Operating circuit and method for operating at least one light-emitting diode depending on a dimming level

Description

[0001] The invention relates to an operating circuit for a lighting device. The invention particularly relates to operating circuits for supplying one or more light-emitting diodes (LEDs) depending on a dimming level.

[0002] With the increasing popularity of light sources such as LEDs and LED modules, operating circuits for such light sources are becoming increasingly important. The operating circuit primarily serves to provide the desired power supply for the light source. Additional functions can be included in the operating circuit, for example, to enable dimming of the light source. The light emission of LEDs depends on the current flowing through the LEDs. For brightness control or brightness regulation, LEDs are therefore typically operated in a mode in which the current flow through the LED is controlled or regulated by an operating circuit.

[0003] For example, document WO 2010 / 025450 A2 discloses a light-emitting diode (LED) lighting system and method. The LED lighting system and method includes an LED controller for precisely controlling a current in an LED system. The LED controller includes components for calculating an actual amount of charge delivered to the LED system based on the current and an active time period of an LED current time period, wherein the LED current time period is clock-modulated at a rate greater than fifty (50) Hz, and using the actual amount of charge to modify and provide a desired target amount of charge to be delivered during a future active time period of the LED current time period.The LED system and method further include components to compare the actual charge amount with a desired charge amount for the active time period and to compensate for a difference between the actual charge amount and the desired charge amount during the future active time period.

[0004] Document WO 2010 / 045666 A1 shows an operating connection for at least one LED. This operating connection, to which a direct voltage or rectified alternating voltage is supplied, provides a supply voltage for at least one LED by means of a coil (L1) and a first switch (S1) that is time-controlled by a control unit (SR). When the first switch (S1) is switched on, energy is temporarily stored in the coil (L1), which is discharged via at least one LED when the first switch (S1) is switched off. The disclosed operating circuit is configured such that the control unit (SR) determines the duration (toff) between a time at which the first switch (S1) is switched off and a time at which the first switch (S1) is switched on again as a function of the voltage across the coil (L1).

[0005] Furthermore, document JP 5942256 B2 discloses a lighting device including a power supply unit that applies a DC voltage to a solid-state light-emitting element that is a light source. The DC voltage is intermittently controlled by controlling the power supply unit, thereby obtaining an average value of a current flowing through the solid-state light-emitting element. A control unit sets a value corresponding to an externally instructed dimming value. When the average value is set to a predetermined reference value or more, the control unit sets a cycle in which the DC voltage is intermittently controlled. The DC voltage application period is changed. When the average value is set to be equal to or less than the reference value, the DC voltage application period is set and the DC voltage stop period is changed.The value is set to a value smaller than the upper limit of the average value and larger than the lower limit of the average value, and the control unit sets the average value to be equal to or smaller than the reference value.

[0006] Switching regulators, particularly step-down converters, can be used to control an array of one or more LEDs. In such an operating circuit, a control device controls a high-frequency clocked, controllable switch. The controllable switch can be a power transistor, for example. When the switch is switched on, current flows through the LED array and a coil, which is thereby charged with energy. The temporarily stored energy in the coil is discharged via the LEDs when the switch is switched off.

[0007] To reduce shifts in the light spectrum at different dimming levels, pulsed operation can be used for brightness control or brightness regulation of LEDs. In this case, pulses of current are supplied to the LEDs by the operating circuit. The LEDs can be dimmed to smaller dimming levels by increasing the time interval between the pulse packets, i.e., by reducing the frequency at which the pulse packets are generated.

[0008] The output current supplied to the LEDs by the operating circuit can have a constant current during the pulse duration, with current ripples superimposed on it. Such current ripples are caused by the clocked switching of the controllable switching device.

[0009] Even with a predefined dimming level, situations can occur in which the number of switching cycles of the controllable switching device varies from one pulse packet to the next. A further switching cycle of the controllable switching device can, for example, be automatically triggered when the output current of the operating circuit reaches a current threshold. Depending on whether this current threshold occurs shortly before or shortly after the end of the specified pulse duration of the pulse packet, the number of switching cycles per pulse packet can vary. Accordingly, the energy supplied to the LEDs for a pulse packet can vary. This can lead to undesirable flickering of the light. Such flickering can be perceived by the human eye and is often perceived as disturbing.

[0010] The invention is based on the object of providing devices and methods that mitigate the described problems. In particular, the object is to provide devices and methods that reduce light flickering without significantly increasing switching losses at high dimming levels.

[0011] According to the invention, an operating circuit for a lighting device and a method are provided having the features specified in the independent claims. The dependent claims define embodiments of the invention.

[0012] According to embodiments of the invention, at least in one dimming level range, e.g., for dimming levels that are lower than a threshold value, a switching frequency of a controllable switching means of an operating circuit is dynamically adapted to the respective dimming level. The current ripples are correspondingly dynamically adapted to the respective dimming level.

[0013] An operating circuit for at least one light-emitting diode (LED) according to one embodiment comprises a coil, a controllable switching means, and a control device. The control device is configured to repeatedly switch the controllable switching means on during a pulse duration in order to temporarily store energy in the coil, and to switch it off in order to discharge energy temporarily stored in the coil via a diode and via the at least one LED, in order to provide a pulse packet to the at least one LED. The control device is configured such that a switching frequency with which the controllable switching means is switched on during the pulse duration or with which the controllable switching means is switched off during the pulse duration depends on a dimming level, at least in one dimming level range.

[0014] The control device can be configured to switch the controllable switching means such that the switching frequency in the dimming level range is a monotonically decreasing function of the dimming level.

[0015] The control device can be configured to switch the controllable switching means such that the switching frequency for dimming levels that are lower than a threshold value is a strictly monotonically decreasing function of the dimming level.

[0016] The control device can be configured to switch the controllable switching means such that the switching frequency is a linearly decreasing function of the dimming level for dimming levels lower than the threshold. For dimming levels higher than the threshold, the switching frequency can be a constant function of the dimming level.

[0017] The control device can be configured to switch the controllable switching means such that an amplitude of current ripples occurring in the pulse packet in the dimming level range depends on the dimming level.

[0018] The control device can be configured to switch the controllable switching means such that the amplitude of the current ripple in the dimming level range is a strictly monotonically increasing function of the dimming level.

[0019] The control device can be configured to switch the controllable switching means such that the amplitude of the current ripple in the dimming level range is a linearly increasing function of the dimming level.

[0020] The control device can be configured to switch the controllable switching means such that a maximum current intensity and a minimum current intensity of current ripples in the dimming level range depend on the dimming level.

[0021] The control device can be configured to switch the controllable switching means such that a maximum current intensity of the current ripple is a linearly increasing function of the dimming level.

[0022] The control device can be configured to switch the controllable switching means such that a minimum current intensity of the current ripple is a linearly decreasing function of the dimming level.

[0023] The control device can be configured to switch the controllable switching means such that an average value of a current intensity of the current ripple is independent of the dimming level.

[0024] The control device can achieve a switching frequency and / or amplitude of the current ripple dependent on the dimming level in various ways. The control device can compare the output current supplied to the at least one LED with a switching threshold and switch the controllable switching means on or off depending on the switching threshold comparison. The control device can determine a time period until the subsequent switching off or on depending on the dimming level. The switching threshold and / or the time period can, for example, be determined based on a characteristic map, calculated by the control device, or specified externally.

[0025] The control device can be configured to switch on the controllable switching means during the pulse duration when a current intensity through the at least one light-emitting diode reaches a first switching threshold value, and to switch it off again after a first time period, wherein the first switching threshold value and the first time period depend on the dimming level.

[0026] The control device can be configured to switch off the controllable switching means during the pulse duration when a current intensity through the at least one light-emitting diode reaches a second switching threshold value, and to switch it on again after a second time period, wherein the second switching threshold value and the second time period depend on the dimming level.

[0027] The control device is configured such that, in different pulse packets, the control device switches the controllable switching means with at least two different switching cycle counts. Such different switching cycle counts can also be deliberately induced for different pulse packets at a fixed dimming level, for example, to make corrections to the output current averaged over several pulse packets. This allows for a fine adjustment of the output current averaged over several pulse packets to a target value.

[0028] The control device is configured such that a switching cycle number of a pulse packet generated for one dimming level and a further switching cycle number of a further pulse packet generated for the same dimming level differ by 1.

[0029] The control device may be a semiconductor integrated circuit. The control device may be configured as a processor, a microprocessor, a controller, a microcontroller, or an application-specific integrated circuit (ASIC).

[0030] The control device may comprise an input for receiving the dimming level or a variable influencing the dimming level.

[0031] The operating circuit may include an input for receiving a DC voltage or a rectified AC voltage.

[0032] The operating circuit may comprise a capacitor connected in parallel with the at least one LED.

[0033] The at least one LED may comprise one or more inorganic and / or organic LED(s).

[0034] According to a further embodiment, a system is provided which comprises the operating circuit and the at least one LED connected to the operating circuit.

[0035] According to a further exemplary embodiment, a method for operating at least one LED by means of an operating circuit is specified. The operating circuit comprises a coil and a controllable switching means. The at least one LED is supplied with energy depending on a dimming level. To generate a pulse packet supplied to the at least one LED, the controllable switching means is repeatedly switched on in order to temporarily store energy in the coil and switched off in order to discharge energy temporarily stored in the coil via a diode and via the at least one LED. A switching frequency with which the controllable switching means is switched on during the pulse duration or with which the controllable switching means is switched off during the pulse duration depends on the dimming level at least in one dimming level range.

[0036] For dimming levels in the dimming level range, the switching frequency can be a monotonically decreasing function of the dimming level.

[0037] At least for dimming levels lower than a threshold, the switching frequency can be a strictly monotonically decreasing function of the dimming level.

[0038] The switching frequency can be a linearly decreasing function of the dimming level for dimming levels lower than the threshold. For dimming levels higher than the threshold, the switching frequency can be a constant function of the dimming level.

[0039] An amplitude of current ripples occurring in the pulse packet may depend on the dimming level in the dimming level range.

[0040] The amplitude of the current ripple can be a strictly monotonically increasing function of the dimming level in the dimming level range.

[0041] The amplitude of the current ripple can be a linearly increasing function of the dimming level in the dimming level range.

[0042] A maximum current and a minimum current of current ripples may depend on the dimming level in the dimming level range.

[0043] The maximum current ripple can be a linearly increasing function of the dimming level.

[0044] The minimum current ripple can be a linearly decreasing function of the dimming level.

[0045] The controllable switching means can be switched in such a way that an average value of a current intensity of the current ripple is independent of the dimming level.

[0046] The output current supplied to the at least one LED can be compared to a switching threshold, and the controllable switching means can be switched on or off depending on the switching threshold comparison. The time until the subsequent switching off or on can be determined depending on the dimming level. The switching threshold and / or the time period can, for example, be determined based on a characteristic map, calculated by a control device, or specified externally.

[0047] The controllable switching means can be switched on during the pulse duration whenever a current intensity through the at least one light-emitting diode reaches a first switching threshold value, and switched off again after a first time period, wherein the first switching threshold value and the first time period depend on the dimming level.

[0048] The controllable switching means can be switched off during the pulse duration when a current intensity through the at least one light-emitting diode reaches a second switching threshold value, and switched on again after a second time period, wherein the second switching threshold value and the second time period depend on the dimming level.

[0049] Different pulse packets have at least two different switching cycle counts. Such different switching cycle counts can be specifically adjusted for different pulse packets at a fixed dimming level, for example, to set the output current averaged over several pulse packets to a specific target value. This allows for fine adjustment of the output current averaged over several pulse packets.

[0050] A switching cycle number of a pulse packet generated for one dimming level and another switching cycle number of another pulse packet generated for the same dimming level differ by 1.

[0051] A direct voltage or a rectified alternating voltage can be supplied to the operating circuit as a supply voltage.

[0052] The method can be carried out automatically with the operating circuit according to one embodiment.

[0053] According to further embodiments of the invention, an operating circuit for at least one light-emitting diode is specified, comprising a coil, a controllable switching means, and a control device. The control device is configured to repeatedly switch the controllable switching means on during a pulse duration in order to temporarily store energy in the coil, and to switch it off in order to discharge energy temporarily stored in the coil via a diode and via the at least one light-emitting diode in order to provide a pulse packet to the at least one light-emitting diode. The control device is configured to switch the controllable switching means off during the pulse duration when a current intensity through the at least one light-emitting diode reaches a second switching threshold value, the second switching threshold value depending on a dimming level.

[0054] The control device can be configured to switch on the controllable switching means during the pulse duration when a current intensity through the at least one light-emitting diode reaches a first switching threshold value, wherein the first switching threshold value depends on the dimming level.

[0055] Methods according to the various embodiments and the effects achieved thereby correspond to the configurations of the operating circuit according to embodiments.

[0056] In devices and methods according to embodiments, a switching frequency with which the controllable switching means is switched on and / or off can be selected depending on the dimming level. In particular, for a dimming level range with small dimming levels, switching frequencies can be selected that are higher than the switching frequency used for larger dimming levels. Due to the higher switching frequency and / or smaller ripple amplitude at small dimming levels, relative fluctuations between the average current strength of pulse packets can be reduced, even if the pulse packets correspond to a different number of switching cycles. At larger dimming levels, a lower switching frequency can be selected since, due to the overall larger number of switching cycles per pulse packet, relative fluctuations by, for example, one switching cycle would not lead to clearly perceptible flickering.The lower switching frequency at higher dimming levels keeps switching losses low, which are caused by the finite switching time of the controllable switching device.

[0057] The invention is explained below with reference to preferred embodiments and the accompanying drawings. In the drawings, identical reference numerals designate identical elements. Fig. 1 shows an operating circuit for light-emitting diodes. Fig. 2 shows pulse packets of an output current of the operating circuit. Fig. Figure 3 shows an enlarged view of a portion of a pulse packet at a first dimming level. Fig. 4 shows an enlarged view of a portion of a pulse packet at a second dimming level that is smaller than the first dimming level, to illustrate the operation of devices and methods according to embodiments. Fig. 5 shows a control signal for controlling a controllable switch. Fig. 6 shows a dependence of a switching frequency on the dimming level in devices and methods according to an embodiment. Fig. Figure 7 shows a change in the duty cycle used to generate pulse packets depending on the dimming level. Fig. 8 shows a dependence of maxima and minima of current ripples on the dimming level in devices and methods according to an embodiment. Fig. 9 shows a dependence of an amplitude of current ripples on the dimming level in devices and methods according to an embodiment. Fig. 10 is a flowchart of a method according to an embodiment. Fig. 11 shows an operating circuit for light-emitting diodes according to an embodiment. Fig. 12 shows an operating circuit for light-emitting diodes according to an embodiment. Fig. 13 and Fig. 14 show sections of pulse packets to explain the operation of embodiments.

[0058] Fig. 1 shows a representation of a system comprising an operating circuit 1 for a light-emitting diode 2. The light-emitting diode 2 can comprise one or more LEDs. The LEDs can be inorganic or organic LEDs. The plurality of LEDs can be connected in series or in parallel. The plurality of LEDs can also be connected in more complex arrangements, for example, in several series circuits connected in parallel. While three LEDs are shown as an example, the light-emitting diode 2 can also comprise just one LED, two LEDs, or more than three LEDs.

[0059] The operating circuit 1 serves to operate the at least one LED 2. The operating circuit 1 is supplied with a supply voltage Vbus, which can be a direct voltage or a rectified alternating voltage. The output of the operating circuit, which is connected to the at least one LED, is connected to a coil 11 and a controllable switching means 13. When the at least one LED 2 is connected to the operating circuit 1, the controllable switching means 13, the coil 11, and the at least one LED 2 are connected in series. A diode 12 is connected in parallel to the at least one LED 2 and the coil 11. A capacitor 15 can be connected between the output terminals, so that the capacitor 15 is connected in parallel to the at least one LED 2.The capacitor 15 is an optional element of the circuit shown and is not required for the basic function, so that the capacitor 15 can be omitted in further embodiments.

[0060] When the controllable switching means 13 is switched on, current flows through the LED(s) 2 and through the coil 11, which is thereby magnetized. When the controllable switching means 13 is switched off, the energy stored in the magnetic field of the coil discharges in the form of a current via the diode 12 and the LED(s) 2. In parallel, the capacitor 15 can be charged at the beginning of the switching on of the controllable switching means 13. During the switching-off phase of the controllable switching means 13 (freewheeling phase), the capacitor 15 can discharge and contributes to the current flow through the LED(s) 2. With appropriate dimensioning of the capacitor 15, this can lead to a smoothing of the current through the LED(s) 2.

[0061] The controllable switching means 13 may be a power switch. The controllable switching means 13 may be a field-effect transistor or a bipolar transistor. The controllable switching means 13 may be an insulated-gate transistor.

[0062] The operating circuit 1 has a control device 14 for clocked switching of the controllable switching means 13. As described with reference to Fig. 2 to Fig. 14, the operating circuit 1 can provide an output current in the form of pulse packets in pulsed operation. The generation of pulse packets does not necessarily have to occur across the entire operating range of the operating circuit 1. For example, at lower dimming levels, pulse packets can be generated to adjust the average current intensity and thus the brightness perceived by the eye. For this purpose, pulse modulation such as pulse width modulation (PWM) can be used, for example, in which a pulse duration of pulse packets and / or a time interval between the pulse packets is adjusted depending on a dimming level.

[0063] As is common in this field of technology, a higher dimming level corresponds to greater brightness, and a lower dimming level corresponds to less brightness. The maximum brightness can correspond to a dimming level of 100%.

[0064] To generate the pulse packet, the control device 14 can repeatedly switch the controllable switching means 13 on and off during a pulse duration. This clocked switching during a pulse packet, in which several switching cycles lie within the pulse duration, must not be confused with the generation of successive pulse packets, which occurs at a lower frequency. As described with reference to Fig. 2 to Fig. 14, the control device 14 controls the controllable switching means 13 at least in a dimming level range, e.g., for low dimming levels that are less than a threshold value, such that a switching frequency and / or a ripple current of the pulse packet depends on the dimming level. For example, the switching frequency may increase below the dimming level threshold for decreasing dimming levels. An amplitude of the ripple current may decrease below the dimming level threshold for decreasing dimming levels.

[0065] The control device 14 can be configured as a processor, a microprocessor, a controller, a microcontroller, or an application-specific integrated circuit (ASIC). The control device 14 can receive a dimming level via an interface, for example from a controller. Alternatively or additionally, the control device 14 can be configured to determine the dimming level depending on at least one sensor signal. For example, for brightness control, an actual brightness can be detected with a sensor, and a dimming level can be determined depending on a comparison of the actual brightness and the target brightness. Alternatively or additionally, the control device 14 can be configured to determine a dimming level depending on the actuation of an actuating element, for example a button, rotary knob, or switch.

[0066] Fig. 2 shows pulse packets 21, 22 of current that are provided by the operating circuit 1 as output current in at least one operating mode and flow through the LED(s) 2. To generate the pulse packets 21, 22, the control device 14 controls the controllable switching means 13. In this case, the controllable switching means 13 is switched on and off several times in a time window, the duration of which determines a pulse duration 27 of the corresponding pulse packet 21, 22.

[0067] The switching frequency with which the cyclically repeating switch-on operations or the cyclically repeating switch-off operations in pulses 21, 22 follow one another depends on the dimming level if the dimming level is lower than a threshold value.

[0068] Consecutive pulse packets 21, 22 are separated by a time interval 28, during which the controllable switching means 13 is not switched in a clocked manner. The rising edge of consecutive pulse packets 21, 22 is separated by a period 29, which defines the slower cyclic repetition of the generation of a pulse packet 21, 22. The ratio of pulse duration 27 to period duration 29 determines the average output current and thus the effectively perceived brightness.

[0069] The period duration 29 is longer than the duration of a switching cycle for the clocked switching of the controllable switching means 13 during the generation of a pulse packet. The period duration 29 can be much longer than the duration of a switching cycle for the clocked switching of the controllable switching means 13 during the generation of a pulse packet.

[0070] A pulsed operation, as in Fig. 2, does not have to occur over the entire operating range of the operating circuit 1. For example, pulsed operation, in which pulse packets are generated, can only occur for lower dimming levels. For higher dimming levels, other dimming methods can be used.

[0071] As in Fig. 2, the pulse packets 21, 22 have a ripple current. As described with reference to Fig. 3 to Fig. 14, in embodiments of the invention, the current ripples can be changed dynamically depending on a dimming level, for example, if the corresponding dimming level is less than a threshold value or is in another predefined dimming level range.

[0072] Fig. 3 shows an enlarged view of a portion of a pulse packet 30 that the operating circuit 1 generates at a first dimming level. Fig. Figure 4 shows an enlarged view of a portion of a pulse packet 35 generated by the operating circuit 1 at a second dimming level that is lower than the first dimming level. The output current of the operating circuit 1 flowing through the LED(s) is shown.

[0073] As in Fig. 3, the controllable switching means 13 can be switched on whenever the current reaches a first switching threshold value 31 of the current. The controllable switching means 13 can be switched on for a period of time t on remain switched on. When the current reaches a second switching threshold 32, the controllable switching means 13 can be switched off. The controllable switching means 13 can remain switched on for a period of time t offremain off. After a switching cycle duration T, the switching operations can be repeated again. This can continue, for example, until the pulse duration 27 of the pulse packet has elapsed when the first switching threshold value 31 is reached.

[0074] Fig. 4 shows a section of a pulse packet when the dimming level is lower than for the dimming level of Fig. 3. The scaling of the coordinate axes is in Fig. 3 and Fig. 4 is selected equal. Shown is the output current of the operating circuit flowing through the LED(s).

[0075] In the Fig. 4, the controllable switching means 13 can be switched on when the current intensity reaches a further first switching threshold value 36 of the current intensity. This further first switching threshold value 36 is greater than the first switching threshold value 31 in the control for the dimming level of Fig. 3. The controllable switching means 13 can remain switched on for a further period 38 which is less than the period t on when controlling the dimming level of Fig. 3. When the current reaches a further second switching threshold 37, the controllable switching means 13 can be switched off. This further second switching threshold 37 is smaller than the second switching threshold 32 for the dimming level of Fig. 3. The controllable switching means 13 can remain switched off for a further period 39 which is less than the period t orr when controlling the dimming level of Fig. 3 is.

[0076] An average value 33 of the output current, which is formed by averaging over a switching cycle, is in the Fig. 3 shown output current for a higher dimming level equal to the average value 33 at the Fig. 4 shows the output current for a lower dimming level. The amplitude 40 of the current ripple decreases as the dimming level decreases and increases as the dimming level increases.

[0077] As in Fig. 3 and Fig. 4, in operating circuits and methods according to embodiments, the generation of the pulse packets can be carried out in such a way that a maximum current strength 32, 37 of the current ripple becomes smaller as the dimming level decreases. In other words, for the dimming level range, e.g., for dimming levels less than a threshold value, the maximum current strength 32, 37 of the current ripple is a monotonically increasing function of the dimming level. The generation of the pulse packets can be carried out in such a way that a minimum current strength 31, 36 of the current ripple becomes larger as the dimming level decreases. In other words, for the dimming level range, e.g., for dimming levels less than the threshold value, the minimum current strength 31, 36 of the current ripple is a monotonically decreasing function of the dimming level.

[0078] The maximum current 32, 37 and / or the minimum current 31, 36 can define switching thresholds at which the control device 14 switches the controllable switching means 13 off or on. If the dimming level is within a specific dimming level range, these switching thresholds can be changed depending on the dimming level such that the amplitude 40 of the current ripple decreases with decreasing dimming level. If the dimming level is within a specific dimming level range, these switching thresholds can be changed depending on the dimming level such that the mean value 33 remains constant as a function of the dimming level.

[0079] The slope of the current edges that Fig. 3 and Fig. 4, depend on the specific design of the circuit components of the operating circuit 1. A change in the current ripple is accompanied by a change in the switching frequency. The switching frequency with which the controllable switching means 13 is switched on during a pulse packet or with which the controllable switching means 13 is switched off during a pulse packet corresponds to the inverse of the duration T of the switching cycle. If the dimming level is within a specific dimming level range, the switching frequency can increase as the dimming level decreases and decrease as the dimming level increases. The switching frequency can be a decreasing function of the dimming level for dimming levels within the dimming level range.

[0080] Fig. 5 shows a control signal for controlling the controllable switching means 13. With the control signal shown, the Fig. 4. A control signal with a logical value of “1” corresponds to Fig. 5 a switched-on controllable switching means 13. A control signal with a logical value “0” corresponds in Fig. 5 a switched-off controllable switching means 13.

[0081] The controllable switching means 13 is switched on at a first switch-on time 41. At the first switch-on time 41, the output current has reached the first switching threshold value 36. The controllable switching means 13 is switched off at a first switch-off time 42. At the first switch-off time 42, the output current has reached the second switching threshold value 37. At the second switch-on time 43, the controllable switching means 13 is switched on again. A time interval between the rising edges of the control signal at successive switch-on times 41, 43 and / or a time interval between the falling edges of the control signal at successive switch-off times corresponds to the inverse of the switching frequency.

[0082] The control device 14 can have various configurations in order to implement the functions described with reference to Fig. 3 to Fig. 5 to realize the change in current ripples and switching frequencies depending on a dimming level and to generate the corresponding control signal for controlling the controllable switching means 13.

[0083] In one embodiment, the control device 14 can be configured to detect when the output current reaches the first switching threshold 36 and when the output current reaches the second switching threshold 37. Accordingly, the control device 14 can switch on the controllable switching means at the switch-on times 41, 43 when reaching the first switching threshold 36 is detected, and switch it off at the switch-off time 42 when reaching the second switching threshold 37 is detected. The first switching threshold and the second switching threshold can be determined by the control device 14 depending on the dimming level. The control device can determine the first switching threshold and the second switching threshold, for example, based on a characteristic map, computationally, e.g. by evaluating functions as shown schematically in Fig. 8, or carry out further computational processing of map-based variables.

[0084] In a further embodiment, the control device 14 can be configured to detect when the output current reaches the first switching threshold 36. The control device 14 can further be configured to determine the first time period 38 for which the controllable switching means should remain switched on at the corresponding dimming level. Accordingly, the control device 14 can switch on the controllable switching means at the switch-on times 41, 43 when reaching the first switching threshold 36 is detected. The control device 14 can determine the switch-off time 42 depending on the switch-on time 41 and the first time period 38 and switch off the controllable switching means 13 at the switch-off time 42. The first switching threshold and the first time period can be determined by the control device 14 depending on the dimming level.The control device can determine the first switching threshold value and the first time duration, for example based on a characteristic map, or computationally, e.g. by evaluating functions as shown schematically in . Fig. 6 and Fig. 8, or perform further computational processing of map-based variables. For example, the first time period can be defined as a known percentage of the inverse of the Fig. 6. The first time period can be determined, for example, by multiplying the inverse of the switching frequency shown in Fig. 6 shown switching frequency can be calculated using a known factor.

[0085] In a further embodiment, the control device 14 can be configured to detect when the output current reaches the second switching threshold 37. The control device 14 can further be configured to determine the second time period 39 for which the controllable switching means should remain switched off at the corresponding dimming level. Accordingly, the control device 14 can switch off the controllable switching means at the switch-off time 42 when reaching the second switching threshold 37 is detected. The control device 14 can determine the switch-on time 43 depending on the switch-off time 42 and the second time period 39 and switch on the controllable switching means 13 at the switch-on time 43. The second switching threshold and the second time period can be determined by the control device 14 depending on the dimming level.The control device can determine the second switching threshold value and the second time period, for example based on a characteristic map, or computationally, e.g. by evaluating functions as shown schematically in . Fig. 6 and Fig. 8, or perform further computational processing of map-based variables. For example, the second time period can be defined as a known percentage of the inverse of the Fig. 6. The second time period can be determined, for example, by multiplying the inverse of the switching frequency shown in Fig. 6 shown switching frequency can be calculated using a known factor.

[0086] Fig. Figure 6 shows, by way of example, the course of a switching frequency 45 of the controllable switching means as a function of the dimming level. A dimming level range 9 comprises low dimming levels, for example, dimming levels that are lower than a threshold value 8, denoted by SW. For dimming levels in the dimming level range 9, the switching frequency is a monotonically decreasing function 47 of the dimming level.

[0087] For dimming levels outside the dimming level range 9, i.e., for dimming levels greater than the threshold value SW, the switching frequency can, for example, be a constant function 46. This can avoid high switching losses in the operating range where larger currents flow. Other configurations are possible in which the switching frequency depends on the dimming level even for larger dimming levels.

[0088] As in Fig. As shown schematically in Figure 6, the switching frequency for dimming levels in the dimming level range 9 can be a linearly decreasing function of the dimming level. Other functional dependencies can be used in which the switching frequency decreases as the dimming level increases.

[0089] While the switching frequency in Fig. 6 is also schematically shown for dimming levels that are greater than the threshold value SW, a different dimming method can also be used outside the dimming level range 9. For example, amplitude dimming can be used.

[0090] The switching frequency at which the controllable switching device is switched during a pulse packet should not be confused with the frequency at which successive pulse packets are generated. The corresponding quantities can, in particular, exhibit different functional dependencies on the dimming level. If T P the pulse duration of a pulse packet, which in Fig. 2 is shown as pulse duration 27, with T NP which in Fig. 2 Time period 28 represented as time period 28 between the falling edge of a pulse packet and the rising edge of the following pulse packet and correspondingly with T P + T NP which in Fig. 2 is called period 29, is 1 / (T P + T NP ) the frequency at which pulse packets are generated. As in Fig. 7 is shown schematically, the product 49 of pulse duration T P and the frequency with which pulse packets are generated, a monotonically increasing function of the dimming level, even if the dimming level is lower than the threshold value SW. This product 49 determines the effective current output averaged over several pulse packets and thus the effective brightness.

[0091] Fig. Figure 8 illustrates the functional dependence of the maximum current 51 on current ripples and the minimum current 52 on current ripples of the pulse packets.

[0092] For dimming levels in the dimming level range 9, the maximum current ripple intensity 51 is a monotonically increasing function of the dimming level. The maximum current ripple intensity 51 can be a linearly increasing function of the dimming level.

[0093] For dimming levels in the dimming level range 9, the minimum current 52 of the current ripple is a monotonically decreasing function of the dimming level. The minimum current 52 of the current ripple can be a linearly decreasing function of the dimming level. The slope of the straight line indicating the minimum current 52 of the current ripple as a function of the dimming level can be equal to the negative of the slope of the straight line indicating the maximum current 51 of the current ripple as a function of the dimming level.

[0094] Other functional dependencies of the maximum current 51 and / or the minimum current 52 can be used.

[0095] The maximum current 51 and / or the minimum current 52 of the current ripple as a function of the dimming level, as shown in Fig. 8 can be used to define switching thresholds. This was done with reference to Fig. 3 to Fig. 5 already described.

[0096] A dependence of the maximum current 51 and / or the minimum current 52 on the dimming level, as shown schematically in Fig. 8, has the effect that the average current intensity remains unchanged over a switching cycle, even when adjusting the switching thresholds, switching frequency, and / or current ripple amplitude. This facilitates the control and / or regulation procedures for determining the pulse duration T P or the frequency 1 / (T P + T NP), which generates successive pulse packets.

[0097] Fig. Figure 9 illustrates the functional dependence of an amplitude 53 of current ripples of the pulse packets. For dimming levels in the dimming level range 9, the maximum current intensity 51 of current ripples is a monotonically increasing function of the dimming level. For dimming levels greater than the threshold value 8, the amplitude of the current ripples can be kept constant. This avoids high switching losses in the operating range where larger currents flow.

[0098] Fig. 10 is a flowchart of a method 60 according to one embodiment. The method can be executed automatically by the operating circuit 1, wherein the control device 14 can execute the corresponding processing steps.

[0099] In step 61, a dimming level is determined. The dimming level can, for example, be received via an interface of the operating circuit 1, be determined depending on a sensor signal, or be determined depending on the actuation of an input element.

[0100] In step 62, a check is made to determine whether the dimming level is less than a threshold. If the dimming level is not less than the threshold, pulse packets can be generated in step 63, for example, whereby the switching frequency of the controllable switching device and the amplitude of the current ripple are independent of the dimming level. Other dimming techniques can also be used.

[0101] If the dimming level is less than the threshold value, the method continues at step 64. A switching frequency and / or at least one switching threshold value and / or at least one time period for which the controllable switching means is to be switched on or off can be determined depending on the dimming level. The determination can be carried out as described with reference to Fig. 3 to Fig. 5 described.

[0102] In step 65, pulse packets are generated with a switching frequency and / or with current ripples that depend on the dimming level.

[0103] If the dimming level changes, steps 61-65 can be repeated. For example, if the dimming level is further reduced from a value lower than the threshold value SW, the switching frequency for the controllable switching means 13 can be increased and / or the amplitude of the current ripple can be reduced.

[0104] Dynamic adjustment of the current ripple and / or the switching frequency of pulse packets depending on a dimming level can be achieved by adjusting switching thresholds. The output current can be detected in different ways, as shown in Fig. 11 and Fig. 12 is illustrated.

[0105] Fig. 11 and Fig. 12 show configurations of operating circuits according to embodiments in which a variable dependent on the output current is detected. This can be compared with at least one switching threshold value that depends on the dimming level, as described with reference to Fig. 3 to Fig. 9. In the operating circuit of Fig. 11 can be used via ohmic voltage dividers 16, 17 to reduce the voltage V across the LED(s) LED The difference between the voltages at the voltage dividers 16, 17 results in the voltage drop across the LED(s) VLED . Knowing the characteristic curve of the LED, the voltage drop across the LED(s) can be used to calculate V LED the current flowing through LED 2 can be determined. Depending on the output voltage of operating circuit 1 determined in this way, a comparison can be made with corresponding switching threshold values.

[0106] When operating Fig. The current flowing through the coil or the voltage drop across the coil can be determined via an inductance 18, which is inductively coupled to the coil 11. Depending on the coil current or the coil voltage of the operating circuit 1 thus determined, the current through the LED 2 can be determined and compared with corresponding switching threshold values.

[0107] By detecting the voltage drop across the coil 11, the voltage drop across the LED(s) V LEDbe closed. When the controllable switching means 13 is open and the coil 11 continues to drive the current through the LED during the demagnetization phase, the voltage drop across the LED(s) corresponds to V LED approximately the voltage drop across coil 11, where the difference between the two voltages is the forward voltage of diode 12, which can either be taken into account in the detection or, in a simple variant, can be neglected. The forward voltage of diode 12 can, for example, be 0.7 V. Knowing the characteristic curve of the LED, the voltage drop across the LED(s) V LED to the current flowing through LED 2.

[0108] Alternatively, to monitor the voltage drop across the LED(s) V LEDTo determine the first switching threshold value 31, 36 for switching on the controllable switching means 13, a direct measurement of the current through the LED 2 can also be carried out.

[0109] As in Fig. 11 and Fig. 12, the control device 14 may comprise an input 19 for receiving a signal DL indicating the dimming level.

[0110] The dynamic adjustment of the current ripple and / or the switching frequency of pulse packets as a function of a dimming level can be used to reduce perceptible flickering even when, at a constant dimming level, different pulse packets have a different number of switching cycles of the controllable switching device. For example, one pulse packet has a number np of switching cycles, and the following pulse packet has a number np ± 1 of switching cycles. The next pulse packet can, for example, again have np switching cycles. Such a "switching" of the number of switching cycles can occur, for example, when the time at which the output current reaches the lower switching threshold approximately coincides with the end of the pulse duration. Such a switching of the number of switching cycles can also be introduced deliberately and in a controlled manner in order to be able to adjust the current averaged over several pulse packets more precisely.

[0111] Fig. 13 shows the end of a pulse packet. At a point 70, the current reaches the first switching threshold. If the pulse duration of the corresponding pulse packet ends at a time 71, shortly before the current reaches the first switching threshold at 70, the controllable switching means is not switched on again. The current ripple 73 is the last current ripple. The current can drop to end the pulse packet. If the pulse duration of the corresponding pulse packet ends at a time 72, shortly after the current reaches the first switching threshold at 70, the controllable switching means is switched on again. Another switching cycle 75 follows, which leads to a current ripple 74. Only after the end of the further switching cycle 75 is the controllable switching means not switched on again to end the pulse packet.

[0112] The time integral over the current of the pulse packet defines the light energy emitted by the LED(s). The corresponding time integral differs by an area of ​​76 for the two cases with switching cycle numbers np and np+1 that differ by one.

[0113] At lower dimming levels, the time integral of the output current calculated over the pulse packet is smaller than at higher dimming levels. The difference 76 between the time integrals is relatively more significant at lower dimming levels compared to the integral over the entire pulse packet than at higher dimming levels. This can lead to undesirable, noticeable flickering.

[0114] Fig. Figure 14 illustrates the effect of devices and methods according to embodiments. The switching frequency is compared to Fig. 13 at low dimming levels. The amplitude of the voltage ripple is reduced. The scaling of the coordinate axes is in Fig. 13 and Fig. 14 elected equally.

[0115] If the pulse duration of the corresponding pulse packet ends at a time 71, shortly before the current intensity at 70 reaches the first switching threshold, the controllable switching means is not switched on again. The current ripple 83 is the last current ripple. The current intensity can drop to end the pulse packet. If the pulse duration of the corresponding pulse packet ends at a time 72, shortly after the current intensity at 70 reaches the first switching threshold, the controllable switching means is switched on again. Another switching cycle 85 follows, which leads to a current ripple 84. Only after the end of the further switching cycle 85 is the controllable switching means not switched on again to end the pulse packet. The difference in the time integrals differs for the two cases with switching cycle numbers np and np+1 differing by one by the area 86, which is smaller than the corresponding area 76 in Fig.13. Flickering can be reduced.

[0116] Increasing the switching frequency at lower dimming levels leads to a correspondingly shorter switching cycle duration. This allows for a reduction in brightness control errors, even when switching between different switching cycle numbers at the same dimming level is not performed. For example, the maximum time difference between the end of a switching cycle and the end of the pulse duration is reduced.

[0117] At higher dimming levels, the time integrals of the output current calculated over a pulse packet are larger overall. A lower switching frequency and a larger current ripple amplitude can be used without causing severe flickering. Switching losses can thus be kept lower by dynamically adjusting the switching frequency.

[0118] While exemplary embodiments have been described with reference to the figures, modifications can be implemented in further exemplary embodiments. Methods and devices according to exemplary embodiments can be used in operating devices for lighting devices, for example in an LED converter.

Claims

[1] Operating circuit for at least one light-emitting diode (2), comprising a coil (11), a controllable switching means (13) and a control device (14) which is designed to repeatedly switch on the controllable switching means (13) to provide a pulse packet (21, 22) to the at least one light-emitting diode (2) during a pulse duration (27) in order to temporarily store energy in the coil (11), and to switch it off in order to discharge energy temporarily stored in the coil (11) via a diode (12) and via the at least one light-emitting diode (2), wherein the control device (14) is arranged to switch on the controllable switching means (13) during the pulse duration (27) when the current intensity through the at least one light-emitting diode (2) reaches a switch-on switching threshold value (31; 36) which depends on the dimming level, wherein the control device (14) is arranged such that in time-sequentially generated pulse packets (21, 22) the control device (14) switches the controllable switching means (13) with at least two different switching cycle numbers, and wherein the control device (14) is arranged such that a switching cycle number of the pulse packet (21, 22) and a further switching cycle number of a further pulse packet (21, 22) differ by 1. [2] Operating circuit for at least one light-emitting diode (2), comprising a coil (11), a controllable switching means (13), wherein the controllable switching means (13) is a field effect transistor, a transistor or an insulated gate transistor, and a control device (14) which is designed to repeatedly switch on the controllable switching means (13) to provide a pulse packet (21, 22) to the at least one light-emitting diode (2) during a pulse duration (27) in order to temporarily store energy in the coil (11), and to switch it off in order to discharge energy temporarily stored in the coil (11) via a diode (12) and via the at least one light-emitting diode (2), wherein the control device (14) is configured to switch off the controllable switching means (13) during the pulse duration (27) when a current intensity through the at least one light-emitting diode (2) reaches a switch-off switching threshold value (32; 37) which depends on a dimming level of the at least one light-emitting diode (2), wherein the control device (14) is arranged such that in time-sequentially generated pulse packets (21, 22) the control device (14) switches the controllable switching means (13) with at least two different switching cycle numbers, and wherein the control device (14) is arranged such that a switching cycle number of the pulse packet (21, 22) and a further switching cycle number of a further pulse packet (21, 22) differ by 1. [3] Operating circuit according to claim 1 or claim 2, wherein the control device (14) is arranged to switch the controllable switching means (13) such that a switching frequency (45) of the controllable switching means (13) is a strictly monotonically decreasing function (47) of the dimming level at least for dimming levels which are less than a threshold value (8). [4] Operating circuit according to one of the preceding claims, wherein the control device (14) is arranged to switch off the controllable switching means (13) during the pulse duration (27) when the current intensity through the at least one light-emitting diode (2) reaches the switch-on switching threshold value (32; 37), and to switch it on again after a first time period (39), wherein the switch-on threshold value (32; 37) and the first time period (39) depend on the dimming level. [5] Operating circuit according to one of the preceding claims, wherein the control device (14) is arranged to switch on the controllable switching means (13) during the pulse duration (27) when the current intensity through the at least one light-emitting diode (2) reaches the switch-off switching threshold value (31; 36), and to switch it off again after a second time period (38), wherein the switch-off threshold value (31; 36) and the second time period (38) depend on the dimming level. [6] Method for operating at least one light-emitting diode (2) depending on a dimming level by means of an operating circuit comprising a coil (11) and a controllable switching means (13), wherein, in order to generate a pulse packet (21, 22) supplied to the at least one light-emitting diode (2), the controllable switching means (13) is repeatedly switched on in order to temporarily store energy in the coil (11) and is switched off in order to discharge energy temporarily stored in the coil (11) via a diode (12) and via the at least one light-emitting diode (2), wherein during the pulse duration (27) the controllable switching means (13) is switched off when the current intensity through the at least one light-emitting diode (2) reaches a switch-on switching threshold value (31; 36) which depends on the dimming level, wherein during the pulse duration (27) the controllable switching means (13) is switched off when a current intensity through the at least one light-emitting diode (2) reaches a switch-off switching threshold value (32; 37) which depends on a dimming level of the at least one light-emitting diode (2), wherein time-sequentially generated pulse packets have at least two different switching cycle numbers. [7] Method for operating at least one light-emitting diode (2) depending on a dimming level by means of an operating circuit comprising a coil (11) and a controllable switching means (13), wherein the controllable switching means (13) is a field-effect transistor, a transistor or a transistor with an insulated gate electrode, wherein, in order to generate a pulse packet (21, 22) supplied to the at least one light-emitting diode (2), the controllable switching means (13) is repeatedly switched on in order to temporarily store energy in the coil (11) and is switched off in order to discharge energy temporarily stored in the coil (11) via a diode (12) and via the at least one light-emitting diode (2), and, wherein during the pulse duration (27) the controllable switching means (13) is switched off when a current intensity through the at least one light-emitting diode (2) reaches a switch-off switching threshold value (32; 37) which depends on a dimming level of the at least one light-emitting diode (2), wherein time-sequentially generated pulse packets have at least two different switching cycle numbers. [8] Method according to claim 6 or 7, wherein during the pulse duration (27) the controllable switching means (13) is switched off when the current intensity through the at least one light-emitting diode (2) reaches the switch-off switching threshold value (32; 37), and is switched on again after a first time period (39), wherein the switch-off threshold value (32; 37) and the first time period (39) depend on the dimming level. [9] Method according to one of claims 6 to 8, wherein during the pulse duration (27) the controllable switching means (13) is switched on when the current intensity through the at least one light-emitting diode (2) reaches the switch-on switching threshold value (31; 36), and is switched off again after a second time period (38), wherein the switch-on threshold value (31; 36) and the second time period (38) depend on the dimming level. [10] Method according to one of claims 6 to 9, wherein a switching frequency (45) of the controllable switching means (13) is a strictly monotonically decreasing function (47) of the dimming level at least for dimming levels which are lower than a threshold value (8).

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