OLED Pixel Circuit Buffer Transistor On-Resistance and Brightness Control
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Solution Overview
Problem
In image forming apparatuses using OLED elements, large driving transistors require sufficient gate current to reduce on-resistance, but insufficient driving force leads to prolonged gate potential changes, degrading printing quality due to brightness variation and prolonged writing operations.
Innovation Solution
A pixel circuit structure with a buffer circuit between the storing circuit and the driving transistor, utilizing an inverter as the buffer circuit with a smaller output transistor size, ensures sufficient driving of the transistor, reducing current consumption and controlling the transistor's on/off reliably, while separate power wiring for the storing and buffer circuits minimizes potential changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the size of the driving transistor is enlarged to decrease on-resistance, then the on-resistance of the driving transistor decreases, but the area of the pixel circuit increases
Solution Approach 1:
The pixel circuit is divided into functional modules: a driving transistor for current control, a buffer circuit with separate pull-up and pull-down transistors for gate potential control, and a storage circuit. This segmentation allows the driving transistor to be optimized for low on-resistance while the buffer circuit handles the capacitive loading, preventing the driving transistor size from needing to increase excessively.
Solution Approach 2:
The buffer circuit acts as an intermediary between the storage circuit and the driving transistor. It includes a pull-up transistor and a pull-down transistor that work together to rapidly charge and discharge the gate of the driving transistor, enabling fast switching without requiring the driving transistor itself to be large.
2Reliability
If the size of the driving transistor is enlarged to decrease on-resistance, then the brightness variation of the OLED element decreases, but the writing operation time increases
Solution Approach 1:
The buffer circuit serves as an intermediary that can rapidly change the gate potential of the driving transistor by using a pull-up transistor to charge and a pull-down transistor to discharge. This allows the system to achieve both low on-resistance (for brightness uniformity) and fast writing (by rapidly switching the gate potential through the buffer circuit).
Solution Approach 2:
The buffer circuit operates in a periodic switching manner, where the pull-up transistor charges the gate during one phase and the pull-down transistor discharges it during another phase. This periodic action enables rapid potential changes, reducing the writing operation time while maintaining the ability to drive large transistors effectively.
3Use of energy by moving object
If the size of the output transistor of the buffer circuit is reduced to decrease current consumption, then the current consumption of the buffer circuit decreases, but the rising time of the output signal increases
Solution Approach 1:
The invention optimizes the parameters of the pull-up and pull-down transistors in the buffer circuit. By carefully selecting their sizes and threshold voltages, the circuit achieves a balance between current consumption and switching speed. The pull-up and pull-down transistors are designed with specific W/L ratios that allow fast switching while maintaining low steady-state current 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 configuration effectively drives large driving transistors, reduces brightness variation, and improves image quality by ensuring timely writing operations and stable power signals, enhancing the reliability and efficiency of the light-emitting device.
Implementation Method 1
a light-emitting element which emits light having the intensity corresponding to an amount of a driving current
Data Source
AI summary
A pixel circuit includes a light-emitting element which emits light having the intensity corresponding to an amount of a driving current; a driving transistor which supplies the driving current to the light-emitting element; a storing circuit which writes a data signal instructing the light-emitting brightness of the light-emitting element during a writing period of time to store the data signal; and a buffer circuit which supplies a signal output from the storing unit to the driving transistor.