Bi-Level LED Driving Method for Luminous Efficacy and Color Stability
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Solution Overview
Problem
The existing driving methods for LEDs, such as amplitude-mode and PWM-mode, face challenges in maintaining color stability and luminous efficacy due to peak emission wavelength shifts, unequal current sharing, and limited dynamic range, which affects their performance in applications like LCD backlighting and dimming, and the lifetime of LED drivers is limited by electrolytic capacitors.
Innovation Solution
The bi-level current driving technique periodically switches the DC current between a high and a low current level, with the low level optionally set to zero, to maintain an average current by adjusting the duty cycle, effectively compensating for the luminous intensity degradation in PWM mode and improving luminous efficacy by narrowing the difference between the two current levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If amplitude-mode driving technique is used to control LED luminous intensity by adjusting forward current, then luminous intensity control is achieved, but color stability deteriorates due to peak emission wavelength shift
Solution Approach 1:
The patent applies periodic action by using pulse-width modulation (PWM) to switch the LED current between high and low levels periodically. The duty cycle of the PWM signal controls the average luminous intensity, while the peak current levels are maintained constant to prevent color shift. This periodic switching allows independent control of brightness and color stability.
2Stability of the object's composition
If PWM driving technique is used to improve color stability by switching current between high and low levels, then color stability is improved, but luminous efficacy deteriorates due to luminous intensity degradation
Solution Approach 1:
The patent optimizes luminous efficacy by carefully selecting and adjusting the high and low current levels in the PWM waveform, as well as the duty cycle. By changing these parameters, the patent achieves better alignment between the electrical input and optical output characteristics of the LED, improving overall luminous efficacy while maintaining color stability through constant peak current levels.
3Adaptability or versatility
If multiple LEDs are connected in series to expand output voltage range, then voltage compatibility is improved, but lifetime deteriorates due to electrolytic capacitor limitations under high temperature
Solution Approach 1:
The patent removes the electrolytic capacitor from the driver circuit by implementing a capacitor-less PWM driving technique. The current waveform is generated using only resistive elements and switching devices, eliminating the lifetime-limiting capacitor component while maintaining the ability to drive multiple LEDs in series with appropriate voltage compatibility.
4Power
If LEDs are connected in parallel to increase current capacity, then current handling is improved, but luminous uniformity deteriorates due to unequal current sharing
Solution Approach 1:
The patent applies periodic PWM switching simultaneously to all parallel LED branches with synchronized duty cycles and constant peak current levels. This ensures that each LED receives the same current waveform despite manufacturing variations, maintaining luminous uniformity across all branches while achieving the required current capacity through parallel connection.
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 technique enhances luminous efficacy by improving the average illuminance and luminous flux per unit electrical power input, offering better color stability and dimming flexibility while reducing the complexity and cost of the driver, and extending the lifetime of LED drivers.
Implementation Method 1
light is produced via the recombination of injected holes and electrons in a semiconductor junction
Data Source
AI summary
A driving method for improving luminous efficacy of a light emitting diode (LED), the method comprising: periodically switching a DC current supplied to the LED between a high current level lH and a low current level lL, the low current level lL being fixed at zero or raised above zero to produce a DC offset; and maintaining an average current at a first value If by adjusting the duty cycle acting on the high current level lH and any one from the grouping consisting of: adjusting the high current level lH and adjusting the low current level lL, and adjusting the high current level lH or adjusting the low current level lL.