Digitally controlled low frequency square wave electronic ballast with resonant ignition and power loop

The paper proposes a digital controller for a low frequency square wave (LFSW) electronic ballast implemented in a dsPIC30F2010 microcontroller 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 stage is a power factor correction (PFC) stage and the second is a full-bridge converter (FB) 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 a 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 (f(0)=25kHz), the Buck converter switching frequency (f(sw)=200kHz) is selected significantly higher than f(0) to attenuate high frequency harmonies and avoid exciting acoustic resonance. Lamp stability is achieved by controlling the inductor current of the LC filter, and power mode control is achieved adjusting the average current and voltage supplied by the PFC stage. A SEPIC converter operating in the continuous conduction mode (CCM) is used for the PFC stage. The microcontroller selects the different operation modes of the FB converter according to the lamp requirements.