Flyback Converter SR Control Using Adaptive Slope Thresholds
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Solution Overview
Problem
Existing synchronous rectification control methods in flyback converters face challenges in distinguishing between normal on-state and parasitic oscillation conditions, leading to false conduction of synchronous rectification transistors, especially in DCM mode, due to varying falling slopes of voltage Vds across different power supply conditions and no-load or light-load scenarios.
Innovation Solution
A synchronous rectification control circuit that includes a turn-on detection circuit with a slope detection circuit and a threshold generation circuit, capable of generating adaptive threshold signals by selecting between first and second threshold reference signals based on the driving signal, to accurately control the switch state of the synchronous rectification transistor, thereby distinguishing between normal and parasitic oscillation conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If synchronous rectification transistor is used to replace rectification diode, then rectification loss is reduced, but false conduction during parasitic oscillation occurs
Solution Approach 1:
The control circuit performs preliminary detection of the voltage difference Vds and its falling slope before enabling the synchronous rectification transistor. By checking whether the falling slope exceeds the threshold in advance, the circuit prevents false conduction during parasitic oscillation while allowing normal operation when the slope condition is met.
Solution Approach 2:
The control circuit continuously monitors the voltage difference Vds across the synchronous rectification transistor and feeds back the falling slope information to the control unit. This feedback mechanism enables real-time adjustment of the transistor's conduction state, preventing false turn-on during parasitic oscillation while maintaining efficient rectification during normal operation.
2Device complexity
If fixed threshold is used for turn-on detection, then circuit is simple, but cannot adapt to different power supply conditions
Solution Approach 1:
The control circuit dynamically adjusts the threshold for turn-on detection based on the detected falling slope of Vds. Instead of using a fixed threshold, the circuit adapts the threshold level according to the actual operating conditions, enabling reliable distinction between normal conduction and parasitic oscillation across different power supply scenarios while maintaining reasonable circuit complexity.
Data Source
AI summary
A synchronous rectification control circuit has a turn-on detection circuit. The turn-on detection circuit has a slope detection circuit and a threshold generation circuit. The slope detection circuit generates a slope representation signal representing a falling slope of the detection signal. The threshold generation circuit generates a threshold signal by selecting one of threshold reference signals according to a driving signal. The turn-on detection circuit generates a comparison result signal by comparing the slope representation signal and the threshold signal for controlling the switch state of the synchronous rectification transistor.