Liquid crystal displays(LCD’s) are widely used in military electronic systems

equipment

and other devices.In such application

cold cathode fluorescent lamps(CCFLs) are commonly used to provide backlight illumination for the transmissive LC cell.For the cockpit display

a high brightness is required to sunlight readability

and on the other hand

a faint light is necessary in the gloomy sky

even the night sky without any ambient light.So that

a multi-tube CCFL backlight

which with up to 4 tubes of CCFL paralleling each other

is usually rugged in the backlight module.On the other hand

the module must be able to dim the CCFLs for the LCD to display faintly in dark sky.So a proper circuit should be provided for lamps-intensity control from zero to full brightness with no hysteresis or pop on.Even now that the CCFL’s brightness is expressed in a ratio of an input current passing through the tube

but it’s difficult for the current-based intensity-control to dim synchronously the CCFLs in low brightness without the tubes crushing out in succession.Furthermore

even if a single tube

for example a snake type tube

is applied

a phenomenon so-called “thermometer effect” will cause a non-uniformity illuminance too.The current-based intensity-control is not fitted in with this multi-tube backlight module.From the electrical point of view

due to the complicated feature of the CCFL in its impedance

for example

the lamp impedance increases almost inversely with the lamp current

it is very difficult to dim the lamp

especially to provide a very wide luminance adjustment range to about 100% down to 0.1% even to 0.05%.The feature of great range for dimming may make the LCD to meet the special requirement for military.In the case of synchronous driving and if designed excellently

the inverter circuitry with pulse width modulating(PWM) may dim the CCFL very well and is great fit for the dimming of the backlight with multi-tube lamps.Beyond that

the relatioship of the CCFL luminance and the PWM duty is linearity and does not cause excessive lamp decay

thereby the LCD may have a high expected reliability which is a very important specification for military electronic equipment.The feature of CCFLs driven by a power supply circuit based on Royer oscillator

which can invert the DC to AC voltage with about 40kHz in frequency and bring low voltage to high voltage about 600～1200V

is researched in detail.The dynamic response and life relative to PWM-based intensity-control circuit is analyzed experimentally.The experimental data indicate that the reansient response of control circuit is a very important factor for wide dimming range.Especially

the responding time for the power ON or OFF must be minished as small as possible.