Driver circuit and method for driving an LED circuit

Abstract

Driver circuit with output terminals for the preferably dimmable control of an LED path (3) consisting of at least one LED, comprising: a clocked converter (1) which has at least one switching element (SK) which is connected on the input side to a voltage (U in ) and which has an electrical parameter (I LED , U out ) of the LED section (3), a control unit (2) which is designed to output a control signal for clocking the at least one switching element of the converter (1) in order to determine the electrical parameter (I LED , U out ) of the LED section (3) in a range above a minimum value (I min , U min ) by means of a feedback variable (7); and the control unit (2) is further designed to be below the minimum value (I min , U min ) of the electrical parameter (I LED , U out) to gradually change the timing of the at least one switching element of the converter (1) and / or the timing of a further switching element (S1) of the driver circuit by means of feedforward control, so that after receiving a switch-off command (6) the electrical parameter (I LED , U out ) of the LED section (3) is initially adjusted to the minimum value (I min , U min ) and then without feedback variable (7) in a range below the minimum value (I min , U min ) to gradually drive to zero with this forward control.

Description

[0001] The invention relates to the control of a lighting device, in particular an LED system with at least one LED, with the aid of a clocked converter. The present invention particularly relates to a driver circuit for operating at least one lighting device, e.g., by means of a clocked converter for operating at least one LED. The present invention further relates to a method for operating lighting devices, an integrated circuit, and a luminaire.

[0002] Driver circuits for operating LEDs are generally known from the prior art. Such a driver circuit is powered by an electronic power source and includes a converter responsible for regulating an electrical parameter, in particular a current or voltage, of the LED circuit.

[0003] DE 10 2014 200 437 A1 discloses an LLC converter as an example of a clocked converter used to control an LED circuit. Here, a half-bridge circuit is clocked using a switching signal from a control unit to supply an LLC resonant circuit with an alternating voltage. On a secondary side of the LLC resonant circuit, a current through an LED circuit is measured and fed to the control unit on the primary side via, for example, an optocoupler or an alternative data transmission method. The current through the LED circuit is a control feedback variable in the control loop of the driver circuit. Thus, an electrical parameter is controlled using a feedback variable.

[0004] A clocked converter for a driver circuit can thus be operated as a constant current source. The converter's clock frequency is varied to control the electrical parameters of the LED circuit. The current through the LED circuit or the voltage drop across the LED circuit is recorded as an actual value and compared with a target value in a control unit. The target value is, for example, a dimming value specification.

[0005] In today's converters, the change in the converter's clock frequency for controlling the electrical parameter using a feedback variable is limited to a minimum value of the electrical parameter. The converter is therefore unable to control the electrical parameter across its entire value range.

[0006] If the LED system is to be completely de-energized after receiving an external or internal switch-off command—i.e., the electrical parameter is effectively regulated to zero—the current method is to regulate the electrical parameter to its minimum value and then turn off the supply voltage. This abrupt switching causes sudden changes in the light output, for example, in the form of flickering and / or flashing effects. This is perceived as annoying by the user. This applies to both switching off and switching on the LED system. These abrupt changes in light output are undesirable and should therefore be avoided.

[0007] To prevent the sudden disappearance or reappearance of the light, one could attempt to further reduce the minimum value. However, due to technical constraints, no usable feedback value can be obtained in these cases. Furthermore, the converters would need to be controlled with very small, unfeasible pulse widths, which would immediately be perceived as flicker.

[0008] US 2012 / 0 120 342 A1 discloses a control circuit for a switching power supply configured to supply a drive voltage to a terminal of a light-emitting element to be controlled intermittently. US 2014 / 0 036 511 A1 discloses a work light with a light-emitting diode (LED) and a related method for controlling the LED work light. US 2014 / 0 354 170 A1 discloses a load control device for controlling the intensity of a lighting load, such as a light-emitting diode (LED) light source. US 2014 / 0 117 874 A1 discloses a power supply device that illuminates a light-emitting element and a luminaire.

[0009] It is therefore an object of the present invention to provide a driver circuit for controlling an LED path in which no abrupt changes in light output occur when the LED path is switched on or off.

[0010] This object is achieved by the driver circuit described in independent claim 1 and by the method described in independent claim 15 for operating an LED array consisting of at least one LED. Preferred embodiments are described in the respective dependent claims.

[0011] A clocked LED driver circuit can have the following: - Output connections to supply an LED track, - a converter clocked by at least one switch, - a control circuit that outputs a control signal to clock the converter.

[0012] Upon receiving a switch-off command, the control circuit is designed to continuously change the converter's timing over a specified time range greater than zero without the use of a feedback signal, such that the light output is gradually reduced and reaches zero at the end of the specified time range. This reduction can be achieved starting from a specified minimum value, above which dimmed operation or undimmed operation with feedback control occurs.

[0013] Alternatively or additionally, the control circuit can be designed to change the clocking of the converter continuously over a predetermined time range greater than zero without using a feedback signal upon receipt of a switch-on command in such a way that the light output is gradually increased and at the end of said time range has a predetermined minimum value above which dimmed operation or undimmed operation with feedback control can take place.

[0014] According to one aspect of the invention, a driver circuit for controlling an LED array consisting of at least one LED is provided. The driver circuit comprises a clocked converter, which is supplied with a voltage on the input side and regulates an electrical parameter of the LED array on the output side. A control unit is configured to change the clocking of the converter in order to regulate the electrical parameter of the LED array to a minimum value using a feedback variable.The control unit is further designed to gradually change the clocking of the converter and / or the clocking of another switching element of the driver circuit by means of feedforward control below the minimum value of the electrical parameter, so that after receiving a switch-off command, the electrical parameter of the LED path is first controlled to the minimum value by means of a feedback variable and then controlled to zero in a range below the minimum value without a feedback variable.

[0015] The driver circuit thus has a control unit that regulates the electrical parameter in a first value range using a feedback variable and in a second value range without using a feedback variable, preferably this feedback variable. The first value range of the electrical parameter preferably extends from a maximum value to a minimum value. The second value range of the electrical parameter extends from a minimum value to a zero value of the electrical parameter. Due to the design of the clocked converter, the minimum value is a value other than zero, for example 10 percent of the electrical parameter, preferably 5 percent of the electrical parameter, more preferably 1 percent of the electrical parameter.

[0016] The electrical parameter is, for example, the current through the LED circuit or the voltage across the LED circuit or the electrical power supplied to the LED circuit.

[0017] The control feedback variable is preferably an electrical variable that can be used to determine the actual value of the electrical parameter of the LED path. In one application, the control feedback variable is the measured current through the LED path. Alternatively or additionally, the control feedback variable is the measured voltage across one of the LEDs or the LED path. The control feedback variable can be measured directly or indirectly—for example, using electrical probes or an electrical coupling. The control feedback variable can also be determined from the emitted light power—for example, using a light power sensor.

[0018] The switch-mode converter is a circuit for voltage and / or current conversion as the basis for supplying power to the LED circuit using a periodically operating electronic switch and at least one energy storage device, such as a coil. The switch-mode converter is, for example, a step-down converter or buck converter, which produces a lower DC voltage at the output than at the input. The switch-mode converter is, for example, a step-up converter or boost converter, which produces a higher DC voltage at the output than at the input. The switch-mode converter is, for example, an inverse converter, or buck-boost converter, which can produce a higher or lower DC voltage at the output than at the input, but this voltage is inverted.

[0019] The control unit is configured to change the converter's clock speed. Clock speed refers, in particular, to the switching frequency—also referred to as the operating frequency—and the switch-on time and / or switch-off time of the converter. The switching frequency indicates the number of times a switching element of the converter switches on and off per second.

[0020] The driver circuit receives a switching command. This occurs, for example, via an interface, in particular via a DALI interface of the control unit from a central controller of the lighting system or via a mains switch. This switching command is intended to either switch the LED system on or off. To prevent a jump in the light output to bridge the second value range, a gradual feedforward control below the minimum value of the electrical parameter takes place. The control feedback variable is ignored, and the electrical parameter is controlled by changing the clock speed of the converter or by changing the clock speed of another switching element in the driver circuit.

[0021] Forward control is gradual, meaning the electrical parameter is controlled step by step by changing the timing to prevent an abrupt change in light output. This means there is no abrupt switching on or off, but rather a gentle control of the electrical parameter in the second value range, which can also be referred to as a "fade-in" or "fade-out." Gradual control occurs, in particular, over a fixed time range and preferably without human perception of the individual stages of the gradual control.

[0022] The gradual change is then a gradual minimization or a gradual increase of a manipulated variable, for example the clocking of the converter or the clocking of the other switching element.

[0023] If the LED path is now to be switched off, according to the invention the electrical parameter is first controlled in the first value range using the control feedback variable to a minimum value of the electrical parameter and then directly controlled in the second value range without using the control feedback variable to a value of zero percent of the electrical parameter.

[0024] If the LED path is now to be switched on, according to the invention the electrical parameter is first controlled in the second value range without using the control feedback variable from zero percent of the electrical parameter to the minimum value and then the electrical parameter is controlled in the first value range using the control feedback variable.

[0025] Detecting the control feedback variable at low values of the electrical parameter in the second value range has so far been too difficult and error-prone, so that control in this range simply does not occur in conventional driver circuits. According to the invention, a feedforward control is used for the second value range of the electrical parameter to prevent an abrupt change in light output.

[0026] In particular, the additional switching element is not a switching element of the converter, but is additionally incorporated into the driver circuit. The additional switching element is, for example, a field-effect transistor, which is operated by the control unit using a switching signal. The timing of the additional switching signal is changed by the control unit in order to regulate the electrical parameter in the second value range, i.e., below the minimum value of the electrical parameter, in feedforward control.

[0027] Controlling below the minimum value is not a possible permanent state of the LED circuit that would have to be maintained for any length of time, but a transient process that can be handled as feedforward control.

[0028] In a preferred embodiment, the additional switching element is arranged parallel to the LED path. For this purpose, the additional switching element preferably has a further coil connected in series, for example in the form of a storage choke, which, due to the clocking of the additional switch, must be designed to be correspondingly large or must be controllable using high switching frequencies. In this way, a short-circuit switch is introduced into the driver circuit. When the electrical parameter reaches zero, the LED path can be truly switched off by permanently closing the switch.

[0029] In an alternative embodiment, the additional switching element is arranged in series with the LED circuit. This series switching element interrupts the supply of the electrical parameter to the LED circuit, thus causing it to switch on or off. In principle, a newer-generation converter uses an LLC resonant circuit. The LLC resonant circuit is essentially a voltage source with an output capacitance in the nanofarrad range. Connecting a switch in parallel leads to undesirable voltage changes in the LED circuit, which in turn are perceived as abrupt changes in light output. Therefore, a switching element in series with the LED circuit is preferably used, in principle an anti-glow field-effect transistor.

[0030] Preferably, the control unit regulates the electrical parameter to 10 percent, preferably 5 percent, and more preferably 1 percent, of the maximum electrical parameter as a minimum value by changing the converter's clock rate and using the feedback variable. The minimum value is, for example, the minimum dimming value that can be regulated by evaluating the feedback variables. Below this minimum value, feedforward control is then applied.

[0031] Preferably, a control characteristic curve is stored in the control unit, with which the clocking of the converter and / or the clocking of the additional switching element of the driver circuit is gradually changed by means of feedforward control below the minimum value of the electrical parameter. The control characteristic curve specifies, for example, the number of steps of the gradual feedforward control. The control characteristic curve specifies, for example, the duration of the gradual feedforward control. The control characteristic curve specifies, for example, the step size of the gradual feedforward control. In this way, various characteristics can be implemented in the control unit, allowing individualized switching on and off.

[0032] The control characteristic is preferably linear. Thus, the timing of the converter or the further switching element is changed in such a way that a linear change in light output is achieved in the second value range, so that no abrupt changes in light output can occur. The linear control characteristic can be achieved, for example, by suitable control using a PWM switching signal by linearly shortening the pulse width of the PWM signal until the zero point of the electrical parameter is reached.

[0033] Alternatively, the control characteristic is non-linear, for example, logarithmic. This changes the clocking of the converter or the other switching element in such a way that a non-linear change in light output is achieved in the second value range. The non-linear control characteristic can, for example, be adapted to the behavior of the human eye, so that the lower values in the second value range, in particular, are set more slowly, since the human eye can better resolve the change in light as the LED section becomes darker. The control is thus designed so that the change in light output is harmonious for the eye.

[0034] Feedforward control below the minimum value is limited to a period of 0.5 to 3 seconds after receiving the switching command or after reaching the minimum value. Thus, this gradual feedforward control of the LED path is designed in such a way that it can be perceived by the human eye as the LED path going out or as a slow brightening of the LED path.

[0035] Preferably, the switching signal for the switching element of the converter and / or the additional switching element is a PWM signal. The timing can then be changed in such a way that the switching frequency is altered and / or the turn-on time and / or the turn-off time. Thus, the timing of the switching elements is modulated, with no difference between the different modulation types.

[0036] Preferably, the clocking of the additional switching element or the clocking of the converter should have a switching frequency greater than 100 Hertz, since clocking below 100 Hz would be perceived as flickering light. Furthermore, from a topological perspective, the minimum pulse width is limited to 1 microsecond. This makes it possible to gradually regulate the electrical parameter by changing the number of pulses themselves or the spacing between the pulses. At the end of the feedforward control, for example, a switching signal with a pulse duration of 1 microsecond and a switching duration (on time + off time) of 10 milliseconds is used to clock the converter or the additional switching element. This corresponds to 10,000 steps in the feedforward control.

[0037] In a preferred embodiment, the converter is an LLC converter. Upon receiving the switching command, the control unit gradually changes the clocking of a half-bridge circuit using feedforward control to change the electrical parameters of the LED circuit. When using an LLC converter, the voltage provided on the output side across the LED circuit must be changed. However, a change in the voltage amplitude changes the operating point of the LLC converter. This change in the operating point is counteracted by gradually changing the clocking of the LLC converter. In this way, the operating point frequency is gradually increased when switched off and gradually decreased when switched on.

[0038] Preferably, a final frequency of, for example, 200 kHz is set so that the feedforward control ends (or begins) when the switching frequency has been gradually changed to this value. For example, the switching frequency obtained at the minimum value using feedback control can be 100 kHz. The switching frequency is then gradually changed to the final switching frequency of 200 kHz to switch off the LED circuit. Conversely, when the lamps are switched on, the final switching frequency of 200 kHz is switched on first, which is gradually reduced until the minimum value is reached. The control change in the control unit of the driver circuit can be enabled by changing the software. The software change then affects the changed response to a switching command, for example a DALI ON / OFF command. The switching frequency is controlled in such a way that it can be adjusted towards the maximum.

[0039] The software adaptation now also makes it possible to further correct the difficult-to-obtain minimum value upwards. For example, a converter that is fundamentally designed to regulate to 1% of the electrical parameter can now be controlled to a minimum value of 10% using feedback variables. Gradual feedforward control then occurs below 10% to 0% of the electrical parameter.

[0040] In a further aspect of the invention, a method for operating lighting devices, in particular LEDs, is proposed. First, a converter is supplied with a voltage on the input side in order to regulate an electrical parameter of the LED path on the output side. Subsequently, a clocking of the converter is changed by means of a control unit in order to regulate the electrical parameter of the LED path to a minimum value using a feedback variable. Finally, the clocking of the converter and / or the clocking of another switching element of the driver circuit is gradually changed by means of feedforward control by the control unit, so that after receiving a switching command, the electrical parameter of the LED path is first regulated to the minimum value using a feedback variable and then gradually regulated without a feedback variable in a range below the minimum value.

[0041] In a further aspect, a computer program product is proposed which implements a method according to the preceding type when running in a computing device.

[0042] In a further aspect, an integrated circuit, in particular ASIC or microcontroller or a hybrid version thereof, is proposed, which is designed to implement a method according to the preceding type.

[0043] In a further aspect, a luminaire is provided, comprising an LED path and a driver circuit according to the type described above for controlling the LED path with a variable electrical parameter.

[0044] The invention and further embodiments and advantages of the invention are explained in more detail below with reference to figures, wherein the figures merely describe exemplary embodiments of the invention. Identical components in the figures are provided with the same reference numerals. The figures are not to be considered to scale; individual elements of the figures may be exaggeratedly large or oversimplified. Fig. 1 shows a first embodiment of a driver circuit according to the invention; Fig. 2 shows a second embodiment of a driver circuit according to the invention; Fig. 3 a current-time characteristic curve of an LED path with conventional control and with control according to the invention; Fig. 4 shows a third embodiment of a driver circuit according to the invention; Fig. 5 shows a fourth embodiment of a driver circuit according to the invention; Fig. 6 shows a further current-time characteristic curve for control according to the invention; Fig. 7 shows a fifth embodiment of a driver circuit according to the invention; Fig. 8 shows a voltage-frequency characteristic curve for the inventive control of an LLC converter.

[0045] Fig. Figure 1 shows a first embodiment of a driver circuit according to the invention. A converter 1 is configured to generate a voltage U IN The converter 1 has at least one switching element S K , which is clocked by a control unit 2 and a switching signal generated by the control unit 2. The converter 1 also has at least one storage unit, for example in the form of a coil, so that the converter can be regarded as a clocked switching regulator. On the output side, the converter 1 is configured to store an electrical parameter, in particular a current I LEDor an electrical voltage U out , to an LED track 3. The converter 1 can regulate the electrical parameter of the LED track 3 by controlling the timing of the switching element S K of the converter 1 is changed by the control unit 2. This control of the electrical parameter is achieved by comparing an actual value of the electrical parameter and a target value, which is provided to the control unit 2, for example, via a DALI interface 6. The control unit 2 receives the actual value by detecting a feedback variable 7, for example, the current measurement or the voltage measurement.

[0046] LED strip 3 is shown here as an example with one LED. The actual wiring of LED strip 3 is not limited. Thus, both the number and colors, as well as the electrical arrangement in series or parallel, can vary for this LED strip 3.

[0047] The control unit 2 receives commands via an interface 6 (not shown in detail) to control the LED strip 3. For example, a DALI interface can be provided on the control unit 2 to receive commands for dimming the LED strip 3. Dimming occurs from a maximum value of the electrical parameter to a minimum value of the electrical parameter using the feedback variable 7, i.e., within a first value range of the electrical parameter. The minimum value I min , U min corresponds to a certain percentage of the electrical parameter.

[0048] The converter 1 is designed to regulate the electrical parameter down to 1% as a minimum value using a feedback variable 7. If a switching command is now transmitted to the control unit 2, for example via the DALI interface or a switch of the lighting unit, the LED path 3 is initially set to the minimum value I min, U min regulated.

[0049] Below the minimum value I min , U min The electrical parameter is controlled by means of gradual feedforward control, ie without using the feedback variable 7. On the one hand, the timing of the converter 1 is changed. On the other hand, the control unit 2 generates a further switching signal, which is provided to a further switching element S1. This further switching element S1 is according to Fig. 1 is arranged in series with the LED circuit 3 and is controlled by a PWM signal. At the end of the control, switch S1 also serves to interrupt the power supply to the LED circuit 3. The PWM signal is gradually changed in the second value range of the electrical parameter, in particular the pulse width T on gradually reduced and / or the switch-off time T offIf the PWM signal is to be gradually increased, a switch-off command should be received. For this purpose, feedback variable 7 is ignored. Alternatively, a switch-on command can be applied to control unit 2 via interface 6. In this case, the electrical parameter is controlled from 0% to the minimum value without feedback variable 7 by gradually changing the timing.

[0050] In this way, the light output of the LED array will not change abruptly due to a switching command. The gradual change in the electrical parameter until the actual zero point of the electrical parameter is reached results in the LED array 3 being switched off, which, to the eye, occurs without flickering or an abrupt change in light output.

[0051] In Fig. 2 shows a second embodiment of a driver circuit according to the invention. Contrary to the serial connection of the further switching element S1, Fig. 2, switch S1 is arranged parallel to LED path 3. This arrangement corresponds to a true short-circuit switch to bridge the power supply. Converter 1 has a significant output capacitance. The parallel arrangement of switching element S1 can cause current / voltage spikes during switching, which in turn could cause flickering. Therefore, it is advantageous to connect a coil L to switch S1. S is connected in series in order to suppress such current / voltage peaks.

[0052] In Fig. Figure 3 shows a comparison of a current-time curve for a conventional control 4 and for a control 5 according to the invention of an LED section 3. In a first value range 8 between the maximum value I max and the minimum value I min of current I LEDThe control is carried out using the feedback variable 7 as an electrical parameter. This control in area 8 is identical for both control 4 and control 5.

[0053] In a second value range 9 between the minimum value I min and the zero value I0 of the current I LED no control takes place according to traditional control 4. This leads from time t x to an abrupt drop in current I LED to the zero value I0. This abrupt current drop causes a disturbing, abrupt change in light output, which is undesirable.

[0054] According to the invention, in the second value range 9 between the minimum value I min and the zero value I0 of the current I LED the control 5 is carried out by forward control of the current I LED . The control characteristic according to control 5 is linear, i.e. the gradual change to control the current I LEDis gradually reduced. This is achieved, for example, by linearly changing the turn-on time of a PWM signal of the additional switching element S1 or by linearly changing the clocking of the converter to change its switching frequency.

[0055] As soon as a switch-off command is received via interface 6, the current I LED using control feedback variable 7 to the minimum value I min and then controlled to zero by means of feedforward control. This feedforward control can - as in Fig. 3 - to achieve the simplest possible control. Alternatively, this control can be non-linear, for example, to achieve the fastest possible switch-off and, if necessary, to exploit the eye's inertia when the light sources 3 become dark.

[0056] This control 5 is controlled by the Fig. 1 and Fig. 2. Either an additional switching element S1 is supplied with a variable PWM signal and / or the clock speed of converter 1 itself is varied accordingly. It should be noted that the clock speed should not fall below a frequency of 100 Hz to avoid flickering.

[0057] In Fig. 4 shows a third embodiment of a driver circuit according to the invention. Here, the Fig. 2 is used, so that the further switch S1 is arranged parallel to the LED section 3. In contrast to Fig. Figure 2 shows the converter 1 in detail. This is a step-down converter, also known as a step-down converter or "buck converter". The buck converter has a series circuit consisting of a converter diode D and the switching element S K The converter 1 is powered by an input voltage U inBetween the anode of diode D and a first contact of switch S K A converter coil L is arranged as a storage element. The LED section 3 is arranged at the output of the converter 1. The control unit 2 influences the switching element S K with a switching signal to provide a constant current I LED This current I LED can be done by changing the timing of the switch S K between I max and I min controlled using feedback variable 7. Control unit 2 compares feedback variable 7 as an actual value with a target value and sets the clocking of converter 1. Clocking refers to both the switching frequency and the on / off time.

[0058] Below the minimum value I min the regulation of the current I LEDby varying the timing of the further switching element S1. Converter 1 therefore continues to supply the current I min , which is now reduced by means of the further switching element S1 without feedback variable 7.

[0059] This situation is very easy to implement with buck converters operating in the so-called continuous mode. A short-circuit switch S1 can be easily inserted here. This simplicity needs to be put into perspective: The switch S K of the buck converter 1 must be able to maintain the minimum current value I min to feed into a short circuit, meaning they can handle very short turn-on times. For discontinuous or boundary mode systems, there is a risk of visible flicker due to interference between the converter frequency and the short-circuit frequency.

[0060] In Fig. 5 shows a fourth embodiment of a driver circuit according to the invention. In contrast to Fig. 4 is in Fig. 5 a series circuit of the switch S1 is used to Fig. 1 discussed advantages.

[0061] In Fig. 6 is a current-time curve for the Fig. 4 and Fig. 5 illustrated embodiments of a driver circuit. Here, the switching signal for the further switching element S1 is from the time t x It can be seen that the further switching element S1 is a PWM signal, which has an increasing switch-off time T1, T2, T3, with a constant switch-on time t on .

[0062] In any case of modulation, the minimum frequency must not fall below 100 Hz, as this would otherwise be perceived as flickering. On the other hand, a minimum pulse width t ONFrom a driver technology perspective, the time must not be less than one microsecond.

[0063] Thus, either the number of pulses t ON be reduced or the interval T1, T2, T3 between the individual pulses can be changed. In this way, the current I LED through the LED circuit 3 is gradually reduced without using a control feedback variable 7. At the end of the gradual control, only a pulse with a pulse width of 1 µs and a period of 10 milliseconds is set as the minimum level. At frequencies above 100 Hz and a minimum pulse width of 1 µs, the current I LED below the minimum value I mincan be decreased or increased in 10,000 steps. Assuming that converter 1 can regulate a minimum value of 1% of the electrical parameter, a 0.0001% reduction can be achieved with each step using the pulse pauses and corresponding pulse widths of the PWM signal, so that in the first step, the electrical parameter is reduced from 1% to 0.9999%.

[0064] Since the control unit 2 has a finite bit width, for example 12 bits, 4,096 steps can be provided as gradual control in the second value range 9.

[0065] Thus, in 4,096 steps of 1%, the minimum value I MINto 0% of the electrical parameter. This occurs in such a gentle manner that it is not perceived as an abrupt change in light output. For practical applications, 100 steps, or a maximum of 1,000 steps, are sufficient, as the human eye cannot resolve a finer light difference.

[0066] In Fig. Figure 7 shows a fifth embodiment of a driver circuit according to the invention. Converter 1 is shown here as an LLC converter. The operation of an LLC converter is known in principle and will be assumed to be known here. Merely for the sake of completeness, it is pointed out that the voltage V applied on the input side in a direct voltage that is split into an alternating voltage by means of a half-bridge circuit. A resonant circuit - consisting of the elements L R , L M , C R- converts this received alternating voltage into an LED voltage U provided on the output side out for the LED circuit 3. For galvanic isolation from the possibly high input voltage U in For the voltage, which is obtained, for example, from a mains voltage of 230 volts, a transformer T consisting of two coils is provided. The secondary coil has a center tap to enable easy rectification.

[0067] According to Fig. 7, the control according to the invention is achieved by changing the operating point of the LLC converter. The control unit 2, for example in the form of a microcontroller, drives the half-bridge circuit using a complementary switching signal. This switching signal has a switching frequency. By changing the frequency of the switching signal at the half-bridge circuit, the output voltage U outcan be varied, thereby varying the light output. The lower the switching frequency, the higher the output voltage and thus the light output at the output of converter 1.

[0068] Thus, by changing the switching frequency of the half-bridge, the voltage U out controlled via the LED section 3. This control takes place up to a minimum value U min the voltage U out by means of a control feedback variable 7. Below the minimum value U min The switching frequency of the half-bridge is controlled without feedback variable 7 in order to avoid an abrupt change in light output.

[0069] For LLC converters 1, a short-circuit switch S1 is not optimal. The LLC converter is essentially a voltage source with a capacitance of several hundred nanofarrads at the output. Therefore, a breaker switch in series with the LED string would be more appropriate (the "anti-glow FET" principle). However, the losses during normal operation must be taken into account here. For this reason, LLC converters 1 usually lack any intelligent control circuitry on the secondary side. This makes controlling such a switch S1 difficult or cumbersome; optocouplers could be used, for example.

[0070] A solution not shown would be to use an auxiliary winding on the transformer T. It would be conceivable to periodically short-circuit an auxiliary winding including the rectifier, and thus to reduce the current I LED from the LLC converter output. At some point the voltage U out secondary side too small to generate a current I through the LED path 3 LEDIn a permanent short circuit, the LLC converter 1 then only pumps reactive power between the resonance capacitor C r and the leakage inductance L r back and forth.

[0071] For safety reasons (Selv), the auxiliary winding might have to be mounted on the primary side of the transfer T. However, this would then have a better coupling than the secondary winding and thus possibly reduce the effective leakage inductance L r If an additional rectifier were added to the secondary of the main winding and periodically short-circuited, no auxiliary winding would be necessary, the problem with the coupling change and the output capacitors would be easily solved, but control via the SELV path would still be necessary.

[0072] An LLC converter 1 is essentially a voltage converter. Large voltage regulation ranges are difficult to handle because zero-voltage or zero-current operation is not possible. The regulation of the voltage U out is thus in the ratio 1 to 2, for example in the range between U min = 50 and U max = 100 Volts. If you now further restrict the voltage range and require that at least enough LEDs be connected to this converter so that the voltage is above 80V, then the current can be set to zero, since at U min = 50V due to the added limit voltages U cut-off no more current can flow.

[0073] As soon as the LED voltage is at least 1 / 6 above the minimum, around 60V at minimum value U min of 50V, the current I LED be set to zero.

[0074] In Fig.Figure 8 shows a voltage-frequency curve for an inventive control of an LLC converter. Here, the output voltage U out maximum when the frequency f of the half-bridge circuit is minimum. With increasing frequency f of the switching signal for the half-bridge circuit of the LLC converter 1, the voltage U out at the output of converter 1 and thus the light output emitted by the LED path 3. The LLC converter 1 can also only be operated up to a minimum value U min of the electrical parameter U out to operate the LED circuit 3 using feedback variable 7. For this minimum value U min an operating frequency f1 is set.

[0075] By applying the control according to the invention, the operating frequency f of the half-bridge circuit of the LLC converter 1 is gradually increased so that the output voltage U outgradually decreases. At a frequency f2, the limit voltage Ucut -off of the LED section 3 is reached and the LED section 3 goes out completely. The operating frequency f2 represents the final frequency, which can be set using the control unit 2.

[0076] All features shown, illustrated and described can be combined with each other as desired. Reference symbol 1 Switched converter 2 control unit 3 LED track, LED 4 Traditional control 5 Control according to the invention 6 switching command, DALI interface 7 Feedback variable 8 First value range 9 Second value range S k Switching element of the converter S1 Additional switching element D converter diode L Converter coil L S switch coil T transformer of the LLC converter ILED Current through the LED track I max Maximum value of the current through the LED path I min Adjustable minimum value of the current through the LED path f1 Working frequency of the converter at minimum value f2 Operating frequency of the converter at the cutoff frequency of the LED U out Voltage across the LED path U max Maximum value of the voltage across the LED path U min Adjustable minimum value of the voltage across the LED section U min U cut-off Turn-off voltage of the LED circuit

Claims

[1] Driver circuit with output terminals for the preferably dimmable control of an LED path (3) consisting of at least one LED, comprising: a clocked converter (1) which has at least one switching element (SK) which is connected on the input side to a voltage (U in ) and which has an electrical parameter (I LED , U out ) of the LED section (3), a control unit (2) which is designed to output a control signal for clocking the at least one switching element of the converter (1) in order to determine the electrical parameter (I LED , U out ) of the LED section (3) in a range above a minimum value (I min , U min ) by means of a feedback variable (7); and the control unit (2) is further designed to be below the minimum value (I min , U min ) of the electrical parameter (I LED , U out) to gradually change the timing of the at least one switching element of the converter (1) and / or the timing of a further switching element (S1) of the driver circuit by means of feedforward control, so that after receiving a switch-off command (6) the electrical parameter (I LED , U out ) of the LED section (3) is initially adjusted to the minimum value (I min , U min ) and then without feedback variable (7) in a range below the minimum value (I min , U min ) to gradually drive to zero with this forward control. [2] Driver circuit according to claim 1, wherein the further switching element (S1) is arranged parallel to the LED path (3), preferably wherein the further switching element (S1) is connected in series with a further coil. [3] Driver circuit according to claim 1, wherein the further switching element (S1) is arranged in series with the LED path (3). [4] Driver circuit according to one of claims 1 to 3, wherein the control unit (2) by changing the timing of the at least one switching element of the converter (1) and by means of the feedback variable (7) the electrical parameter (I LED , U out ) to 10 percent, preferably 5 percent, more preferably 1 percent, of the maximum electrical parameter (I LED , U out ) as minimum value (I min , U min ) regulates. [5] Driver circuit according to one of claims 1 to 3, wherein a control characteristic curve is stored in the control unit (2) with which, below the minimum value (I min , U min ) of the electrical parameter (I LED , U out ) the timing of the at least one switching element of the converter (1) and / or the timing of the further switching element (S1) of the driver circuit can be gradually changed by means of feedforward control. [6] Driver circuit according to claim 5, wherein the drive characteristic curve is linear. [7] Driver circuit according to claim 5, wherein the control characteristic curve is non-linear, for example logarithmic. [8] Driver circuit according to one of claims 1 to 6, wherein the changing of the electrical parameter (I LED , U out ) below the minimum value (I min , U min ) for a period of 0.5 seconds to 3 seconds after receiving the switch-off command (6) or after reaching the minimum value (I min , U min ) is limited. [9] Driver circuit according to one of claims 1 to 8, wherein a switch-on time and / or a switch-off time of the at least one switching element of the converter (1) or of the further switching element (S1) is gradually changed by means of the control unit (2). [10] Driver circuit according to one of claims 1 to 9, wherein the clocking of the further switching element (S1) or the clocking of the at least one switching element of the converter (1) has a switching frequency greater than 100 Hertz. [11] Driver circuit according to one of claims 1 to 10, wherein the timing of the further switching element (S1) or the timing of the at least one switching element of the converter (1) is gradually changed in one thousand steps, preferably one hundred steps. [12] Driver circuit according to one of claims 1 to 11, wherein the converter (1) is an LLC converter and the control unit (2) gradually changes the timing of a half-bridge circuit by means of feedforward control after receiving the switch-off command (6). [13] Driver circuit according to claim 12, wherein the switching frequency for the LLC converter (1) has a fixed predetermined final value, for example 200 kilohertz. [14] Driver circuit according to claim 12 or 13, wherein upon receipt of the switch-off command, the switching frequency of the LLC converter (1) is gradually increased; and when a switch-on command is received, the switching frequency of the LLC converter (1) is gradually reduced. [15] Method for operating an LED track (3) consisting of at least one LED, comprising the following steps: Supplying a converter (1) on the input side, which has at least one switching element (S K ), with a voltage (V in ) to provide an electrical parameter (I LED , U out ) of the LED section (3); Changing a timing of the at least one switching element of the converter (1) by means of a control unit (2) in order to change the electrical parameters (I LED , U out ) of the LED section (3) by means of a feedback variable (7) to a minimum value (I min , U min ) and Changing the timing of the at least one switching element of the converter (1) and / or the timing of a further switching element (S1) gradually by means of feedforward control by the control unit (2), so that after receiving a switch-off command the electrical parameter (I LED , U out ) of the LED section (3) is initially adjusted to the minimum value (I min , U min ) and then without feedback variable (7) in a range below the minimum value (I min , U min ) is gradually regulated. [16] Integrated circuit, in particular ASIC or microcontroller or a hybrid version thereof, which is designed to implement a method according to claim 15. [17] Luminaire, comprising an LED track and a driver circuit according to one of claims 1 to 14 for controlling the LED track (3) with a variable electrical parameter (I LED , U out ).

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