Stability analysis of the three-stage electronic ballast for metal halide lamp

This paper presents a system stability analysis of the three-stage electronic ballast for use in metal halide lamps. After the ignition of this type of lamp, the lamp impedance dramatically drops from an open circuit to a few tens of ohms, which causes the arc in the lamp to extinguish and thus results in ignition failure. This issue will be analyzed from the standpoint of the interactions between different stages in the ballast. For the three-stage electronic ballast, the overall system stability margin can be determined by the impedance-ratio criterion (minor loop gain) between each of the three stages. To simplify the analysis, the DC/AC inverter and the high-intensity-discharge (HID) lamp in the three-stage ballast are modeled together by an equivalent resistance. Therefore, the small-signal model of the three-stage electronic ballast is derived by applying the three-terminal pulse-width modulation (PWM) switch model. Finally, the derived small-signal model is verified by simulations in SIMetrix/SIMPLIS.