Image Number 3 for United States Patent #5034663.
A power supply for a gas discharge lamp includes a current regulator for regulating the current supplied to a lamp and current steering means for steering current from the current regulator in alternating directions through an isolation transformer primary winding, the secondary winding of the transformer being connected with a lamp through an arc striking circuit. The frequency of alternating the current through the transformer is controlled as a function of the lamp voltage in order to regulate the volt-second product of the transformer through all portions of a lamp operating cycle including arc striking. This reduces saturation of the transformer and allows a much smaller transformer to be used. In a preferred embodiment, the current regulation and steering functions are performed in a bridge circuit having FET switches in each leg and in which the transformer primary spans the sides of the bridge. Pairs of FETs in diametrically opposite legs of the bridge are operated in unison, with one FET of each pair controlling lamp current by pulse-width regulation. The current is steered through the transformer primary by alternatingly operating the FET pairs at a frequency established by a voltage-controlled oscillator responsive to transformer voltage.