PWM Pixel Circuit for Display Panel Light Emission Duty Ratio
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Solution Overview
Problem
Conventional inorganic LED display panels face challenges with low light emission duty ratio and high power consumption due to limitations in existing active matrix driving methods, particularly with false contour noise in digital PWM methods and cost issues with CMOS transistors.
Innovation Solution
A display panel configuration with PWM pixel circuits that allow simultaneous data setting and light emission, using a combination of NMOS and PMOS transistors, and a driving method that optimizes light emission duty ratio by controlling the driving time of light emitting elements through a sweep voltage, enabling stable data voltage setting and high light emission duty ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a digital PWM method is used to express gray scale with sub fields, then false contour noise occurs, but increasing the number of sub fields reduces the light emission duty ratio
Solution Approach 1:
The patent uses a periodic sweep signal (triangle wave) to modulate the gate voltage of the control transistor, creating periodic turning-on and turning-off actions. This analog PWM approach allows continuous gray scale control through voltage amplitude variation rather than discrete sub field counting, eliminating false contour noise while maintaining high light emission duty ratio.
2Use of energy by moving object
If a CMOS type transistor is used for analog PWM method, then low power consumption is achieved, but it cannot be applied to oxide TFT and increases cost for LTPS TFT
Solution Approach 1:
The patent employs a single-type transistor (either NMOS or PMOS) instead of CMOS transistor, using a simpler and more manufacturable device structure compatible with oxide TFT and LTPS TFT technologies. While CMOS offers low power consumption, the single-type transistor approach achieves acceptable power consumption with significantly reduced manufacturing complexity and cost, making it suitable for large-scale production.
3Ease of operation
If a conventional single type transistor method is used, then setting of PWM data voltage and light emission cannot be performed simultaneously, but this limits the light emission duty ratio
Solution Approach 1:
The patent stores the PWM data voltage in advance in a storage node (capacitor) connected to the gate of the control transistor. This preliminary action allows the data voltage to be prepared and held ready, enabling simultaneous operation of data setting and light emission phases. The stored voltage is then modulated by the sweep signal during the light emission period, achieving high duty ratio without operational conflicts.
Data Source
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Figure 5A~5B
AI summary
A display panel includes a plurality of pixels arranged in a matrix, the plurality of pixels respectively including a plurality of sub pixels. The plurality of sub pixels respectively includes a light emitting element, and a PWM pixel circuit configured to control a light emitting duration of the light emitting element, based on a pulse width modulation (PWM) data voltage and a sweep voltage. A plurality of PWM pixel circuits included in the display panel are driven, for each of row lines of the plurality of pixels, in an order of a data setting period for setting the PWM data voltage and then a light emitting period in which the light emitting element emits light during a duration corresponding to the set PWM data voltage according to a change of the sweep voltage.