Synchronous Rectification Control Circuit for Flyback Converters
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Solution Overview
Problem
Traditional flyback converters experience high conduction and reverse recovery losses in secondary-side rectifier diodes, leading to reduced efficiency when output current is high and output voltage is low, due to the use of rectifier diodes.
Innovation Solution
A synchronous rectification control circuit that includes control circuits to manage the conduction time of a synchronous rectifier switch based on the drain-source voltage signal and output voltage of the flyback converter, ensuring the synchronous rectifier switch operates only when necessary to avoid negative current and maintain efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a rectifier diode is used in a traditional flyback converter, then the device structure is simple, but conduction losses and reverse recovery losses are high resulting in reduced efficiency
Solution Approach 1:
The patent changes the key parameter of the rectifier element from a diode to a MOSFET power transistor. This parameter change enables synchronous rectification where the MOSFET is actively controlled to conduct during the demagnetization phase, replacing the passive diode conduction. The MOSFET's low on-resistance significantly reduces conduction losses compared to the diode's forward voltage drop, while its controlled switching eliminates reverse recovery losses entirely.
2Loss of energy
If a synchronous rectifier switch is used to improve efficiency, then conversion efficiency is improved, but secondary-side negative current may appear when minimum conduction time is not satisfied
Solution Approach 1:
The patent employs feedback control through a control circuit that monitors the drain-source voltage of the synchronous rectifier switch and the output voltage of the flyback converter. This feedback mechanism dynamically adjusts the gate control signal to ensure the MOSFET conducts for the minimum required time during each switching cycle. By continuously monitoring voltage conditions and adjusting the rectifier switch timing accordingly, the system prevents negative current on the secondary side while maintaining optimal efficiency.
Solution Approach 2:
The control circuit performs preliminary action by generating the gate control signal in advance based on the relationship between the first control signal (derived from drain-source voltage) and a threshold value. This preliminary control ensures that the synchronous rectifier switch is guaranteed to conduct for at least the minimum required time before the primary-side power switch turns on again, preventing the harmful effect of negative current while maximizing efficiency benefits.
3Object-generated harmful factors
If the synchronous rectifier switch conduction time is extended to avoid negative current, then negative current is eliminated, but conduction losses increase due to longer switch operation
Solution Approach 1:
The patent implements dynamic control of the synchronous rectifier switch conduction time rather than a fixed extended conduction period. The control circuit dynamically adjusts the gate control signal duration based on real-time voltage conditions, specifically comparing the first control signal with a threshold value that corresponds to the minimum required conduction time. This dynamic approach ensures the switch conducts exactly the minimum necessary time to prevent negative current, avoiding unnecessary extended conduction that would increase losses, while still eliminating the harmful negative current effect.
Data Source
AI summary
In one embodiment, a synchronous rectification control circuit in a flyback converter, can include: a first control circuit that receives a drain-source voltage signal of a synchronous rectifier switch and a flyback converter output voltage, and generates a first control signal based on a conduction time of a primary-side power switch; a second control circuit configured to receive the drain-source voltage signal, and to generate a second control signal; when the primary-side power switch is turned off, and the first control signal is greater than a threshold value, the second control signal controls a switching operation of the synchronous rectifier switch; and when the primary-side power switch is turned off, and the first control signal is less than the threshold value, the synchronous rectifier switch is configured to stop operation, before the primary-side power switch is turned on again.