LED Dimming Circuit Eliminates Filter Capacitor
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Solution Overview
Problem
Existing LED dimming circuits using TRIAC phase-controlled dimming methods face challenges in maintaining stable current, leading to flicker and reduced efficiency due to the need for a filter capacitor and dummy loads, which affect the reliability and power consumption.
Innovation Solution
A dimming circuit with a double-power-stage structure, where the first power stage controls input current and output voltage, and the second power stage maintains constant current driving for the LED load, eliminating the need for a filter capacitor and dummy loads by using a control circuit to manage the TRIAC conduction angle and generate dimming signals based on input current and output voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a filter capacitor is added to the dimming circuit to smooth the rectified voltage, then the voltage stability is improved, but the circuit complexity increases and LC resonance causes uncontrollable current through the TRIAC
Solution Approach 1:
The patent removes the filter capacitor from the circuit entirely. Instead of smoothing the rectified voltage through capacitance, the invention uses the electronic transformer and TRIAC phase-control to directly regulate the output voltage, extracting the voltage stabilization function from the capacitor and implementing it through active control of the power switching element.
Solution Approach 2:
The patent implements a feedback control mechanism where the microcontroller monitors the output voltage and adjusts the TRIAC triggering angle accordingly. This closed-loop control system maintains stable output voltage without requiring a filter capacitor, as the feedback continuously corrects voltage variations through phase-angle control of the TRIAC.
2Reliability
If a dummy load (holding resistor) is added to maintain minimum holding current for the electronic transformer, then the transformer operates reliably, but power consumption increases due to the resistor dissipating energy
Solution Approach 1:
The patent uses dynamic control of the TRIAC conduction angle to adjust the output current according to the actual LED load requirements. Unlike a fixed holding resistor that continuously dissipates power, the TRIAC-based control dynamically adjusts current flow, maintaining transformer operation reliability only when necessary while minimizing power loss during dimmed states.
Solution Approach 2:
The patent changes the conduction angle parameter of the TRIAC to control the average output current. By varying this parameter from 0 to nearly 360 degrees, the system can maintain sufficient current for transformer operation at high brightness while reducing current (and power loss) at lower brightness levels, eliminating the need for a fixed power-dissipating holding resistor.
3Illumination intensity
If the TRIAC conduction angle is reduced to dim the LEDs, then the light output is reduced, but the current becomes uncontrollable and the TRIAC cannot maintain turned-on state causing flicker
Solution Approach 1:
The patent employs feedback control where the microcontroller monitors the actual output current and voltage, then adjusts the TRIAC triggering angle to maintain stable operation. This feedback mechanism ensures that even at reduced conduction angles for dimming, the TRIAC receives appropriate triggering pulses to maintain its turned-on state, preventing flicker while achieving the desired brightness reduction.
Solution Approach 2:
The patent applies preliminary triggering pulses to the TRIAC control electrode before the main power switch closes. This preliminary action ensures the TRIAC is properly activated and maintains its conductive state throughout the conduction period, even when the conduction angle is reduced for dimming purposes, thereby maintaining current control stability.
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 ensures stable and efficient dimming of LEDs by maintaining input current above the holding current of the electronic transformer, reducing power losses, and avoiding flicker, while accurately controlling the LED driving current and improving system reliability.
Implementation Method 1
By applying the TRIAC phase-controlled operating principle, the supply voltage of the lamp circuit is controlled by controlling the conduction angle of the TRIAC element to chop out a part of the sine wave voltage input from the AC power network, to decrease the average value of the output voltage
Implementation Method 2
which is rectified by the rectifier bridge 107 to generate the DC voltage Vdcin
Data Source
AI summary
The present disclosure relates to dimming circuit and method for LEDs. The dimming circuit obtains a DC voltage from an external AC power supply by using a TRIAC, an electronic transformer, and a rectifier bridge sequentially. The dimming circuit comprises a first power stage circuit, a second power stage, a first control circuit, and a second control circuit. The first power stage circuit has an input terminal configured to receive the DC voltage. The second power stage has an input terminal coupled to an output terminal of the first power stage and an output terminal coupled to an LED load. The first control circuit is configured to generate a first control signal in accordance with a first output voltage generated at the output terminal of the first power stage circuit, a first reference voltage and an upper threshold voltage to maintain an average value of the first output voltage to be consistent with the first reference voltage. The second control circuit is configured to generate a dimming signal in accordance with a first current and the first output voltage to control an operation of the second power stage circuit to maintain an output current of the second power stage circuit to be consistent with an expected driving current represented by the dimming signal. The first current is no less than a holding current of the electronic transformer. An input current of the first power stage circuit is maintained to be consistent with the first current by the first control signal when the first output voltage is in a continuously increasing state and is lower than the upper threshold voltage. The first output voltage decreases continuously and the input current is maintained to be consistent with a second current after the first output voltage reaches the upper threshold voltage.