EP3533291B1 - Step-down converter for a light-emitting diode - Google Patents

Description

TECHNICAL AREA

Various examples of the invention generally relate to a step-down converter for a light-emitting diode with a first switch and a second switch which are operated alternately and periodically in the conductive state by a controller. Various examples of the invention relate in particular to a step-down converter in which the controller operates the first switch and the second switch as a function of a dimming signal.

BACKGROUND

To operate light-emitting diodes, lights typically have an operating device. The operating device typically contains a buck converter, which is set up to reduce the amplitude of a DC supply voltage for operating the light-emitting diode. In addition, the down converter can be set up to change the operation of the light-emitting diode as a function of a dimming signal, which indicates the desired brightness of the lamp.

Conventional step-down converters typically have a switch which is connected in series with a storage choke between a supply voltage connection and an output connection. During an on-time of the switch - i.e. during which the switch is operated in the conductive 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-conductive state - a choke current flows which is fed by the energy previously stored in the storage choke.

A buck converter according to the preamble of claim 1 is from the document US 2015/0373805 A1 known. The pamphlets too WO 2009/138908 A1 and WO 2013 / 028632A1 disclose similar buck converters.

Further buck converters are in the publications JP 2014-127376 A , US 2008/0224625 A1 and DE 10 2009 000602 A1 disclosed.

In principle, different operating modes of the down converter are known. For example, one can be referred to as discontinuous mode Operating mode, the inductor current drops to zero during the switch-off time. In an operating mode referred to as continuous mode, the inductor current does not drop to zero during the switch-off time. In between there is the so-called borderline mode, which corresponds to the transition between intermittent operation and continuous operation.

In the case of reference implementations of step-down converters, it may be necessary to activate intermittent operation as a function of the dimming signal. In particular, in connection with a low desired brightness of the luminaire, it may be necessary to activate the intermittent operation with a particularly long switch-off time. Then the brightness of the lamp is modulated with a comparatively low frequency. The light-emitting diode is typically switched off in the meantime, i.e. the load current can drop to zero. Sometimes this type of operation of the luminaire is also referred to as pulse width modulation. This can have various negative effects on the surroundings of the luminaire: for example, interference with optical devices can occur.

In the case of reference implementations of step-down converters, it may also be necessary to switch between intermittent operation and limit operation depending on the dimming signal. In particular, this can mean that, depending on the dimming signal, there is a particularly strong jump in the frequency with which the switch is switched. This can require complex and expensive control technology.

SUMMARY OF THE INVENTION

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

This object is achieved by the features of the independent patent claims. The features of the dependent claims define embodiments.

According to the invention, a step-down converter for a light-emitting diode comprises a first switch and a second switch. The second switch is connected in series with the first switch between a supply voltage connection and ground. The buck converter also includes a Storage choke. The storage choke is connected in series with the first switch between the supply voltage connection and an output connection. The output connection is set up to output a load current to the light-emitting diode based on a choke current through the storage choke. The buck converter also includes a controller. The controller is set up to operate the first switch and the second switch alternately and periodically in the conductive state as a function of a dimming signal.

In some examples, a step-down converter with 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 arranged at potential. Correspondingly, the second switch is sometimes also referred to as a low-side switch because it is arranged 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 can be connected to a point which is arranged between the first switch and the second switch. The second side of the storage choke can be connected to the output connection. The storage choke can be implemented as a coil with several windings. The storage choke can provide an inductance. Based on the law of induction, the voltage across the storage choke (choke voltage) can thus be equal to the inductance of the storage choke multiplied by the change in the choke current over time. In other words, the storage choke can counteract particularly rapid changes in the choke current

The output connection can, for example, comprise a smoothing capacitor which has the effect that the load current which is output to the light-emitting diode corresponds to a time average value of the inductor current. As a result, the inductor current can be smoothed and a more uniform brightness of the light-emitting diode can be achieved. The output connection could, for example, also have a plug contact, solder contact, clamping contact, etc. in order to establish an electrical connection to the light-emitting diode.

The control 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 implemented at least partially by an analog circuit. The controller could receive the dimming signal via a communication interface, for example.

By using the first switch and the second switch to generate the load current, the brightness of the light-emitting diode can be controlled flexibly as a function of the dimming signal. In particular, it can be unnecessary to activate the intermittent operation when the brightness of the light-emitting diode is low. For example, it would be possible for continuous operation to be activated throughout - i.e. for all brightness levels of the dimming signal.

For example, it is possible for the controller to be set up to operate the second switch in the conductive state for an on time. The on-time of the second switch is dimensioned in such a way that the polarity of the inductor current changes from positive to negative and the voltage at the midpoint of the two switches (midpoint voltage) swings around, i.e. for example from positive to negative polarity or in relation to another reference voltage, such as for example a bus voltage. This can mean that the second switch is operated in the conductive state until the direction of the inductor current is reversed. For example, the inductor current could be fed with negative polarity by discharging a capacitor of the output connection.

Since the inductor current has a negative polarity at least temporarily, a particularly small time average value of the inductor current can be achieved. As a result, a load current of small dimensions can in turn be output to the light-emitting diode. As a result, low brightnesses can also be achieved for corresponding dimming signals for the light-emitting diode. In particular, low brightness levels can be achieved without interrupting the operation of the light-emitting diode in accordance with the pulse-width modulation method. A discontinuous mode can be avoided. Interfering influences on the environment - e.g. a flickering of the light-emitting diode - can be reduced or avoided.

In some examples, the controller can be configured to implement a dead time during which the first switch and the second switch are operated in the non-conductive state. For example, a certain safety area can be provided by the dead time so that short circuits are avoided. For example, the first switch can first be switched to the non-conductive state before the second switch is switched to the conductive state ("break before make"). A corresponding dead time can be designed to be particularly short and, for example, be in the range from 100 ns to 1000 ns.

In further examples, it can be desirable for the dead time to be lengthened or dimensioned to be 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. In this way it can be achieved that after switching the second switch to the non-conductive state - and the continued operation of the first switch in the non-conductive state - the inductor current decreases and finally disappears. It is then possible for the controller to be set up to switch the first switch in a time-synchronized manner with the reversal of the midpoint voltage from the non-conductive state to the conductive state (zero voltage switching, ZVS). In this way, the first switch can optionally be switched without current (zero current switching). Such a current-free switching of the first switch has the advantage of low power dissipation. As a result, the energy consumption of the buck converter can be reduced.

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

For example, the controller could be set up to operate the first switch and the second switch in a regulated manner. The corresponding control loop can take the time mean value of the inductor current into account as a controlled variable. Alternatively or additionally, it would also be possible to take the load current into account as a controlled variable. The corresponding control loop could also take into account a reference variable that is determined based on the dimming signal. For example, it would be possible to use 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 can be possible to set the desired brightness in accordance with the dimming signal particularly precisely.

For example, the controller could be set up to operate the first switch and the second switch in a regulated manner. The corresponding control loop can have at least one Consider the peak value of the choke 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. As an alternative or in addition, it would also be possible to take into account the peak value of the inductor current in the case of negative polarity as a manipulated variable, as a result of which operation can be achieved at a particularly good operating point. Alternatively or additionally, it would be possible, for example, to take into account the duty cycle of the on-time of the first switch and / or of the second switch as a manipulated variable. As an alternative or in addition, 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. As an alternative or in addition, it would also be possible, for example, to take into account the off time of the first switch and / or of the second switch as a manipulated variable.

Sometimes it can be desirable to keep the peak value of the inductor current constant with negative polarity. In this way, current-free switching of the first switch can be achieved - and at the same time the dead time can be comparatively short and fixed.

It can thus be achieved that the voltage at the midpoint of the two switches can swing around and a voltage-free switching on of the first switch is guaranteed.

In some examples, it may be desirable for the controller to be set up to operate the first switch and the second switch in a regulated manner, the peak value of the inductor current being taken into account as a manipulated variable in the case of positive polarity, but the peak value of the inductor current being kept constant in the case of negative polarity . Given a fixed peak value of the inductor current with negative polarity, the voltage-free switching of the first switch can be implemented in a particularly simple manner. In particular, a period of time 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 can also be kept constant at a fixed peak value of the inductor current with negative polarity.

For a regulated operation, the down converter can have a sensor circuit, for example. By means of the sensor circuit it can 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 average value of the inductor current: For this purpose, a low-pass filter could be provided in the sensor circuit, for example. Alternatively or additionally, however, it would also be possible for the measurement signal is indicative of the inductor current with a large bandwidth, which corresponds to the change in the inductor current due to the law of induction based on the inductance of the storage inductor. Alternatively or additionally, it would also be possible for the measurement signal to be indicative of a choke voltage across the storage choke. The sensor circuit can be set up, for example, to bring about a zero point offset between the measurement signal and the inductor current. It can thereby be achieved that the measurement signal has only positive or only negative polarity. A zero crossing of the measurement signal can - despite the zero crossing of the inductor current - be avoided. Such a zero point offset can be implemented, for example, by providing a further current source that provides a reference current. Such a zero point offset could also be achieved, for example, by means of a suitable voltage divider. Since the measurement signal has only positive polarity or only negative polarity, a particularly simple regulation based on the measurement signal can be implemented.

In a further example, an operating device for a luminaire comprises the down converter according to the various examples described herein. Yet another example relates to the luminaire with the operating device which has the down converter.

For example, the operating device could also have an AC / DC converter. The AC / DC converter can be set up to convert an AC supply voltage into the DC supply voltage, which is then fed to the step-down converter. In other examples, however, it would also be possible for the operating device to receive the DC supply voltage directly.

In another example, a method includes receiving a dimming signal for a light emitting diode. The method further comprises, as a function of the dimming signal, the alternating and periodic operation of a first switch of a step-down converter and a second switch of the step-down converter in the conductive state. The second switch is connected in series with the first switch between a supply voltage connection of the step-down converter and ground. The method also includes outputting a load current to the light emitting diode via an output terminal of the buck converter. This takes place based on a choke current of a storage choke of the step-down converter. The storage choke is connected in series with the first switch between the supply voltage connection and the output connection.

For such a method, effects can be achieved which are comparable to the effects which can be achieved for a buck converter according to various examples described herein.

BRIEF DESCRIPTION OF THE FIGURES

• FIG. 1 schematically illustrates an operating device of a luminaire with a step-down converter according to various embodiments.
• FIGs. 2A and 2B schematically illustrate the step-down converter with a first switch and a second switch and a storage choke according to various embodiments.
• FIG. 3 illustrates schematically the operation of the first switch and the second switch alternately and periodically in the conductive state according to various embodiments.
• FIG. 4th illustrates schematically 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 FIG. 4th a dead time is provided.
• FIGs. 5A and 5B schematically illustrate the step-down converter with a first switch and a second switch and a storage choke and a sensor circuit according to various embodiments.
• FIG. 6th Figure 3 is a flow diagram of a method in accordance with various embodiments.
• FIG. 7th Figure 3 is a flow diagram of a method in accordance with various embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is explained in more detail below on the basis of preferred embodiments with reference to the drawings. In the figures, the same reference symbols denote the same or similar elements. The figures are schematic representations of various embodiments of the invention. Elements shown in the figures are not necessarily shown to scale. Rather, the various elements shown in the figures are reproduced in such a way that their function and the general purpose is understandable to a person skilled in the art. Functional units can be implemented as hardware, software, or a combination of hardware and software.

Techniques related to converting a DC supply voltage are described below. The techniques described herein are particularly concerned with stepping down the DC supply voltage, i.e., down-converting. The techniques described herein can 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 areas of application. Examples concern, for example, charge storage devices, power supply units for electronic devices, or other forms of light sources, etc.

In various examples, the supply voltage is converted depending on a dimming signal for the light-emitting diode. The dimming signal can be indicative of a desired brightness of the light-emitting diode. The conversion can output a certain load current to the light-emitting diode, wherein the load current can be greater (lower) for greater (lower) desired brightnesses.

The various techniques described herein are described in particular with reference to a particular architecture of the buck converter: This architecture of the buck converter uses a first switch which is arranged at potential and a second switch which is arranged between potential and ground. In comparison to other architectures of step-down converters that use only a diode instead of the second switch, 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 here make it possible to implement different brightnesses for the light-emitting diode without using pulse width modulation. This can prevent the light-emitting diode from flickering at low levels of brightness. In addition, the techniques described herein can enable simple regulation in which there is no need to switch between continuous operation or limit operation of the step-down converter and intermittent operation of the step-down converter.

FIG. 1 illustrates aspects relating to an operating device 100 for a light-emitting diode 110. For example, the operating device 100 could be part of a lamp. The luminaire could furthermore comprise a housing, heat sink, an emergency battery, etc.

The operating device 100 comprises an AC / DC converter 104, which is set up to convert an AC supply voltage 151 into a DC supply voltage 153. The AC supply voltage 151 is received via a network connection 152. For example, the AC supply voltage 151 could have an amplitude in the range from 100V to 300V. For example, the AC / DC converter 104 could have a rectifier bridge circuit (in FIG. 1 not shown). The AC / DC converter 104 is optional: in other examples, the operating device 100 could receive a DC supply voltage directly.

The operating device 100 also includes a DC / DC converter 101. The DC / DC converter 101 is set up to convert the DC supply voltage 153. In particular, the DC / DC converter 101 is set up to convert the DC supply voltage downwards. Therefore, the DC / DC converter 101 will hereinafter be referred to as the buck converter 101. The light-emitting diode 110 is operated based on the DC supply voltage. For this purpose, a load current can be provided by the step-down converter 101 and output to the light-emitting diode 110.

The down converter 101 is activated by a controller 102. The controller 102 could, for example, implement a regulated operation of the step-down converter 101. As a result, the operation of the light-emitting diode 110 can be stabilized. In addition, the operation of the light-emitting diode 110 can be controlled by external specifications.

In the example of the FIG. 1 the controller 102 receives a dimming signal 161 via a communication interface 103. In the example of FIG FIG. 1 the dimming signal 161 is received via a dedicated transmission medium 162, for example a DALI interface. In other examples, however, it would also be possible for the dimming signal 161 to be received via the network connection 152 (in FIG. 1 not shown). For example, the dimming signal 161 could be modulated onto the AC supply voltage 151. An example would be a phase cut modulation.

The controller 102 can control the operation of the light-emitting diode 110 as a function of the dimming signal 161 as an external specification or control variable. For example, the controller 102 control the down converter 101 in such a way that the load current assumes different values depending on the dimming signal 161.

The dimming signal can, for example, also be specified by a resistor connected to the operating device 100, the resistance value preferably specifying the nominal current of the light-emitting diode. A potentiometer could also be connected as a variable resistor, which would also enable the nominal current to be changed or set.

FIG. 2A and 2B illustrate aspects relating to buck converter 101. In particular, illustrated FIG. 2A the buck converter 101 in greater detail. FIG. 2A FIG. 10 is a circuit diagram of the buck converter 101.

The down converter 101 is set up to receive the DC supply voltage 153 via a supply voltage connection 211. 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 211 and ground 215.

The down converter 101 also includes a storage inductor 212. The storage inductor 212 and the switch 291 are connected in series between the supply voltage connection 211 and an output connection 219 to the light-emitting diode 110. In FIG. 2A a choke current 701 through the storage choke 212 is also illustrated. An orientation of the choke current 701 in the direction of the output connection 219 (as indicated by the corresponding arrow in FIG. 2A shown) is hereinafter referred to as the positive polarity of the inductor current 701.

The output connection 219 has a smoothing capacitor 213 with a resistor 214. Therefore, the load current 702, which is provided based on the inductor current 701 of the light-emitting diode 110, corresponds to a time average value of the inductor current 701.

In FIG. 2A it is also shown 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 conductive state or in the non-conductive state. It is also possible to switch the switch 291 from the conductive state to the non-conductive state and to switch it from the non-conductive state to the conductive state. For example the control signal 601 can be generated by the controller 102. As a result, the controller 102 can operate the switch 291 either in the conductive state or in the non-conductive state.

In FIG. 2A it 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 either in the conductive state or in the non-conductive state. It is also possible to switch the switch 292 from the conductive state to the non-conductive state and to switch it from the non-conductive state to the conductive state. For example, the control signal 602 can be generated by the controller 102. As a result, the controller 102 can operate the switch 292 either in the conductive state or in the non-conductive state.

The controller 102 is set up to operate the switch 291 and the switch 292 alternately and periodically in the conductive state as a function of the dimming signal 161.

Figure 2B shows an alternative implementation of a buck converter. In the Fig.2B is in contrast to Figure 2A the light-emitting diode 110 with the parallel capacitor 213 is not connected to the ground point 215, but to the supply voltage connection 211. The storage choke 212 is magnetized during the on-time of the switch 292. The time phase of the positive rise in the inductor current 701 is the on time of the switch 292. The freewheeling phase, i.e. the phase of demagnetization of the storage inductor 212, takes place via the switch 291. The time phase of the negative increase in the inductor current 701 is the on time of the switch 291

FIG. 3 illustrates aspects relating to operating the switches 291, 292 alternately and periodically in the conductive state. FIG. 3 schematically illustrates the timing of control signal 601 and control signal 602. FIG. 3 further schematically illustrates the resulting time profile of the inductor current 701.

Out FIG. 3 it can be seen that the switch 291 is operated in the conductive state during repeated on times 651. The switch 291 is operated in the non-conductive state during repeated off times 652. The switch 292 is accordingly operated in the conductive state during repeated on times 661. The switch 292 is operated in the non-conductive state during repeated off times 662. In the example of the FIG. 3 is the period 670, with which the switches 291, 272 are periodically operated in the conductive state, is shown.

Out FIG. 3 it can also be seen that the switch 292 is always in the conductive state when 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 alternately operated in the conductive 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 can be achieved that the time average value 712 (horizontal dashed line in FIG. 3 ) of the inductor current 701 - and thus the load current 702 - assumes particularly low values close to zero. As a result, low brightnesses of the light-emitting diode 110 can be achieved.

In the example of the FIG. 3 it could be possible to provide a dead time between the switching of the switches 291, 292 (in FIG. 3 not shown). Such a dead time can avoid short circuits. Such a dead time for avoiding short circuits can be made 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 FIG. 3 these values of the inductor current 701 at which the switches 291, 292 are switched correspond to peak values 751, 752 of the inductor current 701 (compare vertical dashed lines in FIG FIG. 3 ). In some examples it may also be possible to provide a longer dead time.

FIG. 4th illustrates aspects relating to operating the switches 291, 292 alternately and periodically in the conductive state. The example of FIG. 4th basically corresponds to the example of FIG. 3 . However, in the example, the FIG. 4th a comparatively long dead time 670 is provided. In the example of the FIG. 4th 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 and the switch 292 are operated in the non-conductive state. Therefore, the switch 292 is switched from the conductive state to the non-conductive state at a different value of the inductor current 701 than the switch 291, which is switched from the non-conductive state to the conductive state. In particular, the switch 291 is time-synchronized with the reversal of the midpoint voltage of both switches 291 and 292 from the non-conductive state switched to the conductive state. Alternatively or additionally, the switch 291 can be switched from the non-conductive state to the conductive state in a time-synchronized manner with a zero crossing 753 of the inductor current 701. Between the on-time 651 and the off-time 652 there is optionally also a dead time (in FIG. 4th not shown). This dead time between switching off the switch 291 and switching on the switch 292 can be dimensioned in order to avoid a short circuit through both switches 291 and 292.

Operation of switches 291, 292 - for example, according to the implementations of FIG FIGs. 3 and 4th - can be regulated in some examples. For example, the time mean value 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. A deviation between the reference variable and the controlled variable can then be minimized by suitable operation 291, 292.

In principle, different manipulated variables could be taken into account in a corresponding regulation. For example, the duty cycle for the operation of the switch 291 in the conductive state and / or for the operation of the switch 292 in the conductive state could be taken into account as a manipulated variable. For example, the peak value 751 of the inductor current 701 in the case of positive polarity and / or the peak value 752 of the inductor current 701 in the case of negative polarity could be taken into account as a manipulated variable. In some examples, it may be desirable for the peak value 752 of the inductor current 701 to be regulated to a constant value in the case of negative polarity in order to reduce the dead time 670 and at the same time enable the switch 291 to be switched without current.

The FIGs. 5A and 5B illustrate aspects relating to buck converter 101. The examples of FIG FIGs. 5A and 5B basically correspond to the example of FIG. 2A .

In the example of the FIG. 5A a sensor circuit 301 and a sensor circuit 311 are also shown. The sensor circuit 301 is set up to output a measurement signal at the connection 302, which is indicative of a current value of the inductor current 701. The inductor current 701 is detected by means of the resistor 214. The sensor circuit 301 is also set up to output a measurement signal at the connection 303 which is indicative of the time average value 712 of the inductor current 701: a low-pass filter is provided for this purpose. Optionally, it would be possible for the sensor circuit 301 to be set up to provide a zero point offset between the measurement signal at connection 302 and the choke current 701). It can thereby be achieved that the measurement signal does not have alternating polarities - corresponding to the inductor 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 implemented, for example, by means of a power source that can preferably be integrated into the controller. This example is in the Figure 5A shown. Alternatively, for example, the zero point offset can be implemented by means of a pull-up resistor (sometimes also 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 in the Figure 5B shown.

The sensor circuit 311 comprises a coil which is inductively coupled to the storage choke 212. The sensor circuit 311 is set up to output a measurement signal at the connection 312, which is indicative of the throttle voltage and thus also of the voltage at the midpoint of the two switches 291 and 292. Alternatively, for example, the midpoint voltage of the two switches 291 and 292 can also be measured using a voltage divider which taps off the voltage at the midpoint of the two switches 291 and 292. In the example of the Figure 5A the voltage at the midpoint of the two switches 291 and 292 swings to the voltage of the supply voltage connection 211. In the example of the Figure 5B the midpoint voltage of the two switches 291 and 292 can swing around against the voltage at the ground point 215.

FIG. 6th Figure 3 is a flow diagram of a method according to various examples. First, in block 1001, a dimming signal is received. The dimming signal can be indicative of a desired brightness of a light-emitting diode of a lamp. The dimming signal can be received in analog form or digital form, for example. For example, the dimming signal could be received by phase-cutting modulation of an AC supply voltage.

A first switch and a second switch of a step-down converter are then operated alternately and periodically in the conductive 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 that current-free switching of the first switch and / or current-free switching of the second switch is achieved on the basis of a corresponding dimensioning of the dead times.

By switching the switches, a choke current through a storage choke of the step-down converter can be modified. In particular, the storage choke can be alternately charged and discharged by switching the switch.

A load current is then output to the light-emitting diode in block 1003. The load current can 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. 7th Figure 3 is a flow diagram of a method according to various examples. FIG. 7th illustrates details relating to the regulated operation of the first switch and the second switch. For example, the procedure according to FIG. 7th executed as part of block 1002. First, in block 1011, a reference variable is determined based on the dimming signal.

Then, in block 1012, a time average value of the inductor current is determined as a 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 control can be to minimize deviations between the controlled variable and the reference variable. One or more manipulated variables can be changed for this purpose. For example, the peak value of the inductor current could be changed as a manipulated variable with positive polarity. As an alternative or in addition, the peak value of the inductor current could also be changed as a manipulated variable in the case of negative polarity. This is done in block 1013.

Claims (8)

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

   - a supply voltage terminal (211),

   - an output terminal (219) that can be connected to the light-emitting diode (110),

   - a first switch (201, 205, 291),

   - a second switch (202, 206, 292) that is connected between the supply voltage terminal (211) and ground (215) in series with the first switch (201, 205, 291),

   - a storage choke (212), wherein the storage choke (212) and the first switch (201, 205, 291) are connected in series between the supply voltage terminal (211) and the output terminal (219), wherein the output terminal (219) is 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) that is configured to alternately and periodically operate the first switch (201, 205, 291) and the second switch (202, 206, 292) in the conductive state as a function of a dimming signal (161),

   wherein the controller (102) is further configured to operate the second switch (202, 206, 292) in the conductive state for an ON time (661),
   characterized in that the ON time (661) of the second switch (202, 206, 292) is dimensioned such that the polarity of the choke current (701) changes from positive to negative during the ON time (661) of the second switch (202, 206, 292) and a voltage reverses at the midpoint of the series circuit of the first switch (201, 205, 291) and of the second switch (202, 206, 292).
2. The step-down converter (101) according to claim 1,
   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.
3. The step-down converter (101) according to any one of the preceding claims,
   wherein the controller (102) is configured to switch the first switch (201, 205, 291) from the non-conductive state to the conductive state in a manner that is time-synchronized with the reversal of a midpoint voltage between the first switch (201, 205, 291) and the second switch (202, 206, 292).
4. The step-down converter (101) according to any one of the preceding claims,
   wherein the controller (102) is configured to operate the first switch (201, 205, 291) and the second switch (202, 206, 292) in a regulated manner with the time average of the choke current (701) as a control variable, and with a reference variable determined based on the dimming signal (161).
5. The step-down converter (101) according to any one of claims 1 to 3,
   wherein the controller (102) is configured to operate the first switch (201, 205, 291) and the second switch (202, 206, 292) in a regulated manner with at least one peak value of the choke current (701) as a manipulated variable.
6. The step-down converter (101) according to claim 5,
   wherein the controller (102) is configured to operate the first switch (201, 205, 291) and the second switch (202, 206, 292) in a regulated manner with the peak value of the choke current (701) as a manipulated variable given positive polarity and with a constant peak value of the choke current (701) given negative polarity.
7. The step-down converter (101) according to any one of the preceding claims, furthermore comprising:

   - a sensor circuit (301) that is configured to output a measurement signal that is indicative of the choke current (701), wherein the sensor circuit is configured to cause a zero-point offset between the measurement signal and the choke current (701).
8. A method for outputting a load current through an output terminal (219) of a step-down converter (101) to a light-emitting diode (110), wherein the method comprises:

   - receiving a dimming signal (161) for the light-emitting diode (110)

   - as a function of the dimming signal (161): alternately and periodically operating a first switch (201, 205, 291) of the step-down converter (101) and a second switch (202, 206, 292) of the step-down converter (101), wherein the second switch (202, 206, 292) is connected between a supply voltage terminal (211) of the step-down converter (101) and ground (215) in series with the first switch (201, 205, 291), wherein the second switch (202, 206, 292) is operated in the conductive state for an ON time (661), and

   - based on a choke current (701) of a storage choke (212): outputting the load current (702) to the light-emitting diode (110) through the output terminal (219), wherein the storage choke (212) and the first switch (201, 205, 291) are connected in series between the supply voltage terminal (211) and the output terminal (219),

   characterized by dimensioning the ON time (661) of the second switch (202, 206, 292) so that the polarity of the choke current (701) changes from positive to negative during the ON time (661) of the second switch (202, 206, 292) and the voltage reverses at the midpoint of the series connection of the first switch (201, 205, 291) and of the second switch (202, 206, 292).

Images