Abstract:
A liquid cooled LED lighting device includes a sealed housing containing an LED element that emits light. Cooling liquid is contained in the housing to disperse heat generated by the LED element. An enclosure containing compressible material is preferably immovably positioned within the housing and outside of the optical path of the emitted light. The enclosure containing the compressible material compresses in response to expansion of the cooling liquid as it absorbs heat from the LED element. Advantageously, the cooling liquid and the enclosure containing the compressible material act to more efficiently cool the LED element, thereby providing higher light output and increased lifetime of the LED element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. Section 119(e) to U.S. Provisional Application Ser. No. 61/438,389, filed Feb. 1, 2011 and Provisional Application Ser. No. 61/327,180, filed Apr. 23, 2010, which are fully incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a lighting device and more particularly to an LED lighting device. 
       BACKGROUND OF THE INVENTION 
       [0003]    For many illumination applications in LED (light emitting diode) illumination or lighting, an important issue is the removal of heat generated from an LED lighting element of an LED chip. Traditionally, LED chips have been mounted on a metal substrate and the substrate is mounted on a heatsink with cooling fins. A fan can then be used to blow air over the heatsink fins to cool the LED chip. 
         [0004]    However, due to the relatively large distance between the LED chip and the heatsink fins, the cooling efficiency is usually low. As a result, the LED junction operates at higher temperatures, which reduces the light output and lifetime of the LED chip. 
         [0005]    Therefore, it would be desirable to provide an LED light device and method of more efficiently cooling the LED lighting element. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    According to one aspect of the present invention, a liquid cooled LED lighting device includes a sealed housing having a transmissive aperture and an LED element contained in the housing. The LED element has an emitting area that emits light for transmission through the aperture. Cooling liquid is contained in the housing to disperse heat generated by the LED element. Preferably, compressible material enclosed in an enclosure is positioned within the housing and outside of the optical path of the emitted light. The enclosure containing the compressible material compresses in response to expansion of the cooling liquid as it absorbs heat from the LED element. 
         [0007]    Advantageously, the cooling liquid and compressible material act to more efficiently cool the LED element, thereby providing higher light output and increased lifetime. At the same time, use of the compressible material in the housing allows the housing to be made of a completely sealed rigid package. 
         [0008]    According to another aspect of the present invention, a liquid cooled LED lighting device includes a sealed housing having a recycling reflector. The recycling reflector has a reflective surface such that the LED light impinging on the reflective surface reflects back to the emitting area of the LED element. The cooling liquid and compressible material contained in the housing act to disperse heat generated by the LED element. 
         [0009]    According to another aspect of the present invention, a liquid cooled LED lighting device includes an LED element which is attached to the outside of the sealed housing. The cooling liquid and compressible material contained in the housing act to disperse heat generated by the LED element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows an exemplary LED lighting device according to an embodiment of the present invention. 
           [0011]      FIG. 2  shows an LED lighting device having a recycling reflector. 
           [0012]      FIG. 3A  shows an LED array of four LED elements with at least one symmetrically arranged colored pair. 
           [0013]      FIG. 3B  shows an LED array of six symmetrically arranged LED elements. 
           [0014]      FIG. 4  shows a liquid cooled LED lighting device invention in which the light output is recycled to allow higher output intensity according to an embodiment of the present invention. 
           [0015]      FIGS. 5A-5E  shows various types of enclosures that can be used to enclose compressible materials according to the present invention. 
           [0016]      FIG. 6A  shows an LED lighting device having a pump according to an embodiment of the present invention. 
           [0017]      FIG. 6B  shows an LED lighting device having a pump and an LED element in contact with a cooling liquid according to an embodiment of the present invention. 
           [0018]      FIG. 7  shows an LED lighting device having an external pump according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  shows an exemplary LED lighting device according to one embodiment of the present invention. The LED lighting device  2  includes an LED package  4 , heatsink  5 , and cooling liquid  9 . 
         [0020]    The LED package  4  includes at least one LED chip  10  which is typically an LED element having an emitting area that emits light and a substrate  12  on which the chip is mounted. The emitting area includes an optional transparent window  7  that protects the LED chip  10 . The heatsink  5  is attached to the substrate  12  to carry heat away from the LED chip  10 . Such LED packages, for example, are available from Luminus Devices, Inc. of Billerica, Mass. 
         [0021]    Cooling liquid  9  contained in a liquid sealed housing is positioned in close proximity to or near the LED chip  10 . In  FIG. 1 , the boundary of the housing containing the cooling liquid is not shown as it can be used in many different applications that use different types of housings. Preferably, the cooling liquid  9  is in direct contact with the LED chip  10  (i.e., the LED semiconductor itself or the window  7 ) so that any heat generated by the chip will be carried away by the liquid immediately with very little heat resistance. In the case of  FIG. 1 , the cooling liquid  9  is in direct contact with the transparent window  7  of the chip. In cases where the transparent window  7  is absent, the cooling liquid  9  will be in direct contact with the LED semiconductor itself. Preferably, the cooling liquid  9  has low thermal expansion, high heat conductivity, chemically inert, and electrically insulating characteristics. One such liquid is a perfluorinated liquid called Fluorinert™ available from 3M Company of St. Paul, Minn. Other lower cost liquids can be mineral oil, paraffin or the like. 
         [0022]      FIG. 2  shows an LED lighting device with a recycling reflector as disclosed in applicant&#39;s earlier filed application Ser. No. 13/077,006, filed Mar, 31, 2011, which is incorporated herein by reference. The LED lighting device includes an LED package  4 , a driver circuit  3  for driving the LED chips  10 , a recycling reflector  6  such as a recycling collar positioned in front of the LED chip and a transmissive aperture  8  through which the LED light passes. 
         [0023]    The LED chips/elements  10  can be a single chip or multiple chips of white color, single color, or multiple color. For particular applications, they can be arranged such that the optical axis  16  of the transmissive aperture  8  of the recycling reflector  6  goes through the center  20  (see  FIG. 3 ) of the LED elements and the center is also substantially at the proximity of the center of curvature of the recycling reflector. The LED elements  10  are preferably arranged in the same plane and closely positioned to minimize any space between any two emitting areas of the LED elements. The LED elements  10  can emit light of a single color such as red, green and blue or emit white light. The emission angle is typically 180 degrees or less. 
         [0024]    The recycling collar  6  is curved in a concave manner relative to the LED element  10 . The inner surface  14  is a reflective surface such that the LED light that impinges on the inner surface is reflected back to the light source, i.e., LED elements. The reflective surface can be provided by coating the exterior or interior surface of the collar  6  or by having a separate reflective mirror attached to the collar. According to a preferred embodiment, the recycling collar  6  is spherical in shape relative to the center  20  of the LED elements  10  such that the output is reflected back to itself with unit magnification. Thus, it is effectively an imaging system where the LED elements  10  form an image on to itself. Advantageously, substantially all LED light that impinges on the inner spherical reflective surface  14  is reflected back to the light source, i.e., emitting areas of the LED elements  10 . 
         [0025]    As persons of ordinary skill in the art can appreciate, any LED light that does not pass through the transmissive aperture of a conventional illumination system is lost forever. However, by using the curved reflective surface  14 , the LED lighting device of the present invention allows recovery of a substantial amount of light that would have been lost. For example, in an illumination system whose transmissive aperture size captures about 20% of emitted light, the recycling collar  6  allows collection of an additional 20% of the emitted light. Advantageously, that is an improvement of 100% in captured light throughput, which results in a substantial improvement in brightness. 
         [0026]    The LED in the present invention can be a single LED or an array of LEDs. The LED can be white, single color, or composed of multiple chips with single or multiple colors. The LED can also be a DC LED, or an AC LED. 
         [0027]      FIG. 3  shows some of the LED chips that can be used with the present invention.  FIG. 3A  shows an LED array  18  of four colored LED elements  10 . Specifically, the LED array  18  includes one red LED element R emitting red color light, one blue LED element B emitting blue color light arranged at opposite corners and symmetrically about the center  20 , and two green LED elements G 1 ,G 2  emitting green color light arranged at opposite corners and symmetrically about the center  20  of the LED array. The LED array  18  is arranged such that the optical axis  16  of the recycling reflector  6  passes through the center  20  and the center is also substantially at the proximity of the center of curvature of the recycling reflector  6 . 
         [0028]    While the LED array  18  is shown with four LED elements, the present invention can work with at least one LED element. Also, in the case of a pair of LED elements, while it is preferable that the LED elements in the pair emit the same color, they can emit different colors although the efficiency may be lower. Moreover, the size of each LED element in the array can be different from any other LED element. 
         [0029]    It is to be noted that while each LED element  10  is shown as a square, it can be rectangular. Preferably, the total emitting area of the LED array  18  should have the same aspect ratio as the image to be projected. For example, to project a high definition television image whose aspect ratio is 9:16, the total emitting area of the LED array  18  should have the same 9:16 dimension. Similarly, the dimension of the LED array  18  can be, among others, 4:3, 1:1, 2.2:1, which are also popular aspect ratios. 
         [0030]    In the embodiment of  FIG. 3A , the two green LED elements G 1 ,G 2  are imaged on to each other. Specifically, any light from LED element G 1  impinging on the interior reflective surface  14  is reflected back to the symmetrically positioned LED element G 2  and vice versa. For the symmetrically arranged same color LED elements to work well, the driver circuit  3  drives the same color LED elements (e.g., G 1 ,G 2 ) simultaneously. Thus, this arrangement provides high recycling efficiency. On the other hand, light from the blue LED element B is imaged onto the red LED element R and vise versa. Thus, the recycling efficiency is lower for these two colors. 
         [0031]    In order to increase the efficiency with multi-colored LED elements, a symmetric configuration as shown in  FIG. 3B  can be used. In this embodiment, the red chips (LED elements R) are arranged symmetrically with respect to the center  20 . As such, the red chips are imaged onto each other with high recycling efficiency. Similarly, the blue chips (LED elements B) and green chips (LED elements G) are also arranged symmetrically with respect to the center  20  and will be imaged onto each other with high recycling efficiency. 
         [0032]      FIG. 4  shows a liquid cooled LED lighting device invention in which the light output is recycled to allow higher output intensity according to an embodiment of the present invention. In  FIG. 4 , the LED lighting device is an LED light bulb  22  having a sealed housing/bulb  24  and a base  26 . The sealed bulb  24  can be made of plastic, glass or metal. 
         [0033]    An LED mount  28  is attached to the base  26  and provides the rigid support structure for attaching a control circuit  3 , heat sink  5 , substrate  12  and LED chips  10  which are electrically connected to the control circuit. The substrate  12  supporting the LED chip  10  is mounted on the heatsink  5 . The LED mount  28  also has a conduit for carrying electrical wires from the control circuit to an electrical foot contact  32  and screw threaded contact  30 . In operation, line voltage from the electrical contacts  30 , 32  is converted to the desired level for the LED chip  10  by the control/driver circuit  3 . 
         [0034]    Although  FIG. 4  shows a light bulb having an Edison type threaded base connector, any other LED lighting devices such as one having MR-16 type base are also suitable for use with the present invention. 
         [0035]    The bulb  24  has an optically transparent transmissive aperture  8  through which the emitted light from the LED chip  10  passes. The aperture  8  can be a simple optically transparent spherical window or can have a lens such as a focusing lens or collimating lens to obtain a desired output divergence. 
         [0036]    The part of the bulb  24  above the substrate  12  is spherically shaped relative to the center of the LED chip  10  emitting area. A part of the spherical bulb surface around the transmissive aperture  8  is coated with reflective coating  14  for reflecting the emitted light back to the LED chip  10  light emitting area. This functions as the recycling collar  6  as shown in  FIG. 2 . 
         [0037]    According to the invention, the sealed light bulb  24  is filled with cooling liquid  9  for heat sinking. Similar to  FIG. 1 , the sealed cooling liquid  9  is positioned in close proximity to or near the LED chip  10 . As shown, the cooling liquid  9  is in direct contact with the LED chip  10  emitting area so that any heat generated by the chip will be carried away by the liquid immediately with very little heat resistance. 
         [0038]    The LED chip  10  generates heat when emitting light. The heat in turn heats the cooling liquid  9  which expands in volume. Since the cooling liquid  9  is sealed inside the bulb  24 , a relief is needed to prevent explosion due to expansion of the cooling liquid. As shown in  FIG. 4 , compressible material  34  is positioned inside the bulb to absorb the expanding volume of the cooling liquid  9  by compressing. In the embodiment shown, the compressible material  34  is immovably positioned and is outside of the optical path of the emitted light so that it does not interfere with the light being transmitted through the transmissive aperture  8 . If not, the compressible material  34  may travel into the optical path of the light and create distortions and shadows in the light exiting the aperture  8  and may also reduce the light output. 
         [0039]    In  FIG. 4 , the compressible material  34  is attached to the inner surface of the bulb  24 . Alternatively, the compressible material  34  can be immovably attached to the LED mount  28 , heat sink or other parts within the bulb  24  so long as the material is positioned outside of the optical path of the emitted light. In some embodiment the compressible material is contained in a sealed enclosure as shown in  FIG. 4 . 
         [0040]    The compressible material as shown in  FIG. 4  is a pocket of air. The air pocket is contained inside a small sealed balloon enclosure. As the pressure inside the bulb  24  increases, the air pocket  34  will reduce in volume, relieving the pressure inside the light bulb. 
         [0041]    Instead of positioning the compressible material  34  inside the housing  24 , a part of the housing can be made of flexible material such as rubber so that it can expand as the cooling liquid  9  expands. However, this is not a preferred solution because it is difficult to maintain a seal between the flexible material and the rigid housing. Thus, positioning of the compressible material  34  inside the housing  24  according to the present invention allows the housing to be made entirely of rigid, non-expanding material which is completely sealed, thereby improving the reliability and durability of the LED lighting device. 
         [0042]    In an alternative embodiment, the compressible material  34  such as air is contained in an enclosure and is confined within an internal chamber  35  defined by an internal wall  33  having openings so that the fluid  9  flows freely therethrough. In this way, the compressible material  34  do not need to be immovably positioned. Preferably, the wall  33  and therefore the compressible material  34  and its enclosure are outside of the optical path of the emitted light. 
         [0043]    Although the embodiment of  FIG. 4  shows air as the compressible material, any other types of gas, which by nature are compressible, such as nitrogen can be used. In fact, even vacuum can be used so long as the enclosure is sufficiently rigid to withstand the force of vacuum, yet sufficiently flexible to compress due to the external pressure of the expanding cooling liquid  9 . 
         [0044]      FIG. 5  shows various types of enclosures for enclosing compressible materials according to the present invention.  FIG. 5A  is a section of tubing containing air with both ends sealed. The tubing can be rubber, silicone, plastic or the like. 
         [0045]    The shape of the enclosure can be cylindrical as shown in  FIG. 5A , spherical as shown in  FIG. 5B , toroidal as shown in  FIG. 5C , a flat cavity such as a disk as shown in  FIG. 5D , or the like. The air pocket can be independent of the package, or can be attached to the package, or can be integrated with the package. 
         [0046]    As shown in  FIG. 5E , the compressible material  34  can be a collection of small air pockets packed together as a piece of “foam”. Such materials provide the necessary volume of gas required that is easy to handle and that can be cut to size as needed. The foam material can be found in packing cushion materials, for example. Materials that make up these foams could be vinyl, silicone, rubber, etc. The gas inside the pockets can be air, nitrogen, or the like. 
         [0047]    To enhance the efficiency of cooling and heat sinking, a pump  38  can be added to circulate the cooling liquid inside the housing  24 . The pump  38  quickly moves away the hot liquid near the LED chips  10  and replaced it with cooler liquid, thereby increasing the efficiency of cooling in order to reduce the junction temperature of the LED chips. 
         [0048]    In a preferred embodiment, the pump  38  is an ultrasonic pump. Ultrasonic signal is used to drive a transducer such that it generates acoustic waves in the cooling liquid  9 . The configuration of the pump  38  is such that the acoustic wave produces a net flow of liquid. 
         [0049]      FIG. 6A  shows an LED lighting device with such a pump. The liquid sealed housing  24  contains an ultrasonic pump  38  having an inlet  40  on one side and an outlet  42  on another side. The ultrasonic pump  38  is driven by an ultrasonic driver circuit  44  located outside the housing  24  that generates an ultrasonic drive signal. In  FIG. 6A , the substrate  12  and LED chip  10  attached to the substrate are mounted to the outer surface of the housing  24  instead of being attached to the inside of the housing as shown in  FIG. 4 . Cooling fins  50  are attached to the housing  24  to remove heat from the cooling liquid  9 . Preferably, the housing  24  in  FIG. 6A  is made of heat conductive material such as metal or metal alloy. 
         [0050]    The air pocket  34  in  FIG. 6A  is similar to that of  FIG. 4 , except that since the LED chip  10  is attached to the outside of the housing  24 , the air pocket does not have to be immovably attached to the housing  24 . 
         [0051]      FIG. 6B  shows an alternative LED lighting device in which the LED chip  10  and internal heat sink  5  are immersed in the cooling liquid  9  for effective cooling. The compressible material  34  is similar to that of  FIG. 4  and is attached to the interior surface of the liquid sealed housing  24  away from the optical path of the LED chip  10 . Fins  50  are attached to the housing  24  to remove heat from the cooling liquid  9 . Preferably, the housing  24  in  FIG. 6B  is made of heat conductive material such as metal or metal alloy. 
         [0052]    The heatsink  5  is attached to the interior surface of the housing  24  so that the heat from the heatsink can be redistributed throughout the housing. The base  26  attached to the housing  24  couples electrical wires from the LED chip  10  and pump  38  to connectors  46 . The light emitting from the LED chip  10  is transmitted through the aperture/optical window  8 . 
         [0053]      FIG. 7  shows an LED lighting device according to another embodiment of the present invention. An array of LED chips  10  and substrate  12  are mounted on a heatsink  5  attached to the interior surface of the housing  24 . The compressible material  34  is attached to the interior surface of the housing  24  and is positioned outside of the optical path of the emitted light. The housing  24  has an inlet  52  and outlet  54 . A flow tube  56  is coupled between the inlet  52  and outlet  54 . Cooling fins  50  are attached to a portion of the flow tube  56  defining a cooling chamber  58 . A pump such as an ultrasonic pump  38  is connected inline with the flow tube  56  to pump the cooling liquid  9  from the housing  24  to the cooling chamber  58  for efficient heat sinking by the cooling fins. 
         [0054]    The above disclosure is intended to be illustrative and not exhaustive. This description will suggest many modifications, variations, and alternatives may be made by ordinary skill in this art without departing from the scope of the invention. Those familiar with the art may recognize other equivalents to the specific embodiments described herein. For example, although the present invention is shown with a recycling reflector, it can be used without the recycling of light. Also, while the present invention has been shown in the context of an LED as the light source, it can be used with any light source that generates a significant amount of heat in operation. For example, the present invention can be used with laser, arc lamp, or the like. The principles of the present invention can also be applied to any other non-optical applications where heat is generated such as power transistors, microprocessors, inductors, rectifiers and transformers. Accordingly, the scope of the invention is not limited to the foregoing specification.