OLED Pixel Driving Circuit Threshold Compensation
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Solution Overview
Problem
Conventional OLED pixel driving circuits face issues with illumination non-uniformity due to threshold voltage variations resulting from non-uniformity in the fabrication process of driving transistors, leading to degradation and reduced display quality.
Innovation Solution
An OLED pixel driving circuit is designed with a specific configuration of thin-film transistors (TFTs) and capacitors, including P-type and N-type TFTs, and a timing arrangement of scan and illumination signals into data storing and threshold compensation stages, and an illumination stage to manage voltage levels and eliminate threshold voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional OLED 2T1C pixel driving circuit is used with continuous DC-biasing, then the circuit structure is simple, but the OLED threshold voltage increases and brightness steadily declines leading to reduced lifespan
Solution Approach 1:
The patent implements periodic action by alternating between data storing stage (when illumination signal is low) and illumination stage (when illumination signal is high). During the illumination stage, the holding capacitor is discharged through the P-type TFT to reset the threshold voltage, preventing continuous DC-biasing effects and extending OLED lifespan while maintaining simple circuit structure
Solution Approach 2:
The patent changes the voltage level parameter by introducing a high voltage power source VDD (e.g., 10V-20V) during the illumination stage to discharge the holding capacitor and reset the threshold voltage. This parameter change allows the system to overcome threshold voltage increase without complicating the overall circuit structure
2Reliability
If a conventional OLED 5T1C pixel driving circuit is used to compensate for threshold voltage, then display uniformity is improved, but the hardware becomes complicated and voltage level changes are required
Solution Approach 1:
The patent makes the P-type TFT serve multiple functions: it acts as the driving transistor during data storing stage and as the discharge path during illumination stage. This multi-functionality achieves threshold voltage compensation and display uniformity without adding separate hardware components, thus avoiding increased hardware complexity
Solution Approach 2:
The patent performs preliminary action by discharging the holding capacitor through the P-type TFT during the illumination stage before the next data storing stage begins. This preliminary discharge resets the threshold voltage to its initial state, preventing accumulation of threshold voltage shifts and ensuring display uniformity across all pixels without requiring complex compensation circuits
3Reliability
If voltage level of VDD is changed rapidly to compensate threshold voltage, then display uniformity is improved, but charge/discharge time becomes insufficient and current becomes too high
Solution Approach 1:
The patent ensures continuity of useful action by maintaining the illumination signal high during the entire illumination stage, allowing continuous discharge of the holding capacitor through the P-type TFT. This continuous discharge action thoroughly resets the threshold voltage without requiring rapid voltage changes, thus avoiding insufficient charge/discharge time and excessive current while maintaining display uniformity
Data Source
AI summary
An OLED pixel driving circuit includes a first TFT having gate connected to a third node, and having a source and a drain connected to a second node and a first node respectively; a second TFT, having gate receiving a scan signal, and having a source and a drain connected to the first node and the third node respectively; a third TFT, having gate receiving the scan signal, and having a source and a drain connected to the second node and utilized for inputting a data voltage respectively; a fourth TFT, having gate receiving an illumination signal, and having a source and a drain connected to the second node and a DC high voltage power source respectively; a fifth TFT, having gate receiving the illumination signal, and having a source and a drain connected to the first node and an anode of an OLED, and two capacitors.