LED Junction Temperature Monitoring via Self-Sensing
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Solution Overview
Problem
Conventional methods for monitoring LED junction temperature are inaccurate as they rely on external temperature sensors that cannot measure the actual LED PN junction temperature, leading to increased system costs and limited ability to monitor real-time self-heating effects.
Innovation Solution
A method that uses the LED itself as a temperature sensor, combined with a silicon diode, employing both PTAT and CTAT techniques to directly measure the LED junction temperature in real-time, allowing for accurate temperature adjustments during operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external temperature sensors (silicon diodes or thermistors) are placed as close to the LED as possible, then the ambient temperature can be measured, but the actual LED junction temperature cannot be accurately measured
Solution Approach 1:
The LED is made to serve dual functions: as the light-emitting device and as the temperature sensor. By measuring the forward voltage of the LED at two different currents and using the PTAT relationship, the same component performs both illumination and temperature monitoring, eliminating the need for separate temperature sensors and reducing system complexity
Solution Approach 2:
The LED monitors its own junction temperature through self-measurement of its forward voltage characteristics. The system uses the LED's inherent electrical properties (forward voltage variation with temperature and current) to detect its own temperature state, making the LED self-diagnostic without requiring external sensing components
2Measurement precision
If pre-characterized thermal information is used to pad the measured temperature, then estimated temperature variations can be accounted for, but system cost increases significantly
Solution Approach 1:
The patent replaces the mechanical/physical approach of adding more LED lamps to compensate for temperature uncertainty with an electrical measurement approach. Instead of increasing quantity to account for temperature variations, the system uses electrical voltage measurements and PTAT calculations to precisely determine temperature, reducing the need for additional components
3Measurement precision
If the CTAT technique is used with pre-characterized LEDs, then temperature sensing can be performed, but a known starting temperature point is required which is not available in actual use
Solution Approach 1:
The system performs preliminary calibration during manufacturing where the LED's forward voltage is measured at two different currents to establish its specific PTAT characteristics. This pre-characterization stores the necessary relationship data that allows the LED to be used as a temperature sensor in real-time operations without requiring a known starting temperature point
Solution Approach 2:
The patent transitions from static temperature sensing (requiring pre-characterized CTAT coefficients and starting points) to dynamic temperature sensing. By continuously measuring forward voltage at two different currents and calculating temperature based on the PTAT relationship, the system adapts to real-time temperature changes without needing predetermined calibration data during operation
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
Enables accurate real-time monitoring and adjustment of LED junction temperature with reduced component count and cost, improving reliability and performance.
Implementation Method 1
The CTAT approach is based on the principle that the LED forward voltage, VF, has a decreasing rate with increasing temperature
Implementation Method 2
The PTAT method is the basis for band-gap regulator circuits, and takes advantage of the fact that the difference in the forward voltages (VF2−VF1) across a PN junction of a diode taken at two different forward currents, IF2 and IF1, is directly proportional to the absolute temperature
Data Source
AI summary
LED junction temperature is determined in real time using the LED itself as the temperature sensor for directly measuring the LED junction temperature. In addition, temperature measurements from a silicon diode placed in proximity to the LED are also used to complement the temperature measurements from the LED itself. Arbitration is performed among temperature measurements from the LED and temperature measurements from the silicon diode to determine a temperature of the LED junction. The determined LED junction temperature may be used to make adjustments to the LED drive current. Temperature measurements from the LED are made in real time during actual operation by applying snooping currents to the LED during off-times of the PWM cycles of the LED, without interrupting normal operation of the LED.