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ELECTRONIC BALLAST FOR METAL HALIDE LAMPS WITH ACOUSTICAL RESONANCE AVOIDANCE Marcelo Toss, Anderson S. Santos and Fernando Soares dos Reis University Catholic of Rio Grande do Sul PUCRS – PPGE – LEPUC 90619-900 – Porto Alegre – RS – Brazil – Fax +55.51.3320.3500 e-mail: [email protected] Abstract-This paper presents the analyzed, design and implementation of some high frequency electronic ballast for metal halide lamps. Extensive experiments using resonant type electronic ballast have been conducted on 70W metal halide lamps to investigate different control methods for avoiding the phenomenon known acoustic resonance. Besides, a high frequency ballast to operate a different lamp markers for test the methods is also presented. I. INTRODUCTION Metal halide lamps have attracted much attention in recent years, because they offer excellent color rendition, long life and compact volume, in contrast to fluorescent lamps and incandescent lamps. Similarly to fluorescent lamps, the HID lamps present the negative impedance and the device to limit the current should be used. The electromagnetic ballast can be used. However, they have a high weight and size, poor regulation and sensibility to voltage changes [3, 4]. In principle, the use of high frequency electronic ballast can reduce the size and the weight of the ballast and improve the system efficacy. This feature is especially attractive for low-wattage HID lamps because the overall lighting system is expected to be of small size. However, the operation of high-pressure HID lamps with high-frequency current waveforms is hampered by the occurrence of standing pressure waves called acoustical resonance. The phenomenon of acoustical resonance is characteristic of the HID lamps operating at frequencies greater than 1kHz and they appear when the modulation of the power in the lamp exceeds a threshold value. This phenomenon cause fluctuation of the emitted light, variation of the color temperature, variation of lamp voltage and, in worst case, rupture of the discharge tube. Many ballast circuit topologies or control methods [3-5] have been proposed to avoid instability caused by acoustic resonance. There are basically two proposed solutions are the following: 1) Operation inside a frequency range free of acoustic resonance. Typical ballast used in this approach can be: Low frequency ballast: is based on the fact that there isn’t resonance acoustic in frequency below 1kHz.The disadvantages of this method are the complicity of circuit and low efficiency. Tuned high-frequency ballast: require predetermination of the free acoustic resonance zone. However, it is very difficult to select the same window for all lamps marked because the acoustical resonance is a function of tube geometry, gas density, and temperature. It also varies with manufacturing tolerance and by lamp age. Extra high-frequency ballast: its refers to the operation above the maximum resonance frequency range. The disadvantages of this method are the high loss in the switching components and EMI problems because the high frequency discharge arc is a very good antenna radiating noise . Real-time autotracking system: is the same idea of tuned high-frequency, the difference is the real-time autotracking system detect the resonance acoustic and change the frequency. The main problem of this method are detecting a low intensity resonance acoustical, because electrical parameters change are not enough strong to sensitize the controller . 2) Distribute the lamp power spectrum so that the lamp power at the acoustic resonance frequency may not exceed the threshold value. Typical ballast used in this approach can be: Modulation of the switching frequency ballast: the concept is change the frequency to keep the lamp input energy distributed over a frequency range. Ideally this should be done in a frequency range where acoustical resonance are not usually observed. Modulation of phase angle ballast: is based on the idea that constant phase variation disturbs the excitation of acoustic resonance. The disadvantage of this method is the higher level current at each jump phase. Non-sinusoidal voltage in the lamp ballast: the concept is used a high frequency squarewave operation for distribute power spectrum in a theoretically infinite number of harmonics. The problem are the limitation of lower order harmonics and electromagnetic interference (EMI) problems. Comparing the advantages and disadvantages of the technical to eliminate the resonance acoustic above, it can be said that the concept of tuned high frequency ballast together with modulation of the switching frequency ballast is the best choice because de ballast and control are simple and sheep. This paper is organized as follows. Section II presents a description of the proposed ballast. Section III show study about the free acoustic resonance zone for 70W halide lamps from different manufactures. Section IV presents a modulation of the switching frequency ballast used to eliminate the acoustic resonance phenomenon. Finally, the conclusions of this work are presented in section V. II. INVERTER The topology chosen is the half bridge inverter using a resonant LC series C parallel, as can be seen on Figure 1. The LC series C parallel resonant converter has the vantage that the output voltage can be regulated for applied high voltage in start up the HID lamps. The resonant filter circuit design was made based on a method described in . This calculation assumes that the lamp is a pure resistance, as stated previously . S1 E + - D1 Ls DRIVE Cs S2 D2 Cs Lamp Figure 1 High frequency inverter with resonant circuit Using the drive is possible to change the frequency of current lamp to check the free acoustical resonance zone and testing the concept of modulation of the switching frequency ballast. III. THE FREE ACOUSTICAL RESONANCE ZONE According the reference  , the 70W metal halide, have a frequency range free of acoustical resonance inside these frequency: 22-28kHz, 300-400kHz and >1 MHz. The chose the first range, because the other ranges have problem with high loss in the switching components and EMI. The proposed inverter in Section II is used to determine the frequency range free of acoustical resonance. For these, the control change the frequency of the ballast and the light of the lamp is observed. The Figure 2 show the result with lamps for different manufactures. The free frequency windows are very narrow and your position depend on the lamps. For this, is necessary to add other method to avoid the acoustical resonance. Frequency Range Free of Acustical Resonance Sylvânia Metalarc HSI-TD 70W/NDL Osram PowerStart HQI-TS 70W/NDL Philips Powertone MHN-TD 70W GE MQI/70/T6/30 70W Frequency kHz Light Acoustical Resonance The arc flicker Heavy Acoustical Resonance The arc extinguish Free Zone of Acoustical Resonance Figure 2 The 70W metal halide frequency range free of acoustical resonance IV. MODULATION OF THE SWITCHING FREQUENCY The frequency modulation was applied in the ballast drive to elimination de acoustical resonance. In the test, the ballast is turn on and the frequency is adjusted for the frequency with a light and heavy acoustical resonance. When the lamp is under light acoustical resonance the modulation of the switching frequency eliminate all resonance. However, when the lamp is under heavy acoustical resonance the modulation are not enough for completely eliminate the resonance. For all lamp tested, the windows with light and free acoustical resonance is large (18kHz to 25kHz). For this reason the modulation of the switching frequency represent a solution to the problem of acoustic resonance. V. CONCLUSION This paper presented a ballast circuit for HID lamps with acoustic resonance avoidance. The methods for avoidance the resonance are studied and the low cost and efficient method is select and implemented. A prototype is built to demonstrate the methods proposed in this paper. VI. REFERENCES  J. J. de Groot, J. M. van Vliet. “The high pressure sodium lamp”, Philips Technical Library, Macmillan Education, 1986.  T. J. Liang, K. H. Su and W. H. Fu. “High Frequency Electrical Circuit Model of Metal Halide Lamp”. IEEE, pp1163-1165.  Richard Redl and Jon D. P. “A New High Frequency and High Efficiency Ballast for HID Lamps: Topology, Analysis, Design, and Experimental Results”. APEC 1999.  J.Zhou, L. Ma, Z. Qian and K. Hong, “Acoustical Resonance in High Intensity Discharge Lamps and its Possible Solutions”, ’97 International Symposium on Green Lights in China, pp142-149, 1997.  J. Zhou, F. Tao, F. C. Lee, N. Onishi and M. Okawa, “High Power Density Electronic Ballast for HID Lamps” IEEE, pp1875-1880, 2002.  A. S. André, A. J. Perin, C. C. Tavares and J. Moia, “Electronic Ballast for HighPressure Sodium Lamps with Acoustic Resonance Avoidance”. COPEB 2003. F. E. Bisogno, A. R. Seidel, R. Holsbach and R. N. do Prado, “Resonant Filter Applications in Electronic Ballast”. IEEE, pp348-354, 2002.