Wireless Power coupling for Towing Hitch

Abstract

A tow hitch assembly comprises a tow ball and a tow cup, each having a recess formed therein which align when the tow ball 1 is engaged in the tow cup. A primary side power transmission circuit of a w

Description

The present invention relates to improvements in relation to wireless power coupling for delivering power from a towing vehicle to a towed trailer. Towed trailers require a power supply to run onboard lights such as indicators, brake lights etc. Conventionally, the trailer is provided with a power cable with a plug on it which is manually plugged into a socket on the vehicle to provide a hard wire connection between the vehicle and the trailer. However, applicant has developed an automatic trailer coupling system, disclosed in British patent no. GB2554523B, which enables a trailer to align with and connect to a trailer without any manual human intervention, and to complement this system, there is a need for a wireless power coupling system which enables power to be wirelessly transmitted from the vehicle to the trailer, thereby removing the need for human intervention to connect a power connector on the trailer to the vehicle. Applicant’s own earlier British patent No. 2590590 discloses a wireless power transmission for transmitting power from a tow vehicle in which a primary coil is housed in a recess within a tow ball and a secondary coil is housed in a recess in tow cup such that when the tow cup is engaged on the tow ball, the primary and secondary coils are located next to each other so that an alternating current in the primary coil induces a current in the secondary coil which can then be used to charge a battery on the trailer. Typically, the power transmitted by a wireless system can be increased by increasing the size of the primary and secondary coils. However, the size of the recess in each of the tow ball and the tow cup is restricted by the need to maintain the structural integrity of each part, taking account of the loads carried thereby, which, in turn, restricts the size of coils that can be used. This has been found to present problems with delivering a substantially smooth and consistent 20W from the vehicle to the trailer The smaller the coils, the lower the maximum current which can be carried, and using standard circuitry this can result in peak currents at start up, when loads are low, which exceed the primary coil limit and result in damage. Similarly, low load on the secondary coil results in low secondary current which results in high voltage, which can then exceed a level which can cause damage to components in the secondary circuit. It is known in the art to tune the primary and secondary circuits in a wireless transfer system. The standard approach for a wireless power transfer system is to implement series-series tuning, where a capacitor is placed in series with the power transfer coil (an inductor) in both the primary and secondary circuits. Such a circuit composed of a set of capacitors and inductors is known as a resonant network or Matching Circuit. It is typical that the primary side is ‘partially tuned’, to allow the primary system to partially resonate. To tune a resonant network, the capacitance value must be chosen to match with the inductor value. For example, to perfectly match or tune for coils having an inductance of 3.3uH at a frequency of 1 MHz, a network of capacitors with an equivalent value of 7 til nF is required. Under this condition, the system will self-resonate and be at its most efficient. This is the typical setup for the secondary side, where the maximum power transfer condition is fulfilled by designing the system for perfect resonance. This condition must be avoided on the primary side, as perfect resonance will effectively short the input under the no-load condition drawing significant current through the system, causing heating and potentially damage. To address this, a compromise tuning is usually selected where a standard amount of current draw is supplied by partially tuning the system using a capacitance relatively close but greater than that of the resonance condition. This is the typical primary side setup for a wireless power transfer system. However, such a configuration does not adequately limit the voltage on the secondary circuit during a low load state, such as at start up where there is little current drawn and hence maximum voltage induced on the secondary side, risking damage to the secondary circuit components due to excess heat etc. There is therefore a need for a wireless power transmission system which enables adequate power transfer (circa 20Watts) whilst ensuring current and voltage levels are limited to prevent damage to components in the primary and secondary circuits. The particular application of the present invention to wireless transfer through a tow hitch also gives rise to other problems. The tow ball I tow cup system is chosen to allow for necessary differential pitch and roll movement between the vehicle and the trailer as will inevitably occur during travel over any ground which is not perfectly level. This movement is accommodated by allowing the tow cup to move around the tow ball to a limited degree. However, such movement, which is typically constantly occurring to a greater or lesser degree, means that the alignment of the primary and secondary coils housed in the tow ball and tow cup respectively will be constantly changing and hence the flux density in the secondary coil will be constantly changing, varying the coupling therebetween. Such variation in the coupling gives rise to a risk of driving current into the cast iron hitch instead of into the receiver coil. This can be problematic firstly because it can significantly impact efficiency by draining energy from the vehicle in a wasteful manner and secondly because it could drive potentially dangerous currents through the primary system, causing damage. Fully implementing the power transfer coil in the hitch-head introduces challenges that are unique to this system. A primary challenge is the size and restrictions regarding the transfer coils. Both have a maximum size of 22mm in diameter, fixed to a piece of ferromagnetic material. These coils have a hard upper limit for the coils of 3A RMS of current, requiring careful balancing of coil voltage and current to achieve 20W throughput. Conventional wireless charging systems under the Qi standard utilise frequency control mechanisms, whereby the system varies operating frequency to vary the voltage and current throughput by adjusting the relative tuning. However, such systems have been found not to be effective in preventing the above mentioned issues arising due to the movement in the tow hitch. According to a first aspect of the present invention there is provided a secondary side power transmission circuit for a wireless power transfer system of a tow hitch assembly for receiving power transmitted from a primary power transmission circuit, comprising a secondary coil suitable for mounting in the towed side of the tow hitch assembly such that, in use, it substantially aligns with a primary coil of the primary power transmission circuit for receiving power transmitted from the primary coil when the tow hitch assembly is connected together, and further comprising a matching circuit connected to the secondary coil for controlling the output voltage from the secondary coil, said matching circuit including a first capacitance means and a second capacitance means wired in a series I parallel configuration A secondary circuit of a wireless power transfer system according to the first aspect of the invention has the advantage that the provision of a parallel capacitance means within the output circuit acts to constrain the voltage on the secondary side in a high coupling, no-load condition, thereby preventing damage to the secondary circuit due to excessive voltage. Specifically, the addition of the secondary-side parallel capacitance means acts to reflect a lower reactance to the primary-side under no-load conditions, resulting in lower primary current and lower output voltage. The matching circuit furthermore conditions the transferred power to meet the secondary side voltage and current requirements while delivering the required usable output power, preferably 20W. The first and second capacitance means may each be realised, within the scope of the invention, by a single capacitor in each case having the required value or may be formed by a number of capacitors arranged in a configuration which produces the net required capacitance effect. More particularly, the first capacitance means is arranged in series with the secondary coil and the second capacitance means is arranged in parallel with said first capacitance means. The matching circuit preferably further includes a resistor arranged in series with each of said first and second capacitance means and in series with said secondary coil. The values of the said resistor and said first and second capacitance means are chosen to restrict the output voltage to 28V. This resistor is chosen to model the DC resistance of the inductor and will therefore be fixed for a particular a particular pair of coils. Preferably the second capacitance means has a reactance which is in the range of (-)3.5 to (-) 10 times the reactance of the secondary coil, in particular (-)3.8 times the reactance of the secondary coil, and the first capacitance means has a reactance which is (-) 0.25 to (-) 0.72 times the reactance of the secondary coil, in particular (-) 0.5 times the reactance of the secondary coil. In a particularly preferred embodiment, the reactance of the secondary coil is 20.735 jOhms, the reactance of the first capacitance means is -10.6jOhms (15nF), and the reactance of the second capacitance means is -79.5jOhms (2nF). As indicated above these values may be realised by a single capacitor in each case or a number of capacitors arrange to produce the same net value as the single capacitor, the option of the number of capacitors having the advantage that it allows finer adjustment of the net value. The output of the matching circuit preferably feeds AC current to a Full-Bridge Rectifier Circuit, which converts the AC output of the secondary coil into a DC output that can be used by the secondary side systems. In particular, said DC output is fed to a Battery Charging Circuit which regulates and conditions the output voltage and current to safely charge and power a battery mounted in the towed assembly (trailer). The present invention further provides a tow cup having a recess therein in which is mounted a secondary coil of a secondary side power transmission circuit according to the first aspect of the invention. According to another aspect of the present invention, there is provided a primary side power transmission circuit for a wireless power transfer system of a tow hitch assembly for transmitting power transmitted to a secondary power transmission circuit, comprising a DC power supply, a controller, a current sensing unit, a phase-shift inverter and a primary coil suitable for mounting in the towing side of the tow hitch assembly such that, in use, it substantially aligns with a secondary coil of the primary power transmission circuit for transmitting power to the secondary coil when the tow hitch assembly is connected together, wherein the controller is configured to monitor the current in the primary side circuit using the current sensing means and to vary the phase of the AC current delivered to the primary coil using the phase shift inverter in order to limit the current in the primary side circuit. A primary side circuit according to the second aspect of the invention has the advantage that the use of phase shift control enables control of current to prevent damage to the primary side circuit, wasting energy from the primary side source whilst still enabling transfer of the required power to the secondary circuit. Preferably the phase shift inverter is a phase shift full-bridge inverter, the controller being configured to vary the phase of the two sides of the full-bridge inverter with respect to each other. More particularly, the controller is configured to cycle the duty cycle up to increase the voltage driven into the circuit, causing more current draw and increasing the power transmitted to the secondary coil, thereby compensating for lower coupling between the primary and secondary coils, and to cycle the duty cycle down to decrease the voltage driving into the circuit, reducing current and therefore power when the current level is reaching the maximum safe level for the primary cuircuit. Preferably, the primary side circuit further includes a matching circuit connected to the primary coil. The DC power supply is preferably a vehicle battery such as a 12V battery. The primary circuit preferably includes a boost converter which steps the input voltage to the inverter up to 24V. Preferably the primary and secondary coils are less than or equal to 22mm diameter, in one embodiment 20.5mm diameter each. More particularly the diameter of each of the coils is less than or equal to 20mm, less than or equal to 19mm and especially less than 17mm. IN one particularly preferred embodiment, each coil is approximately 16.5mm diameter sat on a ferrite disc of 20mm diameter. The present invention further provides a tow ball having a recess therein in which is mounted a primary coil of a primary side power transmission circuit according to the second aspect of the invention. The present invention still further provides a tow hitch assembly comprising a tow ball and a tow cup, each of the tow ball and the tow cup having a recess formed therein which align when the tow ball is engaged in the tow cup, a primary coil being engaged in the recess in said tow ball and a secondary coil being engaged in the recess in said tow cup, wherein said primary coil is connected to a primary side wireless charging circuit according to the second aspect of the invention and / or wherein said secondary coil is connected to a secondary side wireless charging circuit according to the first aspect of the invention. In order that the invention may be well understood, there will now be described an embodiment thereof, given by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a sectional view of a tow ball of the type with which the present invention may be utilised; Figure 2 is a sectional view of a tow cup of the type with which the present invention may be utilised which is matingly engageable with the tow ball of Figure 1; Figure 3 is a schematic diagram of a wireless transfer system; Figure 4 is a schematic circuit diagram of a prior art matching circuit of a wireless power transfer circuit; Figure 5 is a schematic circuit diagram of a matching circuit of the wireless power transfer circuit according to the invention; Figure 6 is a schematic circuit diagram of a full-bridge inverter which forms part of a primary side circuit according to an aspect of the implementation of the present invention; Figures 7a and 7b are schematic circuit diagrams showing the circuit paths through the inverter of Figure 6 to achieve different phase shifts; Figure 8 are graphs showing the secondary side output voltages variation with different phase shifts; and Figures 9a to 9e are graphs showing the output of the inverter for different phase shifts between the inputs. Referring first to Figures 1 and 2, there are shown a tow ball 1 and mating tow cup 5 of the type in which the present invention may be installed. The tow ball 1 has a ball member 4 which engages in a complementary shaped cavity 6 formed in the tow cup 5 to effect a towing coupling therebetween. A recess 2 is formed in the top of the ball member 4 which has a passage 3 extending therefrom through the tow ball 1. A primary coil of a wireless power transfer system is mounted in the recess 2 with connecting wires extending therefrom through the passage. A similar recess 7 is formed in the top of the tow cup 5 just above the cavity 6 in which is mountable a secondary coil of a wireless power transfer system so that, when the tow ball 1 is engaged with the tow cup 5, the two coils are aligned with and proximate each other for effecting power transfer from primary to the secondary coil. As with the tow ball 1, the tow cup has a passage 8 extending from the recess 7 through the body of the tow cup by means of which the secondary coil can be connected to a circuit in the towed vehicle. A Wireless Power Transfer system 10, depicted in Figure 3, consists of two parts: The primary side system 11 in the present invention, is located on the vehicle. The primary system takes DC power from the vehicle battery 13 and inverts it to AC power with a phase shift Full-Bridge Inverter 16B. The AC power is sent through a Matching Circuit 14 which drives AC current through a Primary Coil. The primary side system also includes a Current Sense Unit 16A, and a Microcontroller Unit (MCU) 17 which generates driver signals for the inverter 16B, which operates in Phase-Shift control to drive a steady current through the Primary Coil 15 as described below. The Matching Circuit 14 comprises capacitors and inductors which acts as an energy tank and filtering network to condition the quasi-sinewave produced by the Inverter into a more sinusoidal current for power transmission. The power is transmitted through the Primary Coil 15 which is mounted in recess 2 of the ball member 4 of the tow ball. This coil acts as an inductor and generates a varying magnetic field (due to the AC driving current) that is the medium through which energy is transferred. The secondary side system 12 in the present invention is attached to a trailer or other towed assembly. A secondary Receiver Coil 18 is mounted in the recess 7 formed in the tow cup 5 and acts as an inductor, capturing the varying magnetic field generated by the primary coil 15 on the vehicle. This coil 18 induces an AC voltage and current in response to the time-varying magnetic field coupled from the Primary Coil 15. A secondary Matching Circuit 19 conditions the transferred power to meet the secondary side voltage and current requirements while delivering the required usable output power - in the present invention 20W. This AC current is driven through a Full-Bridge Rectifier Circuit 20, which converts the AC input into a DC output that can be used by the rest of the system. Finally, a Battery Charging Circuit 21 regulates and conditions the output voltage and current to safely charge and power a battery 22 mounted in the trailer. The dashed line 23 in Figure 3 indicates the physical separation between the primary and secondary circuits and represents the power transmission zone. It will be recognised that there are certain load carrying requirements for the tow ball and tow hitch as each must be able to carry the towing load transmitted therethrough, and this in turn restricts the size of the recesses 2 and 7 which can be formed in both the tow ball 1 and the tow hitch 5. This in turn presents problems with supplying sufficient power to the secondary side of the system - it would be the typical approach to increase the size of the coils to ensure sufficient power is transmitted but in the present application, the recesses are limited to about 22mm diameter and the coils correspondingly to a maximum 20.5mm diameter. IN the preferred embodiment, each of the primary and second coils has a diameter of substantially 16.5mm, each coil sitting on a ferrite disc of substantially 20mm diameter. The preferred embodiment has the following system specifications: 1. Input voltage of 24V 2. Output voltage maximum of 30V 3. Coil current RMS absolute maximum of 3A RMS under all conditions 4. 20W power throughput during system operation. 5. Maximum coil size of 19mm diameter Points 1,2, 3, and 4 (above) are addressed by design of the Matching Circuit. 3 and 5 is addressed by the design of both the Controller and Matching Network. When tuning a WPT system, it is critical to focus on the system corner cases as these represent the most challenging conditions the system must operate within. For the system of the present invention, the corner cases are defined in Table 1. The developed tuning system must be able to operate under all these conditions as well as meeting the requirements set out under points 1, 2, 3, and 4. Table 1. The four corner cases that system operation must be valid under High Load (20Q), Low Coupling (0.01) High Load (20Q), High Coupling (0.6) Low Load (10kQ), Low Coupling (0.01) Low Load (10kQ), High Coupling (0.6) During system startup, where little current is drawn through the system, the load is low and maximum voltage is transferred to the output. Under this case there is a risk of damage due to voltage spikes, which must therefore be controlled. During ongoing system operation, where load is high and coupling is high, the system must be capable of transferring 20W to meet point 4, while adhering to points 2 and 3 so as not to damage the system. In the application of the present invention, whilst the recesses in each of the two ball 1 and the tow cup 5 are aligned when the two parts are in their neutral position, the nature of the design of the tow ball I cup system is to allow roll and pitch movement between vehicle and trailer by allowing the tow cup to roll around the tow ball to a limited degree. This means that, during practical use, the alignment of the two oils will constantly be changing causing the coupling to varying and in some extreme conditions, the coupling may completely fail. If coupling does fail due to extreme trailer position variation, regardless of system loading, the inductance of the primary coil increases, as it is no longer suppressed by the secondary, leading to an increased current draw through the primary coil. Due to the size limitation of the coils, there is a corresponding current limitation - the maximum coil size to fit in the available recess limits the maximum current to approximately 3A. This must be tested such that in the minimum coupling condition, the current driven into the primary coil complies with point 3 so as not to damage the system. The prior art approach for a wireless power transfer system is to implement series-series tuning, where a capacitor Ctx2. C2 is placed in series with the power transfer coil Ltx1, Ltx2 (an inductor) on both the primary and secondary sides. This circuit, composed of a set of capacitors and inductors, is known as a resonant network or Matching Circuit. It is typical that the primary side is ‘partially tuned’, to allow the primary system to partially resonate. To tune a resonant network, the capacitance value Ctx2, C2 must be chosen to match with the inductor value Ltx1, Lrx1. For example, the coils chosen to fit within the hitchball have an inductance of 3.3uH at a frequency of 1 MHz. To perfectly match or tune this, a network of capacitors with an equivalent value of l£lnF is required. Under this condition, the system will self-resonate and be at its most efficient. This is the typical setup for the secondary side, where the maximum power transfer condition is fulfilled by designing the system for perfect resonance. This condition must be avoided on the primary side, as perfect resonance will effectively short the input under the no-load condition drawing significant current through the system, causing heating and potentially damage. To address this, a compromise tuning is usually selected where a standard amount of current draw is supplied by partially tuning the system using a capacitance Ctx2 relatively close but greater than that of the resonance condition. This is the typical primary side setup for a wireless power transfer system. An example of a prior art matching circuit for the primary and secondary side is shown in Figure 4. The primary side is partially series tuned to drive sufficient current into the primary coil Ltx1. The secondary side is fully series tuned to minimise coil impedance and maximise efficiency. This prior art approach is effective to fulfil requirements 1, 3, and 4, but does not fulfil point 2 under the high coupling, no-load condition. Without a load to pull down secondary output voltage, the voltage spikes up to 37V, risking significant damage to the secondary system. This condition is the standard startup condition of the secondary side which will therefore definitely occur. In the present invention, this issue is addressed by implementing a hybrid tuning regime on the secondary side, whereby both series C5 and parallel C4 capacitors are utilised within the resonant loop, as shown in Figure 4. The addition of the parallel capacitance C4 acts to constrain the voltage in the high-load condition, with the value of the Resistor Rrx1 and capacitors C4 and C5 being chosen to restrict the output voltage to 28V, satisfying requirement 2. Specifically, the addition of the secondary-side parallel capacitor C4 acts to reflect a lower reactance to the primary-side under no-load conditions, resulting in lower primary current and lower output voltage. Under all other corner cases, the system also satisfies requirements 1, 3, and 4, driving 20W through the link off a 24V input, and limiting coil current to 1.4 ARMS under the most challenging current draw conditions. Resistor Rrx1 is the DC element of the power transfer inductor (secondary coil 18), and its value will be fixed by the characteristics of the secondary coil 18. Similarly, resistor Rs1 in the primary side circuit is modelled for current measurement capability and effects the tuning but is not critical for the tuning effects. Whilst the illustrated circuit shows C4 and C5 as single capacitors, it will be understood that these merely represent the effective capacitance I reactance effect which is required within the matching circuit, and that within the scope of the invention this may be realised in practice with a single capacitor in each position or with a capacitor array in place of each of the single capacitors C4 and C5, each array producing the same net capacitance / reactance of a single capacitor having that value. IN the preferred embodiment, the secondary coil Lrx1 has a reactance of X1. The parallel capacitor C4 has a reactance X4 in the range of (-) 3.5* X1 to (-) 10*X1, preferably equal to (-) 3.8 * X1. The series capacitor C5 has a reactance X5 which is in the range of (-) 0.25*X1 to (-) 0.72*X1,preferably equal to (-) 0.5*X1. IN one embodiment, X1 is 20.735 jOhms, x4 is -79.5jOhms (2nF), and X5 is -10.6jOhms (15nF). As set out above, the coupling between the two coils will inevitably vary in an unexpected manner during operation of the wireless power transfer system of the invention, as result of which there is the possibility of driving current into the cast iron hitch instead of the receiver coil. This inefficiency will firstly drain energy from the vehicle in a wasteful manner but will also drive potentially dangerous currents through the primary system, causing damage. In the present invention this is mitigated by use of a phase-shifting control method. This differs from the prior art frequency control mechanism implemented by, for example, the Qi standard, under which the system varies operating frequency to vary the voltage and current throughput by adjusting the relative tuning. The phase-shifting mechanism utilised in the present invention functions by varying the phase of the two sides of a full-bridge inverter with respect to each other. This results in a quasi-sinewave output with an ‘on-time’ controllable solely through phase-shift variation. If the two sides of the inverter are in phase, then the current on the two sides will combine constructively and the output current will be doubled. If the two sides are out of phase (180 degrees), then the current on the two sides will combine destructively and the output would be zero. Phase shift between 0 and 180 degrees will result in output current ranging from maximum to zero. Figure 6 shows the construction of a full-bridge circuit embodying the invention, and Figures 7a and 7b how the current flow through the inverter can be varied to alter the phase differential between the terminals A and B. The phase shifting occurs between the two legs of the full-bridge inverter. This allows for the conditioning of the quasi-sinewave that is output from the full bridge between the two legs (Terminals A, and B in Figure 6, 7Aand 7B). Terminals A, and B are used to drive the resonant network used by the power transfer system. Figures 8 and 9Ato 9E illustrate how the output voltage varies with varying phase shift within the inverter, namely between A and B. As can be seen in Figure 8, when the system is at a phase-shift of 90°, the output waveform on-time or duty cycle is 50%. When it as a phase shift of 162°, the output waveform has a duty cycle of 90%. By adjusting the phase shift of the output square waves between the two legs from 0° -180° (0% - 100%), the duty cycle of the quasi-sinewave can be adjusted from minimum output voltage, shown in Figure 9A, to maximum output voltage, shown in Figure 9B. In Figures 9A to 9E, the topmost graph presents the voltage measured in legs A and B with respect to ground, while the bottommost graph represents voltage A with reference to voltage B. This effectively measures the voltage between the terminals and is what is driven into the resonant system. When the voltages are in phase and therefore the same, there is zero potential difference and hence zero current is driven into the resonant system. When the signals are out of phase, there is maximum potential difference between the contacts A and B and hence the maximum current is driven into the resonant system. Phase shifts between 0%-100% (corresponding to 0° to 180°) are available and will vary the voltage ‘on-time’ driven into the resonant system. In Figure 9A, there is 0° phase shift so the two voltages entirely overlay and therefore destructively combine to generate an effective output of zero volts. IN Figure 9B, the A and B waveforms are phase shifted by 180°. They act as mutually exclusive such that when A is on, B is off and vice-versa. These elements combine constructively to generate an effective maximum AC output voltage square wave. A middle ground of 60% duty cycle with a phase shift of 108° (0.6 * 180 = 108) is shown in Figure 9C. The two input square waves partially overlay, generating periods of zero voltage as the input waveforms destructively interfere, and periods of + / - 24V where the waveforms do not destructively interfere. The more the waves overlay, the more destructive interference occurs, causing a reduction in driven voltage. A continuous range of output voltages are available to be driven into the resonant system by varying the phase difference of the two input square waves. More extreme examples of this principle are demonstrated in Figures 9D, and 9E, where 10% (18°) and 90% (162°) phase shifts are introduced. The ‘on-time’ of the system in Figure 9D with an 18° phase shift constitutes 10% of the total potential ‘on-time’, as the small degree of phase shift forces the input square waves into destructive interference for 90% of the period. Conversely, in Figure 9E, the 162° phase shift allows for driving the full voltage of the inverted signal into the resonant system 90% of the time, leading to more power being delivered. As coil current must be controlled, and coil current is proportional to coil voltage, we can condition coil current by controlling coil voltage. This arrangement allows for the fine tuning of coil voltage by varying the phase shift into the resonant network, thereby controlling output voltage through destructive square-wave interference between the inverter legs. The microcontroller 17 is programmed with software which can adjust this phase-shift. By cycling the duty cycle up, an increased voltage is driven into the resonant circuit, resulting in more current being drawn and pushing more power from the primary coil 15 to the secondary coil 18. Inversely, by cycling down the duty cycle, less voltage is driven into the resonant circuit, reducing current draw and driving less power from the primary coil 15 to the secondary coil 18. The microcontroller 17 monitors the current throughput into the primary coil 15 using the current sense unit 16A, the detail of which is known in the art and will not be described in detail here, and adjusts the phase of the duty cycle in response to changes in the current. IN this way, the microcontroller 17 controls the current throughput to a required value. Use of this current-sense and phase control within the resonant loop in the present invention enables full current control and system stability. With this control system implemented, variations in coupling and current draw from the secondary side because of movement between the tow ball 1 and the tow cup 5 can be rapidly compensated for. Additionally, with the inclusion of suitable communications systems, a recorded voltage collapse on the secondary side because of total coil misalignment can allow the system to fully cycle down into a standby mode.

Claims

1. A secondary side power transmission circuit for a wireless power transfer system of a tow hitch assembly for receiving power transmitted from a primary power transmission circuit, comprising a secondary coil suitable for mounting in the towed side of the tow hitch assembly such that, in use, it substantially aligns with a primary coil of the primary power transmission circuit for receiving power transmitted from the primary coil when the tow hitch assembly is connected together, and further comprising a matching circuit connected to the secondary coil for controlling the output voltage from the secondary coil, said matching circuit including a first capacitance means and a second capacitance means wired in a series I parallel configuration.

2. A secondary side power transmission circuit according to claim 1, wherein the matching circuit further includes a resistor arranged in series with each of said first and second capacitance means and in series with said secondary coil.

3. A secondary side power transmission circuit according to claim 1 or claim 2, wherein the second capacitance means has an effective reactance which is in the range of (-) 3.5 to (-) 10 times the reactance of the secondary coil.

4. A secondary side power transmission circuit according to claim 3, wherein the second capacitance means has an effective reactance which is (-) 3.8 times the reactance of the secondary coil.

5. A secondary side power transmission circuit according to any of the preceding claims, wherein the first capacitance means has a reactance which is (-) 0.25 to (-) 0.72 times the reactance of the secondary coil, in particular (-) 0.5 times the reactance of the secondary coil.

6. A secondary side power transmission circuit according to claim 5, wherein the first capacitance means has a reactance which is (-) 0.5 times the reactance of the secondary coil.

7. A secondary side power transmission circuit according to any of the preceding claims, wherein the reactance of the secondary coil is 20.735 jOhms, the net reactance of the first capacitance means is -10.6jOhms (15nF), and the net reactance of the second capacitance means is -79.5jOhms (2nF).

8. A secondary side power transmission circuit according to any of the preceding claims, wherein the output of the matching circuit feeds AC current to a Full-Bridge Rectifier Circuit, which converts the AC output of the secondary coil into a DC output which powers the secondary side systems.

9. A secondary side power transmission circuit according to claim 8, wherein said DC output is fed to a Battery Charging Circuit which regulates and conditions the output voltage and current to safely charge and power a battery.

10. A secondary side power transmission circuit according to any of the preceding claims, wherein the secondary coil has a diameter of less than or equal to 22mm.

11. A secondary side power transmission circuit according to claim 10, wherein the secondary coil has a diameter of less than or equal to 19mm.

12. A secondary side power transmission circuit according to claim 10, wherein the secondary coil has a diameter of substantially 16.5mm sat on a 20mm ferrite disc.

13. A tow cup having a recess therein in which is mounted a secondary coil of a secondary side power transmission circuit according to any of the preceding claims.

14. A primary side power transmission circuit for a wireless power transfer system of a tow hitch assembly for transmitting power transmitted to a secondary power transmission circuit, comprising a DC power supply, a controller, a current sensing unit, a phase-shift inverter and a primary coil suitable for mounting in the towing side of the tow hitch assembly such that, in use, it substantially aligns with a secondary coil of the primary power transmission circuit for transmitting power to the secondary coil when the tow hitch assembly is connected together, wherein the controller is configured to monitorthe current in the primary side circuit using the current sensing unit and to vary the phase of the AC current delivered the primary coil using the phase shift inverter in order to limit the current in the primary side circuit.

15. A primary side power transmission circuit according to claim 14, wherein the phase shift inverter is a phase shift full-bridge inverter, the controller being configured to vary the phase of the two sides of the full-bridge inverter with respect to each other.

16. A primary side power transmission circuit according to claim 15, wherein the controller is configured to cycle the duty cycle up to increase the voltage driven into the circuit, causing more current draw and increasing the power transmitted to the secondary coil, thereby compensating for lower coupling between the primary and secondary coils, and to cycle the duty cycle down to decrease the voltage driving into the circuit, reducing current and therefore power when the current level is reaching the maximum safe level for the primary circuit.

17. A primary side power transmission circuit according to any of claims 14 to 16, wherein the primary side circuit further includes a matching circuit connected to the primary coil.

18. A primary side power transmission circuit according to any of claims 14 to 17, wherein the DC power supply is a vehicle battery.

19. A primary side power transmission circuit according to any of claims 14 to 18, wherein the primary circuit preferably includes a boost converter which steps the input voltage to the inverter up to 24V.

20. A primary side power transmission circuit according to any of claims 14 to 19, wherein the primary coil has a diameter of less than or equal to 22mm diameter.

21. A primary side power transmission circuit according to claim 20, wherein the primary coil has a diameter of less than or equal to 19mm.

22. A primary side power transmission circuit according to claim 20, wherein the primary coil has a diameter equal to substantially 16.5mm sat on a 20mm ferrite disc.

23. A tow ball having a recess therein in which is mounted a primary coil of a primary side power transmission circuit according to any of claims 14 to 22.

24. A tow hitch assembly comprising a tow ball and a tow cup, each of the tow ball and the tow cup having a recess formed therein which align when the tow ball is engaged in the tow cup, a primary coil being engaged in the recess in said tow ball and a secondary coil being engaged in the recess in said tow cup, wherein said primary coil is connected to a primary side wireless charging circuit according to any of claims 14 to 23 and wherein said secondary coil is connected to a secondary side wireless charging circuit according to any of claims 1 to 13.

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