Patent Application
20080158886
Light-emitting diodes (LEDs) are attractive replacement candidates for conventional light sources based on incandescent bulbs and fluorescent light tubes. LEDs have higher energy conversion efficiency than incandescent lights and substantially longer lifetimes than both incandescent and fluorescent light fixtures. In addition, LED-based light fixtures do not require the high voltages associated with fluorescent lights.
LEDs are particularly attractive light sources for backlit displays such as LCD panels that have space constraints. Many mobile electronic devices require a very thin backlight source. LCD displays for use in cellular telephones, PDAs, and laptop computers require a light source for illuminating an LCD panel or keypad. The light source typically consists of a thin two-dimensional flat light pipe that is illuminated from an edge or edges of the thin layer. Light is trapped within the light pipe by internal reflection until the light is scattered by scattering centers on one of the surfaces. The scattered light exits the light pipe through one surface of the light pipe and is used to illuminate a two-dimensional object such as an LCD panel or keypad.
Portable devices place severe constraints on the thickness of the light source. The minimum thickness of the device is set by the combined thickness of the light pipe and the object being illuminated. Ideally, the light source that is used to illuminate the edge of the light pipe is less than this minimum thickness so that the LEDs do not increase the thickness of the device. Since LEDs are inherently small light emitters that can operate on the low voltages available in such portable devices, light sources based on LEDs are of great interest in such applications.
Unfortunately, LEDs have a number of problems that must be overcome to provide a cost-effective solution in such backlight systems. First, LEDs are relatively low power point sources. The backlighting applications require a light source that has a linear geometry and more power than is available from a single LED. Hence, a light source having a relatively large number of individual LEDs must be constructed.
Second, LEDs emit light in narrow optical bands. Hence, to provide a light source that a human observer will perceive as having a particular color, LEDs having different emission spectra must be combined into the same light source or phosphor conversion layers must be utilized to convert some of the LED generated light to light of a different spectrum. For example, an LED that is perceived to emit white light can be constructed by combining the output of LEDs having emission spectra in the red, blue, and green region of the spectrum or by utilizing a blue emitting LED and a layer of phosphor that converts some of the output light to light in the yellow region of the spectrum. For LCD displays, lights that have emission bands in the red, blue, and green regions of the spectrum are typically required. Hence, an LED-based light source must include three types of LEDs and provide for the mixing of the light from three separate sources.
Third, heat dissipation is particularly important in the case of LED-based light sources. The electrical conversion efficiency of an LED decreases with increasing junction temperature in the LED. Hence, any LED-based light source that generates a significant amount of heat must have a good thermal conduction path for removing the heat from the LED.
Finally, cost is of prime importance in most of these applications. In many prior art systems, the light source is constructed from individual LEDs that are incorporated on the printed circuit board (PCB) used to implement other parts of the mobile device. Such custom designs increase the cost of the design as well as the product cycle time. In addition, the efficiency with which the light from the light source is coupled into the light pipe affects the cost of the light source, since sources with poor light coupling efficiency require more LEDs to provide the same level of illumination. The efficiency of coupling also impacts the heat that must be dissipated, since the additional LEDs needed to overcome poor coupling efficiency also generate more heat that must be dissipated.
The present invention includes a light source having a plurality of dies, an LED carrier, and a cover. Each die includes an LED, each die having a top surface, a bottom surface, and one or more side surfaces. The LED carrier includes a metallic core having a top and a bottom surface, the top surface is bonded to a circuit layer, and the dies are bonded to the circuit layer. The cover is bonded to the LED carrier. The cover includes a first opening that allows light from the LEDs to leave the cover, the opening having sides that slant inward such that light leaving a side surface of one of the dies is reflected by one of the sides out of the opening. A dome-shaped encapsulant layer covers each of the dies. The encapsulant layer is spaced from the sides of the opening such that there is an air gap between the encapsulant layers and the sides. The air gap is positioned such that light leaving the side surfaces of the dies passes through the air gap before being reflected by one of the sides. In one aspect of the invention, the bottom surface of the metal core includes an external surface of the light source and the light source can be attached to an external object using holes in the light source such that the bottom surface is in good thermal contact with the external object.
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.
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 another embodiment, the core is constructed from a material having a thermal conductivity greater than 10 W/m.K at 25 degrees Centigrade. 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.
Each LED in light source 30 is encapsulated in a dome-shaped layer of clear material as shown at 60. This layer serves a number of functions. First, the layer protects LED 56 and wire bond 57 from environmental factors. The LED die must be protected from moisture. In addition, the wire bond is fragile. In this regard, it should be noted that if all of the LEDs are encapsulated in a single layer of encapsulant the mechanical stress on the wire bonds when the layer of encapsulant is heated is significantly greater than the stress to which each wire bond is subjected when the dies are individually encapsulated.
Second, the layer can be used as a carrier for phosphors, luminescent materials, or dyes. In phosphor-converted LEDs, all or part of the light emitted by the LED is converted to light of a different spectrum by phosphor particles or luminescent materials that are typically suspended in a layer of clear encapsulant that is applied over the LEDs. In the case of a dye, the light from the LED is filtered to provide light having a more limited spectrum than that generated by the LED.
Finally, and most importantly, the dome-shaped layer improves the extraction of the light from the LED. As noted above, a significant fraction of the light generated in the active layer of the LED is trapped within the LED by internal reflection at the surface of the LED due to the large difference in the index of refraction between the LED and air. This light is either absorbed in the LED material or exits the die through a side surface of the die. The reflector captures the light that exits through the side surface and redirects the light to the forward direction. However, a significant fraction of the trapped light is lost to absorption before the light can exit the die. Hence, it is advantageous to reduce the amount of light that is trapped by internal reflection.
The dome-shaped layer reduces the amount of light that is trapped through two mechanisms. First, the light that is trapped is light that strikes the LED surface at angles greater than the critical angle with respect to the normal to that surface. The critical angle, in turn, depends on the ratio of the indices of refraction between the LED and the material on the other side of the surface. If a material having an index of refraction that is intermediate between that of air and the LED materials is applied to the surface, the critical angle is increased, and hence, a significant amount of the light that would have been lost now passes out of the LED and into the applied layer. Second, the dome shape assures that substantially all of the light that enters the applied layer strikes the boundary between the applied layer and air at angles less than the critical angle with respect to that surface, and hence, escapes the applied layer. If the applied layer did not have the dome shape, a significant fraction of the light that entered that layer would be trapped in that layer.
In effect, the dome-air interface provides the extraction function for assuring that light that enters the encapsulant layer will exit the encapsulant layer. This function, however, assumes that the source of the light that strikes the dome surface is the LED die. Refer now to
The above-described embodiments of the present invention utilize a single layer of encapsulant. However, embodiments in which the encapsulating medium includes multiple layers can also be constructed. Refer now to
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.
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.
The above-described embodiments of the present invention utilize an arrangement having one row of LEDs. However, embodiments having multiple rows of LEDs could also be constructed. Refer now to
As noted above in the discussion of
The above-described embodiments of the present invention utilize a dome-shaped encapsulant layer over each die. For the purposes of this application, a layer will be defined to be “dome-shaped” if the layer has a convex surface opposite the surface of the LED that is emitting the light, and the radius of curvature of the surface is such that light exiting that surface of the LED will strike the surface at angles relative to the normal to the surface that allow the light to escape through the surface of the dome-shaped layer. In general, the light leaving the surface of the LED is confined to a cone of angles about the normal to that surface. The cone of angles is determined by the difference in index of refraction between the material from which the LED is constructed and the encapsulant material. Similarly, the cone of angles about the normal to the dome-shaped surface through which light can exit that layer depends on the index of refraction of the encapsulant material and air. The light that escapes the dome will also depend somewhat on the location on the die at which the light exits the die, since the dies have a finite size.
In the above discussion, a layer is said to encapsulate an object if that layer together with a layer of impermeable material to which the object is attached surrounds the object. Hence, a die that is bonded to a first surface and covered by a layer of material will be defined to be “encapsulated” by the layer of material.
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.
