Device and method for detecting an electric current flow through a diode, and electric drive system

Abstract

The invention relates to the detection of an electric current through a diode which is arranged in parallel with a semiconductor switching element. For this purpose, an output signal of a circuit arrangement is used for saturation monitoring (desat circuit). The output signal is evaluated during periods in which the semiconductor switching element is open.

Description

[0001] Description

[0002] Title:

[0003] Device and method for detecting an electric current flow through a diode, electric drive system

[0004] Technical area

[0005] The present invention relates to a device and a method for detecting an electrical current flow through a diode. In particular, the present invention relates to the detection of an electrical current flow through a diode arranged in parallel with a semiconductor switching element in a half-bridge of an electrical power converter. The present invention further relates to an electric drive system having such a device for detecting the current flow through a diode.

[0006] background

[0007] Electric drive systems are used in numerous applications. For example, such electric drive systems are also used in fully or at least partially electrically powered motor vehicles. A power converter can be used to convert electrical energy from an energy source, such as the traction battery of an electric vehicle, into an electrical voltage suitable for controlling an electrical machine. In addition, so-called safe operating states can also be set in the power converter, for example an active short circuit, in which the switching elements are controlled such that the terminals of the electrical machine are short-circuited. Furthermore, a so-called freewheel can be provided, in which all switching elements in the half-bridges of the power converter are open.

[0008] For example, the publication DE 10 2014222 256 A1 describes a method for switching the operating state of an electrical machine from freewheeling to an active short circuit. In a multi-phase electrical machine, it is proposed that the individual phases be successively switched to the corresponding switching states.

[0009] Disclosure of the invention

[0010] The present invention provides a device and a method for detecting an electric current flow through a diode, as well as an electric drive system having the features of the independent claims. Further advantageous embodiments are the subject of the dependent claims.

[0011] Accordingly, it is provided:

[0012] A device for detecting an electrical current flow through a diode, wherein the diode is arranged in parallel with a semiconductor switching element of a half-bridge. In particular, this can be a half-bridge in an electrical power converter. The device comprises a circuit arrangement for saturation monitoring and a detection device. The circuit arrangement for saturation monitoring of the semiconductor switching element is designed to detect an electrical voltage across the semiconductor switching element. Thus, this circuit arrangement also detects the electrical voltage across the diode arranged in parallel with the semiconductor switching element. Furthermore, the circuit arrangement is designed to provide an output signal corresponding to the detected voltage.The detection device is designed to detect an electric current through the diode using the output signal from the saturation monitoring circuit. In particular, the detection device can detect an electric current through the diode according to the output signal when the semiconductor switching element of the half-bridge arranged in parallel with the diode is open.

[0013] Furthermore, the following is provided: an electric drive system with an electric machine and a power converter. The power converter is electrically coupled to the electric machine. In particular, the power converter comprises at least one half-bridge. The power converter can preferably comprise a half-bridge for each phase of the electric machine. Each of the half-bridges comprises two semiconductor switching elements with a diode arranged in parallel with the respective semiconductor switching element. The power converter is designed to control the electric machine using a DC voltage provided at the power converter. Furthermore, the power converter comprises, for each semiconductor switching element, a device according to the invention for detecting an electric current flow through a diode.

[0014] Finally, it is planned:

[0015] A method for detecting an electrical current flow through a diode. The diode is arranged in parallel with a semiconductor switching element of a half-bridge. Alternatively, the diode can be an intrinsic body diode of the semiconductor switching element in a half-bridge. The method comprises a step of detecting an electrical voltage across the semiconductor switching element and the diode arranged in parallel therewith by means of a circuit arrangement for saturation monitoring of the semiconductor switching element and for providing an output signal corresponding to the detected voltage. The method further comprises a step of detecting an electrical current flow through the diode using the provided output signal when the semiconductor switching element of the half-bridge arranged in parallel with the diode is open.

[0016] Advantages of the invention

[0017] In electrical drive systems in which electrical machines are controlled by a power converter, a freewheeling mode can be set as a so-called safe operating state, in which the switching elements in the half-bridges of the power converter are all open. If the connected electrical machine is in motion, an electrical voltage can be induced in the electrical machine, which is applied to the phase connections and thus also to the power converter. If this induced voltage exceeds a certain value, the body diodes integrated in the switching elements or additional diodes provided in parallel to the switching elements can cause a current to flow from the electrical machine towards the power supply of the power converter. In this case, it may be necessary to set an active short circuit in the power converter instead of the freewheeling mode.To do this, it is necessary to detect the current flow through the diodes parallel to the switching elements in order to then initiate the necessary switching measures if necessary.

[0018] To detect a current flow through a diode parallel to a switching element in the half-bridge of a power converter, the present invention takes advantage of the fact that a circuit arrangement for saturation monitoring of the semiconductor switching element, in particular for monitoring desaturation, can be provided for each semiconductor switching element in the power converter. Even if the term saturation monitoring is generally used in the present description, it is understood that this also includes monitoring desaturation, depending on the semiconductor technology used. Such a circuit arrangement is known, for example, as a Desat circuit (desat detection or desaturation detection). In conventional power converters, this circuit arrangement is preferably used to simulate an electrical voltage applied across a semiconductor switching element during the control of the semiconductor switching element.This can be used, for example, to detect an excessive voltage drop in the controlled semiconductor switching element and, if necessary, to infer aging or a malfunction of the semiconductor switching element or the half-bridge. The signal provided by such a circuit arrangement for saturation monitoring is thus evaluated, in particular, at time intervals during which the semiconductor switching element is controlled, i.e., closed.

[0019] It is now an idea of ​​the present invention to also use such a circuit arrangement, already provided in the power converter, for saturation monitoring of a semiconductor switching element for a further purpose. In particular, the signal from such a saturation monitor can be used during periods in which the semiconductor switching element is open, for example to set a freewheel in the power converter, to detect an electrical current flowing through the diode arranged in parallel with the open semiconductor switching element if a sufficiently high electrical voltage is fed in from a connected electrical machine. In this case, the diode across the open switch becomes conductive and the voltage across the open switch drops into negative territory to the amount of the forward voltage of the diode.The output signal provided by the circuit arrangement thus also drops and falls below a predetermined threshold. This can be interpreted as an indication of a current flow through the diode. Thus, the same circuit arrangement for saturation monitoring can be used for multiple tasks at different time intervals or switching states.

[0020] This makes it possible to reliably detect a current flow through a diode arranged in parallel with a switching element with very little effort. This eliminates the need for additional current sensors or similar devices. This reduces hardware complexity and thus costs.

[0021] According to one embodiment, the detection device comprises a comparator. Such a comparator can be implemented, for example, using an operational amplifier or the like. This comparator is designed to compare the output signal from the saturation monitoring circuit arrangement with a predetermined reference voltage. In particular, the detection device is designed to detect an electrical current through the diode if the comparator determines that the output signal from the saturation monitoring circuit arrangement falls below the predetermined reference voltage.

[0022] According to a further embodiment, the circuit arrangement for saturation monitoring of the semiconductor switching element comprises an analog-to-digital converter. This analog-to-digital converter is designed to determine a digital value corresponding to a voltage value of the output signal from the saturation monitoring circuit arrangement. Furthermore, the detection device is designed to detect an electric current through the diode using the determined digital value. In this way, the processing of the signal from the saturation monitoring circuit arrangement can be carried out in a digital circuit. For example, an application-specific integrated circuit can be used for this purpose.

[0023] According to one embodiment, the device for detecting the current flow further comprises a monitoring device. This monitoring device is designed to monitor the semiconductor switching element using the output signal from the saturation monitoring circuit arrangement when the semiconductor switching element is driven in a conductive state. For example, such a monitoring device can be used to check the proper functioning of the semiconductor switching element in the closed state. If, for example, an electrical voltage drops across a closed semiconductor switching element whose value exceeds a predetermined threshold, this can be an indication of a faulty or aged semiconductor switching element or a fault in the half-bridge.

[0024] According to one embodiment, the detection device and the monitoring device are implemented in a common integrated circuit. For example, the output signal from the saturation monitoring circuit arrangement can be provided to the detection device and the monitoring device in parallel. Depending on the operating mode, only the detection logic needs to be reversed. Alternatively, depending on the operating mode, the output signal can be provided to either of the two devices.

[0025] According to an alternative embodiment, the detection device is designed to detect an electric current through the diode in a first operating mode if a first predetermined threshold voltage is undershot. Furthermore, the detection device is designed to monitor the semiconductor switch in a second operating mode using the output signal from the circuit arrangement for saturation monitoring if the semiconductor switching element is controlled in a conductive state and a predetermined second threshold voltage is exceeded. In other words, both the monitoring of the semiconductor switching element in the open state for detecting a current flow through the parallel diode and the monitoring of the semiconductor switching element in the closed state are carried out by the same components or the same assembly. Only the detection logic is inverted here.Furthermore, a third operating mode can be provided in which the fault reaction is initiated if a current flow through the diode is detected during monitoring. Such a fault reaction can, for example, include initiating an active short circuit.

[0026] According to one embodiment, the first threshold voltage for diode flux detection and the second threshold voltage for saturation detection are the same. This allows for a reduction in the required component count. Likewise, in this case, diode flux detection is triggered before a diode flux occurs, thus enabling faster detection.

[0027] According to an alternative embodiment, the first threshold voltage for diode forward detection and the second threshold voltage for saturation detection can be different. This allows for optimization of the saturation behavior and the diode forward voltage, respectively. This optimization can minimize false positive triggering.

[0028] According to one embodiment, the half-bridge with the switching elements and the diodes is a half-bridge in an electrical power converter. The detection device can be designed to detect an electric current through the diode if an operating state is set in the power converter with the half-bridge in which both switching elements of the half-bridge are permanently open. In particular, this operating state can be a so-called freewheeling operating state, in which the switching elements of all half-bridges of a power converter are open.

[0029] According to one embodiment of the electric drive system, the power converter of the drive system is designed to switch from a freewheel switching state to an active short circuit if a current flow through a diode has been detected in one of the devices for detecting an electric current flow. This can, for example, prevent an excessively strong braking torque of the electric machine, which could occur if electrical energy were to flow from the electric machine through the diodes to a battery connected to the power converter input.

[0030] The above embodiments and further developments can be combined with one another as desired, where appropriate. Further embodiments, further developments, and implementations of the invention also include combinations of features of the invention not explicitly mentioned above or described below with respect to the exemplary embodiments. In particular, those skilled in the art will also add individual aspects as improvements or additions to the respective basic forms of the invention.

[0031] Short description of the drawings

[0032] Further features and advantages of the invention are explained below with reference to the figures. These show:

[0033] Fig. 1: a schematic representation of an electric drive system with a power converter;

[0034] Fig. 2: a schematic representation of a half-bridge of a power converter according to an embodiment;

[0035] Fig. 3: a schematic representation of a basic circuit diagram for

[0036] Detection of a current through a diode according to an embodiment; and Fig. 4: a flowchart underlying a method for detecting a current through a diode according to an embodiment.

[0037] Description of the embodiments

[0038] Figure 1 shows a schematic representation of a basic circuit diagram for an electric drive system, as may form the basis of one embodiment. The drive system comprises an electric power converter 1 and an electric machine 2. The phase connections of the electric machine 2 are electrically coupled to an AC voltage connection of the power converter 1. A DC voltage source, for example, the traction battery of an electric vehicle, can be connected to a DC voltage connection of the power converter 1. The power converter 1 can thus generate an AC voltage from the DC voltage provided on the input side, according to setpoint specifications, which is suitable for controlling the electric machine 2.Furthermore, it is also possible for the electric machine 2 to provide an alternating electrical voltage to the power converter 1 in generator mode, wherein the alternating voltage can be converted by the power converter 1 into a direct voltage suitable for charging a battery connected to the direct voltage connection. The example shown in Figure 1 with a three-phase electric machine 2 serves merely as an example and does not represent a limitation of the present invention.

[0039] The power converter 1 comprises a half-bridge with two switching elements S1 to S6 for each phase of the electric machine 2. Each half-bridge is formed by an upper switching element S1, S3, S5 and a lower switching element S2, S4, S6. The switching elements S1 to S6 can be controlled by a control circuit 10.

[0040] In addition to an operating mode in which the switching elements S1 to S6 are controlled by the control circuit 10 in such a way that electrical energy is exchanged between the DC voltage connection and the electrical machine, so-called safe operating states are also possible. One such safe operating state is, for example, freewheeling. In this case, all switching elements S1 to S6 are open. Accordingly, no power is supplied to the electrical machine 2 by the power converter 1. However, if the electrical machine 2 is in motion, an electrical voltage can be induced in the electrical machine 2 in generator mode, which voltage is therefore also applied to the corresponding connections of the power converter 1. Furthermore, a so-called active short circuit is possible. In this case, either the upper switching elements S1, S3 and S5 or, alternatively, the lower switching elements S2, S4, S6 are closed.As a result, the phase connections of electrical machine 2 are short-circuited.

[0041] Figure 2 shows a schematic representation of a switching element of a half-bridge, such as can be implemented, for example, in the power converter 1 described above. If several such half-bridges are provided in a power converter 1, they can be constructed analogously. The following describes the control of a switching element S1 to S6 as well as the detection of an electric current through a diode D based on a lower switching element S2. However, these explanations also apply analogously to the other lower switching elements S4 and S6 as well as to the upper switching elements S1, S3, and S5.

[0042] The switching elements S1 to S6 can be, for example, insulated gate bipolar transistors (IGBTs) or MOSFETs. In particular, with IGBTs, a diode D can be arranged in parallel with each semiconductor switch. With MOSFETs, especially with SiC MOSFETs, this diode D can also be implemented as an intrinsic body diode.

[0043] To control the switching element S2 (and analogously to control the other switching elements S1, S3-S6), a driver circuit 11 can generate a control signal and provide it to a control terminal of the switching element S2. This driver circuit 11 can be implemented, for example, in the control device 10.

[0044] To monitor the switching behavior of the switching element S2, a

[0045] A circuit arrangement 21 for saturation monitoring / desaturation monitoring may be provided. Such circuit arrangements 21 are also known, for example, as desat circuits or desaturation circuits. Such a circuit arrangement 21 for saturation monitoring simulates the voltage currently present across the switching element S2 at the input of the gate driver. The simulated voltage is limited to an amount of a supply voltage and thus does not rise to the full voltage across the switching element S2. With such a circuit arrangement 21 for saturation monitoring, for example, the voltage drop across the switching element S2 when the switching element S2 is closed can be monitored. In particular, if an excessively high voltage drop is detected in a controlled switching element S2, ieIf a voltage drop exceeds a specified threshold, this may indicate limited conductivity of switching element S2 or excessive current flow through the semiconductor element. This can be used to detect, for example, faulty half-bridge behavior.

[0046] This circuit arrangement 21 for saturation or desaturation monitoring can also be used according to the invention to detect an electrical current through the diode D arranged parallel to the switching element S2 when the switching element S2 is open. In particular, an electrical current through the diode D can thus also be detected when a freewheeling mode is set in the power converter 1, in which all switching elements S1 to S6 are open. Such an electrical current through the diode D in freewheeling mode can be caused, for example, by a voltage induced in the electrical machine 2 if the voltage induced in the electrical machine 2 is so high that electrical energy flows from the electrical machine 2 through the diode D to the DC voltage connection of the power converter 1.

[0047] To detect the aforementioned current flow through diode D, the output signal from circuit arrangement 21 is provided to a detection device 22 for saturation monitoring. This detection device 22 can, for example, compare the output signal from circuit arrangement 21 with a predetermined reference value. If the electrical voltage of the output signal from circuit arrangement 21 falls below this reference value, this can be interpreted as an indication of an electrical current flowing through diode D.

[0048] For example, a comparator can be provided in the detection device 22, which compares the output signal from the circuit arrangement 21 with a predetermined reference value, in particular a reference voltage. This reference voltage can be provided in any desired manner, for example, by means of a resistor divider or the like.

[0049] Alternatively, it is also possible to convert the output signal from circuit arrangement 21 into a digital signal using an analog-to-digital converter and then further process this digital signal. For this purpose, a microcontroller system, an application-specific integrated circuit, or similar can be used.

[0050] As described above, the current flow through diode D is detected when switching element S2 is open, particularly in a freewheeling state. Upon detection of the current flow through diode D, appropriate action can be initiated, if necessary, to prevent or at least limit this current flow through diode D. For example, an active short circuit can be set in converter 1 instead of freewheeling.

[0051] By using the circuit arrangement 21 for saturation monitoring as described above, a current flow through the diode D can be realized without additional sensors, such as a current sensor or the like.

[0052] Figure 3 shows a schematic representation of a basic circuit diagram for monitoring a power converter 1 with a device for detecting a current flow through a diode D according to one embodiment. All embodiments previously described in connection with Figure 2 apply here, where applicable.

[0053] As can be seen in Figure 3, the circuit arrangement 21 for saturation monitoring can be implemented, for example, by a capacitor CI, a diode D1, and the two resistors RI and R2. For example, a first resistor RI is arranged between a supply voltage V and a node E. The capacitor CI is arranged between the node E and a reference potential. Furthermore, a series circuit comprising the second resistor R2 and a diode D1 is provided between the node E and an electrical connection to the phase connection of the semiconductor switching element S2 to be monitored.

[0054] However, this illustrated embodiment of a circuit arrangement 21 for saturation monitoring merely serves to illustrate the basic principle of such a circuit arrangement. It is understood that, depending on the specific application, further adaptations, additions, or modifications can also be made. For example, the circuit arrangement 21 can also be implemented as a resistor chain, with the capacitor CI and the diode D1 being replaced by suitable electrical resistors.

[0055] An output signal is thus available at node E, which can be used in a conventional manner to monitor the switching behavior and the voltage drop across switching element S2 when switching element S2 is closed. Furthermore, the output signal present at node E can also be used to detect a current flow through diode D, which is arranged in parallel with switching element S2. For this purpose, the output signal present at node E can be evaluated by detection device 22 when switching element S2 is open, in particular, for example, in a freewheeling state.

[0056] The output signal of the circuit arrangement 21 at node E can be provided, for example, to a module, in particular an integrated circuit, in which a driver stage for the switching element S2 is provided. This module can, on the one hand, use the output signal from the circuit arrangement 21 when driving the switching element S2 to detect an excessive voltage drop across the switching element S2 when the switching element S2 is closed. Furthermore, when the switching element S2 is open, a current flow through the diode D can be detected by this module.

[0057] For this purpose, two separate units can be provided in the above-mentioned module, for example, a driver IC, wherein one unit forms the detection device 22 for detecting the current flow when the switching element S2 is open, and another unit forms a monitoring device for monitoring the voltage drop across the switching element S2 when the switching element S2 is closed. In this case, a lower trigger threshold can preferably be set for detecting the diode flow. This avoids premature detection, which is associated with the trigger threshold of the monitoring device for monitoring the voltage drop when the switching element is closed.

[0058] Alternatively, it is also possible to implement the monitoring of the voltage drop when the switching element S2 is closed and the detection of the diode flux using a common unit. For example, a logic can be provided for this purpose which switches between the two detection modes described above depending on the application. For example, during normal operation to control the electrical machine 2, the conventional monitoring of the voltage drop across a closed switching element S2 can be carried out. If freewheeling is set in the power converter 1, it is possible to switch accordingly to an operating mode in which a diode flux through the diode D is detected. The trigger threshold, which must be undershot for positive detection, can also be changed in order to select the threshold closer to the actual forward voltage of the diode.

[0059] Figure 4 shows a flowchart underlying a method for detecting an electrical current flow through a diode D according to one embodiment. The method can be used in particular for circuit arrangements comprising a semiconductor switching element and a parallel diode in half-bridges of power converters. In principle, the method can comprise any steps as previously described in connection with Figures 1-3. Analogously, the devices described above can also comprise any components required to implement the method described below.

[0060] In a step 100, an electrical voltage is detected across a semiconductor switching element S1 to S6 and a diode D arranged in parallel thereto. Furthermore, an output signal corresponding to this detected voltage is provided.

[0061] Subsequently, in step 200, an electric current through diode D is detected. This current is detected using the output signal provided in step 100 and comparing it with a threshold value. The current through diode D is only detected when the switching element S1 to S6 arranged in parallel with diode D is open, for example, in a freewheeling mode. Outside of this period, either the output value of circuit arrangement 21 or the output value of detection circuit 22 is ignored or overwritten (blanking).

[0062] If a current flow through diode D is detected in step 200, a suitable reaction can be initiated in a subsequent step. This can, for example, be the setting of an active short circuit as described above. If the corresponding switching elements are closed for this purpose, monitoring of the diode flow is terminated and restarts as soon as the active short circuit is terminated and the switching elements are opened again.

[0063] In summary, the present invention relates to the detection of an electric current through a diode arranged in parallel with a semiconductor switching element. For this purpose, an output signal from a saturation monitoring circuit (Desat circuit) is used. The output signal is evaluated during periods in which the semiconductor switching element is open.

Claims

Claims 1. A device for detecting an electrical current flow through a diode (D), wherein the diode (D) is arranged in parallel with a semiconductor switching element (S1-S6) of a half-bridge, and wherein the device comprises the following: a circuit arrangement (21) for saturation monitoring of the semiconductor switching element (S1-S6), which is designed to detect an electrical voltage across the semiconductor switching element (S1-S6) and the diode (D) arranged in parallel thereto and to provide an output signal corresponding to the detected voltage; and a detection device (22) designed to detect an electrical current through the diode (D) using the output signal from the circuit arrangement (21) for saturation monitoring when the semiconductor switching element (S1-S6) of the half-bridge arranged in parallel with the diode is open.

2. Device according to claim 1, wherein the circuit arrangement (21) for saturation monitoring of the semiconductor switching element (S1-S6) comprises a comparator which is designed to compare the output signal from the circuit arrangement (21) for saturation monitoring with a predetermined reference voltage and to detect an electrical current through the diode (D) if the output signal from the circuit arrangement (21) for saturation monitoring falls below the predetermined reference voltage.

3. Device according to claim 1 or 2, wherein the detection device (22) comprises an analog-to-digital converter which is designed to determine a digital value which corresponds to a voltage value of the output signal from the circuit arrangement (21) for saturation monitoring, wherein the detection device (22) is configured to detect an electric current through the diode (D) using the determined digital value. The device according to one of claims 1 to 3, comprising a monitoring device (11) configured to monitor the semiconductor switching element (S1-S6) using the output signal from the saturation monitoring circuit arrangement (21) when the semiconductor switching element (S1-S6) is driven in a conductive state. The device according to claim 4, wherein the detection device (22) and the monitoring device (11) are implemented in a common integrated circuit.Device according to one of claims 1 to 5, wherein the detection device (22) is designed to detect an electric current through the diode (D) in a first operating mode and to monitor the semiconductor switch (S1-S6) using the output signal from the circuit arrangement (21) for saturation monitoring in a second operating mode when the semiconductor switching element (S1-S6) is driven in a conductive state. Device according to one of claims 1 to 6, wherein the half-bridge with the switching elements (S1-S6) and the diodes (D) are arranged in an electrical power converter (1), and the detection device (22) is designed to detect an electric current through the diode (D) if an operating state is set in the power converter (1) with the half-bridge in which both switching elements (S1-S6) of the half-bridge are permanently open. Electric drive system, comprising. an electrical machine (2); a power converter (1) which is electrically coupled to the electrical machine (2) and which comprises at least one half-bridge, wherein each of the half-bridges comprises two semiconductor switching elements (S1-S6), each with a diode (D) arranged in parallel with a semiconductor switching element (S1-S6), and wherein the power converter (1) is designed to control the electrical machine (2) using a DC voltage provided to the power converter (1); wherein a device for detecting an electrical current flow according to one of claims 1 to 7 is provided in the power converter (1) for each semiconductor switching element (S1-S6). Electric drive system according to claim 8, the power converter (1) is designed to change from a freewheel switching state to an active short circuit if a current flow through a diode (D) has been detected in the device for detecting an electrical current flow.Method for detecting an electrical current flow through a diode (D), wherein the diode (D) is arranged in parallel with a semiconductor switching element (S1-S6) in a half-bridge, and wherein the method comprises the following steps:. Detecting (100) an electrical voltage across the semiconductor switching element (S1-S6) and the diode (D) arranged in parallel thereto by means of a circuit arrangement (21) for saturation monitoring of the semiconductor switching element (S1-S6) and providing an output signal corresponding to the detected voltage; Detecting (200) an electric current through the diode (D) using the provided output signal when the parallel to the semiconductor switching element (S1-S6) of the half-bridge arranged on the diode (D) is open.

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