Abstract
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 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> 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 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> filter resonance frequency ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> = 20 kHz), the buck converter switching frequency ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sw</sub> = 200 kHz) is selected significantly higher than <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> to attenuate high-frequency harmonics and avoid exciting acoustic resonance. Lamp stability is achieved by controlling the inductor current of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> 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.
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