Method for reducing a thermal load on a controllable switching element

Abstract

A method for reducing a thermal load on a switching element of an electronic fuse when switching on a load, wherein (a) a switching element is activated, (b) the switching element is deactivated and (c) the switching element is re-activated after reaching a set value of a switch-off duration, where steps (b) and (c) are repeated until an output voltage reaches a value that falls below a specified difference with respect to an input voltage of an electronic fuse or an output current reaches a specified duration current, where set values of a switch-on duration and/or switch-off current and the switch-off duration are maintained until new set values have been determined based on the output voltage, output current, and/or temperature, a pulse duty factor between the switch-on duration and the switch-off duration is adapted, and the specified maximum allowable temperature increase of the switching element is further observed.

Claims

1. A method for reducing a thermal loading of a controllable switching element of an electronic fuse during a switching-on process of a load (L), the switching element being driven via a drive signal with a predefinable drive period comprising a switched-on duration and a switched-off duration, a temporal profile of at least one of (i) at least one output voltage presenting at the load (ii) an output current flowing into the load and (iii) a temperature of the switching element being ascertained during the switching-on process, and predefined values governing compliance with a predefined maximum permissible increase in a temperature of the switching element within the predefinable drive period being predefined for at least one of (i) the switched-on duration of the switching element and (ii) a switch-off current, and for the switched-off duration of the switching element, the method comprising: a) switching on the switching element; b) switching off the switching element upon at least reaching the predefined value of the switched-on duration or the switch-off current; c) switching on the switching element again upon reaching the predefined value of the switched-off duration; d) and performing steps b to c until an ascertained profile of the output voltage reaches a value at which, with respect to an input voltage of the electronic fuse, a predefinable difference is undershot, or until a predefinable maximum continuous current is at least undershot by the ascertained profile of the output current, wherein the predefined values for at least one of (i) the switched-on duration and (ii) the switch-off current and for the switched-off duration are maintained until, based on at least one of (i) the ascertained profile of the output voltage, (ii) the output current and (iii) the temperature of the switching element, new predefined values are ascertained such that a duty ratio between the switched-on duration and the switched-off duration of the switching element is adapted such that at least continuing compliance with the predefined maximum permissible increase in the temperature of the switching element within the predefinable drive period occurs.

2. The method as claimed in claim 1, wherein a duty ratio between the switched-on duration and the switched-off duration is reduced with at least one of (i) an increase in the output voltage present at the load and (ii) a decrease in the output current flowing into the load relative to the predefinable maximum continuous current.

3. The method as claimed in claim 2, wherein at least one of (i) the temporal profile respectively ascertained for the output voltage, (ii) the output current and (iii) the temperature of the switching element is averaged to ascertain the new predefined values of at least one of (i) the switched-on duration and (ii) the switch-off current, and switched-off duration.

4. The method as claimed in claim 1, wherein at least one of (i) the temporal profile respectively ascertained for the output voltage (ii) the output current and (iii) the temperature of the switching element is averaged to ascertain the new predefined values of at least one of (i) the switched-on duration and (ii) the switch-off current, and switched-off duration.

5. The method as claimed in claim 1, wherein first predefined values for at least one of (i) the switched-on duration and (ii) the switch-off current, and for the switched-off duration and the predefined maximum permissible increase in the temperature of the switching element within the predefinable drive period are determined based on a thermal model of the switching element.

6. The method as claimed in claim 5, wherein the thermal model of the switching element comprises one of a Cauer network and a Foster network.

7. The method as claimed in claim 1, wherein the switching element is switched off upon at least reaching the predefined maximum permissible increase in the temperature of the switching element.

8. The method as claimed in claim 1, wherein the ascertained temporal profile of the temperature of the switching element is compared with a predefinable limit temperature; and wherein the switching element is switched off upon at least reaching said predefinable limit temperature.

9. The method as claimed in claim 1, wherein the temperature of the switching element is determined at one of (i) a housing of the switching element, (ii) in a direct vicinity of the switching element and (iii) immediately directly in the switching element.

10. The method as claimed in claim 1, wherein the drive signal is derived from a predefined signal; and wherein the predefined signal is determined from the temporal profile respectively ascertained for at least one of (i) the output voltage, (ii) the output current and (iii) the temperature of the switching element.

11. The method as claimed in claim 10, wherein the predefined signal is generated by an evaluation unit.

12. The method as claimed in claim 11, wherein the drive signal is generated from the predefined signal by a drive unit.

13. The method as claimed in claim 1, wherein the drive signal comprises a ramped signal.

14. The method as claimed in claim 1, wherein at least one of (i) the temporal profile of at least the output voltage present at the load, (ii) the output current flowing into the load and (iii) the temperature of the switching element is ascertained by a monitoring unit.

15. The method as claimed in claim 1, wherein an impedance is additionally fitted in series with the switching element.

16. The method as claimed in claim 1, wherein the switching element comprises a transistor.

17. The method as claimed in claim 16, wherein the transistor is a MOS-FET.

18. The method as claimed in claim 1, wherein the switching element is formed at least together with a monitoring unit and a drive unit to form an integrated component.

19. The method as claimed in claim 1, wherein the load comprises a capacitive load.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained below in an exemplary manner with reference to the accompanying figures, by way of example, in which:

(2) FIG. 1 shows an electronic fuse for performing the method for reducing a thermal loading of a switching element in accordance with the invention;

(3) FIG. 2 shows an exemplary sequence of the method in accordance with the invention for reducing a thermal loading of a switching element during the switching on of a load; and

(4) FIG. 3 shows by way of example and schematically graphical plots of temporal profiles of a drive signal for the switching element, of an output current, of an output voltage and of a temperature of the switching element during the sequence of the method in accordance with the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

(5) FIG. 1 shows by way of example and schematically an electronic fuse SI having at least one controllable switching element SE, where the fuse is configured for implementing the method in accordance with the invention for reducing a thermal loading of a switching element SE. The electronic fuse SI can be connected, for example, as a protection device between a supply source and an electrical load L. The supply source used is a voltage source, for example, which supplies an input voltage U.sub.E. The electrical load L comprises for example, a resistive portion R.sub.L and a capacitive portion C.sub.L, which can lead to relatively high inrush currents during a switching-on process or at the switch-on instant in the switching element SE of the electronic fuse SI. Furthermore, the electrical load can, e.g., also have an inductive portion. An output voltage U.sub.A of the electric fuse SI is present at the load L. An output current I.sub.A of the electric fuse SI flows into the load L, where a maximum permissible continuous current I.sub.L is settable for ongoing operation of the fuse. The continuous current I.sub.L can, for example, correspond to a “rated current” for the electrical load L or assume a predefinable value, usually below the rated current. During the switching-on process, the output current I.sub.A can assume values lying above the value of the continuous current I.sub.L.

(6) In addition, e.g. line losses, copper and line losses of the fuse SI, or internal resistance of the supply source, are represented as parasitic, ohmic resistance Rp, which at least slightly limits the inrush currents or the current pulse during the switching-on process. For further limiting of the inrush currents, for example, an impedance (not illustrated in FIG. 1) fitted in series with the switching element SE can be provided. Said impedance can be formed, e.g., as an ohmic resistance having a fixed or variable resistance value (e.g., as a thermistor). Alternatively, an inductance can also be used for the impedance, where the inductance prevents an excessively rapid current increase during the process of operationally connecting the load L to the switching element SE.

(7) The electric fuse SI can optionally have an additional protection device SV besides the controllable switching element SE. The additional protection device SV used can be a fusible link, for example, which is intended to respond in the case of a fault (e.g., in the event of a fault such as a short circuit), but is not intended to be triggered by current pulses or inrush currents that arise during an operational switching-on process.

(8) By way of example, a transistor (in particular a metal-oxide-semiconductor field-effect transistor or (MOS-FET) can be used as the switching element SE in the electric fuse. Ideally, an n-channel MOS-FET is used. The switching element SE is driven via a drive signal AS with a predefined drive period, where the drive signal is applied as a gate-source voltage, for example, and in the case of a MOS-FET as switching element SE. The drive period consists of a switched-on duration and a switched-off duration of the switching element and can have, e.g., a duration of from 100 μs to 10 ms (ideally of 1 ms). The drive signal AS can limit, e.g., a maximum possible current increase di.sub.A(t)/dt in the output current I.sub.A flowing into the load L during the switching-on process of the load L. The drive signal AS is generated from a predefined signal VS via a drive unit AE and is configured, for example, as illustrated in FIG. 3, in a ramped manner. The drive signal AS controls the increase in the temperature of the switching element SE during the drive period and optionally the current increase di.sub.A(t)/dt in the output current I.sub.A flowing into the load L, where the ramped waveform or a gradient of the drive signal AS allows, e.g., an increase in the output current I.sub.A through the switching element SE in a regulated manner and thus minimizes an increase in the temperature of the switching element during the respective drive period, in particular within the respective switched-on duration of the switching element, or limits it to a predefined maximum permissible increase in the temperature.

(9) The predefined signal VS is ascertained by an evaluation unit AW on the basis of a temporal profile of the output current I.sub.A and/or the output voltage U.sub.A and/or a temperature of the switching element SE. That is, for generating the predefined signal VS, which also includes predefined values such as switched-off duration, switched-on duration and/or switch-off current for the method in accordance with the invention, at least one of the temporal profiles of output current I.sub.A or output voltage U.sub.A or temperature of the switching element SE is used as a reference variable. The predefined signal VS can be configured as a pulse-width modulated rectangular signal and predefines the respective configuration of the drive signal AS (e.g., gradient and length of the ramped waveform) for example by virtue of the predefined values being taken into consideration.

(10) A monitoring unit UE is provided for ascertaining the temporal profiles of output current I.sub.A and/or output voltage U.sub.A and/or the temperature of the switching element SE. The monitoring unit UE collects, for example, measurement values of output current I.sub.A and/or output voltage U.sub.A and/or temperature of the switching element SE, which are ascertained, e.g., by corresponding sensor or measuring devices T.sub.SE, A, V. In particular, in this case, the temperature of the switching element SE (as illustrated by way of example in FIG. 1) can be determined immediately directly at the switching element SE. Alternatively, the temperature of the switching element SE can also be ascertained indirectly at a housing of the switching element SE. Here, a temperature or an increase in the temperature of the switching element SE, primarily the “junction temperature”, is deduced, for example, based on a measured housing temperature. Such an estimation of the temperature of the switching element SE or of the associated temporal profile can be effected, e.g., by the monitoring unit UE.

(11) Furthermore, the switching element SE can be formed as an integrated component, where the integrated component can comprise at least the monitoring unit UE and the drive unit AW and possibly present sensor or measuring devices T.sub.SE, A, V for, e.g., temperature, current and/or voltage.

(12) FIG. 2 shows by way of example one preferred sequence of the method in accordance with the invention for reducing the thermal loading of a controllable switching element SE of an electric fuse SI as illustrated by way of example in FIG. 1.

(13) The method begins with an initialization or calibration step 100. The calibration step 100 involves determining, based on the a thermal model of the switching element SE, such as a “Cauer” network or “Foster” network, first predefined values for a switched-on duration of the switching element SE and/or a switch-off current and for a switched-off duration of the switching element SE as starting values, where the predefined drive period can be formed, for example, by the first predefined values or starting values for the switched-on duration and the switched-off duration.

(14) By way of example, specific parameters and/or predefinitions of the switching element SE respectively used in the electric fuse SI, such as maximum permissible junction temperature, can be taken into consideration here. Furthermore, the maximum possible current increase in the output current I.sub.A and/or a maximum permissible increase in the temperature of the switching element SE for the predefined drive period (e.g., maximum permissible temperature swing of the junction temperature within a switching cycle of the switching element SE) are/is derived from the thermal model. That is to say that in the calibration step 100, e.g., a maximum permissible temperature swing within the predefined drive period (e.g., of the junction temperature, of the switching element temperature) and/or a maximum permissible current increase di.sub.A(t)/dt in the output current I.sub.A are/is ascertained from the thermal model of the switching element SE for an unfavorable combination of the resistive portions R.sub.L and the capacitive portion C.sub.L of the load L. It is possible to derive therefrom, e.g., the first predefined values for the switched-on duration and/or a switch-off current and for a switched-off duration of the switching element SE and the predefined signal VS generated by the evaluation unit AW. The predefined signal VS (based on the first predefined values) is then forwarded to the drive unit AE and the first drive signal AS for the switching element SE is generated therefrom, e.g., as a ramped drive signal AS.

(15) In a switch-on step 101, the switching element SE is switched on in a manner driven by the drive signal AS and the switching-on process is started. Here, the switching element SE is driven by the drive signal AS such that an increase in the temperature of the switching element SE remains as small as possible or the predefined maximum permissible increase in the temperature of the switching element SE is as far as possible not reached or not exceeded. Care is taken primarily to ensure that in particular a temporal profile of the junction temperature of the switching element SE does not exceed a critical value of the junction temperature and damage to the switching element SE does not occur. To that end, with the drive signal AS, for example, the increase in the temperature of the switching element within a drive period can be limited directly or indirectly by a limitation of the output current I.sub.A flowing into the load L or of the corresponding current increase di.sub.A(t)/dt during the switching on of the load L. In parallel with the switch-on step 101, in a measurement step 102 with the aid of sensor or measuring devices T.sub.SE, A, V, for example, at least one of the temporal profiles of output current I.sub.A and/or output voltage U.sub.A and/or temperature of the switching element SE is ascertained by the monitoring unit UE.

(16) On the basis of the temporal profiles respectively ascertained in the measurement step 102 for output current I.sub.A and/or output voltage U.sub.A and/or temperature of the switching element SE, in an ascertaining step 103, e.g., the evaluation unit AW then determines new predefined values for the switched-on duration of the switching element SE and/or the switch-off current and the switched-off duration of the switching element SE, where a duty ratio between switched-on duration and switched-off duration is thus adapted. In this regard, it is possible, e.g., to lengthen the switched-on duration via a corresponding new predefined value for the switched-on duration and/or the switch-off current to an extent to which the switched-off duration is reduced by a new predefined value for the switched-off duration. Furthermore, care is taken to ensure that the predefined maximum permissible increase in the temperature of the switching element SE within the drive period is at least not reached or is complied with.

(17) To that end, for example, the temporal profiles respectively ascertained for output current I.sub.A and/or output voltage U.sub.A and/or temperature of the switching element SE can be used directly or in averaged form. The individual temporal profiles ascertained for output current I.sub.A and/or output voltage U.sub.A and/or temperature of the switching element SE can be averaged for example mathematically (e.g., by forming the respective arithmetic mean or by forming root-mean-square values) or alternatively via filtering (for example, using low-pass or high-pass filters of arbitrary order) or by means of exclusion or selection of ascertained values from the respective temporal profile.

(18) Furthermore, the measurement step 102 and the ascertaining step 103 are performed in parallel with or largely independently of other steps for performing the method in accordance with the invention. A predefinition checking step 109 then involves checking whether new predefined values for the switched-on duration and/or the switch-off current and the switched-off duration have been determined by the evaluation unit AW.

(19) A termination checking step 104 involves checking whether the ascertained temporal profile of the output voltage U.sub.A has reached a value at which, with respect to the input voltage U.sub.E of the electronic fuse SI, a predefinable difference is undershot. That is, a check is made, for example, to establish whether a difference between input voltage U.sub.E and output voltage U.sub.A (e.g., an average value from the temporal profile ascertained) undershoots a predefinable tolerance value. Alternatively, the termination checking step 104 can also involve checking whether a predefinable maximum continuous current I.sub.L is undershot by the ascertained temporal profile of the output current I.sub.A. An average value from the ascertained temporal profile of the output current I.sub.A, for example, can be used in this case, too. If either the temporal profile of the output voltage U.sub.A reaches a value at which, with respect to the input voltage U.sub.E of the electronic fuse SI, a predefinable difference is undershot or a predefinable maximum continuous current I.sub.L is undershot by the temporal profile of the output current I.sub.A, then the method in accordance with the invention is terminated or ended in an end step 105. The termination checking step 104 can be performed by the evaluation unit AW, for example. The temporal profiles of output voltage U.sub.A and/or output current I.sub.A can be determined in the measurement step 102, for example.

(20) If the termination conditions predefined in the termination checking step 104 are not met, then the switching element SE remains switched on until a switching checking step 106 establishes that the predefined value of the switched-on duration (e.g. the starting value for the switched-on duration of the switching element SE in the case of a first switching cycle of the switching element SE) is at least reached. Alternatively or additionally, the switching checking step 106 can also check whether the predefined value or, in the first switching cycle of the switching element SE, the predefined starting value of the switch-off current is at least reached or exceeded by the respective output current I.sub.A presently ascertained in the measurement step 102. The switching checking step 106 can be performed by the evaluation unit AW, for example.

(21) The switching checking step 106 can alternatively or additionally check whether the predefined maximum permissible increase in the temperature of the switching element SE has been reached or exceeded in the present switching cycle of the switching element SE or in the present drive period of the drive signal AS. The switching element SE can then be switched off. The present increase in the temperature of the switching element SE can for example be determined based on the profile of the temperature of the switching element SE as ascertained in the measurement step 102 or be calculated or estimated, e.g., based on the temporal profile of the output current I.sub.A flowing into the load L as ascertained in the measurement step 102.

(22) Furthermore, in the switching checking step 106, alternatively or additionally, the temporal profile of the temperature of the switching element SE as ascertained in the measurement step 102 can be compared with a predefinable limit temperature for the switching element SE. The switching checking step 106 then checks whether the predefinable limit temperature is at least reached or exceeded by the ascertained temperature profile. The predefinable limit temperature for the switching element SE can be derived from the thermal model of the switching element SE, for example.

(23) If the switching checking step 106 establishes that either the predefined value of the switched-on duration or the predefined value of the switch-off current has been reached or exceeded, then the switching element SE is switched off in a switch-off step 107. Additionally or alternatively, reaching or exceeding the predefined maximum permissible increase in the temperature of the switching element SE within a drive period and/or reaching the predefinable limit temperature by the ascertained temporal profile of the temperature at the switching element SE can also result in the switch-off step 107 being performed or in the switching element SE being switched off.

(24) The switching element SE then remains switched off until the predefined value of the switched-off duration or, in the first switching cycle, the starting value of the switched-off duration is at least reached or exceeded. The reaching of the predefined switched-off duration is checked in a switch-off checking step 108. If the predefined switched-off duration is reached, then the predefinition checking step 109 can ascertain whether new predefined values for the switched-on duration and/or the switch-off current and the switched-off duration have been determined in the ascertaining step 103, e.g., by the evaluation unit AW based on the ascertained temporal profiles of output current I.sub.A and output voltage U.sub.A and/or the temperature of the switching element SE.

(25) If no new predefined values have been ascertained, then the previous predefined values continue to be used at least for the next switching cycle of the switching element SE or pass of the method. The switching element SE is then switched on again in the switch-on step 101, wherein the predefined values and thus the drive signal AS remain unchanged.

(26) If new predefined values for the switched-on duration and/or the switch-off current and the switched-off duration are present, then the previous predefined values are replaced by the new predefined values in an exchange step 110 and are used for the predefined signal VS or output with the latter. That is, the predefined signal VS is adapted, e.g., by the evaluation unit AW based on the new predefined values and is converted into a new or adapted drive signal AS for the switching element SE via the drive unit. The switching element SE is then switched on with the adapted drive signal AS in the switch-on step 101. As a result, for example, an increase in the temperature of the switching element SE within the drive period, in particular during the new switched-on duration, can again be kept below the predefined maximum permissible increase in temperature and/or a possible current increase di.sub.A(t)/dt through the switching element SE can be altered or regulated (as illustrated by way of example in FIG. 3).

(27) Alternatively, the predefinition checking step 109 can also be performed before the switch-off checking step 108. That is, the predefinition checking step 109 and, if appropriate, if new predefined values for switched-on duration and/or switch-off current and switched-off duration of the switching element SE are present, the exchange step 110 are performed before the check as to whether the predefined value of the switched-off duration of the switching element SE has been reached in the switch-off checking step 108.

(28) The method in accordance with the invention is then performed until the termination checking step 104 establishes that one of the termination conditions is met, and is ended with the end step 105, where the switching element SE remains operationally switched on.

(29) FIG. 3 illustrates by way of example and schematically a graphical plot of the temporal profile of the drive signal AS for the switching element SE and corresponding temporal profiles of the output current I.sub.A, the output voltage U.sub.A and the temperature T.sub.J of the switching element SE during the sequence of a plurality of repetitions of the method in accordance with the invention until termination or ending by the termination checking step 104 or the end step 105. Here, the illustration shows by way of example in a topmost diagram the temporal profile of the drive signal AS, in a first middle diagram the temporal profile of the output current I.sub.A, in a second middle diagram the temporal profile of the output voltage U.sub.A and in a bottommost diagram the temporal profile of a temperature T.sub.J of the switching element SE, in particular of a junction temperature T.sub.J. Here, time t is plotted on the respective x-axis and the drive signal AS for the switching element SE and a corresponding profile of the output current I.sub.A and of the output voltage U.sub.A and of the temperature T.sub.J of the switching element SE are respectively plotted on the respective y-axis. The switching element SE undergoes, for example, passes of the method in accordance with the invention or switching cycles S1 to Sn with a predefined, largely constant drive period (consisting of switched-on duration ed1, . . . , edn and switched-off duration ad1, . . . , adn. A switching cycle or the predefined drive period can have for example a duration of from 100 μs to 10 ms) ideally a duration of 1 ms.

(30) The calibration step 100 involves predefining for the method, based on the thermal model of the switching element SE, a first predefined value ed1 for the switched-on duration, a first predefined value ad1 for the switched-off duration and/or a first predefined value I.sub.V1 for the switch-off current, which govern compliance with a maximum permissible increase in the temperature of the switching element SE per drive period, where the increase is likewise determined in the calibration step 100. In the switch-on step 101, the switching element is switched on at a first point in time t0 via the drive signal AS and a first switching cycle S1 is thus started. For this purpose, the drive signal AS can have a ramped waveform. Here, an increase in the temperature of the switching element SE is limited, either directly or indirectly via a limitation of the current increase di.sub.A(t)/dt in the output current I.sub.A through the switching element SE, as illustrated in the first middle diagram for example in a temporal profile of the output current I.sub.A. That is, the current through the switching element SE or the output current I.sub.A increases proportionally to or in a manner regulated by the drive signal AS.

(31) The capacitive portion C.sub.L of the load L is charged by the energy transferred via the switching element SE, an increase in the output voltage U.sub.A as illustrated in the second middle diagram is also effected. The temperature T.sub.J of the switching element SE (as illustrated in the bottommost diagram) and an energy loss converted in the switching element SE also increase analogously to the increase in the output current I.sub.A. Here, care is taken to ensure that the increase in the temperature T.sub.J or the temperature increase turns out to be significantly smaller in comparison with a maximum permissible temperature T.sub.J,max and the predefined maximum permissible increase in the temperature of the switching element SE is complied with.

(32) If it is established at a second point in time t1 in the switching checking step 106 of the first switching cycle S1 that either the first predefined value ed1 for the switched-on duration or the first predefined value I.sub.V1 for the switch-off current is reached or exceeded, then the switching element SE is switched off by the drive signal AS in the switch-off step 107. That is, the drive signal AS is ended as illustrated in the topmost diagram, as a result of which, as illustrated in the first middle diagram, the output current I.sub.A through the switching element goes to a value 0. The output voltage U.sub.A decreases, as illustrated in the second middle diagram, because after the switching element SE has been switched off, the capacitive portion C.sub.L of the load L is discharged again, for example, through the resistive portion R.sub.L of the load L. Furthermore, the temperature T.sub.J of the switching element SE also decreases (as illustrated in the bottommost diagram) because, during the switched-off duration ad1, heating of the switching element SE that arose during the switched-on duration ed1 is at least partly emitted, e.g., to the surroundings, such as housing and subsequently to cooling devices (e.g., heat sink, or copper surfaces).

(33) The charging of the capacitive portion C.sub.L of the load or the increase in the output voltage U.sub.A has the consequence that less energy loss is converted in the switching element SE. That is, the predefined values for switched-on duration ed1, edn and/or switch-off current I.sub.V1, . . . , I.sub.Vn and switched-off duration ad1, . . . , adn can be adapted for at least one following switching cycle S2, . . . , Sn and thus the drive signal AS. At least one of the temporal profiles of output current I.sub.A and/or output voltage U.sub.A and/or temperature T.sub.J of the switching element SE is used for this purpose. New predefined values can be calculated by the evaluation unit AW such that a duty ratio between switched-on duration and switched-off duration of the switching element SE is adapted (e.g., reduced), where the predefined maximum permissible increase in the temperature of the switching element SE within the drive period is still complied with or not reached or exceeded.

(34) Once the switch-off checking step 108 has established that the first predefined value ad1 of the switched-off duration has been reached or exceeded, at a third point in time t2, the switching element SE is switched on by the switch-on step 101 for a second switching cycle S2. The predefinition checking step 109 has established that, e.g., as yet no new predefined values for ed1, edn and/or switch-off current I.sub.V1, . . . , I.sub.Vn, and switched-off duration ad1, . . . , adn are present or have been calculated by the evaluation unit AW. As a result, the first predefined values ed1, ad1, I.sub.V1 continue to be used for the second switching cycle S2. That is, as illustrated in the topmost diagram, the profile of the drive signal AS in the second switching cycle S2 corresponds to the profile in the first switching cycle S1. It is once again evident from the first middle diagram that the output current I.sub.A likewise increases proportionally to the drive signal AS. However, as illustrated in the second middle diagram, the output voltage U.sub.A increases further on account of the further charging of the capacitive portion C.sub.L of the load L in the second switching cycle S2. It is evident from the bottommost diagram that the temperature T.sub.J of the switching element SE also increases further.

(35) If, at a fourth point in time t3, the switching checking step 106 of the second switching cycle S2 establishes that either the first predefined value ed1 for the switched-on duration or the first predefined value I.sub.V1 for the switch-off current is reached or exceeded, then the switching element SE is switched off again by the drive signal AS in the switch-off step 107, until the first predefined value ad1 of the switched-off duration is reached at a fifth point in time t4. The predefinition checking step 109 has then established that new predefined values ed2, I.sub.V2, ad2 for switched-on duration and/or switch-off current and switched-off duration are present or have been calculated by the evaluation unit AW, for example. As a result, the new predefined values ed2, ad2, I.sub.V2 are adopted for a third switching cycle S3 in the exchange step 110.

(36) On the basis of these new predefined values ed2, ad2, I.sub.V2, a new predefined signal VS and thus a new drive signal AS (as illustrated by way of example in the topmost diagram for the third switching cycle S3) are then generated. With the new drive signal AS, the switching element SE is then switched on again in the switch-on step 101. The switching element SE is switched off again in the switch-off step 107 at a sixth point in time t5 (upon the new predefined value ed2 of the switched-on duration being reached or upon the new predefined value I.sub.V2 of the switch-off current being reached) until the new predefined value ad2 of the switched-off duration is reached. Here, it is evident that in the third switching cycle S3, by virtue of the new predefined values, the switched-on duration ed2 has been lengthened to an extent to which the switched-off duration ad2 has been shortened, where the drive period or the duration of the third switching cycle S3 of the switching element SE has remained approximately the same.

(37) The method is performed until, e.g., in an n-th switching cycle of the switching element SE, at a point in time tn, the termination checking step 104 establishes that the ascertained temporal profile of the output voltage U.sub.A has reached a value at which, with respect to the input voltage U.sub.E of the electronic fuse SI, a predefinable difference is undershot. That is, an output voltage U.sub.A or U.sub.L corresponding approximately to the input voltage U.sub.E of the electrical fuse SI is established. Alternatively, the termination checking step 104 can also check whether a predefinable maximum continuous current I.sub.L is reached or undershot by the ascertain temporal profile of the output current I.sub.A. Here, the established output current I.sub.A or I.sub.L corresponds to the established output voltage U.sub.L divided by the resistive portion R.sub.L of the load. The capacitive portion C.sub.L of the load L is fully charged at the point in time tn. If either the temporal profile of the output voltage U.sub.A reaches a value at which, with respect to the input voltage U.sub.E of the electronic fuse SI, a predefinable difference is undershot or a predefinable maximum continuous current I.sub.L is reached or undershot by the temporal profile of the output current I.sub.A, then the method according to in accordance with the invention is terminated or ended in an end step 105 and the switching element SE remains switched on for ongoing operation. It is evident from the bottommost diagram that an approximately constant end temperature T.sub.E for ongoing operation is established at the switching element SE at the point in time tn. The end temperature T.sub.E is usually below a maximum permissible temperature T.sub.J,max, which can be predefined, e.g., switching-element-specifically or by a triggering temperature of an optional additional protection device SV.

(38) Thus, while there have been shown, described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the methods described and the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.