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Thursday, 16 June 2011

A HIGH-POWER-FACTOR SINGLE-STAGE SINGLE SWITCH ELECTRONIC BALLAST FOR COMPACT

COMPACT fluorescent lamps (CFLs) were first introduced in the early 1990s, and are now gradually replacing conventional incandescent lamps in household and commercial lighting. The reason for the CFL’s increasing popularity is that it conserves energy, and subsequently, reduces energy cost when compared to traditional incandescent lamps. Fig. 1 shows the power consumption comparison between CFLs and different types of incandescent lamps. From Fig. 1, it is clear that in order to produce the same amount of light output, CFLs only consumes one-third of the power an incandescent lamp requires and that the CFL’s lifetime is thousand times that of an incandescent lamp [1]. The major difference between fluorescent lamps and incandescent lamps is that fluorescent lamps have negative resistance characteristics, which means that as the fluorescent lamp power increases, the lamp current increases with a decrease in the lamp voltage .

As a result, fluorescent lamps cannot be connected directly to the line, as in the case of incandescent lamps. A lamp current stabilization element called ballast is required in order to provide sufficient voltage for proper lamp ignition and to stabilize the lamp current once the lamp arc is established. To provide a compact and lightweight solution for CFLs, high-frequency electronic ballasts operating at higher frequency than 25 kHz are more suitable than magnetic ballasts. By operating at a higher frequency, the light efficacy can be increased by at least 20% and advanced dimming control can also be implemented with great flexibility. To minimize cost and to ensure that a compact electronic ballast circuit can be installed at the base of a CFL, commercial CFLs normally do not include a power factor correction (PFC) circuit in their electronic ballasts. Fig. 2 is a block diagram of typical electronic ballast used in a commercial CFL. It consists of a diode rectifier and a self-driven half-bridge parallel resonant inverter [2] & [3] with a dc-link capacitor connected in between to provide the required energy to the lamp. The major drawback of this type of circuit configuration is the highly distorted line current drawn at the input. The poor quality of the line current, when reflected back to the utility side, produces a large amount of unwanted harmonics and results in very poor power factor. Although, it has been reported in [4] that the high total harmonic distortion (THD) issue in the line current causes only little concern for the power quality when the CFLs are less than 25% of a building’s total load, this will become a more severe problem

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