Europaisches Patentamt 19 European Patent Office Office europeen des brevets © Publication number : 0 679 050 A1 12 EUROPEAN PATENT APPLICATION © Application number : 95302220.9 © int. ci.6 : H05B 41/29, H05B 41/24 @ Date of filing : 03.04.95 (§) Priority : 18.04.94 US 228826 Inventor : Wharmby, David Osborn 65 Beacon Road (43) Date of publication of application : Loughborough, Leicestershire LE11 2BE (GB) 25.10.95 Bulletin 95/43 Inventor : Ludwig, Gerald Wilbur 112 Glenhill Drive Scotia, New York 12302 (US) @ Designated Contracting States : Inventor : Nerone, Louis Robert DE FR GB IT NL 8058 Tanager Oval Brecksville, Ohio 44141 (US) © Applicant : GENERAL ELECTRIC COMPANY 1 River Road © Representative : Pratt, Richard Wilson et al Schenectady, NY 12345 (US) London Patent Operation G.E. Technical Services Co. Inc. © Inventor : El-Hamamsy, Sayed-Amr Ahmes Essex House 2120 Van Rensselaer Drive 12/13 Essex Street Schenectady, New York 12308 (US) London WC2R 3AA (GB) © Impedance matching circuit for an electrodeless fluorescent lamp ballast. @ An impedance matching circuit for a self- oscillating electrodeless fluorescent lamp bal- last of the type having an inductor connected in series with the parallel combination of a capaci- tor and the lamp's drive coil includes an ad- ditional capacitor connected in series with the drive coil. The capacitance value chosen for the additional capacitor is dependent on : stresses on the parallel capacitor ; matching the im- v»0 pedance of the ballast ; and the impact of the capacitor on the loaded coil phase angle. The additional capacitor reduces the phase angle jHm2 presented to the ballast, thereby lowering the sensitivity of the ballast to component and lamp variations. In addition, the overall impedance of "92 the network is reduced, such that the required inductance of the series inductor is reduced ; hence, the inductor can have fewer turns and FIG. 3 lower conduction losses. The current in induc- tor for the required power level is lower, result- ing in a further reduction in conduction losses as well as a reduction in core losses due to lower flux in the core. Still further, a reduction O of stresses the inductor results in reduced If) on a o operating temperature, and hence increased efficiency, reliability and ballast life. o> LU Jouve, 18, rue Saint-Denis, 75001 PARIS 1 EP 0 679 050 A1 2 Field of the Invention for a required power level is lower, resulting in a fur- ther reduction in conduction losses as well as a reduc- The present invention relates generally to elec- tion in core losses due to lower flux within the core. trodeless lamps (fluorescent and high intensity dis- Still further, a reduction of stresses on the inductor re- charge lamps) and, more particularly, to a circuit for 5 suits in a reduced operating temperature therefor, matching the impedance of an electrodeless fluores- and hence increased efficiency, reliability and ballast cent lamp ballast to that of the lamp's drive coil. life. Background of the Invention Brief Description of the Drawings 10 A self-oscillating resonant circuit is often used as The features and advantages of the present in- an electrodeless lamp (i.e., fluorescent and high in- vention will become apparent from the following de- tensity discharge) ballast because of its simplicity tailed description of the invention when read with the and low cost. Disadvantageously, however, resonant accompanying drawings in which: circuits are, by the nature of their operation, very sen- 15 Figure 1 illustrates a typical electrodeless fluor- sitive to variations in their components and the loads escent lamp configured as a reflector lamp in a they are supplying. Hence, given typical component downlight fixture; and lamp variations, the output power and efficiency Figure 2 schematically illustrates a typical ballast of the ballast may not remain within desired relatively load network for an electrodeless fluorescent narrow limits. 20 lamp; Another problem, particularly for electrodeless Figure 3 schematically illustrates a ballast load fluorescent lamps operated as reflector lamps in network for an electrodeless fluorescent lamp in "downlight" type fixtures (i.e., such that reflected light accordance with the present invention; and is emitted through a lower portion of the lamp oppo- Figure 4 schematically illustrates an alternative site a reflective portion), the ambient temperature in 25 embodiment of a ballast load network in accor- which the lamp operates is relatively high because of dance with the present invention. the confined fixture space. Unfortunately, such high temperatures often approach the thermal limit for bal- Detailed Description of the Invention last components. Accordingly, it is desirable to provide a simple 30 Figure 1 illustrates a typical electrodeless fluor- and cost effective load network for a self-oscillating escent discharge lamp 10 having an envelope 12 con- ballast configured such that the ballast is insensitive taining an ionizable gaseous fill. A suitable fill, for ex- to component and lamp variations. Furthermore, it is ample, for the electrodeless fluorescent lamp of Fig- desirable to improve the efficiency and reliability of ure 1 comprises a mixture of a rare gas (e.g., krypton the self-oscillating circuit by reducing stresses on its 35 and/or argon) and mercury vapor and/or cadmium va- components, thus reducing operating temperature por. A drive coil 14 is situated within, and removable and extending the life of the ballast. from, a re-entrant cavity 16 within envelope 12. For purposes of illustration, coil 14 is shown schematical- Summary of the Invention ly as being wound about a magnetic core 15, i.e., hav- 40 ing a permeability greater than one, which is situated An impedance matching circuit for a self- about an exhaust tube 20 that is used for filling the oscillating electrodeless lamp ballast of the type hav- lamp. Alternatively, however, the coil may be wound ing an inductor connected in series with the parallel about the exhaust tube itself, or may be spaced apart combination of a capacitor and the lamp's drive coil from the exhaust tube and wound about a core of in- includes an additional capacitor connected in series 45 sulating material, or may be free standing, as desired. with the drive coil. The capacitance value chosen for The interior surfaces of envelope 12 are coated in the additional capacitor is dependent on: stresses on well-known manner with a suitable phosphor 18. En- the parallel capacitor, matching the impedance of the velope 12 fits into one end of a base assembly 1 7 con- ballast; and the impact of the capacitor on the phase taining a radio frequency power supply (not shown) angle of the impedance of the loaded drive coil. Ad- so with a standard (e.g., Edison type) lamp base 19 at vantageously, the additional capacitor reduces the the other end. Lamp 10 is illustrated schematically as phase angle presented to the ballast, thereby lower- being installed in a downlight fixture 25 of well-known ing the sensitivity of the ballast to component and type. lamp variations. In addition, the overall impedance of Lamp 10 is illustrated as being of a reflective the network is reduced, such that the required induc- 55 type; that is, light emitted within envelope 12 is re- tance of the series inductor is reduced; hence, the in- flected by a reflector, illustrated as comprising a re- ductor can have fewer turns and lower conduction flective coating 34 on a portion of the interior or ex- losses. As another advantage, current in the inductor terior surface of the envelope, such that light is emit- 2 3 EP 0 679 050 A1 4 ted through an opposing portion 36 of the envelope. tions of the impedance (i.e., resistance and phase an- An exemplary reflective coating is comprised of tita- gle) need to be controlled in addition to control of nia. Adielectric housing, e.g., comprised of plastic, is dead-time and frequency. To this end, the impedance illustrated as being situated around the reflective por- as viewed by the ballast is represented by: tion of envelope 12. 5 In operation, current flows in coil 14 as a result of excitation by a radio frequency power supply. As a re- Zioad = Rload + jtaiM^Rioad radio field is established sult, a frequency magnetic = Rload + jXioad, within envelope 12, in turn creating an electric field which ionizes and excites the gaseous fill contained 10 therein, resulting in an ultraviolet-producing dis- where R!oad represents the resistive part of the ballast charge 23. Phosphor 18 absorbs the ultraviolet radi- load impedance and <|> represents the phase angle. ation and emits visible radiation as a consequence The impedance of the loaded drive coil is represented thereof, which visible radiation is reflected by reflec- as: tive coating 34 through light-emitting portion 36 of 15 Z, = R, + jX,, lamp 10. where R| is the real part and X| is the imaginary part Figure 2 illustrates a typical ballast load network of the loaded drive coil. To optimize operation, as de- for an electrodeless fluorescent lamp such as lamp 1 0 scribed hereinabove, the expression for load impe- of Figure 1. In Figure 2, the resistance and inductance dance Z|0ad is made equal to the expression for the of the arc discharge of lamp 1 0 are represented by Ra 20 loaded impedance of the drive coil Z|, i.e., Z!oad = Z|. and La, respectively. The ballast of Figure 2 is a Class- The parallel capacitor Cp acts as a transformer of D type circuit including two switching devices and the real part of Z|. Letting Zp represent the parallel M2 connected in a half-bridge configuration across a combination of impedance Z| and capacitor Cp results ballast power supply represented as Vb.