LED Forward Voltage Calibration for Temperature Compensation
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Solution Overview
Problem
Conventional LED devices fail to accurately maintain desired luminous flux and color points over temperature variations due to individual LED differences, non-linear relationships between luminous flux and junction temperature, and drive current, leading to issues like brightness banding and flicker.
Innovation Solution
A method for calibrating and compensating individual LEDs by measuring forward voltage with a small drive current, using interpolation techniques to determine necessary drive currents based on stored calibration values, and adjusting drive currents to maintain desired luminous flux and color points, while avoiding undesirable visual artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If temperature sensors and heat sinks are added to the LED chip to measure temperature, then temperature measurement capability is improved, but device cost increases and chip real estate is consumed
Solution Approach 1:
The LED device uses its own forward voltage characteristics to determine junction temperature without requiring external temperature sensors. The forward voltage of an LED naturally varies with junction temperature, and by measuring this forward voltage, the system can derive temperature information that serves the temperature compensation function internally.
Solution Approach 2:
The patent uses forward voltage measurement as an intermediary parameter to indirectly obtain junction temperature information. Instead of directly measuring temperature with sensors, the system measures forward voltage which correlates with temperature, and uses this intermediate measurement to drive the temperature compensation process.
2Reliability
If conventional temperature compensation methods are used with temperature sensors, then temperature compensation is provided, but brightness banding and flicker artifacts occur
Solution Approach 1:
The system continuously monitors forward voltage and uses this feedback to dynamically adjust drive currents for temperature compensation. By measuring forward voltage during operation and comparing it against reference values, the system generates feedback signals that drive real-time compensation adjustments, maintaining stable luminous flux without visual artifacts.
Solution Approach 2:
The patent performs preliminary calibration during manufacturing to establish reference forward voltage values at known temperatures. These pre-established references are stored and used during operation to guide real-time compensation decisions, allowing the system to anticipate and correct temperature drift before it causes visible artifacts.
3Illumination intensity
If drive current is increased to compensate for luminous flux decrease with temperature, then luminous flux is maintained, but individual LED variations cause non-uniform compensation
Solution Approach 1:
The patent applies temperature compensation individually to each LED or LED string by measuring forward voltage separately for each unit. This segmentation allows the system to account for manufacturing variations between individual LEDs, as each LED's unique forward voltage characteristics are measured and compensated independently rather than applying a uniform compensation to all LEDs.
Solution Approach 2:
The system tailors compensation parameters to local conditions of each LED by measuring its specific forward voltage and using this localized information to determine the appropriate drive current adjustment. This local quality approach ensures that each LED receives customized compensation based on its individual characteristics rather than a generic compensation scheme.
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 precise temperature compensation and prevents brightness banding and flicker, ensuring consistent luminous flux and color across temperature changes.
Implementation Method 1
measuring a forward voltage developed across each LED, one LED at a time, during the short durations of time the plurality of LEDs are turned off
Data Source
AI summary
An illumination device comprising a plurality of light emitting diodes (LEDs) and a method for controlling the illumination device, so as to maintain a desired luminous flux and/or a desired color point of the device over variations in temperature and process, is provided herein. According to one embodiment, the method may include measuring a forward voltage developed across a first LED of the illumination device upon applying a first drive current to the first LED, determining a drive current needed to achieve a desired luminous flux from the first LED using the measured forward voltage, a table of stored calibration values correlating forward voltage and drive current to luminous flux at a plurality of different temperatures, and one or more interpolation techniques, and driving the first LED with the determined drive current to produce illumination having the desired luminous flux. The steps of measuring, determining and driving may be performed for each of the plurality of LEDs.