Current-Scaling TFT Circuit for OLED Brightness Uniformity
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Solution Overview
Problem
Current transistor circuits for OLED displays face issues with non-uniform brightness distribution due to time-dependent decay and threshold voltage variations, leading to delayed response times, especially at low gray levels, and reduced aperture ratios due to limited current scaling ratios.
Innovation Solution
A current-scaling active thin film transistor circuit structure that includes multiple switching transistors, a driving transistor, and storage capacitors, where the gates of switching transistors are connected, and storage capacitors are arranged to adjust the current scaling ratio, reducing response time and maintaining aperture ratios by stabilizing the driving current through voltage adjustments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a current driven circuit is used to compensate threshold voltage variation, then brightness uniformity is improved, but response time is seriously delayed at low gray levels
Solution Approach 1:
The patent segments the current path by introducing a dedicated compensation transistor (T202) separate from the driving transistor (T203). This segmentation allows the compensation function to operate independently without interfering with the response time characteristics of the driving transistor, thus resolving the contradiction between brightness uniformity and response time.
Solution Approach 2:
The patent introduces a compensation transistor (T202) as an intermediary element that handles the threshold voltage compensation function. This intermediary component absorbs the time delay issue, allowing the main driving transistor (T203) to maintain fast response characteristics while the compensation transistor handles the brightness uniformity correction.
2Loss of time
If current scaling ratio is increased to improve response time, then response time is reduced, but aperture ratio is reduced due to increased transistor size
Solution Approach 1:
The patent changes the operating parameters of the transistors by introducing a compensation transistor that operates in parallel with the driving transistor. This parameter change allows the system to achieve improved response time without increasing the physical size of the pixel, thereby maintaining the aperture ratio while reducing response time delays.
3Device complexity
If voltage-driven circuit is used, then device complexity is reduced, but brightness uniformity deteriorates due to inability to compensate transistor variations
Solution Approach 1:
The patent implements a self-service mechanism where the compensation transistor (T202) automatically compensates for threshold voltage variations in the driving transistor (T203) through its own operation. This self-compensation mechanism maintains brightness uniformity without requiring complex external control circuits, thus resolving the contradiction between device complexity and brightness uniformity.
Data Source
AI summary
An active TFT circuit structure with current scaling function is disclosed, which includes a current source, a data line, a scan line, a direct current voltage source, capacitors and four transistors, wherein the capacitors form a cascade structure. During the ON-state, the two of the transistors are turn-on based on the voltage provided by the scan line, so that the data current provided by the current source flows through the data line-and the transistor which is one of the opened transistors, thereby arriving an emitting light element and the transistor connected to the emitting light element. When the pixel circuit changes from ON- to OFF-state, the voltage of the node between the storage capacitors reduces due to the feed-through effect of one of storage capacitor, thereby reducing the driving current of the emitting light element. Therefore, it can be achieved the current scaling function.