LED Driver Circuit Using Periodic Current for Junction Temperature
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Solution Overview
Problem
Existing methods for measuring and controlling LED junction temperature are inaccurate due to self-heating effects and require complex thermal diffusion models or discrete table look-up methods, leading to erratic LED performance and high memory requirements.
Innovation Solution
A driver circuit with separate high and low current outputs, using a square wave current sequence to measure the forward bias voltage drop at a low non-zero current, allowing for accurate temperature estimation while the LED is operational, and using analytical functions for smooth control of LED light and color output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a constant current is used to drive the LED for operation, then the LED produces light output, but self-heating effect occurs making temperature measurement inaccurate
Solution Approach 1:
The patent applies periodic action by switching the LED drive current between high current (for light output) and low current (for temperature measurement) in alternating intervals. The control circuit periodically transitions the LED between operational mode and measurement mode, allowing temperature measurement without continuous self-heating while maintaining light output functionality.
Solution Approach 2:
The patent uses preliminary action by measuring the forward voltage of the LED at a low current level before the LED is fully activated to operational current. This preliminary measurement at low current provides temperature data without introducing significant self-heating, and the measurement occurs while the LED is still in a relatively cool state.
2Measurement precision
If a small current is used to measure forward voltage to avoid self-heating, then temperature measurement accuracy improves, but the measurement can only be done when LED is turned off causing time delay
Solution Approach 1:
The control circuit implements periodic switching between measurement current and operational current. During measurement intervals, a low current is applied to measure forward voltage for temperature determination. During operational intervals, high current restores light output. This periodic action eliminates time delay by continuously alternating between measurement and operation without requiring the LED to be completely turned off.
Solution Approach 2:
The patent maintains continuity of useful action by ensuring the LED alternates between measurement mode and operational mode without complete shutdown. The measurement process occurs during low-current intervals while the LED remains biased, and operational mode resumes immediately afterward, ensuring continuous light output functionality without interruption or time delay.
3Measurement precision
If table look-up method is used to map forward voltage to ambient temperature, then temperature estimation is achieved, but LED performance becomes erratic and memory requirements increase
Solution Approach 1:
The patent applies parameter changes by using analytical functions that directly calculate temperature from forward voltage measurements rather than using discrete table look-up values. The analytical approach continuously computes temperature based on the measured forward voltage and known LED characteristics, providing smooth temperature transitions and eliminating the discretization errors inherent in table methods. This reduces memory requirements while improving performance control.
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 accurate and efficient LED junction temperature measurement and control, reducing memory requirements and improving LED performance by enabling precise light output and color management.
Implementation Method 1
the forward voltage of the LED while driving a constant current through it. This method uses the LED itself as a temperature sensor
Data Source
AI summary
A driver circuit (10) for a light emitting diode comprises a first driver circuit (32, 32′, 32′) for generating a first current output for driving the light emitting diode, wherein the first driver circuit has a control switch for interrupting the supply of the first current output. A second driver circuit (50) is for generating a second current output for driving the light emitting diode, and the second driver circuit also has a control switch for interrupting the supply of the second current output. The overall output of the driver circuit comprises a pulse width modulated output current which alternates between a high current (Ihigh) generated by the first driver circuit and a low current (Ilow) generated by the second driver circuit. By providing separate driver circuits for two different current requirements, the circuits can be optimised for each function. For example the high current value can comprise an LED operation current, and the low current value can comprise a non-zero measurement current.