Patent ID: 12255548

DETAILED DESCRIPTION

FIG.1shows an embodiment of a motor vehicle1. The motor vehicle1comprises an electrical circuit device2. The electrical circuit device2forms a high-voltage electrical system of the motor vehicle1and comprises a traction energy storage unit3, an electric traction motor4and an electrical circuit arrangement5designed as a three-phase pulse inverter.

A direct current taken from the traction energy storage unit3can be converted by the circuit arrangement5into a three-phase alternating current for driving the electric traction motor4. It is also possible to convert an alternating current generated by means of the traction motor4in generator operation into a direct current for charging the energy storage device. The structure of the electrical circuit arrangement5is described in more detail below with reference toFIG.2.

The electric circuit device2further comprises an energy storage device6designed as a direct current link capacitor, which is connected between the voltage levels HV+ and HV− of the electric traction energy storage device3. The energy storage device6therefore represents a direct current link capacitor or an X-capacitor.

The electrical circuit device2moreover comprises a control device7, which comprises a driver circuit8and a control unit9. The driver circuit8of the control device7may be spatially contained inside the circuit arrangement5, for example, by arranging the driver circuit8within a housing of the circuit arrangement5. The control device9may also be arranged in the housing or it may be implemented separately and connected to the driver circuit8. For discharging the energy storage device6, the traction energy storage device3can be decoupled, for example, by means of a switching device (not shown here), so that when the energy storage device6is discharged, the traction energy storage device3is not also discharged.

The electrical circuit arrangement5is assigned a cooling device10, by means of which heat generated during operation of the circuit arrangement5, in particular heat generated in switching elements of the circuit arrangement5, can be dissipated. The cooling device10can, for example, be a heat sink and in particular be connected or thermally coupled to a cooling circuit (not shown here) of the motor vehicle1.

In a normal operation of the electrical circuit arrangement5, for example, motor operation or generator operation of the traction motor4, the control terminals of the switching elements Siare driven by the control device7with a control voltage alternating between the switched-off voltage and the maximum switched-on voltage. In this case, the control can, in particular, be performed by means of pulse width modulation (PWM). Moreover, the control device7can control the electrical circuit arrangement5for active discharging of the energy storage device6, as will be described in more detail below.

An active discharging of the energy storage device6may be necessary if, for example, the electrical circuit device2and/or the motor vehicle1has been switched off, for example, as a result of a termination of operation such as switching off after the motor vehicle has been parked or comparable processes. Furthermore, an active discharge may be required as a rapid discharge, for example, upon the occurrence of a fault condition in the electrical circuit arrangement5and/or in the motor vehicle1, as will be explained in more detail below.

A high-voltage tension can be provided by the traction energy storage device3for operating the electrical circuit device2. This high-voltage tension can, for example, be between 250 V and 1500 V, in particular between 360 V and 860 V. During operation of the electrical circuit device2or of the motor vehicle1, the energy storage device6, which is designed as a direct current link capacitor, has a voltage UE which corresponds to the voltage of the high-voltage vehicle electrical system or of the traction energy storage device3. This means that a high voltage is present at the energy storage device6, which must be reduced when the electrical circuit device2or the motor vehicle1is switched off, for example, to ensure that the circuit device2or individual components thereof are safe to touch. In this way, functional safety and protection against contact with the components of the motor vehicle1are also ensured. This active discharging of the energy storage device6can be carried out by the electrical circuit arrangement5, as will be described in more detail below.

FIG.2shows a detailed view of the circuit device2with the electrical circuit arrangement5designed as a three-phase pulse inverter. Also shown are the control device7and the electric traction motor4connected to the electric circuit arrangement5.

The electrical circuit arrangement5comprises three half bridges11,12,13, which are each formed by two of a total of six switching elements S1to S6of the circuit arrangement5. In a normal driving operation of the motor vehicle1, the direct current provided by the traction energy storage device3is converted by means of the half bridges11,12,13, into a three-phase alternating current with the phases U, V and W for operating the electric traction motor4. Moreover, the electrical circuit arrangement5can be used to discharge the energy storage device6. For this purpose, a control voltage UG,iis applied to the control terminals of the switching elements S1to S6of at least one of the half bridges11,12,13, wherein the magnitude of the control voltages UG,iis, in each case, between a threshold voltage Uth,iand a maximum permissible switched-on voltage UG,i_maxof the respective switching element Si.

The switching elements S1to S6each comprise a switchable section, wherein the switchable sections of two switching elements of a half bridge are respectively connected in series. The energy storage device6can be discharged by adjusting the control voltage UG,iof at least two of the switching elements, for example, the control voltages UG,iand UG,4of the switching elements S1and S4of the first half bridge11. The respective electrical resistance of the switchable section of the switching element Sican be adjusted by the control voltage UG,iof the respective switching elements Si. The control voltages UG,iof the switching elements S1to S6can be set by means of the driver circuit8of the control device7as specified by the control device9. The connections between the driver circuit8and the respective control terminals of the switching elements S1to S6are not shown for reasons of clarity.

If the three half bridges11,12,13are used to discharge the energy storage device6, the resistance equivalent circuit of the electrical circuit arrangement5shown inFIG.3results for the discharge of the energy storage device6. The resistance RDS,irespectively designates the electrical resistance of the switchable section of the switching elements S1-S6. However, the half bridges11,12,13do not need to be simultaneously and permanently in a conductive state. The switching elements S1-S6can, in particular, also be operated in a pulse mode between a switched-on state and a switched-off state by means of their control voltages UG,i, as explained in more detail below.

The switching elements S1to S6can, for example, be designed as metal-oxide-semiconductor field-effect transistors, in particular with silicon carbide base. Such SiC-MOSFETs can, for example, have a threshold voltage Uthbetween 3 V and 5 V and a maximum permissible switched-on voltage UG,i_maxbetween 15 V and 18 V. The control voltages UG,ican accordingly be adapted by the control device7to switch the switching elements S1-S6in a transmission mode. The control device7is, in particular, designed to dynamically adapt the control voltages UG,iof the switching elements S1-S6, in particular in the framework of a regulation. Moreover, the ratio between the switched-on duration and the switched-off duration and/or the absolute time durations of the respective switched-on intervals and the switched-off intervals can be adjusted by the control device7during pulse operation of the switching elements S1-S6.

This enables the electrical resistance of the switchable sections of the switching elements S1-S6, in this case the drain-source sections of the switching elements S1-S6, to be set in transmission mode as a function of a discharge current specification which describes the level of a discharge current IE to be set for the energy storage device6in a method for actively discharging the electrical energy storage device6by means of the control voltages UG,i. Additionally, or alternatively, the ratios between the switched-on duration and the switched-off duration in a pulse operation of the individual switching elements S1-S6can also be adjusted as a function of the discharge current IEthat is to be set, which current is described by the discharge current specification.

The control device7can, in particular, regulate the level of the respective control voltages UG,iand/or the ratio between the switched-on duration and the switched-off duration during the active discharge of the energy storage device6as a function of at least one measured variable describing the discharge current. This makes it possible, for example, to set a constant discharge current IEduring active discharging of the energy storage device6. A measured variable describing the discharge current IEcan be obtained directly by measuring the current of the branch of the inverter, which is at the potential HV, for example, by means of a current measuring means15arranged there. The use of such a current measuring means has the advantage that it can, in any case also be used in the regular operation of the circuit for the measurement and/or for the control of one of the phase currents in the phases U, V, W. Alternatively, the current measurement can also be performed in the branch of the inverter that is at potential HV+.

Additionally, or alternatively, a measurement of the energy storage device voltage UEcan also be made by means of a voltage measuring means16, wherein the control voltages UG,ican also be controlled as a function of the energy storage voltage UEin addition or alternatively to the measured value of the discharge current IE. For this purpose, an assignment rule which assigns a resistance value of the switchable path of the respective switching element Sito the respective control voltages UG,ican, for example, be used, such that the discharge current IEresults correspondingly from the energy storage device voltage UEand the resistances RDS,iof the conductively switched drain-source paths of the switching elements. The connections between the current measuring means15, the voltage measuring means16, and the control device7are not shown inFIG.2for reasons of clarity.

In this, the level of the constant discharge current can be specified as a function of degradation information describing the age and/or degradation of the switching elements S1-S6. In particular, with increasing age or increasing degradation, a lower level of the discharge current IEcan be specified, so that the load on the switching elements during active discharging of the energy storage device6is reduced with increasing service life, this in order to be able to achieve a high overall service life of the switching elements S1-S6.

The degradation information can, for example, be stored in the control device7and can, for example, be determined by the control device7itself. The time which has elapsed since assembly of the circuit arrangement or since initial commissioning of the circuit arrangement2, can, for example, be used as the age of the switching elements S1-S6. The cumulative operating time of the switching elements S1-S6or comparable variables can, for example, be used as degradation information. The dependence of the discharge current IE on the age or degradation of the switching elements S1-S6can be described, for example, by means of various stages of aging or degradation stages or in the continuous form of an assignment rule. The corresponding information can be stored, for example, in a memory device of the control device7.

It is moreover possible that during the active discharge of the electrical energy storage device6, the ratio between the switched-on duration and the switched-off duration of the switching elements S1-S6is adjusted using one of the half bridges11to13. In order to limit the heat input in the respective switching elements S1-S6, it can be provided that during the active discharge of the energy storage device6, the switchable paths of the switching elements S1-S6of several, in particular, of all three half bridges, are switched in a pulse mode between a blocking mode and a transmission mode. In so doing, in order to, for example, achieve a constant discharge current IE, the ratios between switched-on durations and switched-off durations for the switching elements S1-S6of a plurality of half bridges can be set in such a way that the switching elements S1-S6of exactly one of the half bridges11,12,13are cyclically alternatingly switched in transmission mode. The switching patterns used for this purpose are shown, for example, inFIG.4.

FIG.4shows a diagram in which the switched-on durations and the switched-off durations for the respective control voltages UG,iof the switching elements S1-S6as a function of time t are represented. The course of the respective control voltages UG,iof the switching elements S1-S6is shown one above the other in individual diagrams in order to make the position of the respective switched-on pulses relative to one another clear.

It is evident that the switching elements S1and S4, which form the first half bridge11, are operated simultaneously in transmission mode in a first time window t1. Correspondingly, in a subsequent time window t2, in which the switching elements S1and S4of the first half bridge11are operated in blocking mode, the switching elements S2and S5of the second half bridge12are operated in transmission mode. In a further time window t3, in which both the switching elements of the first half bridge11and the second half bridge12are operated in blocking mode, the switching elements S3and S6of the third half bridge13are switched to transmission mode. Subsequently, the switching of the respective switching elements S1-S6and the half bridges11-13is repeated cyclically, as shown for the time windows t4to t6. The ratio between the switched-on duration tonand the switched-off duration toffof the switching elements S1-S6is 1:2 or alternatively ton=0.5*toff.

In this manner, it can be achieved, that in the pulse pauses, which is to say during the switched-off phases of the individual switching elements S1-S6, a heat transfer takes place from the switching elements S1-S6into the cooling device10. Thus, a temperature of the switching elements S1-S6, for example, a junction temperature in the switching elements S1-S6, may once again decrease, such that the total heating of the switching elements S1-S6occurring during the active discharge process can be limited, for example to a maximum junction temperature of 200° C. or less.

If a maximum level of the discharge current IEis specified by the discharge current specification, it can thus be provided that the switching elements S1-S6of at least two of the half bridges11to13are cyclically at least temporarily simultaneously switched to transmission mode. To illustrate this process, the control voltages UG,iof the switching elements S1-S6are shown in individual diagrams one above the other inFIG.5. To achieve the maximum discharge current the control voltages UG,iof the switching elements S1-S6are additionally set in such a way that the smallest possible resistances RDS,iare obtained in transmission mode of the switching elements S1-S6. This can, in particular, be achieved by the control voltage UG,icorresponding to the respective maximum permissible control voltage UG,i,maxof the switching elements S1-S6.

It is evident that in part of the time window t1the switched-on phase of the switching elements S1and S4of the first half bridge11overlaps with the switched-on phase of the switching elements S2and S5of the second half bridge12. The second half of the switched-on phase of the switching elements S2and S5of the second half bridge12correspondingly overlaps in part in the time window t1with the switched-on phase of the switching elements S3and S6of the third half bridge13. Subsequently, at the beginning of the time window t3, the switching elements S1and S4of the first half bridge11are switched on again without any further overlap, whereupon the switching pattern described above continues periodically. This enables a higher discharge current to be established due to the parallel switching on of several of the switching elements S1to S6, or the at least temporarily conductive switching of at least two of the half bridges11,12,13.

In such a scenario, it is not possible to regulate a constant charging current IEand, in particular, it is not absolutely necessary inasmuch as rapid discharging is only rarely carried out and then as an emergency measure. Compared with the normal operation for active discharge, which can take place, for example, during a process with a high number of cycles, such as switching off the electrical circuit arrangement2or the motor vehicle1, such a rapid discharge represents an emergency measure, which can be carried out in response to a fault that has occurred in the electrical circuit arrangement2and/or in the motor vehicle1. Such a rapid discharge may, for example, be performed if the motor vehicle1is involved in an accident. In this case, fault information describing the occurrence of the accident or another fault in the motor vehicle1can, for example, be transmitted to the control device7, which correspondingly implements the rapid discharge by actuating the switching elements S1-S6.

German patent application no. 10 2021 129145.3, filed Nov. 9, 2021, to which this application claims priority, is hereby incorporated herein by reference in its entirety.

Aspects of the various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled.