Control device for an inverter, inverter for an asynchronous machine, vehicle and method for operating an inverter

1. Control device for an inverter which has a DC voltage input and a power unit with three half-bridges each being formed by two power switching elements, the control device being arranged to drive the power switching elements in a normal operating mode for converting a DC voltage applied to the DC voltage input into a polyphase AC current provided at an AC current output,

wherein the control device is adapted to evaluate a signal state of a signal indicating a disconnection of a DC voltage source from the DC voltage input, and to control the power switching elements as a function of a result of evaluation for alternately adopting a first switching pattern which effects DC braking and a second switching pattern which effects freewheeling, and

the first switching pattern includes, in one of three half-bridges, one power switching element connected to a first potential of the DC voltage input is in a blocking state and another power switching element is in a conducting state; in another of the three half-bridges, one power switching element connected to a second potential of the DC voltage input is in a blocking state and another power switching element is in a conducting state; and in another of the three half-bridges, both power switching elements are in a blocking state.

2. Control device according to claim 1 , which is further arranged to evaluate, in addition to the signal state, whether the inverter is in a recuperation mode.

3. Control device according to claim 1 , which is further arranged to switch from the first switching pattern to the second switching pattern after a predetermined period of time has elapsed and/or to switch from the second switching pattern to the first switching pattern after a predetermined period of time has elapsed.

4. Control device according to claim 1 , which is further adapted to control the alternate driving in dependence on a voltage value describing a voltage at the DC voltage input.

5. Control device according to claim 4 , which is further adapted to switch from the first switching pattern to the second switching pattern upon detection of reaching of a voltage threshold value by the voltage value and/or to switch from the second switching pattern to the first switching pattern upon detection of reaching of a voltage threshold value by the voltage value.

6. Control device according to claim 4 , which is further adapted to permanently drive the power switching elements to adopt a switching pattern effecting a safe operating condition when the voltage value does not exceed a predetermined voltage threshold when driving the power switching elements with the second switching pattern.

7. Control device according to claim 6 , wherein the switching pattern causing the safe operating condition is the second switching pattern.

8. Inverter for an asynchronous machine, comprising a DC input, a power unit with three half-bridges formed by two power switching elements each, and the control device according to claim 1 .

9. Vehicle comprising the asynchronous machine for driving the vehicle and the inverter according to claim 8 .

10. Control device for an inverter which has a DC voltage input and a power unit with three half-bridges each being formed by two power switching elements, the control device being arranged to drive the power switching elements in a normal operating mode for converting a DC voltage applied to the DC voltage input into a polyphase AC current provided at an AC current output,

wherein the control device is adapted to evaluate a signal state of a signal indicating a disconnection of a DC voltage source from the DC voltage input, and to control the power switching elements as a function of a result of evaluation for alternately adopting a first switching pattern which effects DC braking and a second switching pattern which effects freewheeling, and

to determine the first switching pattern in dependence on at least one current value describing a current flowing at the AC voltage output, and

wherein alternate switching between the first switching pattern and the second switching pattern is controlled for a predetermined period of time in accordance with the first switching pattern or the second switching pattern, the alternate switching being terminated when the DC voltage does not exceed a predetermined voltage threshold during switching of the second switching pattern.

11. Control device according to claim 10 , wherein the first switching pattern describes switching a power switching element connected to a high potential of the DC input to a conductive state when a current from a center tap of the half-bridge formed by the power switching element is positive and a change of the current after time is negative.

12. Control device according to claim 11 , wherein the switching pattern describes the switching of the remaining power switching elements connected to the high potential of the DC voltage input to a blocking state.

13. Control device according to claim 10 , wherein the first switching pattern describes switching a power switching element connected to low potential of the DC input to a conductive state when a current from a center tap of the half-bridge formed by the power switching element is negative and a change in the current after time is positive.

14. Control device according to claim 13 , wherein the first switching pattern describes switching the remaining power switching elements connected to the low potential of the DC input to a blocking state.

15. Method of operating an inverter which has a DC input and a power unit having three half-bridges formed by two power switching elements each, comprising the following steps:

driving the power switching elements in a normal operating mode to convert a DC voltage applied to the DC input into a polyphase AC current provided at an AC output;

evaluating a signal state of a signal indicating a disconnection of a DC voltage source from the DC voltage input; and

driving the power switching elements in dependence on a result of evaluation to alternately adopt a first switching pattern that causes DC braking and a second switching pattern that causes freewheeling,

wherein the first switching pattern includes, in one of three half-bridges, one power switching element connected to a first potential of the DC voltage input is in a blocking state and another power switching element is in a conducting state; in another of the three half-bridges, one power switching element connected to a second potential of the DC voltage input is in a blocking state and another power switching element is in a conducting state; and in another of the three half-bridges, both power switching elements are in a blocking state.