An inverter, its control unit and a method of operation

Abstract

A control circuit for controlling an inverter having a plurality of half - bridges, each half - bridge having a first switching element connected to one potential of a DC link and a second switching element connected to the other potential of the DC link, wherein a control unit is configured to provide a first signal to the first switching element such that the first switching element disconnects upon receiving a request signal, provide a second signal to the second switching element such that the second switching element turns on upon receiving the request signal, and perform a comparison of a measured electrical parameter of the inverter with a predetermined threshold, wherein the control unit is configured to, if a switching criterion is met, the switching criterion including the result of the performed comparison that the measured electrical parameter reaches or exceeds the threshold, provide the second signal such that the second switching element controlled by the second signal disconnects for a predetermined time span.

Description

[0001] FIELD OF THE INVENTION The present invention relates to a control unit for controlling an inverter having a plurality of half-bridges, each half-bridge having a first switching element connected to a potential of a DC link and a second switching element connected to another potential of the DC link, wherein the control unit is configured to provide a first signal to the first switching elements so that they are switched off upon receipt of a request signal, to provide a second signal to the second switching elements so that they are switched on upon receipt of a request signal, and to perform a comparison of a measured electrical parameter of the inverter with a predetermined threshold value. Furthermore, the present invention relates to an inverter for a vehicle and a method for operating an inverter.

[0002] BACKGROUND As is well known, an inverter converts a DC voltage supplied to a DC link into an AC output current for supplying a stator winding of a motor. The inverter includes a control unit configured to provide a signal to a switching element of the inverter so that an AC output current is generated in a normal operating mode. When a request signal (e.g., due to a fault) is received, the control unit switches to a fault mode. In the fault mode, a first switching element connected to the potential of the DC link is disconnected, and a switching element connected to another potential of the DC link is turned on, thereby shortening the stator winding. This switching configuration is generally referred to as an active short circuit. However, most voltage-controlled semiconductor switching elements have a normally-off characteristic. Therefore, in the event of a loss of power supply to a control unit or drive unit of the inverter, all switching elements are permanently disconnected. It is well known that a smoothing capacitor forming a DC link is used to supply power to a control unit or drive unit when power supply is lost. When the smoothing capacitor is discharged to or below a predetermined threshold, energy can be recovered from the motor to recharge the smoothing capacitor.

[0003] Such a control unit is known from EP 2433830 A1, which discloses a controller for a multiphase inverter, the controller comprising upper and lower switches and a smoothing capacitor, which is arranged in a DC link and connected to a three-phase synchronous motor for driving a vehicle. The controller is configured to control the inverter so that the electrical energy from the synchronous motor is recovered during a safe state caused by an active short circuit caused by closing all upper switches or all lower switches. Therein, by opening all switches of the inverter, the synchronous motor operates in a passive generator operation, wherein the smoothing capacitor is charged to a first threshold value of the DC link voltage. The electrical load of the vehicle is powered by the electrical energy of the smoothing capacitor. Above a higher second threshold value, the winding of the synchronous motor is short-circuited by the voltage of the DC link automatically closing all lower switches or all upper switches. After weakening the first threshold value, the active short circuit is released by automatically opening the closed switch. Summary of the invention

[0004] It is an object of the present invention to provide an improved control unit for an inverter.

[0005] According to the present invention, the above object is solved by the subject matter of claim 1. Embodiments of the present invention are defined by the dependent claims.

[0006] According to a first aspect of the present invention, there is provided a control unit as initially described, the control unit being configured to provide a second signal to cause a second switching element controlled by the second signal to open for a predetermined time span if a switching criterion is met, the criterion including the result of a comparison performed, namely the condition that a measured electrical parameter reaches or exceeds a threshold value.

[0007] The control unit according to the first aspect of the present invention avoids a second comparison of the measured electrical parameter for deriving the time span during which the second switching element is open. Instead, a fixed time span is provided. Thus, advantageously, a laborious comparison that requires fast analog measurement is omitted. In addition, the structure of the control unit is made less difficult.

[0008] The control unit according to the first aspect of the present invention may include a timing element configured to provide the time span. The time span may be selected such that the voltage rising during charging of a smoothing capacitor is limited by taking into account the maximum possible charging current and capacitance of the smoothing capacitor. The predefined time span may be selected between 3 μs and 10 μs.

[0009] The control unit may be configured to provide another second signal to cause another second switching element controlled by the additional second signal to open if the switching criterion is met. The control unit may be configured to provide all second signals to cause the other second switching elements to open if the switching criterion is met. The control unit may be configured to provide other second signals to cause a second switching element controlled by the other second signals to close if the switching criterion is met.

[0010] According to a second aspect of the present invention, there is provided a control unit as initially described, the control unit being configured to provide a second signal to cause a second switching element controlled by the second signal to open and to provide the second signal to another second switching element to cause the second switching element to close if a switching criterion is met, the criterion including the result of a comparison performed, namely the condition that a measured electrical parameter reaches or exceeds a threshold value.

[0011] The control unit according to the second aspect of the present invention avoids disconnecting all the second switching elements, which results in a safety pulse-off operation of the inverter, which is undesirable in view of automotive safety requirements. These requirements generally prohibit high braking torques at high rotational speeds, which can be effectively avoided by keeping at least one of the second switching elements turned on. It has been found that disconnecting the switching elements of one half-bridge is sufficient to supply the recuperation energy from the stator windings of the electric machine, supplied by the inverter, to the smoothing capacitor. Furthermore, in the event of a single-point fault where the second switching element controlled by the second signal is permanently disconnected, partial active short-circuiting is employed, which still meets the safety requirements.

[0012] The control unit according to the second aspect of the present invention can be configured to provide a further second signal such that the second switching element controlled by the further second signal is turned on if a switching criterion is met.

[0013] Furthermore, with regard to the control unit according to the second aspect of the present invention, the second switching element controlled by the second signal can be disconnected for a predetermined time span. Thus, a laborious comparison can also be omitted.

[0014] Preferably, the threshold is selected to be lower than the touch voltage permitted for the inverter. The touch voltage is typically defined by 60 V. The threshold is typically selected between 30 V and 50 V. Preferably, the control unit is configured to avoid the DC link voltage rising above the touch voltage. Wherein, the range between the touch voltage and the threshold voltage is wide enough to allow for practical tolerances. Typically, a request signal indicates that a fault has occurred. Once the request signal is received, the inverter and / or the control unit can switch from the normal operating mode to the fault mode. The request signal can be generated by the control unit, or can be obtained by another unit of the inverter or the controller of the vehicle including the inverter. In the normal operating mode of the inverter, the control unit can include a modulation section that is configured to provide a pulse signal such that the switching elements are turned on and off to provide an AC output current, wherein the control unit can be configured to inhibit the pulse signal upon receiving the request signal. Typically, the inverter includes three or more half-bridges. Advantageously, the two control units of the present invention operate at very low rotations and maintain the braking torque at high rotational speeds below the admissible limit.

[0015] Moreover, the control unit can be configured to provide the second signal such that the second switching element is turned on for a predetermined second time span after a first time span has elapsed. Thus, the second time span can also be provided by a timing element. The second time span can be less than or equal to one third of the reciprocal of the maximum electrical frequency that is the frequency of the AC current supplied to the given stator winding.

[0016] Preferably, the measured electrical parameter is the voltage of the DC link. Wherein, the condition may be the result of the comparison performed, i.e., the measured voltage reaches or drops below a threshold. Alternatively, the measured electrical parameter is a current, particularly the current flowing between the half-bridge and the DC link.

[0017] Furthermore, preferably, the control unit includes a control section, which is implemented discretely or by a complex programmable logic device (CPLD) or by a field-programmable gate array (FPGA) or by a microcontroller, and is configured to evaluate the switching criteria and perform a comparison based on a low-voltage analog signal representing the measured electrical parameter and provided to the control section. Implementing the control section discretely or by a CPLD or FPGA allows for fast and robust evaluation and comparison. In particular, the functions of the control unit can be implemented by logic gates. However, it is still possible to implement the functions by software loaded into the microcontroller, if automotive safety requirements permit.

[0018] In addition, the control unit may be configured to perform a second comparison of the measured electrical parameter with a threshold. Thus, the redundancy added to the evaluation of the switching criteria allows for a more robust operation of the inverter. In particular, a higher automotive safety integrity level (ASIL) can be achieved.

[0019] The control unit may include a comparator section for performing the second comparison, which is implemented separately from the control section and is configured to provide a digital signal representing the result of the second comparison. Thus, the redundancy is achieved by an additional hardware device. Preferably, the comparator section (exemplarily by a digital optocoupler) is electrically isolated from the control section. Additionally, the comparison may be based on independent measurements with different measuring devices.

[0020] The switching criteria may include an additional or alternative condition that the second comparison result, i.e., the measured electrical parameter reaches or exceeds, particularly drops below, a threshold.

[0021] Alternatively, the control unit is configured to provide an additional second signal to cause another switching element controlled by the additional second signal to open if the second switching criteria includes the condition that the second comparison results in the measured electrical parameter reaching or exceeding, particularly dropping below, a threshold. In other words, each comparison is assigned to a specific second switching element. This allows for controlling at least one second switching element if another second switching element fails. The control section may also be configured to evaluate the second switching criteria.

[0022] Preferably, one or more switching criteria include an additional condition: the absence of low-voltage power supply for the control unit and / or drive unit of the inverter. Thus, it can be ensured that the control strategy of the control unit according to the present invention occurs only when the smoothing capacitor needs to be recharged. Such a need may be caused by the loss of connection to the vehicle low-voltage battery, especially the connection to terminal 30 (Klemme 30 according to DIN 72552).

[0023] Furthermore, the control unit can be configured to directly provide a first signal when a request signal is received at the signal input port, and to provide a second signal after the delay element has delayed the request signal. This delay ensures that all first switching elements are disconnected before the second switching element is turned on after the request signal has been received. Thus, bridging short circuits can be effectively avoided.

[0024] For the normal operation of the inverter, the control unit can include a modulation section configured to provide a pulse signal such that the switching elements are turned on and off to provide an AC output current through the inverter, wherein the control unit can be configured to inhibit the pulse signal when a request signal is received.

[0025] According to a third aspect of the present invention, there is provided an inverter for a vehicle, including a smoothing capacitor forming a DC link, a plurality of half-bridges, each half-bridge having a first switching element connected to one potential of the DC link and a second switching element connected to the other potential of the DC link, wherein the inverter includes a control unit according to the present invention.

[0026] The inverter can include an interconnected DC / DC converter to convert the DC link voltage into a supply voltage for operating the control unit and / or drive part of the inverter. Generally, the DC / DC converter can provide the supply voltage within a wide range of isolated power supplies, for example, between 30V and 550V. Eventually, the DC / DC converter can supply power to the control unit and / or drive unit down to a vehicle speed of about 10 km / h, because the recovered energy is high enough to sufficiently recharge the smoothing capacitor. Below this speed, disconnecting all the switching elements generates a braking torque smaller than that of an active short circuit. Preferably, the inverter includes a discharging section configured to discharge the smoothing capacitor to a voltage below a threshold value, especially below the touch voltage, when a request signal is received.

[0027] According to a fourth aspect of the present invention, there is provided a method for operating an inverter having a plurality of half - bridges, each half - bridge having a first switching element connected to a potential of a DC link and a second switching element connected to another potential of the DC link, wherein a first signal is provided to the first switching elements such that they are turned off upon receipt of a request signal, a second signal is provided to the second switching elements such that they are turned on upon receipt of the request signal, and a measured electrical parameter of the inverter is compared with a predetermined threshold, wherein if a switching criterion is met, the criterion including the result of the performed comparison that the measured electrical parameter reaches or exceeds the threshold, a second signal is provided such that the second switching elements controlled by the second signal are turned off for a predetermined time span.

[0028] According to a fifth aspect of the present invention, there is provided a method for operating an inverter having a plurality of half - bridges, each half - bridge having a first switching element connected to a potential of a DC link and a second switching element connected to another potential of the DC link, wherein a first signal is provided to the first switching elements such that they are turned off upon receipt of a request signal, a second signal is provided to the second switching elements such that they are turned on upon receipt of the request signal, and a measured electrical parameter of the inverter is compared with a predetermined threshold, wherein if a switching criterion is met, the criterion including the result of the performed comparison that the measured electrical parameter reaches or exceeds the threshold, a second signal is provided such that the second switching elements controlled by the second signal are turned off, and the second signal is provided to another second switching element such that the second switching element is turned on.

[0029] All statements regarding the control unit of the present invention similarly apply to the inverter of the present invention and the method of the present invention, such that the above - mentioned advantages can also be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Further details and advantages of the present invention are disclosed hereinafter, with reference to schematic diagrams which illustrate the following:

[0031] Figure 1 Circuit diagram of a first embodiment of an inverter according to the present invention;

[0032] Figure 2 Block diagram of the control unit of the first embodiment;

[0033] Figure 3 Block diagram of the control unit of a second embodiment of an inverter according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0034] Figure 1It is a circuit diagram of the first embodiment of the inverter 1, which includes a power unit 2, a DC link 3, a control unit 4, a power supply unit 5, and a measurement unit 6. The inverter 1 converts the DC voltage provided by the high-voltage battery 7 and connected to the DC link 3 via the contactor 8 into a three-phase AC output current provided by the power unit 2, and the three-phase AC output current is connected to the stator winding 9 of the motor 10. The motor 10 is a permanent magnet synchronous motor of an electric vehicle.

[0035] The power unit 2 includes three half-bridges 11a-c, each half-bridge having a first switching element 12a-c connected to a first potential 13 which is the high potential of the DC link 3 and a second switching element 14a-c connected to a second potential 15 which is the low potential of the DC link 3. Each switching element 12a-c, 14a-c is formed by a transistor 16 and a freewheeling diode 17 connected in parallel. The transistor 16 is an insulated gate bipolar transistor. Alternatively, the transistor 16 can be a metal oxide semiconductor field effect transistor, transistor. Additionally, the transistor 16 can also be a metal oxide semiconductor field effect transistor, where the diode 17 is formed by the body diode of the transistor 16. The control terminal 18 of each switching element 12a-c, 14a-c is connected to a driver element 19, configured to amplify the signal provided by the control unit 4. The driver element 19 forms the drive unit of the inverter 1.

[0036] The power supply unit 5 obtains a supply voltage of, for example, 12V provided by the low-voltage battery 20 of the vehicle. In addition, the power supply unit 5 includes a DC / DC converter 21 connected to the DC link 3. The power supply unit 5 supplies the operating voltage to the control unit 4 and then to the driver unit, where, for simplicity, the wiring between the power supply unit 5 and the driver unit is not depicted. The output of the DC / DC converter 21 provides a voltage, for example, 0.5V lower than the voltage of the low-voltage battery 20. Due to the diodes 22, 23 interconnected between the output of the DC / DC converter 21 on the anode side and the low-voltage battery 20 and interconnected to the control unit 4 and the driver unit on the cathode side, the higher supply voltage of the low-voltage battery 20 dominates.

[0037] In addition, the measurement unit 6 includes a first voltage measurement device 24 and a second voltage measurement device 25, which are connected in parallel to the smoothing capacitor 26 forming the DC link 3. The voltages measured by the voltage measurement devices 24, 25 are provided to the control unit 4 as measured electrical parameters.

[0038] Figure 2 It is a block diagram of the control unit 4 of the inverter 1. The control unit 4 includes a modulation section 27, a control section 28, an analog isolation amplifier section 29, and a comparator section 30.

[0039] The modulation section 27 is configured to provide a pulse-width modulated pulse signal 31 such that the switching elements 12a-c, 14a-c are switched on and off to provide an AC output current via the inverter 1 in a normal, i.e., fault-free, operating mode. The modulation section is implemented by a microcontroller with software that determines the pulse signal 31. The pulse signal 31 is provided to the control section 28. The modulation section 27 is part of the low-voltage side of the inverter 1.

[0040] The analog insulation amplifier section 29 receives a raw measurement signal from a first voltage measuring device 24 that is part of the high-voltage side of the inverter 1 and provides an analog signal 32 representing the DC link voltage as the measured electrical parameter to the control section 28. The analog insulation amplifier section 29 forms an interface between the high-voltage side and the low-voltage side.

[0041] The comparator section 30 includes a comparator element 33 and a digital optocoupler 34 that forms an interface between the high-voltage side and the low-voltage side. The comparator element 33 receives a second measurement signal from a second voltage measuring device 25 and is part of the high-voltage side of the inverter 1.

[0042] The control section 28 is implemented discretely or via a complex programmable logic device or via a field-programmable gate array separately from the comparator section 30. The control section is part of the low-voltage side of the inverter 1. The control section 28 includes a first sub-section 35 that obtains the pulse signal 31 and the signal 36 from a second sub-section 37. In addition, the first sub-section 35 receives a request signal 38 requesting adoption of a safe state. The request signal 38 can be generated by the control unit 4 itself or can be received from another unit of the inverter 1 or from a controller of the vehicle.

[0043] In the normal operating mode of the inverter 1, the NOT gate 39 of the first sub-section 35 has a logic-zero input signal, which indicates that the request signal 38 has not been received. The output of the NOT gate 39 and the pulse signal 31 for controlling the first switching elements 12a-c are inputs to an AND gate 40 for each first switching element 12a-c, where, for simplicity, Figure 2 only one AND gate 40 is depicted. The first signal 41 output by each AND gate 40 corresponds to the respective pulse signal 31 because the NOT gate 39 provides a logic-1 output as long as the request signal 38 has not been received.

[0044] The pulse signal 31 for controlling the second switching element 14a is an input to the OR gate 42, which also has the request signal 38 delayed by the delay element 43 as an input. Thus, as long as the request signal 38 is not received and the delay time of the delay element 43 has not elapsed, the OR gate 43 outputs the pulse signal 31. The output of the OR gate 43 and the signal 36 are inputs to the AND gate 44. As will be described later, the normal operating mode of the signal 36 is logic 1. Thus, in normal operation, the second signal 45a output by the AND gate 44 corresponds to the pulse signal 31 for controlling the second switching element 14a.

[0045] The pulse signal 31 and the delay request signal for controlling the second switching elements 14b, 14c are inputs to the OR gate 46. Thus, as long as the request signal 38 is not received and the delay time of the delay element 43 has not elapsed, the OR gate 46 outputs the pulse signal 31. Thus, in the normal operating mode, the second signals 45b, 45c correspond to the pulse signals for controlling the second switching elements 14b, 14c.

[0046] The reception of the request signal 38 represented by logic 1, which is input to the delay element 43 and the NOT gate 39, is described in detail below:

[0047] The control unit 4 is configured to provide the first signal 41 to the first switching elements 12a-c such that they are turned off when the request signal 38 is received. Thus, when logic 1 is fed to the NOT gate 39, it outputs logic 0 causing the AND gate 40 to also output logic 0. Thus, the first signal 41 causes the driver element 19 to supply voltage to the control terminals 18 of the first switching elements 12a-c, which turns them off.

[0048] In addition, the control unit 4 is configured to provide the second signals 45a-c to the second switching elements 14a-c such that they are turned on when the request signal 38 is received. Regarding the second switching element 14a, the output of the OR gate 42 is logic 1 after the delay time has elapsed. Since the signal 36 remains logic 1, the output of the AND gate 44 is logic 1 causing the drive element 19 to supply voltage to the control terminal 18 of the second switching element 14a, which turns it on. Regarding the second switching element 14b, the output of the OR gate 46 is logic 1 after the delay time has elapsed. The output of the OR gate 46 causes the driver element 19 to supply voltage to the control terminals 18 of the second switching elements 14b, 14c, which turns them on.

[0049] By adopting the above switching configuration of the switching elements 12a-c, 14a-c, a fully active short circuit (ASC) as a safe state is achieved, in which the stator windings 9 of the motor 10 are short-circuited. Among them, the delay element 43 with a delay time of about 4 μs avoids the short circuit of the half-bridges 11a-c.

[0050] However, since the switching elements 12a-c, 14a-c are normally open, the voltage supplied by the power supply unit 5 is necessary to maintain the ASC. Therefore, in the event of the loss of the supply voltage of the low-voltage battery 20, an emergency voltage supply is achieved by the DC / DC converter 21 that obtains the high voltage of the DC link 3. However, due to safety measures, the contactor 8 may open for the same reason as the trigger request signal 38. Then, when the smoothing capacitor 26 discharges, the DC / DC converter 21 can no longer supply power to the control unit 4 and the drive unit. Therefore, the control unit 4 allows the smoothing capacitor to be periodically charged by the voltage recovered from the stator winding 9.

[0051] To recharge the smoothing capacitor 26, the control unit 4 is configured to provide a second signal 45a such that: if the switching criteria are met, the second switching element 14a is turned off for a predetermined first time span of 4 seconds, and after the first time span has elapsed, it is turned on for a predetermined second time span of 270 seconds. By turning off the second switching element 14a, the two switching elements 12a, 14a of the half-bridge 11a are turned off. This causes the diode 17 to operate as a rectifier that rectifies the recovered voltage from the stator winding 9. The recovered voltage is used to recharge the smoothing capacitor 26. Note that by turning off the switching elements 12a, 14a, a partial ASC is still provided as a safe state.

[0052] To evaluate the switching criteria, the control unit 4, particularly the control section 28, is configured to perform a first comparison of the voltage by the comparator element 47 and receive the analog signal 32 from the analog insulation amplifier section 29. The comparator element 47 is configured to perform a first comparison of the voltage of the DC link 3 represented by the analog signal 32 with a threshold value selected as 40V. Correspondingly, the comparator element 33 of the comparator section 30 performs a second comparison of the voltage of the DC link 3 represented by the measurement signal of the second measuring device 25 with the threshold value. Then, the result of the second comparison performed on the high-voltage side of the inverter is provided to the second sub-section 37.

[0053] The switching criteria include a first condition of the first comparison result - the voltage reaches or drops below the threshold value, a second condition of the second comparison result - the voltage reaches or exceeds the threshold value, and a third condition of the absence of supplying low voltage to the control unit 4 and the drive unit by the low-voltage battery 25. To check the third condition, a signal 48 is provided, which is generated by the control unit 4 or obtained by another unit of the inverter 1 or by the controller of the vehicle.

[0054] To evaluate the switching criterion, the second sub - part 37 includes an AND gate 49 which has the output of the comparator element 47 and the output of the comparator section 30 as inputs. Additionally, the second sub - part 37 includes an AND gate 50 which has the output of the AND gate 49 and the signal 48 as inputs. Thus, when the switching criterion is met, the AND gate 50 outputs a logic 1.

[0055] To ensure that the second switching element 14a is off for a first time span and on for a second time span, the second sub - part 37 includes a timing element 51 which outputs a logic 1 during the first time span and then outputs a logic 0 during the second time span. The output of the timing element 51 is the input of a NOT gate 52 of the output signal 36. Thus, during the first time span, the signal 36 is logic 0, making the second signal 45a logic 0, which turns off the second switching element 14a and the inverter 1 adopts partial ASC. Therein, the smoothing capacitor 26 is re - charged. Correspondingly, during the second time span, the signal 36 is logic 1, making the second signal 45a logic 1, which turns on the switching element 14a again and the inverter 1 adopts full ASC again. Therein, the smoothing capacitor is discharged through the DC / DC converter 21.

[0056] Optionally, the control unit 4 includes a diagnostic section 53 that obtains the output of the AND gate 49. The diagnostic section 53 is configured to evaluate whether the output of the AND gate 49 changes from logic 0 to logic 1 while the smoothing capacitor 26 is initially charged when the inverter 1 is started. Thus, when the voltage of the DC link 3 rises from 0V to the voltage of the high - voltage battery 7, the diagnostic section determines whether the measuring devices 24, 25, the comparison section 30, and the comparator element 47 are operating properly. If no change is detected, the diagnostic section 53 provides a request signal 38 that causes the adoption of full ASC.

[0057] According to a further embodiment of the inverter 1 corresponding to the first embodiment, the second sub - part 37 includes an OR gate instead of the AND gate 49.

[0058] According to a further embodiment of the inverter 1 corresponding to any of the above - mentioned embodiments, the second measuring device 25, the comparator section 30, and the AND gate 49 are omitted. The output of the comparator element 47 is directly fed into the AND gate 50 and optionally into the diagnostic section 53.

[0059] According to a further embodiment of the inverter 1 corresponding to any of the above - mentioned embodiments, the first measuring device 24, the comparator element 47, and the AND gate 49 are omitted. The output of the comparator section 30 is directly fed into the AND gate 50 and optionally into the diagnostic section 53.

[0060] Figure 3It is a block diagram of the control unit 4 of another embodiment of the inverter 1 corresponding to the first embodiment. Below, the differences compared to the first embodiment are described:

[0061] The second switching element 14a is logically controlled by an OR gate 42a and an AND gate 44a corresponding to the OR gate 42 and the AND gate 44 depicted in Figure 1 where the second sub - part provides a signal 36a, which is an input to the AND gate 44a. In contrast to Figure 1 the second switching element 14c is controlled by a similar logic formed by an OR gate 42b and an AND gate 44b, where the third sub - part 37b provides a signal 36b, which serves as an input to the AND gate 44b.

[0062] The second sub - part 37a only includes an AND gate 49a, which takes the output of the comparator element 47 and the signal 48 as inputs. The timing element 51a corresponding to the Figure 1 timing element 51a in takes the output of the AND gate 49a as an input. The NOT gate 52a takes the output of the timing element 51a as an input and supplies the signal 36a to the first sub - part 35. Thus, the second sub - part 37a is configured to evaluate a first switching criterion, which includes a first condition - the first comparison result that the voltage reaches or drops below a threshold, and a third condition - the absence of a low - voltage supply through the control unit 4 of the low - voltage battery 25 and the drive unit. The evaluation result of the first switching criterion is represented by the signal 36a.

[0063] The third sub - part 37b includes an AND gate 49b, a timing element 51b, and a NOT gate 52b corresponding to the second sub - part 37a, where the AND gate 49b takes the output of the comparator part 30 as an input instead of the input of the comparator element 47. Thus, the third sub - part 37b is configured to evaluate a second switching criterion that includes a second condition - the second comparison result that the voltage reaches or drops below a threshold, and the third condition. The evaluation result of the second switching criterion is represented by the signal 36b.

[0064] Therefore, when the voltage of the DC link 3 reaches or drops below a threshold, the inverter 1 according to this embodiment re - charges the smoothing capacitor 26 by using the half - bridges 11a, 11c as rectifiers.

[0065] According to another embodiment corresponding to the Figure 3 embodiment depicted in , the timing elements 51a and 51b are omitted, which results in a fast control loop.

[0066] According to another embodiment corresponding to any one of the foregoing embodiments, the first potential 13 is a low potential, and the second potential 15 is a high potential.

[0067] According to a further embodiment corresponding to any one of the foregoing embodiments, the measuring devices 24, 25 are configured to measure the current flowing in the DC link 3, wherein a comparison with a current threshold is performed.

[0068] According to a further embodiment corresponding to any one of the foregoing embodiments, the control section 28 is implemented by a microcontroller, wherein the logic gates, the comparator element 47, the timing elements 51, 51a, 51b and the delay element 43 are implemented by corresponding software routines of a program loaded into the microcontroller.

Claims

1. A control unit (4) for controlling an inverter (1) having a plurality of half - bridges (11a - c), each of said half - bridges (11a - c) having a first switching element (12a - c) connected to one potential (13) of a DC link (3) and a second switching element (14a - c) connected to the other potential (15) of the DC link (3), and a smoothing capacitor (26) forming the DC link (3), wherein the control unit (4) is configured to provide a first signal (41) to the first switching elements (12a - c) such that the first switching elements (12a - c) turn off upon receipt of a request signal (38), provide a second signal (45a - c) to the second switching elements (14a - c) such that the second switching elements (14a - c) turn on upon receipt of the request signal (38), and perform a comparison of the measured electrical parameters of the inverter (1) with a predetermined threshold, characterized in that, For recharging the smoothing capacitor (26) while receiving the request signal (38), the control unit (4) is configured to: if a switching criterion is met, the switching criterion including the comparison result that the measured electrical parameter reaches or exceeds the predetermined threshold and the condition that there is no low-voltage power supply for the control unit (4), provide one of the second signals (45a-c) such that the second switching element (14a) controlled by one second signal (45a) is turned off for a predetermined time span. The control unit (4) includes a control part (28), and the control part (28) is implemented discretely and configured to evaluate the switching criterion. The control unit (4) is configured to perform a second comparison of the measured electrical parameter with the predetermined threshold.

2. The control unit (4) according to claim 1, wherein the control unit (4) is configured to provide the second signal (45a) such that the second switching element (14a) is turned on for a predetermined second time span after a first time span has elapsed.

3. The control unit (4) according to any one of claims 1-2, wherein the measured electrical parameter is the voltage or current of the DC link (3).

4. The control unit (4) according to any one of claims 1-2, wherein the control part (28) is further configured to perform the comparison based on a low-voltage analog signal (32) representing the measured electrical parameter and supplied to the control part (28).

5. The control unit (4) according to claim 4, further including an analog isolation amplifier part (29), and the analog isolation amplifier part (29) is configured to receive a raw measurement signal from a first voltage measuring device (24) which is part of the high-voltage side of the inverter (1) and provide an analog signal (32).

6. The control unit (4) according to claim 1, wherein the control unit (4) includes a comparator part (30) for performing the second comparison, the comparator part is implemented separately from the control part (28), and is configured to provide a digital signal representing the result of the second comparison.

7. The control unit (4) according to claim 6, wherein the switching criterion includes an additional condition that is the result of the second comparison that the measured electrical parameter reaches or exceeds the predetermined threshold.

8. The control unit (4) according to claim 6, wherein the control unit (4) is configured to, if a switching criterion is met, the switching criterion including the result of the second comparison that the measured electrical parameter reaches or exceeds the predetermined threshold and the condition that there is no low-voltage power supply for the control unit (4), provide another one of the second signals (45a-c) such that one of the second switching elements (14a-c) controlled by the other second signal (45c) is turned off.

9. The control unit (4) according to claim 1, wherein one or more of the switching criteria include an additional condition that there is no drive unit of the inverter (1).

10. The control unit (4) according to claim 1, wherein the control unit (4) is configured to directly provide the first signal (41) when the request signal (38) is received at the signal input port and / or to provide the second signals (45a-c) after the request signal (38) has been delayed by a delay element (43).

11. An inverter (1) for a vehicle, comprising a smoothing capacitor (26) forming a DC link (3); a plurality of half-bridges (11a-c), each having a first switching element (12a-c) connected to one potential (13) of the DC link (3) and a second switching element (14a-c) connected to the other potential (15) of the DC link (3), wherein the inverter (1) comprises a control unit (4) according to any one of claims 1 to 10.

12. The inverter (1) according to claim 11, further comprising an interconnected DC / DC converter (21) to convert the DC link voltage into a supply voltage for operating the control unit (4) and / or the driver part of the inverter (1).

13. A method for operating an inverter (1) having a plurality of half-bridges (11a-c), each of the half-bridges (11a-c) having a first switching element (12a-c) connected to one potential (13) of the DC link (3) and a second switching element (14a-c) connected to the other potential (15) of the DC link (3), wherein a first signal (41) is provided to the first switching elements (12a-c) such that the first switching elements (12a-c) open when the request signal (38) is received, second signals (45a-c) are provided to the second switching elements (14a-c) such that the second switching elements (14a-c) close when the request signal (38) is received, and a comparison of measured electrical parameters of the inverter (1) with a predetermined threshold is performed, wherein for recharging the smoothing capacitor (26) while receiving the request signal (38), if a switching criterion is met, the switching criterion comprising the condition that the comparison result is that the measured electrical parameters reach or exceed the predetermined threshold and the condition that there is no low-voltage supply to the control unit (4), one of the second signals (45a-c) is provided such that the second switching element (14a) controlled by one second signal (45a) opens for a predetermined time span, the control unit (4) comprising a control part (28), the control part (28) being discretely implemented and configured to evaluate the switching criterion, The control unit (4) is configured to perform a second comparison of the measured electrical parameters with the predetermined threshold.

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