A comparative study of random switching schemes for eliminating visible striations in fluorescent lamps

This paper presents a comparative study of using random switching modulation schemes for the resonant inverter in electronic ballasts to eliminate visible striation (appearance of black and white bands along the tube) in fluorescent lamps. As striations can be eliminated by superimposing a small amount of dc current or low frequency ac current to the electrodes to produce composite current waveform through the lamp, the underlying principle of this work is based on the fact that random switching introduces low-frequency harmonics at the inverter output. Although this inherent low-frequency generation characteristics is generally undesirable for switching converters requiring high quality output, it provides a favorable means to eliminate striations in this particular application. Modulation schemes under investigation include random pulse position modulation, randomized pulse width modulation (RPWM) and randomized carrier frequency modulation. Issues addressed include the effectiveness of each scheme in eliminating the striations and the randomness level on lamp flickers. Among the schemes, RPWM is found to be the best choice as it introduces largest amount of low-frequency lamp current. The validity of the analyses Is confirmed experimentally by using a T8 36W prototype.