DC voltage converter and method for operating a DC voltage converter

Abstract

The invention relates to a circuit arrangement and a method for operating a DC voltage converter, in particular a DC voltage converter with a phase-shifted full-bridge topology, wherein energy can be transferred from the primary side to the secondary side even if the voltage on the primary side is lower than the product of the voltage on the secondary side and the transformation ratio of the transformer in the DC voltage converter. In this way, for example, capacitors on the primary side of the DC voltage converter can be discharged to a safe low voltage level.

Description

Field of the Invention

[0001] The present invention relates to a DC voltage converter. Furthermore, the present invention relates to a method for controlling a DC voltage converter. Background Art

[0002] DC voltage converters are applied in many fields. For example, electrical energy can be transmitted between the high-voltage grid and the low-voltage grid of an electric vehicle by means of a DC voltage converter. For this purpose, the DC voltage applied to the input connector of the DC voltage converter can be converted into another DC voltage by means of the DC voltage converter, wherein the magnitude of the voltage at the input can be different from the magnitude of the voltage at the output of the DC voltage converter. In particular, for example, the so-called phase-shifted full-bridge topology is known. Such a DC voltage converter includes a transformer with a fixed turns ratio. Here, the output voltage can be variably adjusted by means of an adjustable phase shift. Here, the limit of the voltage ratio between the input voltage and the output voltage is particularly limited by the turns ratio of the transformer in the DC voltage converter.

[0003] Publication DE 102016200662 A1 describes a bidirectional DC voltage converter for transmitting electrical energy between the high-voltage grid and the low-voltage grid of an electric vehicle. By operating the DC voltage converter in the reverse direction, the intermediate circuit capacitor on the primary side of the DC voltage converter can be charged with the energy on the secondary side of the DC voltage converter. Summary of the Invention

[0004] The present invention discloses a DC voltage converter and a method for operating a DC voltage converter. Other embodiments can be derived from the preferred embodiments.

[0005] Accordingly, it is provided that:

[0006] A DC voltage converter having a first transformer, a first full-bridge circuit, a second full-bridge circuit and a second transformer. The first full-bridge circuit is electrically arranged between a first terminal of the DC voltage converter and the primary side of the first transformer. The second full-bridge circuit is electrically arranged between a second terminal of the DC voltage converter and the secondary side of the first transformer. The primary side of the second transformer is electrically arranged between the second full-bridge circuit and a first connecting element of the second terminal of the DC voltage converter. In addition, a series circuit composed of the secondary side of the transformer, a first switching element and a first diode is provided. This series circuit is electrically arranged between the first connecting element of the second terminal of the DC voltage converter and a second connecting element of the second terminal of the DC voltage converter. In particular, the first diode is arranged along the conducting direction between the first connecting element and the second connecting element of the second terminal of the DC voltage converter. In other words, the anode of the first diode points in the direction of the connecting element with positive polarity, and the cathode of the diode points in the direction of the connecting element with negative polarity of the second terminal. In addition, the first switching element is arranged between the first connecting element of the second terminal and the first terminal of the secondary side of the second transformer, and the first diode is arranged between the second connecting element of the second terminal and the second terminal of the secondary side of the second transformer, wherein the DC voltage converter is designed to: close the first switching element in a first switching interval for charging the secondary side of the second transformer and disconnect the switching elements of the first full-bridge circuit, and in a second switching interval, disconnect the first switching element and respectively close two switching elements in the diagonal path of the first full-bridge circuit.

[0007] In addition, there is provided:

[0008] A method for controlling a phase-shifted full-bridge DC voltage converter, the method having a step for charging the secondary side of a transformer, wherein the transformer is arranged on the output side as a series inductor on the DC voltage converter. In addition, the method includes a step for discharging the electrical energy stored in the secondary side of the transformer through the primary side of the transformer.

[0009] Advantages of the present invention

[0010] The present invention is based on the recognition that: in the voltage range, a DC voltage converter, especially a DC voltage converter having a phase-shifted full-bridge topology, generally only allows energy transfer within a limited voltage range. This voltage range is especially limited by the turns ratio between the primary side and the secondary side of the transformer for a phase-shifted full-bridge DC voltage converter. Here, for a turns ratio of N between the primary side and the secondary side, the voltage on the primary side must be greater than the product of the turns ratio and the voltage on the secondary side.

[0011] Therefore, the concept of the present invention is to provide an extended topology for the phase-shifted full-bridge DC voltage converter in view of this recognition, and this extended topology also allows energy transfer from the primary side to the secondary side beyond the above-mentioned limitations. If the input voltage on the primary side is less than the above limit composed of the product of the turns ratio and the secondary voltage, the topology according to the present invention especially also allows energy transfer from the primary side to the secondary side.

[0012] For this purpose, a transformer is provided instead of a simple series inductor on the secondary side. Here, similar to the traditional series inductor, the primary side of the additional transformer can be connected between the full bridge on the secondary side and the output terminal of the DC voltage converter. The secondary side of the additional transformer, together with the switching element and the diode arranged along the conduction direction, additionally forms a series circuit between the positive terminal and the negative terminal of the secondary side of the DC voltage converter. By closing the switching element, the secondary side of the additional transformer can be charged. By disconnecting the switching element, energy can be transferred from the secondary side of the additional transformer to the primary side, and even when the voltage on the primary side drops below the above limit, energy transfer from the primary side to the secondary side is allowed here through the DC voltage converter.

[0013] In this way, for example, it is possible to discharge the capacitor on the primary side, such as the intermediate circuit capacitor, to a safe voltage level. Thereby, an additional discharge circuit for such an intermediate circuit capacitor can be eliminated.

[0014] Thereby, by discharging the intermediate circuit capacitor according to the topology of the present invention, overhead can be saved and thus the cost for the additional charging voltage can also be saved. In addition, it is also possible to transfer at least part of the electrical energy stored in the intermediate circuit capacitor to the secondary side of the DC voltage converter. The energy from the intermediate circuit capacitor is not completely lost, but can be utilized at least partially on the secondary side of the DC voltage converter. Thereby, the efficiency of the entire system is also improved.

[0015] According to an embodiment, a second diode is arranged in parallel with the first switching element. The second diode is especially arranged in anti-parallel with the first diode. In other words, the cathode of the second diode points in the direction of the positive connection element of the second terminal of the DC voltage converter. The second diode can be, for example, the so-called body diode of the first switching element.

[0016] According to another embodiment, a second switching element arrangement is arranged in parallel with the second diode. In this way, the function of the DC voltage converter can also be additionally improved. For example, by appropriate control of the second switching element, an energy flow in the reverse direction, that is, an energy flow from the secondary side to the primary side of the DC voltage converter, can also be achieved.

[0017] According to one embodiment, the first terminal of the DC voltage converter is designed for electrical coupling with a first voltage source, and the second terminal of the DC voltage converter is designed for electrical coupling with a second voltage source. In particular, the voltage of the first voltage source, that is, the voltage at the first input terminal, can be greater than the voltage of the second voltage source at the second terminal of the DC voltage converter. For example, the first terminal of the DC voltage converter can be coupled to a high-voltage grid, such as the high-voltage grid of an electric vehicle. In this case, the second terminal of the DC voltage converter can be coupled to a low-voltage grid, especially the low-voltage grid of an electric vehicle, for example.

[0018] According to one embodiment, the DC voltage converter is designed to transfer electrical energy from the first terminal of the DC voltage converter in the direction of the second terminal of the DC voltage converter. In particular, the DC voltage converter can be designed to: also transfer electrical energy from the first terminal to the second terminal when the voltage at the first terminal of the DC voltage converter is less than the product of the voltage transformation ratio of the first transformer and the voltage at the second terminal of the DC voltage converter.

[0019] According to one embodiment, the first terminal of the DC voltage converter is designed for coupling with an intermediate circuit capacitor. Here, the DC voltage converter can be designed to discharge the intermediate circuit capacitor. In particular, the DC voltage converter can be designed to discharge the intermediate circuit capacitor below a predetermined voltage threshold. In this way, the intermediate circuit capacitor can be discharged to a safe and non-hazardous voltage level. Here, an additional discharge circuit can be dispensed with. In particular, the intermediate circuit capacitor is discharged to a voltage that is less than the product of the voltage transformation ratio of the first transformer and the voltage at the second terminal of the DC voltage converter.

[0020] As a supplementary solution or alternative, the DC voltage converter can be designed to: even if the voltage on the primary side is less than a predetermined minimum voltage, supply power to a load on the secondary side of the DC voltage converter (1) through the energy on the primary side. Here, the predetermined minimum voltage is preset by the voltage transformation ratio of the transformer. In particular, the minimum voltage can be generated by the product of the voltage on the secondary side and the voltage transformation ratio of the transformer.

[0021] According to one embodiment, the second full-bridge circuit includes two half-bridges each having two switching elements. In addition, the first full-bridge circuit can also include two half-bridges each having two switching elements. The DC voltage converter can be designed here to control the switching elements of the first full-bridge circuit, the switching elements of the second full-bridge circuit, and the switching elements of the first element and, if necessary, also the second switching element. In particular, the control of the switching elements can be carried out, for example, by means of a suitable control device.

[0022] According to one embodiment, the DC voltage converter, in particular the control device of the DC voltage converter, can be designed to close the first switching element and open the switching elements of the first full-bridge circuit in a first switching interval. In addition, in a second switching interval, the first switching element can be opened and two switching elements in the diagonal path of the first full-bridge circuit can be closed respectively. In this way, during the first switching interval, the secondary side of the second transformer can be charged. Subsequently, in the second switching interval, electrical energy can be transferred from the secondary side of the second transformer to the primary side. In this way, even if the voltage on the primary side of the DC voltage converter drops below the product of the turns ratio and the voltage on the secondary side, energy can be transferred from the primary side of the DC voltage converter to the secondary side of the DC voltage converter.

[0023] According to one embodiment, the DC voltage converter, in particular the control device of the DC voltage converter, can be designed to alternately open and close the switching elements of different diagonal paths in the first full-bridge circuit in two successive second switching intervals. In this way, symmetric control of the DC voltage converter and the switching elements in the DC voltage converter can be achieved.

[0024] As long as it makes sense, the above design solutions and improvement solutions can be combined with each other arbitrarily. Other design solutions, improvement solutions, and implementation solutions of the present invention also include combinations of features not explicitly mentioned in the present invention described above or below with respect to the embodiments. In particular, those skilled in the art will also add individual aspects as improvement solutions or supplementary solutions to the corresponding basic forms of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Other features and advantages of the present invention will be explained below with the aid of the drawings. Here:

[0026] Figure 1 A schematic diagram showing a principle circuit diagram of a DC voltage converter according to one embodiment is shown;

[0027] Figure 2Schematic diagram of a time chart showing the switching states and voltage and current profiles in a DC voltage converter according to one embodiment; and

[0028] Figure 3 Schematic diagram of a flowchart, as the method for operating a DC voltage converter according to one embodiment is based on this flowchart. Detailed Description

[0029] Figure 1 Schematic diagram of a principle circuit diagram, as the DC voltage converter 1 according to one embodiment is based on this principle circuit diagram. The DC voltage converter 1 includes, for example, a first terminal A1 having a first connection element A1-1 and a second connection element A1-2. On this first terminal A1, the DC voltage converter 1 can be electrically coupled to a voltage source, such as the high-voltage grid of an electric vehicle, for example. The first terminal A1 can be connected to or separated from the voltage source by means of a suitable disconnect switch, for example. In addition, a capacitor, such as an intermediate circuit capacitor CZ, can be arranged between the two connection elements A1-1 and A1-2, for example.

[0030] Furthermore, the DC voltage converter 1 includes a second terminal A2, which also has a first connection element A2-1 and a second connection element A2-2. On this second terminal A2, the DC voltage converter 1 can be electrically coupled to another voltage source, such as the low-voltage grid of an electric vehicle or a similar voltage source. A capacitor C can also be provided between the first connection element A2-1 and the second connection element A2-2.

[0031] In addition, the DC voltage converter 1 includes a first full-bridge circuit 10. The first full-bridge circuit 10 includes two half-bridges. Here, the first half-bridge includes a series circuit composed of two switching elements 11 and 12, and the second half-bridge includes a series circuit composed of two switching elements 13 and 14. Furthermore, the DC voltage converter 1 includes a second full-bridge circuit 20. The second full-bridge circuit 20 can also include two half-bridges each having two switching elements 21-24. A first transformer T1 is provided between the first full-bridge circuit 10 and the second full-bridge circuit 20. The primary side of the first transformer T1 is connected to the first node of the first half-bridge of the first full-bridge circuit 10 at one terminal. The second terminal of the primary side of the first transformer T1 is connected to the other node of the second half-bridge of the first full-bridge circuit 10. Similarly, the first terminal of the secondary side of the first transformer T1 is connected to the node of the first half-bridge of the second full-bridge circuit 20, and the second terminal of the secondary side of the first transformer T1 is connected to the other node of the second half-bridge of the second full-bridge circuit 20.

[0032] The turns ratio between the primary and secondary sides of the first transformer T1 is, for example, N here. Accordingly, without additional cost, electrical energy can be transferred from the first connection A1 to the second connection A2 by means of the circuit arrangement described thus far, provided that the voltage U_prim at the first connection A1 at least corresponds to the product of the transformation ratio N and the voltage U1_sek at the second output connection A2.

[0033] Furthermore, the second full-bridge circuit 20 is coupled to the second connection A2 of the DC voltage converter 1. Here, a connecting element, for example the second connecting element A2-2, is directly connected to the second full-bridge circuit 20. A series inductor is provided between another connecting element, for example the first connecting element A2-1 of the second connection A2, and the second full-bridge circuit 20. This series inductor can be realized, for example, by means of a second transformer T2. Here, the primary side of the second transformer T2 is arranged between the second full-bridge circuit 20 and the first connecting element A2-1 of the second connection A2. In addition, the secondary side of the second transformer T2, together with the first switching element S1 and the first diode D1, forms a series circuit that is arranged between the first connecting element A2-1 of the second connection A2 and the second connecting element A2-2. Here, the first diode D1 is arranged in the conducting direction, that is to say, the cathode of the first diode D1 points in the direction of the negative polarity, and the anode of the first diode D1 points in the direction of the positive polarity. In particular, the first switching element S1 is arranged between the first switching connecting element A2-1 of the second connection and the first connection of the secondary side of the second transformer T2, and the first diode D1 is arranged between the second connecting element A2-2 of the second connection A2 and the second connection of the secondary side of the second transformer T2.

[0034] Furthermore, a second diode D2 can be arranged in parallel with the first switching element S1. This second diode D2 can be arranged antiparallel to the first diode D1, that is to say, the second diode D2 is arranged in the blocking direction such that the cathode of the second diode D2 points in the direction of the positive polarity at the second connection A2. In addition, a second switching element S2 can be arranged in parallel with the first diode D1.

[0035] The switching elements 21-24 of the second full bridge 20, the first switching element S1, and, if necessary, the switching elements 11-14 of the first full bridge 10 and / or the second switching element S2 can be controlled, for example, by means of a suitable control device 30. The principle and switching sequence for controlling the individual switching elements will be explained in more detail below. If a voltage U_prim smaller than the product of the transformation ratio N of the transformer and the voltage U1_sek at the second terminal A2 of the DC voltage converter 1 is applied to the first terminal A1 on the primary side, energy transfer can also be carried out, in particular, by specifically controlling the first switching element S1 by means of the circuit arrangement described above.

[0036] Figure 2 A schematic diagram showing the switching pattern and voltage / current curves of the control method of the DC voltage converter 1 according to an embodiment is shown. With the control method described below, it is possible, in particular, to transfer electrical energy from the primary side of the DC voltage converter 1 to the secondary side of the DC voltage converter 1 even if the voltage on the primary side is smaller than the product of the transformation ratio N of the transformer and the voltage on the secondary side of the DC voltage converter 1.

[0037] In a first time interval between times t0 and t1, the switching elements 21-24 of the second full bridge 20 and, if necessary, the switching elements 11-14 of the first full bridge 10 are also switched off. In addition, the first switching element S1 is closed. As a result, a current I2_sek with a first rising current intensity flows through the first switching element S1, the secondary side of the second transformer T2, and the first diode D1.

[0038] At time t1, the first switching element S1 is switched off. In addition, in the first full bridge circuit 10, the switching elements 11 and 14 of the diagonal branch are closed. Here, energy is transferred from the secondary side of the second transformer T2 to the primary side of the second transformer T2 and a current flows through the secondary side of the first transformer T1. The voltage U1_sek applied to the secondary side of the first transformer causes energy transfer from the primary side to the secondary side of the DC voltage converter 1. If necessary, the corresponding switching elements 21 and 24 of the second full bridge circuit 20 can be actively controlled for this purpose. As an alternative, the current can also flow through the parallel body diodes.

[0039] At time t2, the switching elements 11 - 14 of the first full - bridge circuit 10 and, if necessary, the switching elements 21 - 24 of the second full - bridge circuit 20 are disconnected. In addition, the first switching element S1 is closed and a new current flows through the secondary side of the second transformer T2. At time t3, the first switching element S1 is disconnected again. In addition, at time t3, the switching elements 12 and 13 in the second diagonal branch of the first full - bridge circuit 10 are closed. Thereby, the secondary side of the second transformer T2 is discharged again through the primary side of the second transformer T2, and now a voltage U1_sek with opposite polarity is applied to the secondary side of the first transformer T1. Here, energy transfer from the primary side to the secondary side also takes place during this time interval.

[0040] Figure 3 A schematic diagram of a flow chart is shown, such as the flow chart on which the method 100 for energy transfer from the primary side to the secondary side of a DC voltage converter according to an embodiment is based. In a first step 110, the secondary side of the second transformer T2 is charged by closing the first switching element S1. Thereafter, in step 120, the secondary side of the second transformer T2 is discharged by disconnecting the first switching element and closing the switching elements in the diagonal branches of the first full - bridge circuit 10. In step 130, the secondary side of the second transformer T2 is charged again by closing the first switching element S1. Finally, in step 140, the secondary side of the second transformer T2 is discharged again, where the first switching element S1 is disconnected again. Here, the switching elements in the second diagonal branch of the first full - bridge circuit 10 are closed. Here, during steps 120 and 140, energy transfer from the primary side to the secondary side of the DC voltage converter 1 can take place. Energy transfer can take place especially if the voltage at the first terminal A1 on the primary side is less than the product of the transformation ratio and the voltage at the second terminal on the secondary side of the DC voltage converter 1.

[0041] In summary, the invention relates to a circuit arrangement and a method for controlling a DC voltage converter, in particular a DC voltage converter having a phase - shifted full - bridge topology, wherein energy transfer from the primary side to the secondary side can also take place if the voltage on the primary side is lower than the product of the voltage on the secondary side and the transformation ratio of the transformer in the DC voltage converter. In this way, for example, the capacitor on the primary side of the DC voltage converter can be discharged to a safe, low voltage level.

Claims

1. A DC voltage converter (1), comprising: a first transformer (T1), a first full-bridge circuit (10) electrically arranged between a first terminal (A1) of the DC voltage converter (1) and a primary side of the first transformer (T1); a second full-bridge circuit (20) electrically arranged between a second terminal (A2) of the DC voltage converter (1) and a secondary side of the first transformer (T1), characterized in that the DC voltage converter (1) has a second transformer (T2), wherein a primary side of the second transformer (T2) is electrically arranged between the second full-bridge circuit (20) and a first connection element (A2-1) of the second terminal (A2) of the DC voltage converter (1), and wherein a series circuit composed of a secondary side of the second transformer (T2), a first switching element (S1) and a first diode (D1) is arranged between the first connection element (A2-1) and a second connection element (A2-2) of the second terminal (A2) of the DC voltage converter (1), and wherein the first diode (D1) is arranged along a conduction direction between the first connection element (A2-1) and the second connection element (A2-2) of the second terminal (A2) of the DC voltage converter (1), wherein the first switching element (S1) is arranged between the first connection element (A2-1) of the second terminal and a first terminal of the secondary side of the second transformer (T2), and the first diode (D1) is arranged between the second connection element (A2-2) of the second terminal (A2) and a second terminal of the secondary side of the second transformer (T2), wherein the DC voltage converter (1) is designed to: close the first switching element (S1) in a first switching interval for charging the secondary side of the second transformer (T2) (110) and disconnect the switching elements (11-14) of the first full-bridge circuit, and disconnect the first switching element (S1) and close two switching elements in diagonal paths of the first full-bridge circuit (10) respectively in a second switching interval.

2. The DC voltage converter (1) according to claim 1, wherein a second diode (D2) is arranged in parallel with the first switching element (S1), and wherein the second diode (D2) is arranged in anti-parallel with the first diode (D1).

3. The DC voltage converter (1) according to claim 1 or 2, wherein a second switching element (S2) is arranged in parallel with the first diode (D1).

4. The DC voltage converter (1) according to claim 1 or 2, wherein the first terminal (A1) of the DC voltage converter (1) is designed to be electrically coupled to a first voltage source, the second terminal (A2) of the DC voltage converter (1) is designed to be electrically coupled to a second voltage source, and wherein the voltage of the first voltage source is greater than the voltage of the second voltage source.

5. The DC voltage converter (1) according to claim 1 or 2, wherein the DC voltage converter (1) is designed to: transfer electrical energy from a first connection (A1) of the DC voltage converter (1) in the direction of a second connection (A2) of the DC voltage converter (1).

6. The DC voltage converter (1) according to claim 1 or 2, wherein the first connection (A1) of the DC voltage converter (1) is designed to be coupled to an intermediate circuit capacitor (CZ), and wherein the DC voltage converter (1) is designed to: discharge the intermediate circuit capacitor (CZ) below a predetermined voltage and / or supply energy to a load on the secondary side of a first transformer (T1) of the DC voltage converter (1) by means of the energy on the primary side of the first transformer (T1), even if the voltage on the primary side of the first transformer (T1) is less than a predetermined minimum voltage, wherein the predetermined minimum voltage is pre-given by the turns ratio of the first transformer (T1).

7. The DC voltage converter (1) according to claim 1 or 2, wherein the first full-bridge circuit (10) comprises two half-bridges, each of the half-bridges having two switching elements (11 - 14), and wherein the second full-bridge circuit (20) comprises two half-bridges, each of the half-bridges having two switching elements (21 - 24); and wherein the DC voltage converter (1) furthermore comprises a control device (30) which is designed to control the switching elements (11 - 14) of the first full-bridge circuit (10), the switching elements (21 - 24) of the second full-bridge circuit (20) and the first switching element (S1).

8. The DC voltage converter (1) according to claim 7, wherein the control device (30) is designed to: alternately open and close the switching elements (11 - 14) of different diagonal paths in the first full-bridge circuit (10) in turn in two successive second switching intervals.

9. A method (100) for controlling a DC voltage converter according to any one of claims 1 to 8 for a phase-shifted full-bridge, having the following steps: charging the secondary side of a second transformer (T2), the second transformer (T2) being arranged on the output side as a series inductor on the DC voltage converter; and discharging the electrical energy stored in the secondary side of the second transformer (T2) via the primary side of the second transformer (T2).

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