Patent ID: 8344644

Claim:
A ballast for a discharge lamp comprising: an inverter circuit functional to convert DC power to supply a rectangular wave AC power at inverter output terminals to power a discharge lamp when connected to the inverter output terminals; a ballast control circuit coupled to the inverter circuit and functional to control the inverter circuit; and the ballast control circuit comprising: a load voltage detection circuit coupled to the inverter output terminals and functional to detect load voltages of both positive and negative polarities when applied to a high-pressure discharge lamp, a comparison and selection circuit coupled to the load voltage detection circuit and functional to compare the detected load voltages of the positive and negative polarities and to select one of the positive and negative polarities, and a switching control circuit coupled to the comparison and selection circuit and functional to respond to the load voltage of the polarity selected by the comparison and selection circuit to control load current in one cycle after a period of detecting the load voltages of both the positive and negative polarities, wherein the load current is controlled as a function of the load voltage of the polarity selected by the comparison and selection circuit.