Inverter circuit and electric motor drive system

Abstract

The present invention relates to an inverter circuit for controlling a single-phase or multi-phase electric machine for an electric drive system, in particular a double-rotor motor, preferably for or in a motor vehicle, having a first and second supply connection via which the inverter circuit can be connected to a DC voltage source, having a load output for coupling the electric machine, having a controllable inverter arranged between the supply connections and the load output, which has a plurality of power switches which are interconnected in such a way as to convert a DC voltage received on the supply side into an AC voltage for driving an electric machine, wherein at least one power switch has a plurality of individual switches arranged in parallel with one another with associated thermal fuses,wherein the thermal fuses are each arranged in the load path of the individual switch assigned to them. The present invention further relates to an electric motor drive device.

Description

FIELD OF THE INVENTIONThe present invention relates to an inverter circuit for driving a single- or polyphase electric machine and to an electromotive drive system.BACKGROUND ARTElectric motor drives have long been used in motor vehicles for a wide variety of applications, for example as actuating drives for window lifters, seat adjusters, parking brakes, sliding roofs, doors, trunk lids and the like. Modern motor vehicles use--inter alia for sustainability reasons and to avoid CO 2- emissions--increasingly electrically powered drives or drive systems. Such drive systems include, for example, one or more electric machines, such as synchronous machines or asynchronous machines, which are supplied by a polyphase alternating voltage using so-called inverters.An inverter, also called an inverter or a rotary converter, is an electrical circuit which converts a DC voltage into an AC voltage. Inverters of this type and their fields of application are generally known in a wide variety of circuit-related variants, so that their general circuit-related design and mode of operation need not be discussed in more detail.Especially in inverters for high-current applications, such as are used for driving motor vehicles, for example, special safety mechanisms are required, in particular in the case of a dysfunctional inverter.A widely used safety measure is, for example, the use of an active short circuit of the supply voltage. However, a comparatively complex inverter topology is required for this purpose, optionally with redundant circuit elements and / or cost-intensive magnets. Since an active short circuit is usually associated with very high currents which may be problematic for the motor vehicle, this must be separately protected, which in turn entails additional outlay.Alternatively, a freewheel with relay (battery contactor) between inverter and DC voltage source (battery) is frequently also used for the safety mechanism, wherein in the event of a fault, the freewheel is actively disconnected by the relay. However, when the battery contactor is closed, very high DC currents can also occur here and, as a result, a high braking torque can occur in the event of uncontrolled generator operation of the electric motor.The present invention is, inter alia, in the field of so-called dual rotor motors, also referred to for short as dual rotor, multiple rotor, dual rotor. These are electric machines having a stator and two rotors connected to one another in a rotationally fixed manner. These dual rotor motors can increase both the torque density and the efficiency of electric drives compared to conventional electric machines having only one rotor. The structure and the mode of operation of such dual rotor motors and of inverter circuits are described, for example, in German patent DE 10 2021 003941 B 4 and in German patent DE 10 2021 003942 B4.Due to their low inductance and the high magnetic flux, such double rotor motors have a low short-circuit resistance, whereby the above-described active short circuit is rather out of the question as a safety mechanism.Against this background, the freewheel with relays between inverter and DC voltage source often represents the only safety mechanism that can be operated at the present time, but the above-described disadvantages must then be accepted.This is a state to be improved.JP 2018-148 164 A describes an inverter circuit for driving a polyphase electric machine for an electric drive system. A controllable inverter includes a plurality of power switches. The circuit breakers here contain a multiplicity of individual switches arranged parallel to one another with associated thermal fuses, which are each arranged in a load path of the associated individual switch.SUMMARY OF THE INVENTIONThe object of the present invention is to provide an improved and in particular a simplified safety concept for protecting an inverter circuit.According to the invention, this object is achieved by an inverter circuit having the features of claim 1 and / or by an electric motor drive system having the features of claim 14.Accordingly, the following is provided:an inverter circuit for controlling a single- or polyphase electric machine for an electric drive system, having a first and second supply terminal via which the inverter circuit can be connected to a direct voltage source, having a load output for coupling the electric machine, having a controllable inverter which is arranged between the supply terminals and the load output and has a multiplicity of power switches which are connected to one another in such a way that a direct voltage picked up on the supply side is converted into an alternating voltage for driving an electric machine, at least one power switch having a multiplicity of individual switches arranged parallel to one another and having associated thermal fuses, the thermal fuses being arranged in each case in the load path of the individual switch respectively associated therewith, and the thermal fuses being formed in each case by using materials having different electrical resistance.an electrical drive system, having an electrical machine, having a DC voltage source and having an inverter circuit according to the invention, which is connected on the supply side to the DC voltage source and is connected via its load output to the electrical machine.The idea underlying the present invention is to provide a safety concept, which can be implemented using comparatively simple means, for protecting the inverter circuit for an electromotive drive. For this purpose, the invention provides, first of all, for the power switch or switches of the inverter of the inverter circuit to be realized with a plurality of individual switches arranged parallel to one another, to which respective corresponding thermal fuses are assigned. The individual switches are designed as identically and preferably identically as possible with regard to their current carrying capacity and line resistance in their load paths.Each individual one of these individual switches with thermal fuse thus functions as a thermal protection device, namely as follows: A load current flowing through a power switch on the load side is typically distributed uniformly over the load paths of the individual switches and thus also through the associated thermal fuses. In the case of a defective individual switch and / or an excessively high load current through the individual switch, the current density in the respectively associated thermal fuse increases, which leads to a sudden temperature increase, for example, due to the resistance there. This melts the thermal fuse and interrupts the load circuit of this individual switch, as a result of which the entire circuit breaker is switched off.In contrast to known solutions, in the case of a fault in the drive system, according to the invention, a free-wheeling or open-wheeling is used as safe state instead of an active short circuit. During the free running or open running, all power switches of the converter are switched off, so that the current in the electric machine falls to zero. In this state, the voltage induced by the still rotating electrical machine is rectified via the half-bridge free-wheeling diodes and charges the intermediate circuit capacitor with the rectified voltage. In order to protect the high-voltage network of the vehicle from this induced voltage and, associated therewith, from an uncontrolled feedback current, in a development, a disconnection switch can preferably be connected into the supply line connected to the positive supply connection. A suitable electromagnetic design of the electric machine can ensure that the induced voltage remains below the breakdown voltage over the entire operating range.This safe state according to the invention covers all relevant applications without causing a dangerous voltage in the drive system or a braking torque at the wheels of the vehicle. The changeover to the freewheel is possible from all possible operating states which the drive unit covers, including operation at maximum speed and / or with very high feedback power.Further advantages of the safety concept according to the invention are the absence of transient overcurrents which could damage the magnets of the dual rotor motor, (virtually) no braking torques and very low drag losses over the entire rotational speed range.Advantageous embodiments and refinements emerge from the further dependent claims and from the description with reference to the figures of the drawing.In one configuration, at least one of the thermal fuses is designed as a fuse. A fuse is an overcurrent protection device which interrupts the circuit by melting a fusible conductor when the current intensity exceeds a specific current value for a sufficiently long time.In a particularly preferred development, at least one fuse is designed as a PCB conductor track or a part thereof. The PCB conductor track formed as a fuse is formed in such a way that, at a predetermined load current through the associated individual switch, this individual switch is switched off by melting the PCB conductor track or a part thereof, as a result of which the inverter is also switched off. The predetermined load current denotes the current which, when the individual switch is short-circuited, flows through the latter and which is also sufficiently large to melt the PCB conductor track at this point and thus interrupt it.According to a preferred embodiment, the thermal fuse is formed as a material thinning, in particular as a tapering of a PCB conductor track on a corresponding printed circuit board. By means of the material thinning, the above-mentioned configuration of the predetermined load current can be set very easily.In an alternative or additional preferred embodiment, the thermal fuse can also be formed by using materials with different electrical resistance. For example, the material in the region of the thermal fuse may have a (significantly) higher electrical resistance than outside thereof.Typically, at least one and preferably each power switch of the inverter has a multiplicity of individual switches arranged parallel to one another (with respect to their load paths) with thermal fuses assigned in each case to these individual switches. Preferably, each circuit breaker has the same number of parallel individual switches with thermal fuses respectively associated therewith. Preferably, at least three individual switches and particularly preferably at least six individual switches are connected in parallel in a circuit breaker.According to a further, particularly preferred aspect of the invention, at least one controllable isolating switch is provided.The at least one controllable isolating switch is arranged between at least one of the supply connections and the load output, for example in the positive and / or negative supply line to the inverter of the inverter circuit. The circuit breaker could also be integrated in the inverter. The controllable isolating switch can be controlled in such a way that the electric machine is disconnected from the DC voltage source and in order to thus protect the inverter circuit or its inverter in the event of a malfunction of the electric machine, for example against an uncontrolled high reverse current or an overvoltage.The circuit breaker preferably has a power transistor, preferably a power transistor with an integrated anti-parallel diode. For example, the circuit breaker can be designed as a power MOSFET, which is arranged, for example, in the positive supply branch. An anti-parallel diode is understood to mean a diode which is connected in parallel with the load path of the respective power transistor and is also connected in a conducting manner from source to drain or emitter to collector of the power transistor. For example, the anti-parallel diode can be formed by the body diode of the power transistor formed as a MOSFET.Preferably, a first diagnostic circuit is provided, which is designed to diagnose a malfunction of an electric machine connected to the load output of the inverter circuit. In the event of a diagnosed malfunction of the electric machine, the first diagnostic circuit controls the circuit breaker in an open state. By isolating the inverter or the inverter circuit from the DC voltage supply, the inverter or the inverter circuit is controlled into a safe state by decoupling. A malfunction can be, for example, such a state in which such a high load current is supplied to the electric machine, in which the inverter circuit generating this load current could damage or would result in the risk of overheating. A malfunction could also be such a state that could have negative effects on the DC voltage source.Preferably, a second diagnostic device is also provided, which is coupled to the circuit breaker and is designed to diagnose the proper functioning of the circuit breaker. The second diagnostic device is further configured to generate a diagnostic signal which contains information about the diagnosed state of the circuit breaker. "Diagnosing" in this context means observing, monitoring and / or checking the proper function. The diagnosing of the proper function may be performed at regular intervals, continuously during the operation of the electric machine, at predetermined times, such as at each restart of the electric machine, or at a predetermined power consumption, for example. Other points in time would also be conceivable.Preferably, a control device coupled to the second diagnostic device is provided. The control device is designed to evaluate the diagnostic signal and, if the diagnosed state of the circuit breaker exceeds a predefined threshold, to initiate a protective measure. This protective measure can be, for example, the opening of the circuit breaker. However, it may sometimes also be that the circuit breaker is defective and therefore can no longer be closed and / or opened at all or can also no longer be controlled, for example. As a protective measure, therefore, a more or less slow shutdown of the downstream inverter into a safe state can also be provided, preferably in conjunction with forwarding the diagnostic signal, for example, to a corresponding superordinate location. The circuit breaker is generally not used, but should be actuated only when there is a risk for the inverter or the inverter circuit. The predefined threshold is therefore defined such that, starting from its exceeding, there is a risk for the inverter or the inverter circuit. This is preferably done by opening the circuit breaker, thereby disconnecting the inverter from the DC supply. Alternatively, an alarm signal can also be output or a corresponding item of information can be output to a superordinate, vehicle-external control device, provided that the diagnosed state of the circuit breaker exceeds a predefined threshold. The diagnosing can take place online, that is to say via a vehicle-internal control device, or offline, that is to say via a vehicle-external control device.The circuit breaker is preferably designed in such a way that the rectified induced voltage at maximum rotational speed of the electric machine is below the breakdown voltage of the inverter. The breakdown voltage is generally determined by the circuit topology of the inverter and in this case in particular by the number, type and interconnection of the various electrical components of the inverter.In a particularly preferred embodiment, the inverter is designed as a fully integrated or at least partially integrated circuit. The circuit breaker can be integrated into the integrated inverter. This allows a circuit which is simplified in terms of production technology and thus more cost-effective to be provided.In a particularly preferred development, at least one free-wheeling diode is provided, which is arranged on the supply side between the first and second supply terminals. This free-wheeling diode offers further protection of the inverter, i.e. by means of the free-wheeling diode an overcurrent at the input of the inverter, which could jeopardize it, can be discharged via the free-wheeling diode.Preferably, the circuit breaker is integrated in the free-wheeling diode. A double protective function can thus be provided in a very cost-effective manner by means of its single semiconductor component.In a preferred embodiment, the circuit breakers or their individual switches with thermal safety can be controlled independently of a circuit breaker.The isolating switch and / or the power switches-and thus also their individual switches-are preferably designed as power MOSFETs, the so-called power MOSFETs. However, other field effect transistors, bipolar transistors, thyristors, relays, IGBTs and the like, which are each capable of switching the correspondingly high load currents, would also be conceivable. The isolating switches or the power transistors are preferably constructed on the basis of silicon (Si) technology or silicon carbide (SiC) technology, since transistors which are particularly resistant to switching can be produced by means of this technology.Preferably, a three or more stage inverter is provided.The inverter particularly preferably has a number of driver stages corresponding to the number of phases. Each driver stage has at least one and preferably at least two controllable power switches. In the case of two power switches per driver stage, one of these is designed as a high-side switch and the other as a low-side switch, with their load paths being connected in series. The center tap between high-side switch and low-side switch is in each case coupled to the load output. During operation of the electric machine, this load output carries a phase of the load current for driving the electric machine.The above embodiments and developments can be combined with one another as desired, if appropriate. Further possible embodiments, developments and implementations of the invention also include combinations of features of the invention described above or below with respect to the exemplary embodiments, which combinations are not explicitly mentioned. In particular, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the present invention.CONTENT INDICATION OF THE DRAWINGThe present invention is explained in more detail below with reference to the exemplary embodiments indicated in the schematic figures of the drawings. The following are shown: FIG. 1 shows a schematic cross-sectional illustration of an electric machine designed as a double rotor motor; FIG. 2 is a block diagram of an electric drive system according to the present invention; FIG. 3 shows a first example of an inverter circuit according to the invention; FIGS. 4-4B show a second example of an inverter circuit according to the invention; FIG. 5 shows a third example of an inverter circuit according to the invention.The accompanying drawings are intended to provide a further understanding of the embodiments of the invention. They illustrate embodiments and, in conjunction with the description, serve to explain principles and concepts of the invention. Other embodiments and many of the advantages mentioned are evident with reference to the drawings. The elements of the drawings are not necessarily shown to scale with respect to each other.In the figures of the drawing, elements, features and components that are the same, functionally the same and act the same-unless stated otherwise-are each provided with the same reference numerals.DESCRIPTION OF EMBODIMENTSFIG. 1 shows a schematic cross-sectional illustration of an electric machine designed as a double rotor motor.The electric machine denoted here by reference sign 10 is preferably designed as a three-phase synchronous machine or synchronous motor. An aspect which is essential for the invention, but is not absolutely necessary, is the fact that the dual rotor motor is furthermore constructed from flux-guiding material from solid material, that is to say they are not laminated. The flux-conducting material is preferably made of solid iron or a corresponding solid iron alloy. It is likewise preferred if the electric machine 10 is a wheel hub motor for an electrically operable motor vehicle. However, other applications would also be conceivable and advantageous.The dual rotor motor 10 comprises the outer rotor 11 and the inner rotor 12, and the stator 13 is arranged between the two rotors 11, 12 in a manner known per se. The stator 13 may preferably, but not necessarily, be a yokeless stator 13. The inner rotor 12 is tubular, wherein a massive, full-volume configuration of the inner rotor 12 would also be conceivable.In the example shown, two magnets 14, 15 with opposite poles are placed between the outer rotor 11 and the stator 13 on the inner surface of the outer rotor 11 in the outer air gap 16. It would be conceivable and advantageous if the magnets 14, 15 are embedded in pocket-shaped recesses of the outer rotor 11 specifically provided for this purpose. However, it would also be conceivable for the magnets 14, 15 to be spaced apart from the outer rotor 11, that is to say not to be attached directly to the inner surface thereof. The lines of flux between the north and south poles of the opposing magnets 14, 15 extend here in the core material of the outer rotor 11.In the example shown, two magnets 18, 19 with opposite poles are likewise placed between the inner rotor 12 and the stator 13 on the inner surface of the inner rotor 12 in the inner air gap 17. Here too, the magnets 18, 19 can be embedded in corresponding pockets of the inner rotor 12 or be spaced apart from the inner rotor 12. The lines of flux between the north and south poles of the opposing magnets 18, 19 extend here in the core material of the inner rotor 12.FIG. 2 is a block diagram of an electric drive system according to the present invention.The electric drive system according to the invention, which is denoted here by reference numeral 20, is preferably-but not necessarily-provided for use in a motor vehicle. The electric drive system 20 according to the invention comprises a DC voltage source 21, an inverter circuit 22 according to the invention having an inverter 23, and a single-phase or polyphase electric machine 10, which is designed, for example, as a double rotor motor, as is illustrated, for example, in FIG. 1.The electric machine 10 is connected on the input side to the inverter circuit 22, which drives the electric machine 10. The inverter circuit 22 has two supply terminals 24, 25, via which the inverter circuit 22 can be connected to the DC voltage source 21, for example a motor vehicle battery or a corresponding rechargeable battery. A first supply potential V 11, for example a positive supply potential, can be tapped off at the first supply terminal 24. A second supply potential V 12, for example a negative supply potential or a reference potential, can be tapped off at the second supply terminal 25. A supply DC voltage VDC=V11-V12 is thus present between the supply terminals 24, 25.A load current IL, for example polyphase, can be tapped at a load output 26 of the inverter circuit 22 or its inverter 23, by means of which the various phases of the electric machine 10 which can be connected via the load output 26 are driven.The inverter circuit 22 can be designed, for example, as a three-stage or multi-stage inverter circuit 12. The inverter circuit 22 has an inverter 23, which is usually referred to as an inverter 23. The inverter 23 is designed to convert the direct voltage VDC aufgenommene on the supply side into an alternating voltage. The inverter 23 is preferably designed as a polyphase inverter 23, wherein the number of phases of the inverter 23 typically corresponds to the number of phases of the electric machine 10. The electric machine 10 is driven via the phase currents ILprovided by the inverter 23 at the load output 26. The electric machine 10 is preferably, but not necessarily, a synchronous machine. In this case, the inverter circuit 22 preferably includes a three-phase inverter 23.Furthermore, a control circuit 28 is provided, which is coupled to a control terminal 29 of the inverter circuit 22 and which is designed to control the function of the inverter circuit 22 or its inverter 23 via a control signal.The control circuit 28 may further include an overcurrent detector configured to detect an overcurrent in, for example, the switches of the inverter 23.According to the invention, the inverter circuit 22 is equipped with a safety concept 27, which is illustrated in FIG. 2 merely as a schematic function block 27. This safety concept 27, its function, its configurations and variants will be described in detail below with reference to FIGS. 3 to 5.FIG. 3 shows a first exemplary embodiment of an inverter circuit according to the invention.In the example shown in FIG. 3, a controllable isolating switch 30 is arranged between the supply terminal 24 and the inverter 23 of the inverter circuit 22. The controllable isolating switch 30, which is part of the safety concept 27 according to the invention, is designed to disconnect the electrical connection between inverter 23 and DC voltage source as required. For this purpose, the circuit breaker 30 is connected with its load path between the supply connection 24 and the inverter 23. If the control terminal of the isolating switch 30 is controlled by means of a suitable control signal S 2, the load path can be interrupted.For controlling the circuit breaker 30, a further control circuit 31, which is only schematically illustrated in FIG. 3 and is typically not a component of the inverter circuit 22, is provided. The further control circuit 31 is preferably a component of the control circuit 28 for controlling the inverter 23 of the inverter circuit 22, wherein a control circuit 31 separate therefrom, for example an external control circuit, would also be conceivable.FIG. 4 shows a second exemplary embodiment of an inverter circuit according to the invention.In FIG. 4, the inverter 23 of the inverter circuit 22 is formed as a three-phase inverter 23. The three-phase inverter 23 has its own driver stage 40 a- 40 cfor each phase of the output-side load current, which driver stages are each arranged between a first supply path having the first supply potential V 11 and a second supply path having the second supply potential V 12. The three driver stages 40 a- 40 ceach have a half bridge consisting of in each case two power switches T 1-T 6. A center tap 41 a- 41 cof a respective driver stage 40 a- 40 cis connected to the load output 26.The half bridge of the first driver stage 40 aincludes a power switch T 1 embodied as a high-side switch and a power switch T 4 embodied as a low-side switch, wherein the center tap 41 ais connected or coupled to the load output 26 between its load paths.The two other driver stages 40b, 40c are designed analogously.In the example of FIG. 4, the above-mentioned safety concept 27 is also implemented in the inverter 23. This safety concept includes two safety elements, which will be explained below with reference to a detail for the circuit breaker T 1 in FIG. 4A.According to a first safety element, the circuit breaker T 1 does not consist of a single switch. Rather, the circuit breaker T 1 has a multiplicity of individual switches T 1 a- T 1 e, in the example shown five individual switches T 1 a- T 1 ewhich are designed as identically as possible. These individual switches T1a-T1e are arranged parallel to one another. This has the effect that the phase current through the power switch T 1 is divided uniformly between the individual switches T 1 a- T 1 e, so that each individual switch T 1 a- T 1 emust switch a correspondingly lower phase current overall.The remaining circuit breakers T 2-T 6 are designed analogously.According to a second safety element, each of the individual switches T 1 a- T 1 eof a respective circuit breaker T 1-T 6 is additionally assigned a thermal fuse 42 a- 42 e. The associated thermal fuses 42 a- 42 eare arranged in a load path of the individual switch T 1 a- T 1 eassociated with them in each case. These thermal fuses 42 a- 42 emay be part of a conductor track 43 connected to the circuit breaker T 1, for example a conductor track to which the first supply potential V 11 is applied. The thermal fuses 42 a- 42 eare branched as lead sections 43 a- 43 efrom the common conductor track 43 and thus connect a respective load terminal of a single switch T 1 a- T 1 eto the common conductor track 43.The conductor track 43 and thus also the lead sections 43 a- 43 econnected to the latter can consist of copper or a copper alloy. Aluminum, silver or alloys of the materials mentioned would also be conceivable. The conductor track 43 or lead sections 43 a- 43 emay be coupled to a cooling body for cooling it or may be directly water-cooled.FIG. 4B shows, by way of example, a possible configuration for such a thermal fuse 42 e. The thermal fuse 42 eis accordingly preferably realized by a (material) thinning 44 ein the lead section 43 efor the individual switch T 1 e. The thinning 44e is selected in accordance with the phase current to be assured. The thinning 44 ein this case forms a thermal resistance in the current-carrying path and is dimensioned such that, starting from a breakdown current predetermined by the width 45 eof the thinning, the temperature increases abruptly and the lead section 43 ethus abruptly fuses in the region of the thinning 44 e. As a result, the current flow in the respective individual switch T 1 eis interrupted.The power switch T 1 and thus also its individual switches are formed using MOS technology, i.e. all individual switches T 1 a- T 1 eare, for example, MOSFETs. The MOSFETs are preferably produced in SiC technology, but can also be produced in conventional silicon technology or GaAs technology.The mode of operation of the safety concept described with reference to FIGS. 4 to 4B is explained in more detail below.A low-impedance fault of a single MOSFET switch T 1 a- T 1 ein one of the half bridges 40 a- 40 ccauses an unbalanced short circuit, which directly leads to very high peak currents in the respectively short-circuited MOSFET, which could lead without a safety concept to thermal damage and possibly to undefined subsequent damage in the inverter 23. In addition, the high losses cause a pulsating braking torque of the electric machine driven by the inverter 23, which may impair the safety and driving stability of the vehicle. Switching to free-wheeling does not reduce these currents, since the body diodes of the respectively still intact MOSFET individual switches T 1 a- T 1 ewere to maintain the current flow.The inventive inverter circuit 22 with safety concept 27 solves this problem by introducing a thermal fuse into each individual MOSFET individual switch T1a-T1e of a half-bridge power switch T1-T6. Each of these MOSFET individual switches T1a-T1e conducts, for example, 80A-120A during normal operation. Since the current density on the thinning 44 eon the conductor track feed line 43 eis already very high in normal operation (typically >400 A / mm 2), the current density in short-circuit operation thus reaches very much higher currents of typically >10,000 A / mm 2. In this fault operation, the temperature in the region of the thinning 44 eincreases abruptly (typically in the range of >1.5 million K / s). This causes the material of the thinning 44 eto melt within a very short time (typically <1 msec). In combination with an overcurrent detection and, associated therewith, a rapid deactivation of all remaining individual MOSFET switches T 1 a- T 1 econtrolled by the control circuit, this ensures a reliable behavior of the drive unit. Available free-wheeling paths in parallel and complementary MOSFETs can prevent the occurrence of any arcs when switching off.The electrical resistance introduced by thinning 44 eis not excessively relevant in normal operation with typically <0.1 mOhm.FIG. 5 shows a third exemplary embodiment of an inverter circuit according to the invention.The inverter circuit 22 here comprises an inverter 23, as is shown for example with reference to FIGS. 4-4B.Furthermore, an input-side EMC filter 50 is provided, which is arranged between the supply terminals 24, 25 and the inverter 23. The EMC filter 50 with its differential mode inductance LDM, the common mode inductances LCM 1, LCM 2 and the capacitors CX, CY is important for the safety concept insofar as it stores a certain energy which influences the disconnection process. In addition, active and passive discharge circuits may be integrated (not shown in FIG. 5 ) in order to reduce the intermediate circuit voltage to safe values in a short time. The intermediate circuit capacitor CDC is furthermore connected between the EMC filter 50 and the inverter 23.In addition to the thermal fuses in the power switches T 1-T 6 of the half bridges 40 a- 40 cof the inverter 23, a further safety concept 27 is additionally implemented here.Accordingly, in order to avoid the large currents and field strengths associated with an active short circuit, a free wheel is used as a safe state. This is made possible by supplementing the inverter circuit 22 with an additional isolating switch 30, which is designed as a MOSFET, and an additional intermediate circuit free-wheeling diode DDC.In normal operation, the circuit breaker 30 is in the closed state and has no influence on the operation of the inverter 23. in the event of faults in the electric machine 10, all power switches T 1-T 6 of the half bridges 40 a- 40 eare switched off. At the same time, the isolating switch 30 is also switched into the open state. This prevents a transfer of energy back from the electric machine 10 to the DC voltage source 21, In this open-circuit operation, the electric machine 10 does not generate any braking torque and therefore no current flows. However, there are some features to be considered in this mode:The induced voltage of the electric machine 10 is rectified via the body diodes of the MOSFET power switches T 1-T 6 and charges the intermediate circuit to the rectified value of the induced voltage.In the field weakening region, the d-axis current disappears and the clamping voltage rises to the induced open circuit voltage of the given rotational speed, which can be higher than the battery voltage of the DC voltage source 21, but the isolating switch 30 does not allow energy to be transferred to the vehicle, so that the stress-free state in the vehicle is ensured even when the electric machine 10 is rotating.The inductive energy stored in the windings of the electric machine 10 must be transferred into the intermediate circuit capacitor CDC without exceeding the maximum breakdown voltage of the intermediate circuit capacitor CDC and inverter 23.The circuit breaker 30 can switch off high feedback currents, for example in the event of a short circuit in the high-voltage power supply system of the vehicle. After the switching off, the inductive energy stored in the supply voltage supply lines and the EMC filter 50 must be dissipated in the intermediate circuit free-wheeling diode DDC.During generator operation, the overvoltage protection of the inverter 23 must ensure as rapid a disconnection as possible in order to prevent an excessive voltage rise in the intermediate circuit.The active discharge circuit must withstand the maximum of the rectified induced voltage and ensure continuous operation even if the electric machine 10 runs out over a relatively long period of time. Suitable strategies for heat protection are also implemented without compromising the safety of the discharge circuit.The term "arranged parallel to one another" refers in the case of circuit breakers and power transistors to their controlled load paths which are arranged parallel to one another. A controlled load path denotes the current-carrying load path or the load path or the output current path. In the case of a bipolar transistor, the controlled load path is the connection between its emitter and collector, and in the case of a field effect controlled transistor, the controlled load path is the connection between its drain and source. The control terminal in such transistors is referred to as the base or gate. In the case of a polyphase inverter, the controlled load path denotes the high current path carrying the phase current.List of reference characters10 ELECTRIC MACHINE 11 OUTER ROTOR 12 INNER ROTOR 13 STATOR 14 MAGNET 15 MAGNET 16 OUTER AIR GAP 17 INNER AIR GAP 18 MAGNET 19 MAGNET 20 ELECTRIC DRIVE SYSTEM 21 DC voltage source 22 inverter circuit 23 inverter, inverter 24 (first) supply terminal 25 (second) supply terminal 26 load output 27 safety concept 28 control circuit 29 control terminal 30 circuit breaker 31 control circuit 40 a- 40 cdriver stages 41 a- 41 ccentre taps 42 a- 42 ethermal fuses 43 conductor track 43 a- 43 efeeding line sections 44 eth thinning 45 ewidth 50 EMC filter CX, CY capacitors CDC DC link capacitor DDC DC link free-wheeling diode IL phase current LDM differential mode inductance LCM 1, lcm2 Common mode inductances S1 Control signal S2 Control signal T1-T6 Power switches T1a-T1e Individual switches V11 First supply potential, positive supply potential V12 Second, negative supply potential, reference potential VDC Direct supply voltage

Claims

Inverter circuit for controlling a single- or polyphase electric machine for an electric drive system, having a first and second supply terminal via which the inverter circuit can be connected to a direct voltage source, having a load output for coupling the electric machine, having a controllable inverter which is arranged between the supply terminals and the load output and has a multiplicity of power switches which are interconnected in such a way that a direct voltage picked up on the supply side is converted into an alternating voltage for driving an electric machine, wherein at least one power switch has a multiplicity of individual switches arranged parallel to one another and having associated thermal fuses, wherein the thermal fuses are each arranged in the load path of the individual switch respectively associated therewith, and wherein the thermal fuses are each formed by using materials having different electrical resistance.Inverter circuit according to Claim 1, characterized in that at least one of the thermal fuses is designed as a fuse.Inverter circuit according to Claim 2, characterized in that at least one fuse is designed as a PCB conductor track which is designed in such a way that, at a predefined load current, the individual switch is switched off by melting the PCB conductor track.Inverter circuit according to one of the preceding claims, characterized in that the thermal fuse is designed as a thin material.Inverter circuit according to Claim 4, characterized in that the thermal fuse is designed as a material thinning as a tapering of a PCB conductor track.Inverter circuit according to one of the preceding claims, characterized in that each of the power switches of the inverter has a multiplicity of individual switches arranged parallel to one another and each having thermal fuses assigned to them.Inverter circuit according to Claim 6, characterized in that each of the power switches of the inverter has at least three individual switches or at least six individual switches with thermal fuses assigned in each case to them.Inverter circuit according to one of the preceding claims, characterized in that at least one free-wheeling diode is provided, which is arranged on the supply side between the first and second supply connections.Inverter circuit according to one of the preceding claims, characterized in that the power transistors are designed as power MOSFETs.Inverter circuit according to Claim 9, characterized in that the power MOSFETs are designed as Si MOSFETs or as SiC MOSFETs.Inverter circuit according to one of the preceding claims, characterized in that a three- or multi-stage inverter is provided.Inverter circuit according to one of the preceding claims, characterized in that the inverter has a number of driver stages which corresponds to the number of phases and each have at least one controllable power switch.Inverter circuit according to one of the preceding claims, characterized in that the inverter circuit is designed to actuate a dual rotor motor for or in a motor vehicle.An electric drive system, having an electric machine, having a DC voltage source, and having an inverter circuit according to one of Claims 1 to 13, which is connected on the supply side to the DC voltage source and is connected via its load output to the electric machine.Inverter circuit according to Claim 14, characterized in that the electric machine is designed as a synchronous machine.Inverter circuit according to Claim 14 or 15, characterized in that the DC voltage source is designed as an accumulator.Inverter circuit according to one of Claims 14 to 16, characterized in that the electric drive system is designed as an electric drive system in or for a motor vehicle.

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