LED Lighting Dimming Circuit Parasitic Voltage Protection
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Solution Overview
Problem
Lighting systems with dimming functions experience malfunctions or failures due to parasitic capacitance between the metallic heatsink and the LED circuit, causing high negative voltages to be applied to the LEDs during PWM modulation, potentially damaging them.
Innovation Solution
Incorporating a diode between the positive and negative output terminals of the switching stage to prevent negative voltages from reaching the lighting module, ensuring the drain terminal of the transistor remains at a constant voltage and preventing voltage exceeding the breakdown voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a switching stage with transistor is used for PWM dimming control, then the lighting system achieves dimming functionality, but parasitic capacitance causes high negative voltages to appear at the drain terminal, potentially damaging the transistor and LEDs
Solution Approach 1:
A diode is introduced as an intermediary component between the drain terminal and the lighting module. This diode acts as a mediator that blocks the harmful negative voltage generated by parasitic capacitance during PWM switching, while allowing the useful positive voltage to pass through to the lighting module for normal operation.
Solution Approach 2:
The circuit design accepts the inevitable presence of parasitic capacitance in the switching stage but converts its harmful effect into a manageable condition. By using the body diode of the transistor or an external protection diode, the harmful negative voltage spike is redirected through a safe path, transforming a potentially destructive phenomenon into a controlled electrical behavior that protects the LED circuit.
2Productivity
If the transistor is used to control PWM switching, then efficient dimming operation is achieved, but the drain terminal voltage may exceed breakdown voltage during negative transitions
Solution Approach 1:
A protection diode is connected in parallel with the transistor's drain-source path, with its cathode at the drain terminal and anode at the source terminal. This diode is positioned beforehand to cushion against negative voltage spikes before they can damage the transistor. During normal PWM operation, the diode remains reverse-biased and non-conductive, but during negative voltage transitions, it becomes forward-biased and conducts, clamping the voltage to a safe level.
Solution Approach 2:
The circuit modifies the voltage parameter at the drain terminal by introducing a clamping mechanism. The protection diode changes the voltage waveform by clipping the negative excursions, effectively altering the voltage parameter from a potentially damaging range to a safe operating range for the transistor and connected LED circuit.
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
Prevents damage to LEDs by blocking negative voltages and maintaining the transistor within safe voltage limits, thereby reducing the likelihood of malfunction or failure in lighting systems with dimming functions.
Implementation Method 1
Incorporating a diode between the positive and negative output terminals of the switching stage to prevent negative voltages from reaching the lighting module
Implementation Method 2
parasitic capacitance between the metallic heatsink and the LED circuit, causing high negative voltages to be applied to the LEDs during PWM modulation
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
There is described a lighting system including an electronic converter, a lighting module (22, 24) and a switching stage (122). The electronic converter includes a transformer (T), a rectifier circuit and an output filter circuit. A capacitance (CY) is connected between the primary winding (T1) and the secondary winding (T2). The lighting module (22, 24) includes a chain of LEDs (22) and a current regulator (24), wherein the chain of LEDs is mounted onto a substrate of a metallic material (206), so that a parasitic capacitance (CS) is present between the lighting module (20) and the substrate of a metallic material (206). The switching stage (122) includes a field-effect transistor (SW) interposed in the negative line which connects the lighting module (22, 24) to the electronic converter, and a control unit configured to drive (CTRL) the gate terminal of the transistor (SW) as a function of a dimming signal. Specifically, the switching stage (122) includes a diode (D), wherein the anode of the diode (D) is connected to the negative output terminal of the switching stage (122) and the cathode of the diode (D) is connected to the positive output terminal of the switching stage (122).