Synchronous Rectifier Control for Backflow Prevention
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Solution Overview
Problem
Voltage transforming apparatuses with synchronous-rectifying switching elements face challenges in performing synchronous rectifying control operations due to backflow of output current, leading to reduced performance and reliability, especially in hybrid vehicles where large output currents generate excessive heat and increase production costs with hall elements, and estimating output current from the primary side is difficult due to intermittent and varying currents.
Innovation Solution
A control apparatus that includes a judging circuit to determine if the output current is below a specified threshold based on the primary-side current, compensating for variations in duty ratio, DC output voltage, and DC input voltage, and an inhibition circuit to prevent synchronous-rectifying control when the output current is low, using a comparator and compensating circuit with a periodic wave signal to accurately switch between permission and inhibition of synchronous rectifying operations.
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 output current may flow back through the elements at low current, lowering performance and reliability
Solution Approach 1:
The control apparatus monitors the output current of the voltage transforming apparatus and provides feedback control. When the output current is small and backflow is detected, the control apparatus inhibits the synchronous-rectifying control operation, preventing damage to the switching elements while maintaining normal operation at higher currents
Solution Approach 2:
The control apparatus dynamically adjusts the operation mode of the synchronous-rectifying switching elements based on the output current level. At low currents, it switches to inhibition mode to prevent backflow, while at higher currents, it enables synchronous rectification to minimize power loss, making the system adaptable to varying load conditions
2Reliability
If a current-detecting resistor is provided in the secondary side to monitor output current, then backflow can be prevented, but the resistor produces large heat at large output currents, increasing apparatus size
Solution Approach 1:
The control apparatus uses the primary-side current as an intermediary parameter to indirectly estimate the output current in the secondary side. By monitoring the primary current and using it to control the synchronous-rectifying operation, the system avoids placing a current-detecting resistor in the secondary side, thereby eliminating the heat generation problem while still preventing backflow
Solution Approach 2:
The invention replaces the physical current-detecting resistor in the secondary side with a control system that uses primary-side current detection and processing. This substitution eliminates the need for a high-power resistor in the secondary circuit, reducing heat generation and apparatus size while maintaining backflow protection functionality
3Temperature
If hall element is used instead of current-detecting resistor, then heat generation is reduced, but production cost increases due to expensive hall element
Solution Approach 1:
The invention extracts the current detection function from the secondary side where it would require expensive hall elements or high-power resistors, and relocates it to the primary side where current detection is more efficient and cost-effective. This extraction allows the system to achieve the same backflow protection functionality using simpler, cheaper components
Solution Approach 2:
The control apparatus creates a control signal based on the primary-side current that effectively copies or represents the secondary-side current conditions. This control signal is then used to inhibit synchronous rectification when backflow is likely, achieving the same protective effect as direct secondary-side current detection would provide
4Device complexity
If output current is estimated from primary-side current without compensation, then simple control is achieved, but accurate judgment is difficult due to intermittent and varying primary current
Solution Approach 1:
The control apparatus performs preliminary actions by detecting the primary-side current and proactively determining the appropriate synchronous-rectifying control operation before the actual secondary current condition becomes critical. This advance detection and control decision-making enables accurate judgment despite the intermittent nature of primary current
Solution Approach 2:
The control apparatus changes the parameter being monitored from direct secondary current to primary-side current, which can be more easily and accurately detected. By establishing a control relationship between primary and secondary sides, the system achieves accurate output current estimation through parameter transformation rather than direct measurement
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 with compensating for a variation of a relationship between the primary side-current and the output current due to variation of duty ratio of the power switching elements, and variation of at least one of the DC output voltage and the DC input voltage of the voltage transforming apparatus.