Pixel Circuit Driver Switch PWM for OLED Degradation
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Solution Overview
Problem
Current OLED technologies face challenges with electrical non-uniformity and degradation, particularly in drive TFTs, which affect pixel brightness and display quality, and existing solutions like current programming struggle with charging time and parasitic capacitance issues as display sizes increase.
Innovation Solution
The proposed solution involves a pixel circuit with a driver switch that uses pulse-width modulation to control the drive current, allowing for efficient operation at lower current levels and compensating for TFT degradation, while minimizing electrical stress on the drive switch TFT.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If current programming is used to compensate for TFT degradation, then threshold voltage shifts are compensated, but charging time increases significantly for low data currents
Solution Approach 1:
The patent employs pulse-width modulation (PWM) to periodically switch the drive TFT on and off, creating effective current control through temporal modulation. This allows the circuit to achieve current programming effects without requiring continuous low-level current charging, thereby reducing charging time while maintaining degradation compensation capabilities
Solution Approach 2:
The patent changes the operating parameters by switching from direct current programming to voltage programming combined with PWM. This parameter transformation allows the circuit to operate in a regime where charging time is reduced while still achieving the desired current control and degradation compensation through the modulation technique
2Loss of time
If dimensional scaling is used to increase programming current, then charging time is reduced, but pixel area increases which is difficult for high resolution displays
Solution Approach 1:
Instead of scaling current through dimensional changes, the patent changes the programming approach from current to voltage, combined with PWM modulation. This allows achieving fast charging without increasing pixel area, as the voltage programming with PWM effectively delivers the necessary charge in shorter time without requiring larger TFT dimensions
3Reliability
If multiple TFTs are used for threshold voltage compensation, then display quality improves, but circuit complexity increases
Solution Approach 1:
The patent implements a self-service mechanism where the drive TFT compensates for its own threshold voltage shifts and degradation through the PWM control scheme. This eliminates the need for additional compensation TFTs, maintaining display quality while avoiding increased circuit complexity
Solution Approach 2:
The drive TFT serves multiple functions simultaneously: it acts as the current source for the OLED, the switching element for PWM modulation, and the compensation element for its own degradation. This multi-functionality reduces the total number of TFTs required while maintaining display quality
Data Source
AI summary
Briefly, in accordance with one or more embodiments, a pixel circuit to drive an electro-optical element of a display backplane comprises an element driver coupled to the electro-optical element, a programming switch coupled to the element driver, and a driver switch coupled to the programming switch and the element driver. The driver switch is capable of controlling when the element driver is turned on or turned off, and is capable of pulse-width modulating the drive current provided to the electro-optical element, for example to provide a lower drive current to the electro-optical element.