1. Field of the Invention
This invention relates to solid state lamps and bulbs and in particular to light emitting diode (LED) based lamps and bulbs capable of providing omnidirectional emission patterns similar to those of filament based light sources.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangement, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package may also include electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In one embodiment of an LED package, a single LED chip is mounted on a reflective cup by means of a solder bond or conductive epoxy. One or more wire bonds connect the ohmic contacts of the LED chip to leads, which may be attached to or integral with the reflective cup. The reflective cup may be filled with an encapsulant material which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly may be encapsulated in a clear protective resin, which may be molded in the shape of a lens to collimate the light emitted from the LED chip. While the reflective cup may direct light in an upward direction, optical losses may occur when the light is reflected (i.e. some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces). In addition, heat retention may be an issue for a package, since it may be difficult to extract heat through the leads.
A conventional LED package may be more suited for high power operations which may generate more heat. In the LED package, one or more LED chips are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount. A metal reflector may be mounted on the submount that surrounds the LED chip(s) and reflects light emitted by the LED chips away from the package. The reflector may also provide mechanical protection to the LED chips. One or more wirebond connections are made between ohmic contacts on the LED chips and electrical traces on the submount. The mounted LED chips are then covered with an encapsulant, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector is typically attached to the carrier by means of a solder or epoxy bond.
LED chips, such as those found in the LED package can be coated by conversion material comprising one or more phosphors, with the phosphors absorbing at least some of the LED light. The LED chip can emit a different wavelength of light such that it emits a combination of light from the LED and the phosphor. The LED chip(s) can be coated with a phosphor using many different methods, with one suitable method being described in U.S. patent application Ser. Nos. 11/656,759 and 11/899,790, both to Chitnis et al. and both entitled “Wafer Level Phosphor Coating Method and Devices Fabricated Utilizing Method”. Alternatively, the LEDs can be coated using other methods such as electrophoretic deposition (EPD), with a suitable EPD method described in U.S. Pat. No. 8,563,339 issued Oct. 22, 2013 to Tarsa et al. entitled “Close Loop Electrophoretic Deposition of Semiconductor Devices”.
In these embodiments the phosphor material is on or in close proximity to the LED epitaxial layers and in some instances comprises a conformal coat over the LED. In these arrangements, the phosphor material can be subjected to direct chip heating which can cause the phosphor material to heat. This elevated operating temperature can cause degradation of the phosphor material over time. It can also cause a reduction in phosphor conversion efficiency and a shift in conversion color.
Lamps have been developed utilizing solid state light sources, such as LEDs, with a conversion material that is separated from or remote to the LEDs. Such arrangements are disclosed in U.S. Pat. No. 6,350,041 issued Feb. 26, 2002 to Tarsa et al., entitled “High Output Radial Dispersing Lamp Using a Solid State Light Source.” The lamps described in this patent can comprise a solid state light source that transmits light through a separator to a disperser having a phosphor. The disperser can disperse the light in a desired pattern and/or change its color by converting at least some of the light through a phosphor. In some embodiments, the separator spaces the light source a sufficient distance from the disperser such that heat from the light source will not transfer to the disperser when the light source is carrying elevated currents necessary for room illumination.
LED based bulbs have been developed that utilize large numbers of low brightness LEDs (e.g. 5 mm LEDs) mounted to a three-dimensional surface to achieve wide-angle illumination. Some of these designs, however, do not provide optimized omnidirectional emission that fall within standard uniformity requirements. Some of these bulbs also contain a large number of interconnected LEDs making them prohibitively complex, expensive and unreliable. This makes these LED bulbs generally impractical for most illumination purposes.
Other LED bulbs have also been developed that use a mesa-type design for the light source with one LED on the top surface and seven more on the sidewalls of the mesa (see GeoBulb®-II provided by C. Crane). This arrangement, however, does not provide omnidirectional emission patterns, but instead provides a pattern that is substantially forward biased. The mesa for this bulb also comprises a hollow shell, which can limit its ability to thermally dissipate heat from the emitters. This can limit the drive current that can be applied to the LEDs. This design is also relatively complex, using several LEDs, and is not compatible with large volume manufacturing of low-cost LED bulbs.