Current-Driven Pixel Circuit for OLED Threshold Voltage Compensation
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Solution Overview
Problem
Conventional pixel circuits for organic EL displays face challenges in maintaining consistent display quality due to variations in threshold voltage of driving TFTs, leading to limitations in compensation periods and degradation in display quality, especially with voltage drops in power supply wiring lines affecting gate terminal potentials.
Innovation Solution
The proposed electric current driving type display device incorporates a pixel circuit design with specific switching elements and capacitors that allow for flexible compensation of threshold voltage variations, maintaining control terminal potentials during light emission and preventing unwanted light emission, thereby enhancing display quality and reducing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a voltage control type drive scheme is adopted in an organic EL display, then the circuit design is simpler, but it is very difficult to suppress variations in the luminance of the organic EL element
Solution Approach 1:
The patent replaces voltage control with current control for driving the organic EL element. By using a current control type drive scheme instead of voltage control, the luminance variations caused by threshold voltage shifts and mobility changes are suppressed, while maintaining reasonable circuit complexity through dedicated current control circuits.
Solution Approach 2:
The patent changes the control parameter from voltage to current. By controlling the current flowing through the organic EL element rather than the voltage applied to it, the system achieves better luminance stability since current control is less susceptible to variations in TFT characteristics such as threshold voltage and mobility.
2Reliability
If an electric current program scheme is used to compensate for variations in TFT characteristics, then variations in threshold voltage and mobility are compensated, but it is difficult to design a pixel circuit and drive circuit due to handling very small amounts of current
Solution Approach 1:
The patent introduces a current control circuit as an intermediary between the data signal and the organic EL element. This current control circuit converts voltage signals into precise current signals, enabling accurate current control while simplifying the overall pixel circuit design by handling current control in a dedicated block rather than directly in the pixel circuit.
Solution Approach 2:
The patent divides the drive circuit into separate functional blocks: a pixel circuit that handles basic switching and signal input, and a dedicated current control circuit that handles the precise current regulation. This segmentation allows each block to be optimized independently, reducing the complexity of the pixel circuit while maintaining accurate current control capability.
3Ease of operation
If the potential of the gate terminal of the driving TFT is affected by voltage drops in power supply wiring lines, then the control becomes simpler, but the display quality degrades due to inability to maintain consistent control terminal potential during light emission
Solution Approach 1:
The patent implements a feedback mechanism through the current control circuit that continuously monitors and adjusts the current flowing through the organic EL element. By using feedback control, the system maintains consistent luminance output despite voltage drops in power supply wiring lines, as the current control circuit compensates for voltage variations by adjusting the drive current accordingly.
Solution Approach 2:
The patent performs preliminary compensation for voltage drops by using the current control circuit to pre-adjust the drive current before it reaches the organic EL element. This preliminary action ensures that the actual current through the element remains stable even when power supply voltage fluctuates, thereby maintaining display quality consistency.
Data Source
AI summary
In a pixel circuit 100, a switching TFT 114, a driving TFT 110, and an organic EL element 130 are provided between a power supply wiring line Vp and a common cathode Vcom and a capacitor 121 and a switching TFT 111 are provided between a gate terminal of the driving TFT 110 and a data line Sj. A switching TFT 112 is provided between a connection point A between the capacitor 121 and the switching TFT 111 and a power supply wiring line Vr, a switching TFT 113 is provided between the gate and drain terminals of the driving TFT 110, and a capacitor 122 is provided between the gate terminal of the driving TFT 110 and the power supply wiring line Vr. Thus, a display device is provided that can freely set a period during which variations in the threshold voltage of a drive element are compensated for, and performs high-quality display by holding a control terminal potential of the drive element during light emission from an electro-optical element.