Patent Grant
8587205
The present invention relates in general to a lighting device comprising a plurality of LEDs as light sources and having only two terminals for receiving power, and more specifically to a LED lighting device having an incandescent lamp color temperature behavior when dimmed. The invention further relates to a kit of parts comprising a LED lighting device and a dimming device.
A traditional light bulb is an example of a lighting device comprising a light source, i.e. the lamp filament, having two terminals for receiving power. When a voltage is applied to such light bulb, a current flows through the filament. The temperature of the filament rises due to Ohmic heating. The filament generates light, having a color temperature related to the temperature of the filament, which may be considered as being a black body. Normally, a lamp has a nominal rating corresponding to a nominal lamp power at nominal lamp voltage, for instance 230 VAC in Europe, and corresponding to a certain nominal color of the emitted light.
Since many decades, people have been used to the light of incandescent lamps of different powers. The light of an incandescent lamp provides a general feeling of well-being. Generally, the lower the power of the incandescent lamp is, the lower the color temperature of the light emitted by the lamp is. As a characterization, the human perception of the light is “warmer” when the color temperature is lower. With one and the same incandescent lamp, the lower the power supplied to the lamp is, which occurs when the lamp is dimmed, the lower the color temperature of the emitted light is.
It is already known that it is possible to dim a lamp, i.e. to reduce the light output. This is done by reducing the average lamp power by reducing the average lamp voltage, for instance by phase cutting. As a result, also the temperature of the filament reduces, and consequently the color of the emitted light changes to a lower color temperature. For instance, in a standard incandescent lamp having 60 W nominal rating, the color temperature is about 2700 K when the lamp is operated at 100% light output while the color temperature is reduced to about 1700 K when the lamp is dimmed to a 4% light output. As is commonly known to a person skilled in the art, the color temperature follows the traditional black body line in a chromaticity diagram. A lower color temperature corresponds to a more reddish impression, and this is associated with a warmer, more cozy and pleasant atmosphere.
A relatively recent tendency is to replace incandescent light sources by lighting devices based on LED light sources, in view of the fact that LEDs are more efficient in converting electric energy to light and have a longer lifetime. Such lighting device comprises, apart from the actual LED light source(s), a driver that receives the mains voltage intended to operate an incandescent lamp and converts the input mains voltage to an operating LED current. LEDs are designed to provide a nominal light output when operated with a constant current having a nominal magnitude. An LED can also be dimmed. This can be done by reducing the current magnitude, but this typically results in a change of the color of the light output. In order to keep the color temperature of the generated light as constant as possible, dimming an LED is typically done by Pulse Width Modulation, also indicated as duty cycle dimming, wherein the LED current is switched ON and OFF at a relatively high frequency, wherein the current magnitude in the ON periods is equal to the nominal design magnitude, and wherein the ratio between ON time and switching period determines the light output.
It is desirable to have a lighting device having one or more LEDs as light source, wherein the dimming behavior of the traditional incandescent lamp is simulated so that, on dimming, the color temperature of the output light also follows a path (preferably close to the black body line) from a higher color temperature to a lower temperature.
Lighting devices capable of such functionality have already been proposed, for instance in US-2006/0273331. Such prior art devices comprise at least two LEDs of mutually different colors, each provided with a corresponding current source, and an intelligent control device, such as a microprocessor, controlling the individual current sources to change the relative light outputs of the respective LEDs. The known device receives an input voltage signal that carries power and a control signal. In the device, the control signal is taken from the input signal and transferred to the intelligent control device, that controls the individual current sources on the basis of the received control data. By changing the ratio between the respective light outputs, the relative contributions to the overall light output is changed and hence the overall color of the overall light output, as perceived by an observer, is changed. Such lighting device, therefore, requires a separate control input signal.
In LED lighting devices, a behavior of the color temperature of the LED light can be obtained which, in dimming conditions, is similar to that of an incandescent lamp, but until now only at the expense of extensive current control, such as e.g. known from DE10230105. The necessity of adding controls to the LED lighting device for the desired color temperature behavior increases the number of components, increases the complexity of the lighting device, and increases costs. These effects are undesirable.
The present invention aims to provide a LED circuit for such LED lighting device, and a LED lighting device comprising such LED circuit, wherein an intelligent control can be omitted and wherein a feedback sensor can be omitted.
It would be desirable to provide an LED lighting device having a color temperature behavior, when dimmed, resembling or approaching the color temperature behavior of an incandescent lamp, when dimmed. It would also be desirable to provide an LED lighting device having an incandescent lamp color temperature behavior, when dimmed, without the need of extensive controls.
According to an aspect of the present invention, an LED lighting device comprises a single dimmable current source and an LED module receiving current from the current source. The LED module behaves as a load to the current source, similar to an array existing of LEDs only. Within the LED module, an electronic circuit senses the current magnitude of the input current, and distributes the current to different LED sections of the LED module on the basis of the sensed current magnitude. No intelligent current control is needed in the current source.
To better address one or more of these concerns, in an aspect of the invention an LED lighting device is provided, comprising a plurality of LEDs, and two terminals for supplying current to the lighting device. The lighting device comprises a first set of at least one LED of a first type producing light having a first color temperature, and a second set of at least one LED of a second type producing light having a second color temperature different from the first color temperature. The first set and the second set are connected in series or in parallel between the terminals. The lighting device is configured to produce light with a color point varying in accordance with a blackbody curve at a variation of an average current supplied to the terminals.
A color temperature behavior of an incandescent lamp may be described by the following relationship:
CT(x%)=CT(100%)*(x/100)1/9.5
where CT(100%) is the color temperature of the light at full power (100% current) of the lamp, CT(x %) is the color temperature of the light at x % dimming of the lamp (x % current, with 0≦x≦100).
In an embodiment, the first set has a varying first luminous flux output as a function of junction temperature of the LED of the first type, and the second set has a varying second luminous flux output as a function of junction temperature of the LED of the second type, and wherein, at varying junction temperatures, the ratio of the first luminous flux output to the second luminous flux output varies. In particular, when the first color temperature is lower than the second color temperature, the lighting device is configured such that, at decreasing junction temperatures, the ratio of the first luminous flux output to the second luminous flux output increases, and vice versa. In such a configuration, e.g. having the first set connected in series with the second set, the first luminous flux output increases relative to the second flux output when the lighting device is dimmed, thereby producing light having a lower color temperature.
In an embodiment, the first set has a first dynamic electrical resistance, and the second set has a second dynamic electrical resistance. When e.g. the first set is connected in parallel with the second set, different luminous flux outputs of the first set and the second set result, which can be designed to produce light having a lower color temperature when dimmed.
In another aspect of the present invention, a lighting kit of parts is provided, comprising a dimmer having input terminals adapted to be connected to an electrical power supply, and having output terminals adapted to provide a variable electrical power. An embodiment of the lighting device according to the present invention has terminals configured to be connected to the output terminals of the dimmer.
Further advantageous elaborations are mentioned in the dependent claims.
These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which:
More generally, when Iin is zero or close to zero, p is equal to a minimum value Pmin which may be equal to zero and q is equal to a maximum value Qmax which may be equal to one. When Iin is at a predetermined nominal (or maximum) level, q is equal to a minimum value Qmin which may be equal to zero and p is equal to a maximum value Pmax which may be equal to one. There is at least a range of input currents where dp/d(Iin) is always positive and dq/d(Iin) is always negative. There may be a range of input currents where p and q are constant. There may be a range of input currents where p=0. There may be a range of input currents where q=0.
In accordance with the present invention, the important issue is that the division circuit is capable of individually changing the current in at least one LED array. There are several ways possible for doing so. For instance, it may be that the two arrays 113, 114 are arranged in parallel, and that the input current is split into a first portion going to first array 113 and a second portion going to second array 114, as illustrated in
The following contains illustrative examples of exemplary implementations embodying the present invention, but it is noted that these examples are not considered to be limiting for the invention. It is noted that in the following only the LED module will be shown; the driver 101 will be omitted for sake of simplicity, since the driver 101 may be implemented by a standard LED driver.
Thus, in the embodiment shown, the collector-emitter path of the transistor 320 is connected in parallel to a portion of the string of white LEDs 341, 342, 343; this can be considered as constituting a total of three strings, one string containing two white LEDs 342, 343 parallel to on string containing one amber LED 371, and these two strings being connected in series to a third string containing one white LED 341. Alternatively the collector-emitter path of the transistor 320 could be connected in parallel to the entire string of white LEDs 341, 342, 343, in which case there would be only two strings. In the example, there are three white LEDs 341, 342, 343 in series, but his could be two or four or more. In this example, the collector line contains only one amber LED, but this line might contain a series arrangement of two or more amber LEDs. In general, it is preferred that the number of amber LEDs connected in series in the collector line is less than the number of series-connected white LEDs in the string parallel to the collector-emitter path of the transistor 320.
The operation is as follows. With increasing input current, the voltage drop over the current sensor resistor 350 rises, thus the voltage between input terminals 111, 112 rises, thus the voltage at the opamp's non-inverting input rises. Since the voltage drop over the string of white LEDs 341, 342, 343 is substantially constant, the voltage rise between input terminals 111, 112 is substantially equal to the rise of voltage drop over the current sensor resistor 350 while the voltage rise at the opamp's non-inverting input is smaller than the voltage rise between input terminals 111, 112, the ratio being defined by the resistors 331, 332 of the voltage divider 320. Thus, the voltage drop over the feedback resistor 360 should be reduced, and hence the current in the collector-emitter path of the transistor 320 is reduced.
The Buck converter is operated in CCM (continuous conduction mode), such that the ripple in Ia is small compared to its average value. The input current Is' of the Buck converter is a switched current, having a peak value equal to Ia and a duty cycle δ. The switched current Is' is supplied from the filter capacitor Cb, and the input current Is to this filter capacitor Cb is in fact the average value of Is'. For the Buck converter operating in CCM and neglecting the current ripple, we can derive Is=δIa. It should be clear that the current in the first LED string 113 is reduced by the input current Is to the filter capacitor Cb, or
Iw=Iin−Is=Iin−δIa.
So, if δ is changed to adapt the amber current Ia, the current Iw through the white LED's also changes. The current source Iin has the same linear dependency on the dim setting as shown in FIG. 2A/B. The input current Iin is monitored by current sensor 116, generating a sense signal Vctrl, and the control circuit 620 changes the duty cycle δ of the Buck converter, and as such changes both the currents Iw and Ia.
In principle, the same white/amber current divisions as shown in FIG. 2A/B can be realized with this embodiment. The advantage compared to the other embodiments is the higher efficiency. The Buck converter inherently has a higher efficiency than a linear current regulator, as the other embodiments of
It is noted that the Buck converter regulating the amber LED current Ia is preferably a hysteretic mode controlled Buck converter.
It is also possible to obtain the desired behavior on the basis of intrinsic characteristics of the LEDs itself.
The one or more LEDs 11 of the first type are selected to have a first luminous flux output as a function of temperature having a gradient which is different from the gradient of a second luminous flux output as a function of temperature of the one or more LEDs 12 of the second type. In practice, the luminous flux output FO variation may be characterized by a so-called hot-coldfactor, indicating a percentage of luminous flux loss from 25° C. to 100° C. junction temperature of the LED. This is illustrated by reference to
The resistor 59 is a negative temperature coefficient, NTC, type resistor, which will compensate relatively slow temperature variations by the variation of its resistance value.
The one or more LEDs 51 of the first type are selected to have a first dynamic resistance (measured as a ratio of a forward voltage across the LED(s) and a current through the LED(s)) which is different from a second dynamic resistance of the one or more LEDs 52 of the second type connected in series with the resistor 59. As a result, a ratio of the current through the one or more LEDs 51 of the first type and the current through the one or more LEDs 52 will be variable. This is illustrated by reference to
The current sources 18, 58 are configured to provide a DC current which may have a low current ripple. For dimming purposes, the current sources 18, 58 may be pulse width modulated. In case of the current source 18 feeding the lighting device 10, the junction temperatures of the LEDs will decrease when dimming. In case of current source 58, the average current during the time that a current flows in the lighting device 50, should be decreased during dimming. Thus, each current source 18, 58 is to be considered as a dimmer having output terminals which are adapted to provide a variable electrical power, in particular a variable current, and the terminals 14, 16 and 54, 56, respectively, are configured to be connected to the output terminals of the dimmer.
In the above it has been explained that in a lighting device sets of LEDs are employed using the natural characteristics of the LEDs to resemble incandescent lamp behavior when dimmed, thereby obviating the need for sophisticated controls. A first set of at least one LED produces light with a first color temperature, and a second set of at least one LED produces light with a second color temperature. The first set and the second set are connected in series, or the first set and the second set are connected in parallel, possibly with a resistive element in series with the first or the second set. The first set and the second set differ in temperature behavior, or have different dynamic electrical resistance. The light device produces light with a color point parallel and close to a blackbody curve.
As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.
The terms “a” or “an”, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.
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.
The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.
Summarizing, in a lighting device, the present invention provides that sets of LEDs are employed using the natural characteristics of the LEDs to resemble incandescent lamp behavior when dimmed, thereby obviating the need for sophisticated controls. A first set of at least one LED produces light with a first color temperature, and a second set of at least one LED produces light with a second color temperature. The first set and the second set are connected in series, or the first set and the second set are connected in parallel, possibly with a resistive element in series with the first or the second set. The first set and the second set differ in temperature behavior, or have different dynamic electrical resistance. The light device produces light with a color point parallel and close to a blackbody curve.
The present invention also relates to a lighting kit of parts, comprising:
a dimmer having input terminals adapted to be connected to an electrical power supply, and having output terminals adapted to provide a variable electrical power; and
a lighting device according to any of the attached claims, wherein the terminals of the lighting device are configured to be connected to the output terminals of the dimmer.
While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims.
For instance, different colors can be used. For instance, instead of amber, it would be possible to use yellow or red. Further, it is noted that in the example the contribution of the white LEDs reduces to zero with reducing input current, but this is not necessary.
Further, while in the above the driver 101 has been described as being capable of receiving dimmed mains from a dimmer 9, it is also possible that the driver 101 is designed for being dimmed by remote control while receiving normal mains voltage. The important aspect is that the driver 101 is acting as a current source and is capable of generating dimmed output current, which is received by the LED module as input current. Thus, the light output level is determined by the driver 101 by generating a certain output current to the LED module, and the color of the light output is determined by the LED module in dependency of the current received from the driver 101.
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 fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.
In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09154950 | Mar 2009 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2010/051053 | 3/11/2010 | WO | 00 | 12/22/2011 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2010/103480 | 9/16/2010 | WO | A |
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