Converter for a drive unit, and drive unit

Abstract

The present invention relates to a converter (10) for a drive unit (8), and to a drive unit (8). The converter (10) comprises at least one circuit breaker (18, 20), at least one driver circuit (28) coupled to the circuit breaker (18, 20), and a control device (30) that is coupled to the at least one driver circuit (28). The at least one driver circuit (28) has a desaturation monitoring circuit (42) having a DESAT diode (44). The at least one driver circuit (28) is configured to detect a measurement voltage and provide same to the control device (30). The measurement voltage comprises a diode voltage across the DESAT diode (44) and an instantaneous voltage prevailing between the drain electrode (36) and the source electrode (40) of the circuit breaker (18, 20) in the on-state. The control device (30) is configured to determine the instantaneous voltage on the basis of the received measurement voltage. The control device (30) is configured to determine a fault threshold value, which is variable over time and dependent on the operating point, for the instantaneous voltage. The control device (30) is configured to output a safe-state signal to the driver circuit (28) if the instantaneous voltage exceeds or falls below the fault threshold value.

Description

[0001] Inverter for a drive unit and drive unit

[0002] The present invention relates to a converter for a drive unit and a drive unit.

[0003] In modern converters, for example in the case of traction converters, topological switches (typically comprising several transistors) with silicon carbide topology (SiC topology) are increasingly used to replace previously common topologies of insulated gate bipolar transistors (IGBT topologies) because the SiC topology offers several advantages.

[0004] For example, the SiC topology enables a reduction in installation space and power losses, at least if the operating point of the converter is selected accordingly. This is primarily due to the fact that in IGBT topologies, a so-called tail current occurs when the corresponding transistor is turned off. This is due to the fact that minority charge carriers must be removed from the n-type drift region of the transistors, which accounts for a significant proportion of the switching losses in IGBT topologies. For SiC topologies, on the other hand, the switching losses when the corresponding transistor is turned off are significantly lower, at least if the operating point is selected accordingly. This allows SiC topologies to be operated at higher switching frequencies than IGBT topologies, while still reducing power losses.In addition to the reduced switching losses, the conduction losses in the medium load range are significantly reduced in SiC topologies compared to IGBT topologies. Since the medium load range is a common application for the underlying converters, the converter's efficiency is significantly increased by switching from IGBTs to SiC.

[0005] To prevent bridge short circuits, safety mechanisms are taken into account. For example, in IGBT topologies, the junction temperature can be measured, and at least the corresponding topological switch, usually the entire corresponding converter, can be transferred to a safe state if a limit temperature is exceeded. However, since the detection of temperature development has latencies (time intervals of 100 ms or longer), a predefined voltage threshold is also provided, with which the forward voltage of the topological switch is compared. If the voltage threshold is exceeded, the topological switch (or the underlying converter) is also transferred to a safe state, creating a much faster safety mechanism (time intervals of 10 ps or shorter).This also provides a safety mechanism for rapid current increases, such as those that occur in the event of a short circuit.

[0006] Since the SiC topology enables a reduction in installation space, several transistors are typically arranged in parallel for a topological switch. Although this is generally also the case with IGBT topologies, the rated current per chip (and thus also per transistor) is generally lower in SiC topologies for manufacturing reasons, which means that a larger number of transistors are connected in parallel in the case of SiC topologies compared to IGBT topologies. This makes it difficult to easily measure the junction temperature, since, for example, not all transistors connected in parallel can be measured independently of one another. Typically, only the temperature of the component that is empirically known to be the hottest is measured. However, this does not guarantee an effective temperature-based protection mechanism for all components.In addition, however, parallel connection also leads to divergences between the junction temperatures of the individual transistors. Therefore, a temperature-based safety mechanism is no longer possible or only inadequately implemented in the case of SiC topologies.

[0007] Furthermore, the detection of the forward voltage is subject to significant tolerances due to the underlying measurement principle and the temperature dependence of the forward resistance. In this regard, the corresponding voltage thresholds have so far been set quite high to ensure proper operation in special transient cases. In other words, the aim is to prevent false triggering during operation. This problem is a consequence of the switch from IGBT topologies to SiC topologies, as IGBT topologies are not equally affected by this due to their low power density (chip area per ampere; correspondingly also area of ​​the topological switch per ampere). This can potentially lead to faults that could damage the topological switch and thus the underlying converter, or overload the high-voltage electrical system of a vehicle equipped with such a converter due to a bridge short circuit.

[0008] In the following, a topological switch is meant whenever a power switch is mentioned. A power switch therefore comprises at least one, but generally several, transistors or chips connected in parallel.

[0009] The invention is based on the object of eliminating or at least mitigating the disadvantages of the prior art. In particular, it is desirable to be able to provide appropriate safety mechanisms for a circuit breaker in order to be able to transfer it to a safe state as needed, regardless of the topology of the underlying circuit breaker.

[0010] The problem is solved by the subject matter of the independent patent claims. Advantageous embodiments are specified in the dependent patent claims and the following description, each of which, individually or in (sub)combination, may represent aspects of the invention.

[0011] According to one aspect, a converter for a drive unit is provided. The converter comprises at least one power switch, at least one driver circuit coupled to the power switch, and a control device coupled to the at least one driver circuit.

[0012] The at least one driver circuit includes a desaturation monitoring circuit (DESAT circuit) with a DESAT diode. The DESAT diode is associated with the at least one power switch and coupled to a drain electrode of the associated power switch.

[0013] The at least one driver circuit is configured to detect a measurement voltage and provide it to the control device. To this end, the driver circuit applies a (small) measurement current to the DESAT diode to place it in a conductive state, thereby enabling the detection of the measurement voltage. The measurement voltage comprises a diode voltage across the DESAT diode and an instantaneous voltage dropped between the drain electrode and the source electrode of the power switch in the on-state. The measurement voltage thus corresponds to the sum of the diode voltage of the DESAT diode and the instantaneous voltage. The instantaneous voltage is a current or momentarily present voltage between the drain electrode and the source electrode of the power switch in the on-state.

[0014] The control device is configured to determine the instantaneous voltage based on the obtained measurement voltage.

[0015] The control device is also designed to determine a time-varying error threshold value for the instantaneous voltage, which depends on the operating point of the converter.

[0016] The control device is additionally configured to output a safe-state signal to the driver circuit if the instantaneous voltage exceeds or falls below the error threshold.

[0017] Unlike previously, the converter does not rely exclusively on a predefined fault threshold for the instantaneous voltage drop across the circuit breaker. Instead, a time-varying fault threshold is determined depending on the converter's operating point.

[0018] Since the converter's operating point essentially depends on an actuator of a higher-level control loop, such as the driver's accelerator pedal position, the operating point changes only very slowly over time. In other words, although the operating point can generally be selected variably, it is "quasi-static" from a rotating field perspective. The "quasi-static" temporal behavior of the operating point makes it possible to set and adjust the fault threshold, particularly in relation to the actual instantaneous voltage drop across the circuit breaker.

[0019] To determine the instantaneous voltage, the measured voltage is adjusted for the diode voltage component. The diode voltage is determined or estimated and subtracted from the measured voltage to obtain the instantaneous voltage. As a result, a reduced tolerance range can be provided for the time-varying fault threshold with respect to the actual instantaneous voltage drop across the circuit breaker. The time-varying fault threshold is thus subject to fewer tolerances. This allows the permissible range for the converter's operating point to be expanded.

[0020] The solutions known from the state of the art, on the other hand, are based exclusively on error thresholds that are constant over time, which is why these solutions must necessarily have greater tolerances than the present solution, which is accompanied by lower efficiency.

[0021] In principle, a topology-independent safety mechanism is ensured, which advantageously requires only a small (computational) effort to determine the time-varying error threshold.

[0022] The narrower tolerance range achieved here can be used to reduce the cooling requirements for the circuit breaker. For example, the cooling temperature requirement of a coolant can be reduced due to the specifically tuned time-varying fault threshold, further increasing the efficiency of the converter. Additionally or alternatively, the reduced cooling requirements can also result in a reduction in installation space.

[0023] For example, the control device comprises at least one processor that performs the computational processes to estimate or determine the diode voltage of the DESAT diode, to determine the instantaneous voltage, and / or to determine the time-varying error threshold. The control device can be embodied as a microcontroller.

[0024] The driver circuit can also be referred to as a gate driver circuit for a gate electrode of the power switch. The driver circuit is generally configured for voltage-based or current-based control of the state of the gate electrode.

[0025] The operating point of the converter can be assigned to a phase current that is determined by a phase voltage output by the at least one circuit breaker. The circuit breaker provides a phase voltage through which the phase current is adjusted according to the machine and operating parameters of the electrical machine connected to the converter. The control device is then also configured to receive or determine the phase current of the circuit breaker. For example, a target torque that is to be provided by the electrical machine coupled to the converter can be transmitted to or specified to the control device, e.g., by a higher-level drive control loop.Based on the target torque, the control device can determine a target phase current that must be provided by the inverter based on a corresponding output phase voltage so that the electric machine can achieve the target torque.

[0026] The phase current is preferably a target phase current of the circuit breaker, an actual phase current, or a time-averaged root mean square phase current (RMS phase current). The corresponding phase current can then be used to determine the time-varying adjusted fault threshold. This advantageously simplifies the determination of the fault threshold. Since the target phase current or the RMS phase current exhibit only slow temporal variations, the effort required to determine the time-varying fault threshold is low. Essentially, the target phase current or the RMS phase current can be regarded as "quasi-static" with respect to the rotating electric field, which is why the respective assigned operating point is also "quasi-static."

[0027] Since the control device can also include a current controller that receives the actual phase current, i.e. the measured phase current, for control, the actual phase current can also be used to detect the operating point.

[0028] The time-varying, operating point-dependent error threshold is therefore determined, among other things, via the phase current.

[0029] In some embodiments, the control device is configured to determine the fault threshold based, among other things, on component properties of the at least one circuit breaker. Thus, the time-varying fault threshold is even more precisely tailored to the topology of the respective at least one circuit breaker.

[0030] For example, the control device is configured to determine the fault threshold based at least on a maximum resistance "R_ds-on-max" between the drain electrode and the source electrode of the respective power switch in the on-state at a maximum permissible junction temperature "TJ-max" of the power switch. This means that a predetermined maximum permissible junction temperature is indirectly taken into account via the maximum resistance in the on-state. The junction temperature of the power switch generally influences the resistance "R_ds-on" of the power switch in the on-state between the drain electrode and the source electrode. Thus, a maximum permissible junction temperature "TJ-max" can be selected. As a result, a maximum resistance in the on-state between the drain electrode and the source electrode is assigned to this maximum permissible junction temperature "TJ-max" ("R_ds-on" for "TJ-max"). “R_ds-on-max”).

[0031] The maximum resistance is taken into account when determining the time-varying fault threshold "U_ds-on-err," which indirectly determines the time-varying fault threshold "U_ds-on-err" as a function of the maximum permissible junction temperature "TJ-max" and thus of the component properties of the circuit breaker. This allows the temperature behavior of the circuit breaker to be taken into account without directly measuring the junction temperature.

[0032] The control device can be configured to determine the error threshold value "U_ds-on-err" based on the product of a phase current assigned to the operating point of the converter and the maximum resistance "R_ds-on-max." The phase current can be, in particular, the target phase current "l_Soll" or the time-averaged effective value of the phase current "l_RMS," i.e., the RMS phase current.

[0033] Optionally, at least one tolerance offset "U_ds-on-tol" can be considered when determining the time-varying error threshold. This means that the time-varying error threshold can be determined as follows, where "I" is the phase current, e.g., the target phase current "l_Soll" or the time-averaged effective value of the phase current "l_RMS":

[0034] U_ds-on-err = (R_ds-on-max • I) + U_ds-on-tol.

[0035] The measurement voltage acquired by the driver circuit can be denoted by "U_adc." It consists of the diode voltage drop across the DESAT diode, "U_desat-diode," and the instantaneous voltage drop across the power switch in the on-state, "U_ds-on-mom":

[0036] U_adc = U_desat diode + U_ds-on-mom.

[0037] The time-varying error threshold "U_ds-on-err" is tuned to the instantaneous voltage drop "U_ds-on-mom" across the power switch in the on-state. Therefore, the measured voltage detected by the driver circuit must be adjusted for the diode voltage component "U_desat-diode."

[0038] In some embodiments, the converter has a B6 bridge with pairs of high-side power switches and low-side power switches. Each power switch is assigned a separate driver circuit. The control device is coupled to all driver circuits. The control device is configured to determine the diode voltages dropped across the DESAT diodes of the respective DESAT circuits based on an averaging of the measurement voltages recorded for the high-side power switch and a low-side power switch assigned to this high-side power switch. Simply put, it can be assumed that the actually occurring measurement voltages of the assigned high-side and low-side power switches partially compensate each other due to the 180° electrical phase shift between them. This leaves only the respective components attributable to the diode voltages.This allows an average value of the diode voltage to be determined for both DESAT diodes.

[0039] The control device is arranged, with respect to each power switch of the converter, to determine the respective instantaneous voltage drop and a respective time-varying fault threshold, in a manner corresponding to the explanations given above.

[0040] Optionally, the control device is configured to determine the diode voltage drop across the DESAT diode based on an interpolation for a phase current of essentially 0 A, i.e., the diode voltage at the zero crossing. The time of the zero crossing can be determined in a variety of ways, e.g., the midpoint between two local extreme values ​​of the measured voltage and / or by taking into account the phase currents of the other circuit breakers. The measured voltage interpolated for this time, i.e., at the zero crossing, drops exclusively across the DESAT diode. In other words, the interpolated value then corresponds to the diode voltage.

[0041] Alternatively or cumulatively, the control device is configured to determine the diode voltage drop across the DESAT diode based on an averaging of a measured voltage recorded for the at least one circuit breaker over a period (of the phase current). Averaged over time, the components of the instantaneous voltage in the measured voltage compensate each other over the period, leaving the component caused by the diode voltage, which can then be determined accordingly.

[0042] The diode voltage drop across the DESAT diode "U_desat-diode" can be assumed to be constant, at least for the subsequent switching cycle of the associated power breaker. This advantageously simplifies the determination of the instantaneous voltage "U_ds-on-mom" dropped across the power breaker. To a good approximation, the determined value of the diode voltage "U_desat-diode" can also be assumed to be constant overall, since this generally only varies due to component variation and / or depending on the installation space temperature. Although the power breaker is generally subject to strong temperature variations, due to the structural distance, these have no effect on the DESAT diode, or only a negligible effect. In other words, the temperature of the DESAT diode can be assumed to be constant, to a good approximation, so that it does not influence the diode voltage U_desat-diode.

[0043] The control device is configured to output a digital control signal to the driver circuit, based on which the gate electrode of the at least one power switch is controlled. The digital control signal is, for example, a pulse-width modulated signal (PWM signal).

[0044] In addition, the control device can be configured to lock the converter based on the safe-state signal. All of the converter's power switches can be locked based on the safe-state signal. In other words, the converter can be de-energized, at least with regard to its outputs, using the safe-state signal.

[0045] In some embodiments, the driver circuit has at least one comparator configured to compare the measurement voltage with a temporally constant error threshold. The comparator is coupled to the control device and configured to transmit a comparison signal to the control device. The control device is configured to output a shutdown signal to the driver circuit based on the comparison signal received from the comparator. As a result, in addition to the temporally variable error threshold, a temporally constant error threshold can also be taken into account in order to consider an additional shutdown threshold. This can be a safety mechanism for the event of a short circuit, which can trigger accordingly quickly, namely within 10 ps or less.

[0046] Preferably, the control device is configured to disable the converter based on the shutdown signal. This means that all power switches of the converter can be disabled and / or at least de-energized with respect to the respective outputs.

[0047] The converter incorporates two safety mechanisms: a time-constant fault threshold for short-circuits and a time-variable fault threshold. These two safety mechanisms are designed for different faults, which is why their tolerances and tripping speeds are different.

[0048] Optionally, the driver circuit includes a galvanic isolation device designed to protect the control device from crosstalk. This allows the control device to be protected from excessively high voltages to prevent damage.

[0049] Alternatively or cumulatively, the converter may also have a separate galvanic isolation between the driver circuit associated with the at least one power switch and the control device.

[0050] In some embodiments, the DESAT circuit comprises at least two DESAT diodes arranged in series, which are coupled to the drain electrode of the respective power switch. This ensures redundancy with regard to the protection mechanism provided by the DESAT diodes. For example, creepage distance requirements can be ensured in this way. These requirements may exist, for example, when the power switch is operated in the high-voltage range, for example at voltages of 100 V or more, preferably 400 V or higher, particularly preferably 800 V or more. In contrast, the driver circuit is typically operated with a low voltage, for example at a voltage of approximately 15 V relative to a reference potential. The DESAT diode(s) decouple these voltage regimes.

[0051] The power switch optionally features a silicon carbide topology. This allows for efficiency gains compared to IGBT topologies through reduced power losses. This allows the advantages of the silicon carbide topology to be utilized while simultaneously ensuring effective protection mechanisms without requiring high computational effort from the control device.

[0052] According to a further aspect, a drive unit is also provided. The drive unit comprises an electric machine and a converter, as previously explained, which is associated with the electric machine.

[0053] According to a further aspect, a vehicle is also provided. The vehicle, in particular, comprises a converter as described above. Optionally, the vehicle can be a motor vehicle.

[0054] For the purposes of the present invention, vehicles (motor vehicles) can include, in particular, land vehicles with an electric machine, namely, inter alia, tricycles, trikes, quads, off-road and road vehicles such as passenger cars, buses, trucks, tractors and other commercial vehicles, and rail vehicles (trains). Preferably, the vehicles can have at least one electric machine used to propel the vehicle. Vehicles can be manned or unmanned. In addition to pure electric vehicles (BEVs), plug-in hybrids (PHEVs) and fuel cell electric vehicles (FCEVs) can also be included. Alternatively, motor vehicles can also include vehicles with an internal combustion engine, which have at least one electric machine that can be operated with an associated converter as described above.

[0055] The invention, as well as further advantageous embodiments and developments thereof, are described and explained in more detail below with reference to the examples shown in the drawings. The features shown in the description and the drawings can be used individually or in any combination according to the invention. They show:

[0056] Fig. 1 is a schematic representation of a drive unit according to the invention with a converter and an electrical machine,

[0057] Fig. 2 is a schematic representation of parts of a converter according to the invention,

[0058] Fig. 3 schematic representations of signal curves in connection with the converter according to the invention, and

[0059] Fig. 4 is a schematic representation of a vehicle with a drive unit according to the invention.

[0060] All features disclosed below with respect to the embodiments and / or the accompanying figures may be combined alone or in any sub-combination with features of the aspects of the present disclosure, including features of preferred embodiments, provided that the resulting combination of features is meaningful to a person skilled in the art.

[0061] Fig. 1 shows a schematic representation of a drive unit 8 according to the invention with a converter 10 and an electric machine 12.

[0062] According to this embodiment, the converter comprises a B6 bridge 14 with three half bridges 16.

[0063] Each half-bridge 16 comprises a first power switch 18, which here is designed as a transistor (n-channel MOSFET) and acts as a high-side switch, and a second power switch 20, which is also designed as a transistor (also an n-channel MOSFET) and acts as a low-side switch. Between the first power switch 18 and the second power switch 20, each half-bridge 16 comprises a center tap 22 for providing a phase voltage U, V, W for the electric machine 12, by which a phase current is driven or established (for example, based on the machine parameters of the electric machine 12).

[0064] Through the three half-bridges 16, the electrical machine 12 is supplied with three corresponding phase voltages, U, V and W. Other topologies, for example a six-phase electrical machine 12, are also possible, which then require corresponding modifications of the converter 10.

[0065] The respective half-bridges 16 are coupled via lines 24, for example, a busbar structure, to a direct current source 26, for example, a high-voltage storage device, whose electrical energy is converted into kinetic energy by the electric machine 12. The electric machine 12 can, for example, serve to drive a vehicle. In this case, a high-voltage voltage HV of 800 V can be provided.

[0066] Each power switch 18, 20 is also assigned a driver circuit 28 ("DRV" for "driver"), which can also be referred to as a gate driver. The respective driver circuit 28 controls the associated power switch 18, 20, as explained in more detail below.

[0067] Depending on the switching positions of the circuit breakers 18, 20, corresponding commutation cells are formed in the converter 10 when the high voltage is applied.

[0068] In addition, the converter 10 includes a control device 30 ("MCU" - micro controller unit) coupled to the driver circuits 28. For clarity, the respective connection of the control device 30 to the driver circuits 28 of the first half-bridge 16 is shown continuously. The other connections between the control device 30 and the driver circuits 28 are merely indicated. The driver circuits 28 receive control signals from the control device 30 to control the switching positions of the power switches 18, 20.

[0069] The control device 30 is configured to indirectly provide corresponding (digital) switching signals for the circuit breakers 18, 20, which are transmitted to the driver circuits 28, which generate the control signals for the circuit breakers 18, 20 based on the switching signals. Based on the received switching signals, the driver circuits 28 provide control voltages to influence the switching positions of the circuit breakers 18, 20.

[0070] The converter 10 comprises output busbars 32, which are coupled, on the one hand, to the center taps 22 of the half-bridges 16, and, on the other hand, to the electric machine 12. Therefore, the illustrated embodiment shows three output busbars 32. The electrical power from the converter 10 is output to the electric machine 12 via the output busbars 32.

[0071] In the embodiment shown, the drive unit 8 comprises phase current sensors 34 arranged in the output busbars 32. The phase current sensors 34 detect the phase currents resulting from the output phase voltages U, V, and W, i.e., the actual values ​​of the phase currents (IJst), and transmit the corresponding values ​​to the control device 30. Based on the detected phase currents, the control device 30 can determine the required states of the circuit breakers 18, 20. In particular, the control device 30 can determine how a duty cycle of a control signal for the circuit breaker 18, 20 is to be adjusted as needed for a subsequent control interval so that the actual phase current IJst can be adjusted to the target phase current l_Soll.

[0072] In Fig. 2, parts of the converter 10 according to the invention are shown in more detail with regard to the structure of the control device 30 and one of the driver circuits 28. In principle, the control device 30 is connected to all driver circuits 28. However, for reasons of clarity, only one of the driver circuits 28 is shown in Fig. 2.

[0073] The functionality of the driver circuit 28 is explained with reference to the individual associated circuit breaker 18. However, this functionality can be transferred to all circuit breakers 18, 20 of the converter 10.

[0074] The power switch 18 is a topological power switch which in this case has a plurality of transistors arranged in parallel (generally at least one transistor), in particular n-channel MOSFETs, whereby the achievable, outputtable phase current can be increased.

[0075] The power switch 18 includes a drain electrode 36, a gate electrode 38, and a source electrode 40. The source electrode 40 is coupled to a reference potential. The same reference potential is assigned to the driver circuit 28, so that it can be used for voltage measurement.

[0076] The driver circuit 28, which controls the power switch 18, also includes a DESAT circuit 42 (desaturation monitoring circuit) which has a DESAT diode 44 which is associated with the drain electrode 36 of the power switch 18.

[0077] The DESAT diode 44 allows the driver circuit 28, which operates at a low voltage, to be protected from the high-voltage range present at the power switch 18. Typically, the driver circuit 28 is operated with voltage amplitudes in the range of 15 V.

[0078] The driver circuit 28 has an analog-to-digital converter 46.

[0079] The driver circuit 28 is configured to detect a measurement voltage U_adc relative to the reference potential. As already explained above, the reference potential is the potential present at the source electrode 40.

[0080] The measuring voltage U_adc represents the sum of the voltages across the DESAT diode

[0081] 44 and the voltage drop across the power switch 18, i.e. between the drain electrode 36 and the source electrode 40.

[0082] According to the present embodiment, the measurement voltage U_adc is detected via the DESAT circuit 42 in the on-state and digitized via the analog-to-digital converter 46 and then provided to the control device 30.

[0083] In addition, the drive unit 8, as explained above, is configured to detect the actual phase current l_lst, which temporarily flows through the power switch 18 (or power switch 20) by the PWM signal applied to the power switch 18 (or power switch 20), for example via the phase current sensor 34. The identification of the conduction state of the power switch 18 can be based on the digital control signal provided by the pulse width modulator 50, as explained in more detail below.

[0084] Both the detected phase current IJst and the detected measurement voltage U_adc are provided to the control device 30.

[0085] Furthermore, according to this embodiment, the control device 30 receives the target phase current l_Soll at an input, which can be provided, for example, by a higher-level drive control loop, for example, depending on an accelerator pedal position. Alternatively, the target phase current l_Soll can also be determined by the control device 30, provided that a target torque to be provided by the electric machine 12 is transmitted to the control device 30 as a specification.

[0086] In general, the control device 30 comprises a current controller 48 which, based on a comparison of the desired phase current l_desired with the actual phase current l_l st, takes into account the rotor position, determines a control signal which is transmitted to a PWM unit 50.

[0087] The PWM unit 50 essentially determines the digital control signal for the driver circuits 28 assigned to the respective power switches 18, 20 in order to adjust the actual phase current to the desired phase current. The control device 30 further includes a software desaturation unit 52 (soft DESAT).

[0088] The Soft-DESAT 52 receives the measuring voltage U_adc and the phase current I, e.g. the target phase current l_target and the actual phase current l_actual, as input variables.

[0089] Based on the phase current, for example, the received actual phase current IJst, the soft DESAT 52 can determine the diode voltage U_desat-diode dropping across the DESAT diode 44, in particular by means of an estimator 54. One of the following methods can be used.

[0090] An averaging of the measured voltages U_adc recorded for the high-side power switch 18 and a low-side power switch 20 assigned to this high-side power switch 18 can be carried out.

[0091] Alternatively, the diode voltage drop across the DESAT diode 44 can determine U_desat-diode based on an interpolation for the phase current of essentially 0 A, i.e. the zero crossing.

[0092] A further alternative provides that the diode voltage U_desat-diode dropping across the DESAT diode 44 is determined based on an averaging of a measurement voltage recorded for the at least one power switch 18, 20 over one period (of the phase current).

[0093] The diode voltage U_desat-diode thus determined is then subtracted from the received measurement voltage U_adc to determine the instantaneous voltage U_ds-on-mom actually dropped across the power switch 18 in the on-state. The instantaneous voltage U_ds-on-mom is fed to a comparator 56 of the soft DESAT 52.

[0094] A time-varying error threshold value U_ds-on-err is used as a comparison value for the comparator 56 of the soft DESAT 52.

[0095] According to this embodiment, the time-varying fault threshold value U_ds-on-err is determined based on component properties of the power switch 18. This means that the soft DESAT 52 knows the component properties of the power switch 18 used, which can be stored, for example, in a memory. In particular, the soft DESAT 52 includes information about a maximum resistance R_ds-on-max of the power switch 18 as a function of the maximum permissible junction temperature TJ-max for the power switch 18.

[0096] The value for the maximum resistance R_ds-on-max of the circuit breaker 18 may have been obtained from a data sheet or from measurements.

[0097] The phase current I received or determined by the Soft-DESAT 52, e.g., the target phase current l_target, is multiplied by the maximum resistance R_ds-on-max. Subsequently, a tolerance offset U_ds-on-tol is taken into account by summation, which reflects an additional, permissible tolerance component (summand) of the error threshold value U_ds-on-err.

[0098] The comparison signal of the comparator 56 is transmitted to the PWM unit 50.

[0099] If the instantaneous voltage U_ds-on-mom of the power switch 18 exceeds (or falls below) the time-varying error threshold value U_ds-on-err, a (digital) control signal is varied by the PWM unit 50. In particular, the digital control signal can be or comprise a safe-state signal, by means of which all power switches 18, 20 coupled to the control device 30 are transferred to a safe state, for example, the blocking state. This would thus block the entire converter 10. The control device 30 can de-energize the converter 10, at least with regard to its output, in particular its outputs.

[0100] In this regard, Fig. 3 shows schematic representations of signal curves 58, 60 in connection with the drive unit 8 according to the invention or the converter 10 according to the invention.

[0101] The upper part of the figure shows the signal waveform 58 of the phase currents I, i.e., lu, lv, Iw, output by the circuit breakers 18 of the converter 10. The current amplitude in amperes is plotted on the vertical axis against time in milliseconds on the horizontal axis.

[0102] The curve of the output phase currents I can be used to determine the point in time at which the output actual phase current 82 (l_lst) is zero for a circuit breaker 18. For example, the dashed line 62 indicates a specific point in time at which one of the phase currents of a specific circuit breaker 18 is 0 A.

[0103] In the corresponding signal waveform 60 in the lower part of the image, the measured voltage U_adc recorded for each combination of power switch 18 and the associated DESAT diode 44 is plotted on the vertical axis in V versus time on the horizontal axis. Note that for better visibility, the horizontal axis of signal waveform 60 is stretched relative to the horizontal axis of signal waveform 58, thus showing only a section that roughly corresponds to the dashed box in signal waveform 58. The time of signal waveform 58 indicated by dashed line 62 is indicated by a corresponding dashed line 64 in signal waveform 60.

[0104] The residual value of the measured voltage U_adc (indicated by the dashed line 66) remaining at this time (dashed line 62) is caused exclusively by the DESAT diode 44. Thus, the portion of the measured voltage U_adc caused by the DESAT diode 44, namely the diode voltage U_desat-diode, can be determined.

[0105] The value of the diode voltage U_desat-diode can be assumed to be constant at least for the subsequent switching cycle of the power switch 18, since the other operating conditions of the power switch 18 and the converter 10 change only negligibly within a single switching cycle, for example the junction temperature and the "quasi-static" desired phase current (or effective value l_RMS of the phase current). The value of the diode voltage U_desat-diode determined in this way can also be assumed to be constant overall to a good approximation, since this generally only varies due to component variation and / or depending on the installation space temperature. Although the power switch 18 is generally subject to strong temperature variations, these have no effect on the DESAT diode 44 due to the structural distance, or only a negligible effect.In other words, the temperature of the DESAT diode 44 can be assumed to be constant to a good approximation, so that it does not influence the diode voltage U_desat-diode.

[0106] As a result, the instantaneous voltage U_ds-on-mom that actually drops across the circuit breaker 18 can be determined from the recorded measurement voltage U_adc for all times of the switching cycle of the circuit breaker 18. With regard to this instantaneous voltage U_ds-on-mom, a fault threshold value U_ds-on-err can be determined using the recorded phase current l_lst, taking into account the component properties, such as the maximum resistance R_ds-on-max of the circuit breaker 18. By comparing the instantaneous voltage U_ds-on-mom with the corresponding time-varying fault threshold value U_ds-on-err, an impermissible operating state of the circuit breaker 18 can then be determined. Advantageously, the time-varying fault threshold value U_ds-on-err is subject to fewer tolerances compared to known approaches from the prior art.This means that a larger parameter range is available for the circuit breaker 18 with regard to the operating point for operating the converter 10. Consequently, increased operating efficiency can be achieved for the converter 10.

[0107] The portion of the measurement voltage U_adc dropped across the DESAT diode 44 (dashed line 66) can, as previously explained, also be determined by temporally averaging two corresponding signal waveforms 58, 60 of measurement voltages U_adc, which are recorded with respect to a corresponding pair of high-side power switch 18 and the associated low-side power switch 20, i.e., UHU and ULU, UHV and ULV, or UHW and ULW. In the temporal average, only the portions of the DESAT diodes 44, i.e., the diode voltages U_desat-diode, remain.

[0108] The time-varying error threshold value U_ds-on-err is represented in the lower signal curve 60 by the dashed lines 72, which have a tolerance range 74 with respect to the measurement voltage U_adc, which here correspondingly reflects the offset U_ds-on-tol as the permissible tolerance component. Correspondingly, the tolerance range 74 represents the difference between the actual resistance of the power switch 18 in the on-state R_ds-on and the maximum resistance R_ds-on-max.

[0109] The above explanations naturally apply analogously to all circuit breakers 18.

[0110] As soon as the determined instantaneous voltage U_ds-on-mom for one of the power switches 18, 20, i.e., one of the voltage signals UHU, ULU, UHV, ULV, UHW, or ULW, intersects one of the dashed lines 72, i.e., the time-varying error threshold value U_ds-on-err, which is detected in the comparator 56, an error occurs, resulting in a signal being output to the PWM unit 50. As already explained above, the time-varying error threshold value U_ds-on-err is continuously recalculated, namely as a function of the operating point or the phase current I of the corresponding power switch 18, 20. This results in the temporal change of the error threshold value U_ds-on-err.

[0111] For example, the error threshold U_ds-on-err is recalculated every 10 ms or every millisecond, whereas the instantaneous voltage U_ds-on-mom is determined at the controller clock rate, e.g., 16,000 times per second, or approximately every 60 ps. This saves computing time and thus power, since the frequency for determining the error threshold is much lower than the controller clock rate.

[0112] Furthermore, the driver circuit 28 includes an additional comparator 76. Using the comparator 76 of the driver circuit 28, the voltage drop across the DESAT diode 44 and the power switch 18 in the on-state is compared with a reference 78, which acts as a time-constant error threshold. The comparator 76 provides the comparison signal for the control device 30, and in particular the PWM unit 50. The comparator 76 and the reference 78 are also considered part of the DESAT circuit 42.

[0113] If the measurement voltage detected via the DESAT diode 44 and the power switch 18 in the on-state exceeds (or falls below) the temporally constant error threshold, the output digital control signal is varied by the PWM unit 50. For example, the digital control signal can then include a shutdown signal based on which all power switches 18, 20 coupled to the control device 30 can be switched off, so that, for example, a blocking state of the converter 10 is reached.

[0114] This creates an additional safety mechanism that is faster than the Soft-DESAT 52 because it is (purely) hardware-based. The additional safety mechanism via the comparator 76 of the driver circuit 28 has a greater tolerance than the safety mechanism of the Soft-DESAT 52 and is particularly suitable for shutting down the converter 10 in the event of a short circuit. Furthermore, the driver circuit 28 also includes an amplification element 80. The amplification element 80 is coupled, for example, via a series resistor, to the gate electrode 38 of the power switch 18.

[0115] The driver circuit 28 also includes a galvanic isolation 82, by means of which the control device 30 is protected from crosstalk between the power switches 18, 20.

[0116] In an alternative, the galvanic isolation 82 can be arranged separately from the driver circuit 28 between the driver circuit 28 and the control device 30.

[0117] Fig. 4 shows a schematic representation of a vehicle 82 with a drive unit 8 according to the invention.

[0118] The vehicle has wheels 84, which in this case are driven by the electric machine 12 of the drive unit 8.

[0119] In this application, reference may be made to quantities and numbers. Unless expressly stated, such quantities and numbers are not to be considered limiting, but rather as examples of the possible quantities or numbers in the context of this application. In this context, the term "plurality" may also be used in this application to refer to a quantity or number. In this context, the term "plurality" means any number greater than one, e.g., two, three, four, five, etc. The terms "about," "approximately," "near," etc., mean plus or minus 5% of the stated value.

Claims

Patent claims 1. A converter (10) for a drive unit (8), comprising at least one power switch (18, 20), at least one driver circuit (28) coupled to the power switch (18, 20), and a control device (30) coupled to the at least one driver circuit (28), wherein the at least one driver circuit (28) has a desaturation monitoring circuit (42) with a DESAT diode (44), wherein the DESAT diode (44) is assigned to the at least one power switch (18, 20) and is coupled to a drain electrode (36) of the assigned power switch (18, 20), wherein the at least one driver circuit (28) is configured to detect a measurement voltage and to provide it to the control device (30), wherein the measurement voltage comprises a diode voltage across the DESAT diode (44) and an instantaneous voltage which is applied between the drain electrode (36) and the source electrode (40) of the circuit breaker (18, 20) drops in the on-state,wherein the control device (30) is configured to determine the instantaneous voltage based on the obtained measurement voltage, wherein the control device (30) is configured to determine a time-variable, operating point-dependent error threshold value for the instantaneous voltage, and wherein the control device (30) is configured to output a safe-state signal to the driver circuit (28) if the instantaneous voltage exceeds or falls below the error threshold value.

2. Converter (10) according to claim 1, characterized in that the operating point is assigned to a phase current of the at least one circuit breaker (18, 20), wherein the control device (30) is set up to receive or determine the phase current of the circuit breaker (18, 20), in particular wherein the phase current is a desired phase current of the circuit breaker (18, 20) or an average effective value of the phase current.

3. Converter (10) according to claim 1 or 2, characterized in that the control device (30) is configured to determine the error threshold value based, among other things, on component properties of the at least one power switch (18, 20).

4. Converter (10) according to claim 3, characterized in that the control device (30) is configured to determine the error threshold value at least based on a maximum resistance between the drain electrode (36) and the source electrode (40) of the respective power switch (18, 20) at a maximum permissible junction temperature of the power switch (18, 20), in particular based on the product of a phase current assigned to the operating point and the maximum resistance.

5. Converter (10) according to one of the preceding claims, characterized in that the converter (10) has a B6 bridge (14) with high-side power switches (18) and low-side power switches (20) assigned to one another in pairs, wherein each power switch (18, 20) is assigned a separate driver circuit (28), wherein the control device (30) is coupled to all driver circuits (28), and wherein the control device (30) is configured to determine the diode voltage dropping across the DESAT diode (44) based on an averaging of the measurement voltages recorded for the high-side power switch (18) and a low-side power switch (20) assigned to this high-side power switch (18).

6. Converter (10) according to claim one of the preceding claims, characterized in that the control device (30) is configured to determine the diode voltage drop across the DESAT diode (44) based on an interpolation for a phase current of substantially 0 A, and / or to determine the diode voltage drop across the DESAT diode (44) based on an averaging of a measurement voltage recorded for the at least one power switch (18, 20) over a period.

7. Converter (10) according to one of the preceding claims, characterized in that the control device (30) is configured to block the converter (10) based on the safe state signal.

8. Converter (10) according to one of the preceding claims, characterized in that the driver circuit (28) has at least one comparator (76) which is configured to compare the measurement voltage with a temporally constant error threshold value, wherein the comparator (76) is coupled to the control device (30) and configured to transmit a comparison signal to the control device (30), and wherein the control device (30) is configured to output a shutdown signal to the driver circuit (28) based on the comparison signal received from the comparator (76), in particular wherein the control device (30) is configured to block the converter (10) based on the shutdown signal.

9. Converter (10) according to one of the preceding claims, characterized in that the driver circuit (28) has a device (82) for galvanic isolation, which is designed to protect the control device (30) from crosstalk.

10. Drive unit (8) comprising an electric machine (12) and a Converter (10) according to one of the preceding claims, which is assigned to the electrical machine (12).

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