CCFL Inverters

A CCFL inverter is a module or a device in a circuit that provides the power supply to a cold cathode fluorescent lamp (CCFL). It is called an inverter because it is supplied by a direct current (DC) source, and provides a different alternating current (AC) voltage output to drive the CCFL. This is usually implemented using a resonant mode power supply called a royer circuit. They are designed to have high efficiency (typically over 80%) as the typical application for CCLF is backlighting for LCDs in battery-powered equipment. An example of the type of product CCFL inverters are required in is a notebook laptop. They can also have additional circuitry to support the control of LCD contrast and backlighting brightness. This is usually provided by integrated digital to analog converters (DACs).

CCFL Tubes require reasonably high power levels that need to be adjustable to manage brightness (100mW to 6W are typical levels). CCFL backlighting tubes require high voltages adjustable AC power sources. This is provided by the CCFL inverter having a self-oscillating, parallel resonant circuit that is currently fed by the back-light controller. The royer oscillator has a feedback circuit that comprises of a diode rectifier that is filtered to generate a DC voltage directly proportional to the oscillating voltages average value. This is divided down and sent to the error amplifier that then uses the error to vary the tube current from the minimum to the maximum. The set point of the error amplifier (reference) is controlled by an inbuilt digital to analog converter (DAC) in the controller.

An inverter requires a transformer; the inverter circuit has gone through several iterations over the years as the use of the secondary side of the transformer to improve the efficiency of the design. Past generations did not use resonance at all; fundamental resonance circuits were eventually adopted once the royer circuit was invented. The latest resonant mode circuits actually utilize a resonant mode of the secondary side of the transformer that is three times that of the primary, known as a third harmonic resonant circuit. This allows lower cost transformers to be used, and allows for flat shape transformers to be utilized.

CCFL tubes have a strike voltage required to start the tube, which can be up to 1500Volts. This is generated by the step-up ratio of the transformer. Great care should be taken with these circuits, as they are very dangerous.
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