Abstract: A high frequency HID ballast includes an inverter for operating the HID lamp at high frequency and an arc instability controller which detects and adjusts the operating frequency of the inverter to avoid acoustic resonance of the discharge arc. The ballast ensures that the power delivered to the lamp remains substantially constant despite variations in the gain of the inverter which occur with changes in the operating frequency. Power control is accomplished by sensing lamp power and controlling a boost converter to vary the bus voltage feeding the inverter. The boost converter is also controlled during lamp ignition, when the load provided by the HID lamp is low, to clamp the bus voltage at a predetermined level. The ballast employs universal techniques suitable for operating HID lamps of different types, wattages and manufacturers despite the occurrence of acoustic resonance/arc instabilities among these lamps over a fairly broad frequency range.

Abstract: A method for detecting arc instabilities in a gas discharge lamp by evaluating the deviations of an electrical parameter of the lamp. A method suitable for detecting frequencies at which acoustic resonance induced instabilities occur evaluates the deviations measured over a range of frequencies. Selection of an operating frequency to avoid acoustic resonance is based on an evaluation of the deviations, such as the frequency at which the lowest standard deviation occurs for the electrical parameter measured. Favorably, the electrical parameter used is the conductivity, which provides high insensitivity to noise, is independent of ballast topology, and enables detection of acoustic resonance at levels not visible to the human eye. A control scheme tracks acoustic resonance during run-up of the lamp and quickly selects a favorable operating frequency after the lamp reaches steady-state. Resonance detection is achieved both during run-up and the steady state substantially without inducing visible flicker.