Flyback Converter Primary-Only Output Current Regulation
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Solution Overview
Problem
Conventional flyback converters face inefficiencies and increased manufacturing complexity due to the need for secondary-side sense resistors and optoisolators for output current regulation, particularly in continuous conduction mode, where primary-only current sensing is not valid.
Innovation Solution
A controller for a flyback converter that calculates a peak sense resistor voltage based on desired average output current, measured on-time, input voltage, transformer turns ratio, and cycle period, allowing the power switch transistor to be cycled off when the sense resistor voltage equals the peak voltage, thereby regulating the average output current independently of conduction mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a secondary-side sense resistor is used to measure output current, then output current regulation accuracy is improved, but efficiency decreases and manufacturing complexity increases
Solution Approach 1:
The patent extracts the current sensing function from the secondary side to the primary side by using the sense resistor voltage on the primary side. The controller calculates the output current based on the primary side sense resistor voltage and transformer parameters, eliminating the need for a secondary side sense resistor and thus removing the associated energy loss and complexity.
Solution Approach 2:
The patent uses the transformer as an intermediary to transfer current information from the secondary side to the primary side. By measuring the sense resistor voltage on the primary side and using the known transformer turns ratio, the controller can determine the output current without direct secondary side sensing, avoiding the need for optocouplers and secondary side components.
2Device complexity
If primary-only current sensing is used during discontinuous conduction mode, then device complexity is reduced, but adaptability decreases when transitioning to continuous conduction mode
Solution Approach 1:
The patent implements a dynamic control strategy where the controller monitors the switching cycle characteristics and adapts its current sensing calculation method based on whether the converter is operating in discontinuous or continuous conduction mode. This allows the system to maintain accurate current regulation across different operating conditions while still using only primary side sensing components.
Solution Approach 2:
The patent creates a universal primary-only current sensing method that functions in both discontinuous and continuous conduction modes. By using the sense resistor voltage on the primary side combined with knowledge of transformer parameters and switching cycle timing, the system achieves mode-independent current measurement capability without requiring separate sensing circuits for different conduction modes.
Data Source
AI summary
Disclosed is a universal primary-only output current regulation for a flyback converter. A controller determines a peak sense resistor voltage responsive to a desired average output current for the flyback converter during a continuous conduction mode of operation. After a power switch transistor is cycled on, the controller monitors a sense resistor voltage to determine when the sense resistor voltage equals the peak sense resistor voltage. The controller switches off the power switch transistor when the sense resistor voltage equals the peak sense resistor voltage to maintain an average output current for the flyback converter equal to the desired average output current.