Control method for a DC voltage converter and DC voltage converter

Abstract

The present invention relates to a control method for a DC voltage converter for transferring energy from a primary side to a secondary side of the DC voltage converter. Here, a special control method is proposed, which can achieve a mainly soft turn-on of the switching elements in the DC voltage converter. Thereby, the losses during the control of the switching elements can be minimized.

Description

Technical Field

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

[0002] DC voltage converters are known for converting a first DC voltage into a second DC voltage having different voltage levels. 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 transfer electrical energy between these two in-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 in-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 of the dependent claims.

[0005] Accordingly, it is provided that:

[0006] A method for controlling a DC voltage converter, in particular a phase-shifted full-bridge DC voltage converter. The DC voltage converter includes a primary-side full bridge and a secondary-side full bridge. A transformer is provided between the primary-side full bridge and the secondary-side full bridge. The method includes steps for controlling the diagonal branches of the secondary-side full bridge. Here, when the primary side of the transformer is short-circuited by means of the primary-side full bridge, the diagonal branches of the secondary-side full bridge are controlled. Furthermore, the method includes subsequent steps for controlling the diagonal branches of the primary-side full bridge.

[0007] Furthermore, it is provided that:

[0008] A DC voltage converter, in particular a phase-shifted full-bridge DC voltage converter. The DC voltage converter includes a full bridge on the primary side, a full bridge on the secondary side, and a transformer arranged between the full bridge on the primary side and the full bridge on the secondary side. The full bridge on the primary side is in particular designed to be coupled to a DC voltage source. The full bridge on the secondary side can be designed to be coupled to a load, in particular a DC voltage load and / or an electrical energy storage. In addition, the DC voltage converter includes a control device. The control device is designed to control the first full bridge and the second full bridge according to the method according to the invention.

[0009] The present invention is based on the recognition that in a conventional control method for a DC voltage converter, such as a phase-shifted full-bridge DC voltage converter, in discontinuous operation (Discontinuous Conduction Mode, DCM), some switching elements are partially hard-switched, that is, when a voltage is applied to the switching element, the corresponding switching element is controlled. Compared with, for example, the case of soft turn-on (where no voltage or only a very small voltage is applied to the corresponding switching element), such hard switching of the switching element results in higher electrical losses.

[0010] Therefore, the idea of the present invention is to take this recognition into account and provide an improved control method for a DC voltage converter, in which the switching elements of the DC voltage converter are preferably soft-switched. Thereby, the electrical losses during the switching process can be reduced. In addition, the switching process with no or only a very small voltage on the switching element also subjects the corresponding switching element to less stress, so that the switching element can be protected and thus its service life can be increased.

[0011] Here, the switching according to the invention of the DC voltage converter can be carried out entirely on the basis of conventional hardware. Therefore, no other components are required and thus no higher costs are incurred.

[0012] The method for controlling a DC voltage converter can in particular be used in so-called discontinuous operation or the English "Discontinuous Conduction Mode (DCM)". The control method can in particular be used for a so-called phase-shifted full-bridge DC voltage converter. Here, in order to adjust the switching state on the secondary side of the DC voltage converter according to the invention, a full bridge with two half-bridges is also required on the secondary side, where each half-bridge has two series-connected switching elements. Such a full bridge on the secondary side of the DC voltage converter can also achieve active rectification in a conventional DC voltage converter.

[0013] Here, according to the present invention, when the transformer is short-circuited on the primary side by at least two switching elements of the full bridge on the primary side, the switching elements of the full bridge on the secondary side can apply a current to the transformer on the secondary side. By means of the current applied in the transformer, the switching elements of the diagonal branch in the full bridge on the primary side can then be softly turned on.

[0014] According to one embodiment, the method includes a first part that has a step of controlling a first diagonal branch in the full bridge on the secondary side when the primary side of the transformer is short-circuited by means of the full bridge on the primary side. Then, the switching elements in the first diagonal branch on the primary side are turned on. Subsequently, the terminals on the primary side of the transformer are electrically connected to each other by means of the full bridge on the primary side, and thereby the primary side of the transformer is short-circuited. In the following process, all the switching elements in the full bridge on the secondary side are turned off. In the second part, the method includes a step of controlling a second diagonal branch in the full bridge on the secondary side when the primary side of the transformer is short-circuited by means of the full bridge on the primary side. Subsequently, the switching elements of the second diagonal branch of the full bridge on the primary side are closed, that is, turned on. Then, the terminals on the primary side of the transformer are electrically connected to each other by means of the full bridge on the primary side, and thereby the primary side of the transformer is short-circuited. Finally, all the switching elements of the full bridge on the secondary side are turned off. Here, the second diagonal branch of the full bridge on the secondary side is complementary to the first diagonal branch of the full bridge on the secondary side. In other words, when the upper switching element in the first half bridge and the lower switching element in the second half bridge are controlled in the first part, the upper switching element in the second half bridge of the full bridge and the lower switching element in the first half bridge of the full bridge are controlled in the second part. The second diagonal branch of the full bridge on the primary side is similarly complementary to the first diagonal branch of the full bridge on the primary side.

[0015] According to one embodiment, after controlling the diagonal branch in the full bridge on the secondary side for a predetermined duration, the diagonal branch in the full bridge on the primary side is activated. Thus, during the predetermined duration while the diagonal branch of the full bridge on the secondary side is being controlled, a current can occur in the turns of the transformer. The current generally rises continuously during the predetermined duration.

[0016] According to one embodiment, the short-circuiting of the primary side of the transformer can be carried out by closing two upper switching elements or two lower switching elements of the full bridge on the primary side. If two upper switching elements or two lower switching elements of the full bridge on the primary side are closed simultaneously, an electrical connection can occur between the terminals on the primary side of the transformer through the corresponding switching elements.

[0017] According to one embodiment, the short - circuiting of the primary side of the transformer is alternately carried out by means of the upper switching elements of the full - bridge on the primary side and the lower switching elements of the full - bridge on the primary side. By alternately controlling the upper or lower switching elements of the full - bridge, the respective switching elements can be evenly loaded. Thereby, as gentle and uniform a heating situation as possible is generated.

[0018] According to one embodiment, once the current in the transformer decays, the switching elements of the full - bridge on the secondary side are turned off. If the current on the secondary side of the transformer decays to zero, then in particular the switching elements of the full - bridge on the secondary side can be turned off.

[0019] According to one embodiment, the switching elements of the full - bridge on the secondary side are turned off before the current in the transformer has decayed completely. In this case, the current can then flow through the diodes arranged in parallel with the switching elements until it has decayed completely. In this way, it can be ensured that when the current on the secondary side of the transformer has decayed completely, the switching elements of the full - bridge on the secondary side are actually turned off.

[0020] As long as it makes sense, the above - mentioned 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 before or below in the description of the embodiments of the present invention. Herein, those skilled in the art will in particular also add individual aspects as improvements or supplements to the corresponding basic forms of the present invention. Description of the Drawings

[0021] The following explains other features and advantages of the present invention with the aid of the drawings. Herein:

[0022] Figure 1 A schematic diagram of a principle circuit diagram is shown as it serves as the basis of a DC - voltage converter according to one embodiment;

[0023] Figure 2 A timing diagram for the sequence of switching states is shown as it serves as the basis of a method for operating a DC - voltage converter according to one embodiment; and

[0024] Figure 3 A schematic diagram of a flow - chart is shown as it serves as the basis of a method for controlling a DC - voltage converter according to one embodiment. Detailed Description of the Invention

[0025] Figure 1Shows 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 includes a full bridge 10 on the primary side and a full bridge 20 on the secondary side. A transformer 30 is provided between the full bridge 10 on the primary side and the full bridge 20 on the secondary side. The full bridge 10 on the primary side includes two half bridges each having two switching elements. The first switching element M1 is arranged between the first input terminal on the primary side of the DC voltage converter and the first node 11. The second switching element M2 is arranged between the first node 11 and the second input terminal on the primary side of the DC voltage converter 1. The first switching element M1 and the second switching element M2 form the first half bridge of the first full bridge 10. The second half bridge of the full bridge 10 includes two switching elements M3 and M4. The third switching element M3 is arranged between the first input terminal on the primary side of the DC voltage converter 1 and the second node 12. The fourth switching element M4 is 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 is connected to the first terminal on the primary side of the transformer 30, and the second node 12 is connected to the second terminal on the primary side of the transformer 30.

[0026] The full bridge 20 on the secondary side also includes two half bridges each having two switching elements. Correspondingly, the fifth switching element M5 is arranged between the first terminal on the secondary side of the DC voltage converter 1 and the third node 21. The sixth switching element M6 is arranged between the third node 21 and the second terminal on the secondary side of the DC voltage converter 1. The seventh switching element M7 is arranged between the first terminal on the secondary side of the DC voltage converter 1 and the fourth node 22. The eighth switching element M8 is arranged between the fourth node 22 and the second terminal on the secondary side of the DC voltage converter 1. A series inductor L can be provided between the connection point of the fifth switching element M5 and the seventh switching element M7 on one side and the first terminal on the secondary side of the DC voltage converter 1 on the other side. In addition, capacitors can be provided not only on the input side but also on the output side. A control device 50 is provided for controlling the switching elements M1 to M4 of the full bridge 10 on the primary side and the switching elements M5 to M8 of the full bridge 20 on the secondary side. The control device 50 can control the switching elements especially according to the method described below.

[0027] Figure 2 Shows a timing diagram of the switching states of the switching elements in the full bridge 10 on the primary side and the full bridge 20 on the secondary side and the subsequent transformer current I.

[0028] As can be seen in the diagram, before time t0, the two lower switching elements M2 and M4 of the primary-side full bridge 10 are switched on and thus closed. This short-circuits the primary side of the transformer 30. Furthermore, shortly before time t0, the switching elements M5 and M8 in the first diagonal branch of the secondary-side full bridge 20 are closed, and the switching elements M6 and M7 in the other diagonal branch are opened. In this way, electrical energy can be fed into the transformer 30 from the secondary side of the DC voltage converter 1, for example, from a capacitor connected to the secondary side, and the current in the transformer 30 begins to rise (here in the negative direction). The slope of this current rise is determined by the series inductance L and the parasitic inductance of the transformer 30.

[0029] At time t0, the first switching element M1 in the primary-side full bridge 10 is closed, and the second switching element M2 is opened. In this way, the first diagonal branch of the primary-side full bridge 10 is active via the first switching element M1 and the fourth switching element M4, while the switching elements M2 and M3 of the other diagonal branch are open. The second switching element M2 is opened first, and after a settling time (Totzeit), the first switching element M1 is closed. The negative transformer current at time t0 enables a soft switching on of the first switching element M1. Subsequently, between time t0 and time t1, the current in the output inductor L increases.

[0030] At time t1, switching elements M3 and M4 are switched. In particular, switching element M4 is opened first, followed by closing of switching element M3. The current in transformer 30 then begins to decay. Subsequently, at the latest at time t2, switching elements M5 and M8 of the active diagonal branch are also opened. To avoid problems that may arise with the precise timing of the disconnection of secondary-side switches M5 and M8, these switches can also be disconnected before time t2. In this case, the residual current can be passively discharged via diodes connected in parallel with the switching elements. At time t3, switching elements M6 and M7 of the diagonal branch of the secondary-side full bridge 20 are closed. The current in transformer 30 then begins to rise. Subsequently, at time t4, the short circuit on the primary side of transformer 30 is opened, and the diagonal branch is closed by switching elements M2 and M3. Switching element M1 can also be opened first to eliminate the primary-side short circuit, and then closing switching element M2 after a rest period. In this case, the further switching sequence is carried out corresponding to the sequence already described above with opposite polarity.

[0031] Here, for a fully soft switching characteristic, either the ripple generated in the output current or the ripple generated in the magnetizing current must provide sufficient energy to recharge the switched capacitor. Typically, the output inductor L is in particular so small here that this does not present a limitation.

[0032] Figure 3 A flow chart is shown as the basis for a method for controlling a DC voltage converter 1 according to an embodiment. Here, the method for controlling the DC voltage converter 1 can in particular include steps as previously described in connection with the control of the DC voltage converter 1 according to Figure 1 and 2 . In addition, the DC voltage converter 1 described previously can also be designed to perform all the steps described below.

[0033] In step S1, the first diagonal branch in the full bridge on the secondary side is controlled. For example, the first diagonal branch can include the switching elements M5 and M6 of the full bridge on the secondary side. When the primary side of the transformer 30 is short-circuited, in particular the switching elements of the diagonal branch on the secondary side are controlled.

[0034] In step S2, the first diagonal branch of the full bridge on the primary side is closed. In step S3, the connecting elements on the primary side of the transformer 30 are electrically connected to each other by means of the full bridge 10 on the primary side, and thereby the primary side of the transformer 30 is short-circuited. In step S4, all the switching elements of the full bridge on the secondary side are opened.

[0035] In the second part of the method, when the primary side of the transformer is short-circuited by means of the full bridge on the primary side, in step S5, the second diagonal branch in the full bridge on the secondary side is controlled. Here, the second diagonal branch of the full bridge on the secondary side is complementary to the first diagonal branch of the full bridge on the secondary side.

[0036] In step S6, the second diagonal branch of the full bridge on the primary side is closed. Here, the second diagonal branch of the full bridge on the primary side is complementary to the first diagonal branch of the full bridge on the primary side. In step S7, the connecting elements on the primary side of the transformer 30 are electrically connected to each other by means of the full bridge 10 on the primary side, and thereby the primary side of the transformer 30 is short-circuited. Finally, in step S8, all the switching elements of the full bridge on the secondary side are opened.

[0037] In order for the energy transfer from the primary side to the secondary side of the DC voltage converter to periodically repeat the steps described above.

[0038] In summary, the present invention relates to a control method for a DC voltage converter for energy transfer from the primary side to the secondary side of the DC voltage converter. Herein, a special control method is proposed, which can achieve mainly soft turn-on of the switching elements in the DC voltage converter. Thereby, the losses during the control of the switching elements can be minimized. In particular, it is proposed to apply a current to the short-circuit state of the primary side of the transformer by turning on the diagonal of the secondary side in advance for an adjustable time period.

Claims

1. A method for controlling a DC voltage converter (1), the DC voltage converter having a primary-side full bridge (10), a secondary-side full bridge (20) and a transformer (30), the transformer being arranged between the primary-side full bridge (10) and the secondary-side full bridge (20), wherein, The method comprises the following steps: When the primary side of the transformer (30) is short-circuited by means of the full bridge (10) on the primary side, controlling (S1) the first diagonal branch in the full bridge (20) on the secondary side; Closing (S2) the first diagonal branch of the full bridge (10) on the primary side; Short-circuiting (S3) the primary side of the transformer (30) by means of the full bridge (10) on the primary side; Opening (S4) all the switching elements in the full bridge (20) on the secondary side; When the primary side of the transformer (30) is short-circuited by means of the full bridge (10) on the primary side, controlling (S5) the second diagonal branch in the full bridge (20) on the secondary side, wherein the second diagonal branch of the full bridge (20) on the secondary side is complementary to the first diagonal branch of the full bridge (20) on the secondary side; Closing (S6) the second diagonal branch of the full bridge (10) on the primary side, wherein the second diagonal branch of the full bridge (10) on the primary side is complementary to the first diagonal branch of the full bridge (10) on the primary side; Short-circuiting (S7) the primary side of the transformer (30) by means of the full bridge (10) on the primary side; and Opening (S8) all the switching elements in the full bridge (20) on the secondary side.

2. The method according to claim 1, wherein After controlling the diagonal branch in the full bridge (20) on the secondary side for a predetermined duration, closing (S2, S6) the diagonal branch in the full bridge (10) on the primary side.

3. The method according to claim 1 or 2, wherein The short-circuiting (S3, S7) of the primary side of the transformer (30) is carried out by closing two upper switching elements (M1, M3) or two lower switching elements (M2, M4) of the full bridge (10) on the primary side.

4. The method according to claim 3, wherein The short-circuiting (S3, S7) of the primary side of the transformer (30) is alternately carried out by means of the upper switching elements (M1, M3) of the full bridge (10) on the primary side and the lower switching elements (M2, M4) of the full bridge (10) on the primary side.

5. The method according to claim 1 or 2, wherein Once the current in the transformer (30) decays, the switching elements (M5-M8) of the full bridge (20) on the secondary side are opened (S4, S8).

6. The method according to claim 1 or 2, wherein Before the current in the transformer (30) decays, the switching elements (M5-M8) of the full bridge (20) on the secondary side are opened (S4, S8).

7. A DC voltage converter (1), comprising: A full bridge (10) on the primary side; A full bridge (20) on the secondary side; A transformer (30) arranged between the full bridge (10) on the primary side and the full bridge (20) on the secondary side, and A control device (50) designed to control the full bridge (10) on the primary side and the full bridge (20) on the secondary side according to the method according to any one of claims 1 to 6.

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