Patent Application
20080074884
The present invention includes a light source and method for making the same. The light source includes a plurality of LEDs, an LED carrier, and a cover. The LED carrier includes a metallic core having a top and bottom surface. The top surface is bonded to a circuit layer having mounting pads for each of the LEDs and a connector that provides connections to circuit conductors connected to the mounting pads. The bottom surface includes an external boundary of the light source. The cover is bonded to the LED carrier. The cover includes a first opening positioned to allow light from the LEDs to leave the cover and a second opening that provides access to the connector. An encapsulant system covers each of the LEDs with a layer of encapsulant material. In one aspect of the invention, the cover includes a cavity, the LED carrier being bonded to an inside surface of the cavity and aligned to the cover by the walls of the cavity. In another aspect of the invention, the encapsulant system includes a layer of clear encapsulant having a first surface in contact with the LEDs and the LED carrier and a second surface that is molded. The molded surface can be flat or shaped to provide optical processing of the light from the LEDs.
The manner in which the present invention provides its advantages can be more easily understood with reference to
Light source 30 includes two main assemblies, a LED carrier 50 and a cover 40. Cover 40 includes a cavity into which LED carrier 50 is inserted. Cover 40 also includes an opening 42 through which light from the LEDs shown at 56 can exit light source 30. The sides of opening 42 are reflective and slanted at an angle to redirect light leaving the LEDs through the side thereof to a direction that allows that light to exit from light source 30. Light source 30 includes a transparent encapsulant member that fills opening 42.
Led carrier 50 is a circuit carrier 59 that is constructed from one or more metal layers that are patterned to provide the connections between the various electronic components in light source 30. The circuit layers are bonded to a metal core 52 that transfers heat from the LEDs to cover 40 and to the underlying structures on which light source 30 is mounted. In one embodiment, the core is constructed from an aluminum alloy. In the embodiment shown in
The connector can be either a male or female connector that is configured to mate to a corresponding connector on a cable or other device in the apparatus in which the light source is utilized. In the above-described embodiments, the connector is positioned to receive the corresponding connector in a direction parallel to the surface of the LED circuit carrier. However, embodiments in which the connector is mounted such that the corresponding connector is received in a direction perpendicular to that surface could also be constructed.
Each LED is connected to two traces within the metal layer. The first connection is provided by a terminal on the bottom of the LED, and the second connection is provided by a terminal on the top of the LED through a wire bond connection 57.
Light source 30 includes three groups of LEDs. The LEDs in each group are connected in series and generate light having the same spectrum. The groups generate light in the red, blue, and green regions of the spectrum. To improve the color uniformity of the output light, the LEDs alternate such that each LED has a neighboring LED of the other two colors. Each group of LEDs is connected to connector 32 by a corresponding trace in the metal layer.
Refer now to
While the embodiment shown in
In one embodiment, X is an additional green LED. The relative efficiency of green LEDs is significantly less than that of red and blue LEDs. Hence in embodiments in which the LEDs are to be operated close to the maximum rated currents, additional green LEDs are needed to provide the same range of colors and still maintain the red and blue LEDs at near the maximum current for those LEDs.
In another embodiment, X is a “white” LED. White LEDs, based on blue LEDs that are covered by a yellow phosphor that converts part of the blue light to yellow light, have a higher power conversion efficiency than white light sources constructed from red, blue, and green LEDs. However, in many applications, a white light source that has a limited range of color tuning around the white light provided by the white LED is useful.
In yet another embodiment, X is an amber or cyan LED. Such light sources have a wider color gamut, and hence are useful in specific applications that require color points in the amber or cyan regions of the color space.
Cover 40 includes a cavity into which LED carrier 50 is inserted such that the bottom surface of LED carrier 50 is flush with the bottom surface of cover 40. This provides an arrangement that maximizes the heat transfer surfaces of light source 30 and the surface to which light source 30 is connected in the final product that utilizes light source 30. Cover 40 is affixed to the LED carrier by encapsulant 31, which is used to fill opening 42 after cover 40 and LED carrier 50 have been assembled. The encapsulant layer bonds to the top surface of LED carrier 50 and the slanted sides of opening 42. Additional adhesive can be applied to the top surface of LED carrier 50 to provide bonding in the other regions of contact if the bonding provided by the encapsulant layer is insufficient.
Light source 30 also includes a number of holes that are provided for mounting light source 30 on other assemblies in the completed product in which light source 30 is utilized. Cover 40 includes holes 41 that are aligned with holes 51 in LED carrier 50 to provide holes through light source 30 that can accommodate a fastener such as a screw. The inside surfaces of holes 41 and/or 51 can be threaded to facilitate such attachment as shown at 48 in
It should be noted that the fasteners can also provide additional bonding between cover 40 and LED carrier 50, as well as additional heat conduction from cover 40 to the underlying substrate on which light source 30 is mounted.
It should also be noted that the holes do not need to go completely through the light source. Either the holes in the cover or the holes in the LED carrier could be blind holes that are threaded to receive a screw.
These holes can also be used during the assembly of the light source to hold cover 40 to LED carrier 50 during the filling of opening 42. The light source is assembled by attaching cover 40 to circuit carrier 50 after all of the LEDs have been affixed to circuit carrier 50 and connected electrically to the various electrical traces. Screws are placed through the holes and tightened to force cover 40 and circuit carrier 50 together. Embodiments in which the holes in only one of the cover or circuit carrier are threaded are of particular use during the assembly operation. The encapsulant is then dispensed into opening 42 and allowed to cure. After the curing is completed, the screws are removed.
Many LEDs emit a significant fraction of the light generated in the die through the side surfaces of the die. This side-emitted light is light that is trapped within the LED due to the difference in index of refraction of the LED materials and the surrounding dielectric material. The trapped light is reflected back and forth between the top and bottom surfaces of the LED until it strikes the surfaces at the edge of the die through which the light escapes.
The embodiments of the present invention discussed above utilize a single opening 42 in cover 40 through which the light from the LEDs exits. The sides of this opening are angled and reflective to re-direct light leaving the sides of the LED dies into the forward direction. Refer again to
Refer now to
The above-described embodiments of the present invention utilize red, green, and blue LEDs to implement a light source that can be tuned to provide a wide range of colors. However, the same general structure can be utilized to provide a light source having a more limited or wider range of colors. For example, the LEDs could be replaced by “white” LEDs that utilize blue emitting LEDs that are covered with a phosphor that converts part of the blue light to yellow light. The resulting output appears to be white to a human observer.
Refer now to
In one embodiment the encapsulant system utilizes a transparent silicone. The silicone provides a low stress encapsulation that has high thermal and photo-stability during the operation of the LEDs. In another embodiment the encapsulant system utilizes thermosetting plastic polymers that are dispensed in liquid form into the opening in the cover and thereafter cured in an oven. These polymers also provide a medium of intermediate refractive index between the air and the LED chip that improves the efficiency of light extraction from the LED chips.
The above-described embodiments of the present invention utilize an encapsulant layer that is filled to the top of the cover and finished with a planar surface. However, the top surface of the encapsulant layer could also be molded. A non-planar molded surface can provide two advantages. First, the molded surface forms a lens that alters the output light profile of the light source. Second, the molded surface improves the extraction of light from the device by reducing the amount of light that is reflected at the encapsulant-air boundary.
Refer now to
The lens could also be cylindrical with the axis of the cylinder parallel to a line through the LEDs. As noted above, in many applications, the light source ideally approximates a conventional linear light source. Such a cylindrical lens improves the approximation of the present invention to a conventional linear source. It should also be noted that other lens shapes including trapezoidal lens and prisms can be constructed by molding the encapsulant.
While the encapsulant lens is shown as being formed above the surface of the cover, embodiments in which the lens is formed within the opening to reduce the thickness of the light source could also be constructed. Such an embodiment is shown in
The encapsulant lens can also be constructed such that the lens do not cover the entire surface of the encapsulant layer. Such an arrangement is shown in
The minimum width of the embodiments discussed above is determined by the size of opening 42 shown in
Refer now to
While connector 123 is shown as being inset in an opening in cover 125 having three sides, it should be noted that sides 126 and 127 are optional. That is, cover 125 could merely terminate leaving the portion of the underlying circuit carrier having the connector pads exposed.
In the above-described embodiments, a single connector has been utilized. However, embodiments having multiple connectors could also be constructed. Such embodiments are particularly useful in designs in which the connectors also provide a means for mounting the light source in a device utilizing the light source. Refer now to
Refer now to
In one embodiment, the cover is constructed from metallic materials to provide high thermal conductivity (typically between 50 to 350 W/m.K) for efficient heat dissipation. Metallic materials are inexpensive and easily formed into various shapes. In addition, such materials can be plated to provide the reflective surfaces discussed above. In one embodiment, the cover is plated with nickel. In one embodiment, the cover is constructed from an aluminum alloy. Aluminum is a cost effective cover material relative to other choices such as ceramics and metal-plated polymers.
In the above-described embodiments of the present invention, the top surface of the cover is smooth except for the openings for the screws and LEDs. However, embodiments in which the surface of the cover is provided with heat fins or other surface area enhancing features to better dissipate heat to the surrounding air could be constructed provided the heat dissipating features do not interfere with the mounting of the light source in the final product. It should be noted that providing the non-light reflecting circuits with a black coating by painting or anodizing could be utilized to further increase the heat transfer without altering the physical profile of the light source.
The above-described embodiments have utilized covers constructed from a metal such as an aluminum alloy. However, embodiments in which the cover is constructed from ceramics, composites, or plastics could also be constructed. Such materials can be plated in the area of the opening to provide a reflective surface.
Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.
