Abstract:
The present invention comprises a compact electrodeless fluorescent lamp that includes a transparent envelope containing a fill of inert gas along with a vaporizable metal such as mercury. An induction coil is operated by a driver circuit, and is positioned inside of a reentrant cavity in the envelope with an adjacent permeable magnetic field manipulation structure having a shunting surface ending at a shunting surface periphery. A thermally and electrically conductive primary cooling structure is positioned adjacent the magnetic field manipulation structure to extend within the shunting surface periphery while being separated from the induction coil thereby. A further component cooling structure is provided to at least partially enclose the driver circuit connected to the induction coil.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to electric lamps, more specifically, to compact electrodeless fluorescent lamps operated at low and intermediate pressures and at frequencies above 20 kHz. 
     Electrodeless compact fluorescent lamps (ECFL) have been recently made available for indoor lighting. The advantage of such lamps is their long operating lifetime which is much longer than that of conventional compact fluorescent lamps employing heating filaments. The visible light is generated by an inductively coupled plasma that, in turn, is produced by a RF electromagnetic field generated in the lamp bulb by an induction coil. 
     A known compact electrodeless fluorescent lamp “Genura” (General Electric Corp.) is operated at a RF frequency of 2.65 MHz and utilizes an induction coil with a ferrite core inserted in a reentrant cavity formed in a transparent bulb container. Genura is marketed as a replacement for an R30 incandescent lamp and is indicated to have 1,100 lumen light output at 23 W of RF power with an operating lifetime of 15,000 hrs. The drawback of the Genura lamp is its high initial cost, and relatively large diameter (80 mm) that is larger than that of a 100-W incandescent lamp (60 mm) having 1500 lumen light output. The latter characteristic imposes some restrictions on the conditions of lamp usage. In addition, the lamp employs an internal reflector and so can be used only in recessed lamp holding fixtures for downward lighting applications. 
     The high initial cost of the Genura lamp is due to the high cost of the driver electronic circuitry because of being operated at a frequency of 2.65 MHz, and which must include a special circuit to prevent electromagnetic interference (EMI). Thus, the use of a lower frequency of approximately 100 kHz is desired to reduce the initial lamp cost. 
     Also, a compact electrodeless fluorescent lamp is desired that is smaller than the Genura lamp (i.e. made with a 60 mm diameter equivalent to that of a A25 bulb) and that can be used in regular fixtures for both upward lighting and downward lighting applications. 
     In a copending U.S. Patent Application entitled “High Frequency Electrodeless Compact Fluorescent Lamp” having Ser. No. 09/435,960 by Chandler et al. and assigned to the same assignee as the present invention, a compact electrodeless fluorescent lamp is disclosed that is operated at relatively “low” frequencies from 50 kHz to 500 kHz. The lamp utilizes a ferrite core and a thin ferrite disk attached to the core bottom both made from MnZn material. A multiple insulated strand wire (Litz wire) is used for the induction coil that is wound in two layers around the ferrite core. 
     Two types of cooling structures that remove the heat generated during operation from the cavity and the ferrite core are described in that application. The first structure comprises a copper tube inside the ferrite core that protrudes along the lamp base down to the Edison socket cup and is welded to a copper cylinder in the Edison socket cup. Such an arrangement provides for the transmission of heat from the cavity/ferrite core to the Edison socket cup and then to the lamp holding fixture. However, this approach has two disadvantages. In many applications, the Edison socket cup does not have a good thermal contact with the fixture, and thus the resulting relatively poor thermal conduction leads to an increase of the ferrite core material operating temperature to values higher than its Curie point. The second disadvantage is the position of the metal (or ceramic) cooling tube in the base center, along its axis, that makes it difficult to place the driver electronic circuitry inside the base. 
     The other structure taught in this application comprises a metal tube inside the ferrite core and a ceramic structure that is thermally connected to the tube. The ceramic structure has a shape of a “skirt” and transfers the heat from the cavity and the core to the atmosphere via convection. 
     Both of these types of cooling structures provide acceptable ferrite core temperatures during operation, that is, temperatures lower than the ferrite material Curie point of 220° C., and sufficiently low temperature inside the lamp base (&lt;100° C.), when the lamp is operated without a lamp holding fixture at an ambient temperature of 25° C. However, neither of these arrangements may always provide the desired operating temperatures when the lamp is inserted in a lamp holding fixture that has the effect of increasing the effective lamp “ambient” temperature up to 50-60° C. Therefore, a more efficient cooling structure is desired for reliable operation of such lamps in a holding fixture. 
     Also, the use of ceramic (alumina) material structure is rather costly so that the initial cost of the lamp may be unacceptably high. The use of materials less expensive than alumina but with the same (or higher) thermal conductivity is desirable to reduce the initial cost of the lamp cooling structure and, hence, of the whole lamp system. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a compact electrodeless fluorescent lamp that includes a transparent envelope containing a fill of inert gas along with a vaporizable metal such as mercury. An induction coil, such as one formed by Litz wire, is operated by a driver circuit, and is positioned inside of a reentrant cavity in the envelope with an adjacent permeable magnetic field manipulation structure having a shunting surface ending at a shunting surface periphery. The magnetic field manipulation structure may comprise a toroid with a disk-like base, and may be formed of a ferrite material. A thermally and electrically conductive primary cooling structure is positioned adjacent the magnetic field manipulation structure to extend within the shunting surface periphery while being separated from the induction coil thereby. The primary cooling structure may comprise a thermally conductive tube, such as a metal tube, for instance copper, placed inside of the cavity extending so as to extend with in the toroid, and may have a finned dissipater provided therewith. 
     A further component cooling structure is provided to at least partially enclose the driver circuit connected to the induction coil. This component cooling structure is separate from the primary cooling structure, and may cool at least an electrolytic capacitor in the driver circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of an embodiment of the present invention showing an electrodeless compact fluorescent lamp having a ferrite operations structure and a cooling structure for the ferrite operations structure; 
     FIG. 2 is the schematic cross-sectional view of another embodiment of the present invention showing an electrodeless compact fluorescent lamp having a ferrite operations structure and an enhanced cooling structure for the ferrite operations structure; 
     FIG. 3 is the schematic cross-sectional view of another embodiment of the present invention showing a lamp having a ferrite operations structure and a cooling structure for the ferrite operations structure, and having a further cooling structure for a driver circuit; 
     FIG. 4 is the schematic cross-section view of an alternative cooling structure for a driver circuit; 
     FIG. 5 is a graph showing a plot of run-up temperatures of a portion of a lamp during operation; and 
     FIG. 6 is a graph showing a plot of the induction coil quality factor in a lamp as a function of the driving frequency. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the cross-section view of the lamp of the present invention in FIG. 1, a transparent bulbous envelope  1  made from glass has a reentrant cavity  2  with an exhaust tubulation  3  located inside the cavity  2  on its axis of substantially radial symmetry. An induction coil  4  made from multiple insulated strand wire (Litz wire) is coiled (two layers) around a permeable, electrically insulative, ferrite material core  5  of a toroidal shape. Litz wire can have from 40 to 150 strands each of gage # 40, and the number of turns is from 40 to 80. In preferred embodiments the number of strands is 60 and the number of turns is 65. The maximum temperature that typically this wire can withstand is 200° C. 
     Ferrite material core  5  is made from manganese zinc, MnZn, material and is positioned within reentrant cavity  2 . The Curie point of this ferrite material is typically 220° C. The core outer diameter is typically about 15 mm and the height is typically about 55 mm. A thin ferrite disk  6  with a central opening is also typically made from MnZn material, though a different ferrite material can be used, and is firmly positioned against the ferrite material core  5  to provide an essentially continuous magnetically permeable material path, or they are together formed as a single unitary ferrite material structure. In preferred embodiments, the diameter of disk  6  is typically about 50 mm, and its thickness is typically about 1.0 mm. Ferrite disk  6  concentrates and orients magnetic fields generated in coil  4  and core  5  during operation so as to in effect shape these magnetic fields to be shunted, or to occur, away from a cooling structure made of copper positioned below it to be further described in the following. Such a result thereby decreases power losses in the cooling structure due to eddy currents and so increases the quality factor, or Q-factor, of the coil/ferrite assembly at resonance during operation. 
     The inert gas (argon, krypton, or the like) fill is at a pressure ranging from 0.1 torr to 5 torr. The mercury vapor pressure (approximately 6 mtorr) is controlled by the temperature of the mercury drop positioned in the cold spot that is located on the inner surface of protrusion  7  of envelope  1  at the top thereof. The inner walls of envelope  1  and cavity  2  are coated with protective coating (alumina or the like)  8  and a phosphor coating  9  represented only in part and in schematic form in FIG.  1 . The inner walls of cavity  2  are further coated with a reflective coating  10 , which is also a single coating provided on the outer walls at the bottom of envelope  1 . 
     A cooling assembly in the embodiment of FIG. 1 is typically made from copper and comprises three parts welded to each other: a tube  11  positioned in the interior opening of ferrite core  5 , a plate  12  with a central opening admitting tube  11 , and a skirting liner  13  at the outer periphery of plate  12 . In this embodiment, the diameter of plate  12  is typically about 50 mm, and its thickness is typically about 2 mm. The interior openings of core  5  and disk  6  are similar in size and are both large enough to accommodate tube  11  therethrough. This cooling structure could be made from an alternative thermally conductive material such as aluminum. 
     Liner  13  can be a right cylindrical shell or a somewhat conical shell. In the preferred embodiments, liner  13  is a right cylindrical shell that typically has about a 45 mm outer diameter, and typically about a 15 mm length. Note that the outer diameters of copper plate  12  and copper liner  13  are smaller than the outer diameter, or periphery, of ferrite disk  6  leaving a peripheral region along the outer edge of disk  6  not reached by copper plate  12  and copper liner  13 . As a result, magnetic fields generated by coil  4  and the ferrite core/disk assembly during operation which penetrate copper parts  12  and  13  to cause eddy currents therein, and hence, power losses, in those copper parts, are much reduced to thereby increase the coil/ferrite/copper structures Q-factor and increase the lamp power efficiency. The wall thickness of liner  13  can typically be about from 0.2 mm to 5 mm. In the preferred embodiments, the thickness of the liner&#39;s walls is 1.5 mm. 
     A plastic material enclosure  14  forms the lamp base and is connected with the bottom of envelope  1  and Edison socket cup 15 . A printed circuit (PC) board  16  with the driver electronic circuitry and the impedance matching network thereon is positioned inside enclosure  14 . In this embodiment, copper plate  12  and copper liner  13  are inside of plastic enclosure  14 . The mains, or main electrical power interconnections in the lamp base, are supplied with standard alternating current from a standard alternating voltage through the lamp holding fixture holding the lamp during usage via the Edison socket cup  15 , and they extend from socket cup  15  to be connected to the driver electronic circuitry located on PC board  16 . 
     Another embodiment of the present lamp invention is shown in cross section in FIG.  2 . Bulbous envelope  1 , cavity  2 , coil  4 , ferrite material core  5 , and ferrite material disk  6  are the same as shown in FIG.  1 . The cooling structure in this embodiment, again made of copper, comprises tube  11 , plate  12 , liner  13 , and a further disk-like dissipater  12   a  with a central opening at which it is welded to tube  11 , and also welded at its lower disk surface to plate  12 . Dissipater  12   a  has fins which help to cool the foregoing copper structure through convection or conduction, or both, and hence, ferrite material core  5 . 
     The heat absorbed by ferrite material core  5  during operation is removed by copper tube  11  and conductively transferred to plate  12  and dissipator  12   a.  A fraction of this heat is dissipated by dissipater  12   a,  and the rest is redirected to liner  13  where it is dissipated in the ambient atmosphere via convection. As a result, the operating temperatures of ferrite core  5  and PC board  16 , where the driver circuitry components are located, are kept substantially lower by the presence of this cooling structure than they would be in its absence. 
     The two embodiments described above provide a relatively low (below the Curie point) operating temperature for ferrite material core  5 . However, the arrangements shown in FIGS. 1 and 2 are not sufficient to reduce the temperature of the circuit component of the driver circuitry that is most sensitive to high temperature, an electrolytic capacitor  17 . Indeed, a portion of the heat transferred to ferrite disk  6  and liner  13  reaches PC board  16  and, hence, reach the components of the driver circuitry including capacitor  17 . To reduce the temperature of capacitor  17 , two further arrangements are provided to lower the operating temperature (below 120° C.) of capacitor  17 . 
     Thus, a further embodiment of the present invention is shown in the cross-section view of FIG. 3. A heat sink  18  made of copper is positioned in lamp base  14  and substantially encloses electrolytic capacitor  17 . Heat sink  18  is shaped as a cylindrical shell and its inner diameter is slightly larger than the parallel extent of capacitor  17 . Electrical insulating material, not shown, having good thermal conductivity (e.g. impregnated Teflon) electrically insulates heat sink  18  from capacitor  17  thereby providing conductive heat removal from the capacitor  17  without allowing sink  18  to electrically interfere with, or permit damage to, the driver circuitry. 
     The height of cylindrical shell sink  18  is slightly more than the length of capacitor  17 . In this embodiment, when the lamp is operated at a driving frequency of 100 kHz, the length of sink  18  is typically about 25 mm. In this embodiment of the present invention, the outer diameter of the sink  18  is typically about 12 mm, and its wall thickness is typically about 1.0 mm. 
     The bottom of heat sink  18  is welded to the bottom of a cup  19  formed of copper that has good thermal contact with Edison socket cup  15 . The outer diameter of cup  19  is typically about 24.5 mm; its height is typically about 7 mm, and the thickness of its wall is typically about 1.0 mm. Plastic enclosure  14  is screwed into the top part of the threads in Edison socket  15  thereby securing them to one another. 
     Heat sink  18  absorbs heat from capacitor  17  and transfers it to copper cup  19  which, in turn, transfers such heat to Edison socket cup  15 . Socket cup  15  is screwed into a socket in the lamp holding fixture during use that is in good thermal contact with the rest of the fixture where the heat is eventually dissipated. 
     A further embodiment of the present invention is shown in cross section view in FIG.  4 . Heat sink  18  is a copper cylindrical shell of the same size as that in the embodiment of FIG.  3 . The heat absorbed by sink  18  is dissipated by cooling radiator  20  with a central opening that has many fins and is welded at that opening to the outer side surface of heat sink  18 . In this arrangement, the heat from the capacitor  17  absorbed by the radiator  20  is transferred to Edison socket cup  15  via convection or conduction, or both. 
     Note that the liner  13  does not have any direct mechanical contact with heat sink  18  to thereby prevent conductive heat transfer from liner  13  to sink  18  and to so keep electrolytic capacitor  17  at temperature below 120° C. If liner  13  was instead mechanically connected to heat sink  18 , the heat from ferrite material core  5  would be transmitted to capacitor  17  via plate  12  and liner  13 , and so increase the capacitor temperature to values higher than 120° C. 
     The above described lamps operate as follows. Bulbous envelope  1 , or bulb  1 , is filled with an inert gas (argon, 1 torr). Mercury vapor pressure in this envelope is controlled by the temperature of the mercury drop in cold spot  7  and is typically around 5-6 mtorr. Standard commercial power line voltage at a frequency of 50-60 Hz with a magnitude of around 120 Volts rms is applied to the driver electronic circuitry assembled and interconnected on and in PC board  16 . A much higher frequency (about 100 kHz) and magnitude voltage is generated by the driver circuitry from the power line voltage and applied to induction coil  4  via an impedance matching network. 
     When the coil high frequency voltage reaches magnitudes of 200-300 V, a capacitive discharge is ignited in bulb  1  along the cavity walls. Further increases in the coil voltage magnitude leads to a transition from a capacitive discharge to an inductively coupled discharge (lamp starting). The transition occurs when the coil voltage exceeds a “transition” value, V tr , in this range. This transition is accompanied with a sharp decrease of the lamp reflected power, a drop of the coil voltage and current, and with a very large increase in the lamp visible light output. 
     The magnitude of V tr  depends on the lamp envelope and cavity sizes, the gas and vapor pressures therein, and the number of turns in induction coil  4 . In the preferred embodiments, the transition voltage, in a lamp operated at 100 kHz, was around 1000 V, and the transition coil current was around 5 A. The coil voltage and current that maintain the inductive discharge, V m  and I m , vary with lamp power and the mercury vapor pressure. After the lamp was operated at a power of about 25 W for 2 hrs, the mercury pressure stabilized and the coil maintaining voltage and current were 350 V and 1.8 A, respectively. 
     About 80% of the total lamp power of 25 W, P lamp , is absorbed by the inductive plasma, represented as P pl , and about 2 W is dissipated in the driver circuitry, P drv . About 2-3 W of the lamp power is dissipated in induction coil  4  and in the ferrite material core  5 , P coil . This power dissipation together with the heat from the plasma via the cavity walls, causes heating of coil  4  and of ferrite material core  5 . Thus, P lamp =P drv +P coil +P pl =P drv +P bulb . The cooling structures described in FIGS. 1-4 provide satisfactory thermal management of the lamps. This result is illustrated in FIG. 5 where temperatures of ferrite material core  5  and capacitor  17  of the lamp according to the embodiment of FIG. 3 are plotted shown as functions of the lamp operating time. It is seen that after operating for 2 hrs, the lamp, operated at 25 W and at a frequency of 100 kHz, has a ferrite core temperature of 186° C., and a capacitor temperature of about 100° C. 
     The bulb power efficiency, η=P pl /(P pl +P coil )=P pl /P bulb , of the lamp operated at 100 kHz and at power of 25 W is 0.9. Such a high power efficiency was achieved due to the high Q-factor achieved for the assembly that comprises coil  4 , ferrite material core  5 , and the associated copper cooling structure. The dependence of the coil Q-factor on the driving frequency is given in the plot of FIG.  6 . It is seen that the Q-factor reaches a maximum (540) at a frequency of about 175 kHz. But even at f=100 kHz, the Q-factor is still high and has there a value of about 460. 
     High lamp power efficiency results in high luminous efficacy for the lamp. The maximum lamp efficacy at the lamp peak light output (about 6 mtorr mercury vapor pressure), is 65 lumens per watt (LPW). After the lamp operates for 2 hrs at a power of 25 W, and the mercury pressure and lamp light output are stabilized, the lamp efficacy dropped to 60 LPW with the total stable light output of 1500 lumens. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.