INVERTER CIRCUIT FOR AN ELECTRICAL AC MOTOR, ELECTRICAL DRIVE AND METHOD FOR CONTROLLING AN ELECTRICAL DRIVE

The present disclosure relates to an inverter circuit for an electrical AC motor with permanent magnets, the circuit comprising a primary energy storage device, a plurality of switching stages, a drive control circuit and an emergency control circuit. The present disclosure further relates to an electrical drive comprising an inverter circuit and at least one AC motor with permanent magnets controlled by the inverter circuit. The present disclosure further relates to a method for controlling an electrical drive.

Document EP 0742 637 A1 relates to a method and device for safe braking of an electrical drive. Using a clock control signal, the reaction speed and braking duration can be optimized.

Embodiments of the disclosure relate to improved circuits and methods for controlling an electrical drive, which are better suited to high power applications. For example, it is desirable to limit the thermal load on switching stages of an inverter circuit during abnormal operation states.

According to a first aspect, an inverter circuit for an electrical AC motor with permanent magnets is provided. The inverter circuit comprises a primary energy storage device, a plurality of switching stages, a drive control circuit and an emergency control circuit. Each switching stage comprises a first semiconductor switch and a second semiconductor switch, the first semiconductor switches of the plurality of switching stages connected between a first terminal of the primary energy storage device and one of a plurality of phase lines of the electrical AC motor with permanent magnets, and the second semiconductor switches of the plurality of switching stages connected between a second terminal of the primary energy storage device and one of the plurality of phase lines. The drive control circuit is configured to selectively switch the first semiconductor switches and the second semiconductor switches to control a torque of the electrical motor during normal operation of the inverter circuit. The emergency control circuit is configured to switch on all of the first semiconductor switches or the second semiconductor switches by providing a short-term control voltage VGSelevated to control gates of the first semiconductor switches or the second semiconductor switches, respectively, to electrically connect the phase lines in case of an abnormal condition of the drive control circuit. The short-term control voltage VGSelevated exceeds a regular switch-on voltage VGSon provided to the control gates by the drive control circuit during normal operation of the inverter circuit .

By providing a short-term control voltage VGSelevated that exceeds a regular switch-on voltage VGSon, the internal resistance of the semiconductor switches can be reduced, resulting in a reduced energy dissipation in the semiconductor switches of the switching stages. This in turn leads to a lower thermal load on the switching stages and helps to sustain a current generated by the electrical motor over a longer time period.

According to at least one embodiment, a maximum transient voltage VGSmax specified for the first semiconductor switches or the second semiconductor switches may be used as the short-term control voltage VGSelevated. Use of the maximum transient voltage VGSmax, e.g. a maximum voltage specified by the semiconductor switch manufacturer, has the advantage that it does not lead to a significantly reduction of the lifetime of the semiconductor switches.

According to at least one embodiment, the emergency control circuit is further configured to alternate between a first switching stage and a second switching stage. In the first switching stage, the short-term control voltage VGSelevated is provided to all of the first semiconductor switches, and in the second switching stage the short-term control voltage VGSelevated is provided to all of the second semiconductor switches. By alternating between activating the first and second semiconductor switches over time, the energy distribution can be spread over multiple semiconductor devices, thus further reducing the electrical and thermal load on individual switches.

According to a second aspect of the disclosure, an electrical drive comprises an inverter circuit according to the first aspect and at least one AC motor with permanent magnets controlled by the inverter circuit. Such an electrical drive can survive an abnormal operation state for a longer time period without the need to provide a stronger inverter circuit for a given motor power or to provide a smaller motor for a given inverter strength.

In at least one embodiment, the electrical drive further comprises an emergency response unit, configured to be activated in case of an abnormal condition of the drive control circuit, the emergency response unit having a predetermined activation time, wherein the emergency control circuit is configured to provide the short-term control voltage VGSelevated to control gates of the first semiconductor switches or the second semiconductor switches during the predetermined activation time. Provision of a relatively slow emergency response unit, such as a braking device, a brake chopper or an isolation relay, in addition to a faster acting emergency control circuit allows the primary energy storage to be protected rapidly during the onset of an abnormal operation condition. At the same time, the emergency response unit can take further actions to protect the inverter circuit itself from damage resulting from relatively high currents generated by the temporarily uncontrolled permanent field excitation motor.

According to a third aspect, a method for controlling an electrical drive is provided. The method comprises: in a normal operating mode Ml, selectively controlling at least one of first semiconductor switches and second semiconductor switches of a first half-bridge and a second half-bridge of an inverter circuit, respectively, to control an electrically AC motor with permanent magnets of the electrical drive; and in an active short circuit (ASC) mode M2, selectively switching on all of the first semiconductor switches or the second semiconductor switches to electrically shorten phase lines of the AC motor, wherein, in the ASC mode, a short-term control voltage VGSelevated is provided to control gates of the first semiconductor switches or the second semiconductor switches, wherein the short-term control voltage VGSelevated exceeds a regular switch-on voltage VGSon provided to the control gates during the normal operation mode Ml.

The devices and methods described above are suitable for high power applications, such as electrical drives of electrical vehicles or industrial equipment.

As detailed above, the present disclosure comprises several aspects of an invention. Every feature described with respect to one of the aspects is also disclosed herein with respect to the other aspects, even if the respective feature is not explicitly mentioned in the context of the specific aspect.

The accompanying figures are included to provide a further understanding. In the figures, elements of the same structure and/or functionality may be referenced by the same reference signs. It is to be understood that the embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.

Figure 1 shows, in a schematic manner, an inverter circuit according to an embodiment of the present disclosure.

Figure 2 shows a circuit diagram of an electrical drive according to an embodiment of the present disclosure.

Figure 3 shows an output characteristic of a single semiconductor switching element.

Figure 4 shows an amount of energy dissipated over time by an inverter circuit. Figure 5 shows, in a schematic manner, a flowchart of a method for controlling an electrical drive.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the figures and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the exemplary embodiments described.

On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention defined by the appended claims.

Figure 1 shows, in a schematic manner, an inverter circuit 10 according to an embodiment of the present disclosure. The inverter circuit 10 comprises a primary energy storage device 12, a plurality of switching stages 14, a drive control circuit 16 and an emergency control circuit 18.

As shown in Figure 1, terminals of the energy storage device 12 and each of the switching stages 14 are connected by two different supply lines 20 and 22 for providing a direct current. Each of the switching stages 14 comprises a first semiconductor switch 24 connected to the positive supply line 20 and a second semiconductor switch 26 connected to the negative supply line 22.

In a normal operation state, the drive control circuit 16 controls each one of the semiconductor switches 24 and 26 of each one of the switching stages 14 to generate a multi-phase AC signal for an external, electrical motor (not shown in Fig. 1), provided through a plurality of phase lines 28. For example, in the case that a three-phase motor is to be controlled by the inverter circuit 10, three phase lines 28 connected to three corresponding switching stages 14 are used. Similarly, in the case of a six-phase electrical motor, six switching stages 14 and six corresponding phase lines 28 are used. During normal operation of the inverter circuit 10, the emergency control circuit 18 remains passive. That is to say, no control signals are provided by the emergency control circuit 18, and its output terminals remain in a high impedance state.

While the inverter circuit 10 is operating, the emergency control circuit 18 monitors the provision of control signals by the drive control circuit 16. In the case of a fault of the drive control circuit 16, the emergency control circuit 18 takes over control of the switching stages 14 of the inverter circuit 10. For example, if the control circuit 16 provides no or irregular output signals, the emergency control circuit 18 will provide control signals for the first semiconductor switches 24 and the second semiconductor switches 26 of all switching stages 14. To avoid any damage to the semiconductor switches 24 and 26, in this emergency operating mode, also referred to as active short circuit (ASC) mode, all phase lines 28 are shortened, either by the set of all first semiconductor switches 24 or all second semiconductor switches 26. This is achieved by providing a short-term control voltage VGSelevated to control gates of the first or second semiconductor switches 24, 26 that exceeds a regular switch-on voltage VGSon provided to the control gates of the first or second semiconductor switches 24 or 26 by the drive control circuit 16 during normal operation of the inverter circuit 10.

Details of the inverter circuit 10 and its operation are explained below with regard to an electrical drive 30. Figure 2 shows a circuit diagram of the electrical drive 30 comprising an inverter circuit 10 according to an embodiment of the present disclosure. In the described embodiment, the inverter circuit 10 of the electrical drive 30 comprises two energy storage devices, a battery 31, for example a rechargeable main battery of an electrical vehicle, and a direct current (DC) link capacitor 42 placed in electrical and physical proximity to the switching stages of the inverter circuit 10. The electrical drive 30 further comprises an alternating current (AC) motor 32 with permanent magnets such as an electrically commutated motor and/or a permanent field excitation motor. In the described embodiment, the AC motor 32 is an electric three-phase permanent-magnet synchronous motor (PMSM) connected to three switching stages 14a to 14c of the inverter circuit 10.

In the embodiment shown in Figure 2, each of the switching stages 14a to 14c comprises two n-channel MOSFETs 34 and 36 respectively (as an alternative MISFETs may exemplarily be used). The first, in Figure 1 upper, n-channel MOSFET 34 of each of the stages 14a to 14c is connected in series with the second, in Figure 2 lower, n-channel MOSFET 36 of the same stage between a positive terminal 38 and a negative terminal 40 of a DC link capacitor 42. Each one of three phase lines 28a to 28c of the three-phase AC motor 32 is connected between the first MOSFET 34 and the second MOSFET 36 of one of the three switching stages 14a to 14c, respectively. Together, the first n-channel MOSFETs 34 of the three switching stages 14a to 14c form a positive half-bridge 44 of the inverter circuit 10. Correspondingly, the second n- channel MOSFETs 36 of the three switching stages 14a to 14c form a negative half-bridge 46 of the inverter circuit 10. During normal operation of the electrical drive 30, switches SI to S6 of the inverter circuit 10 provided by the MOSFETs 34 and 36, respectively, are controlled by a drive control circuit (not shown in Figure 2) by selectively switching on individual switches as required to provide a desired torque of the AC motor 32. For this purpose, each one of the phase lines 28a to 28c can be selectively connected to the positive terminal 38 or the negative terminal 40 of the DC link capacitor 42 and/or another primary energy storage device, such as the battery 31. Suitable methods for the control of a three-phase AC motors 32 are known from the prior art and are therefore not described in detail here. In essence, a rotating electrical field is created within the electrical motor to control the speed and torque of its rotor as desired.

To switch on one of the switches SI to S6, a specific gate- source signal or voltage VGS, in the following referred to as switch-on voltage VGSon, is provided to a terminal 48 of the respective switch. Inversely, to deactivate one of the switches, a second, typically lower gate-source signal VGS, referred to as switch-off voltage VGSoff is provided to the respective gate terminal 48. Depending on the type of the semiconductor switch used, the specific voltages may differ. For example, for the n-channel MOSFETs 34 and 36 shown in Figure 2, VGSon corresponds to a voltage of +15 V, and VGSoff corresponds to a voltage of -4 V with respect to a reference potential such as electrical ground. However, other semiconductor switching elements may also be used. For example, in the case that IGBTs are used as switching elements, the switch-on voltage VGSon usually corresponds to +15 V and the switch-off voltage VGSoff corresponds to -15 V. Typically, such values are specified by the manufacturer of a given semiconductor switch.

If a fault occurs in the electrical drive 30, for example a fault in the drive control circuit 16 used to control the gate terminals 48 of the switches SI to S6, the general control electronic of the electrical drive 30, or some other unexpected event, such as a lack of feedback within the control circuitry, the AC motor 32 will typically be free- spinning, and therefore act as a generator for the circuit shown in Figure 2. In a controlled mode of operation, a current generated by the free-spinning AC motor 32 can be used to recharge the DC link capacitor 42 or other primary energy storage devices to recuperate some of the kinetic energy of the system as electrical energy. For example, for an electrical vehicle, controlled braking of the vehicle may be used to recharge a rechargeable main battery of the vehicle through controlled operation of the inverter circuit 10.

However, if no gate-source voltage VGS is provided to the switches SI to S6, the inverter circuit 10 by default will enter a state where all the MOSFETs 34 and 36 are turned off. In this case, the rotating energy of the motor will flow through the body diodes of the MOSFETs 34 and 36 or, in case IBGTs are used as switches SI to S6, through their anti parallel diodes, and will start charging the DC link capacitor 42 and/or other primary energy storage device 10, such as the battery 31. This is a non-desired behavior because of a danger of overstressing the DC link capacitor 42 or charging the battery 31 with an overcurrent. Thus, in the case that the inverter circuit 10 cannot be controlled in a suitable manner, an emergency control circuit (not shown in Figure 2) will take over control of the switches SI to S6 to avoid an unacceptably high load current to the DC link capacitor 42 and/or battery 31.

For this purpose, all semiconductor switches of one of the positive or negative half-bridges 44 or 46 are activated together, effectively forming a short-circuit between the phase lines 28a to 28c of the AC motor 32. In this way, the rotating energy of the AC motor 32 at the time of the fault is transformed into heat on the switches SI to S6 and dissipated at a cooling system of the inverter circuit 10.

In the example shown in Figure 2, all first n-channel MOSFETs 34 are activated by providing a short-term control voltage VGSelevated to the gate terminals 48 of high-side switches SI, S3 and S5. In the described example, a maximum transient voltage allowed by the individual manufacturer of the MOSFETs 34, referred to as VGSmax, is used. This has the advantage that it does not reduce the lifetime of the semiconductor switches. Using a significantly higher gate control voltage could result in overstressing the gates of the n-channel MOSFETs 34. At the same time, a switch-off voltage VGSoff is provided to the gate terminals 48 of the second n-channel MOSFETs 36 corresponding to low-side switches S2, S4 and S6. However, as detailed later, it is also possible to activate the switches of the negative half-bridge 46 and open the switches of the positive half-bridge 44.

In this ASC mode of operation, a relatively high current flows from the respective phase lines 28 of the AC motor 32 through the MOSFETs 34 in a short-circuit, resulting in a high energy dissipation and thus heating of the semiconductor body of each one of the MOSFETs 34. If the MOSFETs 34 temperature rises above a certain level, destruction of the semiconductor chip will occur due to the elevated temperature. The capability of the MOSFETs 34 and by extension, the inverter circuit 10, to survive such an ASC event is given by the maximum energy which the MOSFETs 34 can sustain. The energy dissipated by the MOSFETs 34 during the

ASC event of time duration tl is given by E _ASC = f _Q RDSonI ² dt.

A typical duration of an ASC event is in the order of tens of milliseconds, e.g. the time it takes for regaining control or stopping the drive 30 by other means as detailed below.

To reduce the internal channel resistance RDSon of the MOSFETs 34, the short-term control voltage VGSelevated provided to the gates 48 of the MOSFETs 34 during the relatively short ASC event is increased with respect to the normal switch-on voltage VGSon. For example, in the described embodiment, a control voltage of 19 V is used, which is 4 V or around 25 % higher than the typical switch-on voltage of 15 V.

It is noted that the specific value of the short-term control voltage VGSelevated to be used in the ASC mode may differ depending on the type of semiconductor switch used. For example, for many commercially available semiconductor switches, a maximum allowable peak voltage or maximum transient voltage VGSmax for short-term provision to the gate terminal 48 may be specified by the respective manufacturer and used as the short-term control voltage VGSelevated.

The effects of such a control method is shown in Figures 3 and 4. Figure 3 shows the output characteristic of a single semiconductor switch for two different gate control voltages, i.e. 15 V and 19 V. Taking a 750 V SiC MOSFET as an example, it can be seen that, for the same current flowing through the semiconductor switch, the voltage drop or bias voltage between the source and drain terminals of the respective switch is lower for the higher control gate supply. For example, the channel resistance RDSon for a current of 180 A is reduced from 21.9 W to 19.2 W, an improvement by 14 %.

This also gives a 14 % margin to the current part of the equation above, because the maximum energy is depending on the MOSFET chip geometry and therefore remains the same. Decreasing the channel resistance RDSon by 14 % allows the

ASC event current to increase to Vl.14 = 1.068 or 6.8 % for a given amount of energy that can be safely dissipated by the switching component. In this context, it is noted that a free-spinning AC motor with permanent magnets essentially acts as a current source. Thus, increasing a gate control voltage for a given speed of the AC motor 32 will result in a reduction of energy being dissipated in the channel region of the corresponding semiconductor switch. The amount of energy that can be safely dissipated by a semiconductor switch during a short-term event, such as an ASC event, may also be specified by the manufacturer, allowing to safely dimension the components of the electrical drive 30.

This is shown in more detail in Figure 4, showing the total amount of energy being dissipated in the inverter circuit 10 during an ASC event. In the example, the ASC event results in a current of 1800 A and last for 10 ms. After this time, other means of control take over as described in further detail below.

After detection of the loss of control at t=0 ms, the emergency control circuit 18 quickly activates corresponding semiconductor switches 24 or 26 by the provision of a corresponding control voltage to the control gates 48 of high-side switches SI, S3 and S5, leading to a rapid increase of a current through these switches. At t=10 ms, the high- side switches SI, S3 and S5 are deactivated again, leading to a decrease of a current through these switches. Note that there is a time lag for both events, caused by the accumulation of charge carriers in a channel of the respective semiconductor switches. The energy E _ASc being absorbed by the inverter circuit 10 is limited to a value determined by the maximum current and length of the ASC event. This energy level differs, depending on the provided gate voltage. In the described example, in case the gate terminals 48 of high-side switches SI, S3 and S5 are biased with 19 V, the inverter circuit 10 absorbs an energy E _ASc of 130 J during the ASC event. In case the gate terminals 48 of high-side switches SI, S3 and S5 are biased with 15 V, the inverter circuit 10 absorbs an energy E _ASc of 160 J during the ASC event, i.e. 30 J or 23 % more energy.

Coming back to the circuit diagram of Figure 2, the electrical drive 30 may comprise further components to avoid any damage on the components of the inverter circuit 10 caused by a prolonged operation of the inverter circuit 10 in the ASC mode. While the ASC emergency mode is very fast to respond, as shown in Figure 4, it does not provide a long term solution for the control of the AC motor 32 in the case that the normal drive control circuit 16 cannot be reactivated quickly. Accordingly, the electrical drive 30 may comprise further components, such as a mechanical brake 50 which can be used to stop the electrical AC motor 32. Alternatively or in addition, one or several isolation relays 52 may be used to permanently disconnect the switching stages 14a to 14c from the battery 31 to avoid any damage to it, for example by overcharging. Moreover, a braking chopper 54 may be used to shorten the positive terminal 38 and the negative terminal 40 of the DC link capacitor 42 using a shunt resistor. Typically, the isolation relays 52 are implemented as electromechanical relays and therefore have higher activation times than the semiconductor switches formed by the MOSFETs 34 or 36. Moreover, the energy dissipated by the inverter circuit 10 can also be distributed over different switch groups as detailed below.

Figure 5 shows a flowchart of a method for operating the electrical drive 30. Initially, the electrical drive 30 is operated in a normal operation mode Ml. In case a failure during normal operation is detected in step 61, the drive unit 30 switches to an ASC mode M2 of operation, wherein an emergency control circuit 18 takes over control of the inverter circuit 10.

In the example shown in Figure 4, in a step 62, all first semiconductor switches 24 of a positive half-bridge 44 are shortened to avoid a feedback of a high current to a primary energy storage device 12. At the same time, the second semiconductor switches 26 of a negative half-bridge 46 are switched off to electrically separate the primary energy storage device 12 from the AC motor 32. This state is maintained for a predetermined amount of time, for example a few milliseconds, e.g. 2 to 5 ms. Thereafter, to avoid overheating of the first semiconductor switches 24, the second semiconductor switches 26 of the negative half-bridge 46 are shortened in a step 63 and the first semiconductor switches 24 of the positive half-bridge 44 are deactivated. In optional steps 64, 65 and 66, one or more other emergency measures may be taken by the emergency control circuit 18.

For example, in step 64, a mechanical brake 50 may be employed to slow down the AC motor 32. Moreover, in a step 65, one or more isolation relays 52 may be controlled to physically disconnect a rechargeable battery 31 from the switching stages 14 of the inverter circuit 10. In a step 66, a braking chopper 54 may be used to electrically shorten terminals 38 and 40 of a DC link capacitor 42.

The embodiments shown in the Figures 1 to 5 as stated represent exemplary embodiments of the improved devices and methods for their operation. Therefore, they do not constitute a complete list of all embodiments according to the improved devices and methods. Actual devices and methods may vary from the embodiments shown in terms of their specific components, configurations, signals and processing steps, for example.

Reference Signs

10 inverter

12 primary energy storage device

14 switching stage

16 drive control circuit

18 emergency control circuit

20 positive supply line

22 negative supply line

24 first semiconductor switch

26 second semiconductor switch

28 phase line

30 electrical drive

31 battery

32 AC motor 34 first (n-channel) MOSFET 36 second (n-channel) MOSFET 38 positive terminal 40 negative terminal 42 DC link capacitor 44 positive half-bridge 46 negative half-bridge 48 gate terminal 50 mechanical brake 52 isolation relay 54 braking chopper

61-66 method steps

Ml normal operation mode

M2 ASC mode

VGS gate-source voltage

VGSon switch-on voltage

VGSoff switch-off voltage

VGSmax maximum transient voltage

VGSelevated short-term control voltage