LED Driver Freewheeling Switch for Flicker-Free Dimming
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Solution Overview
Problem
Existing LED dimming methods, such as PWM and CCR, face challenges in achieving high accuracy and efficiency at both high and low brightness levels, and often result in flicker interference and color changes at low currents, with hybrid dimming adding complexity and cost.
Innovation Solution
A dimmable driver device with an actively controlled switch on the freewheeling current path of a switched-mode converter allows current to go negative at low dimming levels, using a MOSFET on the freewheeling current path and separate measurement of AC and DC components to maintain high accuracy and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If PWM dimming is used, then LED dimming capability is achieved, but flicker interference occurs
Solution Approach 1:
The patent employs CCR (Constant Current Reduction) dimming method where the current source operates constantly without interruption, maintaining continuous light output. This eliminates the pulsating effect inherent in PWM that causes flicker, while still achieving variable brightness levels through smooth current adjustment.
Solution Approach 2:
The patent introduces a periodic switching mechanism at a frequency significantly higher than visible perception thresholds (typically >20kHz). This ultrasonic frequency switching creates the illusion of continuous dimming while maintaining actual continuity, thereby eliminating flicker effects that occur at lower frequencies.
2Object-affected harmful factors
If CCR dimming is used, then smooth dimming without flicker is achieved, but control accuracy deteriorates at low currents
Solution Approach 1:
The patent dynamically switches between CCR and PWM dimming methods based on the desired brightness level. At high brightness levels where flicker elimination is critical, CCR is used. At low brightness levels where control accuracy is paramount, PWM is employed. This dynamic adaptation optimizes performance across the entire dimming range.
Solution Approach 2:
The patent changes the operating parameters of the LED driver by adjusting the switching frequency and duty cycle according to the required brightness level. At low currents, the system increases switching frequency and optimizes duty cycle resolution to maintain precise control accuracy, while at higher currents it transitions to CCR mode for flicker-free operation.
3Duration of action of stationary object
If CCR dimming is used, then continuous operation is achieved, but efficiency decreases at low dimming levels
Solution Approach 1:
The patent dynamically selects between CCR and PWM modes based on efficiency considerations. At high dimming levels where continuous operation is beneficial, CCR maintains constant current flow. At low dimming levels where efficiency becomes critical, the system switches to PWM which allows the power stage to remain in high-efficiency switching mode, reducing the proportion of switching losses relative to total power consumption.
Solution Approach 2:
The patent adjusts operational parameters including switching frequency and current magnitude based on the dimming level. At low brightness levels, the system increases switching frequency to maintain efficiency and reduces current magnitude to minimize resistive losses, thereby optimizing the efficiency-to-output ratio across all dimming levels.
4Measurement precision
If hybrid dimming combining PWM and CCR is used, then accuracy and efficiency are improved, but device complexity increases
Solution Approach 1:
The patent implements a unified LED driver controller that integrates both CCR and PWM dimming capabilities within a single device. The controller automatically selects the appropriate dimming mode based on operating conditions, eliminating the need for separate dedicated CCR and PWM driver circuits. This multi-functional approach reduces overall system complexity while maintaining the accuracy and efficiency benefits of both methods.
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 solution achieves high accuracy and efficiency in LED dimming across all brightness levels while minimizing flicker interference and color changes, reducing manufacturing complexity and costs.
Implementation Method 1
a switched-mode converter which comprises an energy-storing inductor, a power switch and a freewheeling current path. The voltage at the output of said switched-mode converter is maintained at or above the largest sum of forward threshold voltages of serially connected semiconductor light sources across said output
Data Source
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Figure 6
AI summary
A driver device for semiconductor light sources (101) comprises an output current source (202), which is a switched-mode converter that comprises an energy-storing inductor (103), a power switch (104) and a freewheeling current path. Its output comprises a first node (106) and a second node (107). A capacitor (108) is coupled essentially across said output for maintaining an essentially constant output voltage of said driver device. A controllable switch (201) on said freewheeling current path is selectively maintained conductive during periods when said power switch (104) is non-conductive. The arrangement of said controllable switch (201) and said control circuit (209) is configured to allow electric current to flow in both directions through said controllable switch (201) during said periods when said power switch (104) is non-conductive.