OLED Pixel Circuit Compensation for Threshold Voltage Shifts
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Solution Overview
Problem
Active matrix OLED display devices face issues due to threshold voltage shifts in driving TFTs, leading to variations in current and luminance across pixels, even when the same data voltage is applied, due to threshold voltage and supply voltage variations.
Innovation Solution
A display pixel design incorporating a first capacitor, data transistor, control transistor, and driving transistor, where the second node is set to a high supply voltage level, adjusting the data voltage at the first node to account for threshold voltage and supply voltage variations, ensuring consistent current control to the light-emitting diode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional pixel structure with a driving TFT is used, then the device can be manufactured with standard processes, but the luminance varies across pixels due to threshold voltage shifts and supply voltage drops
Solution Approach 1:
The pixel is divided into multiple functional blocks: a driving TFT for current control, a control TFT for voltage adjustment, a data transistor for signal input, and compensation capacitors. This segmentation allows each component to address specific issues (threshold voltage shifts, supply voltage drops) independently, improving luminance consistency without requiring complete redesign of the entire pixel structure.
Solution Approach 2:
A control TFT is introduced as an intermediary component between the data transistor and the driving TFT. This control TFT adjusts the voltage at the gate of the driving TFT based on detected threshold voltage shifts and supply voltage variations, acting as a mediator that compensates for parameter variations and maintains consistent luminance across pixels.
2Reliability
If threshold voltage compensation circuits are added to each pixel, then luminance uniformity improves, but the number of transistors and capacitors per pixel increases
Solution Approach 1:
The compensation capacitors are pre-charged to reference voltages during initialization phases before the actual display operation. This preliminary action stores compensation values that will be applied during operation, allowing the circuit to compensate for threshold voltage shifts without requiring continuous complex calculations or additional active components during the display phase.
Solution Approach 2:
The circuit changes the voltage parameter at the gate of the driving TFT dynamically based on detected variations in threshold voltage and supply voltage. By adjusting this voltage parameter through the control TFT, the system compensates for component variations without adding numerous discrete components, as the compensation is achieved through voltage modulation rather than structural multiplication.
3Power
If supply voltage is increased to compensate for voltage drops, then current drive capability improves, but power consumption increases and threshold voltage effects are exacerbated
Solution Approach 1:
The pixel circuit incorporates feedback mechanisms where the control TFT monitors the actual voltage conditions at the driving TFT gate and adjusts the voltage accordingly. This feedback loop allows the system to maintain adequate current drive capability by applying only the necessary compensation voltage, rather than uniformly increasing the supply voltage across the entire panel, thus avoiding excessive power consumption.
Solution Approach 2:
Instead of increasing the supply voltage globally across the entire display panel, the invention applies voltage compensation locally at each pixel through the control TFT. This local quality approach ensures that each pixel receives the specific voltage adjustment it needs to overcome voltage drops and threshold effects, maintaining current drive capability without unnecessarily increasing overall power consumption.
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 design compensates for threshold voltage and supply voltage variations, ensuring predictable current flow and consistent luminance across pixels, independent of individual TFT characteristics and panel voltage drops.
Implementation Method 1
a first capacitor, C1, coupled between a first node, N1, of the pixel and a second node, N2, of the pixel
Data Source
AI summary
This disclosure relates to a display device that compensates for a threshold voltage of a driving TFT, a voltage drop of a supply voltage source, and a mobility of the driving TFT. The display device can include a plurality of pixels. At least one pixel can include components such as a first capacitor, a second capacitor, a data transistor, a control transistor, an emission transistor, an initialization transistor, a driving transistor and a light emitting diode (LED) among other components.