LED Switched Converter Zero-Crossing Modulation for EMI and Flicker
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Solution Overview
Problem
Switched-mode power converters used for LED loads often experience high levels of electromagnetic interference (EMI) and visible flicker in light output due to improper switching frequency modulation, leading to undesirable jumps in light intensity.
Innovation Solution
A feedback-controlled switched converter that enables/disables a periodic modulation signal only during time periods with amplitudes below a preset threshold, centered at zero-crossings of the modulation signal, to minimize visible flicker and EMI, using a control unit and signal generator to adjust switching parameters based on load current feedback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If switching frequency modulation is applied to reduce EMI, then electromagnetic interference levels are reduced, but visible flicker and jumps in light output occur
Solution Approach 1:
The patent applies periodic switching frequency modulation to reduce EMI, but constrains the modulation to specific time windows centered at zero-crossings of the LED drive current waveform. This periodic action with temporal constraints allows EMI reduction while minimizing visible flicker, as the modulation occurs during periods when human eyes are less sensitive to light changes.
Solution Approach 2:
The patent implements local quality by applying different switching strategies at different times: during zero-crossing periods, frequency modulation is enabled for EMI reduction, while during other periods, stable frequency operation is maintained for light output stability. This spatial-temporal differentiation resolves the contradiction between EMI reduction and flicker prevention.
2Object-affected harmful factors
If switching frequency is modulated to suppress peak EMI levels, then both conducted and radiated EMI are reduced, but visible jumps in light output occur when modulation is activated/deactivated
Solution Approach 1:
The patent prepares for potential modulation activation or deactivation by monitoring system conditions and pre-calculating appropriate modulation parameters. When modulation needs to be activated or deactivated, the system does so during pre-identified safe time windows (zero-crossing periods), preventing sudden light output jumps that would occur with immediate activation/deactivation.
Solution Approach 2:
The patent employs feedback control by continuously monitoring the LED drive current waveform and detecting zero-crossing events. This feedback mechanism triggers frequency modulation activation or deactivation at optimal moments, ensuring that transitions occur when they will not cause visible light output jumps, thus maintaining illumination continuity while still achieving EMI suppression.
3Object-affected harmful factors
If spread-spectrum technique is used to reduce EMI, then electromagnetic interference is reduced, but device complexity increases
Solution Approach 1:
The patent implements spread-spectrum technique by dynamically changing the switching frequency parameter around a nominal value. This parameter variation spreads the EMI energy across a wider frequency spectrum, reducing peak EMI levels. The implementation uses simple frequency modulation based on zero-crossing detection, avoiding complex spread-spectrum hardware while achieving EMI reduction through parameter variation.
Data Source
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AI summary
The invention relates to a feedback-controlled switched converter, comprising: at least one switch, terminals for supplying an LED load, a control unit being supplied with a feedback signal indicating a load current of the LED load. The control unit is configured to: generate an output signal on the basis of the feedback signal, combine the output signal with a periodic modulation signal in order to obtain a control signal, the control signal being configured to set an operation parameter of the at least one switch, apply the control signal to the at least one switch. Moreover, the switched converter comprises means for enabling/disabling the periodic modulation signal, wherein the means for enabling/disabling is configured to enable/disable the periodic modulation signal only at time periods of the periodic modulation signal in which an amplitude of the periodic modulation signal is lower than an amplitude threshold, or wherein the means for enabling/disabling the periodic modulation signal is configured to only enable/disable the periodic modulation signal at first time periods of the periodic modulation signal being centered at zero-crossings of the periodic modulation signal, these first time periods being separated from each other by time periods in which no enabling/disabling of the modulation signal is performed.