OLED Pixel Circuit Threshold Voltage Compensation
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Solution Overview
Problem
In organic light emitting display panels, unequal threshold voltages of driving transistors result in uneven brightness across pixels due to fabrication process variations, leading to inconsistent image quality.
Innovation Solution
A pixel design that includes a capacitor, transfer circuit, switch elements, a driving element, and a light-emitting element, where the driving current is calculated as 1/2·k·(vsg−|vth|)2, independent of the threshold voltage, ensuring consistent brightness by decoupling it from transistor variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional driving transistor design is used, then the pixel structure is simple, but the brightness uniformity deteriorates due to threshold voltage variations
Solution Approach 1:
The patent changes the operational parameters of the driving transistor by applying different voltage levels (ELVDD, ELVSS, compensation voltages) to the gate electrode at different time periods. This dynamic parameter adjustment compensates for threshold voltage variations and achieves uniform brightness without increasing fundamental circuit complexity
Solution Approach 2:
The patent applies preliminary compensation actions by pre-adjusting the gate voltage of the driving transistor before the light-emitting period. The compensation circuit performs preliminary calibration of the driving current based on threshold voltage variations, ensuring uniform brightness before actual operation
2Manufacturing precision
If threshold voltage compensation is implemented, then brightness uniformity is improved, but the circuit complexity increases
Solution Approach 1:
The gate electrode of the driving transistor serves multiple functions: it acts as both the control terminal for normal operation and as a compensation terminal for threshold voltage correction. This multi-functionality eliminates the need for separate compensation circuits, maintaining circuit simplicity while achieving brightness uniformity
Solution Approach 2:
The patent merges the compensation function into the existing driving transistor structure by utilizing the gate electrode for both driving and compensation purposes. The compensation circuit is integrated with the pixel circuit, combining multiple functions into a unified structure that reduces overall complexity
3Ease of manufacture
If simple driving current control is used, then the circuit is easy to manufacture, but image quality deteriorates due to brightness inconsistency
Solution Approach 1:
The patent implements periodic voltage application to the gate electrode, where different voltage levels are applied in different time periods (first period for compensation, second period for light emission). This periodic action enables precise control of driving current while maintaining ease of manufacture through simple timing control
Data Source
AI summary
A pixel and a display panel using the pixel are provided. In the pixel, a driving element provides a driving circuit according to a data signal and a reference voltage to drive a light-emitting element to emit light. The electrical difference of the driving elements due to the fabrication process thereof does not affect the brightness of the light-emitting elements. Moreover, unequal brightness resulted from the equivalent resistance of the power lines is also prevented.