James R. Phillips

Ultracompact LCD backlight inverters: A svelte beast cuts high voltage down to size

The liquid crystal display (LCD) has become ubiquitous. The LCD utilizes a cold cathode fluorescent lamp as a light source to back light the display. The CCFL requires a high voltage AC supplies for operation. Typically, over 1000 V root mean square is required to initiate lamp operation, with sustaining voltages ranging from 200 to 800 V AC . Thus the ability to create devices of varying applications and shapes is greatly enhanced by the use of multilayer piezoelectric transformer (PZT) ceramics. PZT technology is not new. It has been employed before in various forms. PZT-like magnetic devices are basically energy converters. A magnetic transformer operates by converting an electrical input to magnetic energy and then reconverting the magnetic energy back to an electrical output. A PZT has an analogous operating mechanism. It converts an electrical input into mechanical energy and subsequently reconverts this mechanical energy back to an electrical output. The mechanical transport causes the PZT to vibrate, similar to quartz crystal operation, although at acoustic frequencies. The resonance associated with this acoustic activity is extraordinarily high; Q factors over 1000 are typical. This transformer action is accomplished by utilizing properties of certain ceramic materials and structures. A PZTs voltage gain is set by its physical configuration and the number of layers in its construction. This is obviously very different from a magnetic transformer, although some magnetic analogs are turn ratio and core configuration. Also very different and central to any serious drive scheme attempt is that a PZT has a large input capacitance, as opposed to a magnetic transformers input inductance.