Method and monitoring circuit for monitoring a current flowing through a semiconductor switch, device for controlling an electrical machine and electrical drive system

Abstract

The invention relates to a method and a monitoring circuit (100) for monitoring a current flowing through a semiconductor switch (200), wherein an impedance element (110) with an inductive component is connected in series with the semiconductor switch (200), comprising detecting a voltage drop across the impedance element (110) over time; determining a time period during which the voltage lies above a voltage threshold value; and determining that an overcurrent is present if the time period exceeds a time period threshold value.

Description

[0001] The present invention relates to a method and a monitoring circuit for monitoring a current flowing through a semiconductor switch, a device for controlling an electrical machine and an electrical drive system. Background of the invention

[0002] Active power semiconductors used in voltage source inverters (VSIs) can be equipped with a protection circuit to prevent component destruction or damage due to thermal overload in the event of overcurrent, e.g., due to a short circuit in the power stage. A short-circuit protection circuit suitable for this purpose is based on monitoring the voltage drop across the power transistor (e.g., drain-source voltage V DS in the case of unipolar power semiconductors or collector-emitter voltage V CE for bipolar power semiconductors).

[0003] In addition, shunt-based measurements or transistors with a so-called sense terminal can also be used. However, a shunt resistor typically leads to increased losses, and a transistor with a sense terminal results in high component costs, which is why voltage monitoring circuits are widely used and this functionality is also integrated into various gate driver ICs. Disclosure of the invention

[0004] According to the invention, a method and a monitoring circuit for monitoring a current flowing through a semiconductor switch, a device for controlling an electrical machine, and an electrical drive system with the features of the independent patent claims are proposed. Advantageous embodiments are the subject of the dependent claims and the following description.

[0005] The invention aims to improve the detection of short-circuit currents, which can occur, for example, within a half-bridge. The invention can be advantageously used in all power transistors in which a potential short circuit is to be detected. In particular, the invention can be advantageously used in so-called traction inverters, which are used to control electrical machines as traction drives in vehicles. Furthermore, the invention can be integrated into integrated gate driver modules (ASICs), which can be used in all half-bridge-based power converters (including traction inverters) where fast short-circuit detection is advantageous.

[0006] The invention allows for reliable detection of short-circuit and overcurrent events and protection of the power semiconductor through controlled shutdown. In addition, false positive triggering of the short-circuit protection under normal operating conditions is largely reduced or completely avoided. These properties are particularly critical for hard bridge short circuits (HSF, Hard Switching Failure or Type I short circuit). Conventionally, a so-called "blanking time" can be used to mask out a time range where the temporal characteristics or the levels of the detected signals (derived from semiconductor current or voltage, e.g., V DS_on) cannot be clearly distinguished under short-circuit and normal conditions, so that a conclusion about the semiconductor state is not yet possible. Such a "blanking time" represents a significant disadvantage of conventional solutions, particularly through the use of increasingly faster switching components (e.g., SiC MOSFETs or vGaN or GaN HEMTs), and can be avoided with the invention.

[0007] The result of the invention is fast and reliable short circuit detection. This reduces a disadvantage of long delay times, namely the release of large amounts of thermal energy within the chip.

[0008] Improved short-circuit detection and the resulting lower requirements for the short-circuit resistance of the power semiconductor can be profitably used in the design and development of the power transistor. In a SiC MOSFET, for example, the resulting clearance can be used to reduce the specific resistance (R DS,on ·A) or to optimize the reverse recovery behavior of the body diode.

[0009] To this end, the invention makes use of the measure of enabling fast and robust short-circuit detection of a (ohmic-)inductive voltage drop in the current path based on a temporal evaluation and a level evaluation, which is carried out digitally in particular. This (ohmic-)inductive voltage drop can advantageously occur via the connecting elements (packaging and connection technology, AVT) within a power module. To enable clear and reliable evaluation, in addition to the level of the measurement signal (e.g. by a comparator), the time-based curve of the comparator signal (e.g. by digital signal evaluation) can also be recorded and evaluated. This enables a clear distinction between regular switching operations and short-circuit currents across the entire operating characteristic range of the power semiconductor.Compared to a purely threshold-based method, this allows for the differentiation of fast, regular switching operations from short-circuit events. This prevents false detection. At the same time, slow short-circuit events can also be detected (limited by a lower detection threshold; very slow overcurrent events can be additionally detected, for example, by phase current measurement).

[0010] Compared to other voltage monitoring methods (evaluation of the voltage edge) across the power semiconductor (voltage monitoring, DESAT, etc.), a much faster detection is possible here. This is due to the fact that the invention evaluates the effects of the current edge that occurs before the voltage edge. This time advantage contributes significantly to the rapid detection of short circuits. Compared to other methods that use current measurement for detection, the invention is more cost-effective and more reliable for the clear detection of a wide variety of short circuit events.

[0011] Specifically, a method according to the invention for monitoring a current flowing through a semiconductor switch, wherein an impedance element with an inductive component (e.g., ohmic-inductive impedance element) is connected or located in series with the semiconductor switch, comprises detecting a voltage drop across the impedance element over time, determining a time period during which the voltage is above a voltage threshold, and determining that an overcurrent is present if the time period exceeds a time period threshold. By detecting the voltage across an inductive impedance element, a current gradient of a current flowing through the impedance element is indirectly detected and compared with a threshold.

[0012] A monitoring circuit according to the invention for monitoring a current flowing through a semiconductor switch, wherein an impedance element with an inductive component is connected in series with the semiconductor switch, is configured, in particular in terms of circuitry, to carry out a method according to the invention. The monitoring circuit can be implemented as or in an integrated circuit, e.g., an IC (integrated circuit) or ASIC (application-specific integrated circuit), for example, in a gate driver component. The monitoring circuit can be part of a power converter circuit with multiple half-bridges, for example, an inverter, DC / DC converter, mains converter, DC breaker, etc.

[0013] A device according to the invention for controlling an electrical machine comprises a power converter designed to be coupled to an electrical machine having a stator and a rotor and to provide an electrical voltage for controlling the stator of the electrical machine. The power converter comprises at least one monitoring circuit according to the invention. Such a device is also referred to in technical terms as an inverter or traction inverter. The invention can be advantageously used in semiconductor switches of half-bridges of the power converter with a voltage intermediate circuit.

[0014] An electric drive system according to the invention comprises a device according to the invention for controlling an electric machine and an electric machine with a stator and a rotor, which is electrically coupled to the power converter of the device for controlling the electric machine.

[0015] In one embodiment, the period of time during which the voltage is above the voltage threshold is determined using a comparator circuit to which the voltage drop across the impedance element is supplied. In this way, the period of time can be determined simply and reliably.

[0016] In one embodiment, the comparator circuit is configured as a Schmitt trigger, with the voltage drop across the impedance element being supplied to one input and a reference voltage being supplied to the other input. With a Schmitt trigger, a predetermined hysteresis can be provided between the switching on and off of the output signal depending on the input signal, thereby avoiding, in particular, bounce effects that would significantly distort the determination of the time duration.

[0017] In one embodiment, an output of the comparator circuit is connected to a trigger terminal of a counting circuit, wherein the counting circuit is configured to begin a regular incrementation of a counter value upon a signal edge of the first type at the trigger terminal, and to end the regular incrementation of the counter value, i.e., the counting process, upon a signal edge of the second type at the trigger terminal. The signal edge of the first type can (oBdA) be a rising edge, in which case the signal edge of the second type is a falling edge. Expediently, the counting circuit is clocked sufficiently high (i.e., the time between two increments is sufficiently short) to form a meaningful counter value. Using a counting circuit, time measurement can be implemented very easily in the form of a circuit. In other words, the counting circuit can also be referred to as a clock or stopwatch.In application cases, times can be in the three-digit nanosecond range, for example, so that clocking in the MHz range, for example, is advantageous.

[0018] In embodiments of the invention, a signal at the output of the comparator circuit is digitally filtered, for example, to suppress signal bounce. This can further increase the quality and accuracy of the temporal evaluation. Digital filtering can be implemented programmatically using a filter algorithm. The filtering can include, for example, low-pass filtering.

[0019] In one embodiment, it is determined that the duration exceeds the duration threshold when the counter value exceeds a counter threshold. This allows the time to be determined very easily.

[0020] In one embodiment, the counting circuit is configured to reset the counter value to an initial value, in particular zero, upon the second signal edge. Thus, the invention can monitor the semiconductor switch anew during each switching operation.

[0021] In one embodiment, the impedance element with an inductive component is a conductive connecting element, such as an element of the AVT, e.g. a bond wire connection of the semiconductor switch to a circuit carrier (e.g. power substrate, circuit board, etc.) or a busbar. It has been shown that the inductance of already present connecting elements, such as such a bond wire connection, is sufficient to determine a current gradient, while at the same time the ohmic resistance is small enough not to mask the measurement result. Additional impedance elements are therefore advantageously not necessary. As a result, the voltage drop across a current-carrying part of the AVT is used as the measurement signal, i.e. additional components in the current path are not required.

[0022] In one embodiment, the monitoring circuit comprises the impedance element connected in series with the semiconductor switch. Thus, all components used for the invention can be housed in the monitoring circuit.

[0023] The invention can be advantageously applied to a permanent magnet synchronous machine (PSM) or electrically excited synchronous machine (ESM) as an electrical machine, but also to other types of machines that require commutation of the stator current, such as asynchronous machines (ASM), etc.

[0024] Unless otherwise noted, the terms "connected," "connection," and similar refer to electrically conductive connections. Likewise, the term "current" refers to electrical currents.

[0025] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0026] The invention is illustrated schematically in the drawing using exemplary embodiments and is described below with reference to the drawing. Short description of the drawings Fig. 1 shows a schematic representation of a block diagram of an electric drive system according to an embodiment. Fig. 2 shows schematically and in circuit diagram form a monitoring circuit for monitoring a current flowing through a semiconductor switch according to an embodiment of the invention. Fig. Figure 3 shows possible current profiles through a semiconductor switch which can be used in a monitoring circuit according to an embodiment of the invention, such as Fig. 2, can be monitored. Embodiment(s) of the invention

[0027] Fig. 1 shows a schematic representation of a block diagram of an electric drive system 1 with a device 10 for controlling an electric machine 30. The electric drive system 1 comprises an electric machine 30 with a stator, which can be fed by a power converter 11 (so-called inverter), and a rotor. For this purpose, the power converter 11 can be fed, for example, by a DC voltage source such as a battery 20 or the like. The example of a three-phase electric machine 30 shown here serves only to improve understanding and does not represent a limitation of the present invention. Furthermore, any electric machines 30 with a number of electrical phases other than three are of course also possible. For example, it can also be a five- or six-phase electric machine 30 or an electric machine 30 with any other number of phases.

[0028] To control the stator of the electric machine 30, the power converter 11 can convert the DC voltage provided by the battery 20 into a suitable AC voltage. In the case of a three-phase electric machine 30, the power converter 11 can, for example, convert the DC voltage into a three-phase AC voltage. In particular, the amplitude of the AC voltage and / or the value of the output current from the power converter 11 to the stator windings (phases) of the electric machine 30 can be adjusted based on a predetermined setpoint S.

[0029] For example, the power converter 11 may be a power converter with multiple half-bridge circuits. In particular, the power converter 11 may have at least one half-bridge circuit with two semiconductor switches 200 (see FIG. Fig. 2). For example, the power converter 11 for a three-phase electrical machine 30 can have a B6 topology with six semiconductor switches. The semiconductor switches 200 of the power converter 11 can be controlled by the control device 12 using the setpoint S by means of suitable control signals, wherein a control circuit (gate driver) 210 is connected to a monitoring circuit 100, as in Fig. 2. In particular, the control circuit can include the monitoring circuit. In this case, the control device 12 can, for example, provide a control signal for each semiconductor switch 200 of the power converter 11 in order to open or close the corresponding semiconductor switch 200. The control of an upper semiconductor switch of a half-bridge is complementary to the control of the corresponding lower semiconductor switch.

[0030] In Fig. Figure 2 shows a schematic and circuit diagram-like monitoring circuit 100 for monitoring a current flowing through a semiconductor switch 200 according to one embodiment of the invention. The semiconductor switch 200 is embodied here as a MOSFET, but can also be a bipolar transistor, IGBT, etc. A gate terminal is connected to a gate driver 210, and a source terminal is connected to ground via an impedance element with an inductive component 110. In addition to an inductance L, the impedance element 110 also has an ohmic resistance R inherent therein. The impedance element is preferably a connecting element of the AVT. In the present case, a bond wire connection of the semiconductor switch 200 to a circuit carrier is used as the impedance element 110.

[0031] Fig. 3 shows in a diagram 300 possible current profiles 301, 302, 303, 304 through a semiconductor switch, such as the semiconductor switch 200 in Fig. 2, or the impedance element 110. In this case, Fig. 3 a load current or drain-source current I D through the semiconductor switch is plotted against time t. The different current waveforms 301, 302, 303, 304 are characterized by different gradients dI / dt, with a lower gradient (dI / dt)0 and an upper gradient (dI / dt)1 defining a gradient range relevant for overcurrent detection.

[0032] The Fig. 2 and Fig. 3 are described together below.

[0033] The voltage drop across the impedance element 110 is fed to a comparator circuit 120, in particular to a non-inverting input + of the comparator circuit 120, which is implemented here as a Schmitt trigger with a hysteresis. A reference voltage V Ref, which characterizes the current waveform 303 with the lower slope (dI / dt)0, is fed to the inverting input -. The unlabeled resistors are known to be used to set the switching threshold and the hysteresis.

[0034] An output signal waveform 130 of the comparator circuit 120 comprises a series of rectangular pulses depending on the current flowing through the semiconductor switch 200 or the inductive element 110, and is fed to a counting circuit 140. The counting circuit 140 is configured to begin a regular incrementation 142 of a counter value 143 upon a rising signal edge and to output the counter value at an output 144. The counting circuit 140 is further configured to stop the incrementation of the counter value upon a falling signal edge and to reset the counter value 143 to an output value 141, in this case zero, so that the next counting process can begin.

[0035] In a comparison step 150, it is continuously determined whether the time duration exceeds a time duration threshold by continuously determining whether the counter value at the output 144 exceeds a counter threshold 151.

[0036] If it is determined in comparison step 150 that the time duration exceeds the time duration threshold, an overcurrent is detected in block 160. This can be communicated to the gate driver 210 so that it can initiate an appropriate fault action, in particular, disabling the semiconductor switch 200.

[0037] As in Fig. 3, the comparator threshold or voltage threshold can be set via V Refbe set to correspond to a defined gradient threshold (dI / dt). As soon as the measured voltage reaches the voltage threshold or the measured current gradient reaches the gradient threshold, the comparator circuit 120 is triggered, and the comparator output indicates that the current rise is at least equal to or higher than the gradient threshold (dI / dt). This creates a signal that represents a minimum current rise rate dI / dt.

[0038] The signal is then digitally evaluated. In the example shown, a synchronous counter can be started with the presence of the comparator output signal. This counter is started by a rising edge at the comparator output and reset to its starting value by a falling edge when the current rise falls below the gradient threshold of (dI / dt)0. The comparator output signal can be digitally filtered to suppress bouncing and the resulting incorrect counts. This digital, time-based evaluation has increased the information content of the comparator output signal without the need for any further physical measurements.

[0039] It is now known that a current edge with an unknown absolute height dI / dt but with a defined minimum gradient has exceeded the gradient threshold for a certain time Δt. With this information, the following simple relationship can be used to calculate the load current I D must currently flow at least through the semiconductor switch 200: U=LdIdt ID=dIdtΔt

[0040] It is now also possible to calculate backwards from the switching characteristics of the regular operating range of the semiconductor switch 200 and to determine the signal length of the comparator signal from which a short-circuit current or an overload current must be present.

[0041] During design, it is only necessary to ensure that the maximum load capacity of the semiconductor switch 200 must not be exceeded. To this end, the duration threshold value t0 must be selected to be so high that, even with a maximum possible gradient (dI / dt)1 of the short-circuit current, only a drain current I1 results that does not overload the semiconductor switch 200. As an example, exemplary values ​​can be assumed, and for example, for I1 = 1,200 A and a maximum possible gradient of (dI / dt)1 = 18 A / ns, a maximum duration threshold value t0 = 150 ns results. For example, the gradient threshold value (dI / dt)0 can then be derived from this as a function of a maximum possible / permissible current I0 during fault-free operation.

[0042] Alternatively, the gradient threshold value (dI / dt)0 can also be specified by the user, from which the time duration threshold value t0 results depending on the maximum possible / permissible current I0 in fault-free operation.

[0043] This allows the gradient threshold to be potentially exceeded, for example, when a very fast, regular switching operation occurs (low current, high voltage, and very low semiconductor temperature). However, due to the connection to the time axis and the fact that this value is present for only, say, 50 ns, no short circuit is detected (which is also desired and correct). Only when the duration threshold is exceeded is an overcurrent detected, range 400.

[0044] In some embodiments, the duration threshold value t0 is dynamically adjusted. In other words, the duration threshold value t0 is specified as a function of an expected magnitude of the current flowing through the semiconductor switch 200 or as a function of the expected load current, wherein the current threshold value I0 can be adjusted via the duration threshold value t0. This enables operating-point-dependent and thus, on average, even faster detection of short circuits.

[0045] A major advantage of this solution is that the clock speed of the counter circuit 140 only affects the resolution accuracy of the adjustable current trigger threshold I0. Furthermore, after the switching operation is complete, the counter can be set to such a small value that Type II short circuits with sufficient edge steepness can be detected immediately. Very slow Type III short circuits can be detected, for example, using phase current sensors. Alternatively, it is also conceivable to evaluate the typical oscillation of the load current after the switching operation. If this oscillation is absent, a slow short circuit can be assumed.For special operating cases such as an active short circuit, high currents occur outside of normal switching operation, but these have a low gradient and therefore incorrect detection as an unwanted short circuit is unlikely and less problematic than, for example, when monitoring using conventional voltage measurements (e.g. DESAT).

Claims

[1] Method for monitoring a current flowing through a semiconductor switch (200), wherein an impedance element (110) with an inductive component is connected in series with the semiconductor switch (200), comprising: detecting a voltage drop across the impedance element (110) over time; Determining a period of time during which the voltage is above a voltage threshold ((dI / dt)0); Determine that an overcurrent occurs when the time duration exceeds a time duration threshold (t0). [2] Method according to claim 1, wherein the determination of the time period in which the voltage is above the voltage threshold value ((dI / dt)0) is carried out by means of a comparator circuit (120) to which the voltage drop across the impedance element (110) is supplied, the comparator circuit (120) having an output (130). [3] Method according to claim 2, wherein the comparator circuit (120) is connected as a Schmitt trigger, wherein the voltage drop across the impedance element (110) is supplied to an input, and a reference voltage (V Ref ) is fed to the other input. [4] Method according to claim 2 or 3, wherein the output (130) of the comparator circuit (120) is connected to a trigger terminal of a counting circuit (140), wherein the counting circuit (140) is configured to start a regular incrementing of a counter value (143) upon a signal edge of the first type at the trigger terminal, and to stop the incrementing of the counter value (143) upon a signal edge of the second type at the trigger terminal. [5] The method of claim 4, wherein it is determined that the time duration exceeds a time duration threshold (t0) when the counter value exceeds a counter threshold (151). [6] Method according to claim 4 or 5, wherein the counting circuit (140) is arranged to reset the counter value (143) to an initial value (141) at the signal edge of the second type. [7] Method according to one of claims 2 to 6, wherein a signal at the output (130) of the comparator circuit (120) is digitally filtered. [8] Method according to one of the preceding claims, wherein the time duration threshold value (t0) is predetermined as a function of an expected strength of a current flowing through a semiconductor switch (200). [9] Monitoring circuit (100) for monitoring a current flowing through a semiconductor switch (200), wherein an impedance element with an inductive component (110) is connected in series with the semiconductor switch (200), wherein the monitoring circuit (100) is configured to carry out a method according to one of the preceding claims. [10] Monitoring circuit (100) according to claim 9, wherein the impedance element (110) is a conductive connection element. [11] Monitoring circuit (100) according to claim 9 or 10, comprising the impedance element (110) connected in series with the semiconductor switch (200). [12] Device (10) for controlling an electrical machine (30), comprising: a power converter (11) which is designed to be coupled to an electrical machine (30) having a stator and a rotor and to provide an electrical voltage for controlling the stator of the electrical machine (30), wherein the power converter (11) has at least one monitoring circuit (100) according to one of claims 9 to 11. [13] Electric drive system (1), with: a device (10) for controlling an electrical machine (30) according to claim 12, and an electrical machine (30) having a stator and a rotor, which is electrically coupled to the power converter (11) of the device (10) for controlling the electrical machine (30).

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