Motor vehicle drive

Abstract

The invention relates to a drive (10) of a motor vehicle (2), in particular of an ancillary unit (8), having a brushless DC motor (38) with three phases (40). The drive (10) further comprises a converter (26), which has a first connection (28) for a positive pole (20), a second connection (30) for a negative pole (22), and a third connection (32) for a mid-potential (24), and which has two bridge branches (34), each comprising two switching elements (36) electrically connected in series that have a centre tap (48) formed between them in each case. The two bridge branches (34) are each connected between the first connection (28) and the second connection (30), and the centre tap (48) of each bridge branch is routed towards a phase connection (44) of one of the phases (40). The third connection (32) is routed towards a phase connection (44) of the remaining phase (40). The invention also relates to a method for operating a drive (10) and to a circuit (14).

Description

[0001] Page 1

[0002] 2023 223 WO

[0003] Description

[0004] Motor vehicle drive

[0005] The invention relates to a drive system for a motor vehicle. The drive system comprises a frequency converter and a three-phase brushless DC motor. The invention further relates to a method for operating a drive system and to a circuit with such a drive system.

[0006] Motor vehicles have a variety of drive systems that generate movement during operation. One such drive system is a main drive, which propels the entire vehicle. Alternatively, the drive system can be a component of an auxiliary unit, which does not directly contribute to the vehicle's propulsion. Examples of such an auxiliary unit include a pump, such as an oil or water pump, or an electric air conditioning compressor. In another configuration, the auxiliary unit is an actuator, which drives an adjustable component along a defined path. This adjustable component could be, for example, a window pane, a sunroof, a seat, or part of a seat.

[0007] The drive system typically includes an electric motor that generates the movement. For this purpose, the electric motor has several electrical coils that interact with the motor's permanent magnets during operation. The electric motor is, for example, a brushed commutator motor. To increase efficiency, it is increasingly being designed as a brushless direct current (BLDC) motor. In this case, the electrical coils are connected in multiple phases, usually three phases, and a frequency converter is typically used to supply power to these phases. The frequency converter has a bridge circuit, which, according to the number of... (see page 2)

[0008] The inverter is designed with phases. Each phase is assigned a bridge branch, which includes two semiconductor switches. For a three-phase electric motor, a total of six semiconductor switches are therefore required, which is why the inverter's space requirements and manufacturing costs are comparatively high.

[0009] The invention is based on the objective of providing a particularly suitable drive system for a motor vehicle, a particularly suitable method for operating a drive system, and a particularly suitable electrical circuit, advantageously reducing manufacturing costs and / or weight.

[0010] With regard to the drive, this problem is solved according to the invention by the features of claim 1, with regard to the method by the features of claim 4, and with regard to the circuit by the features of claim 5. Advantageous further developments and embodiments are the subject of the respective dependent claims.

[0011] The drive system is a component of a motor vehicle. In other words, the drive system is suitable, specifically designed and configured, to form a component of the motor vehicle in its assembled state. The motor vehicle is primarily intended for land use and preferably has multiple tracks. It is suitably possible to position the motor vehicle essentially freely, preferably on a suitable roadway. For this purpose, the motor vehicle expediently has appropriate wheels. In summary, it is preferably possible to position the motor vehicle on land essentially independently of other conditions. In other words, the motor vehicle is suitably not rail-guided. Preferably, the motor vehicle is a passenger car or a commercial vehicle, such as a truck or bus.

[0012] The drive is, for example, a component of the main drive system of the motor vehicle, by means of which the motor vehicle is thus propelled during operation. Particularly preferably, however, the drive is a component of an auxiliary unit of the motor vehicle, by means of which no direct propulsion of the motor vehicle is generated (page 3). In particular, the drive is a component of an adjustment drive, such as a seat adjustment mechanism or a window regulator. Alternatively, the drive is, for example, a component of a pump, such as a water pump or an oil pump. In another alternative, the drive is, for example, a component of a transmission actuator, and the auxiliary unit is, in particular, a transmission. In yet another alternative, the drive is, for example, a component of an air conditioning compressor. In a further alternative, the auxiliary unit is a steering or braking system.

[0013] The drive system features a three-phase brushless DC motor. In other words, the brushless DC motor (BLDC), which will also be referred to simply as a DC motor or electric motor, has several electrical coils connected to the three phases. Advantageously, each phase is assigned the same number of electrical coils. For example, each phase has exactly one electrical coil. Preferably, however, each phase comprises at least two or more of these coils. Each phase has two phase terminals, which form the ends of each phase. Therefore, if there is a different electrical potential at the two phase terminals of the respective phase, the corresponding electrical coil(s) are energized.

[0014] Advantageously, the three phases are a component of the stator of the DC motor. The electric motor (DC motor) further comprises a rotor to which one or more permanent magnets are suitably attached. The rotor is preferably fixed to a shaft. The shaft is suitably rotatably mounted by means of one or more bearings, particularly with respect to the stator. Preferably, the rotor is arranged such that it is circumferentially surrounded by the stator. The rotor and the stator are suitably designed as (hollow) cylindrical shapes. Preferably, the electric motor is designed as an internal rotor motor.

[0015] The drive further comprises a converter having a first connection, a second connection, and a third connection. The first connection (page 4) is suitable, and in particular designed and configured, for connecting a positive pole, the second connection for connecting a negative pole, and the third connection for connecting a center potential. During operation of the drive, a higher electrical potential is present at the positive pole than at the negative pole or at the center potential. The center potential, in turn, is higher than the electrical potential of the negative pole. For example, the level of the center potential with respect to the negative and positive poles can be arbitrary. Preferably, however, the center potential is located midway between the positive and negative poles.In other words, during operation, the electrical voltage between the center potential and the negative pole is equal to the electrical voltage between the positive pole and the center potential.

[0016] The converter further comprises two bridge branches, each containing two switching elements connected in series. A center tap is formed between the switching elements of each bridge branch. Preferably, each of the switching elements is a semiconductor switch, such as preferably a field-effect transistor, expediently a MOSFET. In particular, the two bridge branches are identical in construction, which is why identical components can be used. Alternatively or in combination with this, each of the bridge branches is formed by means of a corresponding module.

[0017] The two bridge branches are connected between the first and second terminals. The two bridge branches are electrically connected in parallel. The two center taps are each connected to one of the phase terminals of one of the phases. In other words, two of the phases are directly electrically connected to one of the two center taps via their respective phase terminals. Therefore, when operating using the two bridge branches, it is possible to apply either the electrical potential of the positive pole or the electrical potential of the negative pole to the respective assigned phase terminal, which requires actuation of the respective switching elements. Page 5

[0018] The third terminal of the inverter is connected to one of the phase terminals of the remaining phase. Therefore, during operation of the drive, the center potential is always present at this phase terminal.

[0019] The three phases are advantageously connected in a suitable manner so that an electrical voltage can be applied to two different phase terminals via the inverter, resulting in the energization of at least one of the phases. In this configuration, one of the phase terminals is always connected to the center potential. The remaining phase terminals can be connected to either the positive or the negative terminal, thus generating the corresponding electrical voltage.

[0020] Due to this design, only four switching elements are required, thus reducing manufacturing costs. The weight and space requirements of the inverter are also reduced. For example, the inverter has a housing into which the three terminals are integrated. In particular, three additional terminals are integrated into the housing, which are connected to the corresponding phase terminals. Alternatively, the inverter comprises, for example, a B4 inverter, which has the housing with the two bridge branches arranged therein. The first and second terminals, as well as two of the additional terminals, are integrated into the housing. The third terminal, for example, is arranged separately from the housing. This makes it possible to use existing components, further reducing manufacturing costs. The invention also relates in particular to an auxiliary unit with such a drive.

[0021] For example, the three phases are connected in a delta connection. Thus, one of the phase terminals of each of two different phases is at the same electrical potential, and consequently, two of the phases are also electrically connected to the third terminal. The remaining phase terminals of these phases are electrically connected to different center taps. This makes it possible to apply to these two phases the voltage that exists between the positive terminal and the center potential, or (see page 6) between the center potential and the negative terminal. The remaining phase, which is connected to both center taps, can be supplied with the voltage that exists between the positive and negative terminals.

[0022] Particularly preferably, however, the three phases are connected together in a star configuration. Thus, each center tap and the third terminal are electrically connected to only one of the phase terminals. The remaining phase terminals are connected to a common neutral point. This neutral point is, for example, electrically connected to ground or, particularly preferably, floating. In such an embodiment, the electrical voltage present between the positive and negative terminals, between the positive terminal and the center potential, or between the center potential and the negative terminal is applied to each series connection of two phases.

[0023] The drive preferably includes a control circuit by which the switching elements are actuated. This control circuit, also known as the driver circuit, actuates the switching elements using pulse-width modulation. In other words, the control circuit provides a pulse-pause ratio according to which the switching elements are controlled, ensuring they are either electrically conductive or non-conductive. Thus, a pulsed electrical potential is present at the three phase terminals according to the control signal. During the pulses, a constant electrical potential is present, while during the pauses, no electrical potential is present.Due to the inductance provided by the phases, the electrical voltage realized in this way results in an essentially constant electric current through the coils, thus providing a suitable magnetic field.

[0024] The method serves to operate a drive comprising a three-phase brushless DC motor and an inverter. The inverter includes a first connection for a positive pole, a second connection for a negative pole (page 7), and a third connection for a center potential. The inverter further comprises two bridge arms, each with two switching elements connected in series, between which a center tap is formed. The two bridge arms are each connected between the first and second terminals, and their center taps are connected to a phase terminal of one of the phases. The third terminal is connected to a phase terminal of the remaining phase. The drive also includes a control circuit by means of which the switching elements are actuated using pulse-width modulation.

[0025] In this process, a duty cycle for the switching elements is determined based on a power requirement. In other words, a duty cycle or pulse / pause ratio is determined, particularly based on the power requirement. The switching elements are then actuated according to this duty cycle, so that they are either in an electrically conductive or electrically non-conductive state.

[0026] The duty cycle is extended when energizing the phase connected to the third terminal. If multiple phases are connected to the third terminal, the duty cycle is extended for all of them. Advantageously, the duty cycle is adjusted to the electrical voltage between the positive terminal and the center potential, or between the center potential and the negative potential. If the two electrical voltages are equal, the duty cycle for the phase connected to the third terminal is advantageously twice as long as for the phases that are not (directly) electrically connected to the third terminal.Due to the extended duty cycle, the electric current used to power this phase, despite the reduced applied voltage, is essentially the same as the electric current used to power the other phase(s), thus ensuring smooth rotation of the electric motor. This also results in a more uniform torque applied by the electric motor. Page 8.

[0027] The electrical circuit, when assembled, is a component of a motor vehicle. The circuit includes a voltage source that provides an electrical voltage. Furthermore, the circuit features a drive consisting of a three-phase brushless DC motor and an inverter. The inverter has a first terminal for a positive pole, a second terminal for a negative pole, and a third terminal for a center potential. The inverter also has two bridge branches, each with two switching elements connected in series, with a center tap between each of these. The two bridge branches are each connected between the first and second terminals, and their center taps are connected to a phase terminal of one of the phases. The third terminal is connected to a phase terminal of the remaining phase.The voltage source is electrically connected to the inverter, so that an electrical voltage is applied to the inverter via the voltage source. Preferably, other components of the vehicle are also operated via the voltage source.

[0028] For example, the voltage source is a DC voltage source, preferably a battery. The battery comprises two battery modules connected in series. Each battery module is formed, for example, by one or more battery cells, or at least includes them. The battery cells are advantageously connected in series and / or parallel. For example, the battery may have further battery modules. However, it is advantageous for the battery to be configured such that only the two battery modules are present. In particular, the battery also includes other components, such as electronics or electrical circuitry, by means of which, for example, the electrical energy provided for charging the batteries is distributed during operation.

[0029] The series connection of the two battery modules is connected between the positive and negative terminals. Thus, one terminal of one battery module is electrically connected to the first terminal, and one terminal of the other battery module is electrically connected to the second terminal. Therefore, the series connection of the two battery modules (page 9) is connected between the first and second terminals, and each battery module is electrically connected to the third terminal. Since batteries usually already have a series connection of two battery modules, it is possible to use an existing battery for the drive system; only the center potential needs to be provided.

[0030] In particular, the battery provides a comparatively high electrical voltage, for example greater than 200 V. However, it is especially preferred that the battery provides a direct current voltage of 24 V or 48 V, which is thus applied between the positive and negative terminals.

[0031] For example, the two battery modules are different from each other. However, it is particularly advantageous if they are identical in construction, so that the same electrical voltage is present between the center potential and the negative terminal, and between the positive terminal and the center potential. This simplifies the operation of the drive system. It is also possible to use identical components for the battery, which reduces manufacturing costs.

[0032] In an alternative configuration, the voltage source is, for example, an AC voltage source. This AC voltage source provides an alternating voltage, which is, for example, sinusoidal. Specifically, the frequency of the alternating voltage is between 50 Hz or 60 Hz. The AC voltage source has two AC terminals between which the alternating voltage is applied. For example, the AC voltage source is connected to an AC power supply. The AC voltage source is connected to the inverter via a rectifier. The rectifier provides a direct current (DC) voltage, so that despite using an AC voltage source to provide the alternating voltage, the operation of the drive remains essentially unchanged. Page 10

[0033] For example, the rectifier is operated in a regulated manner, thus enabling a comparatively precise adjustment of the electrical voltage applied to the converter. However, the rectifier is particularly preferably unregulated and preferably a diode rectifier. This reduces manufacturing costs. Preferably, the first terminal is connected to both AC voltage terminals via a diode of the rectifier. The reverse bias of the two diodes is directed away from the first terminal. The second terminal is also connected to both AC voltage terminals via a diode of the rectifier. The reverse bias of the two diodes is directed towards the second terminal. In particular, the rectifier thus has a B4 circuit and comprises a diode rectifier.The third terminal of the drive is directly electrically connected to one of the AC terminals via the rectifier, so that the electrical potential provided at this AC terminal is present at the third terminal.

[0034] A capacitor is connected between the first and third terminals, and between the second and third terminals. This capacitor smooths the voltage between the first and third terminals, and between the third and second terminals, resulting in a DC voltage, even though the third terminal is directly connected to one of the AC terminals. This simplifies the wiring, allowing the AC voltage source to be used to operate the drive.

[0035] The further training and advantages explained in connection with the drive can also be applied analogously to the auxiliary unit / the process / the electrical circuit as well as to each other and vice versa.

[0036] Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. The drawing shows: Page 11

[0037] Fig. 1 schematically shows a motor vehicle with a voltage source and with an auxiliary unit that has a drive,

[0038] Fig. 2 shows a schematic diagram of a circuit comprising the voltage source and the drive, and

[0039] Fig. 3 shows a modification of the circuit according to Fig. 2.

[0040] Corresponding parts are marked with the same reference symbols in all figures.

[0041] Figure 1 schematically simplifies the representation of a motor vehicle 2 in the form of a passenger car. The motor vehicle 2 has a total of four wheels 4, which, in their intended state, rest on a road surface (not shown) and are connected to a body 6 of the motor vehicle 2 by means of a chassis. The motor vehicle 2 also includes an auxiliary unit 8, which does not directly contribute to the propulsion of the motor vehicle 2. In the illustrated example, the auxiliary unit 8 is an adjustment drive, such as an electric window regulator. The auxiliary unit 8 has a drive 10, by means of which an adjustment element (not shown) is moved along an adjustment path. A drive train is formed between the adjustment element and the drive 10, which includes, for example, a worm gear, a spindle, and / or a cable drum.The drive 10 is electrically connected to a voltage source 12, which together at least partially form a circuit 14.

[0042] Figure 2 shows a simplified schematic diagram of circuit 14. In this version of circuit 14, the voltage source 12 is formed by a battery 16, which has two battery modules 18. Each battery module 18 has several battery cells that are electrically connected in series. For example, the battery cells of each battery module 18 are arranged in individual housings, or all battery cells are simply enclosed in a common housing of the battery 16. The battery modules 18 are identical in construction and are electrically connected in series between a positive terminal 20 and a negative terminal 22. This series connection provides a DC voltage of 48 V on one side 12, which is thus applied between the two terminals 20 and 22.A central potential 24 is formed between the two battery modules 18, with a DC voltage of 24 V being present between the positive pole 20 and the central potential 24 and between the central potential 24 and the negative pole 22.

[0043] The drive 10 has an inverter 26 with a first terminal 28, a second terminal 30, and a third terminal 32. The positive terminal 20 is connected to the first terminal 28, and the negative terminal 22 is connected to the second terminal 30. The center potential 24 is connected to the third terminal 32. Thus, the series connection of the two battery modules 18 is connected between the first terminal 28 and the second terminal 30, and each battery module 18 is electrically connected to the third terminal 42. The inverter 26 has two identical bridge branches 34, each comprising two switching elements 36 connected in series. In the illustrated example, the switching elements 36 are each designed as a MOSFET, and the bridge branches 34 are connected in parallel to each other between the first terminal 28 and the second terminal 30.

[0044] The drive 10 further comprises an electric motor in the form of a brushless direct current (BLDC) motor 38, which has three phases 40. The phases 40 are components of a stator of the brushless DC motor 38, which also includes a rotor (not shown) with several permanent magnets. Each phase 40 has the same number of electrical coils 42, which are connected between two phase terminals 44 of the respective phase 40. The phases 40 are made in one piece from a common enamelled wire. The phases 40 are connected in a star connection, such that one of the phase terminals 44 of each phase 40 is connected to a star point 46.

[0045] The remaining phase connection 44 of one of the phases 40 is connected to the third connection 32. The remaining phase connections 44 of the other phases 42 are each connected to a center tap 48 of one of the bridge branches (page 13).

[0046] 34. Each center tap 48 is formed between the switching elements 36 of the respective bridge branch 34. In summary, the respective center tap 48 is formed between the electrically series-connected switching elements 36 of each bridge branch 34, and these are each connected to one of the phase terminals 44 of one of the phases 40. The third terminal 32, however, is connected to one of the phase terminals 44 of the remaining phases 40.

[0047] The drive 10 further comprises a control circuit by means of which the switching elements 36 are actuated by pulse width modulation. For this purpose, a switching signal 52 is generated by the control circuit 50, which has a number of pulses 54 and pauses 56 arranged between them. During the pulses 54, the respective switching element 36 is set to the electrically conductive state and during the pauses 56 to the electrically non-conductive state.

[0048] To operate the drive 10, a procedure is carried out in which a power requirement 58 is first determined. Depending on this, a duty cycle for the switching elements 36 is determined, which corresponds to the length of the pulses 54. According to the duty cycle, i.e., according to the pulses 54, the switching elements 36 are then actuated, so that an energization of the individual phases 40 takes place. The sound signals 52 are designed such that, due to the corresponding energization by means of the electrical coils 42, a rotating magnetic field is created, which interacts with the rotor.

[0049] When the two phases 40, which are electrically connected between the two center taps 38, are energized, one of the switching elements 36 of each of the two bridge branches 34 is actuated. Thus, the electrical voltage present between the two poles 20, 22 is applied to the series circuit. If, however, the phase 40 connected to the third terminal 82 is also to be energized, only one of the switching elements 36 is actuated. Therefore, half of this electrical voltage is present in the series circuit consisting of this phase 40 and the phase 40 corresponding to the bridge branch 34 whose switching element 36 was actuated. Therefore, the control circuit on page 14

[0050] The duty cycle for this switching element 36 is extended by 50, namely to twice its original value. The length of the pauses 56, however, remains the same. In summary, the duty cycle is thus extended when the phase 40 connected to the third terminal 32 is energized. Due to the extended duty cycle, i.e., the extended pulses 54, the electric current carried by all phases 40 during operation of the brushless DC motor 38 is the same, so that the resulting magnetic field is also the same. Consequently, the brushless DC motor 38 exhibits a uniform torque.

[0051] Figure 3 shows a modification of the circuit 14, in which the inverter 26, the brushless DC motor 38, the control circuit 50, and their operation and interconnection remain unchanged. The voltage source 12, however, has been modified and is now an AC voltage source 60, which has two AC voltage terminals 62. During operation, an alternating voltage is present between these terminals, for example, with a frequency of 50 Hz. Other frequencies are also possible, for example, 60 Hz or 400 Hz.

[0052] A rectifier 64 is connected between the AC voltage source 60 and the inverter 26. In other words, the AC voltage source 60 is connected to the inverter 26 via the rectifier 64. The rectifier 64 has four diodes 66, with each of the identical diodes 66 connecting the first terminal 28 to each of the AC voltage terminals 62. Thus, the first terminal 28 is connected to both AC voltage terminals 62 via one of the diodes 66. The reverse bias of the diodes 66 is directed away from the first terminal 28. The second terminal 30 is also connected to both AC voltage terminals 62 via the remaining diodes 66. In other words, the second terminal 30 is connected to both AC voltage terminals 62 via one of the diodes 66. The reverse bias of the diodes 66 is directed towards the second terminal 30.The third terminal 32 is directly connected to one of the AC voltage terminals 62 via the rectifier 64. Page 15.

[0053] The rectifier 64 further comprises two identical capacitors 68. One of the capacitors 68 connects the first terminal 28 to the third terminal 32. The remaining capacitor 68 connects the second terminal 30 to the third terminal 32. In other words, one of the capacitors 68 is connected between the first terminal 28 and the third terminal 32, and another between the second terminal 30 and the third terminal 32. This stabilizes the electrical voltage applied between these terminals 28, 30, and 32, ensuring that each is a DC voltage and essentially the same magnitude. Therefore, the drive 10 can be operated in the same manner as in the previous embodiment, even when the AC voltage source 60 is used as the voltage source 12.

[0054] The invention is not limited to the embodiments described above. Rather, other variants of the invention can also be derived by a person skilled in the art without departing from the subject matter of the invention. In particular, all individual features described in connection with the individual embodiments can also be combined with one another in other ways without departing from the subject matter of the invention.

[0055] Page 16

[0056] Reference symbol list

[0057] 2 motor vehicles

[0058] 4-wheeler

[0059] 6 Bodywork

[0060] 8 Auxiliary unit

[0061] 10 Drive

[0062] 12 Voltage source

[0063] 14 Circuit

[0064] 16 Battery

[0065] 18 battery module

[0066] 20 positive pole

[0067] 22 negative pole

[0068] 24 Mid-range potential

[0069] 26 inverters

[0070] 28 first connection

[0071] 30 second connection

[0072] 32 third connection

[0073] 34 Bridge branch

[0074] 36 switching element

[0075] 38 brushless DC motor

[0076] 40 Phase

[0077] 42 electrical coil

[0078] 44-phase connection

[0079] 46 Star point

[0080] 48 Center tap

[0081] 50 Control circuit

[0082] 52 Switching signal

[0083] 54 pulse

[0084] 56 Break

[0085] 58 Performance requirement

[0086] 60 AC voltage source

[0087] 62 AC connection Page 17 Rectifier Diode Capacitance

Claims

Page 18 Claims 1. Drive (10) of a motor vehicle (2), in particular of an auxiliary unit (8), with a brushless DC motor (38) having three phases (40), and with an inverter (26) having a first connection (28) for a positive pole (20), a second connection (30) for a negative pole (22) and a third connection (32) for a center potential (24), and having two bridge branches (34) each with two switching elements (36) connected in series, between which a center tap (48) is formed, wherein the two bridge branches (34) are each connected between the first connection (28) and the second connection (30) and their center tap (48) is connected to a phase connection (44) of one of the phases (40), and wherein the third connection (32) is connected to a phase connection (44) of the remaining phase (40).

2. Drive (10) according to claim 1 , characterized in that the three phases (40) are connected together in a star connection.

3. Drive (10) according to claim 1 or 2, characterized by a control circuit (50) by means of which the switching elements (36) are actuated in pulse width modulation.

4. Method for operating a drive (10) according to claim 3, wherein a duty cycle for the switching elements (36) is determined as a function of a power requirement (58), by means of which the switching elements (36) are actuated, wherein the duty cycle is extended when the phase (40) directed towards the third terminal (32) is energized. Page 19 5. Circuit (14) of a motor vehicle (2), comprising a voltage source (12) which is electrically connected to a drive (10) according to one of claims 1 to 3.

6. Circuit (14) according to claim 5, characterized in that the voltage source (12) is a battery (16) comprising two battery modules (18) connected electrically in series, wherein the series connection is between the first terminal (28) and the second terminal (30), and wherein each battery module (18) is electrically connected to the third terminal (32).

7. Circuit (14) according to claim 6, characterized in that the battery modules (18) are identical in construction to each other.

8. Circuit (14) according to claim 5, characterized in that the voltage source (12) is an AC voltage source (60) with two AC voltage connections (62) which is electrically connected to the converter (24) via a rectifier (64).

9. Circuit (14) according to claim 8, characterized in that the first terminal (28) is electrically connected to both AC terminals (62) via a diode (66) and the second terminal (30) is electrically connected to both AC terminals (62) via a diode (44), and wherein the third terminal (32) is electrically connected to one of the AC terminals (62), and wherein a capacitor (68) is connected between the first terminal (28) and the third terminal (32) and between the second terminal (30) and the third terminal (32).

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