel sub-circuit to control the phase shift of the inductor when operational, in a method such as previously described. Therefore, the controller is configured to control the level of current provided by the parallel sub-circuit to the shared tuning sub-circuit and the power transmission coil between a minimum magnitude and a maximum magnitude.

Because of the parallel arrangement provided by the present invention, the controller is therefore able to control the amount of current provided at the power transmission coil. The controller is able to control this current in large steps by turning on or off different combinations of the inverters in the plurality of parallel sub-circuits. The controller is also able to control the current provided to the power transmission coil by controlling the phase shift of each of the inverters in each parallel sub-circuit. As such, the controller is able to control the phase shift of each inverter to adjust the output current of the parallel sub-circuit with a very fine resolution. By controlling the phase shift of an inverter as well as turning on or off individual inverters, the current at the power transmission coil can be controlled in a continuous manner in response to the requirements for the power at the power transmission coil as determined by the controller.

Because of the parallel arrangement provided by the present invention, the inverters of each parallel sub-circuit are able to be controlled in response to changing power output required at power transmission coil, as determined by the controller based on the input received. The current output of each inverter is able to be controlled with precision between a minimum magnitude and a maximum magnitude through controlling the phase shift of the inverter. The inverter(s) of each parallel sub-circuit is also able to be switched on or off in order to provide its current output at the shared tuning sub-circuit. In this way, the current and therefore the power at the power transmission coil is able to be controlled in a similar way to a gear box in a car.

For example, the minimum current provided at the power transmission coil would involve just one of the parallel sub-circuits inverter(s) turned on and at a phase shift that produces the minimum inverter output voltage Vmin and minimum current Imin into the shared sub-circuit 30. If sub-circuit 30 is series tuned, Imin also flows into the transmission coil. If sub-circuit 30 contains a capacitor in parallel with the transmission coil (for example, parallel or LCL tuned) , part of Imin may flow into the transmission coil. The phase shift of this inverter(s) is then able to be adjusted to produce the maximum inverter magnitude output voltage Vmax, which generates the maximum current Imax (possible by a single inverter) into the shared block 30. Again, the tuning topology of sub-circuit 30 determines the percentage of Imax that flows into the transmission coil. Then, a second parallel sub-circuits inverter(s) is able to be turned on with both the first and second inverters operating at an identical phase shift that produces half of Vmax at each inverter output. This still generates current Imax into the shared block sub-circuit 30 and is equivalent to having just a single inverter on at Vmax due to the fact that currents out of block 28 combine into sub-circuit 30. The phase shift of both inverters are then able to be adjusted to further increase the output voltage of each inverter from Vmax/2 to Vmax, which now doubles the current flowing into the shared sub-circuit 30 to 2*Imax. If more power is required at the power transmission coil, then a third parallel sub-circuits inverter(s) is able to be turned on with all three inverters operating at an identical phase shift that produces 2*Vmax/3 at each inverter output. Therefore, the current flowing into the transmission coil is 2*Imax. Each inverter output voltage can by increased synchronously to Vmax eventually inject 3*Imax into the transmission coil. By adding inverters this way, it avoids inverter output voltage being too low, which would cause excessive inverter currents and inverter losses. The same logic applies when reducing the number of parallel inverters.

An alternative is now described. The method described in the paragraph below different from that above. The above describes a method where all inverter output voltages are kept identical all the time. Now is a described a method where inverter output voltages are different. The minimum current provided at the power transmission coil would involve just one of the parallel sub-circuits inverter(s) turned on and at a minimum phase shift output magnitude. The phase shift of this inverter(s) is then able to be increased using the phase shift up to a maximum magnitude output. Then, a second parallel sub-circuits inverter(s) is able to be turned on and at a minimum phase shift output magnitude, adding to the output of the first operating inverter(s). The phase shift of this second operating inverter(s) is then able to be increased using the phase shift up to a maximum magnitude output. If more power is required at the power transmission coil then, a third parallel sub-circuits inverter(s) is able to be turned on at a minimum phase shift output magnitude, adding to the output of the first and second operating inverter(s). And so on until all of the parallel sub-circuits are turned on. The parallel sub-circuit inverters are able have their output magnitude reduced using phase shift and are able to be turned off in order to reduce the power provided to the power transmission coil.

5.4 Example control embodiment

With reference to Figures 12 to 15, an example embodiment of the control of an inductive power transfer transmitter of the present invention is described. The inductive power transfer transmitter in this example embodiment has two parallel sub-circuits, each comprising an input connected to a DC power source having a fixed voltage, a full bridge inverter, and an LCL type parallel tuning network.

In this example, the coupling coefficient k between primary and secondary coils, as shown in the graph of Figure 12, changes from 0.25 to 0.57 (an increase of 2.28 times) as the inductive power transfer receiver moves from a further position to a closer position. As the coils move further apart, the mutual inductance of the shared tuning sub-circuit changes by a factor of 2.46 from 30.6uH to 12.39uH, as shown in Figure 13. The inductive power transfer transmitter requires the power at the power transmission coil to be constant over this range.

Because of the change in inductance, the inductive power transfer transmitter of the present invention turns off the first of the parallel sub-circuits by shorting the outputs of the corresponding inverter when the coupling coefficient k is higher than 0.44, as shown in Figure 14. The second parallel sub-circuit remains on. Turning off the first parallel sub-circuit reduces the current at the power transmission coil by a factor of 2 compared to having both the first and second parallel sub-circuits turned on. As a result, the inverter output phase does not need to change by too much.

As can be seen in Figure 15, at a coupling coefficient k (k) higher than 0.44, the first parallel sub-circuit is turned off by short-circuiting the output terminals of the corresponding inverter and its output voltage is zero. As a result, output voltage of the second parallel sub-circuit only needs to change between 461V and 700V. In this embodiment there is still current circulating in the switched off first inverter due to the short-circuit condition and voltage in the resonant tank.

The arrangement of the present invention allows for a switching off current of between 25A to 15A lower than a conventional solution, which enables a significant reduction in switching losses for the inductive power transfer transmitter. In a conventional inductive power transfer system, a high switching off current typically occurs in all inverters in the transmitter or primary. In typical inductive power transfer systems, the high switching current occurs because of a much smaller inverter output voltage. In this example embodiment, the first parallel sub-circuit and inverter, when turned off, has minimal or no switching losses because its outputs are shorted and not switching.

6. Variations

The inductive power transfer system described herein can be used in a variety of applications, for charging and/or real-time powering via inductive power transfer.

The embodiments described herein could be used in any suitable inductive power transfer system for any suitable end use. For example, the embodiments could be used in a system that implements inductive power transfer charging of a charge storage device (such as a battery, super capacitor or similar), for example for a vehicle or other electrical equipment. Alternatively, for example, the embodiments could be used in a system that implements real-time powering via inductive power transfer. Non-limiting examples of the sorts of end uses that require wireless power transfer charging or real-time powering where the embodiments might be used comprise: electric vehicles, electric scooters, electric bikes, robots, manufacturing equipment, charge storage devices (e.g. batteries or supercapacitors), or any other suitable electrical systems/devices ("electrical equipment"). The embodiments described can be used in industrial, commercial and/or domestic situations without limitation. The embodiments described are not restricted to just high-power/high current end-use applications.

The controller, and various illustrative logical blocks, modules, circuits, elements, and/or components described in connection with the examples disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic component, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, circuit, and/or state machine. A processor may also be implemented as a combination of computing components, e.g., a combination of a DSP and a microprocessor, a number of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

The control methods or algorithms described in connection with the examples disclosed herein may be embodied directly in hardware, in a software module executable by a processor, or in a combination of both, in the form of processing unit, programming instructions, or other directions, and may be contained in a single device or distributed across multiple devices. A software module may 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 storage medium known in the art. A storage medium may be coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor.

One or more of the components and functions illustrated the figures may be rearranged and/or combined into a single component or embodied in several components without departing from the invention. Additional elements or components may also be added without departing from the invention. Additionally, the features described herein may be implemented in software, hardware, as a business method, and/or combination thereof.

In its various aspects, the invention can be embodied in a computer-implemented process, a machine (such as an electronic device, or a general purpose computer or other device that provides a platform on which computer programs can be executed), processes performed by these machines, or an article of manufacture. Such articles can include a computer program product or digital information product in which a computer readable storage medium containing computer program instructions or computer readable data stored thereon, and processes and machines that create and use these articles of manufacture.

The foregoing description of the invention includes preferred forms thereof. Modifications may be made thereto without departing from the scope of the invention.

7. Advantages

The proposed structure enables a larger variation in the current that flows into the shared sub-circuit and subsequently the transmission coil current while ensuring inverters operate efficiently. The larger variation can be used to compensate for variation in k due to misalignment or change in required power level from the secondary.

The proposed structure allows inverters to be turned off efficiently and safely while also ensuring the remaining inverters can operate efficiently and safely. It does so by using the LCL tuned network which limits the short-circuit current of a disabled inverter and does not detune the resonant tank so the inverter currents of the remaining inverters are optimal.

Having one for each inverter and connecting them in parallel to feed into a shared network comprising transmission coil is beneficial to enable control of current while maintaining circuit characteristics. The power factor and/or impedance and/or other characteristics of the remaining parallel sub-circuits are not altered when an inverter and/or parallel circuit are disconnected (turned off).

The tuned common circuit is important to make inverter impedance largely real. It is also important to keep inverter impedance largely real when inverters are switched off.

Fine tune using phase shift is important as it bridges the gap between different inverter combinations.