TFT Pixel Circuit External Compensation for OLED Brightness Uniformity
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Solution Overview
Problem
Conventional pixel circuits for OLED displays face challenges in achieving uniform brightness due to variations in TFT and OLED characteristics, leading to inadequate compensation methods that are either inaccurate or require excessive transistors, making them unsuitable for high-resolution displays.
Innovation Solution
A pixel circuit with a duplicate voltage supply line configured to sense current through the drive transistor and OLED, using either an analogue or digital external compensation system to adjust the data voltage based on measured current, ensuring accurate compensation for TFT and OLED variations within a compact design suitable for high-resolution displays.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional compensation circuits with multiple transistors are used to compensate for TFT and OLED variations, then compensation accuracy is improved, but the area occupied by the pixel circuit increases, making it unsuitable for high-resolution displays
Solution Approach 1:
The patent extracts the measurement function from the pixel circuit by using a separate test current line that temporarily replaces the emission current path. This allows the pixel circuit to maintain its compact 4T1C structure while enabling external compensation systems to measure and compensate for device variations without adding transistors to the pixel circuit itself.
Solution Approach 2:
The patent introduces a test current line as an intermediary element that enables measurement functionality without requiring additional transistors in the pixel circuit. This intermediary provides a path for external compensation systems to sense current and generate compensated data voltages while the pixel circuit maintains its compact structure.
2Measurement precision
If current is diverted to an external current comparator for measurement, then compensation can be performed, but the OLED properties cannot be measured accurately because the current path is changed
Solution Approach 1:
The patent implements periodic switching between emission mode and test mode. During test mode, the test current line temporarily carries the current for measurement purposes, while during emission mode, the normal emission current flows. This periodic action allows accurate measurement of OLED properties under controlled conditions while maintaining normal display functionality.
Solution Approach 2:
The patent performs current measurement and compensation calculations in advance during test mode before the actual emission phase. The external compensation system uses the measured current to generate compensated data voltages that are then applied during emission, ensuring that compensation is based on accurate pre-measured OLED properties rather than diverted current conditions.
3Adaptability or versatility
If five transistors are used in the pixel circuit for external compensation, then compensation functionality is achieved, but the transistor count is high and the circuit area increases
Solution Approach 1:
The patent makes the existing transistors in the 4T1C pixel circuit perform multiple functions. The same transistors that control emission current also control test current flow during measurement phases. This multi-functionality enables compensation functionality without adding dedicated transistors for measurement, maintaining the compact 4T1C structure.
Solution Approach 2:
The patent merges the emission current path and test current path by using the same pixel circuit transistors for both functions at different times. The test current line is integrated with the existing pixel circuit structure, allowing compensation measurements to be performed using the same transistor components that control display emission, thereby avoiding additional transistors.
4Measurement precision
If measurement time is extended to improve compensation accuracy, then compensation precision is improved, but display refresh rate and productivity decrease
Solution Approach 1:
The patent implements rapid periodic switching between test mode and emission mode within each frame cycle. The test current measurement is performed quickly during a brief test phase, followed by the emission phase that displays the compensated image. This periodic action enables accurate compensation measurements without significantly extending the overall frame time, maintaining high refresh rates.
Solution Approach 2:
The patent rushes through the measurement phase by using a dedicated test current line that enables quick current sensing without requiring complex sequential measurements. The external compensation system rapidly processes the measured current to generate compensated data voltages, minimizing the time spent in test mode and maximizing the time available for emission, thereby maintaining high display productivity.
Data Source
AI summary
A display system includes a pixel circuit and an external compensation system that is operable with the pixel circuit to compensate for differences in a property of the drive transistor and/or light-emitting device. The pixel circuit includes a drive transistor configured to control an amount of current to a light-emitting device; a second transistor having a first terminal connected to the gate of the drive transistor and a second terminal connected to a data voltage input that is supplied by the external compensation system, and a gate of the second transistor is connected to a first control signal; a third transistor having a first terminal connected to a second terminal of the drive transistor and a second terminal connected to a second voltage supply input that is supplied from the external compensation system, wherein a current flow from the second voltage supply input is measured by the external compensation system, and wherein a gate of the third transistor is connected to a second control signal; and a fourth transistor having a first terminal connected to the second terminal of the drive transistor and a second terminal connected to a third voltage supply input, wherein a gate of the fourth transistor is connected to a third control signal. The external voltage compensation system is configured to adjust the data voltage input based on the measured current flow to compensate for a variation in a property of the drive transistor and/or the light-emitting device.