This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/IB13/058339, filed on Sep. 6, 2013, which claims the benefit of U.S. Provisional Patent Application No. 61/697,921, filed on Sep. 7, 2012. These applications are hereby incorporated by reference herein.
The present invention relates to the field of thermal management of lighting devices, and more particularly to light emitting diode (LED) based lighting devices configured to provide thermal management utilizing a light exit element of the lighting device as a heat spreader.
LED based lighting devices, or LED lamps, have become common on the market and are showing great promise to gradually replace incandescent and compact fluorescent lamps throughout the world due to long life-time expectancy, reduced size, and high energy-efficiency with respect to energy and lumen output efficiency as compared to for instance traditional incandescent light bulbs. Utilizing LED based lamps in traffic lights, a city can significantly reduce the energy related cost per year per signal, because a LED lamp uses approximately one-tenth of the electricity that the traditionally used illumination does.
Thermal management of LED lamps is key, since the performance of the LED lamp is often limited in the light output by thermal constraints. Thermal management may be concerned with managing heat produced by the LED lamp itself, as well as external heat sources, or may be related to influence on the LED lamp by the ambient temperature. Generally, the thermal performance determines the maximum light output from the LED lamp, and is further determined mainly by the size of the heated external surface of the LED lamp. As an example, consider a typical retrofit LED lamp comprising at least one LED-based light source arranged in thermal contact with a heat sink, i.e. typically the lamp base. The LED-based light source is arranged for generating light which exits the LED lamp through a light exit element, i.e. an optically transmissive element like for instance a bulb envelope. The light exit element is typically made of a transparent or translucent material, like glass, silicone, and Polycarbonate, PC, which materials all have a low thermal conductance. Therefore heat spreading from the heat sink into the bulb envelope is not effective, and most of the heat produced by the LEDs therefore exits the lighting device via the heat sink.
It is known in the art to increase the heated external surface of the LED lamp by means of providing heat spreading from the heat sink to the light exit element. WO2010/097721 A1 discloses a LED lamp including a LED-based light source configured to emit light and an optically transmissive window optically and thermally coupled to the LED-based light source. Different solutions for configuring the optically transmissive window to in an improved manner radiate heat generated by the LED-based light source to the ambient, as compared to the typical prior art LED lamp as described above, are shown. For instance, the document discloses the optically transmissive window being arranged with one of a coating with predetermined heat conductivity, a compound material, an at least partly integral heat pipe, and a combination of elements including two materials with different thermal conductivities.
In view of the above, an object of the invention is to at least provide an advantageous and alternative solution to thermally control a LED based lighting device by utilizing the light exit element to distribute heat generated by the LED-based light source.
This object is achieved by a lighting device according to the present invention as defined in claim 1. Thus, in accordance with an aspect of the present invention, there is provided a lighting device comprising at least one LED-based light source for generating light, and a light exit element being optically and thermally coupled to the LED-based light source. The light exit element comprises a heat conducting structure arranged for distributing heat generated by the at least one LED-based light source over at least a predetermined sub area of the light exit element. The heat conducting structure may be embedded in, or in physical contact with, or in close proximity to the light exit element, and comprises a set of aligned heat conducting paths. In preferred embodiments of the lighting device, the heat conducting structure comprises heat conducting wires, or a thin patterned heat conducting layer, which both provide simple, yet efficient heat conducting structures which are suitable to be arranged at or embedded in the light exit element without a big influence on the light transmission through the light exit element.
The present inventive concept is based on introducing a heat conducting structure at the light exit element, which conducts heat and effectively spreads the heat over the light exit element and decreases the thermal gradient in the light exit window, and the lighting device overall. The light exit element becomes an integral part of the heat transferring external surface of the lighting device, which increases the possibility to thermally control the lighting device. By utilising the light exit element as an extra heat sink area, the lighting device can take on a more free form factor as compared to traditional LED lighting devices in which the LED heat sink typically occupies a major part of the device.
According to the present inventive concept, the heat conducting structure is advantageously arranged as aligned heat conducting paths/tracks which may be embedded in the light exit window. According to an embodiment of the lighting device, the heat conducting structure comprises a set of heat conductive wires, or is a patterned heat conducting film. The wires or branches of the pattern may be aligned in a predetermined manner to facilitate heat conduction in a predetermined direction or a predetermined distribution within light exit element. There is an advantage of using aligned heat conducting structures over any other heat conducting structure, which is associated with an optimum anisotropy in the thermal conductivity that is obtained in the light exit element. This is needed e.g. if the wires (or patterned branches) are opaque. As an example, a typical light exit element of a LED lamp has a diameter of 5-20 cm, or has a distance from the heat sink of approximately 2.5-10 cm from the heat sink to the centre of the light exit element. Therefore, large thermal gradients occur in the light exit element if the heat spreading from the heat sink to the light exit element is low. When using opaque wire (branch) materials, the opaque wire structure will deteriorate the optical properties of the light exit element, even if the wires are provided with a highly reflective coating, as in some embodiments of the present invention. Maximum heat conduction with minimum material use is wanted for that reason, and this is obtained by arranging the heat conduction material in separate heat conducting paths.
According to an embodiment of the lighting device, at least a main portion of the heat conductive wires or branches of the pattern of the patterned heat conducting film are arranged to transfer heat in a substantially radial direction with respect to the centre of the light exit element. In order to maximize the heat flow in a radial direction with respect to the window centre, maximum thermal anisotropy arranged by alignment of the wires in a radial direction with respect to the centre of the light exit element is the most advantageous solution.
According to embodiments of the lighting device, the spacing between adjacent wires or branches is selected in a range of 5-15 mm, which is advantageous for obtaining optimum uniformity of the temperature distribution in the light exit element. However, a wider spacing between wires or branches may be used if a minimal optical disturbance of the lighting device is required.
According to embodiments of the lighting device, the heat conducting structure may further comprise interconnecting wires or branches between adjacent wires or branches, respectively, thereby providing a meshed heat conducting structure. The interconnecting wires may be added to provide rigidity of the heat conducting structure which may be advantageous during manufacturing or which provides support for the finished light exit element. Further, if the interconnecting wires are heat conductive, the heat spreading within the light exit element is increased.
According to an embodiment of the lighting device, it further comprises a coupling element arranged for thermally coupling the light exit element and the at least one LED-based light source. The coupling element may be at least one heat pipe, a vapour chamber, or at least one heat conductive wire.
The thermal control of the lighting device arrangement according to the first aspect of the present inventive concept, is further applicable for preventing overheating of remote phosphor domes, and for providing an improved mechanical rigidity of small remote phosphor domes. The application of a remote phosphor dome on top of a blue pump LED is a well known method with a relatively high optical efficiency to produce white light. Due to energy loss related to Stokes shift and overall efficiency losses during the down conversion process of blue light (which blue light is produced by the blue pump LED) to yellow light in the phosphor material of the phosphor dome, the remote phosphor dome heats up. An increase in temperature typically leads to decreased lumen performance and an overheated remote phosphor dome. By applying the present inventive concept of a heat conducting structure in the remote phosphor dome, i.e. the light exit element of the lighting device, heat is distributed within the light exit element, and may further be transferred to an overall lamp heat sink of the lighting device, which significantly lowers the internal temperature of the remote phosphor dome.
According to an embodiment of the lighting device, the LED-based light source is a remote phosphor light source comprising a primary LED-based light source and a down conversion phosphor material arranged at the light exit element.
According to an embodiment of the lighting device, it further comprises a heat sink thermally coupled to the light exit element and/or the LED-based light source.
According to the first aspect of the present inventive concept, spreading the heat generated by the LED-based light sources within the light exit element, is in addition to the above, advantageous for outdoor lighting applications in countries having a colder climate or indoor applications in cold environments, such as large walk in freezers, freezer cabinets, ice rinks, sheds and outhouses which in the winter can become freezing inside etc. Since the light output from LEDs is not hot, unlike the output from a halogen lamp for example, ice formation on the light exit element, i.e. the lens of the LED lamp, can occur and obscure the light output from the lighting device. Many countries having a colder climate are less interested in LED lighting in outdoor applications, because the traditional incandescent lamps do not have this problem. By utilizing that the light exit element operates as a heat sink, rather than distributing the heat via a heat sink arranged on the backside of the LED carrier substrate as is traditional, the heat generated by the LEDs can be used to thermally manage the light exit window, and for instance to prevent ice from forming on the lens.
According to an embodiment of the lighting device, it further comprises a temperature sensor and/or timer arranged in communication with a control means for thermally controlling the light exit element by means of a control signal associated with a driving power of the LED-based light source. The control signal may provide one of a pulsed switching of the LED-based light source at a frequency which is undetectable by the human eye but sufficient to heat the light exit element, or a driving power of the LED-based light source selected to provide a light output level from the LED-based light source which is undetectable by the human eye but sufficient to heat the light exit element. Further, a system for thermally controlling a lighting device according to the present invention is disclosed herein in the detailed description.
According to another aspect of the invention, there is provided a method for thermally controlling a lighting device according to the present inventive concept when comprising a temperature sensor and/or a timer comprising:
The control signal may provide one of a pulsed switching of the LED-based light source at a frequency which is undetectable by the human eye but sufficient to heat the light exit element, or a driving power of the LED-based light source selected to provide a light output level from the LED-based light source which is undetectable by the human eye but sufficient to heat the light exit element.
The term LED-based light source includes any light source comprising electroluminescent light generating systems, thus including various semiconductor-based structures that emit light in response to current, light emitting polymers, organic light emitting diodes (OLEDs), electroluminescent strips, etc. Further, a LED-based light source may include LED dies, LED chips, and/or LED packages.
Other objectives, features and advantages will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings.
The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:
An embodiment of a lighting device according to the present inventive concept is now described with reference to
According to the present inventive concept, the light exit element of a lighting device, such as the retrofit LED lamp 100 described with reference to
Optionally, the set of thermally conductive wires 151 are interconnected by supporting wires 154 to create rigidity in a mesh like configuration, as illustrated in
A LED lamp with wired light exit element 150, 171, as described above with reference to
According to an embodiment of the wired light exit element of the invention, a characteristic length of heat spreading within the light exit element, at a right angle from the heat conductive wires, is 4-7 mm effectively. As this is at both sides of the heat conductive wires, the effective, or characteristic heated zone per wire is typically 8-14 mm of width. The thickness of the light exit element is generally selected to be more than 1 mm to completely cover the heat conductive wires. Further, an effective length of the heat conductive wires into the light exit element is restricted, and is defined by the cross section of the wire and the wire spacing. As an example, for a wire diameter of 1 mm in Aluminum, and an interspacing of the wires of 10 mm, the effective length is 35-55 mm (depending on the heat transfer effectiveness at the light exit element). This effective length is applicable for heating up the complete dome or bulb envelope of e.g. a typical retrofit LED lamp. If the complete dome or bulb envelope is heated to the same temperature as the heat sink (when present), the thermal performance, expressed in the thermal resistance from heat spreader to ambient, Rth_spr-amb, is considerably reduced. In an exemplifying embodiment Rth_spr-amb decreases from 9.5 K/W to 5.5 K/W when introducing heat conductive wires in the light exit element in a free ambient bulb environment. For more examples, see Table 1 below.
In
The simulated temperature distribution in a bulb envelope comprising twenty-four Aluminum wires is illustrated in
Table 1 illustrates the simulated thermal resistance Rth in a 10 mm sleeve of the bulb envelope for the regular bulb envelope with no wires, and the wired bulb envelopes with twelve Aluminum wires, twelve Copper wires, and twenty-four Aluminum wires, respectively, at 14.8 W load of the LED lamp at an ambient temperature Tamb25° C. The diameter of the wires was set to 1 mm. The simulated values for the thermal resistance heat spreader to ambient, Rth_spre-amb, and the difference in thermal resistance ΔRth between the regular bulb envelope with no wires, and the wired bulb envelopes with twelve Aluminum wires, twelve Copper wires, and twenty-four Aluminum wires, respectively, are given in the Table.
According to an embodiment of the lighting device, the heat conducting structure in the light exit element is provided as a patterned heat conducting film embedded in the light transmissive material of the light exit element (not shown). Preferably, the pattern of the heat conducting film is arranged as branches arranged to transfer heat in a substantially radial direction with respect to the centre of the light exit element. The spacing between adjacent branches is preferably selected in a range of 5-15 mm. As in the case with wires, interconnecting branches between adjacent branches can optionally be provided in the pattern, such that the heat conducting structure becomes a mesh.
According to an embodiment of the invention, the heat conducting structure is arranged as a honeycomb structure (not shown). Preferably, the honey comb structure is selected to be very open to provide a high anisotropy in the heat conductivity on the scale of every honey comb cell. The anisotropy is advantageous for providing heat distribution over the light exit element area.
According to embodiments of the invention, to optimize the optical behavior of the LED lamp, the external surface of the heat conducting structure is provided with an optically reflective and/or diffuse surface having a high reflectivity index (not shown).
According to an embodiment of the lighting device according to the present invention, which will now be described with reference to
To continue with reference to
The introduction of heat conductive structures into the light exit element that conduct heat from the heat sink into the light exit element, as in the embodiments of the lighting device described above with reference to
Referring now to
In an embodiment of a LED lighting device 400 according to the present invention, see
In a preferred embodiment, at an end portion 404b of each heat conductive wire 404, which end portion 404b is arranged at the light exit window 401, and where heat should be released, the wires 404 are substantially uninsulated, while at the opposite end portion 404a which is closer to the LED-based light sources 402 (heat sources), to avoid heating the substrate 403, the heat conductive wires 404 are at least partly provided with an insulating layer (not shown). The insulating layer can be a polymer coating, see for instance patent U.S. Pat. No. 5,232,737, “Method of coating a metal wire with a temperature and stress resistant polymeric coating”. The heat conductive wires 404 are in an embodiment attached to the edge of the light exit element 401 by means of heat conducting glue.
Referring now to
Lighting devices according to the present invention are applicable in outdoor applications like for instance traffic lights. As previously mentioned, in situations when the lighting devices in outdoor applications (or applications in cold indoor environments) are not activated for a long time, or when the environment is very cold, ice may form on the light exit elements. Referring now to
Although a traffic light is given as an exemplifying embodiment above, it should be recognized that the present inventive concept is applicable in other lighting applications.
The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended claims.
Filing Document | Filing Date | Country | Kind |
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PCT/IB2013/058339 | 9/6/2013 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
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WO2014/037908 | 3/13/2014 | WO | A |
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