OLED Pixel Driving Circuit Threshold Shift Compensation

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Solution Overview

Problem

Existing pixel driving circuits for OLED displays face issues with threshold voltage shifts in switches, leading to incorrect grey level display and increased power consumption, particularly due to the use of a-Si TFTs, which result in reduced aperture ratio and higher complexity in designing high-definition panels.

Innovation Solution

The proposed pixel driving circuit incorporates a first switch, a capacitor, and a second switch with n-type metal oxide semiconductors, where the second switch is controlled by a separate scan signal to manage the discharge of the capacitor's stored data voltage to OLEDs, making the circuit insensitive to threshold voltage shifts and reducing power consumption by adjusting the voltage across the second switch or the number of OLEDs in series.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If a-Si TFT is used in the pixel driving circuit, then the device can be manufactured with lower cost and simpler process, but the threshold voltage shift is larger causing incorrect grey level display

Engineering Contradiction:

Improvemanufacturing simplicityVSAvoidthreshold voltage stability

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

A compensation capacitor (Ccomp) is introduced as an intermediary component between the data storage capacitor (Cdata) and the OLED. This capacitor compensates for the threshold voltage shift in the second switch by storing additional charge that makes up for the voltage loss due to threshold shift, thereby maintaining accurate grey level display despite using a-Si TFT with larger threshold voltage shift

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The invention changes the voltage parameter by introducing a compensation voltage (Vcomp) through the compensation capacitor. This compensation voltage dynamically adjusts the voltage at the OLED cathode to offset the threshold voltage shift effect, ensuring that the current through the OLED remains accurate for proper grey level display

Inventive Principle:

Principle #35Parameter changes

2Reliability

If compensated circuits are added to solve threshold voltage shift, then display accuracy is improved, but the number of components increases reducing aperture ratio

Engineering Contradiction:

Improvegrey level display accuracyVSAvoidaperture ratio

Core Design Contradiction:

ReliabilityVSArea of stationary object

Solution Approach 1:

The compensation function is merged with the existing data storage capacitor (Cdata) and second switch (M2) structure. The compensation capacitor (Ccomp) is connected in parallel with Cdata, and the compensation mechanism integrates into the existing timing sequence of scan signals, avoiding the need for separate compensation circuits and minimizing additional components

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The compensation capacitor serves multiple functions: it compensates for threshold voltage shift, stores additional charge for the OLED, and works within the existing scan signal timing framework. This multi-functionality reduces the need for dedicated compensation components, preserving aperture ratio

Inventive Principle:

Principle #6Universality (Multi-functionality)

3Use of energy by stationary object

If current is reduced to lower power consumption, then power consumption is reduced, but the brightness of OLED decreases

Engineering Contradiction:

Improvepower consumptionVSAvoidOLED brightness

Core Design Contradiction:

Use of energy by stationary objectVSIllumination intensity

Solution Approach 1:

Inventive Principle:

Principle #35Parameter changes

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

This configuration allows for accurate grey level display and reduced power consumption, maintaining brightness while minimizing the impact of threshold voltage shifts and extending OLED lifespan by controlled charging and discharging.

Implementation Method 1

The capacitor C0 is coupled to the second end of the first switch M1 for storing the data signal Data to keep the voltage level of the pixel grey level

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

the organic light emitting diode (OLED) display can be operated without a backlight source

Methodology Applied

Scientific EffectElectroluminescence: Electroluminescence

Data Source

PatentUS9378680B2Pixel driving circuit, driving method thereof and display panel

Publication Date: 2016.06.28 OPTRONIC SCIENCES LLC

AI summary

A pixel driving circuit includes a first switch, a capacitor, a second switch and at least one organic light emitting diode. The first switch includes a first end for receiving data voltage, a control end for receiving a first scan signal, and a second end for outputting the data voltage. The capacitor includes a first end coupled to the second end of the first switch, and a second end. The second switch includes a first end coupled to the second end of the first switch, a control end for receiving a second scan signal, and a second end. The at least one organic light emitting diode includes a first end coupled to the second end of the second switch, and a second end coupled to the second end of the capacitor.