Hid ballast circuit with arc stabilization

Abstract

A HID ballast includes a control circuit that minimizes arc instability associated with acoustic resonance of a lamp energized by the ballast. In one embodiment, the ballast includes an inverter circuit having first and second switching elements coupled in a half bridge configuration. A first control circuit controls the conduction state of the first switching element and a second control circuit controls the conduction state of the second switching element. The second control circuit includes a limiting circuit that limits the pre- and post-ignition energy levels to the lamp. One feature of the invention limits the lamp current amplitude such that the lamp can be driven while in acoustic resonance. In another embodiment, the inverter circuit includes a resonant inductive element and a resonant capacitive element which is coupled in parallel with the lamp. The resonant capacitive element has a temperature-dependent impedance characteristic so as to alter the resonant frequency of the inverter circuit as the circuit heats up prior to ignition of the lamp. As the resonant capacitive element heats up the resonant frequency of the inverter increases which increases a voltage applied to the lamp until an arc is initiated. Another feature of the invention limits the average voltage applied to the lamp before and after ignition. A further feature limits the voltage applied to the lamp on a cycle by cycle basis.