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SUPER SLIM/ULTRA THIN DISPLAY LIGHT BOXES
|Byfort Research Group,Inc.|
Abstract: Cold Cathode Fluorescent Lamps (CCFL) are used as white-light sources to backlight liquid crystal displays (LCDs). CCFLs offer many desirable features, but they also have unique characteristics that must be considered to maximize their usefulness. This application note describes some of these unique CCFL characteristics.
Cold Cathode Fluorescent Lamps (CCFLs) are sealed glass tubes filled with inert gases. When a high voltage is placed across the tube, the gases ionize creating ultraviolet (UV) light. The UV light, in turn, excites an inner coating of phosphor, creating visible light. CCFLs have many desirable features, including:
CCFLs have some unique characteristics that must be considered for maximizing their efficiency, life span, and usefulness. This application note describes some of these CCFL characteristics. Note that the data shown here was collected on a particular CCFL, and that the specific data will change based on the CCFL model used in an application. The general trends described here, however, apply to all CCFLs.
The operating characteristics of CCFLs are strongly influenced by temperature, as shown in Figures 1, 2, and 3. At cold temperatures, lamp brightness drops significantly (see Figure 1) and the voltage required to initially strike (i.e., turn on) the lamps rises significantly (see Figure 2). As illustrated in Figure 3, the lamp exhibits a self-heating characteristic, which directly affects the lamp brightness after the lamp is struck.
CCFL efficiency is greatly affected by the current waveform driving the lamp. Sinusoidal waveforms provide the greatest efficiency. Conversely, nonsinusoidal waveforms with large crest factors are not efficient CCFL drivers. Figure 4 shows two current waveforms with approximately the same RMS current. Although the high-crest-factor waveform has the same RMS current as the sinusoidal waveform, its current excursions beyond the sinusoidal waveform's 150% peak level do not generate additional light, only heat. This means that the electrical power-in to light-out efficiency is greatly reduced in a system operating with a high-crest-factor waveform.
DC offset is another waveform that must be considered when using CCFLs. To reduce the possibility of mercury migration within the lamp, lamp waveforms must have minimal DC offset.
CCFLs are designed to operate at a particular rated current, typically in the 3mARMS to 8mARMS range. Figure 5 shows that decreasing the lamp current reduces the lamp brightness, and increasing the current increases the brightness. Note that this relationship is not linear for higher currents. Near the nominal-rated operational current, the lamp's brightness changes in relation to lamp current at almost a 1:1 ratio; at higher currents, however, this ratio drops to lower than 1:3. It is, consequently, important to operate the lamp near its rated current, because operation very far above that rate will reduce lamp life. Also, in multiple-lamp applications such as LCD TVs and LCD PC monitors, it is important to keep the lamps near the same current (i.e., brightness) level in order to provide uniform light diffusion across the entire LCD panel. In these multiple-lamp applications, the individual lamp-current levels and waveforms must be accurately monitored and tightly controlled, otherwise it is likely that the lamps will exhibit varying levels of brightness.
The CCFL operating and strike voltages required for optimal performance vary with lamp length and diameter. Figure 6 shows how the operating voltage increases with lamp length. Smaller diameter lamps require higher operating voltages.
An unusual characteristic of CCFLs is that they exhibit's negative resistance, which means that lamp voltage drops as current increases (see Figure 7). Negative resistance can vary between individual lamps, causing the lamps to have different currents at any particular voltage level. In multiple-lamp applications, therefore, the most uniform lamp performance will be achieved by providing individual transformers and current control for each lamp.
To create light, the gases
within the CCFLs must first be ionized.
Ionization occurs when a voltage, approximately
1.2 to 1.5 times the nominal-rated operating
voltage, is placed across the lamp for a few
hundreds of microseconds. Before ionization
occurs, the impedance across the lamp is in the
multimega ohm range; in a typical application,
it looks almost completely capacitive. At the
onset of ionization, current begins to flow in
the lamp, its impedance drops rapidly into the
hundreds of kilohms range, and it looks almost
completely resistive. To minimize lamp stress,
the striking waveforms should be symmetrical,
linear sinusoidal ramps without spikes. As noted
above, the voltage required to strike a CCFL
varies with temperature (see Figure 2). The
exact timing of the lamp strike is not highly
repeatable and can vary ?/span>50%, even under the
exact same temperature and biasing
Figure 1. Lamp-brightness temperature dependence.
Figure 2. Strike voltage-temperature dependence.
Figure 3. Lamp self-heating brightness characteristic.
Figure 4. Lamp current-waveform comparison.
Figure 5. Lamp brightness-current dependence.
Figure 6. Lamp voltage-length dependence.
Figure 7. Lamp voltage-to-current relationship.
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