Pixel Driving Circuit Voltage Division for OLED Black Screen
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Solution Overview
Problem
OLED display panels struggle to achieve a pure black screen and reduced contrast due to negative threshold voltage, as existing pixel driving circuits fail to switch off the driving TFT effectively.
Innovation Solution
A pixel driving circuit with a specific configuration including transistors and storage capacitors, where the ratio of the first storage capacitor to the second storage capacitor is greater than 0.2, allowing for effective voltage division and control of the gate voltage of the first transistor to ensure a black screen is achieved, even with negative threshold voltage drift.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the threshold voltage of the IJP OLED device is low (about 2V), then the activation voltage is reduced, but a pure black screen cannot be achieved when negative drift occurs on the TFT threshold voltage
Solution Approach 1:
The patent divides the gate voltage control into two independent parts using two storage capacitors: Cst1 stores the threshold compensation voltage (Vth_comp) and Cst2 stores the data voltage (Vdata). This segmentation allows separate optimization of threshold compensation and data signaling, enabling the gate voltage to be precisely controlled even with low OLED threshold voltage, thus achieving pure black screens while maintaining low activation voltage.
Solution Approach 2:
The patent introduces a compensation transistor (T3) and reference voltage node (Vref) as intermediary elements between the data signal and the OLED gate. The compensation transistor selectively connects the reference voltage to the gate through Cst1, acting as a mediator that adjusts the gate voltage to account for TFT threshold voltage drift, thereby enabling black screen achievement despite low OLED threshold voltage.
2Reliability
If the reference voltage (Vref) is increased to compensate for negative TFT threshold voltage drift, then black screen can be achieved, but the OLED may be activated during capacitor charging
Solution Approach 1:
The patent segments the voltage storage function into two capacitors: Cst1 dedicated to threshold compensation and Cst2 dedicated to data voltage. This segmentation allows the reference voltage Vref to be applied only to Cst1 through the compensation transistor T3, isolating the compensation function from the data signaling path. Consequently, Vref can be optimized for black screen achievement without causing unintended OLED activation during charging, as each capacitor serves its specific function independently.
Solution Approach 2:
The patent performs threshold voltage compensation in advance by storing Vth_comp in Cst1 before the OLED activation phase. The compensation transistor T3 is controlled to connect Vref to Cst1 during the compensation phase, preparing the gate voltage offset beforehand. This preliminary action ensures that when the OLED is supposed to be off, the gate voltage already includes the compensation offset, achieving black screen without requiring elevated Vref during the active phase that could cause unintended activation.
3Device complexity
If a 3T1C pixel driving circuit is used, then the circuit complexity is reduced, but effective threshold voltage compensation and black screen achievement become difficult when negative drift occurs
Solution Approach 1:
The patent extends the 3T1C circuit to 3T2C by adding one transistor (T3) and one capacitor (Cst1). The segmentation of voltage storage functions between Cst1 (threshold compensation) and Cst2 (data voltage) enables effective threshold voltage compensation. The additional transistor T3 acts as a compensation switch that selectively connects Vref to Cst1, providing the necessary control for threshold compensation while maintaining relatively simple circuit operation.
Solution Approach 2:
The patent changes the circuit parameters by adding Cst1 with a specific capacitance value (0.2-2.0 times Cst2) and introducing the compensation transistor T3 with controlled switching. These parameter changes enable the circuit to perform threshold voltage compensation function. The capacitance ratio between Cst1 and Cst2 is optimized to balance the threshold compensation effectiveness and the prevention of unintended OLED activation, achieving reliable black screen performance with minimal circuit complexity increase.
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 ensures a black screen is maintained and contrast is improved by appropriately designing the capacitance ratio and voltage division, preventing activation of the OLED when a black image is required.
Implementation Method 1
the voltage is divided by using the first storage capacitor (Cst1) and the second storage capacitor (Cst2), and the voltage is pulled down and written to the gate of the first transistor (T1)
Data Source
AI summary
A pixel driving circuit and a driving method thereof, and a display panel are provided. The pixel driving circuit includes a first transistor (T1), a second transistor (T2), a third transistor (T3), a first storage capacitor (Cst1), a second storage capacitor (Cst2) and an organic light emitting element (OLED). By appropriately designing capacitance of the two capacitors and dividing a gate voltage of the first transistor (T1), it can be ensured that a black screen is achieved and contrast of a display panel is improved even though a negative drift is seriously caused on a threshold voltage of T1.