LED Drive Circuit With PWM Feedback for Precise Dimming
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Solution Overview
Problem
Existing LED light source control systems face challenges in efficiently regulating the load current and voltage to achieve precise intensity and color control, particularly in dimming applications.
Innovation Solution
A controllable impedance circuit is coupled in series with the LED light source, featuring a first switching device and a feedback circuit that generates a feedback signal indicative of the load current. A control loop circuit, including a second switching device and/or a filter circuit, adjusts the controllable impedance based on the feedback signal to achieve target current control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a feedback circuit and control loop circuit are added to regulate load current, then current control precision is improved, but device complexity increases
Solution Approach 1:
The patent implements a feedback circuit that senses the load current through a sense resistor and feeds the signal back to a control loop circuit. The control loop compares the feedback signal with a reference voltage and adjusts the switching device accordingly to maintain precise current regulation. This closed-loop feedback mechanism directly resolves the contradiction by providing accurate current control through continuous monitoring and adjustment.
Solution Approach 2:
The patent introduces a sense resistor as an intermediary element to convert the load current into a measurable voltage signal. This intermediary allows the control system to indirectly monitor and control the current without directly measuring it, simplifying the control architecture while maintaining precision. The sense resistor acts as a mediator between the current-carrying circuit and the control circuitry.
2Loss of energy
If switching devices are used to control LED current, then energy efficiency is improved, but heat generation increases
Solution Approach 1:
The patent employs pulse-width modulation (PWM) switching to control the LED current. Instead of using linear regulation that continuously dissipates power as heat, the circuit switches the current on and off periodically at high frequency, controlling the average current by adjusting the duty cycle. This periodic switching action dramatically improves energy efficiency by minimizing resistive losses while the short-duration pulses limit heat generation in the switching device.
Solution Approach 2:
The patent changes the operating parameters of the switching device by optimizing its switching frequency and duty cycle. By operating at high switching frequencies, the circuit minimizes the on-time of the switching device, reducing the opportunity for heat generation. Simultaneously, the duty cycle is dynamically adjusted to maintain the desired average LED current while keeping peak currents brief, thereby improving overall energy efficiency and reducing thermal stress on components.
3Illumination intensity
If multiple parallel strings of LEDs are used, then light output is improved, but current balance between strings deteriorates
Solution Approach 1:
The patent implements a feedback mechanism that monitors the total current drawn from the power source and adjusts the switching control to maintain balanced current distribution across parallel LED strings. The control loop circuit receives feedback from the overall current consumption and dynamically adjusts the duty cycle to compensate for impedance variations between strings, ensuring that each string receives appropriate current despite manufacturing tolerances and aging effects.
Solution Approach 2:
The patent employs dynamic current balancing by continuously adjusting the switching parameters in response to changing operating conditions. Rather than using fixed current limiting resistors for each string, the system dynamically modifies the PWM duty cycle based on real-time feedback from the total current measurement. This dynamic approach allows the system to adapt to varying LED forward voltages and maintain current balance across parallel strings as operating conditions change.
Data Source
AI summary
A controllable lighting device may utilize a controllable impedance circuit to conduct a load current through an LED light source. The controllable impedance circuit may be coupled in series with a first switching device, which may be rendered conductive and non-conductive via a pulse-width modulated signal to adjust an average magnitude of the load current. The controllable lighting device may further comprise a control loop circuit that includes a second switching device. The second switching device may be rendered conductive and non-conductive in coordination with the first switching device to control when a feedback signal is provided to the control loop circuit and used to control the LED light source. The control loop circuit may be characterized by a time constant that is significantly greater than an operating period of the load current.