Method and apparatus for detecting a single defective semiconductor switch

Abstract

Method for detecting a single permanently conductive defective semiconductor switch of the semiconductor switches (210, 340) of a semi-controlled H-bridge (100) for setting an excitation current of a separately excited synchronous machine, wherein the semi-controlled H-bridge (100) a bridge branch (200) with one of the semiconductor switches T1 (210) and a diode D3 (230) connected in series in reverse bias and a further bridge branch (300) comprising a further diode D2 (320) in reverse bias in series with a further semiconductor switch T4 (340) and a load branch (400) comprising an excitation winding (460) of the separately excited synchronous machine, wherein the load branch (400) connects a connection point (250) from one of the semiconductor switches T1 (210) and the diode D3 (230) in the first bridge branch (200) and with a further connection point (350) between the further diode D2 (320) and the further semiconductor switch T4 (340) in the further bridge branch (300), and wherein a current measuring device (480) for measuring the current excitation current is arranged in the load branch (400); wherein the semi-controlled H-bridge (100) is coupled to a control device (1200) comprising an excitation current control (1220) which generates two pulse-width modulated control signals (611, 621) of the same frequency, phase-shifted by half a period, for controlling one of the semiconductor switches T1 (210) and the other of the semiconductor switches T4 (340), such that the excitation current flowing through the load branch (400) can be controlled via the pulse-width modulation of the control signals (611, 621) according to a requirement, wherein a duty cycle in normal operation is limited to a range of 50% to 100%, wherein the duty cycle is a ratio of the temporal pulse width of a signal level (612, 622), which puts the correspondingly controlled semiconductor switch (210, 340) into a conducting state, to the period of the control signal (611, 621) and wherein the load branch (400) comprises a current measuring device (480) to measure the excitation current,which is used as a feedback signal by the excitation current control (1220), , the procedure includes the following steps: Detection of the excitation current and a target excitation current, Comparing the target excitation current according to the requirement with the excitation current measured and recorded by the current measuring device to determine a control deviation and Comparing the control deviation with a control deviation threshold and outputting an error signal if the control deviation exceeds the control deviation threshold. where a supply voltage U DC is detected, which is located at the semi-controlled H-bridge (100), and for this supply voltage U DC a standard duty cycle for a The target excitation current value is determined corresponding to the currently recorded measured excitation current value for a minimum possible load branch resistance value of the load branch (400), and an error signal is output if the currently set duty cycle is smaller than the determined standard duty cycle. or for the excitation current value measured at the currently set duty cycle and the A standard duty cycle range (1080) is determined from the measured supply voltage, which includes duty cycles that correspond to the currently measured excitation current value for various possible load branch resistances, and it is checked whether the currently set duty cycle is within the standard duty cycle range (1080), and a short-circuited semiconductor switch is detected if the currently set duty cycle is not within the determined standard duty cycle range (1080).

Description

[0001] The invention relates to a method for detecting a single permanently conductive, defective semiconductor switch of a semi-controlled H-bridge for setting an excitation current of a separately excited synchronous machine. Furthermore, the invention relates to a control device with a detection unit for detecting a single permanently conductive, defective semiconductor switch of such a semi-controlled H-bridge for setting an excitation current.

[0002] A half-controlled H-bridge circuit, also known simply as a half-controlled H-bridge, is frequently used as a control circuit to generate an excitation current for a separately excited synchronous machine. Fig. Figure 1 schematically depicts such a circuit. The circuit 100 comprises one bridge branch 200 and another bridge branch 300. One bridge branch 200 includes a semiconductor switch 210, also designated T1. A reverse-biased diode 230, also designated D3, is connected in series with the semiconductor switch 210. The other bridge branch 300 includes another reverse-biased diode 320, also designated D2. A further semiconductor switch 340, also designated T4, is connected in series with this diode in the other bridge branch 300.

[0003] A load branch 400 is formed between a connection point 250 of one bridge branch 200 between one semiconductor switch 210 and one diode 230, and another connection point 350 of the other bridge branch 300 between the other diode 320 and the other semiconductor switch 340. The load branch comprises an excitation winding 460, for which an ohmic winding resistance 465 and a winding inductance 467 are shown as an equivalent circuit. A current measuring device 480 is also formed in series with the excitation winding 460 in the load branch. The current measuring device 480 is configured to measure the excitation current I exc to measure. An excitation voltage U drops across the excitation winding 460. exc The bridge branch 200 and the further bridge branch 300 are connected to a positive pole 51 of a supply voltage U at one end 201, 301. DCand at the opposite end 202, 302 is connected to a negative terminal 52 of the supply voltage. A DC link capacitor 500 is arranged in parallel to the bridge branch 200. The supply voltage U is applied to this capacitor. DC Additionally, a further current measuring device 180 is provided, which measures the total current I flowing through the semi-controlled bridge circuit 100. DC can measure. A voltage measuring device 190 is designed in parallel with the intermediate circuit capacitor 500, which measures the supply voltage U. DC measures.

[0004] To generate an excitation current I excTo trigger a specific action, for example, in response to a request from a driver in a vehicle powered by a separately excited synchronous machine, the semiconductor switches T1, T4; 210, 340 are each controlled by a pulse-width modulated control signal. When the term "semiconductor switches" is used below, it refers to both semiconductor switch 210, T1 and semiconductor switch 340, T4. When "one of the semiconductor switches" is mentioned, it can refer to either semiconductor switch 210, T1 or semiconductor switch 340, T4.

[0005] The control signals 611 and 621 are square wave signals with the same frequency, but offset by half a period T. PWM are offset from each other (see Fig. 2) A current flows through the semi-controlled semiconductor bridge when both one of the semiconductor switches 210, T1 and the other of the semiconductor switches 340, T4 are conducting simultaneously.

[0006] The control signals each have the same duty cycle, which is 50% or greater. The duty cycle is defined as the ratio of the control signal's time at a level that switches the corresponding semiconductor switch 210, 340 into a conducting state to the total duration of the period T. PWM of the signal understood. The larger the pulse width of the control signals, the longer the periods in which current can be fed into the excitation winding 460.

[0007] In Fig. Figure 2 shows five graphs 610-650 schematically represented one below the other, each plotted against time 601 in units of period T. PWMThe pulse-width modulated signal is shown. The two upper graphs, 610 and 620, show the pulse-width modulated control signal 611 and 621, respectively, for controlling one of the semiconductors 210, T1 and the other semiconductors 340, T4. A level 612, 622 is plotted against time 601. Since the semiconductor switch 210, T1 is closed and conducting when the control signal 611 has a level of 1, and closed and blocking when the level is 0, graph 610 and, analogously, graph 620 can also be interpreted as state graphs, and the control signals 611, 621 as state curves of the corresponding semiconductor switches T1, 210 and T4, 340. Graph 630 shows the excitation voltage 631. In time intervals 635, in which both control signals have a level of 1, a positive voltage, approximately U, falls across the excitation winding. HVThe corresponding voltage is applied. Graph 640 shows the changing excitation current 641. This rises steeply when both semiconductor switches 210, 340, T1, T4 are conducting, and falls somewhat more slowly in the regions where at least one of the semiconductor switches 210, 340 is in a closed state.

[0008] Finally, in the last graph 650, the total current I is shown. DC 651, which flows through the semi-controlled half-bridge, plotted against time 601.

[0009] In the depicted variant according to the state of the art, the two control signals 611, 621 are offset by half the period T. PWMThe timing is offset by half a pulse width modulation (PWM). This offers the advantage of keeping the current fed into and out of the intermediate circuit capacitor 500 low. Furthermore, the frequency of the current ripple in functioning semiconductor switches is twice the switching frequency of the individual semiconductor switches, which is advantageous for the ripple of the excitation current. Additionally, the power dissipation is distributed evenly between the semiconductor switches and diodes. Besides the center-centered pulse width modulation shown here, edge-based pulse width modulation can also be used, in which the control signals 611 and 621 are also offset by half a pulse width modulation period.

[0010] When controlling the semiconductor switches 210, T1; 340, T4, it is essential that the voltage across the excitation winding changes with each on / off operation of one of the semiconductor switches 210, T1 or 340, T4. This voltage is equal to the positive supply voltage, for example, a traction network voltage, or equal to the negative sum of the forward voltages of the diode and semiconductor (approximately -2 V to -3 V). The current then follows a series of exponential functions and, with a positive voltage across the excitation winding, rises relatively steeply, limited by the inductance 367, and falls relatively slowly during the short-circuit phase, i.e., when the semiconductor switches are closed.

[0011] The preceding description assumes that both semiconductor switches 210, T1; 340, T4 switch correctly according to the control signals 611, 621. Since the excitation winding 460 is usually configured as the rotor of a synchronous machine and often has a large inductance 467, it is necessary, in the event of a fault, to short-circuit the excitation winding 460 via one of the two semiconductor switches, either one of the semiconductor switches 210, T1 or the other of the semiconductor switches 340, T4. Due to the current direction, the respective diode 320 or 330 of the other bridge branch 200, 300 conducts. If the short circuit does not occur, the energy from the large inductance of the rotor's excitation winding would flow into the DC link of the inverter. If, in this case of regenerative braking, the energy flows relatively simultaneously, including shortly before or after the start of the regenerative braking process, the following occurs:Shortly after the short circuit, when the battery contactors open and the inverter is thus no longer connected to the traction network battery, the capacity of typical 500 µF DC link capacitors is usually insufficient to prevent an overvoltage that could destroy the main inverter and / or the H-bridge circuit described here, as well as potentially other high-voltage components. Therefore, it is advisable to determine beforehand if one of the semiconductor switches is defective.

[0012] In the event of a fault where one of the two semiconductor switches remains permanently in the open state despite being controlled, the excitation current I exc within a short period of time, usually a few hundred milliseconds, the voltage drops to 0. If both switches are permanently on, the supply voltage U is... DC as the voltage of the excitation winding U excThis causes the current through the excitation winding to rise sharply. An overcurrent detector can reliably detect this fault and disconnect the supply voltage. A more difficult and critical situation arises when one of the two semiconductor switches 210 or 340 no longer switches and remains permanently conductive, thus becoming defective, while the other semiconductor switch continues to switch normally. Such a defect could be caused by a fault in the control circuitry or by a defect in the semiconductor switch 210 or 340 itself.

[0013] DE 10 2014 102 869 A1 describes a rotating electric machine with a first switching element for stopping the supply of an excitation current that energizes a field winding. A detector, based on a voltage at a control terminal of the excitation current, detects how the field winding is supplied with the excitation current via the control terminal. An abnormality detector, based on a detected result and a voltage induced across at least a single-phase stator winding, determines whether an abnormality exists in at least a second switching element connected to an output terminal of the at least single-phase stator winding. A second drive control, based on the result of the abnormality detector's determination, drives the first switching element.

[0014] DE 10 2020 112 704 A1 describes an electrical circuit arrangement for supplying current to a rotor of an electric machine having a rotor winding, comprising a control unit, an input with a first potential connection and a second potential connection, and an output with a first connection and a second connection for connecting the electrical circuit arrangement to a beginning and an end of the rotor winding, wherein the first connection is connected to the first potential connection via a first switching element and to the second potential connection via a first diode, and the second connection is connected to the first potential connection via a second diode and to the second potential connection via a second switching element, wherein the first potential connection can be connected to a center tap of the rotor winding via a third switching element and the second potential connection via a fourth switching element.wherein the control unit is connected to the switching elements and the control unit is designed to keep the third and fourth switching elements permanently open during normal operation and, in the event of a defect in the first or second switching element, to close the third and / or fourth switching element at least temporarily, depending on a state information describing an operating state of the electrical machine to be set via the electrical circuit arrangement.

[0015] US 2013 / 0083434A1 describes a circuit for controlling a load current through an inductor connected to an output terminal of a transistor H-bridge comprising two low-side transistors and two high-side transistors. A current-sensing circuit is coupled to the H-bridge and configured to provide a representation of the load current supplied by the output terminal. A current regulator is configured to generate a modulated signal based on the load current representation and a current setpoint. The modulated signal has a duty cycle. Gate control logic switches the individual transistors of the H-bridge on and off according to the modulated signal. A direction signal supplies the load current to the inductor. The direction signal determines the direction of the load current.An overcurrent detection circuit is coupled to each individual transistor and configured to signal an overcurrent by providing an active overcurrent fault signal when a transistor current through that transistor exceeds a respective maximum value.

[0016] US 2019 / 0305685A1 describes a voltage conversion device that detects an anomaly where the duty cycle supplied to a voltage conversion section does not converge within a normal range. The voltage conversion device includes: an output voltage detection section that detects a value indicating the output voltage of a voltage conversion section; a setting section that sets the duty cycle of a PWM signal supplied to the voltage conversion section to bring the output voltage value closer to a target voltage value; and a drive section that outputs the PWM signal to the voltage conversion section at the duty cycle set by the setting section.An anomaly detection section that detects an anomaly where the duty cycle set by the adjustment section does not converge to a predetermined normal range when the voltage conversion section is in a predetermined stabilized state.

[0017] The invention is based on the objective of providing a method and a device with which a single defective semiconductor switch in a semi-controlled H-bridge for a separately excited synchronous machine can be detected in a simpler manner, in which one of the semiconductor switches is permanently conductively defective.

[0018] The problem is solved according to the invention by a method with the features of claim 1 and by a control device with the features of claim 7. Advantageous embodiments are described in the dependent claims.

[0019] The invention is based on the idea of ​​detecting and exploiting deviations in the control system to identify a state in which a single semiconductor switch is defective. In particular, a method is provided for detecting a single permanently conductive defective semiconductor switch of a semi-controlled H-bridge for setting an excitation current of a separately excited synchronous machine, wherein the semi-controlled H-bridge circuit comprises a bridge branch with one of the semiconductor switches T1 and a diode D3 connected in series in reverse bias, and a further bridge branch comprising another diode D2 connected in reverse bias in series with another of the semiconductor switches T4, as well as a load branch comprising an excitation winding of the separately excited synchronous machine.wherein the load branch connects a connection point from one of the semiconductor switches T1 and the diode D3 in one bridge branch and with another connection point between the other diode D2 and the other semiconductor switch T4 in the other bridge branch, and wherein a current measuring device for measuring the current excitation current is arranged in the load branch; wherein the semi-controlled H-bridge circuit is coupled to a control device comprising excitation current control, which generates two pulse-width modulated control signals of the same frequency, phase-shifted by half a period, for controlling one of the semiconductor switches and the other semiconductor switch, such that the excitation current flowing through the load branch can be controlled via the pulse-width modulation of the control signals according to a requirement (reference variable), wherein a duty cycle in normal operation is limited to a range of 50% to 100%,wherein the duty cycle specifies a ratio of the temporal pulse width of the signal level that puts the driven semiconductor switch into a conducting state to the period of the control signal, and wherein the load branch comprises a current measuring device to measure the excitation current which is used by the excitation current control as a feedback signal (controlled variable), wherein the method comprises the steps of: detecting the excitation current, comparing the required excitation current with the excitation current measured and detected by the current measuring device to determine a control deviation, comparing the control deviation with a control deviation threshold, and outputting an error signal if the control deviation differs from the control deviation threshold by more than a tolerance range.

[0020] If a certain threshold for a control deviation is exceeded, it can be assumed—at least if an excitation current is still measured and has not completely decayed to zero—that one of the semiconductor switches is switching correctly, while the other is defective, specifically in a state where it is permanently conductive. Therefore, if a control deviation occurs above the threshold value, an error signal is output, indicating that one of the semiconductor switches is defective and permanently conductive.

[0021] Accordingly, a control device with a detection device for detecting a single permanently conductive defective semiconductor switch of the semiconductor switches of a semi-controlled H-bridge for setting an excitation current of a separately excited synchronous machine is provided, wherein the semi-controlled H-bridge comprises a bridge branch with one of the semiconductor switches T1 and a diode D3 connected in series in reverse bias, and a further bridge branch comprising a further diode D2 connected in reverse bias in series with another of the semiconductor switches T4, as well as a load branch comprising an excitation winding of the separately excited synchronous machine, wherein the load branch connects a connection point from one of the semiconductor switches T1 and the diode D3 in the first bridge branch and with a further connection point between the further diode D2 and the further semiconductor switch T4 in the second bridge branch.and wherein a current measuring device for measuring the current excitation current is arranged in the load branch; wherein the control device comprises an excitation current control configured to generate two pulse-width modulated control signals of the same frequency, phase-shifted by half a period, for controlling one of the semiconductor switches and the other of the semiconductor switches, such that the excitation current flowing through the load branch can be controlled via the pulse-width modulation of the control signals according to a requirement (reference variable), wherein a duty cycle in normal operation is limited to a range of 50% to 100%, wherein the duty cycle specifies a ratio of the temporal pulse width of the signal level that puts the controlled semiconductor switch into a conducting state to the period of the control signal, and wherein the load branch comprises a current measuring device for measuring the excitation current.which is used by the excitation current control as a feedback signal (controlled variable), wherein the detection device is configured to acquire excitation current measurements from the current measuring device and to compare them with the setpoint excitation current values ​​according to a requirement of the excitation current control, in order to determine a control deviation and to compare the control deviation with a control deviation threshold value and to output an error signal if the control deviation differs from the control deviation threshold value by more than a tolerance range.

[0022] Preferably, the detection device is implemented with a program-controlled processor and a program memory containing program code. When executed, the processor performs the steps of the method described herein in conjunction with a semi-controlled H-bridge described herein. This allows for the simple design of a control device capable of detecting semiconductor switch faults. Due to the limitation of the duty cycle to at least 50% in each of the control signals, the controller can no longer adequately regulate small requested excitation currents. A minimum current is present in the excitation winding because it is energized for half the duration of each control signal period.

[0023] However, the diagnosis is also extended or improved to other areas. For this purpose, a first alternative of the invention provides that a supply voltage U DC is detected, which is applied to the semi-controlled H-bridge, and for this supply voltage U DCA standard duty cycle for a target excitation current value is determined based on the currently measured excitation current value for a minimum possible resistance of the load branch. An error signal is output if the currently set duty cycle is lower than the determined standard duty cycle. The standard duty cycle required to generate a specific excitation current is highly dependent on the ohmic resistance of the excitation winding. This resistance value can vary, for example, due to manufacturing tolerances. Another important influencing factor is the temperature of the excitation winding, which is typically located in the rotor of the synchronous machine. Considering the range of variation in the excitation winding's resistance, and given the supply voltage, it is possible to estimate the required duty cycle to obtain a specific target excitation current value.The simplest estimate assumes that a minimum duty cycle must be exceeded to achieve the lowest possible resistance value if both semiconductor switches of the semi-controlled H-bridge are intact. Such a duty cycle, which determines a target excitation current value, is called the standard duty cycle. If the set duty cycle of the excitation current control is lower, this is a sure sign that one of the semiconductor switches is permanently defective.

[0024] According to a second alternative of the invention, it is taken into account that, based on the fluctuation range of the resistance of the load branch, the rotor resistance, at a given supply voltage, it is possible to estimate the duty cycle for a specific target current. This results in a standard duty cycle range, i.e., a range within which the set duty cycles can lie for the currently detected excitation current value or target excitation current value, assuming intact semiconductor switches. Depending on the actual resistance of the load branch or the excitation winding, which largely determines this resistance, different duty cycles may therefore be required to generate the corresponding excitation current in an intact H-bridge circuit where both semiconductor switches are functioning correctly.

[0025] According to the second alternative of the invention, it is therefore provided that the supply voltage U DCThe system detects the excitation current value measured at the currently set duty cycle or the target excitation current value, and determines a standard duty cycle range for the currently set duty cycle. This range includes duty cycles that correspond to the currently measured excitation current value or the requested target excitation current for various possible load branch resistances. The system then checks whether the currently set duty cycle is within the standard duty cycle range and detects a short-circuited switch if the currently set duty cycle is not within the determined standard duty cycle range.

[0026] As mentioned, the potential range of resistance can be quite large due to manufacturing tolerances and / or a lack of knowledge of the actual temperature of the excitation winding. With aluminum or copper as the main component of the excitation winding, the resistance changes by approximately four thousandths of a Kelvin per temperature difference (α = 4 x 10⁻⁶). -3 / K). The resistance of the excitation winding can be estimated according to the following formula: Rexc(ϑ)=Rexc,nom⋅(1+α(ϑmin−20°C))⋅(1±δ)

[0027] This assumes that the standard resistance of the excitation winding R exc,nom The resistance of the excitation winding at 20°C is given. A manufacturing tolerance δ of approximately 20% is considered realistic. The resistance of the excitation winding with a standard resistance value Rext,nom at 20°C varies in the range of approximately 0.6·Rext,nom to approximately 2·Rext,nom. For an excitation winding that, for example, has a standard resistance R exc,nomIf the resistance is 10 Ω (10 ohms), the resistance range varies from approximately R exc,nom = 6 Ω (6 ohms) up to a maximum value R exc,max = 20 Ω (20 ohms). Here, the minimum value is assumed to be the minimum temperature ϑ. min = -40°C. For the maximum rotor temperature ϑ max A value of 180°C is assumed (ϑ max = 180°C).

[0028] A significant improvement can therefore be achieved in embodiments where the temperature of the excitation winding in the load branch is measured. In a further development, it is therefore provided that the temperature of the excitation winding in the load branch is detected and the resistance of the excitation coil is calculated based on the detected temperature in order to limit the standard duty cycle range.

[0029] Further improvement can be achieved by calibrating the load branch resistance under known temperature conditions of the excitation winding to compensate for uncertainties in the load resistance due to manufacturing tolerances.

[0030] However, measuring the temperature of the excitation winding with a sensor is often difficult because it is integrated into the rotor. Therefore, one embodiment provides for the continuous iterative prediction of the excitation winding temperature based on a temperature model that takes into account the measured and recorded excitation currents and the time intervals between measurements. If the intervals are sufficiently small, the temperature of the excitation winding can be predicted very accurately. For further improvement, additional available measurements can be acquired and incorporated, such as ambient temperature, stator temperature(s), coolant temperatures, etc.The temperature determined using the temperature model is then used to limit the standard duty cycle and thereby enable reliable detection of a defective semiconductor switch, especially at high currents, where the standard duty cycle would otherwise encompass a large number of duty cycles.

[0031] An initial temperature for the temperature model can be derived from a recorded ambient temperature or other measured or calculated temperatures after a prolonged period of machine downtime.

[0032] This ensures that the temperature model is always optimally adapted to the actual temperature of the excitation winding.

[0033] Another way to determine the resistance of the excitation winding as accurately as possible, either additionally or alternatively, is to calculate the resistance of the load branch based on the measured supply voltage and excitation current, as measured for the last duty cycle set within the corresponding standard duty cycle range. This makes it possible to detect a defective permanently conducting semiconductor switch as quickly as possible, even with high required excitation currents. This is because, even with high required excitation currents, the excitation current control must reduce the duty cycle compared to a standard duty cycle, which corresponds to the correct resistance value at a known supply voltage, if one of the two semiconductor switches fails. This method exploits the fact that the time scale over which the resistance of the excitation winding changes due to temperature is larger than the time constant of the current control.However, deviations in the controlled variable excitation current do not occur in these situations. Therefore, monitoring the duty cycle enables more reliable detection of a defective semiconductor switch.

[0034] Knowing if a semiconductor switch is permanently closed due to a fault is particularly advantageous if the excitation winding needs to be short-circuited in the event of a fault. In such a case, it is essential to avoid closing the intact semiconductor switch to create this short circuit.

[0035] The invention is explained in more detail below with reference to a drawing. The drawing shows: Fig. 1 a schematic representation of a semi-controlled H-bridge for generating an excitation current; Fig. Two different graphs plotted against time in units of time per pulse width period, showing the drive signals for the semiconductor switches, a voltage drop across the excitation coil, a variation of the excitation current, and the total current flowing through the entire circuit; Fig. 3 comparable graphs to those of the Fig. 2 in the event that one of the semiconductor switches is permanently defective; Fig. 4 a graphical representation in which an actual excitation current is plotted against a target excitation current, once for the situation in which both semiconductor switches are switching correctly, and once for a situation in which one of the semiconductor switches is permanently defective; Fig. 5 a schematic representation in which the duty cycle for different resistances of the excitation coil winding against a target excitation current is plotted; Fig. 6 a schematic view of a motor vehicle with a separately excited synchronous machine, which is controlled by means of a semi-controlled H-bridge via a control unit, which has a detection device for detecting a permanently conductively defective semiconductor switch; and Fig. 7 a schematic representation of a flowchart of a method for detecting a permanently conductive defective semiconductor switch.

[0036] In Fig. 3 are analogous to the one already described above. Fig. 2 The signal waveforms and determined or resulting measured values ​​for the operating case of a semi-controlled H-bridge are shown, in which one of the semiconductor switches, without loss of generality, is another semiconductor switch 210, T1 according to Fig. Graph 820 shows the level 822 of the control signal 821 for the other semiconductor switch 340, T4, plotted against time 801 in periods of the pulse-width modulated control signal. With an intact semiconductor switch 100, the graph can also be interpreted as a state graph for the other semiconductor switch 340, T4. At level 0, semiconductor switch 340, T4 is open, i.e., non-conducting, and correspondingly at level 1, it is conducting and closed. Graph 810 shows the control signal 811 for one of the semiconductor switches 210, T1, which is represented by a dashed line. The level 812 is plotted against time 801. The switching state 813 of one of the semiconductor switches 210, T1 is shown as a solid line. This is permanently conducting, which corresponds to level 1. The third graph 830 shows the voltage drop U across the excitation coil 460. exc831. Since one of the semiconductor switches 210, T1 is permanently conductive and a minimum duty cycle of 50% is specified for the control signal 811 and also for the semiconductors 340, T4, the supply voltage U DC for at least half the time at the excitation winding 460.

[0037] This results in a minimum excitation current I in the excitation winding 460. exc_min flows. The fourth graph 840 shows the amplitude fluctuation of the excitation current I. exc 841. One frequency of this current change is halved compared to the intact operating state. The fifth graph 850 represents the total current flow I. DC 851.

[0038] In Fig. 4 is the ordinate of the excitation current I exc,ist 910 and as the abscissa the target current I exc,soll 920 is used. The assignment of I is shown graphically. exc,ist to I exc,sollThe situation is shown (solid line 930) in which both semiconductor switches are functioning correctly. The actual and target currents correspond. A dashed line 940 shows the situation in which one of the two semiconductor switches, without loss of generality, one of the semiconductor switches 210, T1, is permanently defective. In the illustrated embodiment, at a minimum duty cycle of 50%, the other semiconductor switch, here the other of the semiconductor switches 340, T4, experiences a minimum excitation current of just under 20 A.

[0039] As with any control system, a certain deviation of the actual value from the setpoint must be tolerated to maintain proper control. This tolerable control deviation is limited by a control deviation threshold value 960. If larger control deviations 970 occur with a steady or slowly changing setpoint, the detection device can determine that one of the semiconductor switches is defective. In the example shown, such a control deviation, which is larger than the control deviation threshold value, occurs for all excitation current requirements that result in a setpoint excitation current in the range of 0 to approximately 17.5 A. This range 980 of implausibly large control deviations is in Fig. Figure 4 is also shown. The semi-controlled H-bridge cannot generate excitation currents in this range, as the minimum current is already just under 20 A. For target currents above the threshold of 17.5 A, a defect in one of the semiconductor switches cannot be detected based on a control deviation. Only when the target current falls below 17.5 A does a detectable control deviation occur.

[0040] However, in the range above 17.5 A in the example shown, detection is possible in at least a large number of situations if the duty cycle, which determines the excitation current control, is also evaluated.

[0041] In Fig. Figure 5 shows the duty cycle of 1010 against the target current of 1020. A dashed line with short dashes indicates a duty cycle of 1030 for the target current when the excitation winding has minimal resistance and both semiconductor switches are functioning correctly. Similarly, a dashed line with long dashes represents the duty cycle of 1040, which is required to generate a target current I. exc,sollThis is required in the case where both semiconductor switches are intact and the excitation winding has its maximum possible resistance. For a target current of, for example, 25 A, duty cycles of 1070 between approximately 59% and 82.5% correspond, depending on the current resistance of the excitation winding. If a duty cycle is detected that lies outside this standard duty cycle range of 1080 (indicated by hatching), the detection circuit can determine that one of the semiconductor switches, in this specific example, one of the semiconductor switches 210, T1, is defective. The solid line shows the duty cycle of 1050, which is necessary to generate the target current when one of the two semiconductor switches, in this specific example, one of the semiconductor switches 210, T1, is permanently defective and the resistance of the excitation winding or the load branch is minimal.In the example shown, with a minimum resistance value of the excitation winding, the minimum current at a duty cycle of 50%, at which the intact semiconductor switches 340 and T4 are switched, is greater than the range shown in the graph. With a minimum resistance value of the excitation winding, the set duty cycle of the excitation current control is therefore always 50% and thus below the standard duty cycle for the minimum resistance 1030, which is set when both semiconductor switches 210, 340, T1, and T4 are switching correctly. A fault can therefore always be detected if a set duty cycle is smaller than the standard duty cycle for the minimum resistance for a corresponding target current that corresponds to the currently measured excitation current.

[0042] Also shown as a dashed line is the duty cycle of 1060 for the target current to be set in the situation where one of the two semiconductor switches 210, 340, T1, T4, in this case one of the semiconductor switches 210, T1, is conductively defective and the excitation winding exhibits maximum resistance. It can be seen that the resulting duty cycles of 1060 for high currents are sometimes within the standard duty cycle range of 1080 if the resistance is not well known.

[0043] Therefore, it is advantageous to limit this standard duty cycle by setting the possible resistance value of the excitation winding to 460 or the load branch to 400 (compare Fig. 1) is limited. In some embodiments, this can be achieved by measuring and recording the actual temperature of the excitation winding 460, which correlates with the resistance value. In other embodiments, the temperature, and consequently the resistance of the excitation winding, is determined by modeling the temperature. For example, the measured excitation current and the resulting power loss across the ohmic resistance 465 are used to continuously model the heating of the excitation coil 460 during operation.

[0044] Furthermore, it is advantageous to calibrate the resistance value of the excitation coil at a known temperature. This can be done, for example, at one end of the production line of the separately excited synchronous machine. Calibration can also be performed again after a prolonged period of inactivity. In a motor vehicle, ambient temperature measurement is generally standard practice, so a measured ambient temperature value can be recorded by the detection device and used as the starting temperature for the excitation winding temperature in the modeling.

[0045] Furthermore, in other embodiments, it is additionally or alternatively possible to continuously determine the resistance value based on a measured and recorded total current and the supply voltage. The resistance value used to determine the standard duty cycle can thus be the resistance last determined for a plausible duty cycle, i.e., a duty cycle that indicates both semiconductor switches are intact. The moment one of the semiconductor switches is permanently defective, the excitation current increases, which the excitation current control compensates for by reducing the duty cycle for the two control signals. The temperature increase of the excitation winding due to the increased excitation current occurs more slowly than the current control can adjust the duty cycle.Even at high currents, a fault that occurs in one of the semiconductor switches due to it being permanently conductive can be detected based on the knowledge of the resistance and the deviation of the setpoint from the standard setpoint.

[0046] In Fig. Figure 6 schematically depicts a motor vehicle 1. This vehicle has a separately excited synchronous machine 10 as its drive motor. Two key influencing factors that determine the required excitation current are the current machine speed, which is proportionally correlated with the vehicle speed, and a torque requested by the driver, referred to here as a request 21. An excitation current for the excitation winding, which is usually located in the rotor, is provided by a semi-controlled H-bridge circuit 100, which is controlled by a control unit 1200. The electrical energy comes from a traction battery 50, which is connected, among other things, to the H-bridge circuit 100. The traction battery 50 has a positive terminal 51 and a negative terminal 52. The control unit 1200 detects a request 21 from a driver, for example, via an accelerator pedal 20.The control unit 1200 comprises a detection device 1240, which is configured to detect a single defective semiconductor switch in the semi-controlled H-bridge circuit 100 and output a fault signal 31. This signal can be output, for example, via a warning lamp in an instrument cluster or any other display device 30 in the vehicle. The fault signal 31 can also be output as an electrical or radio signal for further processing in the control unit 1200 or other control units of the motor vehicle 1. This control unit 1200 is typically implemented as part of a pulse inverter that drives the separately excited synchronous machine.

[0047] Identical technical features are identified by the same reference symbols as in the other figures and are not explained again in detail here. In particular, the H-bridge circuit 100 is essentially the same as the one shown in Fig. The H-bridge circuit 100 is shown in Figure 1. Additionally, only a temperature sensor 490 is shown, which measures the excitation winding temperature T. exc determined and provided as temperature signal 491.

[0048] The control unit 1200 includes an excitation current control 1220. This has a control unit 1221, which sets a target current I that depends on the detected request signal. exc_soll outputs. A signal generation unit 1222 of the excitation current control 1220 generates the control signals StS. T1 and StS T4 These are the pulse-width modulated control signals, which are phase-shifted by half a pulse-width period. These control the semiconductor switches T1, 210 and T4, 340. The two semiconductor switches are, for example, designed as IGBTs. Starting from the supply voltage U DC A medium voltage drops across the excitation winding. Uexc¯=UDC(2TV−1) ab. TV is the duty cycle. The resulting average excitation current Iexc¯=Uexc¯Rexc=UDC(2TV−1)Rexc. Here R gives exc the ohmic resistance of the load branch or the excitation winding 460. Via the measured excitation current I exc , which is recorded as a control variable by the control unit 1200 and its excitation current control 1220, the excitation current I can be exc in accordance with the request signal 21 or the request itself.

[0049] The detection unit 1240 is described below together with the one in Fig. The schematic flowchart of a method for detecting a permanently conductively defective semiconductor switch in the semi-controlled H-bridge 100 is described in more detail in section 7. The detection device firstly detects the setpoint I. exc_soll for the excitation current 2010 and secondly the measured actual value I excThe current measuring device 480 2020. The detection device compares the two values ​​and calculates the control deviation 2030. The control deviation is compared with a control deviation threshold value, and it is checked whether the control deviation threshold value is exceeded 2040. It is understood by those skilled in the art that here, an amount of the control deviation is compared with a control deviation threshold value. If a deviation above the control deviation threshold value is present, an error signal is generated and output 2050, which can be provided as an electronic signal to other control devices or other control units (not shown) and / or output on the display device 30. If no control deviation is detected, the supply voltage U is reduced by the detection device. DC Recorded in 2060. Together with the recorded excitation current I excand with a specification or specifications about the resistance value of the load branch 400 or the excitation winding 460, in particular the ohmic resistance 465, a duty cycle can be determined which should be set by the excitation current control in order to determine the corresponding excitation current I exc to generate. Such a duty cycle is called the standard duty cycle. If the resistance value of the load branch 400 is only known imprecisely, a standard duty cycle range can also be determined, which includes duty cycles that would have to be set at different possible resistance values ​​in order to generate the currently measured excitation current I. excto generate. The detection device 1240 also detects the current duty cycle 2080. The determined standard duty cycle or the determined standard duty cycle range is compared with the actual duty cycle and checked to see if the actual duty cycle is consistent with the standard duty cycle 2090. For example, it is checked whether the actual duty cycle is lower than the smallest possible standard duty cycle. If this is the case, the error signal 2050 is also output. Otherwise, the procedure is executed iteratively. To increase the detection accuracy, the procedure can include optional process steps. For example, if a temperature measuring device 490 is arranged on the excitation winding and a temperature signal T is present, the following steps can be added: exc 491 is provided, this is detected by the detection device 2100. This detected value is for the resistance of the excitation winding R excThis can then be used in process step 2070 to determine the standard duty cycle or standard duty cycle range. Alternatively or additionally, the total current I flowing through the H-bridge circuit can also be measured by the detection device. DC The additional current measuring device 180 is recorded at 2110. Together with the recorded supply voltage U DCThe current resistance of the excitation winding can be calculated from this (2120), whereby this determination is only carried out as long as, during the iterated execution of this procedure in step 2090, it is found that the actual duty cycle is consistent with the standard duty cycle. There are also alternative methods for deriving the resistance value of the excitation winding mathematically from the available electrical measurements. The resistance values ​​of the load branch (400) and the excitation winding (460) determined in this way are also used in determining the standard duty cycle, in order to better determine this value and / or to narrow down the standard duty cycle range.

[0050] Alternatively or additionally, knowledge of the resistance value of the load branch 400 can be improved by calibrating the resistance value at a known temperature of the excitation winding. This can be done, for example, at the end of a production line 2130. Alternatively or additionally, calibration can be performed or repeated after a prolonged standstill of the separately excited synchronous machine 10 2140. For this purpose, for example, a currently measured ambient temperature T can also be used. umg2150. Other signals, such as coolant temperatures, operating states of cooling devices like fans, etc., can also be recorded 2160. In one embodiment, the various values ​​recorded in this way are used to model the temperature of the excitation winding 2170. In addition to the data from calibration, the ambient temperature, and other signals and information indicating the operating state of the vehicle, the measured excitation current values ​​and the time data are used for the modeling in particular to calculate the heating of the excitation winding based on the ohmic losses that occur in it, and then to calculate the resistance from this 2180. The modified modeled resistance value is naturally included in the subsequent modeling, as it significantly determines the thermal losses in the excitation winding.

[0051] Both the excitation current control 1220 and the detection device 1240 are preferably implemented by means of a program-controlled microprocessor 1260, on which program code 1270 is executed and stored in the memory device 1250. It is understood that other embodiments may also have dedicated circuits that implement the excitation current control and / or the detection device.

[0052] Parameters or tables of values ​​used by the detection device to calculate the standard duty cycles or standard duty cycle ranges can also be stored in memory 1250. In particular, the standard duty cycle is calculated using a parameterized formula, one parameter of which is a resistance value for the load branch or the excitation coil, and the target current to be applied is entered as a function variable. However, in another embodiment, the target duty cycles can also be determined using tables stored in the memory.

[0053] If the defect is detected, for example, via an unexpectedly large control deviation or the associated implausible duty cycle, the defective of the two semiconductors can be identified by detecting a positive change in excitation current during the switching cycle when the semiconductor switch should be open. Due to the offset of the control signals and the requirement that the duty cycle be greater than 50% in normal operation, it is ensured that no positive increase in excitation current occurs immediately after one of the semiconductor switches opens if the semiconductor switch is intact, but an increase in excitation current occurs if the semiconductor switch is permanently defective.To identify the defective semiconductor switch, one embodiment checks whether a positive increase in the excitation current change occurs after a nominal switching time for opening one of the semiconductor switches. If this is the case, then the semiconductor switch that should have opened at this nominal switching time is defective.

[0054] It will be understood by those skilled in the art that only an exemplary embodiment is described here; in particular, the additional current measuring device 180 or the temperature sensor 490 are not necessary for all embodiments of the invention. Reference symbol list 1 motor vehicle 10 separately excited synchronous machines 20 Accelerator pedal 21. Drive request signal 30 Display device 31 Error signal 50 traction battery 51 positive pole 52 Negative terminal 100 H-bridge circuit 180 additional electricity measuring devices 190 Voltage measuring device 200 bridge branch 201 an end of the bridge branch 200 202 opposite end of the bridge branch 200 210, T1 semiconductor switch 230, D3 diode 250 connection point 300 more bridge branches 301 an end of the further bridge branch 300 302 opposite end of the further bridge branch 300320, D2 further diode 340, T4 further semiconductor switch 350 connection point 400 load branch 460 Excitation winding 465 ohm winding resistance 467 Winding inductance 480 Current measuring device 490 Temperature sensor 491 Temperature signal 500 Intermediate circuit capacitor 601 Time 610 Graph 611 Control signal (T1) 612 levels 620 Graph 621 Control signal (T4) 622 levels 630 Graph 631 Excitation voltage 640 Graph 641 Excitation current 650 Graph 651 Total current 801 Time 810 Graph 811 Control signal (T1) 812 levels 813 Switching state (of the defective semiconductor switch) 820 Graph 821 Control signal (T4) 822 levels 830 Graph 831 Excitation voltage 840 Graph 841 Excitation current 850 Graph 851 Total current 910 Excitation current 920 target current 930 solid line 940 dashed line 960 Control deviation threshold 970 rule deviation 980 Area of ​​implausibly large control deviations 1010 duty cycle 1020 target current 1030 duty cycle for minimum resistance with intact semiconductor switches 1040 Duty cycle for maximum resistance with intact semiconductor switches 1050 duty cycle at a minimum resistance and a defective semiconductor switch 1060 duty cycle at maximum resistance and a defective semiconductor switch 1070 (standard) duty cycles (for excitation current 25 A) 1080 standard sampling range 1200 Control unit 1220 Excitation current control 1221 Control unit 1222 Signal generation device 1240 Detection device 1250 storage 1260 microprocessor 1270 Program code 2000 Flowchart of a procedure for detecting a defective semiconductor 2010 - 2180 process steps

Claims

[1] Method for detecting a single permanently conductive defective semiconductor switch of the semiconductor switches (210, 340) of a semi-controlled H-bridge (100) for setting an excitation current of a separately excited synchronous machine, wherein the semi-controlled H-bridge (100) a bridge branch (200) with one of the semiconductor switches T1 (210) and a diode D3 (230) connected in series in reverse bias and a further bridge branch (300) comprising a further diode D2 (320) in reverse bias in series with a further semiconductor switch T4 (340) and a load branch (400) comprising an excitation winding (460) of the separately excited synchronous machine, wherein the load branch (400) connects a connection point (250) from one of the semiconductor switches T1 (210) and the diode D3 (230) in the first bridge branch (200) and with a further connection point (350) between the further diode D2 (320) and the further semiconductor switch T4 (340) in the further bridge branch (300), and wherein a current measuring device (480) for measuring the current excitation current is arranged in the load branch (400); wherein the semi-controlled H-bridge (100) is coupled to a control device (1200) comprising an excitation current control (1220) which generates two pulse-width modulated control signals (611, 621) of the same frequency, phase-shifted by half a period, for controlling one of the semiconductor switches T1 (210) and the other of the semiconductor switches T4 (340), such that the excitation current flowing through the load branch (400) can be controlled via the pulse-width modulation of the control signals (611, 621) according to a requirement, wherein a duty cycle in normal operation is limited to a range of 50% to 100%, wherein the duty cycle is a ratio of the temporal pulse width of a signal level (612, 622), which puts the correspondingly controlled semiconductor switch (210, 340) into a conducting state, to the period of the control signal (611, 621) and wherein the load branch (400) comprises a current measuring device (480) to measure the excitation current,which is used as a feedback signal by the excitation current control (1220), the procedure includes the following steps: Detection of the excitation current and a target excitation current, Comparing the target excitation current according to the requirement with the excitation current measured and recorded by the current measuring device to determine a control deviation and Comparing the control deviation with a control deviation threshold and outputting an error signal if the control deviation exceeds the control deviation threshold. where a supply voltage U DC is detected, which is located at the semi-controlled H-bridge (100), and for this supply voltage U DC a standard duty cycle for a The target excitation current value is determined corresponding to the currently recorded measured excitation current value for a minimum possible load branch resistance value of the load branch (400), and an error signal is output if the currently set duty cycle is smaller than the determined standard duty cycle. or for the excitation current value measured at the currently set duty cycle and the A standard duty cycle range (1080) is determined from the measured supply voltage, which includes duty cycles that correspond to the currently measured excitation current value for various possible load branch resistances, and it is checked whether the currently set duty cycle is within the standard duty cycle range (1080), and a short-circuited semiconductor switch is detected if the currently set duty cycle is not within the determined standard duty cycle range (1080). [2] Method according to claim 1, characterized by, that a measured temperature of an excitation winding (460) in the load branch (400) is recorded and the load branch resistance of the excitation coil is calculated on the basis of the measured temperature in order to limit the standard duty cycle range (1080). [3] Method according to any of the preceding claims, characterized by , that the load branch resistance is calibrated under known temperature conditions of the excitation winding (460) to compensate for uncertainties of the load branch resistance due to manufacturing tolerances. [4] Method according to any of the preceding claims, characterized by , that a temperature of the excitation winding (460) is continuously iteratively predicted using a temperature model which takes into account the measured excitation currents and a time interval between measurements. [5] Method according to claim 4, characterized by, that an initial temperature for the temperature model is derived from a measurement of an ambient temperature after a longer standstill of the externally excited synchronous machine. [6] Method according to any of the preceding claims, characterized by , that the load branch resistance of the load branch (400) is calculated on the basis of the detected supply voltage and the detected excitation current, which are measured for the last set duty cycle lying in the associated standard duty range (1080). [7] Control device (1200) with detection device (1040) for detecting a single permanently conductive defective semiconductor switch of the semiconductor switches (210, 340) of a semi-controlled H-bridge (100) for setting an excitation current of a separately excited synchronous machine, wherein the semi-controlled H-bridge (100) a bridge branch (200) with one of the semiconductor switches T1 (210) and a diode D3 (230) connected in series in reverse bias and a further bridge branch (300) comprising a further diode D2 (320) in reverse bias in series with a further semiconductor switch T4 (340) and a load branch (400) comprising an excitation winding (460) of the separately excited synchronous machine, wherein the load branch connects a connection point (250) from one of the semiconductor switches T1 (210) and the diode D3 (230) in the first bridge branch (200) and with a further connection point (350) between the further diode D2 (320) and the further semiconductor switch T4 (340) in the further bridge branch (300), and wherein a current measuring device (480) for measuring the current excitation current is arranged in the load branch (400); wherein the control device (1200) comprises an excitation current control (1220) configured to generate two pulse-width modulated control signals (611, 621) of the same frequency, phase-shifted by half a period, for controlling one of the semiconductor switches T1 (210) and the other of the semiconductor switches T4 (340), such that the excitation current flowing through the load branch (400) can be controlled via the pulse-width modulation of the control signals (611, 621) according to a requirement, wherein a duty cycle in normal operation is limited to a range of 50% to 100%, wherein the duty cycle specifies a ratio of a temporal pulse width of the signal level (612, 622), which puts the correspondingly controlled semiconductor switch (210, 340) into a conducting state, to the period of the control signal (611, 621), and wherein the load branch (400) a current measuring device (480) to measure the excitation current,which is used as a feedback signal by the excitation current control (1220), characterized by , that The detection device (1040) is designed to acquire excitation current measurements from the current measuring device (480) and compare them with target excitation current values ​​according to a requirement of the excitation current control (1220) in order to determine a control deviation and compare the control deviation with a control deviation threshold value and output an error signal if the amount of the control deviation exceeds the control deviation threshold value. and a supply voltage U DC to detect the area adjacent to the semi-controlled H-bridge (100), and for this supply voltage U DC a standard duty cycle for a To determine the target excitation current value corresponding to the currently recorded measured excitation current value for a minimum possible load branch resistance value of the load branch (400), and to output an error signal if the currently set duty cycle is smaller than the determined standard duty cycle, or for the excitation current value measured at the currently set duty cycle and the to determine a standard duty cycle range (1080) from the measured supply voltage, which includes duty cycles that correspond to the currently measured excitation current value for various possible load branch resistances, and to check whether the currently set duty cycle is within the standard duty cycle range (1080), and to detect a short-circuited semiconductor switch if the currently set duty cycle is not within the determined standard duty cycle range (1080). [8] Control device (1200) according to claim 7, characterized by , that the detection device (1240) comprises a program-controlled processor and a program memory containing program code, wherein, when the program code is executed on the processor, the steps according to a method according to one of claims 1 to 6 are performed.

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