This thesis investigates the relevance between the acoustic resonance and power harmonics on a metal halide lamp. First, a sinusoidal current ranging from 20 kHz to 400 kHz is used to drive a 70 W metal halide lamp. Second, a hybrid-current test circuit is designed to generate a current waveform consisting of a low-frequency square-wave and a high-frequency sinusoidal wave. Both of the frequency and the amplitude can be adjusted independently. The test lamp is deliberately driven at its acoustic-resonance eigen-frequencies to observe the effect of the power spectrum on the degree of the acoustic resonance. The experimental results indicate that the occurrence of acoustic resonance is indeed affected by the DC level and related power harmonics. The power harmonic spectrum that elucidates the initiation of acoustic resonance is deduced from the observations. It is found that the power harmonics that excites acoustic resonance can be divided into three categories. The first is independent of the average lamp power; it excites acoustic resonance only if the magnitude of its power exceeds a specific level. The thresholds of power harmonics belong to the second category are proportional to their DC powers. One can also find those remaining power harmonics belong to the third category. The power harmonic spectrum of the acoustic resonance is demonstrated by driving the test lamp with quasi-square-wave and triangle-wave currents. This work helps advance the understanding of the phenomena and mechanism of acoustic resonance in a metal halide lamp.

Conventionally, metal halide lamps were ignited by striking the lamp electrodes several times with high voltage pulses. Such a starting scenario causes uncomfortable light strobes to users. To solve this problem, this thesis attempts to ignite small-wattage metal halide lamps with a single pulse strike. At first, the forms of the high voltage pulses required for breaking down the electrodes are investigated. After being broken down, a continuous current is critical for sustaining the lamp arc. With conventional electronic ballasts, however, the lamp current tends to resonate to zero resulting in break of the lamp arc. This problem can be solved by adding an extra switch to remove the capacitor of the output filter during the ignition stage. An electronic ballast is designed and tested on 70 W metal halide lamps with an associated switch for single pulse striking. Experiments have demonstrated that the proposed ignition criteria can start up the lamps successfully with a single-pulse high voltage.

This study investigates the starting characteristics of metal halide lamps. A laboratory electronic ballast was built to drive metal halide lamps with a programmable low-frequency square-wave current. The lamp current at each stage of the starting transient can be independently adjusted. Experiments were conducted on 150-W metal halide lamps. By examining the waveforms of transient voltage, current and power, the starting period can be classified into four stages, breakdown, glow discharging, glow-to-arc transition, and thermal equilibrium. In addition, the stable operation is defined by observing the variations of the lamp arc, lighting spectrum and luminous output. Based on the investigation results, four starting scenarios are presented and examined to learn the different acceleration schemes. Experimental evidence shows that the starting time of a metal halide lamp can be effectively shortened by increasing the lamp current during the start-up transition. More importantly, a specifically-regulated operating power enables the lamp to further enhance the luminance producing, and hence to greatly reduce the starting transient period.

The research searches for an identification strategy which is able to recognize three small-wattage metal halide lamps rated at powers of 20-W, 35-W and 70-W from three world-wide prominent brands of GE, OSRAM and PHILIPS. A two-stage constant-power starting scenario is adopted to successfully start all three kinds of lamps without causing a tremendous power during the identification process. At the first stage, the tested lamps are started by a constant power of 25 W. The 20-W lamps can be distinguished from the others by their relatively high lamp voltages at the 30th seconds after being ignited. Then, the other lamps are driven up to 35 W to manifest the voltage difference of between the 35-W and 70-W lamps, and thus can be recognized at the 40th seconds. After being made out, the lamps are operated at their rated powers. Eventually, a verification checking with protection is introduced to prevent the tested lamps from over power operation. Experiments have been done on numerous new and aged lamps. The experimental results evidence that the electronic ballast with the proposed identification strategy can recognize three lamps¡¦ rated powers correctly during the starting transition, and drive the lamp to its rated power before entering the steady-state.

Conventionally, metal halide lamps were struck by voltages higher than those required for breaking down the electrodes to ensure successful ignition. These high ignition voltages may hurt the electrodes to some extent, leading to a shorter lamp lifecycle. In practice, the breakdown voltage can be affected by the dark current which occurs when a voltage is applied on lamp before the electrodes have been broken down. A lamp model to account for the dark current is derived from the test results. Three ignition schemes with single-pulse, multiple pulses and step voltage are used for describing the effect of the dark current on the breakdown voltage. Experimental results exhibit that the breakdown voltage can be lowered by applying a higher dark current or allotting more times of dark current to the lamp. The investigation provides useful information for the design of the ignition circuit.

This research attempts to ignite metal halide lamps once with a single-pulse to avoid the problems of uncomfortable light strobes and irregularly high voltage and current stresses on circuit components caused by multiple strikes in conventional electronic ballasts. Metal halide lamps with single-pulse ignition, however, have difficulty in sustaining the lamp arc when operated with a low-frequency square-wave current. Experimental results indicate that the lamp exhibits an extremely small equivalent resistance as the electrode gap has being broken down. In this stage, the ballast has to keep the lamp current not declining to zero in the first half cycle. On the other hand, the lamp acts like open-circuited during commutation when driven by an alternating current. A sufficient energy from the ballast is needed to continue the arc in the next half-cycle. The transition waveform of the lamp arc current after being broken down is analyzed and the required energy for sustaining the lamp arc is calculated accordingly. Based on the investigation results, a starting scenario with appropriately designed circuit parameters for single-pulse ignition can be figured out. The starting scenario has been experimentally implemented on a 70 W metal halide lamp to demonstrate that the metal halide lamp can be successfully started up with single-pulse ignition.

A single-stage high-power-factor electronic ballast with auto frequency searching capability provides a compact and efficient solution for ballasting metal halide lamps. The circuit configuration is originated from the integration of a buck-boost converter and a half-bridge resonant inverter. The buck-boost converter is designed to operate in discontinuous current mode (DCM) to improve the input power factor and at the same time to regulate the output lamp power. The resonant inverter operating at a high frequency is adopted to obtain a high efficiency on the power conversion circuit. The control strategy of auto frequency searching is realized by a microprocessor along with the acoustic resonance detection circuit. To avoid the acoustic resonance, an auto-frequency-searching method is used to search ¡§quite windows¡¨ on operating metal halide lamps with the high-frequency electronic ballast. Provided the acoustic resonance should happen to the lamp on operation, the electronic ballast will automatically change the operating frequency until a stable frequency is located. When the operating frequency has been changed, the duty-ratio of the buck-boost converter is adjusted to regulate the lamp power at the rated value. Experimental tests are carried out on a laboratory with 70-W metal halide lamps to verify the effectiveness of the auto-frequency- searching control.

The acoustic resonance phenomena of metal halide lamps are investigated. A measuring system is set up to examine the effects of the acoustic resonance to the shape of the lamp arc, the lighting energy spectrum, the color temperature, the light output as well as electrical characteristics. Two ballast circuits are built to drive the lamps with the sine-wave current and the square-wave current, respectively. One ballast employs the series resonant inverter to output the sinusoidal lamp current. The other makes use of the full-bridge inverter to drive the lamps with the square-wave current. They are operated over a high-frequency range from 20kHz to 30kHz. For both ballast circuits, the operating frequency and the magnitude of the lamp current can be controlled independently. ¡@¡@The experiments are conducted on the 70-W metal halide lamps. The experimental results show that the spectral energy and the color temperature change more significantly as the acoustic resonance becomes more serious. These effects become less significant when the lamps are driven by the square-wave current. The degree of the acoustic resonance can be identified by detecting the variation of the lamp voltage or the output light. The investigated results of the thesis can provide useful information when a standard of the acoustic resonance is considered.

In this thesis, a novel electronic ballast that includes a repeatedly resonating ignition circuit is proposed for metal halide lamps. The proposed electronic ballast features a two-stage structure that comprises a power factor corrector and a full-bridge inverter used for current control, filtering and ignition. The full-bridge inverter consists of a leg operating at low-frequency with unidirectional switches and a leg operating at high frequency with bidirectional switches. The low-frequency side performs repetitive resonating on the load circuit with inductors and capacitors to accumulate a high voltage for ignition. Adjusting the duty-ratio of the high-frequency side allows for the regulation of the lamp current. The inductors and capacitors in the load circuit function not only producing the high ignition voltage but also filtering out high-frequency components, so that to drive the lamp with a low-frequency square-wave current. The proposed electronic ballast employing the full-bridge inverter with the specially designed control scheme and circuit parameters allows the metal halide lamp to tackle the demanding starting transient and steady state operation. With a simpler circuit structure and a reduced component count, the product cost will be much lower.

Abstract The metal halide lamp is one of high-intensity discharge lamps. It has many advantages such as good color rendering, high efficacy and a variety of color presentations. However, the problems of acoustic resonance, a long transition period of cold starting, and an extremely high ignition voltage for hot restarting should be overcome. The investigation of the thesis is focused on the electrical characteristics of the metal halide lamp operating at high-frequency. Included are the acoustic resonance, starting transient and steady state operation. Various ballast circuits incorporating with ignitors are designed to drive several 70-W metal halide lamps. The operating frequency ranges with acoustic resonance, the ignition voltages for both cold starting and hot restarting, the transition period, the dimming performance, and the lamp equivalent resistance during the lamp life are measured and analyzed. Based on the investigated results, a useful guideline can be provided for the operation and design of the electronic ballast for metal halide lamps. Key words : Metal halide lamp, electronic ballast, acoustic resonance, hot restarting.