ELECTRODE-LESS HIGH-PRESSURE DISCHARGE LAMP

Abstract: The electrode-less high pressure discharge lamp has a lamp vessel (1) which is surrounded by an electric coil (2). The coil (2) has a first and a second outer turn (3, 4) and a third turn (9) in between. The outer turns are thermally coupled to a heat-sink (10), at least one of them in an electrically insulated manner. Also the third turn (9) is coupled to a heat-sink (13), as the case may be in an electrically conductive manner and at earth-potential. An improved cooling of the coil is obtained.

Description: Electrodeless high-pressure discharge lamp. The invention relates to an electrodeless high-pressure discharge lamp comprising: a light-transmitting lamp vessel which is closed in a vacuumtight manner and which has an ionizable filling; around the lamp vessel, an electric coil with a first and a second outer turn each with an end portion which is electrically connected to a first and a second electrical conductor, respectively, extending away from the coil and designed for connection to an electric supply, which coil has a third turn between the outer turns, the first and the second electrical conductor being thermally connected to a heat sink, while at least one of them is electrically insulated from the heat sink by a body which is an electrical insulator as well as a thermal conductor. Such an electrodeless high-pressure discharge lamp is described in the not previously published European Patent Application 94 200 250.2 (PHN 14.376). Heat generated by current passage through the electric conductors and through the coil, and heat radiated from the discharge vessel to the coil render it necessary to cool the conductors in order to prevent the supply unit from assuming too high a temperature. It should also be prevented that the coil assumes a comparatively high temperature and thus a comparatively high electrical resistance. A higher electrical resistance would cause the ohmic losses to increase and thus once more the temperature. In the lamp of the cited Application, cooling is realised in that the electrical conductors are each connected to a heat sink. Heat is thus removed from those conductors through the heat sinks to the environment. The conductor which is the live lead is in thermal contact with the heat sink via an aluminium nitride body here. Aluminium nitride is a good thermal conductor and electrical insulator. This renders said heat sink safe upon contact. In addition, the heat sink of the neutral lead may then be integral with the heat sink of the live lead. In such a lamp as disclosed in EP 0 440 381, the two electrical conductors are each connected to a respective heat sink in an electrically conductive manner. A similar electrodeless high-pressure discharge lamp is also known from US 5,030,889. US 5,039,903 and US 5,042,139 disclose electrodeless high-pressure discharge lamps in which each of the ends of the electric coil is connected via a corresponding heat sink to a ballast, while the electrical conductors extend at a distance from these heat sinks. It is possible in these lamps to circulate water through the coil for cooling the coil. A disadvantage of this is that supply and drain lines are necessary, which requires a drive and the associated power consumption. US 4,910,439 discloses an electrodeless high-pressure discharge lamp in which a forced air flow cools the electric coil. This lamp has the disadvantage that a motor and ducts are necessary for cooling, as well as a power requirement. It is an object of the invention to provide an electrodeless high-pressure discharge lamp of the kind mentioned in the opening paragraph which has reliable means for strongly cooling the electric coil and electrical conductors. According to the invention, this object is achieved in that the third turn is connected to a third electrical conductor which extends away from the coil and is connected at least thermally to a heat sink. The measure in the lamp according to the invention also draws heat away from the coil in a location far removed from the end portions of the outer turns. When the central turn is chosen to be the third turn in a lamp having an odd number of turns, then this location may even be the location farthest removed from the end portions. Additional turns may be connected to a heat sink, if so desired, in coils having more than three turns. In a special embodiment, the third electrical conductor is also connected to a heat sink with electrical conduction and is at earth potential during operation, while the first and the second electrical conductor are each connected to a heat sink with electrical insulation by means of a body which is an electrical insulator as well as a thermal conductor. In this embodiment, the first and the second electrical conductor are then the plus and the minus conductor. The embodiment has the advantage that the heat sink connected to the third electrical conductor is safe to touch. The first and the second electrical conductor may have potentials which are symmetrical relative to earth potential, if the third turn is the central turn of a coil, or alternatively asymmetrical. Contact safety is also realised, however, when the third electrical conductor is connected to a heat sink with electrical insulation by means of a body which is an electrical insulator and a thermal conductor, or when the relevant heat sink is enclosed by an insulator or a housing which is at earth potential. If the third electrical conductor is connected to a heat sink via an electrical insulator, the electrical conductors may all be coupled to the same heat sink. In a favourable embodiment, the lamp is coupled to a light waveguide, for example, to a light pipe. It is favourable when aluminium nitride is used as the electrical insulator and thermal conductor. Aluminium nitride combines a comparatively high electrical resistivity of approximately 10 12 Ωm with a comparatively high thermal conductivity of approximately 150 Wm^K '1 . This renders it highly suitable for use in the thermal, but not electrical connection between a live electrical conductor and an electrically conducting body. The electrodeless high-pressure discharge lamp according to the invention can have a compact shape, in which the use of a fluid, such as air or water, and of the associated circulation means is avoided. Nevertheless, ohmic losses and damage to the electrical supply to be used are effective counteracted. Embodiments of the electrodeless high-pressure discharge lamp according to the invention are shown in the drawing, in which: Fig. 1 shows a lamp in perspective view; Fig. 2 is a diagrammatic cross-section taken on II-II in Fig. 1; Fig. 3 is a diagrammatic cross-section taken on πi-III in Fig. 1; Fig. 4 shows a modification of the coil of Fig. 1 in plan view; Fig. 5 shows the coil of Fig. 4 along the line V in this Figure; Fig. 6 is a cross-section of a second embodiment seen as in Fig. 2; Fig. 7 is a cross-section of the second embodiment seen as in Fig. 3; and Fig. 8 is a cross-section of a third embodiment. In Fig. 1, the electrodeless high-pressure discharge lamp has a light- transmitting lamp vessel 1 which is closed in a vacuumtight manner and which is made, for example, of quartz glass with a volume of 2 cm 3 , but alternatively of ceramic material such as, for example, aluminium oxide. The lamp vessel contains an ionizable filling, for example, of 2.5 mg Nal, 1.5 mg Cel 3 , and 125 mbar Xe. An electric coil 2, for example made of copper, is present around the lamp vessel, with a first and a second outer turn 3, 4 each having an end portion 5, 6. The end portions are electrically connected to a first 7 and second electrical conductor 8, respectively, which extend away from the coil and are designed for connection to an electrical supply. The coil 2 has a third turn 9 between the outer turns 3, 4. The first and the second electrical conductor 7, 8 are thermally connected to a heat sink 10, and at least one of them is electrically insulated from the heat sink 10 by means of a body 11 which is an electrical insulator as well as a thermal conductor. The third tum 9 is connected to a third electrical conductor 12 which extends away from the coil and is connected at least thermally to a heat sink 13. In the Figure, the third electrical conductor 12 is also connected to a heat sink 13 with electrical conduction and is at earth potential (see Fig. 3), while the first and the second electrical conductor 7, 8 are both connected to a heat sink 10 (see Fig. 2) in an electrically insulating manner via a body 11 which is an electrical insulator as well as a thermal conductor, and which is made of aluminium nitride in the Figure. The lamp vessel 1 is accommodated in a housing 20 with a reflector 21 which is closed off with a grid 22 of metal wire. The heat sink 10, 13 has fins 10a, 13a for an improved heat transfer to the surroundings. Whereas a copper coil in a lamp without the measure according to the invention quickly turns black during operation owing to oxidation, the lamp according to the invention exhibits no or a strongly retarded blackening. This already shows the reduced temperature of the coil during operation. In Fig. 2, the conductors 7, 8 are made, for example, of copper and are clamped in in the heat sink 10 between aluminium nitride bodies 11 . They may be connected to a supply via an isolating transformer and may thus be given a positive and negative potential in relation to earth potential. The third electrical conductor 12, for example made of copper, is connected to metal plates 13b of the heat sink 13 with thermal and electrical conduction. The heat sink, and thus the third conductor, is at earth potential. In Figs. 4-8, parts corresponding to parts in the preceding Figures have the same reference numerals. In Figs. 4, 5, the coil is made from a copper plate. An end portion 9a of the third turn 9 is held clamped in by the third electrical conductor 12, which is clamped in in the heat sink 13 between aluminium nitride bodies 14. The first electrical conductor 7 (Fig. 6) is clamped in between metal plates 10b in the heat sink and is connected thereto with electrical and thermal conduction. The second electrical conductor 8 is enclosed between aluminium nitride bodies in the heat sink, connected thereto with thermal conduction but with electrical insulation. The third electrical conductor 12 (Fig. 7) is connected to a heat sink 13 so as to be electrically insulated and thermally conducting, via a body 14 which is an electrical insulator and a thermal conductor. In Fig. 8, the lamp vessel 1 has a coating 1' of ZrC^ at a surface so as to be light-reflecting there. The lamp shown has only one heat sink 10, 13 to which the first conductor is thermally and electrically connected, while the second conductor is connected thereto with thermal conduction and electrical insulation via the body 11, and the third conductor also with thermal conduction and electrical insulation via the body 14. A reflector 21 is present in the heat sink and is in close contact therewith. A light pipe merges into the reflector. The electrical supply unit for the lamp may be accommodated in a housing 26.

Claims: Claims: 1. An electrodeless high-pressure discharge lamp comprising: a light-transmitting lamp vessel (1) which is closed in a vacuumtight manner and which has an ionizable filling; around the lamp vessel, an electric coil (2) with a first and a second outer turn (3, 4) each with an end portion (5, 6) which is electrically connected to a first (7) and a second electrical conductor (8), respectively, extending away from the coil and designed for connection to an electric supply, which coil (2) has a third turn (9) between the outer turns, the first and the second electrical conductor (7, 8) being thermally connected to a heat sink (10), while at least one of them is electrically insulated from the heat sink by a body (11) which is an electrical insulator as well as a thermal conductor, characterized in that the third turn (9) is connected to a third electrical conductor (12) which extends away from the coil and is connected at least thermally to a heat sink (13). 2. An electrodeless high-pressure discharge lamp as claimed in Claim 1, characterized in that the third electrical conductor (12) is connected to a heat sink (13) also with electrical conduction and is at earth potential while the first and the second electrical conductor (7, 8) are each connected to a heat sink (10) with electrical insulation by means of a body (11) which is an electrical insulator as well as a thermal conductor. 3. An electrodeless high-pressure discharge lamp as claimed in Claim 1, characterized in that the third electrical conductor (12) is connected to a heat sink (13) with electrical insulation by means of a body (14) which is an electrical insulator as well as a thermal conductor. 4. An electrodeless high-pressure discharge lamp as claimed in Claim 3, characterized in that the heat sink (13) to which the third conductor (12) is connected is integral with the heat sink (10) to which the first (7) and the second conductor (8) are connected. 5. An electrodeless high-pressure discharge lamp as claimed in Claim 1, 2, 3 or 4, characterized in that aluminium nitride is used as the electrical insulator and thermal conductor. 6. An electrodeless high-pressure discharge lamp as claimed in Claim 1, 2, 3 or 4, characterized in that the lamp is coupled to a light pipe (25).

Inventors: KORBER ACHIM, LOEHN KLAUS, VAN VLIET JOHANNES ADRIANUS JO, KORBER, ACHIM, LOEHN, KLAUS, VAN VLIET, JOHANNES, ADRIANUS, JOSEPHUS, MARIA
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS PATENTVERWALTUNG, PHILIPS ELECTRONICS N.V., PHILIPS PATENTVERWALTUNG GMBH
Application year: 1995
Main classification: H01J65
IPC codes: H01J61, H01J65