Patent Application
20100051976
1. Field of the Invention
The present invention relates to a lighting assembly comprising one or more light emitting diodes. The invention addresses the need for providing adequate galvanic insulation while ensuring sufficient dissipation of heat generated by the light emitting diodes when they are in use.
2. Description of the Related Art
Until a decade ago, light emitting diodes were primarily used in situations requiring lower levels of brightness, such as indicator lights on electronic equipment. The light emitting diodes used for this purpose have a low power consumption and accordingly generate relatively small amounts of heat when in use.
Other uses, requiring greater levels of brightness, include traffic lights and automobile taillights have been developed in the past decade. For these uses, the single color character of light emitting diodes is not a hindrance. In fact, it is used to advantage. Although multiple light emitting diodes are generally used for this kind of application, they can be placed sufficiently far apart that heat dissipation is not a major problem.
It has been suggested to use a combination of light emitting diodes of different colors to produce light that approximates what is considered white light. For a proper mixing of the light from different light emitting diodes it is necessary for optical homogeneity to place these diodes in close physical proximity to each other. Accordingly, dissipation of heat generated by the light within diodes becomes more problematic. For use as a light source competing in brightness with a conventional incandescent light bulb or even halogen light, a lighting assembly based on light emitting diodes requires the use of so-called power LEDs, which further increases the challenge posed by the heat dissipation requirement.
On the other hand, it is important to provide sufficient galvanic insulation of the LED chip assembly from its required heat sink in order to ensure safe handling and use of a light emitting diode based lighting assembly, in particular if the light assembly is to be connected directly to the electric grid as a power source, as distinguished from being connected to a source of low voltage electricity. The needs to provide galvanic insulation and heat dissipation tend to contradict each other.
It is an object of the present invention to provide a LED based lighting assembly that provides sufficient heat dissipation even if the assembly comprises a plurality of light emitting diodes placed closely together, while at the same time providing sufficient galvanic insulation for the lighting assembly to be safely used with the power grid as its power source.
The present invention relates to a LED based lighting assembly comprising:
Preferably, the electric and heat conducting material is a metal. The layer of electric and heat conducting material may have a thickness in the range of from 5 to 40 micrometers, preferably from 10 to 30 micrometers.
The lighting assembly of the present invention is particularly suitable for use with a plurality of light emitting diodes which, in combination, emits white light.
a,
3
b, and 3c show the voltage patterns at different points in the circuit of
The present invention relates to a LED based lighting assembly comprising:
As a general proposition, many electric conducting materials are also heat conducting materials. However, for the power needs of even the bright power LEDs, the electric power requirements are small, on the order of a few Watts. Layers of electric conducting materials used in conjunction with LEDs are therefore generally very thin. This allows for these layers to be deposited by thin-film techniques, such as generally applied in the semiconductor industry for integrated circuits and packaging industry. The deposition of thin layers may be realized by a method such as plating, chemical vapor deposition, or sputtering.
For the purpose of the present invention, such layers are not considered “heat conducting” simply because they are too thin to dissipate a meaningful amount of heat. To be considered a “layer of heat conducting material” within the meaning of the present invention, it is preferred that the layer have a thickness in the range of from 5 to 40 micrometers, preferably from 10 to 30 micrometers. Preferred materials for use in this layer of heat conducting material are metals, in particular silver, gold, copper, and aluminum. Silver is most preferred.
Because of the desired thickness it is generally not convenient to deposit the layer of heat conducting material by means of a thin-film technique. It has been found that lithographic techniques are suitable for depositing heat conducting layers of the thickness desired for the lighting assembly of the present invention. Lithographic techniques have the disadvantage, as compared to thin-film deposition techniques, that the surface of the deposited layer is comparatively rough. It may be necessary to subject the layer to a post-treatment after deposition, to decrease the surface roughness of the layer. Examples of suitable post-treatment techniques include galvanization or lepping (polishing).
The main purpose of the disc of ceramic material is to provide galvanic insulation while maintaining heat conduction. Any ceramic material having the required insulating properties is suitable for this purpose. A secondary purpose of the disc of ceramic material is to provide mechanical strength to the lighting assembly. Ceramic materials having considerable mechanical strength are therefore preferred. Examples of suitable materials include the so-called refractory oxides, in particular aluminum oxide also known as co-fired ceramics.
The term “disc” as used herein refers to a slice of ceramic material of any suitable shape to accommodate the LED or LEDs of the lighting assembly. The shape may be circular, rectangular, square, or any other geometric shape suitable for the purpose. The disc of ceramic material preferably has a thickness in the range of from 200 to 500 micrometers.
The disc has a mounting surface and an attachment surface. The layer of electric and heat conducting material is provided on the mounting surface of the disc. The LED chip or LED chips are mounted on the mounting surface of the disc, in such a way that they are in heat conducting contact with the electric and heat conducting material. The heat dissipation of the lighting assembly may be further improved by providing a heat sink at the attachment surface of the disc.
Mounted on heat conducting layer 13 are LED chips 17. In this preferred embodiment the LED chips are encapsulated in a dome 18 of substantially transparent material, which may be of a type well known in the art. Examples of suitable materials include epoxy materials and polyurethane materials.
For use as a light source replacing, for example, an incandescent light bulb, it is desirable that the lighting assembly emit light having a color rendering index of at least 80, more preferably a color rendering index of at least 90. In addition, it is desirable to provide light with a color temperature in the range of from 2000 to 8000 Kelvin, more preferably in the range of 2000 to 3700 Kelvin.
In a preferred embodiment, the lighting assembly of the present invention comprises a first group of at least four LEDs and a second group of at least one LED. The at least four LEDs of the first group form a rectifier circuit bridge. This arrangement makes it possible to connect the circuit to either an alternating current source or a direct current source, as will be explained in more detail with reference to
When the circuit is connected to an alternating current source, the LEDs of the rectifier bridge emit light that is flashing at a frequency equal to the frequency of the cycles of the alternating current source. This frequency is of a relatively low value, for example 50 hertz in most European countries and 60 hertz in North America. Different from incandescent light bulbs, light emitting diodes stop emitting light immediately when the power supply is interrupted. As a result, a light emitting diode connected to the power grid emits light that the human eye may experience as unstable and flickering. For this reason, it is preferred to use red LEDs emitting light having a peak wavelength in the range of 600 nm to 630 nm for the at least four LEDs in the rectifier bridge which alternate between each other. The human eye is not very sensitive to light in his wavelength range, and therefore no unpleasant flickering is experienced.
The intermittent direct current coming off the rectifier bridge can be used to power the LEDs of the second group. This intermittent direct current has a frequency that is twice the value of that of the connected alternating current source. As result, the LEDs of the second group can be selected to emit light in wavelength areas where the human eye is more sensitive, such as green. In a preferred embodiment the second group comprises at least one green LED, emitting light having a peak wavelength in the range of 500 to 560 nm and at least one blue LED emitting light having a peak wavelength in the range of 460 to 500 nm. In a still further preferred embodiment, the second group consists of three green LEDs and one blue LED.
An important advantage of this multiple LED configuration is the ability to apply same-colored LED chips with a substantial spread in dominant wavelength within the ranges mentioned hereinabove. The spreading allows less critical binning, resulting in a higher yield of the usable chips, and therefore reducing cost. Advanced sorting techniques for the optimal spreading of wavelength within the same embodiment will be desirable.
Shown further in
The brightness of LEDs generally varies with the temperature of the LED, in particular the temperature at the p-n transition layer also called junction (Tj). This temperature is determined by the rate at which heat is generated within the LED, and the rate at which the heat can be dissipated to the environment. As a result, the operating temperature of the LED tends to vary with fluctuations in the ambient temperature. LEDs of different color types respond differently to fluctuations in the ambient temperature. This means that the color of the light emitted by the assembly tends to shift if the ambient temperature goes up or down. Although it is possible to adjust the color temperature by means of potentiometer 51, this requires manual intervention and is therefore not very practical.
Light sensor 54 is provided to ensure continuous and automatic fine-tuning and maintaining of the color temperature through dynamic feedback. If, because of external circumstances such as an increase in ambient temperature, the amount of red light emitted by the LED assembly drops, light sensor 54 acts to reduce the amount of electric power fed to the LEDs of the second group. In the embodiment of
The lumen maintenance over the life time of the red and green LEDs is different. Red LEDs consisting of AlInGaP compound have a much better lumen maintenance over the years of life time than green GaN LEDs. This ageing over the years can also be compensated with the same transistor circuit as used for the heat compensation, since this circuit is based on an optical dynamic feedback. This requires a light sensor in series with potentiometer (51).
For a further improvement of the spectral distribution of the light emitted by the assembly it is desirable to add to the circuit at least one amber LED (not shown), having a peak wave length in the range of 570-590 nm.
A particular advantage of the arrangement shown in
For AC operation the circuit is provided with capacitor 53, which reduces the power received from external AC power source 60, which has a voltage V and a frequency f, to a low voltage intermittent power of the same frequency while maintaining a constant current. For example, AC power of 230 Volts and 50 Hz may be converted to a constant AC current of 350 mA resulting in an AC voltage of 12 Volts over the embodiment.
c shows the current wave form fed to the LEDs of the second group. This is an intermittent DC with a frequency of 2 f, or, in this case, 100 Hz.
It will be understood that the LED chips will emit light only when the voltage of the power feed exceeds a threshold value. Accordingly, even the LED chips of the second group, which are powered by the intermittent direct current of
The capacitance of capacitor 53 may be calculated from the operating voltage Vop and the operating current Iop of the LED assembly, as follows.
V
op
=V
op,21
+V
op,31
+V
op,32
+V
op,24,
wherein Vop,21 is the operating voltage of LED 21, etc. obtained from the manufacturer's specification.
The effective resistance Reff of the assembly is calculated from
R
ef
f=V
op
/I
op
And the reactance Rx of capacitor 52 from
R
x
=R
eff*(Vac−Vop)/Vop
wherein Vac is the voltage of external AC source 60.
Finally, the capacitance C of capacitor 53 is calculated from
C=1/(Rx*2π*f)
If Vac=230 Volts and f=50 Hz, the required capacitor for an assembly having an operating voltage of 12 V and an operating current of 350 mA is 4.9 μF.
Referring again to
Electrostatic discharge is harmful to any type of semiconductor, including light emitting diodes. To protect against electrostatic discharge (EDS), varistors 56 is provided, placed in parallel with the LEDs or the plurality of LEDs. With this provision it is possible to provide an electrostatic discharge protection of at least 8 kV.
To protect the LEDs from the environment and to provide mechanical strength, the LEDs are preferably individually encapsulated in a substantially transparent material. This encapsulation serves also a better optical interface from the light emitting junction to the ambient. Suitable transparent materials are well-known in the art. Preferred for use herein are epoxy materials and polyurethane materials.
Although light emitted by red, green and blue can be mixed to a white light, the spectral distribution is not optimum, because wavelengths between 560 and 600 nm are underrepresented in the spectral distribution. It is desirable to substitute to the lighting assembly at least two red LEDs by amber LEDs emitting light having a peak wavelength in the range of 570 to 590 nm. The amber LEDs are preferably placed in alternating order in the bridge of the red LEDs. For example, LEDs 21 and 22 in
The skilled person will appreciate that the LEDs for use in the lighting assembly may be of the horizontal type or they may be of the vertical type. It is even possible to manufacture the lighting assembly with a combination of LEDs of the horizontal type and often vertical type. This flexibility is another advantage of the lighting assembly of the present invention.
Vertical type LEDs have one electrical connection on the bottom and one at the top of the chip connected to the circuit through a wire bond. The thermal conduction of electric and thermal vertical conducting LEDs is ten times better than horizontal structured LEDs.
A further major advantage of vertical LEDs is the identical planar radiation pattern of the red and green chips resulting in an optimal mix for homogenous white light. The mixing of light of different colors may be further improved by optimizing the geometrical arrangement of the LEDs, for example as shown in
Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06124144.4 | Nov 2006 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP07/62421 | 11/15/2007 | WO | 00 | 5/15/2009 |
