Diodeless start circiut for gas discharge lamp having a voltage divider

Abstract

A ballast circuit for a gas discharge lamp comprises a resonant load circuit incorporating a gas discharge lamp and including first and second resonant impedances whose values determine the operating frequency of the resonant load circuit. A d.c.-to-a.c. converter circuit is coupled to the resonant load circuit so as to induce an a.c. current in the resonant load circuit, and comprises first and second switches serially connected in the mentioned order between a bus conductor at a d.c. bus voltage and ground, and having a common switch node through which the a.c. current flows. A bridge capacitor has one end connected to ground. First and second feedback circuits regeneratively control the first and second switches, respectively, in response to a.c. current in the resonant load circuit. A starting circuit initiates operation of the first and second feedback circuits, and incorporates a voltage-divider network comprising first and second serially connected impedances with a common impedance node, and is coupled between the common switch node and ground. Such circuit includes a starting capacitor coupled between the common impedance node and ground, and a voltage-breakover switch coupled between a non-grounded end of the bridge capacitor and the starting capacitor. Also included in the starting circuit is a transformer winding serially coupled to the voltage-breakover switch so as to conduct a pulse of current when the voltage-breakover switch fires, the winding being coupled to the first and second feedback circuits so as to result in a starting pulse of current in the circuits when the voltage-breakover switch fires.