Operating circuit for leds, having dimming signal comprising high-frequency modulated pulse packet signal with harmonised frequencies

0001The invention relates to an operating circuit with light-emitting diodes as well as a method according to the preamble of the independent patent claims.

Technical area

0002Semiconductor light sources such as, for example, light-emitting diodes have become increasingly interesting for lighting applications in recent years. One of the reasons for this is the fact that decisive technical innovations and great advances in both brightness and light efficiency (light output per watt) of these light sources have been achieved. Light-emitting diodes have also become an attractive alternative to conventional light sources, such as incandescent or gas discharge lamps, due to their comparatively long life.

State of the art

0003Semiconductor light sources are sufficiently known from the state of the art and are hereinafter abbreviated as LEDs (light-emitting diodes). In the following, this term is intended to encompass both light-emitting diodes made of inorganic materials and light-emitting diodes made of organic materials. It is known that the light radiation of LEDs correlates with the current flow through the LEDs.

0004For brightness control, LEDs are therefore basically operated in a mode in which the current flow is controlled by the LED.

0005In practice, switching regulators, for example step-down converters or buck converters, are preferably used for controlling an arrangement of one or more LEDs. Such a switching regulator is, for example, shown in FIG<patcit id="pcit0001" dnum="DE102006034371A1"><text>DE 10 2006 034 371 A1</text></patcit> known. In this case, a control unit controls a high-frequency clocked switch (for example a power transistor). When the switch is switched on, current flows through the LED array and a coil is charged. The intermediate energy of the coil discharges in the switched-off state of the switch via the LEDs (free-running phase). The current through the LED arrangement shows a zigzag-shaped time profile: when the switch is switched on, the LED current shows a rising edge, a falling edge results when the switch is switched off. The time average of the LED current represents the effective current through the LED arrangement and is a measure of the brightness of the LEDs. The average, effective current can be controlled by means of appropriate clocking of the circuit breaker.

0006The function of the operating device is now to set a desired average current flow through the LEDs and to keep the time fluctuation width of the current as low as possible due to the high-frequency switching on and off of the switch (typically in the range above 10 kHz).

0007A large fluctuation range of the current (ripple or ripple) has a disadvantageous effect, especially in the case of LEDs, since the spectrum of the emitted light can change as the current amplitude changes.

0008In order to keep the emitted light spectrum as constant as possible during operation, it is known not to vary the current amplitude in the case of LEDs for brightness control but to use a so-called pulse-width modulation (PWM) method. In this case, low-frequency (typically with a frequency in the range of 100-1000 Hz) pulse packets are supplied with pulse amplitudes (with time average) constant current amplitude to the LEDs by the operating device. The above-mentioned high-frequency ripple is superimposed on the current within a pulse packet. The brightness of the LEDs can now be controlled by the frequency of the pulse packets; The LEDs can, for example, be dimmed by increasing the time interval between the pulse packets.

0009A practical requirement for the operating device is that it can be used as flexibly and in a versatile manner, for example, regardless of how many LEDs are actually connected and operated as a load. The load may also change during operation, for example when an LED fails. In conventional technologies, the LEDs are operated in a so-called 'continuous conduction mode' or non-sweeping mode. That procedure is based on the<figref idrefs="f0001">1a and 1b</figref> (State of the art).

0010Im in the <figref idrefs="f0001">FIG. 1a</figref> Is shown as a basic circuit, a buck converter for operating at least one LED (or several series-connected LEDs), which has a first switch S1. The operating circuit is supplied with a DC voltage or a rectified alternating voltage U0.

0011In the switched-on state of the first switch S1 (during the duration t_on), energy is built up in the coil L1 which discharges at least at least one LED in the switched-off state of the first switch S1 (duration t_off). The resulting temporal current profile is shown in FIG<figref idrefs="f0001">FIG. 1b</figref> (State of the art). Two pulse pulses of the PWM are shown. The current profile within a pulse packet is also shown in an enlarged view. For reasons of color consistency, the amplitude of the ripple should be as small as possible within a pulse packet. This can be done by appropriately selecting the switch-on time t0 and switch-off time t1. For example, these times can be selected such that the first switch S1 is switched on when the current falls below a certain minimum reference value and the switch is switched off when the current exceeds a maximum reference value. However, this method has several disadvantages: on the one hand, in order to achieve the smallest possible ripple, a rapid sequence of switching-on and switching-off processes is necessary. The slope (positive or negative edge) of the current is, in fact, not controllable by the operating device and must be regarded as given since it is determined, inter alia, by the inductance of the coil L1 and by the power consumption of the LEDs.

0012Owing to tolerances in the components of the operating circuit and also due to the limited resolution of the clock units, flicker phenomena or other disturbances can occur.

0013The <patcit id="pcit0002" dnum="US20100156319A1"><text>US 2010/0156319 A1</text></patcit>, Which serves as a basis for the preamble of the independent claims, discloses an operating circuit for an LED lighting system. A pulse width modulation can be used.

0014The <patcit id="pcit0003" dnum="US20030085749A1"><text>US 2003/0085749 A1</text></patcit> Discloses an operating circuit for light-emitting diodes, in which two-fold pulse-width modulation is used. By changing the low-frequency component, the average current can be influenced by an LED module.

DESCRIPTION OF THE INVENTION

0015It is the object of the present invention to provide an improved operating circuit for at least one LED and a method for operating at least one LED which is simple in contrast to the state of the art, which allows a simple maintenance of the current and thus of the LED power.

0016This object is achieved according to the invention by the features of the independent claims. The dependent claims form the central idea of ​​the invention in a particularly advantageous manner.

0017According to a first aspect of the invention, a direct voltage or a rectified alternating voltage is supplied to the operating circuit for at least one LED. A supply voltage for at least one LED is provided by means of a sink and a first switch, clocked by a control unit, an energy being temporarily stored in the coil when the first switch is turned on, which energy is discharged via a diode and above the at least one LED when the first switch is switched off,

0018The control unit controls the first switch with a dimming signal, the dimming signal being generated by a combination of a low-frequency signal and a high-frequency signal, and this connection preferably being arranged within the control unit. The pulse width (TON * LF) and, optionally, the period duration of the low-frequency signal are selected or fixed as an integer multiple of the period duration of the high-frequency signal.

0019A change in the brightness of the LED can be effected by changing the frequency of the low-frequency signal. The frequency is changed by changing the switch-off time of the low-frequency signal. Preferably, the turn-off period of the low-frequency signal is increased when the brightness is to be decreased, and the turn-off period of the low-frequency signal is decreased when the brightness is to be increased.

0020The control unit can select the switch-off time of the first switch so that as little switching losses occur as possible, and nevertheless the current flow through the at least one LED has as small a ripple as possible.

0021For example, the operating circuit has a first sensor unit which generates a first sensor signal dependent on the current flow through the first switch and / or a second sensor unit which detects the demagnetization of the coil and generates a second sensor signal. The second sensor unit can, for example, also detect the voltage or the current through the LED and generate a second sensor signal as a function thereof. The sensor signals are fed to the control unit and processed.

0022For example, the control unit uses a signal from the first sensor unit or a signal from the second sensor unit or a combination of the two signals for determining the on and / or switch-off time and / or the duty factor of the first switch.

0023For example, the control unit switches off the first switch when the current through the first switch exceeds a maximum reference value and, optionally, switches on again when the current through the LED drops below a minimum reference value, for example when the coil is demagnetized and / or the Diode. The time of restarting can also be determined by a predetermined frequency.

0024In a preferred embodiment of the invention, the first sensor unit is a measuring resistor (shunt). In a further embodiment of the invention, the second sensor unit is a secondary winding inductively coupled to the coil or a Hall sensor or the second sensor unit detects the demagnetization of the coil by monitoring the voltage above the first switch by means of an (ohmic) voltage divider.

0025In a further embodiment of the invention, the operating circuit for at least one LED is formed by a resonant or quasi-resonant circuit. A supply voltage for at least one LED is provided by means of at least one first switch clocked by a control unit, the first switch feeding a resonant circuit in the switched-on state, which preferably has at least one coil as inductor. An energy is temporarily stored in the resonant circuit, preferably in the coil, which, when the first switch is switched off, discharges via a diode and above the at least one LED. A capacitor, which is arranged directly or indirectly parallel to the LED, can also be connected between the diode and the LED as a smoothing capacitor. The resonant circuit may also have a resonant capacitor. The coil can also be part of a transformer or, in addition to the coil, a transformer can also be arranged in the resonant circuit. The first switch can also be part of an inverter, for example an alternating clocked half-bridge.

0026The invention also relates to a method for controlling at least one LED.

0027Further preferred embodiments and further developments of the invention are the subject of further subclaims.

0028BRIEF DESCRIPTION OF THE DRAWINGS The present invention is described in more detail below on the basis of preferred exemplary embodiments with reference to the attached drawings.<ul><li><figref idrefs="f0001">FIG. 1a</figref> 10 shows a circuit arrangement according to the known prior art </li><li><figref idrefs="f0001">FIG. 1b</figref> 12 shows a diagram with the temporal progression of the LED current in the circuit arrangement of FIG <figref idrefs="f0001">FIG. 1a</figref> (State of the art)</li><li><figref idrefs="f0002">FIG. 2a</figref> 10 shows a first example of an operating circuit (buck) for LEDs</li><li><figref idrefs="f0002">FIG. 2b</figref> FIG. 12 is a graph showing time-dependent current waveforms and control signals in the FIG <figref idrefs="f0002">FIG. 2a</figref> Shown in FIG</li><li><figref idrefs="f0003">FIG. 3 and FIG. 4</figref> Show specific embodiments of an operating circuit</li><li><figref idrefs="f0004">FIG</figref> 10 shows a modification of the circuit of FIG <figref idrefs="f0002">FIG. 2a</figref> (Buck-boost)</li><li><figref idrefs="f0004">FIG</figref> 10 shows another specific embodiment of an operating circuit</li><li><figref idrefs="f0005">FIG</figref> Shows a first example of an embodiment of a control unit SR according to the invention</li><li><figref idrefs="f0005">FIG</figref> A shows an example of a control according to the invention of an operating circuit for LEDs</li><li><figref idrefs="f0005">FIG. 8b</figref> 10 shows an example of a control of an operating circuit for LEDs according to the prior art</li><li><figref idrefs="f0006">FIG</figref> 10 shows another embodiment of an operating circuit</li></ul>

0029<figref idrefs="f0001">1a and 1b</figref> Show the state of the art.

0030In the <figref idrefs="f0002">FIG. 2a</figref> Is used to operate at least one (or several series-connected and / or parallel-connected) LED. In the illustrated example, for example, two LEDs are connected in series, but, of course, only one or more LEDs can be used.

0031The LEDs or the series-connected and / or parallel-connected LEDs are also referred to as LED sections in the following. An advantage of the present invention is that the operating circuit adapts very flexibly to the type and number of serially connected LEDs. A DC voltage U0 is fed to the circuit, which of course can also be a rectified AC voltage. However, DC voltage U0 is preferably a constant DC voltage, but it can have a small alternating current component as a ripple. The LEDs are connected in series with a coil L1 and a first switch S1.

0032In addition, the circuit arrangement has a diode D1 (the diode D1 is connected parallel to the LEDs and the coil L1) and, optionally, a capacitor C1 connected in parallel with the LEDs. When the first switch S1 is turned on, current flows through the LEDs and through the coil L1, which is thereby magnetized. In the switched-off state of the first switch S1, the energy stored in the magnetic field of the coil discharges in the form of a current via the diode D1 and the LEDs. At the same time, the capacitor C1 is charged at the beginning of the switching-on of the first switch S1. During the switch-off phase of the first switch S1 (free-running phase), the capacitor C1 discharges and contributes to the current flow through the LED path. Given a suitable dimensioning of the capacitor C1, this leads to a smoothing of the current through the LEDS.

0033The first switch S1 is preferably a field effect transistor or also a bipolar transistor. The first switch S1 is switched to high frequency, typically in a frequency range above 10 kHz, preferably above 50 kHz. An advantage of this operating mode is that the first switch S1 is protected during operation when, as explained later, it is preferably switched on when the power applied to it is almost zero. In the state of the art, on the other hand, where the switching operations take place under high power, a high-quality component with a very short switching time must be used for the first switch S1 in order to keep the switching losses in a tolerable frame. An advantage of this operating mode is that for the first switch S1 and the diode D1, a comparatively cheaper component with comparatively somewhat longer switching duration or longer clearing time can also be used.

0034In the circuit of <figref idrefs="f0002">FIG. 2a</figref> A control unit SR is provided, which specifies the clocking of the first switch S1 for controlling the LED power.

0035The control unit SR uses, as input variables, the signals from a first sensor unit SE1 and / or signals from a second sensor unit SE2 to determine the precise switch-on and output time of the first switch S1.

0036The first sensor unit SE1 is arranged in series with the first switch S1 and detects the current flow through the first switch S1. This serves to monitor the current flow through the first switch S1. If the current flow through the first switch S1 exceeds a specific maximum reference value, the first switch S1 is switched off. In an advantageous embodiment, the first sensor unit SE1 can, for example, be a measuring resistor (shunt or current measuring resistor).

0037For monitoring the current flow, the voltage drop can then be tapped off at the measuring resistor (shunt) and, for example, compared with a reference value by means of a comparator.

0038If the voltage drop at the measuring resistor (shunt) exceeds a certain value, the first switch S1 is switched off.

0039The second sensor unit SE2 is arranged within the current branch, which is traversed by the current during the freewheeling phase, preferably in the vicinity or on the coil L1 or else in series or parallel to the LED (for example as a current mirror). With the aid of the second sensor unit SE2, the control unit SR can set a suitable point in time for the switch-on time of the first switch S1.

0040In a possible mode of operation, the first switch S1 is preferably switched on when the current through the coil L1 is zero for the first time or is at least very small, that is to say preferably in the time range when the diode D1 blocks at the end of the freewheeling phase. In this case, the current as low as possible is applied to the switch S1 at the switch-on time of the first switch S1.

0041By recognizing the current zero crossing through the coil L1, nearly loss-free switching is enabled. Preferably, the current through the LEDs shows only low ripple and does not vary greatly. This is due to the smoothing effect of the capacitor C1 connected in parallel to the LEDs. During the phase of a small coil current, the capacitor C1 assumes the supply of the LED.

0042The switch-off duration of the switch S1 can, however, also be predetermined by a fixed predetermined frequency. In this case, the switch-off duration of the switch S1 is obtained from the remaining remaining time of the period duration of the clock frequency of the switch S1 (also referred to as a high-frequency signal in the following), ie the difference between the period duration and the switch-on time of the switch S1.

0043The individual current flows and the optimum switch-on time of the first switch S1 are to be determined using the diagram in FIG <figref idrefs="f0002">FIG. 2b</figref> Will be explained.

0044Similar to diagram in <figref idrefs="f0001">FIG. 1b</figref> The time profile of the current i_L is represented by two pulse packets.

0045The diagram shows the current flow within a PWM pulse packet: The time curve of the current i_L through the coil L1, the time curve of the current i_LED is plotted by the LEDs and the time profile of the state of the first switch S1 First switch S1 is switched off, the switch is closed in state 1, and the signals for the state of switch S1 correspond to the control signal (ie, at the gate) of switch S1). At time t_0, the first switch S1 is closed and a current begins to flow through the LED and the coil L1. The current i_L shows a rise according to an exponential function, wherein a quasi-linear increase of the current i_L can be seen in the region of interest here. I_LED differs from i_L in that part of the current i_L contributes to the charge of the capacitor C1. The opening of the first switch S1 at time t_1 (for example, when a desired maximum reference value is reached) results in the energy stored in the magnetic field of the coil being discharged via the diode D1 and the LEDs or the capacitor C1. The current i_L continues to flow in the same direction, but decreases continuously and can even reach a negative value. A negative current (ie a current flow with the opposite direction) is present as long as the charge carriers, which were previously enriched in the conducting-polarized diode D1, are made from the barrier layer of the diode D1.

0046The current i_LED, on the other hand, decreases only slightly and is maintained, since the capacitor C1 has a smoothing effect. At time t_2, the diode is blocked. The current i_L decreases (but is still negative) and goes to zero. In this phase, parasitic capacitances at the diode D1 and further parasitic capacitances are re-charged in the remaining circuit.

0047The voltages at the node Ux above the first switch S1 and at the coil L1 change very rapidly during this period. The voltage at the node Ux falls to a low value (due to the blocking of the diode D1). An advantageous restarting time t_3 for the first switch S1 is now provided when the current i_L reaches the zero crossing, or at least the proximity of the zero crossing. At this time, the coil L1 is not magnetized. The first switch S1 can be switched on at this time with very low losses since hardly current flows through the coil L1. However, a restart is also possible already at time t_2 or shortly before, since the current through coil L1 is very low in this time range.

0048A second sensor unit SE2 now serves to detect the advantageous switch-on time for the first switch S1. In a first embodiment, for example, the current i_L can be detected by the coil L1. However, this requires relatively complex circuits. The current i_L through the coil L1 can be detected, for example, by means of a Hall sensor. In addition or alternatively, therefore, other / other variables which are suitable for detecting an advantageous switch-on time can be used.

0049In a further embodiment, for example, the magnetization state of the coil L1 can be detected. The second sensor unit SE2 can, for example, be a secondary winding L2 on the coil L1, which taps the voltage across the coil L1. The monitoring of the voltage profile over time on the coil L1 (in particular the 'break-in' shortly after blocking the diode D1 after the time t_2) enables a statement about the advantageous restarting time of the first switch S1. In a simple embodiment, a comparator would be sufficient to detect the attainment of the demagnetization (and thus the zero crossing) on ​​the basis of the exceeding or undershooting of a threshold value.

0050Instead of or in addition to the voltage monitoring on the coil L1, the voltage at the node Ux can be monitored, for example, above the first switch S1. The voltage at node Ux drops significantly from a high value to a low value when the diode is blocked. The signal for reactivating the first switch S1 can therefore be triggered when the voltage Ux is below a certain threshold value. The control unit SR switches the first switch S1 back on when the coil L1 is demagnetized and / or blocks the diode D1. The second sensor unit SE2 can consist of a secondary winding L2 inductively inductively coupled to the coil L1, or of a voltage divider (R1, R2) at the node point Ux.

0051For the operation of the operating circuit and the setting of the current by the LED, however, other control mechanisms are also conceivable, for example, the lowering of a predefined threshold value for the LED current can be a condition for the re-activation. It would also be possible to control only the current detected by the measuring resistor (shunt) RS during the switching-on phase of the switch S1. In this case, for example, the switch-on time of the switch S1 can be changed at a fixed clock frequency The time averaged current can be controlled.

0052The control unit SR uses the information from the first sensor unit SE1 and / or the second sensor unit SE2 for determining the switch-off and switching-on time of the first switch S1 and thus generates a high-frequency signal for direct or indirect control of the LED current. A power regulation is also conceivable by means of the evaluation of the power supplied to the operating circuit.

0053The setting of the brightness can be effected by setting the time-averaged LED power by the control unit SR, preferably in the form of low-frequency PWM signals. The frequency of the low-frequency PWM signal for adjusting the brightness is typically of the order of 100-1000 Hz.

0054The <figref idrefs="f0005">FIG. 8b</figref> Shows a prior art example where a pulse packet (as shown in FIG <figref idrefs="f0001">FIG</figref> Already explained) of high-frequency pulses is cut off by the end of the pulse width of the low-frequency pulse, at a time when a period duration of high-frequency pulses is not completed. The pulse width of the low-frequency pulse determines the width of the pulse packet. Such a clipping can occur both during a falling and a rising edge of a high-frequency pulse.

0055The example of the <figref idrefs="f0005">FIG</figref> Shows the control of the switch S1 according to the invention by the control unit SR, as is described, for example, in the circuit according to the example of FIG <figref idrefs="f0002">FIG</figref> Is applicable. In the<figref idrefs="f0005">FIG</figref> A the low-frequency signal (LF) and the high-frequency signal (HF) as well as the resulting dimming signal (FET) are shown in their exemplary chronological course, the exact mode of operation being described in the following with the aid of the example of FIG <figref idrefs="f0005">FIG</figref> Is explained.

0056A possible implementation of the invention within a control unit SR is shown in FIG <figref idrefs="f0005">FIG</figref> Respectively. The control unit SR controls the first switch S1 with a dimming signal, the dimming signal being generated by a combination of a low-frequency signal and a high-frequency signal. The low-frequency signal and the high-frequency signal can be internally connected in the control unit SR and output at an output of the control unit SR.

0057The low-frequency signal, which can be generated by a low-frequency PWM unit (low-frequency PWM unit), and the high-frequency signal, which can be generated by a high-frequency PWM unit, can be linked via a coupling element be. The coupling element can be formed by a logical link. In the example of<figref idrefs="f0005">FIG</figref> The low-frequency signal generated by a low-frequency PWM unit (low-frequency PWM unit) is supplied to the D input of a D flip-flop. The high-frequency signal, which is generated by a high-frequency PWM unit (high-frequency PWM unit), is fed to a C input of the D flip-flop. The resulting signal of the D flip-flop, which is applied to the output Q, is supplied to an OR connection. The low-frequency signal, which is generated by a low-frequency PWM unit (low-frequency PWM unit), is also supplied to the OR link. The output signal of the OR link is supplied to an AND link. The AND link is also fed high-frequency signal, which is generated by a high-frequency PWM unit (High frequency PWM unit). The signal resulting at the output of the AND link forms the dimming signal which is fed to the gate driver input (gate driver input) of the control unit SR and thus to the gate of the switch S1.

0058However, the low-frequency signal and high-frequency signal can also be combined by means of the parameters such as the period (T_HF) of the high-frequency signal, for example in a digital system, by calculating the parameters for the low-frequency signal.

0059The low-frequency signal and the high-frequency signal are preferably internally connected to the dimming signal and the dimming signal is output at an output of the control unit (SR). The control unit (SR) preferably has means for combining the low-frequency signal and the high-frequency signal in order to select the pulse width (TON * LF) of the low-frequency signal as an integral multiple of the period duration (T_HF) of the high-frequency signal.

0060It would also be possible for a microcontroller, which is external to the control unit (SR), to generate the low-frequency signal by the microcontroller having a low-frequency PWM unit (low-frequency PWM unit). This low-frequency signal can be supplied to the control unit SR, whereby the control unit SR can only have the high-frequency PWM unit (high-frequency PWM unit) for generating the high-frequency signal and the coupling element for linking low-frequency signal and high-frequency signal. Such a system is an example for the<figref idrefs="f0004">FIG</figref> Is explained.

0061When dimming the LED, the frequency of the low-frequency signal can be changed. The change in the frequency of the low-frequency signal can be selected as a function of the change in the pulse width (TON * LF) of the low-frequency signal. The change in the frequency results in this case from an adaptation of the switch-off time duration of the low-frequency signal. Preferably, the length of the switch-off period of the low-frequency signal is increased in order to reduce the brightness or the dimming level.

0062The frequency and the pulse width (TON * LF) of the low-frequency signal can be changed in such a way that the switching-on ratio of the low-frequency signal remains constant with a constant dimming level or brightness level.

0063The low-frequency signal is preferably a low-frequency pulsed, in particular PWM signal, in particular in the range from about 100 Hz to 1000 Hz, preferably in the range from 500 Hz to 1000 Hz. A change in the frequency of the low-frequency signal required for dimming or changing the brightness Can be in the range of, for example, 100 Hz to 200 Hz. The high-frequency signal is preferably a high-frequency pulsed, in particular PWM signal, for example in the region of approximately 50 kHz or more.

0064Due to the combination of the low-frequency signal with the high-frequency signal, it may be necessary that a permanent detection and adaptation of the low-frequency signal with regard to its pulse width must take place in the event of a permanent change in the frequency of the high-frequency signal.

0065Due to the linkage of the low-frequency signal with the high-frequency signal, it may be necessary that the pulse width of the low-frequency signal is only incrementally altered when the brightness is dimmed or changed, the increment length being a complete period duration or a multiple of the complete period duration of the high-frequency signal. An exact graduation of the adjustment of the brightness or of the dimming level can then take place over the length of the switch-off duration of the low-frequency signal.

0066The dimming signal, via which the brightness of the LED is adjusted, is thus formed from pulse packets, preferably as a resultant pulsed, in particular PWM signal, the pulse packets being interrupted by longer pauses.

0067The dimming signal can be dependent on a brightness setting predetermined from outside, for example by a user. This brightness setting can be achieved by the supplied low-frequency signal

0068The low-frequency signal can be dependent on the desired dimming level of the LED. The low-frequency signal can also be predetermined by a further integrated control circuit, such as a microcontroller, which is arranged as a central controller, and is looped through only by the control unit SR. The low-frequency signal can also be preset by a further microcontroller, which is arranged as a central controller, and does not necessarily have to be output or looped through by the control unit SR.

0069The high-frequency signal may depend on the current and / or the voltage through the LED. The high-frequency signal is dependent on a control loop, whereby at least the first switch S1 is clocked by high-frequency control, depending on at least one predetermined setpoint value for a current and / or a voltage within the operating circuit and the comparison with an actual value. For example, the operating circuit can be operated in the hysteretic mode, the switch S1 being switched on and off depending on the attainment of threshold values ​​(eg, when the switch S1 is switched on when the current is zeroed by the coil L1 or a lower limit value for the LED is reached Current and switching off the switch S1 when a current is exceeded by the switch S1). According to the invention, this control loop does not need to take any consideration of the current brightness of the LEDs.

0070Thus, the invention provides the advantage that the control loop for the regulation of the current can be decoupled from the presetting of the brightness by the LED, and nevertheless the control of the switch via a single control signal is possible (whereby the linkage of the high-frequency signal of the control loop with The low-frequency signal for the brightness is preferably internally connected to the control unit SR).

0071The control loop for regulating the current through the LED and thus the high-frequency signal can also be used to compensate for occurring fluctuations in the DC voltage U0. For example, the DC voltage U 0 can have a ripple with a double mains voltage if the DC voltage U 0 is fed, for example, from a 230 V mains with 50 Hz mains frequency via a rectifier and an optionally interposed power factor correction circuit. In this case, the DC voltage U 0 can, for example, have a 100 Hz ripple with approximately 10% amplitude compared to the amplitude of the DC voltage component. The operating circuit with its regulating loop can now be designed in such a way that the high-frequency signal is adapted such that this 100 Hz ripple is not passed on to the LED but is attenuated. This can be done, for example, directly by means of a fast control loop or else by a transmission of the information about the actual amplitude of the DC voltage U 0, whereby the frequency of the high-frequency signal can be adapted as a function of the current amplitude of the DC voltage U 0. Such a control is also referred to as "feedforward" control.

0072Since the period duration of the high-frequency signal thus changes in synchronism with the amplitude of the ripple of the DC voltage U0 (ie the fluctuation of the DC voltage U0) and this continues, the current amplitude of the pulse width (TON * LF) DC voltage U0 can be taken into account. It is also possible to adjust the pulse width (TON * LF) of the low-frequency signal as a function of the current variation of the DC voltage U0, depending on the variation of the DC voltage U0. Thus, the current amplitude of the DC voltage U0 and / or the current waveform of the DC voltage U0 can also be taken into account in the determination of the pulse width (TON * LF) of the low-frequency signal.

0073The invention thus ensures that a pulse packet of high-frequency pulses is not cut off by the end of the pulse width of the low-frequency pulse, but the pulse width of the low-frequency pulse is adapted to the duration of the pulse packet from high-frequency pulses.

0074The control unit SR can be formed by a microcontroller, FPGA, PAL or also an application-specific integrated circuit (ASIC).

0075The control according to the invention is not limited to the topology or circuit arrangement of the invention <figref idrefs="f0003">FIG</figref> , There are also implementations according to the circuits of the <figref idrefs="f0001 f0002 f0003 f0004">FIGS. 1 to 6</figref> possible. For example, this invention can be applied to a buck converter, buck converter, isolated flyback converter, Sepic converter, half-bridge converter, or other topologies and circuit arrangements.

0076The invention relates in principle to operating circuits for at least one LED, which are supplied by means of a switching regulator via at least one clocked first switch S1, the current being influenced by the LED via the frequency and / or the switched-on ratio of the clocked switch S1, and the frequency and / Or the switched-on ratio of the clocked switch S1 is preset by a control unit SR by means of a dimming signal as a drive signal, the dimming signal being generated by a combination of a low-frequency signal and a high-frequency signal. The low-frequency signal (LF) and the high-frequency signal (HF) are preferably linked via a coupling element. The control unit SR can select the pulse width (TON * LF) and, optionally, the period duration of the low-frequency signal as an integer multiple of the period duration of the high-frequency signal. The coupling element thus represents an example of means for linking the low-frequency signal (LF) and the high-frequency signal (HF).

0077The examples of the <figref idrefs="f0005">7 or 8</figref> (And the others, of course, can also be extended), that several operating circuits according to the <figref idrefs="f0005">7 or 8</figref> available. The control units SR of the individual operating circuits can be controlled by a common microcontroller. However, it would also be possible to arrange the function of the central control of the individual operating circuits by a central controller and the regulation of the operation of the operating circuits by the control units SR in a common microcontroller. The individual operating circuits can, for example, control LED strings of different wavelength or color. The microcontroller can be controlled via an interface (wireless or line-connected). Here control signals to adjust the brightness or color, or status information about the SC can hnittstelle be transferred.

0078The invention thus also allows a method for controlling at least one LED, the control unit SR controlling the switch S1 with a dimming signal, and the dimming signal being generated by a combination of a low-frequency signal and a high-frequency signal. The pulse width (TON * LF) and, optionally, the period duration of the low-frequency signal is thereby selected as an integer multiple of the period duration of the high-frequency signal.

0079In a further exemplary embodiment, the operating circuit for at least one LED can be formed by a resonant or quasi-resonant circuit (<figref idrefs="f0006">FIG</figref>). A supply voltage for at least one LED is provided by means of at least one first switch S1, which is clocked by a control unit, the first switch S1 feeding a resonant circuit in the switched-on state, which preferably has at least one coil L1 as inductor. An energy is temporarily stored in the resonance circuit, preferably in the coil L1, which, when the first switch S1 is switched off, discharges via a diode D1 and above the at least one LED. Between the diode D1 and the LED, a capacitor C1 can also be connected as a smoothing capacitor, which is arranged directly or indirectly parallel to the LED. The resonant circuit may also have a resonance capacitor Cr. The coil L1 can also be part of a transformer or, in addition to the coil, a transformer (28, 29) can also be arranged in the resonant circuit. The first switch S1 can also be part of an inverter, for example an alternating clocked half-bridge with the two switches S1 and S3. In the example of FIG<figref idrefs="f0006">FIG</figref> Is exemplarily shown as a so-called LLC converter (series-resonant isolated LLC half-bridge converter). The half-bridge with the alternately clocked switches S1 and S3 can be driven according to the invention with the dimming signal, the two switches S1 and S3 being activated with a 50% duty cycle at high frequency during the pulse width (TON * LF) of the low-frequency signal . The frequency of the high-frequency signal can result from the control loop or the manipulated value for the operating circuit, for example, depending on the current through the first sensor unit, preferably the current through the LED, through one of the two switches S1 or S3 or in the resonant circuit Through the coil L1. The pulse width (TON * LF) and, optionally, the period duration of the low-frequency signal is selected as an integer multiple of the period (T_HF) of the high-frequency signal. The power circuit can also have further filter or smoothing elements at the output, for example an inductor 33. The transformer can also have a plurality of taps on its secondary winding 29 on the secondary side, and thus further diodes such as the diode D1b can also be arranged in order to achieve a higher utilization of the diode Transferred energy.

0080<figref idrefs="f0003">FIG. 3 and FIG. 4</figref> Show specific further embodiments of the invention.

0081In <figref idrefs="f0003">FIG</figref> A special embodiment of the above-described switching arrangement (a buck converter) is shown. The advantageous switch-off instant is detected here by means of detecting the voltage at the node Ux above the first switch S1. This is done by the ohmic voltage divider R1 and R2. The node Ux is connected between the coil L1, the diode D1 and the switch S1.

0082As a voltage divider, for example, a capacitive voltage divider or combined voltage divider, which is constructed from resistance and capacitance, is also possible. The measuring resistor (shunt) RS is used for current detection by the first switch S1. The monitoring of the voltage over time at the node Ux (in particular the 'break-in' shortly after blocking the diode D1 in the vicinity of the instant t_2) enables a statement about the advantageous restarting time of the first switch S1. Instead of or in addition to a voltage monitoring on the coil L1, the voltage at the node Ux can, for example, be monitored above the first switch S1. The voltage at node Ux drops significantly from a high value to a low value when the diode is blocked. The signal for reactivating the first switch S1 can therefore be triggered when the voltage Ux is below a certain threshold value.

0083In circuit arrangement of <figref idrefs="f0003">FIG</figref> A second switch S2 is additionally arranged parallel to the LEDs and the capacitor C1. The second switch S2 is selectively / independently controllable and can be, for example, a transistor (MOSFET or bipolar transistor). If the second switch S2 is closed, the discharging operation of the capacitor C1 is accelerated. The accelerated discharge of the capacitor C1 ensures that the current flow through the LED is as fast as possible towards zero. This is desired, for example, at the end of a PWM package where the current flow through the LED should fall off as quickly as possible, ie the falling edge of the current profile should be as steep as possible (for reasons of color constancy). Preferably, the second switch S2 can be activated and driven at low dimming level, where the PWM packets are very short, and it is important that the current through the LED at the end of a pulse packet rapidly goes to zero. For example, an even lower dimming level can be achieved by suitable control of the second switch S2. A further function of this second switch S2 is that, in the switched-on state, it bridges the LEDs. This is necessary, for example, if the LEDs are to be switched off, ie they are not intended to emit light, but the supply voltage U0 is still present. Without the bridging through the second switch S2, a (though small) current would flow through the LEDs and the resistors R1 and R2 and the LEDs (slightly) would light up.

0084It is to be noted that the arrangement of a second switch S2 parallel to the LEDs and the capacitor C1 for the accelerated discharge of the capacitor C1 or for bridging the LEDs is not restricted to the specific embodiment of the circuit arrangement of FIG <figref idrefs="f0003">FIG</figref> But can be applied as an additional improvement in various embodiments of the invention.

0085<figref idrefs="f0003">FIG</figref> Shows a modification of the circuit in FIG <figref idrefs="f0003">FIG</figref> In that the voltage monitoring is performed on the coil L1. The voltage on the coil S1 can be detected, for example, by means of a secondary winding L2 which is coupled to the coil S1 (or an additional coil L2, which inductively couples to the coil L1). A secondary winding L2 now serves to detect the advantageous switch-on time for the first switch S1. The monitoring of the temporal voltage profile at the coil L1 (in particular the 'break-in' near the blocking of the diode D1 after the instant t_2) enables a statement about the advantageous restarting time of the first switch S1. As already mentioned, this monitoring can also be carried out on the basis of a secondary winding L2.

0086As already mentioned, the determination of the instant of the zero-crossover or the demagnetization can also be carried out by means of a threshold value monitoring (for the lower or a higher threshold value, in the case of a monitoring by means of a secondary winding L2), the polarity of the voltage depends on the winding sense of the secondary winding L2 L1 down).

0087It should be noted that the method for detecting an advantageous switch-on instant for the first switch S1 can, of course, be applied to other switching topologies, for example for a so-called flyback converter or a so-called forward converter.

0088<figref idrefs="f0004">FIG</figref> Shows a modification of the circuit of FIG <figref idrefs="f0002">FIG. 2a</figref> In that the arrangement of the inductor L1, the diode D1 and the orientation of the LED path is modified (buckle boost converter).

0089A possible further development of an operating circuit for LEDs is shown in FIG <figref idrefs="f0004">FIG</figref> Respectively. Detection of the attainment of the demagnetization of the coil L1 by monitoring the voltage at the winding L2 can be carried out by means of a control circuit IC which is available as a standard. This control circuit IC (integrated circuit) corresponds to the control unit SR according to FIG<figref idrefs="f0002 f0003 f0004">FIGS. 2 to 5</figref>, Has an input for detecting the attainment of demagnetization of a coil by monitoring the voltage across a secondary winding provided on the coil. Furthermore, the control circuit IC has an output for controlling a switch and via further monitoring inputs. A first of these monitoring inputs can be used for the presetting of a reference value such as, for example, a reference voltage.

0090A second monitoring input can be used for monitoring the attainment of a maximum voltage or also by means of a voltage measurement at a resistor for monitoring the attainment of a maximum current. A third monitoring input can be used for monitoring a further voltage or also for activating and deactivating the control circuit IC or for controlling the control circuit IC of the controlled switch.

0091According to the <figref idrefs="f0004">FIG</figref> The control circuit IC monitors the current through the first switch S1 during the switching-on phase of the first switch S1 via the measuring resistor (shunt) Rs and the input 4 at the control circuit IC. As soon as the voltage which is tapped off via the measuring resistor (shunt) Rs reaches a certain maximum value, the first switch S1 is opened. The presetting of the voltage required for opening the first switch S1 can be adjusted by the presetting of a reference value (ie a reference voltage) at the input 3 of the control circuit IC. For example, a reference voltage can be preset by a microcontroller, which presets the magnitude of the maximum permissible voltage across the measuring resistor (shunt) Rs and thus the maximum current permitted by the first switch S1.

0092For example, the microcontroller can output a PWM control signal, which is then smoothed by a filter 10 (for example, an RC element) and thus applied as a dc voltage signal with a certain amplitude at the input 3 of the control circuit IC. By changing the duty factor of the PWM control signal of the microcontroller, the amplitude of the signal at the input 3 of the control circuit IC can be adapted.

0093The control circuit IC can recognize the achievement of the demagnetization of the coil L1 via the input 5 on the basis of the monitoring of the voltage at a secondary winding L2 provided on the coil L1. This detection can be used as a re-activation signal.

0094As soon as the demagnetization of the coil L1 has been detected by the control circuit IC, the control circuit IC can switch on the first switch S1 by means of a control via the output 7.

0095The control circuit IC can be activated and / or deactivated by applying a voltage at the input 1. This voltage for activating at input 1 can also switch between a high level and a low level, whereby the control circuit IC is activated at high level and at low level at least the control of the first switch S1 is interrupted. This control of the input 1 can be effected by means of a microcontroller. For example, a low-frequency activation and deactivation of the control circuit IC and thus the activation of the first switch S1 can be achieved in this way and thus the low-frequency control of the operating circuit for dimming the LEDs.

0096A further reference voltage for the control circuit IC can also be preset via the input 1 via the amplitude of the signal applied to this input. This voltage can, for example, also influence the height of the maximum permissible current through the switch or else also the permissible switch-on duration of the first switch S1. The control circuit IC and / or the control circuit IC combines<u>with the</u> Microcontrollers can jointly form the control unit SR.

0097According to the invention, the signal PWM, which is fed to the input 1 of the control circuit IC, can be linked to the drive signal, which is output at the output 7, according to the invention via a coupling element, for example by an ohmic resistance. Optionally, the signal PWM can also be linked with the signal of the output 7 and can not be fed to the input 1.

0098The switched-on duration of the first switch S1 can also depend on a further voltage measurement within the operating circuit. For example, a voltage measurement Vsense can also be supplied to the control circuit IC. Via this voltage measurement, a monitoring or even measurement of the voltage at the node between the coil L1 and the LED can take place via a voltage splitter R40 / R47. This voltage measurement Vsense can be supplied either to a further input of the control circuit IC, as an additional variable, to an already used input of the control circuit IC or also to an input of the microcontroller.

0099Thus, a system can be constructed in which on the one hand simple control for dimming LEDs is enabled by low-frequency PWM, on the other hand a low-loss high-frequency operation of the operating device combined with a constant current through the LED.

0100A microcontroller can be used to specify both the frequency and the duty cycle of a PWM singal for dimming LEDs. In addition, the height of the maximum permissible current can also be predetermined by the first switch S1. The microcontroller can control the dimming of the LEDs by low-frequency PWM via a signal, which is fed to the input 1 of the control circuit IC. Furthermore, the microcontroller can be set via a signal, which is fed to the input 3 of the control circuit IC, the height of the maximum permissible current through the first switch S1 or also the necessary switch-on duration of the first switch S1.

0101The operating circuit may further include a further switch S2 which is arranged such that this second switch S2 can bridge the LED.

0102The second switch S2 can also be arranged such that it can take over or interrupt the current through an existing high-ohm voltage measuring path or a similar existing high-ohmic circuit arrangement from the LED.

0103By connecting the second switch S2 to the LED in parallel, the latter can bridge the LED and thus deactivate it. This method can be used to adjust the brightness (dimming) of the LEDs. A possible alternative variant would be for the dimming to take place via the second switch S2, while the control of the first switch S1 only sets and regulates the current through the LED.

0104However, the control of the two switches S1 and S2 can be used in combination for optimized dimming control. For example, the second switch S2 can additionally be used only for dimming to low dimming levels. The operating circuit is designed on the basis of the existing topology and the control circuit so that the output voltage of the operating circuit (ie the voltage across the LED) is limited to a maximum permissible value. If the LED is bridged by closing the second switch S2, the operating circuit limits the output voltage in such a way that no excessive current can flow, which can lead to a possible destruction. This activation of the second switch S2 can, for example, be used only for dimming to low dimming level.

0105If the buck converter operates fixedly on current source mode (in the so-called hysteretic mode as described in the exemplary embodiments) and runs efficiently, the LED can also be dimmed only with second switch S2, which should be very low-impedance, for certain brightness ranges or operating modes , And the losses are nevertheless low.

0106In addition, the second switch S2 can be controlled such that it can take over the current through an existing high-ohm voltage measuring path or a similar existing high-impedance circuit arrangement from the LED.

0107For example, according to <figref idrefs="f0004">FIG</figref> The first switch S1 is not clocked, no current should flow through the LED. However, due to the existing voltage divider R40 / R47, a small current can flow through the LED. In this case, the second switch S2 can be closed in the event of a desired deactivation of the LED (for example, if no light is to be emitted), so that the current flow through the LED is interrupted or avoided.

0108The second switch S2 can, at least, always be driven following a low-frequency PWM packet in order to bridge or deactivate the LED (during the last discharge edge, ie at the end of a PWM pulse packet).

0109The current can also be interrupted by the LED by arranging the second switch S2 in series with the LEDs.

0110The example of the <figref idrefs="f0004">FIG</figref> (And the others, of course, can also be extended), that several operating circuits according to <figref idrefs="f0004">FIG</figref> available. The control circuits IC or the control units SR of the individual operating circuits are controlled by a common microcontroller. The individual operating circuits can, for example, control LED strings of different wavelength or color. The microcontroller can be controlled via an interface (wireless or line-connected). Control signals for setting the brightness or color or also status information can be transmitted via the interface.