Electrical system and method and computing unit for operating half-bridge for controlling current

Abstract

The invention relates to a method for operating at least one half-bridge for controlling a current, comprising detecting at least one current actual value (210) of the current in the at least one half-bridge, comparing the current actual value with a predefined positive and / or negative threshold value (220, 320, 230, 330), a measure is carried out when the actual current value (210) exceeds a predefined positive threshold value (220, 320) and / or falls below a negative threshold value (230, 330), and the positive and / or negative threshold value (220, 320, 230, 330) is predefined as a function of a current target current value of the current in the at least one half-bridge.

Description

Technical Field

[0001] The invention relates to a method for operating a half-bridge for controlling a current. The invention also relates to a computing unit, a computer program and an electrical system for carrying out the method. Background Art

[0002] A half-bridge comprises two semiconductor switches connected in series and is used in power electronics to control the current flowing through a connected load, such as an electric motor. Typically, a device for monitoring the load current is used to detect defects in such an electrical system consisting of a power electronics device and a load, such as an electrical short circuit in the load or the power electronics device. When a threshold value for the current is exceeded, a fault is detected and, for example, a safe state is activated as a fault response, in particular in order to prevent damage to other components of the system. Depending on the type of connected load, such overcurrent detection may take a certain amount of time, for example in the case of an electric motor, due to the relatively high inductance of the stator winding, until a set threshold value is exceeded and the filtering time and run-through time of the hardware have elapsed. In order to detect faults or electrical short circuits in power electronics more quickly, measures such as so-called DeSat detection of semiconductor switches or VDS monitoring can be provided, which are based on direct voltage monitoring of the power semiconductors.

[0003] WO 2023 / 030761 A1 relates to a method for operating an electric drive system having a power electronic device and having an electric motor that can be controlled by the power electronic device, wherein the drive system can be operated at least in an operating mode as an operating mode and in a freewheeling mode, wherein the power electronic device has at least three half-bridges for controlling the electric motor, wherein at least one current value of a current flowing through one of the half-bridges is monitored, wherein the current value is compared with a predetermined threshold value, and wherein an active short circuit is activated as a safe operating state of the power electronic device when the current value exceeds the threshold value. Provision is made to monitor the operating mode of the electric drive system to determine whether a freewheeling mode exists, and when the presence of a freewheeling mode is detected, the predetermined threshold value is reduced to the freewheeling threshold value. Summary of the invention

[0004] According to the invention, a method for operating a half bridge for controlling a current, an electrical system, and a computing unit and a computer program for carrying out the method are proposed having the features of the independent claims. Advantageous embodiments are the subject matter of the dependent claims and the following description.

[0005] As explained, defects in power electronics (e.g. low-inductance short circuits (with a rapid current rise)) are more likely to be detected by voltage-based monitoring, while defects in the load (higher-inductance short circuits (with a slow current rise)) are more likely to be detected by current-based monitoring. However, there can now be a region in between in which overcurrent detection is not sufficiently effective. In this region, there is the risk that when the phase current is switched off at regular clock cycle switch-off points in time (in the case of PWM control) with the switching speeds usually set, an excessively high phase current is switched off (unrecognized overcurrent) and a critical switch-off overvoltage is thereby reached, which can lead to an avalanche breakdown and thus to undesirable subsequent defects.

[0006] The invention now uses the following measures: the current actual value of the current in at least one half-bridge, in particular the current flowing through the monitored semiconductor switch, is detected and compared with a predetermined positive threshold value and / or a negative threshold value, and appropriate measures are performed when the current actual value exceeds the predetermined positive threshold value or falls below the predetermined negative threshold value. The half-bridge comprises two switching elements connected in series, between which a center tap is arranged. The current in the half-bridge is therefore a current that flows at least through the center tap and through at least one of the two switching elements connected in series. The current is preferably as a load current flowing through a load that can be connected to the center tap, preferably as a current flowing through the load or as a phase current flowing through a phase of the load (for example a motor). Therefore, preferably with the help of suitable measuring devices, the detection of the current in the half-bridge can be carried out at the switching element, at the center tap or also outside the half-bridge at the line of the connected load. However, the positive and / or negative threshold values ​​are not fixedly predetermined here according to the maximum operating point of the electrical system, but rather according to the current rated value of the current in the half-bridge (hereinafter also referred to as phase current). Therefore, the method includes three alternatives in particular, namely monitoring only the positive threshold value, only the negative threshold value or expediently monitoring both threshold values. Monitoring both threshold values ​​shortens the detection time and is particularly advantageous if only one half-bridge of a multi-phase system is monitored, since the phase currents on the individual phases can quickly rise to positive values ​​or drop to negative values, depending on the switching state at the moment of the occurrence of a short circuit. In this way, it is ensured that all fault situations can be detected.

[0007] In the embodiment of the invention, an electrical system having a load and at least one half-bridge is operated so that faults in the electrical system can be detected early. The system can be, for example, an electric drive system, in which the load is an electric motor and the at least one half-bridge is part of a single-phase or multi-phase rectifier (so-called inverter). The current is then the current flowing through the stator winding of the electric motor.

[0008] In particular, the measure can include, for example, switching the electrical system into a safe operating state by triggering an active phase short circuit, in which all high-side switches or all low-side switches of the power electronics are closed and thus, for example, all phase windings of the electric machine are short-circuited as loads.

[0009] The invention provides an advantageous possibility for detecting defects in an electrical system almost without delay. In all cases, timely detection is guaranteed because the corresponding threshold value is always set close to the expected current value. The defect is in particular an electrical short circuit between the phase windings of a load (e.g. an electric motor). The risk of switching off the semiconductor switch too quickly in the presence of an undetected overcurrent is reduced. In addition, the load on the power electronics is reduced when the semiconductor switch is switched off.

[0010] The detection probability and detection speed of short circuits in electrical systems are increased.

[0011] In particular, it is provided that at least one actual current value of the current in at least one half-bridge is detected, the actual current value is compared with a predetermined positive threshold value and / or a negative threshold value, and the positive threshold value and / or the negative threshold value is predetermined continuously, in particular regularly (for example according to clock control) according to the current current rated value, so as to permanently improve the operating safety and availability of the drive system.

[0012] In one embodiment, the current rated value is predetermined as a function of the peak value of the rated current curve in the at least one half-bridge as the current rated value. In other words, the positive threshold and / or the negative threshold are tracked to the peak value or the peak value of the current in the phase, as also in Figure 2 This embodiment is not particularly computationally complex and can be implemented very simply, if necessary also in an existing power electronics unit. False positive detection is not possible. However, in this case, there may be a very large distance between the actual current value and the threshold value near the zero crossing of the current oscillation, which may extend the reaction time in the event of a short circuit.

[0013] In one embodiment, the current setpoint current value is predetermined as a function of the instantaneous value of the setpoint current curve in the at least one half-bridge as the current setpoint current value. This is different from the previously described embodiment, that is, the threshold value of the current oscillation is directly tracked, as also in Figure 3 Compared to the above-described variant using peak values, this embodiment enables faster identification at all operating points and significantly greater limitation of the risk of subsequent faults.

[0014] In one embodiment, the switching-off speed and / or the switching-on speed of the semiconductor switch controlling the current in the at least one half-bridge is predetermined as a function of a positive threshold value and / or a negative threshold value. This allows, in particular, also depending on the operating point, the switching-off speed of the semiconductor switch to be limited or accelerated as much as possible. This operating point-dependent variation of the switching-off speed can be used to a greater extent by means of the invention or can be used in principle to a greater extent without risking critical switching-off overvoltages with subsequent faults. In particular, this is particularly advantageous in combination with the previously described embodiments regarding the monitoring of the instantaneous setpoint value, since the permissible switching-off speed is then not exceeded in any case.

[0015] In one embodiment, the positive threshold value and / or the negative threshold value are additionally predetermined as a function of a tolerance value. In particular, the tolerance value is dimensioned so that it is above the noise or interference of the sensor signal of the detection current value and above the current fluctuations (current ripple) that occur, so that false triggering is avoided.

[0016] In one embodiment, the implementation of the measure includes reducing the switch-off speed and / or the switch-on speed of the semiconductor switch controlling the current in the at least one half-bridge. In other words, when the positive threshold is exceeded or the negative threshold is fallen below, the duration required for the semiconductor switch to transition from the conducting state to the non-conducting state is longer than when the positive threshold is not exceeded or the negative threshold is not fallen below. In the case of this embodiment, in particular, when the current monitoring responds, a safe operating mode is not immediately caused, but the switch-off speed is first reduced. Very small tolerance values ​​or distances between the threshold and the peak value or the instantaneous value can be selected, because a false positive detection does not immediately lead to a system shutdown, but still leads to a tolerable system reaction (slower switching at various points).

[0017] In principle, such an extension of the off-time can also be determined according to the voltage across the semiconductor switch (eg V DSon ), that is, the higher the voltage across the semiconductor switch, the longer the duration required for the semiconductor switch to transition from the conducting state to the non-conducting state.

[0018] The measures of adopting a safe state and reducing the shutdown speed and / or the connection speed can also be combined with different associated current thresholds, so that the shutdown speed is reduced when a first (lower) positive current threshold is exceeded and / or a first (higher) negative current threshold is fallen below, and the safe state is adopted when a second (higher) positive current threshold is exceeded or a second (lower) negative current threshold is fallen below.

[0019] The computing unit according to the invention (for example a control device or a gate driver of a rectifier or inverter) is designed, in particular in terms of programming, to carry out the method according to the invention.

[0020] It is also advantageous to implement the method according to the invention in the form of a computer program or computer program product with a program code for executing all method steps, because this causes particularly low costs, especially when the execution control device is also used for other tasks and therefore exists in any case. Finally, a machine-readable storage medium is provided, on which a computer program as described above is stored. Suitable storage media or data carriers for providing computer programs are especially magnetic, optical and electrical memories, such as hard disks, flash memories, EEPROMs, DVDs, etc. It is also possible to download programs via computer networks (the Internet, intranets, etc.). Such downloading can be done wired or cabled or wirelessly (e.g., via a WLAN network, a 3G connection, a 4G connection, a 5G connection, or a 6G connection, etc.).

[0021] In one embodiment, a digital signal processor (DSP) or a similar microcontroller with signal processing functions can be used as a computing unit for executing the program code. In this case, it is advantageous to use an integrated hardware comparator with a programmable threshold value to monitor the phase current. This threshold value can then be adapted and checked, for example, cyclically with each clock cycle in view of the current maximum value or instantaneous value. Compared to comparing the sampled current values ​​by software, this allows significantly shorter reaction times and is particularly necessary when the current rated value changes rapidly or even when monitoring instantaneous values.

[0022] Further advantages and embodiments of the invention are apparent from the description and the accompanying drawings.

[0023] The invention is schematically illustrated in the drawings with the aid of one embodiment / embodiments and is described below with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A drive system is shown schematically as an electrical system on which the present invention may be based.

[0025] Figure 2 Exemplary current curves through a phase winding of an electric machine or a half bridge of a rectifier and current threshold curves are shown in the case of any desired actuation according to a first embodiment.

[0026] Figure 3 Exemplary current curves through a phase winding of an electric machine or a half bridge of a rectifier and current threshold curves are shown for any desired actuation according to a second embodiment. DETAILED DESCRIPTION

[0027] Figure 1A greatly simplified schematic diagram of an electric drive system 1 is shown, in particular for a vehicle (not shown), where the electric drive system 1 can be used, for example, as a driving or working drive. The electric drive system 1 has a power electronics 2. In the present case, the power electronics 2 is designed as an electric rectifier or inverter for driving an electric motor 3. For this purpose, as symbolically indicated, the power electronics 2 in the present case has three half-bridges for driving the electric motor 3, wherein each half-bridge has two semiconductor switches (e.g. MOSFETs, IGBTs, etc.) connected in series, as is customary.

[0028] The computing unit 4, which is designed in particular as a control device of the vehicle (such as a so-called MCU (English: motor control unit) or inverter controller), is connected to the power electronics 2 and / or the electric machine 3 in terms of communication technology or in the form of data transmission, as indicated by the corresponding double arrows. In particular, the computing unit 4 is designed to control the power electronics so that the power electronics in turn controls the electric machine according to the desired specifications (such as torque, speed, etc.). The computing unit 4 can also be integrated in the power electronics 2.

[0029] In particular, in order to operate the drive system 1, a rotating magnetic field is generated in the stator of the electric machine 3 by means of the power electronics 2, which interacts with the magnetic field of the rotor of the electric machine 3 in the manner of a generator or a motor. Such a rotating magnetic field can be generated by different control schemes, i.e. different switching modes of the semiconductor switches involved in the half bridge of the power electronics 2. Exemplary control schemes are PWM control or block control. In any case, such control schemes lead to a specific changing current curve (ideally sinusoidal) flowing through the phase winding of the electric machine 3. Such a changing current curve is characterized by the peak value of the current oscillation and the frequency or period duration of the current oscillation, which are independent of each other and both of which change continuously during operation.

[0030] exist Figure 2 In the diagram 200, it is shown that Figure 1 2 shows an exemplary curve of the current actual value 210 of the current in the phase winding of the motor of the motor 3 in FIG. 3 over time. Such a current actual value curve can be obtained, for example, when the motor is operated as a motor with different torques. In this case, the current usually flows through the high-side switch or the low-side switch of the first half bridge and flows through the phase winding.

[0031] Furthermore, a curve of a positive threshold value 220 predetermined according to the first embodiment of the present invention over time is shown. In particular, the positive threshold value curve 220 is predetermined according to the current peak value of the actual current value curve 210, which is partially represented by 211. However, it should be pointed out that the peak value is obtained from the rated value curve or from the rated preset, which peak value is present in the calculation unit 4, for example. The corresponding calculation criterion can add a tolerance value to the corresponding peak value. In particular, the size of the tolerance value is specified so that the tolerance value is above the noise or interference of the sensor signal of the detection current value and above the current fluctuation (current ripple) that occurs, so as to avoid false triggering. In addition, a curve of a negative threshold value 230 over time is shown, which detects the deviation of the phase current beyond the negative maximum value. This negative threshold is calculated similarly to the first threshold value 220.

[0032] exist Figure 3 In the diagram 300, the same current actual value curve 210 is shown over time. In addition, a positive threshold value curve 320 is shown, which is predetermined according to the corresponding instantaneous value of the rated current curve. The instantaneous values ​​are not individually indicated by reference numerals, but correspond to the corresponding current rated current values. The corresponding calculation criteria can add tolerance values ​​to the corresponding instantaneous values. In addition, a negative threshold value curve 330 can also be provided here, which monitors the downward deviation of the current actual value from the current rated value. Similarly, the positive values ​​of the curve 320 and the negative values ​​of the curve 330 can be combined into a common curve.

[0033] To implement an embodiment of the present invention, for example, the computing unit 4 or a processor or a microcontroller located in the computing unit 4 may be programmed to execute an embodiment of the method according to the present invention.

[0034] This includes, in particular, regularly (e.g. according to an operating clock) detecting the current actual phase current value, for example by means of a current measuring device (e.g. a shunt resistor or a Hall sensor) and a suitable analog / digital converter (e.g. a Delta-Sigma-ADC) and digitally comparing it with a positive threshold value and / or a threshold value. Alternatively, the current actual phase current value (analog) can be compared with an analog threshold signal, for example, generated by a digital / analog converter of the calculation unit 4, by means of a comparator (e.g. an operational amplifier).

Claims

1. A method for operating at least one half-bridge to control a current, comprising the following steps: detecting at least one current actual value (210) of the current in the at least one half-bridge, comparing the current actual value with a predetermined positive threshold value and / or a predetermined negative threshold value (220, 320, 230, 330), When the current actual value (210) exceeds a predetermined positive threshold value (220, 320) and / or falls below a predetermined negative threshold value (230, 330), a measure is executed, It is characterized in that A positive threshold value and / or a negative threshold value (220, 320, 230, 330) is predefined as a function of a current setpoint value of a current in the at least one half-bridge. 2 . The method according to claim 1 , wherein the current setpoint current value is predefined as a function of a peak value of a setpoint current curve in the at least one half-bridge. 3 . The method according to claim 1 , wherein the current setpoint current value is predefined as a function of an instantaneous value of a setpoint current curve in the at least one half-bridge.

4. The method according to any one of the preceding claims, further comprising: A switching-off speed and / or a switching-on speed of a semiconductor switch of the half-bridge controlling a current in at least one half-bridge is predetermined as a function of a positive threshold value and / or a negative threshold value (220, 320, 230, 330).

5. The method according to any one of the preceding claims, wherein the performing measures comprise: A turn-off speed and / or a turn-on speed of a semiconductor switch of the half-bridge controlling a current in the at least one half-bridge is reduced.

6. The method according to any one of the preceding claims, further comprising: Additionally, a positive threshold value and / or a negative threshold value ( 220 , 320 , 230 , 330 ) is predefined as a function of the tolerance value.

7. The method according to any one of the preceding claims, further comprising: operating an electrical system (1) having a load (3) and the at least one half-bridge, A current flowing through the load is controlled (3).

8. The method of claim 7, wherein performing the measures comprises: For example, the electrical system (1) is brought into a safe state by active short circuit or freewheeling.

9. A computing unit (4) which is designed to carry out all method steps of the method according to any of the preceding claims.

10. A computer program which, when executed on a computing unit (4), causes the computing unit to carry out all method steps of the method according to any one of claims 1 to 8.

11. A machine-readable storage medium having stored thereon the computer program according to claim 10.

12. An electrical system (1) comprising at least one half-bridge and an electrical load (3) which can be controlled by the at least one half-bridge, Furthermore, a current measuring device for detecting at least one current actual value of the current in the at least one half-bridge and a computing unit (4) according to claim 9 are included.

13. The electrical system (1) according to claim 12, comprising an electrical machine as load (3).

Images