A LIGHT EMITTING DIODE, LED, BASED LIGHTING DEVICE ARRANGED FOR EMITTING A PARTICULAR COLOR OF LIGHT, AS WELL AS A CORRESPONDING METHOD

Abstract

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

Claims

1. A Light Emitting Diode (LED) based lighting device arranged for emitting a particular color of light, wherein said LED based lighting device comprises: a power supply unit arranged for providing a Direct Current (DC) bus voltage for powering LEDs; a plurality of parallel cascaded LED channels, wherein each of said LED channels is connected to said bus voltage and comprises at least one colored LED and a switch for activating said corresponding LED channel; a controller arranged for providing control signals to each of said switches in said LED channels for periodically activating said LED channels, wherein each of said control signals has a duty cycle, and wherein said controller is arranged for determining said duty cycles of said control signals based on a received color set point; and a memory comprising a relationship, for each channel, between: bus voltages or currents flowing through said respective channel, and light intensity emitted by said at least one corresponding colored LED of said respective channel; and a single sense resistor for determining a total amount of current flowing through all the LED channels; wherein said controller is further arranged for: determining said DC bus voltage and/or said currents flowing through said respective channels; controlling each of said LED channels for emitting said particular color of light taking into account said relationship; and determining an amount of deficiency in light output of any of said LED channels caused by parasitic effects, originating from a cable resistor between the power supply and the LED channels, by measuring instantaneous currents of said any of said channels and comparing said instantaneous currents with expected currents resulting from the determined duty cycles, and for increasing said corresponding duty cycles based on said determined amount of deficiency.

2. The LED based lighting device in accordance with claim 1, wherein said controller is arranged for: determining a measure related to said determined instantaneous currents of each channel multiplied by an ON-time of said corresponding duty cycle of each channel; comparing said measure with an expected measure related to said determined instantaneous currents of each channel multiplied by an ON-time of said corresponding duty cycle of each channel; determining, for each channel, the increase of duty cycle such that said determined measure will substantially equal said expected measure; increasing, for each channel, said corresponding duty cycle.

3. The LED based lighting device in accordance with claim 1, wherein said controller is arranged for determining said instantaneous currents by: measuring voltages over sensing resistors comprised by any of said plurality of parallel cascaded LED channels.

4. The LED based lighting device in accordance with claim 1, wherein said controller is arranged for determining said instantaneous currents of each channel by: measuring said currents flowing through said respective channels for at least two different DC bus voltages, and measuring said DC bus voltage and determining said respective currents flowing through said respective channels by interpolating said measurements for said at least two different DC bus voltages.

5. The LED based lighting device in accordance with claim 1, wherein said controller is arranged for: providing control signals to each of said switches in said LED channels for periodically activating said LED channels, wherein each of said control signals has a duty cycle, and wherein said controller is arranged for determining said duty cycles of said control signals based on a received color set point for a low lumen output, and scaling said low lumen output to a high lumen output by increasing said duty cycles based on said determined amount of deficiency.

6. The LED based lighting device in accordance with claim 1, wherein said controller is arranged for determining said currents flowing through said respective channels by: measuring said DC bus voltage and calculating said currents by taking into account said measured DC bus voltage, nominal currents flowing through said channels and LED forward voltages of each of said LED's in the channels.

7. The LED based lighting device in accordance with claim 6, wherein said controller is further arranged for measuring said LED forward voltages.

8. The LED based lighting device in accordance with claim 1, wherein said controller is further arranged for measuring an environmental temperature, and wherein said controller is arranged controlling each of said LED channels for emitting said particular color of light taking into account said relationship as well as said temperature.

9. A method of operating a Light Emitting Diode (LED) based lighting device comprising: providing, by a power supply unit, a DC bus voltage for powering LEDs; providing, by a controller, control signals to each of said switches in said LED channels for periodically activating said LED channels, wherein each of said control signals has a duty cycle, and determining, by said controller, said duty cycles of said control signals based on said received color set point; determining, by said controller, said amount of deficiency in light output of each of said LED channels caused by parasitic effects in said LED based lighting device, and increasing, by said controller, said duty cycles based on said determined amount of deficiency.

10. The method in accordance with claim 9, wherein said method further comprises the steps of: determining, by said controller, a measure related to said determined instantaneous currents of each channel multiplied by an ON-time of said corresponding duty cycle of each channel; comparing, by said controller, said measure with an expected measure related to said determined instantaneous currents of each channel multiplied by an ON-time of said corresponding duty cycle of each channel; determining, by said controller, for each channel, the increase of duty cycle such that said determined measure will substantially equal said expected measure; and increasing, by said controller, for each channel, said corresponding duty cycle.

11. A non-transitory computer readable medium having instructions stored thereon which, when executed by a controller of a LED based lighting device, cause said LED based lighting device to implement a method in accordance with claim 9.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0079] FIG. 1 shows an LED based lighting device in accordance with the prior art;

[0080] FIG. 2 shows a flow chart of a method in accordance with the present disclosure;

[0081] FIG. 3 shows a diagram illustrating the principles of the present disclosure.

DETAILED DESCRIPTION

[0082] FIG. 1 shows an LED based lighting device 1 in accordance with the present disclosure.

[0083] Here, a power supply unit 9 is provided for generating the Direct Current, DC, bus voltage 7. The DC bus voltage 7 is typically about 24 Volts DC, but could range to any value. Usually, in order to prevent any hazardous situation, the DC bus voltage 7 is at least lower than about 50V DC. ElectroMagnetic Interference, EMI, filters may be placed close to the output of the power supply unit 9 for reducing any disturbances in the DC bus voltage 7.

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

[0085] Each of the LED's of the different channels 2, 3, 4, 5, 6, may have different current requirements and may have different forward voltages. A forward voltage of a LED is defined as the voltage drop over that specific LED.

[0086] To accomplish that, each of the channels 2, 3, 4, 5, 6 is equipped with a current control element for tuning the current going through the channel. Suppose the DC bus voltage is nominally 24 VDC. The first channel, i.e. the one as indicated with reference numeral 2, may have six LED's each having a forward voltage of 3 VDC. This would accumulate to about 18 VDC voltage drop over the LED's. The remaining voltage, i.e. 24 VDC−18 VDC is 6 VDC is the voltage over the current control element. The resistor value may then be tuned to specify the current flowing through the channel.

[0087] A controller 8 may be present to control the channels 2, 3, 4, 5, 6. More specifically, the controller 8 may provide control signals to the corresponding switches of the channels 2, 3, 4, 5, 6, for either enabling or disabling the corresponding channels 2, 3, 4, 5, 6, for realizing a particular desired color of the total light emitted.

[0088] Typically, these control signals are Pulse Width Modulation, PWM, signals. The duty cycle of these PWM signals may be set by the controller for realizing that the LED based lighting device emits a particular colored light. The ratio between the duty cycles of the control signals determines the particular light color that is actually emitted.

[0089] The controller thus determines the duty cycles of each of the control signals. The controller is further arranged for determining an 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 instantaneous currents of each channel and comparing the instantaneous currents with expected currents resulting from the determined duty cycles, and for increasing the duty cycles based on the determined amount of deficiency.

[0090] The instantaneous currents may be determined in a variety of manners. For example, the Rsense resistor may be used for determining the total amount of current flowing through all the LED channels combined. Using, for example, multiple calibrated bus voltages, the total amount of current may be split into individual currents through the channels that are active at that moment.

[0091] Another option is that the voltages over each of the current control elements are measured, and that the current through a particular channel is determined by dividing the measured voltage by the resistance value of the corresponding current control element.

[0092] The controller may, for example determine a measure related to said determined instantaneous currents of each channel multiplied by an ON-time of said corresponding duty cycle of each channel, compare said measure with an expected measure related to said determined instantaneous currents of each channel multiplied by an ON-time of said corresponding duty cycle of each channel, determine, for each channel, the increase of duty cycle such that said determined measure will substantially equal said expected measure and increase, for each channel, said corresponding duty cycle.

[0093] It is noted that the parasitic aspects of the present disclosure may originate from the resistor as indicates with “Rcable1”. The length of the cable between the power supply unit 9 and the plurality of LED channels 2, 3, 4, 5, 6 may be modelled 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, leads to lower current through each of the LED channels 2, 3, 4, 5, 6.

[0094] FIG. 2 shows a flow chart 51 of a method in accordance with the present disclosure.

[0095] The flow chart 51 starts 52 with obtaining a new target XYZ 53. The new target XYZ 53 indicates the desired color set point of the LED based lighting device. The desired color set point may relate to a particular temperature of the color, for example 4000K, or may relate to a particular RAL color or anything alike.

[0096] The desired color set point is provided to a color algorithm 54 which is executed by the controller. The color algorithm 54 uses the desired color set point for determining the duty cycles of each of the control signals to control the plurality of switches present in each of the plurality of LED channels. In a first instance, the color algorithm 54 determines the ratio between the duty cycles, but assures that the total amount of lumen, i.e. the total amount of light, emitted by the LED based lighting device is relatively low, for example 1 lumen. As such, only the ratios between the duty cycles is calculated but not the intensity of the emitted light.

[0097] In a next step, the low lumen output is scaled to a high lumen output by increase each of the duty cycles accordingly and keeping the ratio between the duty cycles in mind. The ratio between the duty cycles may, however, change during this process due to the parasitic aspects as mentioned above.

[0098] During this process, the amount of deficiency in light output of each of the LED channels caused by parasitic effects in the LED based lighting device is determined by determining instantaneous currents of each channel and comparing said instantaneous currents with expected currents resulting from the determined duty cycles, and for increasing said duty cycles based on said determined amount of deficiency.

[0099] The inputs for the color algorithm may thus be the target color and brightness and may be the properties of the LEDs of the plurality of LED channels. These properties are the color point and flux per primary LED at the given drive current. These parameters could be compensated for the temperature increase due to self-heating of the lamps. Note that in the electronics architecture under discussion, it is the drive current through the LED strings that may vary as a result of fluctuations in the bus voltage.

[0100] In case of a voltage driven system, the target flux may be set to 1 lumen such that the duty cycle ratio between the different channels will be correct.

[0101] However, the brightness will thus be very low. That is the main reason why there is a post-processing step introduced that scales the duty cycles to higher values. The scale may be increased until either duty cycle is 100% or the power of the rated power of the power supply is reached to prevent over-powering of the power supply.

[0102] FIG. 3 shows a diagram 101 illustrating the principles of the present disclosure.

[0103] The procedure is explained with an example having a dominant Rcable1.

[0104] The nominal bus voltage is 24V. At this voltage the depicted LED channel (one of the LED channels e.g. red green or blue), i.e. the line having the reference “I(LED)”, should draw 36 mA if no dominant cable resistor Rcable1 was applied. A deviation from this ideal value can be seen in the graph; Three different phases can be distinguished: [0105] Phase A. All three LED channels of the LED based lighting device are enabled. A high current flows through cable resistor Rcable 1 lowering the voltage over the LED channels, Vx, and reducing the current from its original designed value. In this example, the voltage over the LED channel is 21 V. [0106] Phase B. Two channels are enabled. The load is reduced compared to phase A and hence there is less voltage drop over Rcable 1. Also there is therefore less impact and deviation from the original target, which is 36 mA. In this example, the voltage over the LED channel is 22 V. [0107] Phase C. Single channel enabled—same principle as during phase A and phase B. In this example, the voltage over the LED channel is 23 V. [0108] Phase D. No light output. The voltage over the LED channel should be equal to the bus voltage because the cable resistor Rcable1 does not cause any voltage drop as no current flows through Rcable1. Because there is no voltage drop over the cable resistor Rcable1, the voltage at the LED channel is 24 V.

[0109] The line indicated with reference numeral 102 indicates the Ideal current through the LED channel, i.e. this is what the controller expected if no compensation was applied.

[0110] The line indicated with reference numeral 103 is the required duty cycle to compensate for the loss of light caused by the dominant Rcable1. By measuring the current over time, which equals the total light output, and comparing it to the original target, i.e. current*time of original dashed curve 102, the loss of light output may be determined and compensated by increasing the duty cycle of the corresponding PWM control signal.

[0111] When the current through the LED channels is measured as shown in FIG. 1, a single sense resistor Rsense is used to sense the current that flows through all the LED channels. It is therefore not possible to determine the current flowing through a single LED channel when multiple LED channels are conducting current. The current flowing through a single LED channel e.g. the channel with the red LED, can be measured when only that single LED channel is conducting current. However, even when this single current can be measured, the current flowing through this single, red, LED channel will not be the same when multiple LED channels e.g. red and blue, are conducting current because the total amount of current causes an increase in voltage drop over the cable resistor Rcable1. This causes the bus voltage to drop at the LED channels and therefore the current through the red LED channel will be lower than the current that was measured. It is an insight of the inventors that the current through an LED channel is not only impacted by the voltage drop caused by the current flowing through the LED channel, but also by the additional LED channels that are active at the same time. This can be observed in FIG. 3 as the change in the LED channel voltage Vx when more or less LED channels are active at the same time. Therefore, the controller can be arranged to detect the LED channel voltage Vx at different moments in time when different number of LED channels are conducting current. This allows the controller to relate the voltage drop of the LED channel voltage Vx to the drop of current in each of the LED channels based on the number of channels conducting current at a single moment in time. This relation allows an additional correction of the duty cycles for each of the LED channels.

[0112] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims, In the claims, the word “Comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope thereof.