脉宽调制亮度调节方式下冷阴极荧光灯性能的实验研究

Experimental Research on the Feature of CCFL with PWM Dimming

从电学的角度看 ,由于冷阴极荧光灯 (Coldcathodefluorescentlamp ,CCFL)具有十分复杂的阻抗特性 ,例如 ,灯管的阻抗几乎随工作电流的增加而减小 ,因此对其亮度进行调节也十分困难 ,尤其是大亮度调节比的场合。在对逆变电路进行优良设计并进行同步调光的前提下 ,脉宽调制 (Pulsewidthmodulating ,PWM)可以实现高达 2 0 0 0∶1的亮度调节比。这种大范围亮度调节特性 ,可以满足机载显示器的亮度调节性能要求。另一方面 ,CCFL的亮度输出与PWM的占空比之间具有良好的线性关系。此外 ,还不会对CCFL的寿命造成任何的影响 ,从而保证了军用显示器件的可靠性。实验结果表明 ,PWM方式可以很好地调节多管并列式背光灯组的亮度 。

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 40 kHz in frequency and bring low voltage to high voltage about 600～1 200 V, 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.