A new pixel circuit for "clamped inverter driving" was developed. Since a PMOS inverter is applied instead of the conventional CMOS for the pixel circuit, the circuit is simplified and is composed of only three TFTs. A simulation of gray-scale characteristics verified that this pixel circuit can compensate variation in V th and carrier mobility. A 2.5-inch OLED display based on the new pixel circuit was fabricated by LTPS processing and achieves 6-bit gray-scale and good uniformity.
IntroductionSeveral designs of pixel circuits for organic light-emitting-diode (OLED) displays have been reported [1][2][3]. Regarding the driving scheme for OLED displays, it is known that the voltageprogramming method has advantages of high-speed writing and compatibility with existing driver ICs. However, this scheme has one problem that must be solved; namely, V th and carrier mobility of the TFTs in the pixel circuit must be compensated effectively. At SID'02 and '03, we proposed a "clamped inverter driving" method, which uses sweep-signal comparison to control the duration of light emission [4,5]. This method is a kind of voltage programming. It compensates V th and carrier mobility of a TFT effectively and achieves not only luminous characteristics of 6-bit gray-scale and good uniformity but also gamma correction and peak luminance. The pixel circuit for this driving method, however, requires more than four TFTs. Accordingly, in the present work, we designed a simplified pixel circuit with only three TFTs. Such a pixel circuit composed of fewer TFTs has advantages of high aperture ratio and high yield ratio. We applied it to a 2.5-inch OLED display and evaluated the display characteristics of the OLED panel. Figure 1 shows the configuration of the pixel circuit for the previously developed "clamped inverter driving" scheme, which controls the brightness of the OLED by means of changing the length of the "ON" and "OFF" states of the inverter. TFT T1 and TFT T2 compose the CMOS inverter, and TFT T3 and TFT T4 -Fig. 1 Configuration of previous pixel circuit Fig. 2 Timing chart for OLED driving V data =High Writing mode Illumination mode V data =Low V data =High V data =Low OLED ON OFF ON OFF Vdata Bright Dark Data-line voltage Sweep signal Data signal OLED T4 T3 T1 T2 Illumination line Reset line Data line C1 CMOS inverter
Clamped inverter driving method
A new pixel circuit for the “clamped inverter driving method” reported previously is developed. The pixel circuit is composed of four TFTs, one capacitor and five signal lines including power supply. A 3.5‐inch OLED display with the proposed new pixel circuit has been fabricated by LTPS processing and COG of an existing LCD driver chip. A vertical scanning circuit and selector switches are integrated in the panel. The proposed driving scheme is confirmed as OLED display to produce 6‐bit gray‐scale, gamma correction, and peak luminance with clear full‐color images.
A novel PWM control scheme for the three-phase current source inverter in the photovoltaic power system interconnected to the utility is proposed.The PV-array output power can be adjusted by the modulation factor in the proposed PWM pattern.The harmonic components in the output currents can be decreased sufficiently in order to satisfy the conditions of the Interactive Guideline.Furthermore, a new Maximum Power Point Tracking control is proposed.The inverter output current should be detected and the modulation factor may be controlled so as to obtain the maximum effective current.The inverter output power can be maintained on the Maximum Power Point for the fluctuations of panel temperatures, insolation levels and system voltages.
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