High frequency digital control electronic ballast of metal halide lamp for projector

A novel digital control theory for metal halide lamp electronic ballast used for projector is presented. The LCC resonant ignition strategy is adopted. Considering the tolerance deviation of the passive components, an adaptive digital control method for ignition which improves the reliability and reduces the cost of the system is proposed. Open-loop control is used in the transition stage during ignition to steadily increase the lamp power without large current inrush and voltage stress. A simple closed loop control method is presented to assure the reliable constant lamp power without any power over shooting. High-frequency modulation drive method is adopted to overcome the acoustic resonance problem commonly shown in metal halide lamp. Finally, a 150 W prototype was built in lab, and tests show that the electronics ballast can ignite reliably with a smooth transition stage; steady state constant power can be guaranteed. Total efficiency of the system as high as 93% is achieved without acoustic resonance.