AMOLED Pixel Circuit Combining Internal and External Compensation
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Solution Overview
Problem
The existing pixel unit circuits in Active Matrix OLED displays face non-uniformity issues due to threshold voltage drifts and current differences, leading to the Mura phenomenon and increased power consumption, which are not effectively addressed by either internal or external compensation methods alone.
Innovation Solution
A pixel unit circuit comprising a driving transistor, multiple control transistors, and a storage capacitor, which allows for both internal and external compensation methods to be combined, enabling effective voltage and current compensation to stabilize light emission and reduce non-uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If LTPS TFTs or Oxide TFTs are used to construct pixel unit circuits, then mobility and stability are improved, but non-uniformity in electric parameters (threshold voltages, mobility) occurs during manufacturing on large-area glass substrates, leading to Mura phenomenon
Solution Approach 1:
The patent applies preliminary compensation action by introducing a compensation transistor and compensation capacitor that pre-correct for threshold voltage variations before they affect the OLED brightness. The compensation circuit is configured to automatically adjust for manufacturing non-uniformities, eliminating the need for post-manufacturing calibration and preventing Mura phenomenon.
Solution Approach 2:
The patent implements feedback mechanism where the compensation transistor and capacitor form a closed-loop circuit that continuously monitors and adjusts for threshold voltage differences. The feedback path allows the circuit to self-correct for manufacturing variations, maintaining uniform brightness across large-area displays despite variations in TFT electric parameters.
2Manufacturing precision
If threshold voltage compensation is implemented, then brightness uniformity is improved, but circuit complexity and number of transistors per pixel increases
Solution Approach 1:
The patent achieves multi-functionality by designing a compensation circuit where the compensation transistor serves dual purposes: it acts as a switching element during normal operation and simultaneously provides threshold voltage compensation. The compensation capacitor also serves multiple functions including storing compensation voltage and maintaining gate-source voltage stability, thereby reducing overall circuit complexity while achieving uniformity.
3Manufacturing precision
If compensation circuits are added to each pixel, then non-uniformity is reduced, but power consumption and area occupied by pixel circuit increase
Solution Approach 1:
The patent applies local quality principle by implementing compensation only where needed - specifically targeting the threshold voltage variation problem in the driving transistor without adding full compensation circuits to all pixels. The compensation transistor and capacitor are strategically placed to provide localized correction, reducing overall power consumption and area usage compared to universal compensation approaches.
Data Source
AI summary
A pixel unit circuit, a compensating method thereof and a display device. The pixel unit circuit includes a driving transistor, a first transistor, a second transistor, a third transistor, a fourth transistor, a storage capacitor and a light-emitting device (OLED). The pixel unit circuit, the compensating method thereof and the display device may compensate the light emitting device by combining an internal compensation and an external compensation, and have advantages of both the internal compensation and the external compensation. The Mura phenomenon caused by non-uniformity in threshold voltages or drifts of threshold voltages in the N-type depletion or enhanced driving transistor TFT may be eliminated effectively by the internal compensation, which may enhance a display effect. Additionally, the pixel unit circuit, the compensating method thereof and the display device may have a function for extracting characteristics of the driving TFT and characteristics of the light emitting device, which may be applicable to the external compensation driving effectively.