Buck LED Driver Timing Control for Stable Low-Current Dimming
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Solution Overview
Problem
Existing multichannel LED drivers face challenges in precisely regulating low currents with rapid response and stability, particularly in applications like road and street lighting, due to noise sensitivity and flickering issues in current control methods.
Innovation Solution
A control strategy for buck converters that maintains a constant peak current reference by adjusting the switching time of MOSFETs based on a setpoint, using a simple microcontroller with a PI controller, comparator, and timer, without complex calculations, suitable for low dimming applications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If peak current control mode is used for precise current regulation, then current precision is improved, but noise sensitivity increases and stability deteriorates at low current values
Solution Approach 1:
The patent changes the control parameter from peak current to average current. The microcontroller measures the off-time of the MOSFET and calculates the average current based on this timing information, avoiding the noise sensitivity of peak current measurement while maintaining precision through timing-based control
Solution Approach 2:
The patent replaces direct electrical current measurement with a timing-based measurement system. Instead of measuring current directly (which is noisy), the system measures the off-time duration of the MOSFET switch, which is a clean digital signal that can be accurately captured by the microcontroller's timer
2Object-affected harmful factors
If microcontroller-based control with high bandwidth is used to prevent flicker, then flicker resistance is improved, but computational complexity and noise sensitivity increase
Solution Approach 1:
The system uses the natural switching behavior of the power converter itself to generate the measurement signal. The MOSFET's off-time naturally provides the information needed for current control, eliminating the need for external sensors or complex measurement circuits
Solution Approach 2:
The patent uses periodic switching of the MOSFET at a fixed frequency, with the duty cycle adjusted to control current. This periodic action allows simple timing measurements to accurately represent average current, reducing computational complexity while preventing flicker
3Adaptability or versatility
If PWM dimming is used for multichannel current control, then dimming capability is improved, but stroboscopic effects are generated making it unsuitable for road lighting
Solution Approach 1:
The patent maintains continuous current flow through the LED strings by using a buck converter topology with an inductor that stores energy during the MOSFET on-time and releases it during off-time. This continuous action eliminates the discontinuous current characteristic of PWM that causes stroboscopic effects
Solution Approach 2:
The patent changes the dimming control parameter from duty cycle modulation (PWM) to average current modulation through timing control. By adjusting the off-time duration rather than the duty cycle, the system achieves dimming without the high-frequency switching effects that cause stroboscopic phenomena
Data Source
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AI summary
A converter device (10), such as a buck converter, that can be used for driving LED lighting sources (L) in a multichannel system, comprises: an input node configured to receive an input signal (Vinput), and an output node (OUT) configured to have coupled thereto an electrical load (L); an inductance (Lb) between the output node (OUT) and a driver node (N); and an electronic switch (S1) between the input node (Vinput) and the driver node (N), wherein the driver node (N) is electrically coupled to the input node (Vinput) in response to the electronic switch (S1) being conductive. End-of-current detection circuitry (Cv, Rv, Dz) set between the driver node (N) and the inductance (Lb) generates an end-of-current signal (VI) indicative of the end of the current (ILb) through the inductance (Lb) in response to the electronic switch (S1) being non-conductive. The electronic switch (S1) is switched from non-conductive to conductive, thus facilitating the flow of current (ILb) through the inductance (Lb), with a delay with respect to the end of the current (ILb) through the inductance (Lb), such delay assuming a first value or a second value, higher than the first, in response to a current-reference signal (Iref) being, respectively, higher or else lower than a threshold level.