Using pulse density modulation for controlling dimmable electronic lighting ballasts

摘要

Pulse Density Modulation (PDM) controls light brightness from a fluorescent lamp by applying voltages to the lamp filaments at two or more sequential signal frequencies. A low frequency, an intermediate frequency and a high frequency may be used to control the brightness of the lamp. The lamp gas ionizes to produce light only when the low or intermediate frequency voltage is applied thereto. The lamp gas is not ionized at the high frequency voltage, but the high frequency voltage keeps the lamp filaments warm during low brightness conditions. The low frequency, intermediate frequency, no and/or high frequency voltages have time periods that occur within a modulation frame time period that repeats continuously. The ratio of the low frequency and intermediate frequency time periods, and the no and/or high frequency voltage time periods determine the light output of the fluorescent lamp, and also maintain a proper temperature of the filaments.