| 摘要 |
<p>A ballast circuit for a gas discharge lamp of the type including resistively heated cathodes includes a resonant load circuit incorporating a gas discharge lamp (48) and including first and second resonant impedances (LR, CR) whose values determine the operating frequency of the resonant load circuit. Further included is a d.c.-to.a.c. converter circuit coupled to the resonant load circuit so as to induce an a.c. current in the resonant load circuit. The converter includes first and second switches (S1, S2) serially connected between a bus conductor at a d.c. voltage (VBUS) and ground (14), and has a common node through which the a.c. load current flows. A feedback circuit (RF) provides a feedback signal (VF) indicating the level of current in the resonant load circuit. A high voltage IC drives (16) the first and second switches at a frequency determined by a timing signal which predominantly comprises the feedback signal (VF) during lamp ignition, whereby during lamp ignition the feedback signal causes the high voltage IC to drive the first and second switches towards a switching frequency which promotes resonant operation of the resonant load circuit. A circuit (42) isolates the feedback signal from the timing signal for a predetermined period of time upon energizing of said converter circuit so as to allow the cathodes to become heated during such period of time, prior to lamp ignition. <IMAGE></p> |
