LED Failure Detection Using Multi-Current Voltage Difference
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Solution Overview
Problem
Existing methods fail to accurately detect LED failures in strings, particularly in closed-circuit failures, due to temperature dependencies and the lack of a stable forward voltage during start-up, which can lead to undetected failures and aesthetic or functional issues in applications like automotive lighting.
Innovation Solution
A method and controller that measure the voltage difference between high and low current levels, where minority-carrier diffusion current dominates, to determine a calibration value and detect failures by comparing subsequent measurements to a threshold, allowing for compensation of optical output and potentially eliminating the need for separate temperature sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If voltage across the string is measured at a single current level, then the measurement is simple, but temperature dependencies cause false detection results
Solution Approach 1:
The patent transitions from measuring voltage at a single current level (one-dimensional measurement) to measuring voltage at multiple current levels (multi-dimensional measurement). By introducing the current level dimension, the system can distinguish between temperature-induced voltage changes and failure-induced voltage changes, as the voltage-current relationship differs for operational versus failed LEDs at different current levels.
Solution Approach 2:
The patent changes the measurement parameter from a fixed single current level to multiple variable current levels. By applying different current levels and measuring the corresponding voltage differences, the system creates a voltage-current characteristic profile that enables accurate failure detection independent of temperature variations.
2Loss of time
If forward voltage is measured during start-up phase, then early detection is possible, but the forward voltage is unstable and detection reliability is poor
Solution Approach 1:
The patent implements preliminary calibration measurements during the start-up phase when LEDs are operational, storing these measurements as reference values. This preliminary action establishes a baseline for comparison with subsequent measurements, enabling the system to detect failures even during dynamic conditions like start-up, provided the calibration was performed when LEDs were functioning normally.
Solution Approach 2:
The system uses feedback by comparing subsequent voltage measurements against previously stored calibration values. This feedback mechanism allows the system to detect deviations indicating LED failures while accounting for normal operational variations, thereby improving detection reliability during dynamic phases like start-up.
3Measurement precision
If multiple current levels are used for measurement, then temperature independence is achieved, but the measurement process becomes more complex
Solution Approach 1:
The patent implements periodic measurement cycles where the controller alternates between applying different current levels to the LED string and measuring the corresponding voltages. This periodic action structure organizes the complex multi-level measurements into manageable cycles, making the process systematic and implementable without excessive complexity.
Solution Approach 2:
The patent combines multiple voltage measurements at different current levels into a single diagnostic parameter (voltage difference or ratio). By merging the information from multiple measurements into one composite indicator, the system achieves temperature-independent failure detection while minimizing the complexity of the measurement and processing requirements.
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 approach provides a reliable method for detecting LED failures, reducing the risk of undetected short-circuit failures and enabling accurate detection without additional temperature sensors, thus ensuring consistent performance in LED lighting systems.
Implementation Method 1
A light emitting diode (LED) comprises a p-n junction and is designed to emit light when current passes through the p-n junction
Implementation Method 2
the current through an un-failed LED is dominated by minority-carrier diffusion current
Data Source
Figure 1~2b
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Figure 5~6
AI summary
Disclosed herein is a method of detecting an LED failure in a series-connected string of LEDs using a parameter indicative of a voltage difference between a voltage across the string at a predetermined relatively high current and a voltage across the string at a predetermined relatively low current, wherein at both the relatively low current and the relatively high current levels the current through an un-failed LED is dominated by minority-carrier diffusion current, The method comprises: determining a calibration value of the parameter during a first start-up phase; storing the calibration value in a store; determining a subsequent value of the parameter during a subsequent measurement phase; and, in response to a difference between the calibration value and the subsequent value exceeding a threshold, determining that an LED failure has occurred. The disclose extends to controllers configured to detect an LED failure in a string of LEDs, to LED lighting units comprising such controllers, and to lighting subsystems, for instance automobile lighting subsystems, which include one or more such LED lighting units.