Abstract:
Novel structures for electronically-controlled integral compact fluorescent lamps are described in which the space defined by a folded discharge tube is used to locate a housing for at least a part of the electronic-control circuitry. The housing is provided with ventilation means to allow the escape of heated air from the interior of the housing. This reduces the size of the lamp, and allows temperature sensitive components to be located in a relatively cool region. The housing can be provided with a reflective coating to improve the illumination uniformity, and if made of metal can act as a heat sink.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to novel structures for electronically-controlled integral compact fluorescent lamps, and in particular to such structures that provide improved space efficiency, improved illumination and improved operating life.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electronically-controlled compact fluorescent lamps are becoming more popular as an alternative to a traditional incandescent light-bulbs. They have the advantages of greater efficiency and longer-life. However, there remain a number of drawbacks with existing compact fluorescent lamps that prevent them from obtaining even wider acceptance and use. In particular, existing compact fluorescent lamps are still somewhat larger than incandescent light bulbs, the illumination given out by a compact fluorescent lamp is not uniform, and the lifetime of the compact fluorescent lamp is often limited by the deleterious effects of high-temperatures on the electronic control circuit.  
         PRIOR ART  
         [0003]    These drawbacks can be illustrated with reference to FIGS. 1 and 2 that illustrate conventional compact fluorescent lamp designs. A compact fluorescent lamp comprises a fluorescent discharge tube  1 , and a housing  2  in which is located an electronic control circuit board  3  including control circuitry such as an electronic ballast. A screw connection  4  enables the lamp to be connected to a socket. The fluorescent discharge tube  1  is a single continuous tube that is folded into one of a number of shapes. In FIG. 1 the discharge tube  1  is formed as a spiral around a central axis, while in FIG. 2 the tube  1  is formed as a series of U-shapes symmetrically disposed around a central axis. At either end of the discharge tube  1  there is provided a filament element (not shown) that is connected to the electronic control circuitry. The electronic control circuitry, such as an electronic ballast, is formed on a printed circuit board located within the housing  2  which may be formed of a plastics material. The housing  2  is positioned between the discharge tube  1  and the screw connection  4 .  
           [0004]    Such conventional designs have a number of drawbacks. Firstly the lamps are relatively large, and to a large extent this is because the housing has to be quite large to accommodate the electronic control circuitry. This extra size means that often a compact fluorescent lamp cannot fit into a lamp fitting designed for an incandescent bulb and the compact fluorescent lamp cannot be used to replace directly an incandescent bulb.  
           [0005]    A further disadvantage is that the filaments generate a very high temperature. With an ambient temperature of room temperature the filaments of a compact fluorescent lamp operated at only 23W can reach temperatures of the order of 700° C.-1000° C. With the filaments at these temperatures, the temperature of the glass discharge tube adjacent the filaments can be around 100° C. to 120° C. As is illustrated in FIGS. 1 and 2 there is a temperature gradient along the discharge tube with the temperature decreasing away from the ends of the discharge tube containing the filaments down to around 40° C., but with conventional designs for compact fluorescent lamps the electronic control circuitry in the housing is located closely adjacent the filaments and thus in the hottest part of the lamp. Furthermore such lamps are often ceiling fitted such that the discharge tube is beneath the housing. Since heat rises, and may be trapped in the housing, the temperature in the vicinity of the control circuitry is even higher and the problem is aggravated. Temperature in the housing can also be increased by heat given out by some of the components of the control circuitry, such as switching and conduction losses in the switching power circuit, and core losses in magnetic components.  
           [0006]    These high temperatures reduce the lifetime of the lamp. Even if the components of the electrical control circuitry are rated for operation at such temperatures, they would nonetheless have a still longer lifetime if they were operated at a lower temperature.  
           [0007]    A further disadvantage of such conventional designs is that the illumination from the lamp is not uniform, but generally follows the shape and configuration of the folded discharge tube. It is known that the discharge tube may be surrounded with an opaque housing, but while this does help mitigate the problem it does not completely solve it and the shape of the discharge tube can still be seen through the opaque housing.  
           [0008]    U.S. Pat. No. 5,128,590 shows one attempt to use the inner space defined within the lamp tube more efficiently. In particular, U.S. Pat. No. 5,128,590 describes the use of an inner housing for a ‘detachable’ compact fluorescent lamp. In U.S. Pat. No. 5,128,590 the ballast housing has an extension that occupies the inner space surrounded by the lamp tube. The structure has a plug-in connection and is designed for detachable lamps in which the lamp tube can be separated from the electronic ballast. This structure has the advantages that the inner space surrounded by the lamp tube can be utilized and the surface of the inner housing can reflect light outwardly.  
           [0009]    However, the detachable structure of U.S. Pat. No. 5,128,590 was designed with the intention of the lamp ballast housing being permanently fixed on the ceiling and the lamp tube being replaceable. However, the design of U.S. Pat. No. 5,128,590 may not be suitable for integral compact fluorescent lamps. Despite the mentioned advantages, the proposed structure of U.S. Pat. No. 5,128,590 would suffer a major thermal problem, and in particular a thermal problem that would be particularly serious for integral compact fluorescent lamps in which the lamp housing and the lamp tube are not detachable. If the integral lamp structure is fixed on the ceiling with the screw on top and the lamp tube on bottom, heat will rise and be trapped inside the upper part of the housing. If the lamp position is reversed (with the screw on the bottom and the lamp tube on top), as required in some decorative lamp fixtures, heat will be trapped in the extended housing surrounded by the lamp tube.  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the present invention to provide novel structures for compact fluorescent lamps that overcome or at least mitigate the above problems, and which structures in particular provide compact fluorescent lamps of reduced size, longer operating life and improved illumination distribution. In particular, the present invention provides an improved integrated structure with good thermal properties specifically for ‘integral’ CFL systems, which has both a base housing and inner housing and in which the lamp tube and the ballast are not detachable. The present invention aims to allow heat to flow out of the housing structure of an integral CFL by convection in all lamp positions without affecting the light reflection function of the inner housing.  
           [0011]    According to the present invention there is provided a compact electronically-controlled fluorescent lamp, comprising: a continuous folded discharge tube arranged around a longitudinal axis of the lamp, connecting means for connecting the lamp to a source of electrical power, and a housing comprising a first base portion located between the discharge tube and the connecting means and a second portion located on said axis within a space defined by said folded discharge tube, wherein at least a part of an electronic control circuit for controlling operation of said lamp is located within said housing, characterised in that said lamp is an integral compact fluorescent lamp and in that said housing is formed with ventilation means to allow the escape of heated air from within said housing  
           [0012]    By means of this arrangement a large part of the electronic control circuitry can be located within the housing in the space defined by the discharge tube that would otherwise be wasted. Furthermore temperature sensitive components may be located away from the high temperature filaments and at the same time means are provided to prevent a build-up of heat within the housing.  
           [0013]    Another advantage of the present invention is that the housing may be formed with a reflective outer surface. This may improve the uniformity of the illumination given out by the lamp.  
           [0014]    The discharge tube may be folded in a spiral about the axis, or may be folded so as to present a series of tube sections parallel to the axis and spaced equidistantly on a circle about the axis. In either of these cases, to make maximum use of the space defined by the tube it is preferable for the housing to have a cylindrical form with a circular cross-section.  
           [0015]    In other embodiments the discharge tube is folded so as to present a series of sections arranged in a rectangular configuration about the axis. In such a case the housing may have a cuboid form with a rectangular cross-section.  
           [0016]    The cross-section of the housing could also be polygonal.  
           [0017]    The electronic components forming the control circuit or part thereof provided within the housing may be located on a printed circuit board, which circuit board may be located within the housing such that the axis is perpendicular to the board. Alternatively the printed circuit board may be located within the housing such that the axis is parallel to or lies within said board.  
           [0018]    Viewed from a still further aspect of the present invention there is provided a compact electronically-controlled fluorescent lamp, comprising: a continuous folded discharge tube arranged around a longitudinal axis of the lamp to define a space bounded by said discharge tube, wherein a heat sink means is provided within said space. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which:  
         [0020]    [0020]FIG. 1( a )-( d ) shows plan and side views of a first compact fluorescent lamp in accordance with the prior art,  
         [0021]    [0021]FIG. 2( a )-( d ) shows plan and side views of a second compact fluorescent lamp in accordance with the prior art,  
         [0022]    FIGS.  3 ( a )-( c ) show a side and two sectional views of a first embodiment of the invention,  
         [0023]    [0023]FIG. 4 shows typical air/heat flow paths in the embodiment of FIG. 3( a )-( c ),  
         [0024]    [0024]FIG. 5 shows a side view of a second embodiment of the invention,  
         [0025]    FIGS.  6 ( a )-( c ) show a side view and two end views of a third embodiment of the invention,  
         [0026]    [0026]FIG. 7 shows a side view and an end view of a fourth embodiment of the invention  
         [0027]    [0027]FIG. 8 shows a side view of an integral compact fluorescent lamp according to a fifth embodiment of the invention,  
         [0028]    FIGS.  9 ( a )-( c ) shows three end views of an integral compact fluorescent lamp according to further embodiments of the invention,  
         [0029]    FIGS.  10 ( a )-( d ) shows plan and side views of an integral compact fluorescent lamp according to a further embodiment of the invention,  
         [0030]    FIGS.  11  ( a )-( d ) shows plan and side views of an integral compact fluorescent lamp according to a further embodiment of the invention,  
         [0031]    FIGS.  12 ( a )-( d ) shows plan and side views of an integral compact fluorescent lamp according to a further embodiment of the invention, and  
         [0032]    [0032]FIG. 13( a )-( d ) shows plan and side views of an integral compact fluorescent lamp according to a further embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0033]    Several embodiments of the invention will now be described by way of example with reference to FIGS.  3  to  13 . FIG. 3 will be described firstly and in more detail than the other figures all of which are similar to FIG. 3 and only the differences between those Figures and FIG. 3 will be highlighted. Parts that are unchanged between FIG. 3 and the subsequent Figures will not be discussed in detail.  
         [0034]    FIGS.  3 ( a )-( c ) show three views of a first embodiment of the invention. In these views: FIG. 3( a ) is a side view in section; FIG. 3( b ) is a sectional view through line B-B; and FIG. 3( c ) is a sectional view through line C-C. For the sake of convenience in the following description, it will be assumed that the lamps are to be fitted to a ceiling such that the discharge tube is below the remainder of the lamp, and references to “above” and “below”, “upper” and “lower” should be construed accordingly. It will be understood that the invention is not of course to be regarded as being so limited and the lamp could be fitted otherwise, for example in a decorative lamp fitting such the discharge tube is above the remainder of the lamp. References in this specification to the term “the axis of the lamp”, and the like, mean the longitudinal axis of the lamp about which a lamp would be rotated when being screwed into screw fitting. The format of the subsequent Figures is the same.  
         [0035]    FIGS.  3 ( a )-( c ) show a compact fluorescent lamp formed of a single continuous folded fluorescent discharge tube  10 . In this embodiment (as in the prior art example of FIG. 2) the tube is folded into a spiral about a longitudinal vertical axis of the lamp. The lamp is provided with a screw connector  30  for connection to a lamp fitting. Between the discharge tube  10  and the screw connector  30  is provided a base housing  20 . Within the base housing  20  is located a first part of a circuit board  21  provided with a number of electronic components and from which connecting wires connect the electronic control circuitry to the filaments located at the two ends of the discharge tube  10 .  
         [0036]    Integrally formed with the base housing  20 , and within the space defined within the spiral of the discharge tube, is a second inner housing  23 . In this embodiment the second housing  23  is circular in cross-section and is formed with a part-spherical dome  24  at its end furthest away from the first housing  20 . Within this second housing is located a second part  22  of the electronic control circuitry, such as an electronic ballast, on at least one circuit boards. The components of the electronic control circuitry may be arranged within the base housing and the second housing in any desired manner, but it is particularly preferred if the most temperature sensitive components are located at the far end of the second housing  23  within the part-spherical dome  24 , that is to say as far away from the high-temperatures generated by the filaments as is possible. It is known that even if an electronic component is rated for high temperature use, its expected operating life will still be longer if the component is operated at a lower temperature.  
         [0037]    It will be appreciated that by providing a second housing  23  within the space defined by the discharge tube, otherwise wasted space may be employed, and unlike U.S. Pat. No. 5,128,590 which also uses this space, the provision of ventilation means prevents the build-up of hot air within the housing no matter what the position of the lamp is. This has a number of significant advantages, and in particular, it becomes possible to locate temperature sensitive electronic components in a relatively cooler part of the lamp further away from the heat generated by the filaments, and without the build-up of hot air within the housing, thus increasing the expected operating life of the lamp.  
         [0038]    Referring back to FIG. 3, the second housing  23  is formed of a plastics material, for example the same type of material as the first housing  20  and the non-metallic parts of the screw connection. However, the outer surface of the second housing  23  may be provided with a reflective coating. This has the advantage of improving the diffusion of illumination from the discharge tube  10  so that more uniform illumination is given out by the fluorescent lamp and the shape of the discharge tube is less apparent. A further advantage is that the reflective coating assists in the dissipation of heat from the second housing  23 . A further advantage of the present invention is that the second housing  23 —regardless of whether or not it is provided with a reflective coating—can act as a heat sink for drawing heat away from the filaments and dissipating the heat by convection if it is made of a suitable material such as metal. Indeed the present invention also contemplates the possibility that a heat sink means alone (ie without necessarily acting as a housing for electronic components and without necessarily being reflective) may be located in the space defined by the folded discharge. Such a heat sink means would increase the operating life of the lamp in comparison with the prior art.  
         [0039]    Importantly, it should be noted that both the base housing and the second housing are provided with air vents  25  that enable air to flow through the housings and which prevent an unwanted build-up of air within the housings regardless of the position of the compact fluorescent lamp (that is to say regardless of whether the lamp is ceiling mounted or in any other configuration). Referring to FIG. 3( a ), the base housing is provided with four vents provided at the junction of the base housing and the screw connector. These vents are shown in FIG. 3( c ) and are formed as arcuate slots formed around the circumference of the base housing. The second housing is likewise formed with four arcuate slots shown in FIG. 3( b ) that are located around the circumference of the second housing at the junction between the second housing and its domed end portion. In this regard it should be noted that the domed end portion has a slightly larger circumference than the main body of the second housing to provide an overhang in which the air vents may preferably be provided though this is not essential.  
         [0040]    The advantage of providing such vents  25  is shown in FIG. 4 which corresponds to FIG. 3( a ) but in which the arrows show the typical air flow paths through the base housing and the integrally formed first housing. This flow of air allows heat that might otherwise build up within the base housing and the second inner housing to escape. The precise number and locations of the vents may of course be varied, but by providing the vents at opposite ends of the integral combined housing, a maximum flow of air through the housing is possible.  
         [0041]    [0041]FIG. 5 shows a second embodiment of the invention that is similar to the first embodiment. The main difference is that in this embodiment the electronic components received within the second inner housing  23  are located on a number of horizontal spaced apart circuit boards  26 . Also in FIG. 5, the vents  25  at the end of the second housing distal from the screw connection are provided in L-shaped shoulder portions of the second housing formed at the end of the second housing where the diameter of the second housing is reduced. In all other respects the embodiments of FIGS. 3 and 4 are substantially identical.  
         [0042]    FIGS.  6 ( a )-( c ) show a third embodiment of the invention. This embodiment is similar to the first two embodiments except that in this embodiment the domed end portion of the inner housing is formed of the same diameter as the remainder of the inner housing and does not extend beyond the circumference of the second housing. To provide the vents slots  25 ′ are formed in this domed portion. These slots may be arcuate and arranged in a circular sense (FIG. 6( b )), or they may be linear (FIG. 6( c )). FIGS.  7 ( a ) and ( b ) show a similar embodiment in which the slots are replaced by circular ventilation holes  25 ″.  
         [0043]    It will be understood that the present invention is not limited to the discharge tube being spiral in form. FIG. 8 for example shows another embodiment of the invention in which the discharge tube is folded in a U-shape, otherwise this embodiment is closely related to the embodiments of FIGS. 3 and 5 and it will be noted that ventilation openings are provided at the two ends of the integral housing formed by the base housing and the inner housing located within the space defined by the discharge tube. FIGS.  9 ( a )-( c ) show plan views of the lamp of FIG. 8 viewed from the end distal from the screw connector and each showing different possible combinations of the manner in which the discharge tube is folded and the type of ventilation openings. For example FIG. 9( a ) shows the tube folded into three parallel segments and with the ventilation openings being parallel slots; in FIG. 9( b ) the discharge tube is folded into three segments disposed generally in a triangular configuration and the ventilation openings are arcuate slots; and in FIG. 9( c ) the discharge tube is folded into four segments arranged in a square and the ventilation openings are holes. It should be noted that these combinations of types of folded tubes and types of ventilation openings are purely by way of example, and different combinations of types of folded tubes and types of ventilation openings could also be employed.  
         [0044]    Indeed the present invention could be applied to various types of discharge tube configuration. A number of discharge tube configurations will now be shown with reference to FIGS.  10  to  13 , and although ventilation openings are not explicitly shown in these Figures it will be understood that such ventilation openings may be applied in accordance with the techniques described above with reference to FIGS.  3  to  9 .  
         [0045]    FIGS.  10 ( a )-( d ) shows an embodiment of the invention in which the discharge tube  10  is folded into a series of U-shapes so as to define eight vertical sections of the tube arranged equidistantly around a central longitudinal axis of the lamp. A cylindrical second housing  23  with a domed end  24  is provided in the inner space defined by these eight sections of the discharge tube  20  and receives the major part of the electronic control circuitry as in the embodiment of FIG. 3. Alternatively, the cylindrical second housing having a flat end may be used instead, as illustrated in the embodiments shown in FIGS. 12 and 13 below.  
         [0046]    The embodiment of FIG. 11 is similar to FIG. 10 except that a single vertical circuit board is provided.  
         [0047]    In the embodiments of FIGS. 12 and 13, the discharge tube  10  is folded to define a series of four U-shapes arranged in the form of a square. In this embodiment it is more appropriate for the second housing  23  to be cuboid with a square cross-section in order to make the maximum use of the space defined within the folded discharge tube. As in the previous embodiments the second housing  23  is provided with a reflective coating. The electronic control circuitry may be provided on vertical (FIG. 12) or horizontal (FIG. 13) circuit boards.  
         [0048]    It will be understood that there are a number of other possible ways of implementing the invention. For example a suitably sized cylindrical second housing could be used even when the discharge tube is arranged in a rectangular manner, and similarly a suitably sized cuboid second housing could be used in the space defined by a spiral discharge tube, or circumferentially arranged discharge tube sections. The particular examples shown in the embodiments are preferred, however, as they provide maximum space efficiency. Housings of a triangular or polygonal cross-section may also be useful.  
         [0049]    The reflective coating on the second housing may be a smooth coating, or it may comprise a plurality of individual small reflective elements. The reflective coating may be applied either only to the sides of the second housing, or to the complete second housing including the end of the housing remote from the first housing and screw connection. Furthermore an opaque cover may be provided surrounding the lamp to still further improve the uniformity of the illumination.