Synchronous Rectification Control for Noise Prevention
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Solution Overview
Problem
In pulse width modulation (PWM) control-based power conversion apparatuses, the simultaneous on-state of the power switch on the primary side and the synchronous rectification (SR) transistor on the secondary side during continuous current mode (CCM) leads to noise, potentially damaging internal circuit elements.
Innovation Solution
The SR control circuit turns off the SR transistor on the secondary side before the power switch on the primary side is turned on, using a transformer, SR transistor, and SR control circuit with duty cycle signals to manage the charging and discharging currents, preventing overlapping on periods and thus avoiding noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the power conversion apparatus is operated in continuous current mode (CCM) with synchronous rectification, then conversion efficiency is improved, but noise occurs and internal circuit elements may be damaged due to simultaneous on-state of power switch and SR transistor
Solution Approach 1:
The control method performs preliminary action by detecting the voltage across the SR transistor before the power switch turns on, and proactively turning off the SR transistor in advance. This preliminary detection and shutdown prevents the harmful simultaneous on-state from occurring, while maintaining the efficiency benefits of synchronous rectification during normal operation.
Solution Approach 2:
The control method implements feedback by continuously monitoring the voltage across the SR transistor and using this information to control the timing of SR transistor shutdown. The feedback mechanism allows the system to adaptively adjust the SR transistor off-timing based on real-time voltage conditions, preventing noise while maintaining efficiency.
2Object-affected harmful factors
If the SR transistor is turned off early to avoid simultaneous on-state, then noise is prevented, but the on-time of SR transistor is reduced
Solution Approach 1:
The control method performs preliminary action by detecting the voltage across the SR transistor before the power switch turns on, and proactively turning off the SR transistor in advance. This preliminary detection and shutdown prevents the harmful simultaneous on-state from occurring, while maintaining the efficiency benefits of synchronous rectification during normal operation.
Solution Approach 2:
The control method applies dynamics by making the SR transistor off-timing adaptive rather than fixed. The shutdown timing dynamically adjusts based on the detected voltage conditions and the calculated time required for primary side current to reach zero, optimizing the balance between noise prevention and conduction time utilization.
Data Source
AI summary
A power conversion apparatus including a transformer, a synchronous rectification (SR) transistor and an SR control circuit is provided. A first terminal of a primary side of the transformer receives an input voltage, and a first terminal of a secondary side outputs a DC voltage. A drain terminal of the SR transistor is coupled to a second terminal of the secondary side of the transformer. A source terminal of the SR transistor is coupled to a ground terminal. The SR control circuit receives a signal of the drain terminal of the SR transistor to serve it as a detection signal and generate a duty cycle signal. The SR control circuit converts the duty signal into a charging current and a discharging current so as to charge and discharge an energy-storage device and generate a first voltage. The SR control circuit turns off the SR transistor according to the first voltage.