TECHNICAL

Cold-Cathode Flourescent Lamp Backlights

Operation and Construction

CCFLs work on the principle of glow discharge and require less current and dissipate less heat than the familiar hot cathode fluorescent lamps used in room lighting. CCFLs provide a high intensity uniform white backlight, and are commonly used on medium and high resolution graphics modules.

As CCFL technology evolved into thinner tube diameters, displays changed from direct backlighting to edge-lit designs. Edge-lighting uses a CCFL tube to inject light into the side of a polycarbonate waveguide. The waveguide effectively disperses the light through the principles of total internal reflection. When rays strike one of the diffuser dots on the front surface of the waveguide, light sctatters randomly creating a Lambertian point source. The strategic spacing of the diffuser dots creates a uniform backlight source. Various diffusion, reflector and luminance enhancement films are incorporated into the backlight design to increase the brightness.

Drive Method

CCFLs require a special inverter to create the kick-off voltage and to supply the high frequency sinusoidal drive current needed for proper operation. The four key CCFL parameters that must be considered for specifying an appropriate inverter are:

  • Starting voltage: This is the minimum voltage required to fire the tube ta the lowest operating temperature. It is important to know the worst case starting voltage, because as the tube ages and the temperature drops, the starting voltage increases. If the inverter cannot produce the necessary starting voltage, the CCFL will not light.
  • Operating voltage: This is the voltage across the tube when the lamp is functioning under the specified current levels. This voltage is expressed as a steady-state RMS voltage and reflects the physical characteristics of the tube. Since inverters are designed to function as constant current sources, some variation in operating voltage will be observed.
  • Operating current: This is the nominal RMS current as measured on the return side of the lamp by a current probe. This is the most important parameter of the lamp since it determines the luminous output, power consumption, and the expected life of the backlight. The brightness of the display varies linearly with operating current and the expected life varies exponentially with the operating current.
  • Operating Frequency: This is the frequency of the AC signal used in the drive circuit and can impact the brightness of the tube. Typically, the recommended operationg frequency lies between 20 and 50KHz. Higher frequencies are not converted to light ouput. So any high frequency spikes in the current waveform will be lost resulting in lower lumen-per-watt conversion efficiency.

Dimming 

Dimming may be required if a display system has to operate under a variety of lighting conditions. The simplest way to dim a CCFL backight is to vary the amplitude of the lamp current waveform. This technique is known as analog dimming and can typically provide modulation ratios of only 3:1.

For wider rdimming atios, pulse width modulation (PWM) techniques are used. In PWM dimming, the current to the CCFL is switched on and off at a set frequency. The lamps brightness varies with the duty cycle of the current "on-time". Possible problems associated with PWM dimming include backlight uniformity, perceivable flicker and EMI/RFI.

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