LED Junction Temperature Control via Forward Voltage Feedback

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Solution Overview

Problem

High brightness LEDs in camera flash applications face challenges in maximizing luminous flux output while maintaining junction temperature within safe limits due to variations in starting junction temperature and forward voltage, leading to reduced performance and potential overheating.

Innovation Solution

A method involving measuring the junction temperature and forward voltage of LEDs, determining a drive current based on these measurements, and using a processor-controlled current generator to maintain the junction temperature below a maximum threshold, ensuring optimal operation across varying ambient temperatures and production voltages.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Illumination intensity

If high current is applied to increase luminous flux output, then the LED brightness is improved, but the LED junction temperature increases and lifetime is reduced

Engineering Contradiction:

Improveluminous flux outputVSAvoidLED junction temperature

Core Design Contradiction:

Illumination intensityVSTemperature

Solution Approach 1:

The system performs preliminary measurement of the LED's forward voltage and junction temperature before applying drive current. Based on these measurements, the controller pre-determines the maximum safe drive current that will not exceed the maximum junction temperature threshold, allowing the LED to operate at optimal brightness without risking overheating.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system continuously monitors the LED's forward voltage and junction temperature during operation. The controller uses this feedback information to dynamically adjust the drive current, ensuring the junction temperature remains below the maximum threshold while maximizing luminous flux output. This closed-loop control resolves the contradiction by adapting the current level based on real-time temperature conditions.

Inventive Principle:

Principle #23Feedback

2Reliability

If a fixed design margin is used to prevent overheating, then LED reliability is improved, but LED output and performance are reduced

Engineering Contradiction:

ImproveLED operational safetyVSAvoidLED luminous flux output

Core Design Contradiction:

ReliabilityVSProductivity

Solution Approach 1:

Instead of using a static fixed design margin, the system dynamically determines the appropriate drive current based on real-time measurements of forward voltage and junction temperature. This dynamic approach allows the LED to operate closer to its maximum safe capacity when conditions permit, while automatically reducing current when temperature constraints are approached, thereby maximizing both reliability and performance.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The system changes the operating parameters (drive current, duty cycle) based on measured LED characteristics and thermal conditions. By adjusting these parameters in real-time rather than using a fixed conservative margin, the system achieves both high reliability and maximum luminous flux output under varying operating conditions.

Inventive Principle:

Principle #35Parameter changes

3Device complexity

If variations in forward voltage are ignored, then device complexity is reduced, but junction temperature control precision deteriorates

Engineering Contradiction:

Improvecontrol system simplicityVSAvoidjunction temperature control accuracy

Core Design Contradiction:

Device complexityVSManufacturing precision

Solution Approach 1:

The system incorporates feedback measurement of the actual forward voltage and uses this information to calculate the appropriate drive current. This feedback mechanism compensates for variations in LED characteristics without requiring a complex pre-characterization system, achieving precise temperature control while maintaining reasonable system simplicity.

Inventive Principle:

Principle #23Feedback

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 allows for maximum LED luminous flux output without exceeding the maximum junction temperature, thereby extending the LED's operational lifetime and preventing overheating, while also accounting for variations in LED characteristics and ambient conditions.

Implementation Method 1

Light emitting diodes (LEDs) are used as flash light sources for cameras

Methodology Applied

Scientific EffectElectroluminescence: Electroluminescence

Implementation Method 2

The power that is translated into heat is that portion of the applied power that is not converted to emitted light

Methodology Applied

Scientific EffectJoule heating: Joule Heating

Implementation Method 3

The LED printed circuit board mounting in part determines the thermal resistance of the LED junction to ambient temperature

Methodology Applied

Scientific EffectThermal conduction: Conduction (thermal)

Data Source

PatentUS8816600B2Method of power and temperature control for high brightness light emitting diodes

Publication Date: 2014.08.26 NXP BV

AI summary

Various embodiments relate to a method for driving a light emitting diode (LED) flash including: measuring a junction temperature of the LED by applying a test current to the LED and measuring the LED forward voltage; determining the drive current based upon the measured junction temperature and measured data characteristics of the LED; and applying the drive current to the LED for a specified length of time.