Abstract:
A color tunable light emitting diode is disclosed in the present invention. At least two light emitting chip groups each has a number of light emitting chips mixed with at least one phosphor to produce light with a specific correlated color temperature. By supplying tunable currents to the light emitting chip groups, properly arranging the light emitting chips and providing a suitable substrate, the color tunable light emitting diode can be achieved. The present invention provides a simple and workable method to manufacture tunable light emitting diodes which fulfill the requirement of compact design of modern electronic products.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The following co-pending US patent applications are incorporated herein by reference: US Pub. Nos. 2008/0017962A1, 2008/0017876A1, 2008/0017880A1, 2008/0017963A1, 2008/0191605A1, 2008/0194054A1, 2008/0197370A1, 2009/0032832A1, and 2008/0210963A1. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to a light emitting diode. More specifically, the present invention relates to a tunable correlated color temperature light emitting diode. 
       BACKGROUND OF THE INVENTION 
       [0003]    With the decreasing sources of energy, people are urgently looking for alternative solutions to fulfill the increasing demands worldwide. One of the demands comes from lightening. Therefore, providing economical lightening devices or saving power consumption is a hot topic among these business entities in this field. Light Emitting Diode (LED) is a potential product for people to find out answers. LEDs have advantages of small size, long working life, low power consumption, etc. As to applicability, white light LEDs play the key role. 
         [0004]    White light LEDs are widely used as backlight for mobile phones and liquid crystal displays. They can also be used as a lamp for lightening. However, there is no simple way to produce a LED generating pure white light. White light emitted from LED can be achieved by combination of different light sources or using phosphors. Please refer to  FIG. 1A  to  FIG. 1C . Three conventional methods to get white light with LEDs are provided. As shown in  FIG. 1A . A green, blue and red LEDs are put very close to one another and emit light. The combination of green, blue and red light covers most of visible light spectrum as if it were “white light”, but there are some defects. First, since three LEDs are very close in space, circuit layout is difficult. Besides, red LEDs suffer from decay more seriously than blue and green LEDs. That is to say, if a specific light having a small range of correlated color temperature (CCT) has to keep emitting, the red LED needs to be adjusted often. It is not convenient for product users to do so. An alternative method to tune red LED is using feedback control to adjust the current flowing into it. However, it is still troublesome and increases cost of white light LEDs. 
         [0005]    A second way to provide white light is to use blue LED and a kind of yellow phosphor. Please see  FIG. 1B . A white light LED is made by covering a blue light emitting chip with yellow phosphor and epoxy resin. Light from the blue LED excites the phosphor to generate an excited light. Spectrum of the excited light covers the rest of visible light portion that blue light doesn&#39;t have. Therefore, the combined light seems a white light. The light is not exact white since some wavelengths are not what people sense from daylight. The method is simple in manufacturing and has good color rendering, excellent stability and reliability. However, due to the deviation during packaging process and variation of phosphors, performance of the LEDs compared to the ideal light shows a Gaussian distribution for the same batch of LEDs manufactured. Hence, yield rate of LEDs is low. Heat generated affects phosphor, too. An alternative is to package a red light emitting chip with the blue LED to solve the problem of variation of phosphors, but red light emitting chips have a problem of decay so that workability is limited. 
         [0006]    Similarly, a third conventional method shown in  FIG. 1C  utilizes a blue LED and multiple phosphors. The combined light of each light excited from the phosphors and the blue LED light also covers the visible light spectrum. The method provides stable white light with better color rendering compared to the second method. Nevertheless, stability and reliability are not that good since red phosphor is not stable and decays with time. That leaves room for improvement. 
         [0007]    If a decayed LED or a LED with decayed phosphor can be tuned, a desirable light is obtained as time goes by. Moreover, the way to achieve the desirable light is able to provide customized light sources. A prior art was proposed in US Patent Pub. No. 2008/0278927A1. Please refer to  FIG. 2A  and  FIG. 2B . A color tunable light source  1  forms an array of LED arrangements  2  and  3 . The LED arrangement  2  has LED  9  which generates excitation energy  11  to a region of phosphor material  7  to irradiate light  13 . Combination of the excitation energy  11  and irradiated light  13  forms combined light  4 . Similarly, the LED arrangement  3  has LED  10  which generates excitation energy  12  to a region of phosphor material  8  to irradiate light  14 . Combination of the excitation energy  12  and irradiated light  14  forms combined light  5 . A light output  6  is seen as a white light. 
         [0008]    If the tunable light source  1  shows light with color deviated from a specific color (for example, it is a little reddish compared with the specific correlated color temperature), an adjustment can be done by changing currents provided to each LED arrangement  2 ,  3 . It reveals an easy tuning method for LEDs. However, in practice, LED arrangements to show a specific color is not limited to two. Arrangement itself also plays an important role to CCT displayed. Moreover, installation of current supply device needs to be available in a compact solution. These are the challenges that &#39; 927  can&#39;t solve. However, with a proper package process and a substrate, plus a better LED arrangement, the problems mentioned above can be easily solved. 
       SUMMARY OF THE INVENTION 
       [0009]    This paragraph extracts and compiles some features of the present invention; other features will be disclosed in the follow-up paragraphs. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. 
         [0010]    In accordance with an aspect of the present invention, a color tunable light emitting diode comprises: a substrate; at least two light emitting chip groups, each emitting a correlated color temperature light and comprising: a plurality of light emitting chips for outputting a specific light, formed on the substrate; and at least one phosphor for generating an excited light by excitation of the specific light, attached to the light emitting chips by a fixing agent, wherein the correlated color temperature lights is a mixture of the specific light and the excited light; and a control circuit connected to the light emitting chip groups for supplying tunable currents to the light emitting chip groups respectively to provide a tunable light by combining the correlated color temperature light from the light emitting chip groups. 
         [0011]    Preferably, the substrate is a silicon substrate, a ceramic substrate or a printed circuit board. 
         [0012]    Preferably, the substrate is a cavity substrate or a flat substrate. 
         [0013]    Preferably, the tunable currents each has a fixed magnitude for each of the light emitting chip groups. 
         [0014]    Preferably, the tunable currents each has a periodical change in magnitude for each of the light emitting groups to change correlated color temperature of the specific light. 
         [0015]    Preferably, the periodical change is controlled by a duty cycle of the tunable current flowing therethrough by means of pulse width modulation (PWM). 
         [0016]    Preferably, the fixing agent is silicone. 
         [0017]    Preferably, the light emitting chip groups are arranged as a 2-dimensional array with a periodically alternating sequence of the light emitting chips of each light emitting group in any row or column of the array. 
         [0018]    Preferably, the light emitting chip groups form a plurality of 2-dimensional patterns. 
         [0019]    Preferably, the control circuit is formed on the substrate. 
         [0020]    Preferably, the control circuit is in form of a die bonded on the substrate. 
         [0021]    Preferably, the control circuit is in form of an integrated circuit on the substrate. 
         [0022]    Further in accordance with the other aspect of the present invention, a method of generating desirable light, comprises the steps of: a) attaching at least one phosphor onto a plurality of light emitting chips; b) emitting a specific light by each of the light emitting chips; c) exciting the phosphor by the specific light from the light emitting chip to generate an excited light; d) mixing the specific light of each of the light emitting chips and the excited light to form a correlated color temperature light; and e) supplying tunable currents to each of the light emitting chips to provide a tunable light by combining the correlated color temperature lights of each of the light emitting chips. 
         [0023]    Preferably, the tunable currents each have a fixed magnitude for each of the light emitting groups. 
         [0024]    Preferably, the tunable currents each has a periodical change in magnitude for each of the light emitting groups to change correlated color temperature of the specific light. 
         [0025]    Preferably, the periodical change is controlled by a duty cycle of the tunable current flowing therethrough by means of pulse width modulation (PWM). 
         [0026]    Preferably, step a) uses silicone to attach the phosphor. 
         [0027]    Preferably, the light emitting groups are arranged as a 2-dimensional array with a periodically alternating sequence of the light emitting chips of each light emitting group in any row or column of the array. 
         [0028]    Preferably, the light emitting groups form a plurality of 2-dimensional patterns. 
         [0029]    Preferably, the method further comprising step a 1 ) providing a substrate to support the light emitting chips. 
         [0030]    Preferably, the substrate is a silicon substrate, a ceramic substrate or a printed circuit board. 
         [0031]    Preferably, the substrate is a cavity substrate or a flat substrate. 
         [0032]    Preferably, the tunable currents are provided by a control circuit in form of a die mounted on the substrate. 
         [0033]    Preferably, the tunable currents are provided by a control circuit in form of an integrated circuit on the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1A  illustrates a prior art for producing white light with a green, blue and red light emitting diodes. 
           [0035]      FIG. 1B  illustrates another prior art for producing white light by covering a blue light emitting chip with yellow phosphor and epoxy resin. 
           [0036]      FIG. 1C  illustrates yet another prior art for producing white light by utilizing a blue LED and multiple phosphors. 
           [0037]      FIG. 2A  shows a prior arrangement of LEDs. 
           [0038]      FIG. 2B  details the structures of LED arrangements in  FIG. 2A . 
           [0039]      FIG. 3  illustrates a first embodiment of the present invention. 
           [0040]      FIG. 4A  illustrates the composition of a first light emitting unit in the first embodiment. 
           [0041]      FIG. 4B  is a CIE 1931 color space chromaticity diagram for the first embodiment. 
           [0042]      FIG. 5  is a CIE 1931 color space chromaticity diagram for a second embodiment. 
           [0043]      FIG. 6  illustrates the second embodiment of the present invention. 
           [0044]      FIG. 7A  illustrates the composition of the first light emitting unit in the second embodiment. 
           [0045]      FIG. 7B  illustrates the composition of the second light emitting unit in the second embodiment. 
           [0046]      FIG. 8  shows duty cycles of currents for two light emitting groups in the second embodiment. 
           [0047]      FIG. 9  is a CIE 1931 color space chromaticity diagram for a third embodiment. 
           [0048]      FIG. 10  illustrates the third embodiment of the present invention. 
           [0049]      FIG. 11A  illustrates the composition of the first light emitting unit in the third embodiment. 
           [0050]      FIG. 11B  illustrates the composition of the second light emitting unit in the third embodiment. 
           [0051]      FIG. 11C  illustrates the composition of the third light emitting unit in the third embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0052]    Please refer to  FIG. 3 . A first embodiment is illustrated. According to the present invention, a color tunable light emitting diode  100  comprises five first light emitting units  102 , four second light emitting units  104 , a control circuit die  106  and a ceramic substrate  108 . The control circuit die  106  is bonded on the ceramic substrate  108  to supply tunable currents to the first light emitting units  102  and the second emitting units  104 . The five first light emitting units  102  form a first light emitting group and link to the control circuit die  106  by bonding wires (not shown). The four second light emitting units  104  form a second light emitting group and link to the control circuit die  106  by other bonding wires (not shown). There are nine cavities  1001  in an array on the ceramic substrate  108 . The array has three rows and three columns and is 2-dimensional on the surface of the ceramic substrate  108 . Each of the cavities  1001  is used to support and fix one first light emitting unit  102  or one second light emitting unit  104 . The spacing between adjacent two light emitting units is shorter than 1 mm. The first light emitting unit  102  and the second light emitting unit  104  are arranged in a periodically alternating sequence in any row or column of the array. That is to say, arrangement of light emitting units in any row or column of the array is one first light emitting unit  102  following one second light emitting unit  104 , vice versa. 
         [0053]    A detailed illustration of the first light emitting unit  102  is presented in  FIG. 4A . The first light emitting unit  102  has a light emitting chip  1022  and a plurality of phosphors  1024 . The phosphor  1024  is attached to the light emitting chip  1022  by a silicone  1026 . The light emitting chip  1022  generates a visible blue light with peak light intensity having wavelength around 460 nm on the spectrum. The phosphor  1024  is able to emit a yellow light with peak light intensity having wavelength around 574 nm on the spectrum by excitation of the light from the light emitting chip  1022 . The combined light from the light emitting chip  1022  and the phosphor  1024  seems white to human eyes since it covers the visible light spectrum. 
         [0054]    Please refer to  FIG. 4B . It is a CIE (International Commission on Illumination)  1931  color space chromaticity diagram and used to show how the light emitting chip  1022  works with phosphor  1024 . Any visible color can be found on the diagram with an x chromaticity value and a y chromaticity value. Colors can be defined by the correlated color temperature (CCT). In the first embodiment, light color of the light emitting chip  1022  is shown at point A and light color of the excited phosphor  1024  is shown at point B. In practice, all color light on a dashed line linking point A and point B can be produced by adjusting the current provided to the light emitting chip  1022 . For general use, a white light should have CCT falling within the elliptical area of  FIG. 4B . 
         [0055]    The structure of the second light emitting unit  104  is the same as that of the first light emitting unit  102 . The second light emitting unit  104  has the light emitting chip  1022  and the plurality of phosphors  1024  which are attached to the light emitting chip  1022  by a silicone  1026 . Technically, the light emitting chip  1022  and the second light emitting unit  104  come from the same batch of production. The difference between the first light emitting unit  102  and the second light emitting unit  104  is emitted CCT light and resulted from deviation during attaching process. The particles of the phosphors  1024  are mixed with the silicone  1026  before attaching. However, uniformity of mixture varies. After sorting of the manufactured light emitting chips, the first light emitting unit  102  can generate light with CCT of 5000K and the second emitting unit  104  can generate light with CCT of 6500K under current supply of 20 mA. In order to have a uniform light with CCT of 5200K, the control circuit die  106  provides 8 mA to the first light emitting units  102  and 12 mA to the second emitting unit  104  respectively. Currents are fixed in magnitude all the time. The first embodiment is a utilization of the present invention to solve the problem of quality of light emitting units. 
         [0056]    A second embodiment according to the present invention is described hereinafter with reference to  FIGS. 5-8 . In a more general situation, one light emitting chip can be combined with different phosphors to produce different color light. Please refer to  FIG. 5 . Point A 1  represents a light emitting chip having specific blue light. Points B 1  and B 2  stand for two different phosphors. Each of the phosphors emits a specific excited yellow light by excitation of the blue light from the light emitting chip. Similarly, the combination of light emitting units of different phosphors contributes a variety of color light which falls in the bold-lined quadrangle of  FIG. 5 . 
         [0057]    Please refer to  FIG. 6 . A color tunable light emitting diode  200  comprises twenty first light emitting units  202 , twelve second light emitting units  204 , a control circuit  206  and a silicon substrate  208 . The silicon substrate  208  can be an optical package substrate (OPS). Furthermore, the silicon substrate  208  can include cavities for accommodating the first light emitting units  202  and the second light emitting units  204 . However, the silicon substrate  208  is not limited to have cavities for accommodation of light emitting units. In practice, the silicon substrate  208  can also be a substrate having a flat surface. In other words, while the silicon substrate  208  has a flat surface, the light emitting units may directly be disposed in an array on top of the flat surface of the silicon substrate  208 . By micro-electro-mechanical technology, such as making cavity for dice or wiring, circuit layout (not shown) for connecting each light emitting unit can be formed on the silicon substrate  208 . The twenty first light emitting units  202  form a first light emitting group and link to the control circuit  206  via the circuit layout. The twelve second light emitting units  204  form a second light emitting group and link to the control circuit die  206  via the circuit layout. There are total thirty-two cavities  2001  formed on the silicon substrate  208 . Compared with the first embodiment, arrangement of cavities  2001  is not an array. On the contrary, it has a plurality of 2-dimensional patterns. More specifically, each pattern is similar to one another in shape and composition but different in the spatial orientation. Each of the cavities  2001  is used to support and fix one first light emitting unit  202  or one second light emitting unit  204 . The spacing between adjacent two light emitting units is shorter than 1 mm. The control circuit  206  is for supplying tunable currents to the first light emitting units  202  and the second emitting units  204 . 
         [0058]      FIG. 7A  shows the composition of the first light emitting unit  202 . The first light emitting unit  202  has a light emitting chip  2022  and a plurality of first phosphors  2024 . The first phosphor  2024  is attached to the light emitting chip  2022  by a silicone  2026 . The light emitting chip  2022  generates a visible blue light with peak light intensity having wavelength around 460 nm on the spectrum. The first phosphor  2024  is able to emit a yellow light with peak light intensity having wavelength around 564 nm on the spectrum by excitation of the light from the light emitting chip  2022 . The combined light from the light emitting chip  2022  and the first phosphor  2024  seems white to human eyes since it covers the visible light spectrum. Please refer to  FIG. 7B . The composition of the second light emitting unit  204  comprises a light emitting chip  2022  and a plurality of second phosphors  2044 . The second phosphor  2044  is attached to the light emitting chip  2022  by a silicone  2026 . The second phosphor  2044  is able to emit a yellow light with peak light intensity having wavelength around 578 nm on the spectrum by excitation of the light from the light emitting chip  2022 . 
         [0059]    The first light emitting unit  202  can generate light with CCT of 6500K and the second emitting unit  204  can generate light with CCT of 5000K under current supply of 20 mA. In order to have a uniform light with CCT of 5200K, the control circuit  206  provides periodically changed currents to the first light emitting units  202  and the second emitting unit  204  respectively. Currents are generated from a pulse width modulator designed in the control circuit  206 . Duty cycles of currents for two light emitting groups are shown in  FIG. 8 . Since there are two magnitudes of current (8 mA and 12 mA) alternatively changed for each light emitting unit, the color tunable light emitting diode  200  actually provides a blinking light source. The light from each light emitting unit is white light and people can not perceive blinking phenomenon. A uniform light with CCT of 5200K can be achieved in this way. 
         [0060]    It should be noted that the patterns mentioned in the second embodiment should not be limited to the same shape and composition. One pattern can be totally different from another in shape and design. Meanwhile, patterns can be continuously or discontinuously arranged on a silicon substrate. Most of all, the pattern itself can be randomly designed as long as the lightening effect is satisfactory. 
         [0061]    Design of light emitting unit is not restricted to use only one kind of light emitting chips. A mixture of two or more than two light emitting chips is commonly seen for manufacturing light emitting diodes. The effect of mixture of two groups of light emitting chips can have light spectrum covering the bold elliptical area in  FIG. 9  by changing current. As shown in  FIG. 9 , the dash line A 2 -B 3  and dash line A 3 -B 4  represent two light emitting units with different light emitting chips and phosphors. 
         [0062]    In a third embodiment, a combination using multiple light emitting chips is disclosed. Please see  FIG. 10 . A color tunable light emitting diode  300  comprises three first light emitting units  302 , three second light emitting units  304 , three third light emitting units  306 , a control circuit die  308  and a printed circuit board (PCB)  310 . The control circuit die  308  is bonded on the printed circuit board  310  to supply tunable currents to the first light emitting units  302 , the second light emitting units  304  and the third emitting units  306 . The three first light emitting units  302  form a first light emitting group and link to the control circuit die  308  via the printed circuit board  310 . The three second light emitting units  304  form a second light emitting group and link to the control circuit die  308  via the printed circuit board  310 . The three third light emitting units  306  form a third light emitting group and link to the control circuit die  308  via the printed circuit board  310 . Arrangement of the light emitting groups forms an array. The array has three rows and three columns and is 2-dimensional on the surface of the printed circuit board  310 . The three kinds of emitting units are arranged in a periodically alternating sequence and each one appears once in any row or column of the array. 
         [0063]    A detailed illustration of the first light emitting unit  302  is presented in  FIG. 11A . The first light emitting unit  302  has a first light emitting chip  3022  and a plurality of first phosphors  3024 . The first phosphor  3024  is attached to the first light emitting chip  3022  by silicone  3026 . The first light emitting chip  3022  generates visible blue light with peak light intensity having wavelength around 460 nm on the spectrum. The first phosphor  3024  is able to emit yellow light with peak light intensity having wavelength around 564 nm on the spectrum by excitation of the light from the first light emitting chip  3022 . A first combined light from the first light emitting chip  3022  and the first phosphor  3024  seems white to human eyes since it covers the visible light spectrum. The CCT of the first combined light is 5000K. Please refer to  FIG. 11B . The second light emitting unit  304  has a first light emitting chip  3022  and a plurality of second phosphors  3044 . The second phosphor  3044  is attached to the first light emitting chip  3022  by the silicone  3026 . The second phosphor  3044  is able to emit yellow light with peak light intensity having wavelength around 578 nm on the spectrum by excitation of the light from the first light emitting chip  3022 . A combined light from the first light emitting chip  3022  and the second phosphor  3044  is also white light but has 6500K CCT. 
         [0064]    Please refer to  FIG. 11C . The third light emitting unit  306  has a second light emitting chip  3062  and a plurality of first phosphors  3024  and second phosphors  3044 . The first phosphors  3024  and second phosphors  3044  are attached to the second light emitting chip  3062  by the silicone  3026 . The second light emitting chip  3062  generates visible blue light with peak light intensity having wavelength around 450 nm on the spectrum. A third combined light from the first light emitting chip  3062 , the first phosphors  3024  the second phosphors  3044  is white light with 6000K CCT. 
         [0065]    In order to have a uniform light with CCT of 5500K, the control circuit die  308  provides 8 mA current to every light emitting unit. Currents keep a fixed magnitude. The third embodiment shows that even the current has no deviation in all groups, a desirable light can be generated by arranging three kinds of light emitting units with a single current. It should be noted that the present invention emphasizes currents which can be tuned to have a fixed value. In the third embodiment, the PCB  310  can be replaced with a ceramic substrate with bonding wires for electrical connection. The kinds of light emitting chips can be more than two. Of course, the combination of light emitting units has a great diversity. 
         [0066]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.