Abstract:
A lighting device or a lamp bulb ( 100, 200 ) comprises at least one light source ( 101 ) and a driver assembly ( 108 ), said driver assembly comprising driver electronics; wherein during operation of the light source, a distance between an optical axis ( 120 ) of the lighting device and a heat flow of the lighting device is less than a distance between the optical axis ( 120 ) and a component of the driver electronics having the highest temperature sensitivity.

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/EP2014/067382, filed on Aug. 14, 2014, which claims the benefit of European Patent Application No. 13194175.9, filed on Nov. 23, 2013 and of International Application No. PCT/CN2013/000977, filed on Aug. 22, 2013. These applications are hereby incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates generally to a lighting device, and more specifically to a lighting device or a lamp bulb with a smooth outer appearance and improved thermal performance. The invention also relates to a luminaire with a lamp bulb having a smooth outer appearance and improved thermal performance. 
     BACKGROUND OF THE INVENTION 
     For an optimal thermal performance, a lighting device comprises a heat sink equipped with fins, for example back-reflecting lamp bulbs of type PAR, MR, BR, GU, etc. “PAR” means parabolic aluminized reflector. “MR” means multifaceted reflector. “BR” means bulged reflector, and “GU” refers to a U-shaped lamp with a plug-in lamp base. The light sources of the lamps include conventional halogen filaments or LED light sources. 
     Conventional heat sinks are made of die cast metal, such as aluminum, with high manufacturing and raw material costs. Further, for aesthetic reasons, a non-technical appearance without a visible cooling structure is desired. If the heat sink structure is hidden behind a smooth outer surface, airflow through the cooling structure is preferred for improved thermal performance, which requires inlet and outlet openings. For the desired look-and-feel, these openings should be small. However, a small channel has a high airflow resistance, reducing the cooling performance of the heat sink structure. Since the cooling performance is mainly determined by the amount of air that flows through the cooling structure, also referred to as internal channel, this will reduce the cooling performance of the heat sink. 
     US2012/0044680A1 discloses an illustrator with LED including a rear housing having a cavity. A front housing is disposed in the cavity, wherein the front housing includes through holes. An illuminating module is sandwiched between the rear housing and the front housing. Air holes are formed on the sidewall of the rear housing, so that the cavity can communicate with outside air. 
     An example of a retrofit lamp with a smooth outer appearance may be found in the patent application PCT/IB2013/052999 “Lighting Device with Smooth Outer Appearance”, incorporated herein by reference. As shown in  FIG. 1 , the retrofit lamp  100 ′ comprises at least one light source  101 ′; a heat sink component  104 ′, having a bottom  1043 ′ and a sidewall  1044 ′ extending from the bottom  1043 ′, wherein the bottom  1043 ′ comprises a concave part  1041 ′ and wherein the at least one light source  101 ′ thermally contacts the concave part  1041 ′ of the heat sink component  104 ′; and a cover  103 ′ provided on the sidewall  1044 ′ opposite to the bottom  1043 ′, thereby defining an air chamber  1051 ′ between the cover  103 ′, the sidewall  1044 ′, the bottom  1043 ′ and the concave part  1041 ′. The heat sink component  104 ′ comprises a cover opening  102 ′ and a heat sink opening  106 ′. The air chamber  1051 ′ forms a channel between the cover opening and the heat sink opening to allow a flow of air  105 ′ between the cover opening and the heat sink opening or vice versa. A housing  107 ′ is provided between the heat sink component  104 ′ and the base  109 ′. A driver assembly  108 ′ is provided between the housing  107 ′ and the concave part  1041 ′. When the lamp  100 ′ is mounted in a vertical operating position as shown in  FIG. 2 , the air surrounding the lamp warms up during lamp operation, and the warm air will rise because of natural convection. 
     While the thermal rating, i.e. the maximum temperature for which they are rated to operate without being negatively affected, of most of the components of both the driver assembly and the light source is above 125° C., some of them, such as the electrolytic capacitor(s), are more sensitive to high temperatures. However, the construction of the previous types of lamps results in an unsuitable arrangement, as the thermally sensitive components on the driver assembly  108 ′ are heated by the rising warm air. 
     It is desired to combine optimal heat dissipation with the advantages of a smooth outer appearance of the lighting device. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention, among others, to achieve a lighting device with a smooth appearance, wherein the thermally sensitive electronic components are better protected against high temperatures. 
     To better address one or more of these concerns, in an aspect of the invention, an embodiment of a lighting device is presented which comprises: at least one light source and a driver assembly, said driver assembly comprising driver electronics. During operation of the light source, a distance between an optical axis of the lighting device and a heat flow of the lighting device is less than a distance between the optical axis and a component of the driver electronics having the highest temperature sensitivity. 
     The optical axis extends through the central portion of the lighting device. In the prior art lighting device, the driver assembly is mounted in the central portion. The temperature of the central portion is higher than that of the periphery of the lighting device due to poor heat dissipation in the central portion. In contrast thereto, according to the present invention, a heat flow is provided near the central portion with the warmed airflow during the operation of the light source because of natural convection. According to the invention, the component of the driver electronics having the highest temperature sensitivity is provided at a distance from the central portion. As a result, a light device according to the present invention is advantageous in at least the following two aspects:
         The airflow near the optical axis improves the heat dissipation of the central portion of the lighting device;   The thermally sensitive driver electronics is minimally influenced by the heat flow.       

     In an embodiment, the lighting device further comprises: a heat sink component having a bottom, a sidewall extending from the bottom, and a concave part extending from the bottom into the heat sink component, and wherein the at least one light source thermally contacts the concave part; wherein the heat sink component comprises a heat sink opening, and the heat sink opening comprises a plurality of holes in the bottom of the heat sink component between the concave part and the sidewall; and a cover provided on the sidewall and opposite to the bottom, thereby defining an air chamber between the cover, the sidewall, the bottom and the concave part; wherein the cover comprises a cover opening, and the air chamber forms a channel between the cover opening and the heat sink opening to allow a flow of air between the cover opening and the heat sink opening or vice versa; wherein the driver assembly is arranged in the air chamber. This provides an unobtrusive, hardly visible opening in a side view of the lighting device in the form of a lamp bulb with an ornamental effect. A “chimney” effect may be built up within the channel between the cover opening and the heat sink opening, to achieve improved thermal dissipation. Preferably, a cross section of the channel is larger in surface area than at least one of the cover openings and the heat sink opening. By enlarging the cross section of the air chamber or channel between the inlet and the outlet, which are the cover opening and the heat sink opening, so that the air velocity inside the air chamber or channel is as low as possible, flow losses in the system are minimized. 
     Preferably, in an embodiment, the lighting device further comprises a housing having a plurality of first holes; wherein the concave part has a side surface and a top surface, and wherein the side surface of the concave part forms a portion of the air chamber; wherein a plurality of second holes is formed in the side surface to communicate with the first holes; and wherein a further channel is formed between the first holes and the second holes to allow a further flow of air between the first and the second holes, or vice versa, during operation of the light source, because of a “chimney” effect; the distance between an optical axis of the lighting device and the further flow of air is less than the distance between the optical axis and the driver assembly. The further airflow in the further channel communicates with the airflow in the channel between the cover opening and the heat sink opening at the location of the second holes in the side surface of the concave part. By virtue thereof, the further channel between the first holes and the second holes not only improves thermal dissipation of the central portion of the lighting device, but also provides for additional thermal dissipation to the channel between the cover opening and the heat sink opening. 
     In an embodiment, the driver assembly is arranged in a compartment within the air chamber. The driver assembly may be encased in the compartment by means of potting material to conduct heat from the driver electronics to the heat sink component. By encasing the driver electronics in potting material, the driver electronics will be able to conduct heat more efficiently to the heat sink component, thereby enabling the driver electronics to produce more power, or it may be manufactured from other, less costly materials, materials having a smaller environmental impact or materials producing more heat at the same power. 
     According to another embodiment of the invention, the potting material may advantageously be made from at least one of a group comprising silicon oil, micro silica powder and asphalt, or a mixture thereof, which are materials with a thermal conductivity which is advantageous for use as potting material. 
     In an embodiment, the cover comprises a rim at its outer periphery, and the cover opening comprises a plurality of holes in the rim. 
     In an embodiment, the cover is of a thermally conductive material which thermally contacts the sidewall of the heat sink component. Preferably, the cover comprises a recess which accommodates the at least one light source, and the recess thermally contacts the concave part. In this way, additional thermal contact between the at least one light source and the heat sink component is provided in a convenient and simple way. Preferably, the recess further comprises at least one optical element for the at least one light source. Preferably, the at least one optical element is selected from a group comprising a diffuser, a reflector, a lens, a collimator or a combination thereof. 
     In an embodiment, the sidewall has an intact, smooth, exposed surface. Alternatively, the sidewall may comprise a patch corresponding to a mounting position of the driver assembly. 
     In an embodiment, the at least one light source is thermally coupled to a PCB, which PCB extends into the air chamber; and which PCB has a plurality of PCB openings to allow a flow of air between the cover opening and the heat sink opening or vice versa. The PCB openings may be cut-outs at the edge of the PCB or holes in the PCB. Preferably, the PCB comprises a thermally conductive material, for example, a thick layer of copper, so that the thermal conductivity of the PCB is at least 28 W/mK, measured along the surface of the PCB. 
     This provides the PCB with good thermal conductivity, and therefore the PCB itself can act as a good heat sink. In other words, the airflow can dissipate the heat from the light source via the PCB. In one aspect, this leads to an increase of the thermal performance of the lighting device. In another aspect, this lowers the thermal requirements imposed on all other components in the lighting device, for example, the shell (referred to as heat sink component hereinabove) and the cover can be made of full plastic. It may not need glue, or grease, for the thermal coupling between components. As a full plastic lamp, it may no longer require painting, and there may be much fewer safety concerns for electric shock as compared to a metal housing. The assembly process of the lighting device may also be simplified. In this way, the total cost of the lighting device is substantially reduced. 
     In other embodiments of the lighting device, the at least one light source comprises a LED or an array of LEDs, and the lighting device can be a back-reflecting lamp bulb of type GU, MR, BR or PAR, such as GU10, MR16, BR30, BR40, R20, PAR38, PAR30L, PAR30S, PAR20, etc. 
     According to a second aspect of the invention, a luminaire is provided which comprises a lighting device or a lamp bulb according to the first aspect of the invention. 
     It is noted that the invention relates to all possible combinations of features recited in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the lighting device and the luminaire according to the invention will become apparent from and will be elucidated with respect to the implementations and embodiments described hereinafter and with reference to the accompanying drawings. In the drawings: 
         FIG. 1  shows an example of a retrofit lamp in the current applicant&#39;s unpublished patent application PCT/IB2013/052999; 
         FIG. 2  illustrates the air velocity around and within the retrofit lamp of  FIG. 1  during operation; 
         FIG. 3  shows an exploded view of a lighting device according to an embodiment of the invention; 
         FIG. 4  shows a perspective side view of the lighting device illustrated in  FIG. 3 ; 
         FIG. 5  shows a perspective bottom view of the lighting device illustrated in  FIG. 3 ; 
         FIG. 6  shows a cross sectional view of the lighting device illustrated in  FIG. 3  during operation, taken on the line A-A of  FIG. 5 ; 
         FIG. 7  shows a cross sectional view of the lighting device illustrated in  FIG. 3  during operation, taken on the line B-B of  FIG. 5 ; 
         FIG. 8  shows a perspective top view of the lighting device illustrated in  FIG. 3 ; 
         FIG. 9  shows an exploded view of a lighting device according to another embodiment of the invention; 
         FIG. 10  shows a perspective side view of the lighting device illustrated in  FIG. 9 ; 
         FIG. 11  shows the PCB of the lighting device according to a further embodiment of the invention. 
     
    
    
     It should be noted that items denoted by the same reference numerals in different Figures have the same structural features and the same functions, or are the same signals. If the function and/or structure of such an item have been explained, there is no necessity for repeated explanation thereof in the detailed description. 
     The Figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. 
     DETAILED DESCRIPTION 
     An embodiment of the lighting device according to the present inventive concept is illustrated in  FIG. 3 , and different views of the lighting device are presented in  FIGS. 4 to 8 .  FIG. 3  illustrates a PAR type lamp  100  with LEDs or a LED array representing a light source  101  mounted in the front end opposite to the base  109 . The light source  101  is mounted on a PCB  110  which is thermally coupled to a cover  103  and a heat sink component  104 . There are openings  102  in the cover  103 , and openings  106  in the heat sink component  104 . The cover  103  may act as an additional heat sink component and is thermally coupled to the heat sink component  104  at least along its outer periphery. 
     As shown in  FIG. 6 , the cover  103  has a recess  1031  for accommodating the light source  101 . Alternatively, the light source  101  is provided on the heat sink component  104 , for example on the bottom part of the recess  1031 , and the cover  103  comprises a light exit window where the light from the light source  101  can exit. In addition, suitable optics  130 , for example, a diffuser, a reflector, a lens or a collimator, or a combination of these optical elements, can be included in the recess  1031  of the cover  103 , thus providing a desired optical performance of the lamp  100 . 
     The heat sink component  104  is, in this case, cup-shaped, and has a sidewall  1044  and a bottom  1043  with a concave part  1041  extending from the bottom  1043  into the heat sink component  104 . A housing  107  is provided between the heat sink component  104  and the base  109 . First holes  1071  are provided on the sidewall of housing  107 , and second holes  1042  are provided on the side surface of the concave part  1041 . 
     The cover  103  and the heat sink component  104  are, in this case, assembled so as to establish a good thermal connection at the bottom surface of the recess  1031  and the top surface of the concave part  1041 , in addition to the thermal contact between the sidewall of the heat sink component  104  and the outer periphery of the cover  103 . The heat generated by the light source  101  will, in this case, be conducted to the heat sink component  104  and the cover  103 , in this case also acting as a heat sink, and will be dissipated relatively well at the exposed surfaces of the heat sink component  104  and the cover  103 . The thermal connection between the recessed bottom and the top surface of the concave part can be established via direct attachment or via a thermally conductive medium, such as thermal glue or thermal filler. The thermal connection thickens the base of the heat sink and results in a better temperature distribution under the heat source. 
     As shown in  FIG. 6 , an air chamber  1051  is formed between the cover  103  and the heat sink component  104 . As shown in  FIG. 5 , openings  102  are provided in a rim  1033  around the recess  1031  of the cover  103 , thereby creating a first connection between the air chamber  1051  and ambient air. As shown in  FIG. 6  and  FIG. 8 , openings  106  are provided in the bottom  1043  of the heat sink component  104  adjacent to the sidewall  1044 , thereby creating a second connection between the air chamber  1051  and ambient air. Openings  102  and  106 , together with the air chamber  1051 , form a channel allowing air to flow through the air chamber  1051 , as indicated by means of the dash-lined arrow  105 . Further, a further channel is formed between the first holes  1071  and the second holes  1042  to allow a further flow of air between the first and the second holes, as indicated by means of the dash-lined arrow  1055 . 
     A driver assembly  108 , which includes driver electronics on a PCB, is arranged in the air chamber  1051 . The distance d between the further flow of air  1055  and an optical axis  120  of the lamp  100  is less than the distance D between the optical axis  120  and a component of the driver electronics having the highest temperature sensitivity, for example, an electrolytic capacitor. 
     When the lamp  100  is operated vertically as illustrated in  FIG. 6 , being for instance a down-lighting, a chimney effect will be created in the heat sink structure. The heat source, i.e. the light source  101 , pre-heats the airflow and creates a buoyancy force. The higher the temperature of the air, the larger the driving force will be. This driving force is created by the density difference between hot air and the relatively cold ambient air. In a gravitational field, the hot air becomes less dense and rises, driven by the buoyancy force. Meanwhile, the cold air follows, taking up the space left by hot air, thus creating the airflow. When the air passes through the channel, it has been and will be heated and thus stores a certain amount of energy. As long as the air leaves the channel or air chamber, the heat is carried away. The heat produced by LEDs is mainly removed through the moving air, including both internal (in the chimney channel or air chamber) and external moving air, i.e. outside the lighting device. The further channel between the first holes  1071  and the second holes  1042  helps to remove the heat which normally converged in the central part of a prior art lamp, thus allowing further improved thermal dissipation. The thermally sensitive driver electronics on the driver assembly  108  is arranged in the air chamber  1051 , where cold air enters and causes the temperature around the driver assembly  108  to be lower than at the location of the upper portion of the lamp  100 , which is heated by the rising hot air. As a result, these driver electronics receive minimum influence from the heat flow. 
     In this embodiment, the cross section of the channel between the inlet and the outlet, i.e. openings  102  and  106 , is enlarged, so that the air velocity inside the air chamber  1051  is as low as possible and the overall flow losses in the system are minimized. This is advantageous because it decreases the thermal resistance. 
     As shown in  FIG. 3 , a compartment  1048  is provided to accommodate the driver assembly  108 . The compartment  1048  is provided in the air chamber  1051  (see  FIG. 6 ). The advantage of using the compartment  1048  instead of directly positioning the driver assembly  108  in the air chamber  1051  may be that the driver assembly  108  can be encased in the compartment  1048  by means of potting material to conduct heat from the driver electronics to the heat sink component. The potting material may be silicon oil, micro-silica powder or asphalt, or a mixture of such materials. By encasing the driver electronics in potting material, the driver electronics will be able to conduct heat more efficiently to the heat sink component, whereby the driver electronics may produce more power, or the driver electronics may be manufactured by other, less costly, materials, materials having a smaller environmental impact or materials producing more heat at the same power. Further, in the manufacturing process of the lamp  100 , it may be easier to insulate the driver assembly  108  in the compartment  1048  so as to prevent the safety issue of electric shock. 
     In the embodiment shown in  FIGS. 3 to 8 , an outer surface  1045  of the compartment  1048  acts as a patch to the sidewall  1044  of the heat sink component  104 . Although the sidewall  1044  is no longer a perfectly intact, smooth, exposed surface, such a sacrifice in ornamental appearance may be acceptable to a certain extent, because it may simplify the assembly procedure of the lamp  100 . 
     Alternatively, in the embodiment of lamp  200  shown in  FIG. 9 , a compartment  2048  is mounted completely within the heat sink component  104 . The driver assembly  108  may be encased in the compartment  2048  by means of potting material in the same manner as in the previous embodiment. As shown in  FIG. 10 , which is an outside view of the lamp  200 , the sidewall  1044  of the heat sink component  104  is an intact, smooth, exposed surface, without holes, slots or fins, which provides an ornamental effect. 
     In an alternative embodiment, the compartment  1048  or  2048  is not used. In other words, the driver assembly  108  is mounted directly on the heat sink component  104 , and within the air chamber  1051 . 
     In a further embodiment of the invention, the light source  101  is mounted on a large Printed Circuit Board (PCB)  310  as shown in  FIG. 11 , instead of the PCB  110  as shown in  FIG. 3  of the first embodiment. The PCB  310  extends into the air chamber  1051  like in previous embodiments, and the PCB has a plurality of PCB openings  312  to allow the airflow to pass smoothly through the air channel like in previous embodiments. The PCB openings may be configured as cut-outs at the edge of the PCB or holes  312  as shown in  FIG. 11 . Preferably, the holes  312  are aligned with the openings  102  in the cover  103  so as to allow maximum airflow. 
     The PCB  310  comprises a thermally conductive material, for example, a thick layer of copper, so that thermal conductivity of the PCB is at least 28 W/mK measuring along surface of the PCB. In this embodiment, the PCB  310  acts as a heat sink which can bring additional thermal performance to the lamp or provide solutions with lower cost, for instance a whole plastic lamp. In such whole plastic lamp, low cost engineering plastic is used for the cover  103 , the heat sink component  104  and the housing  107 . 
     A person skilled in the art can understand that other types of back-reflecting lamp bulbs, such as GU, MR, etc., can adopt the same principle to achieve a lamp with a smooth appearance and the advantages of low cost, good manufacturability and high heat dissipation capability. 
     A person skilled in the art can understand that a luminaire can be configured to fit a lighting device or lamp  100 ,  200  according to the above mentioned embodiments. 
     A person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention and that those skilled in the art will be able to design alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps not listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The usage of the words first, second and third, etc., does not indicate any ordering. These words are to be interpreted as names. No specific sequence of acts is intended to be required unless specifically indicated.