DC voltage converter and method for operating a DC voltage converter, as well as charging circuit for an electric vehicle

Abstract

DC voltage converter, in particular a galvanically isolating DC voltage converter. The DC voltage converter comprises a transformer and two bridge circuits, each arranged between the corresponding DC voltage terminals and the transformer. In order to improve zero voltage switching, the invention proposes providing extended dead time in one of the half-bridges when driving the bridge circuits.

Description

[0001] R. 416179

[0002] - 1 -

[0003] Description

[0004] title

[0005] DC-DC converter and method for operating a DC-DC converter as well as charging circuit for an electric vehicle

[0006] Technical field

[0007] The present invention relates to a DC-DC converter and a method for operating such a DC-DC converter. The present invention further relates to a charging circuit for an electric vehicle with such a DC-DC converter.

[0008] background

[0009] DC-DC converters are capable of converting a DC voltage supplied at an input terminal into another DC voltage, possibly with a different voltage level, and supplying this at an output terminal. Furthermore, bidirectional DC-DC converters are known, which can transfer electrical power between two DC terminals in both directions. In particular, so-called galvanically isolated DC-DC converters are known, in which the two DC terminals are galvanically isolated from each other, for example, by means of a transformer and bridge circuits or similar devices.

[0010] Such DC-DC converters, especially galvanically isolated DC-DC converters, can be used, for example, to exchange electrical energy between a high-voltage network and a low-voltage network in an electric vehicle. Furthermore, DC-DC converters in an electric vehicle can also be used to convert a DC voltage supplied by an external energy source into a DC voltage. R. 416179

[0011] - 2 - convert, which is suitable for charging the internal energy storage, for example the traction battery of the electric vehicle

[0012] For example, the publication DE 10 2014 210 283 A1 describes a method for operating a vehicle electrical system with at least two voltage levels, which have different nominal voltages.

[0013] Disclosure of the invention

[0014] The present invention provides a DC-DC converter, a charging circuit for an electric vehicle, and a method for operating a DC-DC converter with the features of the independent claims. Further advantageous embodiments are the subject of the dependent claims.

[0015] Accordingly, the following is planned:

[0016] A DC-DC converter comprising a first DC input, a second DC input, and a transformer. The DC-DC converter further includes a first bridge circuit, a second bridge circuit, and a control unit. The transformer comprises a primary and a secondary side. The first bridge circuit is arranged between the first DC input and the primary side of the transformer. Similarly, the second bridge circuit is arranged between the second DC input and the secondary side of the transformer. The first bridge circuit comprises two half-bridges, each with a first switching element and a second switching element. The first and second switching elements are arranged in series, with the junction between the two switching elements connected to a terminal on the primary side of the transformer.Similarly, the second bridge circuit also comprises two half-bridges, each with a first switching element and a second switching element, arranged in series, with the connection point between the two switching elements being connected to a terminal point on the secondary side of the transformer. Such a circuit is referred to in technical language as R. 416179.

[0017] - 3 - also called a Dual Active Bridge. The control unit is designed to control the first and second switching elements in the half-bridges of the first and second bridge circuits. The control unit is designed to set either a first or a second switching state for pulse-width modulated control of the switching elements in each of the two bridge circuits. In the first switching state, the first switching element is closed and the second switching element is open. In the second switching state, the first switching element is open and the second switching element is closed. Furthermore, the control unit is designed to provide a predetermined dead time during each transition between the first and second switching states in a half-bridge.This interlock dead time also generally serves to prevent a DC link short circuit caused by both transistors of a half-bridge switching on simultaneously during the switching process. During this dead time, both the first and second switching elements of the corresponding half-bridge are open. Furthermore, the control unit is designed to provide an extended dead time for at least one half-bridge. This extended dead time is longer than the initial dead time provided for switching between states in the other half-bridges.

[0018] Furthermore, the following is planned:

[0019] A charging circuit for an electric vehicle with a DC-DC converter according to the invention. The first DC input is designed to be connected to a DC voltage source, for example, a power factor correction stage or another grid feed. The second DC input is designed to be connected to an electrical energy storage device of the electric vehicle, for example, a traction battery or similar.

[0020] Finally, the following is planned:

[0021] A method for operating a DC-DC converter, in particular a DC-DC converter according to the invention. R. 416179

[0022] - 4 -

[0023] The DC-DC converter comprises a first DC input, a second DC input, a transformer, and a first and second bridge circuit. The transformer has a primary and a secondary side. The first bridge circuit is arranged between the first DC input and the primary side of the transformer. Similarly, the second bridge circuit is arranged between the second DC input and the secondary side of the transformer. The first bridge circuit comprises two half-bridges, each with a first switching element and a second switching element. The first and second switching elements are arranged in series, and a connection point between each of the two switching elements is connected to a terminal on the primary side of the transformer.Similarly, the second bridge circuit comprises two half-bridges, each with a first switching element and a second switching element. The first and second switching elements are arranged in series, and a connection point between each of the two switching elements is connected to a terminal on the secondary side of the transformer. The method includes a step for pulse-width modulated control of the switching elements in the first and second bridge circuits. Here, either a first switching state can be set in which the first switching element is closed and the second switching element is open, or a second switching state can be set in which the first switching element is open and the second switching element is closed.During a transition between the first and second switching states, a predetermined dead time is provided, during which both the first and second switching elements are open. The method further includes a step for adjusting the dead time for at least one half-bridge. During this adjustment, an extended dead time is provided for this at least one half-bridge. This extended dead time is longer than the initial dead time provided for the transition between switching states in the other half-bridges.

[0024] Advantages of the invention R. 416179

[0025] - 5 -

[0026] The present invention is based on the finding that for switching operations of semiconductor switching elements, such as those used, for example, in galvanically isolated DC-DC converters, particularly in so-called dual-active-bridge DC-DC converters, it is advantageous to perform a switching operation preferably at the lowest possible electrical voltage across the switching element, especially at zero voltage. To minimize the number of sensors required to determine the optimal switching time, the suitable switching times can, for example, be determined in advance and these values ​​stored, for instance, in a table containing the parameters primary voltage, secondary voltage, and power handling capacity. However, the parameters determined in this way generally refer to idealized components and typically do not take into account the component tolerances encountered in reality.

[0027] However, due to component tolerances inherent in real-world applications, it may not be possible to guarantee that the actual circuit will always achieve zero-voltage switching (ZVS). If such ZVS cannot be fully guaranteed, increased switching losses may occur. In such cases, a correspondingly larger component size or increased cooling capacity for the DC-DC converter may be necessary.

[0028] Based on this finding, the present invention therefore provides a concept for operating a DC-DC converter with actively switching semiconductor switching elements, in which a zero-voltage switching (ZVS) can also be realized for real components with a corresponding component tolerance over a wider operating range. For this purpose, the invention provides for an extended dead time during a commutation process in a half-bridge with two switching elements, if required. During such a dead time, both switching elements of the respective half-bridge are open. In this switching state, an electric current can only flow through the parallel diodes provided internally or externally to the switching elements. This current flow through the corresponding diodes allows the electrical voltage to decay sufficiently so that the new R. 416179

[0029] - 6 -

[0030] The switching state can be set with the lowest possible electrical voltage across the corresponding switching element.

[0031] According to one embodiment, the extended dead time corresponds to a period of at least 10% of one period of the pulse-width modulated control signal used to drive the switching elements in the bridge circuits. In particular, the extended dead time can be at least 15% of such a period. In this way, during this extended dead time, the current flowing through the diodes, which are provided internally or externally in parallel with the switching elements, can decay sufficiently to achieve a zero-voltage switch (ZVS).

[0032] According to one embodiment, the initial dead time provided for the commutation of the remaining half-bridges corresponds to a maximum time span of 5% of the period of the pulse-width modulated drive. Such a dead time is generally sufficient for conventional commutation.

[0033] According to one embodiment, the control device is designed to provide an extended dead time for a half-bridge in the bridge circuit, which, during periodic pulse-width modulated control of the switching elements in the two bridge circuits, exhibits the last switching operation in each period. Investigations have shown that, particularly when the dead time for a commutation operation of this half-bridge is extended, a more reliable zero-value system (ZVS) can be ensured across all operating ranges and all parameter variations.

[0034] According to one embodiment, the control device is designed to provide an extended dead time for each half-bridge in the first bridge circuit and the second bridge circuit if, during periodic actuation of the switching elements of both half-bridges, the last switching operations in the two bridge circuits differ by less than a predetermined time interval. This allows for the possibility of a central control system even if, for example, due to uncertainties in component tolerances or similar factors, complete R. 416179

[0035] - 7 -

[0036] It can be determined with certainty in which of the two bridge circuits the extended dead time is advantageous.

[0037] According to one embodiment, the control device is designed to provide for an extended dead time during pulse-width modulated control by prematurely opening a corresponding switching element. In other words, the closing time of a switching element after the dead time has elapsed remains unchanged, while the extended dead time is achieved by prematurely opening the complementary switching element.

[0038] According to one embodiment, the control device is designed to adjust the switch-on time for closing the switching element, which is opened prematurely for the extended dead time. Since, as previously explained, the extended dead time is achieved by prematurely opening a switching element, this shortens the pulse width of the corresponding pulse. If the switch-on time is also adjusted in the relevant pulse, the shortened pulse width can be at least partially compensated.

[0039] According to one embodiment, the switch-on time can be adjusted by advancing the switch-on time by half the amount by which the switch-off time is advanced to extend the dead time. In other words, the switch-on time is adjusted such that the resulting pulse width is reduced by only half the amount by which the pulse width is reduced by the extended dead time at the end of the pulse.

[0040] 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 respect to the exemplary embodiments, even if not explicitly mentioned. In particular, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic forms of the invention. R. 416179

[0041] - 8 -

[0042] Brief description of the drawings

[0043] Further features and advantages of the invention are explained below with reference to the figures. These show:

[0044] Fig. 1 : a schematic representation of a basic circuit diagram of a DC-DC converter according to one embodiment;

[0045] Fig. 2: a diagram illustrating the control concept in a DC voltage connection according to one embodiment;

[0046] Fig. 3: a diagram illustrating the dead times in a control concept for a DC-DC converter according to one embodiment;

[0047] Figs. 4-6: Diagrams illustrating the dead time extension according to a control concept for a DC-DC converter according to one embodiment; and

[0048] Fig. 7: a flowchart as it may form the basis of a method for operating a DC voltage converter according to one embodiment.

[0049] Description of embodiments

[0050] Figure 1 shows a schematic diagram of a basic circuit diagram of a DC-DC converter 1 according to one embodiment. The DC-DC converter 1 comprises a first DC input 11, a second DC input 12, a transformer 20, a first bridge circuit 30, and a second bridge circuit 40. The first DC input 11 and the second DC input 12 can each have at least one positive terminal and one negative terminal. DC power supplies, for example, can be connected to the two DC inputs 11 and 12. R. 416179

[0051] - 9 -

[0052] The transformer 20 comprises at least one primary side 21 and one secondary side 22. This transformer 20 provides galvanic isolation between the first DC voltage terminal 11 and the second DC voltage terminal 12. The first bridge circuit 30 is arranged between the first DC voltage terminal 11 and the primary side 21 of the transformer 20. Similarly, the second bridge circuit 40 is arranged between the secondary side 22 of the transformer 20 and the second DC voltage terminal 12.

[0053] The first bridge circuit 30 comprises two half-bridges 31 and 32. Each half-bridge 31 and 32 comprises two switching elements 31a and 31b, and 32a and 32b, respectively, arranged in series. The two outer terminals of the two half-bridges 31 and 32 are connected to the corresponding terminals of the first DC input 11. The two connection points where the two switching elements 31a and 31b, and 32a and 32b, respectively, are connected to each other are coupled to corresponding terminals on the primary side 21 of the transformer 20. A resonant element, for example a capacitor C1, can be provided between the connection points of the switching elements 31a and 31b, and 32a and 32b, respectively, and the primary side 21 of the transformer 20.

[0054] The second bridge circuit 40 between the secondary side 22 of the transformer 20 and the second DC voltage terminal 12 is constructed analogously to the first bridge circuit 30. Accordingly, the first half-bridge 41 comprises two semiconductor switching elements 41a and 41b arranged in series. A connection point between the two semiconductor switching elements 41a and 41b is coupled to a terminal point of the secondary side 22 of the transformer 20.

[0055] Similarly, the second half-bridge 42 of the second bridge circuit 40 also comprises two semiconductor switching elements 42a and 42b arranged in series. A connection point between the two semiconductor switching elements 42a and 42b is coupled to another connection point on the secondary side 22 of the transformer 20. The two outer connections of the half-bridges 41 and 42 of R. 416179

[0056] - 10 - the second bridge circuit 40 are connected to corresponding connection points of the second DC voltage connection 12.

[0057] Between the secondary side 22 of the transformer and the second bridge circuit 40, another resonant element, for example a capacitor C2, can be provided.

[0058] Furthermore, a control unit 50 may be provided. This control unit 50 can generate the control signals for closing or opening the semiconductor switching elements 31a, 31b, 32a, 32b, 41a, 41b, 42a and 42b and make them available at the corresponding switching elements.

[0059] By appropriately controlling the switching elements in the first bridge circuit 30 and the second bridge circuit 40, the energy transfer between the first DC voltage terminal 11 and the second DC voltage terminal 12 can be controlled. In particular, pulse-width modulated control can be provided for this purpose. In this case, the switching elements of the two bridge circuits 30 and 40 are controlled based on the same frequency for the pulse-width modulated control.

[0060] Figure 2 shows a schematic representation of a diagram illustrating the timing relationships for controlling the switching elements in the first and second bridge circuits. The upper diagram shows the voltage waveform between the two connection points of the switching elements in the first bridge circuit.

[0061] Figure 30 shows whether, in the two half-bridges 31 and 32, the two upper switching elements 31a and 32a are closed and the two lower switching elements 31b and 32b are open, or whether the two upper switching elements 31a and 32a are open and the two lower switching elements are closed.

[0062] If 31b and 32b are closed, a voltage of at least approximately 0 volts results between the two connection points. However, if in the first half-bridge 31 the upper switching element 31a is open and the lower switching element 31b is closed, while in the second half-bridge

[0063] 32 the upper switching element 32a is closed and the lower switching element R. 416179

[0064] - 11 -

[0065] When 32b is open, a positive voltage is generated between the connection points, which corresponds approximately to the DC voltage at the first DC terminal 11. With a reversed circuit configuration of the two half-bridges 31 and 32, a negative voltage is generated between the two connection points, which also corresponds approximately to the DC voltage at the first DC terminal 11. With periodic control, a positive and a negative voltage pulse alternate with a time interval corresponding to the period T. The width of the voltage pulses is characterized by the parameter Alpha.

[0066] The corresponding curve for the second bridge circuit 40 is shown in the diagram below. The pulse width in the second bridge circuit 40 is characterized by the parameter Beta. The time offset between the pulses in the first bridge circuit 30 and the pulses in the second bridge circuit 40 is specified by the parameter Delta.

[0067] The output voltage level, the electrical power to be transferred, and the direction of energy transfer can all be adjusted by appropriately setting the parameters alpha, beta, and delta. Since the basic control principle for selecting these parameters is considered well-known, it will not be explained in detail here.

[0068] For the control of the switching elements 31a, 31b, 32a, 32b, 41a, 41b, 42a, 42b in the first bridge circuit 30 and the second bridge circuit 40 as described above, two switching states are possible for each of the half-bridges 31, 32, 41, 42. In the first switching state, the upper switching element 31a, 32a, 41a, or 42a is closed, and the corresponding lower switching element 31b, 32b, 41b, 42b is open. Alternatively, the upper switching elements 31a, 32a, 41a, 42a can be open, and the corresponding lower switching elements 31b, 32b, 41b, 42b can be closed. To prevent a possible short circuit between the connection points of the corresponding resistor R416179 during a change between these two switching states,

[0069] - 12 -

[0070] To avoid interference at DC terminal 11 or 12, a so-called dead time is provided for each switch. During this dead time, both switching elements of the corresponding half-bridge 31, 32, 41, 42 are open. In other words, when switching between the two switching states, the closed switching element is opened first, and after a predetermined dead time, the other switching element is then closed.

[0071] Figure 3 shows a schematic representation of the timing diagram of two switching elements in a half-bridge to illustrate the principle of dead time. It can be seen that one switching element is opened at time t_off, and then, after a dead time t_tot, the complementary switching element is closed at time t_on. Typically, a relatively short period is used for such a dead time. For example, the minimum dead time can be a maximum of 5% of the period T in the case of periodic control. However, depending on the application, different durations for the basic dead time are also possible.

[0072] When operating the previously described DC-DC converter 1, it is desirable to switch on the switching elements at the lowest possible applied voltage (Zero Voltage Switching, ZVS). In this case, the resulting switching losses are minimized. However, the implementation of ZVS may not be reliably achievable if, for example, the relative timing of the voltage edges on the primary and secondary sides is unclear due to component tolerances or similar factors. To implement ZVS even under such potentially critical conditions, the concept of extending the dead time during a change of switching states in a half-bridge 31, 32, 41, 42, as described below, can be implemented.

[0073] The basic principle of the invention is explained in more detail below with reference to Figures 4 to 6. In these figures, a switching operation in the first bridge circuit 30 is shown in the upper area, and the corresponding switching operation of the second bridge circuit 40 is shown in the area below. R. 416179

[0074] - 13 -

[0075] In Figure 4, the initial switching times of a pulse in the first bridge circuit 30 are shown by the dashed lines in the upper section. For example, the upper switching element 32a of the second half-bridge would be closed at time t2 and opened at time t4. Since the switching states of the other switching elements are already described in the previous explanations, they are not repeated here. After the upper switching element 32a opens, the complementary switching element 32b closes at a later time after a predetermined dead time.

[0076] As shown in the lower part of Figure 4, the corresponding pulse in the second bridge circuit 40 ends before time t4 - and in particular also at the latest at time t3 as explained below.

[0077] According to the invention, in this case the pulse generated by the first bridge circuit 30 can be prematurely terminated by opening the upper switching element 32a at an earlier time t3. The time for closing the complementary switching element 32b can remain unchanged. This results in an extended dead time between the opening of the upper switching element 32a and the closing of the complementary switching element (here 32b). During this dead time, in which both switching elements 32a and 32b of a half-bridge 32 are open, an electric current can only flow through an internal or external diode arranged in parallel to the semiconductor switching element 32a.

[0078] Since the premature opening of switching element 32a to extend the dead time thus shortens the pulse duration of this pulse, this reduced pulse duration can be at least partially compensated for by switching on this switching element 32a earlier. For example, it is possible to shift the switch-on time t1 forward by approximately half when the dead time is extended, by which time t3 is shifted forward compared to the original time t4 for the dead time reduction. Thus, half of the voltage time area is added at the beginning of the pulse, which is lost at the end of the pulse due to the dead time extension. R. 416179

[0079] - 14 - The resulting pulse profile is shown in the upper part of Figure 4 by the solid line. The control for the corresponding pulse in the second bridge circuit 40 remains unchanged.

[0080] Figure 5 shows a schematic representation for adjusting the control of the switching elements in the bridge circuits 30 and 40 when the switch-off time of the pulse in the second bridge circuit 40 occurs after the switch-off time of the corresponding pulse in the first bridge circuit 30. In this case, the corresponding pulse in the first bridge circuit 30 remains unchanged, while the opening of an upper switching element, for example, switching element 42a, is advanced from time t9 to time t8 in order to implement an extended dead time in the corresponding half-bridge 42. Here, too, the pulse width can be adjusted accordingly by shifting the start of the corresponding pulse from time t7 to time t6, for example, by shifting the start of the pulse forward by half the amount by which the end of the pulse was shifted.

[0081] Finally, Figure 6 shows a corresponding timing diagram for the case where the last switching half-bridge cannot be unambiguously assigned to the first bridge circuit 30 or the second bridge circuit 40. This can occur, for example, if the end of a pulse in one bridge circuit falls within a time range in which the end of the pulse in the other bridge circuit is shifted to accommodate a dead-time extension. In such a case, the intended dead-time extension can, for example, be distributed between the two bridge circuits 30 and 40.

[0082] For extending the dead time in the respective half-bridge 32, 42, an extended dead time can be provided, which corresponds to at least 10%, preferably at least 15%, of a period T for periodic control. In particular, the extended dead time can be, for example, double or triple the initially provided dead time. Depending on the application, suitable adjustments for extending the dead time are of course also possible, deviating from this. R. 416179

[0083] - 15 -

[0084] Finally, Figure 7 shows a flowchart of how a method for operating a DC-DC converter 1 according to one embodiment can be based. In principle, the method can include any steps suitable for implementing a previously described DC-DC converter 1 in one of the embodiments. Similarly, the previously described DC-DC converter 1 can also include any components or units suitable for realizing the method described below.

[0085] The method can generally be applied to a DC-DC converter 1 according to the configuration described above. The switching elements 31a, 31b, 32a, 32b, 41a, 41b, 42a, 42b of the half-bridges 31, 32, 41, 42 in the first bridge circuit 30 and the second bridge circuit 40 are controlled according to a predefined pulse-width modulated control signal. During the control signal, either a first switching state or a second switching state can be set in the individual half-bridges 31, 32, 41, 42. In the first switching state, for example, the upper switching element 31a, 32a, 41a, or 42a can be closed, while the corresponding lower switching element 31b, 32b, 41b, 42b of the respective half-bridge 31, 32, 41, 42 is open.In an alternative second switching state, a complementary configuration is possible in which the upper switching elements 31a, 32a, 41a, 42a are open, while the corresponding lower switching elements 31b, 32b, 41b, 42b are closed. A dead time can be provided during the transition between the first and second switching states, during which both switching elements of the corresponding half-bridge 31, 32, 41, 42 are open.

[0086] In particular, the method provides for an extended dead time for at least one half-bridge during a change between the two switching states. Such an extended dead time is a dead time that is significantly longer than the dead time initially provided for the change between the two switching states. R. 416179

[0087] - 16 -

[0088] In summary, the present invention relates to a DC-DC converter, in particular a galvanically isolated DC-DC converter. The DC-DC converter comprises a transformer and two bridge circuits, each arranged between the corresponding DC voltage terminals and the transformer. To improve zero-voltage switching, it is proposed to provide an extended dead time in one of the half-bridges when driving the bridge circuits.

Claims

R. 416179 - 17 - Claims 1. DC-DC converter (1), comprising: a first DC terminal (11); a second DC terminal (12); a transformer (20) with a primary side (21) and a secondary side (22); a first bridge circuit (30) arranged between the first DC terminal (11) and the primary side (21) of the transformer (20), wherein the first bridge circuit (30) comprises two half-bridges (31, 32) each with a first switching element (31a, 32a) and a second switching element (31b, 32b), wherein the first switching element (31a, 32a) and the second switching element (31b, 32b) are arranged in series;a second bridge circuit (40) arranged between the second DC terminal (12) and the secondary side (22) of the transformer (20), the second bridge circuit (40) comprising two half-bridges (41, 42) each with a first switching element (41a, 42a) and a second switching element (41b, 42b), the first switching element (41a, 42a) and the second switching element (41b, 42b) being arranged in series; and a control device (50) designed to control the first and second switching elements (31a, 31b, 32a, 32b, 41a, 41b, 42a, 42b) in the half-bridges (31, 32, 41, 42) of the first bridge circuit (30) and the second bridge circuit (40); R. 416179 - 18 - wherein the control device (50) is designed to set, for pulse-width modulated control of the switching elements (31 a, 31 b, 32a, 32b, 41 a, 41 b, 42a, 42b) in the first bridge circuit (30) and the second bridge circuit (40) in the half-bridges (31 , 32, 41 , 42) in the first bridge circuit (30) and the second bridge circuit (40), either a first switching state in which the first switching element (31 a, 32a, 41 a, 42a) is closed and the second switching element (31 b, 32b, 41 b, 42b) is open, or a second switching state in which the first switching element (31 a, 32a, 41a, 42a) is open and the second switching element (31 b, 32b, 41 b, 42b) is closed, wherein the control device (50) is designed to provide a predetermined dead time during a change between the first switching state and the second switching state, during which both the first switching element (31 a, 32a, 41 a,42a) as well as the second switching element (31b, 32b, 41b, 42b) are open, wherein the control device (50) is designed to provide an extended dead time for one of the half-bridges (32, 42), wherein the extended dead time is longer than an initial dead time.

2. DC voltage converter (1) according to claim 1, wherein the extended dead time corresponds to a time span of at least 10% of a period of the pulse width modulated control with which the switching elements (31a, 31b, 32a, 32b, 41a, 41b, 42a, 42b) in the respective bridge circuit (30, 40) are controlled.

3. DC voltage drift (1) according to claim 1 or 2, wherein the initial dead time corresponds to a time span of approximately 5% of the period with which the switching elements (31a, 31b, 32a, 32b, 41a, 41b, 42a, 42b) of the bridge circuits (30, 40) are controlled.

4. DC-DC converter (1) according to one of claims 1 to 3, wherein the control device (50) is designed to provide the extended dead time for a half-bridge (32, 42) in the bridge circuit (30, 40), which at R. 416179 - 19 - a periodic control of the switching elements of both bridge circuits (30, 40) exhibits the last switching operation in one period.

5. DC-DC converter (1) according to one of claims 1 to 4, wherein the control device (50) is designed to provide an extended dead time for each half-bridge (32, 42) in the first bridge circuit (30) and the second bridge circuit (40) if, during periodic control of the switching elements (31a, 31b, 32a, 32b, 41a, 41b, 42a, 42b) of both bridge circuits (30, 40), the last switching operations in both bridge circuits (30, 40) differ by less than a predetermined time interval.

6. DC voltage converter (1) according to one of claims 1 to 5, wherein the control device (50) is designed to provide for an extension of the dead time during the pulse width modulated control by prematurely opening a corresponding switching element (31 a, 31 b, 32a, 32b, 41a, 41 b, 42a, 42b).

7. DC voltage converter (1) according to claim 6, wherein the control device (50) is designed to adjust a switch-on time for closing the corresponding switching element (31 a, 31 b, 32a, 32b, 41 a, 41 b, 42a, 42b), which is opened prematurely for the extended dead time.

8. DC voltage converter (1) according to claim 6 or 7, wherein, for adjusting the switch-on time, the switch-on time is advanced by half the value by which the switch-off time is advanced for extending the dead time.

9. Charging circuit for an electric vehicle, comprising: a DC-DC converter (1) according to any one of claims 1 to 8; wherein the first DC voltage connection (11) is designed to be connected to a DC voltage source, and R. 416179 - 20 - wherein the second DC connection (12) is designed to be connected to an electrical energy storage device of the electric vehicle.

10. Method for operating a DC-DC converter (1) comprising a first DC terminal (11), a second DC terminal (12), a transformer (20) having a primary side (21) and a secondary side (22), a first bridge circuit (30) arranged between the first DC terminal (11) and the primary side (21) of the transformer (20), wherein the first bridge circuit (30) comprises two half-bridges (31, 32) each having a first switching element (31a, 32a) and a second switching element (31b, 32b) arranged in series, and a second bridge circuit (40) arranged between the second DC terminal (12) and the secondary side (22) of the transformer (20), wherein the second bridge circuit (40) comprises two half-bridges (41, 42) each having a first switching element (41a, 42a) and a second switching element (41a, 42b). b, 42b) includes those arranged in series,with the steps: pulse width modulated control of the switching elements (31a, 31b, 32a, 32b, 41a, 41b, 42a, 42b) in the first bridge circuit (30) and the second bridge circuit (40), wherein either a first switching state (S1) is set in which the first switching element (31a, 32a, 41a, 42a) is closed and the second switching element (31b, 32b, 41b, 42b) is open, or a second switching state (S2) is set in which the first switching element (31a, 32a, 41a, 42a) is open and the second switching element (31b, 32b, 41b, 42b) is closed, and wherein a predetermined dead time is to be provided in each case when switching between the first switching state (S1) and the second switching state (S2), during which both the first switching element (31a, 32a, 41a, 42a) and the second switching element (31b, 32b, 41b, 42b) are open; Adjusting the dead time for one of the half-bridges (32, 42), wherein an extended dead time is provided for adjusting the dead time, i.e., the dead time is longer R. 416179 - 21 - is, as an initial dead time, which is provided for the change between the switching states of the other half-bridges.

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