A light-emitting diode (LED)-based lighting device configured to emit light of a specific color, and a corresponding method

Abstract

The present invention provides a light emitting diode (LED)-based lighting device configured to emit light of a particular color, the LED-based lighting device comprising: a power supply unit configured to provide a direct current (DC) bus voltage for powering the LEDs; a plurality of parallel cascaded LED channels, each of the LED channels connected to the bus voltage and including at least one color LED and a switch for activating a corresponding LED channel; and a controller configured to provide control signals to each of the switches in the LED channels to periodically activate the LED channels, each of the control signals having a duty cycle, the controller configured to determine the duty cycle of the control signals based on a received color set point, the controller further configured to determine a deficiency in the 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 due to the determined duty cycle, and to increase the duty cycle based on the determined deficiency.

Description

Technical Field

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

Background Art

[0002] For various lighting applications, lighting devices utilizing light-emitting diodes (LEDs) have been developed. LED lamps are designed today for replacement of conventional fluorescent lamps, i.e., for retrofit applications, due to their long life and high energy efficiency. In such applications, retrofit LED lamps are generally adapted to fit into the socket of each lamp fixture being retrofitted. Further, since lamp maintenance is generally performed by the user, retrofit LED lamps should ideally be operable easily with any type of suitable fixture without the need to rewire the fixture.

[0003] The present disclosure relates to multi-channel LED-based lighting devices. Each channel may include a plurality of LEDs capable of emitting light in a specific color. For example, the first channel may be for red emission. The second channel may be for green emission, and the third channel may be for blue emission.

[0004] In such lighting devices, a fixed voltage source may be used to power the LEDs in each channel. The current through each channel can be set at the factory by adjusting a resistor placed in series with the LEDs of a particular channel. One of the disadvantages of such an approach is related to several disturbing factors, such as voltage fluctuations of the power supply, cable length, i.e., impedance, interaction between channels, etc., which can cause errors in the targeted luminous flux and color point.

[0005] More specifically, generally there is a controller that controls a plurality of channels, and each of the switches is configured to enable a particular channel. For example, the first switch can enable the red channel, the second switch can enable the green channel, the third switch can enable the blue channel, and so on. The switches may be supplied with a pulse width modulation (PWM) signal having a specific duty cycle. The frequency of the PWM signal should be selected to exceed the refresh rate of the human eye. This prevents the user from seeing any flicker. By controlling the duty cycle, the contribution of each channel to the total amount of light emitted can be controlled, and thus the color of the light emitted by the LED-based lighting device can also be controlled.

[0006] In many cases, the user may be able to represent or input the color that the LED-based lighting device is desired to emit. As described above, the inventor has found that there can be many disturbing factors that prevent the use of a static duty cycle for each of the PWM signals supplied to the switches.

Summary of the Invention

Problems to be Solved by the Invention

[0007] It would be advantageous to achieve a light-emitting diode (LED)-based lighting device configured to emit light of a specific color. Further, it would be advantageous to achieve a corresponding method.

Means for Solving the Problem

[0008] In a first aspect, there is provided an LED-based lighting device configured to emit light of a specific color. The LED-based lighting device includes - a power supply unit configured to supply a direct current (DC) bus voltage for powering the LEDs, - a plurality of parallel cascaded LED channels, each of the LED channels being connected to the bus voltage and including at least one color LED and a switch for operating the corresponding LED channel, - a controller configured to supply a control signal to each of the switches in the LED channels to periodically operate the LED channels, each of the control signals having a duty cycle, and the controller being configured to determine the duty cycle of the control signal based on a received color set point, The controller is further configured to determine the amount of deficiency in the light output of each of the LED channels caused by parasitic effects in the LED-based lighting device by determining the instantaneous current of each channel and comparing the expected current based on the determined duty cycle with the instantaneous current, and to increase the duty cycle based on the determined amount of deficiency.

[0009] The inventor has found that many parasitic components that may exist anywhere within the electrical circuit of the LED-based lighting device can contribute negatively to the color accuracy of the light emitted by the LED-based lighting device. That is, the difference between the color set point and the actual colored light emitted by the LED-based lighting device increases with an increase in the parasitic components.

[0010] The parasitic components can play an important role when multiple channels are active simultaneously, such that a large current is drawn from the power supply. Generally, when the LED-based lighting device is manufactured, each of the LED channels is calibrated once. Such calibration can be performed in a factory. During such calibration, a single LED channel may be activated and calibrated. Therefore, the calibration may not take into account operating multiple channels simultaneously.

[0011] For example, the bus series resistance, which may represent the resistance of the cable between the power supply unit and the actual LED-based lighting device, can play an important role in the error obtained when multiple channels are operated simultaneously. That is, a large current can cause a large voltage drop across the bus series resistance, thereby potentially reducing the bus voltage. The DC bus voltage may be lower than expected.

[0012] 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. If the bus voltage is lower than expected, and thus the voltage across the current resistor is also lower than expected, the current amount may deviate from the expected current. This leads to a decrease in the amount of current flowing through the channel when the corresponding switch is activated. As a result, the corresponding channel emits less light than was actually expected. When a particular channel emits less light than expected, this can also result in color shift in the case of a system comprising multiple channels.

[0013] According to the present disclosure, the above situation is addressed by increasing the duty cycle of the control signal to the corresponding switch.

[0014] In other words, following the variations caused by the spread of the nominal bus voltage, instantaneous voltage fluctuations can also have a profound impact on the spread of luminous flux and color by the LED-based lighting device. These voltage fluctuations are generally load-dependent and can act as a function of the LED channels being enabled. For each cycle, generally within 1 kHz, combinations of multiple channels can be activated, all of which can have different on-times and result in various current plateaus.

[0015] When many channels are enabled, the load current is the largest, and thus the impact on the power supply unit is the most profound, which results in higher bus voltage fluctuations. Following the behavior of the power supply, parasitic components such as the bus series resistance can also cause higher voltage drops and more current is required from the power supply unit. The behavior of the power supply unit and the values of the parasitic components can be unknown to the controller. The controller may expect measured values of the current of individual channels, collected, for example, during startup or during factory shipment procedures, and they are simply summed when more channels are enabled. This may not be the case due to the influence of the bus voltage fluctuations and parasitic component losses. Therefore, these errors result in deviations in color and luminous flux.

[0016] According to the present disclosure, the controller is further configured to determine the amount of deficiency in the light output of each of the LED channels caused by parasitic effects in the LED-based lighting device, by determining the instantaneous current of each channel and comparing the expected current with the instantaneous current by the determined duty cycle, and increasing the duty cycle based on the determined amount of deficiency.

[0017] It should be noted that due to the parasitic effects in the LED-based lighting device, there is a shortage of light output in any of the LED channels. Such shortages can have multiple effects. For example, the total luminance by the LED-based lighting device can be reduced. Another option is that, for example, when only one LED channel is affected by the parasitic effects, the color set point is shifted.

[0018] In an example, the controller - determines a measure related to the product of the determined instantaneous current of each channel and the on-time of the corresponding duty cycle of each channel, - compares the determined measure with an expected measure related to the product of the determined instantaneous current of each channel and the on-time of the corresponding duty cycle of each channel, - for each channel, determines an increase amount of the duty cycle such that the determined measure is substantially equal to the expected measure, - for each channel, is configured to increase the corresponding duty cycle.

[0019] The above example is directed to a situation where the controller attempts to ensure that the total amount of current flowing through a particular channel is substantially equal to the expected total amount of current.

[0020] For example, consider a situation where the controller expects that the current passing through a particular channel is 35 mA during the on-period of a pulse width modulation (PWM) signal. However, due to any parasitic losses, the actual current obtained is not 35 mA but, for example, 27 mA. The on-period of the PWM signal is, for example, 1.9 ms. Thus, the controller expected 35 mA to flow through a particular channel for 1.9 ms. However, in reality, an amount of 27 mA flowed for 1.9 ms. This effectively results in a reduction in illumination by the LEDs within the corresponding channel.

[0021] In order to compensate for the above, the inventor has noticed that it may be difficult to increase the current amount. This is due to parasitic aspects that reduce the bus voltage, which in turn results in a decrease in the current amount. To counteract the above, the inventor has 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 about 2.4 ms so that the total amount of current passing through each channel remains substantially the same.

[0022] In an example, the controller is - configured to determine the instantaneous current by measuring the voltage across a sense resistor included by any of the plurality of parallel cascade-connected LED channels.

[0023] As described above, the present disclosure is directed to the concept that a momentary decrease in the amount of current due to parasitic aspects is compensated by increasing the duty cycle of the corresponding control signal. The momentary decrease in the amount of current can be determined in several ways. In this example, the momentary amount of current is determined by measuring the voltage across one or more sense resistors included by the LED-based lighting device, e.g., a sense resistor in a supply line towards the plurality of LED channels, or a return line from the plurality of LED channels. Another option is where the voltage across a sense resistor, e.g., a current-setting resistor, present within the actual plurality of LED channels is measured.

[0024] In a further example, the controller is - configured to determine the instantaneous current of each channel by measuring the current flowing through each of the channels for at least two different DC bus voltages, - measuring the DC bus voltage and determining each of the currents flowing through each of the channels by interpolating the measured values for the at least two different DC bus voltages.

[0025] The controller may not know the actual forward voltage drop of the LED in a particular channel, and thus may not know the value of any resistor within the particular channel for setting the current that should flow through the channel. However, the controller can estimate or interpolate these characteristics by measuring the current flowing through each respective channel for at least two different DC bus voltages. This can be done, for example, at the factory.

[0026] Using the obtained values, the current passing through a particular channel can be estimated by interpolating the measured values for the at least two different DC bus voltages. In accordance with the above, during operation of the LED-based lighting device, the controller may determine the instantaneous amount of current flowing through a particular channel by measuring the DC bus voltage.

[0027] In a further example, the controller is configured to - supply a control signal to each of the switches within the LED channel to periodically activate the LED channel, each of the control signals having a duty cycle, and the controller determines the duty cycle of the control signal based on a received color set point for a low lumen output, - scale the low lumen output to a high lumen output by increasing the duty cycle based on the determined deficit amount.

[0028] In the above, two reference steps can be performed sequentially. That is, first, the supplying step may be performed, and then, the scaling step may be performed. This will be described in more detail below.

[0029] The controller may first determine the ratio of the duty cycle 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 set point. 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 adverse effects of the parasitic characteristics are reduced.

[0030] The next step relates to scaling. That is, the total amount of lumen output is increased. In this step, due to the increase in the total amount of current supplied by the power supply unit, parasitic characteristics as described above can play a more important role.

[0031] The above involves an increase in the duty cycle to supply the total amount of lumen output. However, the controller can also, in this way, individually compensate each of the duty cycles based on the parasitic characteristics, thereby also compensating the ratio between the duty cycles. Thus, in this step, the controller can further determine the amount of deficiency in the light output of each of the LED channels caused by the parasitic effects in the LED-based lighting device by determining the instantaneous current of each channel and comparing the expected current with the instantaneous current by the determined duty cycle, and can be configured to increase the duty cycle based on the determined amount of deficiency.

[0032] In a further example, the LED-based lighting device - is a memory, and for each channel, - includes a relationship between the current or bus voltage flowing through each of the channels and - the light intensity emitted by the corresponding at least one color LED of each of the channels. The controller is further configured to determine the current flowing through each of the channels and / or the DC bus voltage, and in consideration of the relationship, control each of the LED channels to emit light of the specific color.

[0033] Each of the LED channels may have a current control element for controlling the amount of current flowing into each of the channels. It should be noted that the current control element controls the amount of current passing through each of the channels based on the DC bus voltage.

[0034] The current control element may be a variable resistor for adjusting the resistance value of the corresponding channel.

[0035] The inventor has insight that the light intensity of a channel is related to the current flowing through that specific channel. The current control element exists to ensure that a predefined amount of current flows through the channel. However, the value of the current control element may be determined based on a nominal, i.e., standard, DC bus voltage. In that case, the variations occurring in the DC bus voltage are not taken into account. Therefore, these variations may result in different light intensities for specific channels.

[0036] It should also be noted that the forward voltage of the LEDs in each of the channels may be different. Therefore, a red LED may have a different forward voltage compared to a green LED and compared to a blue LED. Accordingly, the current control element may vary depending on the channel, and thus the variations in the DC voltage bus may have different effects on each of the channels.

[0037] The inventor has found that the controller, i.e., the one that controls each of the channels, can be beneficial when considering the above-mentioned characteristics. More specifically, the controller can use either the characteristics of the DC bus voltage with respect to the light intensity of each of the channels or the characteristics of the current flowing through the channels with respect to the light intensity of each of the channels to compensate for the characteristics.

[0038] That is, the controller can use the above information to control each of the channels to emit light of a specific color.

[0039] According to the present disclosure, the power supply unit may be configured to receive a main power input supply voltage, for example, 230 Vac or the like, and may be configured to convert the main power input supply voltage into a DC bus voltage for powering the LEDs in each of the channels.

[0040] According to the present disclosure, the memory may be a read-only memory (ROM), a random access memory (RAM), a cache, or the like.

[0041] According to the present disclosure, the controller may be any other control device such as, for example, a microcontroller, or a microprocessor, a field programmable gate array (FPGA), or the like. The microcontroller may receive relevant input signals at some of the available input pins, for example, and may supply output control signals at other available output pins.

[0042] Note that the memory may include, for each channel, the relationship between the current flowing through each of the channels or the bus voltage and the light intensity emitted by the at least one corresponding color LED of each of the channels. This should be understood in a broad sense. Generally, the relationship is directed to the light output by the channel and the electrical characteristics of that channel. This can be expressed in several ways. For example, the bus voltage with respect to the current characteristic or the like. Therefore, the relationship can also be indirectly supplied such that the current flowing through the channel indicates the light emitted by that channel.

[0043] According to the present disclosure, a color LED is an LED that emits a specific color, for example, white, blue, green, red, etc.

[0044] In an example, the controller is configured to determine the current flowing through each of the channels by - measuring the DC bus voltage and calculating 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 of the LEDs in the channel.

[0045] The controller may also be configured to measure the LED forward voltage of the LEDs present in the plurality of LED channels.

[0046] In a further example, the controller is further configured to measure the ambient temperature, and the controller is configured to control each of the LED channels to emit light of the specific color taking into account the relationship and the temperature.

[0047] In a second aspect of the present disclosure, a method of operating a light-emitting diode (LED) - based lighting device according to any of the previous examples is provided. The method includes - supplying, by the power supply unit, a DC bus voltage for powering the LEDs, - A step of supplying a control signal to each of the switches in the LED channel by the controller to periodically activate the LED channel, each of the control signals having a duty cycle; - A step of determining the duty cycle of the control signal by the controller based on the received color set point; - A step of determining, by the controller, the amount of deficiency in the light output of each of the LED channels caused by parasitic effects in the LED-based lighting device; - A step of increasing the duty cycle by the controller based on the determined amount of deficiency.

[0048] Note that the advantages and definitions disclosed as in the embodiments of the first aspect of the present invention also apply to the embodiments of the second aspect of the present invention, which is a method for operating an LED-based lighting device.

[0049] In an example, the method includes - A step of determining, by the controller, an amount related to the product of the determined instantaneous current of each channel and the on-time of the corresponding duty cycle of each channel; - A step of comparing, by the controller, the determined amount related to the product of the determined instantaneous current of each channel and the on-time of the corresponding duty cycle of each channel with an expected amount; - A step of determining, by the controller, for each channel, an increase amount of the duty cycle such that the determined amount is substantially equal to the expected amount; - A step of increasing the corresponding duty cycle for each channel by the controller.

[0050] In a third aspect, there is provided a computer-readable medium having instructions stored thereon which, when executed by a controller of an LED-based lighting device, cause the LED-based lighting device to perform a method according to any of the examples provided above.

[0051] These and other aspects of the present invention will be described and made apparent with reference to the following embodiments.

Brief Description of the Drawings

[0052]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0053] FIG. 1 shows an LED-based lighting device 1 according to the present disclosure.

[0054] Here, a power supply unit 9 for generating a direct current (DC) bus voltage 7 is provided. The DC bus voltage 7 is generally about 24 volts DC, but can be any value. Usually, to prevent any dangerous situations, the DC bus voltage 7 is lower than at least about 50 V DC. To reduce any disturbances in the DC bus voltage 7, an electromagnetic interference (EMI) filter may be arranged near the output of the power supply unit 9.

[0055] In this scenario, the LED-based lighting device 1 has five channels as indicated by reference numerals 2, 3, 4, 5, and 6. Each of the channels 2, 3, 4, 5, and 6 is configured to emit light having a specific color. For example, the channel as indicated by reference numeral 2 is configured to emit red light, the channel as indicated by reference numeral 3 is configured to emit green light, the channel as indicated by reference numeral 4 is configured to emit blue light, the channel as indicated by reference numeral 5 is configured to emit flame white light, and the channel as indicated by reference numeral 6 is configured to emit cool white light.

[0056] Each of the different channels 2, 3, 4, 5, and 6 may have different current requirements and may have different forward voltages. The forward voltage of an LED is defined as the voltage drop across that particular LED.

[0057] To achieve this, each of the channels 2, 3, 4, 5, and 6 is provided with a current control element for adjusting the current passing through the channel. Assume that the DC bus voltage is nominally 24V DC. The first channel, i.e., the channel as indicated by reference numeral 2, may have six LEDs each having a forward voltage of 3V DC. This results in an approximate voltage drop of 18V DC across the LEDs cumulatively. The remaining voltage, i.e., 24V DC - 18V DC, is 6V DC and is the voltage across the current control element. In that case, the resistance value can be adjusted to specify the current flowing through the channel.

[0058] A controller 8 may be present for controlling the channels 2, 3, 4, 5, and 6. More specifically, the controller 8 may supply control signals to the corresponding switches of the channels 2, 3, 4, 5, and 6 to enable or disable the corresponding channels 2, 3, 4, 5, and 6 in order to achieve a specific desired color of the total light emitted.

[0059] Generally, these control signals are pulse-width modulation (PWM) signals. The duty cycle of these PWM signals can be set by the controller to enable the LED-based lighting device to emit light of a specific color. The ratio between the duty cycles of the control signals determines the specific color of light actually emitted.

[0060] Accordingly, the controller determines the duty cycle of each of the control signals. The controller further determines the amount of deficiency in the light output of each LED channel caused by parasitic effects in the LED-based lighting device by determining the instantaneous current of each channel and comparing the expected current based on the determined duty cycle with the instantaneous current, and is configured to increase the duty cycle based on the determined amount of deficiency.

[0061] The instantaneous current can be determined in various ways. For example, a sense resistor can be used to determine the total amount of current flowing through all the combined LED channels. For example, multiple calibrated bus voltages can be used to divide the total amount of current into individual currents flowing through the channels that are active at that time.

[0062] Another option is that the voltage across each current control element is measured, and the current flowing through a specific channel is determined by dividing the measured voltage by the resistance value of the corresponding current control element.

[0063] The controller determines, for example, an amount related to the product of the determined instantaneous current of each channel and the on-time of the corresponding duty cycle of each channel, compares the amount with an expected amount related to the product of the determined instantaneous current of each channel and the on-time of the corresponding duty cycle of each channel, determines, for each channel, an increase amount of the duty cycle such that the determined amount is substantially equal to the expected amount, and may increase the corresponding duty cycle for each channel.

[0064] Note that the parasitic characteristics of the present disclosure can arise from resistors such as those denoted by "Rcable1". The cable lengths between the power supply unit 9 and the plurality of CED channels 2, 3, 4, 5, 6 can be modeled as resistors. Such resistors contribute to a voltage drop such that the bus voltage 7 is lower than the expected bus voltage. This leads to a decrease in the current flowing through each of the LED channels 2, 3, 4, 5, 6.

[0065] Figure 2 shows a flowchart 51 of a method according to the present disclosure.

[0066] The flowchart 51 starts 52 by obtaining a new target XYZ53. The new target XYZ53 indicates a desired color set point of the LED-based lighting device. The desired color set point may be related to a specific color temperature such as 4000K, or may be related to a specific RAL color or the like.

[0067] The desired color set point is supplied to a color algorithm 54 executed by a controller. The color algorithm 54 uses the desired color set point to determine the duty cycle of each of the control signals for controlling a plurality of switches present in each of the plurality of LED channels.

[0068] First, the color algorithm 54 determines the ratio between the duty cycles, ensuring that the total amount of lumens emitted by the LED-based lighting device, i.e., the total amount of light, is relatively low, e.g., 1 lumen. Therefore, only the ratio between the duty cycles is calculated and the intensity of the emitted light is not calculated.

[0069] In the next step, while paying attention to the ratio between the duty cycles, the low lumen output is scaled to a high lumen output by appropriately increasing each of the duty cycles. However, due to the parasitic characteristics as described above, the ratio between the duty cycles can change during this process.

[0070] In an LED-based lighting device, the deficiency in the light output of each LED channel caused by parasitic effects is determined during this step by determining the instantaneous current of each channel and comparing the expected current based on the determined duty cycle with the instantaneous current, and is determined to increase the duty cycle based on the determined deficiency amount.

[0071] Accordingly, the input for the color algorithm may be the target color and brightness, or the characteristics of the LEDs of a plurality of LED channels. These characteristics are the color point and luminous flux per primary LED at a given drive current. These parameters can be compensated for the temperature rise due to the self-heating of the lamp. In the electronic device architecture under consideration, it should be noted that it is the drive current through the LED string that can vary as a result of the variation in the bus voltage.

[0072] In the case of a voltage drive system, the target luminous flux can be set to 1 lumen such that the duty cycle ratio between different channels is correct.

[0073] However, the brightness is, therefore, very low. This is the main reason why a post-processing step of scaling the duty cycle to a higher value is introduced. The scale can be increased until the duty cycle reaches 100% or until the rated power of the power supply is reached to prevent overfeeding of the power supply.

[0074] Figure 3 shows Figure 101 which illustrates the principle of the present disclosure.

[0075] The procedure will be described with an example having a main (dominant) Rcable1.

[0076] The nominal bus voltage is 24V. At this voltage, the line with the LED channel shown (one of the LED channels, for example, red, green, or blue), i.e., the line with the reference sign "I(LED)", should draw 36mA when the main cable resistance Rcable1 is not applied. Deviations from this ideal value can be seen in the graph. Three different phases can be identified. Phase A All three LED channels of the LED-based lighting device are enabled. A high current flows through the cable resistance Rcable1, reducing the voltage Vx across the LED channels and decreasing the current from its original design value. In this example, the voltage across the LED channels is 21V. Phase B Two channels are enabled. The load is reduced compared to Phase A, so the voltage drop across Rcable1 is less. Also, therefore, the deviation and impact from the original target of 36mA are less. In this example, the voltage across the LED channels is 22V. Phase C A single channel is enabled (the same principle as during Phases A and B). In this example, the voltage across the LED channels is 23V. Phase D No light output. Since no current flows through the cable resistance Rcable1 and thus Rcable1 causes no voltage drop, the voltage across the LED channels should be equal to the bus voltage. Since there is no voltage drop across the cable resistance Rcable1, the voltage across the LED channels is 24V.

[0077] The line indicated by the reference sign 102 shows the ideal current through the LED channels, i.e., this is what the controller would expect if no compensation were applied.

[0078] The line indicated by reference numeral 103 is the duty cycle necessary to compensate for the optical loss due to the main Rcable1. By measuring the current over time, which is equal to the total optical output, and comparing it to the original target, i.e., the current * time of the original dashed curve 102, the loss of optical output can be determined and compensated by increasing the duty cycle of the corresponding PWM control signal.

[0079] When the current passing through the LED channels is measured as shown in FIG. 1, a single sense resistor Rsense is used to sense the current flowing through all the LED channels. Therefore, when multiple LED channels are conducting current, it is not possible to determine the current flowing through a single LED channel. The current flowing through a single LED channel, for example, a channel with a red LED, can be measured when only that single LED channel is conducting current. However, even when this single current can be measured, when multiple LED channels, for example, red and blue LED channels, are conducting current, the total amount of current causes an increase in the voltage drop across the cable resistance Rcable1, so the current flowing through this single (red) LED channel is not the same. This reduces the bus voltage in the LED channel, and therefore the current through the red LED channel is lower than the measured current. The inventor's insight is that the current through the LED channel is affected not only by the voltage drop due to the current flowing through that LED channel but also by the influence of additional active LED channels. This can be observed in FIG. 3 as a change in the LED channel voltage Vx when more or fewer LED channels are active simultaneously. Therefore, the controller can be configured to detect the LED channel voltage Vx at different times when different numbers of LED channels are conducting current. This enables the controller to correlate the voltage drop of the LED channel voltage Vx with a decrease in the current in each of the LED channels based on the number of channels conducting current at a single point in time. This relationship enables further correction of the duty cycle for each of the LED channels.

[0080] Those skilled in the art can understand and achieve other modifications to the disclosed embodiments from the study of the drawings, the description, and the appended claims in the implementation of the invention described in the claims. In the claims, the word "comprising" does not exclude other elements or steps, and the singular form does not exclude the plural. A single processor or other unit may perform the functions of a plurality of items recited in the claims. Merely the fact that certain means are recited in mutually different dependent claims does not indicate that these means cannot be used advantageously in combination. A computer program may be stored / distributed on a suitable medium such as an optical storage medium or a solid medium supplied together with or as part of other hardware, but may also be distributed in other forms via the Internet or other wired or wireless telecommunications systems. Any reference signs in the claims should not be construed as limiting the claims.

Claims

1. An LED-based lighting device configured to emit light of a specific color, a power supply unit configured to supply a DC bus voltage for powering the LEDs, a plurality of LED channels connected in parallel cascade, each of the LED channels being connected to the DC bus voltage and including at least one color LED and a switch for operating the corresponding LED channel, the plurality of LED channels being connected in parallel cascade, a controller configured to supply a control signal to each of the switches in the LED channels to operate the LED channels periodically, each of the control signals having a duty cycle, the controller being configured to determine the duty cycle of the control signal based on a received color setpoint, a memory, for each channel, the current flowing through each of the channels or the DC bus voltage, and a memory including the relationship between the current flowing through each of the channels or the DC bus voltage and the light intensity emitted by the corresponding at least one color LED of each of the channels, and an LED-based lighting device having a single sense resistor for determining the total amount of current flowing through all of the LED channels, the controller further determining the current flowing through each of the channels or the DC bus voltage, controlling each of the LED channels to emit the light of the specific color taking into account the relationship, determining the amount of deficiency in the light output of any of the LED channels caused by the parasitic effect resulting from the cable resistance between the power supply unit and the LED channels by measuring the instantaneous current of any of the channels and comparing the expected current with the instantaneous current by the determined duty cycle, and increasing the corresponding duty cycle based on the determined amount of deficiency. An LED-based lighting device configured as such.

2. The controller determines an amount related to the product of the determined instantaneous current of each channel and the on-time of the corresponding duty cycle of each channel, compares the amount with an expected amount related to the product of the expected current of each channel and the on-time of the corresponding duty cycle of each channel, For each channel, determine an increase amount of a duty cycle such that the determined amount substantially equals the expected amount. The LED-based lighting device according to claim 1, configured to increase the corresponding duty cycle for each channel. **Claim 3** The controller is The LED-based lighting device according to claim 1 or 2, configured to determine the instantaneous current by measuring a voltage across a sense resistor included in any one of the plurality of parallel cascaded LED channels. **Claim 4** The controller is Obtain measured values of the current flowing through each of the channels for at least two different DC bus voltages, Measure the DC bus voltage and determine each of the currents flowing through each of the channels by interpolating the measured values for the at least two different DC bus voltages, thereby determining the instantaneous current of each channel. The LED-based lighting device according to any one of claims 1 to 3. **Claim 5** The controller is configured to supply a control signal to each of the switches in the LED channel to periodically operate the LED channel, each of the control signals having a duty cycle, and the controller determines the duty cycle of the control signal based on a received color set point for a low lumen output and increases the duty cycle based on the determined shortage amount to scale the low lumen output to a high lumen output. The LED-based lighting device according to any one of claims 1 to 4. **Claim 6** The controller is further configured to measure an ambient temperature, and the controller is configured to control each of the LED channels to emit the specific color of light in consideration of the relationship and the temperature. The LED-based lighting device according to any one of claims 1 to 5. **Claim 7** A method of operating an LED-based lighting device according to any one of claims 1 to 6, comprising: Supplying, by the power supply unit, a DC bus voltage for powering the LEDs. A step of supplying a control signal to each of the switches in the LED channel to periodically activate the LED channel by the controller, each of the control signals having a duty cycle; A step of determining the duty cycle of the control signal based on the received color set point by the controller; A step of determining the shortage amount in the light output of each of the LED channels caused by parasitic effects in the LED-based lighting device by the controller; A method comprising a step of increasing the duty cycle based on the determined shortage amount by the controller.

8. A step of determining, by the controller, an amount related to the product of the determined instantaneous current of each channel and the on-time of the corresponding duty cycle of each channel; A step of comparing, by the controller, the expected amount related to the product of the expected current of each channel and the on-time of the corresponding duty cycle of each channel with the amount; A step of determining, by the controller, for each channel, an increase amount of the duty cycle such that the determined amount is substantially equal to the expected amount; The method according to claim 7, further comprising a step of increasing the corresponding duty cycle for each channel by the controller.

9. A computer-readable medium having instructions stored on the computer-readable medium, the instructions causing a controller of an LED-based lighting device to perform the method according to any one of claims 7 to 8 when executed.

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