Lamp power supply circuit with feedback circuit for dynamically adjusting lamp current

Claim 1

A ballast circuit for a gas discharge lamp, comprising: (a) a resonant load circuit incorporating a gas discharge lamp and including first and second resonant impedances whose values determine the operating frequency of said resonant load circuit; (b) a d.c.-to-a.c. converter circuit coupled to said resonant load circuit so as to induce an a.c. current in said resonant load circuit, and comprising first and second switches serially connected between a bus conductor at a d.c. voltage and ground, and having a common node through which said bidirectional load current flows; (c) an arrangement for allowing a user to select a setpoint signal during lamp operation that determines the amount of light output by the lamp; and (d) a feedback arrangement for regeneratively controlling said first and second switches; said arrangement including a circuit for sensing a.c. current in said resonant load circuit and producing an a.c. feedback signal in proportion to said a.c. current; a circuit for producing a d.c. feedback signal representing an average value of a lamp output parameter, and a circuit for generating an error signal between said setpoint signal and said d.c. feedback signal; said feedback arrangement further including: (i) a comparator circuit for comparing said a.c. feedback signal with a periodic reference signal, and for producing a comparator output signal 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; (ii) a circuit for generating said periodic reference signal in response to both said comparator output signal and said error signal; and (iii) a conditioning circuit receptive of said comparator output signal for controlling said first and second switches.