Improved current control for HID lamp drivers

HID lamps are typically operated at low frequency (100 Hz to 200 Hz) to avoid damage from acoustic resonance. Accordingly, an electronic ballast normally comprises a buck converter to control the lamp current and an ignition circuit and full-bridge for driving the lamp at low frequency. However, the negative dynamic lamp characteristic in combination with a power electronic driver may give a poorly damped response, often resulting in an oscillatory lamp current. Furthermore, aged HID lamps or reduced power mode operation tends to exhibit reignition voltage overshoot, resulting in reduction of lamp performance during life, or even extinguishing. In the present paper a procedure for obtaining the small-signal dynamic characteristics of metal halide lamps is proposed. Using this dynamic model the lamp-ballast interaction is simulated and analyzed. Then a fuzzy-logic control method is presented to cope with the non-linear behavior and improve the lamp-ballast performance. Finally, simulation results are verified with practical measurements on a laboratory prototype.