Discharging a DC-link capacitor by means of pulsed three-phase short circuits via the inverter half-bridges

Abstract

The invention relates to a concept for discharging a DC-link capacitor on an electrical converter by means of the electric converter. For this purpose, the switching elements of all the half-bridges in the electric converter are used to discharge the DC-link capacitor. To this end, firstly all upper or all lower semiconductor switching elements of the electric converter are closed, and subsequently the complementary semiconductor switching elements are periodically actuated with predetermined pulses. In this way, energy is withdrawn from the DC-link capacitor with each pulse.

Description

[0001] DISCHARGING AN INTERMEDIATE CIRCUIT CAMERAS BY MEANS OF PULSE-PULLED THREE-PHASE SHORT CIRCUITS THROUGH THE INVERTER HALF-BRIDGES

[0002] Technical field

[0003] The present invention relates to a method for discharging a DC link capacitor. The present invention further relates to an electrical power converter and an electrical drive system suitable for discharging a DC link capacitor.

[0004] background

[0005] Electric vehicles typically have an electric drive system in which electrical energy from an electrical energy storage device, such as a traction battery, is converted into an electrical voltage suitable for driving an electric motor by means of a power converter. The DC network that supplies this power converter may include a so-called intermediate circuit capacitor. When the electric drive system is switched off, and especially in the event of a fault (for example, an accident), it is necessary to discharge this intermediate circuit capacitor quickly and reliably to prevent the danger of electric shock. Several approaches already exist for this purpose. For example, German patent application DE 10 2009 055 053 A1 describes a method and a device for discharging an energy storage device in a high-voltage network.It is proposed to discharge the energy storage device, in particular the intermediate circuit capacitor, via a first discharge resistor and to connect a further discharge resistor in parallel depending on a given switching condition.

[0006] Disclosure of the invention

[0007] The present invention provides a method for discharging a DC link capacitor, an electrical power converter, and an electrical drive system with the features of the independent claims. Further advantageous embodiments are the subject of the dependent claims.

[0008] Accordingly, the following is planned:

[0009] A method for discharging an intermediate circuit capacitor, in particular an intermediate circuit capacitor at a DC input of an electrical converter. The electrical converter has several half-bridges. Each half-bridge of the electrical converter comprises a first semiconductor switching element between a first connection point of the DC input and a node of the corresponding half-bridge, and a second semiconductor switching element between the node and a second connection point of the DC input. The method comprises the steps of closing all first semiconductor switching elements in the several half-bridges and then periodically driving the second semiconductor switching elements in the half-bridges. The second half-bridges are driven by pulses with a predetermined pulse duration. Furthermore, the following is provided:

[0010] An electrical power converter with a DC input and multiple half-bridges. The DC input is designed to be connected to an intermediate circuit capacitor. Each of the multiple half-bridges comprises a series connection of two semiconductor switching elements. A first semiconductor switching element is arranged between a first connection point of the DC input and a node of the respective half-bridge, and a second semiconductor switching element is arranged between the node of the respective half-bridge and the second connection point of the DC input. The electrical power converter also includes a control unit. The control unit is designed to close all first semiconductor switching elements in the multiple half-bridges. Furthermore, the control unit is designed to subsequently periodically activate the second semiconductor switching elements in the half-bridges.In this process, the second semiconductor switching elements are each controlled with pulses of a previously determined pulse duration.

[0011] Furthermore, the following is planned:

[0012] An electric drive system comprising a power converter according to the invention, an electric machine, and a DC link capacitor. The electric machine is electrically coupled to the nodes of the electric power converter. For this purpose, an AC voltage connection can be provided on the electric power converter. This AC voltage connection can have an AC voltage connection point for each node of the power converter, which can be connected to a corresponding connection point of the electric machine. The DC link capacitor is electrically coupled to the DC voltage connection. In particular, each connection point of the DC link capacitor is coupled to a corresponding connection point of the DC voltage connection.

[0013] Advantages of the invention

[0014] DC link capacitors, such as those used in electric drive systems, typically require a discharge concept that ensures the capacitor is discharged within a defined time period. Conventional approaches usually require separate circuits for this purpose, which entail additional costs and installation space.

[0015] Furthermore, there are also some concepts for using a single half-bridge in an electrical power converter. However, these concepts may also require additional components, such as sensors for monitoring the current flow. Moreover, the discharge power is limited by the use of a single half-bridge. Therefore, to still be able to comply with the specifications for the time required to discharge the DC link capacitor, further measures are necessary, such as increased heat dissipation of the components involved.

[0016] Based on these findings, the present invention aims to create a concept that enables the efficient and effective discharge of a DC link capacitor entirely without additional hardware components. For this purpose, it is provided that several, preferably all, half-bridges of an electrical power converter are used jointly for discharging the DC link capacitor. For the DC link capacitor discharge according to the invention, either the upper or the lower semiconductor switching elements in all half-bridges are closed to establish an active short circuit (safe operating mode) in the electrical machine. Subsequently, the other semiconductor switching elements in the respective half-bridges are periodically subjected to switch-on pulses. These short switch-on pulses have a predetermined pulse duration.Thus, during each switching pulse, a certain amount of energy is converted at the corresponding semiconductor switching element, thereby discharging the intermediate circuit capacitor.

[0017] By using short switching pulses with a defined pulse duration, the amount of energy transferred per pulse can be precisely controlled. This eliminates the need for complex analysis of the current flow during the discharge process. Furthermore, by using all half-bridges for the discharge process, the DC link capacitor can be discharged significantly faster than if only one half-bridge were used. In particular, the energy stored in the DC link capacitor is distributed across the semiconductor switching elements in all half-bridges. This results in a correspondingly lower load on each individual half-bridge compared to a discharge through only one half-bridge.

[0018] According to one embodiment, the predetermined pulse duration for the periodic activation of the second semiconductor switching elements remains constant throughout the entire discharge process. The pulse duration can, for example, be initially fixed. During the manufacturing of the voltage converter, it is possible to consider the individual characteristics of the installed semiconductor switching elements and to specify a suitable pulse duration for each element, storing this information, for example, in a suitable memory. In this way, component tolerances can be compensated for. This ensures that the electrical energy is distributed as evenly as possible across all half-bridges. Accordingly, in one embodiment, an individual predetermined pulse duration can be specified for each half-bridge.Furthermore, an individual pulse duration can be specified for each semiconductor switching element, that is, for each upper and each lower semiconductor switching element of a half-bridge.

[0019] According to an alternative embodiment, the predetermined pulse duration for driving the second semiconductor switching elements is dynamically adjusted. For example, the predetermined pulse duration can be adjusted using an electrical voltage at the DC voltage terminal that corresponds to the electrical voltage at the intermediate circuit capacitor. Additionally or alternatively, the pulse duration can also be adjusted using the temperature of the first semiconductor switching elements and / or the second semiconductor switching elements. In this way, for example, the electrical energy converted per pulse can be kept at least approximately constant even when the intermediate circuit voltage decreases. Furthermore, the pulse duration can also be adjusted to prevent, for example, thermal overload of the semiconductor switching elements.

[0020] According to another embodiment, the second semiconductor switching elements for discharging the DC link capacitor are controlled with a reduced gate current. In this case, a reduced gate current means that the gate current used to control the semiconductor switching elements during DC link capacitor discharge is lower than the gate current used for normal operation of the converter, for example, when the converter is used to drive an electric machine.

[0021] According to one embodiment, the second semiconductor switching elements in the multiple half-bridges are cyclically activated one after the other. According to an alternative embodiment, the second semiconductor switching elements in the multiple half-bridges can also all be activated simultaneously, i.e., in parallel.

[0022] According to a further embodiment, the method includes a step for detecting an electric current in the half-bridges. Correspondingly, the method can include a step for opening a second semiconductor switching element if the detected electric current in the respective half-bridge exceeds a predetermined threshold. The detection of the electric current and the subsequent opening of the semiconductor switching element upon exceeding the predetermined threshold can, for example, be achieved using current monitoring already implemented in the driver circuits for the semiconductor switching elements. In this context, the threshold for response, i.e., the opening of the second semiconductor switching element, can be lowered compared to the standard value for operational use when discharging the DC link capacitor.

[0023] 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.

[0024] Brief description of the drawings

[0025] Further features and advantages of the invention are explained below with reference to the figures. Figure 1 shows a schematic representation of an electrical power converter according to one embodiment;

[0026] Fig. 2: a schematic representation illustrating the switching states of an electrical power converter according to one embodiment; and

[0027] Fig. 3: a flowchart as it underlies a method for discharging an intermediate circuit capacitor according to one embodiment.

[0028] Description of embodiments

[0029] Figure 1 shows a schematic representation of a principle diagram for an electrical power converter according to one embodiment. Such an electrical power converter can be used, for example, to control an electric machine (not shown here) in an electric drive system. The electrical power converter comprises a DC voltage terminal 20 and an AC voltage terminal 30. The DC voltage terminal 20 can have a first terminal 21, for example a positive one, and a second terminal 22, for example a negative one. The AC voltage terminal 30 can have several terminals 31, 32, 33. Furthermore, the power converter comprises several half-bridges, each with two semiconductor switching elements M1a-M1c, M2a-M2c. Each half-bridge comprises a first semiconductor switching element M1a-M1c and a second

[0030] Semiconductor switching element M2a-M2c, wherein the first semiconductor switching element M1a-M1c and the second semiconductor switching element M2a-M2c are arranged in series. Thus, a first semiconductor switching element M1a-M1c is arranged between the first terminal 21 of the DC voltage terminal 20 and a node K of the corresponding half-bridge. A second semiconductor switching element M2a-M2c is arranged between the corresponding node K and the second terminal 22 of the DC voltage terminal 20. The nodes K of the half-bridges are each connected to a terminal 31, 32, 33 of the AC voltage terminal 30.

[0031] A DC network or DC voltage source can be connected between the connection points 21, 22 of the DC terminal 20. This DC network can, in particular, include a so-called intermediate circuit capacitor 40. This intermediate circuit capacitor 40 can also be a capacitor arrangement with more than one capacitor. Furthermore, an electrical energy storage device 41, for example, the traction battery of an electric vehicle, can also be connected to the DC network connected to the DC terminal 20. This electrical energy storage device 41 can, for example, be connected to the DC terminal 20 via a suitable switching device 42.

[0032] For example, an electric machine (not shown) can be connected to AC terminal 30. The electrical converter can then use the DC voltage provided at DC terminal 20 to generate an AC voltage suitable for driving the electric machine connected to AC terminal 30. However, any other functionalities of the electrical converter are also possible.

[0033] For the operation of the electrical power converter, the semiconductor switching elements M1a-M1c and M2a-M2c are controlled in a suitable manner. For this purpose, corresponding driver circuits 11ac and 12a-c can be provided for each semiconductor switching element M1a-M1c and M2a-M2c, such that each driver circuit 11ac or 12a-c can provide a control signal at a control terminal of a corresponding semiconductor switching element M1a-M1c or M2a-M2c. The control can be voltage- or current-controlled. For example, the corresponding driver circuit 11 ac, 12a-c can provide an electric current or a current profile (for example, a defined curve or sequence for a current) at the respective control terminal of the corresponding control connection of the respective semiconductor switching element M1a-M1 c, M2a-M2c to close the switching element M1 a-M1 c, M2a-M2c.For the sake of simplicity and clarity, Figure 1 shows only one driver circuit 11ac, 12a-c for each of the upper semiconductor switching elements M1a-M1c and the lower semiconductor switching elements M2a-M2c. However, it is understood that each semiconductor switching element M1a-M1c, M2a-M2c has its own corresponding driver circuit 11ac, 12a-c.

[0034] To implement a desired control concept, suitable control signals can be provided to the driver circuits 11 ac, 12a-c by a control unit 10. These control signals can indicate to the individual driver circuits 11 ac, 12a-c whether the corresponding semiconductor switching element M1a-M1 c, M2a-M2c should be opened or closed.

[0035] Optionally, current sensors 13 can be provided in the paths of the individual half-bridges. These current sensors 13 can monitor an electric current in the respective half-bridge and provide a sensor signal corresponding to the detected current to the driver circuits 11 ac, 12a-c and / or the control unit 10. This allows, for example, the implementation of a protection function in which, if an electric current exceeds a predefined threshold, a shutdown occurs or at least the semiconductor switching elements M1 a-M1 c, M2a-M2c of the corresponding half-bridge are opened. Furthermore, temperature sensors 14 can also be provided in the half-bridges, in particular at the semiconductor switching elements M1 a-M1 c, M2a-M2c, to detect a temperature at the respective position. This temperature can also be provided to the driver circuits 11 ac, 12a-c and / or the control unit 10.In this way, the control of the semiconductor switching elements M1 a-M1 c, M2a-M2c can be adjusted according to the currently detected temperature, if necessary a power reduction can be carried out if a temperature threshold is exceeded or possibly a complete shutdown of the electrical power converter can be carried out above a temperature threshold.

[0036] In addition to the conventional control of the semiconductor switching elements M1a-M1c, M2a-M2c, which is considered known here and is therefore not explained in detail, the power converter according to the invention provides a further operating mode which is suitable for actively discharging the intermediate circuit capacitor 40 by means of suitable control of the semiconductor switching elements M1a-M1c, M2a-M2c, without the need for any further components.

[0037] The following explains this operating mode for discharging the DC link capacitor 40. In the described embodiment, the lower semiconductor switching elements M2a-M2c are permanently closed at the second terminal of the DC voltage connection 20, and the upper semiconductor switching elements M1a-M1c are pulsed at the first terminal of the DC voltage connection. It should be understood, however, that a reverse operation is also possible, in which the upper semiconductor switching elements M1a-M1c are permanently closed and the lower semiconductor switching elements M2a-M2c are pulsed. It is also possible to switch between these two operating strategies for discharging the DC link capacitor 40.For example, during the first discharge of the DC link capacitor 40, the lower switching elements M2a-M2c can be closed and the upper switching elements M1a-M1c can be clocked, and during a subsequent discharge, the upper switching elements M1a-M1c can be permanently closed while the lower switching elements M2a-M2c are clocked. Furthermore, any other strategies for switching between the two operating strategies are also possible.

[0038] Figure 2 shows a schematic representation of a principle diagram to illustrate the concept according to the invention for discharging a DC link capacitor 40 by means of an electrical power converter according to one embodiment. The circuit arrangement corresponds to the circuit arrangement previously described in connection with Figure 1. It should be noted again that in this embodiment the concept is described based on closed lower semiconductor switching elements M2a-M2c and switched upper semiconductor switching elements M1a-M1c, but a complementary operating strategy is also possible in principle.

[0039] To discharge the intermediate circuit capacitor 40 at the DC terminal 20, the lower semiconductor switching elements M2a-M2c are first permanently closed. This creates an electrical connection between the terminals 31, 32, 33 at the AC terminal 30. This corresponds to an active short circuit of an electric machine connected to the AC terminal 30. The control unit 10 can then cause the driver circuits 11ac for the upper semiconductor switching elements M1a-M1c to periodically apply short drive pulses to these elements. These drive pulses can, in particular, be pulses with a constant current during the pulse duration. However, this current can be lower than the current used in conventional operation to fully open the semiconductor switching elements M1a-M1c.In this way, the electric current through the respective semiconductor switching elements M1 a-M1 c can be limited during the pulse duration. In principle, it is possible, for example, to simultaneously apply the periodic pulses to all upper semiconductor switching elements M1 a-M1 c in parallel. Alternatively, it is also conceivable to apply the pulses to the upper semiconductor switching elements M1 a-M1 c cyclically, that is, alternately and sequentially, so that each of the upper semiconductor switching elements M1 a-M1 c becomes electrically conductive for the predetermined pulse duration.

[0040] The pulse duration for controlling the upper semiconductor switching elements M1a-M1c can, for example, be kept constant throughout the entire discharge process. In principle, it is possible to use the same pulse duration for all upper semiconductor switching elements (and alternatively, also for all lower semiconductor switching elements). Alternatively, it is also possible to define an individual pulse duration for each semiconductor switching element M1a-M1c (or alternatively for lower semiconductor switching elements M2a-M2c). In this way, component tolerances can be compensated for, for example, so that at least approximately the same amount of energy is dissipated at each semiconductor switching element M1a-M1c, M2a-M2c with each pulse. The information about the pulse duration can, for example, be stored in a suitable memory 10a of the control unit 10.

[0041] Alternatively, the pulse duration for driving the upper semiconductor switching elements M1a-M1c during the discharge process can be dynamically adjusted. For example, the pulse duration can be adjusted based on the current voltage across the DC link capacitor and thus at the DC voltage terminal 20. In this way, for instance, by increasing the pulse duration as the DC link voltage decreases, at least approximately the same amount of electrical energy can be dissipated for each pulse. Furthermore, it is also possible, for example, to monitor the temperature of one or more semiconductor switching elements M1a-M1c, M2a-M2c and to use this temperature to adjust the pulse duration. This allows, for example, the consideration of temperature-dependent properties of the semiconductor switching elements M1a-M1c, M2a-M2c.Similarly, for example, excessive heating of the semiconductor switching elements M1a-M1c, M2a-M2c can be detected and appropriate measures can be taken to prevent damage to the semiconductor switching elements Mia-M1c, M2a-M2c.

[0042] The relationships or characteristic curves for adjusting the pulse duration as a function of the DC link voltage and / or temperature can, for example, be stored as a formulaic preset in memory 10a of the control unit 10. Alternatively, it is also possible to store data from suitable characteristic curves or similar in the form of a lookup table or in another suitable format in memory 10a. If necessary, further values ​​can also be interpolated between the support points stored in a table.

[0043] Optionally, the electrical power converter can also include current monitoring in the current paths of the half-bridges. For example, current sensors 13 can detect an electric current in the half-bridges and provide it to the driver circuits 11 ac, 12a-c and / or the control unit 10. For example, such current monitoring can also be provided in conventional electrical power converters to initiate a shutdown of the control signal for the semiconductor switching elements M1 a-M1 c, M2a-M2c when a threshold value is exceeded by the driver circuits 11 ac, 12a-c.

[0044] If such a shutdown upon exceeding a predetermined current value is already implemented, and in particular if the threshold for initiating the shutdown can be adjusted, then the threshold for switching off the control signals of the semiconductor switching elements M1a-M1c and M2a-M2c can be lowered during the discharge of the DC link capacitor 40. This provides additional protection through shutdown based on the monitored current in the half-bridges. Furthermore, with such monitoring of the electric current in the half-bridges for discharging the DC link capacitor 40, a comparatively longer pulse duration can also be selected. If, during such a pulse duration, the electric current in a half-bridge rises above the (lowered) threshold of the current monitoring, an automatic shutdown occurs in this case by the corresponding driver circuit 11ac and / or the control unit 10.This allows for the selection of a longer pulse duration for fixed pulses, ensuring that even with decreasing DC link voltage, the maximum possible amount of energy per pulse can be extracted from the DC link capacitor. Alternatively, for example, with dynamic pulse length adjustment using the current DC link voltage and / or a temperature at the semiconductor switching elements M1a-M1c, M2a-M2c, the characteristic curve with fewer data points can also be specified. In this case, the additional current monitoring ensures that the electrical current during DC link capacitor discharge in the half-bridges always remains within the values ​​specified by the automatic shutdown threshold.

[0045] Figure 3 shows a flowchart illustrating a method for discharging a DC link capacitor using an electrical converter described above. The method can, in principle, include any steps previously described in connection with the electrical converter. Alternatively, the electrical converter described above can also include any components suitable for carrying out the method described below.

[0046] If the intermediate circuit capacitor 40 at the input terminal 20 of an electrical converter is to be discharged, then in a first step S1 all lower semiconductor switching elements M2a-M2c (or alternatively all upper semiconductor switching elements M1a-M1c) are closed. Then, in step S2, the upper semiconductor switching elements M1a-M1c (or alternatively all lower semiconductor switching elements M2a-M2c) are periodically activated. This activation is performed, in particular, with pulses of a predetermined pulse duration. Furthermore, a fixed, constant electric current can be supplied to the respective semiconductor switching elements M1a-M1c for this activation.

[0047] In this way, a certain amount of energy is extracted from the intermediate circuit capacitor 40 with each pulse, thus discharging the intermediate circuit capacitor 40 below a predetermined threshold. The process can be continued until a given termination or end condition is met, for example, until the intermediate circuit voltage falls below a predetermined threshold or a predetermined time interval has elapsed.

[0048] Pulses with the same duration can be used to control the upper semiconductor switching elements M1a-M1c throughout the entire discharge process. Alternatively, the pulse duration can be dynamically adjusted during the discharge process. For example, the pulse duration can be adjusted depending on the voltage at the intermediate circuit capacitor (and thus at the DC terminal 20) and / or the temperature at the semiconductor switching elements M1a-M1c and M2a-M2c.

[0049] In summary, the present invention relates to a concept for discharging a DC link capacitor in an electrical converter by the electrical converter itself. Here, the switching elements of all half-bridges in the electrical converter are used to discharge the DC link capacitor. For this purpose, all upper or lower semiconductor switching elements of the electrical converter are first closed, and then the complementary semiconductor switching elements are periodically activated with predetermined pulses. Thus, energy is extracted from the DC link capacitor with each pulse.

Claims

Claims 1. Method for discharging an intermediate circuit capacitor (40) at a DC voltage terminal (20) of an electrical converter with multiple half-bridges, wherein in each half-bridge a first semiconductor switching element (M1a-M1c) is arranged between a first connection point (21) of the DC voltage terminal (20) and a node (K) of the half-bridge and a second semiconductor switching element (M2a-M2c) is arranged between the node (K) of the half-bridge and a second connection point (22) of the DC voltage terminal (20), comprising the steps: Closing (S1) of the first semiconductor switching elements (M1 a-M1c) in the multiple half-bridges; periodic activation (S2) of the second semiconductor switching elements (M2a-M2c) in the half-bridges, wherein the second semiconductor switching elements (M2a-M2c) are each activated with pulses with a predetermined pulse duration.

2. Method according to claim 1, wherein the predetermined pulse duration for the periodic activation of the second semiconductor switching elements (M2a-M2c) during the discharge process is constant.

3. Method according to claim 1 or 2, wherein an individual predetermined pulse duration is specified for each half-bridge.

4. Method according to any one of claims 1 to 3, wherein the predetermined pulse duration for driving (S2) the second semiconductor switching elements (M2a-M2c) is adjusted using an electrical voltage at the DC voltage terminal (20) and / or a temperature of the second semiconductor switching elements (M2a-M2c) and / or the first semiconductor switching elements (M1a-M1c).

5. Method according to one of claims 1 to 4, wherein the control (S2) of the second semiconductor switching elements (M2a-M2c) for discharging the intermediate circuit capacitor (40) is carried out with a reduced gate current.

6. Method according to one of claims 1 to 5, wherein the second semiconductor switching elements (M2a-M2c) in the multiple half-bridges are cyclically controlled one after the other.

7. Method according to one of claims 1 to 5, wherein the second semiconductor switching elements (M2a-M2c) in the multiple half-bridges are controlled simultaneously.

8. Method according to any one of claims 1 to 7, comprising the steps Detection of an electric current in the half-bridges; and Opening of a second semiconductor switching element (M2a-M2c) if the detected electric current in the respective half-bridge exceeds a predetermined threshold.

9. Electrical power converter, comprising: a DC terminal (20) designed to be connected to an intermediate circuit capacitor (40); several half-bridges, wherein in each half-bridge a first semiconductor switching element (M1a-M1c) is arranged between a first connection point (21) of the DC terminal (20) and a node (K) of the half-bridge, and a second semiconductor switching element (M2a-M2c) is arranged between the node (K) of the half-bridge and a second connection point (22) of the DC terminal (20); and a control device (10) designed to close the first semiconductor switching elements (M1a-M1c) in the multiple half-bridges, and subsequently periodically control the second semiconductor switching elements (M2a-M2c) in the half-bridges, wherein the second semiconductor switching elements (M2a-M2c) are each controlled with pulses with a predetermined pulse duration.

10. Electric drive system comprising: an electric converter according to claim 9; an electric machine electrically coupled to the nodes of the electric converter; an intermediate circuit capacitor (40) electrically coupled to the DC voltage connection (20).

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