Introduction
A cold cathode fluorescent lamp (CCFL) is a sealed glass tube filled with an 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 with low cost and high efficiency (power goes in and out)
Long life (>25K hours)
Stable and predictable operating brightness can be easily changed to light weight
In order to maximize CCFL efficiency, longevity and availability, it must take into account 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 vary depending on the CCFL module used in the application. However, the general trends described here apply to all CCFLs.
Temperature effect
As shown in Figures 1, 2 and 3, the operating characteristics of the CCFL are largely affected by temperature. At low temperatures, the lamp brightness drops very significantly (see Figure 1), and the voltage required to turn on the lamp (for example, 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 turned on.
Figure 1. Lamp-Brightness Temperature Dependency
Figure 2. Qihui voltage-temperature dependence
Figure 3. Self-heating brightness characteristics of the lamp
Lamp current
The CCFL efficiency is largely affected by the current waveform driving the lamp. Sinusoidal waveforms provide the best efficiency. Conversely, a non-sinusoidal waveform with a large crest factor is not a highly efficient CCFL drive signal. Figure 4 shows two current waveforms with similar RMS currents. Although a high crest factor waveform has the same RMS current as a 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 converting the electrical power into a luminance output is much reduced in systems operating under high crest factor waveforms.
Figure 4. Lamp Current - Waveform Comparison
DC offset is another waveform problem that must be considered when using CCFL. To reduce the possibility of mercury migration within the lamp, the lamp waveform must have a minimum DC offset.
The CCFL is designed to operate at a specific rated current, typically ranging from 3mARMS to 8mARMS. Figure 5 shows that reducing the lamp current reduces the brightness of the lamp, and increasing the lamp current increases the brightness of the lamp. Note that this relationship is not linear at higher currents. When approaching the nominal rated operating current, the brightness of the lamp varies in a nearly 1:1 ratio with the lamp current; however, at higher currents the ratio drops to less than 1:3. Therefore, it is important to operate the lamp close to its rated current, as operating at temperatures well above its rating will reduce lamp life. Similarly, in multi-lamp applications such as LCD TVs and LCD PC monitors, it is important to maintain the lamp close to the same current (eg, brightness) amplitude in order to provide uniform light spread across the LCD panel. In these multi-lamp applications, the individual lamp current amplitudes and waveforms must be accurately monitored and tightly controlled, otherwise the lamp will likely exhibit different brightness.
Figure 5. Lamp Brightness - Current Dependency
Lamp voltage
The CCFL operation and ignition voltage required to achieve optimum performance are based on lamp length and diameter. Figure 6 shows how the operating voltage is increased according to the length of the lamp. Lamps with smaller diameters require higher operating voltages.
Figure 6. Lamp Voltage-Length Dependency
The CCFL has an unusual characteristic that exhibits a 'negative resistance', that is, when the current increases (see Figure 7), the lamp voltage decreases. Negative resistance varies with the lamp, which causes different lamps to have different currents at any given voltage. Therefore, each lamp uses a separate transformer and current control circuit to achieve the most uniform lamp characteristics.
Figure 7. Lamp voltage vs. current
Light bulb
Before the ionization occurs, the impedance on the lamp is several megaohms; in a typical application, it can be seen as purely capacitive. At the beginning of the ionization, the current begins to flow into the tube, and its resistance quickly drops to a few hundred kilohms, which can be seen as purely resistive. To reduce lamp pressure, the start-up waveform should be a symmetric, linear sine wave ramp with no spikes. As noted above, the voltage required to start the CCFL varies according to the temperature (see Figure 2). Even at exactly the same temperature and bias conditions, the exact timing of the lamp's ignition is not highly repeatable and can vary by ±50%.
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