System and method for regulating an open circuit voltage of an HID lamp ballast

摘要

A lamp ballast is provided for igniting and operating an HID lamp. A buck inverter circuit includes a pair of high-frequency and a pair of low-frequency switches. A capacitor and an inductor are series-connected between a first node between the low frequency switches and a second node between the high frequency switches. A lamp igniting circuit is coupled between the first node and a first lamp output terminal, and a second lamp output terminal is coupled to a third node between the capacitor and the inductor. A control circuit turns on and off the switches to regulate low frequency square wave voltage generated across the lamp output terminals to be less than an input DC voltage from the positive rail, wherein the buck inverter operates in either of a critical discontinuous mode or a discontinuous mode during an open circuit operation, and in a critical discontinuous mode during other operations.