OLED Inrush Current Limiting Circuit Design
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Solution Overview
Problem
Organic light emitting diode (OLED) arrangements experience high inrush currents when turned on, which can damage the devices due to their low impedance connection, and existing solutions like negative temperature coefficient resistors have limitations such as high power dissipation and non-resettable nature.
Innovation Solution
Incorporating a passive circuit with a series inductor that limits current during the initial switch-on interval more than subsequent intervals, utilizing a spiral inductor on a flex foil potentially sandwiched between ferrites and mu metals, to manage inrush currents effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a circuit limits current during the first time interval more than during the second time interval, then protection against inrush current is improved, but device complexity increases
Solution Approach 1:
The patent applies parameter changes by using a negative temperature coefficient resistor (NTC) whose resistance value changes with temperature. During the first time interval (cold state), the NTC presents high resistance to limit inrush current. During the second time interval (heated state), the NTC resistance decreases automatically, reducing current limitation and improving efficiency. This temperature-dependent parameter change resolves the contradiction by providing protection when needed while minimizing impact when not needed.
Solution Approach 2:
The patent employs dynamics through a switchable resistor configuration that can transition between different resistance states. The circuit includes a resistor that can be switched in series during the first time interval to provide strong current limitation, and then switched out or bypassed during the second time interval. This dynamic switching capability allows the system to adapt its protection level based on operational requirements, balancing reliability and complexity.
2Ease of operation
If a passive circuit is used to limit current, then ease of operation is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent applies self-service through the negative temperature coefficient resistor, which automatically adjusts its resistance based on its own temperature without requiring external control. The NTC resistor self-regulates the current limitation function by heating up during operation and naturally reducing its resistance, eliminating the need for complex control circuits or precise manual adjustment mechanisms.
Solution Approach 2:
The passive circuit uses parameter changes in the NTC resistor's temperature-resistance relationship to achieve current limitation. By selecting an NTC with appropriate thermal and electrical characteristics, the circuit achieves effective inrush current protection without requiring high-precision manufacturing tolerances on other components.
3Manufacturing precision
If an active circuit is used to limit current, then manufacturing precision is improved, but ease of operation decreases
Solution Approach 1:
The patent uses an intermediary approach by introducing a switchable resistor as a mediator between the power source and the organic light emitting device. This switchable element provides precise current control when activated during the first time interval, and can be bypassed or deactivated during the second time interval. The intermediary component enables accurate manufacturing precision requirements to be met while maintaining operational simplicity through automated or timer-based switching.
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 solution effectively prevents damage from high inrush currents while maintaining efficiency by providing necessary protection only when needed and reducing power losses, with the inductor offering higher impedance initially and lower impedance as current stabilizes, thus preserving the OLED's performance.
Implementation Method 1
An embodiment of the organic light emitting diode arrangement is defined by the circuit comprising a negative temperature coefficient resistor. A negative temperature coefficient resistor has a relatively large resistance value when being cold and has a relatively small resistance value when being heated up.
Implementation Method 2
An embodiment of the organic light emitting diode arrangement is defined by the circuit comprising a series inductor. A series inductor has a relatively large impedance value when a current starts flowing through this inductor and has a relatively small impedance value when the current has been flowing through this inductor for a while
Data Source
Figure 1~2
Figure 3~4-1
Figure 4-2~5
AI summary
Organic light emitting diode arrangement Organic light emitting diode arrangements (1) are, to protect them against an effect of a switch-on, provided with circuits (31-36) for, during a first time interval that follows a switch-on, limiting a current through the organic light emitting diode arrangement (1) more and for, during a second time interval that follows the first time interval, limiting the current less. The circuit (31-36) may be passive such as a negative temperature coefficient resistor (31) or a series inductor (32) possibly with a freewheel diode (40) or may be active such as a switchable resistor (33) that is not bridged during the first time interval and that is bridged during the second time interval or a switchable resistor that is bridged in response to a detection of a value of the current exceeding a threshold value or such as a part of a converter (63) that is controlled in response to a detection of a value of the current.