Electronic Ballasts for CFL Operating at Frequencies Above of 1 MHz: Design Considerations and Behavior of the Lamp I.

Transcription

1 Electronic Ballasts or CFL Operating at Frequencies Above o 1 MHz: Design Considerations and Behavior o the Lamp I. INTRODUCTION Nowadays, the trends in lighting aim toward the development o more eicient lamps and with greater useul lie[1]. One o the strategies to increase the useul lie o discharge lamps is the elimination o the cathodes. Since, the lost o the emissive substance in the cathodes is an indicative o this parameter. The principal waste o the cathodes is during the lamp starting, but i the temperature o the cathode is increasing above o the recommended practice, the lost o emissive substance is signiicant too. However, when one lamp is dimmed, the current through the cathodes is diminished and the plasma inside the lamp is cooled and low range o dimming are reached. The solution to this problem is to maintain constant the current through the lamp and, in this way, the plasma is not cooled at the same proportion []. The elimination o the cathodes avoids this problems and permits to increase the useul lie o the lamp. Based on this principle, electrodeless lamps have been developed and they produce the discharge inside the lamp using an inductive or capacitive coupling [1][3,4]. In order to produce the electrical discharge under these conditions, these lamps have specials orms and requires a dedicated ballast or this application. Since these lamps are not very commercial, their cost is more elevated than the cost o a standard lamp. Nevertheless, the operation o common compact luorescent lamps (CFL) at very high requency (VHF), above o 1 MHz, that could present additional advantages such as: size reduction o the passive elements in the resonant tank and improvements o the behavior o the lamp. For example; it has been ound that the breakdown voltage on solids and gases diminish according the requency is increased [5, 6]. Thereore, i the operation requency o the lamp is increased the necessary starting voltage is lower, and the sputtering o the emissive substance in the cathode will be lower increasing the useul lie o the lamp. Also, with the VHF operation o the lamp is possible to avoid the use o the cathodes, like the electrodeless lamps, or use it in hybrid orm, in other words, limiting their inluence in the useul lie o the lamps, since it permits a greater range o dimming without overheating the cathodes and increasing the eiciency o the lamp reducing the losses in the cathodes. The use o common CFL operating at VHF, above o 1 MHz, as a strategy to increase the useul lie o the lamps an their eiciency oers a intermediate solution among the use o luorescent electrodeless lamps with high cost and the standard CFL operating at HF with low cost. In this paper the inluence o the requency on the starting voltage and the dimming will be evaluated and the eects o the requency on magnetics and drivers are considered. To validate only the eects o the requency over this parameters the cathodes were not heated and were short-circuited. This paper is organized on ollowing way: irst, a revision o topologies with the capability to operate at VHF is presented. Ater, some specials design considerations about the operation at VHF will be presented. Next, experimental results o the behavior o one luorescent lamp operating at 1.34 MHz and.13 MHz will be shown, and inally, the conclusions will be mentioned. II. REVIEW OF TOOLOGIES Electronic ballasts operating at VHF have been developed mainly or electrodeless lamps and they are based on the class D and class E ampliiers used in sel-excited mode [7-9]. For the operation at VHF the commutations with zero losses is necessary. The class E ampliier presents by itsel zero voltage switching (ZVS). On the other hand, the resonant tank o the

2 class D ampliier must be designed to present zero current switching (ZCS) at the on-o and o-on transitions o the switch, in other words, the current through the switches must be in phase with the voltage across the switch. To obtain this condition the series impedance o the resonant tank is zero and the lamp can present instabilities. For this reason the class E ampliier was chosen as the best option. Also, the class E ampliier use only one switch and the circuitry control is more simple. Fig. 1 shows the basic coniguration o the class E ampliier. In order to adapt the class E ampliier as electronic ballast or CFL, the series resonant tank o the class E ampliier must be changed by a parallel resonant tank. For this project the capacitive impedance inverter (CII) was chosen due the capability o this tank to ignite the lamp rom low voltage [10]. Fig. shows the class E ampliier with the CII adapted as electronic ballast. This topology was used in this paper to evaluate the behavior o the lamp when is operated at VHF. III. DESIGN CONSIDERATIONS. When one converter is operated at VHF the parasitic o passive and active elements become relevant. Following, the inluence o more relevant parasitic o active and passive elements on the class E ampliier operation is considered. A. Eect o the output parasitic capacitance o the MOSFET (Coss). In class E ampliiers there are a commitment between the switching requency, C 1 and L 1. I L 1 = then the switching requency is minimum and can be expressed as [11]: s min = (1) V The maximum switching requency is when the ripple o current in L 1 is maximum and the minimum value o this current is zero, under this condition the maximum switching requency is [1]: s max = () V Equations (1) y () permits to calculate the maximum and minimum switching requency in unction o the speciications and the value o C 1. This capacitance is in parallel with the switch and must include the output parasitic capacitance (Coss) o the MOSFET. According to (1) and () i s is very high then C 1 can become equal to Coss. The value o this capacitance depends on the voltage V DS o the MOSFET and its behavior is no-lineal. To avoid this problem C1 must be suiciently greater than Coss to absorb the no-linearity o this capacitance. In this work the MOSFET IRF840 was chosen as switch, and it has Coss=310 pf. Based on this vale the selected value or C 1 was C 1 4Coss=1. nf. Since C 1 =C 1 +Coss, the value o C 1 will be 80 pf. In order to design the class E ampliier an intermediate value o s was selected. Thereore, this expression was used: s = (3) 17.33V B. Eect o the input parasitic capacitance o the MOSFET (Ciss). In order to operate at VHF it is necessary to charge and discharge quickly the input parasitic capacitance Ciss o the MOSFET. The gate driver must have very low resistances to charge and discharge this capacitances. Conventional drivers used

3 in power converters works at 1 MHz as maximum. Other solution is to use sel-oscillating circuits, but their design is more complicated [13]. In this case we use the solution proposed in [14] with a Schmitt-Trigger IC used as clock signal. The diagram is shown in ig. 3. More details will be given in the inal version o this paper. C. Design o magnetic elements At VHF the parasitic currents induced inside the core o the inductors are greater and the core-losses are higher. These losses saturate the magnetic lux density and the value o the inductor is not predictable. The value o the inductance in air-core inductors is more stable and with low cost. For this reason the inductors used in this project were air-core inductors. Due the VHF operation the size o this inductors is small. In the inal version o this paper more details about the design o this kind o inductors will be provided. D. Design procedure Based on the guidelines indicated above to design at VHF, the ollowing design procedure was established. 1. Speciications.- The used CFL lamp have the ollowing characteristics: L =3 W, I LRMS =45 ma. The class E ampliier will commute at sub-optimum regimen, thereore the design power will be greater than the lamp power, on this way the parameters sensitive o the class E will be lower. In this case, a design power o n =40W will be used. Two designs will be implemented, one above o 1 MHz and other above o MHz. The Value o C 1 will be the value mentioned on section III.E.. Evaluation o the source voltage Vcc. Equation (3). 3. Evaluation o L1[1]. 4. Evaluation o the series resonant tank o the class E ampliier[1]. 5. Evaluation o the capacitive impedance inverter elements [10]. The results using commercial values or the elements are shown in table I. Table I. Calculated values or the 1 and MHz designs Fs(MHz) V CC (V) L 1 (uh) L (uh) C (nf) C 3 (nf)* and and *The values indicated in this column are two capacitors connected in parallel. IV. EXERIMENTAL RESULTS Experimental results obtained with the designed ballast are shown in igures 4-7, to adjust the power deliver to the lamp, the requency o each ballast was adjusted to reach the nominal power o the lamp maintaining ZVS, the adjusted values o the requency were 1.34 and.13 MHz or each respective ballast. Also, to avoid the eects o the warm in the cathodes, they were short-circuited. Fig. 4 shows the obtained results with the ballast operating at 1.34 MHz and Fig. 5 the obtained results with the ballast operating at.13 MHz. Figs. 4a and 5a show the current and voltage through the switch. The voltage plot indicates that the class E presents ZVS. The current in ig 4a is divided in three times, t1 is when the current low through the MOSFET, t is when the current low through C1 and t3 is when the current low through the parasitic diode o the MOSFET. Figs. 5a and 5b show the lamp current and voltage, both igs. show that the current and voltage are sinusoidal. Figs. 4c and 5c shows the starting voltage, Fig.

4 5c shows the starting voltage is lower at.34 MHz than the starting voltage at 1.34 MHz, the reduction is 0%. Furthermore, the starting time is greater at.34 MHz than 1.34 MHz. Fig. 6 and Fig. 7 shows the dimming results at 1.34 MHz and.13 MHz. Figs. 6a and 7a shows the dynamic impedance o the lamp. In this igs can be seen that the dynamic impedance is most lineal at.34 MHz than 1.34 MHz. Figs. 6b and 7b shows the equivalent resistance o the lamp (R L ) vs the lamp power. In these igs can be seen that or low power R L at 1.34 MHz is greater than R L at.13 MHz. On the other hand, Figs. 6c and 7c shows that the power actor o the lamp presents almost any changes at 1.34 MHz and.13 MHz, respectively. V. CONCLUSIONS In this paper the behavior o the lamp operating above o 1 MHz was analyzed. The starting voltage and the dimming or one CFL o 3 W was compared at 1.34 MHz and.34 MHz. Experimental results indicate that the starting voltage is lower or higher requencies, dimming results was not conclusive and not signiicant changes was observed. Design considerations about the VHF operation were commented and an analysis o the eect o the parasitics o the active and passive elements o the ballast were included. The converter used to drive the lamp at VHF was the class E ampliier using a capacitive impedance inverter as resonant tank. REFERENCES [1] B. Cook. New Developments and Future Trends in High-Eiciency Lighting. Engineering Science and Education Journal, October 000, pp: [] E. Tetri. Eect o Cathode Heating on Lamp Lie in Dimming Use. IEEE Industry Application Society Annual Meeting, IAS 01, pp: [3] V. A. Godyak. Radio Frequency Light Sources, IEEE Industry Application Society Annual Meeting, IAS 00, pp [4] D. O. Wharmby. Electrodeless Lamps or Lighting: a Review IEE roceedings, Vol. 140, No. 6, November 1993, pp: [5] K. Elanseralathan, M. Joy Thomas, G. R. Nagabhushana. Breakdown o Solid Insulating Materials under High Frequency High Voltage Stress. roceedings o the 6 th International Conerence on roperties and Applications o Dielectric Materials, June 000, Xi an, China, pp: [6] W. G. Dunbar, D. L. Schweickart, J. C. Horwath, L. C. Walko. High Frequencies Breakdown Characteristics o Various Electrode Geometries on Air. ower Modulator Symposium, 1998, pp: 1-4. [7] N. Yunoue, K. Harada, Y. Ishihara, T. Todaka, F. Okamoto. A Sel-Excited Electronic Ballast or Electrodeless Fluorescent Lamps Operated at 10 MHz, IEEE Industry Application Society Annual Meeting, IAS 98. [8] H. Miyazaki, H. Shoji, Y. Namura. High-Frequency Class-D Converter Driving with Feedback Capacitors or Electrodeless Fluorescent Lamps, IEEE Industry Application Society Annual Meeting, IAS 98. [9] H. Kido, S. Makimura, S. Masumoto. A Study o Electronic Ballast or Electrodeless Fluorescent Lamps with Dimming Capabilities, IEEE Industry Application Society Annual Meeting, IAS 01, pp [10] M. once, J. Arau, J. M. Alonso and M. Rico-Secades. Analysis o the class E ampliier used as electronic ballast with dimming capability or photovoltaic applications. International Journal o Electronics, Vol. 88, No 7, July 001, pp [11] M. once. Sistemas de Alimentación para Lámparas de Descarga Basados en Ampliicadores Clase E, h. D. Thesis, CENIDET, México. In spanish. [1] C. H. Li, Y. O. Yam. Maximum Frequency and Optimum erormance o Class-E ower Ampliiers. IEE roc. Circuits Devices Systems, Vol. 141, No. 3, (June 1994), pp [13] L. R. Nerone. Analysis and design o sel-oscillating class E ballast or compact luorescent lamps, IEEE Transactions on Industrial Electronics, Vol. 48, No. 1, February 001, pp [14] R. Redl, B. Molnár, (y) N. O. Sokal. Class E Resonant Regulated DC/DC ower Converters: Analysis o Operations, and Experimental Results at 1.5 Mhz. IEEE Transactions on ower Electronics, Vol. E-1, No., (Abril 1986), pp

5 L 1 L C L 1 L C 3 nf nf N +1V 47uF R V cc M 1 C 1 R L V cc M 1 C 1 C V IN C V OUT 10 ohms To MOSFET Fig. 1. Basic coniguration o the class E ampliier. Fig.. Class E ampliier with the CII used as electronic ballast N3906 Fig. 3. Driver used to control the MOSFET. Fig. 4. Obtained results with the ballast operating at 1.34 MHz. (a) switch voltage and current, (b) lamp voltage and current, (c) Starting voltage and current. Scales are indicated in the graph. Fig. 5. Obtained results with the ballast operating at.13 MHz. (a) switch voltage and current, (b) lamp voltage and current, (c) Starting voltage and current. Scales are indicated in the graph. V Lrms (v) R L (ohms) F.. (p.u.) I Lrms (ma) 0 L (%) 0.7 Fig. 6. Dimming results with the ballast operating at 1.34 MHz. (a) Dynamic impedance, (b) Instantaneous impedance (c) ower Factor. VLrms (v) ILrms (ma) RL (ohms) L (%) F.. (p.u.) L (%) 0.7 Fig. 7. Dimming results with the ballast operating at.13 MHz. (a) Dynamic impedance, (b) Instantaneous impedance (c) ower Factor. L (%)