ACTIVE FILTER FOR SINGLE-PHASE CHARGING USING A SWITCHING POWER SUPPLY

Description

FIELD OF THE INVENTION

[0001] The present invention relates to an active filter using a bi-directional switch-mode converter for an electric vehicle on-board charger.BACKGROUND OF THE INVENTION

[0002] Electric vehicles include a battery for powering an electric drive system. Plug-in electric vehicles are a category of electric vehicles having a rechargeable battery pack that can be recharged from the electrical grid. Because most electrical grids provide an AC voltage, and because most rechargeable battery packs require a DC voltage, power from the electrical grid must first be converted into a suitable DC voltage when recharging an electric vehicle.

[0003] A variety of circuits exist for converting an AC voltage from an electrical grid into a DC voltage for a rechargeable battery. One known circuit includes a dual-active-bridge (DAB) converter. The DAB converter receives a DC voltage from a front end rectifier and outputs to a DC link capacitor. In operation, its DC input is inverted by a primary-side bridge and rectified by a secondary-side bridge. The DAB converter is typically operated by a controller to regulate the magnitude of the DC output in accordance with battery charging requirements. Reference may be made to US 9 729 066 B1 which relates to electric power conversion apparatus having single-phase and multi-phase operation modes. An AC / DC conversion apparatus includes first, second, and third AC / DC conversion modules operated by a controller in two modes of operation. In the first mode, the input AC signal is 3-phase and each of the three modules are enabled to handle a respective one of the input phases. In the second mode, the input AC signal is single phase and the first and second modules are enabled to deliver output power based on the single-phase AC input, while the controller actuates an H-bridge switches in the third module to which active filter circuitry is connected, to reduce an AC component in the output signal. The active filter circuitry can be selectively connected to the H-bridge switches when single-phase operation is desired. Reference may be made to MEHTA P ET AL: "SWITCHED - CAPACITOR FILTERS", 26 June 1989, PROCEEDINGS OF THE ANNUAL POWER ELECTRONICS SPECIALISTS CONFERENCE, MILWAUKEE, NEW YORK, IEE, US, PAGE(S) 392 - 399, which relates to an active filter using switched-capacitor circuits. It discloses a novel technique for building 'active filters' using switched-capacitor (s-c) circuits. The technique is used to control input current harmonics in a phase-controlled converter.

[0004] Electrical grids exist as three-phase electrical grids or single-phase electrical grids, often changing between the two. While high-power charging is more ideally suited to a three-phase electrical grid than a single-phase electrical grid, the latter may be the only option available. It is therefore desirable to have an on-board charger that can support both grid types. In particular, it is desirable to have an on-board charger that removes second-order harmonics during single-phase charging to provide a purely DC output from a single-phase electrical grid.SUMMARY OF THE INVENTION

[0005] The present invention is defined by the appended independent claims, to which reference should now be made. Specific embodiments are defined in the dependent claims. In one arrangement described herein, an on-board charger having a dual-purpose switch-mode converter for electrical connection to a three-phase electrical grid and a single-phase electrical grid is provided. When connected to a three-phase electrical grid, three switch-mode converters, for example three DAB converters, convert a rectified voltage to produce direct current at the output. When connected to a single-phase electrical grid, the third switch-mode converter is bi-directional such that it is operable to filter the power output of the first and second switch-mode converters and thereby maintain direct current at the output. The on-board charger includes a filter capacitor in electrical communication with the third switch-mode converter through a relay. The relay is responsive to a controller to couple the third switch-mode converter to the filter capacitor when the single-phase electrical grid is detected and to couple the third switch-mode converters to the third phase of the electrical grid when the three-phase electrical grid is detected.

[0006] In one arrangement, the on-board charger includes a first DAB converter, a second DAB converter, and a third DAB converter, each being operable to convert a rectified voltage into a DC component when coupled to a three-phase electrical grid. When coupled to a single-phase electrical grid, however, the input stage of the third DAB converter is coupled to a filter capacitor as an active filter to remove harmonics from the DC output of one or both of the first and second DAB converters. In particular, the filter capacitor draws power from the DC output through the third DAB converter if the DC output is greater than a power reference and supplies power to the DC output through the third DAB converter if the DC output is less than the power reference. The input stage of each DAB converter is electrically coupled to a rectifier, optionally a full-bridge rectifier. A relay alternatively couples the input stage of the third DAB converter to the rectifier or to the filter capacitor, depending on whether a single-phase electrical grid or a three-phase electrical grid is detected.

[0007] In another arrangement, a method of operation is provided. The method includes determining the presence of a single-phase electrical grid or a three-phase electrical grid at the input of the on-board charger. If a three-phase electrical grid is detected, a controller operates a first rectifier and a first DAB converter to convert a first phase of the AC input into a first DC component, operates a second rectifier and a second DAB converter to convert a second phase of the AC input into a second DC component, and operates a third rectifier and a third DAB converter to convert a third phase into a second DC component. If a single-phase electrical grid is detected, the controller causes the third DAB converter to decouple from the third rectifier and couple to a filter capacitor. The controller further causes the third DAB converter to operate as an active filter of the DC output of the first and second DAB converters, such that the filter capacitor draws power from the DC output through the third DAB converter if the DC output is greater than a power reference and supplies power to the DC output through the third DAB converter if the DC output is less than the power reference.

[0008] In single-phase operation, the third DAB converter functions as the switch-mode and inductive elements of an output active filter to cancel second-order harmonics in a single-phase system. By using one DAB converter as an active filter, the hardware utilization of the entire system is maximized while the number of additional hardware and software requirements are minimized. The inductive component of the active filter is provided by the transformer of the DAB converter, and the output sensors to control the reactive power are already in place for three-phase operation. Furthermore, because the control algorithm for the DAB converter can be modulated bi-directionally through zero power, only a new power references need be applied to the same open loop algorithm, meaning the control loop maintaining the capacitor voltage can be designed using the same transfer function as the other two modules.

[0009] An advantage of positioning a relay between a grid rectifier and the converter modules is that the relay allow the grid rectifier to short its unused phase to neutral and add an additional conduction path to the neutral line. When switching from three-phase to single-phase operation, if one converter is used as an active filter while the other two work in parallel to supply power to the output, the neutral connection must handle the return current of both phase connections. By shorting the neutral to the third, unused phase, through the unused rectifier, balance is restored to the current distribution between the neutral and phase connections.

[0010] These and other features and advantages of the present invention will become apparent from the following description of the invention, when viewed in accordance with the accompanying drawings and the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a circuit diagram of a three-phase / single-phase, single stage on-board charger for a vehicle battery. Figure 2 is a circuit diagram of a first alternative embodiment of the on-board charger of Figure 1. Figure 3 is a circuit diagram of a second alternative embodiment of the on-board charger of Figure 1. Figure 4 includes three embodiments of a relay for disconnecting the electrical grid and connecting the filter capacitor. Figure 5 includes six embodiments of relay placement to switch electrical connection to the DAB module between the electrical grid and the filter capacitor. Figure 6 is a circuit diagram of an on-board charger including a filter capacitor between a full bridge rectifier and a full-bridge DAB primary. Figure 7 is the circuit diagram of Figure 6 including third and fourth relays coupled between the filter capacitor and the output. Figure 8 is a graph of instantaneous grid voltage, current, and power sourced by a single phase of the grid under unity power-factor operation. Figure 9 is a graph of idealized operation of a three-phase / single-phase, single stage on-board charger, using one phase as an output active filter. DETAILED DESCRIPTION OF THE CURRENT EMBODIMENT

[0012] Referring now to Figure 1, a three-phase / single-phase on-board charger (OBC) is illustrated and generally designated 10. As discussed below, one DAB module of the OBC 10 is optionally used as the switch-mode and inductive elements of an output active filter and employs a single relay on the primary, high-side DC linkage to dynamically switch from a three-phase configuration when only a single-phase connection is detected.

[0013] More specifically, the OBC 10 is coupled to a three-phase electrical grid or a single-phase electrical grid 12. The OBC 10 includes a controller 14 for providing control signals to each of three DAB module 16, 18, 20, each with eight controllable switches. The controllable switches of the DAB modules 16, 18, 20 are operated by the controller 14 to provide a DC voltage to a battery 22, for example a vehicle battery. The controller 14 is also operable to detect a single-phase connection and a three-phase connection and is operable to control operation of one or more relays 24 coupled to a filter capacitor 32. Three grid synchronous rectifiers 26, 28, 30 provide a varying DC voltage to each DAB module 16, 18, 20.

[0014] During three-phase operation, each DAB module 16, 18, 20 is responsible for power factor correction of a single alternating current. The resulting DC output is achieved through the combination of all three DAB outputs. When applied to a three-phase grid, the isolated AC / DC system can take advantage of the 120° shift between each phase of the grid voltage. When the phase shift is 120° and the amplitude is equivalent between each voltage sinusoid, the grid is said to be balanced. This means that the sum of all three-phase voltages at any time t will be equal to zero as will, by extension, the phase currents and all their harmonics, given a unity power factor.

[0015] During single-phase operation, and to cancel the second-order harmonic in a single-phase system, one DAB module 20 from the three-phase system is dynamically repurposed as the switch-mode and inductive elements of an output active filter. Once connected to the filter capacitor 32, optionally a capacitor bank, the solo DAB module is operated bi-directionally, about 180° out of phase with the other two DAB modules 16, 18 to achieve active filtering. While three possible embodiments are shown in Figures 1 through 3, the embodiment of Figure 1 is most advantageous for its lower conduction losses and additional separation from the grid, and is therefore discussed in further detail below.

[0016] By using one module 20 as an active filter, the hardware utilization of the entire system is maximized while the number of additional hardware and software requirements are minimized. Only the relay mechanism 24, filter capacitor 32, and a voltage sensor 34 across the filter capacitor 32 are required in addition to the existing hardware for three-phase operation. No additional PWM channels, only one additional ADC channel, and potentially as few as one additional GPIO output are required from the primary controller. The inductive component of the active filter is provided by the transformer and the output sensors to control the reactive power are already in place for three-phase operation. Furthermore, because the control algorithm can be modulated bi-directionally through zero power, only a new power references need be applied to the same open loop modulation algorithm, meaning the control loop maintaining the capacitor voltage can be designed using the same transfer function as the other two modules.

[0017] An on-board charger 10 in accordance with a second embodiment is illustrated in Figure 2. This is embodiment differs from the embodiment of Figure 1 in that the primary-side full bridge of each DAB 16, 28, 20 now includes a matrix converter having multiple bidirectional semiconductor switches for each phase of the electrical grid 12. In addition, the filter capacitor 32 is connected to the input side of the matrix converter for the third DAB, or "Matrix DAB 20" as depicted in Figure 2. As also shown in Figure 3, the filter capacitor 32 is connected to the input side of the third grid synchronous rectifier 30. The embodiments of Figures 2 and 3 are otherwise functionally similar to the embodiment of Figure 1, in that each Matrix DAB or each DAB module is responsible for power factor correction of a single alternating current when connected to a three-phase grid, with one Matrix DAB or DAB module being repurposed as the switch-mode and inductive element of an active filter when connected to a single-phase grid.

[0018] The relay mechanism 24 may be any device that breaks the grid connection and simultaneously connects the filter capacitor. This may be realized with a single device, such as a single-pole, double-throw type mechanical relay, or with a combination of devices, such as complementary switched, solid-state or mechanical relays. Figure 4 shows three device agnostic embodiments, in which a relay is made up of one or more devices. While either the grid, the filter capacitor, or neither may be connected to the DAB module 20, both being connected to the DAB simultaneously would be considered a fault condition. A relay in the form of Figure 4(a) and Figure 4(c)may be preferred for this reason.

[0019] Implementation of the relays may have a variety of embodiments since only the electrical circuit must be broken to disconnect either the grid or the filter capacitor, as opposed to all physical connections. Figure 5 shows six embodiments of relay placement that will sufficiently break electrical connection to either the grid or the filter capacitor from the DAB module. Figure 5a, 5c, and 5e, while viable, are less desirable as the rectifier will add thermal losses during the active filter configuration that can be avoided by using the configures in Figures 5b, 5d, and 5f. Of those, Figure 5d and Figure 5f are most desirable from a cost and space optimization standpoint while Figure 5b may be preferred from an isolation or EMI perspective. The embodiment of Figure 1 shows the relay placement in Figure 5d, while Figure 6 shows the relay placement of Figure 5b. Figure 7 shows a further embodiment in which the filter capacitor 32 is coupled to the output rail voltage through third and fourth relays 25. In this embodiment, a total of four relays are used, which allow for the filter capacitor 32 to be switched between the output rail voltage in three-phase mode and the input DC linkage during single-phase mode. Consequently, the filter capacitor 32 is used both in three-phase operation and single-phase operation. In three-phase operation, switches Sr7 and Sr8 are closed such that the filter capacitor 32 reduces the output current ripple to the battery 22 and generally increases the ability to absorb transients on the grid so that they are not seen by the battery. In single-phase operations, switches Sr7 and Sr8 are open. The configuration in Figure 7 therefore allows for maximum utilization of the filter capacitor 32 irrespective of the grid configuration. Depending upon the particular system, connecting the filter capacitor 32 to the output rail voltage could increase the output capacitance five to six times, while also greatly increasing the ability to supply transient demands on the output bus from other systems connected to the battery.

[0020] An additional advantage of placing the relays on the DC linkage between the unfolded rectifier and the DAB primary full-bridge is that it allows the rectifier to short its unused phase to neutral and add an additional conduction path to the neutral line. When switching from the three-phase and single-phase operation, if one DAB module is used as an active filter while the other two work in parallel to supply power to the output, the neutral connection must handle the return current of both phase connections. By shorting the neutral to the third, unused phase, through the unused rectifier, balance is restored between the neutral and phase connections.

[0021] The principle of operation of the OBC 10 will now be described. The control algorithm used to modulate the power of each phase's DAB module allow the instantaneous power through each to be controlled to any reference waveform. Therefore, the power reference supplied to the third DAB module 20, being used as the active filter, need merely be formulated to destructively interfere with the undesired output harmonics created by the power modulation of the other DAB modules. To illustrate this concept, Figure 8 shows the power command needed to draw 14 kW, average power, from a 277 V RMS single-phase grid with unity power factor. This would therefore be the power reference applied to the first two DAB modules (16), (18) in Figure 1. Assuming an idealized system, the output power, P out , would equal the input power, P grid , as a 120 Hz sine wave with a 14 kW amplitude and 14 kW DC offset. The desired output power is the average power output, P avg , plotted in Figure 9. Therefore, the power that must be supplied to the output to cancel the output harmonic is P act = P avg - P out . If P out is represented by the expression 14k*sin(120πt)+14kW, and if P avg is represented by the expression 14 kW, then P act is equal to - 14k*sin(120πt), which has an RMS value of P avg √ 2 W and a net contribution of 0W. During the negative portion of P act in Figure 9, the filter DAB module 20 is drawing power away from the output and storing that energy to the filter capacitor 32. This is the region in which the PFC DABs are supplying more power than the desired average. During the region where P act is positive, the filter DAB module 20 is pulling the stored energy out of the filter capacitor 32 and delivering it to the output. This is the region where the PFC DAB modules 16, 18 are delivering less power than desired. In this way, the load only sees the average power as the desired DC output.

Claims

1. An on-board charger (10) for a vehicle comprising: a first switch-mode converter (16) being operable to convert a first rectified voltage into a first DC component; a second switch-mode converter (18) being operable to convert a second rectified voltage into a second DC component; a third switch-mode converter (20) being operable to convert a third rectified voltage into a third DC component, the third switch-mode converter (20) being bidirectional such that the third switch-mode converter (20) is operable as an active filter to remove harmonics from a DC output of the first (16) and second (18) switch-mode converters when coupled to a single-phase electrical grid, wherein the third switch mode converter (20) includes a primary-side full bridge, an isolation transformer, and a secondary-side full bridge; and a first rectifier (26), a second rectifier (28), and a third rectifier (30) electrically coupled in series with the first switch-mode converter (16), the second switch-mode converter (18), and the third switch-mode converter (20), respectively; and a filter capacitor (32) selectively coupled between the third rectifier (30) and the primary-side full bridge of the third switch-mode converter (20), wherein the filter capacitor (32) draws power from the DC output through the third switch-mode converter (20) if the DC output is greater than a power reference and supplies power to the DC output through the third switch-mode converter (20) if the DC output is less than the power reference.

2. The on-board charger (10) of claim 1 wherein each of the first, second, and third switch-mode converters (16, 18, 20) includes a dual-active-bridge topology.

3. The on-board charger (10) of claim 1 wherein the first rectifier (26), the second rectifier (28), and the third rectifier (30) each include a full bridge topology.

4. The on-board charger (10) of claim 1 further including a relay (24) to alternatively couple the primary-side full bridge of the third switch-mode converter (20) to the third rectifier (30) and the filter capacitor (32).

5. The on-board charger (10) of claim 4 further including a controller (14) to detect a single-phase AC input voltage and a three-phase AC input voltage and to control operation of the relay (24).

6. The on-board charger (10) of claim 5 wherein each of the first, second, and third switch-mode converters (16, 18, 20) provide unity power factor when coupled to the three-phase AC input voltage.

7. The on-board charger (10) of claim 5 wherein the third switch-mode converter (20) is operated 180° out of phase relative to the first and second switch-mode converters (16, 18) when the single-phase AC input voltage is detected by the controller (14).

8. The on-board charger (10) of claim 5 further including a voltage sensor (34) across the filter capacitor (32), the voltage sensor (34) being coupled to the controller (14).

9. The on-board charger (10) of claim 1 wherein the power reference is selected such that the third switch-mode converter (20) destructively interferes with the harmonics of the DC output.

10. A method for converting an AC input voltage into a DC output voltage comprising: providing an on-board charger (10) including a first rectifier (26), a second rectifier (28), and a third rectifier (30) electrically coupled in series with a first switch-mode converter (16), a second switch-mode converter (18), and a third switch-mode converter (20), respectively, wherein the third switch mode converter (20) includes a primary-side full bridge, an isolation transformer, and a secondary-side full bridge, the on-board charger (10) further including a filter capacitor (32) selectively coupled between the third rectifier (30) and the primary-side full bridge of the third switch mode converter (20); in response to a three-phase AC input being detected at the on-board charger (10), operating the first rectifier (26) and the first switch-mode converter (16) to convert a first phase of the AC input into a first DC component, operating the second rectifier (28) and the second switch-mode converter (18) to convert a second phase of the AC input into a second DC component, and operating the third rectifier (30) and the third switch-mode converter (20) to convert a third phase of the AC input into a third DC component; and in response to a single-phase AC input being detected at the on-board charger (10), filtering a DC output of at least one of the first and second switch-mode converters (16, 18) through the third switch-mode converter (20) to reduce harmonics in the DC output, wherein filtering the DC output includes de-coupling the third switch-mode converter (20) from the third rectifier (30) and coupling the primary-side full bridge of the third switch-mode converter (20) to the filter capacitor (32), such that the filter capacitor (32) draws power from the DC output if the DC output is greater than a power reference and supplies power to the DC output if the DC output is less than the power reference.

11. The method of claim 10 wherein the first rectifier (26), the second rectifier (28), and the third rectifier (30) each include a full bridge topology.

12. The method of claim 10 wherein the first switch-mode converter (16), the second switch-mode converter (18), and the third switch-mode converter (20) each include a dual-active-bridge topology.

13. The method of claim 10 wherein the filter capacitor (32) includes a capacitor bank, the capacitor bank being coupled to the third switch-mode converter (20) through a relay (24).

14. The method of claim 10 further including: coupling the DC output of the on-board charger (10) to a vehicle battery (22); or coupling the filter capacitor (32) to a DC output of the on-board charger (10) through first and second switches (Sr5, Sr6) when a three-phase AC input is detected to reduce current ripple in the DC output of the on-board charger (10), optionally including decoupling the filter capacitor (32) from the DC output of the on-board charger (10) through the first and second switches (Sr5, Sr6) when a single-phase AC input is detected.