Device and method for detecting current flowing through diode, and electric drive system

Abstract

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

Description

Technical Field

[0001] The present invention relates to a device and a method for detecting the current flowing through a diode. In particular, the present invention relates to the detection of the current flowing through a diode, which is arranged in parallel with a semiconductor switching element in a half-bridge of a current converter. The present invention also relates to an electric drive system having such a device for detecting the current flowing through a diode. Background Art

[0002] Electric drive systems have a variety of applications. For example, such electric drive systems are also used in fully electric or at least partially electric motor vehicles. Here, the electrical energy of an energy source (such as the traction battery of an electric vehicle) can be converted into a voltage suitable for controlling the motor by means of a converter. In addition, a so-called safe operating state, such as an active short circuit, can be provided in the converter, in which the switching elements are controlled so that the connection of the motor is short-circuited. In addition, a so-called no-load condition can be provided, in which all the switching elements in the half-bridge of the converter are open.

[0003] For example, the published text DE 102014222256A1 describes a method for switching the operating state of a motor from no-load to active short circuit. It is proposed here that in a polyphase motor, the individual phases are sequentially switched to the corresponding switching states. Summary of the Invention

[0004] The present invention provides a device and a method for detecting the current flowing through a diode and an electric drive system having the features described in the independent claims. Further advantageous embodiments are the subject matter of the dependent claims.

[0005] Therefore, it is provided that:

[0006] A device for detecting the current flowing through a diode, wherein the diode is arranged in parallel with a semiconductor switching element of a half-bridge. Here, in particular, it can be a half-bridge in a current converter. The device includes 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 the voltage across the semiconductor switching element. Therefore, the circuit arrangement also detects the voltage across the diode arranged in parallel with the semiconductor switching element. In addition, the circuit arrangement is designed to provide an output signal corresponding to the detected voltage. The detection device is designed to detect the current flowing through the diode using the output signal of the circuit arrangement for saturation monitoring. In particular, when the semiconductor switching element arranged in parallel with the diode in the half-bridge is open, the detection device can detect the current flowing through the diode according to the output signal.

[0007] In addition, an electric drive system having an electric machine and a converter is provided. The converter is electrically connected to the electric machine. The converter in particular includes at least one half-bridge. The converter preferably may include a half-bridge for each phase of the electric machine. Each half-bridge respectively includes two semiconductor switching elements together with diodes arranged in parallel with the respective semiconductor switching element. The converter is designed to control the electric machine in the case of using the DC voltage provided at the converter. In addition, for each semiconductor switching element, the converter includes means according to the invention for detecting the current flowing through the diode.

[0008] Finally, there is provided:

[0009] A method for detecting the current flowing through a diode. Here, the diode is arranged in parallel with the semiconductor switching element of the half-bridge. Alternatively, the diode may be the intrinsic body diode of the semiconductor switching element in the half-bridge. The method includes the following steps: detecting the voltages on the semiconductor switching element and on the diode arranged in parallel with the semiconductor switching element by means of a circuit arrangement for saturation monitoring of the semiconductor switching element; and providing an output signal corresponding to the detected voltage. In addition, the method includes the following steps: when the semiconductor switching element arranged in parallel with the diode of the half-bridge is turned on, detecting the current flowing through the diode in the case of using the provided output signal.

[0010] Advantages of the invention

[0011] In an electric drive system for controlling an electric machine by means of a converter, no-load can be set to a so-called safe operating state, in which the switching elements in the half-bridges of the converter are all turned on. If the connected electric machine is in motion, a voltage is induced in the electric machine, which voltage is applied to the phase terminals and thus also to the converter. If this induced voltage exceeds a certain value, due to the body diode integrated in the switching element or an additional diode arranged in parallel with the switching element, a current flow of the electric machine in the direction of the voltage source of the converter occurs. In this case, it may be necessary to provide an active short circuit in the converter instead of no-load. For this purpose, it is necessary to identify the current flowing through the diode arranged in parallel with the switching element in order to subsequently initiate the necessary switching measures if necessary.

[0012] In order to detect the current flowing through a diode connected in parallel with a switching element in a half-bridge of a converter, the present invention makes use of the fact that a circuit arrangement for saturation monitoring, in particular desaturation monitoring, of a semiconductor switching element can be provided in the converter for each semiconductor switching element. Although the term "saturation monitoring" is generally used in this specification, it should be understood that depending on the semiconductor technology used, desaturation monitoring is also included here. Such a circuit arrangement is, for example, referred to as a desaturation circuit (English: desatDetection or desaturationDetection). In a conventional converter, such a circuit arrangement is preferably used to reproduce the voltage present on the semiconductor switching element during the control of the semiconductor switching element. This voltage can be used, for example, to detect an excessive voltage drop in the controlled semiconductor switching element and, if necessary, to conclude whether the semiconductor switching element or the half-bridge is aging or faulty. Therefore, in particular, the signal provided by such a circuit arrangement for saturation monitoring is evaluated in the time interval during which the semiconductor switching element is controlled (i.e., closed).

[0013] Here, one concept of the present invention is to also use such a circuit arrangement for saturation monitoring of semiconductor switching elements already provided in the converter for other purposes. Specifically, during the time period when the semiconductor switching element is opened to set, for example, no-load in the converter, the signal of such saturation monitoring can be used to detect the current flowing through the diode connected in parallel with the opened semiconductor switching element when a sufficiently high voltage is fed in by the connected motor. In the said case, the diode conducts via the opened switch and the voltage across the opened switch drops to a negative value until the value of the forward voltage of the diode is reached. Therefore, the output signal provided by the circuit arrangement also decreases and drops below a predetermined threshold value. This can be evaluated as an indication of the current flowing through the diode. Therefore, the same circuit arrangement for saturation monitoring can be used for multiple tasks in different time intervals or switch states.

[0014] In this way, the current flowing through the diode connected in parallel with the switching element can be reliably identified without effort. Additional current sensors, etc. can be dispensed with hereby. Thereby, the hardware cost is reduced and thus the cost is lowered.

[0015] According to an embodiment, the detection device includes a comparator. Such a comparator can be implemented, for example, by means of an operational amplifier, etc. The comparator is designed to compare the output signal of the circuit arrangement for saturation monitoring with a predetermined reference voltage. Specifically, the detection device is designed to detect the current flowing through the diode once it is determined by the comparator that the output signal of the circuit arrangement for saturation monitoring is lower than the predetermined reference voltage.

[0016] According to another embodiment, a circuit arrangement for saturation monitoring of a semiconductor switching element includes an analog-to-digital converter. The analog-to-digital converter is designed to determine a digital value corresponding to the voltage value of the output signal of the circuit arrangement for saturation monitoring. In addition, the detection device is designed to detect the current through the diode using the determined digital value. In this way, the signal of the circuit arrangement for saturation monitoring can be processed in a digital switching circuit. For example, an application-specific integrated circuit can be used for this purpose.

[0017] According to one embodiment, the device for detecting current further includes a monitoring device. The monitoring device is designed to perform monitoring of the semiconductor switching element using the output signal of the circuit arrangement for saturation monitoring when the semiconductor switching element is controlled in the conducting state. For example, such a monitoring device can be used to check whether the semiconductor switching element is operating properly in the closed state. For example, if the voltage across the closed semiconductor switching element drops and its value exceeds a predetermined threshold, this may indicate a fault or aging of the semiconductor switching element, or a fault in the half-bridge.

[0018] According to one embodiment, the detection device and the monitoring device are implemented in a common integrated circuit. In this case, for example, the output signal of the circuit arrangement for saturation monitoring can be provided in parallel to the detection device and the monitoring device. Depending on the operating mode, only the detection logic needs to be inverted. Alternatively, depending on the operating mode, the output signal can be selectively provided to one of the two devices respectively.

[0019] According to an alternative embodiment, the detection device is designed to detect the current through the diode in a first operating mode if a first predetermined threshold voltage is exceeded. In addition, the detection device is designed to perform monitoring of the semiconductor switching element using the output signal of the circuit arrangement for saturation monitoring in a second operating mode if the semiconductor switching element is controlled in the conducting state and a second predetermined threshold voltage is exceeded. In other words, the monitoring of the semiconductor switching element in the open state to detect the current flowing through the parallel diode, as well as the monitoring of the semiconductor switching element in the closed state, are performed by the same component or the same assembly. Here, only the detection logic is inverted. In addition, a third operating mode can be provided, in which a fault reaction is initiated if a current flowing through the diode is detected during the monitoring. For example, such a fault reaction can include initiating an active short circuit.

[0020] According to one embodiment, the first threshold voltage for diode current detection and the second threshold voltage for saturation detection are the same. This can reduce the required component cost. Also, in this case, the diode current detection is triggered before the diode current occurs, so faster detection can be achieved.

[0021] According to an alternative embodiment, the first threshold voltage for diode current detection can be different from the second threshold voltage for saturation detection. This enables an optimization of the saturation behavior, i.e., the forward voltage of the diode, accordingly. By means of this optimization, false alarm triggering can be minimized.

[0022] According to one embodiment, a half-bridge having a switching element and a diode is a half-bridge in a current converter. Here, the detection device can be designed to detect the current through the diode when a running state is set in the converter having the half-bridge, in which two switching elements of one half-bridge are always open. This running state can in particular be a so-called no-load running state, in which the switching elements of all half-bridges of the converter are open.

[0023] According to one embodiment of an electric drive system, the converter of the drive system is designed to switch from a no-load switching state to an active short circuit if a current flowing through the diode is detected in one of the devices for detecting current flow. For example, in this way, an excessive braking torque of the motor can be avoided, which can occur once the electrical energy of the motor flows through the diode to the battery connected to the input of the converter.

[0024] Where reasonable, the above design solutions and improvements can be combined with each other arbitrarily. Other design solutions, improvements and implementations of the present invention also include combinations of features of the present invention described previously or below for the embodiments that are not explicitly mentioned. In particular, those skilled in the art can also add individual aspects as improvements or supplements to the respective basic forms of the present invention. Description of the Drawings

[0025] The following explains other features and advantages of the present invention with reference to the drawings. Shown here are:

[0026] Figure 1 : A schematic diagram of an electric drive system having a converter;

[0027] Figure 2 : A schematic diagram of a half-bridge of a converter according to an embodiment;

[0028] Figure 3 : A schematic diagram of a basic circuit diagram for detecting the current flowing through a diode according to an embodiment; and

[0029] Figure 4 : A flowchart based on the method for detecting the current flowing through a diode according to an embodiment. Detailed Description of the Embodiments

[0030] Figure 1A schematic diagram showing a basic circuit diagram of an electric drive system that can serve as the basis for an embodiment. The drive system includes a current converter 1 and an electric motor 2. The phase terminals of the electric motor 2 are electrically connected to the AC voltage terminals of the converter 1. A DC voltage source (e.g., the traction battery of an electric vehicle) can be connected to the DC voltage terminals of the converter 11. Thus, the converter 1 can generate an AC voltage according to a rating from the DC voltage provided on the input side, and this AC voltage is suitable for controlling the electric motor 2. In addition, the electric motor 2 can also supply an AC voltage to the converter 1 in the generator operating mode, where the AC voltage can be converted into a DC voltage by the converter 1, and this DC voltage is suitable for charging the battery connected to the DC voltage terminals. Here, Figure 1 The example of the three-phase electric motor 2 shown is only by way of example and does not represent a limitation of the present invention.

[0031] For each phase of the electric motor 2, the converter 1 includes a half-bridge having two switching elements S1 to S6. Here, each half-bridge is formed by the upper switching elements S1, S3, S5 and the lower switching elements S2, S4, S6. Here, the switching elements S1 to S6 can be controlled by means of a control circuit 10.

[0032] In addition, in addition to the operating mode in which the switching elements S1 to S6 are controlled by means of the control circuit 10 so that electrical energy is exchanged between the DC voltage terminals and the electric motor, a so-called safe operating state is also possible. For example, one such safe operating state is no-load. Here, all the switching elements S1 to S6 are open. Accordingly, no power is supplied through the converter 1 at the electric motor 2. However, if the electric motor 2 is in motion, a voltage can be induced in the electric motor 2 in the generator operating mode, and this voltage is thus also applied to the corresponding terminals of the converter 1. In addition, so-called active short-circuit is also possible. In this case, either the upper switching elements S1, S3 and S5 or, alternatively, the lower switching elements S2, S4, S6 are closed. Thereby short-circuiting the phase terminals of the electric motor 2.

[0033] Figure 2 A schematic diagram of the switching elements of the half-bridge is shown, as can be implemented, for example, in the above-mentioned converter 1. If a plurality of such half-bridges are provided in the converter 1, they can be constructed in a similar manner. The control of the switching elements S1 to S6 and the detection of the current through the diode D are described below based on the lower switching element S2. However, these implementations are also similarly applicable to the other lower switching elements S4 and S6 and the upper switching elements S1, S3 and S5.

[0034] The switching elements S1 to S6 can be, for example, bipolar transistors (IGBTs) or MOSFETs having insulated gate terminals. Here, in particular in the case of IGBTs, a diode D can be arranged in parallel with each semiconductor switch. In the case of MOSFETs, especially SiC-MOSFETs, the diode D can also be implemented by an intrinsic body diode.

[0035] In order to control the switching element S2 (and similarly also the other switching elements S1, S3 - S6), a control signal can be generated by the drive circuit 11 and supplied to the control terminal of the switching element S2. The drive circuit 11 can be implemented, for example, in the control device 10.

[0036] In order to monitor the switching behavior of the switching element S2, a circuit arrangement 21 for saturation monitoring / desaturation monitoring can be provided. Such a circuit arrangement 21 is also known, for example, as a Desat-circuit or a Desaturation-circuit. Here, such a circuit arrangement 21 for saturation monitoring reproduces the voltage currently applied to the switching element S2 at the input of the gate driver. Here, the reproduced voltage is limited to the value of the supply voltage and thus does not rise to the full voltage on the switching element S2. For example, when the switching element S2 is closed, the voltage drop across the switching element S2 can be monitored using such a circuit arrangement 21 for saturation monitoring. In particular, when an excessive voltage drop is detected in the controlled switching element S2, that is, if the voltage drop exceeds a predetermined threshold, this may indicate a limited conductivity of the switching element S2, firstly, or an excessive current through the semiconductor element. For example, in this way, a fault behavior of the half-bridge can be detected.

[0037] According to the invention, the circuit arrangement 21 for saturation monitoring or desaturation monitoring can also be used to identify the current flowing through the diode D arranged in parallel with the switching element S2 when the switching element S2 is open. Thus, in particular, when the converter 1 is set to no-load, where all the switching elements S1 to S6 are open, the current through the diode D is also detected. When the voltage induced in the motor 2 is high enough that the electrical energy of the motor 2 flows through the diode D up to the DC voltage terminals of the converter 1, for example, such a current through the diode D in no-load can be caused by the voltage induced in the motor 2.

[0038] In order to detect the above-mentioned current flowing through the diode D, the output signal of the circuit arrangement 21 for saturation monitoring is supplied to the detection device 22. The detection device 22 can compare the output signal of the circuit arrangement 21 with, for example, a predetermined reference value. If the voltage of the output signal of the circuit arrangement 21 is lower than the reference value, this can be evaluated as an indication of the current through the diode D.

[0039] For example, a comparator can be provided in the detection device 22, which compares the output signal of the circuit arrangement 21 with a predetermined reference value (in particular a reference voltage). This reference voltage can be provided in any way, for example by means of a resistive voltage divider or the like.

[0040] Alternatively, the output signal of the circuit arrangement 21 can also be converted into a digital signal by means of an analog-to-digital converter, and then this digital signal can be further processed. For this purpose, a microcontroller system, an application-specific integrated circuit or the like can be used.

[0041] As described above, when the switching element S2 is open, especially in the no-load state, the current flowing through the diode D is detected. Here, when the current flowing through the diode D is detected, appropriate measures can be taken as required to prevent or at least limit the current flowing through the diode D. For example, for this purpose, an active short circuit can be provided in the converter 1 instead of no-load.

[0042] Therefore, by using the circuit arrangement 21 for saturation monitoring as described above, it is possible to achieve the current flowing through the diode D without an additional sensor system (such as a current sensor or the like).

[0043] Figure 3 A schematic diagram showing a basic circuit diagram of the converter 1 monitored by means of a device for detecting the current flowing through the diode D according to an embodiment is shown. Here, all the embodiments made before are applicable where applicable. Figure 2 All the embodiments made before are applicable.

[0044] As can be seen from Figure 3 For example, the circuit arrangement 21 for saturation monitoring can be realized by means of a capacitor C1, a diode D1 and two resistors R1 and R2. Here, for example, the first resistor R1 is arranged between the supply voltage V and the node E. The capacitor C1 is arranged between the node E and the reference potential. In addition, a series circuit composed of the second resistor R2 and the diode D1 is provided between the electrical connection of the node E and the phase terminal of the semiconductor switching element S2 to be monitored.

[0045] However, the shown embodiment of the circuit arrangement 21 for saturation monitoring is only used to illustrate the basic principle of such a circuit arrangement. It can be understood that further adjustments, additions or modifications can also be made according to the specific application situation. For example, the circuit arrangement 21 can also be realized by means of a resistor chain, where the capacitor C1 and the diode D1 are replaced by suitable resistors.

[0046] Accordingly, an output signal can exist at node E, which can be used to monitor the switching behavior and the voltage drop across switching element S2 when the switching element S2 is closed based on conventional methods. In addition, the output signal at node E can also be used to detect the current flowing through diode D arranged in parallel with switching element S2. For this purpose, when the switching element S2 is open, especially in the no-load state for example, the output signal at node E can be evaluated by the detection device 22.

[0047] The output signal of the circuit arrangement 21 at node E can be provided, for example, to a component, especially an integrated circuit, for use, where a drive stage for the switching element S2 is provided. On the one hand, this component can use the output signal of the circuit arrangement 21 when controlling the switching element S2 in order to detect an excessive voltage drop across the switching element S2 when the switching element S2 is closed. In addition, when the switching element S2 is open, the current flowing through diode D can be detected by this component.

[0048] For this purpose, two independent units can be provided in the above-mentioned component (such as a driver IC), where one unit forms the detection device 22 for detecting the current when the switching element S2 is open, and the other unit forms a monitoring device for monitoring the voltage drop across the switching element S2 when the switching element S2 is closed. Here, a numerically smaller trigger threshold can preferably be set for the detection of the diode current. This avoids premature detection, which is accompanied by the trigger threshold of the monitoring device for monitoring the voltage drop when the switching element is closed.

[0049] Alternatively, the monitoring of the voltage drop when the switching element S2 is closed and the detection of the diode current can also be implemented by a common unit. For example, for this purpose, a logic circuit can be provided, which switches between the above two detection modes according to the application situation. Thus, for example, during normal operation, in order to control the motor 2, the conventional monitoring of the voltage drop across the closed switching element S2 can be performed. If a no-load is set in the converter 1, the operating mode for detecting the diode current flowing through diode D can be switched accordingly. Here, the trigger threshold (below which a positive detection must occur) can also be changed in order to select a threshold closer to the actual forward voltage of the diode.

[0050] Figure 4 A flowchart of a method for detecting the current flowing through diode D according to an embodiment is shown. This method is particularly applicable to a circuit arrangement having a semiconductor switching element and a parallel diode in a converter half-bridge. In principle, this method can include any steps previously described in conjunction with Figures 1 to 3 already described. Similarly, the device described above can also include any components required to implement the method described below.

[0051] In step 100, the voltages on semiconductor switch elements S1 to S6 and the diodes D arranged in parallel therewith are detected. Furthermore, an output signal corresponding to the detected voltage is provided.

[0052] Then, in step 200, the current flowing through the diode D is detected. This detection of the current is carried out in a situation where the output signal provided in step 100 is used and compared with a limit value. At the same time, the detection of the current flowing through the diode D is only carried out when the switch elements S1 to S6 arranged in parallel with the diode D are open (for example, in the no-load mode). Outside this time period, the output value of the circuit arrangement 21 or the output value of the detection circuit 22 will be ignored or overwritten (blanked).

[0053] If a current flowing through the diode D is detected in step 200, an appropriate reaction can be initiated in the next step. For example, this can be the previously described setting of an active short circuit. If the corresponding switch element is closed for this purpose, the monitoring of the diode current ends and restarts once the active short circuit ends and the switch element is open again.

[0054] In summary, the present invention relates to a detection of the current flowing through a diode arranged in parallel with a semiconductor switch element. For this purpose, the output signal of a circuit arrangement (Desat circuit) for saturation monitoring is used. Here, the output signal is evaluated during the time period when the semiconductor switch element is open.

Claims

1. A device for detecting the current flowing through a diode (D), wherein, the diode (D) is arranged in parallel with the semiconductor switch elements (S1 to S6) of a half-bridge, and wherein the device comprises the following components: a circuit arrangement (21) for saturation monitoring of the semiconductor switch elements (S1 to S6), the circuit arrangement being designed to acquire the voltage across the semiconductor switch elements (S1 to S6) and the voltage across the diode (D) arranged in parallel with the semiconductor switch elements, and to provide an output signal corresponding to the detected voltage; and a detection device (22), the detection device being designed to detect the current flowing through the diode (D) using the output signal of the circuit arrangement (21) for saturation monitoring when the semiconductor switch elements (S1 to S6) of the half-bridge arranged in parallel with the diode (D) are open.

2. The device according to claim 1, wherein, the circuit arrangement (21) for saturation monitoring of the semiconductor switch elements (S1 to S6) comprises a comparator, the comparator being designed to compare the output signal of the circuit arrangement (21) for saturation monitoring with a predetermined reference voltage and to detect the current flowing through the diode (D) once the output signal of the circuit arrangement (21) for saturation monitoring is below the predetermined reference voltage.

3. The device according to claim 1 or 2, wherein, the detection device (22) comprises an analog-to-digital converter, the analog-to-digital converter being designed to determine a digital value corresponding to the voltage value of the output signal of the circuit arrangement (21) for saturation monitoring, wherein the detection device (22) is designed to detect the current flowing through the diode (D) using the determined digital value.

4. The device according to any one of claims 1 to 3, the device having a monitoring device (11), the monitoring device being designed to perform monitoring of the semiconductor switch elements (S1 to S6) using the output signal of the circuit arrangement (21) for saturation monitoring when the semiconductor switch elements (S1 to S6) are controlled in the conducting state.

5. The device according to claim 4, wherein, the detection device (22) and the monitoring device (11) are implemented in a common integrated circuit.

6. The device according to any one of claims 1 to 5, the detection device (22) being designed to detect the current flowing through the diode (D) in a first operating mode and to perform monitoring of the semiconductor switches (S1 to S6) using the output signal of the circuit arrangement (21) for saturation monitoring in a second operating mode when the semiconductor switch elements (S1 to S6) are controlled in the conducting state.

7. The device according to any one of claims 1 to 6, wherein, the half-bridge together with the switch elements (S1 to S6) and the diode (D) arrangement is in a current converter (1), and The detection device (22) is designed to detect the current through the diode (D) when a running state is set in the converter (1) having a half-bridge, in which two switching elements (S1 to S6) of the half-bridge are always open.

8. An electric drive system having: an electric motor (2); a converter (1) that is electrically coupled to the electric motor (2) and includes at least one half-bridge, where each half-bridge respectively includes two semiconductor switching elements (S1 to S6) together with a diode (D) arranged in parallel with the respective semiconductor switching element (S1 to S6), and wherein, the converter (1) is designed to control the electric motor (2) using a DC voltage provided at the converter (1); wherein, for each semiconductor switching element (S1 to S6), a device for detecting current flow as claimed in any one of claims 1 to 7 is provided in the converter (1).

9. The electric drive system according to claim 8, wherein, the converter (1) is designed to change from the no-load switching state to active short-circuit when a current flowing through the diode (D) is detected in the device for detecting current flow.

10. A method for detecting the current flowing through the diode (D), wherein, the diode (D) is arranged in parallel with the semiconductor switching elements (S1 to S6) in the half-bridge, and the method includes the following steps: detecting (100) the voltages on the semiconductor switching elements (S1 to S6) and on the diode (D) arranged in parallel with the semiconductor switching elements by means of a circuit arrangement (21) for saturation monitoring of the semiconductor switching elements (S1 to S6); and providing an output signal corresponding to the detected voltages; detecting (200) the current through the diode (D) using the provided output signal when the semiconductor switching elements (S1 to S6) arranged in parallel with the diode (D) in the half-bridge are open.

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