Method for determining a transfer characteristic of a semiconductor switch

Abstract

The present invention relates to a method for determining a transfer characteristic of a semiconductor switch, comprising: a first step (100) for switching on a semiconductor switch in a switched-off state, a second step (200) for measuring a collector current of the semiconductor switch in order to record a profile of the collector current during the switching-on process, a third step (300) for measuring a gate voltage at a gate terminal of the semiconductor switch in order to record a profile of the gate voltage during the switching-on process, a fourth step (400) for determining a profile of an internal gate voltage by subtracting an influence of an internal gate resistance of the semiconductor switch and / or an influence of an inductive coupling caused by wiring connected to the semiconductor switch,from the profile of the measured gate voltage and a fifth step (500) for determining a compensated transfer characteristic of the semiconductor switch based on a relationship between the profile of the internal gate voltage and the profile of the collector current.,

Description

Prior ArtThe present inventionPower semiconductor switches such as IGBTs and MOSFETs are known from the prior art, the transmission characteristic of which is determined in a high current range, in particular in a short-circuit current range on the basis of a plurality of individual measurements with different gate-emitter voltages and with different voltages across a switching path of the power semiconductor switches.The plurality of individual measurements is usually carried out in a steady state of the power semiconductor switches with a substantially constant current.Disclosure of the InventionThe method according to the invention for determining a transmission characteristic of a semiconductor switch offers, among other things, the advantage that a particularly rapid determination of the transmission characteristic of the semiconductor switch is made possible, since a sufficiently precise transmission characteristic of the semiconductor switch can already be determined on the basis of an evaluation of an individual switch-on process of the semiconductor switch.In a first step of the method according to the invention, the semiconductor switch which is in a switched-off state is switched on.The semiconductor switch is, for example, an IGBT, a MOSFET or a bipolar transistor. The semiconductor switch is particularly advantageously a power semiconductor switch, without thereby being restricted to the aforementioned semiconductor types. It should be noted that a semiconductor switch designed as an IGBT is described as a representative in the following description. IGBT-specific designations of the terminals of the semiconductor switch are therefore not to be considered as a restriction, since it is obvious to the person skilled in the art with which terminals differently designed semiconductor switches correspond.It should be further noted that the semiconductor switch can be a semiconductor switch designed as a chip and / or module and / or can have a corresponding packaging in the form of a housing.The semiconductor switch is switched on on the basis of a control signal which, in the case of an IGBT used, can be, for example, a voltage pulse which is applied to a gate of the IGBT and which preferably corresponds to at least one duration which is required for completing the switching-on process of the semiconductor switch.It should be further noted that during the execution of the method according to the invention, a DC voltage provided by a voltage source is applied to a switching path of the semiconductor switch (i.e. a collector-emitter path in the case of an IGBT). A configuration of the voltage source is fundamentally not restricted, so that it can be designed, for example, in the form of a capacitor, which behaves substantially like a voltage source during the relatively short switch-on process of the semiconductor switch.The initiation of the switch-on process and / or at least part of the steps described below for measuring and recording measured values can be carried out, for example, on the basis of an evaluation unit which can be connected at least partially to respective sensors for measuring the measured values according to the invention in terms of information technology and / or which can be connected to a storage unit for storing measured values determined according to the invention.In a second step of the method according to the invention, a collector current of the semiconductor switch is measured in order to record a profile of the collector current during the switch-on process, wherein a time period from a start of a switch-on time of the semiconductor switch until a substantially static switch-on state is reached in which substantially all switch-on transients are concluded is preferably recorded.Such a recording of the profile of the collector current and further profiles of further measured variables described below is carried out, for example, on the basis of an A / D converter with a correspondingly high sampling rate in order to obtain a sufficient number of sampling values for ascertaining the transmission characteristic explained below. Accordingly, the higher the number of samples in the aforementioned period, the higher the accuracy in obtaining the transfer characteristic. The generated sampling values are stored, for example, in the above-mentioned memory unit. It should be noted that methods other than the above description may be used to record the course of the collector current.In a third step of the method according to the invention, a gate voltage (i.e. a gate-emitter voltage using the example of a semiconductor switch embodied as an IGBT) is measured at a gate terminal of the semiconductor switch in order to record a profile of the gate voltage during the turn-on operation. The measured values or sampled values determined during the recording can likewise be stored, for example, in the above-mentioned memory unit.In a fourth step of the method according to the invention, a profile of an internal gate voltage (i.e. an internal gate-emitter voltage using the example of a semiconductor switch embodied as an IGBT) is determined by subtracting an influence of an internal gate resistance of the semiconductor switch and / or an influence of inductive coupling, which is caused by a wiring connected to the semiconductor switch, from the profile of the measured gate voltage. In other words, curves of voltage drops caused by the internal gate resistor and / or the inductive coupling are subtracted from the curve of the measured gate voltage in order to obtain a compensated voltage curve representing the internal gate voltage curve during the switch-on process. It is particularly preferred to compensate both the influence of the gate resistor and the influence of the inductive coupling in order to be able to determine a particularly precise transmission characteristic of the semiconductor switch.In a fifth step of the method according to the invention, a transmission characteristic which accordingly corresponds to a compensated transmission characteristic of the semiconductor switch is ascertained on the basis of a relationship between the profile of the internal gate voltage and the profile of the collector current.The dependent claims show preferred developments of the invention.Further preferably, the transmission characteristic of the semiconductor switch is determined in particular for a collector current which essentially represents a short-circuit case. In other words, the gate of the semiconductor switch is preferably driven with a voltage which generates a substantially minimum volume resistance of the collector-emitter path of the IGBT. In this way, a transmission characteristic of the semiconductor switch can be determined accordingly for the particularly relevant short-circuit case, which can occur, for example, if the semiconductor switch is arranged in a half bridge in conjunction with a further semiconductor switch and the further semiconductor has a short circuit.In one embodiment of the present invention, the influence of the internal gate resistance is determined on the basis of a measurement of the internal gate resistance carried out in advance using a predefined alternating signal.In a particularly advantageous embodiment of the present invention, the influence of the internal gate resistor is determined by measuring a gate current during the switch-on process (e.g. by means of a measuring resistor which is connected between the gate and a control unit which controls the gate) in order to record a profile of the gate current during the switch-on process. On the basis of the profile of the gate current, a gate voltage value is then determined at a point in time at which the gate current has a maximum value. This evaluation is carried out, for example, on the basis of the aforementioned evaluation unit, which can read out and evaluate measured values stored, for example, in the memory unit over the course of the gate current. Then, a value of the internal gate resistance is calculated based on a quotient of the gate voltage value and the maximum gate current.Further preferably, an increase in the gate voltage is determined at the point in time at which the gate current has the maximum value, since an influence of undesired, in particular parasitic, inductances at this point in time substantially corresponds to a value of zero. Subsequently, a value for a gate capacitance of the semiconductor switch is determined based on the slope of the increase in gate voltage (du / dt) and based on the maximum gate current. Then, a value of a voltage drop across the gate capacitance is determined based on a charge amount introduced into the gate capacitance from the start of the turn-on operation to the time of the maximum gate current, and based on the gate capacitance. Finally, the value of the internal gate resistor is determined by subtracting from the gate voltage value the value of the voltage drop across the gate capacitance and dividing a result thereof by the maximum gate current.The inductive coupling described above is preferably calculated or simulated on the basis of information about a dimensioning and about an arrangement of the wiring, such that the influence of the inductance on the profile of the measured gate voltage can be compensated as described above, in order to be able to determine the internal gate voltage with a particularly high accuracy. Specifically, a voltage drop Ugengenerated by the inductive coupling can be determined based on the equation: Ugen=Uge-M*dc / dt, where Ugeis the measured gate voltage, M is the determined inductance, and Icis the collector current.In addition, it is advantageously possible to additionally take into account an influence of a Miller capacitance of the semiconductor switch when ascertaining the internal gate voltage of the semiconductor switch, in order to further increase an accuracy in ascertaining the transmission characteristic.In a particularly preferred embodiment, the compensated transmission characteristic is determined and / or checked for plausibility on the basis of at least one further embodiment of the method according to the invention. It is particularly preferred for the method according to the invention to be repeated multiple times and with respectively different collector-emitter voltages and / or with respectively different gate voltages and / or with respectively different external gate resistance values. For example, by comparing a plurality of determined transmission characteristics for respectively different collector-emitter voltages, which are determined, for example, respectively during turn-on processes until the saturation operation of the semiconductor switch is reached, it is possible to determine whether these transmission characteristics can be represented substantially by a common transmission characteristic. If this is not the case, this allows conclusions to be drawn, for example, about an inhomogeneous current distribution and / or a not negligible influence of heating of the semiconductor switch. Furthermore, it is possible on the basis of a multiple execution of the method according to the invention to verify whether a linear dependence between the collector-emitter voltage and the collector current is to be expected according to the semiconductor theory.Furthermore, it is advantageously possible to generate a characteristic curve field on the basis of a multiplicity of determined compensated transmission characteristics of the semiconductor switch, the characteristic curves of which field correspond in each case to different collector-emitter voltages.The ascertained transmission characteristic according to the invention for the internal gate voltage can be used advantageously, inter alia, for creating a semiconductor model in a simulation program (e.g. PSPICE, etc.).Brief Description of the DrawingsHereinafter, embodiments of the invention will be described in detail with reference to the accompanying drawings. In the drawing, the following is: FIG. 1 shows a flow chart representing an exemplary embodiment of a method according to the invention; FIG. 2 shows exemplary curves of gate voltages of a semiconductor switch measured and compensated according to the invention; FIG. 3 shows exemplary current and voltage profiles of a semiconductor switch measured according to the invention; and FIG. 4 shows a compensated transmission characteristic of a semiconductor switch determined according to the invention.Embodiments of the InventionFIG. 1 shows a flow chart representing an exemplary embodiment of a method according to the invention for determining a transmission characteristic of a semiconductor switch, wherein the semiconductor switch is a power semiconductor switch designed as an IGBT.In step 100 of the method according to the invention, the semiconductor switch is switched on on on the basis of an evaluation unit, which is designed, for example, as a microcontroller, in conjunction with a driver unit, wherein the semiconductor switch is in a switched-off state until the evaluation unit and the driver unit are actuated.It should be noted that, at least during the duration of the switch-on process of the semiconductor switch, a predefined voltage is present at a collector-emitter path of the semiconductor switch, said predefined voltage being provided by a voltage source.The actuation is effected here in such a way that the semiconductor switch is in saturation in the switched-on state.In step 200 of the method according to the invention, a collector current of the semiconductor switch is measured in order to record a profile of the collector current during the switch-on process in a memory unit connected to the evaluation unit.In step 300 of the method according to the invention, a gate voltage at a gate terminal of the semiconductor switch is measured in order to record a profile of the gate voltage in the memory unit during the switch-on operation.In step 400 of the method according to the invention, a profile of an internal gate voltage is determined by subtracting an influence of an internal gate resistance of the semiconductor switch and an influence of inductive coupling, which is caused by a wiring connected to the semiconductor switch, from the profile of the measured gate voltage. Here, an influence of a gate capacitance of the semiconductor switch is also calculated and compensated.In step 500 of the method according to the invention, a compensated transfer characteristic of the semiconductor switch is determined on the basis of a relationship between the profile of the internal gate voltage and the profile of the collector current.On the basis of a plurality of compensated transfer characteristics determined on the basis of different collector-emitter voltages, it is checked on the basis of the following equation whether a dependence of the collector-emitter voltage on the collector current is linear: Ic(Uce)=Ic(Uce=0V)*(1+const*Uce), wherein Ic is the collector current and Uce is the collector-emitter voltage. A value represented by "constant" can be determined accordingly on the basis of the plurality of determined transmission characteristics, so that corresponding transmission characteristics can subsequently be derived for arbitrary fixed collector-emitter voltages.In addition, on the basis of the plurality of compensated transmission characteristics of the semiconductor switch determined (by repeatedly carrying out the method according to the invention), a characteristic curve field is generated, the characteristic curves of which correspond in each case to different collector-emitter voltages.FIG. 2 shows exemplary curves of gate voltages Uge measured according to the invention and compensated gate voltages Ugecomp1, Ugecomp2of a semiconductor switch.The curve Ugecomp1 is obtained by subtracting an influence of the internal gate resistance and the gate capacitance determined according to the invention on the measured gate voltage Uge from the curve of the curve of the measured gate voltage Uge.The curve Ugecomp2is obtained by subtracting an influence of the inductive coupling determined according to the invention on the measured gate voltage Ugefrom the compensated curve Ugecomp1. The curve Ugecomp2thus corresponds to the profile of the internal gate voltage Ugeint.FIG. 3 shows exemplary current and voltage characteristics of a semiconductor switch measured according to the invention.FIG. 3 shows a profile of a gate voltage Uge measured according to the invention, a profile of a gate current Ige measured according to the invention, wherein the gate current Ige has a maximum value Igemax, which corresponds to a measured gate voltage Uge(Igemax) at the maximum gate current Igemax, on the basis of which an influence of a gate capacitance of the semiconductor switch on the measured gate voltage Uge can be determined. This influence leads to a voltage drop Ucge across the gate capacitance. Further, the voltage drop Urgint across the internal gate resistor of the semiconductor switch is shown.FIG. 4 shows a compensated transfer characteristic Ic(Ugeint) of a semiconductor switch determined according to the invention, i.e. a relationship between an internal gate-emitter voltage Ugeint of the semiconductor switch and a collector current Ic of the semiconductor switch.

Claims

Method for determining a transmission characteristic of a semiconductor switch, comprising: - a first step (100) for switching on a semiconductor switch which is in a switched-off state, - a second step (200) for measuring a collector current (Ic) of the semiconductor switch in order to record a profile of the collector current (Ic) during the switching-on operation, - a third step (300) for measuring a gate voltage (Uge) at a gate terminal of the semiconductor switch in order to record a profile of the gate voltage (Uge) during the switching-on operation, - a fourth step (400) for determining a profile of an internal gate voltage (Ugeint) by subtracting an influence of an internal gate resistance of the semiconductor switch and / or an influence of an inductive coupling (M), which is effected by a wiring connected to the semiconductor switch, from the course of the measured gate voltage (Uge), and - a fifth step (500) for determining a compensated transfer characteristic (Ic(Ugeint)) of the semiconductor switch on the basis of a relationship between the course of the internal gate voltage (Ugeint) and the course of the collector current (Ic).Method according to claim 1, wherein the semiconductor switch is - an IGBT, a MOSFET or a bipolar transistor, and / or - a power semiconductor switch.Method according to one of the preceding claims, wherein the transmission characteristic of the semiconductor switch is determined for a collector current (Ic) which substantially represents a short-circuit case.Method according to any of the preceding claims, wherein the influence of the internal gate resistance is determined based on a pre-performed measurement of the internal gate resistance using a predefined alternating signal.Method according to one of the preceding claims, wherein the influence of the internal gate resistance is determined by - measuring a gate current (Ige) during the switch-on process in order to record a profile of the gate current (Ige) during the switch-on process, - determining a gate voltage value (Uge(Igemax)) at a point in time at which the gate current (Ige) has a maximum value (Igemax), and - calculating a value of the internal gate resistance on the basis of a quotient from the gate voltage value (Uge(Igemax) and the maximum gate current (Igemax).Method according to Claim 5, wherein - an increase in the gate voltage (Uge) is determined at the point in time at which the gate current (Ige) has the maximum value (Igemax), - a value for a gate capacitance of the semiconductor switch is determined on the basis of the increase in the gate voltage (Uge) and the maximum gate current (Igemax), - a value of a voltage drop (Ucge) across the gate capacitance is determined on the basis of a charge quantity which is introduced into the gate capacitance and the gate capacitance from the start of the switch-on operation to the point in time of the maximum gate current (Igemax), and - the value of the internal gate resistance is determined, by subtracting from the gate voltage value (Uge) the value of the voltage drop (Ucge) across the gate capacitance and dividing a result thereof by the maximum gate current (Igemax).Method according to one of the preceding claims, wherein the inductive coupling (M) is calculated on the basis of information about a dimensioning and an arrangement of the wiring.Method according to one of the preceding claims, wherein an influence of a Miller capacitance is additionally taken into account in determining the internal gate voltage (Ugeint) of the semiconductor switch.Method according to one of the preceding claims, wherein - the compensated transmission characteristic Ic(Ugeint) is determined and / or checked for plausibility on the basis of at least one further embodiment of the method according to the invention, and / or - the method according to the invention is executed with different collector-emitter voltages (Uce) and / or with different gate voltages (Uge) and / or with different external gate resistance values.Method according to one of the preceding claims, wherein a characteristic field is generated on the basis of a multiplicity of determined compensated transmission characteristics Ic(Ugeint) of the semiconductor switch, the characteristic curves of which correspond in each case to different collector-emitter voltages (Uce).

Images