An electronic ballast uses solid state electronic circuitry to provide the proper starting and operating electrical conditions to power discharge lamps. An electronic ballast can be smaller and lighter than a comparably-rated magnetic one. The ballast may be "potted" (filled) with a resin to protect the circuit boards and components from moisture and vibration. An electronic ballast is usually quieter than a magnetic one, which produces a line-frequency hum by vibration of the transformer laminations.
Electronic ballasts are often based on the SMPS topology, first rectifying the input power and then chopping it at a high frequency. Advanced electronic ballasts may allow dimming via pulse-width modulation or via changing the frequency to a higher value. Ballasts incorporating a microcontroller (digital ballasts) may offer remote control and monitoring via networks such as LonWorks, DALI, DMX512, DSI or simple analog control using a 0-10 V DC brightness control signal. Systems with remote control of light level via a wireless mesh network have been introduced.
Electronic ballasts usually supply power to the lamp at a frequency of 20,000 Hz or higher, rather than the mains frequency of 50 - 60 Hz; this substantially eliminates the stroboscopic effect of flicker, a product of the line frequency associated with fluorescent lighting (see photosensitive epilepsy). The high output frequency of an electronic ballast refreshes the phosphors in a fluorescent lamp so rapidly that there is no perceptible flicker. The flicker index is used for measuring perceptible light modulation ranges from 0-1, with 0 indicating lower possibility of flickering and 1 indicating the highest. Lamps operated on magnetic ballasts have a flicker index between 0.04-0.07 while digital ballasts have a flicker index of below 0.01.
Because more gas remains ionized in the arc stream, the lamp operates at about 9% higher efficacy above approximately 10 kHz.Lamp efficacy increases sharply at about 10 kHz and continues to improve until approximately 20 kHz. Trials are ongoing in some Canadian provinces to assess cost savings potential of digital ballast retrofits to existing street lights.
With the higher efficiency of the ballast itself and the higher lamp efficacy at higher frequency, electronic ballasts offer higher system efficacy for low pressure lamps like the fluorescent lamp. For HID lamps there is no improvement of the lamp efficacy in using higher frequency, but for these lamps the ballast losses are lower at higher frequencies and also the light depreciation is lower, meaning the lamp produces more light over its entire lifespan. Some HID lamp types like the ceramic discharge metal halide lamp have reduced reliability when operated at high frequencies in the range of 20 - 200 kHz; for these lamps a square wave low frequency current drive is mostly used with frequency in the range of 100 - 400 Hz, with the same advantage of lower light depreciation.
The original document from http://www.gpusi.com/resource/What-is-an-electronic-ballast-1366598587.html