Efficiency optimization by parallel-serial switching over dimming interface

Abstract

The invention relates to a driver circuit for driving a plurality of light sources. The driver circuit includes a driver adapted to provide a first current to the first light source and a second current to the second light source, a controller for controlling the driver. The controller is arranged to receive a dimming signal indicative of a dimming level of the light output of the light source. The controller is arranged to convert the received dimming signal to a discrete number of dimming levels representing the dimming levels. The discrete dimming level corresponds to an amount of current to be provided by the driver to the first light source and / or the second light source. At a first discrete dimming level, the controller is arranged to allow a first current to be provided to the first light source and to prevent a second current from being provided to the second light source. At the second discrete dimming level, the controller is arranged to allow the first current to be provided to the first light source and the second current to be provided to the second light source.

Description

TECHNICAL FIELD

[0001] The present invention relates to a driver circuit. The present invention further relates to a lighting device. BACKGROUND

[0002] For the lighting industry, the energy efficiency requirements become more challenging, especially with the new Ecodesign label introduced on September 1, 2021. This new label follows the trend of increasing energy efficiency of lighting products. Up to that point, more and more light sources achieved a label rating of A+ or A++, making it impossible for customers to see any light efficiency difference between products. With the new label, light sources again become more distributed within the label range. This also means that, for example, a lamp rated A++ in the old system is now marked as C. Therefore, it is desirable to further increase the energy efficiency of light sources.

[0003] Especially at tunable light sources, it is highly desirable to increase the efficiency. At dimming, especially deep dimming, the fixed losses in the driver, for example caused by the control circuit, become a major part of the losses in the lighting device. The driver is usually designed for its rated power, so the fixed losses also depend on the rated power of the driver. Usually, a driver with a lower rated output power also has a lower fixed power loss. This obviously affects the total power that can be provided to the load. A driver with a higher rated power can provide more power for its output, but this comes with more fixed losses. Therefore, it is desirable to provide a lighting device that can operate with very high efficiency while providing a good dimming function. SUMMARY

[0004] It is an object of the present invention to provide a solution that allows performing dimming while maintaining a good overall efficiency.

[0005] To provide such a solution, in a first aspect of the invention, a driver circuit for driving a plurality of light sources is provided. The driver circuit comprises:

[0006] a driver adapted to provide a first current to a first light source and a second current to a second light source;

[0007] a controller for controlling the driver;

[0008] wherein the controller is arranged to receive a dimming signal indicative of a dimming level for a light output of the light source,

[0009] wherein the controller is arranged to convert the received dimming signal into a discrete number of dimming levels representing the dimming level, wherein the discrete dimming levels correspond to an amount of current to be provided by the driver to the first light source and / or the second light source, wherein:

[0010] At a first discrete dimming level, the controller is arranged to allow a first current to be provided to the first light source and to prevent a second current to be provided to the second light source;

[0011] At a second discrete dimming level, the controller is arranged to allow a first current to be provided to the first light source and a second current to be provided to the second light source.

[0012] A driver is provided that can provide regulated currents to multiple light sources. A first light source and a second light source can receive currents from the driver. A controller is provided to receive a dimming signal and to provide discrete dimming levels. Examples of dimming signals are phase-cut dimming signals, 0 to 10V dimming signals, DALI dimming signals, DMX dimming signals or wireless dimming signals. The controller translates the received dimming signal into a plurality of discrete dimming levels. The discrete dimming levels are used to determine the amount of current to be provided to the first light source and / or the second light source.

[0013] The absolute maximum efficiency of an LED is achieved at a specific current density, i.e. above and below this current density, the efficiency decreases. The inventors insight is that the current density of an LED needs to be optimized for the entire dimming range in order to have maximum efficiency over the dimming range. The present invention provides the optimal current density by turning off the LED at lower dimming levels. To provide the turn-off event, discrete dimming levels are introduced. The discrete dimming levels can be used to provide levels that can provide active light source variation.

[0014] The first discrete dimming level can be a low dimming level. At this dimming level, a low amount of light output is required. The controller can be used to allow the driver to provide a first current to the first light source and to prevent a second current to be provided to the second light source. The driver does not need to provide an additional second current and only maintains the first current through the first light source. The total current provided by the driver is reduced while the total amount of active light sources is also reduced. The current density in the first light source is not affected by turning off the second light source or removing the second current. Therefore, the driver provides a lower current to the light sources, effectively reducing the light output of the light sources while maintaining a high current density in the active light sources.

[0015] The second discrete dimming level can be a high dimming level. At this dimming level, a high amount of light output is required. The controller can be used to allow the driver to provide a first current to the first light source and a second current to the second light source. The driver provides the first current and the second current. Therefore, the total current provided by the driver is increased while the total amount of light sources is also increased. The current density in the first light source is not affected by the introduction of the second light source and the second current. The increase in current results in the light sources providing more light while maintaining a good efficiency.

[0016] Preferably, the driver can only provide a steady current corresponding to the level of current set at the discrete dimming level. In this way, the current density in the first light source and / or the second light source can remain constant and at a level that allows the light source to efficiently emit light.

[0017] In yet another example, at a third discrete dimming level, the controller is arranged to prevent the first current from being provided to the first light source and to provide the second current to the second light source.

[0018] Another discrete dimming level can be provided in which the driver provides the second current to the second light source but no current to the first light source. This allows the controller to determine, based on the provided dimming level, to change between powering only the first light source, powering only the second light source and powering both the first light source and the second light source. As will be explained later, powering only the first light source or only the second light source can have many benefits.

[0019] In yet another example, between two discrete dimming steps, the driver is arranged to provide a variable current to only one of the first light source or the second light source based on the dimming signal while maintaining the current through the other of the first light source or the second light source constant.

[0020] Preferably, between two discrete dimming levels, only one light source can provide a varying current from the driver. This can mean that for example the driver provides a fixed current to the first light source, preferably at the optimal current density of the first light source. Preferably, the driver can then additionally provide a current to the second light source which can vary based on the dimming signal. By providing a fixed current to the first light source, preferably at the optimal current density of the first light source, the first light source is operated at the highest efficiency. The second light source can be operated at a possibly lower efficiency. However, the overall efficiency is still improved and the variable current in the second light source allows for higher dimming levels. Between two discrete dimming levels, the controller can provide additional dimming levels which allow the driver providing variable power to the light sources to change its output power according to the dimming signal.

[0021] In yet another aspect, a lighting device is provided. The lighting device comprises:

[0022] a driver circuit;

[0023] a first light source, and

[0024] a second light source.

[0025] The lighting device benefits most from the invention as the overall efficiency of the lighting device is improved mainly by providing current to the light sources allowing for more efficient light emission.

[0026] Preferably, the light source is a semiconductor light source. Examples of semiconductor light sources are LEDs, laser diodes and vertical cavity surface emitting lasers, VCSELs. Preferably, the LED is formed as a filament.

[0027] In yet another example, the first light source and the second light source are coupled in a parallel configuration.

[0028] Preferably, the first light source and the second light source are coupled in parallel. This allows the driver to provide easy current control and distribution between the first light source and the second light source.

[0029] In yet another example, the driver circuit comprises a first switch coupled in series with the second light source, wherein the controller is arranged to open and close the first switch, and wherein when the first switch is open, the first current flows through the first light source, and wherein when the first switch is closed, the second current flows through the second light source.

[0030] As a very simple way of regulating the current through the first light source and the second light source, the first switch is coupled in series with the second light source. When the first switch is open, the current path through the second light source is blocked, so the current can only flow through the first light source. Thus, the opening of the first switch can be used at a first discrete dimming level to allow the driver to provide the first current to the first light source and to prevent the second current to be provided to the second light source. When the first switch is closed, the second current can flow through the second light source. In addition, the first current can still flow through the first light source. Thus, the closing of the first switch can be used at a second discrete dimming level to allow the driver to provide the first current to the first light source and the second current to the second light source. In this example, preferably, the first light source and the second light source have substantially the same forward voltage.

[0031] In yet another example, the driver circuit comprises a first switch coupled in series with the first light source and a second switch coupled in series with the second light source, wherein the controller is arranged to open and close the first switch and the second switch, wherein when the first switch is closed, the first current flows through the first light source, and wherein when the second switch is closed, the second current flows through the second light source.

[0032] In this example, both light sources are coupled in series with a respective switch. Closing the respective switch places the respective light source in activity. If the first switch is closed, a first current will flow through the first light source. If the second switch is closed, a second current will flow through the second light source. If both switches are closed, depending on the configuration of the light sources, the first current can flow through the first light source and the second current can flow through the second light source. If both switches are open, no current can flow through either light source. If the driver provides a current, this can lead to a risk. An additional switch can be provided to shunt all light sources. The current from the driver now has a current path that does not go through any light source. Alternatively, the driver can be set to not generate a current when both switches are open. Preferably, in any case, one of the two switches is closed so that a current path for the driver is ensured.

[0033] In yet another example, the driver circuit comprises a series combination of a first switch and a second switch between outputs of the driver, wherein the first light source is coupled in parallel with the first switch and the second light source is coupled in parallel with the second switch.

[0034] Instead of a parallel configuration, the light sources can also be coupled in series. A first shunt switch is provided across the first switch and a second shunt switch is provided across the second switch.

[0035] When the first shunt switch is open and the second shunt switch is closed, the driver provides a first current to the first light source. No current is provided to the second light source. Thus, the opening of the first switch and the closing of the second switch can be used at a first discrete dimming level to allow the driver to provide the first current to the first light source and to prevent the second current from being provided to the second light source.

[0036] When the first shunt switch is closed and the second shunt switch is open, the driver provides a second current to the second light source. No current is provided to the first light source. Thus, the closing of the first switch and the opening of the second switch can be used at a third discrete dimming level to allow the driver to provide the first current to the first light source and the second current to the second light source.

[0037] When the first shunt switch and the second shunt switch are open, the driver provides a first current to the first light source and a second current to the second light source. Thus, the opening of the first switch and the second switch can be used at a second discrete dimming level to allow the driver to provide the first current to the first light source and the second current to the second light source. The first current can be the same as the second current. In this configuration, a current path for the driver is always ensured. When the first light source and the second light source have the same optimal current density, the controller can adjust the amplitudes of the first current and the second current so that they allow the current density in the light sources to reach the optimal current density. This allows the light sources to generate light efficiently.

[0038] In yet another example, the controller is arranged to sense the current provided by the driver, wherein the controller is arranged to control the first switch and / or the second switch based on the amplitude of the current provided by the driver.

[0039] The controller can be used to control the driver. Discrete dimming levels can be translated into currents. The controller can sense the current provided by the driver. The amplitude of the current can be taken as an indication of at what discrete dimming level the light source will operate. If the sensed current is, for example, high, the controller can allow a first current to flow through the first light source and a second current to flow through the second light source. If the sensed current is, for example, relatively low, the controller can only allow the first current to flow through the first light source and prevent any second current from flowing through the second light source. It can be considered that, in this case, the driver does not provide enough current to support the second current, so only the first current is provided.

[0040] In yet another example, the forward voltage of the first light source is less than the forward voltage of the second light source.

[0041] The forward voltage of the first light source can be different from the forward voltage of the second light source. This can be because different types of light sources are used, such as different LED types. Preferably, the forward voltage of the first light source is less than the forward voltage of the second light source, such that only one light source is active at the first discrete dimming level, regardless of the configuration of the switches.

[0042] In yet another example, at the first discrete dimming level, the driver is arranged to provide a first current to the first light source, such that the current density through the first light source allows the first light source to emit light at close to the highest efficiency of the first light source.

[0043] Preferably, the amplitude of the current of the first light source is such that the first light source operates at an optimal current density. During the first discrete dimming level, the first light source operates at its optimal efficiency, so the light generated during dimming is generated in an efficient manner.

[0044] In yet another example, at the second discrete dimming level, the driver is arranged to:

[0045] provide a first current to the first light source, such that the current density through the first light source allows the first light source to emit light at close to the highest efficiency of the first light source, and

[0046] provide a second current to the second light source, such that the current density through the second light source allows the second light source to emit light at close to the highest efficiency of the second light source.

[0047] Preferably, the amplitude of the current to the first light source is such that the first light source operates at its optimal current density and the amplitude of the current to the second light source is such that the second light source operates at its optimal current density. During the second discrete dimming level, the first light source and the second light source operate at their optimal efficiency, thus the light generated (which can be full power) during this dimming level is generated in an energy efficient manner.

[0048] In yet another example, the first light source generates a different color or color temperature than the second light source.

[0049] Preferably, the first light source and the second light source generate different colors. For example, the first light source can generate warm white light and the second light source can generate cool white light or vice versa. The light sources can also generate light with different colors. The light sources can emit, for example, red light, green light or blue light. This allows the color output of the light sources to vary based on the discrete dimming levels.

[0050] In yet another example, the lighting device further comprises at least three light sources configured in parallel, wherein the number of dimming levels is based on the number of light sources.

[0051] Preferably, more discrete dimming levels can be required to allow more dimming steps to be introduced over the entire dimming range. This can be achieved by introducing additional light sources which can be controlled by the controller to receive their respective current. These currents can be provided at different discrete dimming levels, for example a fourth discrete dimming level. Different combinations of active light sources during different discrete dimming levels allow the light output of the lighting device to vary over all dimming ranges. Preferably, the discrete dimming levels are set such that they represent a mimic of regular incandescent bulb dimming. This means that as the current of the light sources is reduced, the light sources generating a relatively cooler white light are deactivated such that by reducing the current of the light sources, the overall color temperature warms up.

[0052] In yet another example, the light generated by the first light source is emitted at a first surface and the light generated by the second light source is emitted at a second surface, wherein the first surface at least partially does not overlap with the second surface.

[0053] In addition or as an alternative to varying the light intensity, color or color temperature of the light sources, a beam shaping mechanism can be introduced. When the first light source emits light at a different surface than the surface at which the second light source emits light, the discrete dimming levels can be used to illuminate different surfaces. The overall beam angle of the lighting device can be adjusted based on the discrete dimming levels. BRIEF DESCRIPTION OF DRAWINGS

[0054] Examples of the application will now be described with reference to the accompanying drawings, in which:

[0055] Figure 1 An example of a circuit diagram of a lighting device is shown.

[0056] Figure 2 Another example of a circuit diagram of a lighting device is shown.

[0057] Figure 3 An example of a graph showing the relationship between light source current and dimming level is shown.

[0058] Figure 4 Another example of a circuit diagram of a lighting device is shown.

[0059] Figure 5 An example of an implementation of a lighting device is shown. DETAILED DESCRIPTION

[0060] The present application will be described with reference to the accompanying drawings.

[0061] 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 application. These and other features, aspects, and advantages of the apparatus, systems and methods of the present application will become better understood from the following description, appended claims, and accompanying drawings. It should also be understood that the drawings are only schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the drawings and that they refer to same or like components.

[0062] Figure 1An example of a circuit diagram of a lighting device is shown. The lighting device has a driver circuit. The driver circuit has a driver 1 which is arranged to power a first light source LED1 and a second light source LED2. A first switch M1 is coupled in series with the second light source LED2. The series configuration of the first light source LED1 and the second light source LED2 with the first switch M1 is coupled in parallel to an output of the driver 1. The series configuration of the first light source LED1 and the second light source LED2 with the first switch M1 can be coupled in parallel to each other. A controller 2 is provided to control the first switch M1. Preferably, the controller 2 can also be used to control the driver 1. This allows one controller 2 to provide control for the first switch M1 and the driver 1. Alternatively, the controller 2 comprises multiple components for different purposes, e.g. controlling the first switch M1 separately from the driver 1. The controller 2 can be an integral part of the driver. The driver 1 can be arranged to receive a mains voltage Mains. The mains voltage Mains is an AC voltage, thus can be rectified by a rectifier circuit. The rectifier circuit can be part of the driver. The controller 2 can receive a dimming signal from an external device which is arranged to provide the dimming signal. Examples of a dimming signal are a phase-cut dimming signal, a 0 to 10 V dimming signal, a DALI dimming signal, a DMX dimming signal or a wireless dimming signal. The controller 2 receives the dimming signal and converts the dimming signal into discrete dimming levels. This means that the controller 2 translates the dimming signal into dimming steps. Preferably, the number of dimming steps is based on the number of light sources provided when the first light source LED1 and the second light source LED2 provide a similar light output. In the provided example, two light sources are provided. There are at least three discrete dimming steps available. One discrete dimming step can be set to 0% output power, another discrete dimming step can be set to 33% output power, another discrete dimming step can be set to 66% output power and another discrete dimming step can be set to 100% output power. In this case, the driver 1 can be configured to not generate a current, such that in the dimming step of 0% output power no light source is powered.

[0063] At the first discrete dimming level, the driver 1 can provide a current with an amplitude corresponding to the first current. The controller 2 can open the first switch M1, such that the first current flows through the first light source LED1. In this case, no current can flow through the second light source LED2. At the first discrete dimming level, the driver circuit provides the first current to the first light source LED1 and no current to the second light source LED2.

[0064] At the second discrete dimming level, the driver can provide a current of an amplitude corresponding to the sum of the first current and the second current. The controller 2 can close the first switch M1. This allows the second current to flow through the second light source LED2. Preferably, the forward voltage of the first light source LED1 is approximately equal to the forward voltage of the second light source LED2. In this case, closing the first switch M1 allows the first current to flow through the first light source LED1 and the second current to flow through the second light source LED2.

[0065] The driver 1 can be arranged to regulate the first current such that it will correspond to a current density in the first light source LED1 that allows the first light source LED1 to emit light with a very high efficiency, preferably such that the efficiency of the first light source LED1 is maximized. Preferably, the first current has an amplitude that provides a current density in the first light source such that the first light source generates light with a predetermined efficiency. Preferably, the first current allows the first light source to operate at or close to its maximum efficiency. The driver 1 can also be arranged to regulate the second current such that it will correspond to a current density in the second light source LED2 that allows the second light source LED2 to emit light with a very high efficiency, preferably such that the efficiency is maximized. Preferably, the second current has an amplitude that provides a current density in the second light source such that the second light source generates light with a predetermined efficiency. Preferably, the second current allows the second light source to operate at or close to its maximum efficiency. In an example where the first light source LED1 has approximately the same power requirements as the second light source LED2, the amplitudes of the first current and the second current can be the same. The driver 1 can then be configured to provide only two current levels, i.e. the first current or the sum of the first current and the second current.

[0066] Figure 2A further example of a circuit diagram of a lighting device is shown. The lighting device has a driver circuit. The driver circuit has a driver 1 which is arranged to power a first light source LED1 and a second light source LED2. A first switch M1 is coupled in series with the first light source LED1. A second switch M2 is coupled in series with the second light source LED2. The series configuration of the first light source LED1 and the first switch M1 and the series configuration of the second light source LED2 and the second switch M2 are coupled in parallel to an output of the driver 1. The series configuration of the first light source LED1 and the first switch M1 and the series configuration of the second light source LED2 and the second switch M2 can be coupled in parallel to each other. A controller 2 is provided to control the first switch M1 and the second switch M2. Preferably, the controller 2 can also be used to control the driver 1. This allows one controller 2 to provide control for the first switch M1 and the driver 1. Alternatively, the controller 2 comprises multiple components for different purposes, e.g. controlling the first switch M1 separately from the driver 1. The controller 2 can be an integral part of the driver. The driver 1 can be arranged to receive a mains voltage Mains. The mains voltage Mains is an AC voltage, thus can be rectified by a rectifier circuit. The rectifier circuit can be part of the driver. The controller 2 can receive a dimming signal from an external device which is arranged to provide the dimming signal. Examples of a dimming signal are a phase-cut dimming signal, a 0 to 10 V dimming signal, a DALI dimming signal, a DMX dimming signal or a wireless dimming signal. The controller 2 receives the dimming signal and converts the dimming signal to a discrete dimming level. This means that the controller 2 translates the dimming signal to a dimming step. Preferably, the number of dimming steps is based on the number of light sources provided when the first light source LED1 and the second light source LED2 provide a similar light output. In the provided example, two light sources are provided. There are three discrete dimming steps available. One discrete dimming step can be set to 0% output power, another discrete dimming step can be set to 33% output power, another discrete dimming step can be set to 66% output power and another discrete dimming step can be set to 100% output power. In this case, the driver 1 can be configured to not generate a current, such that in the dimming step of 0% output power no light source is powered.

[0067] The controller 2 can be arranged to control the first switch M1 and the second switch M2 based on the discrete dimming level. The driver 1 can be arranged to adjust the current to the light sources based on the discrete dimming level. The driver 1 can receive the discrete dimming signal from the controller 2.

[0068] At the first discrete dimming level, the controller 2 can close the first switch Ml and open the second switch M2. The driver 1 can then provide a current to the light source corresponding to the first current. By closing the first switch Ml and opening the second switch M2, the controller 2 allows the first current to flow through the first light source LED 1. The driver 1 determines the amplitude of the first current, and thereby the current density present in the first light source LED 1. Preferably, the first current is set to an amplitude that allows the current density in the first light source LED 1 to be at its optimal current density, to allow the first light source LED 1 to emit light in its most efficient manner. The first discrete dimming level allows the lighting device to emit light with only the first light source LED 1. This means that a relatively low light output is provided, i.e. a dimmed light output.

[0069] At the second discrete dimming level, the controller 2 can close the first switch Ml and close the second switch M2. The driver 1 can then provide a current to the light source corresponding to the sum of the first current and the second current. Preferably, the first light source LED 1 and the second light source LED 2 have the same forward voltage. This allows the current provided by the driver 1 to be distributed between the light sources without the need for additional regulation. In this example, the first current flows through the first light source LED 1 and the second current flows through the second light source LED 2. Preferably, the amplitude of the current to the first light source LED 1 is such that the first light source LED 1 operates at its optimal current density, and the amplitude of the current to the second light source LED 2 is such that the second light source LED 2 operates at its optimal current density.

[0070] At the third discrete dimming level, the controller 2 can open the first switch Ml and close the second switch M2. By opening the first switch Ml and closing the second switch M2, the controller 2 allows the second current to flow through the second light source LED 2. The driver 1 determines the amplitude of the second current, and thereby the current density present in the second light source LED 2. Preferably, the second current is set to an amplitude that allows the current density in the first light source LED 2 to be at its optimal current density, to allow the first light source LED 2 to emit light in its most efficient manner. The third discrete dimming level allows the lighting device to emit light with only the second light source LED 2. This means that a relatively low light output is provided, i.e. a dimmed light output.

[0071] At the first discrete dimming level, the first light source LED1 is powered by the driver 1 and the second light source LED2 is not powered by the driver 1. At the third discrete dimming level, the first light source LED1 is not powered by the driver 1 and the second light source LED2 is powered by the driver 1. At the second discrete dimming level, the first light source LED1 and the second light source LED2 are powered by the driver 1. The first and second dimming levels can be used for dimming purposes. The third dimming level can be used to provide a different desired effect. The first light source LED1 and the second light source LED2 can have the same forward voltage, but can provide for example different color temperatures. The first light source LED1 can be an LED providing warm white light and the second light source can be an LED providing cool white light. The LEDs can for example be blue LEDs with a phosphor layer converting the blue light to warm white light or cool white light. At the first discrete dimming level, the first light source LED1 is powered, so the lighting device generates warm white light at a dimmed light output. At the third discrete dimming level, the second light source LED1 is powered, so the lighting device generates cool white light at a dimmed light output. At the second discrete dimming level, the first light source LED1 and the second light source LED2 are powered. The lighting device generates a combination of warm white light and cool white light.

[0072] Conventionally, to provide a dimmed light output, a lower (first) current is provided to both light sources. This results in a drop in current density in both light sources, effectively reducing the efficiency of both light sources. It is the insight of the inventor that by simultaneously reducing the number of light sources and the current, the current density remains approximately the same, allowing dimming to be performed efficiently.

[0073] Preferably, the light output generated by the lighting device at the second discrete dimming level is higher than the light output at the first discrete dimming level.

[0074] In the provided example, two light sources are provided. More light sources can be provided connected in parallel to the first light source LED1 and / or the second light source LED2. These additional light sources can be provided with their own corresponding series connection switch. More light sources can allow for the introduction of more discrete dimming levels, which can increase the resolution in dimming steps. The driver 1 can be adapted to provide a corresponding current for each of the discrete dimming levels. The more discrete dimming levels, the more current levels the driver 1 can need to provide. Therefore, the driver 1 can be adapted to provide a plurality of discrete current levels to provide current to the light sources corresponding to the discrete dimming levels.

[0075] Figure 3A graph showing the relationship between dimming level and current through the powered light source according to another example is shown. In this example, the driver 1 is adapted to provide a current that can be increased based on the dimming level. The driver 1 can increase or decrease the current to the light source. This can impact the efficiency of the light source, as a change in current will cause the current density to deviate from the optimal current density. The driver 1 can be arranged to limit the current change so that not too much efficiency is lost in the light source. Preferably, the deviation of the current through the light source from the maximum current provided to the light source can not exceed 50%. This means that the current through the at least one light source can be varied between 100% and 50% of the current.

[0076] The way the discrete dimming levels are used is different from the previous example. The discrete dimming levels are used to determine whether additional light sources need to be activated or whether another light source is to be prevented from being powered. The controller 2 determines the discrete dimming level based on the received dimming level. The controller 2 determines how many light sources to activate based on the determined discrete dimming level. In the provided example, the controller 2 determines which switches to close and which switches to open.

[0077] In Figure 3 eight light sources are used. The eight light sources form eight parallel channels and preferably have approximately the same forward voltage. Preferably, they also have approximately the same current requirement. The dimming level is represented as a DALI dimming level ranging from 0 to 253. In this example, the dimming curve is a logarithmic dimming curve. This can provide the benefit that the dimming behavior of the lighting device follows the behavior of the human eye. The human eye has a logarithmic sensitivity to changes in light intensity. At a dimming level of 0, the controller 2 can configure that no light source is connected to the driver 1. No power is provided to the light source, so the driver 1 can be configured to not provide a current to the light source.

[0078] Between the dimming levels of 1 and 175, a single light source is provided with current by the driver. The current starts at a minimum current of zero mA and gradually increases to 85 mA. In this example, the dimming level is used by the driver 1 to determine the increasing current to the light source and by the controller 2 to determine how many light sources to provide. In this case, the single light source that is active can not be operated at or close to its optimal current density, as the current of this single light source varies from 0 mA to 85 mA. To further optimize the efficiency, a trade-off can be made on the dimming level. As an example, a current below 42.5 mA (50% of the total current) can not be provided so that not too much impact is made on the efficiency of the light source. However, if the maximum current of 85 mA corresponds to the optimal current density in the active light source, the impact of reducing the current and thus the current density in the light source will be less than the impact of for example increasing the current and thus raising the current density above the optimal current density.

[0079] Between dimming levels of 176 and 205, the controller 2 can decide that another discrete dimming level has been reached, thus allowing current from the driver 1 to be provided to an additional light source. In this example, two light sources can be powered. As there are now two, rather than one, light sources receiving current from the driver 1, the current through the light sources will approximately halve, as can be seen in the steps at dimming level 176, where the current for each light source drops to approximately 45mA. However, the driver 1 is still providing 85mA. Increasing the dimming level to 205 allows the driver 1 to increase the current in the light sources further back to 85mA. The driver 1 can provide a total current of 170mA at the dimming level of 205.

[0080] Between dimming levels of 206 and 216, the controller 2 can decide that another discrete dimming level has been reached. In this example, three light sources can be powered. As there are now three light sources powered by the driver 1, the total current provided by the driver 1 is divided between the three light sources. Then at the dimming level of 206, 170mA will result in approximately 57mA for each light source. Increasing the dimming level to 216 allows the driver 1 to increase the current in the light sources further back to 85mA. Then, the driver 1 will provide 255mA for all active light sources.

[0081] Between dimming levels of 217 and 226, the controller 2 can decide that another discrete dimming level has been reached. In this example, four light sources can be powered. As there are now four light sources powered by the driver 1, the total current provided by the driver 1 is divided between the four light sources. Then at the dimming level of 217, 255mA will result in approximately 64mA for each light source. Increasing the dimming level to 226 allows the driver 1 to increase the current in the light sources further back to 85mA. Then, the driver 1 will provide 340mA for all active light sources.

[0082] Between dimming levels of 227 and 234, the controller 2 can decide that another discrete dimming level has been reached. In this example, five light sources can be powered. As there are now five light sources powered by the driver 1, the total current provided by the driver 1 is divided between the five light sources. Then at the dimming level of 227, 340mA will result in approximately 68mA for each light source. Increasing the dimming level to 234 allows the driver 1 to increase the current in the light sources further back to 85mA. Then, the driver 1 will provide 425mA for all active light sources.

[0083] Between the dimming levels of 235 and 242, the controller 2 can decide that another discrete dimming level has been reached. In this example, six light sources can be powered. Since now six light sources are powered by the driver 1, the total current provided by the driver 1 is distributed among the six light sources. Then at the dimming level of 235, 425mA will result in approximately 71mA per light source. Increasing the dimming level to 242 allows the driver 1 to further increase the current in the light sources back to 85mA. Then, the driver 1 will provide 510mA for all active light sources.

[0084] Between the dimming levels of 243 and 248, the controller 2 can decide that another discrete dimming level has been reached. In this example, seven light sources can be powered. Since now seven light sources are powered by the driver 1, the total current provided by the driver 1 is distributed among the seven light sources. Then at the dimming level of 243, 510mA will result in approximately 73mA per light source. Increasing the dimming level to 248 allows the driver 1 to further increase the current in the light sources back to 85mA. Then, the driver 1 will provide 595mA for all active light sources.

[0085] Between the dimming levels of 249 and 253, the controller 2 can decide that another discrete dimming level has been reached. In this example, eight light sources can be powered. Since now eight light sources are powered by the driver 1, the total current provided by the driver 1 is distributed among the eight light sources. Then at the dimming level of 249, 595mA will result in approximately 74mA per light source. Increasing the dimming level to 253 allows the driver 1 to further increase the current in the light sources back to 85mA. Then, the driver 1 will provide 680mA for all active light sources. At this discrete dimming level, all eight light sources are active. At the dimming level of 253, all light sources are provided with 85mA, thus, the luminaire will emit the possible maximum light.

[0086] In the provided example, eight light sources are used. It is clear that this is only one example, where other examples can use more or less light sources, preferably at least two light sources.

[0087] Figure 4An example of a lighting device is shown. The lighting device has a driver circuit. The driver circuit has a driver 1 which is arranged to supply power to a first light source LED1 and a second light source LED2. The first light source LED1 is coupled in series with the second light source LED2. The series configuration of the first light source LED1 and the second light source LED2 is coupled between the outputs of the driver 1. A first switch M1 is coupled in parallel with the first light source LED1. A second switch M2 is coupled in parallel with the second light source LED2. When closed, the switches can act as shunt switches, effectively shunting the corresponding light source. The driver 1 is arranged to provide a current to the light sources. A controller 2 is provided to control the first switch M1 and the second switch M2. Preferably, the controller 2 can also be used to control the driver 1. This allows one controller 2 to provide control for the first switch M1 and the driver 1. Alternatively, the controller 2 comprises multiple components for different purposes, for example controlling the first switch M1 separately from the driver 1. The controller 2 can be an integral part of the driver. The driver 1 can be arranged to receive a mains voltage Mains. The mains voltage Mains is an AC voltage, and can therefore be rectified by a rectifier circuit. The rectifier circuit can be part of the driver. The controller 2 can receive a dimming signal from an external device which is arranged to provide the dimming signal. Examples of dimming signals are phase-cut dimming signals, 0 to 10 V dimming signals, DALI dimming signals, DMX dimming signals or wireless dimming signals. The controller 2 receives the dimming signal and converts the dimming signal to discrete dimming levels. This means that the controller 2 translates the dimming signal to dimming steps. Preferably, the number of dimming steps is based on the number of light sources provided when the first light source LED1 and the second light source LED2 provide similar light output. In the provided example, two light sources are provided. There are three discrete dimming steps available. One discrete dimming step can be set to 0% output power, another discrete dimming step can be set to 33% output power, another discrete dimming step can be set to 66% output power, and another discrete dimming step can be set to 100% output power. In this case, the driver 1 can be configured to not generate a current, such that in the dimming step of 0% output power no light source is powered.

[0088] The controller 2 can be arranged to control the first switch M1 and the second switch M2 based on the discrete dimming levels. The driver 1 can be arranged to adjust the current to the light sources based on the discrete dimming levels. The driver 1 can receive the discrete dimming signal from the controller 2. The teachings provided in the example of light sources coupled in parallel can also be applied to the teachings in this example of light sources coupled in series.

[0089] When the first switch Ml is open and the second switch M2 is closed, the driver 1 provides the first current to the first light source LEDl. The second light source LED2 is shunted by the second switch M2, so no current flows through the second light source LED2. Thus, opening the first switch Ml and closing the second switch M2 can be implemented at a first discrete dimming level.

[0090] When the first switch Ml is open and the second switch M2 is open, the driver 1 provides the first current to the first light source LEDl and the second current to the second light source LED2. The first light source LEDl and the second light source LED2 are not shunted by the first switch Ml and the second switch M2, respectively, so current flows through the first light source LEDl and the second light source LED2. Thus, opening the first switch Ml and opening the second switch M2 can be implemented at a second discrete dimming level.

[0091] When the first switch Ml is closed and the second switch M2 is open, the driver 1 provides the second current to the second light source LED2. The first light source LEDl is shunted by the first switch Ml, so no current flows through the first light source LEDl. Thus, closing the first switch Ml and opening the second switch M2 can be implemented at a third discrete dimming level.

[0092] In this example, it can be desirable that the first current and the second current are the same.

[0093] Figure 5 An example is shown of an implementation of a lighting device. The light sources in the lighting device can be provided with their own optics, allowing specific beam angles to be provided. In this example, four beam angles are provided, namely, φ1, φ2, φ3 and φ4. Each beam angle is generated with a different combination of light sources. In this example, four light sources can be used, each for a corresponding beam angle. Each beam angle can also correspond to a discrete dimming level.

[0094] As an example, at a first discrete dimming level, a first current can be provided to a first light source LEDl. This would then result in a light output with a beam angle of φ1. At a second discrete dimming level, a second current can be provided to a second light source LED2. This would then result in a light output with a beam angle of φ2. At a third discrete dimming level, a third current can be provided to a third light source. This would then result in a light output with a beam angle of φ3. At a fourth discrete dimming level, a fourth current can be provided to a fourth light source. This would then result in a light output with a beam angle of φ4.

[0095] Other combinations are possible. As an example, at the fourth, third or second discrete dimming level, all or part of the other light source can be powered, resulting in a light output with a beam angle of φ4, φ3 or φ2, but due to all or part of the other light source also being provided with a corresponding current, the light output at other angles is also increased. Preferably, the amplitude of the current provided by the driver 1 to the light source allows the light source to be powered at its optimal current density. In this example, the dimming signal can be used to provide different beam angles of the light output of the lighting device according to the discrete dimming levels. Additionally, the dimming signal can be used to vary the amount of light generated by the lighting device according to the discrete dimming levels.

[0096] In the provided example, the driver 1 can provide the current to the light source in different ways. To keep the current at a level that realizes the optimal current density in the first light source LED 1 and / or the second light source LED 2, while allowing the light source to emit less light, the current can be pulse width modulated. Thus, the amplitude of the current to the light source is not altered, but the average current to the light source is varied based on the duty cycle of the current. This effectively reduces the light output of the light source without affecting the efficiency of the light source. Preferably, the frequency of the PWM current is higher than the frequency perceptible by the human eye, for example 200 Hz. More preferably, the frequency is higher than 1 kHz.

[0097] In the provided example, the driver can be provided as a switched mode power supply. Examples of switched mode power supplies are a boost converter, a buck converter, a buck-boost converter, a flyback converter or a resonant converter.

[0098] In the provided example, for simplicity, the discrete dimming levels can be evenly distributed over the entire dimming range. It is to be understood that this is only one option to convert the dimming levels to discrete dimming levels. Alternatively, the discrete dimming levels can be distributed such that in a low dimming sub-range, more discrete dimming levels are provided than in a high dimming sub-range, or vice versa.

[0099] In the provided example, the dimming level can be linearly related to the power required by the light source. Other relationships, such as logarithmic or non-linear, can also be envisaged and result in the desired effect.

[0100] As a definition of the dimming level, it is understood how much power the driver circuit needs to provide to the light source. At a dimming level of 100%, it is expected that the driver circuit provides 100% of its rated power. At a dimming level of 0%, it is expected that the driver circuit provides 0% of its rated power. Dimming levels between 100% and 0% can be scaled linearly or non-linearly from the rated power of the driver circuit. In linear scaling, a dimming level of 50% can be related to 50% of the rated power of the driver circuit. In non-linear scaling, a dimming level of 50% can for example be related to 25% of the rated power of the driver circuit.

[0101] In the provided example, the controller 2 is arranged to prevent current to flow to any light source. It is clear that the controller 2 can achieve this by using the switches provided in the example.

[0102] In the provided example, the first light source LED1 and / or the second light source LED2 can be a semiconductor light source. Examples of semiconductor light sources are LEDs, laser diodes and vertical cavity surface emitting lasers, VCSELs. Preferably, the LEDs are formed as filaments.

[0103] In the provided example, the discrete dimming levels can be translated to current amplitudes generated by the driver 1. The driver 1 generates this current and provides it to the light sources. The controller 2 senses the amplitude of the provided current and determines, based on this sensing, the amount of light sources that need to be connected to the driver 1 so that the current provided by the driver 1 is properly distributed between the light sources.

[0104] In the provided example, the dimming signal can be a digital signal. The digital signal can for example be derived from a DALI signal. When using multiple lighting devices, the discrete dimming levels of each lighting device will be similar to each other, so lighting devices that receive the same digital dimming command will generate a similar light output. When an analog dimming signal is provided, the tolerances of the electronic components can provide a deviation in light output between the lighting devices.

[0105] 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. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A lighting device comprising: a first light source (LED1); a second light source (LED2), and a driver circuit comprising: a driver (1) adapted to provide a first current to the first light source (LED1) and to provide a second current to the second light source (LED2); a controller (2) for controlling the driver (1); wherein the controller (2) is arranged to receive a dimming signal, the dimming signal being indicative of a dimming level for a light output of a light source, wherein the controller (2) is arranged to convert the received dimming signal into a discrete number of dimming levels, wherein each discrete dimming level corresponds to an amount of current to be provided by the driver (1) to the first light source (LED1) and / or the second light source (LED2), wherein: at a first discrete dimming level, the controller (2) is arranged to allow the first current to be provided to the first light source (LED1) and to prevent the second current to be provided to the second light source (LED2), wherein the first current has an amplitude to provide a current density in the first light source (LED1) such that the first light source (LED1) generates light with a predetermined efficiency; at a second discrete dimming level, the controller (2) is arranged to allow the first current to be provided to the first light source (LED1) and the second current to be provided to the second light source (LED2).

2. The lighting device according to claim 1, wherein at a third discrete dimming level, the controller (2) is arranged to prevent the first current to be provided to the first light source (LED1) and to provide the second current to the second light source (LED2).

3. The lighting device according to any one of the preceding claims, wherein between two discrete dimming steps, the driver (1) is arranged to provide a variable current to only one of the first light source (LED1) or the second light source (LED2) based on the dimming signal, while maintaining the current through the other one of the first light source (LED1) or the second light source (LED2) constant.

4. The lighting device according to any one of the preceding claims, wherein the second current has an amplitude to provide a current density in the second light source (LED2) such that the second light source (LED2) generates light with a predetermined efficiency.

5. The lighting device according to any one of the preceding claims, wherein the first light source (LED1) and the second light source (LED2) are coupled in a parallel configuration.

6. The lighting device according to claim 5, wherein the driver circuit comprises a first switch (M1) coupled in series with the second light source (LED2), wherein the controller (2) is arranged to open and close the first switch (M1), and wherein when the first switch (M1) is open, the first current flows through the first light source (LED1), and wherein when the first switch (M1) is closed, the second current flows through the second light source (LED2).

7. The lighting device according to any one of the claims 1 to 5, wherein the driver circuit comprises a first switch (Ml) coupled in series with the first light source (LEDl) and a second switch (M2) coupled in series with the second light source (LED2), wherein the controller (2) is arranged to open and close the first switch (Ml) and the second switch (M2), wherein the first current flows through the first light source (LEDl) when the first switch (Ml) is closed, and wherein the second current flows through the second light source (LED2) when the second switch (M2) is closed.

8. The lighting device according to any one of the preceding claims, wherein the driver circuit comprises a series combination of a first switch (Ml) and a second switch (M2) between outputs of the driver (1), wherein the first light source (LEDl) is coupled in parallel with the first switch (Ml) and the second light source (LED2) is coupled in parallel with the second switch (M2).

9. The lighting device according to any one of the claims 6 to 8, wherein the controller (2) is arranged to sense a current provided by the driver (1), wherein the controller (2) is arranged to control the first switch (Ml) and / or the second switch (M2) based on an amplitude of the current provided by the driver (1).

10. The lighting device according to any one of the preceding claims, wherein a forward voltage of the first light source (LEDl) is less than a forward voltage of the second light source (LED2).

11. The lighting device according to any one of the preceding claims, wherein at the first discrete dimming level, the driver (1) is arranged to provide the first current to the first light source (LED2) such that a current density through the first light source (LEDl) allows the first light source (LEDl) to emit light close to a highest efficiency of the first light source (LEDl).

12. The lighting device according to any one of the preceding claims, wherein at the second discrete dimming level, the driver (1) is arranged to: provide the first current to the first light source (LEDl) such that a current density through the first light source (LEDl) allows the first light source (LEDl) to emit light close to a highest efficiency of the first light source (LEDl), and provide the second current to the second light source (LED2) such that a current density through the second light source (LED2) allows the second light source (LED2) to emit light close to a highest efficiency of the second light source (LED2).

13. The lighting device according to any one of the preceding claims, wherein the first light source (LEDl) generates a color or color temperature different from a color or color temperature of the second light source (LED2).

14. The lighting device according to any one of the preceding claims, further comprising at least three light sources, wherein a number of dimming levels is based on a number of light sources. ​ ​ 15. The lighting device according to any one of the preceding claims, wherein light generated by the first light source (LED1) is emitted at a first surface and light generated by the second light source (LED2) is emitted at a second surface, wherein the first surface at least partially does not overlap with the second surface.

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