WO2019053160A1

WO2019053160A1 - Charging an electric energy store of a motor vehicle

Info

Publication number: WO2019053160A1
Application number: PCT/EP2018/074807
Authority: WO
Inventors: Bernhard Sofaly; Thomas Paesler
Original assignee: Conti Temic Microelectronic Gmbh
Priority date: 2017-09-18
Filing date: 2018-09-13
Publication date: 2019-03-21
Also published as: DE102017216468A1

Abstract

The invention relates to a method for charging an electric energy store (10) of a motor vehicle with electric energy from a vehicle-external charging station (14), in which the charging station (14) provides single-phase alternating electric current (18) in relation to a predefined electric reference potential (16), wherein the electric energy store (10) is coupled electrically to the vehicle-external charging station (14) by means of a clocked energy converter (20), wherein the clocked energy converter converts the electrical energy by means of at least two parallel-connected series circuits (22, 24) each comprising two switching elements (28, 30), for which purpose respective centre taps (32, 34) of the series circuits (22, 24) are coupled electrically to the charging station (14). According to the invention, the switching elements (28, 30) are controlled between one of at least two energy storage potentials of the electric energy store (10) and the electric reference potential (16), at least as a function of an electric reference voltage (38).

Description

description

Charging an electrical energy storage of a power ^¬ vehicle

The invention relates to a method for charging an electrical energy storage device of a motor vehicle with electrical energy from a vehicle external charging station, wherein the charging station provides a single-phase electrical AC voltage with respect to a predetermined electrical reference potential, wherein the electrical energy storage by means of a clocked energy converter with the vehicle external charging station electrically is coupled, wherein the clocked energy converter, the electrical energy by means of at least two parallel-connected series circuits of two

Switching elements converts, for which purpose respective center terminals of the series circuits are electrically coupled to the charging station. The invention further relates to a charging ^{¬ device} for charging an electrical energy storage device of a motor vehicle with electrical energy from a vehicle external charging station, with a charging station connection for supplying a single-phase AC electrical power with respect to a predetermined electrical reference potential, an energy storage port for electrically coupling the

electrical energy storage, a clocked energy converter with at least two series circuits of two

Switching elements, wherein the series circuits are connected in parallel and electrically coupled to the energy storage terminal, wherein each of the at least two series circuits has a central terminal and wherein the central terminals are electrically coupled to the charging station terminal. Finally, the invention also relates to a motor vehicle with an electrical system which has a DC voltage intermediate circuit and an electrical energy store connected to the DC voltage intermediate circuit, a charging connection for connecting to a vehicle-external charging station providing a single-phase electrical AC voltage and one to the charging connection and the electrical energy storage device coupled charging device for charging the electrical energy storage with electrical energy from the charging station.

Charging an electrical energy store of a

Motor vehicle with electrical energy from a vehicle ^¬ external charging station is basically well known in the art, so that a separate documentary evidence for this need not. As a rule, a motor vehicle comprises an electrical vehicle electrical system in order to be able to supply electric devices which can be connected to the electrical vehicle electrical system as well as units of the electrical vehicle electrical system in a predeterminable manner. The electrical system thus serves the electrical power distribution. In general, the electrical system includes a DC voltage intermediate circuit to which the electrical energy storage is connected. The electrical energy storage is used to store excess energy in the electrical system, as well as to provide for a demand for electrical energy for the electrical system. The electrical energy storage is often formed for this purpose by an accumulator or the like, which is able to chemically store electrical energy reversibly. In addition, the electrical energy store may also include an electrical capacitor.

Of particular importance is the electric energy storage in an electrically driven motor vehicle, which comprises an electric drive device by means of which the motor vehicle can be driven in the intended driving operation. As a rule, such a drive device comprises an inverter connected to the vehicle electrical system, in particular to an intermediate circuit of the electrical system, to which in turn a usually rotating electrical machine is connected, which is capable of providing the mechanical drive power. Usually, for this purpose a three-phase electrical machine is used, for example in the manner of a synchronous ^¬ machine, an induction machine and / or the like. In an electrically driven motor vehicle, which may be, for example, an electric vehicle or hybrid vehicle, it is usually necessary to charge the electrical energy storage regularly so that it can provide electrical energy for the intended driving operation. For this purpose, the motor vehicle has one with the electric

Energy storage electrically coupled charging device, which is connected, for example, to the DC voltage intermediate circuit of the onboard ^¬ network in order to establish the electrical connection to the electrical energy storage can. In addition, the charging device is connected to the charging port of the

Motor vehicle electrically connected, via which an energy-related connection to the vehicle external charging station can be made.

Usually, the vehicle-external charging station is formed by a charging station, for example a charging station or the like. The vehicle-external charging station can be either lei ^¬ tion bound or wireless, for example by means of an alternating magnetic field or the like, be coupled to the charging port of the motor vehicle to promote electrical energy from the charging station to the electrical energy storage can. To produce an energy-technical coupling, the charging device is usually a switched energy converter, for example in the manner of a rectifier, the gels ^¬ may be sionally also coupled to a DC / DC converter and / or the like. In addition, as a rule, due to the electromagnetic compatibility and / or the

electrical safety galvanic isolation provided which separates the vehicle external charging station galvanically from the electrical system of the motor vehicle. The galvanic isolation is often formed by a transformer.

Frequently, the vehicle-external charging station for the purpose of energy supply provides an electrical alternating voltage, which relates to a predetermined electrical reference potential is, for example, the ground potential, another suitable ground potential and / or the like. Since the electrical energy is usually taken from a public power supply network or is provided by this, the AC voltage usually has an AC voltage frequency, which corresponds to that of the public power distribution network ent ^¬ . In Europe and much of Asia, this is predominantly a frequency of about 50 Hz. In America, however, the frequency is usually 60 Hz.

Depending on the configuration of the vehicle-external charging station a single-phase or three-phase a Wech ^¬ selspannung is usually provided. In a wired power engineering coupling special requirements relating to electrical safety and electromagnetic compatibility to be ^¬ shall be respected. For this reason, a corresponding filter circuit is usually for example la- destationsseitig and / or power the vehicle side, so that the diesbezüg ^¬ union prescribed requirements can be met. A corresponding requirement is, for example, compliance with a maximum permissible leakage current. The Ab ^¬ leitstrom is an electric current that flows in the normal operation of an electrical system in an undesirable current path. The current path is often by one

Protective conductor formed, which is usually electrically connected to the electrical reference potential. The electrical reference potential is often the ground potential.

In many electrical applications, the leakage current is essentially determined by the filter circuit, which serves to comply with the electromagnetic compatibility, in particular with regard to conducted radio interference. To suppress such conducted radio interference, Y-capacitors are often used in the filter circuit, which electrically couple a respective potential to be suppressed with the reference potential. This causes that when exposed to an AC voltage and a corresponding alternating current flows through the Y-capacitor, which at least partially determines the leakage current. In addition, the leakage current can also be dependent on further, in particular capacitive, couplings.

Especially with a connection of a charging station, which provides a single-phase AC voltage, the effort in terms of electromagnetic compatibility and electrical safety, in particular with respect to the leakage current, proves to be significant. For physical reasons, the corresponding expense for a three-phase AC voltage is significantly lower. Especially when connected to a charging station that provides a single-phase AC voltage, so are special Anfor ^¬ changes to take into account that require a high technical expenditure.

The invention has for its object to improve the situation with respect to a leakage current when charging an electrical energy storage of a motor vehicle.

As a solution, the invention proposes a method, a charging device and a motor vehicle according to the independent claims. Further advantageous embodiments will become apparent from the features of the dependent claims.

With regard to a generic method, it is in particular proposed that the switching elements are controlled at least as a function of an electrical reference voltage between one of at least two energy storage potentials of the electrical energy store and the electrical reference potential.

With regard to a generic charging device, it is proposed, in particular, that this has a control unit which is designed to switch the switching elements at least as a function of an electrical reference voltage between one of at least to control two energy storage potentials at the energy storage connection and the electrical reference potential.

With regard to a generic motor vehicle, it is proposed in particular that the charging device is designed according to the invention.

The invention is based on the finding that by suitably controlling the cycled energy converter, in particular its switching elements arranged in a predetermined circuit structure, it is possible to already fundamentally reduce the generation of a leakage current. The invention uses the idea that the AC voltage of the charging station is usually related to a reference potential, usually the ground potential. The charging station, the Wech ^¬ selspannung usually of two electrical connections provided, wherein one of the electrical connections, a zero terminal and the second electrical connections is a phase terminal. In most cases, the zero terminal is electrically connected to the ground potential in the charging station itself or by a subsequent electrical infrastructure to which the charging station is connected to the ground potential as the reference potential. Since during charging, the charging station is electrically conductively connected to the charging terminal of the motor vehicle, these electric potentials are respectively in the motor vehicle before ^¬ hands. In general, therefore, the reference potential of the charging station with an electrical reference potential of the

Motor vehicle, for example, a mass, electrically connected. In general, the electrical reference potential of the motor vehicle is formed by a motor vehicle mass, which is connected via a separately guided connection cable to the reference potential of the charging station. As a result, dangerous electrical states can be reduced or avoided during the charging of the electrical energy store. The invention makes use of the fact that the electrical energy store is usually connected with its energy storage connection to the DC voltage intermediate circuit and thus provides the intermediate circuit DC voltage with its at least two electrical energy storage potentials. The energy storage connection therefore generally also includes the DC voltage intermediate circuit at the same time.

The DC intermediate circuit has for line-connected suppression of the energy storage potentials of the electrical energy storage device connected Y capacitors that electrically couple each of the at least two energy storage potentials that also correspond to the DC link potentials with the motor vehicle-side reference potential or the vehicle mass. As a result, a conducted radio interference suppression can be achieved or improved.

Since the motor vehicle in the intended driving operation with the charging station and thus with the reference potential of the charging station, in particular the ground potential, is not electrically coupled, and special requirements for the off ^¬ leitstroms island operation of the electrical system of the motor vehicle generous than stationary electrical facilities, As a rule, the Y capacitors have a comparatively large capacitance value. However, since the DC link voltage is subject to relatively small voltage fluctuations during normal operation as DC voltage, the leakage current is correspondingly low despite the large capacitance value. A common capacity of such Y-capacitors is for example about lyF.

The situation changes when the charging station provides a single-phase AC voltage for charging the electrical energy storage device and the motor vehicle is connected thereto for the purpose of charging the energy storage device. On the basis of the bridge rectification principle during charging, namely, if no galvanic isolation is provided, the energy storage potentials vary over that Reference potential in the rhythm of the AC voltage frequency. As a result, of course, the motor vehicle side

Y capacitors with this AC voltage acted upon accordingly. This results in charging a significant leakage current, which exceeds the allowable limit for stationary systems to which the motor vehicle is connected when charging the electrical energy storage. In the prior art elaborate filters and compensation measures are provided here.

The invention is based on the further idea of reducing the fluctuation of the energy storage potentials with respect to the reference potential during charging of the electrical energy store by controlling the semiconductor switches. It can thereby be achieved that the leakage current can be correspondingly reduced on the motor vehicle side, in particular between the circuit circle side. The effects of the invention can thus be achieved in particular when the electrical energy store is charged by a charging station which merely provides a single-phase AC voltage. For a three-phase AC voltage is already due to the underlying

Rectification principle of the leakage current motor vehicle side basically already much lower, so that the above problem with respect to the leakage does not occur here or can be already reduced by other simple cost-effective measures already in a suitable manner. Basically, however, the inventive principle can also be used here to reduce the leakage current. Nevertheless, the effect according to the invention is particularly suitable for charging stations which provide a single-phase AC voltage.

The clocked power converter, which is designed preferably in the manner of an inverter has at least two pa ^¬ rallelgeschaltete series circuits each consisting of two switching elements. The two series circuits are also connected to the electrical energy storage, for example, to the DC voltage intermediate circuit, if the elekt ^¬ rische energy storage also at Gleichspannungszwischen- circle is connected. However, it can also be provided that the electrical energy storage for the purpose of charging from the DC voltage intermediate circuit is disconnected and electrically coupled to the Ladeein ^¬ direction.

Each of the series circuits has a corresponding center terminal provided at a junction of the respective two switching elements of the respective series circuit. Each of the two center terminals is electrically connected to one of the potentials provided by the charging station, between which is the AC voltage provided by the charging station for the purpose of charging.

The clocked energy converter further comprises an energy converter control which is connected to the switching elements and operates the switching elements in a clock mode in order to be able to provide the energy conversion in a desired manner. For this purpose, switching patterns or clock patterns for the switching elements are provided, by means of which the desired energy conversion can be realized.

The switching elements of the energy converter are preferably semiconductor switching elements which may be formed, for example, by transistors such as bipolar transistors, field-effect transistors, in particular metal oxide semiconductor field effect transistor (MOSFET), isolated gate bipolar transistor (IGBT) or the like. The transistors are operated in a switching mode. The switching operation of the semiconductor switching element means that in the on state during the switch-on between terminals of the semiconductor switching element between which the switching path is formed, a very low electrical resistance is provided so that a high current flow at very low residual voltage is possible. In the off

State during the turn-off, the switching path of the semiconductor switching element, however, is high impedance, that is, it provides a high electrical resistance, so also At high, applied to the switching path electrical voltage substantially no or only a very small, especially negligible, current flow is present. This differs from a linear operation, which is usually not used in clocked energy converters. The switching operation preferably provides only for the on ^¬ switching state and the off state.

Thus, not only the switching elements for reducing the leakage current can be controlled by means of the energy converter control, but it is also possible to be able to set the converted power in a predeterminable manner. This makes it possible to operate the electrical energy storage in the most favorable operating condition for the purpose of charging. As a result, an aging of the electrical energy store can be kept as low as possible and at the same time, if appropriate, a high efficiency with respect to the electrical charging can be achieved. According to the invention, the switching elements are controlled at least as a function of the electrical reference voltage between one of at least two energy storage potentials of the electrical energy store and the electrical reference potential. Since the at least two energy storage potentials are coupled together in a predetermined manner by the electrical energy store, the reference voltage need only be taken into account with respect to one of the two energy storage potentials.

Unlike in the conventional art, according to the invention not only exclusively a conversion power of the clocked energy converter is set, but the operation of its

Switching elements is adjusted taking into account the reference voltage so that the leakage current can be influenced. For this purpose, the electrical reference voltage is detected, for example by using a voltage sensor or the like. The reference voltage also represents a measure of the electrical voltage with which the Y-capacitors, which comprises the electrical system of the motor vehicle and the electrical Energy storage are electrically coupled, are acted upon. This reference voltage thus also represents a measure of which the leakage current is dependent. It should be noted that due to the physical properties of the Y capacitors, the leakage current may also be frequency-dependent. Incidentally, this also explains why at a Be ^¬ zugsspannung, which is essentially only a DC voltage, the leakage current is correspondingly low. With the invention it is thus possible, by means of a suitable control concept which uses predetermined switching pattern for the switching elements to the leakage current corresponding to redu ^¬ decorate. In addition, with the invention, a control in the manner of a control can be achieved, in which the electrical reference voltage detected by a suitable voltage sensor and a control unit of the control unit of the charging ^¬ device is supplied by means of the switching pattern for the switching elements adapted in a suitable manner or be provided so that the reference voltage has the smallest possible ripple, preferably is substantially constant. The suitable switching patterns can be determined by means of simulation methods. With decreasing ripple of the reference voltage, the leakage current caused by the Y capacitors is reduced at the same time. With the invention, for example, depending on the reference voltage, a suitable switching pattern for respective switching elements can be selected.

In particular, a Y capacitor is a capacitor specially designed for the function of radio interference suppression, which must meet special technical requirements, in particular with respect to its dielectric strength, its current carrying capacity and / or the like. These capacitors are electronic components which have to meet special requirements, in particular with regard to the radio interference suppression, as is the case for example with standards relating to electromagnetic compatibility, such as EN 61000, or also with regard to electrical safety. Y capacitors are usually covered by standardization, for example according to IEC 60364. These are capacitors which are usually connected between the phase or the zero connection and the reference potential, for example a touchable, in particular protective earthed, part, whereby they can bridge a usually required due to electrical safety basic insulation. You must therefore meet very high, especially standardized, requirements. Y capacitors are commonly used to suppress common mode noise. Incidentally, this is a special constellation in which the reference potential, as it can be provided, for example, by the protective conductor, may be used for purposes other than protective earthing and thus protection against electric shock. This differs from the X-capacitors also used for the purpose of radio interference suppression of conducted radio interference, which are commonly used to suppress push-pull interference. In order not to jeopardize the protective function of the reference potential or of the protective conductor, increased protection requirements are imposed both on X capacitors and on Y capacitors.

In order to be able to realize the method according to the invention, the charging device is also proposed with the invention. The charging device is preferably designed for arrangement on or in the motor vehicle. It is in particular designed to be connected to the DC voltage intermediate circuit of the electrical system of the motor vehicle, and in particular when the electrical energy storage is connected to this intermediate circuit and thus can be acted upon directly by the charging device with electrical charge. In addition, it is of course also possible to connect the charging device with its energy storage connection directly to the electrical energy storage. It can be provided to electrically disconnect the DC intermediate circuit during charging of the electrical energy storage device from the electrical energy storage, for example by means of electromechanical switching elements, which are provided by a contactor and / or the like. However, the leakage current need not only be caused by the Y capacitors, but it can equally or additionally be effected by parasitic ka ^¬ pacitive couplings the reference potential with one or both of the energy storage potentials of the electrical energy storage or the DC link potentials of Gleichspan ^¬ voltage intermediate circuit. Parasitic capacities are often present due to design and can be avoided even with the most careful construction hardly or only insufficiently. In general, however, the leakage current is essentially determined by the Y capacitors, because in reality their electrical capacitance is usually significantly greater than the electrical capacitance of parasitic capacitances.

The charging device has the control unit, by means of which the switching elements can be controlled in a suitable manner. Preferably, the control unit comprises the energy converter ^¬ control. However, it can also be provided that the control unit represents a separate structural unit which is connected to the

Energy converter control is connected and the energy converter control supplied with corresponding control signals for the switching elements. The control unit is preferably an electronic circuit, which may be connected to a voltage sensor, in particular for detecting the reference voltage, or may even include this. In addition, the control unit may also include a program-controlled computer unit, which may also be combined with the electronic circuit. By means of the computer program, the computer unit can provide the desired functionality.

It proves particularly advantageous if the AC voltage is provided at a predetermined AC voltage frequency and only a spectral portion of the electrical reference voltage in a predetermined range of the AC voltage frequency is taken into account for controlling the switching elements. The range of the AC voltage frequency includes, for example, a range that is determined by a maximum deviation from the AC voltage frequency by about 10%, preferably 5%, be ^¬ be provided. Preferably, the predetermined range corresponds exactly to the AC voltage frequency. This has the advantage that it is precisely the frequencies that are particularly unfavorably to be filtered that can be compensated for in the range of the AC voltage frequency. The AC voltage frequency is, for example, 50 Hz or 60 Hz, depending on in which region and in which energy supply network the charging station is connected. In isolated networks, moreover, a frequency of 400 Hz may be provided, such as at

On-board networks on ships, in aircraft and / or the like. Especially due to the low frequency only with great effort to reduce leakage currents in the range of 50 Hz or 60 Hz can thus be reduced, whereby a related effort in terms of filtering can be reduced accordingly. Moreover, these Ver ^¬ direction drive course is particularly suitable for control purposes for which the switching pattern for the switching elements are changed depending on the detected reference voltage.

In addition, it is proposed that the switching elements are controlled such that the electrical reference voltage is substantially constant. This embodiment takes into account that only very small or negligible leakage currents are caused at a substantially constant reference voltage, in particular by the Y capacitors. This results from the functionality of capacitors.

According to a further development, it is proposed that the

Switching elements are controlled such that the electrical reference voltage corresponds to 40% to 60%, preferably 50%, an electrical energy storage voltage between the at least two energy storage potentials of the electrical energy storage. If the electrical energy store is connected directly to the DC voltage intermediate circuit, this voltage corresponds to the DC link voltage. This embodiment has the advantage that a voltage load of the components relative to the electrical reference potential can be uniform. In addition, effects that result in, for example, differential mode noise can be reduced. It is further proposed that one of the electrical

AC potentials of the electrical AC voltage is electrically coupled to the electrical reference potential, wherein the center terminals of the two series circuits are coupled to the two provided by the charging station connections or electrical potentials. A power-frequency component of the voltages, for example at a frequency of approximately 50 Hz, which are set at the two center connections of the two series circuits with respect to the reference potential, essentially follows the power-frequency component of the two connected electrical potentials with respect to the reference potential. Furthermore, the voltage drops across the line filter chokes can be taken into account when setting the voltages at the two center terminals of the two series circuits, which can lead to a nearly complete elimination of the mains frequency components of the leakage currents.

In a conventional power supply in Germany, for example, a public power grid, in which the zero terminal is fixed to the reference potential, while the phase connection performs the full working voltage compared to the reference potential, an advantageous control of the circuit breaker would be characterized in that the series circuit, the center port electrically coupled to the reference potential, about 40% to 60%, preferably about 50%, the energy storage voltage or optionally the DC link voltage, and the other series circuit performs the full stroke of the charging voltage, ie sinusoidally, for example, between 10% and 90% of the energy storage voltage with the Mains frequency oscillates. In a further advantageous embodiment, the voltage drops across the line filter chokes can be superimposed on these two voltages. The two series circuits of the clocked energy converter are preferably operated according to the invention so that it provides an AC voltage with a full amplitude at the AC frequency at one of the two center connections with respect to the DC link voltage available at the clocked power converter, whereas the other series circuit its central terminal provides an opposite phase partial voltage of the AC voltage at the AC frequency. This can be achieved that the corresponding AC component of the reference ^¬ voltage is reduced, thereby consequently, the leakage current is reduced accordingly.

As a rule, each of the at least two energy storage potentials of the electrical energy store is electrically coupled via a respective Y capacitor to the electrical reference potential. Thus, the charging device can determine which is electrically coupled to the central terminals to the reference potential, it is further proposed that the switching elements each having an inverse diode, wherein the switching elements are turned off, and respective electrical Schaltelem ^¬ relaxations are recorded and evaluated to determine which of the Central terminals is electrically coupled to the reference potential. This embodiment makes use of the fact that drive circuits provided for the switching elements generally comprise a switching state detection. The switching state detection is based on detecting a switching element voltage applied to the switching element. If the detected switching element voltage is smaller than a predetermined comparison value, this is output as a switched-on state. If, however, the detected switching element voltage greater than the comparison value, this will be as a power-down state ^¬. This makes it possible to detect the respective switching state of the switching elements. Since a respective diode is connected in parallel to each switching element, thus also the

Switching state of the respective diode can be determined because the switching elements are themselves in the designed switching state. In this embodiment, the clocked energy converter works like a single-phase bridge rectifier. The comparison value is preferably chosen at an electrical voltage of about 4V. However, it may also be chosen in a range from about 2.5 V to about 8 V, in deviation. In this case, this embodiment is based on the fact that a switching state of the switching element, in which the middle connection of the series connection is electrically coupled to the phase connection, corresponds to FIG

AC frequency oscillates back and forth, whereas the switching state of a switching element in the series circuit whose center terminal is connected to the zero terminal, constantly outputs the off state. The thus determined connection situation of the charging station to the clocked energy converter can be used to be able to control the switching elements of the two series circuits in a suitable manner, as has already been explained in detail above.

According to a further development it is proposed that the laser is destationsanschluss coupled to the switched energy converter galvanic ^¬ cally. Namely, the invention makes it possible to dispense with galvanic isolation, as is conventional in the prior art. In the prior art, the galvanic separation also serves, among other things, to prevent leakage currents. For this purpose, a suitable transformer is usually provided, by means of which the electrical isolation can be achieved. However, such a transformer is costly and also requires a large amount of space. With the invention, it is possible to avoid the galvanic separation and at the same time to meet the requirements, which may be at least partially justified by the standardization.

It is further proposed that the motor vehicle has an electric drive device, which has a three-phase electrical machine for driving the motor vehicle and an output connected to the electric machine and the vehicle power supply inverter, which is formed, provide a three-phase AC power supply system for the electric machine bereitzu ^¬, wherein the inverter is configured to provide the clocked power converter of the charging device. This development uses an existing already in an electrically driven motor vehicle anyway existing inverter to provide the clocked energy converter for the purpose of charging the electrical energy storage. Thus, it is not necessary to provide a separate clocked energy converter in this development. As a result of the fact that the inverter is already designed for three-phase operation, this inverter generally also includes a switching element structure, as is required for the cycled energy converter, which is used to carry out the method according to the invention. As a result, effort, space and weight can be saved. This is particularly advantageous in motor vehicles. It is also proposed that the clocked energy converter is electrically coupled via phase windings of the electric machine to the charge port. In this case, the electric machine is at least partially included in the conversion process. This makes it possible to operate the clocked energy converter as a boost converter or buck converter. It proves to be particularly advantageous if stator windings of the electric machine are connected to a star point in a proper driving operation, with the neutral point being resolved for the purpose of charging, so that respective winding connections released thereby can take over the function of the center connections of the cycled energy converter. As a result, additional effects due to the inductances provided by the windings can be used advantageously for the method according to the invention.

The invention also includes developments of the method according to the invention, which have features as they have already been described in connection with the developments of the charging device according to the invention or the motor vehicle according to the invention and vice versa. For this reason, the corresponding developments are not described here again. Embodiments of the invention are be ^¬ wrote. This shows:

Fig. 1 is a schematic diagram of an electrical wiring system of an electric vehicle as a force ^¬ vehicle with an electric drive device in a designated driving operation;

Fig. 2 is a schematic diagram view as in FIG. 1, but in which the electric vehicle for charging a

Accumulator of the electrical system is connected to a charging station, which provides a three-phase ^AC voltage network ^¬ ; Fig. 3 is a schematic diagram of electrical voltages applied to AC terminals of an inverter of the drive means when charging in accordance with Fig. 2; Fig. 4 in a schematic diagram representation

PWM pattern for one of the AC voltages shown in FIG. 3;

5 is a schematic diagram of electrical see voltages as Figure 3, abut the input terminals of the inverter when connecting to a charging station, which provides a single-phase AC voltage ^¬ ; Fig. 6 is a schematic diagram representation as shown in FIG. 4 of

PWM patterns for adjusting the voltages according to FIG. 5;

7 shows a schematic circuit diagram representation like FIG. 2, in which the vehicle electrical system for charging the energy store is connected to a charging station, which provides a single-phase AC voltage; FIG. 8 shows a schematic view like FIG. 5 for the circuit according to FIG. 7; FIG.

Fig. 9 is a schematic representation as shown in FIG. 6 for the

Providing a voltage according to FIG. 8;

Fig. 10 is a schematic diagram of Fig. 7, wherein the switching elements of the inverter are controlled in accordance with a switching pattern of the invention, as illustrated with reference to Fig. 9;

FIG. 11 is a schematic equivalent circuit diagram for a simulation of the operation of the circuit of FIG. 7; FIG.

FIG. 12 shows a schematic diagram of the conditions during operation of the circuit according to FIG. 7; FIG.

Fig. 13 is a schematic equivalent circuit diagram of the

Circuit according to FIG. 10 for simulation purposes;

FIG. 14 is a view like FIG. 12 for the equivalent circuit diagram of FIG. 13; FIG. FIG. 15 is a schematic equivalent circuit diagram for further improved common-mode rejection; FIG.

FIG. 16 shows an illustration as in FIG. 14 for the equivalent ^switching image representation according to FIG. 15; FIG.

17 is a schematic equivalent circuit diagram for

Determining which of the inverter terminals is electrically connected to a zero terminal of the charging station;

18 is a schematic diagram of a simulation of the circuit of FIG. 17 to determine which of the inverter terminals is connected to the neutral connection of the charging station is electrically coupled; and

FIG. 19 is an illustration like FIG. 18, in which the recognition is further clarified.

The exemplary embodiments explained below are preferred embodiments of the invention. In the exemplary embodiments, the described components of the respective embodiment each represent individual, independently to be considered features of the invention, which further develop the invention independently and thus individually or in any other than the combination shown to be considered part of the invention. Furthermore, the described embodiments can also be supplemented by further features of the invention already described.

In the figures, functionally identical elements are each provided with the same reference numerals.

Fig. 1 shows a schematic diagram representation of a DC intermediate circuit 54 of an electrical system of an electric vehicle not further illustrated as an electrically driven motor vehicle, in which an electrical An ^¬ drive device 56 is connected. The electric drive device 56 comprises a three-phase electrical machine 58, which in the present case is designed as a synchronous machine and which serves to drive the electric vehicle.

To the electric machine 58, an inverter 60 is closed at ^¬ formed to provide a three-phase alternating voltage network ^¬ for the electric machine 58th The inverter 60 comprises a control unit 52, by means of the inverter 60 is controlled with respect to the alternating-voltage network in a suitable manner, so that the electrical machine 58 to provide the desired drive function during the determination ^¬ proper driving operation of the electric vehicle can. For this purpose, the inverter has three inverter connections, which are explained in more detail below, to which non-designated stator windings of the electrical machine 58 are connected. The stator windings of the electric machine 58 are connected while driving at a common star point 62. The inverter 60 is further connected to the DC voltage intermediate circuit 54.

At the DC voltage intermediate circuit 54 is also a high ^¬ voltbatterie 10 is connected as an electrical energy storage, which provides electrical energy for the intended driving operation of the electric vehicle. Thus, the high-voltage battery 10 ^¬ at the same time also provides the intermediate circuit direct voltage of the DC voltage intermediate circuit 54th

Electrical potentials of the high-voltage battery 10, which represent energy ^¬ storage potentials are on respective

Y capacitors 42, 44 coupled to a reference potential, which is presently formed by a mass 40 of the electric vehicle. By the Y-capacitors 42, 44, a radio interference with respect to common mode noise is achieved. The Y capacitors 42, 44 cooperate in this regard with a current-compensated inductor 64. In the present embodiment, the electrical capacitance of each of the two Y capacitors 42, 44 is about 1 i iF. The electric capacity of the Ka ^¬ Y-capacitors 42, 44 may be needed, even thereof chosen differently.

In two diagrams, shown one above the other in a left-hand region in FIG. 1, a respective electrical voltage of the potentials of the high-voltage battery 10 with respect to the mass 40 is shown. It can be seen from the two schematic diagrams that a reference voltage 38, which is formed between a positive electrical potential of the high-voltage battery 10 and the ground 40, corresponds to approximately half of the DC ^link voltage. Accordingly, an electrical voltage results between the negative electrical potential of the high-voltage battery 10 and the mass 40. In two in a right portion adjacent to the circuit diagram shown charts electrical potentials of a star ^¬ point 62 with respect to the negative potential of the high-voltage battery 10 are shown. The star point 62 is formed by respective winding terminals of windings of the electric machine 58. The left of the two diagrams shows an electrical voltage between the two potentials with a resolution in the range of a clock frequency of the inverter 60. The diagram shown to the right shows the electrical potential relative to an operating frequency of the electric machine 58.

During normal driving operation, the common mode potential on the intermediate circuit side of the high-voltage battery 10 via the Y-capacitors 42, 44 is clamped to the ground 40. In contrast, the push-pull potential on the AC side of the electric machine 58 jump around the mass 40, in time with the switching frequency of the inverter 60. The inverter 60 has in this case three series circuits 22, 24, 26 of two semiconductor switches 28, 30, in the present case are formed by Isolated Gate Bipolar Transitors (IGBT). The series circuits 22, 24, 26 are paral ^¬ lelgeschaltet and subsequently closed to the DC voltage intermediate circuit 54th Each of the three series circuits 22, 24, 26 has a respective center terminal 32, 34, 36, which provide the inverter terminals to which the Stän ^¬ derwicklungen the electrical machine 58 are connected. The inverter 60 is thus designed in the manner of a B6 inverter bridge and provides the three-phase alternating voltage network for driving.

Fig. 2 shows in a schematic diagram representation based on Fig. 1, as the electrical system for charging the

High-voltage battery 10 is connected to a vehicle external charging station 12, which provides electrical energy for charging the high-voltage battery 10. The charging station 12 is a stationary charging station and the electric vehicle is used for close to the charging station 12 in the area of the charging station 12 parked. About a not further shown wired ^¬ bound connection, in this case a connection cable, the electrical system, as will be explained further below, connected to the charging station 12. The charging station 12 provides a three-phase AC voltage with a nominal voltage of about 230 V per phase at an AC voltage frequency of about 50 Hz. The three phases are inserted ver ^¬ respectively to each other by 120 °.

In the present embodiment, a galvanic connection between the charging station 12 and the electrical system of the electric vehicle is provided. Accordingly, a protective conductor connection or a reference potential 16, here ground, the loading station 12 to the ground 40 of the Elect ^¬ rofahrzeugs is electrically coupled.

The loading station 12 also comprises a customized for three-phase alternating voltage network trained filter is carried out by means of which a filtering of the terminals of the charging station 12, so that the prescribed limits for reactions un ^¬ be undershot.

In order to connect the charging station 12 to the electrical system of the electric vehicle, it is provided in this case that the

Star point 62 of the electric machine 58 is opened and the corresponding terminals of the stator windings of the electric machine 58 are connected to the respective phase terminals of the La ^¬ tion 12. As a result, a charging station connection 48 of the electric vehicle is provided.

The stator windings of the electric machine 58 and the inverter 60 can then be used to supply electrical energy to the high-voltage battery 10. For this purpose, the IGBTs 28, 30 of the inverter 60 are suitably controlled by the control unit 52 of the inverter 60. The inverter 60 thus provides a clocked energy converter, which makes use of the inductance of the stator windings the electric machine 58 realized an energy conversion in a suitable manner. As a result, the high-voltage battery 10 can be supplied with electrical energy from the charging station 12. The respective reference voltage 38 is again shown in two schematic diagrams in a left-hand area in FIG. 2. It can be seen that the Be ^¬ zugsspannung 38 is substantially equal, as they correspond also in the embodiment shown in Fig. 1 operating situation. Only slight fluctuations occur when charging by means of a three-phase alternating voltage. Accordingly, the conditions at the central connections 32, 34, 36 set in such a way as they are already shown for Fig. 1. This is illustrated by two superimposed diagrams in a central region in the circuit diagram of FIG. 2. In the right-hand area of FIG. 2, the three phases of the AC voltage provided by the charging station 12 are shown.

By means of a charging station provided on the net filter or filter circuit 120 can be prevented that the reference potential 16 jumps neither with respect to the DC link side nor with respect to the charging station side AC voltage side. Depending on a respective switching pattern for the respective IGBTs 28, 30, which use a respective PWM pattern to realize the respective push-pull voltage with respect to the power frequency of 50 Hz, different noise components are generated. The filter circuit 120 of the charging station 12 is designed adapted for these disturbances.

4 shows a schematic diagram of the time profile of a PWM pattern of the series circuit 24 of the inverter 60 with a graph 66 and the associated integrated AC voltage with a graph 68. FIG. 3 shows the three corresponding AC voltages at the three input terminals of the inverter 60 with three graphs 70, 72 and 74. In the graphs 3 and 4, the abscissa is respectively a time axis in which the time is given in milliseconds. The ordinate is a voltage axis indicating the voltage in volts. FIG. 7 shows a schematic diagram of a diagram as in FIG. 2, but here the vehicle electrical system of the electric vehicle is connected to a charging station 14, which provides a single-phase AC voltage 18. As a result, only the series circuits 22 and 24 are now activated by the inverter 60 with respect to the energy conversion. Together with the

Stator windings of the electric machine 58, a clocked energy converter 20 is thereby formed. The clocked energy converter 20 has an energy storage connection 50, with which it is connected to the DC voltage intermediate circuit 54 and thus also to the high-voltage battery 10. Thus, the clocked energy converter 20 forms a charging device 46, which is designed to be connected to the charging station 14.

The center terminals 32 and 34 of the series circuits 22, 24 are respectively electrically connected to a phase terminal L and a neutral terminal N of the charging station 14. On the charging station side, an alternating voltage of 230 V at 50 Hz is provided at the phase connection L in relation to the zero connection N. The La ^¬ destation 14 further also has a filter circuit 120th

Moreover, a not shown Energiever ^¬ supply, to which the charger 14 is connected, the neutral connection N are electrically coupled with a protective conductor connection as an electrical reference potential sixteenth The reference potential 16 is presently the ground potential. The other components and their functionalities correspond to those which have already been explained for the embodiment according to FIGS. 2 to 4, for which reason reference is made to the relevant explanations. By means of the control unit 52, the IGBTs 28, 30 of the

Series circuits 22, 24 controlled so that a corresponding energy conversion can be realized. Thus, the load destation taken 14 electrical energy and the high-voltage battery ^¬ be supplied 10th

In the middle of Fig. 7, two schematic diagrams arranged one above the other are shown in the circuit diagram, as shown in FIG. From these diagrams, it can be seen that the common mode signal at the input terminals of the inverter 60 is compensated here. The upper of the two diagrams shows the common mode potential with respect to the negative potential of the high-voltage battery 10 with respect to the PWM pattern, whereas the lower schematic diagram represents the common-mode potential with respect to the AC frequency.

In the left area - as well as in Fig. 2 - also two schematic diagrams are shown one above the other. The two superposed diagrams respectively represent the reference voltage 38 with respect to the positive potential of the high-voltage battery 10 with respect to the ground 40 and the negative potential with respect to the ground 40. The ground 40 is electrically coupled to the reference potential 16.

It can be seen that the electrical potentials fluctuate in phase with approximately half the mains voltage amplitude with respect to the ground 40 with the AC voltage frequency. The resulting reference voltage 38 is thus superimposed with a comparatively high alternating voltage, so that a correspondingly high leakage current is dissipated by the Y capacitors 42, 44 into the mass 40. Due to the comparatively high capacity for Y capacitors, the leakage current can have an inadmissibly high value. There are therefore provided by the filter circuit 120 of the charging station 14 appropriate measures that prevent a correspondingly high leakage current can occur. FIGS. 5 and 6 further clarify the functionality in single-phase charging operation in this operating mode. Fig. 5 corresponds to the illustration, as has already been explained to Fig. 3. Graphs 78 and 80 show the respective voltages at the input terminals of the inverter 60 which are integrated at the center terminals 32, 34 which provide the respective input terminals. The illustration in FIG. 5 essentially shows the AC frequency component without further spectra, which may be supplemental, inter alia, by the PWM operation. Fig. 6 shows, in a protracted representation corresponding PWM signals with graphs 82, 84, wherein the voltage profile illustrated with the graph 78, the PWM signal is too ^¬ sorted, which is illustrated with the graph 84 and the graph 80 shown Voltage curve that

PWM signal represented by graph 82. Since the PWM signal for the series circuit 24 whose center terminal 34 is connected to the neutral terminal N is inverted to the PWM signal which is applied to the center terminal 32 of the series circuit 22 which is electrically coupled to the phase terminal L, the respective common mode voltages However, a change ^¬ voltage component of the common mode voltage on the network side is not zero, because the zero terminal N is electrically coupled to the reference potential 16. Because the charging station side

Common mode filter of the filter circuit 120 is unable to adequately filter the relatively low AC frequency frequency of about 50 Hz, the phase terminal L and the neutral terminal N are connected directly to the inverter terminals. The AC frequency related common mode component of the inverter outputs follows the AC frequency common mode component on the charging station side. Since there is no common mode component of the inverter terminals with respect to the negative potential of the high voltage battery 10, the common mode voltage on the DC link side follows

Common mode component on the charging station side. FIG. 10 now shows a schematic circuit diagram as in FIG. 7, wherein, however, the schematic diagrams additionally shown in FIG. 10 illustrate the leakage current resulting from the different control according to the invention as well as common-mode potentials. The circuit structure therefore corresponds to that which has already been explained with reference to FIG. 7, for which reason reference is additionally made to the relevant explanations. The idea of the invention is to use a PWM pattern for single-phase charging of the high voltage battery 10, because it said before ^¬ problems in single-phase charging can be solved. FIGS. 8 and 9 show diagrams corresponding to FIGS. 5 and 6, which likewise illustrate voltage profiles, as already explained with reference to FIGS. 5 and 6.

With the graphs 86 and 88, in FIG. 8, respective voltages at the inverter terminals corresponding to the illustration of FIG. 5 are shown. FIG. 9 shows a time-magnified resolution, wherein graphs 86 and 88 are also shown. In addition, supplementally assigned PWM signals with graphs 90, 92 are shown. In this case, the PWM signal according to the graph 90 is assigned to the voltage profile according to the graph 86, and the PWM signal according to the graph 92 is assigned to the voltage profile according to the graph 88.

It can be seen in FIG. 10 in the schematic diagrams superimposed in the left region that the reference voltage 38, in contrast to FIG. 7, is essentially a DC voltage. Accordingly, the negative potential of the high ^¬ voltbatterie 10 relative to the ground 40 is a DC voltage. As a result, the Y capacitors 42, 44 are now essentially each subjected to a constant voltage, so that the leakage current is considerably reduced by these capacitors.

By the control principle of the invention can thus reduce the aforementioned problem with respect to the leakage current, if not even completely avoided. At the inverter terminals, which are formed by the central terminals 32, 34, set against the embodiment of FIG. 7 other voltages. It can be seen that in the upper diagram in the central region of FIG. 10, the common mode potential with respect to the negative potential of the high-voltage battery 10 fluctuates with the clock frequency of the inverter 60. It can be seen from the schematic diagram below that the common-mode potential with respect to the negative potential of the high-voltage battery 10 has a significant spectral component at the AC frequency, in which case an AC voltage of approximately 115 V is present. Overall, a reduction of the leakage current in the range of the AC voltage frequency can thus be achieved with the invention. Although here in contrast to the embodiment according to FIGS. 5 to 7 the

Suppress clock frequency of the inverter 60 in addition to, however, this frequency compared to the AC voltage frequency is considerably higher, for example, 10 kHz, which is why this high frequency can be filtered in a suitable manner with relatively little filter effort.

In the following figures 11 to 20 various simulations and their results are shown, with which the effect of the invention is further clarified.

FIG. 11 shows a schematic equivalent circuit diagram for a simulation of a symmetrical full bridge with a common mode current or leakage current to the reference potential 16 with a frequency of approximately 50 Hz in the case of a symmetrical full bridge operation with a suitable PWM pattern. 12 shows a schematic voltage or current-time diagram in which graphs 94 and 96 show AC voltages at the inverter terminals of the inverter 60, with only the spectral component in the range of 50 Hz with respect to the negative potential of the high-voltage battery 10 is shown. A graph 98 shows the leakage current to the reference potential 16, which is detected by the Y capacitors 42, 44 and optionally further Y capacitors of the line filter. As the filter circuit 120 is effected. The electrical capacitance of the Y capacitors of a respective phase of the network filter or the filter circuit 120 is generally about 22 nF.

Fig. 13 shows a schematic equivalent circuit diagram like Fig. 11, in which now the clocked energy converter 20 is operated according to the invention. FIG. 14 shows a representation like FIG. 12, in which graphs 100 and 102 show the corresponding AC voltages at the center terminals 32, 34 of the clocked energy converter 20 for a spectral range around the AC frequency of 50 Hz. A graph 104 shows the corresponding leakage current. Compared to the illustration according to FIGS. 11 and 12, it can be seen that the leakage current can be almost completely reduced by the control principle of the invention.

Fig. 15 shows a further schematic Ersatzschaltbilddar ^¬ position based on the equivalent circuit representations of the FIGS. 11 and 13, wherein caused to further reduce the leakage current, an expected voltage drop across filter inductances of the line filter or the filter circuit 120 of the La ^¬ destation 14 in the direction of the zero terminal N is added by the leakage current or the charging current to the voltage at the corresponding inverter terminal.

FIG. 16 shows the corresponding effects in a diagram as in FIGS. 12 and 14.

In the schematic diagram of FIG. 16, graphs 106 and 108 again represent the corresponding AC voltages at the center terminals 32, 34 of the clocked energy converter 20. With a graph 110, however, again the leakage current is shown. It is also apparent here that the leakage current is considerably reduced compared to the simulation according to FIGS. 11 and 12.

In the embodiments, inverter 60 is configured to include a portion of the clocked power converter 20 of the charging device 46 to provide. The clocked energy converter 20 thus comprises in this case the series circuits 22, 24 of the alternating ^¬ richters 60 whose central terminals 32, 34 are connected to the corresponding terminals of the charging station 14, namely the phase connection L and the neutral terminal N. Thus, for the operation of the switched power converter 20, not only the Wech ^¬ selrichter 60, but also its control unit 52 is used, which thus also comprises a power converter control, which is designed for the inventive process control.

FIG. 17 schematically shows an equivalent circuit diagram in which only inverse diodes D 1, D 2, D 3, D 4 connected in parallel are illustrated with the IGBTs 28, 30. This substitute circuit diagram is intended to explain how the charging device 46 is enabled to determine which of the connections of the clocked energy converter 20, namely which of the central connections 32, 34, is electrically coupled to the phase connection L and which of the central connections 32, 34 to the neutral connection N. is. This is useful for the control of the clocked energy converter in the sense of the method according to the invention.

Basically, of course, it is possible to measure the ent ^¬ speaking voltages at each of the central terminals 32, 34 relative to the reference potential 16. For other applications in which such a voltage can not be measured, a state information of the inverter 60 or of the clocked energy converter 20 provided by the inverter 60 can be used, which by means of corresponding not shown driver units corresponding to the corresponding IGBTs 28, 30 be controlled, can be determined. From the information obtained in this way, it can be determined which of the central connections 32, 34 is electrically connected to the phase connection L and which is electrically connected to the zero connection N.

Fig. 18 shows a corresponding timing diagram as shown in FIGS. 12, 14, 16, with which the above-mentioned measurement method is to be ^¬ interpreting further light ver. Because the DC link side of the AC The inverter 60 and the clocked energy converter 20 via the Y capacitors 42, 44 connected to the ground 40 and consequently to the reference potential 16, the voltage across the IGBT's 28, 30, in Fig. 17 only by their respective diodes Dl, D2, D3, D4 are shown, very different for connection to the phase terminal L and the neutral terminal. This is with Fig. 18 clarifies. For this purpose, this embodiment uses a function of the corresponding driver units, which provide a status signal, depending on a switching state of the respective IGBT 28, 30th

The state signal is dependent on an electrical voltage measured at a switching path of the respective IGBT 28, 30. If this voltage is less than approximately 4 V, which corresponds to a predetermined reference voltage, this is signaled as a switched-on state with a suitable signal. Accordingly, a detected voltage greater than about 4 V is reported as being off with an appropriate signal. In this case, this embodiment makes use of the fact that the respective switched-off state of the respective IGBT 28, 30, which is electrically coupled to the phase connection L, jumps back and forth, whereas a corresponding state of the IGBTs 28, 30, the are electrically coupled to the neutral terminal N, constantly indicate the turned off state.

When a DC voltage due to passive rectification is detected across the inverse diodes D1, D2, D3, D4, which is not increased by any other DC voltage source, the rectified DC voltage is coupled to the reference potential 16. Without the Y-capacitors 42, 44 of the DC intermediate circuit 54, the rectified voltage would rise only to the maximum peak voltage of the supplied AC voltage. Because the Y-capacitors 42, 44 act on the DC link side like bootstrap capacitors when the AC side is not symmetrical with respect to the

Reference potential 16, the DC voltage increases to about twice the peak voltage of the AC voltage when the Phase terminal L or the neutral terminal N are coupled to the Bezugspo ^¬ potential 16. Fig. 18 illustrates this.

A graph 114 in FIG. 18 shows the corresponding voltage of the IGBTs 28, 30 which are coupled to the phase connection L. The graph 112 shows the situation for the IGBTs 28, 30 connected to the neutral terminal N. A graph 116 represents the reference potential 16. From FIG. 18 it can be seen that it can be easily determined that the diodes D 1 and D 3 are coupled to the phase connection L. Accordingly, their IGBTs 28, 30 are electrically coupled to the phase terminal L. This can be used 30 to control the IGBT's 28, in order to realize the invention Ver ^¬ direction drive.

FIG. 19 shows, in a schematic representation like FIG. 18, detection areas 118 that can be used to be able to determine the assignment of the connections provided by the charging station 14 to the reference potential 16.

Overall, the exemplary examples show how the inventive process it ^¬ leakage particularly when charging an electrical energy accumulator of a motor vehicle ^¬ with a single-phase AC voltage of a laser can be destation reduced. The embodiments are merely illustrative of the invention and are not intended to limit this.

_c

35

Reference sign list

10 high-voltage battery

12 charging station

14 charging station

16 reference potential

18 alternating voltage

20 energy converters

22 series connection

24 series connection

26 series connection

28 semiconductor switches

28 IGBT

30 IGBT

32 middle connection

34 middle connection

36 middle connection

38 reference voltage

40 mass

42 Y capacitor

44 Y capacitor

46 Charging device

48 charging station connection

50 energy storage connection

52 control unit

54 DC intermediate circuit

56 drive device

58 electric machine

60 inverters

62 star point

64 throttle

66 graph

70 graph

72 graph

74 graph

76 graph

78 graph

80 graph _{3 g}

82 graph

84 graph

86 graph

88 graph

90 graph

92 graph

94 graph

96 graph

98 graph

100 graph

102 graph

104 graph

106 graph

108 graph

110 graph

112 graph

114 graph

116 graph

118 detection area

120 filter circuit

L phase connection

N zero connection

Dl to D4 inverse diodes

Claims

claims

1. A method for charging an electrical energy store (10) of a motor vehicle with electrical energy from a vehicle-external charging station (14), wherein the charging station (14) provides a single-phase electrical AC voltage (18) with respect to a predetermined electrical reference potential (16), wherein the electrical energy store (10) by means of a ge ^¬ clocked energy converter (20) is electrically coupled to the vehicle-external laser destation (14), wherein the switched power converter, the electrical energy by means of at least two parallel-connected series circuits (22, 24) from j in each case two Switching elements (28, 30) converts, for which purpose respective center connections (32, 34) of the series connections (22, 24) to the charging station (14) are electrically coupled,

d a d u r c h e c e n c i n e s that

the switching elements (28, 30) at least dependent on an electrical reference voltage (38) ge ^¬ controls between one of at least two energy storage potentials of the electric energy storage (10) and the reference electrical potential (16).

2. The method according to claim 1, characterized in that the AC voltage (18) is provided with a predetermined AC voltage frequency and for controlling the switching elements (28, 30) only a spectral portion of the electrical reference voltage (38) in a predetermined range of the AC voltage frequency is taken into account.

3. The method according to claim 1 or 2, characterized in that the switching elements (28, 30) are controlled such that the electrical reference voltage (38) is substantially constant.

4. The method according to any one of the preceding claims, characterized in that the switching elements (28, 30) are so ge ^¬ controls that the electrical reference voltage (38) 40% to 60%, preferably 50%, of an electrical energy storage Voltage between the at least two energy storage potentials of the electrical energy store (10) corresponds.

5. The method according to any one of the preceding claims, characterized in that one of the electrical potentials (N) of the alternating electrical voltage with the electrical reference potential (38) is electrically coupled, wherein the Mittelan ^{¬ connections} (32, 34) of the two series circuits (22, 24) are coupled to the two terminals (L, N) provided by the charging station (14).

6. The method according to any one of the preceding claims, characterized in that the switching elements (28, 30) each have an inverse diode (Dl, D2, D3, D4), wherein the switching elements (28, 30) are turned off and respective electrical

Switching element voltages are detected and evaluated to determine which of the center terminals (32, 34) is electrically coupled to the reference potential (16).

7. Charging device (46) for charging an electrical

Energy store (10) of a motor vehicle with electric power from a vehicle-external charging station (14) with a charging station terminal (48) for feeding a single-phase electrical AC voltage (18) with respect to a predetermined electrical reference potential (16), a Energiespeicheran ^¬ circuit (50) for electrically coupling the electrical energy store (10), a clocked energy converter (20) having at least two series circuits (22, 24) each of two switching elements (28, 30), wherein the series circuits (22, 24) connected in parallel and with the energy storage connection (50 ), each of the at least two series circuits (22, 24) having a central terminal (32, 34), and wherein the central terminals (32, 34) are electrically coupled to the charging station terminal (48), characterized by a control unit (52 ), which is formed, the switching elements (28, 30) at least depending on an electrical reference voltage (38) between one of at least two energy storage potentials on Energy storage connection (50) and the electrical Bezugspo ^¬ potential (16) to control.

8. Charging device according to claim 7, characterized in that the charging station connection (48) is galvanically coupled to the clocked energy converter.

9. motor vehicle with an electrical vehicle electrical system, the a DC voltage intermediate circuit (54) and a connected to the DC voltage intermediate circuit (54) electrical

Energy storage (10), a charging station (48) for connection to a vehicle external, a single-phase AC electrical voltage (18) providing charging station (14) and with the charging station (48) and the electrical energy storage (10) electrically coupled charging device (46) for charging the electrical energy store (10) with electrical energy from the charging station (14), characterized in that the charging ^¬ device (46) is designed according to claim 7 or 8.

10. Motor vehicle according to claim 9, characterized by an electric drive device (56), which has a three-phase electrical machine (58) for driving the motor vehicle and an electrical machine (58) and the electrical system connected inverter (60), the is configured to provide a three-phase alternating voltage network for the electrical machine (58), wherein the inverter (60) is designed to provide the clocked energy converter (20) of the charging device (46).

11. Motor vehicle according to claim 10, characterized in that the clocked energy converter (20) via phase windings of the electric machine (58) to the charging station terminal (48) is electrically coupled.