A circuit (20) for powering a load (50) includes a rectifier circuit (200), a voltage clamping circuit (300), and a DC-to-DC converter (400) such as a buck-boost converter. Voltage clamping circuit (300) is coupled between the rectifier circuit (200) and the DC-to-DC converter (400), and functions to prevent the input voltage (V.sub.IN) of the DC-to-DC converter (400) from exceeding a predetermined acceptable level, so as to protect the DC-to-DC converter (400) from damage due to transients in a voltage (V.sub.AC) provided by an AC line source (40). Preferably, voltage clamping circuit (300) is realized by an arrangement that includes a voltage divider circuit (320), a voltage sensing circuit (340), and an energy-limiting circuit (360), and is well-suited for use in power supplies and in electronic ballasts for powering gas discharge lamps.