Control method for direct current voltage converter and direct current voltage converter

Abstract

The invention relates to the control of a DC voltage converter (1), such as, for example, a phase-shifted full-bridge DC voltage converter, wherein it is possible to switch between two predetermined switching sequences. In this way, a uniform load of the switching elements (M1-M4) of the full bridge (10) on the primary side of the DC voltage converter (1) can be achieved. Thus, for example, for the first switching sequence (S1), the switching elements (M1, M3) of the first half-bridge of the full bridge (10) on the primary side can be hard-switched, while for the second switching sequence (S2), the switching elements (M2, M4) of the second half-bridge of the full bridge (10) on the primary side can be hard-switched.

Description

Field of the Invention

[0001] The present invention relates to a method for controlling a DC voltage converter. Furthermore, the present invention relates to a DC voltage converter, in particular a phase-shifted full-bridge DC voltage converter. Background Art

[0002] DC voltage converters are known for converting a first DC voltage into a second DC voltage having a different voltage level. The group of DC voltage converters particularly also includes so-called phase-shifted full-bridge DC voltage converters. As long as active switching elements are also used on the secondary side, these DC voltage converters are also able to achieve power transfer from the secondary side to the primary side under certain framework conditions. For example, a DC voltage converter can be used to couple the high-voltage power grid of an electric vehicle to the low-voltage power grid of the vehicle and to transfer electrical energy between these two on-vehicle power grids of such an electric vehicle.

[0003] Publication DE 10 2016 200 662 A1 discloses a bidirectional DC voltage converter for energy transfer between the high-voltage power grid and the low-voltage power grid of an electric vehicle. The converter includes at least one transformer for electrically separating the two on-vehicle power grids, an electronic switch for reversing the windings of the transformer, and a control device for controlling the switch. The DC voltage converter should in particular be able to charge the intermediate circuit capacitor on the high-voltage side. Summary of the Invention

[0004] The present invention discloses a method for controlling a DC voltage converter and a DC voltage converter having the features of the independent claims. Other embodiments are the subject matter of the dependent claims.

[0005] Accordingly, it is provided that:

[0006] A method for controlling a DC voltage converter. The DC voltage converter includes a full bridge on the primary side and a transformer. The transformer is arranged between the full bridge on the primary side of the DC voltage converter and the secondary side. The method for controlling the DC voltage converter alternately controls a first diagonal branch and a second diagonal branch of the full bridge on the primary side. Here, the first diagonal branch is complementary to the second diagonal branch of the full bridge on the primary side. Here, the switching elements of the full bridge can be alternately controlled according to a first switching sequence and a second switching sequence. In the first switching sequence, between the control of the first diagonal branch of the full bridge on the primary side and the control of the second diagonal branch of the full bridge on the primary side, the primary side of the transformer is short-circuited by means of the upper switching element of the full bridge on the primary side. In the second switching sequence, between the control of the first diagonal branch of the full bridge on the primary side and the control of the second diagonal branch of the full bridge on the primary side, the primary side of the transformer is short-circuited by means of the lower switching element of the full bridge on the primary side. In addition, the primary side of the transformer can also be short-circuited by means of the upper switching element or the lower switching element between the control of the second diagonal branch and the control of the first diagonal branch.

[0007] Furthermore, it is provided that:

[0008] A DC voltage converter, in particular a phase-shifted full-bridge DC voltage converter, having a full bridge on the primary side and a transformer. The transformer can be arranged between the full bridge on the primary side of the DC voltage converter and the secondary side. The full bridge on the primary side can be arranged between the DC voltage connection on the primary side of the DC voltage converter and the primary side of the transformer. In addition, the DC voltage converter includes a control device. The control device can be designed to control the switching elements of the full bridge on the primary side according to the method according to the invention.

[0009] The present invention is based on the following recognition: In the conventional control of a DC voltage converter, such as a phase-shifted full-bridge DC voltage converter for example, the switching elements in the primary-side full bridge are controlled according to a fixedly pre-given switching sequence. Here, the switching elements of one half-bridge of the full bridge usually have to be hard-switched, that is to say, a significant voltage drop is applied across the switching elements during the turn-on process. In contrast, the switching elements of the other half-bridge can, if necessary, be soft-switched, that is to say, no significant voltage drop is applied across the switching elements during the turn-on process. Since usually higher losses occur during the hard-switching process, the switching elements that have to be hard-switched are subjected to a stronger load, and thus these switching elements heat up more severely than the switching elements that are only soft-switched. This leads on the one hand to an uneven load on the switching elements involved. In addition, the maximum power limit of the DC voltage converter is restricted due to the components that are subjected to the strongest load. Therefore, in the method described above, power limitation may occur based on the losses on the hard-switched switching elements.

[0010] Therefore, the concept of the present invention is to take this recognition into account and provide a DC voltage converter and a method for operating a DC voltage converter, which can achieve a more uniform load on the components, in particular the switching elements in the primary-side full bridge. For this purpose, it is provided that the switching sequence for controlling the switching elements in the primary-side full bridge is modified. By means of the modified switching sequence, it is in particular possible to not always operate the same switching elements in a hard-switched or soft-switched manner. By switching between the hard-switching process and the soft-switching process for each individual switching element, the load and the loss power generated thereby can be evenly distributed to all the switching elements involved in this way. In this way, a uniform load and heating of all the switching elements in the primary-side full bridge of the DC voltage converter are achieved.

[0011] By evenly distributing the loss power to each individual switching element in this way, it is possible, if necessary, to also use switching elements with higher turn-on losses. In addition, the design of the cooling is also less critical.

[0012] According to one embodiment, in the first switching sequence, between the control of the second diagonal branch in the full bridge on the primary side and the control of the first diagonal branch in the full bridge on the primary side, the electrical connection of the connecting element on the primary side of the transformer is carried out by means of the lower switching element of the full bridge on the primary side. Similarly, in the second switching sequence, between the control of the second diagonal branch in the full bridge on the primary side and the control of the first diagonal branch in the full bridge on the primary side, the electrical connection of the connecting element on the primary side of the transformer is carried out by means of the upper switching element of the full bridge on the primary side. In this way, the electrical connection of the connecting element on the primary side of the transformer is carried out respectively between each control of the diagonal branches in the full bridge on the primary side. In other words, between two control processes of the diagonal branches in the full bridge on the primary side, the short circuit of the primary side of the transformer is carried out respectively by the upper or lower switching element of the full bridge on the primary side.

[0013] According to one embodiment, the conversion between the first switching sequence and the second switching sequence is carried out according to a predetermined number of switching cycles. For example, the control of the diagonal branches can be regarded as a switching cycle accordingly. As an alternative, the control of the first diagonal branch first and then the control of the second diagonal branch can also be regarded as a switching cycle accordingly. As an alternative, the conversion between the first switching sequence and the second switching sequence can also be carried out according to a predetermined time interval or according to any other criterion. For example, the temperature on one or more switching elements of the first full bridge can also be monitored, and the conversion between the first switching sequence and the second switching sequence is carried out when a predetermined limit temperature is reached. Of course, the replacement from the second switching sequence back to the first switching sequence can also be carried out according to the same or corresponding criterion.

[0014] According to one embodiment, in the method for controlling the DC voltage converter, the DC voltage converter is controlled in a discontinuous conduction mode (English "Discontinuous Conduction Mode (DCM)"). In this operating mode, electrical energy is fed into the transformer through the diagonal branches of the DC voltage converter on the primary side for a predetermined duration, so that an increasing current appears in the transformer. Subsequently, the primary side of the transformer is short-circuited and the current in the transformer drops to 0 amperes. In a further process, this process is repeated by controlling the complementary diagonal branches in the full bridge on the primary side with the opposite sign.

[0015] The full bridge on the primary side can include a first half bridge and a second half bridge. Herein, the first half bridge can include a first upper switching element and a first lower switching element. The second half bridge can similarly include a second upper switching element and a second lower switching element. The first upper switching element can be arranged between a first connection element of the DC voltage connection on the primary side and a first node. The first lower switching element can be arranged between the first node and a second connection of the DC voltage connection on the primary side. The second upper switching element can similarly be arranged between the first connection element of the DC voltage connection on the primary side and a second node. The second lower switching element can be arranged between the second node and a second connection element of the DC voltage connection on the primary side. Additionally, the first node can be connected to a first connection element of the primary side of the transformer, and the second node can be connected to a second connection element of the primary side of the transformer. In this way, the first upper switching element and the second lower switching element form a first diagonal branch. The second upper switching element and the first lower switching element similarly form a second diagonal branch that is complementary to the first diagonal branch.

[0016] As long as it makes sense, the above design solutions and improvement solutions can be arbitrarily combined with each other. Other design solutions, improvement solutions, and implementation solutions of the present invention also include combinations of features not explicitly mentioned before or below regarding the embodiments of the present invention. Herein, those skilled in the art will particularly add individual aspects as improvements or supplements to the corresponding basic forms of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The following explains other features and advantages of the present invention with the aid of the drawings. Among them:

[0018] Figure 1 A schematic diagram of a principle circuit diagram as the basis of a DC voltage converter according to an embodiment is shown;

[0019] Figures 2 - 5 Principle circuit diagrams respectively showing different switching states during the operation of a DC voltage converter according to an embodiment are shown; and

[0020] Figure 6 A schematic diagram of a flowchart as the basis of a method for controlling a DC voltage converter according to an embodiment is shown. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] Figure 1The figure shows a schematic diagram of a principle circuit diagram as the basis of a DC voltage converter 1 according to an embodiment. The DC voltage converter 1 can be, for example, a phase-shifted full-bridge DC voltage converter. The DC voltage converter includes a full bridge 10 on the primary side and a full bridge 20 on the secondary side. A transformer 30 is arranged between the full bridge 10 on the primary side and the full bridge 20 on the secondary side. In principle, it is also feasible to provide a passive rectifier, for example, by means of four diodes instead of the full bridge 20 on the secondary side. The switching elements M1 to M4 of the full bridge 10 on the primary side and, if necessary, the switching elements M5 to M8 of the full bridge 20 on the secondary side can be controlled, for example, by a control device 50.

[0022] The full bridge 10 includes two half bridges each having two switching elements. In the first half bridge, the first switching element M1 can be arranged between a first input terminal on the primary side of the DC voltage converter and a first node 11. The second switching element M2 can be arranged between the first node 11 and a second input terminal on the primary side of the DC voltage converter 1. In the second half bridge, the third switching element M3 can be similarly arranged between a first connecting element on the primary side of the DC voltage converter 1 and a second node 12. The fourth switching element M4 can be arranged between the second node 12 and the second input terminal on the primary side of the DC voltage converter 1. The first node 11 can be connected to a first terminal on the primary side of the transformer 30. The second node 12 can be similarly connected to a second terminal on the primary side of the transformer 30. As long as the DC voltage converter 1 also has a full bridge 20 on the secondary side, then this full bridge can be constructed on the secondary side of the transformer 30 and the terminals on the secondary side of the DC voltage converter 1 in a manner similar to the full bridge 10 on the primary side. Capacitors can be respectively arranged between two connecting elements of the terminals on the primary side of the DC voltage converter 1 and / or between two connecting elements of the terminals on the secondary side of the DC voltage converter. In addition, a series inductor L can be arranged between the full bridge 20 on the secondary side and the connecting elements on the secondary side of the DC voltage converter 1.

[0023] The first switching element M1 of the first half bridge thus forms a first upper switching element. The second switching element M2 of the first half bridge thus forms a first lower switching element. The third switching element M3 of the second half bridge forms a second upper switching element, and the fourth switching element M4 of the second half bridge forms a second lower switching element. Thus, the first upper switching element M1 and the second lower switching element M4 form a first diagonal branch. The second upper switching element M3 and the first lower switching element M2 similarly form a second diagonal branch complementary to the first diagonal branch.

[0024] In the following Figures 2 to 5depicts different switching states as they can be set by means of the full bridge 10 on the primary side.

[0025] Figure 2 Shows a schematic diagram of the full bridge 10 on the primary side, in which the switching elements M1 and M4 of the first diagonal branch are controlled. The current can accordingly flow from the first connecting element of the joint on the primary side via the first switching element M1 and the primary side of the transformer 30 and the second lower switching element M4 to the second connecting element of the joint on the primary side of the DC voltage converter 1. Thereby, a rising current appears in the transformer 30.

[0026] Figure 3 Shows the switching state of the full bridge 10 on the primary side, in which the two upper switching elements M1 and M3 are closed and the two lower switching elements M2 and M4 are open. In this switching state, the two connecting elements on the primary side of the transformer 30 are electrically connected to each other by the two upper switching elements M1 and M3 and thus the primary side of the transformer 30 is short-circuited. In this switching state, the current set in the transformer 30 can continuously decrease until 0 amperes.

[0027] Figure 4 Shows the switching state of the full bridge 10 on the primary side, in which the second diagonal branch consisting of the first lower switching element M2 and the second upper switching element M3 is closed. Accordingly, in this switching state, the second diagonal branch complementary to the diagonal branch according to Figure 2 is controlled. Thereby, a current can be formed in the transformer 30, the direction of flow of which is opposite to the direction of flow of the current according to Figure 2 .

[0028] Finally, in Figure 5 a switching state is shown in which the two lower switching elements M2 and M4 are closed and the two upper switching elements M1 and M3 are open. Accordingly, the primary side of the transformer 30 is short-circuited by the two lower switching elements M2 and M4. Thereby, the current can flow through the two lower switching elements M2 and M4 until the current in the transformer 30 decays to 0 amperes.

[0029] For the following explanation, the switching state according to Figure 2 is referred to as switching state 1, the switching state according to Figure 3 is referred to as switching state 2, the switching state according to Figure 4 is referred to as switching state 3, and the switching state according to Figure 5 is referred to as switching state 4.

[0030] In order to transfer electrical energy from the primary side of the DC voltage converter 1 to the secondary side, for example, a current can be applied to the transformer 30 periodically in a series of switching states 1-2-3-4-1-2-3-4... with alternately positive and negative flow directions. Here, when the first upper switching element M1 is turned on for the switching state 1 and when the first lower switching element M2 is closed for the third switching state 3, the full primary-side input voltage U_prim is respectively applied across the corresponding switching elements M1, M2. Thus, the switching elements M1, M2 of the first half-bridge must be hard-switched. In contrast, when the switching elements M3 and M4 of the second half-bridge are closed, significantly smaller voltages are respectively applied across the switching elements M3, M4, so that these switching elements can be soft-switched respectively. Therefore, if the DC voltage converter 1 is constantly controlled with the above-described switching sequence 1-2-3-4, the losses on the switching elements M1, M2 of the first half-bridge will be higher than the losses on the two switching elements M3, M4 of the second half-bridge.

[0031] Conversely, if the DC voltage converter 1 is controlled with the alternative switching sequence 1-4-3-2-1-4-3-2..., in this switching sequence, the switching elements M3 and M4 of the second half-bridge are hard-switched, while the switching elements M1, M2 of the first half-bridge are soft-switched. In this case, higher power losses are generated on the switching elements M3 and M4 of the second half-bridge than on the switching elements M1 and M2 of the first half-bridge.

[0032] Therefore, in order to achieve an as-uniform-as-possible distribution of the losses in the first full bridge 10, it is possible to switch between the two above-described switching sequences. For example, it is possible to switch periodically between the first switching sequence (1-2-3-4) and the second switching sequence (1-4-3-2) according to a predetermined number of switching processes. Of course, any other arbitrary pre-specification for the periodic alternation between these two switching sequences is also feasible. Thus, for example, it is possible to switch between these two switching sequences accordingly according to a predetermined time interval. As an alternative, for example, the temperature of the switching elements can also be monitored. For this purpose, for example, temperature sensors or temperature models can be considered for calculating the temperature. If the temperature of the switching elements on a half-bridge exceeds a pre-given threshold, then subsequently, for example, the alternative switching sequence can be switched to in order to relieve the corresponding switching elements. Of course, any other arbitrary mode of switching between the switching sequences is also feasible.

[0033] Figure 6The figure shows a schematic diagram of a flowchart as the basis of a method for controlling a DC voltage converter according to an embodiment. For this method, all the explanations made before in connection with the DC voltage converter 1 apply. In addition, the DC voltage converter 1 described before can also be designed accordingly to perform all the steps described next in connection with the method.

[0034] In a first step S1, the DC voltage converter can be controlled with a first switching sequence. For this first switching sequence, the first diagonal branch and the second diagonal branch are controlled alternately. In between, the electrical connection of the two joints on the primary side of the transformer is made either through the upper switching elements M1, M3 or through the lower switching elements M2, M4 respectively. In the first switching sequence, as described before, for example, by the switching sequence 1 - 2 - 3 - 4, the first diagonal branch composed of, for example, the first upper switching element M1 and the second lower switching element M2 can be controlled first. Subsequently, the two upper switching elements M1 and M3 can be controlled in the switching state 2. Next, the second diagonal branch composed of the second upper switching element M3 and the first lower switching element M2 can be controlled. Finally, the two lower switching elements M2 and M4 are controlled. Then, the first diagonal branch can be controlled again according to the first switching state 1.

[0035] In a second switching sequence S2, the conversion between the first diagonal branch and the second diagonal branch is carried out by controlling the two lower switching elements M2 and M4. Correspondingly, the conversion between the second diagonal branch and the first diagonal branch is carried out by controlling the two upper switching elements M1 and M3.

[0036] Here, the conversion between the first switching sequence in step S1 and the second switching sequence in step S2 can be carried out regularly, for example, periodically. As described before, for this purpose, any mode, such as conversion over a predetermined number of switching processes, conversion according to a predetermined time period, etc. is feasible.

[0037] In particular, the DC voltage converter can be operated in discontinuous conduction mode (DCM).

[0038] In summary, the present invention relates to the control of a DC voltage converter, such as, for example, a phase-shifted full-bridge DC voltage converter, wherein it is possible to switch between two pre-given switching sequences. In this way, an even loading of the switching elements of the full bridge on the primary side of the DC voltage converter can be achieved. Thus, for example, for the first switching sequence, the switching elements of the first half-bridge of the full bridge on the primary side can be hard-switched, while for the second switching sequence, the switching elements of the second half-bridge of the full bridge on the primary side are hard-switched.

Claims

1. A method for controlling a DC voltage converter (1), the DC voltage converter having a primary-side full bridge (10) and a transformer (30), the transformer being arranged between the primary-side full bridge (10) and the secondary side of the DC voltage converter (1), in, Alternately controlling a first diagonal branch and a second diagonal branch, wherein the first diagonal branch is complementary to the second diagonal branch; Wherein, the first switch state includes operating the first diagonal branch; The second switching state includes electrically connecting two connecting elements on the primary side of the transformer (30) to each other through two upper switching elements (M1, M3) of the full bridge (10) on the primary side; wherein the third switch state includes manipulating the second diagonal branch; The fourth switching state includes electrically connecting two connecting elements on the primary side of the transformer (30) to each other through two lower switching elements (M2, M4) of the full bridge (10) on the primary side; The full bridge (10) on the primary side is controlled alternately according to a first switching sequence (S1) and a second switching sequence (S2), the first switching sequence (S1) includes a sequence of first, second and third switching states, and the second switching sequence (S2) includes a sequence of first, fourth and third switching states.

2. The method according to claim 1, wherein: The switchover between the first switching sequence (S1) and the second switching sequence (S2) is performed according to a predetermined number of switching cycles.

3. The method according to claim 1 or 2, wherein: The DC voltage converter (1) is operated in a discontinuous operating mode.

4. A DC voltage converter (1) having: a full bridge (10) on the primary side; and a transformer (30) arranged between the full bridge (10) on the primary side and the secondary side of the DC voltage converter (1), in, The primary-side full bridge (10) is arranged between the primary-side DC voltage connection and the primary side of the transformer (30), and The DC voltage converter (1) comprises a control device (50) which is designed to actuate the switching elements (M1-M4) of the primary-side full bridge (10) according to the method according to any one of claims 1 to 3.

5. A DC voltage converter (1) according to claim 4, wherein: The full bridge (10) on the primary side comprises a first half bridge and a second half bridge, wherein the first half-bridge comprises a first upper switching element (M1) and a first lower switching element (M2), and the second half-bridge comprises a second upper switching element (M3) and a second lower switching element (M4), wherein the first upper switching element (M1) is arranged between a first connecting element of the DC voltage connection on the primary side and a first node (11), and the first lower switching element (M2) is arranged between the first node (11) and a second connecting element of the DC voltage connection on the primary side, wherein the second upper switching element (M3) is arranged between a first connection element of the DC voltage connection on the primary side and a second node (12), and the second lower switching element (M4) is arranged between the second node (12) and a second connection element of the DC voltage connection on the primary side, and The first node (11) is connected to a first connecting element on the primary side of the transformer (30), and the second node (12) is connected to a second connecting element on the primary side of the transformer (30).

6. A DC voltage converter according to claim 4 or 5, comprising a secondary-side full bridge (20), which is arranged between the secondary side of the transformer (30) and a secondary-side DC voltage connection of the DC voltage converter (1).

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