Comparing magnetic and piezoelectric transformer approaches in CCFL applications

Cold cathode fluorescent lamps (CCFLs) are commonly used as a backlight source for color liquid crystal displays (LCDs) used in notebook computers and portable electronic devices. These lamps require a high ac voltage for ignition and operation. The required ignition voltage is typically double the operating voltage and increases at cold temperatures. Figure 1 shows the operating voltage of lamps with 3-mm diameters and various lengths (100 mm, 150 mm, and 250 mm). Lamp voltage is primarily dependent on length and is fairly constant with current, giving a nonlinear characteristic. Lamp current is roughly proportional to brightness or intensity and is the controlled element of the backlight supply. The lamp requires a sinusoidal voltage to provide the best electrical-to-optical energy conversion. The lamp and display enclosures used for next-generation portable products are becoming increasingly narrow, generating the need for a low-profile CCFL power supply. Advances in both magnetic and ceramic piezoelectric transformers (PZTs) have enabled efficient and smaller backlight converters to be built. The choice of transformer depends on several factors including cost, size, and efficiency. For example, a magnetic transformer may be thicker, heavier, and less efficient than a PZT at a particular power level; but it has the advantages of lower cost and the ability to function over a wider range of load conditions. The PZT transformer has the advantages of inherently sinusoidal operation, high strike voltage (which can By Eddy Wells Power Management Products Systems Specialist