WO2021094126A1

WO2021094126A1 - A light emitting diode, led, based lighting device arranged for emitting a particular color of light as well as a corresponding method

Info

Publication number: WO2021094126A1
Application number: PCT/EP2020/080742
Authority: WO
Inventors: Johannes Maria THIJSSEN; Berend Jan Willem Ter Weeme; Henricus Marius Joseph Maria Kahlman
Original assignee: Signify Holding B.V.
Priority date: 2019-11-14
Filing date: 2020-11-03
Publication date: 2021-05-20

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 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 characterized in that each of said plurality of parallel cascaded LED channels comprises a linear current regulator arranged for converting, when said corresponding switch is activated, a varying DC bus voltage to a constant current

Description

A Light Emitting Diode, LED, based lighting device arranged for emitting a particular color of light as well as a corresponding method

FIELD OF THE INVENTION

The present invention generally relates to the field of lighting and, more specifically, to a Light Emitting Diode, LED, based lighting device which is arranged to emit a particular color of light. The present invention further relates to a method of operating the LED based lighting device.

BACKGROUND OF THE INVENTION

Lighting devices have been developed that make use of Light Emitting Diodes, LEDs, for a variety of lighting applications. Owing to their long lifetime and high energy efficiency, LED lamps are nowadays also designed for replacing traditional fluorescent lamps, i.e. for retrofit applications. For such an application, a retrofit LED lamp is typically adapted to fit into the socket of the respective lamp fixture to be retrofitted. Moreover, since the maintenance of a lamp is typically conducted by a user, the retrofit LED lamp should ideally be readily operational with any type of suitable fixture without the need for re-wiring the fixture.

The present disclosure is related to multi-channel LED based lighting devices. Each channel may comprise a plurality of LED’s that are capable of emitting light at a particular color. For example, a first channel may be directed to emit red colored light. A second channel may be directed to emit green colored light and a third channel may be directed to emit blue colored light.

In such lighting devices, a fixed voltage source may be used to power the LEDs in each of the channels. The current through each of the channels may be set in the factory by tuning a resistor which is placed in series with the LEDs of a particular channel. One of the downsides of such an approach is related to several disturbing factors such as voltage variations of the power source, cable lengths, i.e. impedances, interactions between channels which may cause errors for the targeted flux and color point.

More specifically, typically, there is a controller to control a plurality of switches, wherein each of the switches is arranged to enable a particular channel. For example, a first switch may enable a red channel, a second switch may enable a green channel, a third switch may enable a blue channel, etc. The switches may be provided width Pulse Width Modulation, PWM, signals having particular duty cycles. The frequency of the PWM signals should be chosen such that it exceeds the refresh rates of the human eye. This would prevent a user from seeing any flickering. By controlling the duty cycle, the contribution of each of the channels to the total amount of light emitted can be controlled, and thus also the color of the light that is emitted by the LED based lighting device.

Often, a user may express, or input, the color that he/she wants the LED based lighting device to emit. As mentioned above, the inventors have found that many disturbing factors may exist which prevents the use of static duty cycles for each of the PWM signals fed to the switches.

SUMMARY

It would be advantageous to achieve a Light Emitting Diode, LED, based lighting device arranged for emitting a particular color of light. It would further be advantageous to achieve a corresponding method.

In a first aspect, there is provided 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 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 may also be based on an intensity level, i.e. dimming, setting; characterized in that each of said plurality of parallel cascaded LED channels comprises a linear current regulator arranged for converting, when said corresponding switch is activated, a varying DC bus voltage to a constant current.

In prior art situations, the current is set by using resistors in each of the LED channels. The voltage over the resistor then effectively determines the current through a particular LED channel. The voltage over the resistor is, however, dependent on the actual bus voltage as the voltage over the LEDs are considerably static. The total voltage over the LEDs comprises the summing of the forward voltages of each of the LEDs. As such, the voltage over the resistor equals the bus voltage minus the total voltage over the LEDs. These resistors may be dimensioned in such a way that a desired current flows through an LED channel when the DC bus voltage has a nominal value.

The inventors have found that the deviation in the DC bus voltage may lead to a reduced, or increased, current through the corresponding LED channel. The amount of current flowing through an LED channel is dimensioned based on a nominal DC bus voltage. However, with increasing DC bus voltage, or decreasing DC bus voltage, the amount of current may not equal the actual desired, or nominal current, through the LED channel. Deviations in the nominal DC bus voltage may, for example, be caused by manufacturing tolerances, load dependencies, or anything alike.

As such, the inventors have found to include, in an LED channel, a linear current regulator arranged for converting, when said corresponding switch is activated, a varying DC bus voltage to a constant current.

In accordance with the present disclosure, linear current regulators may provide a constant current starting from a varying input voltage source. They may be used for replacing discrete-component LED driving solutions in low-voltage applications including 5 V, 12 V or 24 V supplies, providing benefits in terms of precision, integration and reliability. An external resistor may be used to set the actual amount of current with a particular precision.

One of the advantages of the presented disclosure is that the dependency of the actual amount of current flowing through a particular LED channel and the DC bus voltage is removed. In the prior art situation, there is a direct dependency between the amount of current through a particular LED channel and the DC bus voltage. The linear current regulator eliminates such a direct relationship.

A linear current regulator may be implemented by active electronic components, for example transistors, having current-stable nonlinear output characteristic when driven by steady input quantity; i.e. the DC bus voltage. These circuits may behave as dynamic resistors changing their present resistance to compensate current variations. For example, if the load increases its resistance, the transistor decreases its present output resistance, and vice versa, to keep up a constant total resistance in the circuit.

The common emitter configuration driven by a constant input current or voltage and common source, i.e. common cathode, driven by a constant voltage naturally behave as current sources, or sinks, because the output impedance of these devices is naturally high. The output part of the simple current mirror is an example of such a current source. The common base, common gate and common grid configurations can serve as constant current sources as well.

A Junction Gate Field-Effect Transistor, JFET, can also be made to act as a current source by tying its gate to its source. The current then flowing is the drain current for zero bias of the FET. These can be purchased with this connection already made and in this case the devices are called current regulator diodes or constant current diodes or current limiting diodes. An enhancement-mode N-channel Metal-Oxide-Semiconductor Field-Effect Transistor, MOSFET, can be used in a linear current regulator circuit.

Following the above, a linear current regulator may be implemented as a constant-current source or sink which may be formed from one component: a JFET with its gate attached to its source. Once the drain-source voltage reaches a certain minimum value, the JFET enters saturation where current is approximately constant.

It is noted that in may be preferred to implement a linear current regulator in accordance with the present disclosure using discrete components instead of integrated solutions. The reason is that integrated solutions may not be commercially available that could handle sufficient power for power the LED based lighting device. If such integrated solutions do become available, then, of course, these could be used as well.

It is further noted that, in accordance with the present disclosure each of the plurality of parallel cascaded LED channels may further be divided in one, two or more parallel sub-channels. Each sub-channel may comprise a switch and/or each sub-channel may comprise a linear current regulator. Alternatively, one switch may be provided for all parallel sub-channels and/or one linear current regulator may be provided for all parallel sub-channels combined.

In an example, the linear current regulator comprises two PNP transistors, wherein an emitter of a first of said two PNP transistors is connected to a base of a second of said two transistors, and wherein a collector of said second of said two PNP transistors is connected to a base of said first of said two PNP transistors.

The above described example is advantageous as it is a relatively simple implementation, wherein the implementation does cause for a fairly constant current.

In an example, the linear current regulators are embodied in one or more Integrated Circuits, ICs, wherein the switches may also be embodied in said one or more ICs. In a further example, the power supply unit is arranged for providing a 24V DC bus voltage, and wherein each of said LED channels have seven LEDs.

The above described example is directed to a pragmatic embodiment in practice.

In a further example, the at least one of said plurality of LED channels comprises a voltage absorber for compensating lower forward voltages of corresponding LEDs in said channel such that voltages over said linear current regulators are substantially the same.

Typically, the control of the switches, i.e. the duty cycles of the control signals that are provided to the switches, is tuned to a nominal DC bus voltage. By adding voltage absorbers in one or more LED channels, the system may be made more robust against non- nominal DC bus voltages. The voltage absorbers may ensure that the effects of a reduced, or increased, DC bus voltage are more or less the same over each of the LED channels.

The voltage absorbers may, for example, be any of a diode or a resistor. Preferably, the voltage absorber is a diode or a Schottky diode as these types of devices have a substantially fixed amount of voltage drop.

In a second aspect, there is provided a method for operating an LED based lighting device in accordance with any of the previous claims, wherein said method comprises the steps of: providing, by said controller, said 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 said received color set point; converting, by said linear current regulator, when said corresponding switch is activated, said varying DC bus voltage to said constant current.

It is noted that the advantages and definitions as disclosed with respect to the embodiments of the first aspect of the invention also correspond to the embodiments of the second aspect of the invention, being the method of operating an LED based lighting device.

In an example, said linear current regulator comprises two PNP transistors, wherein an emitter of a first of said two PNP transistors is connected to a base of a second of said two transistors, and wherein a collector of said second of said two PNP transistors is connected to a base of said first of said two PNP transistors.

In a further example, the linear current regulators are embodied in one or more Integrated Circuits, ICs. The switches may also be embodied in said one or more ICs.

In an example, the power supply unit is arranged for providing 24V DC bus voltage, and wherein each of said LED channels have seven LEDs.

In a further example, at least one of said plurality of LED channels comprises a voltage absorber, and wherein said method comprises the step of: absorbing, by said voltage absorber, a voltage for compensating lower forward voltages of corresponding LEDs in said channel such that voltages over said linear current regulators are substantially the same.

In yet another example, the voltage absorber is any of a diode and/or a resistor. In an example, the voltage absorber is a Schottky diode having a substantially constant forward voltage for different currents.

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

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows an LED based lighting device in accordance with the prior art; Fig. 2 shows an LED based lighting device in accordance with the present disclosure;

Fig. 3 discloses an example of a linear current regulator in accordance with the present disclosure;

Fig. 4 discloses a graph of current through different LED channels;

Fig. 5 discloses another graph of current through different LED channels.

DETAILED DESCRIPTION

Figure 1 shows an LED based lighting device 1 in accordance with the present disclosure.

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.

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.

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.

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

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.

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 color light that is actually emitted.

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 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.

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 current through the channels that are active at that moment. 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.

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.

It is noted that the parasitic aspects of the present disclosure may originate from the resistor as indicated with “Rcablel”. 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.

Figure 2 shows an LED based lighting device 21 in accordance with the present disclosure.

Here, each of the LED channels 2, 3, 4, 5, 6 comprises a linear current regulator 22, 23, 24, 25, 26. It is noted that, in accordance with the present disclosure, it is not necessary that each of the LED channels 2, 3, 4, 6, 6 comprises such a linear current regulator. Advantages of the present disclosure may already be obtained when one LED channel comprises such a linear current regulator.

Figure 3 discloses an example of a linear current regulator in accordance with the present disclosure.

In the following description, the present disclosure is compared to a prior art reference, which is six LEDs in series and one or more resistors in each LED channel for setting the current through the LED channel. Light output of each string is controlled via PWM control signals to a corresponding switch in the LED channel.

In the present disclosure each LED channel may have seven LEDs and a linear current regulator. As a result, a larger portion of the DC bus Voltage is used for generating light. As a rule of thumb, the Vf of an LED is 3 V. In the prior art situation, 6*3V/24V = 75% of the input power is used for light generation, the other 25% is absorbed in the resistors for setting the current. In the example of the present disclosure, 7*3V/24V = 87,5% of the energy is used for light generation.

A linear current regulator may be connected in each LED channel. As can be seen in figure 2, the linear current regulator may be placed after the PWM, no reference or ground connection is required. An advantage is that there is no standby power for the current source. Figure 3 only discloses one example of a linear current regulator.

In another option, the linear current regulator may be formed from one component, i.e. a JFET with its gate attached to its source. Once the drain-source voltage reaches a certain minimum value, the JFET enters saturation where current is approximately constant. Due to the large variability in saturation current of JFETs, a source resistor may be included which allows the current to be tuned down to a desired value.

Another example is directed to a bipolar junction transistor, BJT, implementation, wherein a Zener voltage stabilizes, i.e. R1 and DZ1, drives an emitter follower loaded by a constant emitter resistor sensing the load current. The external load of this current source may be connected to the collector so that almost the same current flows through it and the emitter resistor. The transistor may adjust the output current so as to keep the voltage drop across the constant emitter resistor almost equal to the relatively constant voltage drop across the Zener diode. As a result, the output current is almost constant even if the load resistance and/or voltage vary.

One aspect of the above described implementations is that the thermal compensation may not be ideal. In bipolar transistors, as the junction temperature increases the base-emitter Voltage, Vbe, drop decreases. In the two previous circuits, a decrease in Vbe will cause an increase in voltage across the emitter resistor, which in turn will cause an increase in collector current drawn through the i.e. the LEDs. The end result is that the amount of 'constant' current supplied is at least somewhat dependent on temperature. The circuit shown in figure 3 may overcome a potential thermal problem.

Figure 4 discloses a graph 61 of current through different LED channels. The current setting, i.e. I CW, I_WW and I lime as indicated in the figure, is such that with these settings the different LED channels may match the target flux for the LED channels. In this way, a controller can set the desired colors of the LED based lighting device.

The horizontal axis represents the supply voltage, i.e. the DC bus voltage. The vertical axis represents the LED drive current scaled to the nominal required current. Nominally, the operating window may be in the flat part of the curve, above the threshold. Even slightly below the threshold the luminous flux variation is smaller than with the reference.

In an aspect of the present disclosure, there is a risk of color spread if one or more of the LED channels is below the threshold. In this case, the luminous flux for one LED channel may decrease while the luminous flux for other strings essentially may remain constant. This would result in a color shift.

An example of this color shift is given in figure 4. Suppose the supply voltage, i.e. DC bus voltage, of the LED based lighting device is at the left vertical line. At this point, the relative current drop with respect to the target is equal for lime and warm white, WW. So, although the absolute flux decreases in these LED channels, the ratio between these channels may remain equal. However, in this example, the current of the LED channel as indicated with CW drops more than in the other LED channels, leading into a shift in the ratio of the fluxes in the channels, which leads to a color shift towards more green and warm white.

Differences in the threshold voltage occur due to binning, LED type, and design drive current.

In an additional example this problem is solved by adding a voltage absorber, which is a component that adds a voltage drop to a LED channel with a lower threshold. The intention of this absorber is to shift the different curves in such a way that the ratio of the currents of the channels remains constant as good as possible.

So in other words: The system is improved in such a way that I_ww/I_lime and I_cw/I_lime show a limited variation for a so large as possible deviation from the nominal supply voltage, i.e. DC bus voltage.

This is shown in figure 5, wherein the tipping points of the three channels are matched by means of an additional voltage absorber.

In this figure, the cool white string has the highest threshold, i.e. 22,5V. The warm white string has a threshold of 21,8V, a difference of 0,7V. It is not possible to lower the threshold of the cool white string without changing other specs, but the threshold of the warm white string can be made higher by adding one or more voltage absorbers, for example diodes like Schottky diode or by adding a resistor in the string with a voltage drop of 0,7V. A combination of a resistor and diode is also possible. This can be used to further tune the curve at the left of the threshold. As a result, both strings show similar behavior for voltage variations which helps to reduce color shift. A similar action can be done with the lime channel. In Figure 5, there is still some deviation in relative currents between the LED channels. This can be further optimized by fmetuning the required voltage shifts. The voltage absorber can be a diode or e.g. a resistor. An advantage of a Schottky diode is that the Vf is nearly constant for different currents, but not every Vf is available. An advantage of the resistor is that the value can easily be chosen in such a way that the voltage drop can be defined with good accuracy, but it slightly varies with changes in current.

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.

Claims

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 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 each of said plurality of parallel cascaded LED channels comprises a linear current regulator arranged for converting, when said corresponding switch is activated, a varying DC bus voltage to a constant current, characterized in that at least one of said plurality of LED channels comprises a voltage absorber for compensating lower forward voltages of corresponding LEDs in said channel such that voltages over said linear current regulators are substantially the same.

2. An LED based lighting device in accordance with claim 1, wherein said linear current regulator comprises two PNP transistors, wherein an emitter of a first of said two PNP transistors is connected to a base of a second of said two transistors, and wherein a collector of said second of said two PNP transistors is connected to a base of said first of said two PNP transistors.

3. An LED based lighting device in accordance with claim 1, wherein said linear current regulators are embodied in one or more Integrated Circuits, ICs.

4. An LED based lighting device in accordance with claim 3, wherein said switches are also embodied in said one or more ICs.

5. An LED based lighting device in accordance with any of the previous claims, wherein said power supply unit is arranged for providing a 24V DC bus voltage, and wherein each of said LED channels have seven LEDs.

6. An LED based lighting device in accordance with claim 5, wherein said voltage absorber is any of: a diode; a resistor.

7. An LED based lighting device in accordance with claim 6, wherein said voltage absorber is a Schottky diode having a substantially constant forward voltage for different currents.

8. A method for operating an LED based lighting device in accordance with any of the previous claims, wherein said method comprises the steps of: providing, by said controller, said 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 said received color set point; converting, by said linear current regulator, when said corresponding switch is activated, said varying DC bus voltage to said constant current.

9. A method in accordance with claim 8, wherein at least one of said plurality of LED channels comprises a voltage absorber, and wherein said method comprises the step of: absorbing, by said voltage absorber, a voltage for compensating lower forward voltages of corresponding LEDs in said channel such that voltages over said linear current regulators are substantially the same.