OLED Pixel Circuit Separating Initialization from Threshold Compensation
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Solution Overview
Problem
As OLED displays increase in size, the differences in threshold voltages among driving transistors lead to non-uniform image quality, and the existing methods of initializing and compensating for these voltages simultaneously result in reduced contrast ratio and increased initialization time due to unwanted light emission during initialization.
Innovation Solution
A pixel circuit design that separates the initialization time from the threshold voltage compensation period, utilizing a specific configuration of N-channel metal-oxide semiconductor (NMOS) transistors and light emitting control signals to manage the voltage and current applied to the OLED, ensuring uniform brightness and improved contrast ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If initialization and threshold voltage compensation are performed simultaneously, then the initialization process is completed, but unwanted light emission occurs during initialization reducing contrast ratio
Solution Approach 1:
The patent divides the initialization and threshold voltage compensation processes into separate time periods. The initialization period (first time period) initializes the storage capacitor without causing light emission, while the threshold voltage compensation period (second time period) compensates for transistor threshold voltages. This temporal segmentation prevents the contrast ratio degradation that would occur if both processes were simultaneous, as the OLED remains off during initialization.
2Area of stationary object
If the OLED display size is increased, then the display area is enlarged, but threshold voltage differences among driving transistors increase causing non-uniform image quality
Solution Approach 1:
The patent implements a dedicated threshold voltage compensation period that occurs before the display of actual image data. During this preliminary action phase, the circuit measures and compensates for threshold voltage differences in the driving transistors by adjusting the stored voltage values. This preliminary compensation ensures that subsequent image display achieves uniform brightness across the entire large display area, overcoming the threshold voltage variations that naturally occur with larger transistor arrays.
3Device complexity
If initialization is performed without separating time periods, then the process is simpler, but light emission during initialization reduces image quality
Solution Approach 1:
The patent employs periodic action by structuring the pixel circuit operation into distinct time periods: an initialization period where the storage capacitor is initialized and the OLED is kept off, followed by a threshold voltage compensation period, and then the display period. This periodic structure, controlled by scan signals and light emitting control signals, prevents light emission during initialization while maintaining relatively simple circuit architecture using standard transistor and capacitor components.
Data Source
AI summary
A pixel circuit and an organic light-emitting diode (OLED) display using the pixel circuit is provided. The pixel circuit includes: an OLED; a third N-channel metal-oxide semiconductor (NMOS) transistor coupled to a data line and a first scan line and configured to apply a data signal to a first node; a storage capacitor having one terminal coupled to the first node and the other terminal coupled to a second node; a fourth NMOS transistor coupled between a first power and the second node and configured to apply a voltage of the first power to the second node; a first NMOS transistor having a first electrode, a second electrode, and a gate electrode coupled to the second node; and a second NMOS transistor coupled between the second node and the first electrode of the first NMOS transistor and configured to diode-connect the first NMOS transistor.