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How to Backlight an LCD

High-voltage ac lighting makes a low-voltage dc app work--go figure!

Randy Frank -- Design News, October 25, 2004

Power Trip: Power requirements for proper operation of the cold cathode fluorescent (CCF) tube that backlights a liquid crystal display (LCD).

Cold cathode fluorescent (CCF) tubes, the light sources used for backlighting the latest liquid crystal display (LCD) modules, require careful engineering to provide the performance and service life users expect. The power requirements for the LCDs themselves—which are used for flat panel TVs, computers, medical equipment, aircraft instrumentation, and even gas pumps—are reasonably straightforward since they use standard dc supply voltages, such as 5 or 12V. But the LCDs are transmissive and require a built-in light source transmitted from the rear of the display to read them.

Most LCDs use cold cathode fluorescent (CCF) tubes to backlight the display. These tubes are not unlike the fluorescent tubes used for office building lighting, just smaller in size. These narrow-diameter tubes are very bright and can be configured as single, double, or L-shaped edge lights.

Matching Power Matters

The CCF tube ultimately determines the readability of the display. Matching the power of the inverter to the tube is essential to a successful design, as mismatching can reduce life and even cause a catastrophic failure "The whole reliability is really dependent upon how long and how well that tube lasts," says John Peterson, president, Endicott Research Group, a maker of dc-ac inverters for powering backlit flat panel displays.

The starting or applied ac voltage required to ignite the gas in the tube is a shocking specification. A fluorescent tube may require 1,500V ac to start and then 700V ac to operate. If the minimum starting voltage is not provided, the tube will not start. The dc-ac inverter is a constant current source so an excess voltage cannot occur. With most of today's electronic designs, engineers work with low dc voltages. Packaging, creepage, and clearance distances are among the design considerations.

The minimum starting voltage can change under various circumstances with starting temperature being one of the more critical factors. "Old and cold" are critical terms, according to Bryon Cole, ERG's regional sales manager who has worked on many customer applications. When the lamp is new it may take considerably less voltage to start, but with age the voltage requirements can increase producing a worst-case scenario.

The inverter must have sufficient voltage to handle the voltage drop that could occur between the inverter and the tube. This is not trivial, since the operating frequency of the lamps is 40 to 50 kHz, and stray capacitance at these frequencies can significantly reduce the voltage from the inverter.

The impedance of a typical 2.5W, 6-inch CCF tube is 50,000 to 70,000V. Both voltmeters and oscilloscopes can load the output if they were used without considering their effect on the measurement. This means that a detailed analysis must be performed to avoid unexpected voltage drops. Sufficient margin is factored into the inverter's design under normal circumstances but also requires consideration by the user to avoid extreme cases. In many instances a bit of help from the high voltage supply experts is required to validate the layout of the boards in the application.

Engineers at ERG treat any problem in the design stage as an application issue. They identify stray capacitance as a cause of early failures 90 percent of the time. The capacitance can be from misplaced metal, wires too long, and, in general, problems for high voltage that are not problems for low-voltage designs.

Constant Current: Endicott Research Group’s DMA dc-ac inverter provides up to 12W for powering two CCF tubes. The two-transistor inverter is essentially a constant current ac source.
Efficiency Expert: An external pulse width modulated (PWM) control signal to the inverter (usually less than 1 kHz) controls the brightness of the CCF tube and provides efficient operation under dimming conditions.

Effeciency is Also the Key

Efficiency is another issue requiring the output current to precisely match the display's specification. Overdriving the display can produce very good image in the short term but the overall life is compromised. In some cases this meets the overall design objectives but in most cases reduced operating life is not acceptable.

For example, a Sharp 10.4-inch display specification provides a minimum life of 50,000 hrs with 6 mA but this drops to only 30,000 hrs if the current is 7 mA—just 1 mA higher. Complicating the issue, the current and frequencies measurements are very difficult since most customers do not have equipment to measure 5 mArms at 40 kHz. Using specially designed current probes and D-A converters, ERG validates that a particular inverter is a good match for a specific CCF tube.

No Strays: Testing the dc-ac inverter’s no-load starting voltage requires minimizing stray capacitance and any external loading that would reduce the inverter’s output.

Their test uses a non-contact current probe where the wiring is run through a transformer in the primary. The transformer generates a small voltage that is read by an oscilloscope, the signal is chopped into 512 pieces, a D-A conversion is performed, and the rms value is calculated. This allows current measurements without contacting the device since the contact would add stray capacitance and distort the reading. The inverter's circuit is designed to produce 5 mA so the measurement validates that this is performing properly in the application.

Another important design factor is observing that the waveform's distortion to minimize electromagnetic interference (EMI). The dc-ac inverter produces a pure sine wave but distortion occurs due to dynamics in the CCF tube. The current measurement also identifies if unacceptable distortion is present. Analyzing the pieces for the waveform has improved as the PC's performance has improved. Today ERG can compare point by point to a perfect sine wave at the appropriate frequency and generate a distortion factor so customers know in advance that their inverter matches the CCF tube requirements in their display.

Analyze This: The waveform (typical shown) and analysis of the output of the dc-ac inverter reveals both the critical current rating and the potential distortion.

Reach Contributing Editor Randy Frank at r.frank@ieee.org.

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