LED Dimming via Dynamic PWM Frequency and Duty Cycle Control
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Solution Overview
Problem
Illumination devices using LEDs face limitations in light output level, color quality, spatial and angular variations, high costs due to color control electronics, and poor dimming characteristics, especially at low light levels, where current dimming control schemes like PWM exhibit unsmooth transitions and visible flicker.
Innovation Solution
An LED-based illumination system that controls the average current by periodically switching it between high and low states, adjusting the duration of the switching period and the ratio of time in the high state to the low state, using a dimming control engine with a microprocessor to generate digital control signals, allowing for smooth transitions and improved dimming performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If pulse width modulated (PWM) dimming control schemes are used to achieve deep dimming, then dimming capability is improved, but unsmooth transitions and visible flicker occur at low light levels
Solution Approach 1:
The patent dynamically adjusts the PWM frequency based on the desired dimming level. At higher dimming levels, a lower frequency is used, while at lower dimming levels, the frequency is increased. This dynamic frequency adjustment ensures smooth transitions across the entire dimming range without visible flicker or unsmooth transitions at low light levels.
Solution Approach 2:
The patent changes the PWM frequency parameter as a function of the dimming level. By varying the frequency parameter dynamically according to the operating point, the system maintains smooth transitions and eliminates flicker artifacts that would otherwise occur at low duty cycles with fixed frequency PWM dimming.
2Stability of the object's composition
If a large number of pulses are used in PWM period to produce smooth transitions at low intensities, then transition smoothness is improved, but clock frequency increases or PWM period increases resulting in visible flicker
Solution Approach 1:
Instead of using a fixed high clock frequency for all dimming levels, the system dynamically selects the appropriate clock frequency based on the current dimming level. This dynamic approach allows smooth transitions at low intensities without unnecessarily increasing the clock frequency across the entire operating range, thereby avoiding visible flicker.
Solution Approach 2:
The patent changes the clock frequency parameter dynamically as a function of the dimming level. At low dimming levels where smooth transitions are critical, the frequency is increased. At higher dimming levels, a lower frequency suffices. This parameter change strategy achieves smooth transitions without the penalty of continuously high clock frequencies that would cause visible flicker.
3Device complexity
If constant current reduction (CCR) dimming control schemes are used, then implementation simplicity is improved, but deep dimming capability is limited to no less than 10% of normal light output
Solution Approach 1:
The patent employs periodic pulse width modulated action to achieve deep dimming. By switching the current on and off periodically with adjustable duty cycles, the system can achieve light output levels below 10% of normal while maintaining LED operational reliability. The periodic switching nature of PWM allows precise control of average current without the limitations of CCR dimming.
Data Source
AI summary
An LED based illumination device is dimmed by controlling an average current supplied to the LED based illumination device. The currently supplied to the LED may be supplied by an LED driver that is in communication with a dimming control engine. The dimming control engine may receive an indication of a desired average current level. The dimming control engine controls the LED driver to periodically switch a current supplied to an LED of the LED based illumination device from a high state to a low state over a switching period, wherein both a duration of the switching period is adjusted and a ratio of a time in the high state to a time in the low state is adjusted as the average current supplied to the LED based illumination device transitions from a first average current level to the desired average current level.