Power supply device

Abstract

A power supply device for a discharge lamp has a rectifier for rectification of an AC voltage from an AC voltage source into an output DC voltage, a smoothing capacitor connected across the rectifier for providing a smoothed DC voltage, and an inverter which receives the smoothed DC voltage to provides a high frequency output to the lamp. The inverter includes a composite resonant circuit with first and second resonant systems each including an impedance element. The lamp is connected across the impedance element of the first resonant system. A balancing circuit is included to combine the high frequency powers fed respectively through the first and second resonant systems in a direction of canceling the high frequency powers with each other to provide a combined high frequency power. A feedback arrangement is included to superimpose the combined high frequency power upon the output DC voltage of the rectifier so as to feedback the resulting power to an input of the inverter for flowing a charge current for the smoothing capacitor. The first and second resonant systems are arranged to vary an amount of the combined high frequency power provided from the balancing means in response to a varying load condition of the lamp for reducing the canceling effect at the balancing means as the high frequency power supplied to the lamp increases.