Synchronous Rectifier Control via Primary Current Inhibition
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Solution Overview
Problem
Voltage transforming apparatuses with synchronous-rectifying switching elements face challenges in performing synchronous-rectifying control due to backflow issues, especially when output currents are small, leading to performance and reliability concerns, and the use of current-detecting resistors or hall type current sensors increases costs and complexity.
Innovation Solution
A control apparatus that judges the output current based on the primary-side current flowing through the transformer's primary coil immediately before the off-operation of power switching elements, inhibiting synchronous-rectifying control when the current is below a specified threshold, thereby reducing the impact of surge noise and allowing more accurate switching between permission and inhibition of synchronous-rectifying control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If synchronous-rectifying switching elements are used instead of diodes, then power loss is reduced, but backflow occurs at low output current reducing reliability
Solution Approach 1:
The control apparatus determines the output current before activating the synchronous-rectifying control operation. By assessing current conditions in advance, the system prevents backflow by inhibiting the control operation when output current is below a threshold, while allowing it when current is sufficient, thus maintaining both low power loss and high reliability
Solution Approach 2:
The system continuously monitors output current and uses this feedback to dynamically control the synchronous-rectifying operation. When output current falls below a predetermined threshold, the control operation is inhibited to prevent backflow; when current is above the threshold, the operation proceeds to minimize power loss, creating a closed-loop control system that balances efficiency and reliability
2Reliability
If a current-detecting resistor is used to monitor output current, then backflow can be prevented, but the apparatus becomes large and expensive
Solution Approach 1:
The control apparatus uses the primary-side current as an intermediary parameter to infer the output current condition on the secondary side. Instead of directly measuring the difficult-to-access secondary current with expensive sensors, the system measures the easily-accessible primary current and uses it to determine whether to inhibit synchronous-rectifying control, thereby preventing backflow without requiring complex current-detecting resistors or Hall sensors
Solution Approach 2:
The invention replaces the physical current-detecting resistor or Hall effect sensor with an electrical control mechanism that uses primary-side current measurement. This substitution eliminates the need for bulky, expensive current sensing components on the secondary side while achieving the same backflow prevention function through control logic based on primary current characteristics
3Device complexity
If primary-side current is used to estimate output current, then cost is reduced, but switching noise makes accurate judgment difficult
Solution Approach 1:
The system determines the output current condition at a timing before the synchronous-rectifying control operation is activated. By making the determination in advance, the control apparatus avoids the switching noise that occurs during the actual switching operation, enabling accurate judgment of primary-side current and precise control decisions without being affected by noise interference
Data Source
AI summary
The control apparatus for controlling a voltage transforming apparatus having a transformer, power switching elements disposed in a primary side, and synchronous-rectifying switching elements disposed in a secondary side includes a judging circuit making a judgment as to whether or not an output current of the voltage transforming apparatus is smaller than a specified current on the basis of a primary-side current of the transformer and an inhibition circuit inhibiting the synchronous-rectifying switching elements from performing their synchronous-rectifying control operation when the judging circuit judges that the output current is smaller than the specified current. The judging circuit makes the judgment on the basis of the primary-side current flowing through the primary coil of the transformer immediately before the power switching elements are turned off.