Abstract
ABSTRACT Due to the rapid progress in the increasing size made in liquid crystal displays (LCD), many researches have been directed toward the study of adopting cold cathode fluorescent lamp (CCFL) as the back light source for large-panel LCDs. In recent years, winding transformers are widely used in the CCFL drivers. However, there are still some drawbacks remaining to be overcome. For example, the high turn-ratio implies worse insulation, higher temperature rise, and thicker dimension. In addition, the magnetic flux leakage also causes electromagnetic interference (EMI) problems. Hence, in this dissertation a high performance piezoelectric transformer (PT) based driver for CCFLs is proposed to overcome those drawbacks. First, a simple approach is proposed for obtaining the accurate parameters of the equivalent circuit model of a CCFL for simulation, analysis and controller design. A HP4194A instrument is used to obtain the parameters of the PT automatically. A simple modification is then made to correct only the corresponding equivalent turn-ratio whose value is obtained by applying the rated power to the PT. The other parameters are kept unchanged. Experimental results show that rather close agreement between the simulation and the experimental results can be achieved. Second, a PQ plane design oriented approach is proposed for designing the LC filter. Both the output real and reactive power capability as well as the effect of input DC voltage variations can be seen clearly from the PQ plane trajectories. This will obviate the trial-and-error design approach. Third, an automatic frequency selection circuit is proposed for controlling the switching of the driver such that a near optimal frequency can always be located without worrying about variations of PT and CCFL characteristics. This robust controller can not only keep the driver operated at the much better efficiency but also can increase greatly the yield of the PT drivers during mass production. Fourth, in order to overcome the thermometer effect of CCFLs, a bi-frequency dimming phase-shift controller is adopted. However, the frequency of the corresponding burst-mode signal should be restricted between 100Hz and 200Hz approximately. Fifth, to achieve an almost full range dimming control, a tri-frequency controller is proposed. It turns out that the frequency range of the corresponding burst mode signal can now be varied from 100Hz up to 3kHz approximately. The light intensity can be adjusted very smoothly and linearly, say, from 5% up to 100%. Excellent stability and nice efficiency, more than 84%, can be achieved simultaneously. Finally, for multi-CCFLs drivers, in order to achieve equal light intensity, a passive balance circuit is integrated with the filter circuit to reduce the component numbers and cost. Experimental results, up to four-CCFLs, verify the effectiveness of the proposed driver. In addition to the high performance, the proposed driver seems rather attractive for practical application.