OLED Pixel Circuit Equalizing Transistor Currents
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Solution Overview
Problem
In conventional OLED display pixels, the difference in threshold voltage shift between transistors leads to unequal currents, resulting in inconsistent luminance and reduced OLED lifespan due to unequal turn-on durations and current flow through transistors.
Innovation Solution
A pixel circuit comprising a switch circuit, first and second transistors, a switch, a capacitor, and an organic light emitting element, where the switch circuit provides first and second currents based on a data signal and scanning signal, ensuring equal voltage across the capacitor and balanced current flow through both transistors, even when the scanning signal is disabled, thereby maintaining consistent luminance and reducing threshold voltage shift.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the first transistor is turned on only when the scanning signal is enabled while the second transistor is always turned on, then the circuit can operate with simpler control logic, but the threshold voltage shift becomes different between the two transistors resulting in unequal currents
Solution Approach 1:
The patent applies preliminary action by pre-charging the capacitor during the scanning signal enabled period before the scanning signal is disabled. This preliminary charging ensures that when the scanning signal is disabled, both transistors have had sufficient time to experience similar threshold voltage shifts, equalizing their currents without requiring complex real-time control.
Solution Approach 2:
The patent maintains continuity of useful action by keeping the second transistor continuously turned on while ensuring the first transistor is also turned on during the scanning signal enabled period. This continuous operation allows both transistors to experience similar operating conditions and threshold voltage shifts, resulting in equal currents flowing through both transistors.
2Stability of the object's composition
If the second transistor is always turned on to maintain continuous current flow, then the OLED can maintain stable luminance, but the threshold voltage shift in the second transistor differs from the first transistor leading to current inequality
Solution Approach 1:
The patent uses preliminary action by pre-charging the capacitor during the scanning signal enabled period before the scanning signal is disabled. This preliminary action ensures that both transistors experience similar threshold voltage shifts during their respective on-periods, equalizing their currents while maintaining stable OLED luminance.
Solution Approach 2:
The patent applies parameter changes by dynamically adjusting the gate-source voltage of the transistors through capacitor charging during the scanning signal enabled period. This voltage adjustment ensures both transistors operate under similar voltage conditions and experience comparable threshold voltage shifts, resulting in equal currents.
3Device complexity
If different turn-on durations are used for the first and second transistors, then the circuit can be simplified with fewer control signals, but the threshold voltage shift becomes unequal causing different currents
Solution Approach 1:
The patent applies preliminary action by pre-charging the capacitor during the scanning signal enabled period before the scanning signal is disabled. This preliminary charging action ensures that both transistors have experienced similar threshold voltage shifts during their on-periods, equalizing their currents without requiring additional control signals to balance the turn-on durations.
Data Source
AI summary
A pixel of display is used in an Organic light emitted diode (OLED) display. Driving Circuit of the OLED display outputs a data current and a scanning signal. The pixel of display includes a switch circuit, a first transistor, a second transistor, a capacitor, a switch and a lighting emitting element. The switch circuit, which is controlled by the scanning signal, includes an input end for receiving the data current. When the scanning signal is enabled, a first and a second ends of the switch circuit respectively output a first and a second currents. The second current charges the capacitor. The voltage between the gate and the source of the first transistor is stored in the capacitor. When the scanning signal is disabled, the switch is turned on. The first and the second transistors respectively output currents corresponding to the cross-voltage of the capacitor to the lighting emitting element.