DC voltage converter and method for operating the DC voltage converter

Abstract

The invention relates to a DC voltage converter for transferring power from a high voltage network to a low voltage network. For this purpose, a circuit configuration which can be operated alternatively as an active-clamp flyback converter or an active-clamp buck converter is used.

Description

[0001] title

[0002] DC-DC converter and method for operating the DC-DC converter

[0003] Description

[0004] Technical area

[0005] The present invention relates to a DC-DC converter, in particular a DC-DC converter for transmitting energy between a high-voltage network and a low-voltage network. Furthermore, the invention relates to a method for operating the DC-DC converter.

[0006] State of the art

[0007] Vehicles that are fully or at least partially electrically powered usually have a so-called traction battery, which provides electrical energy to drive the vehicle. Such traction batteries generally have an output voltage of several hundred volts, for example 400 volts. In addition, the vehicles have a so-called low-voltage network, to which additional consumers, such as lighting, auxiliary drives, on-board computers, or similar, are connected. The low-voltage network of a vehicle is usually fed by electrical energy from the high-voltage network. This requires converting the electrical voltage upstream of the high-voltage network to the voltage level of the low-voltage network. In addition, galvanic isolation between the high-voltage network and the low-voltage network is usually required.

[0008] The document DE10 2016 220 679 A1 discloses a

[0009] DC-DC converter and a method for controlling a DC-DC converter with a so-called phase-shift full-bridge DC-DC converter. In particular, this document proposes reducing the number of lossy switching operations by appropriately controlling the DC-DC converter.

[0010] Disclosure of the invention

[0011] The present invention discloses a DC-DC converter, in particular a DC-DC converter for energy transmission between a high-voltage network and a low-voltage network, having the features of independent patent claim 1. Further advantageous embodiments are the subject of the dependent patent claims. Furthermore, the invention relates to a method for operating the DC-DC converter.

[0012] Accordingly, it is provided:

[0013] A DC-DC converter for transmitting energy between a high-voltage network and a low-voltage network. The DC-DC converter comprises an input terminal, an output terminal, a transformer, a first switching element, a third switching element, a fourth switching element, a fifth switching element, and a capacitor. The input terminal is designed to be coupled to a first DC-voltage network, a first DC-voltage source, or a first DC-voltage sink. The output terminal is designed to be coupled to a second DC-voltage network, a second DC-voltage source, or a second DC-voltage sink. The transformer has a primary side and a secondary side. A series circuit comprising the first switching element and the third switching element is connected between a first terminal element of the input terminal and a second terminal element of the input terminal.A series circuit comprising the primary side, or the primary winding, of the transformer and the fourth switching element is connected in parallel with the third switching element; a series circuit comprising the fifth switching element and the capacitor is connected in parallel with the fourth switching element. Preferably, the first connection element of the input terminal is the positive connection pole, and the second connection element of the input terminal is the negative connection pole of the input terminal.

[0014] The present invention is based on the finding that a low-voltage network of an electric or hybrid vehicle, preferably a second DC voltage network, is generally supplied with energy from a high-voltage network, preferably a first DC voltage network. This typically requires galvanic isolation between the high-voltage network and the low-voltage network. Furthermore, the present invention is based on the finding that, with the progressive development of vehicles, the performance and, in particular, the battery capacity of electrically powered vehicles are increasing. In this context, traction batteries with a higher output voltage are also increasingly being used. Therefore, DC-DC converters with sufficient dielectric strength are also required for the coupling between the high-voltage network and the low-voltage network.DC-DC converters that can be used over the widest possible input voltage range are also desirable.

[0015] It is therefore an idea of ​​the present invention to take this finding into account and create a DC-DC converter that can meet the above-mentioned requirements. To this end, the following invention creates a circuit concept for a DC-DC converter for coupling a high-voltage network with a low-voltage network, which has a very simple circuit topology. The DC-DC converter can convert the input DC voltage into a predetermined output DC voltage over a very wide input voltage range. The circuit concept according to the invention makes it possible to ensure sufficient dielectric strength, especially at high input voltages or DC input voltages, even with conventional components. Due to the relatively simple circuit topology, the DC-DC converter can be implemented particularly cost-effectively.Furthermore, the DC-DC converter according to the invention also requires relatively little installation space. According to one embodiment, the DC-DC converter comprises a rectifier. The rectifier is coupled to the secondary side, or the secondary winding, of the transformer. Furthermore, the rectifier is designed to rectify a voltage present on the secondary side of the transformer. The rectified voltage can then be provided at the output terminal of the DC-DC converter. The electrical voltage on the secondary side of the transformer can be rectified in any desired manner using an active or passive rectifier. Using the transformer and subsequent rectification, it is thus possible to realize a galvanically isolated energy transfer from the high-voltage network to the low-voltage network.

[0016] According to one embodiment, the rectifier comprises a rectifier diode. Alternatively, the rectifier can also comprise a second switching element, preferably a semiconductor switch, in particular with a rectifier diode provided in parallel with the semiconductor switch. In this case, the second switching element can be actively controlled, wherein controlling the second switching element rectifies the electrical voltage on the secondary side of the transformer. Such active rectification by means of a second switching element can reduce power loss and thus increase efficiency. Alternatively, a particularly cost-effective rectification can be achieved using a rectifier diode.Preferably, a series circuit comprising a second switching element and the secondary side, or the secondary winding, of the transformer is connected between a first connection element and a second connection element of the output connection of the DC-DC converter. This is preferably a simple circuit topology of the secondary side of the DC-DC converter. Preferably, the first connection element of the output connection is the positive connection pole, and the second connection element of the output connection is the negative connection pole of the output connection. Preferably, the DC-DC converter is bidirectionally operable if the rectifier comprises a second switching element.

[0017] According to one embodiment, the first switching element, the second switching element, the third switching element, the fourth switching element, and / or the fifth switching element each comprise a semiconductor switch. In particular, a diode, in particular a so-called body diode, can be provided in parallel with each semiconductor switch. The semiconductor switch can be, for example, a transistor, in particular an insulated gate bipolar transistor (IGBT). Of course, any other semiconductor switches, such as MOSFETs, silicon carbide switches, or gallium nitride switches, are also possible.

[0018] According to one embodiment, the diode of the fifth switching element is arranged opposite the diode of the first switching element. The diode of the fifth switching element is preferably oriented such that a current flow from the capacitor toward the first terminal element of the input terminal is prevented when the fifth switching element is open. The diode of the fifth switching element is preferably oriented such that it prevents a discharge of the capacitor through the closed fourth switching element. Preferably, the fifth switching element is then switched off and its diode is blocked. In this way, a discharge of the capacitor can be prevented when the fifth switching element is open.

[0019] According to one embodiment, the DC-DC converter comprises a control device. The control device can be designed to control the first switching element, the second switching element, the third switching element, the fourth switching element, and / or the fifth switching element. Furthermore, if an active switching element, the second switching element, is provided in the rectifier of the DC-DC converter, the control device can also control this switching element of the DC-DC converter. In this way, the control of the individual switching elements can be specifically synchronized.

[0020] According to one embodiment, the control device is designed to open the third switching element and the fifth switching element in a first operating mode and to simultaneously control the first switching element and the fourth switching element in a clocked manner and / or to open the fourth switching element and close the fifth switching element in a second operating mode. Furthermore, in the second operating mode, the first switching element and the third switching element can each be controlled alternately in a clocked manner. In this way, in the first operating mode, a DC voltage conversion between the high-voltage side and the low-voltage side can take place on the basis of a so-called flyback converter, in particular an active-clamp flyback converter, and with the same circuit topology in a further operating mode, the circuit can be operated as an active-clamp buck converter.This allows the DC-DC converter to be controlled appropriately over a wide voltage range.

[0021] Preferably, in the first operating mode, after the first and fourth switching elements are switched off after a dead time has elapsed, the second, third, and / or fifth switching elements are switched on. This advantageously reduces the losses caused by the switching elements. Subsequently, the second, third, and / or fifth switching elements are switched off or opened again. After another dead time has elapsed, the cycle begins again with the first and fourth switching elements being switched on.

[0022] Preferably, in the second operating mode, the second switching element is switched off while the first switching element is switched on. Further preferably, after the first switching element is switched off and a dead time has elapsed, the second switching element is switched on while the third switching element remains switched on. This advantageously reduces losses due to the second switching element. After a further dead time has elapsed, the cycle begins again from the beginning with the first switching element being switched on.

[0023] According to one embodiment, an input voltage at the input terminal of the DC-DC converter for controlling the second operating mode is higher than the input voltage for controlling the first operating mode. The selection of a suitable operating mode for DC-DC conversion can be carried out, for example, by means of the control device for controlling the individual switching elements, preferably depending on the input voltage at the input terminal. Furthermore, the invention relates to a method for operating the DC-DC converter in a first or a second operating mode. The first operating mode comprises the method steps: opening the third switching element and the fifth switching element, and clocked simultaneous controlling of the first switching element and the fourth switching element.Preferably, clocked simultaneous control of the first switching element and the fourth switching element means that, depending on a clock or signal specified by a control system, the first switching element and the fourth switching element are switched on simultaneously and then switched off again, and are switched on and off again repeatedly depending on the clock or signal. The second operating mode comprises the following process steps:

[0024] Opening the fourth switching element and closing the fifth switching element as well as clocked alternating control of the first switching element and the third switching element. Preferably, clocked alternating control of the first switching element and the third switching element means that, depending on a clock or signal predetermined by a control system, the first switching element is switched on and the third switching element is switched off, and then the first switching element is switched off and the third switching element is switched on, and depending on the clock or signal, the first switching element and the third switching element are complementarily switched on and off again repeatedly. Dead times are preferably provided for the switching processes so that a short circuit between the connection elements of the input connection via the first and the third switching element is prevented.

[0025] The above embodiments and developments can be combined with each other as desired, where appropriate. Further embodiments, developments, and implementations of the invention also include combinations of features of the invention not explicitly mentioned above or described below with respect to the exemplary embodiments. 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. Brief Description of the Drawings

[0026] Further features and advantages of the invention are explained below with reference to the figures.

[0027] Figure 1: a block diagram of a DC-DC converter according to an embodiment;

[0028] Figure 2: a flowchart of a method for operating the DC-DC converter

[0029] Description of embodiments

[0030] Figure 1 shows a schematic representation of a block diagram of a DC-DC converter 1 according to one embodiment. The DC-DC converter 1 can be connected on the input side via the input terminal 10, for example, to a first DC voltage network 2, a first DC voltage source, a first DC voltage sink, or a high-voltage network. On the output side, the DC-DC converter 1 can be connected via the output terminal 13, for example, to a second DC voltage network 3, a second DC voltage sink, a second DC voltage source, or a low-voltage network. The DC-DC converter 1 can preferably have, for example, an input terminal 10 for connection to a high-voltage network. At this input terminal 10, electrical energy can be provided, for example, by a traction battery of an electric vehicle.An electrical voltage, an input voltage U in, is preferably applied to the input terminal 10. The DC-DC converter 1 can convert this input voltage U in into another electrical DC voltage and provide it as an output DC voltage U out at an output terminal 13. If not already connected to the input terminal 10 in another way, for example by a connected.

[0031] DC voltage source 2, a first intermediate circuit capacitor (not shown in Figure 1) is preferably provided between the potentials of a first connection element 11 and a second connection element 12 of the input connection 10.

[0032] The DC-DC converter 1 comprises, in addition to the input terminal 10 and the output terminal 13, a transformer T. The transformer T has a primary side Pri, or primary winding, and a secondary side Sek, or secondary winding. Furthermore, the DC-DC converter 1 has four switching elements S1, S3, S4 and S5 as well as a capacitor C between the input terminal 10 and the primary side Pri of the transformer T. A rectifier 40 is preferably provided on the secondary side of the DC-DC converter 1. The rectifier 40 preferably serves to rectify an AC voltage resulting at the secondary side Sek of the transformer. The rectifier 40 preferably comprises a switching element S2. The DC-DC converter 1 preferably has the switching element S2 between the output terminal 13 and the secondary side Sek of the transformer T.

[0033] The input terminal 10 of the DC-DC converter 1 comprises the first connection element 11 and the second connection element 12. An input DC voltage U Jn can be provided accordingly between the first connection element 11 and the second connection element 12. A series circuit comprising the first switching element S1 and the third switching element S3 is connected between the first connection element 11 of the input terminal 10 and the second connection element 12 of the input terminal 10. A series circuit comprising the primary side Pri of the transformer T and the fourth switching element S4 is connected in parallel with the third switching element S3. A series circuit comprising the fifth switching element S5 and the capacitor C, a clamping capacitor, is connected in parallel with the fourth switching element S4. The order of the fifth switching element S5 and the capacitor C within the series circuit parallel to the fourth switching element S4 is preferably irrelevant.

[0034] As already mentioned above, a rectifier 40 is provided between the secondary side Sek of the transformer T and the output terminal 13 of the DC-DC converter 1. The rectifier 40 can, for example, be a passive diode, which is provided between a terminal on the secondary side Sek of the transformer T and a connection element of the output terminal 13. Alternatively, active rectification can also be carried out by means of a second switching element S2, in particular a semiconductor switching element, which is arranged between a terminal on the secondary side Sek of the transformer T and a connection element of the output terminal 13.

[0035] A control device 50 may be provided to control the switching elements, in particular the first, third, fourth, and / or fifth switching elements S1, S3-S5, and optionally the second switching element S2 in the rectifier 40. The functional principle and the switching sequence for controlling the switching elements are explained in more detail below.

[0036] In a first operating mode, the DC-DC converter 1 can be operated in particular as a so-called active-clamp flyback converter. The DC-DC converter is preferably operated in the first operating mode when an input voltage of up to, for example, 500 V is present at the input terminal. At lower input voltages, the first operating mode is advantageously selected, in which all five switching elements are switched, preferably to reduce line losses. Furthermore, the first operating mode makes it possible to keep the output voltage constant even at very low input voltages; it is even possible to increase the output voltage. The first and fourth switching elements, S1 and S4, switch on and off simultaneously. If the first and fourth switching elements, S1 and S4, are switched on, the transformer T is magnetized. Energy is preferably stored in the air gap of the transformer core.The second, third and fifth switching elements, S2, S3 and S5, are switched off. In the next step, the first and fourth switching elements, S1 and S4, are switched off. During the following dead time, all switching elements are switched off. The primary current flows from the transformer through the diode of the fifth switching element S5 into the capacitor C and through the diode of the third switching element S3. During this dead time, the secondary current flows through the diode of the second switching element S2 towards the first terminal element 14 of the output terminal 13. Preferably, after this dead time, the second, third and / or fifth switching elements S2, S3 and S5 are switched on. Advantageously, the losses of the switching elements are minimized. Thus, the energy stored in the transformer is transferred to the secondary side. After that, if not already done, the first, second, third, fourth and fifth switching elements are switched off.After a further dead time, the cycle of the first operating mode begins again. By adjusting the frequency and / or pulse width of the clock or signal and thus the switching on and off of the first and fourth switching elements, the energy transfer, in particular the DC output voltage U out resulting at the output terminal 13, can be regulated. If not already connected to the output terminal by another means, for example by connected loads, a second intermediate circuit capacitor is preferably provided between the potentials of the first connection element 14 and the second connection element 15 of the output terminal 13 (not shown in Figure 1). This second intermediate circuit capacitor preferably smooths a resulting pulsating secondary current.

[0037] In a second operating mode, the DC-DC converter 1 operates as a galvanically isolated active-clamp buck converter. The DC-DC converter is preferably operated in the second operating mode when an input voltage of, for example, above 500 V up to, for example, 1000 V or 1400 V is present at the input terminal. The second operating mode is advantageously selected for higher input voltages, in which only two or three switching elements are switched, preferably to reduce switching losses. The second operating mode is suitable for higher input voltages, since the output voltage cannot always be kept constant in the second operating mode, particularly at low input voltages. In this operating mode, the first switching element S1 and the third switching element S3 switch on and off alternately at a frequency and / or pulse width to be determined and specified. The fourth switching element S4 is permanently off during the second operating mode.The fifth switching element S5 is permanently switched on during the second operating mode. As long as the first switching element S1 is switched on, the transformer T is magnetized in the same direction as during the flyback. The primary winding or primary side Pri of the transformer and the capacitor C are connected in series. The input voltage U Jn applied to the input terminal 10 is therefore divided between the primary winding Pri of the transformer and the capacitor C. The voltage load on the transformer T is thereby advantageously minimized and kept low. Advantageously, therefore, for the second operating mode, despite the significantly higher input voltage, no increased insulation requirements need to be taken into account for the transformer T compared to those in the first operating mode. In this step, the second and third switching elements, S2 and S3, are switched off. Subsequently, the first switching element S1 is switched off.During the subsequent dead time, the primary current flows from the transformer through the closed fifth switching element S5 into the capacitor C and through the diode of the third switching element S3. After this dead time, the third switching element S3 is switched on. Thus, the capacitor C is connected in parallel with the primary winding of the transformer, and these two components are discharged into the secondary side of the transformer. Preferably, the second switching element S2 is switched on with the third switching element S3, preferably simultaneously. After a time or cycle determined and specified by the control system, the third switching element, and if applicable the second switching element, are switched off. After the subsequent additional dead time, the cycle of the second operating mode starts again.By adjusting the frequency and / or pulse width of the clock or the signal and thus the alternating switching on and off of the first switching element S1 and the third switching element S3, the energy transfer, in particular the DC output voltage U out resulting at the output terminal 13, can be regulated.

[0038] In particular, since the rectifier 40 is designed as a simple, unidirectional rectifier, it is necessary for the configuration of the circuit arrangement of the DC-DC converter 1 that the magnetization of the transformer T takes place in the same direction in both the first operating mode and the second operating mode. This magnetization task is performed by the first and fourth switching elements, S1 and S4, in the first operating mode, and by the first switching element S1 in the second operating mode. In both cases, the transformer T is demagnetized when the energy is transferred to the secondary side Sek of the transformer T. This occurs when the first and fourth switching elements, S1 and S4, or the first switching element S1, are switched off and, accordingly, the fifth switching element S5 is switched on or its diode conducts.

[0039] Advantageously, the first, third, fourth, and fifth switching elements can be designed such that their respective blocking voltage corresponds to the maximum value of the applied input voltage. This advantage results from two different effects. Firstly, from the use of two different topologies for different voltage ranges: an active clamp flyback topology for a lower voltage range and an active clamp buck topology for an upper voltage range. Secondly, the advantage results from the special arrangement of the first, third, fourth, and fifth switching elements, the capacitor, and the primary side of the transformer on the primary side of the DC-DC converter.

[0040] In the second operating mode, in active clamp buck operation, when the first switching element S1 is switched off, the polarity of the voltage at the primary winding of the primary side Pri of the transformer T is reversed. This voltage is equal to or almost equal to the voltage across the capacitor C. However, the voltage across the capacitor C does not reverse its polarity. As a result, the primary current continues to flow through the diodes of the fifth and third switching elements S5 and S3. The voltage drop across the diode of the third switching element S3 is very low and can therefore be neglected. The voltage at the connection point K between the first switching element S1, the primary side Pri of the transformer T and the third switching element S3 is therefore almost 0V when the first switching element S1 is open. The maximum blocking voltage for the first switching element S1 is the input voltage Uin applied to the input terminal 10.The electrical mesh consisting of the primary winding of the primary side Pri of the transformer T, the capacitor C and the diodes of the third and fifth switching elements S3 and S5 has an external voltage of almost 0V, since the sum of the rotated primary voltage on the primary side Pri of the transformer T and the voltage of the capacitor C results in zero externally. When the first switching element S1 is switched on, the input voltage U Jn applied to the input terminal 10 is above the third switching element S3. The maximum blocking voltage for the third switching element S3 is therefore the input voltage U Jn applied to the input terminal 10. When the fourth switching element S4 is switched off, its maximum blocking voltage is limited or clamped to the maximum voltage of the capacitor C by the diode of the fifth switching element S5. When the fourth switching element S4 is switched on (as is the case in the first operating mode), the fifth switching element S5 is switched off.In this case, the maximum blocking voltage of switching element S5 also results in the maximum voltage of capacitor C. During operation, the voltage across capacitor C is significantly lower than the input voltage U Jn present at input terminal 10. In the second operating mode, the first switching element S1 is preferably the main switch. The third switching element S3 is preferably the clamping switch in the second operating mode.

[0041] In this application, the transformer is preferably always magnetized and demagnetized in the same direction in both operating modes.

[0042] In active-clamp flyback operation, or in the first operating mode, the maximum blocking voltage of the first and third switching elements S1 and S3 is also the input voltage U Jn applied to input terminal 10. Preferably, the first and fourth switching elements S1 and S4 are the main switches. Preferably, the third and fifth switching elements S3 and S5 are the clamping switches in the first operating mode.

[0043] In this application, the transformer is preferably always magnetized and demagnetized in the same direction in both operating modes.

[0044] For example, the circuit arrangement described can be used for a

[0045] DC-DC converter 1 can thus be used, for example, for traction batteries with a relatively low voltage level, for example, voltages up to 500 volts. For DC-DC conversion with higher input voltages, for example, above 500 volts up to 800 or possibly 1000 volts, the same DC-DC converter 1 can be operated in the second operating mode. This enables simple and cost-effective DC-DC conversion for DC input voltage over a wide voltage range with relatively little circuit complexity.

[0046] It is further advantageous that on the primary side of the DC-DC converter 1, two of the first, third, fourth, and fifth switching elements are always arranged in series between the first connection element 11 of the input terminal 10 and the second connection element 12 of the input terminal 10. If one of the switching elements S1, S3, S4, S5 is short-circuited or defective, the other switching element S1, S3, S4, S5 connected in series can block and advantageously prevent a short circuit of the input terminal 10. This advantageously results in increased operational reliability of the DC-DC converter 1.

[0047] The secondary side of the DC-DC converter preferably comprises a sixth switching element S6. The sixth switching element S6 is preferably connected between a connection element 14 or 15 and one end of the series connection comprising the secondary side Sec of the transformer T and the rectifier 40. Advantageously, a series connection comprising two switching elements S2, S6 also results on the secondary side of the transformer. As described with regard to the primary side, this results in increased operational reliability of the DC-DC converter 1, since if one of the switching elements on the secondary side is defective, a switching element can still prevent a short circuit of the output connection 13. If a sixth switching element S6 is provided, the second intermediate circuit capacitor (not shown in Figure 1) is preferably connected in parallel to the series connection comprising the secondary side Sec of the transformer T and the rectifier 40.

[0048] Advantageously, a change from the first operating mode to the second operating mode, or vice versa, is carried out by means of suitable regulation and control without interruption during operation. Figure 2 shows a flow diagram of a method 100 for operating the DC-DC converter. The method begins with step 105. The DC-DC converter 1 is preferably operated in a first operating mode or a second operating mode depending on an input voltage U in applied to the input terminal 10. The first operating mode comprises the method steps: opening 110 of the third switching element S3 and the fifth switching element S5 and clocked simultaneous control 120 of the first switching element S1 and the fourth switching element S4.The second operating mode comprises the following method steps: opening 130 of the fourth switching element S4 and closing the fifth switching element S5, as well as clocked, alternating activation 140 of the first switching element S1 and the third switching element S3. The method ends with step 145.

[0049] Preferably, in the first operating mode, after the first and fourth switching elements S1, S4 are switched off, the second, third, and / or fifth switching elements S2, S3, S5 are switched on after a dead time has elapsed. This advantageously reduces the losses through the switching elements. Subsequently, the second, third, and / or fifth switching elements S2, S3, S5 are opened or switched off again. After another dead time has elapsed, the cycle begins again from the beginning with the first and fourth switching elements S1, S4 being switched on.

[0050] Preferably, in the second operating mode, the second switching element S2 is switched off while the first switching element S1 is switched on. Further preferably, after the first switching element S1 is switched off and a dead time has elapsed, the second switching element S2 is switched on while the third switching element S3 is switched on. This advantageously reduces the losses caused by the second switching element S2. After a further dead time has elapsed, the cycle begins again from the beginning with the first switching element S1 being switched on.

[0051] In summary, the present invention relates to a

[0052] DC-DC converter for energy transfer from a high-voltage network to a

[0053] Low-voltage network. A simple circuit configuration is proposed for this purpose, which can be operated alternatively as an active-clamp flyback converter or an active-clamp buck converter.

Claims

Claims 1. A DC-DC converter (1) for transmitting energy between a high-voltage network (HV) and a low-voltage network (LV), comprising: an input terminal (10) designed to be coupled to a first DC-voltage network (2); an output terminal (13) designed to be coupled to a second DC-voltage network (3); a transformer (T) having a primary side (Pri) and a secondary side (Sek); a capacitor (C); a first switching element (S1), a third switching element (S3), a fourth switching element (S4), and a fifth switching element (S5); wherein a series circuit comprising the first switching element (S1) and the third switching element (S3) is connected between a first terminal element (11) of the input terminal (10) and a second terminal element (12) of the input terminal (10);wherein a series circuit comprising the primary side (Pri) of the transformer (T) and the fourth switching element (S4) is connected in parallel with the third switching element (S3); wherein a series circuit comprising the fifth switching element (S5) and the capacitor (C) is connected in parallel with the fourth switching element (S4); 2. DC voltage converter (1) according to claim 1, comprising a rectifier (40) coupled to the secondary side (Sec) of the transformer (T) and which is designed to rectify a voltage applied to the secondary terminal (Sec) of the transformer (T).

3. DC-DC converter (1) according to claim 2, wherein the rectifier (40) comprises a rectifier diode or a second switching element (S2) which is designed to rectify the voltage applied to the secondary terminal (Sek) of the transformer (T).

4. DC-DC converter (1) according to one of claims 1 to 3, wherein the first switching element (S1), the third switching element (S3), the fourth switching element (S4) and / or the fifth switching element (S5) each comprise a semiconductor switch with a body diode.

5. DC-DC converter (1) according to claim 4, wherein the body diode of the fifth switching element (S5) is arranged opposite to the body diode of the first switching element (S1).

6. DC voltage converter (1) according to one of claims 1 to 5, with a control device (50) which is designed to control the first switching element (S1), the second switching element (S2), the third switching element (S3), the fourth switching element (S4) and / or the fifth switching element (S5).

7. DC voltage converter (1) according to claim 6, wherein the control device (50) is designed in a first operating mode to open the third switching element (S3) and the fifth switching element (S5) and to control the first switching element (S1) and the fourth switching element (S4) simultaneously in a clocked manner, and or in a second operating mode to open the fourth switching element (S4), to close the fifth switching element (S5) and to control the first switching element (S1) and the third switching element (S3) alternately in a clocked manner.

8. DC voltage converter (1) according to wherein a value of an input voltage (UJn) at the input terminal (10) for driving in the second operating mode is higher than a value of the input voltage (U Jn) for driving in the first operating mode.

9. Method (100) for operating a DC-DC converter according to one of claims 1 to 8 in a first or in a second operating mode, wherein the first operating mode comprises the method steps: Opening (110) the third switching element (S3) and the fifth switching element (S5); Clocked simultaneous control (120) of the first switching element (S1) and the fourth switching element (S4); wherein the second operating mode comprises the method steps: Opening (130) the fourth switching element (S4) and closing the fifth switching element (S5); Clocked alternating control (140) of the first switching element (S1) and the third switching element (S3).

Images