OLED Pixel Bias Circuit for Flicker-Free Variable Refresh Switching
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Solution Overview
Problem
In organic EL display devices with a variable refresh rate system, still image display at a low refresh rate leads to flicker due to the hysteresis characteristic of thin film transistors, and switching to high refresh rate for moving images is delayed without proper refresh operation.
Innovation Solution
A display device with at least two operation modes (low and high refresh modes) that includes a bias application circuit to apply a bias voltage to the driving transistor, allowing quick switching between modes while suppressing flicker.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a variable refresh rate system is adopted to reduce power consumption during still image display, then power consumption is reduced, but flicker occurs due to the hysteresis characteristic of thin film transistors
Solution Approach 1:
The patent applies a bias voltage to the driving transistor before the refresh operation to compensate for the hysteresis characteristic. This preliminary action ensures that the transistor's threshold voltage is adjusted in advance, preventing the flicker that would otherwise occur during the low refresh rate operation while still maintaining power savings.
Solution Approach 2:
The patent changes the voltage parameter applied to the driving transistor by introducing a bias voltage during the non-refresh frame period. This parameter change compensates for the hysteresis effect of the thin film transistor, allowing the system to operate at low refresh rates without producing flicker, thus resolving the contradiction between power consumption and flicker suppression.
2Use of energy by moving object
If refresh operation is stopped during non-refresh frame periods to reduce power consumption, then power consumption is reduced, but switching response to moving images is delayed
Solution Approach 1:
The patent maintains the bias voltage application during non-refresh frame periods, which keeps the driving transistor in a ready state. When switching to a refresh frame period for moving image display, the transistor is already prepared with the appropriate voltage characteristics, enabling faster response without requiring a full refresh operation to initialize the transistor state.
3Use of energy by moving object
If the organic EL element is turned off during non-light emission periods in refresh frame periods, then power consumption is reduced, but the turning off is visually recognized as flicker
Solution Approach 1:
The patent ensures continuous bias voltage application to the driving transistor across both refresh and non-refresh frame periods. This continuous action maintains the transistor's operational characteristics, ensuring smooth transitions between light emission and non-light emission periods, thereby preventing visible flicker while still allowing the organic EL element to be turned off for power savings.
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
Enables quick mode switching from low to high refresh rate without flicker, reducing power consumption and response delay in organic EL display devices.
Implementation Method 1
since a thin film transistor as a driving transistor in a pixel circuit has a hysteresis characteristic, flicker is still visually recognized when driven low
Implementation Method 2
The organic EL element is a self-luminous display element that emits light with luminance corresponding to a current flowing therethrough
Data Source
AI summary
In a current drive type display device of a variable refresh rate system such as an organic EL display device, an on-bias voltage line for supplying an on-bias voltage Vobs to be applied to a driving transistor in a pixel circuit is provided in a display portion in order to reduce an influence of a hysteresis characteristic of the driving transistor on display luminance. Therefore, when an operation mode is switched from the low refresh mode to the high refresh mode, among pixel circuits in the same column connected to the same data signal line, a pixel circuit to which data is written and a pixel circuit to which the on-bias voltage is applied can be mixed. This enables quick switching from the low refresh rate to the high refresh rate.