Secondary-Side Rectifier Control for Isolated Power Supplies
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional isolated power supplies face issues with cross-conduction between primary and secondary side circuits, leading to degraded conversion efficiency and safety risks, particularly due to misjudged natural resonance of inductors and delayed turn-off of synchronous rectifiers.
Innovation Solution
A control circuit for an isolated power supply that includes a primary side controller and a secondary side controller. The secondary side controller detects the voltage from the secondary side winding and provides control signals to the synchronous rectifier, ensuring it turns on only when the voltage drops to a predetermined level after the primary transistor switch is turned off, and turns off when the voltage rises back to another predetermined level, with adaptive adjustment of these voltages based on the comparison of time intervals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the synchronous rectifier is controlled to turn on based on detected slope and/or value of secondary side winding voltage, then the rectifier can operate automatically, but cross-conduction between primary and secondary side circuits occurs
Solution Approach 1:
The patent implements a feedback mechanism where the controller detects the voltage across the secondary winding and uses this information to control the synchronous rectifier's turn-on and turn-off timing. The controller monitors when the voltage drops to a first predetermined value (turn-on signal) and when it rises to a second predetermined value (turn-off signal), creating a closed-loop control system that prevents cross-conduction while maintaining automatic operation.
Solution Approach 2:
The patent applies preliminary action by ensuring the synchronous rectifier is turned off before the primary transistor switch turns on. By detecting the voltage rise to the second predetermined value and turning off the rectifier in advance, the system prevents cross-conduction from occurring, addressing the reliability issue before it can manifest.
2Productivity
If the synchronous rectifier is turned off at a later time than the primary power transistor, then continuous conduction is maintained, but cross-conduction risk increases
Solution Approach 1:
The controller continuously monitors the secondary winding voltage and uses this feedback to precisely time the rectifier's turn-off. By detecting when the voltage reaches the second predetermined value, the system determines the optimal moment to turn off the rectifier, balancing continuous conduction needs with cross-conduction prevention.
Solution Approach 2:
The system performs preliminary action by turning off the synchronous rectifier before the primary transistor switch turns on in the next switching cycle. This advance turn-off, triggered by the voltage reaching the second predetermined value, eliminates the cross-conduction window while maintaining efficient continuous conduction operation.
3Device complexity
If natural resonance of the inductor is used as turn-on signal, then simple control is achieved, but false turn-on occurs degrading efficiency
Solution Approach 1:
Instead of relying on simple inductor resonance detection, the patent implements feedback-based voltage monitoring across the secondary winding. The controller detects the actual voltage waveform and identifies the precise moment when voltage drops to the first predetermined value, providing accurate turn-on signaling that avoids false triggers while maintaining operational efficiency.
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 proposed control circuit effectively prevents cross-conduction between primary and secondary side circuits, thereby enhancing the safety performance and overall conversion efficiency of the isolated power supply.
Implementation Method 1
a transformer coupled between the primary and secondary side circuits
Implementation Method 2
a synchronous rectifier in the secondary side circuit is turned on or off typically independently of control logic
Data Source
AI summary
The present invention provides a control circuit of an isolated power supply and an isolated power supply. A secondary side controller in the control circuit can turn on and off a synchronous rectifier based on a voltage from a secondary side winding, enabling adaptive turn-on and turn-off control of the synchronous rectifier. Specifically, when the voltage from the secondary side winding reaches a second predetermined voltage, an instruction for turning off the synchronous rectifier is provided. The second predetermined voltage is adjusted based on a comparison between a time interval for the previous switching period and a predetermined reference interval. In this way, a delay from a turn-off time of the synchronous rectifier to a turn-on time of a primary transistor switch is substantially maintained at the predetermined reference interval. This prevents cross-conduction of the primary and secondary side circuits, ensuring safe operation of the isolated power supply.