US Patent 7355904 Method and apparatus for drain pump operation

A method and apparatus are provided for improved noise reduction from switching on and off drain pumps (202) in a high voltage generator. The drain pumps (202) are divided into groups (204) and activation of the groups (204) of drain pumps (202) is staggered (304, 310). In addition, when drain pumps are switched on and off for power conservation or to maintain a steady state high voltage level, the groups (204) of drain pumps (202) are switched on and off in response to various predetermined high voltage levels (410, 412, 414, 416), with different voltage levels for different groups (204) of drain pumps (202).