Configurable ac / DC converter arrangement with parallel and series modes for extended DC voltage output

Abstract

The present invention provides an flexible electric AC to DC power converter suited for supplying DC power to e.g. an electrolyzer such as for Power-to-X applications. The power converter has an AC input terminal (2) and a DC output terminal (6). First and second converter modules (10, 20) each comprises at least one active three-phase converter bridge comprising an arrangement of semiconductor-based electric switches. First and second controllable electric switches (15, 25) serve to connect or disconnect inputs (12) of the respective first and second converter modules (10, 20) to the electric input terminal (2). A controllable electric output switch arrangement (42, 44) is configured for connecting the electric output terminal (6) to one of: the input (22) of the second converter module (20), and the converter output (4). In a first mode of operation, a high DC voltage mode, the first and second controllable electric switches (15, 25) are controlled to connect the respective inputs (12, 22) of the first and second converter modules (10, 20) to the electric input terminal (2), and wherein the controllable electric output switch arrangement (42, 44) is controlled to connect the electric output terminal (6) to the converter output (4). In a second mode of operation, a low DC voltage mode, the first controllable electric switch (15) is controlled to connect the input (12) of the first converter module (10) to the electric input terminal (2), the second controllable electric switch (20) is controlled to disconnect the input (22) of the second converter module (20), and wherein the controllable electric output switch arrangement (42, 44) is controlled to connect the electric output terminal (6) to the input (22) of the second converter module (20), so as to allow the second converter module (20) to operate as a Pulse Width Modulated DC-DC Buck converter.

Description

[0001] CONFIGURABLE AC / DC CONVERTER ARRANGEMENT WITH PARALLEL AND SERIES MODES FOR EXTENDED DC VOLTAGE OUTPUT

[0002] FIELD OF THE INVENTION

[0003] 5 The present invention relates to power electronics. More specifically, the invention relates to an electric power converter which has a dynamic configuration that allows an extended DC power capability output. More specifically, the invention relates to an AC to DC power converter, such as suitable for wind turbine applications such as for providing DC power to an electrolyzer for Power-to-X generation or other DC objects or devices requiring a precharging functionality.

[0004] BACKGROUND OF THE INVENTION

[0005] Providing DC power for e.g. electrolyzers based on electric power from an AC

[0006] 15 source based on an AC-DC converter normally requires pre-charging to allow operation at low DC voltages. This is due to diode bias in the rectifier circuit of the AC-DC converter.

[0007] Such pre-charging can be provided by a resistive pre-charging (dissipating heat)

[0008] 20 or by adding a separate AC-DC converter serving to provide the pre-charging. Both of these solutions add components to the converter system, and both of these solutions have a relatively low reliability due to these extra components.

[0009] SUMMARY OF THE INVENTION

[0010] 25

[0011] It is an object of the present invention to provide an AC-DC power converter capable of operating at a wide range of DC voltages with a high efficiency and with a limited number of components.

[0012] In a first aspect, the invention provides an electric AC to DC power converter arranged to receive an electric AC voltage at an electric input terminal and to generate an electric DC voltage at an electric output terminal, the power converter comprising:

[0013] - a first converter module comprising at least one active three-phase converter

[0014] 35 bridge comprising an arrangement of semiconductor-based electric switches, - a first controllable electric switch, such as a circuit breaker, serving to connect or disconnect an input of the first converter module to the electric input terminal,

[0015] - a second converter module comprising at least one active three-phase converter bridge comprising an arrangement of semiconductor-based electric switches,

[0016] - a second controllable electric switch, such as a circuit breaker, serving to connect or disconnect an input of the second converter module to the electric input terminal,

[0017] - a converter output connected to outputs of both of the first and second converter modules, and

[0018] - a controllable electric output switch arrangement configured for connecting the electric output terminal to one of: the input of the second converter module, and the converter output, wherein in a first mode of operation, the first and second controllable electric switches are controlled to connect the respective inputs of the first and second converter modules to the electric input terminal, and wherein the controllable electric output switch arrangement is controlled to connect the electric output terminal to the converter output, and wherein in a second mode of operation, the first controllable electric switch is controlled to connect the input of the first converter module to the electric input terminal, the second controllable electric switch is controlled to disconnect the input of the second converter module, and wherein the controllable electric output switch arrangement is controlled to connect the electric output terminal to the input of the second converter module, so as to allow the second converter module to operate as a Pulse Width Modulated DC-DC Buck converter.

[0019] Such power converter is advantageous, since it allows the power converter to operate at high DC output voltages (e.g. 1 kV) as well as low DC output voltages (down to zero V) since it has a dynamic configuration with two modes of operation.

[0020] In the first mode of operation, a high DC voltage mode, the two converter modules both operate as AC-DC converter, thereby cooperating to generate high DC power up to high DC voltages. In the second mode of operation, a low DC voltage mode, the second converter module is operated as a DC-DC Buck converter which allows operation at very low DC voltages which can be required by certain types of loads, for example an electrolyzer for hydrogen production.

[0021] Still, such wide range of DC operating voltages is possible without adding separate pre-charging components.

[0022] The power converter according to the invention is further easy to scale to a desired power rating, since it can be built of a plurality of identical converter modules operating in parallel.

[0023] Especially, the invention utilizes the property of an active three-phase bridge converters that allows the same hardware to be operated in an AC-DC mode as well as in a DC-DC mode. Especially, in DC-DC mode, the phases of such three- phase bridge converter can be Pulse Width Modulated to a desired output voltage by adjusting the PWM period. Especially, the phases can be operated with the same duty cycle but phase shifted (interleaved). In preferred embodiments, the three-phase bridge converter are two-level converters, thus one converter module can be implemented by a stack of six interconnected semiconductor switches.

[0024] The power converter is suited as an AC-DC converter for a three-phase AC output of an electric generator, such as an electric generator of a wind turbine. More specifically the power converter is highly suited as electric interface between a wind turbine generator and an electrolyzer system, such as for Power-to-X applications, or between a grid MVAC, a transformer and a Power-to-X application involving an electrolyzer.

[0025] In the following, preferred features and embodiments will be described.

[0026] Preferably, the power converter comprises a controller configured to control the first and second controllable electric switches and the controllable electric output switch arrangement, thereby switching between the first and second mode of operation of the power converter. Especially, the controller may be configured to sense a DC voltage at the converter output, and / or at the electric output terminal, and to enter the first or second mode of operation according to the sensed DC voltage. More specifically, the controller may be configured to enter the second mode of operation when the sensed DC voltage is below a predetermined threshold value, and wherein the controller is configured to enter first mode of operation otherwise. Especially, said threshold value may be selected to a voltage level where the first and second converter modules are capable of controlling a DC power flow to the electric output terminal, such as a threshold of about 100-600 V, but the selected threshold may in a specific application depend on various parameters.

[0027] In preferred embodiments, the first converter module comprises a plurality of active three-phase converter bridge modules, such as two-level three-phase converters, in a parallel connection between the input of the first converter module and the converter output. For example, 2-10 identical active three-phase converter bridge modules, such as two-level three-phase converters, may constitute the first converter module.

[0028] In preferred embodiments, the second converter module comprises a plurality of active three-phase converter bridge modules, such as two-level three-phase converters, in a parallel connection between the input of the second converter module and the converter output. For example, 2-10 identical active three-phase converter bridge modules, such as two-level three-phase converters, may constitute the first converter module.

[0029] Especially, both of the first and second converter modules may comprise a plurality of active three-phase converter bridge modules, such as two-level three- phase converters, such as 2-10 identical two-level three-phase converters. For example, both converter modules each comprises two two-level three-phase converters. In specific examples, the first and second converter modules may comprise the same number of active three-phase converter bridge modules, such as two-level three-phase converters, but it may be preferred that the two converter modules have different numbers of two-level three-phase converters, for example the first converter module may include more active three-phase converter bridge modules than the second converter module.

[0030] Thus, by adding further identical sets or stacks of active three-phase converter bridge modules, such as two-level three-phase converters, to the first and second power modules, the power converter can be scale to the desired power handling. The electric input terminal is configured to receive a three-phase electric AC input.

[0031] The first and second controllable electric switches or circuit breakers are thus also preferably three-pole switches.

[0032] In some embodiments, the power converter comprises a third converter module comprising at least one active three-phase converter bridge comprising an arrangement of semiconductor-based electric switches, further comprising a third controllable electric switch serving to connect or disconnect an input of the third converter module to the electric input terminal, and wherein an output of the third converter module is connected to the converter output. In these embodiments, an extra switch or circuit breaker is required, that adds complexity, but in this way it is possible to further scale power handling of the power converter. It is to be understood that further than sets of controllable electric switch and converter module can be added, thus having a total of four or more sets, if preferred.

[0033] Preferably, the controllable electric output switch arrangement comprises two controllable electric switches which are operated to opposite switch function so as to switch the output between the first and second modes of operation.

[0034] In some embodiments, the controllable electric output switch arrangement comprises a two-pole, a three-pole four-pole or a six-pole controllable electric switch, such as comprising two two-pole, four-pole or six-pole controllable electric switches. In general, more poles could be considered to deviate the "bulk" current by more "branches".

[0035] The power converter according to any of the preceding claims, wherein the power converter is configured to generate a DC output voltage of at least 500 V, such as at least 1 kV, such as 1-3.3 kV. However, in general the power converter topology can be scaled with respect to both voltage and power handling.

[0036] In a second aspect, the invention provides a wind turbine comprising:

[0037] - an AC electric power generator with an AC electric output, such as a fixed frequency output, and - a power converter according to the first aspect, wherein the AC electric output of the AC electric power generator is connected to the electric input terminal of the power converter.

[0038] Especially, the electric output terminal of the power converter may be connected to a DC electric power input of an electrolyzer system, such as an electrolyzer system configured to generate hydrogen.

[0039] In a third aspect, the invention provides an electrolyzer system, such as an electrolyzer system configured to generate hydrogen, the system comprising:

[0040] - an electrolyzer with a DC electric power input, and

[0041] - the power converter according to the first aspect, wherein the electric output terminal of the power converter is connected to the DC electric power input of the electrolyzer.

[0042] Especially, the power converter is connected with its electric AC input terminal to one of: 1) an AC power source with a fixed frequency AC output, 2) an AC power source with a variable frequency AC output. The fixed frequency AC power source can be such as a wind turbine generator or an electric grid. The variable frequency AC power source can be a wind turbine generator or other power generator.

[0043] In a fourth aspect, the invention provides a method for converting an electric AC input to an electric DC output, the method comprising:

[0044] - providing a first converter module comprising at least one active three-phase converter bridge comprising an arrangement of semiconductor-based electric switches,

[0045] - providing a second converter module comprising at least one active three-phase converter bridge comprising an arrangement of semiconductor-based electric switches,

[0046] - operating the first and second converter modules in a first mode of operation, wherein the inputs of the first and second converter modules are connected to the electric input terminal, and wherein the electric output terminal is connected to outputs of both of the first and second converter modules, and

[0047] - operating the first and second converter modules in a second mode of operation, wherein the input of the first converter module to connected to the electric input terminal, wherein the electric output terminal is connected to the input of the second converter module, and wherein the second converter module to operate as a Pulse Width Modulated DC-DC Buck converter.

[0048] The method preferably comprises sensing a DC voltage at the electric output terminal, and controlling electric switches for selectively operating the first and second converter modules in the first or second mode of operation in response to said sensed DC voltage.

[0049] Features and embodiments of the mentioned aspects of the present invention may each be combined with each other. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0050] BRIEF DESCRIPTION OF THE FIGURES

[0051] The present invention and in particular preferred embodiments thereof will now be disclosed in more detail with regard to the accompanying figures. The figures show ways of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

[0052] FIG. 1 illustrates a block diagram of a power converter embodiment,

[0053] FIG. 2a and 2b illustrate electric states of the controllable switches and power flow in the two modes of operation of the power converter embodiment of FIG. 1, FIG. 3 illustrates a preferred active three-phase converter bridge module in the form of a two-level three-phase converter module,

[0054] FIG. 4 illustrates a preferred embodiment with two times two two-level three- phase converter modules,

[0055] FIG. 5 and 6 illustrate two different ways of operating the second converter module (Referring to FIG. 1) in case of two-level three-phase converter modules being used and in case of a two-pole output switch arrangement,

[0056] FIG. 7 illustrates an example of operating the second converter module (Referring to FIG. 1) in case of two-level three-phase converter modules being used and in case of a two-pole output switch arrangement, FIG. 8a and 8b illustrate application of the power converter for Power-to-X applications based on power from a wind turbine and a MVAC grid, and FIG. 9 illustrates steps of a method embodiment.

[0057] DETAILED DESCRIPTION OF EMBODIMENTS

[0058] FIG. 1 illustrates a block diagram of an AC to DC power converter power embodiment with an AC input terminal 2 and a DC output terminal 6. For simplicity, the diagram is single lined, even though in preferred embodiments, the power converter is arranged to operate on a three-phase electric AC input.

[0059] The power converter has two converter modules 10, 20 each comprising one or more active three-phase converter bridge modules having an arrangement of semiconductor-based electric switches operated according to a Pulse Width Modulation scheme.

[0060] A first controllable electric switch or circuit breaker 15 serves to connect or disconnect an input 12 of the first converter module 10 to the electric input terminal 2, while a second controllable electric switch or circuit breaker 25 serves to connect or disconnect an input 22 of the second converter module 20 to the electric input terminal 2.

[0061] A converter output 4 is connected to outputs 14, 24 of both of the first and second converter modules 10, 20.

[0062] A controllable electric output switch arrangement with two switches 42, 44 is configured for connecting the electric output terminal 6 to one of: the input 22 of the second converter module 20, and the converter output 4.

[0063] In a first mode of operation, the first and second controllable electric switches 15, 25 are controlled to connect the respective inputs 12, 22 of the first and second converter modules 10, 20 to the electric input terminal 2, and wherein the controllable electric output switch arrangement 42, 44 is controlled to connect the electric output terminal 6 to the converter output 4. In the first mode of operation, both converter modules 10, 20 operate as AC-DC converter and in this mode, the power converter can handle maximum DC output power.

[0064] In a second mode of operation, the first controllable electric switch 15 is controlled to connect the input 12 of the first converter module 10 to the electric input terminal 2, the second controllable electric switch 25 is controlled to disconnect the input 22 of the second converter module 20, and the controllable electric output switch arrangement 42, 44 is controlled to connect the electric output terminal 6 to the input 22 of the second converter module 20, so as to allow the second converter module 20 to operate as a Pulse Width Modulated DC- DC Buck converter. In the second mode of operation, the first converter module 10 operates as AC-DC converter, while the second converter module 20 operates as DC-DC converter, and thus allows the power converter to control DC power output also as low DC voltages.

[0065] A controller 30 controls operation of the controllable switches 15, 25, 42, 44 and thus controls whether the power converter operates in the first or second mode of operation. The controller 30 decided about the mode of operation in response to sensing the voltage at the converter output 4 and preferably also in response to a voltage sensed at the output 6. If this voltage is below a threshold value, then the controller 30 operates the switches 15, 25, 42, 44 to enter the first mode of operation. The second mode of operation is entered in case the sensed voltage exceeds the threshold value.

[0066] In this way, the power converter configuration shown in FIG. 1 provides a flexible AC-DC power conversion which especially allows applications where a DC load connected to the output terminal 6 may be required to operate at very low DC voltages, for example an electrolyzer such as for Power-to-X applications.

[0067] FIG. 2a and 2b illustrate electric states of the controllable switches and power flow in the two modes of operation of the power converter embodiment of FIG. 1.

[0068] In FIG. 2a, the switches 15, 25, 42, 44 are set to operate according to the first mode of operation. Here, both of the first module 10 are connected to operate as AC-DC converters, i.e. both converter inputs 12, 22 are connected to the input terminal 2, and the converter output 4 is connected to the output terminal 6.

[0069] In FIG. 2b, the switches 15, 25, 42, 44 are set to operate according to the second mode of operation. Here, the first converter module 10 operates as AC-DC converter, while the second converter module 20 operates as DC-DC converter. Thus, only input 12 is connected to the input terminal 2, while the input 22 is connected to the output terminal 6.

[0070] FIG. 3 illustrates a preferred active three-phase converter bridge circuit to be used both for the first and second converter modules 10, 20 of the embodiment of FIG. 1. The circuit is a two-level three-phase converter module, where three sets of two semiconductor switches S1-S6 are connected between three input terminals to the left, one for each electric phase, and with input inductors LI. A capacitor Cl is connected across two output terminals to the right.

[0071] In one mode, the switches S1-S6 are operated according to a PWM switching scheme to act as an AC-DC converter. In another mode, the same circuit can be operated as DC-DC Buck converter. Thus, the identical circuit as shown in FIG. 3 can be used both for the first and second converter module 10, 20 in the embodiment of FIG. 1.

[0072] FIG. 4 illustrates a preferred embodiment with two times two two-level three- phase converter modules between a three phase AC input to the left, and a DC output at terminals + and -, shown to the right. Referring to the embodiment of FIG. 1, two circuits as shown in FIG. 3 are used to implement the first converter module 10, while two circuits as shown in FIG. 3 are used to implement the second converter module 20.

[0073] FIG. 5 and 6 illustrate two different ways of operating the two-level three-phase converter modules implementing the second converter module, i.e. the two lower two-level three-phase converter modules of FIG. 4 or module 20 in FIG. 1, in the second mode of operation in case of a two-pole output switch arrangement, which allows two electric terminals to be used for interleaving to generate the DC output at terminals + and -. The configuration shown in FIG. 5 provides a high efficiency, since only few components are series connected, while the configuration shown in FIG. 6 provides a higher filter capability.

[0074] FIG. 7 illustrates an example of operating the two-level three-phase converter modules implementing the second converter module, i.e. the two lower two-level three-phase converter modules of FIG. 4, or module 20 of FIG. 1, in the second mode of operation in case of a four-pole output switch arrangement, which allows up to six electric terminals to be used for interleaving to generate the DC output at terminals + and -.

[0075] FIG. 8a illustrates an example of a wind turbine 70 according to the invention with typically two or three rotor blades 78 which serve to drive an electric generator 72 located inside the nacelle 76 on top of a tower 74. Such wind turbine 70 may generate an electric power of at least 1 MW, such as 2-20 MW, or more. In block diagram format it is illustrated that the electric generator 72 provides an electric input to an electric power converter 80 which is configured for converting a three- phase electric AC input from the generator 72 of the wind turbine 70 and to output an electric DC output for powering an electrolysis process 90.

[0076] FIG. 8b illustrates a block diagram format application of the electric power converter 180 which is configured for converting a three-phase electric MVAC input from an electric grid via a transformer and to output an electric DC output for powering an electrolysis process 190, such as a part of a Power-to-X application.

[0077] FIG. 9 illustrates steps of a method embodiment, i.e. a method for converting an electric AC input to an electric DC output. First, providing 110 a first converter module comprising at least one active three-phase converter bridge comprising an arrangement of semiconductor-based electric switches. Next, providing 112 a second converter module comprising at least one active three-phase converter bridge comprising an arrangement of semiconductor-based electric switches. Further, operating 114 the first and second converter modules in a first mode of operation, wherein the inputs of the first and second converter modules are connected to the electric input terminal, and wherein the electric output terminal is connected to outputs of both of the first and second converter modules. Further, operating 116 the first and second converter modules in a second mode of operation, wherein the input of the first converter module to connected to the electric input terminal, wherein the electric output terminal is connected to the input of the second converter module, and wherein the second converter module to operate as a Pulse Width modulated DC-DC Buck converter. Further, sensing 118 a DC voltage at the electric output terminal, and controlling 120 electric switches for selectively operating the first and second converter modules in the first or second mode of operation in response to said sensed DC voltage.

[0078] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

CLAIMS1. An electric AC to DC power converter arranged to receive an electric AC voltage at an electric input terminal (2) and to generate an electric DC voltage at an electric output terminal (6), the power converter comprising:- a first converter module (10) comprising at least one active three-phase converter bridge comprising an arrangement of semiconductor-based electric switches,- a first controllable electric switch (15) serving to connect or disconnect an input (12) of the first converter module (10) to the electric input terminal (2),- a second converter module (20) comprising at least one active three-phase converter bridge comprising an arrangement of semiconductor-based electric switches,- a second controllable electric switch (25) serving to connect or disconnect an input (22) of the second converter module (20) to the electric input terminal (2),- a converter output (4) connected to outputs (14, 24) of both of the first and second converter modules (10, 20), and- a controllable electric output switch arrangement (42, 44) configured for connecting the electric output terminal (6) to one of: the input (22) of the second converter module (20), and the converter output (4), wherein in a first mode of operation, the first and second controllable electric switches (15, 25) are controlled to connect the respective inputs (12, 22) of the first and second converter modules (10, 20) to the electric input terminal (2), and wherein the controllable electric output switch arrangement (42, 44) is controlled to connect the electric output terminal (6) to the converter output (4), and wherein in a second mode of operation, the first controllable electric switch (15) is controlled to connect the input (12) of the first converter module (10) to the electric input terminal (2), the second controllable electric switch (25) is controlled to disconnect the input (22) of the second converter module (20), and wherein the controllable electric output switch arrangement (42, 44) is controlled to connect the electric output terminal (6) to the input (22) of the second converter module (20), so as to allow the second converter module (20) to operate as a Pulse Width Modulated DC-DC Buck converter.

2. The power converter according to claim 1, comprising a controller (30) configured to control the first and second controllable electric switches (15, 25) and the controllable electric output switch arrangement (42, 44).

3. The power converter according to claim 2, wherein the controller (30) is configured to sense a DC voltage at the converter output (4), preferably also at the electric output terminal output (6), and to enter the first or second mode of operation according to the sensed DC voltage.

4. The power converter according to claim 3, wherein the controller (30) is configured to enter the second mode of operation when the sensed DC voltage is below a predetermined threshold value, and wherein the controller (30) is configured to enter first mode of operation otherwise.

5. The power converter according to claim 4, wherein said threshold value is selected to a voltage level where the first and second converter modules (10, 20) are capable of controlling a DC power flow to the electric output terminal (6).

6. The power converter according to any of the preceding claims, wherein the first converter module (10) comprises a plurality of two-level three-phase converters in a parallel connection between the input (12) of the first converter module (10) and the converter output (4).

7. The power converter according to any of the preceding claims, wherein the second converter module (20) comprises a plurality of two-level three-phase converters in a parallel connection between the input (22) of the second converter module (20) and the converter output (4).

8. The power converter according to any of the preceding claims, wherein the electric input terminal (2) is configured to receive a three-phase electric AC input.

9. The power converter according to any of the preceding claims, comprising a third converter module comprising at least one active three-phase converter bridge comprising an arrangement of semiconductor-based electric switches,further comprising a third controllable electric switch serving to connect or disconnect an input of the third converter module to the electric input terminal (2), and wherein an output of the third converter module is connected to the converter output (4).

10. The power converter according to any of the preceding claims wherein the controllable electric output switch arrangement (42, 44) comprises a two-pole, three-pole, a four-pole or a six-pole controllable electric switch.

11. The power converter according to any of the preceding claims, wherein the power converter is configured to generate a DC output voltage of at least 500 V, such as at least 1 kV.

12. A wind turbine (70) comprising:- an AC electric power generator (72) with an AC electric output, and- a power converter (80) according to any of claims 1-11, wherein the AC electric output of the AC electric power generator (72) is connected to the electric input terminal of the power converter (80).

13. An electrolyzer system (90, 190), such as an electrolyzer system configured to generate hydrogen, the system (90, 190) comprising:- an electrolyzer with a DC electric power input, and- the power converter (80, 180) according to any of claims 1-11, wherein the electric output terminal of the power converter (80, 180) is connected to the DC electric power input of the electrolyzer.

14. A method for converting an electric AC input to an electric DC output, the method comprising:- providing (110) a first converter module comprising at least one active three- phase converter bridge comprising an arrangement of semiconductor-based electric switches,- providing (112) a second converter module comprising at least one active three- phase converter bridge comprising an arrangement of semiconductor-based electric switches,- operating (114) the first and second converter modules in a first mode of operation, wherein the inputs of the first and second converter modules are connected to the electric input terminal, and wherein the electric output terminal is connected to outputs of both of the first and second converter modules, and- operating (116) the first and second converter modules in a second mode of operation, wherein the input of the first converter module to connected to the electric input terminal, wherein the electric output terminal is connected to the input of the second converter module, and wherein the second converter module to operate as a Pulse Width modulated DC-DC Buck converter.

15. The method according to claim 14, comprising- sensing (118) a DC voltage at the electric output terminal, and- controlling (120) electric switches for selectively operating the first and second converter modules in the first or second mode of operation in response to said sensed DC voltage.

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