Dual active bridge DC / DC converter with phase shift control for bidirectional power transfer or capacitor discharge using reactive power

Abstract

The invention relates to a circuit concept for using a DC / DC converter for discharging an DC link capacitor. In particular, a DC / DC converter of a charging circuit for charging a traction battery in an electric vehicle can be used for this purpose.

Description

[0001] Description

[0002] title

[0003] Dual Active Bridge DC / DC Converter with Phase Shift Control for Bidirectional Power Transfer or Capacitor Discharge via Reactive Power

[0004] Technical field

[0005] The present invention relates to a voltage converter arrangement and a charging circuit for an electrical energy storage device with such a voltage converter arrangement. The present invention further relates to a method for operating a voltage converter arrangement, in particular a voltage converter arrangement in a charging circuit for an electrical energy storage device.

[0006] background

[0007] Vehicles that are fully or partially electrically powered can have an electric drive system in which electrical energy from a (high-voltage) DC network is used to operate an electric motor. This DC network may include a so-called intermediate circuit capacitor. This capacitor stabilizes the DC voltage. When the electric vehicle is switched off, and especially in the event of a malfunction such as an accident, this intermediate circuit capacitor should be discharged to prevent potential hazards from electric shock. Additional discharge circuits or similar devices may be provided for this purpose.

[0008] For example, the publication EP 2 516 197 B1 describes a method and a device for discharging an energy storage device in a high-voltage network. In particular, it is proposed to first provide a discharge resistor and to connect a further discharge resistor in parallel to this first discharge resistor when the voltage across the capacitor to be discharged falls below a threshold value.

[0009] Disclosure of the invention

[0010] The present invention provides a voltage converter arrangement, a charging circuit for an electrical energy storage device, and a method for operating a voltage converter arrangement with the features of the independent claims. Further advantageous embodiments are the subject of the dependent claims.

[0011] Accordingly, the following is planned:

[0012] A voltage converter arrangement comprising a first terminal, a second terminal, and a DC-DC converter. The first terminal is designed to be coupled to a first DC network. This first DC network may include a first capacitor. This first capacitor may be located between a positive terminal and a negative terminal of the first DC network. The second terminal of the voltage converter arrangement is designed to be coupled to a second DC network. This second DC network may include a second capacitor. This second capacitor may be located between a positive terminal and a negative terminal of the second DC network. Each DC-DC converter in the voltage converter arrangement comprises a transformer, a first full bridge, and a second full bridge.The first transformer comprises a primary winding and a secondary winding. The first full bridge is arranged between the first terminal of the voltage converter assembly and the primary winding of the transformer. The second full bridge of the DC-DC converter is arranged between the second terminal of the voltage converter assembly and the secondary winding of the transformer. Each full bridge can comprise two half-bridges. Each half-bridge can comprise a series connection of two switching elements, in particular two semiconductor switching elements. The at least one DC-DC converter of the voltage converter assembly is designed to transfer electrical energy from the first terminal to the second terminal in a first operating mode.Furthermore, the at least one DC-DC converter is designed to transfer electrical energy from the second terminal, in particular the second capacitor at the second terminal, towards the first terminal in a second operating mode. The switching elements of the first full bridge and the switching elements of the second full bridge are periodically controlled by pulse-width modulated signals in both the first and second operating modes. In the first operating mode, the switching times for controlling the switching elements in the first full bridge are shifted by less than a quarter of a period relative to the switching times for controlling corresponding switching elements in the second full bridge.In the second operating mode, the switching times for controlling the switching elements in the first full bridge are shifted by at least a quarter of a period compared to the switching times for controlling the corresponding switching elements in the second full bridge.

[0013] Furthermore, a charging circuit for an electrical energy storage device with a voltage converter arrangement according to the invention is provided. The first terminal of the voltage converter arrangement is designed to be connected to a DC voltage source. The second terminal of the voltage converter arrangement is designed to be connected to the electrical energy storage device to be charged. Optionally, the charging circuit can include a rectifier circuit designed to be connected to a single-phase or multi-phase AC voltage source at an AC voltage terminal. In this case, the rectifier circuit is further designed to convert the AC voltage provided at the AC voltage terminal into a DC voltage and to provide this DC voltage at the first terminal of the voltage converter arrangement.

[0014] Finally, the following is planned:

[0015] A method for operating a voltage converter arrangement, in particular a voltage converter arrangement according to the invention. The voltage converter arrangement comprises a first terminal, a second terminal, and at least one DC-DC converter. The first terminal is designed to be connected to a first DC-DC network. This first DC-DC network may include a first capacitor arranged between a positive terminal and a negative terminal of the first DC-DC network. The second terminal of the voltage converter arrangement is designed to be connected to a second DC-DC network. The second DC-DC network may include a second capacitor arranged between a positive terminal and a negative terminal of the second DC-DC network.Each of the at least one DC-DC converters comprises a transformer, a first full bridge, and a second full bridge. The transformer includes a primary winding and a secondary winding. The first full bridge is arranged between the first terminal of the corresponding DC-DC converter and the primary winding of the transformer. The second full bridge is arranged between the second terminal of the corresponding DC-DC converter and the secondary winding of the transformer. The method comprises a step for transferring electrical energy from the first terminal to the second terminal of the voltage converter arrangement in a first operating mode. Furthermore, the method comprises a step for transferring electrical energy from the second terminal, in particular from the second capacitor at the second terminal, towards the first terminal of the voltage converter arrangement in a second operating mode.In both steps, the switching elements in the first and second full bridges are periodically controlled with pulse-width modulated signals. In the first operating mode, the switching times for controlling the switching elements in the first full bridge are shifted by less than a quarter of a period relative to the switching times for controlling the corresponding switching elements in the second full bridge. In the second operating mode, the switching times for controlling the switching elements in the first full bridge are shifted by at least a quarter of a period relative to the switching times for controlling the corresponding switching elements in the second full bridge.

[0016] Advantages of the invention

[0017] The present invention is based on the understanding that a galvanically isolated DC-DC converter is often provided in DC electrical systems with an intermediate circuit capacitor. For example, such a DC-DC converter may be present in a charging circuit through which an electrical energy storage device in the DC network, such as a traction battery of an electric vehicle, can be charged from an external energy source. Therefore, one aspect of the present invention is to provide a concept for discharging the intermediate circuit capacitor in order to discharge the intermediate circuit capacitor in the DC network using such a DC-DC converter.

[0018] A particular feature of the concept according to the invention for discharging the intermediate circuit capacitor is the ability to control such a galvanically isolated DC-DC converter in such a way that the energy extracted from the intermediate circuit capacitor during discharge can preferably be converted into thermal energy within the transformer and, if necessary, the switching elements of the full bridges. This allows for rapid discharge of the intermediate circuit capacitor while maintaining the current-carrying capacity and thermal limits of all components involved in the discharge process.

[0019] According to the concept of the invention, the voltage converter arrangement with at least one DC-DC converter can be arranged between a first DC network and a second DC network. In the first operating mode, energy transfer can take place according to a conventional control of the switching elements in the full bridges of the DC-DC converters, for example, to transfer electrical energy from the first terminal to the second terminal. The DC-DC converters can be, for example, so-called dual-active-bridge converters or resonant converters.

[0020] The first full bridge between the first terminal and the primary side of the transformer, and the second full bridge between the second terminal and the secondary side of the transformer, can each comprise two half-bridges, each with two switching elements connected in series, in particular semiconductor switching elements. The nodes where the two switching elements of a half-bridge are connected can each be connected to a terminal of a transformer winding. The outer terminals of the half-bridges can each be connected to a terminal of the respective DC-DC converter. The individual switching elements are controlled by a periodic pulse-width modulated (PWM) control signal.For conventional power transmission, the times at which a semiconductor switching element in the first full bridge and a corresponding semiconductor switching element in the second full bridge are activated are only slightly phase-shifted, i.e., by less than a quarter of a period. Corresponding semiconductor switching elements in this context means that a switching element to be activated in the first half-bridge is assigned an analogously arranged switching element in the second half-bridge. By varying the pulse width for activating the switching elements in the first full bridge and the pulse width for activating the switching elements in the second full bridge, it is possible, for example, to reduce the reactive currents in the transformer and to enable smooth switching of the switching elements.By varying the phase shift for the timing of controlling corresponding switching elements in the first full bridge and the second full bridge, the power flow in the DC-DC converter can be adjusted.

[0021] In particular, this also makes bidirectional energy transfer possible, so that, for example, in a third operating mode, electrical energy can also be transferred from the second connection to the first connection.

[0022] Furthermore, a second operating mode is provided for the discharge of an intermediate circuit capacitor in the second DC network according to the invention, in which the phase shift, i.e., the time interval between the activation of a switching element in the first full bridge and the activation of a corresponding switching element in the second full bridge, is at least in the range of approximately one-quarter of the period of the periodic pulse-width modulated control. In the range of a phase shift of approximately half a period, only a small amount of electrical power is transferred between the terminals of the DC-DC converter. The precise time difference can be adjusted so that a constant or at least sufficiently high electrical voltage is maintained at the first terminal of the voltage converter arrangement.Even with low power transmission, a high voltage-time area is applied to the transformer's leakage inductance and, if applicable, to the resonant inductances, resulting in a high reactive current in the transformer. This leads to increased losses in the transformer and potentially also in the switching elements of the full bridges.

[0023] According to one embodiment, the at least one DC-DC converter is designed to transfer electrical energy from the second terminal to the first terminal in a third operating mode. Analogous to the first operating mode, in the third operating mode the switching times for controlling the switching elements in the first full bridge are also shifted by less than a quarter of a period relative to the switching times for controlling corresponding switching elements in the second full bridge. In this way, bidirectional operation is possible in the first operating mode or, alternatively, in the third operating mode.

[0024] Energy transfer between the first connection and the second connection is achieved.

[0025] According to one embodiment, the at least one DC-DC converter comprises a galvanically isolated resonant converter. In this embodiment, the resonant converter is operated outside its resonant frequency in the second operating mode for discharging the intermediate circuit capacitor at the second terminal.

[0026] According to one embodiment, the voltage converter arrangement comprises several DC-DC converters. These multiple DC-DC converters can be arranged in parallel to each other between the first and second terminals. This increases the energy transfer between the first and second terminals.

[0027] According to one embodiment, in the second operating mode, when more than one DC-DC converter is used in the voltage converter arrangement, the switching times for activating the switching elements in the full bridges of one DC-DC converter are phase-shifted relative to the switching times for activating the corresponding switching elements in the full bridge of another DC-DC converter. This reduces the resulting reactive currents and thus the load on the capacitor at the first terminal of the voltage converter arrangement.

[0028] The above embodiments and further developments can be combined with one another as appropriate. Further embodiments, further developments, and implementations of the invention also include combinations of features of the invention described previously or subsequently with regard to the exemplary embodiments, even if not explicitly mentioned. In particular, those skilled in the art will also add individual aspects as improvements or additions to the respective basic forms of the invention.

[0029] Brief description of the drawings

[0030] Further features and advantages of the invention are explained below with reference to the figures. These show:

[0031] Fig. 1: a schematic representation of a principle diagram of a voltage converter arrangement according to one embodiment; Fig. 2: a schematic representation of a timing diagram to illustrate the switching times in a first operating mode of a voltage converter arrangement according to one embodiment;

[0032] Fig. 3: a schematic representation of a timing diagram to illustrate the switching times in a second operating mode of a voltage converter arrangement according to an embodiment;

[0033] Fig. 4: a schematic representation of a principle diagram of a voltage converter arrangement according to a further embodiment;

[0034] Fig. 5: a schematic representation of a principle diagram of a charging circuit with a voltage converter arrangement according to one embodiment; and

[0035] Fig. 6: a flowchart as it underlies a method for operating a voltage converter arrangement according to one embodiment.

[0036] Description of embodiments

[0037] Figure 1 shows a schematic representation of a principle diagram of a voltage converter arrangement 1 according to one embodiment. The voltage converter arrangement 1 comprises a first terminal 10 and a second terminal 20. A first terminal 11, for example a positive terminal, can be connected to a first terminal of a first DC voltage network. Correspondingly, a second terminal 12 of the first terminal 10, for example a negative terminal, can be connected to another terminal of the first DC voltage network. A first capacitor C1 can be provided between the first terminal 11 and the second terminal 12 of the first terminal 10.

[0038] Similarly, a first connection point 21 of the second connection 20, for example a positive connection point, can be connected to a corresponding connection point of a second DC network. Likewise, a second connection point 22, for example a negative connection point, can be connected to a corresponding second connection point of the second DC network. The second DC network can have a capacitor C2, for example an intermediate circuit capacitor or link capacitor, between the first connection point and the second connection point.

[0039] At least one DC-DC converter 30 is arranged between the first terminal 10 and the second terminal 20 of the voltage converter arrangement 1. The DC-DC converter 30 is, in particular, a galvanically isolated DC-DC converter with a transformer 33. A first full bridge 31 is arranged between the first terminal 10 and a primary side of the transformer 33. Similarly, a second full bridge 32 is provided between the secondary side of the transformer 33 and the second terminal 20. Each full bridge comprises two half-bridges with two series-connected switching elements M1 to M8, in particular semiconductor switching elements.

[0040] Thus, a first switching element M1 can be arranged between the first connection point 11 of the first terminal 10 and a first node K1, and a second switching element M2 can be arranged between the first node K1 and the second connection point 12 of the first terminal 10. Similarly, a third switching element M3 can be arranged between the first connection point 11 of the first terminal 10 and a second node K2, and a fourth switching element M4 can be arranged between the second node K2 and the second connection point 12 of the first terminal 10.

[0041] Similarly, in the second full bridge 32, a fifth switching element M5 can be arranged between the first connection point 21 of the second terminal 20 and a third node K3, and a sixth switching element M6 can be arranged between the third node K3 and the second connection point 22 of the second terminal 20. Finally, a seventh switching element M7 can be arranged between the first connection point 21 of the second terminal 20 and a fourth node K4, and an eighth switching element M8 can be arranged between the fourth node K4 and the second connection point 22 of the second terminal 20.

[0042] The two connection points of the primary side of transformer 33 can be connected to the first node K1 and the second node K2, respectively. Similarly, the two connection points of the secondary side of transformer 33 can be connected to the corresponding nodes K3 and K4.

[0043] Optionally, resonant elements such as inductors L and / or capacitors C can be provided between nodes K1 and K2 and the primary side of the transformer 33 and / or between the third and fourth nodes K3, K4 and the corresponding connection points of the secondary side of the transformer 33. In this way, for example, a resonant converter can be implemented.

[0044] For the operation of the DC-DC converter 30 in the voltage converter arrangement 1, the switching elements M1 to M8 can be controlled according to a modulation scheme with pulse-width modulated control. Since the underlying control method is considered known for conventional operation, it will not be explained in detail here; instead, only some essential points are outlined below.

[0045] Figure 2 shows a schematic representation of a timing diagram to illustrate the control of the switching elements M1 to M8 for a conventional power transmission between the first terminal 10 and the second terminal 20. The upper half shows the control signals for the primary switching elements M1 to M4 in the first full bridge 31. Below are the corresponding signals for controlling the associated switches M5 to M8. By varying the pulse widths t1 for the primary switching elements M1 to M4 and / or the pulse widths t2 for the secondary switching elements M5 to M8, the resulting reactive current in the transformer 33 can be adjusted, and, if necessary, a soft start-up of the switching elements M1 to M8 can be enabled. The phase shift between a control signal for the primary switching elements M1 to M4 and a corresponding control signal for the secondary switching elements M5 to M8, respectively, allows for the adjustment of the switching current.The corresponding time shift t3 allows the power flow between the first terminal 10 and the second terminal 20 to be adjusted. In conventional control systems, this phase shift lies between -90° (maximum negative power transfer) and +90° (maximum positive power transfer). This corresponds to a time difference of ± one-quarter of a period T. Larger phase shifts are usually avoided in conventional operation because they reduce power transfer while increasing reactive currents in the transformer 33.

[0046] This region, which, as mentioned above, is generally avoided in conventional control, can, however, be deliberately used to discharge electrical energy stored in the DC link capacitor C2 at the second terminal 20 of the voltage converter arrangement 1. A corresponding timing diagram is shown as an example in Figure 3. As can be seen in Figure 3, in such an operating mode, in which the DC link capacitor C2 is to be discharged at the second terminal 20 of the voltage converter arrangement 1, the pulse width modulated control can be at least a quarter of a period T. This corresponds to a phase shift of more than ±90°. Near approximately 90°, i.e., t3 approximately equal to T / 4, only a small amount of power is transferred between the two terminals 20, 10 of the voltage converter arrangement 1. Preferably, the phase shift is set such that a primary-side voltage is constant or...The voltage remains sufficiently high for lossy operation. If the capacitor C1 at the first terminal 10 is initially not charged or not sufficiently charged, a phase shift with a time difference t3 can initially be selected, in which a (small) energy transfer from the second terminal 20 to the first terminal 10 takes place.

[0047] Such a modulation scheme can result in a high voltage-time area across the transformer's leakage inductance and, if applicable, the resonant inductances L, even with low power transfer, leading to a high reactive current in transformer 33. This results in increased losses in transformer 33 and the switching elements M1 to M8. The magnitude of the resulting reactive current and the associated losses can be adjusted by appropriately varying the pulse widths t1 and / or t2. Likewise, if required, a soft turn-on at a current zero crossing for the switching elements M1 to M8 can be achieved by varying these parameters t1 and t2, thus preventing thermal overload of the switching elements M1 to M8.

[0048] If a resonant converter is used for the DC-DC converter 30, such a resonant converter can preferably be operated above its resonant frequency to discharge the intermediate circuit capacitor C2, particularly when the inductive component of the resonant network dominates. Figure 4 shows a schematic representation of a basic circuit diagram of a voltage converter arrangement 1 according to a further embodiment. The voltage converter arrangement 1 according to Figure 4 differs from the previously described voltage converter arrangement 1, in particular, in that several DC-DC converters 30 are connected in parallel. The previously stated descriptions regarding the construction and operating strategy apply to the individual DC-DC converters 30.In such a configuration with two or more DC-DC converters 30 connected in parallel, the switching elements M1 to M8 in the individual DC-DC converters 30 can be controlled with a phase shift relative to each other (interleaved). In this way, the resulting current that loads the capacitor C1 at the first terminal 10 of the voltage converter arrangement can be reduced overall. Thus, even with a relatively small capacitor C1 at the first terminal 10, a high-power discharge of the intermediate circuit capacitor C2 can be achieved.

[0049] Figure 5 shows a schematic representation of a charging circuit with a voltage converter arrangement 1 according to one embodiment. Such a charging circuit can be used, for example, to charge an electrical energy storage device 4, such as a traction battery in an electric vehicle. In such a charging circuit, a DC voltage supplied at the first terminal 10 can be adapted by means of the DC voltage converter 30 to a DC voltage suitable for charging the electrical energy storage device 4 connected at the second terminal 20. Optionally, a rectifier circuit 2, for example an active power factor corrector (PFC) circuit, can also be provided. Such a rectifier circuit 2 can convert a single-phase or multi-phase AC voltage supplied by an AC voltage source 3, for example a power supply network, into a DC voltage.In such a circuit arrangement, the previously described voltage converter arrangement 1 for discharging the intermediate circuit capacitor C2 can be seen as part of a complete charging circuit, which, for example, can be provided as a so-called on-board charger (OBC) in electric vehicles. The inventive concept for controlling the switching elements M1 to M8 thus enables such a charging circuit to be used for actively discharging the intermediate circuit capacitor C2.

[0050] Figure 6 shows a flowchart illustrating a method for operating a voltage converter arrangement 1 according to one embodiment. The voltage converter arrangement 1 can be, in particular, the voltage converter arrangement already described in connection with Figures 1 to 5. Specifically, the voltage converter arrangement 1 can be part of a charging circuit for an electrical energy storage device, especially for charging a traction battery in an electric vehicle. Thus, the method can, in principle, include any steps previously described in connection with one of the voltage converter arrangements 1. Conversely, the voltage converter arrangements 1 described previously can also include any components suitable for implementing the method described below.

[0051] The method can comprise a first step S1, in which, in a first operating mode, electrical energy is transferred from the first terminal to the second terminal. Furthermore, the method can comprise a second step S2, in which, in a second operating mode, electrical energy is transferred from the second terminal, in particular from a capacitor C2 connected to the second terminal, towards the first terminal 10. The switching elements M1 to M8 of the first and second full bridges 31, 32 are periodically controlled by pulse-width modulated signals. In the first operating mode, the switching times for controlling the switching elements M1 to M4 in the first full bridge 31 are shifted by at least a quarter of a period T relative to the switching times for controlling the corresponding switching elements M5 to M8 in the second full bridge 32.In the second operating mode, however, the switching times for controlling the switching elements in the first full bridge 31 are shifted by more than a quarter of a period T compared to the switching times for controlling the corresponding switching elements in the second full bridge 32.

[0052] In summary, the present invention relates to a circuit concept for using a DC-DC converter for discharging an intermediate circuit capacitor. In particular, a DC-DC converter of a charging circuit for charging a

[0053] Traction battery used in an electric vehicle.

Claims

Claims 1. Voltage converter arrangement (1) comprising: a first terminal (10) designed to be coupled to a first DC network, the first DC network comprising a first capacitor (C1) arranged between a positive terminal and a negative terminal of the first DC network; a second terminal (20) designed to be coupled to a second DC network, the second DC network comprising a second capacitor (C2) arranged between a positive terminal and a negative terminal of the second DC network; and at least one DC-DC converter (30), each comprising: a transformer (33) having a primary winding and a secondary winding; a first full bridge (31) arranged between the first terminal and the primary winding of the transformer (33);a second full bridge (32) arranged between the second terminal and the secondary winding of the transformer (33); wherein at least one DC-DC converter (33) is designed to transfer electrical energy from the first terminal (10) to the second terminal (20) in a first operating mode, and to transfer electrical energy from the second capacitor (C2) at the second terminal (20) towards the first terminal (10) in a second operating mode, wherein the switching elements (M1 - M4) in the first full bridge (31) and the switching elements (M5 to M8) in the second full bridge (32) are each controlled by periodic pulse-width modulated signals, wherein in the first operating mode switching times for controlling the switching elements (M1 - M4) in the first full bridge (31) are shifted by less than a quarter of a period (T) relative to switching times for controlling corresponding switching elements (M5 - M8) in the second full bridge,and wherein in the second operating mode the switching times for controlling the switching elements (M1 - M4) in the first full bridge (31) are shifted by at least a quarter of a period (T) compared to the switching times for controlling corresponding switching elements (M5 - M8) in the second full bridge (32).

2. Voltage converter arrangement (1) according to claim 1, wherein the at least one DC voltage converter (30) is designed to transfer electrical energy from the second terminal (20) to the first terminal (10) in a third operating mode, and wherein in the third operating mode the switching times for controlling the switching elements (M1 - M4) in the first full bridge (31) are shifted by less than a quarter of a period (T) compared to switching times for controlling corresponding switching elements (M5 - M8) in the second full bridge (32).

3. Voltage converter arrangement (1) according to claim 1 or 2, wherein the at least one DC voltage converter (30) comprises a galvanically isolating resonant converter, and wherein the resonant converter is operated outside its resonant frequency in the second operating mode.

4. Voltage converter arrangement (1) according to one of claims 1 to 3, comprising several DC voltage converters (30) arranged in parallel to each other between the first terminal (10) and the second terminal (20).

5. Voltage converter arrangement (1) according to claim 4, wherein in the second operating mode the switching times for controlling the switching elements (M1 - M8) in the full bridges (31 , 32) of a DC voltage converter (30) are phase-shifted relative to the switching times for controlling the corresponding switching elements (M1 - M8) in the full bridges (31 , 32) of a further DC voltage converter (30).

6. Charging circuit for an electrical energy storage device (4), comprising: a voltage converter arrangement (1) according to one of claims 1 to 5, wherein the first terminal (10) of the voltage converter arrangement (1) is designed to be connected to a DC voltage source, and wherein the second terminal (20) of the voltage converter arrangement (1) is designed to be connected to the electrical energy storage device (4).

7. Charging circuit according to claim 6, comprising a rectifier circuit (2) designed to be connected to an AC voltage source (3) at an AC voltage terminal, converting an AC voltage provided at the AC voltage terminal into a to convert DC voltage and to provide the DC voltage at the first terminal (10) of the voltage converter arrangement (1).

8. Method for operating a voltage converter arrangement (1) comprising: a first terminal (10) designed to be coupled to a first DC network, the first DC network comprising a first capacitor (C1) arranged between a positive terminal and a negative terminal of the first DC network, a second terminal (20) designed to be coupled to a second DC network, the second DC network comprising a second capacitor (C2) arranged between a positive terminal and a negative terminal of the second DC network, and at least one DC voltage converter (30), each comprising a transformer (33) having a primary winding and a secondary winding, and a first full bridge (31) arranged between the first terminal (10) and the primary winding of the transformer (33).and a second full bridge (32) connecting the second terminal (20) and the, The secondary winding of the transformer (33) is arranged; the method comprising the following steps: Transfer (S1) electrical energy from the first terminal (10) to the second terminal (20) to be transmitted in a first operating mode, and Transfer (S2) electrical energy from the second capacitor (C2) at the second terminal (20) towards the first terminal (10) in a second operating mode, wherein the switching elements (M1 - M4) in the first full bridge (31) and the switching elements (M5 - M8) in the second full bridge (32) are each controlled by periodic pulse-width modulated signals, wherein in the first operating mode switching times for controlling the switching elements (M1 - M4) in the first full bridge (31) are shifted by at least a quarter of a period (T) compared to switching times for controlling corresponding switching elements (M5 - M8) in the second full bridge (32), and wherein in the second operating mode the switching times for controlling the switching elements (M1 - M4) in the first full bridge (31) are shifted by more than a quarter of a period (T) compared to the switching times for controlling corresponding switching elements (M5 - M8) in the second full bridge (32).

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