Cold-cathode fluorescent lamp circuits

A cold-cathode fluorescent lamp, or CCFL, is a type of fluorescent tube that produces light without heating its electrodes the way an ordinary fluorescent lamp does. Like other fluorescent lamps, it works by passing an electric discharge through a gas inside the tube, generating ultraviolet light that a phosphor coating converts to visible light. The “cold cathode” design uses electrodes that do not rely on a heated filament, which allows the tubes to be made thin and gives them a long operating life.

CCFLs require a special driving circuit because the lamp needs a high voltage to start the discharge and then a controlled current to run steadily. This is provided by an inverter, which takes a low direct-current supply and converts it into a high-frequency alternating output stepped up by a transformer to the hundreds of volts the lamp needs. Operating at high frequency keeps the transformer and other parts small and makes the lamp light efficiently and without visible flicker. The circuit must also limit and regulate the lamp current, since the discharge would otherwise increase uncontrollably.

For many years CCFLs were the standard backlight for liquid-crystal displays in laptops, monitors and televisions, where the thin tubes illuminated the screen evenly from behind or along an edge. They were also used in scanners and for decorative and instrument lighting. In most displays they have since been replaced by light-emitting diode (LED) backlights, which are more efficient and contain no gas, but CCFL inverter circuits remain a clear example of high-voltage, high-frequency lamp driving.

Frequently asked questions

Why does a CCFL need a special circuit?

It requires a high voltage to start the discharge and a regulated current to run, supplied by a high-frequency inverter and step-up transformer.

What were CCFLs mainly used for?

For many years they backlit liquid-crystal displays in laptops, monitors and televisions, as well as scanners and decorative lighting.

What replaced CCFL backlights?

Light-emitting diode (LED) backlights, which are more efficient and contain no gas, now dominate most displays.