Driver circuit and method for controlling a semiconductor switch

Abstract

The invention relates to a driver circuit (12) and a method for actuating a semiconductor switch (10), wherein during the course of a deactivation process of the semiconductor switch, a first measurement value is ascertained for the period of time (tV) lasting from the beginning of the deactivation process until a collector-emitter voltage threshold which can be defined is reached, a second measurement value for a voltage (UCEmax) which occurs during the course of the deactivation process and which maximally drops across the semiconductor switch is ascertained, a value pair consisting of the first and second measurement value is stored at least up until a subsequent deactivation process, for a subsequent deactivation process, an evaluation variable is ascertained from the value pair, and a control measure is carried out on the basis of the evaluation variable in order to reduce the change of the current flowing through the semiconductor switch over time.

Description

[0001]202310296 1 Description Driver circuit and method for controlling a semiconductor switch When semiconductor switches such as IGBTs are switched off, switch-off overvoltages occur due to connected inductances. The connected inductances can be either inductive components or stray inductances, i.e. parasitic elements such as line inductances. The resulting switch-off overvoltages can be higher than the maximum permissible blocking voltage of the semiconductor switch and thus reduce its service life or, in extreme cases, even destroy it directly. Since the level of the switch-off overvoltages is influenced, among other things, by the switch-off speed, the switch-off overvoltages are more significant in newer IGBT generations because these components are designed for faster switching.Fast switching is generally preferred because it reduces switching losses. It is known that excessive turn-off overvoltages can be determined based on the time derivative of the collector current (dI). C / dt) or based on the time derivative of the voltage across the semiconductor switch (collector-emitter voltage (dU CE / dt)). This detection can take place directly in the driver circuit of the semiconductor switch, and countermeasures such as temporarily re-switching the semiconductor switch can also be initiated directly there. This means that the countermeasures are implemented quickly. In earlier generations of IGBTs, this meant that sufficient reaction speed was available to initiate a countermeasure in the remaining time of – roughly – 1 µs after one of the measurement methods described and to prevent damage to the semiconductor switch. 202310296 2 However, with modern semiconductor switches, due to their high switching speed, the time remaining after one of the measurement methods described above to initiate countermeasures is shorter than before and can be in the range of less than 100 ns.This time may be too short, even if the driver circuit reacts, to reliably rule out damage to the semiconductor switch. As a general countermeasure, switching off can be delayed regardless of the specific situation of the semiconductor switch. However, this is very unattractive, as it negates the advantage of fast switching, namely the reduction in switching losses. The object of the present invention is to provide a driver circuit for a semiconductor switch that avoids the disadvantages mentioned above, thus enabling an improved response to a possibly excessively high switching voltage. A further object is to provide a corresponding method for controlling a semiconductor switch. This object is achieved by a driver circuit having the features of claim 1. With regard to the method, a solution lies in the method having the features of claim 14.The driver circuit according to the invention for controlling a semiconductor switch comprises a contact for connection to a control terminal (gate terminal) of the semiconductor switch. It further comprises a device in the form of a circuit which is designed to measure a value dependent on the level of the current flowing through the switch during a turn-off process of the semiconductor switch. Depending on the specific semiconductor switch, the current flowing through the switch is the collector current or the drain current. 202310296 3 In a first alternative, this value is the level of the gate current at a time after the start of the turn-off process. In a second alternative, this value is a time period from the start of the turn-off process until a definable threshold value of the voltage drop across the semiconductor switch is reached.Depending on the specific semiconductor switch, the voltage is referred to as the collector-emitter voltage or the source-drain voltage. The device is further configured to record a second measured value for a maximum voltage drop across the semiconductor switch occurring during the turn-off process. The device is configured to determine an evaluation variable for a subsequent turn-off process using the first and second measured values ​​and, based on the evaluation variable, to implement a control measure to reduce the temporal change in the current flowing through the semiconductor switch. "Reduction" here means a reduction in the magnitude of the temporal change, i.e., a slowing down of the current change, regardless of its direction.In the method according to the invention for controlling a semiconductor switch, a measurement of a value dependent on the magnitude of the collector current is taken during a turn-off process of the semiconductor switch. In a first alternative, this value is the magnitude of the gate current at a time after the start of the turn-off process. In a second alternative, this value is a time period from the start of the turn-off process until a definable threshold value of the collector-emitter voltage is reached. Furthermore, a second measured value is recorded for a maximum voltage drop across the semiconductor switch occurring during the turn-off process, an evaluation value is determined for a subsequent turn-off process using the first and second measured values, and based on the evaluation value, a control measure is taken to reduce the temporal change in the current flowing through the semiconductor switch.It is expedient to store a pair of values ​​from the first and second measured values ​​at least until a subsequent shutdown process. Advantageously, the times at which a measurement is available using the method and driver circuit according to the invention that provide information about the occurrence of an excessive shutdown overvoltage are significantly earlier than those achievable with other known methods. For example, the dU can be CE / dt measurement should only take place when the collector-emitter voltage has already risen to approximately half the nominal voltage. In contrast, in the second alternative, the threshold value can be set to a significantly lower value. The invention therefore enables a much earlier detection of a high collector current and therefore permits a reaction to an excessively high collector current even in very fast-switching semiconductor switches. Another advantage is that by determining the evaluation variable using the second measured value, it can be determined how strong the intervention in the switch-off process must be, especially during normal operation, i.e. without an error being present. Advantageously, calibration to the properties of the specific semiconductor switch is unnecessary. The variation between semiconductor switches could otherwise result in the driver circuit having to be adapted to the individual switch.The influence of other disturbances such as temperature is also insignificant, even if this influence is significant or even predominant compared to the influence of the collector current on the measured values. The prerequisite is that the disturbances exhibit only a slight change between two successive shutdown processes. However, since the shutdown processes follow one another at frequencies of 10 kHz or more, this is met for the temperature. Advantageous embodiments of the driver circuit according to the invention and of the method according to the invention emerge from the claims dependent on claim 1. The embodiment according to claim 1 can be combined with the features of one of the subclaims or, preferably, with those of several subclaims.Accordingly, the following additional features can be provided: The device can be designed to store value pairs from a respective first and second measured value for two or more, in particular consecutive, shutdown processes and, during a subsequent shutdown process, to determine a value for an expected maximum voltage dropping across the semiconductor switch during the shutdown process as an evaluation variable from two of the stored value pairs and a first measured value for the subsequent shutdown process. This can be done, for example, by linear interpolation. In a more specific example, value pairs from the delay time and the measured maximum voltage across the semiconductor switch are stored. The expected maximum voltage for the subsequent shutdown process is then determined from the value pairs and the current delay time in the subsequent shutdown process.The device can be designed to temporarily switch the semiconductor switch back on as a control measure. Likewise, the device can be designed to change the gate voltage as a control measure. Furthermore, the device can be designed to switch to a higher gate resistance as a control measure. The measures mentioned result in the collector current dropping less sharply or even rising again, at least for a short period of time. The collector-emitter voltage correspondingly rises less sharply or falls for the short period of time. This period is available for the further dissipation of the inductively stored energy and thus ensures a reduction in the resulting overvoltage when the semiconductor switch is subsequently finally switched off.The device can be designed to send a signal to a higher-level controller depending on the size of the value. This ensures that the higher-level controller of the device of which the semiconductor switch is a part, for example a converter, can react. For example, the input of such a signal can be responded to with emergency operation. The device can be designed to use a maximum of 10% of the nominal voltage of the semiconductor switch as the threshold value for the collector-emitter voltage, in particular a maximum of 5%. If such a small value is used, the measuring period ends earlier and the value is available earlier. The period of time available for a reaction to a potentially excessive collector current is thereby increased. The device can be designed to determine the start of a shutdown process based on a threshold value for the gate voltage.For example, with a typical wiring of the gate contact, a gate voltage falling below 14 V can be used as a trigger to determine the start of the shutdown process. The device can be designed to determine the start of a shutdown process based on a control command from the higher-level controller. In a further alternative, the device is designed to determine the start of a shutdown process based on the value of a control voltage, wherein the control voltage is a 202310296 7 voltage value between an output stage of the driver circuit and a gate resistor. The device can be designed to compare value pairs from the first and second measured values ​​or a value derived therefrom for shutdown processes that are separated from one another by a time period of at least 1 s, in particular at least 1 day.In other words, the value pairs or, for example, the quotient of the first and second measured values ​​can be stored for an extended period of time. A comparison is then made between values ​​from a current shutdown process and a shutdown process from some time ago. Other values, such as the temperature of the semiconductor switch, can also be included in the comparison, or the comparison can be carried out only for values ​​where the temperature is essentially the same. The comparison can advantageously be used to determine aging of the semiconductor switch, since, under otherwise identical ambient conditions, there is a fixed relationship between the delay time and the maximum overvoltage. This relationship can vary depending on the semiconductor switch.Because no absolute value is sought, but rather values ​​of the respective semiconductor switch are compared with each other, the variation between individual units is irrelevant. The device can be designed to define a period in which the control measure can be carried out by means of a delay circuit. For example, an output signal that triggers the control measure can be grounded by a semiconductor switch outside of this period. The period itself is started by another signal and ended by a delay, for example, defined by the circuitry. This delay can be determined, for example, by an RC element with a subsequent Schmitt trigger.202310296 8 Alternatively, the device can be designed to define a period in which the control measure can be carried out by means of a voltage measurement at a Zener diode that is connected to the collector-emitter voltage by means of a voltage divider. This voltage measurement has proven advantageous in tests because the design is very simple and generates a clearly pronounced voltage plateau during the period of the collector-emitter voltage rise, by means of which the period of meaningful intervention in the shutdown process can be very precisely defined. The driver circuit and the method can advantageously be used in a circuit with a semiconductor switch such as an IGBT switch. The invention is not limited to IGBTs, but can equally be applied to MOSFETs, in particular silicon carbide (SiC)-based MOSFETs.In a known manner, references to the collector are to be understood as drain, and references to the emitter as source. Further advantages and features can be found in the following description of exemplary embodiments with reference to the figures. In the figures, the same reference numerals designate the same components and functions.: Figure 1 shows a block diagram of an IGBT with a driver circuit and a higher-level controller connected to the driver circuit, Figure 2 shows a diagram with highly schematic current and voltage curves during a turn-off process of the IGBT, Figure 3 shows a diagram with measurement results for the collector-emitter voltage at different collector currents, 202310296 9 Figure 4 shows a diagram with measurement data on the dependence of a delay time when turning off the IGBT on the collector current, Figure 5 shows a diagram with highly schematic voltage curves during a turn-off process of the IGBT, Figure 6 shows a schematic sequence of a method for controlling the IGBT, Figure 7 shows a further diagram with measurement results for the collector-emitter voltage at different collector currents. Figure 1 shows a block diagram of a circuit section that includes an IGBT 10.The IGBT 10 is exemplary of all semiconductor switches for which the invention can be applied. The IGBT 10 can be part of a circuit that forms, for example, a half-bridge or a converter. For this purpose, the IGBT 10 is connected to other IGBTs or other semiconductor switches and is controlled by a higher-level controller 20 to achieve the purpose of the circuit. For this purpose, the IGBT 10 is switched, for example, using pulse width modulation with a frequency of 10 kHz. The hardware-related control of the IGBT 10, i.e., the application of voltage and current to its gate terminal 11, is performed by a driver 12. For this purpose, the driver 12 is connected on the one hand to the gate terminal 11 of the IGBT 10 and on the other hand to the controller 20.In addition to implementing control signals from the controller 20, the driver 12 is also responsible for controlling processes that require such rapid intervention that feedback to the controller 20 and waiting for control signals from there would take too long. These tasks include a reaction to a possible overvoltage that occurs across the IGBT 10 during a turn-off process of the IGBT 10. The reason for such an overvoltage lies in inductively stored energy, which counteracts the current change during the turn-off process 202310296 10 and, depending on the specific design of the circuit, for example, existing capacitances and the size of the stored energy, leads to higher or lower overvoltage. If the voltage across the IGBT 10 exceeds its blocking capacity, this can lead to its damage or destruction. If the turn-off speed, ieIf the temporal change in current is not too high compared to the time in which the inductively generated overvoltage dissipates automatically, the resulting overvoltage also remains low. Modern IGBTs, however, are being optimized for ever faster switching processes in order to reduce the switching losses that occur. A blanket artificial extension of the turn-off time counteracts this improvement and is therefore not normally an option. In known driver circuits, the turn-off speed is therefore limited, if necessary, to values ​​that prevent a damaging overvoltage by briefly switching the IGBT 10 on again as needed depending on the situation. In order to be able to switch on depending on the situation, the emerging overvoltage must be detected. In known driver circuits, this is done, for example, by measuring dU. CE / dt, i.e. the temporal change of the collector-emitter voltage or dI C / dt, i.e., the temporal change in the collector current. These measurements are only possible when the respective changes occur during the turn-off process and are then large enough to allow reliable measurements. This is only possible approximately in the middle of the turn-off reaction, i.e., the actual drop in current or the actual rise in the collector-emitter voltage. Towards the end of this reaction, the overvoltage already occurs, if one occurs. Figure 2 shows a highly schematic and simplified turn-off process for any IGBT. The voltage curve 21 of the collector-emitter voltage and the current curve 22 of the collector current are shown. The overvoltage occurs from time 23, as can be seen from the voltage curve 21. A 202310296 11 measurement of the current change dI C / dt is possible at time 24, which is only slightly before the onset of the overvoltage. A measurement of the voltage change dU CE / dt is possible at time 25. Time 25 is still slightly before time 24 and therefore allows slightly more time until the onset of the overvoltage at time 23. Depending on the switching speed of the IGBT in question, the time remaining for a reaction, i.e. the interval between time 23 and one of the times 24, 25, can be in the range of µs and be sufficient to limit the speed of the current change due to a switching action of the driver 12. For particularly fast IGBTs, however, the time remaining for a reaction can also be in the range of only 100 ns. This time is too short for a reaction even for the driver 12, since even the small parasitic inductances are too high to switch the IGBT in such a time frame. In a first exemplary embodiment of the invention, the driver 12 therefore performs a modified measurement.This measurement is based on the finding that the delay time, which extends from the start of a turn-off process until a certain collector-emitter voltage is reached, depends on the collector current level. This relationship is illustrated in Figure 3. Figure 3 shows a series of measurements of the switching processes of an IGBT at various collector current values. The measurement curves 31a…d show the course of the gate-emitter voltage U. GE during a turn-off process, which lasts approximately 2 µs. The measurement curve 31a is at a collector current of I C = 225 A and the measurement curve 31d at a collector current of I C = 850 A. The corresponding curves 32a…d of the collector-emitter voltage U CE are also plotted. It can be seen that with an exact match of the start of the turn-off process at approximately t = 100 ns, the rise in the collector-emitter voltage U CEat different collector currents I 202310296 12 C is shifted in time. If the time at which the collector-emitter voltage U CE a threshold value 33, for example 50 V, is reached, four different times 34a…d result, the position of which is directly dependent on the collector current I C From this temporal position, the collector current I C From the level of the collector current, it can be concluded whether a fault is present and whether it is caused by an excessively high collector current I C excessive overvoltage is to be expected. It is advantageous to select a relatively low threshold value 33, for example, only 20% or even 10% or even 5% of the nominal voltage. When measuring dU CE / dt, however, a useful value can only be achieved when the voltage U CEhas risen to approximately half the nominal voltage, i.e., later. Thus, by evaluating the delay time until the threshold value 33 is reached, a faster measurement results, and thus an increased reaction time for countermeasures. The delay time measurement is conveniently implemented using a circuit with an operational amplifier. A capacitor is charged and retains its charge until the voltage value has been transmitted via an analog-to-digital converter to a microcontroller used in the driver 12. Figure 4 shows the relationship between the resulting delay times t V and the collector current I C at turn-off for an exemplary 3.3 kV IGBT. As can be seen from Figure 3, the delay time t V with increasing collector current I CHowever, this graph, like the curves shown in Figure 3, is subject to individual variation and would not exactly match for two specific IGBTs. Furthermore, there is a clear dependence of the delay time t V from the temperature of the IGBT. 202310296 13 The driver 12 is now designed so that in addition to measuring the delay time t V also a measurement of the maximum voltage U occurring across the IGBT 10 during the turn-off process CEmax The two values ​​are shown in Figure 5 based on the curve of gate voltage U GE and collector-emitter voltage U CE for a shutdown process. Overvoltages are always present, ie the maximum voltage is always slightly higher than the ultimately applied steady-state voltage, which in real use could be, for example, an intermediate circuit voltage U ZKIn Figure 3, the intermediate circuit voltage is 700 V. As can also be seen from Figure 3, the overvoltage increases with the collector current I c The process performed by the driver is shown schematically in Figure 6. The process used is largely cyclical and is therefore largely carried out for each shutdown process i, j, k. At the beginning of an exemplary shutdown process k, the delay time t Vk measured. For a switch-off process i, the determined delay time t Vi and the determined maximum voltage U CEmax,i a pair of values ​​(t Vi , U CEmax,i ). At least two such pairs of values ​​(t Vi , U CEmax,i ) and (t Vj , U CEmax,j) for two shutdown processes are stored in memory locations 601, 602. These are regularly overwritten in the course of later steps of the method according to Figure 6 and are therefore assumed to be present here. The shutdown processes i, j can be consecutive shutdown processes, but the two shutdown processes i, j can also be separated from each other by other shutdown processes. In the shutdown process k currently being considered, in a second step 62 the delay time t Vk calculated to the now expected maximum voltage U max,k This is done by linear extrapolation. The following calculations are performed: 202310296 14 Δt V1 = t Vj - t Vi Δt V2 = t Vk - t Vi ΔU1= U CEmax,j - U CEmax,i U max,k = U CEmax,i + ΔU1‧ Δt V2 / Δt V1It is understood that some of these calculations can be carried out in advance after recording the values ​​for the previous shutdowns i, j. In particular, the quotient ΔU1 / Δt V1 already after recording the corresponding values, i.e. before the switch-off process k, so that during the switch-off process k only the multiplication with Δt V2 and the summation remains to be done. Since the threshold value for the end of the delay time t V If a value of 50 V is used for the collector-emitter voltage, for example, an extrapolated value for the expected overvoltage U is available at a relatively early point in time. max,kin the currently occurring shutdown process k. This extrapolated value is advantageously not determined by an absolute conversion from the available data, but is calculated only from measured values ​​determined shortly before. The value is therefore completely independent of any variation in the switch type used, since these influences are also part of the measured values ​​from which U max,k The value for the maximum voltage U max,kis also independent of the temperature of the IGBT 10 if the value pairs used for the extrapolation, and thus the turn-off processes i, j, are not too far back in time. However, since switching occurs at frequencies of 10 kHz or more, the preceding turn-off processes ideally lie only in the range of a few 100 µs. At higher switching frequencies, the intervals are even smaller. A significant temperature change of the switch is not to be expected during such periods. Since the values ​​for the turn-off processes i, j were determined at the same or almost the same temperature, the absolute temperature of the IGBT 10 also plays no role in determining the maximum voltage U m- ax,k . It is therefore possible to use the determined value for the expected maximum voltage U max,k directly in the driver 12, whether they meet the dielectric strength U Sof the switch used. In addition to a comparison with the dielectric strength U S of the IGBT 10 can also be compared with another threshold value for the maximum voltage U max,k For example, for safety reasons, a threshold value U SD which is a few percent below the dielectric strength, for example U SD = U S ‧ 0.96. In a third step 63, a comparison of the calculated maximum voltage U max,k with the threshold value. If the expected maximum voltage U max,k the threshold U SD not, then intervention in the shutdown process is unnecessary, since the shutdown process does not lead to a dangerous overvoltage. In this case, the method continues with the fourth step 64. In the fourth step 64, the actually achieved maximum collector-emitter voltage U CEmax,kof the IGBT 10 for the turn-off process k is measured. This measurement is based on charging a capacitor with the aid of an amplifier circuit, the voltage of which is transmitted to the microcontroller via an analog-to-digital converter. Since the measured value for the maximum voltage is not absolutely necessary before a subsequent turn-off process, i.e., before the expiration of another 100 µs, there is no particular urgency for this measurement. 202310296 16 In a fifth step 65, the value pair from the delay time t Vk and the maximum collector-emitter voltage U CE- max,kof the IGBT 10 is stored in one of the two memory locations 601, 602. In doing so, a previously existing value pair is typically overwritten, if one already existed. It goes without saying that no value pairs are available for the first shutdown processes and therefore the second step cannot be carried out either. In order to always have current value pairs in the memory locations 601, 602, the storage between these two memory locations 601, 602 can alternate or, during storage, the value pair from the second memory location 602 can first be moved to the first memory location 601 before the newly measured value pair is written to the second memory location. There are also alternative embodiments in which value pairs from two consecutive shutdown processes are not necessarily stored.For example, storing a counter for shutdown events can ensure that the stored value pairs originate from shutdown events that are separated from each other by one or more other shutdown events, for example, five. This can advantageously increase the accuracy of the extrapolation, since it can then be expected that Δt. V1 and ΔU1 are greater in magnitude, in other words, there are greater differences in the measured values ​​of the two shutdown processes i, j. After the fifth step, the method is continued with the next shutdown process i, j, k and then starts again with the first step 61 for the then current shutdown process. If the comparison in the third step 63 shows that the expected maximum voltage U max,k the threshold U SDexceeds , then in a sixth step 66, an intervention in the shutdown process is initiated. In one embodiment, such an intervention includes a re-switching of the IGBT 10 for a strictly limited period of time. This re-switching ensures that the inductively driven current is no longer blocked at the switch for a short time and that the built-up voltage can dissipate again. It is understood that this re-switching must be terminated after a short time. In an alternative embodiment, a re-switching with a limited intensity can also occur, in which the gate voltage U GEof the IGBT 10 is controlled so that the IGBT 10 only becomes conductive to a limited extent. This re-switching must also be carried out as quickly as possible in order to keep the resulting electrical losses as low as possible. Since the intervention in the switch-off process limits the actually occurring maximum collector-emitter voltage U CEmax,k is falsified and thus an extrapolation from these values ​​would be incorrect, no storage is made in the memory locations 601, 602 in the event of an intervention. Instead, the measured value for the delay time for this shutdown process is also discarded. The process is then resumed during the next shutdown process. Through the extrapolation, it is known how far the expected maximum voltage U max,kexceeds the threshold value. This allows the intensity of the intervention, ie its duration and / or the value for the gate voltage, to be adapted to the expected overvoltage. This way, the generation of additional electrical losses can be reduced to the necessary minimum to protect the IGBT 10. If control of the intervention from the extrapolation is not necessary, for example, if the duration of the intervention is determined differently, then to simplify the process it may also be sufficient to use the threshold value U SD for the maximum voltage and the stored value pairs a threshold value t SD for the delay time t V to be calculated. If the 202310296 18 threshold t SD for the delay time in the switch-off process k is exceeded, i.e. t Vk < t SD , then it is expected that the maximum voltage generated will exceed the threshold value U SDfor the voltage. The formulas for extrapolation are: U SD = U CEmax,i + ΔU1‧ (t SD - t Vi ) / Δt V1 t SD = t Vi + (U SD - U CEmax,i ) ‧ Δt V1 / ΔU1t SD can thus be determined from the two stored value pairs. The extrapolation in the shutdown process k in the second step 62 is omitted and is replaced by the simpler comparison of the delay times t Vk , t SD in the third step. With the delay time t V a quantity is determined which is influenced by the collector current I C , but does not correspond to this, because it is also determined by other significant influencing factors. A delay time t V An alternative such quantity, which has been found in measurements on IGBTs, is the gate current I G. In a second embodiment of the invention, the driver 12 therefore performs an alternative measurement. This measurement is based on the finding that the level of the gate current I G , which results after the expiration of a measuring time 72 after the start of a switch-off process, from the level of the collector current I C This relationship is illustrated in Figure 7. Figure 7 shows a series of measurements of switching operations of an IGBT 10 at different values ​​for the collector current I C . The measurement curves 71a…d show the course of the gate current I G during a turn-off process, which lasts approximately 2 µs. The measurement curve 71a is at a collector current of I C = 225 A and the measurement curve 71d at a collector current of I C = 850 A. The corresponding curves 32a…d 202310296 19 of the collector-emitter voltage U CEare also plotted. To actually determine the gate current, an integration of the current measured at a shunt resistor is performed within a measurement period, also called the measurement window, thus determining a charge quantity. This measurement window is adapted to the IGBT type used. For example, the measurement window can cover a period of 200 ns to 300 ns after the start of the turn-off process. The specific time 73 at which the measurement is performed results from the measurement time elapsed after the start of the turn-off process at time 74. Figure 7 shows that at the beginning of the turn-off process, the gate current curves 71a…d are not yet distinguishable, but after the measurement time has elapsed at time 73, a clear dependence on the collector current I C The gate current I G is subsequently calculated in steps 62, 65 of the method analogously to the delay time t Vused. The advantages of this approach also apply to the gate current I G as for the delay time t VAs already explained, the described procedure has several advantages. The estimation of the expected maximum collector-emitter voltage and thus also of the overvoltage allows a very early decision to be made during normal operation, i.e., away from short circuits or similar, as to whether intervention in the shutdown process is necessary, and also to what extent the intervention in the shutdown process is necessary. Furthermore, this estimation is independent of batch variation, component variation, and the temperature of the switch, since the estimation only uses values ​​measured shortly beforehand. It therefore works without calibration and only requires the setting of a few threshold values. The threshold for the end of the delay time measurement can be set very roughly, for example, to 50 V. The threshold for the permitted maximum voltage is directly related to the dielectric strength of the switch and otherwise requires no calibration.This makes it very easy to optimize the shutdown processes during normal operation without the need for calibration. While the value pairs stored in memory locations 602, 602 should ideally be very up-to-date, i.e., regularly overwritten, it is advantageous to store a selection of these value pairs for the longer term. This can be done in driver 12 or elsewhere. The value pairs can be combined with other available measured values. In one embodiment, one such value pair is stored in a database every minute, together with values ​​for the temperature of IGBT 10 and the current power of a power converter in which IGBT 10 is used. Taking into account all values ​​stored at a time, a comparison of the relationship between delay time t. V and maximum collector-emitter voltage U CEmaxcarried out. A change in this ratio with other operating data remaining the same indicates aging of the IGBT 10. Thus, permanent monitoring can be carried out using the data stored in this way. This allows service times (e.g., cleaning of the air coolers) and failure times (e.g., due to aging of the IGBT: bond wire lifting) to be determined more accurately. 202310296 21 List of Reference Symbols 10 IGBT 11 Gate connection 12 Driver circuit 20 Higher-level controller 21 Collector-emitter voltage profile 22 Collector current profile 23 Time of overvoltage occurrence 24 Time of dI C / dt measurement 25 Time of dU CE / dt measurement 31a…d Gate-emitter voltage curve 32a…d Collector-emitter voltage curve 33 Threshold value 34a…d Times 71a…d Gate current curve 72 Measurement time 73 Time 74 Start of turn-off process U ZK DC link voltage UGE Gate-emitter voltage t V Delay time U CE Collector-emitter voltage, voltage across the semiconductor switch U CEmax maximum voltage across the semiconductor switch i, j, k shutdown process 61…65 first to fifth step 601, 602 storage location

Claims

202310296 22 claims 1. Driver circuit (12) for controlling a semiconductor switch (10), comprising a contact for connection to a control terminal (11) of the semiconductor switch (10) and a device which is designed, in the course of a switch-off process (i, j, k) of the semiconductor switch (10), to measure a first measured value (t V ), wherein the value is either - the level of the gate current (71a…d) at a time (72) or in a time window after the start (74) of the switch-off process (i, j, k) or - a time period from the start (74) of the switch-off process (i, j, k) until a definable threshold value (33) of the voltage drop across the semiconductor switch (10) (U CE), characterized in that the device is designed - a second measured value for a maximum voltage drop (U CEmax ), - for a subsequent switch-off process (k) with the first and second measured values ​​(t V , U CEmax ) to determine an evaluation variable, - based on the evaluation variable, to carry out a control measure for reducing the temporal change in the current flowing through the semiconductor switch (10).

2. Driver circuit (12) according to claim 1, wherein the device is designed to determine, for two or more, in particular consecutive, switch-off processes (i, j), value pairs from a respective first and second measured value (t V , U CEmax ) and, during a subsequent switch-off process (k), to calculate from two of the stored value pairs and a first measured value (t V) for the following shutdown process (k) a value for an expected maximum during the shutdown process over the 202310296 23 semiconductor switch (10) to determine the voltage drop as an evaluation variable.

3. Driver circuit (12) according to one of the preceding claims, in which the device is designed to carry out a temporary re-switching of the semiconductor switch (10) as a control measure.

4. Driver circuit (12) according to one of the preceding claims, in which the device is designed to carry out a change in the gate voltage (U GE).

5. Driver circuit (12) according to one of the preceding claims, in which the device is designed to carry out a switch to a higher gate resistance as a control measure.

6. Driver circuit (12) according to one of the preceding claims, in which the device is designed to send a signal to a higher-level controller (20) as a function of the evaluation variable.

7. Driver circuit (12) according to one of the preceding claims, in which the device is designed to use as a threshold value (33) of the collector-emitter voltage (U CE ) at most 10% of the nominal voltage of the semiconductor switch (10).

8. Driver circuit (12) according to one of the preceding claims, in which the device is designed to determine the start (74) of a turn-off process (i, j, k) based on a threshold value for the gate voltage (U GE9. Driver circuit (12) according to one of the preceding claims, in which the device is designed to determine the start (74) of a switch-off process (i, j, k) based on a control command from the higher-level controller (20). 202310296 24 10. Driver circuit (12) according to one of the preceding claims, in which the device is designed to determine the start of a switch-off process (i, j, k) based on the value of a control voltage, wherein the control voltage is a voltage value between an output stage of the driver circuit (12) and a gate resistor.

11. Driver circuit (12) according to one of the preceding claims, in which the device is designed to determine a period in which the control measure can be carried out by means of a voltage measurement at a Zener diode, which is connected to the collector-emitter voltage (U CE) is connected.

12. Driver circuit (12) according to one of the preceding claims, in which the device is designed to generate pairs of values ​​from the first and second measured values ​​(t V , U CEMax) or a value derived therefrom for shutdown processes separated by a time period of at least 1 s, in particular at least 1 day.

13. Semiconductor switch (10), in particular IGBT (10), MOSFET or SiC-based MOSFET, with a driver circuit (12) according to one of the preceding claims.

14. Method for controlling a semiconductor switch (10), in which, in the course of a switch-off process (i, j, k) of the semiconductor switch (10), a determination of a first measured value for a value dependent on the level of the current flowing through the switch is carried out, wherein the first measured value either represents the level of the gate current (71a…d) at a time (72) after the start (74) of the switch-off process (i, j, k) or a time period from the start (74) of the switch-off process (i, j, k) until a definable threshold value (33) of the voltage drop across the switch (U CE), characterized in that a second measured value (U CEmax ) for a maximum voltage across the semiconductor during the turn-off process (i, j, k) 202310296 25 switch (10) dropping voltage (U CEmax ) is recorded, for a subsequent switch-off process (k) with the first and second measured values ​​(t V , U CEmax ) an evaluation value is determined and, based on the evaluation value, a control measure is carried out to reduce the temporal change in the current flowing through the semiconductor switch (10).

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