WO2018077599A1

WO2018077599A1 - Step-down converter for a light-emitting diode

Info

Publication number: WO2018077599A1
Application number: PCT/EP2017/075619
Authority: WO
Inventors: Lukas Saccavini; Frank Lochmann
Original assignee: Tridonic Gmbh & Co Kg
Priority date: 2016-10-31
Filing date: 2017-10-09
Publication date: 2018-05-03
Also published as: AT17349U1; DE102016221398A1; EP3533291B1; EP3533291A1

Abstract

The present invention relates to a step-down converter (101) for a light-emitting diode (110), comprising a first switch (201, 205, 291) and a second switch (202, 206, 292) which is connected in series with the first switch (201, 205, 291) between a supply voltage connection (211) and earth (215). The step-down converter (101) also comprises a storage choke (212) which is connected in series with the first switch (201, 205, 291) between the supply voltage connection (211) and an output connection (213, 219). The step-down converter (101) also comprises the output connection (213, 219) which is designed to output a load current (702) to the light-emitting diode (110) on the basis of a choke current (701) through the storage choke (212). The step-down converter (101) also comprises a controller which is designed to operate the first switch (201, 205, 291) and the second switch (202, 206, 292) in the conductive state alternately and periodically depending on a dimming signal.

Description

Down converter for a light emitting diode

TECHNICAL FIELD Various examples of the invention generally relate to a down converter for a light emitting diode having a first switch and a second switch which are operated by a controller alternately and periodically in a conducting state. In particular, various examples of the invention relate to a buck converter in which the controller operates the first switch and the second switch in response to a dimming signal.

BACKGROUND

For operating light-emitting diodes, lights typically have an operating device. The operating device typically includes a buck converter, which is configured to lower a DC supply voltage in amplitude to operate the light emitting diode. In addition, the buck converter may be configured to alter the operation of the light emitting diode in response to a dimming signal indicating the desired brightness of the light. Conventional downconverters typically include a switch connected between a supply voltage terminal and an output terminal in series with a storage choke. During an on-time of the switch - i. During which of the switches is operated in the conducting state, a choke current flows through the storage choke, which is fed by the supply voltage, and energy is stored in the storage choke. During an off-time of the switch - during which the switch is operated in the non-conducting state - flows, a choke current, which is fed by the previously stored in the storage inductor energy.

In principle, different modes of operation of the buck converter are known. For example, in a mode called discontinuous mode, the inductor current may drop to zero during the off-time of the switch. In a mode called continuous mode, the inductor current does not decrease to zero during the off-time of the switch. In between there still exists the so-called borderline mode, which corresponds to the transition between the intermittent operation and the continuous operation.

For reference implementations of buck converters, it may be necessary to enable gap operation in response to the dimming signal. In particular, it may be necessary in connection with a low desired brightness of the luminaire to activate the lopsided operation with a particularly long off-time of the switch. Then the brightness of the luminaire is modulated with a comparatively low frequency. The LED is typically switched off in the meantime, i. the load current can drop to zero. Sometimes such operation of the luminaire is also referred to as pulse width modulation. This can have various negative effects on the environment of the luminaire: for example, interference with optical devices may occur.

In reference implementations of buck converters, it may further be necessary to switch between the lopsided operation and the boundary operation depending on the dimming signal. In particular, this may mean that, depending on the dimming signal, a particularly strong jump occurs in the frequency with which the switch is switched. This may require a complicated and expensive control technology. SUMMARY OF THE INVENTION

Therefore, there is a need for improved down-converters and techniques for down-converting a supply voltage for a light emitting diode. In particular, there is a need for such techniques that overcome at least some of the above disadvantages and limitations.

This object is solved by the features of the independent claims. The features of the dependent claims define embodiments. In one example, a down converter for a light emitting diode includes a first switch and a second switch. The second switch is connected between a supply voltage terminal and ground in series with the first switch. The buck converter also includes a storage choke. The storage inductor is connected in series with the first switch, for example between the supply voltage connection and an output connection. The output port is configured to be based on a throttle current to output a load current to the light emitting diode through the storage choke. The buck converter also includes a controller. The controller is arranged to alternately and periodically conduct the first switch and the second switch in response to a dimming signal in the conducting state.

In some examples, a buck converter having a first switch and a second switch according to the above example is also referred to as a synchronous converter.

Sometimes the first switch is also referred to as a high-side switch because it is on potential. Accordingly, the second switch is sometimes referred to as a low-side switch because it is located between potential and ground. For example, it would be possible for the first switch and / or the second switch to be implemented by a semiconductor switch element. Examples of such semiconductor switch elements include: a transistor; a bipolar transistor; a field effect transistor; a metal oxide field effect transistor; an insulated gate field effect transistor.

For example, one side of the storage choke may be connected to a point located between the first switch and the second switch. The second side of the storage throttle may be connected to the output port. The storage choke can be implemented as a coil with multiple windings. The storage choke can provide an inductance. Based on the law of induction, the voltage across the storage inductor (inductor voltage) can be equal to the inductance of the storage inductor multiplied by the time change of the inductor current. In other words, the storage choke can counteract particularly rapid changes in the throttle current

For example, the output terminal may include a smoothing capacitor that causes the load current that is output to the light emitting diode to correspond to a time average value of the inductor current. As a result, the inductor current can be smoothed and it can be a more uniform brightness of the LED can be achieved. The output terminal could, for example, continue to have a plug contact, solder contact, terminal contact, etc., to produce an electrical connection of the light emitting diode.

The controller can be implemented, for example, as an application-specific integrated circuit (ASIC) or as a microcontroller. The controller could also be implemented as an FPGA or processor. The control could also be at least partially by an analog circuit be implemented. The controller could receive the dimming signal, for example via a communication interface.

By using the first switch and the second switch for generating the load current, the brightness of the light emitting diode can be controlled flexibly in response to the dimming signal. In particular, it may be unnecessary to activate the lopsided operation at low brightness of the light emitting diode. For example, it would be possible for continuous operation to be continuous - i. for all brightness levels of the dimming signal - is activated.

For example, it would be possible for the controller to be configured to operate the second switch in an on-time state. The on-time of the second switch may be dimensioned such that the polarity of the inductor current changes from positive to negative and the voltage at the midpoint of the two switches (midpoint voltage) changes, i. e.g. from positive to negative polarity or with respect to another reference voltage, such as a bus voltage. This may mean that the second switch is operated in the conductive state until the direction of the inductor current reverses. For example, the reactor current with negative polarity could be fed by discharging a capacitor of the output terminal.

Since the inductor current has a negative polarity at least at times, a particularly small-dimensioned average time value of the inductor current can be achieved. As a result, once again a small-sized load current can be output to the light-emitting diode. As a result, even low levels of brightness for corresponding dimming signals for the light-emitting diode can be achieved. In particular, low brightnesses can be achieved without interrupting the operation of the light emitting diode according to the pulse width modulation method. A discontinuous mode can be avoided. Interference with the environment - ie, for example, a flickering of the LED - can be reduced or avoided. In some examples, the controller may be configured to implement a deadtime during which the first switch and the second switch are operated in the non-conductive state. For example, a certain security area can be provided by the dead time, so that short circuits are avoided. For example, first the first switch can be switched to the non-conductive state before the second switch in the conductive state (English, "break betoken make"). A corresponding dead time can be particularly short dimensioned and, for example, in the range of 100 ns to 1000 ns.

In further examples, it may be desirable that the dead time is extended or dimensioned comparatively long. For example, the dead time could not be less than 5% of the on time of the second switch, optionally not less than 10% of the on time of the second switch, further optionally not less than 25% of the on time of the second switch. It can thus be achieved that, after the second switch has been switched to the non-conducting state-and the continuous operation of the first switch in the non-conducting state-the inductor current decreases and finally disappears. Then, it is possible that the controller is arranged to switch the first switch from the non-conductive state to the conductive state in time-synchronized with the switching of the midpoint voltage from zero-voltage switching (ZVS). Such an optional power-free switching of the first switch can be done (English, zero current switching). Such current-free switching of the first switch has the advantage of low power loss. This can reduce the power consumption of the buck converter.

It would be possible for a controlled operation of the first switch and the second switch to be implemented by the controller. This means that the controller could implement a loop. It can thereby be achieved that the brightness of the light-emitting diode can be set particularly accurately and stably by generating the load current.

For example, the controller could be configured to operate the first switch and the second switch in a controlled manner. In this case, the corresponding control loop can take into account the time mean value of the inductor current as a controlled variable. Alternatively or additionally, it would also be possible to consider the load current as a control variable. The corresponding control loop could also take into account a reference variable which is determined based on the dimming signal. For example, it would be possible by means of a look-up table to determine the reference variable based on the dimming signal in such a way that it can be compared directly with the load current as a controlled variable. In this way, it may be possible to set the desired brightness in accordance with the dimming signal with particular precision.

For example, the controller could be configured to operate the first switch and the second switch in a controlled manner. In this case, the corresponding control circuit can take into account at least one peak value of the throttle current as a manipulated variable. For example, the peak value of the inductor current with positive polarity could be taken into account as a manipulated variable. Alternatively or additionally, it would also be possible to take into account the peak value of the inductor current at negative polarity as a manipulated variable, whereby the operation can be achieved in a particularly good operating point. Alternatively or additionally, it would be possible, for example, to consider the duty cycle of the on-time of the first switch and / or the second switch as a manipulated variable. Alternatively or additionally, it would also be possible, for example, to take into account the on-time of the first switch and / or the second switch as a manipulated variable. Alternatively or additionally, it would also be possible, for example, to take into account the off-time of the first switch and / or the second switch as a manipulated variable. Sometimes it may be desirable to keep the peak value of the reactor current at negative polarity constant. Thus, a power-free switching of the first switch can be achieved - and at the same time the dead time are comparatively small and fixed dimensioned.

This can be achieved that the voltage at the center of the two switches can swing around and a voltage-free switching of the first switch is guaranteed.

In some examples, it may be desirable for the controller to be configured to operate the first switch and the second switch in a controlled manner, taking into account the peak value of the positive current inductor current as the manipulated variable, but keeping the peak negative inductance inductor current constant , At a fixed peak value of the reactor current at negative polarity can be achieved that the voltage-free switching of the first switch can be implemented particularly easily. In particular, a time period between the switching of the second switch from the conductive state to the non-conductive state and the switching of the first switch from the non-conductive to the conductive state at a fixed peak value of the reactor current at negative polarity can also be kept constant.

For a controlled operation, the buck converter may for example comprise a sensor circuit. By means of the sensor circuit it may be possible to obtain a measurement signal which is indicative of the controlled variable. For example, the measurement signal could be indicative of the inductor current. For example, the measurement signal could be indicative of the time mean value of the inductor current: For this purpose, for example, a low-pass filter could be provided in the sensor circuit. However, it would alternatively or additionally also be possible for the measurement signal to be indicative of the inductor current with a large bandwidth which corresponds to the change in the inductor current on the basis of the inductance of the storage inductor, is. Alternatively or additionally, it would also be possible for the measurement signal to be indicative of a choke voltage via the storage choke. The sensor circuit may be configured, for example, to effect a zero offset between the measurement signal and the inductor current. It can thereby be achieved that the measuring signal has only positive or only negative polarity. A zero crossing of the measuring signal can - be avoided despite the zero crossing of the inductor current. Such zero offset may be implemented, for example, by providing another current source that provides a reference current. Such a zero point offset could also be achieved, for example, by a suitable voltage divider. Since the measuring signal has only positive polarity or only negative polarity, a particularly simple control based on the measuring signal can be implemented.

In another example, an operating device for a luminaire includes the buck converter according to the various examples described herein. Yet another example relates to the luminaire with the operating device having the buck converter.

For example, the operating device could further include an AC / DC converter. The AC / DC converter may be configured to convert an AC supply voltage to the DC supply voltage, which is subsequently applied to the down converter. In other examples, it would also be possible for the operating device to directly receive the DC supply voltage.

In another example, a method includes receiving a dimming signal for a light emitting diode. The method further includes, in response to the dimming signal, alternating and periodically operating a first switch of a buck converter and a second buck converter switch in the on state. In this case, the second switch - for example, between a supply voltage terminal of the buck converter and ground - connected in series with the first switch. The method also includes outputting a load current to the light emitting diode via an output terminal of the buck converter. This is done based on a choke current of a storage choke of the buck converter. The storage choke is connected in series between the supply voltage terminal and the output terminal to the first switch. For such a method, effects can be achieved that are comparable to the effects that can be achieved for a down-converter according to various examples described herein. The features and features set out above, which are described below, can be used not only in the corresponding combinations explicitly set out, but also in other combinations or isolated, without departing from the scope of the present invention. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically illustrates an operating device of a luminaire with a buck converter according to various embodiments. FIGs. 2A and 2B schematically illustrate the buck converter with a first switch and a second switch and a storage choke according to various embodiments.

FIG. FIG. 3 schematically illustrates the operation of the first switch and the second switch alternately and periodically in the conducting state according to various embodiments.

FIG. FIG. 4 schematically illustrates the operation of the first switch and the second switch alternately and periodically in the conductive state according to various embodiments, wherein in the example of FIG. 4 is provided a dead time. FIGs. 5A and 5B schematically illustrate the buck converters having a first switch and a second switch and a storage choke and a sensor circuit according to various embodiments.

FIG. 6 is a flowchart of a method according to various embodiments.

FIG. 7 is a flowchart of a method according to various embodiments. DETAILED DESCRIPTION OF EMBODIMENTS The above-described characteristics, features, and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more clearly understood in connection with the following description of the embodiments which will be described in detail in conjunction with the drawings. Hereinafter, the present invention will be described with reference to preferred embodiments with reference to the drawings. In the figures, like reference characters designate the same or similar elements. The figures are schematic representations of various embodiments of the invention. Elements shown in the figures are not necessarily drawn to scale. Rather, the various elements shown in the figures are reproduced in such a way that their function and general purpose will be understood by those skilled in the art. Functional units can be implemented as hardware, software or a combination of hardware and software. Hereinafter, techniques related to the conversion of a DC supply voltage will be described. In particular, the techniques described herein involve lowering the DC supply voltage, ie, downconverting. The techniques described herein may be used in particular in connection with the operation of light-emitting diodes. In other examples, however, it would also be possible for the techniques described herein to be used in other applications. Examples include, for example, charge storage, power supplies for electronic devices, or other forms of lighting, etc.

In various examples, the supply voltage is converted as a function of a dimming signal for the light-emitting diode. The dimming signal may be indicative of a desired brightness of the light emitting diode. The conversion can output a specific load current to the light emitting diode, whereby the load current can be larger (lower) for larger (lower) desired brightnesses. The various techniques described herein are described in particular with respect to a particular architecture of the buck converter: This buck converter architecture uses a first switch that is placed on potential and a second switch that is located between potential and ground. In comparison to other architectures of down-converters which use only one diode instead of the second switch, a particularly energy-efficient operation can be achieved in this way: in particular, the voltage drop across the diode can be avoided by using the second switch.

The techniques described herein make it possible to implement different brightnesses for the light emitting diode without the use of pulse width modulation. This can do that It should be avoided that the LED flickers at low brightness levels. In addition, the techniques described herein may allow for simple control in which there is no need to switch between a continuous operation of the buck converter and a bucking operation of the buck converter.

FIG. 1 illustrates aspects relating to a driver 100 for a light emitting diode 1 10. For example, the driver 100 could be part of a light fixture. The lamp could further comprise a housing, heat sink, a backup battery, etc. The operating device 100 includes an AC / DC converter 104 that is configured to convert an AC supply voltage 151 into a DC supply voltage 152. The AC supply voltage 151 is received via a grid connection 152. For example, the AC supply voltage 151 could have an amplitude in the range of 100V to 300V. For example, the AC / DC converter 104 could include a rectifier bridge circuit (not shown in FIG. 1). The AC / DC converter 104 is optional: in other examples, the operating device 100 could directly receive a DC supply voltage.

The operating device 100 also includes a DC / DC converter 101. The DC / DC converter 101 is configured to convert the DC supply voltage 152. In particular, the DC / DC converter 101 is configured to down-convert the DC supply voltage. Therefore, the DC / DC converter 101 will hereinafter be referred to as downconverters 101. Based on the DC supply voltage, the light emitting diode 1 10 is operated. For this purpose, a load current from the down converter 101 can be provided and output to the light emitting diode 110. The down converter 101 is driven by a controller 102. For example, the controller 102 could implement a regulated operation of the buck converter 101. Thereby, the operation of the light emitting diode 1 10 can be stabilized. In addition, the operation of the light emitting diode 1 10 can be controlled by external requirements. In the example of FIG. 1, the controller 102 receives a dimming signal 161 via a communication interface 103. In the example of FIG. 1, the dimming signal 161 is received via a dedicated transmission medium 162, eg a DALI interface. However, in other examples, it would also be possible for the dimming signal 161 to be received via the power line 152 (not shown in FIG. 1). For example, the dimming signal 161 could be applied to the AC supply voltage 151 are modulated. An example would be a phase-cut modulation.

The controller 102 may control the operation of the light emitting diode 110 in response to the dimming signal 161 as an external command. For example, the controller 102 could drive the buck converters 101 such that the load current takes on different values depending on the dimming signal 161.

The dimming signal can also be predetermined, for example, by a resistor connected to the operating device 100, the resistance value preferably prescribing the rated current of the light-emitting diode. It could also be a potentiometer connected as a variable resistor, which would also allow a change or adjustment of the rated current. FIG. FIGS. 2A and 2B illustrate aspects with respect to down converter 101. In particular, FIG. 2a shows the buck converter 101 in more detail. FIG. 2 is a circuit diagram of the down converter 101.

The down converter 101 is configured to receive the DC supply voltage 152 via a supply voltage terminal 21 1. A field effect transistor 201 with a freewheeling diode 205 implements a switch 291. A field effect transistor 202 with a freewheeling diode 206 implements a switch 292. The switch 291 and the switch 292 are connected in series between the supply voltage terminal 21 1 and ground 215. The buck converter 101 also includes a storage inductor 212. The storage inductor 212 and the switch 291 are connected in series between the supply voltage terminal 21 1 and an output terminal 219 to the light emitting diode 1 10. In FIG. 2a, a throttle current 701 is illustrated by the storage throttle 212. An orientation of the inductor current 701 toward the output terminal 219 (as indicated by the corresponding arrow in FIG. 2a) will be referred to hereinafter as the positive polarity of the inductor current 701.

The output terminal 219 has a smoothing capacitor 213 with resistor 214. Therefore, the load current 702 provided based on the reactor current 701 of the light emitting diode 110 corresponds to a time average value of the reactor current 701. In FIG. FIG. 2A also shows that a control signal 601 is applied to a control contact of the field-effect transistor 201 of the switch 291. By means of the control signal 601 it is possible to operate the switch 291 either in the conducting state or in the non-conducting state. Further, it is possible to switch the switch 291 from the conductive state to the non-conductive state and to switch from the non-conductive state to the conductive state. For example, the control signal 601 may be generated by the controller 102. Thereby, the controller 102 may selectively operate the switch 291 in a conductive state or a non-conductive state.

In FIG. 2a is also shown that a control signal 602 is applied to a control contact of the field effect transistor 202 and thus switch 292. By means of the control signal 602 it is possible to operate the switch 292 optionally in the conducting state or in the non-conducting state. Further, it is possible to switch the switch 292 from the conductive state to the non-conductive state and to switch from the non-conductive state to the conductive state. For example, the control signal 602 may be generated by the controller 102. This allows the controller 102 to selectively operate the switch 292 in a conductive or non-conductive state.

In some examples, the controller 102 is configured to alternately and periodically conduct the switch 291 and the switch 292 in a conductive state in response to the dimming signal 161.

Fig. 2B shows an alternative implementation of a buck converter. In FIG. 2B, in contrast to FIG. 2A, the light-emitting diode 110 is connected to the parallel capacitor 213 not against the ground point 215 but against the supply voltage terminal 21 1. The storage choke 212 is magnetized during the on-time of the switch 292. The time phase of the positive rise of the choke current 701 is the on-time of the switch 292. The freewheeling phase, ie the phase of the demagnetization of the storage choke 212, takes place via the switch 291. The time phase of the negative increase of the inductor current 701 is the on time of the switch 291.

FIG. Figure 3 illustrates aspects relating to the operation of the switches 291, 292 alternately and periodically in the conducting state. FIG. 3 schematically illustrates the timing of the control signal 601 and the control signal 602. FIG. 3 also schematically illustrates the resulting time profile of the inductor current 701. From FIG. 3, it is seen that the switch 291 is operated in a conductive state during repeated on times 651. The switch 291 is operated in non-conductive state during repeated off times 652. The switch 292 is accordingly operated during repeated times 661 in the conducting state. The switch 292 is operated during repeated off-times 662 in the non-conductive state. In the example of FIG. 3, the period duration 670 at which the switches 291, 272 are operated periodically in the conducting state is shown.

From FIG. 3, it can further be seen that the switch 292 is in the conductive state whenever the switch 291 is in the non-conductive state. In addition, the switch 291 is always in the conductive state when the switch 292 is in the non-conductive state. Accordingly, the switches 291, 292 are operated alternately in the conducting state. In particular, the on time 661 of the switch 292 is dimensioned such that the polarity of the inductor current 701 changes from positive to negative at time 755. By implementing the inductor current 701 with an at least temporarily negative polarity, it is possible for the mean time value 712 (horizontal dashed line in FIG. 3) of the inductor current 701-and thus the load current 702-to reach particularly low values close to zero. As a result, low brightnesses of the light emitting diode 1 10 can be achieved.

In the example of FIG. 3, it may be possible to provide a dead time between the switching of the switches 291, 292 (not shown in FIG. Such a dead time can avoid short circuits. Such a dead time to avoid short-circuits can be dimensioned particularly short: In particular, the switching of the switches 291, 292 takes place essentially at the same values of the inductor current 701. In the example of FIG. 3, these values of the inductor current 701 at which the switches 291, 292 are switched correspond to peaks 751, 752 of the inductor current 701 (see vertical dashed lines in FIG. 3). In some examples, it may also be possible to provide a longer dead time.

FIG. FIG. 4 illustrates aspects relating to the operation of the switches 291, 292 alternately and periodically in the conducting state. The example of FIG. 4 basically corresponds to the example of FIG. 3. However, in the example of FIG. 4 a comparatively long dead time 670 is provided. In the example of FIG. 4, the dead time 670 is approximately 25% of the on time 661 and approximately 20% of the off time 652. During the dead time 670, both the switch 291, FIG also the switch 292 operated in the non-conductive state. Therefore, at another value of the reactor current 701, the switch 292 is switched from the conductive state to the non-conductive state, as the switch 291, which is switched from the non-conductive state to the conductive state. More specifically, the switch 291 is switched from the non-conductive state to the conductive state in time-synchronized with the reversal of the center-point voltage of both switches 291 and 292. Alternatively or additionally, the switch 291 may be switched from the non-conducting state to the conducting state in a time-synchronized manner with a zero crossing 753 of the inductor current 701. Optionally, there is also a dead time between on-time 651 and off-time 652 (not shown in FIG. This dead time between the switching off of the switch 291 and the switching on of the switch 292 can be dimensioned to avoid a short circuit through both switches 291 and 292.

The operation of the switches 291, 292 - for example, according to the implementations of FIGS. 3 and 4 - can be regulated in some examples. For example, the time average 712 of the inductor current 701 could be taken into account as a controlled variable, since this can be directly proportional to the load current 702. For example, the dimming signal 161 or a variable derived therefrom could be taken into account as a reference variable. Then, by suitable operation 291, 292, a deviation between the reference variable and the controlled variable can be minimized.

In principle, different variables could be taken into account in a corresponding regulation. For example, the duty cycle for the operation of the switch 291 in the on state and / or for the operation of the switch 292 in the on state could be taken into account as a manipulated variable. For example, the peak value 251 of the positive current inductor current 701 and / or the peak value 752 of the negative polarity inductor current 701 could be taken into account as the manipulated variable. In some examples, it may be desirable that the peak value 752 of the inductor current 701 be controlled to a constant value at negative polarity to reduce the dead time 670 while allowing for currentless switching of the switch 291.

The FIGs. 5A and 5B illustrate aspects relating to the buck converter 101. The examples of FIGS. 5A and 5B basically correspond to the example of FIG. Second

In the example of FIG. 5A, a sensor circuit 301 and a sensor circuit 31 1 are also shown. The sensor circuit 301 is set up to provide a measurement signal at the terminal 302 which is indicative of a current value of the inductor current 701. The detection of the inductor current 701 is effected by means of the resistor 214. The sensor circuit 301 is further configured to output at terminal 303 a measurement signal which is indicative of the time average 712 of the inductor current 701: for this purpose a low-pass filter is provided. Optionally, it would be possible for the sensor circuit 301 to be arranged to cause a zero offset between the measurement signal at the terminal 302 and the inductor current 701). It can thereby be achieved that the measuring signal does not have changing polarities, corresponding to the choke current 701: this can simplify the determination of the peak values 751, 752 and / or an implementation of the control loop.

The zero point offset can be realized for example by means of a current source, which can preferably be integrated into the controller. This example is shown in FIG. 5A. Alternatively, for example, the zero point offset can be realized by means of a pull-up resistor (sometimes referred to as a pull-up resistor), which is preferably connected to a supply voltage such as the supply voltage Vcc of the operating device. This example is shown in FIG. 5B.

The sensor circuit 31 1 comprises a coil, which is inductively coupled to the storage inductor 212. The sensor circuit 31 1 is set up to output at terminal 312 a measurement signal which is indicative of the choke voltage and thus also of the voltage at the midpoint of the two switches 291 and 292. Alternatively, for example, the center point voltage of the two switches 291 and 292 can also be measured via a voltage divider which picks up the voltage at the midpoint of the two switches 291 and 292. In the example of FIG. 5A, the voltage at the midpoint of the two switches 291 and 292 oscillates to the voltage of the supply voltage terminal 21 1. In the example of FIG. 2B, the midpoint voltage of the two switches 291 and 292 may swing against the voltage at the ground point 215.

FIG. 6 is a flowchart of a method according to various examples. First, a dimming signal is received in block 1001. The dimming signal may be indicative of a desired brightness of a light emitting diode of a luminaire. The dimming signal can be received, for example, in analog form or digital form. For example, the dimming signal could be received by phase-slicing an AC supply voltage. Subsequently, a first switch and a second switch of a buck converter are operated alternately and periodically in the conducting state. It would optionally be possible to provide dead times during which both the first switch and the second switch are operated in the non-conductive state. For example, it would be possible to achieve current-free switching of the first switch and / or current-free switching of the second switch on the basis of a corresponding dimensioning of the dead times.

By switching the switches, a choke current can be modified by a storage choke of the buck converter. In particular, by switching the switches, the storage inductor can be alternately charged and discharged.

Subsequently, in block 1003, a load current is output to the light emitting diode. The load current may correspond, for example, to an average value of the inductor current. The load current is fed alternately by the supply voltage and the storage choke.

FIG. 7 is a flowchart of a method according to various examples. FIG. Figure 7 illustrates details related to the controlled operation of the first switch and the second switch. For example, the method of FIG. 7 as part of block 1002. First, in block 101 1, determining a command variable based on the dimming signal.

Then, in block 1012, determining a time average of the inductor current as the controlled variable. Alternatively or additionally, the load current could also be taken into account as a controlled variable. It is then possible to compare the controlled variable with the reference variable. The aim of the regulation may be to minimize deviations between the controlled variable and the reference variable. For this purpose, one or more manipulated variables can be changed. For example, the peak value of the inductor current with positive polarity could be changed as a manipulated variable. Alternatively or additionally, the peak value of the inductor current with negative polarity could also be changed as a manipulated variable. This is done in block 1013.

Of course, the features of the previously described embodiments and aspects of the invention may be combined. In particular, the features may be used not only in the described combinations but also in other combinations or per se, without departing from the scope of the invention.

Claims

1 . A down converter (101) for a light emitting diode (110) comprising:

a first switch (201, 205, 291),

a second switch (202, 206, 292) connected between one

Supply voltage terminal (21 1) and ground (215) in series with the first switch (201, 205, 291) is connected,

a storage choke (212) connected in series with the first switch (201, 205, 291),

an output terminal (213, 219) configured to output a load current (702) to the light emitting diode (110) based on a choke current (701) through the storage choke (212), and

- A controller (102) which is adapted to the first switch (201, 205, 291) and the second switch (202, 206, 292) in response to a dimming signal (161) alternately and periodically in the conductive state to operate.

2. down converter (101) according to claim 1,

wherein the controller (102) is arranged to operate the second switch (202, 206, 292) in the on state for an on-time (661), the on-time (661) of the second switch (202, 206, 292) is dimensioned such that the polarity of the inductor current (701) changes from positive to negative.

3. down converter (101) according to claim 2,

wherein the controller (102) is configured to implement a dead time during which the first switch and the second switch are operated in the non-conductive state.

4. down converter (101) according to any one of the preceding claims,

wherein the controller (102) is arranged to time-shift the first switch (201, 205, 291) in time with the reversal of a mid-point voltage between the first switch (201, 205, 291) and the second switch (202, 206, 292) -conducting state to the conducting state.

5. down converter (101) according to any one of the preceding claims,

wherein the controller (102) is arranged to operate the first switch (201, 205, 291) and the second switch (202, 206, 292) in a controlled manner with the time average value of Throttle current (701) as a controlled variable and with a reference based on the dimming signal (161) command variable.

6. Down converter (101) according to one of the preceding claims,

wherein the controller (102) is arranged to operate the first switch (201, 205, 291) and the second switch (202, 206, 292) in a controlled manner with at least a peak value of the inductor current (701) as the manipulated variable.

7. down converter (101) according to claim 6,

wherein the controller (102) is arranged to operate the first switch (201, 205, 291) and the second switch (202, 206, 292) in a controlled manner with the peak value of the

Reactive current (701) with positive polarity as a manipulated variable and with a constant peak value of the inductor current (701) in negative polarity.

The buck converter (101) of any one of the preceding claims, further comprising:

a sensor circuit (301) arranged to output a measurement signal indicative of the inductor current (701), the sensor circuit being arranged to effect a zero offset between the measurement signal and the inductor current (701).

9. A method comprising:

Receiving a dimming signal (161) for a light-emitting diode (1 10),

in dependence on the dimming signal (161): alternately and periodically operating a first switch (201, 205, 291) of a down converter (101) and a second one

Switch (202, 206, 292) of the buck converter (101) connected between a

Supply voltage terminal (21 1) and ground (215) in series with the first switch (201, 205, 291) is connected, in the conductive state, and

- Based on a reactor current (701) of a storage inductor (212) connected in series with the first switch (201, 205, 291): outputting a load current (702) to the light emitting diode (1 10) via an output terminal (213, 219).

10. The method according to claim 9,

the method being carried out by the down converter (101) according to any one of claims 1-8.