Study On Digital Electronic Ballast For Low Power Metal Halide Lamp

With the implementation of green lighting project, gas discharge lamps have been widely used in many applications including: home lighting, outdoor lighting and automotive lighting. However, traditional electromagnetic ballast cannot satisfy the demand of modern lighting system, due to drawback including large size, heavy weight, low power factor, flicker, poor power regulation and sensitive to line-voltage dips. As the new efficient lighting electrical appliance, electronic ballast is the critical issue in the green lighting project.Digital electronic ballast for low power metal halide lamp is selected as the research subject of this dissertation and several problems such as basic lamp characteristic, topology, control method and ignition technology are addressed. Some novel methods and ideas are proposed based on analysis and experiment. The main contents and achievements are listed below.Combining the Half-bridge dual Buck topology with parallel-loaded resonant (PLR) circuit, a novel two-stage low-frequency-square-wave (LFSW) digtal electronic ballast with resonant ignition is proposed. Inverter operates in the mode of PLR duiring ignition stage and external ignitor is removed. Inverter operates in mode of Half-bridge dual Buck converter in steady state. Lamp voltage and current are low-frequency-square-wave. Lamp arc is stable without acoustic resonance. Operating principle and parameter design method are represented. To solve the problem of resonant parameter tolerance and inductor saturation, an adaptive iterating ignition soft start-up control strategy is proposed and the effect is satisfied.The key factor affecting the power desity is the passive elements. The most effective way of enhancing the power desity is to increase the operating frequency. However, the Buck converter operating in CCM mode is not applicable when operating frequency is beyond 100 kHz because of the reverse recovery of diode and EMI problem. Meanwhile, the MOSFET is in hard switch mode and the turn-on loss is significant. To solve these problems, an adaptive ZVS dual Buck quasi-square-wave (QSW) dual Buck converter is proposed. The principle of QSW is explained in detail. Accurate voltage gain transfer function, bound of ZVS and parameter design method are given. Adaptive ZVS control strategy guarantees the zero voltage turn-on of switch during the whole process the lamp operation and is effective on various lamps of different manufactures too. Microcontroller AT90PWM2 for lighting helps to improve the performance and simplify the hardware circuit. The prototype of 70W digital electronic ballast for metal halide lamp proves the validity of the proposed topology and control strategy. Operating frequency is beyond 100 kHz and the whole system efficency is 92%.The start-up process is complex because of the characteristic of metal halide lamp. Corresponding control method is applied to the lamp according to features of each stage. One cycle peak current control strategy is proposed to limit the excessive current during commutation period and run-up stage. One cycle peak current contrl strategy and adaptive ZVS control strategy play an important role in each stage. The adaptive ZVS control strategy can not only guarantee the zero voltage turn-on of switch but also satisfy the rapid response requirement in ignition transient state. The average input power control method based on the adjustment of turn-on time is simple and effective. The experiments with different lamps prove the feasibility and accuracy of constant power control strategy.Resonant ignition method enjoys the simple circuit and flexible control but it can not avoid the excessive current. Meanwhile, resonant ignition method is not proper for most of LFSW topology such as single stage topology. In practice, pulse ignition method is adopted in most of the LFSW ballast. The requirements for the pulse ignition circuit are introduced and a new dual frequency trigger pulse ignition circuit is proposed. Circuit model is analysised in detail and the influence of circuit parameters on pulse amplitude and width is given. Peak current is limited and amplitude variation caused by transformer parasitic capacitor torlance is restricted by the auxiliary inductor. The auxiliary inductor and ransformer parasitic capacitor form another resonant circuit. As a result, an igniting pulse sequence with two different frequencies is applied to the lamp and both pulse amplitude and width are guaranteed.