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A digital control architecture is presented for electronic ballasts that provides a phase sweep for reliable, soft lamp ignition and a smooth transition to lamp current regulation mode. The controller is based on an inner phase loop for fast regulation of the resonant tank operating point and an outer current loop for lamp current regulation. The inner loop operates on a simple digital control law that computes the required gate timing relative to the inductor current positive zero crossing. Phase control provides reliable drive of the resonant converter in the presence of large dynamic changes in the load impedance during lamp ignition and warm up and natural tracking of component variations with temperature and time. The primarily digital approach provides programmability for broad application, insensitivity to process and temperature variations, realization in low cost CMOS processes and few external components. Experimental results are presented for an integrated ballast controller fabricated in a 0.8 mum CMOS process used in a 400 W, 150 kHz HID electronic ballast.
Date of Publication: Jan. 2008