CCFT parameters
The following are the main parameters of a cold cathode fluorescent tube that have to be considered when designing the driving d.c.–a.c. inverter or in replacing the tube:
● Starting voltage (typical values 2000–3000 Vpeak): Also known as the dis- charge voltage, the starting voltage is the minimum voltage required to ignite, i.e. start the tube. The starting voltage is usually 50% higher than its operating voltage. The starting voltage is the primary parameter which determines the ‘end of life’ for the tube. The older is the tube, the higher is its starting voltage.
● Operating voltage (typical values 2500–3500 VP-p): This is the voltage across the tube when it has been lit. It is a key parameter in the design of the d.c.–a.c. inverter.
● Tube current: The current through the tube determines, to a large extent, its brightness. It also indirectly determines the tube’s useful life. In gen- eral, the tube’s life is the square of its current. If the current increases by 20% above its normal value, its life span decreases by 40%. Higher than normal current also results in excessive heat.
● Frequency (typical value: 40–60 kHz): Frequency generally has no effect on the brightness of the tube, its efficiency or its useful life. However, it does have an impact on the compatibility between the tube, the display itself and the graphic information displayed by the tube.
● Waveform: An undistorted current and voltage sinusoidal waveforms are required to avoid radiated electric noise that may impact on the sys- tem and surrounding environment introduced by a distorted sine wave. Although the a.c–d.c. inverters produce pure sine waves, the dynamic nature of the tube distorts both the voltage and the current waveforms.
● Impedance: A high impedance is presented by the CCFT assembly which is in the region of 50–70 kQ.
Tube brightness control
The CCFT requires a sine wave with an amplitude of few thousand volts and a frequency in the region of 50–70 kHz. This is provided by a d.c.–a.c. converter which is essence an oscillator. While maximum brightness may be obtained by turning the tube fully on, in most application, there is a need to reduce the lamp’s brightness. There are two basic methods for dim- ming the CCFT. The first simply reduces the tube current either directly or indirectly by reducing the voltage applied to it. The second method main- tains a constant current but turns the lamp on and of to control its bright- ness (Figure 11.24). If the inverter is turned on for longer periods than it is off, a brighter light is produced and vice versa. This technique employs a pulse-width modulated (PWM) waveform to turn the inverter oscillator on and off. A typical tube drive and control signals are shown in Figure 11.25.