In accordance with one embodiment of the present invention, there is provided a switch-mode power supply to generate the heating current for a hot-filament electron-emitting cathode. The power supply directly couples, without an output transformer, the output from a full-bridge converter that operates at an output frequency in the range from ten Hz to ten kHz. A connection to a reference potential that minimizes the potential fluctuation of the cathode is provided by one of the direct-current inputs to the converter.