Multichannel Current Source PWM Dimming Color Locus Stability
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Solution Overview
Problem
Dimmable constant current sources used in lighting systems, particularly those employing RGB LEDs, experience color locus shifts when dimming, leading to undesirable changes in perceived color tone due to variations in current intensity across channels.
Innovation Solution
A multi-channel dimmable constant current source with separate adjustable dimming devices for each output, utilizing pulse width modulation (PWM) to maintain a constant maximum current value, thereby minimizing color locus drift. Each output has a first dimming device with a switch controlled by a first PWM signal and a second dimming device with a switch controlled by a second PWM signal, with a control unit adjusting duty cycles to keep the maximum current constant, typically within 5% fluctuation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If constant current dimming is used to dim LED channels, then individual channel dimming is achieved, but color locus shifts occur causing undesirable color tone changes
Solution Approach 1:
The patent applies periodic pulse width modulation (PWM) signals to control the switching of LEDs in each channel. By switching LEDs on and off at high frequency with variable duty cycles, the patent achieves precise dimming control while maintaining stable color characteristics, as the maximum current during the on-state remains constant regardless of duty cycle adjustments.
Solution Approach 2:
The patent changes the control parameter from direct current magnitude to pulse width modulation duty cycle. By maintaining constant maximum current amplitude and varying only the duty cycle of PWM signals, the patent achieves dimming functionality while preventing color locus shifts that would occur with direct current magnitude changes.
2Illumination intensity
If current intensity is varied to dim LED channels, then dimming functionality is achieved, but the maximum current value changes causing color point position shifts
Solution Approach 1:
The patent uses periodic PWM switching to achieve dimming through duty cycle variation rather than maximum current reduction. The LEDs are switched on and off periodically, and the average illumination intensity is controlled by adjusting the proportion of on-time within each period, while the peak current during on-states remains constant to maintain stable color characteristics.
Solution Approach 2:
The patent introduces dynamic time-based control through PWM signals, transforming the static current control approach into a dynamic temporal control method. By modulating the timing characteristics (duty cycle) rather than the amplitude of current, the patent achieves dimming functionality while maintaining stable color properties.
Data Source
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AI summary
The current source (47) has outputs (CH1, CH2) providing constant currents (I1, I2) for operating electrical loads (10, 20). A dimming device (3) is provided for the outputs to conduct part of currents (I1', I2') flowing to the outputs to a reference potential (GND). The dimming device has switches (17, 27) controlled by a set of PWM signals (short1, short2). Another dimming device (4) has switches (12, 22) controlled by another set of PWM signals (open1, open2). A control unit controls the switches based on dimming position, and a current regulator is connected with a current modulator (1).