AT14654U1 - LED operating circuit with start-up circuit - Google Patents

Abstract

The invention provides an operating circuit for driving an LED track (LS), comprising: a primary-side supplied with a supply voltage isolated clocked converter, in particular a flyback converter having a clocked switching element (S1) on its primary side, wherein connections for the LED circuit (LS) are supplied starting from the secondary side of the converter, a start-up circuit (AS), starting from which the clocked switching element is controlled control unit (S1) for the duration of a start-up phase, and a secondary side is arranged to be arranged, the clocked Actuate switching element (S1) in particular after the start-up phase and which is further adapted to disable the start-up circuit (AS) after the start-up phase.

Description

description

LED OPERATING CIRCUIT WITH INTERCONNECTION

The invention relates to an operating circuit for controlling at least one LED track having at least one LED. In particular, the operating circuit has a clocked converter, in particular a flyback converter (also referred to as isolated flyback converter) and a start-up circuit. The operating circuit is divided by an electrically insulating barrier into at least one primary side and one secondary side. The clocked converter is configured to transmit electrical energy from the primary side through inductive coupling to the secondary side of the operating circuit. A Steuerein¬heit is arranged on the secondary side of the operating circuit. The operating circuit is thus subdivided into two, in particular galvanically separated regions, and the clocked converter couples these regions separately.

The starting point of the invention is not published at the time of the present application German patent application 10 2012 215 481.7. In this application is already in Figs. 3 and 4 show an operating circuit with a secondary side control unit.

Fig. 1, which corresponds to the Fig. 3 from the above-mentioned German application, already shows in a block diagram a primary side with a DC or AC supplied operating circuit. If necessary, the alternating current is rectified by a rectifier. About a clocked with an actively controlled switching element S1 converter (flyback converter) secondary side, the LED track LS is supplied. The clocked converter has for this purpose a transformer T1, T11 for potential-separated energy transmission. It will also be appreciated that a control unit S, shown in Fig. 1 as " ASIC " is alternatively verwirklicht by an integrated circuit IC or a microcontroller on the secondary side is arranged.

The control unit S controls the switch of the clocked converter also potential-separated, in particular by inductive coupling by means of a second transformer T2. The control by the second transformer T2 is effected such that the clocked switching element S1 of the clocked converter, in particular a gate of a field-effect transistor (for example FET or MOSFET), is correspondingly driven. Thus, the control unit S can be arranged both Sekundär¬seitig and still drive the primary side arranged clocked switching element S1 of the clocked converter. Thus, the control unit S can adjust the clock or the tone time of the clocked converter. The second transformer T2 can also be used, for example, as a so-called "coreless transformer". (Air coil) or be designed as an optocoupler.

In order to supply the control unit S electrically in a start-up phase, it is to be ensured that a supply can also take place without direct electrical supply via the clocked converter. Providing a direct electrical supply to the control unit until such time as the clocked converter is operating properly, i.e. is operated with a certain clock, as shown in Fig. 1, carried over a Wider¬stand Rstanupi. Although this cuts through the potential separation, the resistance is dimensioned so that safety regulations (for example SELV regulations) are observed (SELV = safety extra low voltage or safety extra-low voltage, ie a small electrical voltage, due to their small height and insulation in comparison the circuit of higher voltage on the primary side offers special protection against electric shock and is so small that electrical body currents normally remain without consequences).

In the context of this SELV determination, a high-impedance non-potential-separated connection is permitted. For example, the resistance Rstartup1 can be dimensioned in the megohm range.

As soon as the control unit properly clocks the switch of the clocked converter, a voltage supply of the control unit is subsequently carried out via an inductive coupling, which is shown in FIG. 1 as an auxiliary winding T12 of the transformer T1, TL.

Further, in Fig. 2, which corresponds to the Fig. 4 from the above-mentioned German application, a capacitive start-up control of the control unit S shown. In this case, however, the transmission of the starting voltage or of the starting energy for the control unit S is effected capacitively via the capacitors Clnki and CLnk2. [0009] In the capacitive supply during the start-up phase, a defined change in the input voltage of the capacitor CLnki takes place through a time circuit T, which outputs at its output a variable with a predetermined frequency (adjustable by an external RC element, not shown). Typically, the signal output by the timing circuit T moves between the value of the supply voltage and the value zero. The timing circuit T thus converts a smoothed rectified supply voltage into a voltage which changes in a defined manner and which serves as the supply for the capacitor Clnki but also for the capacitor CLnk2.

In this case, the frequency of the timing circuit T and the capacitance of the capacitor CLnkiso be selected that sets a desired starting current to the control unit. The starting current typically has a value in the range of 10-100 μΑ. Correspondingly low, the frequency with which the timing circuit T supplies the capacitor Clnki is selected. An inductive transfer of the starting current from the primary side to the secondary side is also known. Alternatively, an optocoupler can also be used for the control (gate drive) of the clocked switching element S1. This is then supplied with power, for example, on its secondary side.

Based on the above-described prior art, the invention has now set itself the goal of a power supply of the secondary side and in particular a Bereitstel¬lung a supply voltage of the secondary side arranged steuergli¬chen and thereby costs for the circuit and the switching / Reduce power losses. In particular, an electrical supply to the secondary-side control unit should be made possible without the need for auxiliary windings or additional transformers. In particular, transformers that comply with the SELV regulations cause relatively high costs for a circuit.

The invention therefore provides a solution to this problem as claimed in the independent claims. Further developments of the invention are the subject of the dependent claims.

In a first aspect, the invention provides an operating circuit for driving a LED path, comprising: a potential-separated clocked converter supplied on the primary side with a supply voltage, in particular a flyback converter, which has a clocked switching element on its primary side wherein connections for the LED path are supplied starting from the secondary side of the converter, [0015] a primary-side start-up circuit from which the clocked switching element is driven for the duration of a start-up phase, and [0016] a secondary side arranged Control unit, which is set up to control the clocked switching element, in particular after the start-up phase, and which is further configured to deactivate the start-up circuit after the start-up phase. Preferably, no control unit is provided on the primary side of the converter for controlling the pulsed switching element, i. only on the secondary side of the converter is a control unit provided for controlling the clocked switching element.

The control unit can be supplied electrically from the secondary side of the converter.

For the control unit can be retrievable threshold, in particular changeable or programmable stored. The control unit can detect a supply voltage delivered starting from the secondary side of the converter, in particular by a first supply circuit, and deliver a signal to a driver circuit as a function thereof.

The driver circuit may be connected to the start-up circuit and in particular to a second supply circuit supplying the starting circuit.

The driver circuit may activate / deactivate a first switching element, and preferably alternatively or additionally, a second switching element.

The first switching element can deactivate the second supply circuit during its activation / deactivation, in particular by outputting a signal to a third switching element.

The second switching element can deactivate the start-up circuit during its activation / deactivation and, in particular, a reset input of the start-up circuit to ground.

For activating / deactivating the first switching element and / or the second switching element, the driver circuit may output an output signal whose amplitude lies in particular above a breakdown voltage of a zener diode.

The driver circuit can output the output signal if it is supplied by the Steuerein¬heit an input signal.

The zener diode may be connected to a gate terminal of the first switching element and / or the second switching element. Preferably, the first switching element and / or the second switching element can be activated / deactivated upon output of the output signal.

When activating / deactivating the first switching element, the third switching element can be deactivated / activated.

The driver circuit may receive the input signal from a control unit and transmit it by means of a transmission element, e.g. an opto-coupler, an inductive coupling, a capacitive coupling, and / or a resistor, transmitted from a secondary side to a primary side of the operating circuit and optionally filter and / or amplify.

The start-up circuit can electrically supply the control unit for the duration of the start-up phase.

The control unit can clock the switching element.

The starting circuit can be connected by means of a coupling element, e.g. an opto-coupler, an inductive coupling, a capacitive coupling and / or a resistor, be connected to the control unit across an electrically insulating barrier.

Between the connecting element and the control unit, a third supply circuit can be provided, which optionally filters, amplifies, rectifies and / or smoothes a signal output by the starting circuit.

The coupling element and / or the transmission element can be a high-impedance resistor, e.g. greater than 1 Ω.

The operating circuit can supply the secondary-side control unit after the start-up phase only from the secondary side of the converter.

In the start-up phase, supply to the secondary-side control unit can be effected exclusively by the start-up circuit.

The particular passive driver circuit may be provided for driving the switching element. When actuated by the control unit, the driver circuit can operate the clocked switching element, in particular the gate of a transistor, with a clock set by the control unit.

The converter can bridge the electrically insulating barrier of the operating circuit. The electrically insulating barrier can be a galvanically insulating barrier or a SELV barrier.

The control unit may deactivate the start-up circuit when the supply voltage supplied to the control unit exceeds the threshold value.

The start-up circuit may be a clock generator, a timer, timer / pulse generator, a Zeitschal¬tung and / or an oscilator.

In a further aspect, the invention provides an LED module or operating device for driving an LED track, comprising an operating circuit as described above and an LED track supplied therewith.

In yet another aspect, the invention provides a method of operating an LED link operating circuit, wherein a floating-type switched converter, in particular a flyback converter, has a clocked switching element on its primary side and is supplied with a supply voltage on the primary side in which terminals for the LED track are supplied from the secondary side of the converter, from a starting circuit, the clocked switching element is driven for the duration of a start-up phase, and wherein a secondary-side control unit controls the clocked switching element, in particular after the start-up phase and deactivates the start-up circuit after the start-up phase. In particular, no control unit is provided on the primary side of the converter for controlling the clocked switching element.

The invention will now be described with reference to the figures. In the drawings: Fig. 1 shows a first circuit arrangement according to the prior art.

Fig. 2 shows a second circuit arrangement according to the prior art.

Fig. 3 shows schematically a first embodiment of an inventive

Circuitry.

FIG. 4 schematically shows a drive according to the first embodiment from FIG. 1.

FIGS. 5a and 5b show by way of example a concrete embodiment of circuit parts as they can be used in the circuit arrangement according to the invention.

FIGS. 6a and 6b schematically show a second embodiment of a erfindungsge¬ MAESSEN circuit arrangement and a corresponding control.

Fig. 3 shows an example of a first embodiment of the invention. A switching element S1, which of course is e.g. may be formed as a switch or transistor, here is the clocked element of a clocked converter (flyback converter) with a transformer TR.

The invention now makes use of the fact that in this topology of a clocked converter the energy path of the clocked converter (transformer TR, switching element S1, output diode Dout and output capacitor Cout) can be used for the transmission of energy from the primary side of the operating circuit to the secondary side , In this case, a starting circuit AS (timer / timer or pulse generator) is provided on the primary side of the operating circuit, which activates the switching element S1 and, in particular, a gate of the switching element S1. By this control, the switching element S1 switches clocked on and off. Energy is transferred from the primary side to the secondary side of the operating circuit. A supply circuit V1 supplying the control unit SE is provided on the secondary side, which supplies the control unit SE and which in particular is connected downstream of the output diode Dout.

If the control unit SE is adequately supplied, that is, in particular if its supply voltage exceeds a predetermined threshold value, then the control unit SE assumes control of the switching element S1 on the primary side.

For this purpose, as shown in Fig. 4, a driver circuit DRV is provided which receives Steuersig¬nale of the control unit SE on the secondary side and transmits to the primary side, spielsweise by means of inductive or capacitive coupling. Preferably, the driver circuit DRV also ensures that the transmitted signals are aufbere¬tet on the primary side. For example, as a driver circuit DRV a so-called E-driver circuit (E-drive) or a so-called C-driver circuit (C-Drive) can be used.

However, the transmission of the control signals for driving the switching element S1 on the primary side may result in that the driving of the switching element S1 by means of the starting circuit AS comes into conflict with the control by the control unit SE.

In order to avoid this, the driver circuit DRV transmits a switch-off signal to the start-up circuit AS as well as to a second supply circuit V2 supplying the start-up circuit AS. This is shown schematically in FIG. 4. The control unit SE transmits a control signal DRV | N on the secondary side to the driver circuit DRV, shown as an example as a result of voltage pulses. The at least one output signal DRVOUt can then be supplied to the second supply circuit V2 (1) in order to deactivate / activate it. At the same time, the at least one output signal DRVout for activating / deactivating the startup circuit AS (2) and used to clock the switching element S1 (3).

As start-up circuit AS can in particular a cheap integrated circuit (IC, ASIC, ...) are used, which is known for example as a 555 timer. As a second Versorgungsschal¬tung V2, for example, a Zener diode can be used with a series resistor. In order to deactivate or disconnect the supply of the starter circuit AS, the connection between the zener diode and the series resistor must be disconnected.

A schematic yet more detailed primary-side arrangement of the starting circuit AS and the second supply circuit V2 is shown in FIGS. 5a and 5b. It should be noted that although the inventive solution dispenses with the provision of an additional supply of the secondary-side control unit SE from the primary side, two supply circuits are used.

However, this is nevertheless advantageous, since a very favorable and possibly inefficient supply circuit V2 can be used on the primary side since, as explained later, this supply circuit V2 is used only during a start-up phase of the operating circuit. On the other hand, however, there are the high costs of voltage supply on the primary side and transmission via the galvanically insulating barrier (SELV barrier) for supplying the secondary side, since such a transmission requires at least one inert, SELV-standard transformer.

The operation of the in Figs. 5A, 5B is as follows: [0058] The circuit parts are supplied by way of example on the basis of a rectified AC voltage. If the driver circuit DRV outputs a signal DRVout, that is to say if on the secondary side a corresponding control by the control unit SE takes place with an input signal DRVin which exceeds the breakdown voltage of a Zener diode D5 (shown in FIG. 5b as a pulsed voltage signal with amplitude 9V), the voltage on transistor Q1 rises (shown in FIG. 5b as a voltage rise to, for example, 2.1 V at the gate of transistor Q1).

The transistor Q1 is activated, i. E. turned on when the breakdown voltage of the Zener diode is reached or exceeded. After activating transistor Q1, the current flowing through resistors R1, R2 is dissipated directly to ground, so that no more current flows across a resistor R8, which is connected in series with resistors R1 and R2. The series connection of the resistors R1, R2 and R8 is connected on the one hand to the output of a rectifier and on the other hand to ground.

The activation of the transistor Q1 reduces the voltage at a gate of a second transistor Q2, as a result of which the second transistor Q2 is deactivated, that is to say it is switched off. As a result, the second supply circuit V2 is deactivated, which is exemplarily formed by a series connection of the resistors R3-R6 to the second transistor Q2 and a Zener diode D2 and is connected in parallel with the series connection of the resistors R1, R2 and R8. By deactivating the transistor Q2, therefore, the second supply circuit V2 is switched off. As a result, the supply voltage Vcc of the starter circuit AS decreases slowly, depending on the dimensioning of a buffer capacitor C2, whereby a shutdown of the starting circuit AS takes place. The buffer capacitor C2 is otherwise fed by the second supply circuit V2 when the transistor Q2 is active.

In order to achieve a fast turn-off of the starter circuit AS, upon reaching the breakdown voltage of the zener diode D5, not only the transistor Q1 is activated, i. E. is turned on, but also a third transistor Q3 whereby the voltage at a RESET input of the starting circuit AS suddenly drops, whereby the starting circuit AS is immediately deactivated.

It should be noted at this point that the diode D6 is advantageous because when the starting circuit AS is deactivated, the output OUT of the starting circuit AS is not in a so-called tri-state. In the absence of the diode D6, signals output by the driver circuit DRV could be diverted via the ground of the starter circuit AS instead of being supplied to the clocked switching element S1 of the clocked converter.

In Figs. Figures 6A and 6B show an alternative embodiment of the invention.

As illustrated in FIG. 6A, the starting circuit AS is provided on the primary side of the driving circuit. Starting from the starting circuit AS, a supply voltage can be transmitted to the secondary side by means of a coupling element KE, for example by means of an inductive or capacitive coupling or a transmission by means of a suitable resistor. This means that in particular the signal generated by the starting circuit AS is transmitted via the SELV barrier or the electrically insulating barrier. In the case of transmission by means of a resistor, as already shown for the prior art, the galvanic isolation can be broken again the relevant provisions, for example the SELV provisions, are complied with.

A further supply circuit V3 is then provided on the secondary side, which in particular rectifies the signal transmitted by the coupling element KE, in particular one / of a pulsed voltage / current. This further supply circuit V3 generates a supply voltage for the control unit SE.

If a sufficiently high voltage, for example a voltage which is above a certain threshold value, is applied to the control unit SE, ie sufficient electrical power is generated by the starting circuit AS and transmitted by the coupling element KE, then the control unit becomes SE the driver circuit DRV by means of the signal DRV | Nansteuern to clock the primary-side switching element S1. The clocked converter then begins to operate and its output voltage increases.

As shown by way of example in FIG. 6b, the starting circuit AS generates a pulsed signal, whereby a voltage is output on the secondary side of the coupling element KE.

The further supply circuit V3 provides a correspondingly adapted, e.g. rectified voltage to the control unit SE. With sufficient supply to the control unit SE, e.g. When a threshold is reached by the supplied voltage, the control unit SE outputs a signal DRV | N to the driver circuit DRV. The driver circuit DRV generates on the primary side at least one output signal DRVOUt- The at least one output signal DRVOut can then be supplied to the second supply circuit V2 (1) in order to deactivate / activate it. At the same time, the at least one output signal DRVout for activating / deactivating the starting circuit AS (2) and for timing the switching element S1 (3) can be used. After the starting circuit AS clocks the switching element S1, the power transmitted by the clocked converter increases on the secondary side of the circuit arrangement and correspondingly also the voltage supplied by the supply circuit V1 to the control unit SE, which then serves to supply the control unit SE.

The voltage applied to the control unit SE supply voltage is thus provided in a start-up phase by the primary-side starting circuit AS. As shown in the figures, the supply circuit V1 may be arranged after an output diode Dout of the clocked converter to provide a supply voltage for the control unit SE.

The primary-side starting circuit AS and the second supply circuit V2 supplying it can then be deactivated, as already described for FIGS. 5A and 5B. In particular, as soon as the breakdown voltage of the zener diode D5 is reached, the starting circuit AS can be switched off by means of the transistors Q1 and Q2. Fast turn off occurs if the corresponding timing, e.g. As a 555 timer, again via the third transistor Q3 and the resistor Rreset- [0071] Consequently, according to the invention, in a start-up phase, the clocked switching element S1 of the clocked converter is operated starting from the starting circuit AS. If, on the secondary side, there is a sufficient voltage which is provided by the clocked converter and exceeds a certain threshold value, the control unit SE assumes the timing of the switching element S1. In one embodiment, during the start-up phase, i. until the threshold value is reached, a supply to the secondary-side control unit via the starting circuit AS (for example a pulse generator, an oscillator, etc.) is supplied. As soon as the corresponding threshold value is reached or exceeded, the secondary-side control unit SE assumes the timing of the primary-side converter switch S1. To this end, the secondary side circuit transmits a clock indicative signal to the primary side of the operating circuit. By this signal, the primary-side start-up circuit AS and, if necessary, a supply circuit supplying it is also deactivated.

The threshold value can also be based on the state of charge of an energy store, e.g. Obtain a capacitor through which sufficient energy can be stored on the secondary side to supply the control unit SE after being supplied by the means until the switching element S1 is clocked by the control unit SE as provided.

Claims (25)

Claims 1. Operating circuit for driving an LED track (LS), comprising: - a floating isolated transactivated converter, in particular a flyback converter, which has a clocked switching element (S1) on its primary side, wherein Terminals for the LED track (LS) are supplied starting from the secondary side of the converter, - a start-up circuit (AS), starting from the clocked switching element (S1) is driven for the duration of a start-up phase, and - a secondary-side control unit ( SE), which is set up to control the switched switching element (S1) in particular after the startup phase and which is further configured to deactivate the startup circuit (AS) after the startup phase, in particular only on the secondary side of the converter controlling a control unit ¬ tion of the clocked switching element (S1) is provided. 2. An operating circuit according to claim 1, wherein the control unit (SE) is electrically supplied from the secondary side of the converter. 3. Operating circuit according to claim 1 or 2, wherein for the control unit (SE) retrievable a threshold value, in particular changeable or programmable is stored, and / or wo¬bei the control unit (SE) is adapted, starting from the secondary side of the converter, in particular by to detect a supply voltage supplied to a first supply circuit (V1), and to supply a signal to a driver circuit (DRV) depending thereon. 4. Operating circuit according to claim 3, wherein the driver circuit (DRV) is connected to the start-up circuit (AS) and in particular to a start-up circuit (AS) supplying the second supply circuit (V2). 5. Operating circuit according to claim 3, wherein the driver circuit (DRV) a first switching element (Q1) and preferably, alternatively or additionally, a second switching element (Q2) is activated / deactivated. An operation circuit according to claim 4 and 5, wherein the first switching element (Q1), when activated / deactivated, deactivates the second supply circuit (V2), in particular by outputting a signal to a third switching element (Q3). 7. Operating circuit according to claim 5, wherein the second switching element (Q2) deactivates the start-up circuit (AS) during its Akti¬vierung / deactivation and in particular a reset input (RESET) of the starting circuit (AS) connects to ground. 8. operating circuit according to claim 5, wherein the driver circuit (DRV) for activating / deactivating the first switching element (Q1) and / or the second switching element (Q2) outputs an output signal (DRV0ut) whose amplitude is in particular above a breakdown voltage of a zener Diode (D5) is located. 9. Operating circuit according to claim 8, wherein the driver circuit (DRV) outputs the Ausgangssig¬nal (DRVout) when it is zu¬geführt by the control unit (SE) an input signal (DRV, N). 10. Operating circuit according to claim 8, wherein the Zener diode (D5) with a gate terminal of the first switching element (Q1) and / or the second switching element (Q2) is ver¬bunden, and wherein preferably the first switching element (Q1 ) and / or the second switching element (Q2) are activated / deactivated when the output signal (DRV0Ut) is output. 11. An operation circuit according to claim 6, wherein upon activation / deactivation of the first switching element (Q1) the third switching element (Q3) is deactivated / activated. 12. Operating circuit according to claim 9, wherein the driver circuit (DRV) receives the input signal (DRVin) from a control unit (SE) and this by means of a Übertragungsele¬ments, in particular an optocoupler, an inductive coupling, a capacitive coupling, and / or a resistor, transmits from the secondary side to a primary side of the operating circuit and optionally filters and / or amplified. 13. Operating circuit according to one of the preceding claims, wherein the starting circuit (AS) electrically supplies the control unit (SE) for the duration of the start-up phase. 14. Operating circuit according to one of the preceding claims, wherein the control unit (SE) the switching element (S1) clocks. 15. Operating circuit according to one of the preceding claims, wherein the starting circuit (AS) by means of a coupling element (KE), in particular an opto-coupler, an inductive coupling, a capacitive coupling and / or a resistor, across an electrically insulating barrier away with the control unit ( SE) is connected. 16. Operating circuit according to one of the preceding claims, wherein between the connection element (KE) and the control unit (SE) a third supply circuit (V3) is provided which optionally filters a signal output by the start-up circuit (AS), amplified, rectified and / or smoothed. 17. Operating circuit according to claim 12 and / or 15, wherein the transmission element and / or the coupling element (KE) is a high-impedance resistor, in particular greater than 1 Ω. 18. Operation circuit according to one of the preceding claims, wherein the Betriebsschaltungso is configured so that the secondary-side control unit (SE) is supplied after the start-up phase le¬diglich from the secondary side of the converter. 19. Operating circuit according to one of the preceding claims, wherein in the start-up phase, a supply of the secondary-side control unit (SE) exclusively by the start-up circuit (AS). 20. Operation circuit according to claim 3, wherein the particular passive driver circuit (DRV) for driving the switching element (S1) is provided and wherein the driver circuit (DRV) is adapted to, when actuated by the control unit (SE) the clocked switching element (S1) , in particular the gate of a transistor, with a clock set by the control unit (SE). 21. An operating circuit according to any one of the preceding claims, wherein the converter bridges the electrically insulating barrier of the operating circuit, and wherein the electrically insulating barrier is a galvanically insulating barrier or a SELV barrier. 22. Operating circuit according to one of the preceding claims, wherein the control unit (SE), the start-up circuit (AS) deactivated when the control unit (SE) supplied Versor¬gungsspannung exceeds the threshold. 23. Operation circuit according to one of the preceding claims, wherein the starting circuit (AS) is a clock generator, timer, pulser and / or oscillator. An LED module or operating device for operating an LED track (LS), comprising an operating circuit according to one of the preceding claims and a LED track (LS) supplied therewith. 25. A method for operating an operating circuit for an LED track (LS), wherein: - a potential-separated clocked converter, in particular a flyback converter having a clocked switching element (S1) on its primary side, and is supplied with a supply voltage on the primary side, terminals for the LED track (LS) aus¬hend supplied from the secondary side of the converter, starting from a primary-side starting circuit (AS) starting the clocked Schaltele¬ment (S1) is driven for the duration of a start-up phase, and wherein - a secondary side arranged control unit (SE) controls the clocked switching element (S1) in particular after the startup phase and the startup circuit (AS) is deactivated after the startup phase, wherein in particular only on the secondary side of the converter, a control unit for controlling the clocked switching element (S1) is provided. 4 sheets of drawings