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Introduction Cold cathode fluorescent lamps (CCFL) are sealed glass tubes filled with inert gas. When a high pressure is applied to the tube, the gas ionizes to produce ultraviolet (UV) light. The UV light excites the internal phosphor coating to produce visible light. CCFL has many very good features, including: Excellent white light source, low cost, high efficiency (power input to light output), long life (> 25K hours), stable and predictable operation, brightness can be easily changed, light weight, to make CCFL efficient, To maximize life and usability, we must consider some of its unique characteristics. This application note describes some of these CCFL features. It should be noted that the data given here is collected on a specific CCFL, and the detailed data will change depending on the CCFL module used in the application. However, the general trends described here apply to all CCFLs.
The influence of temperature is shown in Figures 1, 2, and 3. The operating characteristics of CCFL are largely affected by temperature. At low temperatures, the brightness of the lamp decreases significantly (see Figure 1), and the Voltage required to start the lamp (eg, turn on) rises significantly (see Figure 2). As shown in Figure 3, the self-heating characteristics exhibited by the lamp will directly affect the brightness of the lamp after the lamp is started. 
Figure 1. Lamp-brightness temperature dependence 
Figure 2. Enlightenment voltage-temperature dependence 
Figure 3. Lamp self-heating brightness characteristics. Lamp current CCFL efficiency is largely affected by the current waveform driving the lamp. The sinusoidal waveform provides the best efficiency. Conversely, a non-sinusoidal waveform with a large crest factor is not an efficient CCFL drive signal. Figure 4 shows two current waveforms with similar RMS currents. Although the high crest factor waveform has the same RMS current as the sinusoidal waveform, its current drift beyond the 150% peak amplitude of the sinusoidal waveform does not produce additional light, but only heat. This means that the efficiency of a system operating on a high crest factor waveform whose electrical power is converted into brightness output is much reduced. 
Figure 4. Comparison of lamp current waveforms
DC offset is another waveform issue that must be considered when using CCFL. To reduce the possibility of mercury migration in the lamp, the lamp waveform must have a minimum DC offset.
CCFL is designed to operate at a specific rated current, with a typical value range of 3mARMS to 8mARMS. Figure 5 shows that reducing the lamp current will decrease the lamp brightness, and increasing the lamp current will increase the lamp brightness. Note that this relationship is not linear at higher currents. When the nominal rated operating current is approached, the brightness of the lamp changes in a ratio of almost 1: 1 with the lamp current; but at higher currents the ratio drops to less than 1: 3. Therefore, it is important to operate the lamp at near its rated current, because operation far beyond its rated value will reduce lamp life. Similarly, in multi-lamp applications such as LCD TV and LCD PC monitors, in order to provide uniform light diffusion across the LCD panel, it is important to keep the lamps close to the same current (eg, brightness) amplitude. In these multi-lamp applications, the current amplitude and waveform of a single lamp must be accurately monitored and strictly controlled, otherwise, it is likely that the lamps will exhibit different brightness. 
Figure 5. Lamp brightness-current dependence. The CCFL operation and ignition voltage required by the lamp voltage to achieve the best performance is determined by the lamp length and diameter. Figure 6 shows how the operating voltage increases according to the lamp length. Lamps with smaller diameters require higher operating voltages. 
Figure 6. Lamp voltage-length dependence
CCFL has a non-ordinary characteristic that behaves as 'negative resistance', that is, when the current increases (see Fig. 7), the lamp voltage decreases. Negative resistance varies from lamp to lamp, which causes different lamps to have different currents at any particular voltage. Therefore, using a separate transformer and current control circuit for each lamp will achieve the most uniform lamp characteristics. 
Figure 7. The relationship between lamp voltage and current. The gas in the lamp CCFL must be ionized before it can emit light. Ionization occurs when the voltage on the lamp reaches approximately 1.2 to 1.5 times the nominal rated operating voltage and lasts for a few hundred microseconds. Before ionization occurs, the impedance on the lamp is in the order of several megaohms; in typical applications, it can be considered almost purely capacitive. At the beginning of ionization, the current began to flow into the lamp, and its resistance quickly fell to hundreds of kiloohms, which can be regarded as purely resistive. In order to reduce the lamp pressure, the start waveform should be a symmetrical, linear sine wave ramp without spikes. As noted above, the voltage required to start CCFL varies according to temperature (see Figure 2). Even under the same temperature and bias conditions, the exact timing of the lamp's ignition is not highly repeatable, and it can vary by ± 50%.
CCFL features
Abstract: Cold cathode fluorescent lamps (CCFL) are used as white light sources to provide backlight for liquid crystal displays (LCDs). CCFL can provide many good features, but also consider their unique characteristics in order to maximize their usability. This application note describes some unique characteristics of CCFL.