OLED Pixel Circuit Intermediate Voltage Switching
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Solution Overview
Problem
Organic light-emitting display devices for VR devices face challenges in reducing inrush current and electric current requirements during global shutter operation, which can lead to visual distortions and increased electromagnetic interference.
Innovation Solution
The implementation of a pixel circuit with a first and second switch, and a driving transistor, where the OLED is turned on and off by coupling the driving transistor to different voltage levels, and the OLED's electrode is controlled to an intermediate voltage to mitigate voltage fluctuations and inrush current, using a global shutter control circuit with a first switch, a second switch, and a controller to manage these transitions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the OLED is turned on and off directly using standard voltage switching, then the switching operation is simple, but large inrush current occurs causing electromagnetic interference and visual distortions
Solution Approach 1:
An intermediate voltage node is introduced between the OLED and the second switch. During the non-emission period, the second switch connects the OLED cathode to the intermediate voltage node (at voltage Vint) instead of directly to VSS. This intermediate node acts as a mediator that gradually transitions the voltage level, preventing sudden voltage changes and the associated inrush current when switching between emission and non-emission states.
Solution Approach 2:
The voltage parameter at the OLED cathode is changed from direct switching between VSS and VDD to switching between VSS and Vint (intermediate voltage). By modifying the voltage transition parameters to include an intermediate level, the patent reduces the voltage swing magnitude during switching, thereby reducing inrush current while maintaining the global shutter functionality.
2Reliability
If the OLED is kept on during the non-emission period for simple control, then the control operation is easy, but visual distortions occur due to incomplete pixel turn-off
Solution Approach 1:
The control period is segmented into two distinct phases: a non-emission period (first period) where the OLED is completely turned off and data is written/hold, and an emission period (second period) where the OLED emits light. This segmentation allows complete pixel turn-off during the non-emission period, preventing visual distortions from incomplete switching, while the control circuit manages this through systematic timing signals.
Solution Approach 2:
During the non-emission period before the emission period, the control circuit performs preliminary actions including writing display data to the pixel circuit and holding it in the capacitor. This preliminary data preparation ensures that when the emission period begins, the pixels are ready to emit with correct data, and the complete turn-off during this preliminary period prevents any visual distortion from residual light.
3Object-affected harmful factors
If standard voltage switching is used without intermediate voltage, then the circuit operation is simple, but voltage fluctuations cause inrush current and EMI
Solution Approach 1:
An intermediate voltage node is introduced between the OLED and the second switch. During the non-emission period, the second switch connects the OLED cathode to the intermediate voltage node (at voltage Vint) instead of directly to VSS. This intermediate node acts as a mediator that gradually transitions the voltage level, preventing sudden voltage changes and the associated inrush current when switching between emission and non-emission states.
Solution Approach 2:
The intermediate voltage node provides a cushioning effect by absorbing the voltage transition stress. During switching transitions, particularly when turning the OLED off, the intermediate voltage acts as a buffer that prevents abrupt voltage changes, thereby cushioning against inrush current and electromagnetic interference before the full voltage transition occurs.
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 reduces the inrush current by half and minimizes voltage fluctuations, improving the display's performance by ensuring simultaneous pixel emission and reducing electromagnetic interference.
Implementation Method 1
An organic light-emitting display device employs a self-luminous element using a thin emission layer between the electrodes
Data Source
AI summary
A display device includes a driving transistor, an organic light emitting diode (OLED), a first switch, and a second switch. The driving transistor has a first terminal (e.g., a drain terminal) and a second terminal (e.g., a source terminal). The OLED includes a first terminal coupled to the second terminal of the driving transistor. The first switch is configured to couple the first terminal of the driving transistor to a first voltage (e.g., VDD) to turn on the OLED, and to couple the first terminal to an intermediate voltage to turn off the OLED. The second switch is configured to couple a second electrode of the OLED to a second voltage (e.g., VSS) to turn on the OLED, and to couple the second electrode of the OLED to the intermediate voltage to turn off the OLED. The intermediate voltage is in between the first voltage and the second voltage.