필드 시퀀셜 구동 방법을 이용한 저소비전력 액정 디스플레이

Abstract

최근 휴대용 및 옥외용 디스플레이 활용 제품 수요가 증가하고 있으며, 이들 중 액정표시장치를 이용한 디지털 간판인 공공 정보 표시장치의 수요가 지속적으로 증가하고 있다. 공공 정보 표시장치는 버스정류장, 쇼핑센터, 음식점, 공항 등 많은 장소에 설치되어 다양한 환경조건에서 24시간 운영되고 있다. 따라서 공공정보 디스플레이는 저 소비전력 및 고휘도 특성이 요구되고 있다. 더불어 많은 나라에서 에너지 규제를 제정하고 있으며, 이에 따라 화질열화 없이 고휘도 표시가 가능한 저 소비전력 액정 표시장치 기술 개발이 요구되고 있다. 본 논문은, 저 소비전력 액정표시장치를 구현하기 위하여 필드 시퀀셜 구동방법과 RGBW 화소를 사용한 구동방법의 기술적 문제에 대하여 논의하고, 이 문제점들을 해결하기 위한 새로운 방식의 필드 시퀀셜 구동방법을 제안한다. 필드 시퀀셜 액정표시장치의 색섞임 감소를 위해 동시구동이 가능한 화소 구조 제안하고, RGB CF를 사용한 부화소 및 W 부화소로 이루어진 화소를 사용하는 두가지 방식의 하이브리드 필드 시퀀셜 구동방식을 제안한다. 첫번째 방법은 RGBW 화소 사용 구동방법에서 발생하는 동시대비비문제를 해결하기 위해 RGW 화소에 YB 필드 시퀀셜 구동을 적용하는 방법으로 제안하였고, 두번째 방법은 야외에서 사용되는 공공정보 표시장치의 화질열화를 개선하기 위하여 다양한 환경조건에서 동작 가능한 RGBWWW 화소구조에 RGB 필드 시퀀셜 구동을 적용한 방법을 제안한다. 첫째로, 액정표시장치의 투과율을 증가시키며 색섞임이 없는 필드 시퀀셜 액정표시장치를 구현하기 위하여, 동시구동이 가능한 화소 구조 및 구동방법을 제안한다. 제안된 화소 구조는 데이터 저장부와 액정 구동부로 이루어 지며, 각 화소의 데이터 저장부에 저장되어 있는 데이터 전압은 동시에 액정 구동부에 인가되어 모든 화소가 동시에 구동된다. 추가적으로 데이터 저장부에 저장된 데이터의 전압 승압 기능을 적용하여, 소스드라이버 IC의 구동전압 영역의 증가 없이 제안된 필드 시퀀셜 액정표시장치의 투과율은 유지된다. 제안한 방식의 특성 검증을 위하여 HSPICE 모의실험 실시 및 15.6인치 투명디스플레이 쇼케이스를 제작하여 특성을 평가하였다. HSPICE 모의실험 결과 제안한 화소는 동시 구동 가능함이 확인 되었고, 투과율, 대비비, CIE-1931색공간상의 색재현율은 16.7%, 4000이상, 85.6% 이다. 이 결과들은 현재 상용화되어 있는 투명디스플레이들의 특성보다 우수한 결과이다. 또한, 색섞임의 개선을 확인 하기 위하여 색 균일도를 측정하였으며, 전체 패널상의 색 편차 Δx와 Δy는 0.002보다 작게 측정되었고 이는 제안된 화소 구조를 사용함으로써 색섞임 현상이 개선되어 전체 패널의 색 균일도가 개선됨을 보여준다. 두 번째로, RGW 화소 구조에 YB 필드 시퀀셜 구동을 적용하여 고휘도, 저 소비전력 특성을 가지며, RGBW 화소 구조가 가지는 동시대비 문제를 해결할 수 있는 구동방법을 제안한다. 또한 제안된 구동방법을 사용시 화질개선을 위한 RGB 데이터를 RGBY 데이터로 변환하는 알고리즘을 제안하였으며 Kodak lossless true color image suite를 사용한 MATLAB시뮬레이션을 통하여 성능을 검증하였다. 제안된 RGBY 색 변환 알고리즘은 평균 0.99의 ΔE*00 값과 평균 1.89의 S factor값을 가지며, 이 결과는 RGB 화소 구조를 사용하는 기존 액정표시장치와 비교해서 평균 휘도가 1.89배 밝아짐을 나타낸다. 제안된 구동방식을 검증하기 위하여 YB 백라이트를 사용하는 46인치 액정표시장치를 만들어 패널 투과율, 대비비, 색재현율을 측정하였으며, 각각 12.3%, 4000이상, 76.1%로 측정되었다. 또한 제작된 액정표시장치의 소비전력은 700니트의 밝기에서 75W로써, 이 결과는 RGB 화소를 사용하는 기존 액정표시장치 소비전력의 46.4 % 값으로써 제안된 구동방법은 저 소비전력 특성을 가짐을 검증하였다. 마지막으로, 옥외용 공공정보 표시장치로 사용하기 위한 RGBWWW 화소구조에 필드 시퀀셜 구동을 적용한 액정표시장치 및 구동방법을 제안한다. 제안된 구동방법으로 외부광량이나 온도 같은 구동조건에 따라 두 가지 구동방식으로 동작한다. 밝은 환경에서 사용되는 첫번째 구동방식은 W 부화소에 RGB 필드 시퀀셜 구동 적용 및 RGB 부화소를 통한 이미지 표현 방법의 하이브리드 필드 시퀀셜 구동방법으로써, W 부화소를 RGB 화소처럼 사용하여 동시대비 문제없이 패널의 투과율을 증가시키는 구동방법이다. 두번째 구동방식은 밤이나 겨울 같이 기온이 낮은 환경에서 동작하는 구동방법으로써, 저온에서 액정 응답특성이 느려짐으로 인해 발생하는 화질왜곡을 방지하기 위하여 RGB CF를 사용한 부화소만을 사용하는 구동방법이다. 제안된 방법의 평가를 위하여, 15.6인치 패널을 제작하였으며, 측정된 투과율, 대비비, 색재현율은 각각 14.5%, 4000이상, 82.5%로 측정되었다. 이 결과는 제안된 구동방법을 사용하면 기존 액정표시장치와 비교할 경우 같은 소비전력에서 두배 이상 밝은 영상을 표현하거나, 같은 밝기상태에서 소비전력을 50%이상 감소 시킬 수 있다. 상기의 결과들에서 보여주듯이, 제안된 방법들은 다양한 고휘도 저 소비전력 특성이 요구되는 액정표시장치의 제품에 사용할 수 있음을 보였다. |Demand of applications for mobile and outdoor display products is increasing as of late. Especially, demand for public information displays (PIDs), which are digital signage using liquid crystal displays (LCDs) are increasing consistently. They are used all day long under various environmental conditions in many public places such as bus stops, shopping malls, quick service restaurants, and airports. These products require high-brightness and low-power consumption for outdoor usage. Moreover, most countries are strengthening compulsory regulation on power consumption, and to meet the strict power consumption regulations, a method to reduce power consumption of LCD is strongly needed along with a method to maintain high-brightness without degrading the image quality. The technical issues for low-power LCD using field sequential (FS) driving method and RGBW sub-pixels are discussed in this dissertation. Further, new FS driving methods to solve the technical problems for various low-power LCD applications are proposed. A pixel structure and simultaneous driving methods are proposed to reduce color mixing artifact for FS-LCD. Two kinds of hybrid field sequential driving methods that use pixel consisting of sub-pixels using RGB CF and white sub-pixels, are proposed. First, a hybrid FS driving method using RG sub-pixels with CF and W sub-pixels is presented to eliminate simultaneous contrast problem caused by using RGBW pixel structure. Second, a hybrid FS driving method using RGBWWW sub-pixels to operate under various hash environment conditions for outdoor PIDs is presented. The pixel structure and simultaneous driving method for FS-LCD are proposed to improve the transmittance of LCD panel and to reduce color mixing artifacts. The proposed pixel consists of a memory stage and a liquid crystal (LC) driving stage, and all pixels are simultaneously operated as programmed data in the memory stage is transferred to the LC driving stage. In addition, data voltage boosting method with memory capacitor is adopted to maintain the transmittance of FS-LCD without increasing the driving voltage of source driver IC. To verify the performance of the proposed pixel, HSPICE simulation is conducted and 15.6-inch FS-LCD embedded with the proposed pixel is fabricated for experimental vehicles. HSPICE simulation results show that the proposed pixel works properly, and the measured transmittance, contrast ratio, and color gamut are 16.7%, over 4000, and 85.6% compared to NTSC in the CIE-1931 color space, respectively. The measurement results of the proposed transparent PID show higher transmittance and wider color gamut compared to conventional commercial PIDs. Furthermore, to verify the reduction in the color mixing artifact, color variations in the x and y-axis of the CIE-1931 chromaticity diagram (Δx and Δy) are measured. The Δx and Δy are smaller than 0.002 across the whole display panel. Therefore, the proposed transparent PID shows superior color uniformity by reducing color mixing artifacts. A RGW pixel with YB FS driving method is proposed to reduce power consumption of LCD while maintaining high brightness. In addition, an RGB to RGBY color conversion algorithm is developed to improve image quality of the proposed driving method and the algorithm is verified by MATLAB simulation with 24 images of Kodak lossless true color image suite. The proposed RGBY color conversion algorithm has an average ΔE*00 of 0.99, and an average S factor of 1.89, respectively. These results describe that average brightness is increased 1.89 times compared to that of conventional LCD using RGB sub-pixel structure. A 46-inch LCD panel is fabricated and its transmittance, contrast ratio, color gamut are measured, and they are 12.3%, over 4000, 76.1% in CIE-1931 color space, respectively. Also, the measured power consumption of fabricated PID has 75 W at 700 nits, and this result is 46.4% lower than that of the conventional TFT-LCD using RGB pixel. Finally, a new driving method using RGBWWW pixel structure with FS driving is proposed to improve the transmittance of LCD for outdoor PID. The proposed driving method performs in either of two operation modes, which is selected according to the amount of sunlight and the temperature. Mode one is a hybrid field sequential (FS) mode in which W sub-pixels are operated in order to increase the transmittance without simultaneous contrast problem under very bright condition. Mode two is a low temperature mode in which only RGB sub-pixels are operated in order to prevent image distortion under low temperature condition. For experimental verification, 15.6-inch FS-LCD using the proposed pixel structure is fabricated. The measured results of the fabricated LCD show that transmittance, contrast ratio, and color gamut are 14.5%, over 4000, and 82.5%, respectively. These results show that the proposed hybrid driving methods consumes 50% less power with the same brightness or has twice the luminance with the same power consumption compared with conventional TFT-LCD products. As the results show, it is expected that the proposed methods can be used in various high brightness LCD applications with low-power consumption.; Demand of applications for mobile and outdoor display products is increasing as of late. Especially, demand for public information displays (PIDs), which are digital signage using liquid crystal displays (LCDs) are increasing consistently. They are used all day long under various environmental conditions in many public places such as bus stops, shopping malls, quick service restaurants, and airports. These products require high-brightness and low-power consumption for outdoor usage. Moreover, most countries are strengthening compulsory regulation on power consumption, and to meet the strict power consumption regulations, a method to reduce power consumption of LCD is strongly needed along with a method to maintain high-brightness without degrading the image quality. The technical issues for low-power LCD using field sequential (FS) driving method and RGBW sub-pixels are discussed in this dissertation. Further, new FS driving methods to solve the technical problems for various low-power LCD applications are proposed. A pixel structure and simultaneous driving methods are proposed to reduce color mixing artifact for FS-LCD. Two kinds of hybrid field sequential driving methods that use pixel consisting of sub-pixels using RGB CF and white sub-pixels, are proposed. First, a hybrid FS driving method using RG sub-pixels with CF and W sub-pixels is presented to eliminate simultaneous contrast problem caused by using RGBW pixel structure. Second, a hybrid FS driving method using RGBWWW sub-pixels to operate under various hash environment conditions for outdoor PIDs is presented. The pixel structure and simultaneous driving method for FS-LCD are proposed to improve the transmittance of LCD panel and to reduce color mixing artifacts. The proposed pixel consists of a memory stage and a liquid crystal (LC) driving stage, and all pixels are simultaneously operated as programmed data in the memory stage is transferred to the LC driving stage. In addition, data voltage boosting method with memory capacitor is adopted to maintain the transmittance of FS-LCD without increasing the driving voltage of source driver IC. To verify the performance of the proposed pixel, HSPICE simulation is conducted and 15.6-inch FS-LCD embedded with the proposed pixel is fabricated for experimental vehicles. HSPICE simulation results show that the proposed pixel works properly, and the measured transmittance, contrast ratio, and color gamut are 16.7%, over 4000, and 85.6% compared to NTSC in the CIE-1931 color space, respectively. The measurement results of the proposed transparent PID show higher transmittance and wider color gamut compared to conventional commercial PIDs. Furthermore, to verify the reduction in the color mixing artifact, color variations in the x and y-axis of the CIE-1931 chromaticity diagram (Δx and Δy) are measured. The Δx and Δy are smaller than 0.002 across the whole display panel. Therefore, the proposed transparent PID shows superior color uniformity by reducing color mixing artifacts. A RGW pixel with YB FS driving method is proposed to reduce power consumption of LCD while maintaining high brightness. In addition, an RGB to RGBY color conversion algorithm is developed to improve image quality of the proposed driving method and the algorithm is verified by MATLAB simulation with 24 images of Kodak lossless true color image suite. The proposed RGBY color conversion algorithm has an average ΔE*00 of 0.99, and an average S factor of 1.89, respectively. These results describe that average brightness is increased 1.89 times compared to that of conventional LCD using RGB sub-pixel structure. A 46-inch LCD panel is fabricated and its transmittance, contrast ratio, color gamut are measured, and they are 12.3%, over 4000, 76.1% in CIE-1931 color space, respectively. Also, the measured power consumption of fabricated PID has 75 W at 700 nits, and this result is 46.4% lower than that of the conventional TFT-LCD using RGB pixel. Finally, a new driving method using RGBWWW pixel structure with FS driving is proposed to improve the transmittance of LCD for outdoor PID. The proposed driving method performs in either of two operation modes, which is selected according to the amount of sunlight and the temperature. Mode one is a hybrid field sequential (FS) mode in which W sub-pixels are operated in order to increase the transmittance without simultaneous contrast problem under very bright condition. Mode two is a low temperature mode in which only RGB sub-pixels are operated in order to prevent image distortion under low temperature condition. For experimental verification, 15.6-inch FS-LCD using the proposed pixel structure is fabricated. The measured results of the fabricated LCD show that transmittance, contrast ratio, and color gamut are 14.5%, over 4000, and 82.5%, respectively. These results show that the proposed hybrid driving methods consumes 50% less power with the same brightness or has twice the luminance with the same power consumption compared with conventional TFT-LCD products. As the results show, it is expected that the proposed methods can be used in various high brightness LCD applications with low-power consumption.