OLED Pixel Circuit Compensation for Threshold Voltage and IR Drop
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Solution Overview
Problem
The existing pixel circuit structures in AMOLED displays face issues with non-uniform brightness due to threshold voltage variations and IR drops, leading to mura phenomena and efficiency degradation over time, especially in large-sized displays.
Innovation Solution
A pixel circuit structure comprising P-type TFT transistors, a capacitor, and an OLED, where the driving current is independent of the threshold voltage of the TFT and the power supply voltage, utilizing a pre-charging, compensation, and light-emitting stage operation to maintain consistent current flow, thereby compensating for threshold voltage non-uniformity and IR drops.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If LTPS TFT is used to provide corresponding currents to OLED devices, then mobility and steady character are improved, but non-uniformity in electrical parameters such as threshold voltage occurs due to crystallization process limitations
Solution Approach 1:
The patent applies a preliminary compensation action by introducing a compensation transistor and capacitor that pre-adjust for threshold voltage variations before the main driving transistor operates. The compensation transistor is configured to generate a compensation signal that counteracts the non-uniformity in threshold voltage of the driving TFT, thereby preventing brightness non-uniformity before it occurs during normal operation.
Solution Approach 2:
The patent implements a feedback mechanism where the compensation transistor and capacitor form a feedback loop that continuously monitors and adjusts for threshold voltage variations in the driving TFT. The compensation circuit responds to the actual threshold voltage deviations and provides real-time correction, ensuring uniform brightness across all pixels despite manufacturing variations.
2Ease of manufacture
If power lines on the backboard have certain resistance and driving currents are provided by ARVDD, then power supply is simplified, but voltage drop (IR Drop) occurs causing current differences in different areas
Solution Approach 1:
The patent applies preliminary compensation by configuring the compensation transistor and capacitor to anticipate and counteract IR drop effects before they manifest as brightness non-uniformity. The compensation circuit pre-adjusts the driving current to compensate for expected voltage drops in power lines, ensuring uniform current distribution across different display areas.
Solution Approach 2:
The compensation circuit functions as a feedback mechanism that detects and responds to voltage drops in the power supply lines. By monitoring the actual voltage at the pixel level and adjusting the driving current accordingly, the system maintains uniform current distribution despite the simplified power supply architecture with resistive power lines.
3Duration of action of stationary object
If OLED device operates for a long time, then light emitting function is maintained, but threshold voltage increases and light emitting efficiency decreases
Solution Approach 1:
The patent implements a feedback-based compensation mechanism where the compensation transistor and capacitor continuously monitor and adjust for threshold voltage drift in the OLED device over time. As the OLED operates and its threshold voltage changes, the compensation circuit detects this drift and adjusts the driving current to maintain constant brightness, effectively compensating for degradation throughout the device's operational lifetime.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution effectively reduces current fluctuations caused by threshold voltage and IR drop variations to less than 3%, and compensates for OLED degradation by adjusting the current to maintain brightness, significantly improving display uniformity and efficiency.
Implementation Method 1
An Organic Light Emitting Diode (OLED), as a current-type light emitting device
Data Source
Figure 1a~2b
Figure 2c~3
Figure 4~5
AI summary
The present invention provides a pixel structure of an organic light emitting display device and driving method thereof. The pixel structure comprises first to fifth thin film transistors, a capacitor and an OLED device, wherein a radio of width to length of the first thin film transistor is set so as to compensate a brightness loss due to the degradation of the organic light emitting display device. Following steps are performed for the pixel structure in a refresh process of each frame of images: during a pre-charging period, the scan line and a first control signal (EM) are at a low level, a second control signal (EMD) is at a high level; during a compensation period, the scan line is at a low level, the first control signal (EM) and the second control signal (EMD) are at a high level; and during a light emitting period, the scan line is at a high level, the first control signal (EM) and the second control signal (EMD) are at a low level.