Average Current Control Circuit for Flicker-Free LED Dimming
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Solution Overview
Problem
LED lamp drivers face challenges in accurately controlling average current to prevent flicker and ensure seamless dimming, as existing solutions struggle to reject low-frequency ripple and maintain high power factor while complying with regulatory standards.
Innovation Solution
A control circuit and method that regulate the average output current through a buck converter using a transconductance amplifier, integrating capacitor, and zero crossing detection circuit, independent of switching frequency, to ensure stable operation in both continuous and discontinuous conduction modes, and a voltage feedforward circuit to compensate for propagation delays.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a switching voltage regulator is used to drive LED, then the LED can be powered with controlled current, but low-frequency ripple in the average current causes flicker in the emitted light
Solution Approach 1:
The patent implements a feedback control mechanism using a transconductance amplifier that senses the current through the LED and adjusts the switching transistor gate signal accordingly. The amplifier compares the sensed current with a reference and modifies the duty cycle to maintain constant average current, thereby eliminating flicker caused by low-frequency ripple while preserving stable light output.
2Illumination intensity
If the average current flowing through the LED is reduced to enable dimming, then the light intensity decreases, but maintaining accurate current control becomes more difficult
Solution Approach 1:
The patent employs a dynamic control approach where the transconductance amplifier continuously adjusts its gain and operating point based on the instantaneous current level. This allows the circuit to maintain high measurement precision and control accuracy across the entire dimming range, from full brightness down to low intensity levels, by adapting to changing operating conditions rather than using fixed parameters.
3Reliability
If the switching frequency is increased to eliminate flicker, then the light output becomes stable, but the circuit complexity and power losses increase
Solution Approach 1:
The patent introduces a transconductance amplifier as an intermediary element between the switching transistor and the LED load. This intermediary actively shapes the current waveform and cancels out low-frequency ripple components, allowing the system to operate at lower switching frequencies without flicker. The amplifier mediates the switching action to produce clean, stable current delivery to the LED, reducing both complexity and losses associated with high-frequency operation.
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 accurate and flicker-free control of LED current, independent of input voltage and switching frequency, while maintaining high power factor and low harmonic distortion, thus extending LED lifespan and meeting regulatory requirements.
Implementation Method 1
a sense resistor coupled to a current path of the power transistor
Implementation Method 2
a transconductance amplifier configured to produce a sense current based on a current flowing through the current path of the first transistor
Implementation Method 3
integrating the sense current with an integrating capacitor to generate a first voltage
Implementation Method 4
an inductor coupled to the current path of the power transistor
Data Source
AI summary
A control circuit includes an output terminal configured to be coupled to a control terminal of a transistor that has a current path coupled to an inductor; a transconductance amplifier configured to produce a sense current based on a current flowing through the current path of the transistor; and a first capacitor. The control circuit is configured to turn on the transistor based on a clock signal, integrate the sense current with an integrating capacitor to generate a first voltage, generate a second voltage across the first capacitor based on a first current, generate a second current based on the second voltage, generate a third voltage based on the second current, turn off the transistor when the first voltage becomes higher than the third voltage; discharge the integrating capacitor when the transistor turns off; and regulate an average output current flowing through the inductor based on the first current.