Method and system to enable highest efficacy and deep dimming in color light

Abstract

Provided is a driving arrangement for an LED lighting system comprising a first LED arrangement and a second LED arrangement. Specifically, the first LED arrangement is driven by an amplitude modulation (AM) circuit, and the second LED arrangement is driven by a pulse width modulation (PWM) circuit. A controller is provided that controls the AM circuit and the PWM circuit based on a target brightness value. Accordingly, at high brightness levels, the AM circuit drives the first LED arrangement (and optionally the PWM circuit drives the second LED arrangement) to provide the target light output. At low brightness levels, only the PWM circuit is controlled to drive the second LED arrangement. Thus, high efficiency lighting is provided by the AM circuit driving the first LED arrangement, while deep dimming and a good colour consistency is provided by the PWM circuit driving the second LED arrangement.

Description

[0001] Method and system to enable highest efficacy and deep dimming in color light

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of lighting, and in particular, to the field driving arrangements for lighting.

[0004] BACKGROUND OF THE INVENTION

[0005] It is well known that white light can be generated by white LEDs (e.g., phosphor converted blue light LEDs) or by a combination of red (R), green (G), and blue (B) LEDs. It is also well known that phosphor converted white LEDs can have good colour rendering index (CRI) levels. In contrast, white light generated by a combination of R, G and B LEDs will have a much lower CRI, especially for direct emission R, G and B. The CRI level of these RGB-based light sources (e.g., <80) are insufficient for general lighting applications, where CRI of 80 or 90 is required.

[0006] Furthermore, energy efficiency of lighting systems is an increasingly important parameter, both for saving costs and for reducing environmental impact. In recent regulations, (e.g. Single Lighting Regulation for EMEA market, 2021), lighting devices must be labelled according to their efficiency, and efficiency requirements have been increased. Devices categorized in the highest efficiency categories get a green label, which is attractive for energy conscious consumers.

[0007] Amplitude modulation (AM) dimming is the most common method for dimming lighting systems. However, in cases where colour consistency is of high importance, pulse width modulation (PWM) dimming is preferred. Unfortunately, PWM drivers are typically less efficient than AM dimming drivers. Thus, for applications where efficiency of the lighting system is of high importance, AM dimming is preferred.

[0008] Of course, it would be desirable to have a dimmable lighting system having good colour consistency, whilst also being highly efficient. There therefore exists a need for a driving arrangement that facilitates these aims.

[0009] SUMMARY OF THE INVENTION

[0010] The invention is defined by the claims. According to an aspect of the invention, there is provided a driving arrangement for an LED lighting system comprising a first LED arrangement and a second LED arrangement, the driving arrangement comprising: an amplitude modulation, AM, circuit configured to only drive the first LED arrangement; a pulse width modulation, PWM, circuit configured to only drive the second LED arrangement; and a controller configured to:

[0011] - receive a target brightness value describing a target light output of the LED lighting system;

[0012] - control the AM circuit to drive the first LED arrangement based on the target brightness value responsive to the target brightness value being above a non-zero lower brightness level threshold; and

[0013] - control the PWM circuit to drive the second LED arrangement based on the target brightness value.

[0014] Provided is a driving arrangement for an LED lighting system comprising a first LED arrangement and a second LED arrangement. Specifically, the first LED arrangement is driven by an amplitude modulation (AM) circuit, and the second LED arrangement is driven by a pulse width modulation (PWM) circuit. A controller is provided that controls the AM circuit and the PWM circuit based on a target brightness value. Accordingly, at high brightness levels, the AM circuit drives the first LED arrangement (and optionally the PWM circuit drives the second LED arrangement) to provide the target light output. At low brightness levels, only the PWM circuit is controlled to drive the second LED arrangement. Thus, high efficiency lighting is provided by the AM circuit driving the first LED arrangement, while deep dimming and a good colour consistency is provided by the PWM circuit driving the second LED arrangement.

[0015] Put another way, the first LED arrangement (comprising one or more LEDs) is controlled by AM dimming and a second LED arrangement (comprising one or more LEDs) is controlled by PWM dimming. For a higher target brightness level, both the first and second LED arrangement may be driven simultaneously to produce a target light output of the LED lighting system. Accordingly, a desired colour point may be achieved based on LEDs from the first and second LED arrangement, whilst also providing efficient lighting by virtue of the efficient AM dimming. Below a lower brightness level threshold (which is non-zero), only the PWM circuit is controlled, meaning that only the second LED arrangement produces light. PWM enables deeper dimming than AM, without exhibiting the effects of artifacts that AM may produce when driving an LED arrangement at a low brightness level.

[0016] Thus, the controller ensures that a target light output (i.e. target light brightness and colour) of the LED lighting system is achieved by appropriately controlling the PWM circuit and the AM circuit to drive their respective LED arrangements. At high brightness levels, this target light output may be achieved by the first LED arrangement, or a combination of the first LED arrangement, and the second LED arrangement. At low brightness levels (i.e. below the lower brightness level threshold), this target light output is achieved by only the second LED arrangement.

[0017] The PWM circuit may be any hardware capable of performing PWM and thus providing a selected current to the first LED arrangement according to a control signal sent by the controller. The current provided to the first LED arrangement must be suitable for driving the first LED arrangement (i.e. driving LEDs of the first LED arrangement). Similarly, the AM circuit may be any hardware capable of performing AM and thus providing a selected current to the second LED arrangement according to a control signal sent by the controller. The current provided to the second LED arrangement must be suitable for driving the second LED arrangement (i.e. driving LEDs of the second LED arrangement).

[0018] The target brightness value may be any parameter / signal indicative of the amount of light that is to be output by the LED lighting system. The target brightness value may thus be indicative of a level of desired dimming. That is, the target brightness value may be provided in a form of a percentage or proportion of the maximum brightness level of light that can be produced of the LED lighting system.

[0019] The lower brightness threshold defines the brightness level below which the first LED arrangement does not contribute toward the target brightness level. Thus, this defines the level below which the AM circuit ceases driving the first LED arrangement to produce light. Therefore, below this level, the light out is produced only by the second LED arrangement driven by the PWM circuit.

[0020] A colour gamut area spanned by the second LED arrangement may comprise at least a colour gamut area spanned by the first LED arrangement.

[0021] A colour gamut area is a range of colors that a particular device can produce. Thus, rephrased, the second LED arrangement may be controllable to produce light of a colour range, that at least includes the colour range producible by the first LED arrangement. This may ensure consistency of a colour profile of output light from the LED lighting system independent of the target brightness level. Further, if the colour gamut area spanned by the second LED arrangement is greater than the colour gamut area spanned by the first LED arrangement, the second LED arrangement may be used to enhance the colour of the first LED arrangement at any target brightness value.

[0022] In some cases, the first LED arrangement may comprise a white LED, and the second LED arrangement comprises a red LED, a green LED, and a blue LED. In addition, the second LED arrangement may further comprise a white LED.

[0023] Alternatively, the first LED arrangement may comprise a first white LED, and the second LED arrangement may comprise a second white LED.

[0024] To be clear, a white LED may be an LED capable of emitting white light, such as a phosphor converted blue LED. The red, green, and blue LEDs may be LEDs capable of outputting the respective colour. Of course, each of the first and second LED arrangement may comprise a plurality of each LED type.

[0025] Optionally, the controller may further obtain a target colour value describing a colour of light to be output by the LED lighting system. This may be the case, for example, when the first and second LED arrangements comprise LEDs of more than one colour. In this case, the controller may further be configured to control the AM circuit and / or PWM circuit to drive the first and / or second LED arrangements based on the target colour value.

[0026] In some embodiments, the controller may be configured, responsive to the target brightness value being above the lower brightness level threshold, to balance control of the AM circuit to drive the first LED arrangement and the PWM circuit to drive the second LED arrangement simultaneously to provide the target light output of the LED lighting system.

[0027] That is, when above the lower brightness threshold, both the first and second LED arrangement generate light simultaneously. The controller controls the AM circuit and the PWM circuit such that the sum / net total of the light output by the first LED arrangement and the second LED arrangement meets the target light output level defined by the target brightness value.

[0028] Also the controller may be configured, responsive to the target brightness value being in a transition range, and as the target brightness level decreases, to: control the AM circuit to drive the first LED arrangement to gradually decrease a light output from the first LED arrangement to zero light output at the lower brightness level threshold; and control the PWM circuit to drive the second LED arrangement based on the target brightness level to compensate for the decrease in the light output of the first LED arrangement.

[0029] The transition range defines the range of target brightness values in which the proportional contribution to the overall light output of the LED lighting system by the first LED arrangement decreases relative to the second LED arrangement, as the target brightness level decreases. This transition area means that there may not be a sharp change in the light output from the first and second LED arrangement to just the second LED arrangement as the target brightness level decreases toward the low brightness level threshold. In the transition range, the light output by the LED lighting system is increasingly generated by the second LED arrangement driven by PWM, while the contribution of the first LED arrangement driven by AM decreases.

[0030] The lower brightness level threshold may be equal to or greater than the minimum brightness level of the AM circuit.

[0031] That is, the lower brightness level threshold may be greater than a minimum brightness level that may be output by the first LED arrangement driven by the AM circuit.

[0032] In some embodiments, the controller is configured to control the PWM circuit to drive the second LED arrangement based on the target brightness value responsive to the target brightness value being below an upper brightness level threshold. The lower brightness level threshold may be less than the upper brightness level threshold.

[0033] The upper brightness level threshold may thus define a threshold beyond which the second LED arrangement does not contribute toward the light output by the LED lighting system. This may be beneficial, for example, when it is desired to avoid producing high levels of light by driving using a PWM scheme, which are typically less efficient than using an AM scheme. Therefore, at the highest brightness levels (above the upper brightness level threshold), the light output by the LED lighting system may be solely from the first LED arrangement driven by the AM circuit.

[0034] Of course, this may not be desirable when the second LED arrangement is used to provide a desired colour of light that cannot be provided by the first LED arrangement alone, but this may depend on the application of the LED lighting system.

[0035] Furthermore, the controller may be further configured to control the AM circuit to switch-off responsive to the target brightness value being below the non-zero lower brightness level threshold. According to another aspect of the invention, there is provided an LED lighting system comprising a first LED arrangement, a second LED arrangement, and the driving arrangement according to a disclosed embodiment.

[0036] Further, there is provided a lamp comprising the LED lighting system described herein.

[0037] There is also provided a luminaire comprising the lamp described herein.

[0038] According to a further aspect of the invention, there is provided a method for driving an LED lighting system comprising a first LED arrangement and a second LED arrangement, the method comprising: receiving a target brightness value describing a target light output of the LED lighting system; controlling an amplitude modulation, AM, circuit to drive the first LED arrangement based on the target brightness value responsive to the target brightness value being above a non-zero lower brightness level threshold; and controlling a pulse width modulation, PWM, circuit to drive the second LED arrangement based on the target brightness value.

[0039] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

[0040] BRIEF DESCRIPTION OF THE DRAWINGS

[0041] For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0042] Fig. 1 presents a graph showing the contribution by a first LED arrangement and a second LED arrangement to the total light output of a lighting system as a function of a target brightness value according to an embodiment of the invention;

[0043] Fig. 2 presents another graph showing the contribution by a first LED arrangement and a second LED arrangement to the total light output in a transition area of a lighting system according to an embodiment of the invention;

[0044] Fig. 3 is a simplified block diagram of an LED lighting system and a driving arrangement according to an embodiment of the invention; and

[0045] Fig. 4 is a flow diagram of a method for driving an LED lighting system according to another embodiment of the invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] The invention will be described with reference to the Figures.

[0047] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

[0048] 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. If the term "adapted to" is used in the claims or description, it is noted the term "adapted to" is intended to be equivalent to the term "configured to".

[0049] According to proposed concepts, a number of possible solutions may be implemented separately or jointly. That is, although these possible solutions may be described below separately, two or more of these possible solutions may be implemented in one combination or another.

[0050] Provided is a driving arrangement for an LED lighting system comprising a first LED arrangement and a second LED arrangement. Specifically, the first LED arrangement is driven by an amplitude modulation (AM) circuit, and the second LED arrangement is driven by a pulse width modulation (PWM) circuit. A controller is provided that controls the AM circuit and the PWM circuit based on a target brightness value. Accordingly, at high brightness levels, the AM circuit drives the first LED arrangement (and optionally the PWM circuit drives the second LED arrangement) to provide the target light output. At low brightness levels, only the PWM circuit is controlled to drive the second LED arrangement. Thus, high efficiency lighting is provided by the AM circuit driving the first LED arrangement, while deep dimming and a good colour consistency is provided by the PWM circuit driving the second LED arrangement.

[0051] By way of explanation, there are 3 major disadvantages of AM dimming over PWM dimming: (i) The minimum achievable level of AM dimming is around 4%. In many applications, it is desired to reach a 0.2% dimming level. This 0.2% is the dimming level of the combined LED channels. For sufficient resolution each channel should be able to dim to 0.02%. In contrast, PWM dimming is capable of achieving this dimming level.

[0052] (ii) The colour point of the LED shifts with current and temperature. Maintaining good colour consistency with AM dimming is a complex task, especially when using nonphosphor converted RGB LEDs.

[0053] (iii) When using an AM circuit to drive filaments (i.e. many LEDs in series), AM dimming results in visible light artefacts. That is, sections of single filament and / or individual filaments connected in series will show non-uniform brightness level at lower AM dimming level.

[0054] Nevertheless, AM dimming is typically more energy efficient than PWM dimming. Thus, while PWM dimming may provide better colour consistency and deep dimming, it may not be suitable for applications in which efficiency is of importance. As energy efficiency becomes increasingly important to save cost and to achieve climate aims, AM dimming may be preferred.

[0055] Generally, it is proposed to drive a first LED arrangement using an AM circuit / AM dimming to provide light at higher target brightness levels, and to drive a second LED arrangement using a PWM circuit / PWM dimming at lower brightness levels to ensure deep dimming and improved efficiency. In some cases, the second LED arrangement may be used to compensate for any shift in the colour point of the first LED arrangement and / or artifacts resulting from AM dimming, thus ensuring good colour consistency at all target brightness levels.

[0056] In addition, there may be a transition region as a target brightness decreases (or a target dimming level increases), in which a light output provided by the first LED arrangement and AM circuit is decreased while a light output provided by the second LED arrangement and PWM circuit is increased to compensate for the decrease in the output of the first LED arrangement. In other words, the AM circuit and the PWM circuit may be used to drive respective LED arrangements simultaneously in some cases, so that the output of the first LED arrangement is not instantaneously replaced by the second LED arrangement, resulting in a more continuous light output of the lighting system as a whole as a target brightness level changes.

[0057] Furthermore, disclosed embodiments propose a lighting system comprising both white LED and RGB LEDs to cover a full dimming range. For higher light / brightness levels, the white LEDs in a first LED arrangement are driven using AM dimming. At low light levels, the RGB LEDs in a second LED arrangement are used with PWM dimming. As a result, there is provided a high efficiency AM circuit for providing white light at full power, while not compromising light quality at lower brightness levels. The PWM dimming may enable dimming to well below 4% of maximum brightness, without causing issues associated with low CRI due to PWM dimming (in low lighting conditions, colour rendering is usually not important for the lighting application).

[0058] In this case, RGB LEDs are used to generate white light for dimmed conditions. In this way, in normal use (high brightness) white LEDs driven using AM provide (at least most ol) the light output in an efficient manner. In a (deep) dimmed state, the RGB LEDs provide (at least most ol) the light output. The resulting low CRI in this deep dimmed state is usually not important for human perception. In a transition region, a mix of the light outputs may be used.

[0059] Put another way, there is provided a method to use a first LED arrangement comprising white LEDs in full to medium power settings (i.e. brightness levels) of an LED lighting system using AM dimming, and a second LED arrangement using RGB LEDs with PWM dimming for low power settings (i.e. brightness levels) of the LED lighting system. In the transition region of AM to PWM dimming, both the first LED arrangement and the second LED arrangement (i.e. both white LEDs and RGB LEDs) can be used to smoothen the transition from RGB to white and vice versa.

[0060] In Figs. 1 and 2, the flux generated (i.e. light output) by a first LED arrangement is represented by the solid black line, the flux generated by the second LED arrangement is represented by the dotted black line, and the overall flux generated by the lighting system (i.e. the combination of the light output by the first and second LED arrangement) is represented by the dashed grey line. It should be noted that Figs. 1 and 2 present an example of linear dimming, but other smooth and monotonically decreasing curves may be used.

[0061] With reference to Figs. 1, disclosed embodiments of the invention may enact at least one or more of the following steps:

[0062] (i) At a target brightness value of 100% (i.e. full power setting), only the first

[0063] LED arrangement driven by an AM circuit may be on. The first LED arrangement may comprise only white LEDs; (ii) With a reducing target brightness value (i.e. as the lighting system is dimmed), the AM circuit reduces the current through the first LED arrangement, thus reducing the light output of the lighting device;

[0064] (iii) When dimmed further, the current is further reduced;

[0065] (iv) At an upper brightness level threshold, that may be a point above minimum dimming / brightness level that can be supplied by the AM circuit / dimmable driver (e.g. at 20%), the current through the first LED arrangement is reduced and the current through the second LED arrangement is increased to maintain the target brightness value. Ideally, the dimming / brightness curve should remain smooth.

[0066] The second LED arrangement may comprise RGB LEDs. Thus, a target colour point for this target brightness value may be achieved by increasing the PWM duty cycles of the RGB channels;

[0067] (v) At a lower brightness level threshold, potentially corresponding to a minimum dimming level of the AM circuit / driver (e.g., 10%), the AM circuit may be switched off, setting the current through the first LED arrangement to zero; and

[0068] (vi) When dimming further (i.e. with reducing target brightness level further), the RGB driver duty cycle is reduced until the minimum level of the RGB driver.

[0069] Fig. 2 presents a zoomed in version of the graph of Fig. 1 around the transition between the light output of the first LED arrangement and the second LED arrangement. In this case, the transition may not be linear, but instead the contribution of the second LED arrangement may plateau after an initial increase as the target brightness level decreases, and as the contribution of the first LED arrangement settles toward zero light output.

[0070] Of course, the transition region shown in Figs. 1 and 2 are merely exemplary.

[0071] The transition region may begin at 100%, with a contribution to the overall light output of the lighting system provided by the PWM-driven second LED arrangement being non-zero at full brightness output (for example, to provide a good colour output and compensate for potential artifacts). However, to improve efficiency, the second LED arrangement may begin contributing at 10-50% dimming / brightness level.

[0072] While it has been stated that the first LED arrangement may comprise white LEDs and the second LED arrangement may comprise RGB LEDs, embodiments of the disclosed invention are not restricted as such. For example, the second LED arrangement may (alternatively or additionally) comprise white LEDs. That is, the second LED arrangement may comprise only white LEDs, or a combination of the RGB and white LEDs. In order to provide good colour consistency for all target brightness values, the colour gamut area spanned by the LEDs forming the second LED arrangement should at least cover the colour gamut area of the LEDs forming the first LED arrangement. In other words, the colour of the LEDs of the first LED arrangement can always be made by the right combination of the LEDs of the second LED arrangement. For example, when the first led arrangement comprises white LEDs, the second LED arrangement may comprise white LEDs, RGB LEDs, Lime RB LEDs, RGB White LEDs, etc.

[0073] Accordingly, the disclosed lighting system may output any colour within the colour gamut area of the second LED arrangement. To output light of various colors in the colour gamut area of the second LED arrangement, the second LED arrangement may be driven selectively by the PWM to provide a light output of between 100% and 0% power. To increase the total light output, the first LED arrangement is driven by the AM circuit while the second LED arrangement is driven by the PWM circuit to provide the target colour point. This results in a lighting system with improved efficiency and CRI.

[0074] Moving on, Fig. 3 presents a simplified block diagram of an LED lighting system 200 with a driving arrangement 100 according to an embodiment of the invention. The LED lighting system 200 comprises a first LED arrangement 210 and a second LED arrangement 220. The LED lighting system 200 is dimmable, such that the brightness of light output by the LED lighting system 200 may be varied, either automatically or based on input by a user.

[0075] In some embodiments, the first LED arrangement 210 may comprise one or more white LEDs, and the second LED arrangement 220 may comprise one or more red LEDs, one or more green LEDs, and one or more blue LEDs. The second LED arrangement 220 may further comprise a white LED. The first LED arrangement 210 may alternatively or additionally comprise one or more red LEDs, one or more green LEDs, and one or more blue LEDs. In these cases, the LED lighting system 200 may be capable of outputting light of a plurality of different colors. Of course, alternative colour LEDs may be utilized.

[0076] Alternatively, the first LED arrangement 210 comprises at least one first white LED, and the second LED arrangement 220 comprises at least one second white LED. In this case, the LED lighting system 200 may be a monocolor lighting system. Of course, colors other than white LEDs may be used.

[0077] In preferred embodiments, a colour gamut area spanned by the second LED arrangement 220 comprises at least a colour gamut area spanned by the first LED arrangement 210. As a result, a consistent colour may be produced regardless of the target brightness value.

[0078] The driving arrangement 100 comprises an AM circuit 110 configured to drive the first LED arrangement 210. That is, an AM controller / circuit is provided that controls the dimming (and therefore the brightness) of light output by the first LED arrangement 210 (based on a control signal from the controller 130). The AM circuit 110 may control all of the LEDs of the first LED arrangement 210 as a collective, or individually. In the case that the first LED arrangement 210 comprises LEDs of more than one colour, then the AM circuit 110 controlling each of the LEDs individually may allow for the AM circuit 110 to control a colour of light output by the first LED arrangement 210.

[0079] The driving arrangement 100 also comprises an PWM circuit 120 configured to drive the second LED arrangement 220. That is, a PWM controller / circuit is provided that controls the dimming (and therefore the brightness) of light output by the second LED arrangement 220 (based on a control signal from the controller 130). The PWM circuit 120 may control all of the LEDs of the second LED arrangement 220 as a collective, or individually. In the case that the second LED arrangement 220 comprises LEDs of more than one colour, then the PWM circuit 120 controlling each of the LEDs individually may allow for the PWM circuit 120 to control a colour of light output by the second LED arrangement 220.

[0080] The driving arrangement 100 further comprises a controller 130 configured to control the light output by the LED lighting system 200 by controlling the AM circuit 110 and the PWM circuit 120.

[0081] Specifically, the controller 130 is configured to receive / obtain a target brightness value describing a target light output of the LED lighting system 200. The target brightness value may be obtained / received, for example, from a dimmer switch or from a software application (e.g., a smartphone app).

[0082] The target brightness value describes the target / desired brightness or intensity of the light to be output by the LED lighting system 200.

[0083] The controller 130 may further receive / obtain a target colour value describing a colour of light to be output by the LED lighting system 200. This may be the case when one or more of the LED arrangements comprises LEDs one two or more colors (i.e. wherein one of the LED arrangements is not monochrome).

[0084] The controller 130 is configured to control the AM circuit 110 to drive the first LED arrangement 210 based on the target brightness value responsive to the target brightness value being above a non-zero lower brightness level threshold. Accordingly, for any target brightness values above the non-zero lower brightness level, the AM circuit 110 drives the first LED arrangement 210 to generate light based on the target brightness value (describing overall brightness of the LED lighting system 200).

[0085] Furthermore, the controller 130 is configured to control the PWM circuit 120 to drive the second LED arrangement 220 based on the target brightness value. Accordingly, the PWM circuit 120 drives the second LED arrangement 220 to generate light based on the target brightness value (describing overall brightness of the LED lighting system 200).

[0086] The controller 130 controls the individual contributions of the outputs of the first LED arrangement 210 and second LED arrangement 220 to the overall light output. The balance between the individual contributions may depend on the individual application. For example, for energy efficient applications, the first LED arrangement 210 may provide most (or all) of the output light at high brightness values to take advantage of the superior efficiency of the AM circuit 110 compared to the PWM circuit 120. For the greatest colour consistent applications, both the first LED arrangement 210 and the second LED may both provide the output light at high brightness values.

[0087] Furthermore, when a target colour value is provided, the controller 130 is configured to control the AM and PWM circuit 120 to drive the first LED arrangement 210 and second LED arrangement 220 further based on the target colour value.

[0088] In specific embodiments, the controller 130 is configured, responsive to the target brightness level being in a transition range, and as the target brightness level decreases, to control the AM circuit 110 to drive the first LED arrangement 210 to gradually decrease a light output from the first LED arrangement 210 to zero light output at the lower brightness level threshold; and control the PWM circuit 120 to drive the second LED arrangement 220 based on the target brightness level to compensate for the decrease in the light output of the first LED arrangement 210.

[0089] In essence, this means that in a transition range (e.g., as a target brightness value decreases from 20% to 10% as shown in Fig. 1 and 2), the proportional contribution of the second LED arrangement 220 increases to 100% of the target light output while the proportional contribution of the first LED arrangement 210 decreases to zero light output. Of course, the exact relationship may not be linear, and may taper in any continuous, monotonic manner.

[0090] Moreover, the system depicted in Fig. 3 may be provided in the form of a lamp, which may be employed in a luminaire. Fig. 4 is a flow diagram of a method for driving an LED lighting system according to another embodiment of the invention.

[0091] In step 310, a target brightness value is received / obtained. The target brightness value describes a target light output of the LED lighting system, as outlined above.

[0092] In step 320, an AM circuit is controlled in order to drive the first LED arrangement responsive to the target brightness value being above a non-zero lower brightness level threshold. That is, the first LED arrangement is driven to produce light responsive to the target brightness value being above a certain threshold. The driving is based on the target brightness value, such that an overall target light output of the LED lighting system may be achieved (when considering contribution of the second LED arrangement).

[0093] In step 330, a PWM circuit is controlled in order to drive the second LED arrangement. That is, the second LED arrangement is driven to produce light by the PWM circuit. The driving is based on the target brightness value, such that an overall target light output of the LED lighting system may be achieved (when considering contribution of the first LED arrangement).

[0094] In some cases, the PWM circuit may be controlled to drive the second LED arrangement responsive to the target brightness value being below an upper brightness level threshold. Thus, light may only be produced by the second LED arrangement at lower brightness levels.

[0095] Overall, the method provides a means for ensuring control of an LED lighting system in an efficient manner (by virtue of the AM driving), whilst also ensuring deep dimming (by virtue of the PWM driving).

[0096] 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.

[0097] Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A driving arrangement (100) for an LED lighting system (200) comprising a first LED arrangement (210) and a second LED arrangement (220), the driving arrangement (100) comprising: an amplitude modulation, AM, circuit (110) configured to drive only the first LED arrangement (210); a pulse width modulation, PWM, circuit (120) configured to drive only the second LED arrangement (220); and a controller (130) configured to:- receive a target brightness value describing a target light output of the LED lighting system;- control the AM circuit to drive the first LED arrangement (210) based on the target brightness value responsive to the target brightness value being above a non-zero lower brightness level threshold; and- control the PWM circuit to drive the second LED arrangement (220) based on the target brightness value, wherein at a target brightness value of 100%, only the first LED arrangement (210) driven by the AM circuit (110) is on.

2. The driving arrangement of claim 1, wherein the controller (130) is configured, responsive to the target brightness value being above the lower brightness level threshold, to balance control of the AM circuit (110) to drive the first LED arrangement (210) and the PWM circuit (120) to drive the second LED arrangement (220) simultaneously to provide the target light output of the LED lighting system (200).

3. The driving arrangement of any of the preceding claims, wherein the controller (130) is configured, responsive to the target brightness value being in a transition range, and as the target brightness level decreases, to:control the AM circuit (110) to drive the first LED arrangement (210) to gradually decrease a light output from the first LED arrangement to zero light output at the lower brightness level threshold; and control the PWM circuit (120) to drive the second LED arrangement (220) based on the target brightness level to compensate for the decrease in the light output of the first LED arrangement.

4. The driving arrangement of any of the preceding claims, wherein the lower brightness level threshold is greater than the minimum brightness level of the AM circuit (HO).

5. The driving arrangement of any of the preceding claims, wherein the controller (130) is configured to control the PWM circuit (120) to drive the second LED arrangement (220) based on the target brightness value responsive to the target brightness value being below an upper brightness level threshold.

6. The driving arrangement of any of the preceding claims, wherein the controller (130) is further configured to: receive a target colour value describing a colour of light to be output by the LED lighting system (200); control the AM circuit (110) to drive the first LED arrangement (210) further based on the target colour value; and control the PWM circuit (120) to drive the second LED arrangement (220) further based on the target colour value.

7. The driving arrangement of any of the preceding claims, wherein the controller (130) is further configured to control the AM circuit (110) to switch-off responsive to the target brightness value being below the non-zero lower brightness level threshold.

8. An LED lighting system (10), comprising a first LED arrangement (210), a second LED arrangement (220), and the driving arrangement (100) of any of the preceding claims.

9. The LED lighting system (10) according to claim 8, wherein a colour gamut area spanned by the second LED arrangement (220) comprises at least a colour gamut area spanned by the first LED arrangement (210).

10. The LED lighting system (10) according to any of the claims 8 to 9, wherein the first LED arrangement (210) comprises a white LED, and the second LED arrangement (220) comprises a red LED, a green LED, and a blue LED.

11. The LED lighting system (10) according to any of the claims 8 to 10, wherein the second LED arrangement (220) further comprises a white LED.

12. The LED lighting system (10) according to any of the claims 8 to 11, wherein the first LED arrangement (210) comprises a first white LED, and the second LED arrangement (220) comprises a second white LED.

13. A lamp comprising the LED lighting system (10) of any of the claims 8 to 12.

14. A luminaire comprising the lamp of claim 13.

15. A method (300) for driving an LED lighting system comprising a first LED arrangement (210) and a second LED arrangement (220), the method comprising: receiving (310) a target brightness value describing a target light output of the LED lighting system; controlling (320) an amplitude modulation, AM, circuit to drive the first LED arrangement based on the target brightness value responsive to the target brightness value being above a non-zero lower brightness level threshold; and controlling (330) a pulse width modulation, PWM, circuit to drive the second LED arrangement based on the target brightness value.

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