WO2014172734A1

WO2014172734A1 - Operating circuit for leds

Info

Publication number: WO2014172734A1
Application number: PCT/AT2014/000092
Authority: WO
Inventors: Philip JERMYN
Original assignee: Tridonic Gmbh & Co Kg
Priority date: 2013-04-26
Filing date: 2014-04-28
Publication date: 2014-10-30
Also published as: AT15534U1; DE102013207718A1

Abstract

The invention relates to a method for operating an LED series using a switch controller (81) controlled by a control unit (80), in order to generate a current for the LED series, said switch controller (81) being designed for the constant operation of the LED series, preferably at a constant current. In order to dim the LED series, the input side of the switch controller (81) is supplied with a current such that the functionality of the switch controller (81) is switched off.

Description

The present invention relates to a circuit and to a method for operating light sources, in particular light-emitting diodes (LEDs) by means of, for example, switching regulators for providing an operating current or a. Operating voltage for the LEDs. More particularly, the invention relates to circuits or converter modules that enable dimmable operation of LEDs that are dimmable by a phase dimmer (triac). Triacs are still widely used as an incandescent dimming infrastructure.

In principle, it is already known to use switching regulators, such as buck converters, for driving an LED track having the LEDs. In this case, a control unit controls a clocked semiconductor power switch, by means of which in the

switched-on state, a coil is magnetized. The energy built up in the coil discharges in the coil

switched off state of the switch via the LED ^■ route.

Typically, such buck converters will be so

operated that the the LED track available

adjusted current to a constant value. A dimming of the LED track is therefore not achievable.

Therefore, an object of the invention is to provide a circuit that enables dimming of an LED track, especially in the case of an LED track operated by, for example, a buck converter, such as in the case of a LED retrofit lamp.

This problem is solved according to the independent claims. Further aspects of the invention are dealt with in the dependent claims.

In a first aspect, the invention proposes a method for operating an LED route based on a switching controller controlled by a control unit for generating a current for the LED route. Of the

Switching regulator is designed for a constant operation of the LED route with preferably constant current. The switching regulator is dimmed for dimming the LED track

supplied on the input side with such a current that the functionality of the switching regulator is turned off.

Preferably, for dimming the LED track the

Supply current for the switching regulator reduced until this supply current is no longer sufficient, the

To operate switching regulator such that the current for the LED track remains constant.

Preferably, the switching regulator is such for a

constant operation of the LED track configured that the current through the LED track is kept constant as long as the switching regulator receives sufficient power at its input. In a further aspect, the invention proposes a circuit for operating an LED track, comprising a switching controller controlled by a control unit for constant operation of the LED track with preferably constant current. The circuit includes a converter for providing a supply current for the

Switching regulator. To dim the LED path, the converter supplies the switching regulator (81) with such a supply current that the functionality of the

Switching regulator is turned off.

Preferably, the dimming of the LED path of the supply provided by the converter supply current is reduced until this supply current is no longer sufficient to operate the switching regulator such that the current for the LED route remains constant.

Preferably, the switching regulator is designed for a constant operation of the LED track such that the current through the LED track is kept constant, as long as the switching regulator receives sufficient current at its input, in particular above a threshold value.

When the functionality of the switching regulator is turned off, the LED path is preferably dimmed by the converter changing or reducing the supply current accordingly.

Preferably, the control unit is designed to ensure a constant operation, preferably with a constant current of the LED track.

Preferably, the switching regulator has an as

Energy storage serving coil and a switch for demagnetizing the coil and on. The functionality of the switching regulator is switched off in such a way that the switch no longer upsets and demagnetizes the coil. Preferably, the switching regulator is designed as a down converter.

Preferably, the circuit has a

Voltage limiting circuit to limit the supply voltage provided to the switching regulator.

If the supply voltage is too high, the voltage limiting circuit preferably ensures that an electrical parameter of the converter is reduced or that a measurement of an electrical parameter of the converter is falsified.

Preferably, the converter is designed in the form of a flyback converter. Preferably, the electrical characteristic of the converter is the current through the primary winding of the converter.

Preferably, the voltage limiting circuit comprises a zener diode disposed at the output of the converter.

Preferably, the voltage limiting circuit in series with the Zener diode comprises an optical transmitter of an opto-coupler. With a conductive Zener diode, the optical receiver of the optocoupler ensures that a measurement of an electrical parameter of the converter is falsified.

Preferably, the switching regulator and the LED track are arranged in a LED retrofit lamp.

The invention can be used in particular in the field of so-called. Retrofit LED lamps, which come as a replacement, for example, incandescent or halogen lamps used. Retrofit lamps accordingly have connecting sockets with which they are in known lamp sockets

introduced, e.g. screwed or plugged in, can be.

These aspects relate both to one

inventive method as well as on a

inventive circuit. The invention provides that by means of a converter or a drive circuit for dimming in a

Constant current mode is switched, and more specifically, the power supply for the buck converter is reduced until the product of the 12 volts

Supply and the power provided is no longer sufficient to operate the buck converter so that the LED track is constantly supplied. Thus, when operating below a predetermined current threshold, the buck converter operation is interrupted and the buck converter switch is turned on continuously. Once through this

Measure, ie in particular by greatly reducing the power supply, the functionality of the Buck converter is turned off, the LED track behaves as if it were essentially directly, ie without down converter, connected to the 12 volt terminals of the converter. By correspondingly changing the power supply, the LED track can then be dimmed. The invention is therefore that LED retrofit lamps, which have a down converter for a constant operation of the LED track, the input side are supplied with egg such a current that turns off the functionality of the buck converter that the connected R16 lamp behaves as if the LED track would be operated directly on the terminals.

Further aspects of the invention will be explained below with reference to the drawings. These show in particular in:

Fig. 1 shows an exemplary dimmer driver on the

Basis of a PFC controller with fixed output power;

Fig. 2 is an exemplary input current waves orm;

FIG. 3 shows an exemplary power factor corrected buckler; FIG.

Fig. 4 is an existing PFC model;

Fig. 5 shows an example fixed driver

Output power with PFC;

Fig. 6 shows an example of a modification of

inventive circuit,

Fig. 7 shows a detailed illustration of a

Section of the circuit shown in Fig. 6,

8 shows an operating circuit for LEDs, for example for a LED retrofit lamp. Single-stage, fixed-output, single-ended lockout drive (IC) controllers with built-in power factor correction are known, but this kind of controller proves to be a simple and elegant dimmer solution with very good performance, as described below.

In particular, the invention is directed to providing a smooth and flicker-free dimming that can be achieved in conjunction with most dimmers, whereby the amount of parts and material lists can be reduced and which can function simultaneously with different ICs. In addition, no high-voltage electrolytic capacitor must be present.

To this end, the circuit present in the converter module for controlling the at least one LED may provide a power factor correction (PFC) function, e.g. In particular, the invention uses the high power factor of a PFC circuit When a flyback converter is used, the current / voltage on the secondary side of the flyback converter driving the at least one LED mirrors, reflected by phase angle or phase section at the input of the primary side of the converter dimming reflected.

There is the problem that the PFC circuit sometimes does not absorb enough power that the (triac) dimmer used for phase angle or phase dimming requires as a holding current. Bleed circuits are known to remedy this symptom and to ensure the required holding current through their loss of power. Usually, these bleed circuits are formed as tracks, which can be selectively operated or interrupted. The problem can be solved by using only passive components to fabricate the bleed circuits, and in particular an ohmic resistor RIO and a capacitor CIO as shown in the exemplary LED actuator circuit shown in FIG.

Furthermore, two additional earthing resistors Rl, R9 are connected to the input lines of the circuit, which are referred to as current / phase wire and zero line in Fig. 1, which is advantageous for the damping of the ring at the input of the circuit by an interaction of Phase control / triac dimmers with inductive identifier and a PFC circuit with capacitive switch is generated.

Fig. 1 shows an arrangement with additional passive components for attenuating a ringer Rl, R9 of the radio interference suppression (RFI) of the dimmer with switching capacity. This in turn ensures even, flicker-free dimming. Thus, Fig. 1 shows a circuit for a converter module with a fixed output power PFC controller. The PFC function circuit is realized by the use of a circuit integrated in the control unit, e.g. the flyback converter driver, realized with an integrated switch.

The circled in Fig. 1 components represent the additional components for damping the ringer Rl, R9 and for the production of the passive Bleeding RIO, CIO. However, the circuit shows very little complexity of the converter even using the latest fixed output control units. Furthermore, an auxiliary winding is provided, which is inductively coupled to the converter and in which current / voltage is generated as a function of the mains voltage of the LED actuator. This generated voltage is supplied to the control unit as a negative current / voltage. The generated voltage also indicates the current / voltage through a shunt resistor connected in series with a switch built into the control unit. The switch-off threshold is adaptively changed by the control unit, ie depending on the current state of the mains voltage, resulting in the PFC function. In order to simplify dimming operations as a function of a leading phase angle of the mains voltage, a feedback signal is fed back from a detection device, for example an auxiliary winding AUX of the converter, to the control unit. In particular, the current is measured by the detection means to determine the current in the converter, preferably the current through the primary side of a flyback converter. As a result, the control unit directly or indirectly determines the current / voltage, for example through the primary side of the flyback converter. This particular current / voltage represents the signal fed back to the controller. The controller may be an IC, ASIC or an icrocontroller. If the converter is designed, for example, as a flyback converter, the circuit of the converter module can provide an energy store (capacitor) on the secondary side of the converter, wherein the flyback converter is an inductive. Separation between on its primary side arranged components and arranged on the secondary side components. The auxiliary winding as shown for example in Fig. 5 can also be located on the secondary side.

A flyback converter with constant on-time (T _on ) could realize the PFC function even without a feedback supplied to the driver IC from the auxiliary winding. However, the power factor would not be so good. An example of a control unit used is the HVLED805 from ST Microelectronics.

A single stage, power factor corrected controller may also include a small DC bus capacitor in the range of about 100nF. Therefore, a flyback converter receives as its input non-smoothed, rectified mains Hal cycles, which can fall to 0V. The use of such an integrated driver circuit (IC) allows excellent dimming performance with most so-called leading or trailing edge (dimming) dimmers. The circuit usually fits on a single-sided board and, as the process works with virtually any fixed PFC controller, the designs can be easily adapted to applications that use the latest low-cost IC. While in Fig. 1, a possible output ripple is controlled only by an output capacitor, the simple structure of the circuits also leaves room for a large capacitor, with which the ripple can be kept below 40% to 60%, in particular 50%. As a control factor, the control unit generates current / voltage which is used to change the current of the flyback converter on its primary input side. In particular, a dimming operation depends on the phase gating when the peak current, namely, the current at the flyback switch is changed, and the peak current is controlled so that it is in principle proportional to the amplitude of the present input voltage.

Consequently, depending on the phase angle, a current characteristic could be generated, as shown by way of example in FIG. 2.

It should be noted that the converter module circuit can also be used with other converters, such as, e.g. Down converter structures. In addition, the control unit may have a power factor correction (PFC), but this is not mandatory. However, an improvement in the power factor of the converter module circuit is desirable, and therefore, a method for applying a power factor correction in any control unit will be described below, particularly for control units having an external peak current measuring resistor.

In particular, if a control unit is used only with source input (eg, NXP SSL21083, which has only one source input to the HVLED815, for example) to determine a current / voltage on the shunt resistor, another approach may be used. In Fig. 3, such Control unit shown in a construction with a buck converter.

At the source input of the control unit shown in Fig. 3, a relatively low impedance of e.g. about 1 to 5 Ω, preferably 2.7 Ω, connected. If a relatively high negative current / negative voltage were detected by the detector at this input, the power loss would be unduly high.

Thus, the power loss is limited by significantly reducing the voltage at the detector. This is achieved by reducing the turns of, for example, the auxiliary winding to only a few turns, for example 1 to 5 turns, in particular approximately two turns. As a result, the current / voltage at the detecting means can be reduced, and auc_h_.be.i, at a lower impedance. the power loss can be reduced.

Specifically, the control unit includes a control system having a current sense resistor for measuring a peak inductor current in the time when the main switch is turned on.

In a device without PFC, the switch is generally turned off when the voltage drop across the voltage measuring resistor reaches a predetermined value. The peak current flowing through the inductor represents a known amount of energy, and this, in conjunction with a known switching frequency, forms the basis for power output control or current control. The power factor associated with such a model tends to be poor, even if an energy storage capacitor is small. This is because the on-time can be relatively long when the mains voltage is low {during mains current values). The reason for this is that the inductor current takes longer to reach the required peak value. Consequently, the average input current is high during these times and low during the mains voltage spikes. This is exactly the opposite of the current waveform required for a high power factor.

To correct this, the detector may be added to the throttle. This is to provide a negative current / voltage while the main switch is turned on. This current / voltage is then applied to the CS input of the current sensor of the control unit via a second resistor. In Fig. 3, an example is shown in which the method has been applied to a cost-effective buck converter. Thus, Figure 3 shows a power factor corrected buck converter. The negative current / voltage provided by the auxiliary winding varies depending on the instantaneous line voltage. When the main switch is turned on, a current flows through the resistor R3, and due to the changing negative current / voltage, the current / voltage is increased when the input current / voltage has reached its peak and is turned on Mains voltage valleys strongly reduced. The currents through the resistor R3 conduct the current away from the measuring resistor R4 and thus increase the final peak current by an amount that depends on the input voltage at that time. The result is an input current that is largely constant over most of each half cycle of the line voltage, thus providing a greatly improved power factor.

Again, the resulting typical current waveform is shown in FIG. 2, in this case a typical power factor correction input current waveform.

Existing applications, however, require the current sense input of the controller to be disconnected from the actual current carrying terminal, as illustrated in FIG. 4, which is a typical example of an existing PFC model.

In Fig. 1, the measuring resistor R5 is not directly connected to the CS input of the current measurement of the control unit. Instead, the wire resistor R7 is connected. Thus, the impedance is sufficiently increased that the method can operate using the normal auxiliary winding present in a typical flyback converter. By providing a separate winding, as shown in Figure 3, with a small number of turns, the method can be generalized to work with any driver IC having an external current sense resistor.

This solution is particularly suitable for buck converter applications because the turns of the auxiliary winding can easily be wound around a normal drum inductor. In summary, the in Fig. 3 shown Established a very cost effective and widely applicable method of power factor correction that provides power factors of 80% to 90%, especially about 85%, and a total harmonic distortion (THD) of about 25-45%, in particular 30% to 40%. This structure can also be easily adapted to control units and can also be combined with the structure of Fig. 1. Another example of an inventive circuit is shown in FIG. 5, which shows a schematic diagram of a modified PFC model. While power factor correction can be achieved in several ways, one common technique is to contribute to the peak current sense circuit of the control unit. from a detector via an external resistor, as indicated above.

While the switching transistor of the converter is turned on, the output from the detector is negative and proportional to the mains input voltage at that time. The supply from the detector causes the peak current reached by the primary winding to be increased by an amount proportional to the instantaneous line voltage. The contribution to the peak current actually dominates with the result that the power consumed by the converter becomes approximately sinusoidal and provides a high power factor. In summary, the wiring to the control unit is changed to compensate for changes or variations in the line voltage that result in changes in the light output emitted by the at least one LED. For this reason, a Zener diode shown in Fig. 5 is provided in the feedback path from the detection means to the converter to the control unit to clamp the negative current / negative voltage ^■ fed back to the control unit to a maximum and / or predetermined value. Above this value, the voltage remains disconnected and thus constant and changes or fluctuations outside the clamping range are disregarded. Instead of a Zener diode, the clamping can also be simplified by built-in devices of the control unit and / or the software of the control unit.

This improves the control characteristics with respect to stability of the output current / voltage of the converter, taking into account a slight decrease of the power factor as described below.

A disadvantage of the method described can be seen in the fact that the converter output is dependent on the mains voltage. The output control is poor in the fluctuation of the input voltage. However, this can be improved by a control with only a minimal degradation of the power factor as set forth below.

As shown in Fig. 5, the original PFC was achieved by the resistor R8 connected directly between the auxiliary winding and the current measuring input of the control unit and the high voltage side of the resistor R7. Fig. 5 now shows some additional components in the illustrated box. In particular, a shunt reference U2 of the control unit (eg, 2.5V) clamps the voltage on the low voltage side of the controller Resistor R8 to a maximum size of a negative voltage, eg 1 to 5 V, in particular 2.5 V negative. This voltage is then applied to the current sense input of the driver IC via resistor R27. ^'

The power factor correction is still present, but the maximum amount is limited by the claimed voltage. Thus, increases in the mains voltage can not further increase the maximum peak current.

In connection with Figs. 6 to 8, a further circuit construction is shown, which is particularly suitable for the operation of LED retrofit lamps. Fig. 6 shows a modified circuit construction of a converter 60 for LEDs. This circuit construction is based on the variants described above. Shown is a converter in the form of a flyback converter or flyback converter. FIG. 7 shows a detailed illustration of a portion of the converter shown in FIG. 6.

FIG. 8 shows a typical structure of an operating circuit 82 for LEDs and in particular a MR16 LED retrofit lamp. Such LED retrofit lamps are typically powered by a 12 volt supply and have a connection socket (not shown) that can be incorporated into a corresponding socket for halogen MR16 lamps. Such LED retrofit lamps typically includes an input-side bridge circuit Nl ^λ consisting of four diodes for rectifying an AC voltage such as a mains voltage. A capacitor Cl ^Λ is the bridge circuit Nl downstream. The circuit according to Fig. 8 further comprises a switching regulator, in which the power supply of the LED track is ensured by means of at least one periodically operating electronic switch Ql ^x and at least one energy storage.

In the embodiment of Fig. 8, the switching regulator is configured as a buck converter 81 or Buck converter, alternatively the switching regulator is also designed e.g. could be configured as a boost converter or boost converter. The rectified voltage is applied to a series circuit consisting of the LED track, the energy store in the form of a coil LI * and the switch Ql * preferably in the form of a semiconductor power switch such. of a MOSFET.

The LED track comprises three LEDs connected in series. Alternatively, the LEDs may also be connected in parallel or according to a serial and parallel arrangement. The LEDs can be OLEDs. Furthermore, it may be, for example, monochromatic LEDs, color-converted white LEDs and / or RGB LED modules. A freewheeling diode Dl * is arranged parallel to the LED path and coil LI ^Λ . In parallel to the LED track, a capacitor C2 is connected.

In the switched-on state of the switch Ql ^v , a current flows through the LED track and the coil LI ^Λ . During this switch-on phase, therefore, the current through the coil LI ^{Λ increases} . During a subsequent freewheeling phase, ie in the off state of the switch Ql ^x , the energy stored in the coil LI ^λ discharges in the form of a current through the LED path. It is a control circuit 80 is provided which controls the switch Ql ^Λ high-frequency or as the manipulated variable of the control of the LED power and the LED current, the timing of the switch Ql ^Λ pretends. The control circuit 80 may be configured as a control and / or regulating circuit, in particular in order to keep the current constant through the LED path.

For performing the current control, e.g. between the switch Ql and ground, a measuring resistor may be provided for detecting the current through the coil Li. Alternatively or additionally, e.g. in the freewheeling path consisting of the diode Dl of the LED track and the coil LI another measuring device, such. a measuring resistor, be provided for measuring the current through the coil Li.

As long as the buck converter 81 receives sufficient power at its input, it keeps the current through the LED path constant. Accordingly, no dimming can take place.

Therefore, the invention provides that by means of the converter 60, the LED track can be dimmed. For this purpose, the operating circuit 82 is connected to the output A of the converter 60. At output A, a 12 volt supply for circuit 82 is provided.

The in Figs. 6, 7 illustrated flyback converter is based on an inductance in the form of a primary winding L3, which is connected in series with a switch 71. The switch 71 is preferably controlled by the control unit 70 according to the above embodiments. In the embodiment of Figs. 6, 7, the switch 71 is part of the control unit 70. Alternatively, the switch 71 may also be arranged outside the control unit 70. The primary winding L3 is connected to terminal D of the control unit

70 connected. In series with the primary winding L3 and the switch 71, a measuring resistor RH is provided. Preferably, the measuring resistor RH is arranged between the switch 71 and ground. The current sense terminal CS of the control unit 70 is used to return a signal that supplies the current through the converter 60, i.e., the signal S0. the current through the primary winding L3 or through the switch

71 reproduces. A resistor is provided between the terminal CS and the center between the switch 71 and the measuring resistor RH. The flyback converter or the switch 71 can then be operated in such a way that it is regulated to a desired primary-side current. The flyback converter can be operated by the control unit 71 in accordance with the above embodiments.

The control unit 70 is, for example, an LED driver HV LED 815 from ST Microelectronics as a control IC, which is designed as a dimmable driver for LED retrofit lamps. As already explained, the control unit 80 together with the down converter 81 tries to keep the current constant through the LED path, so that no dimming can take place. Therefore, the invention provides that the converter 60 is switched to a constant current mode for dimming. More specifically, the power supply to buck converter 81 is reduced until the product of the 12 volt supply and the power provided is no longer sufficient to operate the buck converter 81 such that the LED track is constantly supplied. Thus, when the buck converter 81 is operated below a predetermined current threshold, the operation of the buck converter 81 is interrupted and the switch Q1 of the buck converter 81 is turned on continuously, it is continuously turned on.

As soon as the functionality of the buck converter 81 is switched off by this measure (strong reduction of the power supply), the LED path behaves as if it were essentially directly, i. without down converter 81, would be connected to the 12 volt terminals A. By changing the power supply, the LED track can be dimmed.

In the converter 60 and in the supply circuit of Fig. 6, a voltage limiting circuit 61 is shown. The voltage limiting circuit 61 is preferably provided for the secondary side of the converter 60, on the other hand, as the supply of current to the load is reduced, the secondary side voltage, in particular the secondary side voltage at the output A, would increase.

The voltage limiting circuit 61 comprises a series circuit connected between the terminals of the output A of a Zener diode Z3, a resistor R29 and an optical transmitter 62 of an optocoupler. The optical receiver 63 of the optocoupler is connected between a supply terminal VCC of the control unit 70 and the st ommernungsanschluss CS. If not dimming, ie, if the dimmer is set to maximum brightness, the output current of the converter 60 is more than sufficient to drive the LED path. Thus, the output voltage at the output A will begin to increase until the Zener diode Z3 becomes conductive. As soon as the Zener diode 23 becomes conductive, the optical transmitter 62 of the optocoupler is activated. As a result, the optical receiver 63 of the optocoupler connects the supply terminal VCC to the sensor terminal CS.

This increases the voltage at the current detection interface CS. This is interpreted by the controller 70 as if the current through the switch 71 and through the primary winding were too high. The control unit 70 responds accordingly and reduces the current through the primary winding by appropriate control of the switch 71. Ultimately, this feedback through the voltage limiting circuit 61 results in the output voltage at the output A being limited to approximately 12V. For this, e.g. the zener diode has a zener voltage of 11V.

This feedback is advantageous in that the converter or IC is still operated normally, in which a high power factor correction can be maintained. The behavior of the triac dimmer remains stable. The solution according to the invention is also an inexpensive way of dimming e.g. a LED retrofit lamp. As a converter, a flyback converter can be provided, but also alternative topologies can be used.

Claims

Claims 1. A method of operating an LED route from a control unit (80)

Switching regulator (81) for generating a current for the LED path, wherein the switching regulator (81) is designed for a constant operation of the LED path with preferably constant current,

wherein for dimming the LED path of the switching regulator (81) is supplied on the input side with such a current that the functionality of the switching regulator (81)

is turned off.

2. The method according to claim 1,

wherein for dimming the LED path of the supply current for the switching regulator (81) is reduced until this supply current is no longer sufficient to operate the switching regulator (81) such that the current for the LED route remains constant.

3. The method according to claim 1 or 2,

wherein the switching regulator (81) is designed for a constant operation of the LED track such that the current through the LED track is kept constant as long as the switching regulator (81) receives sufficient power at its input.

4. A circuit for operating an LED track, comprising: - one controlled by a control unit (80)

Switching regulator (81) for a constant operation of the LED path with preferably constant current, a converter (60) for providing a

Supply current for the switching regulator (81),

wherein for attenuating the LED path, the converter (60) supplies the switching regulator (81) with such a supply current that the functionality of the switching regulator (81) is switched off.

5. A circuit according to claim 4,

wherein for dimming the LED path of the converter (60) made available supply current as long

is reduced until this supply current is no longer sufficient to operate the switching regulator (81) such that the current for the LED route remains constant.

6. A circuit according to any one of claims 4 to 5,

wherein the switching egier (81) is designed for a constant operation of the LED track such that the current through the LED track is kept constant as long as the switching regulator (81) at its input sufficient current, in particular above a threshold receives.

7. A circuit according to any one of claims 4 to 6,

wherein when the functionality of the switching regulator (81) is turned off, the LED path is dimmed by the converter (60) changing the supply current accordingly.

8. A circuit according to any one of claims 4 to 7,

wherein the control unit (80) is adapted to ensure a constant operation with preferably a constant current of the LED track.

9. A circuit according to any one of claims 4 to 8, wherein the switching regulator (81) has a coil (LI) serving as energy storage and a switch (Ql) for demagnetizing and demagnetizing the coil (LI),

wherein the functionality of the switching regulator (81) is turned off in such a way that the switch (Q1 *) no longer up-and-demagnetizes the coil (LI *).

10. A circuit according to any one of claims 4 to 9,

wherein the switching regulator (81) as a down converter

is designed.

11. A circuit according to any one of claims 4 to 10,

comprising a voltage limiting circuit (61) for limiting the supply voltage provided to the switching regulator (81).

12. A circuit according to claim 11,

where at too high supply voltage, the

Voltage limiting circuit (61) ensures that an electrical characteristic of the converter (60) is reduced or that a measurement of an electrical characteristic of the converter (60) is falsified.

13. A circuit according to claim 12,

wherein the converter (60) is designed in the form of a flyback converter, and

wherein the electrical characteristic of the converter (60) is the current through the primary winding of the converter (60).

14. A circuit according to any one of claims 11 to 13,

wherein the voltage limiting circuit (61) comprises a zener diode (Z3) arranged at the output (A) of the converter (60).

15. A circuit according to claim 14,

wherein the voltage limiting circuit (61) in series with the Zener diode (Z3) comprises an optical transmitter (62) of an opto-coupler,

wherein at Zener conductive diode (Z3) of the optical

Receiver (63) of the optocoupler ensures that a measurement of an electrical characteristic of the converter (60) is falsified.

16. A circuit according to any one of claims 4 to 15, wherein the switching regulator (81) and the LED track are arranged in a LED retrofit lamp.