Synchronous Rectifier Timing for Main Switch Zero-Voltage Turn-On
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Solution Overview
Problem
Switching power supplies, particularly flyback converters, face limitations in switching frequency, volume reduction, and efficiency due to turn-on losses under continuous conduction mode, quasi-resonant mode, or discontinuous conduction mode, with existing solutions either being limited in application or increasing system complexity.
Innovation Solution
A zero-voltage-switching control circuit that controls the synchronous rectifier switch to turn on before the main power switch, allowing the drain-source voltage of the main power switch to decrease to zero, thereby reducing conduction losses, and is applicable across various converter topologies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the main power switch is turned on during conventional switching modes (continuous conduction mode, quasi-resonant mode, or discontinuous conduction mode), then the switching power supply can operate and transfer power, but turn-on losses occur that limit switching frequency, increase volume, and reduce efficiency
Solution Approach 1:
The synchronous rectifier switch is turned on in advance during the demagnetization period before the main power switch needs to be turned on. This preliminary action prepares the circuit by establishing current flow through the synchronous rectifier switch, which enables the main power switch to turn on at zero voltage, thereby eliminating turn-on losses and allowing higher switching frequencies without energy loss penalties
2Loss of energy
If the synchronous rectifier switch is turned on before the main power switch, then zero-voltage-switching is achieved and conduction losses are reduced, but the control circuit complexity increases
Solution Approach 1:
The control circuit uses feedback signals from the secondary side of the transformer to detect the demagnetization period and control the timing of the synchronous rectifier switch. By monitoring the voltage across the synchronous rectifier switch and using this feedback to trigger turn-on at the appropriate moment, the circuit achieves zero-voltage-switching without requiring complex external control mechanisms
Solution Approach 2:
The synchronous rectifier switch controls its own turn-on timing based on the natural demagnetization process of the transformer. The circuit uses the inherent voltage changes during demagnetization to automatically trigger the synchronous rectifier switch, eliminating the need for external complex control circuits while achieving optimal switching timing
3Loss of energy
If existing zero-voltage-switching solutions are implemented, then turn-on losses are reduced, but the solutions are limited to specific converter topologies or modes
Solution Approach 1:
The control method is designed to be universally applicable to various converter topologies including flyback converters operating in continuous conduction mode, quasi-resonant mode, and discontinuous conduction mode. By using the synchronous rectifier switch's inherent characteristics and voltage feedback rather than topology-specific control mechanisms, the solution achieves zero-voltage-switching across multiple converter types and operating modes without requiring separate control circuits for each application
Data Source
AI summary
A zero-voltage-switching control circuit for a switching power supply having a main power switch and a synchronous rectifier switch, is configured to: control the synchronous rectifier switch to be turned on for a first time period before the main power switch is turned on and after a current flowing through the synchronous rectifier switch is decreased to zero according to a switching operation of the main power switch in a previous switching period of the main power switch; and where a drain-source voltage of the main power switch is decreased when the main power switch is turned on, in order to reduce conduction loss.