A new approach is proposed in this paper for the tracking of the optimal operating frequency in a Class E backlight inverter using the phase-locked loop (PLL) technique. First, a new single-stage backlight module is introduced to simplify the circuit and to raise the system efficiency. A piezoelectric transformer (PT) is used to drive the cold cathode fluorescent lamp (CCFL) to eliminate the downside of a conventional transformer and to reduce the dimension 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 removed and, hence, the system efficiency and stability is improved. The feedback variable proposed is a phase angle rather than a lamp current amplitude traditionally used. A simplified model, along with its design procedure, is next presented. The complete analysis and design considerations are detailed. Finally, it is rather encouraging to observe that the experimental results match our analytical solutions closely.

Low-profile inverter power supplies are increasingly required for backlight systems of liquid crystal displays (LCDs). A great deal of attention has been focused on the application of piezoelectric transformers (PTs) to such power supplies. To miniaturize PT inverters still further, PTs need to have sufficient high voltage-step-up-ratio, which can be achieved by a multilayered PT. First, this paper describes a method for simulating such performance using a distributed constant equivalent circuit model. The results of the simulation for a multilayered PT operated in the third order longitudinal vibration mode show that the resistance of internal electrodes causes the dominant loss factor. Next, a power inverter incorporating the multilayered PT was fabricated. This power inverter can be operated over a wide input DC voltage range from 7-20 V. Regarding a conventional inverter drive circuit, when input DC voltage range was extended, the inverter efficiency remarkably decreased. For the reason, we developed a new inverter circuit, which is equipped with an automatic drive voltage control circuit to maintain the drive voltage to the PT at a constant value. As a result, the fabricated power inverter exhibited more than 90% overall efficiency and 3.5 W output power, which is enough to light up a 12.1-inch color LCD. The maximum luminance efficiency on a light transmission plate of the backlight was as high as 30 cd/m2/W.

Steady-state characteristics of the push-pull inverter with a piezoelectric transformer are analyzed. The piezoelectric transformer operating in the 3rd-order longitudinal vibration mode is used in place of a conventional magnetic transformer to produce a high output voltage to light up a cold cathode fluorescent lamp. The circuit operation, the load characteristics, the efficiency and the ZVS conditions are analyzed using equivalent circuits. These analytical results are confirmed by experiments. An example of the output current control is also shown.

Low-profile, miniaturized and highly efficient power inverters are required to light up backlights, which include cold cathode fluorescent lamps (CCFLs), in color liquid crystal displays (LCDs), replacing conventional power inverters with electromagnetic transformers. The object of this study is to actualize a power inverter to which a novel multilayered piezoelectric ceramic transformer operating in the third order longitudinal mode is applied. The piezoelectric transformer has a symmetrical structure in the lengthwise direction and its generating part operates in a piezoelectric stiffened mode in order to increase both energy conversion efficiency and power density. This transformer has great advantages. Namely, all the electronic terminals in this transformer can be connected at the vibration nodes of the transformer, which contributes to the guarantee of stable transformer performances at high power operation, and this transformer is superior in impedance matching against the backlight load at steady state, because the output impedance of this transformer is much lower than that of conventional Rosen type transformers. Then a power inverter with the transformer was fabricated. In this power inverter, a separately excited oscillation circuit was adopted to drive the transformer with high efficiency, and the transformer drive frequency was controlled by detecting the backlight current in order to adjust the backlight luminance properly. As a result, the fabricated power inverter exhibited more than 90% overall efficiency and 4. 5-W output power, which is enough power to light up a 9. 4 inch color LCD, including the stray capacitance loss resulting from CCFL mounting. The luminance value on a light transmission plate of the backlight was more than 2000 cd/m2.

A piezoelectric transformer (PT) converter with PWM control is presented. The combination of an active-clamp circuit and a resonant circuit makes it possible to control the output voltage of the PT converter with PWM at a constant switching frequency. The PT converter circuit is evaluated using an AC analysis, and a design procedure is presented. The PT converter implemented on a printed circuit board is experimentally evaluated and a good controllability is successfully achieved.

A power converter with a new piezoelectric transformer is presented. The piezoelectric transformer, made of lead titanate solid solution ceramic, is operated with a thickness extensional vibration mode. This transformer can operate at high frequency, over several megahertz, with about 90% high efficiency. The resonant frequency for the transformer is 2 MHz. The power converter with the transformer applies the theory for a class-E switching converter using an electromagnetic transformer. Maximum output power was obtained when the switching frequency was slightly higher than the resonant frequency. 4.4 W output power was successfully obtained with 52% efficiency at 2.1 MHz switching frequency.