Method and device for determining a switching current in an electronic current and / or voltage converter; electronic current and / or voltage converter

Abstract

Method for determining a switch current in an electronic current and / or voltage converter, in particular in an inverter, comprising: - energizing (101) a measuring section (11) with a switch current flowing through a semiconductor switching element of a half-bridge installed in a current and / or voltage converter, wherein the measuring section (11) has a parasitic inductance (14) and a parasitic resistance (16); - measuring (102) a voltage drop across the measuring section (11); - integrating (103) the voltage drop by means of an active low-pass filter (21) in order to obtain a profile of an integrated voltage as a function of an energization time; - determining (104) three data points from the time course of the integrated voltage and reading out three associated value pairs for the respective data points; - inserting (105) the value pairs into a system of equations which describes the integrated voltage as a function of the energization time, wherein the function contains one or more coefficients which are dependent on the magnitude of the switch current, preferably on a pulse height of the switch current, further preferably additionally on the parasitic inductance and / or a gain factor of the active low-pass filter; - Calculating (106) the coefficient(s) to determine the magnitude or pulse height of the switch current.

Description

The invention relates to a method and a device for determining a switch current in an electronic current and / or voltage converter, in particular in an inverter. The electronic current and / or voltage converter, in particular the inverter, serves for energizing an electric drive in an electric vehicle or hybrid vehicle. The invention further relates to an electronic current and / or voltage converter, in particular an inverter, having such a device or in connection with the same.In the prior art, pure electric vehicles and hybrid vehicles are known, which are driven exclusively or supportingly by one or more electric machines as drive units. In order to supply the electric machines of such electric vehicles or hybrid vehicles with electrical energy, the electric vehicles and hybrid vehicles comprise electrical energy stores, in particular rechargeable DC voltage sources, which are designed, for example, as electrical batteries. However, the electric machines generally require an alternating voltage. Therefore, a so-called inverter is usually connected between a battery and an electric machine of an electric vehicle or of a hybrid vehicle in order to enable a conversion of a battery-side direct current into a polyphase alternating current.Such inverters typically include semiconductor switching elements, typically formed of transistors. It is known to configure the semiconductor switching elements as half bridges. Each half bridge comprises a high-side switching position (hereinafter "high-side") with a higher electrical potential and a low-side switching position (hereinafter "low-side") with a lower electrical potential. The highside and the lowside may each include one or more single switch / semiconductor switching elements connected in parallel.Depending on the design of the semiconductor switching elements, the semiconductor switching elements can then be switched to conduct current or to block current by applying current or voltage to them. In this way, the semiconductor switching elements are preferably switched in accordance with a pulse width modulation (PWM) in order to enable a sinusoidal time profile of the phase currents. In this way, an input-side direct current can be converted into a polyphase output current (alternating current) with a plurality of phase currents, with which an electric drive (electric machine) of an electric vehicle or hybrid vehicle is energized.During operation of the inverter, it is essential to determine the switch currents flowing through the semiconductor switching elements as accurately as possible and, in the case of abnormal behavior, to adjust these switch currents using suitable control methods so that the switch currents move in a desired range. In particular, jumps in the switch currents should lead to an impairment of the semiconductor switching elements and, associated therewith, also of the entire inverter.For example, DE 10 2016 222 214 A1 discloses a measurement of a current through a bridge module for electromotive adjustment systems of vehicle components such as window lifters, seat adjusters, door and sliding roof drives or radiator vents, and pumps or interior fans, which corresponds to the motor current. US 2004 / 0 056 661 A1 also discloses a current measurement for a bridge module which is provided for operation of an electric motor.In the inverters known from the prior art, however, a sufficiently accurate current determination is not possible. In particular, the switch current is described only with insufficient accuracy in the mathematical models used in the known inverters for describing the current profile, so that tracking or monitoring of the switch current in real time is difficult.It is an object of the invention to provide a method and a device for determining a switch current in an electronic current and / or voltage converter, in particular in an inverter, in which the aforementioned disadvantages are at least partially overcome.This object is achieved according to the invention by the method, the device and the electronic current and / or voltage converter according to the independent patent claims. Advantageous embodiments and further developments of the invention are evident from the dependent claims.The invention relates to an electronic current and / or voltage converter, in particular an inverter, for energizing an electric drive in an electric vehicle or a hybrid vehicle. The electronic current and / or voltage converter comprises a plurality of half bridges, which each correspond to one of the current phases of the polyphase alternating current on the output side. For example, the inverter comprises three half bridges, wherein each half bridge is associated with an associated current phase of the three-phase output current. In the case of an inverter or an inverter, the output current is an alternating current which is generated on the basis of an input-side direct current by means of targeted switching processes of the semiconductor switching elements. The present invention is described below using the example of an inverter, wherein the electronic converter can alternatively be a rectifier for converting an input voltage into an output voltage different from the input voltage.The half bridges each comprise a high side and a low side. The highside and the lowside each comprise one or more parallel-connected semiconductor switching elements. The high side allows current flow between an AC power terminal and a DC plus power terminal. The lowside allows current flow between the AC power terminal and a DC minus power terminal. The highside and the lowside are connected in series with each other. Depending on the configuration of the semiconductor switching elements, the semiconductor switching element can then be switched to conducting current or to blocking current by applying voltage to the signal contact. In this way, the semiconductor switching elements are preferably switched in accordance with a pulse width modulation (PWM) in order to enable a sinusoidal time profile of the phase currents.The method according to the invention is designed to determine a switch current in the inverter. The switch current is an electric current which flows through a semiconductor switching element installed in the electronic current and / or voltage converter, in particular in an inverter. The method is carried out by means of an associated device which is installed in the electronic current and / or voltage converter, in particular in the inverter, or is connected thereto. The method comprises energizing a measurement path with the switch current. The measurement path is preferably part of the circuit of the electronic current and / or voltage converter or of the inverter. The measurement path has a parasitic inductance and a parasitic resistance. The method comprises measuring a voltage drop across the measurement path. Two voltage tap points, between which the parasitic inductance and the parasitic resistance are connected in series, are selected for this purpose in order to measure the voltage drop between the two voltage tap points. The method also includes integrating the voltage drop by means of an active low pass filter. The active low-pass filter is contained in the device according to the invention for determining the switch current. By integrating the measured voltage drop over the measurement path, a profile of an integrated voltage can subsequently be determined as a function of an energization time. The time profile of the integrated voltage is a voltage-time diagram in which the integrated voltage is plotted as a function of the energization time. The method according to the invention further comprises defining three data points from the time profile of the integrated voltage and reading out three associated value pairs for the respective data points. The three data points can be selected arbitrarily on the voltage-time diagram. Each pair of values includes a first value for the integrated voltage and a second value for the energization time of the respective data points. Additional data points (i.e., more than three data points) may be used to determine the switch current. The method additionally includes determining the switch current by inserting the value pairs into a system of equations that describes the integrated voltage as a function of the energization time. Preferably, a pulse height of the switch current to be determined is thereby determined. The function of the equation system is a mathematical function of the energization time with one or more coefficients, which are dependent on the amount of the switch current, preferably on the pulse height of the switch current. By inserting the value pairs into the equation system, the coefficients of the mathematical function can therefore be determined. From this, the amount, preferably the pulse height, of the switch current can be determined.In this way, the switch current can be determined more accurately than in the methods known from the prior art. In particular, the result of the switch current is completely independent of the thermal behavior of the measurement path, in particular of the parasitic resistance and / or of the parasitic inductance.The mathematical function of the equation system preferably comprises a summation of an exponential function and a linear function of the energization time, wherein the exponential function is factorized with a first coefficient, wherein the linear function is factorized with a second coefficient. The exponential function is preferably a natural exponential function with an exponent that is a product of the energization time and a component-specific constant of the active low-pass filter. The component-specific constant of the active low-pass filter is furthermore preferably an additive reversal of a reciprocal of a product of a resistance value and a capacitance value of the active low-pass filter. The first coefficient and / or the second coefficient are preferably dependent on the parasitic resistance, a pulse height of the switch current and a slope of the switch current occurring on account of the parasitic inductance.The summation can furthermore contain a constant as a further summand which is dependent on the parasitic resistance, a pulse height of the switch current and a slope of the switch current occurring on account of the parasitic inductance.In addition, the first coefficient, the second coefficient and / or the constant may additionally be dependent on the parasitic inductance and / or a gain factor of the active low-pass filter.The device according to the invention for determining a switch current, which flows through a semiconductor switching element installed in the inverter, is designed to carry out the method according to the invention. The device comprises an active low-pass filter and a scanning unit. The device can be designed as an analog or digital circuit. Furthermore, the device can be designed as a component of a microcontroller or can interact with a microcontroller. The device can additionally be embodied as a programmable logic circuit and / or field-programmable gate array.The invention further relates to an inverter for an electric drive of an electric vehicle or of a hybrid vehicle, wherein the device according to the invention is installed in the inverter or the inverter is connected to the device according to the invention. This results in the advantages already described in connection with the method according to the invention also for the inverter according to the invention.The invention is explained below by way of example with reference to embodiments shown in the figures.The following are shown: FIG. 1 shows a schematic circuit diagram of a device for determining a switch current; FIG. 2 shows a schematic block circuit diagram of a method for determining the switch current by means of the device from FIG. 1.Identical items, functional units and comparable components are denoted by the same reference numerals across the figures. These objects, functional units and comparable components are identical in terms of their technical features, unless the description explicitly or implicitly reveals otherwise.FIG. 1 shows a schematic circuit diagram of a device 10 for determining a switch current. The circuit diagram shows a semiconductor switching element 12 which is installed in an inverter, not shown in detail here, for energizing an electric drive in an electric vehicle or a hybrid vehicle. The semiconductor switching element 12 may be a MOSFET or an IGBT. The semiconductor material underlying the semiconductor switching element 12 may preferably be a so-called wide bandgap semiconductor (Wide Bandgap Semiconductor), such as silicon carbide (SiC) or gallium nitride (GaN). In operation of the inverter, a current flows from a positive electrode to a negative electrode of the semiconductor switching element 12 forming the switch current in certain switching states of the semiconductor switching element 12. For safe operation of the inverter, it is essential to determine the switch current as accurately as possible. For this purpose, according to a method 100 according to the invention (see FIG. 2 ), a measurement path 11 electrically connected to the semiconductor switching element 12 is selected, which measurement path comprises a parasitic inductance 14 and a parasitic resistance 16 connected in series with the latter. In a method step 101, the switch current is applied to the measuring section 11. In a further method step 102, two voltage tap points S and S' are selected, between which the measurement path 11 lies, in order to measure a voltage drop V S'S over the measurement path.In a further method step 103, the measured voltage drop is integrated by means of an active low-pass filter 21 over an energization time t in order to obtain a profile of an integrated voltage V int as a function of the energization time t. The time profile of the integrated voltage V int is preferably present in the form of a voltage-time diagram. The active low-pass filter 21 forms a processing unit of the apparatus 10 and is shown by way of example in FIG. 1 and comprises an RC element composed of a low-pass resistor 18 and a low-pass capacitor 20. A current flowing through the low-pass resistor 18 is supplied to the positive input, the negative input being connected to a point between two resistors 24, 26 of a voltage divider connected at the output of the active low-pass filter 21.Then, in a further method step 104, three data points are defined from the time profile or the voltage-time diagram of the integrated voltage V int and an associated value pair is read out for the respective data points. The value pair contains a first value for the integrated voltage V int and a second value for the energization time t. This means that three value pairs can be taken from the voltage-time diagram. More than three data points may also be established. The reading out of the value pairs is effected via a sampling unit 31, which, as shown here by way of example, comprises an A / D converter 32 (ADC) and a timer 34. The timer 34 is signal-connected to a control terminal (gate electrode) of the semiconductor switching element 12 to read the energization time t.In a further method step 105, the read-out (at least) three value pairs are inserted into a system of equations which describes the integrated voltage V int as a function of the energization time t. The function contains one or more coefficients which are dependent on the magnitude of the switch current, preferably on a pulse height Lithographically of the switch current. The function preferably has the following form (equation (1)): wherein A, B, C and D are constant parameters.More preferably, the parameters A, B, C and D are defined as follows:wherein the following definitions apply:U PT1: the integrated voltage;t: the energization time;R S'S: the parasitic resistance of the measuring path;L S'S: the parasitic inductance of the measurement path;diagrammatically: a pulse height of the switch current;K: an amplification factor of the active low-pass filter;- δ: the slope of the switch current;T PT1: a component-specific time constant of the active low-pass filter, which is dependent on a resistance value and a capacitance value of the active low-pass filter.In the above definitions of the parameters A, B, C and D, the variables L S'S, K and T PT1, are component-specific constants of the measurement path 11 or of the active low-pass filter 21. On account of the number of unknowns, namely three, three data points are sufficient to completely determine the parameters and thus the three unknown variables R S'S, Lithographically and δ, which takes place in a further method step 106. Here, two assumptions are made: first, that the parasitic resistance R S'S, of the measurement path 11 which is generally dependent on the temperature does not change in all three data points and thus behaves as a constant; second, that the curve of the switch current has an initial square jump from 0 to the pulse height Ull and a subsequent increase with a constant gradient δ. The first assumption is taken into account in that the three data points have a time difference from one another which lies in a range between 0 and 10 μs (microsecond or 10 -6 second, respectively), preferably between 0 and 1 μs. The second assumption is taken into account in that the data points are only selected after the switching operation, at which the switch current was applied to the measurement path.In this way, the switch current or its pulse height Lithographically can be determined with high accuracy independently of the thermal behavior of the parasitic resistance R S'S of the measurement path 11, which is advantageous compared to the methods known from the prior art.Reference numerals denote reference numerals10 Device 11 Measuring path 12 Semiconductor switching element 14 Parasitic inductance 16 Parasitic resistance 18 Low-pass resistance 20 Low-pass capacitance 21 Active low-pass filter 22 Comparator 24, 26 Resistors 31 Scanning unit 32 A / D converter 34 Timer 101- 106 Method steps

Claims

Method for determining a switch current in an electronic current and / or voltage converter, in particular in an inverter, comprising: - energizing (101) a measurement path (11) with a switch current which flows through a semiconductor switching element of a half bridge installed in a current and / or voltage converter, wherein the measurement path (11) has a parasitic inductance (14) and a parasitic resistance (16); - measuring (102) a voltage drop across the measurement path (11); - integrating (103) the voltage drop by means of an active low-pass filter (21) in order to obtain a profile of an integrated voltage as a function of an energization time; - defining (104) three data points from the profile over time of the integrated voltage and reading out three associated value pairs for the respective data points; inserting (105) the value pairs into a system of equations which describes the integrated voltage as a function of the energization time, wherein the function contains one or more coefficients which are dependent on the magnitude of the switch current, preferably on a pulse magnitude of the switch current, further preferably additionally on the parasitic inductance and / or a gain factor of the active low-pass filter; calculating (106) the coefficient or coefficients in order to determine the magnitude or the pulse magnitude of the switch current.Method according to Claim 1, wherein the function comprises a summation of an exponential function and a linear function of the energization time.Method according to Claim 2, wherein the exponential function has an exponent which is a product of the energization time and a component-specific constant of the active low-pass filter (21).The method of claim 3, wherein the component-specific constant of the active low-pass filter is an additive inversion of a reciprocal of a product of a resistance value and a capacitance value of the active low-pass filter.Method according to one of Claims 2 to 4, wherein the exponential function is factorised with a first coefficient which is dependent on the value of the parasitic resistor (16), a pulse height of the switch current and a gradient of the switch current which occurs on account of the parasitic inductance (14).Method according to one of Claims 2 to 5, wherein the linear function is factorised with a second coefficient which is dependent on the value of the parasitic resistor (16), a pulse height of the switch current and a gradient of the switch current which occurs on account of the parasitic inductance (14).Method according to one of Claims 2 to 6, wherein the summation contains, as a third summand, a constant which is dependent on the value of the parasitic resistor (16), a pulse height of the switch current and a gradient of the switch current which occurs on account of the parasitic inductance (14).Method according to one of claims 1 to 7, wherein the equation system has the following function: U PT1( t) = A · e D·t+ B · t + C, wherein A, B, C and D are constant parameters.The method of claim 8, wherein the parameters A, B, c and D are defined as follows: A = R S'S ⋅ I ^ ⋅ K ⋅ L S'S R S'S - T PT1 T PT1 + R S'S ⋅ K ⋅ d i Last d t ⋅ ( T PT1 - L S'S R S'S ) B = R S'S ⋅ K ⋅ d i Last d t C = R S'S ⋅ I ^ ⋅ K + R S'S ⋅ K ⋅ δ ⋅ ( L S'S R S'S - T PT1 ) D = - 1 T PT1 wherein the following definitions apply: - U PT1: the integrated voltage; - t: the energization time; R S'S: the parasitic resistance of the measurement path; L S'S: the parasitic inductance of the measurement path; Lithographically: a pulse height of the switch current; K: a gain factor of the active low-pass filter; δ: the slope of the switch current; T PT1: a component-specific time constant of the active low-pass filter, which is dependent on a resistance value and a capacitance value of the active low-pass filter.Device (10) for determining a switch current in an electronic current and / or voltage converter, in particular in an inverter, comprising an active low-pass filter (21) and a scanning unit (31) for carrying out the following method steps: - energizing (101) a measurement path (11) with a switch current which flows through a semiconductor switching element of a half-bridge installed in a current and / or voltage converter, wherein the measurement path (11) has a parasitic inductance (14) and a parasitic resistance (16); - measuring (102) a voltage drop across the measurement path (11); - integrating (103) the voltage drop by means of the active low-pass filter (21) in order to obtain a profile of an integrated voltage as a function of an energizing time; defining (104) three data points from the temporal profile of the integrated voltage and reading out three associated value pairs for the respective data points; inserting (105) the value pairs into a system of equations which describes the integrated voltage as a function of the energization time, wherein the function contains one or more coefficients which are dependent on the magnitude of the switch current, preferably on a pulse magnitude of the switch current, further preferably additionally on the parasitic inductance and / or a gain factor of the active low-pass filter; calculating (106) the coefficient or coefficients in order to determine the magnitude or the pulse magnitude of the switch current.Electronic current and / or voltage converter, in particular inverter, for an electric drive of an electric vehicle or of a hybrid vehicle, wherein the electronic current and / or voltage converter comprises a device (10) for determining a switch current according to claim 10 or is connected to such a device (10).

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