Method and device for power control or voltage control

Abstract

Method for power control or voltage control for supplying an electrical consumer (10) with the aid of a power output stage (12), wherein the power control or voltage control is carried out via pulse-width modulation (PWM) with an adjustable duty cycle, wherein in a measuring phase under different operating conditions described by at least two operating parameters of the power output stage (12) a measured effective voltage across the consumer is determined, wherein the duty cycle of the pulse-width modulation is changed such that the measured effective voltage corresponds to a predetermined effective target voltage,wherein the duty cycles thus determined are stored as a function of the associated operating conditions, and wherein, in an application phase for the present operating conditions, a corrected duty cycle is calculated from the stored duty cycles and set for power control or voltage control, characterized in that at least the temperature of the power output stage and the value of a supply voltage and an age of the power output stage (12) are taken into account as operating parameters.

Description

State of the art

[0001] The invention relates to a method for power control or voltage control for supplying an electrical consumer with the aid of a power output stage, wherein the power control or voltage control is carried out via pulse width modulation (PWM) with an adjustable duty cycle.

[0002] The invention further relates to a device for power control or voltage control for supplying an electrical consumer with a power output stage, wherein the power control or voltage control is carried out via pulse width modulation (PWM) with an adjustable duty cycle and wherein a control unit is assigned to the power output stage.

[0003] To adjust power, effective voltage, or effective current, it is common practice to control electrical loads using pulse-width modulation (PWM). In this case, a constant supply voltage of the electrical load, for example, is switched on and off at a specified duty cycle (TV). The ratio of the on-time to the off-time can be used to adjust the electrical power supplied to the load, the effective voltage, and the effective current.

[0004] The switching process can be performed via a correspondingly controlled power amplifier. The duty cycle can be controlled or specified in a regulated manner.

[0005] In controlled systems, the duty cycle is adjusted under the assumption of ideal behavior of the electronic components used and thus the temporal progression of the voltage supplied to the load. Deviations from this ideal behavior, for example, due to a limited edge slew rate or delay times, particularly in the output stage, have a direct impact on the power supplied to the load or the effective current and voltage. These deviations can be due to the response behavior of the electronic components used, which may be influenced by component tolerances. The edge slew rate may also be limited by electromagnetic compatibility (EMC) requirements.

[0006] Deviations from the ideal behavior have a particularly strong impact at small duty cycles of pulse-width modulation. Small duty cycles can arise, for example, from electrical consumers operating with different supply voltages. For example, DE102010001004A1 describes a method for controlling actuators within an on-board power system that has different operating voltages or temporal on-board power system voltage changes. The actuator(s) are controlled with different pulse-width modulated control signals, whereby the pulse width and period of the control signals can be adjusted independently of one another and are adapted depending on the currently applied on-board power system voltage.

[0007] Exhaust gas sensors, such as lambda sensors, as used in modern internal combustion engines to monitor and regulate exhaust gas composition, often have an electric heater for setting a predefined operating temperature of the exhaust gas sensor, which is, for example, in the range of 800°C. The heating power is adjusted by pulse-width modulation, which is designed for a 12V supply voltage, for example. The possible use of the exhaust gas sensor in a 24V system results in very small duty cycles. Deviations from the ideal behavior of the pulse-width modulation are compensated for by a temperature controller during controlled operation above a certain cell temperature.In controlled operation, for example when a measured value from the exhaust gas probe is not available for operating a control system, the described deviations from the ideal behavior can lead to strong temperature deviations with a correspondingly high risk of failures due to incorrect operation.

[0008] In another operating mode, known as protective heating, the lambda sensor is set to a low target temperature in the range of 100°C to 200°C during a heat-up phase. The purpose of protective heating is to evaporate any water in or on the lambda sensor and in the exhaust tract. This minimizes the risk of ceramic breakage of the lambda sensor element due to excessive thermomechanical stresses during the subsequent heat-up process with higher effective heater voltage. In order to maintain the specified target temperature sufficiently accurately, the effective voltage applied to the heater must be precisely adjusted. For this purpose, the duty cycle of the voltage applied to the heater is calculated from a supply voltage value measured in a control unit and the desired effective voltage across the heater resistor, assuming an ideal rectangular voltage curve.For protective heating, this results in very small duty cycles in the range of 0.5 - 2%. Deviations in the voltage curve from the ideal rectangular waveform lead to deviations between the setpoint and the actual value of the effective voltage applied to the heater. At the necessary small duty cycles, this leads to unacceptably high deviations of the lambda sensor temperature from the target temperature.

[0009] To enable controlled operation at low duty cycles, fast (in terms of high edge steepness and short delay times) and tightly toleranced power stages can be used for the control. However, this leads to unacceptably high EMC emissions and increased component costs.

[0010] Another option is to use a DC / DC converter instead of pulse-width modulation. This leads to increased manufacturing costs. Additional disadvantages include the large space required for the DC / DC converter and the high heat loss generated during this process.

[0011] Measuring and setting a suitable mean or effective current value also results in high costs and a high space requirement, as well as high hardware expenditure.

[0012] It is therefore an object of the invention to provide a method which enables precise and cost-effective voltage or power control of an electrical consumer via pulse width modulation even at small duty cycles.

[0013] A generic power control is disclosed in US 5 932 938 A. Further methods for power control or voltage control are known from JP 2011 - 55 602 A.

[0014] It is a further object of the invention to provide a corresponding device. Disclosure of the invention

[0015] The object of the invention relating to the method is achieved in that in a measuring phase under different operating conditions described by at least two operating parameters of the power output stage a measured effective voltage is determined across the consumer, that the duty cycle of the pulse width modulation is changed in such a way that the measured effective voltage corresponds to a predetermined effective target voltage, that the duty cycles thus determined are stored as a function of the associated operating conditions and that in an application phase for the existing operating conditions a corrected duty cycle is calculated from the stored duty cycles and set for power control or voltage control.The measured effective voltage is determined during the measurement phase with a suitable time resolution, allowing the voltage curve to be determined over the entire turn-on pulse, from which the measured effective voltage across the load can be determined. The measurement can be performed separately for each power stage or, advantageously, for a predefined portion of the power stages in a production batch. During the application phase in regular operation, voltage measurement across the load is no longer necessary. Since the corrected duty cycle is calculated from the stored duty cycles, interpolation over a large data field can be avoided, allowing the corrected duty cycle to be determined quickly and with minimal hardware effort.The corrected duty cycle takes into account deviations of the actual voltage curve from the ideal square wave, additionally taking into account the influence of the existing operating conditions, so that a specified effective voltage can be set precisely even with small duty cycles.

[0016] According to a particularly preferred embodiment of the invention, it can be provided that, in order to calculate a corrected duty cycle, the dependence of the determined duty cycles on various operating parameters is at least approximately described by adapted mathematical functions with adapted coefficients, that the functions and the associated coefficients are stored, and that, in the application phase, the corrected duty cycle is determined for the existing operating parameters using the stored functions and the stored coefficients. To calculate the corrected duty cycle, only the functions and the coefficients need to be stored, which results in a very low memory requirement.Calculating using mathematical functions allows for a very quick determination of the duty cycle depending on the current operating conditions under which the power amplifier is operating. The mathematical functions can be determined from the determined dependencies of the duty cycle on various operating conditions that influence the voltage curve, for example, using appropriate fit functions or regression analyses.

[0017] In order to take two operating parameters into account when correcting the duty cycle, it can be provided that, in order to calculate the corrected duty cycle, a dependency of the determined duty cycles on a first operating parameter for different values ​​of a second operating parameter is at least approximately described by a first set of a first mathematical function and associated first coefficients, that a dependency of the first coefficients on the second operating parameter is at least approximately described by a second set of a second mathematical function and associated second coefficients, that the second coefficients and the mathematical functions are stored,that in an application phase for the present second operating parameter, the first coefficients associated with the present second operating parameter are determined using the stored second coefficients and the second mathematical function, and that the corrected duty cycle for the present first operating parameter is calculated using these first coefficients and the first mathematical function.

[0018] Therefore, only the first and second functions and the second coefficients need to be saved. The prerequisite for this is that the dependence of the determined duty cycles on the first operating parameter for different values ​​of the second operating parameter can be approximated by a common function with adjusted first coefficients in each case. Accordingly, a second mathematical function must be found with which the dependence of the first coefficients on the second operating parameter can be described by adjusting the second coefficients. The first set of the first mathematical function results from the first mathematical function and the adjusted first coefficients in each case. The second set of the second mathematical function results from the second mathematical function and the adjusted second coefficients in each case.

[0019] The duty cycle corrected in this way takes into account the influence of the first and second operating parameters on the resulting voltage curve.

[0020] The influence of any number of operating parameters on the duty cycle can be corrected by determining an additional set of a further function and associated additional coefficients for each additional operating parameter.

[0021] It can be provided that a dependency of the coefficients determined for a previously considered operating parameter for different values ​​of a further operating parameter is at least approximately described by a further set of a further mathematical function and associated further coefficients, that the further coefficients and the further mathematical function are stored and that in an application phase for the current value of the further operating parameter the coefficients determined for the previously considered operating parameter are determined.

[0022] The effective voltage across the load can be easily determined by digitally recording a voltage curve across the load during the measurement phase and then determining the measured effective voltage across the load from this voltage curve. The recording can be performed using a sufficiently fast analog / digital converter, allowing the representation of an edge curve that deviates from a rectangular shape. Since the voltage curve is only determined during the measurement phase, the control unit does not need to be equipped with a corresponding analog / digital converter. The voltage curve can be determined at an appropriately equipped measuring station for the respective power output stages.

[0023] Deviations from the ideal rectangular voltage waveform due to a limited edge steepness or delay times have a noticeable impact on the effective voltage, especially at low duty cycles. Therefore, a duty cycle correction can be performed for a duty cycle of less than 5%, especially for a duty cycle of less than 2%. No correction is performed for larger duty cycles. Accordingly, the functions and coefficients only need to be determined for low duty cycles.

[0024] The parameters that dominate the switching behavior of the power amplifier, such as edge steepness or delay times, are generally strongly influenced by the supply voltage and the temperature of the power amplifier. Furthermore, the switching behavior can be affected by aging effects.

[0025] According to the invention, at least the temperature of the power output stage and the value of a supply voltage and an age of the power output stage are taken into account as operating parameters.

[0026] The dependence of the determined duty cycle or coefficients on the relevant operating parameters, in particular on the supply voltage and temperature, often follows the curve of second-order polynomials. Therefore, second-order polynomials may be used as functions to describe the dependence of the determined duty cycles on the operating parameters and / or to describe the dependence of the coefficients on the operating parameters.

[0027] The dependencies of the determined duty cycle and the coefficients can depend on the specified effective target voltage. If the load is operated at different effective voltages, it can therefore be provided that coefficients and / or functions are determined for different effective target voltages.

[0028] The object of the invention relating to the device is achieved in that the control unit contains a memory for storing functions and associated coefficients for describing the dependence of a corrected duty cycle on operating parameters of the power output stage, and in that the control unit contains at least one program sequence for calculating the corrected duty cycle for current values ​​of the operating parameters using the stored coefficients and functions. The device thus enables the implementation of the described method.

[0029] The method and the device can preferably be used for power control or voltage control of an electric heater of an exhaust gas probe in the exhaust duct of an internal combustion engine.

[0030] The invention is explained in more detail below using an exemplary embodiment illustrated in the figures. They show: Fig. 1 an electrical circuit for controlling the power of an electrical consumer, Fig. 2 Signal waveforms of a pulse width modulation with a short duty cycle, Fig. 3 first curves of determined duty cycles, Fig. 4 first curves of first coefficients as a function of temperature, Fig. 5 second curves of determined duty cycles, Fig. 6 third curves of determined duty cycles, Fig. 7 fourth curves of determined duty cycles, Fig. 8 second courses of fitted coefficients.

[0031] Fig. Figure 1 shows an electrical circuit for controlling the power of an electrical consumer 10 according to the prior art. In the exemplary embodiment, the electrical consumer 10 corresponds to an electric heater for a lambda probe in the exhaust duct of an internal combustion engine.

[0032] A supply voltage is provided by a voltage source 14, which is connected to ground 15 on one side.

[0033] The electrical circuit contains a control unit 11, to which a power output stage 12 and a TV detector 13 are assigned. The power output stage 12 is implemented as an NMOS field-effect transistor (FET) with a drain terminal 12.1, a source terminal 12.2, and a gate terminal 12.3. The source terminal 12.2 is connected to the ground terminal of the voltage source 14. The drain terminal 12.1 is connected to the load 10, and the gate terminal 12.3 is connected to the TV detector 13. The voltage source 14 is connected to the control unit 11 and, on one side, to the load 10.

[0034] To control the electrical power supplied to load 10, a pulse-width modulated signal (PWM signal) is applied to gate terminal 12.3. While the PWM signal is on, the field-effect transistor switches on, and a current flows from voltage source 14 through load 10 and power output stage 12 to ground 15. During off-times, the current flow is interrupted. The power converted in load 10 is specified via the duty cycle TV of the PWM signal. The duty cycle TV is determined by TV detection 13, taking the existing supply voltage into account. Gate terminal 12.3 is controlled by TV detection 13 such that a specified effective voltage is applied to load 10.

[0035] According to the illustrated structure of the electrical circuit, the power output stage 12 in the exemplary embodiment is designed as a low-side switch to ground 15.

[0036] Fig. 2 shows signal curves of the pulse width modulation with a short duty cycle, as they are achieved with a Fig. 1 during operation of an electric heater of a lambda probe in the exhaust duct of an internal combustion engine. A control signal 22, a load current curve 23, and a drain-source voltage 24 are plotted against a common signal axis 20 and a time axis 21. Fig. 1, the control signal 22 is output from the TV detection circuit 13 to the gate terminal 12.3 of the power output stage 12. The load current waveform 23 corresponds to the current waveform through the load 10, while the drain-source voltage 24 represents the voltage waveform across the field-effect transistor of the power output stage 12.

[0037] The control signal 22 is an ideal square wave. Due to a limited edge steepness or delay times of the power output stage 12, the load current waveform 23 and the drain-source voltage 24 deviate from the ideal square wave. Especially at low duty cycles, this deviation leads to a noticeable deviation in the effective voltage drop across the load compared to a specified effective target voltage. The electrical power converted in the load deviates from the target value.

[0038] Short duty cycles with duty cycles in the range of 0.5% to 2% are provided, for example, in the protective heating mode, in which water contained in or on the lambda sensor or in the exhaust tract is evaporated at comparatively low temperatures in the range of 100°C to 200°C. Very short duty cycles occur particularly in the truck sector with its 24V on-board voltage, which is high compared to a car's 12V. Protective heating largely prevents ceramic breakage, which can occur due to excessive thermomechanical stresses during the subsequent heating process with a higher effective heater voltage to the operating temperature of the lambda sensor of, for example, 800°C. To ensure that the lambda sensor does not become too hot or too cold during protective heating, precise adherence to the specified effective voltage is very important.

[0039] Currently, the duty cycle for the lambda sensor heater is calculated from the supply voltage measured in the control unit 11 and the desired effective voltage across the heater resistor, assuming an ideal rectangular voltage curve. Fig. 2, the deviation of the drain-source voltage 24 and the load current 23 from the ideal rectangular shape results in large deviations between the desired effective voltage and the actually applied effective voltage at small duty cycles, resulting in high temperature deviations.

[0040] According to the invention, it is therefore provided to correct the duty cycle, particularly for small duty cycles, in such a way that, despite a deviation of the voltage curve from the ideal rectangular shape, the effective voltage actually applied across the load corresponds to the predetermined effective target voltage. For this purpose, a measurement during a measuring phase is used to determine the required duty cycle for the power output stage 12 used, at which the measured effective voltage corresponds to a predetermined effective target voltage. During the measurement, the voltage across the load is recorded with a suitable time resolution, and the effective voltage is calculated from the measured values. The duty cycle is adjusted by a control loop until the desired effective target voltage is reached.

[0041] The parameters that dominate the switching behavior of the power output stage 12, such as the edge steepness and delay times, are highly dependent on the conditions under which the power output stage is operated. Key operating parameters include the supply voltage and the temperature of the power output stage 12. Therefore, the measurement is repeated at different values ​​of the relevant operating parameters, for example, at different temperatures and supply voltages, resulting in a multidimensional characteristic map of corrected duty cycles.

[0042] In a subsequent application phase during operation of the load 10 and the power output stage 12, a corrected duty cycle is calculated from the determined duty cycles and the measured operating parameters using a calculation method. Therefore, no measurement of the voltage across the load 10 is necessary during operation. The number of stored measured values ​​depends on the required setting accuracy of the effective value and the individual switching behavior of the respective power output stage 12.

[0043] Fig. Figure 3 shows initial curves of duty cycles determined during a measurement phase. This includes a first curve TV korr T1 32.1, a second course TV korr T2 32.2 and a third course TV korr T3 32.3 opposite an axis TV korr 30 and an axis U batt 31 applied.

[0044] Fig. 3 describes together with Fig. 4 in one embodiment, the correction of the duty cycle of a power output stage 12 as a function of the two operating parameters temperature of the power output stage 12 and level of the supply voltage U batt .

[0045] The first course TV korr T1 32.1 corresponds to the determined duty cycles as they occur at different supply voltages U batt for a first temperature T1. The second curve TV korr T2 32.2 and the third course TV korr T3 32.3 correspond to the determined duty cycles as they occur at different supply voltages U batt for a second temperature T2 and for a third temperature T3.

[0046] In a first process step, for each temperature T1, T2, T3, the respective curve is determined by a suitable function depending on the supply voltage U battThe curves shown in the example can be approximated by second-degree polynomials according to a first theorem of a mathematical function: For T1 applies: TVkorr(u)=a1u2+b1u+c1 For T2 applies: TVkorr(u)=a2u2+b2u+c2 For T3 applies: TVkorr(u)=a3u2+b3u+c3

[0047] The following applies to the first coefficients a, b and c: a i = a(T i ); b i = b(T i ); c i = c(T i ). The value of the supply voltage is denoted by u.

[0048] Fig. Figure 4 shows the first curves of the first coefficients as a function of temperature. This includes the adjusted coefficients a (T) 42, b (T) 43 and c (T) 44, as they are calculated according to Fig. 3, are plotted as a function of temperature against a coefficient axis 40 and a temperature axis 41.

[0049] In a second step, these curves of the first coefficients are approximated by a second set of a second mathematical function. The curves shown in the exemplary embodiment can in turn be approximated by second-degree polynomials: a(T)=s2T2+s1T+s0 b(T)=r2T2+r1T+r0 c(T)=q2T2+q1T+q0 s i , r i and q i represent the second coefficients.

[0050] Using the first function and the second function and the associated first and second coefficients, a corrected duty cycle can be calculated for each permissible temperature and supply voltage. For this purpose, only the two functions and the second coefficients s0, s1, s2, r0, r1, r2, q0, q1, q2 need to be stored in the control unit 11. Using these second coefficients and the second mathematical function, the first coefficients are initially calculated in the application phase as a function of the current temperature of the power output stage 12. Using these first coefficients and the first mathematical function, the corrected duty cycle is then determined for the current supply voltage.

[0051] The functions with which the Fig. 3 and Fig. The curves shown in Figure 4 are not limited to polynomials or second-degree polynomials. The curves within a diagram simply need to be approximated using a similar function by adjusting the respective coefficients.

[0052] The method is not limited to the dependence of two operating parameters. It can be applied to any number of parameters, as long as appropriate functions can be found to approximate the curves.

[0053] In the Fig. 5, Fig. 6, Fig. 7, Fig. 8 will be the Fig. 3, Fig. 4 described procedure is extended to another operating parameter, the age of the power output stage 12.

[0054] Fig. 5 shows second curves of duty cycles determined during a measurement phase, Fig. 6 shows third curves of determined duty cycles and Fig. Figure 7 shows fourth curves of determined duty cycles, each depending on the supply voltage and the temperature of the power output stage 12 as parameters. The representations correspond to the Fig. 3 described representation.

[0055] In Fig. 5 shows the curves of the duty cycles for a first age A1 of the power output stage 12. For this purpose, a first curve TV korr T1 A1 33.1, a second course TV korr T2 A1 33.2 and a third course TV korr T3 A1 33.3 opposite the TV axis korr 30 and the axis U batt 31 applied.

[0056] In Fig. 6 shows the curves of the duty cycles for a second age A2 of the power output stage 12. For this purpose, a first curve TV korr T1 A2 34.1, a second course TV korr T2 A2 34.2 and a third course TV korr T3 A2 34.3 opposite the TV axis korr 30 and the axis U batt31 applied.

[0057] In Fig. 7 shows the curves of the duty cycles for a third age A3 of the power output stage 12. For this purpose, a first curve TV korr T1 A3 35.1, a second course TV korr T2 A3 35.2 and a third course TV korr T3 A3 35.3 opposite the TV axis korr 30 and the axis U batt 31 applied.

[0058] The Fig. 5, Fig. 6, Fig. 7, from the supply voltage U Batt The dependent curves can be approximated by a second-degree polynomial by adjusting the coefficients accordingly. The adjustment is made for each age A1, A2, and A3 for the first temperature T1, the second temperature T2, and the third temperature T3 of the power output stage 12.

[0059] The coefficients thus determined can again be plotted as a function of the temperature of the power output stage 12 and described by a suitable function with adapted coefficients.

[0060] Accordingly, Fig. 8 second curves of fitted coefficients as they are derived from the Fig. 5, Fig. 6, Fig. 7. The representation corresponds to that in Fig. 4, limited to the curves of the coefficient a. This is plotted as coefficient a A1 (T) 45 for the first age, as coefficient a A2 (T) 46 for the second age, and as coefficient a A3 (T) 47 for the third age of the performance end stage 12.

[0061] The curves of the coefficient a can in turn be approximated by a suitable further function with adapted further coefficients for the different ages of the power output stage 12.

[0062] According to the Fig.The curves of coefficient a shown in Figure 8 can also be used to represent coefficients b and c and approximate them using functions with adjusted coefficients. From these functions and coefficients, as well as the previously determined functions, a corrected duty cycle can be determined during the application phase, taking into account the age and temperature of the power output stage as well as the supply voltage level.

[0063] According to the procedure shown, any number of additional operating parameters can be taken into account.

[0064] The method can be performed for any effective nominal voltage that must be maintained with high accuracy. If measurements are performed at different effective nominal voltages, the duty cycles for the intermediate effective values ​​can also be interpolated.

[0065] For heating a lambda sensor, only one measurement is required at the effective target voltage required for protective heating. During the subsequent heating process, significantly larger duty cycles result, so determining the duty cycle assuming an ideal square-wave voltage provides sufficient accuracy.

[0066] The method enables precise adjustment of an effective voltage across a load, whereby the influences of various operating parameters, such as the supply voltage and the temperature of the power output stage 12, can be compensated. No voltage measurement during operation is necessary. Depending on the manufacturing tolerances of the power output stage 12 used, it is not necessary to measure each power output stage 12 individually; instead, for example, only one power output stage 12 per production batch needs to be measured. The method can be applied to all types of power output stages 12 (discrete, integrated, high-side, low-side). It can be used in both passenger cars and trucks. Only a small number of parameters and functions need to be stored in the control unit, thus requiring only a small amount of memory.The functions enable a quick determination of the correct duty cycle, since no lengthy interpolation over a large data field is necessary.

Claims

[1] Method for power control or voltage control for supplying an electrical consumer (10) with the aid of a power output stage (12), wherein the power control or voltage control is carried out via a pulse-width modulation (PWM) with an adjustable duty cycle, wherein in a measuring phase under different operating conditions described by at least two operating parameters of the power output stage (12) a measured effective voltage across the consumer is determined, wherein the duty cycle of the pulse-width modulation is changed such that the measured effective voltage corresponds to a predetermined effective target voltage,wherein the duty cycles thus determined are stored as a function of the associated operating conditions, and wherein, in an application phase for the existing operating conditions, a corrected duty cycle is calculated from the stored duty cycles and set for power control or voltage control, , characterized by that at least the temperature of the power output stage and the value of a supply voltage and an age of the power output stage (12) are taken into account as operating parameters. [2] Method according to claim 1, characterized bythat in order to calculate a corrected duty cycle, the dependence of the determined duty cycles on various operating parameters is at least approximately described by adapted mathematical functions with adapted coefficients, that the functions and the associated coefficients are stored and that in the application phase the corrected duty cycle is determined for the existing operating parameters using the stored functions and the stored coefficients. [3] Method according to claim 1 or 2, characterized bythat, in order to calculate the corrected duty cycle, a dependency of the determined duty cycles on a first operating parameter for different values ​​of a second operating parameter is at least approximately described by a first set of a first mathematical function and associated first coefficients, that a dependency of the first coefficients on the second operating parameter is at least approximately described by a second set of a second mathematical function and associated second coefficients, that the second coefficients and the mathematical functions are stored,that in an application phase for the present second operating parameter, the first coefficients associated with the present second operating parameter are determined using the stored second coefficients and the second mathematical function, and that the corrected duty cycle for the present first operating parameter is calculated using these first coefficients and the first mathematical function. [4] Method according to one of claims 1 to 3, characterized by that for each additional operating parameter, another set of another function and associated additional coefficients are determined. [5] Method according to one of claims 1 to 4, characterized bythat a dependency of the coefficients determined for a previously considered operating parameter for different values ​​of a further operating parameter is at least approximately described by a further set of a further mathematical function and associated further coefficients, that the further coefficients and the further mathematical function are stored and that in an application phase for the current value of the further operating parameter the coefficients determined for the previously considered operating parameter are determined. [6] Method according to one of claims 1 to 5, characterized by that in the measuring phase a voltage curve across the consumer (10) is digitally recorded and that the measured effective voltage across the consumer (10) is determined from the voltage curve. [7] Method according to one of claims 1 to 6, characterized bythat a correction of the duty cycle is carried out if the duty cycle is less than 5%, in particular if the duty cycle is less than 2%. [8] Method according to one of claims 1 to 7, characterized by that second-degree polynomials are used as functions to describe the dependence of the determined duty cycles on the operating parameters and / or to describe the dependence of the coefficients on the operating parameters. [9] Method according to one of claims 1 to 8, characterized by that coefficients and / or functions are determined for different effective target voltages. [10] Device for power control or voltage control for supplying an electrical consumer (10) with a power output stage (12), wherein the power control or voltage control is carried out via pulse width modulation (PWM) with an adjustable duty cycle and wherein a control unit (11) is assigned to the power output stage (12), characterized by that the control unit (11) contains a memory for storing functions and associated coefficients for describing the dependence of a corrected duty cycle on operating parameters of the power output stage (12), and that the control unit contains at least one program sequence for calculating the corrected duty cycle for current values ​​of the operating parameters using the stored coefficients and functions, and that the operating parameters include at least the temperature of the power output stage, the value of a supply voltage, and an age of the power output stage (12). [11] Application of the method and device according to one of claims 1 to 10 for power control or voltage control of an electric heater (14) of an exhaust gas probe in the exhaust gas duct of an internal combustion engine.

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