Capacitive Inverter Circuit for OLED Luminance Uniformity
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Solution Overview
Problem
In organic EL display devices using the active matrix system, variations in threshold voltage and mobility of transistors lead to uneven luminance and increased power consumption due to the inherent characteristics of existing inverter circuits, which affect the uniformity and efficiency of the display.
Innovation Solution
The proposed inverter circuit incorporates specific configurations with multiple transistors and capacitive elements to minimize the time overlap of transistor activation, reducing through current and power consumption, and utilizes advanced capacitive element connections to mitigate the influence of threshold voltage variations on output voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional inverter circuit with two transistors is used in the write scan circuit, then the circuit structure is simple, but through current increases and power consumption increases due to transistor activation overlap
Solution Approach 1:
The inverter circuit is segmented into multiple transistor stages (first inverter circuit with T1-T2, second inverter circuit with T3-T4, and output stage with T5-T6) rather than using a simple two-transistor design. This segmentation allows for controlled transistor activation sequences that prevent simultaneous conduction, thereby reducing through current while maintaining circuit functionality.
Solution Approach 2:
The circuit employs periodic control of transistor activation through capacitive coupling mechanisms. The capacitors C1-C3 store and transfer charge in a sequential manner, creating a timing sequence where transistors are activated and deactivated in a periodic fashion. This ensures that transistors in series do not conduct simultaneously, eliminating through current while maintaining the inverter function.
2Device complexity
If threshold voltage variations of transistors are not compensated, then the circuit operation is simple, but output voltage varies and luminance uniformity deteriorates
Solution Approach 1:
The circuit implements a feedback mechanism through capacitive coupling where the output voltage variations are fed back to the gate of the drive transistor via capacitors C1-C3. When output voltage varies due to transistor threshold voltage variations, the capacitors transfer this voltage change to the gate, creating a compensating effect that stabilizes the drive transistor operation and maintains luminance uniformity across pixels.
Solution Approach 2:
The circuit changes the operating parameters of the transistors dynamically through capacitive charge transfer. The capacitors C1-C3 store charge at specific voltage levels and transfer it to adjust the gate-source voltage of transistors, effectively compensating for threshold voltage variations. This parameter adjustment ensures consistent drive transistor operation despite manufacturing variations, improving luminance uniformity.
Data Source
AI summary
An inverter circuit includes: first and second transistors connected between first and second voltage lines; a fifth transistor having a drain connected to a fifth voltage line and a source connected to a gate of the second transistor; a first capacitive element between a gate and the source of the fifth transistor; a second capacitive element between a first input terminal and the source of the fifth transistor; and the third capacitive element between a second input terminal and the source of the fifth transistor. A first pulse signal into the first input terminal has a phase advanced more than a second pulse signal into the second input terminal. The second pulse signal is switched while the gate of the fifth transistor and the first voltage line are connected. The first pulse signal is switched while the gate of the fifth transistor and the first voltage line are unconnected.