Observer Based Ceramic HID Lamp Control

Abstract

High intensity discharge (HID) lamps are typically operated at low frequency to avoid damage from acoustic resonance. Accordingly, an electronic ballast normally comprises a buck converter to control the lamp current magnitude, and a bridge to commutate the lamp current at a low frequency. These functions can be combined in a stacked buck converter [1]. The resulting system performance depends on the dynamics of the lamp as well as the ballast, the so-called lamp-ballast interaction [2]. Competitor lamps, production spread, reduced power operation and lifetime effects lead to a wide spread in lamp parameters. Some lamp-ballast combinations tend to be poorly damped, resulting in oscillatory lamp current. In such a system, the negative incremental lamp impedance may interact with the power electronic driver. Furthermore, lamp ageing and reduced power mode operation both tend to increase re-ignition voltage overshoot, which in turn may lead to reduced lifetime, or prematurely extinguishing lamps. Because of considerable non- linearities, feedback control based on the gas discharge electrical terminal quantities can improve the lamp-ballast performance only to a limited extent. Therefore, to bring the electronic HID lamp-ballast system performance a step further, feedback of the physical lamp states is proposed in this paper. However, direct feedback of the physical lamp states is not practical due to either economic or physical constraints. For this reason, to construct unmeasured system states using a known set of system states and parameters, a ceramic metal halide lamp observer is proposed. The HID lamp observer directly enhances system performance because it allows more sophisticated control schemes that make use of physical quantities like the electrode sheath voltage and cold spot temperature, which heretofore were not accessible. The ceramic metal halide observer is based on energy balance equations that include plasma and arc tube wall dynamics. Finally, simulations and practical measurements are included to validate the observer based ceramic metal halide lamp control.