Light emitting diode, LED, based lighting device arranged for emitting light of a specific color and corresponding method

Abstract

A light emitting diode, LED, based lighting device arranged for emitting a specific color of light, wherein the LED based lighting device comprises: a power supply unit arranged for providing a direct current, DC, bus voltage for powering the LEDs; a plurality of parallel cascaded LED channels, wherein each of the LED channels is connected to the bus voltage and comprises at least one colored LED and a switch for activating the corresponding LED channel; a controller arranged for providing a control signal to each of the switches in the LED channels to periodically activate the LED channels, wherein each of the control signals has a duty cycle, and wherein the controller is arranged for determining the duty cycle of the control signals based on a received color set point, wherein the controller is further arranged for determining an amount of deficiency of light output of each of the LED channels caused by parasitic effects in the LED based lighting device by determining an instantaneous current of each channel and comparing the instantaneous current with an expected current resulting from the determined duty cycle, and for increasing the duty cycle based on the determined amount of deficiency.

Description

TECHNICAL FIELD

[0001] The present invention relates generally to the field of lighting, and more specifically to a light emitting diode (LED) based lighting device arranged to emit a specific color of light. The present invention also relates to a method of operating an LED based lighting device. BACKGROUND

[0002] Lighting devices utilizing light emitting diodes, LEDs, have been developed for various lighting applications. Due to the long lifetime and high energy efficiency of LED lamps, LED lamps are now also designed for replacing traditional fluorescent lamps, i.e. for retrofit applications. For such applications, the retrofit LED lamps are typically adapted to fit into the lamp holder of the respective lamp to be retrofitted. Furthermore, since the maintenance of the lamps is typically performed by the user, the retrofit LED lamps should ideally be easy to operate with any type of suitable fixture without the need for rewiring the fixture.

[0003] The present disclosure relates to a multi-channel LED based lighting device. Each channel can comprise a plurality of LEDs capable of emitting a specific color of light. For example, a first channel can be directed to emit red light. A second channel can be directed to emit green light, and a third channel can be directed to emit blue light.

[0004] In such lighting devices, a fixed voltage source can be used to power the LEDs in each channel. The current through each channel can be set in the factory by tuning a resistor placed in series with the LEDs of a specific channel. One of the drawbacks of this approach relates to several disturbing factors, such as voltage variations of the power supply, cable length (i.e. impedance), interaction between channels, the latter possibly leading to errors in the target flux and color point.

[0005] More specifically, typically, there is a controller for controlling a plurality of switches, where each switch is arranged to enable a specific channel. For example, a first switch can enable a red channel, a second switch can enable a green channel, a third switch can enable a blue channel, etc. The switches can be provided with a pulse width modulation, PWM, signal having a specific duty cycle. The frequency of the PWM signal should be chosen to exceed the refresh rate of the human eye. This will prevent the user from seeing any flickering. By controlling the duty cycle, it is possible to control the contribution of each channel to the total amount of emitted light, and thus also the color of the light emitted by the LED based lighting device.

[0006] Typically, the user can express or input a color that he / she wants the LED based lighting device to emit. As mentioned above, the present inventors have found that there can be a number of disturbing factors that prevent the use of a static duty cycle for each PWM signal fed to the switches. SUMMARY

[0007] It is advantageous to implement a light emitting diode (LED) based lighting device arranged for emitting a specific color of light. It is also advantageous to implement a corresponding method.

[0008] In a first aspect, a light emitting diode, LED, based lighting device arranged for emitting a specific color of light is provided. The LED based lighting device comprises:

[0009] - a power supply unit arranged for providing a direct current, DC, bus voltage for powering the LEDs;

[0010] - a plurality of parallel cascaded LED channels, wherein each of the LED channels is connected to the bus voltage and comprises at least one colored LED and a switch for activating the corresponding LED channel;

[0011] - a controller arranged for providing a control signal to each of the switches in the LED channels for periodically activating the LED channels, wherein each of the control signals has a duty cycle, and wherein the controller is arranged for determining the duty cycle of the control signals based on a received color setpoint;

[0012] wherein the controller is further arranged for determining an amount of deficiency of light output of each of the LED channels caused by parasitic effects in the LED based lighting device by determining an instantaneous current of each channel and comparing the instantaneous current with an expected current resulting from the determined duty cycle, and for increasing the duty cycle based on the determined amount of deficiency.

[0013] The inventors have found that a number of parasitic components that can be present anywhere within the circuitry of an LED based lighting device can contribute in a negative way to the accuracy of the color of light emitted by the LED based lighting device. That is, the difference between the color setpoint and the actual colored light emitted by the LED based lighting device is increased by the parasitic components.

[0014] When multiple channels are active at the same time, the parasitic categories can dominate, such that a large current is drawn from the power supply. Typically, when an LED based lighting device is manufactured, each LED channel is calibrated once. This calibration can be done in the factory. During calibration, a single LED channel can be activated and calibrated. Hence, the calibration can not take into account the activation of multiple channels at the same time.

[0015] For example, the bus series resistance, which can represent the resistance of the cable between the power supply unit and the actual LED-based lighting device, can play a dominant role in the obtained errors when multiple channels are activated at the same time. That is, a large current can cause a significant voltage drop over the bus series resistance, thereby reducing the bus voltage. The DC bus voltage can become lower than expected.

[0016] The current through each channel can be set by a current resistor. The duty cycle set for a particular channel depends on the amount of current expected to flow through the particular channel. When the bus voltage is lower than expected, the amount of current can deviate from the expected current, and therefore the voltage over the current resistor is also lower than expected. This will result in a reduced amount of current flowing through the channel when the corresponding switch is activated. The result is that the light emitted by the corresponding channel is less than the light actually expected. If the light emitted by a particular channel is less than expected, this can also result in a color shift in the case of a system with multiple channels.

[0017] According to the present disclosure, the above-mentioned situation is handled by increasing the duty cycle of the control signal to the corresponding switch.

[0018] In other words, in addition to the variations caused by the nominal bus voltage spread, transient voltage fluctuations can also have a severe impact on the flux and color spread of the LED-based lighting device. These voltage fluctuations are typically load-dependent and can therefore act as a function of the enabled LED channels. Within each cycle, which is typically 1 kHz, a combination of multiple channels can be active, all of which can have different on-times, resulting in various current plateaus.

[0019] When many channels are enabled, the load current is highest and therefore the impact on the power supply unit is most severe, which results in higher bus voltage fluctuations. In addition to the power supply behavior, parasitic components such as the bus series resistance can also cause a higher voltage drop, from which more current will be required from the power supply unit. The behavior of the power supply unit and the value of the parasitic components can be unknown to the controller. The controller can expect measurements of the individual channel currents, which are collected for example during a start-up or factory process, which are simply added together when more channels are enabled. Due to the effects of bus voltage fluctuations and parasitic component losses, this can not be the case. As a result, these errors introduce color and flux deviations.

[0020] According to the present disclosure, the controller is further arranged to determine an amount of deficiency of light output of each of the LED channels caused by parasitic effects in the LED-based lighting device by determining an instantaneous current through each channel and comparing the instantaneous current to an expected current resulting from the determined duty cycle, and to increase the duty cycle based on the determined amount of deficiency.

[0021] It is noted that due to parasitic effects in the LED-based lighting device, a deficiency in light output occurs in any of the LED channels. This deficiency can have multiple effects. For example, the overall brightness of the LED-based lighting device can be reduced. Another option is to change the color set point in case for example only one LED channel is affected by the parasitic effects.

[0022] In one example, the controller is arranged for:

[0023] - determining a measure related to the determined instantaneous current of each channel multiplied by the corresponding duty cycle on-duty time of each channel;

[0024] - comparing the measure to an expected measure related to the determined instantaneous current of each channel multiplied by the corresponding duty cycle on-duty time of each channel;

[0025] - for each channel, determining an increase of the duty cycle such that the determined measure will be substantially equal to the expected measure;

[0026] - increasing the corresponding duty cycle for each channel.

[0027] The above example is directed to a situation in which the controller aims to ensure that the total amount of current flowing through a particular channel is substantially equal to an expected total amount of current.

[0028] For example, consider a situation in which the controller expects a current of 35 mA through a particular channel during the on-period of a pulse width modulated, PWM, signal. However, due to all kinds of parasitic losses, the actual obtained current is not 35 mA, but for example 27 mA. The on-period of the PWM signal is for example 1.9 ms, so the controller expects 35 mA to flow through the particular channel in 1.9 ms. However, in reality, 27 mA flows for 1.9 ms, which actually results in less illumination of the LED in the corresponding channel.

[0029] To compensate for the above problem, the inventors have realized that it can be difficult to increase the amount of current. This is caused by parasitic aspects, which cause a bus voltage drop, which in turn causes a reduced amount of current. To overcome the above problem, the inventors have found that increasing the duty cycle, i.e. increasing the on-time of the control signal. In this example, the on-time can be increased to approximately 2.4 ms, such that the total amount of current through the corresponding channel remains substantially the same.

[0030] In one example, the controller is arranged for determining the instantaneous current by:

[0031] - measuring a voltage over a sense resistor comprised by any of the plurality of parallel connected LED channels.

[0032] As mentioned above, the present disclosure relates to the idea of compensating for a decrease in the amount of instantaneous current caused by parasitic aspects by increasing the duty cycle of the corresponding control signal. The decrease in the amount of instantaneous current can be determined in several ways. The present example determines the amount of instantaneous current by measuring the voltage over one or more sense resistors comprised by the LED-based lighting device, for example sense resistors in the supply line towards the plurality of LED channels or in the return line from the plurality of LED channels. Another option is to measure the voltage over a sense resistor (e.g. a current set resistor) present in the actual plurality of LED channels.

[0033] In another example, the controller is arranged for determining the instantaneous current for each channel by:

[0034] - measuring the current flowing through the respective channel for at least two different DC bus voltages, and

[0035] - measuring the DC bus voltage and determining the respective current flowing through the respective channel by interpolating the measurements of the at least two different DC bus voltages.

[0036] The controller can not be aware of the actual forward voltage drop of the LEDs in a particular channel, and thus also not of the value of any resistor in a particular channel for setting the current flowing through the channel. However, the controller can estimate or interpolate these aspects by measuring the current flowing through the respective channel for at least two different DC bus voltages. This can for example be performed in the factory.

[0037] Using the obtained values, the current through a particular channel can be estimated by interpolating the measurements of the at least two different DC bus voltages. After the above, during operation of the LED-based lighting device, the controller can determine the amount of instantaneous current flowing through a particular channel by measuring the DC bus voltage.

[0038] In yet another example, the controller is arranged for:

[0039] - providing a control signal to each of the switches in the LED channel to periodically activate the LED channel, wherein each of the control signals has a duty cycle, and wherein the controller is arranged for determining the duty cycle of the control signal based on a received color setpoint for a low lumen output, and

[0040] - scaling the low lumen output to a high lumen output by increasing the duty cycle based on the determined deficiency.

[0041] In the above, the two reference steps can be performed one after the other. That is, first the providing step is performed, and subsequently the scaling step. This is explained in more detail below.

[0042] The controller can first determine a ratio of the duty cycles of each of the control signals for controlling each of the plurality of LED channels. The ratio of the control signals can be important for achieving the correct color setpoint. At this stage, the total lumen output of the LED-based lighting device can be intentionally kept low. The goal here is to have the correct ratio between the different duty cycles of the control signals. By intentionally keeping the total amount of lumen output low, the negative effects of the parasitic aspects are reduced.

[0043] The next step involves scaling. That is, the total amount of lumen output will be increased. As mentioned above, due to the increase in the total amount of current to be provided by the power supply unit, the parasitic aspects can play a more dominant role in this step.

[0044] The above requires an increase of the duty cycles to provide the total amount of lumen output. However, the controller can thus also compensate each duty cycle individually based on the parasitic aspects, and thereby also the ratio between the duty cycles. In this step, the controller can thus be further arranged for determining an amount of light output of each of the LED channels caused by parasitic effects in said LED-based lighting device by determining an instantaneous current of each channel and comparing said instantaneous current with an expected current resulting from the determined duty cycle, and for increasing said duty cycle based on the determined amount of deficiency.

[0045] In a further example, the LED-based lighting device further comprises:

[0046] - a memory comprising, for each channel, a relationship between:

[0047] - the total bus voltage or current flowing through said respective channel, and

[0048] - the light intensity emitted by said at least one respective colored LED of said respective channel;

[0049] wherein the controller is further arranged for determining the DC bus voltage and / or the current flowing through said respective channel and for controlling each of the LED channels to emit said specific color of light taking into account said relationship.

[0050] Note that each LED channel can comprise a current control element for controlling the amount of current flowing to the respective channel, wherein the current control element controls the amount of current through the respective channel based on the DC bus voltage.

[0051] The current control element can be a tuning resistor for tuning the resistance value of the corresponding channel.

[0052] The inventors have realized that the light intensity of a channel is related to the current flowing through that particular channel. There is a current control element for ensuring that a predetermined amount of current flows through the channel. However, the value of the current control element can be determined based on a nominal, i.e. standard, DC bus voltage. Variations occurring in the DC bus voltage are thus not taken into account. These variations can therefore result in different light intensities for a particular channel.

[0053] It is further noted that the forward voltage of the LED in each channel can be different. In this way, a red LED can have a different forward voltage than a green LED and than a blue LED. It is thus possible that the current control element is different for each channel, and it is thus possible that variations in the DC voltage bus will have a different impact on each channel.

[0054] The inventors have found that it can be beneficial if the controller, i.e. the controller controlling each channel, takes the above aspects into account. More specifically, for compensating aspects, the controller can use the DC bus voltage to compensate the light intensity characteristic of each channel, or use the current flowing through the channel to compensate the light intensity characteristic of each channel.

[0055] That is, the controller can use the above information to control each channel to a specific color of light.

[0056] According to the present disclosure, the power supply unit can be arranged for receiving a mains input supply voltage, e.g. 230 Vac or any similar voltage, and can be arranged for converting this mains input supply voltage to a DC bus voltage for powering the LED in each channel.

[0057] According to the present disclosure, the memory can be a read-only memory, ROM, a random access memory, RAM, a cache or any similar memory.

[0058] According to the present disclosure, the controller can for example be a microcontroller or any other control device, e.g. a microprocessor, a field programmable gate array, FPGA, or any similar device. For example, the microcontroller can receive the relevant input signals at some available input pins, and can provide the output control signals at other available output pins.

[0059] It is noted that for each channel, the memory can comprise a relation between a bus voltage or current flowing through the respective channel and an intensity of light emitted by the at least one corresponding colored LED of the respective channel. This is to be understood broadly. Typically, the relation relates to light output by a channel and an electrical characteristic of that channel. This can be expressed in several ways. For example, a bus voltage versus current characteristic, or any similar characteristic. Since the current through a channel is indicative of the light emitted by that channel, the relation can also be provided indirectly.

[0060] According to the present application, a colored LED is an LED that emits a specific color, such as white, blue, green, red, etc.

[0061] In one example, the controller is arranged for determining the current flowing through the respective channel by:

[0062] - measuring the DC bus voltage and calculating the current by taking into account the measured DC bus voltage, a nominal current flowing through the channel, and an LED forward voltage of each of the LEDs in the channel.

[0063] The controller can further be arranged for measuring an LED forward voltage of an LED present in the plurality of LED channels.

[0064] In another example, the controller is further arranged for measuring an ambient temperature, and wherein the controller is arranged for controlling each of the LED channels for emitting the specific color of light taking into account the relation and the temperature.

[0065] In a second aspect of the present disclosure, a method of operating a light emitting diode, LED, based lighting device is provided. The method comprises the following steps:

[0066] - providing, by the power supply unit, a DC bus voltage for powering the LEDs;

[0067] - providing, by the controller, a control signal to each of the switches in the LED channels for periodically activating the LED channels, wherein each of the control signals has a duty cycle, and

[0068] - determining, by the controller, the duty cycle of the control signals based on the received color setpoint;

[0069] - determining, by the controller, the deficiency in light output of each of the LED channels caused by parasitic effects in the LED based lighting device, and

[0070] - increasing, by the controller, the duty cycle based on the determined deficiency.

[0071] It should be noted that the advantages and limitations disclosed in relation to the embodiments of the first aspect of the application also correspond to the embodiments of the second aspect of the application, i.e. the method for operating an LED-based lighting device.

[0072] In one example, the method further comprises the steps of:

[0073] - determining, by the controller, a measure related to the determined instantaneous current of each channel multiplied by the corresponding duty cycle on time of each channel;

[0074] - comparing, by the controller, the measure to an expected measure related to the determined instantaneous current of each channel multiplied by the corresponding duty cycle on time of each channel;

[0075] - determining, by the controller, for each channel, an increase of the duty cycle such that the determined measure will be substantially equal to the expected measure;

[0076] - increasing, by the controller, for each channel, the corresponding duty cycle.

[0077] In a third aspect, there is provided a computer readable medium having stored thereon instructions which, when executed by a controller of an LED-based lighting device, cause the LED-based lighting device to perform the method according to any of the examples provided above.

[0078] These and other aspects of the application will become apparent in view of the embodiments described below. BRIEF DESCRIPTION OF DRAWINGS

[0079] Figure 1 An LED-based lighting device according to the prior art is shown;

[0080] Figure 2 A flowchart of a method according to the present disclosure is shown;

[0081] Figure 3 A diagram illustrating the principles of the present disclosure is shown. DETAILED DESCRIPTION

[0082] Figure 1 An LED-based lighting device 1 according to the present application is shown.

[0083] Here, a power supply unit 9 is provided for generating a direct current, DC, bus voltage 7. The DC bus voltage 7 is typically about 24 volts DC, but can be in a range of any value. Typically, to prevent any hazardous situations, the DC bus voltage 7 is at least below about 50 V DC. An electromagnetic interference, EMI, filter can be placed close to the output of the power supply unit 9 for reducing any interference in the DC bus voltage 7.

[0084] In the present approach, the LED-based lighting device 1 comprises five channels, as indicated by reference numerals 2, 3, 4, 5, 6. Each of the channels 2, 3, 4, 5, 6 is arranged for emitting light having a specific color. For example, the channel indicated by reference numeral 2 is arranged for emitting red light, the channel indicated by reference numeral 3 is arranged for emitting green light, the channel indicated by reference numeral 4 is arranged for emitting blue light, the channel indicated by reference numeral 5 is arranged for emitting flame white light, and the channel indicated by reference numeral 6 is arranged for emitting cool white light.

[0085] Each LED of the different channels 2, 3, 4, 5, 6 can have different current requirements and can have different forward voltages. The forward voltage of an LED is defined as the voltage drop over that specific LED.

[0086] To achieve this, each of the channels 2, 3, 4, 5, 6 is equipped with a current control element for tuning the current through the channel. It is assumed that the DC bus voltage is nominally 24 V DC. The first channel, i.e. the channel indicated by reference numeral 2, can have six LEDs, each having a forward voltage of 3 V DC. This will accumulate a voltage drop of about 18 V DC over the LEDs. The remaining voltage, i.e. 24 V DC - 18 V DC, is 6 V DC, which is the voltage over the current control element. The resistor value can then be tuned to specify the current flowing through the channel.

[0087] There can be a controller 8 to control the channels 2, 3, 4, 5, 6. More specifically, the controller 8 can provide control signals to the respective switches of the channels 2, 3, 4, 5, 6 for enabling or disabling the respective channels 2, 3, 4, 5, 6 to achieve a specific desired color of the total light emitted.

[0088] Typically, these control signals are pulse width modulated, PWM, signals. The duty cycle of these PWM signals can be set by the controller for achieving that the LED-based lighting device emits a specific color of light. The ratio between the duty cycles of the control signals determines the specific color of light that is actually emitted.

[0089] Hence, the controller determines a duty cycle for each control signal. The controller is further arranged for determining an amount of deficiency in light output of each LED channel caused by parasitic effects in the LED-based lighting device by determining an instantaneous current for each channel and comparing the instantaneous current to an expected current resulting from the determined duty cycle, and for increasing the duty cycle based on the determined amount of deficiency.

[0090] The instantaneous current can be determined in a number of ways. For example, a Rsense resistor can be used to determine the total amount of current flowing through all LED channels combined. For example, using a number of calibrated bus voltages, the total amount of current can be split into individual currents through the channels active at that moment in time.

[0091] Another option is to measure the voltage over each current control element and determine the current through a particular channel by dividing the measured voltage by the resistance value of the respective current control element.

[0092] For example, the controller can determine a measured value related to the determined instantaneous current of each channel multiplied by the on-time of the respective duty cycle of each channel, compare the measured value to an expected measured value related to the determined instantaneous current of each channel multiplied by the on-time of the respective duty cycle of each channel, determine an increase of the duty cycle for each channel such that the determined measured value will be substantially equal to the expected measured value, and increase the respective duty cycle for each channel.

[0093] It is noted that the parasitic aspect of the invention can originate from a resistor, as indicated by "Rcable1". The length of the cable between the power supply unit 9 and the plurality of LED channels 2, 3, 4, 5, 6 can be modeled as a resistor. Such a resistor contributes to a voltage drop, such that the bus voltage 7 is lower than the expected bus voltage. This in turn results in a lower current through each LED channel 2, 3, 4, 5, 6.

[0094] Figure 2 A flowchart 51 of the method according to the invention is shown.

[0095] The flowchart 51 starts 52 with obtaining a new target XYZ 53. The new target XYZ 53 indicates a desired color setpoint of the LED-based lighting device. The desired color setpoint can relate to a specific temperature of a color, for example 4000 K, or can relate to a specific RAL color or any similar color.

[0096] The desired color setpoint is provided to a color algorithm 54 executed by the controller. The color algorithm 54 uses the desired color setpoint to determine a duty cycle for each control signal to control the plurality of switches present in each of the plurality of LED channels.

[0097] In a first example, the color algorithm 54 determines the ratio between the duty cycles, but ensures that the total amount of lumens emitted by the LED-based lighting device, i.e. the total amount of light, is relatively low, for example 1 lumen. In this way, only the ratio between the duty cycles is calculated, without calculating the intensity of the emitted light.

[0098] In a next step, the low lumen output is scaled to a high lumen output by increasing each duty cycle accordingly and remembering the ratio between the duty cycles. However, due to parasitic aspects as described above, the ratio between the duty cycles can change during this process.

[0099] In this process, the amount of light output per LED channel caused by parasitic effects in the LED-based lighting device is determined by determining the instantaneous current per channel and comparing it to the expected current resulting from the determined duty cycle, and used to increase the duty cycle based on the determined amount of deficiency.

[0100] The input of the color algorithm can thus be the target color and brightness, and can be the characteristics of the LEDs of the multiple LED channels. These characteristics are the color point and the flux per primary LED at a given driving current. These parameters can compensate for the temperature rise due to self-heating of the lamp. Note that in the electronic architecture discussed, the driving current through the LED string can vary due to fluctuations in the bus voltage.

[0101] In the case of a voltage-driven system, the target flux can be set to 1 lumen, so that the duty cycles between the different channels will be correct.

[0102] However, the brightness will thus be very low. This is the main reason why a post-processing step is introduced that scales the duty cycles to higher values. This ratio can be increased until the duty cycles are 100% or the power of the power supply is reached to prevent over-supply of the power supply.

[0103] Figure 3 A figure 101 illustrating the principles of the present disclosure is shown.

[0104] The process is explained with an example having a dominant Rcable1.

[0105] The nominal bus voltage is 24 V. At this voltage, the depicted LED channel (one of the LED channels, for example red, green or blue), i.e. the line with reference "I(LED)" should draw 36 mA if no dominant cable resistor Rcable1 is applied. Deviations from this ideal value can be seen in the figure; three different phases can be distinguished:

[0106] Phase A. Enable all three LED channels of the LED-based lighting fixture. A high current flows through the cable resistor Rcable1, reducing the voltage Vx on the LED channel and decreasing the current from its original design value. In this example, the voltage on the LED channel is 21V.

[0107] Phase B. Both channels are enabled. Compared to Phase A, the load is reduced, resulting in a smaller voltage drop across Rcable1. Therefore, the impact and deviation from the original target (i.e., 36mA) are also smaller. In this example, the voltage on the LED channel is 22V.

[0108] Phase C. Enable a single channel – the same principle as during Phases A and B. In this example, the voltage on the LED channel is 23V.

[0109] Stage D. No light output. The voltage on the LED channel should be equal to the bus voltage because the cable resistor Rcable1 does not cause any voltage drop when no current flows through it. Since there is no voltage drop across the cable resistor Rcable1, the voltage at the LED channel is 24V.

[0110] The line indicated by reference numeral 102 in the attached figure represents the ideal current through the LED channel, which is what the controller expects if no compensation is applied.

[0111] The line indicated by reference numeral 103 represents the duty cycle required to compensate for the optical loss caused by the dominant Rcable1. The loss of optical output can be determined and compensated by increasing the duty cycle of the corresponding PWM control signal by measuring the current (which equals the total optical output) over time and comparing it to the original target—current * time of the original dashed curve 102.

[0112] When Figure 1The shown measurement of the current through the LED channel uses a single sense resistor Rsense to sense the current through all LED channels. Thus, when multiple LED channels conduct current, it is not possible to determine the current through a single LED channel. When only a single LED channel conducts current, the current through the single LED channel, for example the channel with the red LED, can be measured. However, even when this single current can be measured, when multiple LED channels, for example red and blue, conduct current, the current through this single red LED channel will not be the same, because the total amount of current causes an increased voltage drop over the cable resistor Rcablei. This causes the bus voltage at the LED channel to drop, and thus the current through the red LED channel will be lower than the measured current. The inventors have insight that the current through an LED channel is not only influenced by the voltage drop caused by the current through the LED channel, but also by the additional LED channels that are active at the same time. This can be observed in Figure 3 the figure, in which the change in LED channel voltage VX when more or less LED channels are active at the same time. Thus, when different numbers of LED channels conduct current, the controller can be arranged to detect the LED channel voltage VX at different moments in time. This allows the controller to relate the voltage drop of the LED channel voltage VX to the current drop in each LED channel based on the number of channels that conduct current at a single moment in time. This relationship allows for an additional correction of the duty cycle of each LED channel.

[0113] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit can fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program can be stored / distributed on a suitable medium, such as an optical storage medium or a solid-state storage medium supplied together with or as part of other hardware, but can also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A lighting device based on a light-emitting diode (LED), arranged to emit light of a specific color, wherein the LED-based lighting device comprises: - A power supply unit, which is arranged to provide a DC bus voltage for powering the LEDs; - Multiple parallel cascaded LED channels, wherein each of the LED channels is connected to the bus voltage and includes at least one colored LED and a switch for activating the corresponding LED channel; - A controller, the controller being arranged to provide a control signal to each of the switches in the LED channel to periodically activate the LED channel, wherein each of the control signals has a duty cycle, and wherein the controller is arranged to determine the duty cycle of the control signal based on a received color setpoint; as well as - The memory, for each channel, includes the following relationships: - The bus voltage or current flowing through the corresponding channel, and - The light intensity emitted by the corresponding colored LED of the respective channel; and - A single sensing resistor (Rsense) is used to determine the total current flowing through all LED channels; The controller is further arranged to: - Determine the DC bus voltage and / or current flowing through the corresponding channel; - Take into account the aforementioned relationship to control each of the LED channels for emitting light of the specific color; as well as - By correlating the voltage drop of the LED channel with the current drop in each LED channel based on the number of channels conducting current at a single moment, the insufficient amount of light output of any LED channel caused by the parasitic effect of the cable resistor (Rcable1) between the power supply and the LED channel is determined, and the corresponding duty cycle is increased based on the determined insufficient amount. The duty cycle is increased based on the actual number of channels in use.

2. The LED-based lighting device according to claim 1, wherein, The more channels enabled, the higher the current flowing through the cable resistor (Rcable1) and the single sensing resistor (Rsense). The higher the current flowing through the cable resistor (Rcable1), the lower the voltage drop across the LED channel. The controller is configured to: - Determine a measurement related to the on-time of the determined instantaneous current of each channel multiplied by the corresponding duty cycle of each channel; - Compare the measured value with the expected measured value, which is related to the determined instantaneous current of each channel multiplied by the on-time of the corresponding duty cycle of each channel; - For each channel, determine the increase in duty cycle such that the determined measurement will be substantially equal to the expected measurement; - For each channel, increase the corresponding duty cycle.

3. The LED-based lighting device according to any one of the preceding claims, wherein the controller is arranged to determine the instantaneous current in such a way as: - Measure the voltage across the sensing resistor included in any one of the plurality of parallel cascaded LED channels.

4. The LED-based lighting device according to claim 1 or 2, wherein the controller is arranged to determine the instantaneous current of each channel in such a way as: - Measure the current flowing through the corresponding channel for at least two different DC bus voltages, and - Measure the DC bus voltage and determine the corresponding current flowing through the corresponding channel by interpolating the measured values ​​of the at least two different DC bus voltages.

5. The LED-based lighting device according to claim 1 or 2, wherein the controller is arranged to: - A control signal is provided to each of the switches in the LED channel to periodically activate the LED channel, wherein each of the control signals has a duty cycle, and wherein the controller is arranged to determine the duty cycle of the control signal based on a received color setpoint for low lumen output. - The low-lumen output is scaled to a high-lumen output by increasing the duty cycle based on the determined deficiency.

6. The LED-based lighting device of claim 1, wherein the controller is arranged to determine the current flowing through the respective channel in such a way as: - Measure the DC bus voltage and calculate the current by taking into account the measured DC bus voltage, the nominal current flowing through the channel, and the LED forward voltage of each LED in the channel.

7. The LED-based lighting device of claim 6, wherein the controller is further arranged to measure the forward voltage of the LED.

8. The LED-based lighting device according to claim 1 or 2, wherein the controller is further arranged to measure ambient temperature, and wherein the controller is arranged to take into account the relationship and the temperature to control each of the LED channels to emit light of the specific color.

9. A method of operating a lighting device based on a light-emitting diode (LED) according to any one of the preceding claims, wherein the method comprises the following steps: - The power supply unit provides the DC bus voltage for powering the LEDs; - The controller provides a control signal to each of the switches in the LED channel to periodically activate the LED channel, wherein each of the control signals has a duty cycle, and - The controller determines the duty cycle of the control signal based on the received color setpoint; - Based on the number of channels conducting current at a single moment, the voltage drop of the LED channel is correlated with the current drop in each LED channel. The controller determines the insufficient light output of each LED channel caused by the parasitic effect originating from the cable resistor (Rcable1) between the power supply and the LED channel. - The controller increases the duty cycle based on the determined deficiency.

10. The method of claim 9, wherein the method further comprises the following steps: - The controller determines a measurement related to the on-time of the determined instantaneous current of each channel multiplied by the corresponding duty cycle of each channel; - The controller compares the measured value with a expected measured value, which is related to the determined instantaneous current of each channel multiplied by the on-time of the corresponding duty cycle of each channel; - For each channel, the controller determines an increase in the duty cycle such that the determined measurement will be substantially equal to the expected measurement; - For each channel, the controller increases the corresponding duty cycle.

11. A computer-readable medium having instructions stored thereon, the instructions, when executed by a controller of an LED-based lighting device, causing the LED-based lighting device to perform the method according to any one of claims 9-10.

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