A single-stage low-frequency electronic ballast for HID lamps featuring high power factor

This paper proposes a high power-factor electronic ballast for metal halide lamps. In the proposed circuit, two buck-boost converters and a buck converter are integrated with a full-bridge inverter of which the four active switches are shared. Two active switches are switched at high frequency, while the others are switched at low frequency. The buck converter is used to step down the dc-link voltage of the full-bridge inverter and filter out the high-frequency voltage components to drive lamps with a low-frequency square-wave (LFSW) voltage. The buck-boost converters are operated at discontinuous-conduction mode (DCM) to perform the function of power-factor-correction (PFC) to ensure nearly a unity power factor at the input line. By tactful arrangement of diodes, the dc-link capacitors are charged in series and discharged in parallel. It helps to operate the high-frequency active switches at a high duty ratio to have a small peak value of the inductor current of the PFC converters, and then smaller conduction losses. Detail operation modes and the design equations are provided. A prototype electronic ballast for a 70 W metal halide lamp is built and tested. Experimental measurements have proven that the proposed ballast has the advantages of high power factor, and free of acoustic resonance. By integrating the active switches of the converters and the inverter, the product cost of the proposed electronic ballast can be effectively reduced.