CONTROL DEVICE FOR AN INVERTER, INVERTER FOR A VEHICLE, VEHICLE AND METHOD OF OPERATING AN INVERTER

Abstract

The invention relates to a control device for an inverter which includes three half-bridges each having a first power switching element connected to a first DC voltage potential and a second power switching element connected to a second DC voltage potential. The control device is arranged for driving the power switching elements for converting a DC voltage present between the DC voltage potentials into a polyphase AC current in a normal operating mode and for transferring the inverter from the normal operating mode into a safe operating mode. The control device is further set up to alternately drive the power switching elements in the safe operating mode for switching single-phase active short circuits and for switching two-phase active short circuits.

Claims

1. A control device (2) for an inverter (1) comprising three half-bridges (9u, 9v, 9w) each having a first power switching element (11u, 11v, 11w) connected to a first DC voltage potential (10) and a second power switching element (13u, 13v, 13w) connected to a second DC voltage potential (12), wherein the control device (2) is arranged for driving the power switching elements (11u, 11v, 11w, 13u, 13v, 13w) to convert a DC voltage present between the DC voltage potentials (10, 12) into a polyphase AC current in a normal operating mode and for transferring the inverter (1) from the normal operating mode to a safe operating mode, characterized in that in that the control device (2) is further set up to drive the power switching elements (11u, 11v, 11w, 13u, 13v, 13w) alternately in the safe operating mode for switching single-phase active short circuits and for switching two-phase active short circuits.

2. The control device according to claim 1, which is further arranged to drive a respective first power switching element (11u, 11v, 11w) for conducting when switching a single-phase active short circuit and to drive respective two first power switching elements (11u, 11v, 11w) for conducting when switching a two-phase active short circuit.

3. The control device according to claim 2, which is further adapted not to drive the first power switching element (11u, 11v, 11w) driven for conduction when switching the single-phase active short circuit for conduction when switching the two-phase active short circuit.

4. The control device according to claim 1, which is further adapted to first drive the power switching elements (11u, 11v, 11w, 13u, 13v, 13w) to switch a single-phase active short circuit at the start of the safe operating mode.

5. The control means according to claim 4, which is further arranged to determine phase current values of the polyphase alternating current and to select a power switching element (11u, 11v, 11w, 13u, 13v, 13w) for the first single-phase active short circuit whose half-bridge (9u, 9v, 9w) carries the largest phase current in terms of magnitude when transferred to the safe operating mode.

6. The control device according to claim 1, which is further arranged to switch the single-phase active short circuits each for a first time period and the two-phase active short circuits each for a second time period different from the first time period.

7. The control device according to claim 1, wherein the inverter (1) comprises three further half bridges each having first power switching elements and having second power switching elements, wherein the control device (2) is further arranged to drive the power switching elements of the further half bridges in safe operating mode for switching single-phase active short circuits, when the power switching elements (11u, 11v, 11w, 13u, 13v, 13w) of the first half bridges (9u, 9v, 9w) are driven for switching the two-phase active short circuit and for switching two-phase active short circuits when the power switching elements (11u, 11v, 11w, 13u, 13v, 13w) of the first half bridges (9u, 9v, 9w) are driven for switching the single-phase active short circuit.

8. Inverter (1) for a vehicle (22), comprising three half-bridges (9u, 9v, 9w) each having a first power switching element (11u, 11v, 11w) connected to a first DC voltage potential (10) and having a second power switching element (13u, 13v, 13w) connected to a second DC voltage potential (12), and a control device (2) according to claim 1.

9. A vehicle (22) comprising an electric machine (8) adapted to drive the vehicle (22), and an inverter (1) according to claim 8 adapted to power the electric machine (8).

10. A method of operating an inverter (1) comprising three half-bridges (9u, 9v, 9w) each having a first power switching element (11u, 11v, 11w) connected to a first DC potential (10) and having a second power switching element (13u, 13v, 13w) connected to a second DC potential (12), comprising the steps of: Driving the power switching elements (11u, 11v, 11w, 13u, 13v, 13w) in a normal operating mode to convert a DC voltage present between the DC potentials (10, 12) into a polyphase AC current; Transferring the inverter (1) from the normal operating mode to a safe operating mode; and alternate switching of single-phase active short circuits and two-phase active short circuits by the power switching elements (11u, 11v, 11w, 13u, 13v, 13w) in safe operating mode.

Description

[0023] Further advantages and details of the present invention will be apparent from the embodiments described below and from the drawings. These are schematic representations and show:

[0024] FIG. 1 A circuit diagram of a first embodiment of the inverter according to the invention with an embodiment of the control device according to the invention;

[0025] FIG. 2 a pulse diagram over time for driving power switching elements of the inverter shown in FIG. 1;

[0026] FIG. 3 curves of phase currents and a torque during operation of the inverter shown in FIG. 1;

[0027] FIG. 4 a locus of phase currents in dq coordinates during operation of the inverter shown in FIG. 1;

[0028] FIG. 5 curves of phase currents and a torque during operation of a prior art inverter;

[0029] FIG. 6 a locus of space vector currents during operation of the prior art inverter; and

[0030] FIG. 7 a schematic sketch of an embodiment of the vehicle according to the invention.

[0031] FIG. 1 is a circuit diagram of an embodiment of an inverter 1 with an embodiment of a control device 2.

[0032] In addition, the inverter 1 comprises a DC voltage input 3, an AC voltage output 4, a power unit 5, and a DC link capacitor 6 connected in parallel with the DC voltage input 3. The inverter 1 converts a voltage U applied to the DC voltage input 3 and provided by a high-voltage battery 7 into a polyphase, in this case three-phase, AC current provided at its AC current output 4. An electric machine 8, here exemplarily in the form of a permanently excited synchronous machine, is connected to the AC output 4.

[0033] The power unit 5 comprises three half-bridges 9u, 9v, 9w, each formed by a series connection of first power switching elements 11u, 11v, 11w connected to a first DC voltage potential 10 of the DC voltage input 3 and second power switching elements 13u, 13v, 13w connected to a second DC voltage potential 12 of the DC voltage input 3. Exemplarily, in FIG. 1, the first DC potential 10 is the potential provided for connection to a negative terminal of the high-voltage battery 7, and the second DC potential 12 is the potential provided for connection to a positive terminal of the high-voltage battery 7. However, the potential provided for connection to the negative terminal and the power switching elements connected thereto may also be used as the second DC voltage potential and the second power switching elements, respectively, and the potential provided for connection to the positive terminal and the power switching elements connected thereto may be used as the first DC voltage potential and the first power switching elements, respectively, without any further modifications or restrictions.

[0034] Each power switching element 11u, 11v, 11w, 13u, 13v, 13w comprises an insulated gate bipolar transistor (IGBT) 14 and a freewheeling diode 15 connected in parallel therewith. Alternatively, a respective power switching element 11u, 11v, 11w, 13u, 13v, 13w may be implemented by a power MOSFET. A center attack 16 of a respective half-bridge 11u, 11v, 11w is connected to the AC output 4 at which phase currents I.sub.u, I.sub.v, I.sub.w of the polyphase AC current are provided to the electric machine 8.

[0035] The control device 2 is arranged to control the power switching elements 11u, 11v, 11w, 13u, 13v, 13w in a normal operating mode for converting the DC voltage U applied to the DC voltage input 3 into the polyphase AC current applied to the AC current output 4. For driving, the control device 2 is connected to a control input 17 of a respective power switching element 11u, 11v, 11w, 13u, 13v, 13w.

[0036] When a fault condition is detected by an external control device 18, the transfer of the inverter 1 from the normal operation mode to a safe operation mode is initiated. The control device is arranged to alternately trigger the power switching elements 11u, 11v, 11w, 13u, 13v, 13w in the safe operating mode for switching single-phase active short circuits and for switching two-phase active short circuits. The control device applies this switching strategy as soon as it receives a signal 19 indicating the transfer to the safe operating mode from the control device 18.

[0037] A single-phase active short circuit is generally characterized by a first power switching element 11u, 11v, 11w or a second power switching element 13u, 13v, 13w being driven to conduct, while all other power switching elements 11u, 11v, 11w, 13u, 13v, 13w are driven to block. In contrast, in a two-phase active short circuit, generally two first power switching elements 11u, 11v, 11w or two second power switching elements 13u, 13v, 13w are driven to conduct, while all remaining power switching elements 11u, 11v, 11w, 13u, 13v, 13w are driven to block.

[0038] FIG. 2 is a pulse diagram over time t for driving the power switching elements 11u, 11v, 11w, 13u, 13v, 13w of the inverter 1. Here, a pulse waveform 20u is assigned to the first power switching element 11u, a pulse waveform 20v is assigned to the first power switching element 11v, and a pulse waveform 20w is assigned to the first power switching element 11w. Similarly, a pulse waveform 21u is associated with the second power switching element 13u, a pulse waveform 20v is associated with the second power switching element 21v, and a pulse waveform 21w is associated with the second power switching element 13w.

[0039] At a time t.sub.0, the control device 2 receives the signal 19 and then terminates the normal operating mode shown for times t<t.sub.0. The control unit 2 first determines which phase current I.sub.u, I.sub.v, I.sub.w is the largest in terms of magnitude at time t.sub.0 on the basis of setpoint values specified for the normal operating mode. In the present case, this is the phase current I.sub.w (cf. FIG. 3). By means of the half-bridge 9w assigned to this phase current, a single-phase active short-circuit is first switched for a first period of time between the time t.sub.0 and a time t.sub.1. For this purpose, the control device 2 controls the first power switching element 11w for conducting and the other power switching elements 11u, 11v, 13u, 13v, 13w for blocking.

[0040] Then, for a second period of time between the time t.sub.1 and a time t.sub.2, the control device 2 controls the other two first power switching elements 11u, 11v to conduct and the remaining power switching elements 11w, 13u, 13v, 13b to block. This pulse sequence continues periodically after time t.sub.2.

[0041] FIG. 3 shows the curves of the phase currents I.sub.u, I.sub.v, I.sub.w and a torque M of the electric machine 8 over time t, whereby the time axes in FIG. 3 are compressed by a factor of 10 compared to those in FIG. 2. FIG. 2 thus shows the pulse diagram over a duration of approx. 1 ms, whereas FIG. 3 shows the curves over a duration of approx. 10 ms. The current and torque values shown result from a purely exemplary configuration.

[0042] It is obvious that the switching strategy described above leads to a rapid decay of the phase currents I.sub.u, I.sub.v, I.sub.w, whereby harmful current peaks are avoided. It can also be seen from the torque M curve that the torque M is rapidly reduced to a value of around 0 Nm from time t.sub.0 onwards and only insignificant braking torques occur.

[0043] FIG. 4 is a locus of space vector currents .sub.Id, I.sub.q resulting from a dq transformation of phase currents I.sub.u, I.sub.v, I.sub.w. Obviously, the space vector currents I.sub.d, I.sub.q are guided on a very direct path close to the zero vector to realize the safe state.

[0044] For comparison, FIG. 5 shows curves of the phase currents I.sub.u, I.sub.v, I.sub.w and the torque M over time t, and FIG. 6 shows a locus curve of the space vector currents I.sub.d, I.sub.q in dq coordinates if a complete, i.e. triple, active short circuit is switched instead of alternating between the single-phase active short circuit and the two-phase active short circuit as known in the prior art. Obviously, this results in considerable overshoots of the phase currents I.sub.u, I.sub.v, I.sub.w and undesirable torque changes. The locus curve also shows that the space vector currents I.sub.d, I.sub.q only approach a steady state with a q component close to zero in a damped-oscillating manner.

[0045] While in the previously described embodiment example the time periods during which the single-phase active short circuit or the two-phase active short circuit is switched were substantially equal in length, it is possible in other embodiment example that the ratio of the time periods differs therefrom, for example a ratio of 60:40 is selected.

[0046] According to another embodiment, the inverter 1 shown in FIG. 1 has a total of six half-bridges for providing a six-phase alternating current for the electric machine 8. In this case, the first three half-bridges 9u, 9v, 9w are controlled in the safe operating mode as previously described and the other three half-bridges (not shown) are controlled differently in such a way that the two-phase active short circuit is switched first and then the single-phase active short circuit is switched. The switching strategy between the first half-bridges 9u, 9v, 9w and the other three half-bridges is therefore in opposite directions.

[0047] FIG. 7 is a schematic sketch of an embodiment of a vehicle 22 comprising, analogously to FIG. 1, an inverter 1 according to one of the embodiments described above, an electric machine 8, a high-voltage battery 7, and a control unit 18 which, as a higher-level control unit, provides a signal 19 for activating a safe operating mode.