Microcontroller power mode stabilized power factor correction stage for electronic ballast applied to metal halide lamps

This paper presents the design considerations, control strategy and resulting performance of a two-stage ballast system; power factor correction (PFC) and resonant inverter (RI), for metal halide lamps (MHL). Medium and long term stability, with regard to utility line perturbations and lamp aging, is achieved by the PFC stage controlled in power mode, while the short term stability is assured by the high output impedance of the LCC inverter, operating in open loop at constant switching frequency. Power mode control requires over voltage and short circuit protection. The LCC resonant inverter is designed in such a way that the maximum power supplied to the lamp is inherently limited, provided that the input voltage is also limited. Hence, the protection requirements are reduced to limit only the maximum voltage supplied by the PFC stage to the inverter stage. By boosting the PFC output voltage with respect to the nominal value during the protection mode two benefits are achieved: 1) the ignition performance using the resonance of the LCC circuit is improved, since it is not necessary to reach the highest voltage gain frequency which is more sensitive to frequency or component tolerance. Hot re-ignition performance is also improved since the required voltage gain is also less sensitive to component thermal deviation. 2) The warm-up time is clearly reduced in comparison to electromagnetic ballast or other non-controlled ballast since the RI input voltage increases when the lamp power is below the target value to reach the power level imposed by the reference