Self-oscillating dimmable gas discharge lamp ballast

Abstract

A ballast includes a resonant load circuit having a resonant inductance and a resonant capacitance. The load circuit couples a.c. current to a gas discharge lamp. A self-oscillating complementary converter circuit of the ballast induces a.c. current in the resonant load circuit. The converter circuit includes a pair of switches serially connected between a d.c. voltage bus and a reference bus. Respective reference nodes of the switches are interconnected at a common node through which the induced a.c. current flows, and respective control nodes of the switches are substantially directly interconnected. A gate drive circuit controls the switches. The gate drive circuit includes a drive winding connected to the control nodes. The drive winding is mutually coupled to the resonant inductance for sensing current therein, and an inductor is serially connected to the drive winding and to the common node. A clamping circuit is directly coupled across terminals of the inductor of the gate drive circuit and controls voltage across the inductor in response to an error signal. The error signal represents the difference between a user-selectable signal and a feedback signal that represents a time-averaged value of a lamp operating parameter. The ballast circuit of the invention allows a user to adjust the output of the lamp while it operates. The ballast circuit improves over conventional ballast circuits by eliminating a bulky mutually coupled winding, thereby reducing ballast mass and volume.