Valley Locking Circuit for Quasi-Resonant Flyback Noise Reduction
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Solution Overview
Problem
Flyback switching power supplies operating in quasi-resonant mode face challenges with valley frequency hopping, leading to noise issues when the power switch alternately turns on at adjacent valley values within the audible frequency range, especially when load characteristics change.
Innovation Solution
A control circuit and method that includes an output current sampling circuit to generate a feedback signal and a valley locking circuit, which switches to a corresponding valley based on the output current feedback signal and multiple valley switching threshold voltages, ensuring reliable locking and reducing noise by adjusting the operating frequency in response to load changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the power supply operates in quasi-resonant mode with automatic valley detection, then conversion efficiency is improved and EMI is reduced, but valley frequency hopping occurs causing audible noise
Solution Approach 1:
The patent applies dynamics by making the valley selection adaptive rather than fixed. The controller dynamically switches between different valley points (first valley, second valley, third valley) based on real-time load conditions. When load is light, it selects higher frequency valleys; when load increases, it transitions to lower frequency valleys to avoid audible noise while maintaining efficiency. This dynamic adaptation resolves the contradiction between efficiency and noise.
Solution Approach 2:
The patent changes the operating parameter (valley selection point) based on load conditions. By monitoring load current and switching between multiple predefined valley thresholds, the system adjusts the resonant frequency parameter to avoid the audible range (20Hz-20kHz) while maintaining optimal efficiency. This parameter change strategy allows the system to escape the harmful frequency hopping effect.
2Loss of energy
If the power switch turns on at valley values to achieve zero voltage switching, then power losses are reduced, but frequency instability occurs under varying load conditions
Solution Approach 1:
The patent segments the resonant frequency range into multiple discrete valley points (first valley, second valley, third valley, etc.), each corresponding to different load conditions. Instead of relying on a single fixed valley detection, the system divides the operating range into multiple stable zones. This segmentation allows the controller to select the appropriate valley based on load current, thereby maintaining frequency stability across varying loads while preserving zero voltage switching benefits.
Solution Approach 2:
The patent implements feedback by continuously monitoring the load current and using it to determine which valley point to lock onto. The controller compares the actual load condition with predefined thresholds and adjusts the valley selection accordingly. This closed-loop feedback mechanism ensures that the system maintains stable operation at the optimal valley point for each load condition, preventing frequency instability while maintaining low power losses.
3Adaptability or versatility
If multiple valley points are available for switching, then adaptability to different loads is improved, but complexity of valley selection increases
Solution Approach 1:
The patent applies dynamics by implementing a load-current-based decision logic that automatically selects the appropriate valley point. The controller dynamically adjusts the valley selection based on real-time load measurements, transitioning between predefined valley thresholds as load conditions change. This dynamic approach provides high load adaptability without requiring complex algorithms, as the selection logic follows simple if-then rules based on current thresholds.
Solution Approach 2:
The patent changes the selection parameter (valley point) based on load current thresholds. By defining multiple valley points with associated current thresholds, the system creates a parameter-based selection mechanism that is both adaptable and simple to implement. The controller only needs to compare the measured load current against predefined thresholds to determine the appropriate valley, avoiding the need for complex optimization algorithms while maintaining high adaptability across different load conditions.
Data Source
AI summary
A control circuit can include: an output current sampling circuit configured to sample a current through a power switch of a switching power supply operating in a quasi-resonant mode, and to generate an output current feedback signal that represents an output current of the switching power supply; and a valley locking circuit configured to receive the output current feedback signal and a plurality of valley switching threshold voltages, where the valley locking circuit is switched to a corresponding valley from a valley currently being locked when the output current feedback signal is increased or reduced to one of the plurality of valley switching threshold voltages.