AC-DC convertor and a control scheme thereof

Abstract

A control system 400 for controlling a single-stage AC-DC converter of an on-board charger of a vehicle generates a control signal 418 to compensate for a bridge of the single-stage AC-DC converter to reduce the second harmonic of the output current. The control system comprises one or more processors 412, 414, 416 to receive an input signal 390 indicative of an output current of the single-stage AC-DC converter and generate, in dependence on the received input signal, a second harmonic component signal comprising a representation of a second harmonic of the output current. A control signal to compensate for the second harmonic of the current is generated in dependence on the second harmonic component signal. The generated control signal is output to control switching of a secondary-side bridge of the single-stage AC-DC converter. The single-stage AC-DC converter may comprise a primary-side bridge, the secondary-side bridge comprising a plurality of switches each configured to open and close in dependence on the control signal generated by the control system, the secondary-side bridge comprising a first output terminal and a second output terminal.

Description

TECHNICAL FIELD The present disclosure relates to a single-stage AC-DC convertor and a control scheme thereof. Aspects of the invention relate to a control system for controlling a single-stage AC-DC convertor of an on-board charger of a vehicle, to a single-stage AC-DC convertor of an on-board charger of a vehicle, to an AC-DC converter of an on-board charger of a vehicle for three-phase supply, and to a vehicle comprising the control system, the single-stage AC-DC convertor or the AC-DC converter, and to a method for controlling a single-stage AC-DC convertor of an on-board charger of a vehicle. BACKGROUND It is known to provide an on-board charger (OBC) as part of an electric vehicle such as a battery electric vehicle (BEV) or plug-in hybrid electrical vehicle (PHEV). The OBC typically includes an AC-DC convertor to convert an external AC power supply or source to DC power to charge a high voltage battery of the vehicle. Interest in single-stage AC-DC converters has been increasing rapidly in recent times, because of its capability to achieve high efficiency and high-power density, and to reduce use of bulky electrolytic capacitors. However, during single-phase and split-phase AC-DC conversion with power factor correction, in addition to the DC current output, a large second harmonic component may be present. Removing this second harmonic power is challenging and it may typically be overcome by utilising a large capacitor. However, the commonly used electrolytic capacitor is large in size, has a short expected life span and may cause reliability issues. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system for controlling a single-stage AC-DC converter of an onboard charger of a vehicle, a single-stage AC-DC converter, a three-phase AC-DC converter, a vehicle comprising the control system, the single-stage AC-DC converter or the three-phase AC-DC converter and to a method for controlling a single-stage AC-DC converter of an on-board charger of a vehicle. According to an aspect of the present invention, there is provided a control system for controlling a single-stage AC-DC convertor of an on-board charger of a vehicle, the control system comprising one or more processors collectively configured to: receive an input signal (160) indicative of an output current of the single-stage AC-DC convertor (200); generate, in dependence on the received input signal (160), a second harmonic component signal comprising a representation of a second harmonic of the output current; and generate, in dependence on the second harmonic component signal, a control signal (418) to compensate for the second harmonic of the output current. According to an aspect of the present invention, there is provided control system for controlling a single-stage AC-DC convertor of an on-board charger of a vehicle, the control system comprising one or more processors collectively configured to: receive an input signal indicative of an output current of the single-stage AC-DC convertor; generate, in dependence on the received input signal, a second harmonic component signal comprising a representation of a second harmonic of the output current; generate, in dependence on the second harmonic component signal, a control signal to compensate for the second harmonic of the output current; and output the generated control signal to a secondary-side bridge of the single-stage AC-DC convertor to control a switching of the secondary-side bridge of the single-stage AC-DC convertor. The control system provides feedback of second harmonic distortion present in the output current of the single-stage AC-DC converter. Secondary-side bridges of single-stage AC-DC converters comprise a plurality of switches, for example four switches, which are controlled by respective control signals. By modifying the control signals based on the second harmonic noise present in the output current, the noise may be substantially reduced. According to another aspect of the invention, generating the second harmonic component signal comprises: filtering the received input signal around the second harmonic of the output current. Filtering the received input signal around the second harmonic provides a signal including only the second harmonic output distortion with the fundamental output current and other harmonics removed or reduced. This allows a control signal to be generated, the control signal controlling the switching of the secondary-side bridge of the converter, which is focussed on reducing the second harmonic. The filtering may be achieved using a bandpass filter with a pass-band centred on the second harmonic frequency of the output current. The second harmonic frequency has twice the frequency of the fundamental component of the output current or input current. Generating the second harmonic component signal may further comprise determining the inverse of the second harmonic of the output current. Determining the inverse value or function of the second harmonic component of the output current may comprise comparing the second harmonic component signal to a reference signal, where the reference signal is a signal with an amplitude of zero. By subtracting the filtered input signal from 0, an inverse of the second harmonic component signal is generated. Other methods to determine an inverse may be used. Using an inverse of the second harmonic component signal to control the switching of the secondary-side bridge results in the switching of the bridge cancelling or reducing the second harmonic component. Generating the control signal to compensate for the second harmonic of the output current may comprise: providing the generated second harmonic component signal to a proportional resonator, PR, loop; and generating the control signal in dependence on an output of the PR loop. A PR loop or proportional resonator circuit may comprise a proportional loop and a resonant loop. The PR loop may configured to resonate at the frequency of the second harmonic to compensate for the second harmonic in the output current One or more of the processors may be collectively configured to: combine the generated control signal with a shifted carrier wave signal to generate a plurality of switching control signals to respectively open and close a plurality of switches of the secondary-side bridge; and output the plurality of switching control signals to the plurality of switches of the secondary-side bridge. According to another aspect of the invention, there is provided a single-stage AC-DC convertor of an on-board charger of a vehicle, the single-stage AC-DC convertor comprising: the control system; a primary-side bridge; the secondary-side bridge comprising a plurality of switches each configured to open and close in dependence on the control signal generated by the control system, the secondary-side bridge comprising a first output terminal and a second output terminal. The primary-side bridge comprises a plurality of primary switches controllable to open and close. The first output terminal may be a positive output terminal and the second output terminal may be a negative or neutral output terminal. The first output terminal and second output terminal act to provide the output DC current of the single-stage AC-DC converter. The current at the output terminals may be sensed and provided to the control system as the input signal, such that the control system can or is configured to receive the input signal indicative of an output current of the single-stage AC-DC convertor. The single-stage AC-DC convertor may comprise: a transformer comprising a primary winding coupled to the primary-side bridge and a secondary winding coupled to the secondary-side bridge; a capacitor coupled between a midpoint of secondary winding and the second output terminal of the secondary-side bridge. Providing a capacitor coupled between the mid-point of the secondary winding and the second output terminal allows the second bridge to provide both buck and boost operation, storing charge in the capacitor. This charge may alternatively be used to provide a current sink or current source to the output terminals of the single-stage AC-DC converter, allowing the second harmonic ripple in the output current to be significantly reduced. The single-stage AC-DC convertor may comprise a relay, the relay coupled between the midpoint of the secondary winding and the capacitor. The relay is configured or controllable by the control system to close or remain closed when the single-stage AC-DC converter is coupled to a single-phase power system or a split-phase power system. When the single-stage AC-DC converter is part of a larger converter system, such as a three-phase AC-DC converter, the relay may be configured to open or remain open when the AC-DC converter is coupled to a three-phase power system. The second harmonic of the output current is present when the power system is single or split-phase. Closing the relay allows the capacitor to source or sink current, and reduce the output current ripple at the second harmonic frequency. When the relay is open, the capacitor is not connected to the secondary winding of the transformer and does not perform this action. The single stage AC-DC converter may further comprise an input inductor configured to receive an input current. It may additionally or alternatively comprise an output capacitor coupled to the first output terminal and second output terminal of the second bridge, the capacitor configured to output the output DC current of the single-stage AC-DC converter. The capacitor may act to provide a stable DC output voltage. According to another aspect of the invention, a three-phase AC-DC converter of an on-board charger of a vehicle is provided, the three-phase AC-DC convertor comprising: a first single-stage AC-DC convertor; a second single-stage AC-DC convertor; a third single-stage AC-DC convertor. A system comprising thee single-stage AC-DC converters may act to convert single-phase, split-phase or three-phase power. When the input to the three-phase AC-DC converter is a single-phase or split-phase power system, the same two input phase connections (live and neutral for single-phase; first live and second live for split-phase) are connected to the input terminals of all three of the single-stage converters. When the input to the three-phase AC-DC converter is a three-phase power system, each of the phases is coupled to a different one of the three single-stage AC-DC converters. Including three single-stage converters in this manner provides greater flexibility, in that the converter may be used for a multitude of different power system inputs. The respective first output terminals of the first, second and third single-stage AC-DC convertors may be coupled together and the respective second output terminals of the first, second and third single-stage AC-DC converters may be coupled together. Coupling the output terminals together provides a single output current and voltage from the converter. According to another aspect of the invention, vehicle comprising the control system, or the single-stage AC-DC converter, or the three-phase AC-DC converter is provided. According to another aspect of the invention, a method for controlling a single-stage AC-DC convertor of an on-board charger of a vehicle is provided, the method comprising: receiving an input signal indicative of an output current of the single-stage AC-DC convertor; generating, in dependence on the received input signal a second harmonic component signal comprising a representation of the second harmonic of the output current; generating, in dependence on the second harmonic component signal, a control signal to compensate for a second harmonic of the output current; and outputting the generated control signal to a secondary-side bridge of the single-stage AC-DC convertor to control a switching of the secondary-side bridge of the single-stage AC-DC convertor. Generating the second harmonic component signal may comprise filtering the received input signal around the second harmonic of the output current. Generating the control signal may comprise providing the generated second harmonic component signal to a proportional resonator, PR, loop; and generating the control signal in dependence on an output of the PR loop. The method may further comprise combining the generated control signal with a shifted carrier wave signal to generate a plurality of switching control signals to respectively open and close a plurality of switches of the secondary-side bridge; and outputting the plurality of switching control signals to the plurality of switches of the secondary-side bridge. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a block diagram illustrating a control system according to an embodiment of the present invention; Figure 2 shows a block diagram illustrating a single-stage AC-DC convertor according to an embodiment of the present invention; Figure 3A shows a schematic diagram illustrating a single-stage AC-DC convertor according to an embodiment of the present invention; Figure 3B shows a schematic diagram illustrating the second bridge of a single-stage AC-DC convertor according to an embodiment of the present invention; Figures 4A-4B show block diagrams illustrating a control scheme of the secondary-side bridge of the single-stage AC-DC converter according to an embodiment of the present invention; Figure 5 shows an alternative block diagram illustrating the control scheme of Figure 4A according to an embodiment of the present invention; Figures 6A-6B show block diagrams illustrating a control scheme for generating a carrier wave and controlling the primary-side bridge of the single-stage AC-DC converter according to an embodiment of the present invention; Figure 7 shows an AC-DC converter comprising three single-stage AC-DC converters according to an embodiment of the present invention; Figures 8A-8B show charts illustrating the output current of a single-stage AC-DC convertor having second harmonic distortion in the output current; Figures 9A-9B show charts illustrating the output current of a single-stage AC-DC convertor having reduced second harmonic distortion in the output current according to an embodiment of the present invention; Figure 10A shows a flow chart illustrating a method according to an embodiment of the present invention; Figure 10B shows a flow chart illustrating a method of configuring the system according to an embodiment of the present invention; and Figure 11 shows a vehicle in accordance with an embodiment of the invention. DETAILED DESCRIPTION The present disclosure relates to a single-stage AC-DC convertor, such as a single-stage AC-DC convertor for use with an on-board charger (OBC) of a vehicle such as a battery electric or plug-in hybrid electrical vehicle, and to control schemes thereof. As is described above, single-stage AC-DC convertors may achieve high efficiency and high power density. Single-stage AC-DC convertors are therefore particularly beneficial in certain situations, such as the exemplified use in vehicles, where the single-stage AC-DC convertor may be provided as part of an OBC to convert AC electrical power from an electrical power source, such as an electricity grid, to DC electrical power to charge a high voltage battery of the vehicle. However, during single-phase and split-phase AC-DC conversion with power factor correction, in addition to the DC current output, a large second harmonic component may be present. The present disclosure relates to single-stage AC-DC convertor design and control to mitigate and reduce the second harmonic component in the output current. The single-stage AC-DC convertor comprises bridges, each comprising a plurality of switches which are opened and closed by control signals. The control signals provided to these switches may be modified in dependence on the second harmonic content of the output current. By modifying these control signals, the second harmonic content in the output may be reduced. A reduction in second harmonic content allows the vehicle and on-board charger to operate more efficiently. A single stage AC-DC converter may in some examples be composed of two interleaved totem-pole circuits which are switched at fixed 50% duty and fixed frequency resulting in a ripple-free sinusoidal grid current with very small filter inductors. The secondary bridge switches may be 180° phase-shifted and modulated using the signal ds(t)=0.5|sin(urgt)|, where ds(t) is the duty cycle of the secondary bridge and cog is the angular frequency of the input voltage of the single-stage AC-DC converter. The active power is controlled by the phase shift between the primary-side and secondary-side bridges. The control is simple and easy to implement since the phase-shift angle between the control signals of the primary-side and secondary-side bridges is the only control variable. The power equation is a non-linear function with phase shift and duty. A traditional single-stage AC-DC convertor may be controlled by a control scheme similar to one described as follows. The single-stage AC-DC converter may use 2-cascaded current control and voltage control loops which comprises two Proportional-Integral (PI) linear controllers to regulate the output voltage and the input current. In this example, a measured output voltage is compared to a reference output voltage, and the difference between these signals is given to the PI controller to generate a control signal to minimize the input difference. The difference between these signals may be considered to be an error voltage, or the difference between desired or expected output voltage and actual output voltage. A Phase-Locked Loop (PLL) is a control system used to synchronize the input current with the input / grid voltage. The PLL of this example may provide real-time angle data by taking the reference from the grid voltage, which is then used to generate a DQ transformation of the input current. Where the input is a three-phase input, the DQ transformation converts the 3-phase quantities (voltages or currents) from a stationary reference frame (abc) to a rotating reference frame (dq). The d-axis aligns with the direct component of the three-phase system, and the q-axis is perpendicular to the d-axis. This transformation simplifies the analysis and control of the system, making it easier to manage the rotating components. The measured input current may then be compared with the reference input current to generate a signal indicative of the difference between the measured input current and the reference input current. This signal may be provided to a current PI controller to generate a phase shift control signal for a carrier generator to subsequently generate a carrier and shifted carrier wave signal, which are used for generating pulse width modulation (PWM) signals for switches of bridges of the single-stage AC-DC convertor. The carrier wave signal is used to control the primary-side bridge. The shifted carrier wave signal is a carrier wave signal with a phase-shift compared to the carrier wave signal and is used to control the secondary-side bridge. There is a need to improve the control system to reduce the effect of second harmonic distortion in the output current. A control system 100 in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figure 1 and a single-stage AC-DC convertor 200 in accordance with an embodiment of the present invention is described herein with reference to accompanying Figure 2. As shown in Figure 11, the control system 100 and / or the single-stage AC-DC convertor 200 can be installed in a vehicle 900. With reference to Figure 1, there is illustrated a control system 100 for a single-stage AC-DC convertor, such as a single-stage AC-DC convertor of an on-board charger of a vehicle. The control system 100 comprises one or more controller 110. The control system 100 is for controlling a single-stage AC-DC convertor of a vehicle such as the single-stage AC-DC convertor 200 of Figure 2, which is described in detail below. The control system 100 as illustrated in Figure 1 comprises one controller 110, although it will be appreciated that this is merely illustrative. The controller 110 comprises processing means 120 and memory means 130. The processing means 120 may be one or more electronic processing device 120 which operably executes computer-readable instructions. The memory means 130 may be one or more memory devices 130. The memory means 130 is electrically coupled to the processing means 120. The memory means 130 is configured to store instructions, and the processing means 120 is configured to access the memory means 130 and execute the instructions stored thereon. The controller 110 comprises an input means 140 and an output means 150. The input means 140 may comprise an electrical input of the controller 110. The output means 150 may comprise an electrical output of the controller 110. The input means 140 is arranged to receive an input signal 160 indicative of an output current of the single-stage AC-DC convertor. The processing means 120 is configured to generate, in dependence on the received input signal 160, a second harmonic component signal comprising a representation of a second harmonic of the output current. The processing means 120 is further configured to generate, in dependence on the second harmonic component signal, a control signal to compensate for the second harmonic of the output current. The output means 150 is arranged to output the generated control signal 170 to the secondary side bridge of the single-stage AC-DC convertor to control a switching of the bridge of the single-stage AC-DC convertor. In this way, the control of the secondary bridge is modified in dependence on the second harmonic of the output current. Further, the processing means 120 may further be configured to combine the generated control signal 170 with a shifted carrier wave signal generated by a carrier generator, as will be described below, to generate a plurality of switching control signals, each switching control signal being configured to control a switch among a plurality of switches of the secondary-side bridge of the single-stage AC-DC convertor to open or close. The processing means 120, and / or the carrier generator, may be configured to generate the carrier wave signal and / or the shifted carrier wave signal in dependence on a phase shift control signal, to control a phase shift between a primary bridge and a secondary bridge, to thereby control the active power of the single-stage AC-DC convertor as explained above. The phase shift control signal may be determined in dependence on a comparison of circuit operating characteristics or may be received from a proportional integral, PI, controller. The generation of the carrier wave signal and the shifted carrier wave signal may be performed in a number of different ways. The switching control of the secondary bridge may then be modified to reduce the effects of second harmonic distortion. It should be understood that the control means 100 may be configured to perform each of the functions explained above where there are multiple single-stage AC-DC converters. In the example where the input current is single-phase current, but there are three single-stage converters, the control means 100 may be arranged to perform each of the functions explained above for each of the single-stage converters respectively. For example, in a single-stage AC-DC convertor adapted for three-phase power supply, there may be provided three parallel single-stage AC-DC convertors such as those shown in Figure 2, each having a respective bridge to be controlled as described above. Where the input is as single-phase, it may be provided to each of the three parallel single-stage AC-DC converters, with each of the converters controlled to remove the second harmonic. Where the input is a three-phase current, the second harmonic is not present in the output current, and therefore the control modification outlined here is not needed. The same control signal 170 may be provided to each of the three parallel single-stage AC-DC converters. Figure 2 illustrates a single-stage AC-DC convertor 200 according to an embodiment of the invention. As will be explained, the single-stage AC-DC convertor 200 of Figure 2 may operate under the control of the control system 100 of Figure 1. The control system 100 may form part of the single-stage AC-DC convertor 200 of Figure 2, or may be external to the single-stage AC-DC convertor 200 and configured to communicate with the single-stage AC-DC convertor 200. As will be explained, the control system 100 of Figure 1 may be considered to include, form part of, or communicate with one or more of the primaryside bridge controller 280 or the secondary-side bridge controller 290. The single-stage AC-DC convertor 200 of Figure 2 comprises an AC input 210, an input inductor 220, a primary-side or first bridge 230, a primary-side or first bridge controller 280, a transformer 240, a secondary-side or second bridge 250, a secondary-side or second bridge controller 290, an output capacitor 260, and a DC output 270. The AC input 210 is configured to receive an input current. The input current is an alternating current. In some examples, the input current may be a current input from an electricity grid, or from a part of an on-board charger of a vehicle connected to the electricity grid. The input may be single-phase, split-phase or three-phase. To allow operation with both single-phase and three-phase inputs, three single-stage AC-DC converters as shown in Figure 2 may be provided. If the input is a three-phase input, there may be provided a single-stage AC-DC convertor 200 for each phase of the input power, configured to receive the different phases of the input current, and to output DC power to the same DC output. If the input is single-phase or splitphase, then the same phase may be coupled to each of the single-stage AC-DC converters. The input inductor 220 comprises one or more inductors. The input inductor 220 is electrically connected to the AC input 210 and is configured to receive AC electrical power from the AC input 210, and is further electrically connected to the primaryside bridge 230 and configured to provide electrical power to the primary-side bridge 230. The input inductor may be any suitable type of inductor, such as an iron core inductor, and powder core inductor or a ferrite core inductor. The input inductor 220 is used to help shape the input current waveform and improve the power factor of the system. This helps to ensure that the power drawn from the electrical load is more sinusoidal and in phase with the input voltage. It also helps improve the input grid harmonics (total harmonic distortion - THD). A powdered core may be used to achieve a wide range of DC bias inductance. The primary-side bridge 230 comprises a bridge connected to a primary terminal or primary side of the transformer 240 or an AC side input, and to the input inductor 220. The primary-side bridge 230 may comprise a plurality of switches operable to open or close in dependence on the receipt of a control signal based on a carrier wave signal (as described previously). The primary-side bridge 230 may include any number of switches, but in some examples includes 4,6 or 8 switches. The primaryside bridge may include a number of diodes in place of some switches, or multiple switches in parallel in place of each switch to increase the current rating of the system. Each of the switches of the primary-side bridge 230 may open or close independently or at different times in dependence on control signals received from the primary-side bridge controller 280. The primary-side bridge 230 may operate under the control of the primary-side bridge controller 280. The primary-side bridge controller 280 may be configured to generate and output a control signal or a plurality of control signals to the primary-side bridge and the switches that the primary-side bridge 230 comprises. This control signal may be generated in a manner that would be understood by the person skilled in the art based on the traditional control scheme of a single-stage AC-DC convertor explained above. For example, based on a carrier wave signal, the primary-side bridge controller 280 may control the primary side bridge 230 to switch at fixed 50% duty and fixed frequency. The primary-side bridge 230 may be controlled in a number of different ways based on the carrier wave signal. The transformer 240 comprises any suitable type of transformer. The transformer receives an AC input at a primary side, terminal or winding and provides an AC output at a secondary side, terminal or winding. The primary side or winding of the transformer 240 is coupled to the primary-side bridge 230. The secondary side or winding of the transformer 240 is coupled to the secondary bridge 250. The transformer 240 may act to modify the voltage provided at the secondary winding relative to the voltage received at the primary winding. For example, the transformer 240 may step-down or step-up the voltage. The primary and secondary windings may each comprise a number of turns, with the number of turns selected based on a desired voltage transformation. The transformer 240 further provides isolation between the primary bridge 230 and the secondary bridge 250. The secondary-side bridge 250 comprises a bridge electrically connected to a secondary terminal or secondary side of the transformer 240 and to the output capacitor 260. Similar to the primary-side bridge, the secondary-side bridge comprises a plurality of electrical switches each independently configured to open or close in dependence on receipt of a control signal, or a switching control signal, from the secondary-side bridge controller 290. The secondary-side bridge 250 may also comprise any number of switches, but in some examples comprises 4 switches. The secondary-side bridge controller 290 is configured to generate and output control signals to the secondary-side bridge 250 to control the switching of the secondary-side bridge 250 based on a shifted carrier wave signal. The shifted carrier wave signal is a phase-shifted version of the carrier wave signal used to control the primary side bridge 230, with the phase shift chosen as described previously. The phase shift may be chosen or controlled to provide the power factor correction and output voltage control. So as to reduce the effects of a second harmonic in the output current, the secondary-side bridge controller 290 is configured to generate and output the control signals to the secondary-side bridge 250 in dependence on the second harmonic content of an output current of the single-stage AC-DC converter 200. The modulation or control of the secondarybridge switches is therefore modified in dependence on the second harmonic content to compensate for the second harmonic content. The secondary-side bridge controller 290 may generate the control signal to compensate for the second harmonic of the output current, to reduce the output noise current of the single-stage AC-DC convertor 200. It should be understood that the secondary-side bridge controller 290 may operate in the same way as the control system 100 of Figure 1, explained above, to generate and output the control signal to the secondary-side bridge 250. The output capacitor 260 comprises one or more capacitors. The output capacitor 260 is electrically connected between the secondary-side bridge 250 and the DC output 270. The output capacitor 260 may be configured to smooth and / or filter a rectified DC voltage. The DC output 270 comprises an electrical output and is configured to output a DC signal converted by the single-stage AC-DC convertor 200 from the AC input 210. The DC output 270 is electrically connected to the output capacitor 260 and may further be electrically connected to one or more electrical loads. The one or more electrical loads may be electrical loads of a vehicle, and may in some examples comprise a high voltage battery or an electrical bus of the vehicle. The DC output 270 may output the converted electrical power in dependence on a requirement of the one or more electrical loads. It should be understood that one or more components of the single-stage AC-DC convertor 200 illustrated in Figure 2 may be omitted. For example, the primary-side bridge controller 280 and the secondary-side bridge controller 290 may be provided as a single controller in an example. In an example, the control system 100 of Figure 1 may implement the function of the primary-side bridge controller 280 and / or the secondary-side bridge controller 290. It should be understood that either or both of the primary-side bridge controller 280 and the secondary-side bridge controller 290 may be implemented as a single module or as a plurality of modules, and may each comprise multiple controllers or processors. Figure 3A illustrates a schematic diagram of the single-stage AC-DC convertor 200 according to an embodiment of the invention. The single stage AC-DC converter of Figure 3A includes more detail of an implementation of the circuit elements of Figure 2. As shown in Figure 3A, the AC input 210 comprises a first input terminal 310 and a second input terminal 320. The first input terminal 310 may be coupled to a positive or line voltage of a single-phase power system and the second input terminal 320 may be coupled to a neutral voltage of the single-phase power system. Where the power system is a split-phase power system, the first input terminal 310 may be coupled to a first phase voltage and the second input terminal 320 may be coupled to a second phase voltage. Where the power system is a three-phase power system, the first terminal 310 may be coupled to one of the phases of the three-phase power system (one of the first, second or third phase) and the second input terminal 320 may be coupled to neutral. The AC input 210 is coupled to the input inductors 220. As shown in Figure 3, the input inductors 220 may comprise two inductors, a first input inductor Lg 1 and a second input inductor Lg2. The first input terminal 310 is coupled to the primary side bridge 230 via the input inductors 220 and the second input terminal is coupled to the primary side bridge 230. The primary side bridge 230 includes four switches or switching devices S1-S4. The switches may be MOSFET devices, or any other suitable power transistor. The primary side bridge 230 further comprises a first diode D1 and a second diode D2. The first diode D1 and the second diode D2 may instead be replaced by two furthers switches or switching devices. Each of the switches S1-S4 may be replaced by a plurality of switches connected in parallel to increase the current rating of the bridge. The primary bridge 230 is coupled to the primary side of the transformer 240. The secondary side of the transformer 240 is coupled to the secondary side bridge 250. The secondary side of the transformer 240 comprises a second winding with a midpoint in the second winding. As such, the second or secondary winding may be considered to comprise two windings - a first winding 330 and a second winding 340 - connected or coupled at a midpoint. The secondary side bridge 250 comprises four switches or switching devices S5-S8. Each of the switches S5-S8 may be replaced by a plurality of switches connected in parallel to increase the current rating of the bridge. The secondary side bridge 250 further comprises a first relay 350 and a first capacitor 360. The first relay 350 is coupled between the midpoint of the secondary winding of the transformer 240 and a first terminal of the first capacitor 360. The second terminal of the first capacitor 360 is coupled to the DC output 270. The DC output 370 comprises a first (positive) output terminal 370 and a second (negative or neutral) output terminal 380. The second terminal of the first capacitor 360 is coupled to the second (negative or neutral) output terminal 380. As such, the midpoint of the secondary winding of the transformer 240 is coupled to the negative or neutral output terminal 380 by the first relay 350 and the first capacitor 360. The first relay 350 may be any suitable power relay or switch. Figure 3B shows a subsection of the schematic diagram of Figure 3A, including the connection of the secondary-side bridge 250 of the AC-DC converter 200. As is shown in Figure 3B, the output current 390 of the single-stage AC-DC converter 200 is provided to or at the first output terminal 370. The control system 100 may receive the input 160 indicative of the current at the first output terminal 370 by sensing the first output terminal 370 using any suitable means, such as a current shunt. The relay 350 may be configured to close (connecting the midpoint of the secondary side of the transformer 240 to the capacitor 360) or open (disconnecting the midpoint of the secondary side of the transformer 240 to the capacitor 360). The relay 350 may be controllable by a control signal 392. The control signal may be generated or provided by the controller 100. The controller 100 may receive an input signal indicative of the power-system type coupled to the input 210 of the AC-DC converter (single-phase, split-phase or three-phase) and output the control signal 392 in dependence on the power-system type. Where the AC input 210 is a single-phase or split-phase system, the control system 100 may output a control signal 392 instructing the relay 350 to close or remain closed. Where the AC input 210 is a three-phase system (or a phase of a three-phase system) the control system 100 may output a control signal 392 instructing the relay 350 to be opened or remain open. By closing the relay, the secondary side bridge 250 is configured to act as a buck-boost converter, storing and providing charge from the capacitor 360 to reduce or remove the second harmonic distortion in the output current at output terminal 370. When the relay is open, the secondary side bridge 250 may act in a traditional manner. Where the input is a three-phase input, and each phase is provided to a respective one of three AC-DC converters 200 with the output terminals coupled, the second harmonic distortion is not present. As such, the relay 350 may remain open where the input is a three-phase input. The first relay 350 may be removed and instead a direct connection may be provided between the mid-point of the secondary winding of the transformer 240 and the first capacitor 360. This allows the system to compensate for the second harmonic where the input is a single-phase or split-phase. Figure 4A and 4B illustrates a control scheme 400 for controlling switching of the secondary-side bridge 250 of the single-stage AC-DC converter 200 according to an embodiment of the present invention. Figure 4A illustrates a first scheme 410 for generating a control signal 418. Figure 4B illustrates a second scheme 420 for controlling switching of a second bridge of the single-stage AC-DC converter based on the control signal 418. It should be understood that although they are illustrated separately, the first scheme 410 and the second scheme 420 operate together, with the output of the first scheme 410 provided as an input to the second scheme 420. The control schemes 400 may be performed by the control system 100 according to Figure 1 or the secondary bridge controller 290 according to Figure 2. In the first scheme 410, an input signal 390 indicative or representative of the output current of the single-stage AC-DC converter 200 is received by or provided to the input of a filter 412. The filter 412 acts to generate, in dependence on the received input signal, a second harmonic component signal comprising a representation of a second harmonic of the output current. The filter 412 may act or be configured to filter the received input signal around the second harmonic of the output current. Filtering the received input signal around the second harmonic provides a signal including only the second harmonic output distortion, with the fundamental output current and other harmonics removed. The filter 412 may comprise a band-pass filter with a pass-band centred on the second harmonic frequency of the output current, however other filter types may be used. The second harmonic frequency is a frequency twice that of the fundamental component of the frequency of the input current. The second harmonic component signal is output by the filter 412 and provided to an inverse circuit or system 414. The inverse circuit 414 acts to determine the inverse of the second harmonic component of the output current. Figure 5 shows one way in which the inverse system 414 may be implemented. As shown in Figure 5, the inverse system 414 may comprise a comparator or summing block 414. The comparator 414 receives the second harmonic component signal output by the filter 412 and a reference signal 413. The reference signal may be, for example, a ground reference or other reference, such as a reference with an amplitude of zero. The comparator 414 subtracts the second harmonic component signal from the reference signal. By subtracting the filtered input signal from the reference signal, an inverse of the second harmonic component signal is generated. Other methods to determine an inverse maybe used. Using an inverse of the second harmonic component signal to control the switching of the bridge results in the switching of the bridge cancelling or reducing the second harmonic component, rather than adding to the second harmonic component. The output of the inverse circuit 414 is provided to and received at the input of a PR loop 416. A PR loop (a proportional resonator loop or a proportional resonator circuit) comprises a proportional and a resonant section. The PR loop 416 is configured to resonate at a specific frequency. The PR loop is configured to resonate at the frequency of the second harmonic of the output current 390. The PR loop 416 generates a control signal 418 to compensate for the second harmonic of the output current. This control signal may be outputted to the secondary-bridge 250 of the single-stage AC-DC converter to control the switching of the secondary bridge of the single-stage AC-DC converter. The control signal may be provided to the second control scheme 420. The control signal 418 is provided to an input of a summing block or comparator 422. The comparator 422 adds the control signal 418 to a fixed duty cycle reference signal 421. In some examples, the fixed duty cycle reference 421 is 50%, but it should be understood that other duty cycles are within the scope of the disclosure. By adding the control signal 418 to the duty cycle reference signal 421, the duty cycle reference signal 421 is modified to take into account and reduce the second harmonic content of the output current. An output of the comparator 422 is provided to the inverting inputs of a first comparator 424 and a second comparator 426. A shifted carrier wave signal 428 is provided to the non-inverting input of the first comparator 424. An inverted shifted carrier wave signal 432 that is 180° out of phase with the shifted carrier wave signal 428 is provided to the non-inverting input of the second comparator 426. The signal 432 may be generated by providing the shifted carrier wave signal 428 to a NOT logic gate or inverter 430, with the output of the logic cate 430 provided as the input to the non-inverting input of the second comparator 426. In this way, the first comparator and second comparator receive modified versions of the shift carrier wave signal 428 which are 180° out of phase with each other. The comparators 424, 426 generate respective control signals each providing control of a respective one of the switches of the secondary bridge 250. The first comparator 424 generates a first control signal 434 to control the fifth switch S5 of the second bridge 250. An inverted version of the first control signal 434 can be generated using a NOT logic gate 434 or other suitable means, generating a second control signal 438 to control the sixth switch S6 of the second bridge 250. The second comparator generates a third control signal 440 to control the seventh switch S7 of the second bridge 250. An inverted version of the third control signal 440 can be generated using a NOT logic gate 442 or other suitable means, generating a fourth control signal 444 to control the eighth switch S8 of the second bridge 250. As such, a control signal is generated for each of switch S5, switch S6, switch S7 and switch S8 of the secondary bridge 250. The shifted carrier signal 428 is a signal comprising a carrier wave at a carrier frequency. The carrier frequency may be chosen in dependence on the operating characteristics of the single-stage AC-DC converter, as would be understood by the person skilled in the art. Figure 6A illustrates a control scheme 600 which may be used to generate the shifted carrier signal 428 used in the control scheme 420 to control the secondary-side bridge 250 and a carrier wave signal 622 which may be used to control the switching of the primary-side bridge 230 of the single-stage AC-DC converter 200. Whilst the shifted carrier signal 428 may be generated in the manner shown in Figure 6A, other methods to generate a shifted carrier signal may be used. The modulation of the shifted carrier wave supplied to the switches of the secondary bridge may be used to reduce the effects of the second harmonic content of the output current using the control scheme of Figure 4. An output voltage 604 of the single-stage AC-DC convertor 200 and a reference voltage 606 are compared using a comparator or summing circuit 602 and a difference 608 between the two voltages is provided to a first proportional integral, PI, controller 610. The difference signal 608 may represent an error between the actual output voltage 604 of the single-stage AC-DC converter and the reference voltage 606, where the reference voltage 606 is representative of a desired output voltage of the single-stage AC-DC converter 200. The first PI controller 610 may be a PI linear controller which is configured to compare the output voltage and the reference voltage and to generate a reference frequency signal. The first PI controller 610 then outputs the reference frequency signal. The first PI controller 610 may be known as a voltage PI controller. The output reference frequency signal is then compared with a measured input current 614 of the single-stage AC-DC convertor 200 and the difference 616 is provided to a second PI controller 618. In some examples, this difference 616 may be referred to as an error signal, and may be indicative of a difference between the measured input current and the reference frequency signal. In one example, the measured input current is obtained by a phase-locked loop, PLL, providing real-time angle data by taking a reference from the grid voltage and using the angle data to generate a DQ transformation of the input current, as described above. The second PI controller 618 is configured to generate a phase shift control signal in dependence on the comparison 616 of the measured input current and the reference frequency signal, and to output the phase shift control signal to a carrier wave generator 620. The phase shift control signal may be indicative of a target phase shift between the primary-side and secondary-side bridges 230,250, which as explained above, may be used to control the active power of the single-stage AC-DC convertor 200. The carrier wave generator 620 is configured to generate a carrier wave signal 622 and a shifted carrier wave signal 428, The carrier wave signal 622 and the shifted carrier wave signal 428 are used to control the switching of the primary-side bridge 230 and the secondary-side bridge of the single-stage AC-DC convertor 200 respectively. The shifted carrier wave signal 428 may be used in the previously described control scheme 400, in combination with a determination of the second harmonic content of the output current 390 and the capacitor 360, to reduce the effects of second harmonic output distortion. Figure 6B shows a scheme for generating the control signals for the primary side bridge 230 of the single-stage AC-DC converter 200. These control signals are generated in a similar manner to those of the secondary side bridge 250, however no representation of the second harmonic of the output current is used. A duty cycle reference signal 632 is provided to the inverting inputs of a first comparator 634 and second comparator 636. As described previously, the duty cycle reference signal may be a signal with a duty cycle of 50%, however any suitable duty cycle may be used. The carrier wave signal 622 is provided to the non-inverting input of the first comparator 634, and an inverted carrier wave signal that is 180° out of phase with the carrier wave signal 622 is provided to the non-inverting input of the second comparator 636. The inverted carrier wave signal may be generated using a NOT logic gate 638 or inverter. The comparators 634, 636 generate respective control signals each providing control of a respective one of the switches of the primary bridge 230. The first comparator 634 generates a first control signal 640 to control the first switch S1 of the primaryside bridge 230. An inverted version of the first control signal 640 can be generated using a NOT logic gate 642 or other suitable means, generating a second control signal 644 to control the second switch S2 of the primary-side bridge 230. The second comparator 636 generates a third control signal 646 to control the third switch S3 of the primary-side bridge 230. An inverted version of the third control signal 646 can be generated using a NOT logic gate 648 or other suitable means, generating a fourth control signal 650 to control the fourth switch S4 of the primary-side bridge 230. As such, a control signal is generated for each of switch S1, switch S2, switch S3 and switch S4 of the primary-side bridge 230. Figure 7 is a schematic diagram of an AC-DC converter 700 suitable for converting any of single-phase, split-phase or three-phase inputs. The AC-DC converter comprises a first single-stage AC-DC converter 702, a second single-stage AC-DC converter 704 and a third single-stage AC-DC converter 706. Each of the single-stage AC-DC converters 702-706 may represent one of the single-stage AC-DC converters 200 described previously, with each of the single-stage AC-DC converters 702-706 comprising a first input terminal 310, a second input terminal 320, a first output terminal 370 and a second output terminal 380. The AC-DC converter comprises a first input node 708, a second input node 710, a third input node 712 and a fourth input node 714. The first input node 708 may be referred to as a first live input node, the second input node 710 may be referred to as a neutral input node, the third input node 712 may be referred to as a second live input node and the fourth input node 714 may be referred to as a third live input node. As well as the respective first relays 350 in each of the single-stage AC-DC converters 702-704, the AC-DC converter 700 may comprise a second relay 716 and a third relay 718 coupling the first input terminals 310 of each of the converters 702- 704. Where the input power system is a single-phase system or a split-phase system, the first live supply of the single or splitphase system may be coupled to the first live input node 708 and the neutral supply of the single-phase system (or the second live supply of the split-phase system) may be coupled to the neutral input node 710. Closing the second relay 716 and the third relay 718 results in the same single-phase or split-phase input being provided to all three of the AC-DC converters 702-706. The control system 100 may determine the power system input type and, where the supply input is single-phase or split phase, close the second relay 716 and the third relay 718. This allows all three AC-DC converters 702-706 to be used in parallel, allowing each of the converters to be specified to a lower current input than if only a single converter was used. As noted previously, where the input is single-phase or split-phase, the control system may further close the first relay 350 included in each of the respective single-stage AC-DC converters 702-706 to allow the system to reduce the impact of the second harmonic. Where the input power system is a three-phase system, the second relay 716 and third relay 718 may be configured by the control system to remain open or be opened. The first live of the three-phase system is coupled to the first input node 708, the neutral is coupled to the neutral input node 710, the second live is coupled to the second live input node 712 and the third live is coupled to the third live input node 714. The respective first output terminals 370 are coupled to one another and the respective second output terminals 380 are coupled to one another. This provides a first output node 720 and a second output node 722 configured to provide a DC output. By providing three parallel single-stage AC-DC converters 702-706, the system 700 may convert any of single-phase, split-phase or three-phase power to DC whilst also reducing the impact of second harmonic distortion in the output current. Figures 8A-8B illustrate the output current of a single-stage AC-DC convertor having high second order distortion (for example a conventional single-stage AC-DC converter). Figures 9A-9B illustrate the output current of a single-stage AC-DC convertor 200 having reduced second order distortion according to an embodiment of the present invention. As can be seen in the output current of Figure 8A, the second harmonic distortion results in a large current ripple at twice the fundamental frequency of the input current. Figure 8B shows a frequency response or Fourier transform of the current ripple present in the output current 390, showing a large current spike 810 at twice the fundamental frequency. In Figures 9A and 9B, the current ripple is significantly reduced, with the second harmonic content represented by the spike 910 in the frequency response of Figure 9B being significantly reduced (note that Figure 9B shows the current in milliamps, whereas figure 8B shows the current in amps) by the use of capacitor 360 and the control schemes 400. The control of the second harmonic component in the output current may be arranged as insufficient to reduce that component to such a degree that it is no longer detectable by the comparator 422 in the control signal 418. If that were the case then control of the second harmonic would potentially undesirably oscillate between minimum and maximum values as the harmonic periodically appeared and disappeared at the output. Figure 10A illustrates a method 1000 according to an embodiment of the present invention. The method 1000 of Figure 10A may be performed by any of the control system 100 of Figure 1, the single-stage AC-DC convertor 200 of Figure 2, the control schemes of Figures 4A and 4B, or the AC-DC convertor 500 of Figure 7. The method 1000 includes receiving, at step 1010, an input signal indicative of an output current 390 of the single-stage AC-DC convertor 200. At step 1020, the method 1000 further comprises generating, in dependence on the received input signal, a second harmonic component signal comprising a representation of a second harmonic of the output current 390. In some examples, generating a second harmonic component signal comprises filtering the received input signal around the second harmonic of the output current. The second harmonic of the output current may be a frequency equal to twice that of the frequency of the input current. The filtering may be achieved using a bandpass filter with a pass-band centred on the second harmonic frequency of the output current. Generating the second harmonic component signal may further comprise determining the inverse of the second harmonic of the output current. At step 1030, the method 1000 further comprises generating, in dependence on the second harmonic component signal, a control signal to compensate for the second harmonic of the output current. Generating the control signal may comprise providing the generated second harmonic component signal to a proportional resonator, PR, loop and generating the control signal in dependence on an output of the PR loop. The method 1000 further comprises outputting, at step 1040, the generated control signal to a secondary-side bridge of the single-stage AC-DC convertor to control a switching of the secondary-side bridge of the single-stage AC-DC convertor. Outputting the generated control signal may comprise combining the generated control signal with a shifted carrier wave signal to generate a plurality of switching control signals to respectively open and close a plurality of switches of the secondary-side bridge 250 and outputting the plurality of switching control signals to the plurality of switches of the secondary side bridge 250. Figure 10B illustrates a method 1050 according to an embodiment of the present invention. The method 1050 of Figure 10B may be performed by any of the control system 100 of Figure 1, the single-stage AC-DC convertor 200 of Figure 2, the control schemes of Figures 4A and 4B, or the AC-DC convertor 500 of Figure 7. The method 1050 comprises determining, at step 1060, the input power-system type coupled to the input terminals of the single-stage AC-DC converter. Determining the input power-system type may comprise receiving a signal indicative of the power system type (whether the power-system is single-phase, split-phase or three-phase) or sensing the input terminals to determine the input power-system type. At step 1070, the method 1050 comprises configuring, in dependence on the determined input power-system type, the first relay 350, and the second and third relays 716, 718, to open or close, as described above. Figure 11 illustrates a vehicle 1100 according to an embodiment of the present invention. The vehicle 1100 may comprise at least one of the control system 100 of Figure 1, the single-stage AC-DC convertor 200 of Figure 2 or the three-phase single-stage AC-DC convertor 700 of Figure 7. In some examples, the vehicle 1100 is a battery electric vehicle or a plug-in hybrid vehicle, and further comprises an on-board charger configured to charge a battery of the vehicle. The control schemes described above reduce the second harmonic present in the output current of the single-stage AC-DC converter. Where the input voltages are of the single-stage AC-DC converter are imbalanced or harmonics are present, there may be an increased total harmonic distortion (THD) of the input grid current. The control schemes described above may be modified to further reduce the THD of the input grid current by reducing the odd harmonics present in the input current as well as to reduce the second harmonic content of the output current. This may be achieved by modifying the duty cycle signal 421 provided to the second bridge control scheme. The duty cycle signal 421 may be modified to compensate for one or more harmonics of the input current based on a comparison of the input signal and a reference frequency signal, to reduce a THD of the single-stage AC-DC convertor 200. The modified duty cycle signal 421 may be modified using one or more PR loops. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A control system for controlling a single-stage AC-DC convertor of an on-board charger of a vehicle, the control system comprising one or more processors collectively configured to:receive an input signal indicative of an output current of the single-stage AC-DC convertor;generate, in dependence on the received input signal, a second harmonic component signal comprising a representation of a second harmonic of the output current;generate, in dependence on the second harmonic component signal, a control signal to compensate for the second harmonic of the output current; andoutput the generated control signal to a secondary-side bridge of the single-stage AC-DC convertor to control a switching of the secondary-side bridge of the single-stage AC-DC convertor.

2. The control system according claim 1, wherein generating the second harmonic component signal comprises: filtering the received input signal around the second harmonic of the output current.

3. The control system according to claim 1 or 2, wherein generating the second harmonic component signal further comprises determining the inverse of the second harmonic of the output current.

4. The control system of any preceding claim, wherein generating the control signal to compensate for the second harmonic of the output current comprises:providing the generated second harmonic component signal to a proportional resonator, PR, loop; and generating the control signal in dependence on an output of the PR loop.

5. The control system of any preceding claim, wherein the one or more processors are collectively configured to: combine the generated control signal with a shifted carrier wave signal to generate a plurality of switching control signals to respectively open and close a plurality of switches of the secondary-side bridge; andoutput the plurality of switching control signals to the plurality of switches of the secondary-side bridge.

6. A single-stage AC-DC convertor of an on-board charger of a vehicle, the single-stage AC-DC convertor comprising:the control system of any preceding claim;a primary-side bridge;the secondary-side bridge comprising a plurality of switches each configured to open and close in dependence on the control signal generated by the control system, the secondary-side bridge comprising a first output terminal and a second output terminal.

7. The single-stage AC-DC convertor according to claim 6, the single-stage AC-DC convertor comprising:a transformer comprising a primary winding coupled to the primary-side bridge and a secondary winding coupled to the secondary-side bridge;a capacitor coupled between a midpoint of secondary winding and the second output terminal of the secondaryside bridge.

8. The single-stage AC-DC convertor according to claim 7, the single-stage AC-DC convertor comprising: a relay, the relay coupled between the midpoint of the secondary winding and the capacitor.

9. An AC-DC converter of an on-board charger of a vehicle, the AC-DC convertor comprising:a first single-stage AC-DC convertor according to any of claims 6-8;a second single-stage AC-DC convertor according to any of claims 6-8;a third single-stage AC-DC convertor according to any of claims 6-8.

10. The AC-DC converter of an on-board charger of a vehicle as claimed in claim 9, wherein the respective first output terminals of the first, second and third single-stage AC-DC convertors are coupled together and the respective second output terminals of the first, second and third single-stage AC-DC converters are coupled together, whereinwhen the AC-DC converter has a three-phase supply, a respective live terminal of the three-phase supply is connected to a live terminal of the first, second and third single-stage AC-DC convertors and a neutral terminal of the three-phase supply is connected to a neutral terminal of the first, second and third single-stage AC-DC convertors;when the AC-DC converter has a single-phase supply, a live terminal of the single-phase supply is connected to each live terminal of the first, second and third single-stage AC-DC convertors and a neutral terminal of the single-phase supply is connected to the neutral terminal of the first, second and third single-stage AC-DC convertors; andwhen the AC-DC converter has a split-phase supply, a first live terminal of the split-phase supply is connected to a live terminal of the first, second and third single-stage AC-DC convertors and a second live terminal of the split-phase supply is connected to a neutral terminal of the first, second and third single-stage AC-DC convertors.

11. A vehicle comprising the control system according to any of claims 1 -5, or the single-stage AC-DC converter according to any of claims 6-8, or the AC-DC converter according to any of claims 9-10.

12. A method for controlling a single-stage AC-DC convertor of an on-board charger of a vehicle, the method comprising:receiving an input signal indicative of an output current of the single-stage AC-DC convertor;generating, in dependence on the received input signal a second harmonic component signal comprising a representation of the second harmonic of the output current;generating, in dependence on the second harmonic component signal, a control signal to compensate for a second harmonic of the output current; andoutputting the generated control signal to a secondary-side bridge of the single-stage AC-DC convertor to control a switching of the secondary-side bridge of the single-stage AC-DC convertor.

13. The method according to claim 12, wherein generating the second harmonic component signal comprises filtering the received input signal around the second harmonic of the output current.

14. The method according to claim 12 or claim 13, wherein generating the control signal comprises: providing the generated second harmonic component signal to a proportional resonator, PR, loop; and generating the control signal in dependence on an output of the PR loop.

15. The method according to any of claims 12-14, the method further comprising:combining the generated control signal with a shifted carrier wave signal to generate a plurality of switching control signals to respectively open and close a plurality of switches of the secondary-side bridge; andoutputting the plurality of switching control signals to the plurality of switches of the secondary-side bridge.21

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