Flyback Converter Switching Control Circuit for ZVS
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Solution Overview
Problem
Flyback converters face significant switching losses due to high drain-source voltage when turning on the main switch, especially at high input voltages or low transformer turns ratios, which complicates achieving zero-voltage switch (ZVS) and affects efficiency.
Innovation Solution
A switching control circuit for flyback converters that includes a first voltage generating circuit to sample the input voltage and a synchronous rectification control circuit to adjust the on-time of the rectifier switch, thereby controlling the absolute value of the negative current flowing through the secondary winding to reduce the drain-source voltage of the main switch, ensuring ZVS under varying input conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the main switch is turned on at high input voltage or low transformer turns ratio, then the converter can operate at higher input voltages, but the drain-source voltage remains high causing large switching losses
Solution Approach 1:
The patent applies preliminary action by generating a negative current in advance through the synchronous rectification control circuit before the main switch turns on. This negative current discharges the parasitic capacitor of the main switch, reducing the drain-source voltage to zero before switching occurs, thereby achieving zero-voltage switching and eliminating switching losses at high input voltages
Solution Approach 2:
The patent changes the parameter of drain-source voltage by dynamically adjusting the on-time of the rectifier switch based on the input voltage level. The synchronous rectification control circuit modifies the conduction time to control the magnitude of negative current, which directly changes the discharge effect on the parasitic capacitor and achieves voltage reduction for ZVS
2Loss of energy
If the on-time of the rectifier switch is increased to generate larger negative current, then the drain-source voltage is reduced more effectively, but large loop currents are generated
Solution Approach 1:
The patent employs feedback by using the first voltage sampling signal (representing input voltage) as feedback to the synchronous rectification control circuit. The control circuit adjusts the on-time of the rectifier switch based on this feedback signal, creating a closed-loop control system that automatically optimizes the negative current magnitude to achieve ZVS while preventing excessive loop currents
Solution Approach 2:
The patent applies dynamics by making the on-time of the rectifier switch variable rather than fixed. The synchronous rectification control circuit dynamically adjusts the conduction time based on real-time input voltage conditions, allowing the system to optimize the negative current generation adaptively - generating sufficient current for ZVS when needed while limiting current when input voltage is low
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces switching losses by adaptively controlling the negative current, allowing for ZVS across different input voltages and preventing large loop currents, thereby enhancing the efficiency of the flyback converter.
Implementation Method 1
the negative current can be used to discharge a parasitic capacitor of the main switch to reduce a drain-source voltage of the main switch
Implementation Method 2
a rectifier switch coupled to a secondary winding of the transformer
Data Source
AI summary
A switching control circuit for a flyback converter having a main switch coupled to a primary winding of a transformer and a rectifier switch coupled to a secondary winding of the transformer, can include: a first voltage generating circuit configured to generate a first voltage sampling signal representing information of an input voltage; a synchronous rectification control circuit configured to adjust an on-time of the rectifier switch according to the first voltage sampling signal in order to adjust an absolute value of a negative current flowing through the secondary winding; and where the negative current is configured to discharge a parasitic capacitor of the main switch in order to reduce a drain-source voltage of the main switch.