Backlight design for large LCD TV screens

Today’s “sweet spot” panels of greater than 30 inches are quickly moving to greater than 40 inches as manufacturing efficiencies and production line capabilities continue to increase. Consumer interest for larger screens presents manufacturers, and their design engineers, with new technical challenges to improve picture quality while reducing cost.

Larger LCD display backlight systems typically require 20 to 40 lamps, either cold cathode fluorescent lamps (CCFL) or external electrode fluorescent lamps (EEFL), to provide the high intensity illumination required to display full motion video. In today’s TV systems the backlight unit (BLU) represents from 30 percent to 50 percent of the total cost of the panel components.

To power these lamps, today’s backlight inverters must be able to drive backlight systems with high efficiency, low interference, reliable start up, reliable protection, and tight control of lamp current distribution under multi-lamp environments. To do so, backlight inverters must evolve continuously.

LCD Backlight Configurations
Most LCD backlights use straight lamps. In screens below 20 inches, lamps are normally mounted at the top and bottom edges of the panel, a low-cost solution. But issues of cross- coupling between the lamps, uniformity of illumination, and heat concentration grow as panel sizes increase. So the multiple lamps in larger LCD TV systems are spaced evenly behind the panel.

The wiring scheme also affects display performance and cost. In general, single-ended wiring is the most popular for screens below 32 inches, for lower cost. In this configuration, the drive voltage is applied only to the hot end of the lamps, with the other end grounded to the chassis.

Voltage on the hot end can be a common single phase, or multiple phases. Single phase offers simplicity in inverter design and lower cost. But the high voltage field interference on the LCD panel may result in “water wave” distortions. For higher picture quality, an interleaved two-phase design in which voltages between adjacent lamps are alternated by 180 degrees is preferred. In this arrangement the high voltage field is largely localized between two adjacent lamps, significantly reducing interference to the LCD display. Fig.1 shows a typical configuration.

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Figure 1 - Typical Multi-CCFL Architecture

As screen sizes increase, the single-ended lamp configuration becomes inadequate to maintain even brightness over the entire lamp--due largely to the increasing lamp length, driving voltage, and the parasitic capacitance along the lamp. The solution is a floating lamp structure in which the high voltages are applied to both ends of the lamp, with a 180-degree phase difference of their fundamental frequency.

There are other BLU configurations in practice today. U-shaped CCFL and EEFL are in mass production. However, as both require higher driving voltage than CCFL, their application is currently limited to screens below 32 inches.

Next: The Technical Challenges