Cold cathode fluorescent lamp power supply based on piezoelectric transformers

This work discusses design and implementation of a switching power supply for low power cold cathode fluorescent lamps (CCFLs), used in high brightness, back-lighted, liquid crystal displays (LCDs). The power supply design is based on piezoelectric transformers. The use of piezoelectric transformers (PZTs), instead of conventional magnetic ones, allows a significant reduction of the converter´s size and component count. The particular features of the piezoelectric transformer allow the designer to directly connect the lamp to the secondary side of the device without the need for the typical ballast capacitor. Besides, the start up over-voltage is automatically generated with no need for input voltage amplitude or frequency modulation. On the other hand, the peculiar PZT secondary side arrangement and its high voltage, low current operation, call for particular care in the implementation of the LCD dimming circuit. The paper describes the different power supply components, particularly focusing on the PZT characteristics, outlines the design procedure for the output stage of the power supply (i.e. a resonant half-bridge inverter) and discusses the LCD brightness control strategy. Experimental results coming from a preindustrialised prototype of the converter are presented.