Analysis, Design and Experimentation of a Closed-Loop Metal Halide Lamp Electronic Ballast

This paper presents the analysis, design and experimentation of a closed-loop metal halide lamp ballast, which supplies the lamp with a low-frequency square waveform. First of all, a discussion related to the lamp parameter that should be regulated (power or current) is presented, since this issue has not been well addressed in previous literature. Experiments demonstrated that lamp current can be chosen to be kept constant during lamp life, because, on one hand, it can attenuate the effect of lumen depreciation with lamp ageing, and, on the other hand, the lamp power does not reach over-rated values during lamp life. In order to analyze the open-loop characteristic and design a suitable controller for the closed-loop operation, a dynamic model of the lamp-ballast system is developed,. Therefore, considering the dynamic of the lamp current sensor, a PI controller is proposed to minimize the steady state error and provide a phase margin of about 80°: (fully stable system). Experimental results using a flyback-based ballast supplied from a non-regulated DC voltage source (100 – 150V) are presented to validate the theoretical analysis. It is demonstrated that the lamp current is kept constant at rated value (0.42A) within the whole input voltage range. The time response of the complete closed-loop system is about 5 ms.