Abstract:
A lighting device or a lamp bulb ( 100, 200 ) with a smooth appearance comprises at least one light source ( 101 ); a heat sink component ( 104, 204 ), having a bottom ( 1043 ) and a side wall ( 1044 ) extending from the bottom ( 1044 ), wherein the bottom ( 1043 ) comprises a protrusion ( 1041 ) and wherein the at least one light source ( 101 ) thermally contacts the protrusion ( 1041 ) of the heat sink component ( 104, 204 ); and a cover provided on the sidewall ( 1044 ) opposite to the bottom ( 1043 ), thereby defining an air chamber ( 1051, 2051 ) between the cover, the side wall ( 1044 ), the bottom ( 1043 ) and the protrusion ( 1041 ).

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/IB2013/052999, filed on Apr. 16, 2013, which claims the benefit of Chinese Application PCT/CN2012/074472, filed Apr. 20, 2012 and European Patent Application No. 12168673.7, filed on May 21, 2012. 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. The invention also relates to a luminaire with a lamp bulb having a smooth outer appearance. 
     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 casting 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 side wall of the rear housing, so that the cavity can communicate with outside 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 and with the advantages of low cost, good manufacturability and high heat dissipation capability. 
     To better address one or more of these concerns, in an aspect of the invention, an embodiment of a lighting device is presented, comprising: at least one light source; a heat sink component, having a bottom and a side wall extending from the bottom, wherein the bottom comprises a protrusion, and wherein the at least one light source thermally contacts the protrusion of the heat sink component; and a cover provided on the sidewall opposite to the bottom, thereby defining an air chamber between the cover, the side wall, the bottom and the protrusion. The protrusion provides an increased surface area of the heat sink component, leading to improved thermal properties of the heat sink and furthermore it provides a part of the enclosure of the air chamber. In another embodiment, the width or diameter of the protrusion is the same as the width or diameter of the bottom, and hence the air chamber is enclosed by the protrusion, the side wall and the cover, because in this case the total area of the bottom is the protrusion. Thus, in this case the side wall effectively extends from the protrusion. 
     Preferably, the cover is of a thermally conductive material which thermally contacts the side wall of the heat sink component. In a further embodiment, the cover comprises a recess which accommodates the at least one light source, and the recess thermally contacts the protrusion. In this way an additional thermal contact between the at least one light source and the heat sink component is provided in a convenient and simple way. Further, a part of the cover may comprise a light exit window which may comprise an optical element, such as diffuser, a lens, etc. 
     Preferably, the protrusion has a side surface and a top surface, and the side surface forms a portion of the air chamber. Thus, the top surface of the protrusion does not form a portion of the air chamber. In this embodiment, the side surface of the protrusion is part of the enclosure of the air chamber, together with the side walls of the heat component, the cover and a part of the bottom. In the case that the whole bottom area is a protrusion, the air chamber is defined and enclosed by the side surface of the protrusion, the side wall of the heat sink component and the cover. 
     Preferably, the cover comprises a first opening, the heat sink component comprises a second opening, and the air chamber forms a channel between the first opening and the second opening to allow a flow of air between the first and second opening or vice versa. This provides for additional cooling and a further improved heat sink capacity of the heat sink component. The protrusion provides an increased air flow cooling area of the heat sink component with respect to the state of the art in which no protrusion is defined. Preferably, a cross section of the channel is larger than at least one of the first opening and the second opening. By enlarging the cross section of the air chamber or channel between the inlet and the outlet, so that the air velocity inside the air chamber or channel is as low as possible, flow losses in the system are minimized. 
     In an embodiment, the bottom of the heat sink component is substantially circular and the protrusion is also substantially circular. In this case, the side wall also will have a substantially circular cross-section and also the cover will be substantially circular. 
     In an embodiment, the protrusion encompasses at least a part of an electronic component. The electronic component drives the at least one light source. In this way, space is saved by using the protrusion to enclose at least a part of the electronic component. In this embodiment, the electronic component thus is not part of the air chamber but is situated outside the air chamber in another chamber between the protrusion of the heat sink component and a base of a lamp which comprises the lighting device. Preferably, electrical contacts are provided between the electronic component and the at least one light source via through holes in the heat sink component. 
     There is a larger area for thermal coupling by virtue of the protrusion and therefore improved thermal performance of the lighting device. In an embodiment, the cover is additionally mechanically attached to the protrusion, next to the mechanical attachment (and thermal connection) to the side wall of the heat sink component. Furthermore, the air can flow alongside the protrusion, which further improves the thermal performance of the lighting device. 
     Preferably, the side wall of the heat sink component has the shape of the side walls of a cup. 
     The heat sink component and optionally also the cover can be made of sheet metal, such as aluminum plates, using a low-cost metal stretching process, such as deep drawing. Alternatively, the heat sink component and optionally also the cover can be made of plastic, using a stretching or injection/molding process. Compared to the conventional heavy die-cast heat sink, the cost of both raw material and manufacturing can be decreased, and the weight of the final product can be reduced. 
     According to an embodiment of the lighting device, the cover comprises a rim at its outer periphery, and the first opening comprises a plurality of holes near the rim. Advantageously, the rim provides a mechanical attachment to the side wall. 
     According to another embodiment of the lighting device, the heat sink component comprises a first and a second opening, and the air chamber forms a channel between the first opening and the second opening to allow a flow of air between the first and second opening or vice versa, and wherein the first opening is a slit in the side wall of the heat sink component. 
     The first opening is thus designed as holes in the cover or a hardly visible narrow slit in the side wall, resulting in an unobtrusive opening in the main view of the lighting device in the form of a lamp bulb. This may provide an ornamental effect to the bulb. 
     According to a further embodiment of the lighting device, the second opening of the lighting device preferably comprises a plurality of holes in the bottom of the heat sink component between the protrusion and the side wall of the heat sink component. This provides an unobtrusive, hardly visible opening in the main view of the lighting device in the form of a lamp bulb with an ornamental effect. 
     Preferably, the side wall of the heat sink component has an intact smooth exposed surface, without holes, slots or fins, which provides an ornamental effect in the main view of the lighting device in the form of a lamp bulb. The exposed surface has a relatively good heat dissipation capacity. 
     According to yet another embodiment of the lighting device, the second opening comprises a plurality of holes disposed in the side wall of the heat sink component adjacent to the bottom of the heat sink component. 
     Preferably, the heat sink component and the cover are thermally coupled at least through engagement between a bottom portion of a recess in the cover and a top surface portion of the protrusion of the heat sink component. The heat generated by the light source and/or the electronic driving component can be conducted via the heat sink component and the cover and transported to the surrounding air via the exposed surfaces. 
     In a further embodiment, the at least one light source is thermally coupled to a PCB. The PCB extends into the air chamber, and the PCB has a plurality of PCB openings to allow the flow of air between the first and second 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 thermal conductivity of the PCB is at least 28 W/mK measuring along 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 air flow can dissipate the heat from the light source via the PCB. In one aspect, this brings additional thermal performance to the lighting device. In another aspect, this lowers the thermal requirements to all other components in the lighting device, for example, the shell (or call heat sink component as above) and the cover can be made of full plastic. Thus, the design of the lighting device is eased. It may not need glue, or grease, for the thermal coupling between components. As a full plastic lamp, painting may no longer be needed, and there may be much less safety concerns of electric shock due to metal housing. The process of assembly of the lighting device may also be simplified. By this way, the total cost of the lighting device is greatly decreased. 
     In other embodiments of the lighting device, the recess of the cover can further comprise a reflector. In yet 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 second aspect of the invention, a luminaire is provided which comprises a lighting device or lamp bulb according to the first aspect of the invention with a smooth outer appearance. 
     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 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 a lighting device according to an embodiment of the invention; 
         FIG. 2  shows a top view of the lighting device illustrated in  FIG. 1 ; 
         FIG. 3  shows a bottom view of the lighting device illustrated in  FIG. 1 ; 
         FIG. 4  shows a side view of the lighting device illustrated in  FIG. 1 ; 
         FIG. 5  shows a schematic sectional side view of the lighting device illustrated in  FIG. 1 ; 
         FIG. 6  illustrates the air velocity around and within the lighting device illustrated in  FIG. 1  during operation; 
         FIG. 7  shows a lighting device according to another embodiment of the invention; 
         FIG. 8  shows a schematic sectional side view of the lighting device illustrated in  FIG. 7 ; 
         FIG. 9  shows an explosive view of a lighting device according to a further embodiment of the invention; 
         FIG. 10  shows the PCB of the lighting device in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the lighting device according to the present inventive concept is illustrated in  FIG. 1 , and different views of the lighting device are presented in  FIGS. 2 to 5 .  FIG. 1  shows a PAR 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 thermally coupled to a cover  103  and a heat sink component  104 . There are holes  102  in the cover  103 , and holes  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. 5 , 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. The heat sink component  104  is, in this case, cup-shaped, and has a side wall  1044  and a bottom  1043  with a protrusion  1041  provided in the bottom  1043  of the heat sink component  104 . The protrusion  1041  is adapted for receiving and partly enclosing an electronic driving component  108  which is adapted to provide energy to the light source  101 . Furthermore, a housing  107  is provided between the heat sink component  104  and the base  109 . The housing  107  can be made of plastic and provides a safety shield for the electronic driving component  108 . 
     The cover  103  and the heat sink component  104  are, in this case, assembled with a good thermal connection at the recess bottom  1032  and the protrusion top surface  1042 , in addition to the thermal contact between the side wall 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  1032  and the protrusion top surface  1042  can be established via direct attachment or via a thermally conducting 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. 
     An air chamber  1051  is formed between the cover  103  and the heat sink component  104 . As shown in  FIG. 2 , first holes  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. Furthermore, second holes  106  are provided in the bottom  1043  of the heat sink component  104  adjacent to the side wall  1044 , thereby creating a second connection between the air chamber  1051  and ambient air. First and second holes  102  and  106 , together with the air chamber  1051 , form a channel allowing air to flow through the air chamber  1051 , as the dash-lined arrow  105  indicates. When the lamp  100  is operated as is illustrated in  FIG. 5 , in this case a down-lighting, a chimney effect will be created in the heat sink structure, as is illustrated in  FIG. 6  which shows the air flow inside and outside the lamp  100 , wherein the arrows indicate the direction (direction of arrow) and the speed (size of arrow) of the airflow. 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 becomes, 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 transported 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. 
     At the same time, radiation heat transfer is also a significant source for dissipating the generated heat in addition to natural convection. Both the rim  1033  of the cover  103  and the side wall  1044  of the cup-shaped heat sink component  104  are exposed to ambient air, and allow radiation heat transfer. 
     The air flow direction  105  is upwards in  FIG. 5 . However, a person skilled in the art can understand that the air flow direction  105  can be reversed in situations where the lamp  100  operates in another direction. The chimney effect can be built up within the air chamber  1051  of the heat sink component because of a temperature gradient, and will force the air to flow through the air chamber  1051 . 
     In this embodiment, the cross section of the channel between the inlet, i.e. first or second holes  105 , 106 , and outlet, i.e. second or first holes  106 ,  105 , 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 is shown in  FIG. 4 , which is an outside view of the lamp  100 , the side wall  1044  of the heat sink component  104  is an intact smooth exposed surface, without holes, slots or fins, which provides an ornamental effect. Comparing to the construction of the prior art lamp foot, e.g., in US2012/0044680A1, which is not possible to have holes in the bottom, the cooling effect of the lamp  100  is improved further, because the cooling is implemented by extending the path for the air to flow and this is done by moving the holes to the bottom of the heat sink. As shown in  FIG. 4 , a part of the bottom  1043  of the heat sink is not covered by the housing  107 , to allow air flow through the holes. 
       FIG. 7  and  FIG. 8  show another embodiment of the invention, wherein a BR lamp  200  has a narrow slit  202  in side wall  2044  of heat sink component  204  adjacent or near to a rim  2033  of a cover  203  instead of the first holes  102  in the cover  103  in the first embodiment. The narrow slit  202  is hardly visible, while, also in this embodiment, the internal structure (air chamber  2051 ) is much wider. Second holes  206  are provided in the bottom  2043  of the cup-shaped heat sink component  204 . Also in this embodiment, the side wall  2044  is an intact smooth exposed surface, without holes, slots or fins, which provides an ornamental effect. In addition, a reflector  211  is included in the recess  2031  of the cover  203  providing a desired optical performance of the lamp  200 . 
     The protrusion of the heat sink component  204  is relatively small in height compared to the height of the protrusion of the heat sink component  104  of the first embodiment. And, therefore, in this case the electronic driving component is accommodated in the housing. The airflow within air chamber  2051  formed between the cover  203  and the heat sink component  204  provides an optimal thermal performance. 
     In a further embodiment of the invention as shown in  FIG. 9 , the light source  301  is thermally coupled to a big Print Circuit Board (PCB)  310 . The PCB  310  extends into the air chamber, and the PCB has a plurality of PCB openings to allow the air flow goes fluently between the first opening  302  of the cover  303  and second opening  306  of the shell  304 . The PCB openings may be cut-outs at the edge of the PCB or holes  312  as shown in  FIG. 10 . Preferably, the holes  312  are aligned with the holes  302  in the cover  303  so as to allow maximum air flow. 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  300  or provide solutions with lower cost. 
     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 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 constructed 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.