Low-frequency square-wave electronic ballast with resonant ignition using digital mode and power control

The paper proposes the use of a single full-bridge (FB) inverter and LC filter in HID electronic ballast, realizing both the functions of a resonant circuit for lamp ignition and a current controlled low frequency square wave inverter for normal operation. Ignition is achieved by resonance when the converter is controlled as a FB inverter at the resonant frequency of the LC filter. After ignition the converter operates as a LF square wave 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, the Buck converter switching frequency is selected significantly higher to attenuate high frequency harmonics and avoid exciting acoustic resonances. System stability is achieved by controlling the input current to the FB (output current of the PFC front-end), resulting in a power mode control provided that the voltage supplied by the PFC stage is constant. A digital control circuit selects the different operation modes of the FB converter according to the lamp requirements.