OLED Pixel Driving Circuit Threshold Voltage Compensation
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Solution Overview
Problem
In organic light emitting diode (OLED) pixel circuits, the difficulty in maintaining identical threshold voltages for transistors in a current mirror configuration leads to inconsistent drive currents, degrading display quality due to threshold voltage drift during the fabrication process.
Innovation Solution
The implementation of an organic light emitting diode pixel driving circuit with an external circuit and intra-pixel circuits, where each intra-pixel circuit includes a signal loading module, a driving transistor, and an OLED, allowing the gate of the driving transistor to be connected with its drain during signal loading, ensuring the gate voltage is the sum of the image data signal and the threshold voltage, thereby making the drain current independent of the threshold voltage when the OLED is driven.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If transistors T2 and T3 are arranged in a current mirror configuration adjacent to each other on the substrate, then the circuit can generate drive current for the OLED, but the threshold voltage of the two TFTs cannot be made substantially the same due to TFT parameter variations in the fabrication process, leading to inconsistent drive currents and degraded display quality
Solution Approach 1:
The patent changes the circuit configuration from a conventional current mirror to a source follower configuration where the driving transistor's gate is connected to its drain. This parameter change in circuit topology makes the drive current independent of the transistor's threshold voltage, thereby resolving the issue of threshold voltage non-uniformity affecting display quality
Solution Approach 2:
The patent replaces the traditional current mirror mechanism with a source follower mechanism. Instead of relying on matched transistor parameters (which are difficult to achieve due to fabrication variations), the new mechanism uses the transistor's inherent source follower characteristics to generate stable drive current that is independent of threshold voltage variations
2Reliability
If the gate of the driving transistor is connected with its drain during signal loading, then the drain current becomes independent of the threshold voltage, but the circuit requires additional switching control to manage the connection states
Solution Approach 1:
The patent merges the gate and drain connections of the driving transistor during the signal loading phase, creating a unified node that simultaneously serves as both gate and drain. This merging eliminates the need for separate gate and drain terminals, simplifying the control mechanism while achieving threshold voltage independence
Solution Approach 2:
The patent employs periodic switching action to alternate between connection states. During the signal loading phase, the gate and drain are connected; during the signal output phase, they are disconnected. This periodic switching enables the circuit to achieve drive current consistency while managing the necessary control complexity through time-multiplexed operation
3Area of stationary object
If multiple pixel elements share the same external circuit, then the number of devices and size of pixel elements are reduced, but the circuit must efficiently manage signal loading for multiple pixels simultaneously
Solution Approach 1:
The patent segments the pixel array into groups that share common external circuits. Each pixel element within a group uses the same external circuit resources, but the signal loading is segmented into discrete phases controlled by scan signals. This segmentation allows efficient resource sharing while maintaining independent control over each pixel's signal loading process
Solution Approach 2:
The patent implements preliminary action by pre-configuring the external circuits to serve multiple pixel elements. The signal loading process is prepared in advance with dedicated scan signals for each pixel row, allowing the shared external circuits to efficiently manage multiple pixels through pre-synchronized control signals rather than reacting to each pixel individually
Data Source
AI summary
An organic light emitting diode pixel driving circuit includes an external circuit and multiple intra-pixel circuits. Each of the intra-pixel circuits includes a signal loading module, a driving transistor and an organic light emitting diode. Each of the signal loading modules is configured to store an image data signal and the threshold voltage of the driving transistor as a drive signal and load the drive signal to the gate of the driving transistor in a signal loading phase, and to control the driving transistor by the drive signal stored in the signal loading phase and a signal at a source of the driving transistor to drive the organic light emitting diode to emit light in a light emitting phase. The external circuit is configured to load a first power supply signal to the source of the driving transistor in the light emitting phase.