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

Abstract

Operating circuit for at least one light-emitting diode (2), comprising a coil (11), a controllable switching device (13) and a control device (14) which is configured to repeatedly switch on the controllable switching device (13) during a pulse duration (27) in order to provide a pulse packet (21, 22) to the at least one light-emitting diode (2) in order to temporarily store energy in the coil (11), and to switch off the 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 on the controllable switching device (13) during the pulse duration (27) when the current through the at least one light-emitting diode (2) reaches a switch-on switching threshold value (31; 36) which depends on a dimming level, wherein the pulse duration (27) of the pulse packets (21, 22) and / or a time interval of the pulse packets (21, 22) is adjustable depending on the dimming level by means of a pulse modulation, for example a pulse width modulation, and where the product of the pulse duration (27) and the frequency at which pulse packets are generated is a monotonically increasing function of the dimming level.

Description

[0001] The invention relates to an operating circuit for a light source. The invention relates in particular to operating circuits for supplying power to a light-emitting diode (LED) or several LEDs depending on a dimming level.

[0002] With the increasing prevalence of light sources such as LEDs and LED modules, operating circuits for these light sources are gaining in importance. The operating circuit primarily serves to provide the desired power supply to the light source. Additional functions can be incorporated into the operating circuit, for example, to enable dimming. The light emission of LEDs depends on the current flowing through them. Therefore, for brightness control or regulation, LEDs are typically operated in a mode where the current flow through the LED is controlled or regulated by an operating circuit.

[0003] Switching regulators, especially buck converters, also known as step-down converters, can be used to control an array of one or more LEDs. In such a circuit, a control device operates a high-frequency switched controllable switch. The controllable switch could be, for example, a power transistor. When the switch is on, current flows through the LED array and an inductor, which is thereby charged with energy. The energy stored in the inductor is discharged through the LEDs when the switch is off.

[0004] To reduce shifts in the light spectrum at different dimming levels, LEDs can be used for brightness control. In this method, the LEDs are supplied with pulsed current via the operating circuit. The LEDs can be dimmed to lower levels by increasing the time interval between the pulses, i.e., by decreasing the frequency at which the pulses are generated.

[0005] The output current supplied to the LEDs by the operating circuit can exhibit a constant average current during the pulse duration, superimposed with current ripple. Such current ripple is caused by the clocked switching of the controllable switching device.

[0006] Even with a predefined dimming level, situations can arise where the number of switching cycles of the controllable switching device varies from one pulse packet to the next. For example, another switching cycle of the controllable switching device can be triggered automatically when the output current of the operating circuit reaches a current threshold. Depending on whether this current threshold is shortly before or shortly after the end of the defined pulse duration of the pulse packet, the number of switching cycles per pulse packet can vary. Consequently, the energy supplied to the LEDs for a pulse packet can also vary. This can lead to undesirable flickering of the light. Such flickering can be perceived by the human eye and is often considered bothersome.

[0007] US 2006 / 0197720 A1 describes a hysteresis LED current control circuit in which a sense element and amplifier drive a window comparator / logic controller that switches a switching element on and off to maintain the LED current between lower and upper threshold values. The circuit uses an energy storage device as an inductor, a diode, and a switching element; timing diagrams and flowcharts illustrate the hysteresis control method as well as alternative implementations.

[0008] WO 2010 / 025450 A2 discloses an LED lighting system that determines the actual amount of charge supplied to the LEDs during each dimming active phase and uses this information to adjust the target charge for a future active phase at dimming frequencies > 50 Hz. The system distinguishes between a dimming controller and a current controller; the current controller operates at a switching frequency, while the dimming operates at its own rate (PWM or delta-sigma), with a time-based controller determining state change times from detected comparison events.

[0009] US 2012 / 0286686 A1 describes a lighting device with two superimposed switching controls: a first high-frequency switching control for the converter and a second, lower interval control that intermittently stops the switching; the second frequency is varied when the LED current changes. An output detection circuit combines current and voltage feedback, uses a bypass current through a Zener diode as a bias at low dimming levels, and enables stable dimming down to near zero; the topology is applicable to buck, boost, flyback, and buck-boost operation.

[0010] JP 5942256 B2 relates to a luminaire control system that, at higher dimming levels, fixes the interval cycle and varies the DC voltage application time, while at lower dimming levels, it fixes the application time and varies the blocking time to minimize color and brightness fluctuations. A reference threshold between the upper and lower limits of the average LED current defines the switching point between the two strategies; above this threshold, the peak current is optionally varied depending on the dimming level, and below it, the blocking time is adjusted incrementally according to the dimming level.

[0011] The invention is based on the objective of providing devices and methods that reduce the problems described. In particular, the objective is to provide devices and methods that reduce light flicker without significantly increasing switching losses at high dimming levels.

[0012] According to the invention, an operating circuit for a light source and a method with the features specified in the independent claims are provided. The dependent claims define embodiments of the invention.

[0013] According to exemplary embodiments of the invention, at least in one dimming level range, e.g., for dimming levels below a threshold value, the switching frequency of a controllable switching device of an operating circuit is dynamically adjusted to the respective dimming level. The current ripple is correspondingly dynamically adjusted to the respective dimming level.

[0014] An operating circuit for at least one light-emitting diode (LED) according to an exemplary embodiment comprises a coil, a controllable switching device, and a control unit. The control unit is configured to repeatedly switch on the controllable switching device during a pulse duration to temporarily store energy in the coil and to switch it off to discharge the energy temporarily stored in the coil via a diode and the at least one LED. The control unit is configured such that the switching frequency at which the controllable switching device is switched on or off during the pulse duration depends on a dimming level at least within a dimming level range, wherein the product of the pulse duration (27) and the frequency at which pulse packets are generated is a monotonically increasing function of the dimming level.

[0015] The control device can be configured to switch the controllable switching device in such a way that the switching frequency in the dimming level range is a monotonically decreasing function of the dimming level.

[0016] The control device can be configured to switch the controllable switching device such that the switching frequency for dimming levels below a threshold is a strictly monotonically decreasing function of the dimming level.

[0017] The control device can be configured to switch the controllable switching device such that the switching frequency for dimming levels below the threshold is a linearly decreasing function of the dimming level. For dimming levels above the threshold, the switching frequency can be a constant function of the dimming level.

[0018] The control device can be configured to switch the controllable switching device in such a way that the amplitude of current ripples occurring in the pulse packet in the dimming level range depends on the dimming level.

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

[0020] The control device can be configured to switch the controllable switching device in such a way that the amplitude of the current ripple in the dimming level range is a linearly increasing function of the dimming level.

[0021] The control device can be configured to switch the controllable switching device in such a way that a maximum current and a minimum current of current ripple in the dimming level range depend on the dimming level.

[0022] The control device can be configured to switch the controllable switching device in such a way that the maximum current of the current ripple is a linearly increasing function of the dimming level.

[0023] The control device can be configured to switch the controllable switching device in such a way that a minimum current ripple is a linearly decreasing function of the dimming level.

[0024] The control device can be configured to switch the controllable switching device in such a way that an average current value of the current ripple is independent of the dimming level.

[0025] The control unit can achieve a switching frequency and / or current ripple amplitude dependent on the dimming level in various ways. The control unit can compare the output current supplied to the at least one LED with a switching threshold and switch the controllable switching device on or off depending on the threshold comparison. The control unit can determine a time period until the next switch-off or switch-on, depending on the dimming level. The switching threshold and / or the time period can, for example, be determined based on a characteristic curve, calculated by the control unit, or specified externally.

[0026] The control device can be configured to switch on the controllable switching device during the pulse duration when a current 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.

[0027] The control device can be configured to switch off the controllable switching device during the pulse duration when a current through the at least one light-emitting diode reaches a second switching threshold, and to switch it on again after a second time period, wherein the second switching threshold and the second time period depend on the dimming level.

[0028] The control unit can be configured such that, in different pulse packets, it switches the controllable switching device with at least two different switching cycle counts. Such different switching cycle counts can also be deliberately generated 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 fine-tuning of the output current averaged over several pulse packets to a target value.

[0029] The control device can be configured such that the number of switching cycles of a pulse packet generated for one dimming level and the number of switching cycles of another pulse packet generated for the same dimming level differ by 1.

[0030] The control unit can be an integrated semiconductor circuit. The control unit can be designed as a processor, a microprocessor, a controller, a microcontroller, or an application-specific integrated circuit (ASIC).

[0031] The control device may include an input for receiving the dimming level or a quantity that influences the dimming level.

[0032] The operating circuit can include an input for receiving a DC voltage or a rectified AC voltage.

[0033] The operating circuit can include a capacitor connected in parallel to at least one LED.

[0034] At least one LED can comprise one or more inorganic and / or organic LED(s).

[0035] According to a further embodiment, a system is provided that includes the operating circuit and the at least one LED connected to the operating circuit.

[0036] In a further embodiment, a method for operating at least one LED by means of an operating circuit is described. The operating circuit comprises an inductor and a controllable switching device. 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 device is repeatedly switched on to temporarily store energy in the inductor and switched off to discharge the energy temporarily stored in the inductor via a diode and via the at least one LED. A switching frequency at which the controllable switching device is switched on or off during the pulse duration depends on the dimming level in at least one dimming level range, wherein the product of the pulse duration (27) and the frequency at which pulse packets are generated is a monotonically increasing function of the dimming level.For dimming levels within the dimming level range, the switching frequency can be a monotonically decreasing function of the dimming level.

[0037] At least for dimming levels that are smaller 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 below the threshold. For dimming levels above the threshold, the switching frequency can be a constant function of the dimming level.

[0039] The amplitude of current ripples occurring in the pulse packet can 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] The maximum and minimum current of current ripples can depend on the dimming level within the dimming level range.

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

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

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

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

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

[0048] The controllable switching device can be switched off during the pulse duration if a current 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 can have at least two different switching cycle counts. Such different switching cycle counts can be specifically set for different pulse packets at a fixed dimming level, for example, to adjust the output current averaged over several pulse packets to a specific target value. This allows for fine-tuning of the output current averaged over multiple pulse packets.

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

[0051] The operating circuit can be supplied with either a direct current voltage or a rectified alternating current voltage.

[0052] The procedure can be executed automatically using the operating circuit according to an exemplary embodiment.

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

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

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

[0056] In devices and methods according to exemplary embodiments, the switching frequency at which the controllable switching device is switched on and / or off can be selected depending on the dimming level. In particular, for a dimming level range with low dimming levels, switching frequencies can be selected that are higher than the switching frequency used for higher dimming levels. The higher switching frequency and / or smaller ripple amplitude at low dimming levels can reduce relative fluctuations in the average current of pulse packets, even if the pulse packets correspond to a different number of switching cycles. At higher dimming levels, a lower switching frequency can be selected because, due to the overall larger number of switching cycles per pulse packet, relative fluctuations of, for example, one switching cycle would not lead to a clearly perceptible flicker.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 the figures and preferred embodiments. In the figures, identical reference numerals denote identical elements. Fig. Figure 1 shows an operating circuit for light-emitting diodes. Fig. Figure 2 shows pulse packets of an output current from the operating circuit. Fig. Figure 3 shows an enlarged view of a section of a pulse packet at a first dimming level. Fig. Figure 4 shows an enlarged representation of a section of a pulse packet at a second dimming level that is smaller than the first dimming level, to illustrate the mode of operation of devices and methods according to exemplary embodiments. Fig. Figure 5 shows a control signal for controlling a controllable switch. Fig. Figure 6 shows a dependence of a switching frequency on the dimming level for devices and methods according to an exemplary embodiment. Fig. Figure 7 shows a change in the duty cycle used to generate pulse packets, depending on the dimming level. Fig. Figure 8 shows a dependence of maxima and minima of current ripples on the dimming level for devices and methods according to an exemplary embodiment. Fig. Figure 9 shows a dependence of the amplitude of current ripples on the dimming level in devices and methods according to an exemplary embodiment. Fig. Figure 10 is a flowchart of a process according to an exemplary embodiment. Fig. Figure 11 shows an operating circuit for light-emitting diodes according to an exemplary embodiment. Fig. Figure 12 shows an operating circuit for light-emitting diodes according to an exemplary embodiment. Fig. 13 and Fig. Figure 14 shows sections of pulse packets to illustrate the operation of exemplary embodiments.

[0058] Fig. Figure 1 shows a diagram of a system comprising an operating circuit 1 for a light source 2. The light source 2 can comprise one or more light-emitting diodes (LEDs). The LEDs can be inorganic or organic. The multiple LEDs can be connected in series or in parallel. The multiple 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 source can also have only one LED, two LEDs, or more than three LEDs.

[0059] Operating circuit 1 serves to operate at least one LED 2. A supply voltage Vbus, which can be a DC voltage or a rectified AC voltage, is supplied to operating circuit 1. The output of the operating circuit, which is connected to the at least one LED, is connected to an inductor 11 and a controllable switching device 13. When the at least one LED 2 is connected to operating circuit 1, the controllable switching device 13, the inductor 11, and the at least one LED 2 are connected in series. A diode 12 is connected in parallel with the at least one LED 2 and the inductor 11. A capacitor 15 can be connected between the output terminals, so that the capacitor 15 is connected in parallel with 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 device 13 is switched on, current flows through the LED(s) 2 and through the coil 11, which is thereby magnetized. When the controllable switching device 13 is switched off, the energy stored in the magnetic field of the coil is discharged as a current through the diode 12 and the LED(s) 2. Simultaneously, the capacitor 15 can be charged at the beginning of the switch-on phase of the controllable switching device 13. During the switch-off phase of the controllable switching device 13 (freewheeling phase), the capacitor 15 can discharge and contribute 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 device 13 can be a power switch. The controllable switching device 13 can be a field-effect transistor or a bipolar transistor. The controllable switching device 13 can be a transistor with an insulated gate electrode.

[0062] The operating circuit 1 has a control device 14 for the clocked switching of the controllable switching element 13. As with reference to Fig. 2 to Fig. As described in detail in section 14, 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 operating circuit 1. For example, pulse packets can be generated at lower dimming levels to adjust the average current and thus the brightness perceived by the eye. For this purpose, pulse modulation such as pulse-width modulation (PWM) can be used, in which the pulse duration and / or the time interval between pulse packets are set depending on the dimming level.

[0063] As is typical in this area of ​​technology, a higher dimming level corresponds to greater brightness, and a lower dimming level corresponds to less brightness. 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 device 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. As described in detail in section 14, the control device 14 controls the controllable switching device 13 at least within a dimming level range, e.g., for low dimming levels below 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 can increase below the dimming level threshold for decreasing dimming levels. The amplitude of the ripple current can decrease below the dimming level threshold for decreasing dimming levels.

[0065] The control unit 14 can be configured as a processor, a microprocessor, a controller, a microcontroller, or an application-specific integrated circuit (ASIC). The control unit 14 can receive a dimming level via an interface, for example, from a controller. Alternatively or additionally, the control unit 14 can be configured to determine the dimming level based on at least one sensor signal. For example, for brightness control, the current brightness can be detected by a sensor, and a dimming level can be set based on a comparison of the current brightness and the target brightness. Alternatively or additionally, the control unit 14 can be configured to determine a dimming level based on the actuation of an actuator, such as a push button, rotary knob, or switch.

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

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

[0068] Successive pulse packets 21, 22 are separated by a time interval 28, during which the controllable switching device 13 is not clocked. The rising edge of successive 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 29 is longer than the duration of a switching cycle for the clocked switching of the controllable switching device 13 during the generation of a pulse packet. The period 29 can be much longer than the duration of a switching cycle for the clocked switching of the controllable switching device 13 during the generation of a pulse packet.

[0070] A bustling operation, like the one in Fig. The operation shown in Figure 2 does not necessarily have to apply across the entire operating range of the operating circuit 1. For example, pulsed operation, in which pulse packets are generated, can be used only for lower dimming levels. Other dimming methods can be used for higher dimming levels.

[0071] As in Fig. As shown schematically in Figure 2, pulse packets 21 and 22 exhibit a ripple current. As referenced in Figure 2, pulse packets 21 and 22 exhibit a ripple current. Fig. 3 to Fig. As described in detail in section 14, the current ripple in embodiments of the invention can be dynamically changed depending on a dimming level if the corresponding dimming level is, for example, smaller than a threshold value or lies in another predefined dimming level range.

[0072] Fig. Figure 3 shows an enlarged representation of a section of a pulse packet 30 that generates the operating circuit 1 at a first dimming level. Fig. Figure 4 shows an enlarged view of a section 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. As shown in Figure 3, the controllable switching device 13 can be switched on each time the current reaches a first switching threshold value 31. The controllable switching device 13 can be switched on for a time period t. on The controllable switching device 13 can remain switched on. When the current reaches a second switching threshold value 32, the controllable switching device 13 can be switched off. The controllable switching device 13 can remain switched on for a time period t. offThe device remains switched off. After a switching cycle duration T, the switching operations can be repeated. This can continue, for example, until the pulse duration 27 of the pulse packet has elapsed when the first switching threshold 31 is reached.

[0074] Fig. Figure 4 shows a section of a pulse packet when the dimming level is lower than the dimming level of Fig. 3 is. The scaling of the coordinate axes is in Fig. 3 and Fig. 4 are chosen equally. The output current of the operating circuit, flowing through the LED(s), is shown.

[0075] At the in Fig. In the pulse packet shown in Figure 4, the controllable switching device 13 can be switched on each time the current reaches a further first switching threshold value 36. 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 device 13 can remain switched on for a further period of time 38, which is less than the period of time t. on in the control for the dimming level of Fig. 3. When the current reaches a further second switching threshold value 37, the controllable switching device 13 can be switched off. This further second switching threshold value 37 is smaller than the second switching threshold value 32 for the dimming level of Fig. 3. The controllable switching device 13 can remain switched off for a further period of time 39, which is less than the period of time t. off in the control for 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 case of the Fig. The output current shown in section 3 for a larger dimming level is equal to the average value of 33 at which in Fig. Figure 4 shows the output current for a smaller dimming level. The amplitude of the current ripple decreases as the dimming level decreases and increases as the dimming level increases.

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

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

[0079] The slope of the current flanks, which in Fig. 3 and Fig. The switching frequency, as shown schematically in Figure 4, depends on the specific configuration 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 at which the controllable switching device 13 is switched on or 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. For dimming levels within this range, the switching frequency can be a decreasing function of the dimming level.

[0080] Fig. Figure 5 shows a control signal for controlling the controllable switching device 13. The control signal shown is used to operate the device in Fig. The four pulse packets shown are implemented. A control signal with a logical value "1" corresponds to... Fig. 5 a switched-on controllable switching device 13. A control signal with a logical value “0” corresponds in Fig. 5 a switched-off controllable switching device 13.

[0081] The controllable switching device 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 device 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 device 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 comply with the requirements referred to in Fig. 3 to Fig. 5 to realize the described change in current ripples and switching frequencies depending on a dimming level and to generate the corresponding control signal to control the controllable switching device 13.

[0083] In one configuration, the control unit 14 can be set up 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 unit 14 can switch on the controllable switching device at switch-on times 41 and 43 when the first switching threshold 36 is detected, and switch it off at switch-off time 42 when the second switching threshold 37 is detected. The first and second switching thresholds can be determined by the control unit 14 depending on the dimming level. The control unit can determine the first and second switching thresholds, for example, using a characteristic map or computationally, e.g., by evaluating functions such as those schematically shown in Fig. 8 are shown, or perform computational processing of characteristic map-based quantities.

[0084] In a further embodiment, the control unit 14 can be configured to detect when the output current reaches the first switching threshold 36. The control unit 14 can also be configured to determine the first duration 38 for which the controllable switching device is to remain switched on at the corresponding dimming level. Accordingly, the control unit 14 can switch on the controllable switching device at switch-on times 41 and 43 when the first switching threshold 36 is detected. The control unit 14 can determine the switch-off time 42 based on the switch-on time 41 and the first duration 38, and switch off the controllable switching device 13 at switch-off time 42. The first switching threshold and the first duration can be determined by the control unit 14 depending on the dimming level.The control unit can determine the first switching threshold 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 are shown, or a computational processing of characteristic-map-based quantities can be performed. For example, the first time duration can be a known percentage of the inverse of the in Fig. The switching frequency shown in section 6 can be determined. The first time duration can be determined, for example, by multiplying the inverse of the value shown in the diagram. Fig. The switching frequency shown in section 6 can be calculated using a known factor.

[0085] In a further embodiment, the control unit 14 can be configured to detect when the output current reaches the second switching threshold 37. The control unit 14 can also be configured to determine the second time period 39 for which the controllable switching device is to remain switched off at the corresponding dimming level. Accordingly, the control unit 14 can switch off the controllable switching device at the switch-off time 42 when the reaching of the second switching threshold 37 is detected. The control unit 14 can determine the switch-on time 43 based on the switch-off time 42 and the second time period 39, and switch on the controllable switching device 13 at the switch-on time 43. The second switching threshold and the second time period can be determined by the control unit 14 depending on the dimming level.The control unit can determine the second switching threshold and the second 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 are shown, or a computational processing of characteristic-map-based quantities can be performed. For example, the second time duration can be a known percentage of the inverse of the in Fig. The switching frequency shown in 6 can be determined. The second time period can be determined, for example, by multiplying the inverse of the frequency shown in Fig. The switching frequency shown in section 6 can be calculated using a known factor.

[0086] Fig. Figure 6 shows an example of the switching frequency 45 of the controllable switching device as a function of the dimming level. A dimming level range 9 includes low dimming levels, for example, dimming levels that are less than a threshold value 8, which is 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 SW, the switching frequency can, for example, be a constant function 46. This avoids high switching losses in the operating range where larger currents flow. Other configurations are possible in which the switching frequency also depends on the dimming level for larger dimming levels.

[0088] As in Fig. As schematically represented 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 relationships can be used where the switching frequency decreases as the dimming level increases.

[0089] While the switching frequency in Fig. Figure 6 schematically shows dimming levels greater than the threshold SW. Outside of dimming level range 9, another dimming method can be used. For example, amplitude dimming can be used.

[0090] The switching frequency at which the controllable switching device is switched during a pulse packet must not be confused with the frequency at which successive pulse packets are generated. The corresponding quantities can also exhibit different functional dependencies on the dimming level. If with T P the pulse duration of a pulse packet, which is in Fig. 2 is represented as pulse duration 27, with T NP the in Fig. 2. Time period 28, represented as time duration 28, is the time period 28 between the falling edge of one pulse packet and the rising edge of the following pulse packet, and correspondingly with T. P + T NP the in Fig. The time period represented as period 29 is 1 / (T P + T NP ) the frequency at which pulse packets are generated. As in Fig. As shown schematically in section 7, the product 49 of pulse duration T is P and the frequency at which pulse packets are generated, a monotonically increasing function of the dimming level, even if the dimming level is less than the threshold 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 dimming level range 9, the maximum current 51 of current ripples is a monotonically increasing function of the dimming level. The maximum current 51 of the current ripples can be a linearly increasing function of the dimming level.

[0093] For dimming levels in 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 line that represents 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 line that represents 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 described in Fig. Figure 8, which can be used to define switching thresholds, was developed 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. As shown in Figure 8, the effect is that the average current remains unchanged over a switching cycle even when the switching thresholds, switching frequency, and / or amplitude of the current ripple are adjusted. This simplifies the control procedures for determining the pulse duration T. P or the frequency 1 / (T P + T NP), with which successive pulse packets are generated.

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

[0098] Fig. Figure 10 is a flowchart of a process 60 according to an exemplary embodiment. The process can be executed automatically by the operating circuit 1, with the control unit 14 being able to perform 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 set depending on a sensor signal, or be determined depending on the actuation of an input element.

[0100] Step 62 checks whether the dimming level is below a threshold. If the dimming level is not below the threshold, pulse packets can be generated in step 63, where 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 lower than the threshold, the procedure continues at step 64. In this step, a switching frequency and / or at least a switching threshold and / or at least a time period for which the controllable switching device is to be switched on or off can be determined, depending on the dimming level. This determination can be carried out as described in [reference to...]. Fig. 3 to Fig. 5 described below.

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

[0103] Steps 61-65 can be repeated if the dimming level is changed. For example, if the dimming level is further reduced starting from a value lower than the threshold SW, the switching frequency for the controllable switching device 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. Output current can be measured in various ways, as shown in... Fig. 11 and Fig. 12 is illustrated.

[0105] Fig. 11 and Fig. Figure 12 shows embodiments of operating circuits according to exemplary implementations in which a quantity 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 with reference to Fig. 3 to Fig. 9 was explained. During the operating circuit of Fig. 11 can use ohmic voltage dividers 16, 17 to divide the voltage V dropping across the LED(s). LED The voltage drop across the LED(s) can be determined. The difference in voltages across the voltage dividers 16 and 17 yields the voltage V.LED Knowing the characteristic curve of the LED, the voltage drop V across the LED(s) can be used to determine LED The current flowing through LED 2 can be inferred. Depending on the output voltage of the operating circuit 1 determined in this way, a comparison with corresponding switching threshold values ​​can be made.

[0106] During the operating circuit of Fig. The current flowing through the coil or the voltage drop across the coil can be determined via an inductor 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 deduced and compared with corresponding switching threshold values.

[0107] By measuring the voltage drop across coil 11, the voltage drop V across the LED(s) can be determined. LEDto be closed. When the controllable switching device 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 The voltage drop across coil 11 is approximately equal to the difference between the two voltages, where the difference is the forward voltage of diode 12. This value can either be taken into account in the measurement or, in a simpler version, 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) can be used to determine the voltage. LED inferences can be made about 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 device 13, a direct measurement of the current through the LED 2 can also be carried out.

[0109] As in Fig. 11 and Fig. As shown schematically in Figure 12, the control device 14 can include an input 19 for receiving a signal DL indicating the dimming level.

[0110] Dynamically adjusting the current ripple and / or switching frequency of pulse packets as a function of a dimming level can be used to reduce perceptible flickering, even when different pulse packets exhibit a different number of switching cycles of the controllable switching device at a constant dimming level. For example, one pulse packet might have np switching cycles, and the following pulse packet might have np ± 1 switching cycles. The next pulse packet might again have np switching cycles. Such a "switching" of the number of switching cycles can occur, for example, when the point 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. Figure 13 shows the end of a pulse packet. At point 70, the current reaches the first switching threshold. If the pulse duration of the corresponding pulse packet ends at time 71, shortly before the current reaches the first switching threshold at 70, the controllable switching device 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 time 72, shortly after the current reaches the first switching threshold at 70, the controllable switching device is switched on again. Another switching cycle 75 follows, leading to a current ripple 74. Only after the end of this further switching cycle 75 is the controllable switching device not switched on again to end the pulse packet.

[0112] The time integral of the pulse packet current defines the light energy emitted by the LED(s). The corresponding time integral differs by an area of ​​76 for the two cases with a switching cycle count 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 in 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 an undesirable, perceptible flicker.

[0114] Fig. Figure 14 illustrates the effect of devices and methods according to exemplary embodiments. The switching frequency is compared to... Fig. 13 is increased 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 were equally elected.

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

[0116] Increasing the switching frequency at low dimming levels results in a correspondingly shorter switching cycle duration. This reduces errors in brightness control, even when no switching between different numbers of switching cycles occurs at the same dimming level. 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 via a pulse packet are larger overall. A lower switching frequency and a larger amplitude of the current ripple can be used without causing significant 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 embodiments. Methods and devices according to these embodiments can be used in control gear for light sources, 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 device (13) and a control device (14) which is configured to repeatedly switch on the controllable switching device (13) during a pulse duration (27) in order to provide a pulse packet (21, 22) to the at least one light-emitting diode (2) in order to temporarily store energy in the coil (11), and to switch off the 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 on the controllable switching device (13) during the pulse duration (27) when the current through the at least one light-emitting diode (2) reaches a switch-on switching threshold value (31; 36) which depends on a dimming level, wherein the pulse duration (27) of the pulse packets (21, 22) and / or a time interval of the pulse packets (21, 22) is adjustable depending on the dimming level by means of a pulse modulation, for example a pulse width modulation, and where the product of the pulse duration (27) and the frequency at which pulse packets are generated is a monotonically increasing function of the dimming level. [2] Operating circuit for at least one light-emitting diode (2) comprising a coil (11), a controllable switching device (13), and a control device (14) which is configured to repeatedly switch on the controllable switching device (13) during a pulse duration (27) in order to provide a pulse packet (21, 22) to the at least one light-emitting diode (2) in order to temporarily store energy in the coil (11), and to switch off the 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 device (13) during the pulse duration (27) when a current through the at least one light-emitting diode (2) reaches a switch-off threshold value (32; 37) which depends on a dimming level of the at least one light-emitting diode (2), wherein the pulse duration (27) of the pulse packets (21, 22) and / or a time interval of the pulse packets (21, 22) is adjustable depending on the dimming level by means of a pulse modulation, for example a pulse width modulation, and where the product of the pulse duration (27) and the frequency at which pulse packets are generated is a monotonically increasing function of the dimming level. [3] Operating circuit according to claim 1 or claim 2, wherein the control device (14) is configured 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 that are less than a threshold value (8). [4] Operating circuit according to one of the preceding claims, wherein the control device (14) is configured to switch off the controllable switching device (13) during the pulse duration (27) when the current 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), where the switch-on threshold (32; 37) and the first time duration (39) depend on the dimming level. [5] Operating circuit according to one of the preceding claims, wherein the control device (14) is configured to switch on the controllable switching device (13) during the pulse duration (27) when the current through the at least one light-emitting diode (2) reaches the switch-off threshold value (31; 36), and to switch it off again after a second time period (38), where the switch-off threshold (31; 36) and the second time duration (38) depend on the dimming level. [6] Operating circuit according to one of the preceding claims, wherein the control device (14) is configured 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. [7] Operating circuit according to claim 6, wherein the control device (14) is configured such that the number of switching cycles of one pulse packet (21, 22) and the number of switching cycles of another pulse packet (21, 22) differ by 1. [8] 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 device (13), wherein, in order to generate one of the pulse packets (21, 22) supplied to the at least one light-emitting diode (2), the controllable switching device (13) is repeatedly switched on to temporarily store energy in the coil (11) and switched off 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 device (13) is switched on when the current 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 pulse duration (27) of the pulse packets (21, 22) and / or a time interval of the pulse packets (21, 22) is adjustable depending on the dimming level by means of a pulse modulation, for example a pulse width modulation, and where the product of the pulse duration (27) and the frequency at which pulse packets are generated is a monotonically increasing function of the dimming level. [9] 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 device (13), wherein, to generate one of the pulse packets (21, 22) supplied to the at least one light-emitting diode (2), the controllable switching device (13) is repeatedly switched on to temporarily store energy in the coil (11) and switched off 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 device (13) is switched off when a current through the at least one light-emitting diode (2) reaches a switch-off threshold value (32; 37) which depends on a dimming level of the at least one light-emitting diode (2), wherein the pulse duration (27) of the pulse packets (21, 22) and / or a time interval of the pulse packets (21, 22) is adjustable depending on the dimming level by means of a pulse modulation, for example a pulse width modulation, and where the product of the pulse duration (27) and the frequency at which pulse packets are generated is a monotonically increasing function of the dimming level. [10] Method according to claim 8 or 9, wherein during the pulse duration (27) the controllable switching device (13) is switched off when the current through the at least one light-emitting diode (2) reaches the switch-off threshold value (32; 37), and is switched on again after a first time period (39), where the switch-off threshold (32; 37) and the first time duration (39) depend on the dimming level. [11] Method according to any one of claims 8 to 10, wherein during the pulse duration (27) the controllable switching device (13) is switched on when the current through the at least one light-emitting diode (2) reaches the switch-on threshold value (31; 36), and is switched off again after a second time period (38), where the switch-on threshold (31; 36) and the second time duration (38) depend on the dimming level. [12] Method according to any one of claims 8 to 11, 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 that are smaller than a threshold value (8). [13] Method according to any one of claims 8 to 12, wherein time-sequentially generated pulse packets have at least two different switching cycle numbers.

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