Patent ID: 7180251

Claim:
A power circuit for starting and operating a gas discharge lamp with ac-voltage, wherein the power circuit is provided with: at least a first and a second lamp output terminal between which, in use, the gas discharge lamp is connected, a dc-voltage input, with at least two input terminals, a series connection of at least two switching elements such as, for instance, power transistors, which is connected between the input terminals of the dc-voltage input, a control circuit arranged for delivering control pulses to the switching elements for alternately and not in an overlapping manner bringing said switching elements into conduction, at least one lamp coil which is connected, on the one side, to the junction of said switching elements and which, on the other side, is connected with the first lamp output terminal, at least one resonance capacity, at least comprising one or more capacitors while the resonance capacity is connected, on one side, to the first lamp output terminal and, on the other side, is connected to one or both input terminals, at least one coupling capacity, at least comprising one or more capacitors, the coupling capacity having a capacity value which is considerably greater than the capacity value of the resonance capacity, the coupling capacity being connected on the one side with the second lamp output terminal and being connected on the other side with one or both terminals of said dc-voltage input and a variable frequency oscillator which, in use, delivers a signal to said control circuit with a frequency which is determinative of the control frequency with which said switching elements are brought into conduction periodically and not in an overlapping manner, characterized in that the power circuit is arranged such that for starting the lamp, the resonance frequency of the resonance circuit of the power circuit, formed by at least the lamp coil and the resonance capacity, divided by the frequency of the control pulses for alternately and not in an overlapping manner bringing said switching elements into conduction for starting the gas discharge lamp, is slightly smaller than an odd positive integer, preferably 3%–40% smaller, more preferably 10% to 25% smaller than the odd positive integer.