PWM control of electronic ballast for high-pressure Na lamps in comparison to fluorescent lamps. Introduction to quasi-optimum control

The design of electronic ballasts to drive high and low-pressure lamps should fulfil both the high-voltage necessities for the start-up and the current limiting requirements in steady-state operation. The full-bridge parallel resonant power converter reduces the current stress compared to the half-bridge power converter, so it is presented as a solution to drive lamps over 100 W. The analysis of the full-bridge control based on pulsewidth modulation is presented for HPS and fluorescent lamps. As the inverter input voltage is usually quite high compared to the lamp voltage requirements in steady-state operation, the quality factor of the resonant tank is low, resulting in an appreciable level of reactive current. The PWM dimming control at fixed frequency is proposed when the total current level is acceptable. In this case, while maintaining the transistors´ turn-on at zero voltage, the PWM control improves the power converter efficiency by reducing the reactive current. When the reactive current causes an important efficiency reduction, a quasi-optimum control method is proposed that consists of fixing, at each control point, the frequency vs. duty-cycle relation that minimizes the reactive current while maintaining soft turn-on