Effective white tuning for solid-state luminaires

Abstract

The invention relates to an apparatus and method for white tuning with and without color correction (e.g., Duv correction) of luminaires with and without dimming function, wherein one or more color-correcting light sources (e.g., LEDs) are connected in series with one string of two parallel-connected luminaire strings (e.g., LED filaments) and are disabled when the one string reaches a maximum duty cycle of the dimming function. The one string may comprise luminaires with higher correlated color temperature (CCT) and the other string may comprise luminaires with lower CCT. The lower-CCT string may comprise a larger number of light sources compared to the higher-CCT string so that the higher-CCT string is turned off when the parallel-connected lower-CCT string is turned on. As another option, an overall current through both luminaire strings is sensed and compared with a reference value. Based on the result, a current source in one of the luminaire strings is controlled for white tuning.

Description

[0001] 2024PF80519

[0002] 1

[0003] EFFECTIVE WHITE TUNING FOR SOLID-STATE LUMINAIRES

[0004] FIELD OF THE INVENTION

[0005] The invention relates to the field of white tuning for solid-state luminaires, such as - but not limited to - light emitting diode (LED) and filament LED luminaires for use in different applications for home, office, retail, hospitality and industry.

[0006] BACKGROUND OF THE INVENTION

[0007] A luminaire can be any type of lighting unit or lighting fixture that comprises one or more light sources (e.g., visible or non-visible (infrared (IR) or ultraviolet (UV)) light sources) for illumination and / or communication purposes and optionally other internal and / or external parts necessary for proper operation of the lighting, e.g., to distribute the light, to position and protect the light sources and ballast (where applicable), and to connect the luminaires to a power supply.

[0008] Solid-state lighting (SSL) is a type of lighting that uses semiconductor lightemitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination rather than electrical filaments, plasma (used in arc lamps such as fluorescent lamps), or gas. Solid state electroluminescence is used in SSL, as opposed to incandescent bulbs (which use thermal radiation) or fluorescent tubes. Compared to incandescent lighting, SSL creates visible light with reduced heat generation and less energy dissipation.

[0009] A driver circuit is required to deliver a highly stable constant load current to the luminaire(s) irrespective of variations in the luminaire characteristics or the supply voltage while complying with increasingly stringent regulations covering input power requirements such as power factor and total harmonic distortion. Pulse modulation (e.g., pulse width modulation (PWM)) is commonly applied to the load current flowing through the luminaire(s) to control colour and / or different brightness of the luminaire(s).

[0010] Filament SSL luminaires (e.g., filament LEDs) have become popular due to their artistic and nostalgic look. As such types of luminaires became more efficacious, it was possible to install driver and light sources (e.g., LEDs) in a housing that looks like a glass2024PF80519

[0011] 2

[0012] incandescent lightbulb, without a visible heatsink. However, these filament SSL luminaires do not dim like incandescent lamps.

[0013] Tuneable lighting differs from traditional single color temperature lighting by allowing users to adjust the color temperature of the light. For example, white tuning provides the ability to control the color temperature output of a light source (e.g., LED or LED string), e.g., to dim to warm LED lights. It can be realized through compatible tuneable controllers using common PWM wiring or through proprietary protocols such as 0-10V, digital addressable lighting interface (DALI), digital multiplex (DMX), or wireless communication via Bluetooth, Wi-Fi, ZigBee or the like.

[0014] White tuning of SSL light sources may utilize at least two white light sources, a warm white and a cool white. Depending on the difference in color temperatures between the two white light sources, a correction of the resultant chromaticity may be required.

[0015] SUMMARY OF THE INVENTION

[0016] It is an object of the present invention to provide white tuning for SSL light sources without adding much hardware complexity.

[0017] This object is achieved by a luminaire as claimed in claim 1, by a method as claimed in claim 14, and by a computer program product as claimed in claim 15.

[0018] According to a first aspect, a luminaire is provided, that comprises:

[0019] a first current branch with a first string of solid-state light sources of a higher color temperature;

[0020] a second current branch with a second string of solid-state light sources of a lower color temperature; and

[0021] a white tuning circuit arranged in one of the first and second current branches and configured to perform white tuning by modifying the contribution of light generated by the one of the first and second current branches.

[0022] Furthermore, according to a second aspect, a method of performing white tuning in a luminaire having a first current branch with a first string of solid-state light sources of a higher color temperature and a second current branch with a second string of solid-state light sources of a lower color temperature is provided, the method comprising:

[0023] performing white tuning in one of the first and second current branches by modifying a contribution of light by the one of the first and second current branches.

[0024] Moreover, according to a third aspect, a computer program product is provided, that comprises code means for producing the steps of the method of the second2024PF80519

[0025] 3

[0026] aspect when run on a controller circuit (e.g., driver / converter controller, switch drive circuit, etc.) of a driver circuit of the luminaire.

[0027] Accordingly, a one-channel solution for white tuning can be achieved by modifying a contribution of light by the one of the first and second current branches.

[0028] Opposite switching behavior of the luminaire strips of the current branches can be achieved by providing different total string forward voltages connected in parallel.

[0029] White tuning with color correction can be achieved by connecting one or more correction light sources of different color temperature in series with one of the parallel luminaire strings and disabling the correction light sources when a 100% or 0% duty cycle has been reached.

[0030] Additionally, a two-channel solution for white tuning can be achieved by controlling correlated color temperature (CCT) in the first branch and Duv in the second branch. The two-channel solution is advantageous in that a chromaticity point above or below the black body curve can be achieved.

[0031] As an alternative to the above color correction approach, the color temperature of the luminaire strips may be reduced with a desired mutual characteristic to a minimum value when dimmed, thereby performing white tuning to mimic the behavior of an incandescent lamps (Dim-to-warm).

[0032] Thus, white tuning can be achieved without color correction in one branch (e.g., if the original correlated color temperature (CCT) range is narrow, e.g. 3000K to 5000K) by mixing two distinct CCTs to produce a third CCT between the original two, or by introducing color correction with one or more non-white light sources in one branch (e.g., if the CCT range is wider, e.g., 2700K to 6500K, 1400K to 3000K, or 1400K to 6500K) to move a chromaticity point, that has shifted below the black body curve during white tuning, back to a close location to the black body curve. According to a first option of any of the first to third aspects, the number of solid-state light sources of the first string and the number of solid-state light sources of the second string may be selected so that a total forward voltage of the first string is lower than a total forward voltage of the second string and the second string is switched off when the first string is switched on, and vice versa.

[0033] According to a second option of any of the first to third aspects, which may be combined with the first option, the white tuning circuit may comprise one or more solid-state light sources of a color temperature different from the higher and lower color temperatures, that are controlled in their luminance based on a ratio between the luminance of the first string and the luminance of the second string.2024PF80519

[0034] 4

[0035] According to a third option of any of the first to third aspects, which may be combined with the first or second option, the luminaire may comprise an average detector for determining an average value of a pulse-modulated control signal that determines the current flowing through the one of the first and second current branches.

[0036] According to a fourth option of any of the first to third aspects, which may be combined with any one of the first to third options, the white tuning circuit may comprise a transistor connected in parallel to the one or more solid-state light sources of different color temperature and controlled by the average value to change the current through one or more solid-state light sources of different color temperature in dependence on the average value.

[0037] According to a fifth option of any of the first to third aspects, which may be combined with the third option, the white tuning circuit may comprise a parallel connection of first and second correction strings of solid-state light sources, wherein the first and second correction strings may be phased in and out by a transistor depending the average value of the average detector, wherein the first correction string may comprise the one or more solid-state light sources of different color temperature, and wherein the second correction string may comprise additional solid-state light sources of the higher color temperature.

[0038] According to a sixth option of any of the first to third aspects, which may be combined with the fifth option, the white tuning circuit may be configured to gradually replace correction light generated by the first correction string by light of the higher color temperature generated by the second correction string, as a target color temperature approaches the higher color temperature.

[0039] According to a seventh option of any of the first to third aspects, which may be combined with the fifth or sixth option, an inverse of the pulse-modulated control signal may be supplied to the average detector, wherein the transistor may be connected in series with the first correction string.

[0040] According to an eighth option of any of the first to third aspects, which may be combined with any one of the third to seventh options, the different color temperature of the one or more solid-state light sources of different color temperature may comprise at least one of Lime, green, Cyan and Amber.

[0041] According to a ninth option of any of the first to third aspects, the white tuning circuit may comprise a difference amplifier for determining a difference between a predetermined reference value and a measured value of the current flowing through the first and second current branches, and a current source controlled by the determined difference2024PF80519

[0042] 5

[0043] and arranged to feed its output current through the one of the first and second current branches.

[0044] According to a tenth option of any of the first to third aspects, that may be combined with the ninth option, the output current of the current source may show a quadratic dependence on a dimming factor of the current flowing through the first and second current branches.

[0045] According to an eleventh option of any of the first to third aspects, that may be combined with any one of the first to tenth options, the higher color temperature may range between 2700K and 7000K and the lower color temperature may range between 1400K and 2000K.

[0046] According to a twelfth option of any of the first to third aspects, that may be combined with any one of the first to eleventh options, the first and second strings may be filament strings, wherein the luminaire may comprise a screw base for housing a driver circuit and may be configured to generate the current flowing through the first and second current branches.

[0047] It is noted that the circuits and / or components of the luminaire may be implemented based on discrete hardware circuitries with discrete hardware components, integrated circuits, or arrangements of integrated modules, or based on signal processing devices or integrated circuits controlled by software routines or programs stored in memories, written on a computer readable media, or downloaded from a network, such as the Internet.

[0048] It shall be understood that the luminaire of claim 1, the method of claim 14, and the computer program product of claim 15 may have similar and / or identical preferred embodiments, in particular, as defined in the dependent claims.

[0049] It shall further be understood that a preferred embodiment of the invention can also be any combination of the dependent claims or above embodiments with the respective independent claim.

[0050] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0051] BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In the following drawings:

[0053] Fig. 1 shows schematically characteristics of two parallel LED strings driven by a common driver;

[0054] Fig. 2 shows schematically a standard chromaticity diagram;2024PF80519

[0055] 6

[0056] Fig. 3 shows schematically a chromaticity diagram of the two parallel LED strings of Fig. 1 without color correction;

[0057] Fig. 4 shows schematically a chromaticity diagram of the two parallel LED strings of Fig. 1 with a color correction shift by additional Lime and Amber LEDs along a line joining a Lime-Amber correction point;

[0058] Fig. 5 shows schematically a chromaticity diagram of the two parallel LED strings of Fig. 1 with a plurality color correction shifts for different color temperatures along the line joining the Lime- Amber correction point;

[0059] Fig. 6 shows schematically a block diagram of a luminaire with one-channel white tuning according to various embodiments;

[0060] Fig. 7 shows schematically a circuit diagram of a luminaire with one-channel white tuning according to a first embodiment;

[0061] Fig. 8 shows schematically a circuit diagram of a luminaire with one-channel white tuning according to a second embodiment;

[0062] Fig. 9 shows schematically a filament luminaire with two filament LED strings;

[0063] Fig. 10 shows schematically two filament LED strings with different color temperature of the filament luminaire of Fig. 9;

[0064] Fig. 11 shows schematically a circuit diagram of a filament luminaire with filament LED sub-strings controlled for improved dimming characteristic; and

[0065] Fig. 12 shows schematically a diagram with an improved dimming characteristic obtained by combining individual dimming characteristics of the filament LED substrings.

[0066] DETAILED DESCRIPTION OF EMBODIMENTS

[0067] Various embodiments of the present invention are now described, which are applicable to luminaires of a solid-state lighting system, such as semiconductor LEDs, semiconductor lasers, vertical -cavity surface emitting lasers (VCSELs), organic lightemitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination or light sources in visible or non-visible light spectra.

[0068] More specifically, the following embodiments are directed to LED luminaires. They can be implemented in connection with any type of LED module or board and are applicable to various kinds of drivers or converters of luminaires.2024PF80519

[0069] 7

[0070] It is noted that - throughout the present disclosure - the structure and / or function of blocks or circuit components with identical reference numbers that have been described before are not described again, unless an additional specific functionality is involved. Moreover, only those structural elements and functions are shown, which are useful to understand the embodiments. Other structural elements and functions are omitted for brevity reasons.

[0071] The term 'white light' encompasses a range of actual colors. Light referred to as 'white' may vary from a very cold bluish white (e.g., as produced by an arc lamp) through warm reddish whites (e.g., as produced by a candle). These colors are referred to as 'white,' but may be differentiated by a characteristic referred to as 'color temperature.' The higher the value of color temperature, the more blue the 'white' light appears.

[0072] As described in more detail below with reference to Fig. 2, the colors of light that are accepted by the human eye as white of a given color temperature may be referred to as a correlated color temperature (CCT) value. 'White' beams with a common value of CCT will appear as the same color temperature to the human eye, but with a tint towards either magenta or green. The amount of tint can be described by a value referred to as 'Duv' (sometimes referred to as 'Delta uv', 'Delta (u,v)', '+ / - green', or 'plus-or-minus green'). For consistency, this disclosure will use Duv throughout to refer to this parameter but it should be understood that the parameter could equally be labeled or called '+ / - green' or any other synonym.

[0073] In some cases, a luminaire may be designed such that the CCT of the light emitted by the luminaire is adjustable. For example, an LED luminaire may emit a warm white light (e.g., 2700K-3000K or even lower) at one setting, a cool white light (e.g., 5000K-7000K) at another setting or a white light with a CCT between warm and cool white lights at yet another setting. For example, white light tuning may be accomplished by using a combination of warm white light and cool white light, resulting in a combined light with a resultant CCT that is a combination of the CCT of the warm white light and the CCT of the cool white light.

[0074] White light tuning can be achieved by providing a luminaire with a first string of LEDs configured to emit a warm white light having a warm CCT and a second string of LEDs configured to emit a cool white light having a cool CCT. The luminaire may also include color correction LEDs that emit a light of another color. A flux of the light of the other color may be controlled based on a flux of the cool white light or a flux of the warm2024PF80519

[0075] 8

[0076] white light. The flux of the warm white light and the flux of the cool white light may change proportionally with respect to each other.

[0077] Embodiments described below provide a one- or two-channel solution by having different string voltages connected in parallel. A white tuning circuit or element is connected in series with one or two of the at least two white LEDs or LED strings and is controlled to obtain an improved color appearance during a dimming process.

[0078] Fig. 1 shows schematically characteristics of two parallel LED strings driven by a common driver.

[0079] In the diagram of Fig. 1, the vertical axis corresponds to the forward current If through the LED string and the horizontal axis corresponds to the forward voltage Vf across the LED string. The volt-current characteristic of each LED string determines the current at a specific voltage as shown in the chart of Fig. 1. The left-hand characteristic corresponds to a string of series-connected high-CCT LEDs (CCT1 = 6500K (Kelvin)) and the right-hand characteristic corresponds to another string of low-CCT LEDs (CCT2 = 1400K) connected in parallel. The low-CCT LED string has a larger number of LEDs (and thus a higher total forward voltage) compared to the high-CCT LED string, so that the higher-voltage string (CCT2 = 1400K) is turned off when the parallel-connected lower voltage string (CCT1 = 6500K) is turned on.

[0080] The two LED strings with different CCT are driven from the same constant current driver. Since they are connected in parallel, the source current supplied by the driver will split between them in a ratio determined by the common forward voltage.

[0081] When the 6500K LED string is turned on, the lower forward voltage will turn off the higher voltage 1400K LED string. Thus, when a switch opens the current path through the 6500K LED string, the voltage across the parallel-connected 1400K LED string will decrease to the lower forward voltage and the 1400K LED string will be turned off. In other words, the 1400K string voltage will be forced to reduce causing the forward current to drop to near zero, effectively turning it off.

[0082] Fig. 2 shows schematically a standard chromaticity diagram, i.e., an xy chromaticity diagram 50 according to the 1931 standard of the International Commission on Illumination (CIE). The diagram 50 shows a boundary 52, which encompasses all colors viewable by the human eye. The boundary 52 indicates the range of colors from a saturated blue in the bottom left corner, through a saturated red in the bottom right comer, and a saturated green at the top left peak of the curve. Line 54 is referred to as the 'Planckian locus' (or 'black body' curve) and indicates the color emitted by an incandescent black body at2024PF80519

[0083] 9

[0084] various temperatures. The black body curve 54 is limited to colors that are considered as 'white'. Some example color temperatures (or temperatures of the incandescent black body) are labeled on the diagram 50. For example, line 56 is the line for a color temperature of 6000K. Other dashed lines in the diagram 50 represent other color temperatures.

[0085] The example color temperatures are represented in the diagram 50 as lines, rather than as single points because, while the actual white point for a given color temperature lies exactly on the black body curve 54, the human vision system is flexible and perceives as the same white of a given color temperature lightly saturated colors that lie close to the black body curve 54, but not exactly on it. The value of color temperature of light on the dashed lines that are accepted by the eye as white of a given color temperature may be referred to as the CCT value. 'White' beams with a common value of CCT will appear as the same color temperature to the human eye, but with a tint towards either magenta (for points below the black body curve 54) or green (for points above the black body curve 54). The dashed lines in Fig. 2 such as line 56 are referred to as 'CCT isotherms'. Every point along a CCT isotherm has the same value of CCT, but differs in its amount of tint, which difference is shown as a distance from the black body.

[0086] Thus, CCT is the color temperature white position along the black body curve 54, such as the intersection point of the CCT isotherm 56 with the black body curve 54, while Duv is the distance between an actual white color point and the black body curve 54 along a given CCT isotherm. As a convention, points that are above the black body curve 54 have positive Duv values, while points that are below the lack body curve 54 have negative Duv values. Points on a CCT isotherm that have a more positive value of Duv are perceived as having a larger amount of green tint, while those that have a more negative value of Duv are perceived as having a larger amount of pink or magenta tint.

[0087] Another standard chromaticity diagram is the CIE 1976 u’v’ chromaticity diagram, which may be regarded as a more uniform color space where equal distances correspond to equal chromaticities. The full CIE 1976 u’v’ color space describes the same range of colors as the 1931 xy chromaticity diagram 50 shown in Fig. 2 (i.e., all colors visible to the human eye). The axes in the CIE 1976 u’v’ color space are adjusted via a linear projective transform such that CCT isotherms are now shown as normal to the black body curve 24. Because the CCT isotherms are normal to the black body curve 54 in the u’v’ chromaticity diagram, the CCT value and the CCT isotherms can be used as orthogonal coordinates to uniquely refer to any white point. The two coordinates are referred to as CCT and Duv.2024PF80519

[0088] 10

[0089] The diagrams of the following Figs. 3 to 5 correspond to CIE 1976 u’v’ diagrams.

[0090] Fig. 3 shows schematically a chromaticity diagram of the two parallel LED strings of Fig. 1 without color correction.

[0091] In the chromaticity diagram of Fig. 3, the locations of Cyan (Cy) light, Lime (Li) light and Amber (Am) light are shown. The CCT of a combined light resulting from a combination of the high-CCT and low-CCT lights is located on a straight line joining a CCT of the warm white light (1400K) and a CCT of the cool white light (6500K). Typically, the chromaticity of the resultant white light moves away from the black body curve 54 as the combined CCT changes from the CCT (1400K) of the warm or the cool white light towards the halfway point between the warm and cool white lights.

[0092] As can be gathered from Fig. 3, a linear transition from a low CCT to a high CCT, in this case from 1400K to 6500K, departs significantly from the black body curve 54. This is shown by the dotted line between the 1400K point and the 6500K point. Thus, a linear change from the high-CCT (e.g., 6500K) LED string to the low-CCT (e.g., 1400K) LED string would lead to significant Duv deviation from the intermediate CCT values (1600K, 2700K, 3000K, 3500K, 4000K and 5000K) on the black body curve 54.

[0093] Achieving white light color tuning in a cost effective and reliable manner while keeping the curve of the combined white light relatively close to the black body curve 54 can be challenging.

[0094] The following embodiments provide solutions that enable effective white light color tuning that results in a light that is relatively close to the black body curve 54.

[0095] According to embodiments, LED light with e.g. Lime color and Amber color may be used for Duv correction, wherein the Lime and Amber LEDs are not provided in an independent LED string but are instead connected in series with one branch that includes e.g. the high-CCT (e.g., 6500K) string.

[0096] As an example, the number of high-CCT LEDs, low-CCT LEDs, Lime LEDs and Amber LEDs in the parallel LED strings may be selected to achieve a desired amount of color correction. More specifically, the ratio of Lime and Amber light output to that of the high-CCT and low-CCT white LEDs can set by the number of corresponding LEDs. For example, the ratio may be 1400K / 6500K / Lime / Amber may be set as 8 / 3 / 673 in number of LEDs driven from the same current source. That is, eight high-CCT LEDs, three low-CCT LEDs, six Lime LEDs and three Amber LEDs may be combined for a desired color correction control.2024PF80519

[0097] 11

[0098] In the above example, a current source (LED driver) may be provided, that supplies a constant total current of 100 mA through both parallel LED strings, which may be dimmed by a dimming function of the current source. The LEDs may be connected in combination of series and parallel for light output and string voltage matching.

[0099] Fig. 4 shows schematically a chromaticity diagram of the two parallel LED strings of Fig. 1 with a color correction shift by additional Lime and Amber LEDs along a line joining a Lime-Amber correction point (CP). The location of the correction point can be set by the relative number of Lime and Amber LEDs.

[0100] At a 100% duty cycle current (full-time on state) supplied through the high-CCT (6500K) LED string, the Lime-Amber correction point will shift the chromaticity of the 6500K light along the line joining the 6500K point to the Lime- Amber correction point.

[0101] Fig. 5 shows schematically a chromaticity diagram of the two parallel LED strings of Fig. 1 with a plurality of color correction shifts for different color temperatures along the line from the 6500K point to the Lime-Amber correction point.

[0102] Thus, the color-corrected transition line between the low-CCT (e.g., 1400K) point and the high-CCT (e.g., 6500K) point will be, for its most part, above the black body curve 54. The color-corrected CCT point is thus shifted along the line joining the Lime-Amber correction point from its maximum position towards the 6500K point, as the resultant CCT value of the white light LEDs is increased from 1400K to 6500K (as indicated by the arrows in Fig. 5). The most upper dotted line in the diagram of Fig. 4 shows the maximum shift for the 1600K point.

[0103] In alternative embodiments, Lime and / or Cyan can be used in combination with Amber to establish a correction point. If both Lime and Cyan are used on one end and Amber on the other end, the correction line can be extended to the left to thereby better position the correction point.

[0104] Fig. 6 shows schematically a block diagram of a luminaire with one-channel color correction according to various embodiments.

[0105] A driver circuit (DR) 10 may comprises a first power conversion stage and a second power conversion stage with a controller that controls a controllable switch (e.g., MOSFET) to generate a constant load current through an output channel with a high-CCT luminaire string (LHCCT) 22 and a low-CCT luminaire string (LLCCT) 24. The control of the luminaire current by a dimmer circuit or function (DIM) 11 may be achieved by a current sensing function (not shown) that may be implemented e.g. by a sensing resistor Rs for measuring the actual load current.2024PF80519

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[0107] The first power conversion stage of the driver 10 may be supplied with power via supply terminals. More specifically, a power supply AC voltage (e.g., a power grid voltage of 110 or 220V at a mains frequency of 50 or 60Hz) may be supplied to an electromagnetic interference (EMI) filter (not shown) which is an electronic device that attenuates electromagnetic interference from the power system to limit the noise in the system and lower a risk of malfunctioning of the luminaire driver.

[0108] The first power conversion stage may thus serve to convert the input AC voltage into a DC voltage while meeting power intake requirements such as power factor (PF) and total harmonic distortion (THD). A smoothening capacitor may be applied between output terminals of the first power conversion stage to reduce a voltage ripple. The output voltage with reduced ripple may then be converted by the controller and the controllable switch of the second power conversion stage into a ripple-free (mains-ripple-free) constant current that is switched by the controllable switch.

[0109] As the second power conversion stage is designed as a current source, it operates to keep constant its output current through the output channel, while its output voltage can vary instantly.

[0110] In addition, the controller of the driver 10 may be configured to generate a switching control signal 100 (e.g., a pulse width modulation (PWM) signal) and applies the switching control signal 100 to a control terminal (e.g., gate electrode of a MOSFET) of a controllable switch (SW) 40 to switch on / off the constant load current through the high-CCT luminaire string 22 to control the luminance of the high-CCT luminaire string 22. The driver 10 may comprise input terminals (e.g., parallel or serial digital or analog input) to control application or generation of the switching control signal 100 for the controllable switch 40 based on an output of a white tuning control function.

[0111] The current through the high-CCT luminaire string 22 may be rapidly switched on and off at a high frequency. The human eye cannot perceive this rapid switching, so the high-CCT luminaire string 22 appears to be continuously lit. The brightness level is determined by the ratio of the "on" time to the "of1time within each cycle. The duty cycle is the percentage of on-time during each cycle. A higher duty cycle means that the high-CCT luminaire string 22 is on for a longer period, resulting in brighter light. Conversely, a lower duty cycle produces dimmer light. The frequency of the on / off cycles may be set high enough (usually above 1000 Hz) to avoid visible flicker.2024PF80519

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[0113] Thus, when the luminaire current is dimmed down, the PWM period remains the same while the PWM on-time is shortened to reduce the effective load current and hence the brightness of the generated light or light color.

[0114] Other pulse modulation schemes (e.g., pulse frequency modulation) may be used for the dimming process.

[0115] As shown in Fig. 6, the left branch with the high-CCT luminaire string 22 comprises a serially connected controllable white tuning circuit (WT) 26 that may be configured to add light of one or more different colors to achieve a desired shift of the CCT transition to better match with the black body curve 54 of Figs. 3 to 5. The white tuning circuit may thus comprise one or more light sources (e.g., LEDs) of different color(s) (e.g., Lime and Amber), that are controlled in their luminance based on the ratio between high-CCT and low-CCT light. Thereby, in the example of Fig. 5, the resultant combined CCT of the two strings 22, 24 is moved along the line between the 6500K point and the Lime-Amber correction point (CP).

[0116] As explained above with reference to Fig. 1, if the forward voltage over the high-CCT luminaire 22 is selected to be lower than the forward voltage over the low-CCT luminaire 24, the low-CCT luminaire 24 is automatically switched off when the high-CCT luminaire 22 is switched on, and vice versa. Thus, the ratio between high-CCT and low-CCT light is defined by the duty cycle of the PWM signal 100.

[0117] However, note that the opposite on / off switching of the two parallel luminaire branches may as well be achieved by providing respective switching elements in each luminaire branch (e.g., controlled by inverse switching control signals or with inverse switching behavior).

[0118] The duty cycle of the switching process can be determined by an average detector (AVD) 30, which may be implemented by an integration function (e.g., an RC low pass filter or the like) that outputs the average value (e.g., direct current (DC) component of the PWM signal). The obtained average value or signal can then be used to control the current through the luminaires of the white tuning circuit 26 to obtain a desired shift of the CCT value with respect to the black body curve 54(i.e., location of the correction point between the Lime CCT and the Amber CCT), as shown in Fig. 5.

[0119] In an alternative embodiment, the control signal for the white tuning circuit 26 may be obtained by a software routine of a software-controlled processor provided in the controller of the driver 10 of Fig. 6.2024PF80519

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[0121] The white tuning circuit may as well be provided in the other luminaire branch with corresponding luminance control.

[0122] More specific examples of the white tuning circuit 26 of Fig. 6 are described below with reference to Figs. 7 and 8, where the white correction or white tuning is achieved by adding light of Lime and Amber LEDs and continuously reducing the added Lime and Amber light during an increase of the current through the branch that comprises a high-CCT LED string (6500K) and Lime and Amber LEDs.

[0123] Fig. 7 shows schematically a circuit diagram of a luminaire with one-channel color correction according to a first embodiment.

[0124] The circuit of Fig. 7 describes an exemplary way of reducing the current through two serially connected correction LEDs (Lime-Amber) by means of a first transistor QI (e.g., metal oxide semiconductor field effect (MOSFET) transistor) connected in parallel to the correction LEDs of the white tuning circuit 26 in dependence on the duty cycle of the string current.

[0125] The left luminaire branch comprises the white tuning circuit 26 and a high-CCT luminaire string 22 of seven high-CCT (e.g., 6500K) LEDs with a total forward voltage Vf-65, through which a pulse-switched luminaire current les is flowing. The right luminaire branch comprises a low-CCT luminaire string 24 of ten low-CCT (e.g., 1400K) LEDs with a total forward voltage Vf-i4, through which a pulse-switched luminaire current Ii4 is flowing. The high-CCT forward voltage Vf-65 of a total of nine LEDs is thus smaller than low-CCT forward voltage Vf-i4 of a total of ten LEDs.

[0126] In the first embodiment of Fig. 7, the PWM control signal 100 is applied to a second transistor Q2 (e.g., MOSFET transistor) to switch on / off the high-CCT current 165 according to a duty cycle of the PWM control signal 100.

[0127] Starting from a 100% duty cycle 1400K string current Ii4, the Lime- Amber light contribution will be zero, since the high-CCT string (6500K plus Lime-Amber LEDs) current Ls is zero (0% duty cycle). As the high-CCT string current les increases, the correction via the Lime-Amber light of the white tuning circuit 26 will also increase.

[0128] To achieve this, the PWM control signal 100 is applied to an average detector circuit 30 with amplifying (operational amplifier), rectifying (diode) and integrating (RC lowpass filter) function, which generates an output voltage across the capacitor C that corresponds to the average value (e.g., DC component) of the PWM control signal 100 and that is applied to the gate of the first transistor QI .2024PF80519

[0129] 15

[0130] At some point (e.g., near 2700K on the black body curve 54), the correction light of the Lime / Amber LEDs will reach its maximum value. As the duty cycle of the high-CCT current Ls increases further, the average value of the PWM control signal 100 will reduce the ON-resistance of the drain-source path of transistor QI and thereby reduce the current flowing through the parallel path of the Lime- Amber LEDs of the white tuning circuit 26. When the high-CCT string current Ls reaches a duty cycle of 100%, the Lime- Amber LEDs of the white tuning circuit 26 will be shunted (bypassed) by the on-state (low-resistance drain-source path) of the first transistor QI (which corresponds to the 6500K point in the diagram of Fig. 5).

[0131] As a result, the transition curve from the high-CCT light to the low-CCT light will be improved as explained in connection with Fig. 5.

[0132] Fig. 8 shows schematically a circuit diagram of a luminaire with one-channel color correction according to a second embodiment.

[0133] There are cases where the lumen output requirement conflicts with the forward voltage of the parallel high / low-CCT luminaire strings 22 and 24. For example, the 6500K lower string forward voltage Vf-65 required to switch off the 1400K luminaire string 24 may result in a too low lumen output (too low brightness).

[0134] Therefore, in the second embodiment, the white tuning circuit 26 is configured to gradually replace the correction light (e.g., three Lime LEDs serially connected to two Amber LEDs) by a further 6500K LED string 22 as the target CCT approaches the 6500K CCT.

[0135] In the second embodiment, the high-CCT luminaire string 22 is composed of a common 6500K LED string 22 (three LEDs in the circuit of Fig. 8) of forward voltage Vfi-65 and two correction strings of the white tuning circuit 26, that are phased in and out depending on the duty cycle (i.e., average output of the average detector circuit 30) of the high-CCT current Ls. The first correction string is a Lime-Amber string with a forward voltage Vf-LA and the second correction string is a 6500K LED string 22 (seven LEDs in Fig. 8) with a forward voltage Vf2-65, where Vf-LA < Vf2-65. As a result, the maximum forward voltage of the high-CCT branch is Vfi-65 + V12-65 = Vf-65. The number of LEDs (nine LEDs in Fig. 8) of the 1400K LED string is selected so that Vf-65 < Vf-i4.

[0136] In the second embodiment, the current PWM control signal 100 is fed via an inverter and the average detector circuit 30 to the first transistor QI operating in the linear region and connected in series with the first correction string (Lime-Amber LED string). As the duty cycle of the current Ls in the high-CCT string increases, the corresponding average2024PF80519

[0137] 16

[0138] output voltage applied to the gate terminal of the first transistor QI decreases which increases the ON-resistance of its drain-source path and gradually reduces the current through the Lime-Amber LED string. This will gradually replace the Lime-Amber correction light with high-CCT light of the parallel second correction string (6500K LED string).

[0139] As a result, the transition curve from the high-CCT light to the low-CCT light will be improved as explained in connection with Fig. 5, while ensuring sufficient brightness (lumen) of the light output.

[0140] Due to their low hardware complexity, embodiments described herein are particularly suitable for filament luminaires with filament LED strings, where hardware circuits (e.g., the above driver 10, dimmer 11, average detector circuit 30 and components of the white tuning circuit 26) are placed in a screw base to appear as traditional incandescent lamps. Implementation of the embodiments described herein may therefore be advantageously integrated in filament luminaires, due to their low additional hardware requirements. Preferably, due to the small size of the white tuning circuit 26, it may be integrated on at least one of the filaments.

[0141] Fig. 9 shows schematically a filament luminaire 90 with two filament LED strings (double filaments) 94 and a screw base 92.

[0142] Filament LED luminaires are close retrofits of low-power traditional incandescent lamps, where the shape of the filament is maintained.

[0143] Fig. 10 shows schematically two filament LED strings 96, 98 of a double filament 94 with different CCT of the filament luminaire of Fig. 9. The two filament strings may comprise an incandescent-CCT LED filament 96 (having a CCT (e.g., 2700K) that corresponds to (resembles) an incandescent lamp) and a low-CCT (e.g., 1600K) LED filament 98.

[0144] In the following third embodiment, white tuning without color correction is proposed to obtain a dim-to-warm filament LED luminaire as a desirable luminaire for nostalgic filament look.

[0145] Fig. 11 shows schematically a circuit diagram of a filament luminaire according to a third embodiment with filament LED strings 22, 24 controlled for improved dimming characteristic.

[0146] According to the third embodiment, a white tuning circuit is configured to control a current through a high-CCT luminaire string (LED filament) to reduce the total CCT to a minimum value when dimmed, thereby mimicking the behavior of incandescent lamps.2024PF80519

[0147] 17

[0148] A dimmable LED driver (driver 10 plus dimmer function 11) can be provided in the screw base 92 of the filament luminaire with a glass or plastic housing (as shown in Fig. 9). The LED driver 10 may comprise a circuit (e.g., controller) that measures the total current supplied to two parallel high-CCT and low-CCT LED (filament) strings 22, 24, which is controlled by the dimmer function 11.

[0149] The white tuning circuit comprises a difference amplifier 112 that interfaces with the control input of a current source 110 connected in series with the high-CCT LED string 22 (e.g., incandescent LED CCT). The current source 110 may be a transistor (bipolar junction transistor (BJT) or MOSFET) operating in the active region.

[0150] The two filament LED strings 22, 24 comprise a high-CCT LED string 22 with incandescent CCT (e.g., 3200K to 2600K) and a low-CCT LED string (2000K to 1400K) and are configured to replace an existing single LED filament of conventional filament luminaires.

[0151] The circuit of Fig. 11 includes an element or circuit (e.g., resistor Rs of Fig.

[0152] 11) that measures a total driver current Io through both LED filament strings 22 and 24, that is controlled by the dimmer 11. The measured value of the current is fed to the difference amplifier 112 which subtracts it from a reference value Ref (e.g., the maximum total driver current Io for d= 1 ) and uses the difference as a control value for controlling the current source 110 feeding the high-CCT (e.g., 2700K) LED string 22. The low-CCT (e.g., 1600K) LED string 24 receives the remaining difference of the current value.

[0153] The dimmer function 11 may operate according to a dimming function f(d), where d is a linear dimming action variable, for example in the range between 0 and 1. f(d) may be linear, logarithmic or any other specific function. For example, a linear dimming function f(d) may be k*d where k is a real constant. A simple linear function may be f(d)=d where in this case k=l. A logarithmic example may be f(d)=a+b*ln(d) where a and b are real numbers.

[0154] The current I22 through the high-CCT LED string 22 is controlled by the white tuning circuit (difference amplifier 112 and current source 110) to follow the dimming factor d and therefore corresponds to Io*d*f(d). In the above linear case, the high-CCT current I22 therefore corresponds to d2Io and shows a quadratic dependence on the dimming factor d.

[0155] Since the current I24 in the low-CCT LED string 24 corresponds to the difference between the main current f(d)*Io and the high-CCT string current I22, it corresponds to Io*f(d)-Io*d*f(d). In the linear case, this corresponds to d*Io-d2*Io.2024PF80519

[0156] 18

[0157] Fig. 12 shows schematically a diagram of dimming function f(d) versus dimming factor d with the improved dimming characteristic obtained by combining the above individual dimming characteristics of the high-CCT and low-CCT filament LED strings 22 and 24, as obtained by the white tuning circuit.

[0158] As shown in Fig. 12, with increasing dimming factor d between 0 and 1, the high-CCT current I22 though the high-CCT LED string 22 for colder white light (incandescent CCT, e.g., 2700K)) is increased on a sub-linear scale (e.g., d2) and the low-CCT current I24 through low-CCT LED string 24 for warmer white light (e.g., 1600K) is decreased on a sub-linear scale (e.g., (d-d2)). Thus, at low dimming factors (d < 0.5) the warmer low-CCT white light prevails, while at high dimming factors (d > 0.5) the colder high-CCT white light prevails, to thereby resemble a dim-to-warm characteristic of a conventional incandescent lamp.

[0159] It is noted that the third embodiment of Fig. 11 may as well comprise a color correction function with non-white color correcting LEDs as described in connection with the first and second embodiments.

[0160] It is further noted that the above average detector circuit 30, white tuning circuit 26, difference amplifier 112 and current source 110 of the above embodiments may be implemented as analog circuits with analog input and output values or as digital circuits with digital input and output values. In the latter case, the white tuning circuit may comprise a digital-to-analog converter (DAC) for generating the analog control signal for the first transistor QI or the current source 110, respectively.

[0161] To summarize, apparatus and methods for white tuning with and without color correction (e.g., Duv correction) of luminaires with and without dimming function have been described, wherein one or more color-correcting light sources (e.g., LEDs) are connected in series with one luminaire string of two parallel-connected luminaire strings (e.g., LED filaments) and are disabled when the one string reaches a maximum duty cycle of the dimming function. The one string may comprise luminaires with higher correlated color temperature (CCT) and the other string may comprise luminaires with lower CCT. The lower-CCT string may comprise a larger number of light sources compared to the higher-CCT string so that the higher-CCT string is turned off when the parallel-connected lower-CCT string is turned on. As another option, an overall current through both luminaire strings is sensed and compared with a reference value. Based on the result, a current source in one of the luminaire strings is controlled for white tuning.2024PF80519

[0162] 19

[0163] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments concerning LED luminaires. The proposed embodiments can be applied in connection with any type of SSL devices. Moreover, the white tuning function with or without color correction can be implemented by various discrete circuits or signal processing functions or processor routines.

[0164] More specifically, elements and components mentioned in the above embodiments may be embedded in one or more integrated circuits (ICs), e.g., application specific ICs (ASICs) or programmable logic arrays (PLAs) or the like.

[0165] The above embodiments may be implemented in (e.g., integrated or combined with) various high-efficiency products such as office luminaires, outdoor lighting, LED strips, color-tuneable spots or the like.

[0166] The proposed single-channel white tuning may alternatively be based on a flux of green light (instead of the Lime light proposed in the above embodiments, which is a phosphor-converted green light) controlled based on a flux of the cool white light or a flux of the warm white light, wherein the flux of the warm white light and the flux of the cool white light may change proportionally with respect to each other. The solution may enable tracking the black body curve anywhere between two CCTs, such as, for example, 1500K to 7000K or 1400K to 6500K, by including, for example, an Amber LED along with the green light LED, and adjusting the ratio of the green and Amber lights, resulting in a moving correction light, to correct the chromaticity of the resultant of the lights at the two ends (1400K / 1500K and 6500K / 7000K).

[0167] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in the text, the invention may be practiced in many ways, and is therefore not limited to the embodiments disclosed. It should be noted that the use of particular terminology when describing certain2024PF80519

[0168] 20

[0169] features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to include any specific characteristics of the features or aspects of the invention with which that terminology is associated.

[0170] A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0171] The described white tuning procedures can be implemented as program code means of a computer program and / or as dedicated hardware of the receiver devices or transceiver devices, respectively. The computer program may be stored and / or distributed on a suitable medium, such as an optical storage medium or a solid-state medium, supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

2024PF8051921CLAIMS:

1. A luminaire (303) comprising:a first current branch with a first string (22) of solid-state light sources of a higher color temperature;a second current branch with a second string (24) of solid-state light sources of a lower color temperature; anda white tuning circuit (26; 110, 112) arranged in one of the first and second current branches and configured to perform white tuning by modifying the contribution of light generated by the one of the first and second current branches,wherein the white tuning circuit (26) comprises one or more solid-state light sources (Li, Am) of a color temperature different from the higher and lower color temperatures, that are controlled in their luminance based on a ratio between the luminance of the first string (22) and the luminance of the second string (24),wherein the luminaire comprises an average detector (30) for determining an average value of a pulse-modulated control signal (100) that determines the current flowing through the one of the first and second current branches,wherein the white tuning circuit (26) comprises a parallel connection of first and second correction strings of solid-state light sources, wherein the first and second correction strings are phased in and out by a transistor (QI) depending the average value of the average detector (30), wherein the first correction string comprises the one or more solid-state light sources (Li, Am) of different color temperature, and wherein the second correction string comprises additional solid-state light sources of the higher color temperature.

2. The luminaire of claim 1, wherein the number of solid-state light sources of the first string (22) and the number of solid-state light sources of the second string (24) is selected so that a total forward voltage of the first string (22) is lower than a total forward voltage of the second string (24) and the second string (24) is switched off when the first string (22) is switched on, and vice versa.2024PF80519223. The luminaire of claim 1 or 2, wherein the white tuning circuit (26) comprises a transistor (QI) connected in parallel to the one or more solid-state light sources (Li, Am) of different color temperature and controlled by the average value to change the current through one or more solid-state light sources (Li, Am) of different color temperature in dependence on the average value.

4. The luminaire of claim 1, wherein the white tuning circuit (26) is configured to gradually replace correction light generated by the first correction string by light of the higher color temperature generated by the second correction string, as a target color temperature approaches the higher color temperature.

5. The luminaire according to any of the preceding claims, wherein an inverse of the pulse-modulated control signal (100) is supplied to the average detector (30), and wherein the transistor (QI) is connected in series with the first correction string.

6. The luminaire of any of the preceding claims, wherein the different color temperature of the one or more solid-state light sources (Li, Am) of different color temperature comprises at least one of Lime, green, Cyan and Amber.

7. The luminaire of claim 1, wherein the white tuning circuit comprises a difference amplifier (112) for determining a difference between a predetermined reference value (Ref) and a measured value of the current flowing through the first and second current branches, and a current source (110) controlled by the determined difference and arranged to feed its output current through the one of the first and second current branches.

8. The luminaire of claim 7, wherein the output current of the current source (110) shows a quadratic dependence on a dimming factor of the current flowing through the first and second current branches.

9. The luminaire of any one of the preceding claims, wherein the higher color temperature ranges between 2700K and 7000K and the lower color temperature ranges between 1400K and 2000K.2024PF805192310. The luminaire of any one of the preceding claims, wherein the first and second strings (22, 24) are filament strings (96, 98) and wherein the luminaire (90) comprises a screw base (92) for housing a driver circuit (10) and configured to generate the current flowing through the first and second current branches.

11. A method of performing white tuning in a luminaire having a first current branch with a first string (22) of solid-state light sources of a higher color temperature and a second current branch with a second string (24) of solid-state light sources of a lower color temperature, the method comprising:performing white tuning in one of the first and second current branches by modifying a contribution of light by the one of the first and second current branches.

12. A computer program product comprising code means for producing the steps of claim 11 when run on a controller circuit of a driver circuit (10) of the luminaire.

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