DC-DC converter and method for operating a DC-DC converter

Abstract

The invention relates to a DC-DC converter and to a method for operating a DC-DC converter, in particular a galvanically isolating DC-DC converter. A PWM modulation for controlling the switching elements in the DC-DC converter is proposed, in which further degrees of freedom are created by introducing an additional pulse per period duration. These further degrees of freedom can, for example, improve the chance of a soft switching of the switching elements.

Description

[0001] R.415025-IP1

[0002] - 1 -

[0003] Description

[0004] title

[0005] DC-DC converter and method for operating a DC-DC converter

[0006] Technical field

[0007] The present invention relates to a DC-DC converter and a method for operating such a DC-DC converter.

[0008] background

[0009] DC-DC converters are capable of converting an input DC voltage provided at an input terminal into another DC voltage and outputting this additional DC voltage at an output terminal. The voltage level of the additional DC voltage can differ from the voltage level of the input DC voltage. In particular, galvanically isolated DC-DC converters are known, which, for example, use a transformer to provide galvanic isolation between the input and output terminals.

[0010] Such DC / DC converters can be used, for example, in fully or at least partially electrically powered motor vehicles to exchange electrical energy between two power supply networks, such as a low-voltage on-board network and a high-voltage on-board network.

[0011] The publication DE 10 2009 212 888 A1 describes a DC-DC converter and a control method for a DC-DC converter. In particular, a specific R.415025-IP1 is mentioned.

[0012] - 2 -

[0013] A control method has been proposed which makes it possible to switch on the switching elements in the DC voltage converter predominantly softly.

[0014] Disclosure of the invention

[0015] The present invention provides a DC-DC converter and a method for operating a DC-DC converter with the features of the independent claims. Further advantageous embodiments are the subject of the dependent claims.

[0016] Accordingly, the following is planned:

[0017] A DC-DC converter comprising a first DC input, a second DC input, a transformer, a first bridge circuit, a second bridge circuit, and a control device. The first DC input can be configured to connect to a first DC network. The second DC input can be configured to connect to a second DC network. The voltage level at the first DC input can differ from the voltage level at the second DC input. The transformer comprises a primary and a secondary terminal. The first bridge circuit comprises at least one half-bridge. Each half-bridge of the first

[0018] The bridge circuit comprises at least one first switching element and one second switching element. The switching elements can be, in particular, semiconductor switching elements such as MOSFETs or IGBTs. The first switching element is arranged between a first connection point of the first DC voltage terminal and a node of the respective half-bridge. The second switching element is arranged between the node of the respective half-bridge and a second connection point of the first DC voltage terminal. Each node of a half-bridge in the first bridge circuit is electrically coupled to a connection point of the primary terminal of the transformer. The second bridge circuit comprises at least one half-bridge. Each half-bridge of R.415025-IP1

[0019] - 3 - The second bridge circuit comprises at least one first switching element and one second switching element. The first switching element is arranged between a first connection point of the second DC voltage terminal and a node of the respective half-bridge. The second switching element is arranged between the node of the respective half-bridge and a second connection point of the second DC voltage terminal. Each node of a half-bridge of the second bridge circuit is electrically coupled to a connection point of the secondary terminal of the transformer. The control device is designed to control the switching elements in the first bridge circuit and the second bridge circuit.In particular, the control unit is designed to control the switching elements of the first bridge circuit in a first operating mode using control signals according to a first PWM modulation with a predetermined period, and to control the switching elements of the second bridge circuit using control signals according to a second PWM modulation with the same predetermined period. The period of the first PWM modulation corresponds to the period of the second PWM modulation. The control signals for at least one half-bridge of the second bridge circuit include one additional control pulse per period. In other words, a control signal is generated for at least one half-bridge of the second bridge circuit that has two pulses per period.The first operating mode is specifically designed to transfer electrical energy from the first DC voltage terminal towards the second DC voltage terminal.

[0020] Furthermore, the following is planned:

[0021] A method for operating a DC-DC converter. The DC-DC converter comprises a first DC input, a second DC input, a transformer with a primary and a secondary input, a first bridge circuit with at least one half-bridge, and a second bridge circuit with at least one half-bridge. Each half-bridge of the first bridge circuit R.415025-IP1

[0022] - 4 - Each half-bridge comprises at least one first switching element and one second switching element. The first switching element is arranged between a first connection point of the first DC voltage terminal and a node of the respective half-bridge. The second switching element is arranged between the node of the respective half-bridge and a second connection point of the first DC voltage terminal. Each node of a half-bridge of the first bridge circuit is electrically coupled to a connection point of the primary terminal of the transformer. Each half-bridge of the second bridge circuit comprises at least one first switching element and one second switching element. The first switching element is arranged between a first connection point of the second DC voltage terminal and a node of the respective half-bridge.The second switching element is arranged between the node of the respective half-bridge and a second connection point of the second DC voltage terminal. Each node of a half-bridge in the second bridge circuit is electrically coupled to a connection point of the transformer's secondary terminal. In a first operating mode, the method involves controlling the switching elements of the first bridge circuit with control signals according to a first PWM modulation with a predetermined period. Furthermore, the method involves controlling the switching elements of the second bridge circuit with control signals according to a second PWM modulation with the same predetermined period. Thus, the period for the control signals of the switching elements in the first bridge circuit and the period for controlling the switching elements in the second bridge circuit are the same.The control signals for at least one half-bridge of the second bridge circuit include an additional control pulse per period.

[0023] Advantages of the invention

[0024] The present invention is based on the finding that, in the case of a so-called hard switch-on of a transistor, the switching losses are higher than in the case of a so-called soft switch-on, in which the electrical R.415025-IP1

[0025] - 5 -

[0026] The voltage across the transistor should be as low as possible. Therefore, for the operation of DC-DC converters, especially galvanically isolated DC-DC converters such as dual-active-bridge converters or resonant converters, it is advantageous to switch the switching elements in the DC-DC converter preferably softly, i.e., to close them at the lowest possible electrical voltage across the switching element to be switched.

[0027] However, if a highly coupled transformer is used in a galvanically isolated DC-DC converter, such soft switching may not be achievable across the entire operating range using conventional control methods. In particular, if similar switching transitions occur simultaneously on the input and output sides (for example, voltage transitions with a positive slope on both sides), conventional control methods may prevent both switching transistors from being softly switched.

[0028] Based on this understanding, the present invention aims to create a concept that offers additional degrees of freedom for the operation of the DC-DC converter. This makes it possible to achieve smooth switching of the switching elements in the DC-DC converter across its entire operating range, particularly in the case of a galvanically isolated DC-DC converter. Since the concept according to the invention can be used to control the switching elements in the DC-DC converter across its entire operating range, there is no need to differentiate between cases that might otherwise require switching between different control methods depending on the operating point. Thus, the proposed concept allows for optimized control of the DC-DC converter.The additional degrees of freedom make it possible, for example, to implement smooth switching of the switching elements across the entire operating range. R.415025-IP1.

[0029] - 6 -

[0030] For the control method according to the invention, the switching elements in the half-bridges on the input or primary side and the output or secondary side of the DC-DC converter are controlled based on control signals for pulse width modulation (PWM). The control signals are generated based on periodic PWM signals, each containing one pulse with a predetermined pulse duration per period. According to the invention, it is provided that an additional pulse per period is added to the control signals for at least one half-bridge, preferably all half-bridges, on the output or secondary side of the DC-DC converter. This additional pulse creates further degrees of freedom.These additional degrees of freedom make it possible, for example, to implement the control of the switching elements in the DC-DC converter in such a way that smooth switching of the switching elements can be achieved across the entire operating range.

[0031] According to one embodiment, the control signals for a half-bridge of the second bridge circuit are designed such that two control pulses are provided within one period. The first control pulse does not begin immediately at the start of the period. The second control pulse does not continue until the end of the period. Therefore, a voltage-free interval is provided between the edges of the period and the two control pulses. This voltage-free interval is not the usual dead time within the half-bridge, during which both switching elements are briefly switched off, but rather a freewheeling phase in which the switching elements of the half-bridge are driven in the same direction, with no voltage applied to the transformer. These voltage-free intervals prevent the control signals from exhibiting abrupt transitions at the period boundaries.This allows for a more even distribution of switching operations and reduces unwanted high-frequency signal components, which has a positive effect on the electromagnetic emission behavior of the converter. R.415025-IP1.

[0032] - 7 -

[0033] According to one embodiment, the second bridge circuit comprises several half-bridges. In particular, the control signals for at least one half-bridge of the second bridge circuit can each include an additional control pulse per period in the first operating mode. In this way, identical control signals with the additional control pulse are generated for all half-bridges of the second bridge circuit. These identical control signals can be calculated and generated particularly easily.

[0034] According to one embodiment, the second bridge circuit comprises several half-bridges, wherein in the first operating mode only exactly one of the half-bridges receives an additional drive pulse per period. The remaining half-bridges are each operated with only one drive pulse per period. This measure results in a modified loss distribution between the transistors of the bridge circuit. At the same time, the described drive leads to an improvement in electromagnetic emission behavior, since even harmonics in common-mode emission are reduced. The embodiment thus allows for an advantageous combination of simplified drive, targeted loss distribution, and improved EMC performance.According to a further embodiment, the control signals for the two switching elements of a half-bridge are designed such that the additional control pulse is assigned in successive periods to the switching element that is switched to conduction in the respective half-wave. This design results in a symmetrical control pattern, which leads to a uniform voltage waveform at the output node of the half-bridge. The resulting symmetrical behavior has a beneficial effect on electromagnetic compatibility, since, in particular, even-numbered harmonics in common-mode emission can be attenuated.

[0035] According to one embodiment, the additional control pulse has a pulse duration of at least five percent of the period. An additional control pulse with a correspondingly long pulse duration can, on the one hand, be relatively R.415025-IP1

[0036] - 8 - can be easily integrated into the control signal and also has a significant influence on the operating behavior.

[0037] According to one embodiment, the control device is designed to adjust the pulse duration of the additional control pulse using the current operating point of the DC-DC converter. Additionally or alternatively, the switching point, i.e., the start, end, and / or midpoint of the additional control pulse, can also be adjusted using the current operating point. In particular, the characteristics of the additional control pulse can be adapted such that smooth switching of the switching elements in the DC-DC converter is always achieved during operation.

[0038] According to one embodiment, an edge of the additional control pulse in a control signal for the second bridge circuit coincides at least approximately with an edge of a PWM pulse of the control signal for the first bridge circuit. In particular, for example, a rising edge in the control signals for the second bridge circuit can coincide at least approximately with a complementary, i.e., for example, falling edge in the control signals for the first bridge circuit. The deviations in the edges can, for example, be limited or set to a predetermined range or a predetermined duration.

[0039] According to one embodiment, the control device is designed to operate in a second operating mode for electrical energy transfer from the second DC terminal to the first DC terminal. In this second operating mode, an additional control pulse per period can be generated in the control signal for at least one half-bridge of the first bridge circuit. In other words, the inventive principle of an additional control pulse in at least one output-side half-bridge can also be applied to inverse energy transfer from the second DC terminal to the first DC terminal. Alternatively, R.415025-IP1

[0040] - 9 - however, even when transmitting from the second DC terminal to the first DC terminal, an additional control pulse per period is included in the control signals for the second bridge circuit.

[0041] According to one embodiment, the first bridge circuit and / or the second bridge circuit comprises a full bridge circuit. The full bridge circuits can each comprise two half bridges. Thus, a separate half bridge can be provided for each connection point of the primary and / or secondary terminal of the transformer.

[0042] According to one embodiment, the DC-DC converter comprises a dual active bridge converter. Alternatively, the DC-DC converter can also comprise a resonant converter. Such DC-DC converters have already proven their worth in many applications for galvanically isolated electrical power transfer between two DC voltage sources. In particular, such DC-DC converter arrangements are very well suited for the control concept according to the invention.

[0043] According to one embodiment, the DC-DC converter further comprises at least one third DC terminal and a third bridge circuit. The transformer also includes multiple secondary terminals. In particular, a secondary terminal is provided for each bridge circuit on the secondary side of the transformer. The second bridge circuit is arranged between a first secondary terminal of the transformer and the second DC terminal. Similarly, the third bridge circuit is arranged between the second secondary terminal of the transformer and the third DC terminal. The control signals for the second bridge circuit can include the additional control pulses described above, while the control signals for the third bridge circuit do not include any additional control pulses. In principle, more than two secondary-side bridge circuits with corresponding R.415025-IP1 are also possible.

[0044] - 10 -

[0045] DC connections are possible. In this case, at least one secondary-side bridge circuit is operated without an additional drive pulse.

[0046] According to one embodiment of the method, the control signals are provided in such a way that the additional control pulse does not last immediately at the beginning or immediately until the end of a period. This creates voltage-free periods at the period boundaries, thereby avoiding abrupt transitions and improving the E-MV behavior.

[0047] According to a further embodiment of the method, the second bridge circuit comprises several half-bridges, wherein in the first operating mode only an additional drive pulse per period is provided for exactly one half-bridge. The remaining half-bridges are each operated with a basic pulse, thereby altering the loss distribution. Simultaneously, the described control method can achieve an improvement in the electromagnetic emission characteristics.

[0048] According to a further embodiment of the method, the additional control pulse is assigned in successive periods to the switching element of the half-bridge that is switched to conduction in the respective half-wave. In this way, a symmetrical control signal is obtained, which can be used to attenuate even harmonics in common-mode emission.

[0049] The above embodiments and further developments can be combined with one another as appropriate. Further embodiments, further developments, and implementations of the invention also include combinations of features of the invention described previously or subsequently with regard to the exemplary embodiments, even if not explicitly mentioned. In particular, those skilled in the art will also add individual aspects as improvements or additions to the respective basic forms of the invention.

[0050] Brief description of the drawings R.415025-IP1

[0051] - 11 -

[0052] Further features and advantages of the invention are explained below with reference to the figures. These show:

[0053] Fig. 1 : a schematic representation of a basic circuit diagram of a DC-DC converter according to one embodiment;

[0054] Fig. 2: a schematic representation to illustrate the voltage conditions in a conventional DC-DC converter;

[0055] Fig. 3: a schematic representation of the control signals of a conventional DC-DC converter;

[0056] Fig. 4: a schematic representation to illustrate the control signals and voltage relationships in a DC-DC converter according to one embodiment;

[0057] Fig. 5: a schematic representation to illustrate the voltage and current conditions in a DC-DC converter according to a further embodiment;

[0058] Fig. 6: a schematic representation to illustrate the control signals and voltage relationships in a DC-DC converter according to one embodiment;

[0059] Fig. 7: a schematic representation of a basic circuit diagram of a DC-DC converter according to a further embodiment; and

[0060] Fig. 8: a flowchart as it may form the basis of a method for controlling a DC voltage converter according to one embodiment.

[0061] Description of embodiments R.415025-IP1

[0062] - 12 -

[0063] Figure 1 shows a schematic diagram of a basic circuit diagram of a DC-DC converter 1 according to one embodiment. The DC-DC converter 1 comprises a first DC voltage terminal 11 and a second DC voltage terminal 12. The first DC voltage terminal 11 can be electrically coupled to a first DC voltage network, a DC voltage source, or the like. The second DC voltage terminal 12 can be electrically coupled to a second DC voltage network, a DC voltage load, or the like. For example, the DC-DC converter 1 can be used between a high-voltage electrical system and a low-voltage electrical system in an electric vehicle. In this way, electrical energy can be exchanged between the high-voltage and low-voltage electrical systems.

[0064] Although the concept according to the invention is described below primarily for electrical energy transfer from the first DC voltage terminal 11 towards the second DC voltage terminal 12, the present invention is not limited to this. Rather, it is also fundamentally possible, for example for bidirectional DC-DC converters, to carry out electrical energy transfer in the opposite direction from the second DC voltage terminal 12 towards the first DC voltage terminal 11. In particular, bidirectional DC-DC converters are also possible which can realize electrical energy transfer both from the first DC voltage terminal 11 towards the second DC voltage terminal 12 and in the opposite direction from the second DC voltage terminal 12 towards the first DC voltage terminal 11.

[0065] The DC-DC converter 1 further comprises a transformer 30, a first bridge circuit 21, and a second bridge circuit 22. Each bridge circuit 21, 22 can comprise one or more half-bridges, each with two switching elements, in particular semiconductor switching elements such as MOSFETs or IGBTs. For example, a first half-bridge of the R.415025-IP1

[0066] - 13 - The first bridge circuit 21 comprises a first switching element M1 and a second switching element M2, wherein the first switching element M1 is arranged between a first connection point 11a, for example the positive connection point of the first DC voltage terminal 11, and a node. The second switching element M2 can be arranged between the node and a second connection point 11b, for example the negative connection point of the first DC voltage terminal 11. The node of the half-bridge can be electrically coupled to a connection point of the primary side of the transformer 30.Similarly, another half-bridge with the switching elements M3 and M4 can also be provided between the first connection point 11 a and the second connection point 11 b of the first DC voltage connection 11, wherein the node at which these two switching elements M3 and M4 are connected to each other is electrically coupled to another connection point of the primary side of the transformer 30.

[0067] Similarly, the bridge circuit 22 can also comprise several half-bridges, each with two switching elements M5, M6, M7, M8, arranged between the secondary side of the transformer 30 and the second DC voltage terminal 12, analogous to the concept described above. In the case of a resonant converter, further optional components 41, 42 may be provided between the bridge circuits 21 and 22 and the transformer 30. However, the basic principle of a resonant converter is considered well-known and is therefore not explained further here.

[0068] For control, i.e. for opening or closing the switching elements M1 to M8, a control device can generate 50 suitable control signals and provide them at each of the switching elements M1 to M8.

[0069] Figure 2 shows a schematic representation of the voltage waveforms between the nodes of the half-bridges of the bridge circuits 21, 22, i.e., at the outputs of the respective bridge circuits 21, 22. In the upper diagram, the voltage waveform U_p for the first bridge circuit 21 between the first DC voltage terminal 11 and the primary side of the transformer 30 R.415025-IP1 is shown.

[0070] - 14 - shown. Below is the voltage waveform U_s for the second bridge circuit 22 between the second DC voltage terminal 12 and the secondary side of the transformer 30. As can be seen in these diagrams, with conventional control using a predetermined period T, both the pulse width A for the electrical voltage at the output of the first bridge circuit 21 and the pulse width B for the electrical voltage at the output of the second bridge circuit 22 can be varied. Furthermore, the time offset D between the pulses at the output of the first bridge circuit 21 and the pulses at the output of the second bridge circuit 22 is available as an additional parameter. In the example shown here, the parameter D characterizes the time offset between the midpoints of the respective pulses. However, this offset can also be referenced to the edges of the respective pulses.

[0071] Figure 3 shows a schematic representation of the corresponding control signals for the switching elements M1 to M8 for such a conventional control as may be provided, for example, in a dual active bridge converter.

[0072] Figure 4 shows a schematic representation of the waveforms of the control signals for the switching elements M1 to M8 and the resulting electrical voltages U_p and U_s at the outputs of the bridge circuits 21, 22 according to a possible embodiment of the present invention. In the waveforms shown here, the control signals for the switching elements M1 to M4 on the primary and input sides of the DC-DC converter 1 correspond to the control signals of a conventional pulse-width modulation. The control signals each comprise one pulse per period T.

[0073] The control signals shown below for the switching elements M5 to M8 on the secondary and output sides of the DC-DC converter 1 each also include an additional control pulse in the ranges 110 and 120. R.415025-IP1

[0074] - 15 -

[0075] These additional control pulses in ranges 110 and 120 provide one further degree of freedom for the operation of the DC-DC converter. These additional degrees of freedom can relate, in particular, to the width d_B and the (relative) position d_D of the resulting gap in the voltage waveform at the output of the second bridge circuit 22. These additional degrees of freedom can be used, for example, to adjust the switching operations in the bridge circuits 21 and 22 so that the switching elements M1 to M8 can always be switched smoothly.

[0076] For example, these resulting additional degrees of freedom make it possible to avoid the occurrence of identical switching transitions at the same time, regardless of the output power. In particular, a consistent switching sequence for switching elements M1 to M8 can be maintained across the entire operating range of the DC-DC converter 1. This simplifies the control of the DC-DC converter 1 and also prevents abrupt changes in the control behavior that could occur when switching between different control concepts.

[0077] By using identical additional control pulses for the control signals of the switching elements M5 to M8 in all half-bridges of the second bridge circuit 22, the generation of the control signals can generally be simplified even further. However, in principle, even the use of additional control pulses for the control signals of just one half-bridge in the second bridge circuit 22 can gain additional degrees of freedom and thus enable advantageous control of the DC-DC converter 1.

[0078] Furthermore, it is also possible to provide additional control pulses for the control signals in the half-bridges on the primary side, i.e., for the switching elements M1 to M4 of the first bridge circuit 21. In particular, it is possible to provide such additional control pulses for only one or, depending on the configuration, only for a subset of the half-bridges present in the first bridge circuit 21. R.415025-IP1

[0079] - 16 -

[0080] In the embodiment shown in Figure 4, it can be seen that the double pulses within a period are arranged between the period boundaries. The start of the first control pulse is not directly at the beginning of the period, and the end of the second control pulse is not directly at the end of the period. This results in short voltage-free intervals at the period boundaries. These voltage-free intervals are not the usual dead times within the half-bridge, in which both switching elements are briefly switched off, but rather phases in which no voltage is applied to transformer 30, for example, by switching the switching elements in the same direction. This arrangement contributes to a smoother voltage curve and avoids sharp edges precisely at the period boundaries.

[0081] Figure 5 shows a further schematic representation illustrating the modified modulation scheme with additional drive pulses. Reference numeral 210 denotes the resulting output voltage U_p at the output of the first bridge circuit 21. Reference numeral 220 denotes the resulting output voltage U_s at the output of the second bridge circuit 22. The primary-side transformer current is represented by reference numeral 230. The additional pulses are labeled with reference numerals 221 and 222. In the embodiment shown here, an edge of the secondary-side additional pulses coincides with an edge of the primary-side pulses.

[0082] As explained above, the described concept of additional control pulses for the switching elements M5 to M8 of the secondary-side bridge circuit 22 can also be applied to an inverse power transfer from the second DC voltage terminal 12 to the first DC voltage terminal 11. For such a power transfer, additional control pulses are provided for the switching elements M1 to M4 of the primary-side bridge circuit 21. Accordingly, the power transfer example shown in Figure 5 in R.415025-IP1 results in

[0083] - 17 -

[0084] In the reverse direction, a current shape is advantageous if one edge of the secondary-side additional pulses coincides with the other edge of the primary-side pulses.

[0085] Alternatively, in reverse operation of the DC-DC converter 1, additional control pulses can be provided in the control signals for the switching elements M5 to M8 of the secondary-side bridge circuit 22 for energy transfer from the second DC terminal 12 to the first DC terminal 11. This can be done in addition to or as an alternative to the previously described additional control pulses in the control signals for the primary-side bridge circuit 21.

[0086] For setting the additional degrees of freedom d_B and d_D, parameters beyond those already mentioned, such as soft start-up, can be considered. For example, the parameterization of the additional control pulses can also take into account properties such as electromagnetic interference, transformer load, and similar factors. In particular, the parameterization of the control pulses can be adapted depending on an operating point or similar. Any concept is possible for this, such as predefined parameterizations in the form of a previously calculated table or even formulaic specifications.

[0087] Figure 6 shows a pulse diagram illustrating a further embodiment of the invention. In this embodiment, the control signals provided by the control unit 50 are designed such that only the switching elements of one half-bridge of the second bridge circuit 22 are supplied with double pulses. The selected half-bridge thus has three control pulses within one period, while the other half-bridges are each operated with only one control pulse.

[0088] The pulse pattern shown illustrates that in the positive half-wave two control pulses are assigned to the switching element M5, M7 that is switched to conduction in this half-wave, while in the negative half-wave two R.415025-IP1

[0089] - 18 -

[0090] The control pulses are assigned to the complementary switching elements M6 and M8 of the same half-bridge. This creates a symmetrical sequence of control signals within the period, while simultaneously avoiding abrupt transitions at the period boundaries.

[0091] The control method shown results in a uniform voltage waveform at the secondary terminal of transformer 30 and significantly improves the converter's electromagnetic interference (EMI) performance. In particular, even harmonics in the common-mode emission spectrum are attenuated, for example, the frequency components at 200 kHz, 400 kHz, or 600 kHz at a switching frequency of 100 kHz. This significantly reduces common-mode interference in the relevant frequency range. Differences from the previously described embodiment arise primarily in the loss distribution between the switching elements M5 to M8 of the second bridge circuit 22, which is altered by limiting the circuit to a half-bridge, but remains within a practically acceptable range.

[0092] In the embodiment shown in Figure 6, the first control pulse does not begin immediately at the start of the period, and the second control pulse does not end immediately at the end of the period. This creates short voltage-free intervals at the edges of the period, thus avoiding abrupt transitions at the period boundaries and resulting in a more uniform voltage curve.

[0093] Figure 7 shows a schematic diagram of a basic circuit of a DC-DC converter 1 according to a further embodiment. The DC-DC converter 1 according to Figure 7 differs from the previously described DC-DC converter 1 in particular in that several secondary-side bridge circuits 22, 23 are provided in this embodiment. Accordingly, the transformer 30 also comprises several secondary-side connections in this embodiment. The transformation ratios for the individual secondary-side connections can be selected as appropriate depending on the application. R.415025-IP1 is connected between each secondary-side connection of the transformer 30 and a corresponding secondary-side DC voltage connection 12, 13.

[0094] - 19 - each bridge circuit 22, 23 is provided. In principle, more than two secondary-side bridge circuits 22, 23 and corresponding DC voltage connections 12, 13 are also possible.

[0095] For the operation of this DC-DC converter 1, the control signals for a secondary-side bridge circuit 22 between the first secondary-side terminal of the transformer 30 and the corresponding DC voltage terminal 12 may, for example, include the previously described additional control pulses, while the control signals for the further secondary-side bridge circuit 23 between the second secondary-side terminal of the transformer 30 and the corresponding DC voltage terminal 13 do not include such additional control pulses. If there are more than two secondary-side bridge circuits, at least one bridge circuit is not provided with additional control pulses.

[0096] Figure 8 shows a flowchart illustrating a possible method for operating a DC-DC converter 1 according to one embodiment. The DC-DC converter 1 can, for example, be a DC-DC converter 1 according to the structure described above. Accordingly, the method described below can also include any steps or substeps suitable for implementing the previously described concept. Similarly, the previously described DC-DC converter 1 and the control concept described in this context can include any components suitable for implementing the method described below.

[0097] For the operation of the DC-DC converter, an operating mode can be provided in which electrical energy is transferred from the first, or primary-side, DC voltage terminal 11 to the second, or secondary-side, DC voltage terminal 12. In this mode, the switching elements M1 to M4 of the primary-side bridge circuit 21 are controlled by the first control signals of a PWM modulation. Furthermore, the switching elements M5 to M8 of the secondary-side bridge circuit 22 are controlled by the second control signals of a PWM modulation. The first and R.415025-IP1

[0098] - 20 - The second control signals have the same predetermined period. Furthermore, in the second control signals for the switching elements M5 to M8 of the secondary-side bridge circuit 22, an additional control pulse per period is provided for at least one half-bridge. Preferably, such an additional control pulse is provided in all control signals for the switching elements M5 to M8 of the secondary-side bridge circuit 22.

[0099] For the operation of a bidirectional DC voltage converter 1, a further operating mode can be provided in which the additional control pulses are provided in the control signals for the switching elements M1 to M4 of at least one half-bridge of the primary-side bridge circuit 21.

[0100] In summary, the present invention relates to a DC-DC converter and a method for operating a DC-DC converter, in particular a galvanically isolated DC-DC converter. A PWM modulation for controlling the switching elements in the DC-DC converter is proposed, in which further degrees of freedom are created by inserting an additional pulse per period. These additional degrees of freedom can, for example, improve the possibility of smooth switching of the switching elements.

Claims

R.415025-IP1 - 21 - Claims 1. DC-DC converter (1), comprising a first DC terminal (11); a second DC terminal (12); a transformer (30) comprising a primary terminal and a secondary terminal; a first bridge circuit (21), comprising at least one half-bridge, wherein each half-bridge of the first bridge circuit (21) comprises a first switching element (M1, M3) and a second switching element (M2, M4), wherein the first switching element (M1, M3) is arranged between a first connection point (11a) of the first DC terminal (11) and a node of the respective half-bridge, and the second switching element (M2, M4) is arranged between the node of the respective half-bridge and a second connection point (11b) of the first DC terminal (11), and wherein each node of a half-bridge of the first bridge circuit (21) is electrically coupled to a connection point of the primary terminal of the transformer (30);a second bridge circuit (22) with at least one half-bridge, wherein each half-bridge of the second bridge circuit (22) comprises a first switching element (M5, M7) and a second switching element (M6, M8), wherein the first switching element (M5, M7) is arranged between a first connection point (12a) of the second DC voltage connection (12) and a node of the respective half-bridge and the second switching element (M6, M8) is arranged between the node of the respective half-bridge and a second; R.415025-IP1 - 22 - connection point (12b) of the second DC voltage connection (12), and wherein each node of a half-bridge of the second bridge circuit (22) is electrically coupled to a connection point of the secondary connection of the transformer (30); and a control device (50) designed to control, in a first operating mode, the switching elements (M1-M4) of the first bridge circuit (21) each with control signals according to a first PWM modulation with a predetermined period, and to control the switching elements (M5-M8) of the second bridge circuit (22) each with control signals according to a second PWM modulation with the predetermined period, wherein the control signals for at least one half-bridge of the second bridge circuit (22) include an additional control pulse per period.

2. DC-DC converter (1) according to claim 1, wherein the control signals for the half-bridge of the second bridge circuit, which includes the additional control pulse per period, are designed such that two control pulses are provided in one period, wherein the first control pulse does not start immediately at the beginning of the period and the second control pulse does not continue immediately until the end of the period, so that a voltage-free period is provided between the edges of the period and the control pulses.

3. DC-DC converter (1) according to claim 1 or 2, wherein the second bridge circuit (22) comprises several half-bridges, and wherein the control signals for at least one of the several half-bridges of the second bridge circuit (22) in the first operating mode comprise an additional control pulse per period. R.415025-IP1 - 23 - 4. DC voltage converter (1) according to claim 3, wherein the control signals for exactly one half-bridge of the second bridge circuit (22) in the first operating mode comprise an additional control pulse per period.

5. DC voltage converter (1) according to claim 4, wherein the control signals for the two switching elements of the exactly one half-bridge are designed such that the additional control pulse is alternately assigned to one of the two switching elements of the half-bridge in successive periods.

6. DC voltage converter (1) according to one of the preceding claims, wherein the additional control pulse has a pulse duration of at least 5 percent of the period.

7. DC voltage converter (1) according to one of the preceding claims, wherein the control device (50) is designed to set a switching time and / or the pulse duration of the additional control pulse using a current operating point of the DC voltage converter (1).

8. DC voltage converter (1) according to one of the preceding claims, wherein an edge of the additional control pulse in a control signal for the second bridge circuit (22) corresponds at least approximately to an edge of a PWM pulse of the control signal for the first bridge circuit (21).

9. DC voltage converter (1) according to one of the preceding claims, wherein the control device (50) is designed to transfer electrical energy from the second DC voltage terminal (12) to the first DC voltage terminal (11) in a second operating mode. R.415025-IP1 - 24 - 10. DC-DC converter (1) according to any of the preceding claims, wherein the DC-DC converter (1) comprises a dual-active-bridge converter or a resonant converter.

11. DC-DC converter (1) according to one of the preceding claims, comprising a third DC voltage terminal (13); and a third bridge circuit (23); wherein the transformer (30) comprises several secondary terminals, wherein the second bridge circuit (22) is arranged between a first secondary terminal of the transformer (30) and the second DC voltage terminal (12), and wherein the third bridge circuit (23) is arranged between the second secondary terminal of the transformer (30) and the third DC voltage terminal (13), and wherein no additional control pulses are provided in the control signals for the third bridge circuit (23).

12. Method for operating a DC-DC converter (1) with a first DC voltage terminal (11), a second DC voltage terminal (12), a transformer (30) with a primary terminal and a secondary terminal, a first bridge circuit (21) with at least one half-bridge and a second bridge circuit (22) with at least one half-bridge, wherein each half-bridge of the first bridge circuit (21) comprises a first switching element (M1, M3) and a second switching element (M2, M4), wherein the first switching element (M1, M3) is located between a first connection point (11a) of the first DC voltage terminal (11) and a node of the respective R.415025-IP1 - 25 - half-bridge is arranged and the second switching element (M2, M4) is arranged between the node of the respective half-bridge and a second connection point (11b) of the first DC voltage terminal (11), and wherein each node of a half-bridge of the first bridge circuit (21) is electrically coupled to a connection point of the primary terminal of the transformer (30), and wherein each half-bridge of the second bridge circuit (22) comprises a first switching element (M5, M7) and a second switching element (M6, M8), wherein the first switching element (M5, M7) is arranged between a first connection point (12a) of the second DC voltage terminal (12) and a node of the respective half-bridge and the second switching element (M6, M8) is arranged between the node of the respective half-bridge and a second connection point (12b) of the second DC voltage terminal (12),and wherein each node of a half-bridge of the second bridge circuit (22) is electrically coupled to a connection point of the secondary terminal of the transformer (30), and wherein in a first operating mode the method controls the switching elements of the first bridge circuit (21) each with control signals according to a first PWM modulation with a predetermined period (S1), and controls the switching elements of the second bridge circuit (22) each with control signals according to a second PWM modulation with the predetermined period (S2), wherein the control signals for at least one half-bridge of the second bridge circuit (22) comprise an additional control pulse per period.

13. Method according to claim 12, wherein the control signals for at least one half-bridge of the second bridge circuit (22), which includes an additional control pulse per period, are provided such that the additional control pulse is neither R.415025-IP1 - 26 - begins immediately at the beginning of the period and ends completely at the end of the period.

14. Method according to claim 12 or 13, wherein the second bridge circuit (22) comprises several half-bridges, and wherein the control signals in the first operating mode comprise one additional control pulse per period for exactly one half-bridge of the second bridge circuit (22).

15. Method according to one of claims 12 to 14, wherein in the half-bridge of the second bridge circuit (22), whose control signals comprise an additional control pulse per period, the additional control pulse is applied alternately to the first switching element (M5, M7) and to the second switching element (M6, M8) of this half-bridge.

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