Digitally Controlled Low-Frequency Square-Wave Electronic Ballast With Resonant Ignition and Power Loop

This paper proposes a digital controller for a low-frequency square-wave (LFSW) electronic ballast that includes the ignition sequence, a double control loop, and the selection of the positive and negative operation modes. The whole ballast is a two-stage circuit, where the first part is a power factor correction (PFC) stage and the second is a full-bridge (FB) converter used for both ignition and square-wave drive. Ignition is achieved by approaching the resonant frequency of the LC filter when the lamp is in the off state and the FB is working as a resonant inverter. After ignition, the converter operates as an LFSW inverter by controlling the FB to act alternately as a buck converter supplying positive or negative current. While ignition occurs at the LC filter resonance frequency (f_o = 20 kHz), the buck converter switching frequency (f_sw = 200 kHz) is selected significantly higher than f_o to attenuate high-frequency harmonics and avoid exciting acoustic resonance. Lamp stability is achieved by controlling the inductor current of the LC filter, and power mode control is achieved by adjusting the average current and voltage supplied by the PFC stage. The solution is experimentally validated for different types of 150-W high-intensity discharge lamps. A coupled-inductor single-ended primary inductance converter operating in continuous conduction mode is used for the PFC stage.