1. Field of the Invention
The present invention relates to a power conversion circuit for driving fluorescent lamps, such as, for example, cold cathode fluorescent lamps or hot cathode fluorescent lamps, and more particularly relates to a lamp inverter using square wave signals for more efficient operation.
2. Description of the Related Art
Fluorescent lamps are used in a number of applications where light is required but the power required to generate the light is limited. For example, fluorescent lamps are used for back lighting or edge lighting of liquid crystal displays (LCDs), which are typically used in display systems for flat panel computer monitors, notebook computers, hand held computers, LCD television, web browsers, automotive and industrial instrumentation, and entertainment systems. The fluorescent lamps in the display systems need to have long life and high operating efficiency.
A power conversion circuit is generally used for driving a fluorescent lamp. The power conversion circuit accepts a direct current (DC) input voltage and provides an alternating current (AC) output voltage to the fluorescent lamp. The power conversion circuit typically uses resonant drive methods, and the AC output voltage is a sinusoidal waveform.
One problem with a sinusoidal waveform is that lamp efficiency may be poor. Lamp efficiency in terms of light output versus power provided to the fluorescent lamp degrades with increasing lamp current crest factor. The lamp current crest factor is defined as a ratio of the peak lamp current level to the root mean square (RMS) lamp current level. The light output of the fluorescent lamp is proportional to the RMS lamp current level and is inversely proportional to the lamp current crest factor.
A pure sine wave has a crest factor of approximately 1.414. Many power conversion circuits with resonant topologies achieve lamp current crest factors in the range of 1.5 to 1.6. A pure DC waveform provides a lowest possible crest factor of 1.0. However, a DC lamp current is not viable because the operating life of the fluorescent lamp is shortened due to mercury migration.