Traction network and method for actively discharging an intermediate circuit capacitor of a traction network of an electric vehicle

Abstract

The invention relates to a traction network (1) of an electric vehicle, 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 leave the active discharge circuit (5) activated until the high-voltage battery (2) is to be switched on again or until the voltage at the intermediate circuit capacitor (4) falls below a first threshold value, wherein at least one half-bridge (H1-H3) of the inverter (6) is then switched through for an active short circuit until the high-voltage battery (2) is to be switched on again, and to a method.

Description

[0001] The invention relates to a traction network and a method for actively discharging an intermediate circuit capacitor of a traction network of an electric vehicle.

[0002] 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.

[0003] 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.

[0004] Further active discharge circuits are known from DE 10 2020 132 571 B3 or US 2022 / 0393571 A1.

[0005] The invention is based on the technical problem of creating a traction network and providing a method for actively discharging an intermediate circuit capacitor in order to improve the process of active discharging and the guarantee of contact protection.

[0006] The solution to the technical problem is provided by a traction network having the features of claim 1 and a method having the features of claim 5. Further advantageous embodiments of the invention emerge from the subclaims.

[0007] The traction network of an electric vehicle comprises a high-voltage battery, an inverter with half-bridges, at least one intermediate circuit capacitor, at least one active discharge circuit for the at least one intermediate circuit capacitor, and at least one control unit for controlling the active discharge circuit. The control unit is designed to leave the active discharge circuit activated until the high-voltage battery is to be reconnected. Alternatively, the control unit is designed to activate the active discharge circuit until the voltage at the intermediate circuit capacitor falls below a first threshold value, in which case at least one half-bridge of the inverter is switched on for an active short circuit until the high-voltage battery is to be reconnected. The active discharge circuit can be deactivated or remain activated during the active short circuit.This reliably prevents an intermediate circuit voltage above 60 V from building up again due to feedback (particularly from an electric machine). This is based on the knowledge that undesirable feedback can occur. If the active discharge circuit is then deactivated, it must then be reactivated, whereby the voltage on the intermediate circuit capacitor can build up again. This is now prevented according to the invention, since either the active discharge circuit itself or the active short circuit of at least one half-bridge of the inverter reliably prevents a voltage build-up. The first threshold value is preferably below 60 V. The first threshold value is preferably between 55 V and 30 V. The lower the first threshold value is selected, the lower the peak current that flows due to the active short circuit, so that the transistors of the half-bridges are less stressed.

[0008] For example, the active discharge circuit has at least one switching element in series with an ohmic resistor, with at least one further switching element connected in parallel to the series circuit. The switching element with the ohmic resistor is then initially switched on until a first threshold value is undershot, at which point the at least one further switching element is switched on to create an active short circuit. Several switching elements or transistors can also be connected in series so that the dielectric strength of the transistors does not have to be too high. Alternatively, the further switching element can be omitted, so that the active discharge circuit consists only of a series connection of switching element and resistor.

[0009] In a preferred alternative embodiment, the active discharge circuit is formed by at least one half-bridge of the inverter, wherein the control unit is configured to control the at least one half-bridge in linear or pulsed mode for active discharge until the first threshold is reached and then to connect the at least one half-bridge for an active short circuit. The advantage of this embodiment is that no additional components are required; only the software in the control unit is adapted.

[0010] In a further embodiment, the control unit is designed such that all half-bridges of the inverter are switched through for the active short circuit in order to divide the continuous current. It can also be provided that all half-bridges are controlled in linear or pulsed operation to reach the first threshold value. Furthermore, the half-bridges can be controlled alternately for both pulsed or linear operation and the active short circuit. This also distributes the load evenly across all half-bridges. It can also be provided that the method is carried out with one half-bridge, with this being saved, and when the method is to be carried out again, a different half-bridge is selected in order to symmetrize the load on the half-bridges.

[0011] In a further embodiment, a temperature and / or current sensor and / or an overcurrent / short-circuit detection device (e.g. DESAT - desaturation protection) is assigned to the at least one half-bridge, wherein the control unit is designed such that, if a threshold value for the temperature and / or current and / or voltage at the switching element is exceeded, the half-bridge is blocked or operated in linear or pulsed mode. This prevents damage to the half-bridge, and the temperature and / or current sensor and / or the overcurrent / short-circuit detection device is already present in most inverters, so that no further components are required. Only the threshold value for the current or voltage needs to be adjusted, since this threshold value must be selected to be lower for the continuous current in an 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 so that the previously active half-bridge can 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.

[0012] The method for actively discharging an intermediate circuit capacitor of a traction network of an electric vehicle, wherein the traction 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 and at least one control unit for controlling the active discharge circuit, comprises the following steps: a) Receiving a signal by the control unit that the intermediate circuit capacitor should be discharged, b1) Control of the active discharge circuit by the control unit, whereby the control remains active until the high-voltage battery is to be switched on again or b2) Controlling the active discharge circuit until the voltage at the intermediate circuit capacitor falls below a first threshold value, whereby the active discharge circuit is then deactivated and at least one half-bridge of the inverter is switched through for an active short circuit until the high-voltage battery is to be switched on again.

[0013] With regard to the further procedural details, reference is made in full to the preceding statements.

[0014] The invention is explained in more detail below using preferred embodiments. The figures show: Fig. 1 is a schematic block diagram of a traction network in a first embodiment, Fig. 2 a schematic block diagram of a traction network in a second embodiment and Fig. 3. a schematic flow diagram of a method for actively discharging an intermediate circuit capacitor.

[0015] In the Fig. Figure 1 shows a highly schematic diagram of a first embodiment of a traction network 1. The traction network 1 has a high-voltage battery 2, which is connected to the rest of the traction network 1 via contactors 3. The traction 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 in parallel with the intermediate circuit capacitor 4. The active discharge circuit 5 has a first branch, which consists of a series circuit of a switching element in the form of a transistor T1 and a nonreactive resistor R. A series circuit of two second transistors T2, T3 is arranged in a second branch. Both branches are parallel to the intermediate circuit capacitor 4.Furthermore, the traction network 1 has an inverter 6 with three half-bridges H1-H3, wherein two switching elements S1-S6 are schematically shown in each half-bridge H1-H3, which are usually designed as transistors. However, designs are also possible in which four switching elements are used per half-bridge H1-H3, for example to enable 3L operation. However, designs with a different number of half-bridges are also possible (e.g. six). The center taps of the half-bridges H1-H3 are then connected to an electric machine (not shown). Finally, the traction network 1 also has a voltage measuring device 7 and a control unit 8.

[0016] For example, in the event of a crash, the control unit 8 receives a command to actively discharge the intermediate circuit capacitor 4, whereby the control unit 8 or another control device also opens the contactors 3. The control unit 8 also receives the measured values ​​from the voltage measuring device 7. In a first step, the transistor T1 is then switched on, while the other transistors T2, T3 remain blocked. The intermediate circuit capacitor 4 is then discharged via the ohmic resistor R, whereby the ohmic resistor limits the current so that the transistor T1 is not damaged. If the voltage across the intermediate circuit capacitor 4 then falls below a first threshold value (e.g. 50 V), the other two transistors T2, T3 are switched on, resulting in an active short circuit and the intermediate circuit capacitor 4 being completely discharged very quickly. The transistor T1 can remain switched on or be blocked.The first threshold value is selected such that the short-circuit current flowing is not too high to damage the two transistors T2 and T3. If feedback occurs during discharge (e.g., from the electric motor), which recharges the intermediate circuit capacitor 4, this voltage is immediately reduced. If the high-voltage battery 2 is then to be reconnected, the control unit 8 deactivates the active discharge circuit 5 and blocks all transistors T1-T3. The contactors 3 are closed, with the intermediate circuit capacitor 4 being precharged beforehand via a precharging circuit (not shown).

[0017] In an alternative embodiment, the two transistors T2, T3 can be omitted, in which case, after reaching the first threshold value, at least one half-bridge H1-H3 is switched into the active short circuit (e.g. S1, S2 permanently closed).

[0018] In Fig. Figure 2 shows an alternative embodiment of a traction 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. In addition to or as an alternative to the current sensors 9, a total current sensor can also be used to detect the direct current flowing into the inverter. 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. Furthermore, at least one half-bridge H1-H3 is controlled in pulsed or linear mode. For example, the switching element S2 is permanently closed, and the switching element S1 is controlled in a pulsed manner to be blocked or switched on (in linear mode, slowly from blocked to switched on) so that the flowing current does not destroy the switching elements S1, S2.This continues until the voltage at the intermediate circuit capacitor 4 reaches a first threshold. Subsequently, the half-bridge or all half-bridges H1-H3 are switched into active short-circuit mode (all switching elements S1-S6 are permanently closed). This also effectively counteracts a voltage build-up at the intermediate circuit capacitor 4 due to feedback. The current and temperature sensors 9, 10, which are usually already installed in inverters 6, can then be used for monitoring purposes to ensure that the switching elements S1-S6 are not overloaded. For this purpose, the software in the gate driver modules only needs to be slightly modified. For example, an overcurrent threshold must be lowered, since, in contrast to pulse-controlled inverter operation, a continuous current now flows in the case of an active short-circuit.

[0019] It should be noted that if the voltage supply to the control unit 8 (including the gate drivers) is provided via a DC / DC converter from the high-voltage side, this can no longer supply the control unit 8 if the voltage at the intermediate circuit capacitor 4 drops below a certain voltage level (e.g., if the voltage is less than 20 V), resulting in the control unit 8 being switched off. If feedback then occurs, the voltage rises again, and the control unit 8 switches back on, allowing active discharging again.

[0020] To solve this problem, the power supply of the control unit 8 can also be taken over by an on-board battery.

[0021] In the Fig. 3 is a flowchart for a method of active discharge for an embodiment according to Fig.2. In a first step ST1, the control unit 8 receives the request for active discharge. In a second step ST2, at least one half-bridge H1-H3 is then operated in pulsed or linear mode. In a third step ST3, a check is made as to whether the voltage at the intermediate circuit capacitor 4 is less than a first threshold value. If this is not the case, step ST2 is continued. If, however, this is the case, in a fourth step ST4, one or all half-bridges H1-H3 are switched into active short circuit mode, and in a fifth step ST5, an overcurrent value for the gate drivers is adjusted (e.g., reduced to 100 A). The fifth step ST5 can also take place at the same time as or before the fourth step ST4. In a sixth step ST6, the active short circuit is then maintained. In a step ST7, a query is then made as to whether there is a request to reconnect the high-voltage battery 2.If this is denied, the active short circuit is maintained (step ST6). Otherwise, the active short circuit is terminated (step ST8) and the overcurrent value is reset to the values ​​for pulse-alternating operation (step ST9). List of reference symbols 1 traction 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 T1-T3 transistors R resistance ST1-ST6 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

[0004] US 2022 / 0393571 A1

[0004]

Claims

[1] Traction network (1) of an electric vehicle, 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), characterized by that the control unit (8) is designed in such a way that the active discharge circuit (5) remains activated until the high-voltage battery (2) is to be switched on again or until the voltage at the intermediate circuit capacitor (4) falls below a first threshold value, wherein at least one half-bridge (H1-H3) of the inverter (6) is then switched through for an active short circuit until the high-voltage battery (2) is to be switched on again. [2] Traction network according to claim 1, characterized bythat the active discharge circuit (5) is formed by at least one half-bridge (H1-H3) of the inverter (6), wherein the control unit (8) is designed to control the at least one half-bridge (H1-H3) in linear or pulsed operation for active discharge until the first threshold value is reached and then to switch the at least one half-bridge (H1-H3) through for an active short circuit. [3] Traction network according to one of the preceding claims, characterized by that the control unit (8) is designed such that all half-bridges (H1-H3) of the inverter (6) are switched through for the active short circuit. [4] Traction network according to one of the preceding claims, characterized bythat a temperature and / or a current sensor (9, 10) and / or an overcurrent / short-circuit detection device is assigned to the at least one half-bridge (H1-H3), wherein the control unit (8) is designed such that, when a threshold value for the temperature and / or the current and / or the voltage at the switching elements (S1-S6) is exceeded, the half-bridge (H1-H3) is blocked or operated in a linear or pulsed mode. [5] Method for actively discharging an intermediate circuit capacitor (4) of a traction network (1) of an electric vehicle, wherein the traction network (1) has a high-voltage battery (2), an inverter (6) with half-bridges (H1-H3), at least one intermediate circuit capacitor (4) and at least one control unit (8) for controlling the active discharge circuit, comprising the following steps: a) receiving a signal by the control unit (8) that the intermediate circuit capacitor (4) is to be discharged, b1) Control of the active discharge circuit (5) by the control unit (8), whereby the control remains active until the high-voltage battery (2) is to be switched on again, or b2) Activating the active discharge circuit (5) until the voltage at the intermediate circuit capacitor (4) falls below a first threshold value, wherein the active discharge circuit is then deactivated and at least one half-bridge (H1-H3) of the inverter (6) is switched through for an active short circuit until the high-voltage battery (2) is to be switched on again. [6] Method according to claim 5, characterized by that the active discharge circuit (5) is formed by at least one half-bridge (H1-H3) of the inverter (6), wherein the control unit (8) controls the at least one half-bridge (H1-H3) in linear or pulsed operation for active discharge until the first threshold value is reached and then switches the half-bridge (H1-H3) into the active short circuit. [7] Method according to claim 5 or 6, characterized by that all half-bridges (H1-H3) of the inverter (6) are switched through for the active short circuit. [8] Method according to one of claims 5 to 7, characterized by that a temperature and / or current sensor (9, 10) and / or an overcurrent / short-circuit detection device is assigned to the at least one half-bridge (H1-H3), wherein the control unit (8) blocks the half-bridge (H1-H3) or operates it in a linear or pulsed mode when a threshold value for the temperature and / or the current and / or the voltage is exceeded.

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