Discharge lamp driving circuit

Abstract

A circuit for operating a discharge lamp at a low frequency AC voltage while repetitively interrupting at a high frequency the voltage component to be applied to the lamp. The circuit includes at least one switching transistor for repetitively interrupting the voltage to be applied to the lamp at the high frequency, for example, 40 KHz so as to allow the use of a light and less bulky inductor as the current limiting conductor to be connected in series with the lamp. A bridge inverter is provided to have at least one pair of switching transistors for alternately reversing a DC voltage to apply the resulting AC voltage to the lamp at the low frequency, for example, 100 Hz low enough to stably operate the lamp without suffering from acoustic resonance. The high frequency component is bypassed through a bypass capacitor connected in parallel with the lamp. By reason of the inverter producing the low frequency AC voltage, the circuit requires all-off period during which the switching transistors of the bridge inverter are simultaneously off for preventing the short circuiting of the power source. An oscillation-inducing inductor is connected in series with the lamp in parallel relation to the bypass capacitor so as to form a series oscillating circuit which causes to flow an alternating oscillating current to the lamp circuit throughout the all-off period. The resulting oscillating lamp current continues for a longer time to thereby retard the deionization of the lamp. Accordingly, the all-off time can be extended to such an extent as to give greater flexibility in designing the circuit which is free from short circuiting of the power source as well as from the increased reignition voltage.