Single-stage AC-DC convertor and a control scheme thereof

Abstract

A single-stage AC-DC converter 200 of an on-board charger of a vehicle has a control system comprising one or more processors to receive an input signal indicative of an input current of the converter. The received input signal is compared to a reference frequency signal and a control signal is generated, in dependence on the comparison, to compensate for at least one harmonic of the reference frequency signal within the input current. The generated control signal is output to control switching of a bridge of the single-stage AC-DC converter. The single-stage AC-DC converter may comprise an input inductor 220 configured to receive the input current, a primary bridge 230 comprising a plurality of primary switches controlled by a primary bridge control signal, a transformer 240, a secondary bridge 250 comprising a plurality of secondary switches controlled by the control signal generated by the control system, and an output capacitor configured to output an output DC current.

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 a vehicle comprising the control system or the single-stage AC-DC convertor, and to a method for controlling a control system 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 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, due to voltage imbalances or the presence of harmonics in a power supply to the single-stage AC-DC convertor such as a grid power supply, a total harmonic distortion (THD) may increase in the input grid current. 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 convertor of an on-board charger of a vehicle, a single-stage AC-DC convertor of an on-board charger of a vehicle, a vehicle comprising the control system or the single-stage AC-DC convertor, and a method for controlling a control system for controlling a single-stage AC-DC convertor of an on-board charger of a vehicle as claimed in the appended claims. 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 indicative of an input current of the single-stage AC-DC convertor; compare the received input signal to a reference frequency signal; generate, in dependence on the comparison, a control signal to compensate for at least one harmonic of the reference frequency signal within the input current; and output the generated control signal to a bridge of the single-stage AC-DC convertor to control a switching of the bridge of the single-stage AC-DC convertor. Advantageously, a total harmonic distortion, THD, of the single-stage AC-DC convertor arising due to the presence of harmonics or grid voltage imbalance can be reduced by generating and outputting the control signal to compensate for the at least one harmonic of the reference frequency signal within the input current. That is, at least one harmonic associated with a frequency signal used as a reference for the input current can be compensated for, thus reducing distortion arising due to the presence of the harmonic within the input current. The control system comprises one or more controllers collectively comprising at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to: receive an input signal indicative of an input current of the single-stage AC-DC convertor; compare the received input signal to a reference frequency signal; generate, in dependence on the comparison, a control signal to compensate for at least one harmonic of the reference frequency signal within the input current; and output the generated control signal to a bridge of the single-stage AC-DC convertor to control a switching of the bridge of the single-stage AC-DC convertor. In some examples, the bridge is a secondary bridge of the AC-DC convertor. The secondary bridge may comprise a bridge electrically connected between a transformer of the AC-DC convertor and a DC output of the AC-DC convertor. Control of an active power of the AC-DC convertor may be based on a phase shift between a primary bridge and the secondary bridge, and the active power control may still be performed while reducing the THD of the AC-DC convertor. In some examples, a frequency signal of the input signal is compared to the reference frequency signal. Harmonics may be present at integer multiples of the frequency of the input signal, and thus by comparing a frequency signal of the input signal to the reference frequency signal, a difference between the input signal and the reference signal at the frequencies of one or more harmonics may be identified. This difference may be used to compensate for the identified harmonics. In some examples, the input signal includes an angle generated by a Phase-Locked Loop, PLL, circuit at a particular instant. The input signal may be monitored and harmonics may be compensated for dynamically during operation of the AC-DC convertor. In some examples, the angle comprises an x-phase angle generated by the PLL circuit, wherein x-phase comprises one or more of R-phase, Y-phase or B-phase. In some examples, the one or more processors are collectively configured to: generate a first signal in dependence on the comparison of the received input signal to the reference frequency signal; output the generated first signal to one or more proportional resonator, PR, loops; and generate the control signal in dependence on an output of each of the one or more PR loops. Advantageously, the first signal may be provided to the PR loops to compensate for the at least one harmonic. I n some examples, each PR loop is pre-configured or tuned to output a signal to compensate for a harmonic at a specific frequency. In some examples, the first signal is indicative of a difference between the frequency signal of the input signal and the reference frequency signal. In some examples, the generated first signal is output to a plurality of PR loops; and the one or more processors are collectively configured to sum an output of each of the PR loops and to generate the control signal in dependence on the sum of the outputs of each of the plurality of PR loops. Advantageously, THD associated with multiple harmonics in the input signal can be compensated for. In some examples, the one or more processors are collectively configured to combine the output of each of the PR loops with the input signal to generate the control signal. The control signal is advantageously generated to account or compensate for the presence of harmonics in the input signal, based on the output of the PR loops. In some examples, to generate the control signal, the one or more processors are collectively configured to generate the reference frequency signal in dependence on the input signal; and the reference frequency signal is indicative of a reference input current expected to be provided to the single-stage AC-DC convertor. Advantageously, the reference signal may be used as described above to compensate for harmonics of the input current. In some examples, the one or more processors are collectively configured to: combine the generated control signal with a carrier wave signal or a shifted carrier wave signal to generate a plurality of switching control signals to respectively open or close a plurality of switches of the bridge; and output the plurality of switching control signals to the plurality of switches. Advantageously, the switching of the bridge can be controlled to reduce the THD in the AC-DC convertor. Further, the power control of the AC-DC convertor can be implemented based on a phase shift between the carrier wave signal and the shifted carrier wave signal. In some examples, the bridge is a primary bridge; and the one or more processors are collectively configured to generate the carrier wave signal in dependence on a phase shift control signal. In some examples, the bridge is a secondary bridge; the one or more processors are collectively configured to generate the shifted carrier wave signal in dependence on a phase shift control signal; and the shifted carrier wave signal is phase shifted relative to the carrier wave signal. In some examples, the one or more processors are collectively configured to generate the phase shift control signal in dependence on the comparison of the received input current and the reference frequency signal, or to receive the phase shift control signal from a proportional integral, PI, controller. In some examples, the phase shift between the carrier wave signal and the shifted carrier wave signal is determined to control an active power of the single-stage AC-DC convertor. In some examples, the input current comprises three-phase input current, and the one or more processors are collectively configured to compare the received input current to a respective reference frequency signal, generate the control signal and output the control signal for each of the three phases of the three-phase input current. Advantageously, the present invention can be utilised to reduce THD with single-phase or three-phase input power. In some examples, the control system further comprises the one or more PR loops. 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 indicative of an input current of the single-stage AC-DC convertor; in dependence on the received input signal, generate a control signal to compensate for at least one harmonic of a frequency signal within the input current; and output the generated control signal to a bridge of the single-stage AC-DC convertor to control a switching of the bridge of the single-stage AC-DC convertor. According to another aspect of the present 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 of any preceding paragraph; at least one bridge comprising a plurality of switches each configured to open or close in dependence on the control signal generated by the control system; and at least one proportional resonator, PR, loop, each configured to generate an output signal to compensate for a harmonic of the reference frequency signal within the input current. In some examples, the at least one PR loop comprises a plurality of PR loops; and each of the plurality PR loops is configured to generate an output signal to compensate for a respective harmonic among a plurality of predetermined harmonics of the reference frequency signal within the input current. Advantageously, a plurality of harmonics within the input current can be compensated for and THD can be further reduced. In some examples, the plurality of predetermined harmonics of the reference frequency signal within the input current comprise low-order, odd harmonics of the reference frequency signal within the input current. The low-order, odd harmonics of the reference frequency signal within the input current may contribute the most to an increase in THD, and thus by providing the plurality of PR loops to be tuned to these harmonics, THD can be reduced efficiently by balancing the number of PR loops in the device (which influences device size, power requirements, and other factors) with a reduction in THD. In some examples, the plurality of predetermined harmonics of the reference frequency signal within the input current comprise one or more of first, third, fifth, seventh harmonics. These may be harmonics which contribute the most to THD of the single-stage AC-DC convertor. In some examples, the single-stage AC-DC convertor further comprises an input inductor configured to receive the input current; a primary bridge comprising a plurality of primary switches each configured to open or close in dependence on a primary bridge control signal; a transformer electrically connected to the primary bridge; a secondary bridge comprising a plurality of secondary switches each configured to open or close in dependence on the control signal generated by the control system; and an output capacitor configured to output an output DC current. In some examples, the single-stage AC-DC convertor further comprises one or more of a phase-locked loop, PLL, configured to synchronize one or more of a current or a voltage of the input current with the reference signal; a voltage control Proportional-Integral, PI, controller configured to compare a measured voltage and a reference voltage and output the first signal in dependence on the comparison; a current control PI controller configured to compare the first signal and the input signal and output the phase shift control signal to a carrier generator in dependence on the comparison; or a carrier generator configured to generate the carrier and shifted carrier wave signals. According to another aspect of the present invention, there is provided a vehicle comprising the control system or the single-stage AC-DC convertor of any of the preceding paragraphs. According to another aspect of the present invention, there is provided a method for controlling a control system 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 input current of the single-stage AC-DC convertor; comparing the received input signal to a reference frequency signal; generating, in dependence on the comparison, a control signal to compensate for at least one harmonic of the reference frequency signal within the input current; and outputting the generated control signal to a bridge of the single-stage AC-DC convertor to control a switching of the bridge of the single-stage AC-DC convertor. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. 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 2A and 2B show respectively a block diagram and a schematic diagram illustrating single-stage AC-DC convertors according to an embodiment of the present invention; Figures 3A-3C show block diagrams illustrating a control scheme for a first bridge and a second bridge of a single-stage AC-DC convertor according to an embodiment of the present invention; Figure 4 shows a block diagram illustrating a control scheme of a bridge of the single-stage AC-DC convertor according to an embodiment of the present invention; Figure 5 shows a block diagram illustrating a control scheme of a single-stage AC-DC convertor according to an embodiment of the present invention; Figures 6A-6C show charts illustrating a single-stage AC-DC convertor having high total harmonic distortion according to an embodiment of the present invention; Figures 7A-7B show charts illustrating a single-stage AC-DC convertor having reduced total harmonic distortion according to an embodiment of the present invention; Figure 8 shows a flow chart illustrating a method according to an embodiment of the present invention; and Figure 9 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 electric vehicle, and to control schemes thereof. As is described above, single-stage AC-DC convertors may achieve high efficiency and high power density, and can reduce a need for large or bulky electrolytic capacitors. 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. A power transfer of the single-stage is controlled by a phase shift between a primary-side bridge and a secondary-side bridge of the single-stage AC-DC convertor, as will be explained in more detail below. However, when grid voltages are imbalanced or harmonics are present in the input power, a total harmonic distortion (THD) in the input grid current may increase. The present disclosure relates to single-stage AC-DC convertor design and control to mitigate and reduce THD due to these factors. This is primarily achieved through the provision of one or more proportional resonator controllers (PR controllers, also known as PR loops) to the control of one of the bridges of the single-stage AC-DC convertor. The one or more PR loops are each tuned to compensate for specific harmonics of the input current, and thus the THD of the input current can be reduced. The bridges of the single-stage AC-DC convertor each comprise a plurality of switches which are opened and closed by control signals. The output of the one or more PR loops may be used for generating these control signals to control the switching of the bridges. 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(wgt) |. Here, ds(t) is the duty cycle of the secondary bridge, and cog is the angular frequency of the input voltage. 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 is the only control variable. The power equation is a non-linear function with phase shift and duty. When the input voltages are imbalanced or harmonics are present, there may be an increased THD of the input grid current. 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 a 2-cascaded current control and voltage control loop 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. This voltage control output signal may be a reference value of the input current. A Phase-Locked Loop (PLL) is a control system used to synchronize the input current with the input (or 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. The DQ transformation is often employed in the analysis and control of three-phase AC systems. The 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. This 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. However, due to voltage imbalances or harmonics in the input current, the THD may be increased in the input current. The present invention aims to overcome this problem of the traditional control scheme by introducing one or more PR loops to compensate for one or more harmonics of the input current, as explained in detail with reference to the Figures below. It should be understood that a single-stage AC-DC convertor described below may be used with single-phase or three-phase AC input power. For example, a single-stage AC-DC convertor for use with three-phase input power may be considered to comprise three single-stage AC-DC convertors of the type used with single-phase input power. A parallel output connection of the three modules provides DC output current despite the pulsating current of each module at twice the mains frequency. 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 9, 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 input current of the single-stage AC-DC convertor. The processing means 120 is configured to compare the received input signal 160 to a reference frequency signal. The processing means 120 is further configured to generate, in dependence on the comparison, a control signal 170 to compensate for at least one harmonic of the reference frequency signal within the input current. In some examples, a frequency signal of the input signal is compared to the reference frequency signal. In some examples, the input signal may include an angle generated by a phase-locked loop (PLL) circuit at a particular instant. The angle may comprise one or more of an x-phase angle, where x-phase comprises one of R-phase, Y-phase or B-phase. As will be discussed, in some examples the reference frequency signal may be predetermined. In other examples the reference frequency signal may be determined by the processing means 120 based on the input signal 160 and / or based on a measured voltage and a reference voltage. The reference frequency signal may be indicative of an expected input current which would be expected to be input to the single-stage AC-DC convertor in ideal or near-ideal conditions from the grid or other AC power source. The reference frequency signal is discussed in more detail with respect to Figure 5, which should be taken in combination with the description of Figure 1. The processing means 120 may further be configured to generate a first signal in dependence on the comparison of the received input signal to the reference frequency signal. The generated first signal may be indicative of a difference between the frequency signal of the input signal and the reference frequency signal. The generated first signal may otherwise be understood as an error signal, and may be indicative of an error in the input signal compared to the reference signal. For example, the generated first signal may correspond to differences between the received input signal and the reference frequency signal due to voltage imbalance or harmonics in the input current. The processing means 120 may comprise one or more proportional resonator (PR) controllers or PR loops, or may be configured to communicate with one or more PR controllers or PR loops. The generated first signal may be provided to the one or more PR loops, and an output of the one or more PR loops may be received by the processing means 120. The output of the one or more PR loops may be indicative of a compensation to be applied to the input signal 160 to compensate for one or more harmonics of the reference frequency signal within the input current. That is, in some examples the output of the one or more PR loops may be added to the input signal to compensate for the one or more harmonics. In some examples, where there is a plurality of PR loops, each PR loop is respectively tuned or pre-configured to resonate at a particular frequency to compensate for a particular harmonic of the input current. In this example, the processing means 120 may sum the output of each of the respective PR loops. In some examples, the PR loops are configured to compensate for a predetermined group of harmonics which contribute the most to increasing THD of the single-stage AC-DC convertor. These may be low-order, odd harmonics, such as the third, fifth and / or seventh harmonics. It should be understood that any number of PR loops may be provided to compensate for any number of harmonics, although the number of PR loops used may depend on any size requirements of the device. It should be understood that PR loops may also be known as PR controllers, and may take any suitable known form to provide the output to compensate for the at least one harmonic. The PR loops are discussed in more detail later. In some examples, the processing means 120 may further be configured to combine the first signal with the input signal to generate the control signal 170. Further, the processing means 120 may further be configured to combine the generated control signal 170 with a carrier wave signal or 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 a 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 the received input signal 160 and the reference frequency signal, or may be received from a proportional integral, PI, controller. The output means 150 is arranged to output the generated control signal 170 to a bridge of the single-stage AC-DC convertor to control a switching of the bridge of the single-stage AC-DC convertor. The output means 150 may further be arranged to output the plurality of switching control signals to a plurality of switches of the bridge of the single-stage AC-DC convertor. It should be understood that the control means 100 may be configured to perform each of the functions explained above in a similar way for single-phase input current or three-phase input current. In the example where the input current is three-phase current, the control means 100 may be arranged to perform each of the functions explained above for each phase of the three-phase current 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. Figure 2 illustrates a single-stage AC-DC convertor 200 according to an embodiment of the invention. Figure 2A illustrates the single-stage AC-DC convertor 200 in block diagram form, and Figure 2B shows a schematic diagram illustrating the single-stage AC-DC convertor 200. 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 a primary-side bridge controller 280 or a 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 bridge 230, a primary-side bridge controller 280, a transformer 240, a secondary-side bridge 250, a secondary-side bridge controller 290 comprising one or more PR loop 295, 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 current may be single-phase or three-phase input. As explained previously, if the input current is 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 same input current, to process different phases of the input current, and to output DC power to the same DC output. For the single-phase input, one AC-DC converter is provided and for the three-phase input three single-phase AC-DC converters are provided; one for each phase of the input. The input current may be sinusoidal in nature with some harmonics due to the presence of non-linear load generated due to switching by the power electronics converters. The input inductor 220 comprises one or more inductor. 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 primary-side bridge 230 and configured to provide electrical power to the primary-side bridge 230. The input inductor 220 is configured to reduce a ripple in the input current and to thereby improve a power factor of the single-stage AC-DC convertor 200. Based on various factors such as the application and core material of the AC-DC convertor 200, the input inductor 220 can be of different types, for example: an iron core inductor; a powder core inductor; or a ferrite core inductor. It should be understood that other types of inductor may be used. The input inductor 220 is provided 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 (THD). For a power factor correction application, a powdered core inductor may be used to get the wide range of DC bias inductance. The primary-side bridge 230 comprises a bridge connected to a primary terminal 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. The primary-side bridge 230 may include any number of switches, but in some examples includes 4, 6 or 8 switches. The primary-side 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. In an example, a 3rd part of the primary side bridge which is connected neutral can include diodes or MOSFETs / IGBT's. 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 to control switching of the primary-side bridge 230 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 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 may act to modify the voltage provided at the secondary winding relative to the voltage received at the primary winding. For example, the transformer 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 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 controller 290 comprises one or more PR loops 295 (or alternatively, may be configured to communicate with one or more external PR loops). 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. In some examples, the secondary-side bridge controller 290 is configured to generate and output the control signals to the secondary-side bridge 250 in dependence on a comparison of an input current received by the AC input 210 and a reference frequency signal. The reference frequency signal may be predetermined or may be determined by the secondary-side bridge controller 290, similar to that explained above in respect of Figure 1. The secondary-side bridge controller 290 may generate the control signal to compensate for one or more harmonics of the input current based on the comparison of the input signal and the reference frequency signal, to reduce a THD of the single-stage AC-DC convertor 200. It should be understood that the secondary-side bridge controller 290 may operate in the same ways as the control system 100 of Figure 1, explained above, to generate and output the control signal to the secondary-side bridge 250. The one or more PR loops 295 comprise circuitry configured to resonate at specific frequencies in order to mitigate current harmonics and improve performance of the single-stage AC-DC convertor 200. The PR loops 295 may further be configured to apply proportional control to a fundamental frequency to mitigate harmonics in a waveform. For example, the PR loops 295 may each be configured to resonate at a specific frequency corresponding to commonly found harmonic frequencies of electrical systems. For example, the specific frequency may correspond to a harmonic commonly found in the expected current provided at the AC input 210, such as a grid current. The harmonics should be understood to occur at integer multiples of the frequency of the input current. The PR loops are configured to generate an output that can be combined with the input current to minimise or cancel out the respective harmonic. In some examples, the harmonics that contribute the most to the THD of the system (that is, which introduce a greatest difference between the input signal and the reference frequency signal) are low-order, odd harmonics, such as the first, third, fifth, and seventh harmonics. The PR loops 295 may therefore be configured to tuned to generate an output to compensate for these harmonics. It should be understood that the PR loops 295 may be configured to resonate at any particular frequency, and that any number of PR loops 295 may be provided to compensate for different harmonics. The output capacitor 260 comprises one or more capacitor. The output capacitor 260 is electrically connected between the secondaryside bridge 250 and the DC output 270. The output capacitor 260 may be configured to smooth and / or filter a rectified DC voltage. The output capacitor 260 is provided to reduce the ripple in the output voltage. The capacitance value depends on the output voltage ripple requirement. For example, a smoother output voltage is achieved if the capacitance value is high, but this will also increase the size and cost of the capacitor. The DC output 270 comprises an electrical output and is configured to output electrical energy converted from the AC input 210 to DC electrical power by the single-stage AC-DC convertor 200. 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. It should also be understood that, while in Figure 2 the PR loops 295 are provided in the secondary-side bridge controller 290, in another example the PR loops 295 may be provided externally to either bridge controller. Figure 2B 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 200 of Figure 2B includes more detail of an implementation of the circuit elements of Figure 2A. As shown in Figure 2B, the AC input 210 comprises a first input terminal 211 and a second input terminal 212. The first input terminal 211 may be coupled to a positive or line voltage of a single-phase power system and the second input terminal 212 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 211 may be coupled to a first phase voltage and the second input terminal 212 may be coupled to a second phase voltage. Where the power system is a three-phase power system, the first terminal 211 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 212 may be coupled to neutral. The AC input 210 is coupled to the input inductors 220. As shown in Figure 2B, the input inductors 220 may comprise two inductors, a first input inductor Lg1 and a second input inductor Lg2. The first input terminal 211 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 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 DC output 270 comprises a first (positive) output terminal 271 and a second (negative or neutral) output terminal 272. As is shown in Figure 2B, the output current of the single-stage AC-DC converter 200 is provided to or at the first output terminal 271. The control system 100 may receive the input 160 indicative of the current at the first output terminal 271 by sensing the first output terminal 271 using any suitable means, such as a current shunt. Figure 3 illustrates a control scheme 300 for controlling switching of bridges of a single-stage AC-DC convertor 200 according to an embodiment of the present invention. The single-stage AC-DC convertor 200 may be the single-stage AC-DC convertor 200 of Figure 2. Figure 3A illustrates a first scheme 310 for generating a carrier wave signal and a shifted carrier wave signal according to an embodiment of the disclosure. Figure 3B illustrates a second scheme 320 for controlling switching of a first bridge of the single-stage AC-DC convertor 200. Figure 3C illustrates a third scheme 330 for controlling switching of a second bridge of the single-stage AC-DC convertor 200. It should be understood that although they are illustrated separately, the first scheme 310, the second scheme 320 and the third scheme 330 may be implemented in the same single-stage AC-DC convertor 200 to control different parts of the single-stage AC-DC convertor 200. In the first scheme 310 illustrated in Figure 3A, an output voltage of the single-stage AC-DC convertor 200 and a reference voltage are compared and a difference 311 between the two voltages is provided to a first proportional integral, PI, controller 312. The first PI controller 312 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 indicative of a reference current of the input current. The first PI controller 312 then outputs the reference frequency signal. The first PI controller 312 may be known as a voltage PI controller. The output reference frequency signal is then compared with a measured input current of the single-stage AC-DC convertor 200 and the difference 313 is provided to a second PI controller 314. In some examples, this difference 313 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 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 314 is configured to generate a phase shift control signal in dependence on the comparison 313 of the measured input current and the reference frequency signal, and to output the phase shift control signal to a carrier wave generator 315. 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 315 is configured to generate a carrier wave signal 316 and a shifted carrier wave signal 317. As will be explained, the carrier wave signal 316 and the shifted carrier wave signal 317 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. Figure 3B illustrates how the carrier wave signal 316 is used to control the switching of a plurality of switches of a first bridge, which in some examples is the primary-side bridge 230, by outputting a plurality of switching control signals 324. In the second scheme 320 illustrated in Figure 3B, a plurality of pulse width modulation (PWM) signals 324 are generated for the plurality of switches. Although control signals 324 for six switches are illustrated in Figure 3B, it should be understood that a different number of switches may be used. As shown in Figure 3B, the carrier wave signal is provided to the second scheme 320 and in conjunction with a fixed duty 321 used to control the switching of the plurality of switches. In some examples, the fixed duty 321 is 50%, but it should be understood that other duties are within the scope of the disclosure. A plurality of comparators 322a, 322b, 322c may be provided as shown in Figure 3B to generate PWM signals for the switches. The carrier wave generated by the PI controller is then compared with specific values (for example, 0.5) and based on the logic, a high or low signal is output. These PWM signals will be used to drive the switches of the converter. One or more NOT gate 323 may also be provided to generate a complementary carrier signal of the input carrier signal. The operation of blocks 322c, and 324e-f are the same as that of the blocks present on the left-hand side. However, the input coming to the 322c may be different in some examples. For example, the input to the comparator 322c shown on the right-hand side of Figure 3B may be 'Vga' and '0'. This will lead to generating different PWM signals of the connected switches. It should be understood that in the example of Figure 3B, the second scheme 320 is used to control the switching of switches 324 of a primary-side bridge 230 such as that of Figure 2, in the case where the novel bridge control method of the present disclosure is applied to the secondary-side bridge 250. However, in another example, the second scheme 320 may be adapted to generate the control signals for the switches 324 based on a modified control signal, such as one generated by the control system 100 of Figure 1. Figure 3C illustrates a third scheme 330 for generating control signals 339 for a plurality of switches of a second bridge such as the secondary-side bridge 250 of Figure 2. In the example of Figure 3C, the shifted carrier wave signal 317 is combined with a control signal 336 generated based on real-time angle data 331 of the input current. In the example of Figure 3C, the real-time angle data 331 of the input current is processed based on the active power control equation described above. That is, the sine of the real-time angle data 331 is taken at block 332, which is then multiplied by a fixed ratio 334 at block 333. The fixed ratio 334 in one example is 0.5, but may be different in other examples. The modulus of the output of block 333 is taken at block 335 to generate the control signal 336. In other words, the control signal 336 of Figure 3C may be generated based on the equation ds(t)=0.^sin(ujgt)\. As explained above, this reflects a traditional control scheme of a secondary-side bridge 250 in which THD may be increased. As shown in Figure 3C, the control signal 336 and the shifted carrier wave signal 317 may be used in combination with a plurality of amplifiers 337,338 to output a plurality of control signals 339 to a plurality of switches to control the switching of the plurality of switches. Figure 4 illustrates a modified scheme 400 for generating a control signal 485 to control the switching of a bridge of the single-stage AC-DC convertor 200 according to an embodiment of the present invention. It should be understood that the modified scheme 400 of Figure 4 can be considered to replace the part of the third scheme 330 shown in Figure 3C to generate the control signal 336. That is, considering Figure 4 and Figure 3C, the control signal 485 generated in the scheme 400 of Figure 4 can be provided to the plurality of switches 339 of the secondary-side bridge 250 in combination with the shifted carrier wave signal 317 via the amplifiers 337, 338 to control the switching of the plurality of switches 339 of the secondary-side bridge 250. In other words, the modified scheme 400 of Figure 4 may be performed by the secondary-side bridge controller 290 of Figure 2 or the control system 100 of Figure 1. As shown in Figure 4, the sine of the real-time angle data 331 of the input current may be taken in block 410 as in block 332 of Figure 3C. However, in the modified scheme 400 of Figure 4, the reference frequency signal 420 and the input signal 430 are compared, and a difference between the reference frequency signal 420 and the input signal 430 are provided to one or more PR loops 440. Figure 4 illustrates 3+n PR loops 440 (illustrated by the dotted lines connected to the fourth PR loop). However, it should be understood that any number of PR loops 440 may be used. As has been discussed, each PR loop 440 is configured to resonate at a respective specific frequency. Each PR loop 440 may be configured to resonate at a different frequency to other PR loops 440. The resonant frequencies of the PR loops 440 may correspond to frequencies of harmonics of the input current (or to harmonics of the reference frequency signal), as has been discussed above. Each PR loop 440 provides an output to compensate for an associated harmonic, and these outputs are then summed 450. In some examples, each of the PR loops 440 receives an input of one of the harmonics of the current. In this example, a bandpass filter may be provided at the input of each of the PR loops 440 that band-passes around the respective harmonic frequency for the respective PR loop 440, so that the PR loop 440 only receives an input related to the respective harmonic frequency. The sum 450 of the outputs of the PR loops 440 is then combined with the sine 410 of the real-time angle data 331 to produce a modified sine wave 460. This is then multiplied by a fixed duty 470 and the modulus 480 of this multiplication is taken to generate a control signal 485. The control signal 485 generated by the modified scheme 400 of Figure 4 may be provided to the secondary-side bndge 250 in combination with the shifted carrier wave signal 317 in the same way as shown in Figure 3C. Figure 5 is a block diagram illustrating a single-stage AC-DC convertor 500 according to an embodiment of the present invention. The single-stage AC-DC convertor 500 of Figure 5 may be considered similar to the single-stage AC-DC convertor 200 of Figure 2, except that the flow of electrical power and signals around the single-stage AC-DC convertor 500 is shown in Figure 5, and a particular example implementation of the bridge controllers is shown. The single-stage AC-DC convertor 500 of Figure 5 may operate under the control system 100 of Figure 1, and may also include the generation of control signals in a similar way to that shown in Figures 3 and 4, as will be explained below. The single-stage AC-DC convertor 500 of Figure 5 includes an AC input 501. The AC input 501 may be similar to the AC input 210 described in respect of Figure 2. The AC input 501 is configured to receive an alternating current electrical supply. For example, the AC input 501 may receive a supply of electrical energy from an electric vehicle charger, although the present invention is not limited thereto. The AC input 501 is configured to output an input current 502 to an input inductor 504 and to a phase-locked loop (PLL) controller 511. The AC input 501 is also configured to output an input signal 503 indicative of the input current 502 to the input inductor 504 and to a DQ transformation block 514. As explained above, the DQ transformation is often employed in the analysis and control of three-phase AC systems. The 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 input inductor 504 comprises one or more suitable inductor of any known type, and is configured to receive the input current 502 and the input signal 503 from the AC input 501. The input inductor 504 may be similar to the input inductor 220 of Figure 2. The input inductor 504 is configured to operate as would be understood to reduce a ripple in the input current 502, and to improve a power factor of the single-stage AC-DC convertor 500. The input inductor 504 is configured to output electrical power to a primary-side bridge 505. The PLL controller 511 may be similar to the PLL controller described with respect to the earlier Figures, and comprises circuitry to synchronize the input current and / or voltage with the reference signal. The PLL controller 511 is configured to receive the input signal 503 from the AC input 501. In this example, the input signal 503 further comprises information relating to a reference voltage of the input current. The PLL controller 511 is configured to output real-time angle data 513 in dependence on the reference voltage of the input current 502 included in the input signal 503. It should be understood that the real-time angle data 513 comprises angle-data generated by the PLL controller 514 at a particular instant, and may be represented by “Theta_x” in some examples, where “Theta_x” is an x-phase angle generated by the PLL controller 514, where x-phase can be one of R-phase, Y-phase or B-phase. The PLL controller 511 is configured to output the real-time angle data 513 to the DQ transformation block 514. The PLL controller 511 may be further configured to output input current phase information 512. The input current phase information 512 may comprise “lp_phase” being the sin(theta) component of grid voltage. This is used to generate the PWM signals for the secondary bridge switches. The lp_phase may be multiplied by 0.5 and its modulus is compared with the shifted carrier wave and shifted carrier 180 phase shift to get the PWM signals necessary fordriving the secondary side switches. The DQ transformation block 514 is configured to receive the input signal 503 and the real-time angle data 513. The DQ transformation block 514 is configured to output an input phase current feedback signal 516 to one or more PR loops 517. The input phase current feedback signal 516 is based on the input current 502 and the input signal 503 received at the AC input 501, and may be indicative of a measured phase current at an input of the primary-side bridge 505. The DQ transformation block 514 may further be configured to output a second input phase current feedback signal 515 in some examples. The output of the DQ transformation block 514 in some examples may comprise two outputs when a grid current (e.g., “Lgrid”) is converted of DQ frame. The two outputs may be referred to as “Igrid_g_fb” and “lgrid_d_f / b”, where lgrid_q_fb is the measured quadrature component of the input current and lgrid_d_f / b is the measured direct component of the input current. In this example, the second input phase current feedback signal 515 may comprise lgrid_q_fb and the input phase current feedback signal 516 may comprise lgrid_d_f / b. The single-stage AC-DC convertor 500 of Figure 5 further comprises a transformer 506, a secondary-side bridge 507, an output capacitor 508 and a DC output 509 that operate similarly to those components described in Figure 2. The DC output 509 is configured to output a measured voltage 510 to a first PI controller 519. The measured voltage 510 may be indicative of a voltage of electrical power at an output of the DC output 509. The first PI controller 519 comprises a PI controller similar to those explained previously (for example in Figure 3A), and is configured to receive the measured voltage 510 and to further receive a reference voltage 518. The reference voltage 518 may be indicative of an expected voltage to be output by the DC output 509 based on the input current 502. The first PI controller 519 is configured to compare the measured voltage 510 to the reference voltage 518 and to output a reference frequency signal 520 based on the comparison. The reference frequency signal 520 may be output to the one or more PR loops 517 and to a second PI controller 522. The one or more PR loops 517 comprise one of more PR loops of any suitable type for resonating at a particular frequency, and providing an output to compensate for at least one harmonic of the input current. The one or more PR loops 517 may be similar to those described above in respect of Figures 2 and 4. In some examples, the PR loops 517 comprise a plurality of PR loops, each being configured to resonate at a different frequency corresponding to a respective harmonic of the input current. The PR loops 517 may be preconfigured to resonate at specific frequencies. In some examples, the specific frequencies may be selected based on known harmonics of the input current 502. The PR loops 517 are configured to receive the input phase current feedback signal 516 and the reference frequency signal 520 and to compare the two received signals. The PR loops 517 may then process a difference between the two received signals to generate an output to compensate for the differences arising due to harmonics in the input signal 503. Where a plurality of PR loops 517 are provided, the output of each PR loop may be summed. The PR loops 517 are configured to provide an output signal 521 to a second PWM generator 529. The second PI controller 522 is configured to receive the input phase current feedback signal 516 and the reference frequency signal 520 and to compare the two received signals. The second PI controller 522 is configured to output phase shift information to a carrier generator 523 in dependence on this comparison. The carrier generator 523 is configured to generate a carrier wave signal 524 and a shifted carrier wave signal 525 in dependence on the phase shift information received from the second PI controller 522. The carrier wave signal 524 is output to a first PWM generator 527 and the shifted carrier wave signal 525 is output to the second PWM generator 529. The output of the carrier generator 523 to the first and second PWM generators 527, 529 in Figure 5 can be considered similar to the output of the carrier wave signal 316 and the shifted carrier wave signal 317 explained in respect of Figure 3A. The carrier wave signal 524 and the shifted carrier wave signal 525 may be phase shifted relative to each other, and the phase shift may control the active power of the single-stage AC-DC convertor 500 as has previously been explained. The first PWM generator 527 is configured to generate a plurality of first switching control signals 528 for controlling switching of a plurality of electrical switches of the primary-side bridge 505, in dependence on the received carrier wave signal 524 and a fixed duty 526. The plurality of first switching control signals 528 indicate to the switches of the primary-side bridge 505 to open or close. In some examples, the fixed duty 526 may be 50%, but the present invention is not limited thereto. The second PWM generator 529 is configured to generate a plurality of second switching control signals 530 for controlling switching of a plurality of electrical switches of the secondary-side bridge 507, in dependence on the received shifted carrier wave signal 525 and the output signal 521 of the PR loops 517. The plurality of second switching control signals 530 indicate to the switches of the secondary-side bridge 505 to open or close. Control signals 530 for controlling the switching of the secondary-side bridge 507 are therefore generated by the single-stage AC-DC convertor 500 illustrated in Figure 5 which take into account a difference between an input signal 503 and a reference frequency signal 520, to compensate for harmonics of the input current 502 by controlling the switching of the secondary-side bridge 507 using the output 520 of the PR loops 517. The single-stage AC-DC convertor 500 of Figure 5 therefore achieves a reduction in THD associated with one or more harmonics or voltage imbalance in the input current 503. Figures 6A-6C illustrate system information of a single-stage AC-DC convertor having high THD (for example, a conventional single-stage AC-DC convertor) due to voltage imbalance or harmonics in an input current. Figures 7A and 7B illustrate system information of a single-stage AC-DC convertor 200, 500 having reduced THD according to an embodiment of the present invention. As can be seen in Figure 6A, imbalances can exist in input voltages which cause distortion and imbalance in the convertor waveforms 610,620, 630. This results in a high THD as illustrated in Figures 6B and 6C. As can be seen in Fourier transforms of the waveforms shown in Figure 6C, current spikes exist at multiplies of the frequency of the input current, which correspond to harmonics in the input current. In Figures 7A and 7B, when the single-stage AC-DC convertor 200,500 according to the present invention is used, the convertor waveforms 710, 720,730 exhibit lower THD and show smoother sine waves. As can be seen in Figure 7B, the spikes associated with harmonics in the input current are significantly reduced. Figure 8 illustrates a method 800 according to an embodiment of the present invention. The method 800 of Figure 8 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 3A, 3B and 4, or the single-stage AC-DC convertor 500 of Figure 5. The method 800 includes receiving 810 an input signal 503 indicative of an input current 502 of the single-stage AC-DC convertor 200, 500. In some examples, the input signal 503 comprises a frequency signal. In some examples, the input signal includes real-time angle information generated by a PLL controller 511 at a particular instant. The method 800 further comprises comparing 820 the received input signal 503 to a reference frequency signal 520. In some examples, the frequency signal of the input signal 503 is compared to the reference frequency signal 520. In some examples, the method 800 may further comprise generating a first signal in dependence on the comparison of the received input signal 503 to the reference frequency signal 520, and outputting the generated first signal to one or more PR loops 517. In some examples, the generated first signal is output to a plurality of PR loops 517 and the method further comprises summing an output of the plurality of PR loops 517. In some examples, each PR loop 517 is configured to resonate at a specific frequency to compensate for an associated harmonic of the input current 502. In some examples, the reference frequency signal is indicative of a reference input current expected to be provided to the single-stage AC-DC convertor 200, 500. The method 800 further comprises generating 830, in dependence on the comparison, a control signal to compensate for at least one harmonic of the reference frequency signal within the input current. In an example, generating 830 the control signal comprises generating a plurality of switching control signals. In an example, generating 830 the control signal comprises generating the control signal in dependence on an output signal received from one or more PR loops 517. The method 800 further comprises outputting 840 the generated control signal to a bridge of the single-stage AC-DC convertor to control a switching of the bridge of the single-stage AC-DC convertor. In some examples, the bridge is a secondary-side bridge 505 of a single-stage AC-DC convertor 200, 500. In some examples, the method 800 comprises combining the output of the PR loops 517 with a shifted carrier wave signal generated by a carrier generator 523. The shifted carrier wave signal may be generated by the carrier generator 523 in dependence on a phase shift control signal generated by a PI controller. It should be understood that the method 800 may further comprise other operations of the control system 100 of Figure 1, the single-stage AC-DC convertor 200 of Figure 2, the schemes 300, 400 of Figure 3 or Figure 4, or the single-stage AC-DC convertor 500 of Figure 5 described above. Figure 9 illustrates a vehicle 900 according to an embodiment of the present invention. The vehicle 900 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 single-stage AC-DC convertor 500 of Figure 5. In some examples, the vehicle 900 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 effects of odd harmonics or the total harmonic distortion (THD) of the input grid current. However, a second harmonic distortion may be present in the output current. The control schemes described above may be modified to reduce the second harmonic content of the output current as well as to reduce the THD of the input grid current. This may be achieved by modifying the control signal 336 provided to the second bridge control scheme 330. The control signal 336 may be modified to compensate for the second harmonic of the output current by providing a signal representing the second harmonic components to a PR loop, and modifying the control signal 336 based on the output of the PR loop. 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 input current of the single-stage AC-DC convertor;compare the received input signal to a reference frequency signal;generate, in dependence on the comparison, a control signal to compensate for at least one harmonic of the reference frequency signal within the input current; andoutput the generated control signal to a bridge of the single-stage AC-DC convertor to control a switching of the bridge of the single-stage AC-DC convertor.

2. The control system of claim 1, wherein the one or more processors are collectively configured to:generate a first signal in dependence on the comparison of the received input signal to the reference frequency signal; output the generated first signal to one or more proportional resonator, PR, loops; and generate the control signal in dependence on an output of each of the one or more PR loops.

3. The control system of claim 2, wherein the generated first signal is output to a plurality of PR loops; and the one or more processors are collectively configured to sum an output of each of the PR loops and to generate the control signal in dependence on the sum of the outputs of each of the plurality of PR loops.

4. The control system of claim 2 or 3, wherein the one or more processors are collectively configured to combine the output of each of the PR loops with the input signal to generate the control signal.

5. The control system of any preceding claim, wherein to generate the control signal, the one or more processors are collectively configured to generate the reference frequency signal in dependence on the input signal; andwherein the reference frequency signal is indicative of a reference input current expected to be provided to the single-stage AC-DC convertor.

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

7. The control system of claim 6, wherein the bridge is a primary bridge; and wherein the one or more processors are collectively configured to generate the carrier wave signal in dependence on a phase shift control signal.

8. The control system of claim 6, wherein the bridge is a secondary bridge;wherein the one or more processors are collectively configured to generate the shifted carrier wave signal in dependence on a phase shift control signal; andwherein the shifted carrier wave signal is phase shifted relative to the carrier wave signal.

9. The control system of any preceding claim, wherein the input current composes three-phase input current, and the one or more processors are collectively configured to compare the received input current to a respective reference frequency signal, generate the control signal and output the control signal for each of the three phases of the three-phase input current.

10. 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;at least one bridge comprising a plurality of switches each configured to open or close in dependence on the control signal generated by the control system; andat least one proportional resonator, PR, loop, each configured to generate an output signal to compensate for a harmonic of the reference frequency signal within the input current.

11. The single-stage AC-DC convertor of claim 10, wherein the at least one PR loop comprises a plurality of PR loops; and wherein each of the plurality PR loops is configured to generate an output signal to compensate for a respective harmonic among a plurality of predetermined harmonics of the reference frequency signal within the input current.

12. The single-stage AC-DC convertor of claim 11, wherein the plurality of predetermined harmonics of the reference frequency signal within the input current comprise low-order, odd harmonics of the reference frequency signal within the input current.

13. The single-stage AC-DC convertor of any of claims 10 to 12, comprising:an input inductor configured to receive the input current;a primary bridge comprising a plurality of primary switches each configured to open or close in dependence on a primary bridge control signal;a transformer electrically connected to the primary bridge;a secondary bridge comprising a plurality of secondary switches each configured to open or close in dependence on the control signal generated by the control system; andan output capacitor configured to output an output DC current.

14. A vehicle comprising the control system of any of claims 1 to 9 or the single-stage AC-DC convertor of any of claims 10 to 13.

15. A method for controlling a control system 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 input current of the single-stage AC-DC convertor;comparing the received input signal to a reference frequency signal;generating, in dependence on the comparison, a control signal to compensate for at least one harmonic of the reference frequency signal within the input current; andoutputting the generated control signal to a bridge of the single-stage AC-DC convertor to control a switching of the bridge of the single-stage AC-DC convertor.

Images