Lamp ballast circuit with cathode preheat function

摘要

A ballast circuit for a gas discharge lamp (12) contained within a resonant load circuit (16) has resistively heated cathodes (12A, 12B). A d.c.-to-a.c. converter circuit (14) supplies a.c. current to the resonant load circuit. The converter circuit comprises first and second switches serially connected between a bus conductor at a d.c. voltage and a reference conductor, and has a common node through which the a.c. current flows. In an arrangement for controlling the converter switches, a comparator circuit (18) compares a signal on a first input node with a periodic reference signal on a second input node, and produces a comparator output signal V22 that changes state when a first one of the compared signals becomes greater than the second of the compared signals, and that further changes state when the second of the compared signals then becomes greater than the first of the compared signals. A circuit generates the periodic reference signal VR in response to the comparator output signal. A first circuit (25) produces a signal on the first input node upon initial converter energisation, for preventing lamp ignition while the lamp cathodes become heated. A second circuit for producing a signal on the first input node for allowing lamp ignition comprises a feedback circuit (RF) for sensing a.c. current in the resonant load circuit and producing a feedback signal (VF) in proportion to the a.c. current. The feedback signal is coupled to the first input node after a predetermined period of time from initial energising of the converter circuit, during which period the lamp cathodes become heated.