Patent Application
20140265811
1. Field
The present disclosure relates generally to light-emitting diode (LED) bulbs, and more particularly, to an LED bulb that has a phosphor structure in an index-matched, thermally conductive liquid.
2. Related Art
Traditionally, lighting has been generated using fluorescent and incandescent light bulbs. While both types of light bulbs have been reliably used, each suffers from certain drawbacks. For instance, incandescent bulbs tend to be inefficient, using only 2-3% of their power to produce light, while the remaining 97-98% of their power is lost as heat. Fluorescent bulbs, while more efficient than incandescent bulbs, do not produce the same warm light as that generated by incandescent bulbs. Additionally, there are health and environmental concerns regarding the mercury contained in fluorescent bulbs.
Thus, an alternative light source is desired. One such alternative is a bulb utilizing an LED. An LED comprises a semiconductor junction that emits light due to an electrical current flowing through the junction. Compared to a traditional incandescent bulb, an LED bulb is capable of producing more light using the same amount of power. Additionally, the operational life of an LED bulb is orders of magnitude longer than that of an incandescent bulb, for example, 10,000-100,000 hours as opposed to 1,000-2,000 hours.
While there are many advantages to using an LED bulb rather than an incandescent or fluorescent bulb, LEDs have a number of drawbacks that have prevented them from being as widely adopted as incandescent and fluorescent replacements. One drawback is that an LED, being a semiconductor, generally cannot be allowed to get hotter than approximately 120° C. As an example, A-type LED bulbs have been limited to very low power (i.e., less than approximately 8 W), producing insufficient illumination for incandescent or fluorescent replacements.
One approach to alleviating the heat problem of LED bulbs is to fill an LED bulb with a thermally conductive liquid, to transfer heat from the LEDs to the bulb's shell. The heat may then be transferred from the shell out into the air surrounding the bulb.
Another drawback to an LED bulb is that LEDs tend to produce light that has a relatively narrow emission band with respect to the entire visible color spectrum. For example, one type of LED, based on gallium nitride (GaN), efficiently emits light over a relatively narrow emission profile centered at a peak wavelength in the blue region of the visible spectrum (approximately 450 nm). GaN LEDs are typically used because they can provide significantly brighter output light than other types of LEDs. However, the relatively narrow emission band, having a primarily blue color, may not produce the desired illumination qualities.
A phosphor material can be used to control the light emitted from an LED. Typically, a phosphor material absorbs LED light at a first wavelength, and emits light at a second wavelength. The phosphor material can be selected so that the emitted wavelength provides the desired color properties. It may be advantageous for an LED bulb to use a phosphor material to convert the convert the narrow band of emitted wavelengths from an LED into a broader or color-shifted emission spectrum. In particular, it may be advantageous to use a phosphor material to increase the wavelength of the light emitted from an LED bulb to produce a red-shifted emission spectrum.
In one exemplary embodiment, a liquid-filled LED bulb includes a base and a shell connected to the base to form an enclosed volume. The liquid-filled LED bulb also includes a plurality of LEDs attached to the base and disposed within the enclosed volume. A thermally conductive liquid is held within the enclosed volume and has a first index of refraction. The LED bulb also includes a ring structure disposed within the enclosed volume and immersed in the thermally conductive liquid. The ring structure has a second index of refraction that matches the first index of refraction. A phosphor material is disposed on, or dispersed within, the ring structure. A first amount of the thermally conductive liquid is disposed between the LEDs and the ring structure and a second amount of the thermally conductive liquid is disposed between the ring structure and the shell. In some cases, the second index of refraction of the ring structure is matched within 10 percent of the first index of refraction of the thermally conductive liquid. In some cases, at least one of the plurality of LEDs has a third index of refraction, and the first index of refraction of the thermally conductive liquid is matched within 40 percent of the third index of refraction of the at least one LED.
In some embodiments, the ring structure is configured to facilitate a flow of the thermally conductive liquid from the LED to an inner surface of the shell. The flow of thermally conductive liquid may be caused, at least in part, by passive convection. In some embodiments, an inside surface of the ring structure is positioned a fixed distance from a light-emitting face of at least one of the plurality of LEDs to facilitate a flow of the thermally conductive liquid from the LED to an inner surface of the shell. In some embodiments, an outside surface of the ring structure is positioned a fixed distance from an inner surface of the shell to facilitate a flow of the thermally conductive liquid from the LED to an inner surface of the shell.
In some embodiments, the phosphor material is configured to absorb light produced by the LEDs having a first wavelength and is configured to emit light at a second wavelength, the second wavelength being longer than the first wavelength. The phosphor material may be disposed on an external surface of the ring structure. The phosphor material may also be disposed on an internal surface of the ring structure.
The following description is presented to enable a person of ordinary skill in the art to make and use the various embodiments. Descriptions of specific devices, techniques, and applications are provided only as examples. Various modifications to the examples described herein will be readily apparent to those of ordinary skill in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the various embodiments. Thus, the various embodiments are not intended to be limited to the examples described herein and shown, but are to be accorded the scope consistent with the claims.
As used herein, an “LED bulb” refers to any light-generating device (e.g., a lamp) in which at least one LED is used to generate light. Thus, as used herein, an “LED bulb” does not include a light-generating device in which a filament is used to generate the light, such as a conventional incandescent light bulb. It should be recognized that the LED bulb may have various shapes in addition to the bulb-like A-type shape of a conventional incandescent light bulb. For example, the bulb may have a tubular shape, globe shape, or the like. The LED bulb of the present disclosure may further include any type of connector; for example, a screw-in base, a dual-prong connector, a standard two- or three-prong wall outlet plug, bayonet base, Edison Screw base, single-pin base, multiple-pin base, recessed base, flanged base, grooved base, side base, or the like.
For convenience, all examples provided in the present disclosure describe and show an LED bulb being a standard A-type form factor bulb. However, as mentioned above, it should be appreciated that the present disclosure may be applied to LED bulbs having any shape, such as a tubular bulb, a globe-shaped bulb, or the like.
Shell 101 may be made from any transparent or translucent material such as plastic, glass, polycarbonate, or the like. Shell 101 may include dispersion material spread throughout the shell to disperse light generated by LEDs 103. The dispersion material prevents LED bulb 100 from appearing to have one or more point sources of light.
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In
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It may also be desirable for thermally conductive liquid to have a large coefficient of thermal expansion to facilitate passive convective flow. As used herein, “passive convective flow” refers to the circulation of a liquid without the aid of a fan or other mechanical devices driving the flow of the thermally conductive liquid. A more detailed description of passive convective flow in an LED bulb is provided in Section 3, below.
A phosphor material is deposed on or dispersed within the ring structure 140. As mentioned above, a phosphor material can be used to absorb light produced by the LEDs 103 at a first wavelength and emit light at a different, second wavelength. Accordingly, a phosphor material can be used to control the wavelength of light emitted from the LED bulb 100.
To improve optical efficiency of the LED bulb, the optical materials of the LED bulb 100 may be selected to minimize back reflection. In general, back reflection refers to an amount of light that is reflected back toward a source due to an optical interface between two materials. The amount of back reflection that occurs at an optical interface depends, at least in part, on the difference in the indices of refraction for the two materials at the optical interface. Typically, when the difference between the indices is reduced, the amount of back reflection is also reduced.
To minimize back reflection at the optical interface between the LEDs 103 and the thermally conductive liquid, the thermally conductive liquid is selected to have an index of refraction that is between the index of refraction of the LED and the index of refraction of air (1.0). In some cases, the thermally conductive liquid has an index of refraction that is matched within 40 percent of the index of refraction of the LED. For example, an LED made from GaN may have an index of refraction of approximately 2.2. A thermally conductive liquid may be selected having an index of refraction of approximately 1.45, which is within approximately 35 percent of the index of refraction of the LED.
To reduce back reflection at optical interfaces between the thermally conductive liquid and the ring structure 140, the ring structure 140 has an index of refraction that matches the index of refraction of the thermally conductive liquid. The index of refraction of the ring structure is considered matched if the index of refraction is matched within 10% of the index of the thermally conductive liquid. For example, if the thermally conductive liquid has and index of refraction of approximately 1.45, the index of refraction of the ring structure 140 may have an index of refraction ranging between approximately 1.3 and 1.6 and still be considered matched to the index of the liquid.
The index of refraction of the ring structure 140, as discussed herein, does not include the index of refraction of the phosphor material. In some cases, the phosphor material integrated with the ring structure 140 may have a different index of refraction than the ring structure 140. For example, the phosphor material may have an index of refraction of approximately 1.8. In some cases, the phosphor material is selected to have an index of refraction that matches the index of refraction of the thermally conductive liquid.
There are a number of advantages to using a ring structure that is spatially separated from the surface of the LED and that is index-matched to a surrounding thermally conductive liquid. One advantage is that the ring structure provides a relatively large surface area for applying a phosphor material, as compared to the surface area of the LED. Light emitted from an LED 103 propagates along a roughly conical light path that becomes wider as the light travels further from the LED 103. That is, as the light propagates away from the LED, approximately the same number of emitted photons is spread over a larger area. By placing the phosphor material further along the conical light path, a larger area of phosphor can be used for approximately the same number of photons. In some cases, the effectiveness of the phosphor material can be improved by increasing the phosphor to photon ratio.
Another advantage of using a spatially separated ring structure that is index-matched to a surrounding thermally conductive liquid is that light reflected back from the phosphor is at least some distance away from the surface of the LED. As described above, the back reflection may occur due to different indices at the optical interface between materials. The phosphors may also emit light back toward the LEDs, when the phosphors are stimulated. This light emitted by the phosphors at a different wavelength may also be considered reflected light. The further away these reflections occur, the less light that will be reflected back on (and absorbed by) the surface of the LED. Reducing light absorbed by the LED may reduce the thermal load on the LED and increase the overall light output of the LED bulb.
As described in more detail below, the thermally conductive liquid provides cooling for the LED via passive convective currents. The thermally conductive liquid also provides cooling for the phosphor material integrated with the ring structure. To allow for liquid cooling, the ring structure may also facilitate the flow of passive convective currents in the thermally conductive liquid. Exemplary ring structures that facilitate passive convective flow are described in more detail below in Section 3.
An LED bulb 100 using a thermally conductive liquid to cool the LEDs can produce light equivalent to standard incandescent bulbs. In some embodiments, the LED bulb 100 may use 6 W or more of electrical power to produce light equivalent to a 40 W incandescent bulb. In some embodiments, the LED bulb 100 may use 20 W or more to produce light equivalent to or greater than a 75 W incandescent bulb. Depending on the efficiency of the LED bulb 100, between 4 W and 16 W of heat energy may be produced when the LED bulb 100 is illuminated.
As described above, there are multiple advantages to integrating a phosphor material with a structure that is spatially separated from the LED. There are also advantages to using a phosphor structure that is immersed in an index-matched, thermally conductive liquid. When such a structure is used in a liquid-filled LED bulb, the structure may be placed within the LED bulb in a way that does not significantly alter the performance of the LED bulb. Specifically, the ring structure may be shaped to capture light emitted by the LEDs without significantly interfering with passive convective currents of the thermally conductive liquid. In this way, the ring structure can be used to alter the optical qualities of the light emitted by the LED bulb while facilitating flow of the thermally conductive liquid. Exemplary ring structure shapes are depicted in
As mentioned above, a liquid-filled LED bulb may use a thermally conductive liquid to transfer heat from the LEDs to the bulb's shell via passive convection. To facilitate passive convective flow, the thermally conductive liquid may have a large coefficient of thermal expansion.
Due to the heat exchange between the LEDs 103, LED mounts 105, shell 101, and base 113, passive convective currents tend to flow as indicated by the arrows in
As shown in
Because cells of the thermally conductive liquid within the dead zones 130 do not significantly participate in the convective heat transfer, the dead zones 130 may be suitable locations for placing a structure containing a phosphor material.
For example, as explained in more detail below, a ring structure including a phosphor material can be placed in the dead zones 130 to alter the spectral emissions of the LED bulb. By placing the ring structure in or near the dead zones 130, the ring structure can be used to alter the emission spectrum of the LED bulb 100 while facilitating convective heat transfer by the thermally conductive liquid. That is, in some embodiments, the ring structure can be placed within enclosed volume 111 of the LED bulb 100 in a location that does not significantly block or impede the passive convective flow of the thermally conductive liquid.
A ring structure 140 positioned as shown in
The ring structure 140 is positioned with respect to the LEDs 103 so that the light emitted from the LEDs 103 can be absorbed by the phosphor material integrated with the ring structure 140. In some cases, the ring structure 140 is shaped and placed in a location so that the ring structure 140 can intercept nearly all the light traveling on a path from the LEDs 103 to the shell 101. One advantage of such a ring structure 140 is that the light distribution produced by the LED bulb 100 appears more uniform. Exemplary ring structure embodiments are shown in
Additionally, because the ring structure is spatially separated from the LEDs 103, the phosphor material is not in direct contact with the LEDs 103. Keeping the phosphor material away from the LEDs 103 (and other hot elements in the LED bulb) may increase the life of the phosphor material.
In some embodiments, the ring structure 140 may be configured to facilitate the passive convective flow of the thermally conductive liquid when the LED bulb is in multiple orientations. For example,
In some cases, the dimension of the inner surface of the ring structure 142 is sized to provide a gap between the inner surface of the ring structure 142 and the LEDs, when the ring structure 142 is installed in the LED bulb. In some cases, the gap between the LEDs and the inner surface of the ring structure 142 is at least 2 mm.
The ring structure 142 depicted in
The ring structure 144 of
A phosphor material can be deposited on the inside or outside surface of any ring structure embodiment (140, 142, or 144) included in the embodiments discussed above. In some cases, the ring structure (140, 142, or 144) is hollow and the phosphor material is deposited on the inside of the ring structure (140, 142, or 144). In some cases, the phosphor material is dispersed within the ring structure material.
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and it should be understood that many modifications and variations are possible in light of the above teaching.
