EP2019569A1 - Method for dimming light emitted by LED lights, in particular in the cabin of a commercial airplane - Google Patents

Abstract

The method involves providing control cycle for current flow time periods (tr, tg, tb) that are presettable over multi-colored LED arrays (12r, 12g, 12b) independent of each other, and subjecting the control cycle to variation of cycle length. The cycle length is varied when one of the arrays is supplied with current flow for a small time period for respective operating period lengths. The variation of the cycle length is utilized when a hardware technical short current flow time period arises in one of the LED arrays.

Description

The invention relates to a method according to the preamble of the main claim. From the DE 10 2005 016 729 B3 the dimming of the light emitted by a white light emitting diode (LED) is known in successive working periods with each other of equal length, in each of which a high-frequency chopping of the current flowing through the diode during the switch-on periods in the successive operating periods takes place. The shorter the turn-on period in the work period, the less constant current pulses flow across the LED, the lower the brightness of the emitted light.

To change the color impression of an LED light usually the light emission of LED arrays in the primary colors red, green and blue are superimposed on each other in different intensity, for which the individual arrays for dimming independent control of their Arraystrom time periods experienced in the working periods.

However, only a dimming ratio of the order of 1: 1000 between dark and light can be achieved with this. This is no longer sufficient for e.g. Color constant variable twilight impressions (such as a time-stretched transition from starry sky to sunrise when lighting in a passenger cabin) with gamut color correction (compensation for the shift to warmer light color at transition to lower brightness), if the RGB light-emitting diode arrays are already working very dimmed, so lowest adjustable brightness; For this purpose, a dimming ratio which is stronger by at least an order of magnitude is to be striven for even less activation before complete switch-off.

Because the gamut color correction required for high-quality, color-constant lighting effects requires very short current flow times via light-emitting diodes. It can then be compensated for that the color locations of LEDs vary within a production lot. In order to achieve nevertheless nevertheless a certain basic color, already at the manufacturing adjustment or later in the enterprise (over Photodiodes compensated) the two other primary colors mixed with low intensities, resulting for the respective color from the in the CIE standard color chart (in the so-called color shoe) inscribed color triangle for the LEDs. For example, a guaranteed color locus "blue, unsaturated" is gamut corrected by generating the green LED at 5% and the red LED at 2% in addition to the full drive (100%) of the blue LED. In order to represent this color location at low brightness, for example dimmed to 1%, with a drive cycle of 3 ms, blue has a turn-on time of 1% of the full cycle, ie 30 μs, for green 1% of 5% equal to 0, 05% (1.5μs) and for red 1% of 2% equal to 0.02% (0.6μs current flow over the red LED).

Energizing LEDs with such short pulses will cause many problems. Thus, these short pulses have fundamental frequencies of a few hundred kilohertz, which can lead to disturbing interference (EMI phenomena) with frequencies assigned to certain radio services (such as the emergency radio to 200 kHz); too short turn-off times hinder the discharge of the self-capacitance of the LEDs; and with low-cost components can not be realized sufficiently fast switching current sinks. In circuit terms, such extreme LED dimming would only be possible with very fast and therefore expensive processors of high coding depth for the fine subdivision of the working period, including high-performance high-frequency transistors for the current sinks in the R, G and B diode series circuits, ie with rarely acceptable circuit technology Effort.

In recognition of these circumstances, the present invention is based on the technical problem to further develop a method of the generic type that even with limited processor capacity and in conjunction with current sinks in inexpensive available, as conventional bipolar circuitry extremely low, i. light-dim dimming settings on LEDs can be set reproducibly and then also sensitively varied.

This object is achieved by the essential features specified in the main claim. Thereafter, a sort of superimposed low frequency frequency modulation subject driving cycle is provided for the LEDs. With particular lengthening of the cycle is reduced, despite not further reduced current flow period, the current integral over the cycle, so without having to further reduce the duty cycle of the working period for the thus occurring further reduction in the emission of LEDs.

This solution is realized particularly advantageously in that the cycle is subdivided into a working period with a time-limited current flow and at least one currentless, here called idle period.

The currentless idle period in the (overall) cycle, ie between two work periods spaced apart by an idle period, can in turn be varied to even finer-level dimming, for example by succession of a different number of idle periods of equal length and / or by length variations of the idle periods.

In order to avoid a color change and a jump in brightness when changing the number or the length of the idle periods in a cycle, this switching is expediently just at the end of a cycle of work period and idle periods. In addition, the pulse duration in the individual LED arrays can be set to temporarily constant cycle current integral so that no jump in the current integral, ie no brightness fluctuation and no abrupt color change, occur at this moment.

Finally, the length of the operating periods in which the current pulses of constant length occur can also be varied in the successive cycles in order to influence the radiation integral current integral over the cycle, without having to further shorten the current flow periods for further dimming down ,

Decisive for the solution according to the invention is thus that the shortest in bipolar technology for the current sinks and with a processor-acceptable coding depth still easily manageable current flow time to further dimming does not need to be further reduced, but then can remain constant, because now the cycle of Art a superimposed frequency modulation is extended. Now, the resulting current flow is changed over the variation of the cycle lengths for the diode arrays, in particular even further reduced, without having to change the current flow time itself and, in particular, to further reduce it. This eliminates an increase in the coding depth of the current sinks of the arrays-controlling processor to finer gradation of the current flow periods and thus also leads to no higher-frequency control of the current sinks themselves, which is why the introduced hardware technology continues to be used despite much increased dimming ratio.

This visually noticeably improves light resolution and color gamut (the illustrated compensation for color locus shifts in one LED by minimal current flow variations in the other two LEDs). The required and inventively achieved dimming ratio of much greater than 1: 10,000, which would not be achievable in analog circuit technology, thus allowing a high brightness dynamics while ensuring high color fidelity to the lowest Brightnesses of light emission, in which the human eye, adapting to the currently relatively brightest color, reacts with particular color sensitivity.

Fig.1

ein vereinfachtes Schaltbild für individuelle Farbansteuerung einer Leuchte mit LED-Arrays in den drei Grundfarben rot, grün und blau,

Fig.2

Zeitdiagramme einer Ansteuerung der Arrays nach Fig.1 mit Zyklen aus abwechselnden Folgen von Arbeits- und Leerperioden untereinander gleicher Längen für stark herabgedimmten Leuchtenbetrieb,

Fig.3

eine Variation der Ansteuerung nach Fig.2 durch variable Längen von Leerperioden, insbesondere für farbkorrigierbaren gleitenden Helligkeitsübergang zwischen ganz abgeschaltetem und nur minimal eingeschaltetem Leuchtenbetrieb, und

Fig.4

im Gegensatz zu Fig.2 und Fig.3 variable Längen der Arbeitsperioden zur Variation der Stromintegrale, hier ohne Einschieben von Leerperioden .

Additional alternatives and developments of the solution according to the invention will become apparent from the other claims and, also with regard to their advantages, from the following description of a preferred embodiment for realizing the method according to the invention. In the drawing shows

Fig.1

a simplified circuit diagram for individual color control of a luminaire with LED arrays in the three basic colors red, green and blue,

Fig.2

Timing diagrams of an activation of the arrays after Fig.1 with cycles of alternating sequences of working and idle periods with one another equal lengths for heavily diminished luminaire operation,

Figure 3

a variation of the control after Fig.2 by variable lengths of idle periods, in particular for color-correctable sliding brightness transition between completely switched off and only minimally switched on luminaire operation, and

Figure 4

in contrast to Fig.2 and Fig.3 variable lengths of the working periods for varying the current integrals, here without insertion of idle periods.

In the Fig.1 symbolically sketched light 11 each has an individually brightness controllable array 12 (12r, 12g and 12b) of series circuits red, green and blue emitting light emitting diodes 13; is not taken into account in this sketch, that for color fine correction or to influence the color saturation in addition a brightness controllable white light array of blue-emitting but with phosphor-coated LEDs is useful. Each array 12 is connected to a supply voltage 14 (of typically 55 volts) against device ground 15, to the latter via a constant current sink 16 in the form of a bipolar transistor in emitter circuit with its emitter resistor 17.

A commercially available micro-processor 18 with a coding depth of typically 2 exp 4 = 16-bit time resolution within a working period Ta switches through the transistors of the constant-current sinks 16 independently of each other over a time span tr, tg, tb. The length of these individual current flow periods t determines in each case via the cyclic current time integral the resulting array current intensity and thus the intensity (brightness) of the associated red, green and blue superimposed color emissions. This current color mixture from the three arrays 12 gives the light color emitted by the luminaire 11. The currently desired color mixture and its intensity is determined by a higher-level, external control signal 19 for the individual current flow periods t.

Insofar as a temperature- or current-dependent Farbortdrift is to be expected (as in red and green emitting light emitting diodes 13r, 13g), in the programming of the processor 18 or in the external signal 19 an adjusted gamut Farbortkorrektur given by minimal variation of periods t ,

In order to reduce the current integral in the respective array 12, the current flow time t may be reduced stepwise within a period Ta, which is typically 3 milliseconds corresponding to a repetition frequency of 333 Hertz. For a large resolution, ie for small increments, the working period Ta must be correspondingly finely subdivided, the processor 18 thus having a correspondingly high coding depth for setting even very short time periods t, which makes it much more expensive. Such a narrow-pulse control of the current sinks 16 would also become too high-frequency for the operation of constant-current transistors in the inexpensive bipolar technology.

Therefore, at the latest then to a frequency modulation (approximately according to Fig.2 ) of all currently set current integrals a working period Ta switched when in at least one of the arrays of current flow t - especially for lack of processor-dependent finer resolution - should not be further reduced. Now the currently pending - but within the given processor coding depth still individually variable - array current integrals for an additional, namely even stronger dimming further reduced by a working period Ta (at least) an electroless empty period To, so initially no renewed control of current sinks 16 follows before with the passage of a drive cycle of now Z = Ta + To again a working period Ta starts with current flow time t. Since the temporal current flow integral as a whole, even during the period Ta not changed current flow time t, over the extended cycle Z decreases overall, the radiated brightness is reduced, without having to increase the coding depth for this purpose in the processor 18. Compared to the hitherto highest achievable dimming of about 0.1%, this means an increased resolution of the current flow through the arrays 12 by at least a factor of 10 and therefore also improved possibilities of influencing the light location even at extremely small dimming values. In addition, according to Figure 3 in order to further vary cycle lengths Z 'and thus the resulting current integral without affecting the time periods t, the idle periods To are changed (shortened and extended). This results in a more refined screening of the current flow integral and thus an increase in the light color impression, especially at the lowest brightness levels, with the same coding depth.

If the idle periods To have shrunk to zero, the current integrals can still be varied without changing the time periods t, influencing the lengths of the now immediately consecutive and thus already the cycle lengths Z, compared to the periods t very low frequency working periods Ta out of the processor 18, as in Figure 4 outlined. A fluent change of control to Figure 3 on the one after Figure 4 allows due to the increasingly fine resulting current screening so to speak a dynamic transition from low to lowest brightness while compensating for the otherwise occurring Farbortverschiebungen in the radiation of the individual arrays 12, until finally in the completely off state of the light radiation into - without the functional limits of the processor 18th overwhelming, as frequency critical short, current flow time t would be required t.

On the other hand brighter radiation from the lamp 11, so less strong dimming, is not critical to the operation of the processor 18 because of the then extended current flow periods t. Therefore, a variation of the cycle lengths Z for influencing the current integral through the arrays 12 can be completely dispensed with, and it is switched to conventional operation with variable periods t in the immediate succession of a fixed period grid Ta (ie, even without intermediate periods To). Such a switch from variable to rigid cycles Z = Ta is expediently carried out at the end of a cycle Z in order to avoid color changes that would otherwise have to be compensated for again immediately over the individual time periods t.

In the time charts of the Fig.2 to Fig.4 is taken into account that the variable current flow periods tr, tg and tb occurring within the working periods Ta, T'a should be used as offset as possible from one another at the beginning of the period, around the middle of the period and before the end of the period.

Such interlacing avoids visually disturbing stroboscopic effects, such as may occur when colors are driven sequentially such that only one of the primary colors is always lit; or in general, when light is generated at very low frequency (substantially below 100 Hz).

From a high-frequency (typically 400 Hz having) AC on-board electrical system 20, a power supply 21 is fed with voltage converter 22 for providing the supply voltage 14. By a buffer 23 of high capacity (and not considered in the drawing voltage regulation) load changes are intercepted. In particular, the energy stored in the buffer 23 is available when an LED is just turned on during the voltage zero crossing of the electrical system 20. The buffer 23 is then until the next zero crossing of On-board network 20 recharged. In order to avoid thereby dependent on the efficiency of bromine phenomena, the buffer 23, typically an electrolytic capacitor, must be sized quite large, which represents a significant cost factor. However, the turn-on interleaving of the diodes reduces the stress on the power supply 21, so that a cheaper smaller buffer 23 can be used.

With a working period Ta of on average 3 ms length (corresponding to 333 Hz) arises with the on-board frequency of 400 Hz a beating of 67 Hz, which can be well balanced without additional circuitry. Above all, this repetition rate is so high that a flicker of light due to beat phenomena due to driving of light sources in adjacent frequency bands does not occur.

In order to dim the brightness of the mixed-color light from an LED luminaire 11 with LED arrays 12r, 12g, 12b of different color emissions, in particular in the passenger cabin of a commercial aircraft, the first and then conventionally constant working period lengths Ta become different over the different arrays 12 adjustable current flow time periods tr, tg, tb - starting from the nominal current (typically around 25 mA) for maximum brightness - gradually reduced until in one of the arrays 12 a modulation (a dimming level) of typically only 1% of the normal brightness is reached. Before then occur in the array drive frequency components that can lead to about the frequency of the on-board network 20 about to beat phenomena, or if the coding depth of the current-controlling processor 18 and the response of the Konstantstromsenken 16 behind the LED arrays 12 further dimming by more Shortening of the current flow times t in any case no longer allow one of the arrays 12, according to the invention to further and more finely graded dimming an extension of the cycles Z by extending the working periods Ta and / or by inserting constant or variable lengths of no-current periods To between successive periods Ta, namely to further reduce the current integrals in the arrays 12 over the current cycle Z even without further shortening an already critically short current flow time t itself, if necessary, adjusting the current flow periods t to the desired Abstrahl-In intensity and color in the other arrays 12. This allows with the introduced circuit technology for the Konstantstromsenken 16 in the LED arrays 12 and without increasing the coding depth in the processor 18 for the stage current flow timing t fine color correction for constant mixed color impression even at extremely small brightness, up to a smooth transition to the light-off situation; or vice versa when switching on despite very slow dimming color constant mixed color light from the LED lamp 11. This achieved with conventional hardware in the result extremely finely achieved effective current variation gamut color correction (ie compensation of the reduction in current to longwave occurring color locus shift in the standard color panel, by slightly affecting the brightness of the blended Basic colors) even at the lowest brightness, as well as a compensation age-related, color-dependent different brightness loss in the various LED arrays 12th

LIST OF REFERENCE NUMBERS

11

Light (with 12)

12

Array (from 13)

13

LED (LEDs)

14

Supply voltage (for 12)

15

Device mass (of 11)

16

Constant current sink (in series with 12)

17

Emitter resistance (at 16)

18

processor

19

Control signal (at 18 for t and possibly for T)

20

board network

21

Power supply (on 20)

22

Voltage transformer (in 21)

23

Buffer (in 21 between 22 and 11)

t

Time periods (tr, tg, tb for 12r, 12g, 12b during Ta)

T, T '

Periods (Ta = working period; To = empty period)

Z, Z '

Cycles (Ta or Ta + To)

Claims (10)

Method for dimming the light emitted by LED lights, in particular in the passenger cabin of a commercial aircraft, by changing LED current flow periods during cyclically consecutive working periods, characterized by a drive cycle for multicolored LED arrays independently definable current flow periods, which is subjected to a variation of its cycle length. Method according to the preceding claim, characterized in that the cycle length is varied when at least one of the LED arrays is energized for a short period of time in relation to the respective working period length. Method according to the preceding claim, characterized in that the variation of the cycle length begins when in at least one of the LED arrays a hardware-technically shortest possible current flow time period occurs. Method according to one of the preceding claims, characterized in that the lengths of the operating periods in which the current flow periods occur, are varied. Method according to at least one of the preceding claims, characterized in that the sequence of cycles is composed in each case of the succession of an energized working period and at least one currentless idle period. Method according to the preceding claim, characterized in that lengths of the idle periods are varied. Method according to one of the preceding claims, characterized in that switching between different cycle lengths takes place in each case at the end of a cycle. Method according to one of the preceding claims, characterized in that the time period of the respective current flow in the LED arrays with respect to the beginning of a working period offset in time with each other. Method according to the preceding claim, characterized in that the current flow in one of the LED arrays begins at the beginning of each working period but in a different color LED array before the end of the respective working period. Method according to one of the two preceding claims, characterized in that the current flow time period in a further one of the LED arrays is in each case time-symmetrical to the middle of the working period.

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