The elimination of the temperature effect on a piezoelectric transformer in a backlight inverter based on the phase-locked loop technique

A new method is proposed in this paper to eliminate the temperature effect on a piezoelectric transformer (PT) residing inside a half-bridge backlight module which utilizes the phase-locked loop (PLL) technique. The PT is adopted to drive a cold cathode fluorescent lamp (CCFL) to eliminate the downside of a conventional transformer and to reduce the dimension, particularly the thickness, of the backlight module. Next, a PLL is embedded in the backlight system, as a feedback mechanism, to track the optimal operating frequency of the PT so that the PT's temperature effect is resolved and, hence, the overall power efficiency, along with the system stability, is improved. The feedback variable proposed in this paper is a phase angle rather than a lamp current amplitude which is conventionally used. A simplified model and its design procedure are also introduced. The complete analysis and design considerations are detailed. Finally, it is encouraging to observe that the experimental results match our analytical solutions rather well.