| Summary: | 博士 === 國立臺灣科技大學 === 電機工程系 === 97 === Fluorescent lamps are used generally in lighting domain. Due to the demand of energy saving and lighting quality, it is necessary to adopt the electronic ballast in the lighting fixture. Besides, in order to gain high power factor, the active power factor correction circuit is usually used in the electronic ballast. However, due to the considerations of cost-effective and market-competitive, the valley-filled PFC circuit is extensively applied in the small or compact fluorescent lamps. It has merits of high power, simple structure and low cost. However, it also makes the lamp has an over high current crest factor, which is harmful to the electrodes of lamp and shorten the lifetime of lamps.
In this dissertation, a PWM method to improve the lamp current crest factor of the valley-filled electronic ballast is presented. The proposed method controls switching duty ratio by feeding forward the dc-link voltage waveform of the valley-filled filter. It decreases the duty ratio with rising of dc-link voltage. Therefore, the proposed method can restrain the lamp peak current and improve the lamp current crest factor. The control block diagram and basic theory are introduced to explain the improving principle. Furthermore, the functions of lamp effective current and peak current can be calculated to gain the minimum current crest factor. The control circuit is simple, economy and unnecessary to use the feedback control and current sensor. Moreover, the proposed method can achieve soft-switching (zero voltage switching) in whole modulation range. Additionally, fixed switching frequency operation provides good filer effect and simplified EMI filter design. Finally, the computer simulation and circuit implementation verify the proposed method. The experimental results demonstrate the lamp current crest factor was reduced from 1.9 to 1.58, the ac line power factor is 96% and the input current THD is 29.3%.
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