High-voltage network and method for actively discharging an intermediate circuit capacitor

Abstract

The invention relates to a high-voltage network (1) comprising a high-voltage battery (2), an inverter (6) with half-bridges (H1-H3), at least one intermediate circuit capacitor (4), at least one active discharge circuit (5) for the intermediate circuit capacitor (4) and at least one control unit (8) for controlling the active discharge circuit (5), wherein the control unit (8) is designed such that, upon detection of a request signal, the active discharge circuit (5) is activated until the voltage at the intermediate circuit capacitor (4) has fallen below a first threshold value, wherein the control unit (8) is designed such that, after the first threshold value has been fallen below, at least one half-bridge (H1-H3) is switched into the active short circuit, and to a method for actively discharging an intermediate circuit capacitor (4).

Description

[0001] The invention relates to a high-voltage network and a method for actively discharging an intermediate circuit capacitor.

[0002] A typical high-voltage network is, for example, the traction network of an electric vehicle. However, such high-voltage networks can also be used for other purposes, such as in solar systems or other auxiliary systems of an electric vehicle.

[0003] In certain situations, a DC link capacitor must be actively discharged relatively quickly to ensure touch protection, i.e., the residual voltage remaining after the specified time must be less than 60 V. One such situation is a crash, for example, in which the high-voltage battery is disconnected from the traction network via contactors or other switching elements, and the DC link capacitor is discharged to below 60 V via an active discharge circuit. The active discharge circuit is then deactivated.

[0004] Various approaches to designing an active discharge circuit are known. One approach involves connecting at least one transistor in series with an ohmic resistor between the HV+ and HF- lines. The transistor is blocked during normal operation and is switched on for active discharge, with the discharge then occurring via the resistor that limits the current. Another approach involves using at least one half-bridge of an inverter as an active discharge circuit. In this case, one switching element is permanently switched on and the other switching element is controlled in linear or pulsed mode to limit the discharge current. The switching elements are typically transistors.

[0005] Further active discharge circuits are known from DE 10 2020 132 571 B3 or US 2022 / 0393571 A1.

[0006] WO 2022 / 171385 A1 discloses a control device for a three-phase inverter of a vehicle drive motor. The control device is configured to control an active discharge of an intermediate circuit capacitor of the inverter. For this purpose, the lower half-bridge switches of the inverter are switched on simultaneously during the active discharge, and one of the upper half-bridge switches of the inverter is switched on alternately. This continues until the voltage at the intermediate circuit capacitor falls below a threshold value.

[0007] DE 10 2020 216 327 B3 discloses a method for operating an inverter arranged between at least one vehicle battery and a drive motor of an electrically powered vehicle, wherein the inverter is connected to the vehicle battery via an intermediate circuit and the intermediate circuit has at least one storage device for electrical energy and a discharge circuit for rapid discharge of the intermediate circuit. In this case, a connection of the intermediate circuit to the vehicle battery is only permitted if the discharge circuit is not considered to be thermally loaded, and the discharge circuit is not considered to be thermally pre-loaded after a predetermined waiting time has elapsed. To initiate a rapid discharge, it is first checked whether the intermediate circuit is connected to the vehicle battery, and a rapid discharge is only carried out if there is no connection between the intermediate circuit and the vehicle battery.This is done by evaluating a current and / or voltage gradient at the beginning of the discharge.

[0008] The invention is based on the technical problem of creating a high-voltage network and providing a method for actively discharging an intermediate circuit capacitor in order to improve the process of active discharging and to ensure contact protection.

[0009] The solution to the technical problem is provided by a high-voltage network having the features of claim 1 and a method having the features of claim 7. Further advantageous embodiments of the invention emerge from the subclaims.

[0010] The high-voltage network comprises a high-voltage battery, an inverter with half-bridges, at least one intermediate circuit capacitor, at least one active discharge circuit for the intermediate circuit capacitor, and at least one control unit for controlling the active discharge circuit. The control unit is designed to activate the active discharge circuit upon detection of a request signal until the voltage at the intermediate circuit capacitor falls below a first threshold. The control unit is further designed such that, once the first threshold is undershot, at least one half-bridge is switched into the active short circuit. This allows the voltage at the intermediate circuit capacitor to be reduced to 0 V. This is a permanent short circuit that is maintained at least until the intermediate circuit capacitor is completely discharged or the switching elements can no longer be controlled.The active short circuit is preferably maintained permanently until a request is made to reconnect the high-voltage battery. The active discharge circuit can, in principle, be any active discharge circuit known from the state of the art.

[0011] In one embodiment, the control unit is configured such that all half-bridges are simultaneously switched into active short-circuit mode after the first threshold is exceeded. This distributes the short-circuit current among the three half-bridges, so that the load is evenly distributed among the three half-bridges.

[0012] In another embodiment, the active discharge circuit remains activated after the first threshold is undershot. Although the majority of the current flows through the active short circuit, a small portion also flows through the active discharge circuit, thus slightly reducing the load on the half-bridge(s). This applies to embodiments where the inverter's half-bridges do not form the active discharge circuit.

[0013] In a further embodiment, the control unit is designed to evaluate a current and / or voltage gradient at the intermediate circuit capacitor after detecting the request signal, wherein if the gradient is less than a second threshold value, the activated active discharge circuit is deactivated again or is not activated. This prevents the active discharge from not being carried out if the high-voltage battery is inadvertently not switched off or disconnected. When the contactors or other switching or isolating elements of the high-voltage battery are opened, the current or voltage at the intermediate circuit capacitor must no longer increase but must decrease. This can be checked before activating the discharge circuit. However, since feedback is also possible, the active discharge circuit is preferably activated at least briefly and the gradient evaluated.If the voltage does not drop significantly, the active discharge circuit is deactivated again.

[0014] In a further embodiment, gate drivers are assigned to the switching elements of the inverter, to which at least one backup capacitor is assigned, wherein the voltage supply to the gate drivers is provided via the high-voltage lines. This eliminates the need for a separate or redundant voltage supply from a low-voltage vehicle electrical system. The voltage supply has, for example, a DC / DC converter that reduces the high-voltage voltage to a suitable voltage of, for example, 40 V. If the voltage on the high-voltage side then drops below a threshold value of, for example, 35 V, the DC / DC converter can no longer ensure the voltage supply to the gate drivers. However, the backup capacitance then allows the switching elements to continue operating, so that the active discharge can continue until the voltage at the backup capacitor collapses.Therefore, the support capacitance is preferably dimensioned in such a way that, after the voltage on the intermediate circuit capacitor drops below the switch-off threshold, it can maintain the voltage for the gate drivers until the voltage on the intermediate circuit capacitor is zero.

[0015] In a further embodiment, a temperature and / or current sensor is assigned to the at least one half-bridge, wherein the control unit is configured to block the half-bridge or operate it in linear or pulsed mode when a threshold value for the temperature and / or current is exceeded in the active short circuit. This prevents damage to the half-bridge, and the temperature and / or current sensor is already present in most inverters, so that no additional components are required. Only the current threshold needs to be adjusted, since this must be selected to be lower for the continuous current in the active short circuit than the current in normal operation, where the half-bridge is controlled in a pulsed manner. For example, the current threshold is 100 A.It can also be provided that if one half-bridge is blocked due to temperature, another half-bridge is switched into active short-circuit mode, allowing the previously active half-bridge to cool down. Furthermore, it can also be provided that if initially only one half-bridge was in active short-circuit mode and the current threshold is reached, one or both of the other half-bridges are switched through to divide the current.

[0016] The method for actively discharging an intermediate circuit capacitor of a high-voltage network, wherein the high-voltage network has a high-voltage battery, an inverter with half-bridges, at least one intermediate circuit capacitor, at least one active discharge circuit for the intermediate circuit capacitor, at least one control unit for controlling the active discharge circuit and switching elements for isolating the high-voltage battery from the rest of the high-voltage network, comprises the following steps: a) Detection of a request signal by the control unit that the active discharge of the intermediate circuit capacitor should be carried out, b) Activation of the active discharge circuit by the control unit, c) Detecting the voltage at the DC link capacitor; if the voltage falls below a first threshold, at least one half-bridge of the inverter is switched into active short circuit. The active short circuit is permanently switched in such a way that it is maintained until the DC link capacitor is discharged or another termination condition is met.

[0017] With regard to further details, reference is made in full to the preceding statements.

[0018] A preferred field of application of the invention is a traction network of an electric vehicle.

[0019] The invention is explained in more detail below using preferred embodiments. The figures show: Fig. 1 a schematic traction network in a first embodiment, Fig. 2 a schematic traction network in a second embodiment and Fig. 3 a flowchart of a method for actively discharging an intermediate circuit capacitor.

[0020] In the Fig. 1 shows a highly schematic illustration of a high-voltage network 1 in the form of a traction network of an electric vehicle in a first embodiment. The high-voltage network 1 has a high-voltage battery 2, which is connected to the rest of the traction network 1 via contactors 3. The high-voltage network 1 also has at least one intermediate circuit capacitor 4. The intermediate circuit capacitor 4 can also consist of a series circuit of two capacitors. An active discharge circuit 5 is arranged parallel to the intermediate circuit capacitor 4. The active discharge circuit 5 has a branch consisting of a series circuit of a switching element in the form of a transistor T1 and an ohmic resistor R. The branch is parallel to the intermediate circuit capacitor 4. The high-voltage network 1 also has an inverter 6 with three half-bridges H1-H3, wherein two switching elements S1-S6, which are usually designed as transistors, are shown schematically in each half-bridge H1-H3.However, designs are also possible where four switching elements are used per half-bridge H1-H3, for example, to enable 3L operation. The center taps of the half-bridges H1-H3 are then connected to an electric motor (not shown). Finally, the high-voltage network 1 also has a voltage measuring device 7 and a control unit 8.

[0021] For example, in the event of a crash, the control unit 8 receives a request signal to actively discharge the intermediate circuit capacitor 4. This request signal is transmitted, for example, by a battery management control unit (not shown) or another higher-level control unit. The control unit 8 also receives confirmation (e.g., as a CAN message) from the battery management control unit that it has opened the contactors 3 or has otherwise disconnected the high-voltage battery 2 from the rest of the high-voltage network 1. In addition, the control unit 8 receives measured values ​​from the voltage measuring device 7. In a first step, the transistor T1 is switched on. The control unit 8 checks the voltage gradient at the intermediate circuit capacitor 4. If the voltage drops sufficiently quickly, i.e., if the gradient is greater than a threshold value, the control unit 8 concludes that the contactors 3 are actually open and continues the discharge process.Otherwise, the active discharge is aborted. If the voltage at the intermediate circuit capacitor 4 then reaches a first threshold, at least one half-bridge H1-H3, preferably all three half-bridges H1-H3, are switched into active short-circuit mode (S1-S6 permanently closed), so that the remaining voltage is reduced very quickly to 0 V. The active discharge circuit 5 can remain activated. The first threshold can be selected differently. The higher the first threshold, the faster the discharge is completed. However, the current limits of the switching elements S1-S6 must be observed, since the short-circuit currents through the half-bridges H1-H3 increase with a higher first threshold. The first threshold can also be selected to be higher than the contact voltage of 60 V, for example, 70 V. Preferably, however, the first threshold is set below the contact voltage and is, for example, 59 V.The advantage of this is that the active short circuit of half-bridges H1-H3 only occurs when the voltage is below the touch voltage, thus reducing safety requirements. Preferably, the active short circuit remains active until a request is received to reconnect high-voltage battery 2.

[0022] In Fig. Figure 2 shows an alternative embodiment of a high-voltage network 1, wherein the active discharge circuit 5 is fully integrated into the inverter 6. Current sensors 9 and temperature sensors 10 are also shown. The measured values ​​are also fed to the control unit 8. If the intermediate circuit capacitor 4 is to be actively discharged again, the contactors 3 are opened again. Then, at least one half-bridge H1-H3 is controlled in pulsed or linear mode, and the voltage gradient is initially evaluated again. Rotating pulsed mode, as in WO 2022 / 171385 A1, is also possible. If the voltage then reaches the first threshold again, at least one, preferably all, half-bridges H1-H3 are switched into the active short circuit, so that the voltage drops to 0 V.

[0023] If the voltage supply for the gate drivers of the switching elements S1-S6 is provided via the high-voltage side (e.g., via a DC / DC converter), at least one backup capacitor must be provided to ensure that the gate drivers can continue to switch the switching elements S1-S6 for a sufficient time to fully discharge the intermediate circuit capacitor 4. Otherwise, the active discharge will fail, and the intermediate circuit capacitor 4 will still have a small residual voltage, which is then dissipated by passive discharge.

[0024] Most of the current and temperature sensors 9, 10 already installed in inverters 6 can then be used for monitoring purposes to ensure that the switching elements S1-S6 are not overloaded. This requires only slight modifications to the software in the gate drivers. For example, an overcurrent threshold for the active short circuit must be lowered, since, in contrast to pulse-controlled inverter operation, a continuous current now flows in the active short circuit. If the current or temperature then exceeds a limit, the affected switching elements S1-S6 are blocked, or the system switches back to pulsed or linear operation.

[0025] In the Fig.3 shows a highly simplified flow diagram of the method. In a step ST1, the control unit 8 receives a request for active discharge and a message that the high-voltage battery 2 has been disconnected from the rest of the high-voltage network 1. In a second step ST2, the control unit 8 starts the active discharge. In a third step ST3, it is checked whether the gradient of the voltage at the intermediate circuit capacitor 4 is greater than a threshold value. If this is not the case, the active discharge is terminated in a fourth step ST4. Otherwise, the active discharge is continued (step ST5). In a sixth step ST6, the voltage at the intermediate circuit capacitor 4 is continuously measured and compared with a first threshold value. As long as the first threshold value has not yet been reached, the active discharge is continued (step ST5).If the first threshold is undershot, all half-bridges are simultaneously switched into active short-circuit in a seventh step ST7. List of reference symbols 1 high-voltage network 2 high-voltage batteries 3 contactor 4 DC link capacitor 5 Discharge circuit 6 inverters 7 Voltage measuring device 8 Control unit 9 Current sensor 10 Temperature sensor H1-H3 half bridges S1-S6 switching elements R resistance T1 transistor ST1-ST7 steps QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2020 132 571 B3

[0005] US 2022 / 0393571 A1

[0005] WO 2022 / 171385 A1 [0006, 0022] DE 10 2020 216 327 B3

[0007]

Claims

[1] High-voltage network (1), comprising a high-voltage battery (2), an inverter (6) with half-bridges (H1-H3), at least one intermediate circuit capacitor (4), at least one active discharge circuit (5) for the intermediate circuit capacitor (4) and at least one control unit (8) for controlling the active discharge circuit (5), wherein the control unit (8) is designed to activate the active discharge circuit (5) upon detection of a request signal until the voltage at the intermediate circuit capacitor (4) has fallen below a first threshold value, characterized by that the control unit (8) is designed such that after the first threshold value is undershot, at least one half-bridge (H1-H3) is switched into the active short circuit. [2] High-voltage network according to claim 1, characterized by that the control unit (8) is designed such that all half-bridges (H1-H3) are switched into the active short circuit. [3] High-voltage network according to claim 1 or 2, characterized by that the control unit (8) is designed in such a way that the active discharge circuit (5) remains activated after the threshold value is undershot. [4] High-voltage network according to one of the preceding claims, characterized by that the control unit (8) is designed such that, after detecting the request signal, it evaluates a current and / or voltage gradient at the intermediate circuit capacitor (4), wherein, if the gradient is less than a second threshold value, the activated active discharge circuit (5) is deactivated again or is not activated. [5] High-voltage network according to one of the preceding claims, characterized by that the switching elements (S1-S6) of the inverter (6) are assigned gate drivers to which at least one support capacitance is assigned, the voltage supply of the gate drivers being provided via the high-voltage lines. [6] High-voltage network according to claim 5, characterized bythat the at least one support capacitance is dimensioned such that, after the voltage at the intermediate circuit capacitor (4) drops below a switch-off threshold, it can maintain the voltage for the gate drivers until the voltage at the intermediate circuit capacitor (4) is zero. [7] Method for actively discharging an intermediate circuit capacitor (4) of a high-voltage network (1), wherein the high-voltage network (1) has a high-voltage battery (2), an inverter (6) with half-bridges (H1-H3), at least one intermediate circuit capacitor (4), at least one active discharge circuit (5) for the intermediate circuit capacitor (4), at least one control unit (8) for controlling the active discharge circuit (5) and switching elements for isolating the high-voltage battery (2) from the rest of the high-voltage network (1), comprising the following steps: a) detecting a request signal by the control unit (8) that the active discharge of the intermediate circuit capacitor (4) is to be carried out, b) Activation of the active discharge circuit (5) by the control unit (8), c) detecting the voltage at the intermediate circuit capacitor (4), wherein if a first threshold value is undershot, at least one half-bridge (H1-H3) of the inverter (6) is switched into the active short circuit. [8] Method according to claim 7, characterized by that all half-bridges (H1-H3) are switched into active short circuit. [9] Method according to claim 7 or 8, characterized by that after the first threshold value is undershot, the active discharge circuit (5) remains activated. [10] Method according to one of claims 7 to 9, characterized bythat the control unit (8) evaluates a current and / or voltage gradient after detecting the request signal, wherein if the gradient is less than a second threshold value, the activated active discharge circuit (5) is deactivated again or is not activated.

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