Impedance Matching Circuit for an Electrodeless Fluorescent Lamp Ballast

Europaisches Patentamt 19 European Patent Office

Office europeen des brevets © Publication number : 0 679 050 A1

12 EUROPEAN PATENT APPLICATION

@ 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)

@ An impedance matching circuit for a self- oscillating electrodeless fluorescent lamp ballast of the type having an inductor connected in series with the parallel combination of a capacitor and the lamp's drive coil includes an additional 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 inductor for the required power level is lower, resulting 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>

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 further 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 invention 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. An impe- in the expression: dance matching circuit is connected to the circuit 25 Zl ■7 - so„„ that node between switching devices and M2. Specif- 1 + jcoCpZi , ically, the matching circuit includes an inductor L2 connected in series with a parallel matching capacitor Cp. 1 Ri,load Q, Q, The capacitor Cp is coupled in parallel with the drive coX, 1 + coil 14. The capacitance of parallel capacitor Cp is se- 30 lected to match the resistive portion of the loaded coil lm(Z|oad) " lm(ZP} where Q, = JandL2 = to adjuxt impedance to the required ballast load resistance. Ri co The inductance of series inductor L2 is then selected the phase angle. so that the impedance presented to the ballast has an Disadvantageously, the sensitivity of the circuit optimized phase angle to satisfy zero-voltage switch- 35 of Figure 2 to component and lamp variations is rela- ing and power output conditions. A feedback circuit, tively high, in large part due to the light loading of the which comprises a feedback transformer T^ the gate drive coil. That is, the phase angle of the impedance capacitances of switching devices and M2, and of the loaded drive coil is very large, e.g., close to or padding capacitors Cg1 and Cg2 in parallel with the above 80°. This large phase angle is due to the weak gate capacitances of devices and M2, respectively, 40 coupling between the discharge and the drive coil. is provided such that the ballast oscillates at the de- Coupling improvements, however, are limited by the sign frequency and has a dead-time (i.e., time be- geometry of the lamp, the need to maximize light out- tween on and off states of a switching device) which put, and cost. results in zero-voltage, or lossless, switching (i.e., The feedback circuit as configured in Figure 2 switching of devices and M2 with zero voltage 45 makes the circuit relatively insensitive to variations in thereacross). Blocking capacitors Cb are provided for the line voltage. However, the power variation due to blocking dc voltage into the drive coil and for filtering; typical component and lamp variations would be un- their capacitance values are such that they do not af- acceptable in a practical system. Although it would be fect the impedance matching at the frequency of op- possible to operate such a circuit in a controlled feed- eration. Exemplary feedback circuit control to ach- 50 back loop wherein power is maintained constant, ieve the desired dead-time and zero-voltage switch- such an approach would require fairly expensive con- ing is described by Louis R. Nerone and A-Haq Qur- trol circuitry and would therefore be impractical. eshi in "Mathematical Modeling and Optimization of As an additional disadvantage of the circuit of the Electrodeless, Low-Pressure, Discharge Sys- Figure 2, the inductance of inductor L2 is relatively tem", Transactions of IEEE, PESC 1 993, pp. 509-514, 55 large (e.g., 51 p.H). As a result, the voltage across in- which is incorporated by reference herein. ductor L2 is larger than needed for proper operation To accomplish zero-voltage switching while deliv- of the lamp/ballast system. In addition, the phase an- ering the required power, the real and imaginary por- 3 5 EP 0 679 050 A1 6 gle between the midpoint voltage vc and the load cur- ballast life. rent iL is relatively large which means that, to deliver Figure 4 illustrates an alternative embodiment of a certain amount of power from a given bus voltage the ballast load network of Figure 3. In particular, the VbUs> the load current has to be even higher. A higher network of Figure 4 does not include blocking capac- current means higher conduction losses. 5 itors Cb because the added series capacitance Cs ad- In accordance with the present invention, a bal- vantageously provides the dc blocking function. last load network is provided which reduces the load While the preferred embodiments of the present phase angle <|> presented to the ballast. Figure 3 illus- invention have been shown and described herein, it trates a ballast load network according to the present will be obvious that such embodiments are provided invention comprising a capacitor Cs added in series 10 by way of example only. Numerous variations, with the lamp's drive coil 14. A lower limit on the ca- changes and substitutions will occur to those of skill pacitive value of capacitor Cs is dependent on stress- in the art without departing from the invention herein. es on the parallel capacitor Cp and the need for impe- Accordingly, it is intended that the invention be limited dance matching to achieve an inductive phase angle only by the spirit and scope of the appended claims. for the series combination of capacitor Cs and the 15 drive coil. An upper limit on the capacitive value is dependent on the impact of the capacitor Cs on the load- Claims ed coil phase angle. The impedance Z!s of the series combination of 1 . A ballast for an electrodeless lamp of a type hav- series capacitor Cs and the loaded drive coil is repre- 20 ing a drive coil situated proximate a light- sented as: transmissive envelope for exciting an arc discharge in an ionizable fill contained therein when excited current Zis - Ri + jXis by an alternating power supply, comprising: = Rl+j(Xi at least two devices connected coCs' 25 switching in a bridge configuration; and an impedance matching circuit coupled to Therefore, the impedance Zps of the parallel combin- said bridge configuration of switching devices, ation is: said matching circuit comprising an inductance Zis 30 connected in series with a parallel connection of Zps-7 _ so that 1 + jcoCpZis , a parallel matching capacitor and said drive coil, said matching circuit further comprising a series Q, Q, matching capacitor connected in series with said = J_ oad p drive coil, said series matching capacitor having coX, 1 + Q,2 35 a capacitance value selected to optimize the where Q!s = phase angle of the impedance of said ballast so as to avoid sensitivity to ballast component and lamp variations and to minimize said inductance. Example 40 2. The ballast of claim 1, comprising an electrode- An exemplary ballast load network (Figure 3) in- less fluorescent lamp ballast. cludes components having the following values: L2 = 33 nH, Cp = 71 0 pF, and Cs = 1 .5 nF. 3. An electrodeless fluorescent lamp, comprising: Advantageously, the matching circuit (L2, Cp, and a light-transmissive envelope containing of the invention reduces the ballast load Cs) present 45 an ionizable, gaseous fill for sustaining an arc phase angle, significantly lowering sensitivity of the discharge when subjected to a radio frequency circuit to component and lamp variations. In addition, magnetic field and for emitting ultraviolet radia- the overall impedance of the network is reduced, tion as a result thereof, said envelope having an such that the inductance of inductor L2 is reduced; L2 interior phosphor coating for emitting visible radi- thus has fewer turns and lower conduction losses. As 50 ation when excited by said ultraviolet radiation; another the overall load at the advantage, impedance a reflector situated about a reflector por- midpoint vc has a lower phase angle so that the cur- tion of said envelope for reflecting said visible ra- rent in inductor L2 for the required power level is lower, diation through a light-emitting portion of said en- resulting in both a reduction of conduction losses as velope situated opposite said reflector portion; well as a reduction in core losses due to lower flux in 55 and the core. Still further, a reduction of stresses on in- a drive coil situated proximate said envel- ductor results in reduced L2 a operating temperature ope for providing said radio frequency magnetic for and hence increased and L2, efficiency, reliability field when excited by a radio frequency energy 4 EP 0 679 050 A1 source via a ballast, said ballast comprising at least two switching devices connected in a bridge configuration and an impedance matching circuit coupled to said bridge configuration of switching devices, said matching circuit comprising an in- 5 ductance connected in series with a parallel connection of a parallel matching capacitor and said drive coil, said matching circuit further comprising a series matching capacitor connected in series with said drive coil, said series matching ca- 10 pacitor having a capacitance value selected to optimize the phase angle of the impedance of said ballast so as to avoid sensitivity to ballast component and lamp variations and to minimize said inductance. 15

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European Patent EUROPEAN Application Number Office SEARCH REPORT EP 95 30 2220

DOCUMENTS CONSIDERED TO BE RELEVANT category i v imuon oi aocument with indication, where appropriate, Relevant CLASSIFICATION OF THE of relevant passages to claim APPLICATION qnt.C1.6) tH-A-U 498 497 (PHILIPS NV) 12 August 1992 1,2 H05B41/29 Y * column 6, line 41 - column 6, line 50; 3 H05B41/24 figure 2 *

EP-A-0 222 441 (PHILIPS NV) 20 May 1987 1 * figure 1 *

Y EP-A-0 541 344 (GEN ELECTRIC) 12 May 1993 I * abstract; figure 1 *

A US-A-4 383 203 (STANLEY CHARLES A) 10 May 1,2 1983 * abstract; figures 1,3 *

TECHNICAL FIELDS SEARCHED (Int.CI.6) H05B HOD

l ne present search report has been drawn up tor all claims umt of conpieooa of Ike searcA THE HAGUE 31 May 1995 Speiser, P I : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date V : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document