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Wei-Chung Cheng, Member, IEEE, Massoud Pedram, Fellow, IEEE - page 3 / 8





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blue phosphors produces the desired three-band white light. Otherwise, the displayed image will be color-shifted. The CCFL converts electrical energy into visible light, which is called the gas discharge phenomenon. When a high voltage is applied to the electrodes turning on the lamp, electrical arcs are generated that ionize the gas and allow the electrical current to flow. The collision among the moving ions injects energy to the mercury atoms. The electrons of the mercury atoms receive energy and jump to a higher energy level followed by emitting ultraviolet photons when falling back to their original energy level. The ionized gas conducts the electrical current. The impedance of the gas conductor, unlike that of the metal conductor having a linear behavior, decreases as the current increases. Therefore, the CCFL has to be driven by an alternative current (AC) to avoid a potential explosion. A DC-AC inverter is usually used to drive a CCFL in battery-powered applications. A DC-AC inverter is basically a switching oscillator circuit that supplies high- voltage AC current from a low-voltage battery. The nominal AC frequency of modern CCFL is in the range of 50-100 kHz to avoid flickering. The nominal operate

voltage has to be higher than 500 VRMS ionized.

to keep inert gas

To conserve energy in battery-powered applications, dimming control is a desired feature for DC-AC inverters. Different methods of dimming CCFL have been used, including linear current, pulse-width-modulation, and current chopping [6]. In a DC-AC inverter with dimming control, an analog or digital input signal is exposed for adjusting the CCFL illumination. Most well designed DC- AC inverters have high electrical efficiency (>80%) and linear response of output electrical power to input power. Most fluorescent lamps, however, have low optical efficiency (<20%) and non-linear response of output optical power versus input power [7].

A. CCFL Characteristics

The CCFL illumination is a complex function of the driving current, ambient temperature, warm-up time, lamp age, driving waveform, lamp dimensions, and reflector design [7]. For CBCS, only the driving current is controllable. Therefore, we model the CCFL illumination as a function of the driving current only and ignore the other parameters.

The typical relationship between the CCFL illumination and the driving power is shown in Fig. 3a. The CCFL illumination increases monotonically as the driving power increases before reaching 80% of the full driving power. Beyond 80%, the CCFL illumination starts to saturate. This saturation phenomenon is because the enclosed ionized gas has been fully discharged and cannot release more photons. Additionally, the increased temperature and

pressure inside the tube inhibit further discharge [7][8]. This observation suggests that the decreased optical efficiency of CCFL in the saturated region is not favored by power-aware applications.

B. CCFL Illumination/Power Characterization

We use a stepwise function of illumination to characterize the power consumption of CCFL as a

function of illumination:

t n a b PS t bPLi ac g Pb kli h (b)






Bs b1



The backlight factor b[0,1] represents the normalized backlight illumination, which is dynamically controllable by the CBCS policy.

The analog or digital dimming control input of the DC- AC inverter is not always linearly proportional to the output backlight illumination. Careful calibration is needed to derive the correct mapping between the backlight factor b and dimming control input q(b). A precision luminance meter such as [9] provides accurate absolute illuminance readings. These expensive meters,







laboratories. We find that the absolute illuminance readings are not required to calibrate the CCFL in the backlight scaling applications. An accurate photographic light meter can serve the purpose so far as it is capable of sensing minor illuminance variance. We use the light meter as a weight scale and adjust the backlight and TFT- LCD simultaneously while maintaining the same illuminance. We start with measuring the illuminance for the maximum CCFL backlight b=1 when applying







transmissivity x=0[0,255]. The transmissivity x is obtained by displaying a pure gray image, in which Red=Green=Blue=x for every pixel. The transmissivity x is increased until the light meter can sense a variation and report a different reading. Then reduce the backlight factor b by reducing the dimming control q until the meter reports the previous reading. Since the change of the TFT- LCD grayscale (transmissivity) is known, the change of the backlight is asserted to be the same. Record q as the dimming control value for the backlight factor b=(255- x)/256. At the same time, the power consumption of the backlight Pbacklight is also measured and recorded. Repeat the above procedure for x=0,1...255. After interpolation, we can obtain q(b) and Pbacklight(b). The results for a color

backlit TFT-LCD [10] are shown in Fig. 3a. Plugging


(4), the following parameters are obtained: PLin=0.4991, PSat=0.1489, CLin=0.1113, CSat=0.6119, Bs=0.8666


This power model will be incorporated in Section IV to solve the optimal CBCS problem.

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