OLED Pixel Circuit Low-Frequency Luminance Drift
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Solution Overview
Problem
OLED display devices experience a gradual increase in luminance when driven at low frequencies due to threshold voltage shifts, leading to increased power consumption and reduced efficiency, especially in portable devices like smartphones.
Innovation Solution
The design of an OLED display device pixel that includes a capacitor and transistors configured to perform normal and low-frequency driving operations, with specific initialization and threshold voltage compensation periods, and a unique application of gate-source voltage to prevent luminance increase, utilizing PMOS and NMOS transistors to reduce leakage current and maintain constant luminance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If the OLED display device is driven at low frequency to reduce power consumption, then power consumption is reduced, but luminance gradually increases due to threshold voltage shifts
Solution Approach 1:
The patent applies preliminary action by initializing the capacitor and applying compensation voltages before the low-frequency driving period begins. The first transistor initializes the capacitor to a reference voltage, and the fourth transistor receives compensation voltages that preemptively counteract expected threshold voltage shifts, preventing luminance drift before it occurs during the extended low-frequency operation.
Solution Approach 2:
The patent utilizes parameter changes by dynamically adjusting the gate-source voltage of the fourth transistor (driving transistor) based on detected threshold voltage shifts. The compensation voltage applied to the gate of the fourth transistor changes in response to measured threshold voltage variations, thereby maintaining constant luminance despite operating at low driving frequencies where threshold voltage drift is pronounced.
2Reliability
If gate-source voltage with high absolute value is applied to driving transistor during low frequency driving, then threshold voltage shifts in negative direction, but subsequent positive shift increases luminance
Solution Approach 1:
The patent implements feedback by detecting the threshold voltage of the fourth transistor and using this information to adjust the gate-source voltage applied to the transistor. The detection circuit measures threshold voltage shifts, and this detected information feeds back to the voltage application circuit, which modifies the compensation voltage to counteract the measured drift, thereby stabilizing luminance despite repeated high absolute value voltage applications.
Solution Approach 2:
The patent applies preliminary anti-action by introducing a compensation voltage that acts in opposition to the expected threshold voltage shift before the shift fully manifests. The compensation voltage is applied to the gate of the fourth transistor in advance and in the opposite direction of the anticipated threshold voltage drift, preventing the harmful luminance increase before it occurs.
3Ease of operation
If capacitor is initialized with high absolute value gate-source voltage, then capacitor is initialized, but driving transistor experiences threshold voltage shift
Solution Approach 1:
The patent applies local quality by making a distinction between the initialization voltage applied to the capacitor and the gate-source voltage applied to the driving transistor. The first transistor initializes the capacitor with an appropriate voltage, while the fourth transistor (driving transistor) receives a specially designed compensation voltage that is tailored to minimize threshold voltage shifts. This localized differentiation of voltage application strategies allows effective capacitor initialization while protecting the driving transistor from harmful voltage-induced threshold shifts.
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
The solution effectively prevents the gradual increase in luminance at low frequency driving, reducing power consumption and maintaining efficiency, suitable for use in portable devices by ensuring consistent light emission across different driving frequencies.
Implementation Method 1
an organic light emitting diode (OLED) display device
Data Source
AI summary
A pixel of an OLED display device includes a capacitor coupled between first and second nodes, first and second transistors, each including a gate receiving a respective initialization signal, a first terminal receiving a first power supply voltage, and a second terminal coupled to the capacitor, a third transistor including a first terminal coupled to a data line and a second terminal coupled to the first node, a fourth transistor including a gate coupled to the second node, a first terminal receiving the first power supply voltage, and a second terminal coupled to a third node, a fifth transistor including a first terminal coupled to the third node and a second terminal coupled to the second node, sixth and seventh transistors receiving a scan signal, eighth and ninth transistors receiving an emission signal, and an OLED.