OLED Pixel Driving Circuit for Stable Data Voltage Writing
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Solution Overview
Problem
In OLED devices, the power supply voltage is reduced during the writing process due to wiring line resistance, leading to incorrect brightness emission, as the driving current flows through the power supply line, affecting the conduction state of the driving transistor and the brightness of the electro-optical device.
Innovation Solution
An electronic circuit is designed with a driving transistor, a capacitor, and switching devices to control the conduction state and intercept the driving current, preventing power supply voltage reduction by isolating the driven device from the power supply line during writing and ensuring correct voltage recording in the capacitor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the data voltage is recorded in the capacitor during the writing process, then the data voltage can be stored for driving the OLED device, but the power supply voltage is reduced due to wiring line resistance when driving current flows, causing incorrect brightness emission
Solution Approach 1:
The patent applies preliminary action by turning off the second switching device before the writing process to prevent driving current from flowing during data voltage recording. This anticipatory action eliminates the voltage drop caused by wiring line resistance before it can occur, ensuring accurate data voltage storage in the capacitor without being affected by subsequent current-driven voltage losses.
Solution Approach 2:
The patent segments the timing of current flow and voltage recording by using the second switching device to separate these two operations. The driving current is supplied only during specific time periods when data writing is not occurring, while during writing operations, the switching device isolates the capacitor from voltage fluctuations caused by current flow, thus resolving the contradiction between reliable voltage recording and energy loss.
2Productivity
If the driving current flows during the writing process, then the OLED device can be driven, but the electric potential of the power supply line changes, causing the voltage maintained in the capacitor to be incorrect
Solution Approach 1:
The second switching device is turned off in advance during the writing process to prevent driving current from flowing at that time. This preliminary action ensures that the power supply line's electric potential remains stable during data voltage recording, allowing the capacitor to accurately maintain the intended voltage without being affected by current-induced potential changes.
Solution Approach 2:
The patent implements periodic action by alternately controlling the second switching device to turn on and off at different time periods. During writing periods, the switching device is off to ensure voltage accuracy; during emission periods, it is on to supply driving current. This periodic control resolves the contradiction by separating the conflicting requirements of continuous driving and precise voltage maintenance into distinct time phases.
3Loss of information
If the capacitor is connected to the power supply line during writing, then the data voltage can be recorded, but the voltage changes when driving current flows due to wiring line resistance
Solution Approach 1:
The patent applies the taking out principle by using the second switching device to extract or remove the harmful effect of wiring line resistance from the data writing process. By turning off the switching device during writing operations, the circuit path that would cause voltage drop due to line resistance is removed, allowing the capacitor to store data voltage accurately without being affected by the resistive effects of the power supply line.
Solution Approach 2:
The second switching device acts as an intermediary element between the capacitor and the power supply line. It controls the connection state to isolate the capacitor from the harmful effects of the power supply line resistance during writing operations, while still allowing connection during emission operations. This intermediary component resolves the contradiction by mediating the interaction between the capacitor and the resistive power supply line at different time phases.
Data Source
AI summary
At a writing time, a first transistor 412 is turned on so that a data signal Xj is supplied to one end of a capacitor 420. At this time, since a second transistor 414 is turned off, driving current does not flow to an organic light emitting diode (OLED) device 430. A power supply voltage Vdd is supplied to the other end of a capacitor through a power supply line L. However, since the driving current does not flow at the writing time, the power supply voltage Vdd is not reduced by the wiring line resistance of the power supply line L. On the other hand, at an emission time, the first transistor 412 is turned off and the second transistor 414 is turned on. Therefore, the driving current is supplied to the OLED device 430.