| Summary: | 博士 === 臺灣大學 === 電子工程學研究所 === 96 === Piezoelectric transformer (PT) has the advantages of high power density, small size, high efficiency and low EMI. Moreover, it is suitable for the cold cathode fluorescent lamp (CCFL) backlight inverter because of the thin requirement of flat panel display (FPD) and load dependent voltage gain for CCFL striking. However, piezoelectric transformer has the drawbacks of the narrow bandwidth and high sensitivity. A specific feedback control is required for the piezoelectric transformer.
Phase control is beneficial on tracking the operating point of piezoelectric transformer. It fixes the phase delay of piezoelectric transformer by using a Phase-Lock-Loop (PLL), and the control phase is analyzed with the constant lamp voltage assumption. However, circuit delays in the control loop increase the control errors. Piezoelectric transformer has a large equivalent input capacitor and hence an inductor is usually used in series to generate lagging current for zero-voltage-switching (ZVS). This inductor and the input capacitor also create a band pass filter. As a result, it provides variant phase error near the operating frequency.
A quasi-modal piezoelectric transformer is introduced to take over the filter function. Thus a high order harmonics ZVS design is presented to achieve ZVS with smaller inductance. It provides the ZVS current with high order harmonics of input voltage. Thus the LC tank has less influence at fundamental frequency.
A control system for piezoelectric transformer based CCFL backlight inverter is designed. It includes a PLL for tracking the operating point and a high order harmonic ZVS design for small phase delay. Furthermore, burst mode control is utilized for dimming function. It can provide a wide dimming range with steady voltage gain and load for piezoelectric transformer. Finally, a start-up circuit utilizing the variant voltage gain of piezoelectric transformer is designed for the high striking voltage of CCFL. A control IC including these control strategies is designed for verification of the concept, and the conversion efficiency of the piezoelectric transformer based CCFL backlight inverter is about 85%.
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