Isolated Power Supply Rectifier Timing to Prevent Cross-Conduction
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Solution Overview
Problem
Isolated switched-mode power supplies face challenges in controlling synchronous rectifiers to prevent simultaneous conduction with primary main transistor switches, leading to reliability issues and efficiency degradation due to false turn-ons and lagged turn-offs, especially in discontinuous conduction mode.
Innovation Solution
A control circuit for isolated power supplies that includes a primary original signal generator and a secondary control signal generator, which synchronize the turn-on and turn-off of the secondary synchronous rectifier with the primary main transistor switch based on voltage feedback and secondary winding signals, adjusting the turn-off threshold based on freewheeling and dead times to prevent simultaneous conduction and optimize efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the secondary synchronous rectifier is controlled independently based on secondary voltage detection, then the control design is simple and transportable, but simultaneous conduction with the primary main transistor switch occurs leading to reliability degradation and efficiency loss
Solution Approach 1:
The patent merges the control of the secondary synchronous rectifier with the primary main transistor switch by sharing the same control circuit and control signal. The control circuit generates a control signal that simultaneously controls both switches to turn on and turn off together, eliminating the independence between secondary and primary control that caused simultaneous conduction risks.
Solution Approach 2:
The patent introduces a control circuit as an intermediary that coordinates the operation of both primary and secondary switches. This control circuit receives feedback signals and generates synchronized control signals, acting as a mediator that ensures the secondary synchronous rectifier and primary main transistor switch operate in coordination rather than independently.
2Loss of energy
If the secondary synchronous rectifier is turned off later to ensure complete current transfer, then current transfer is complete, but simultaneous conduction occurs causing breakdown risk and efficiency degradation
Solution Approach 1:
The control circuit is designed to turn off the secondary synchronous rectifier at the exact same instant as the primary main transistor switch by generating synchronized control signals. This preliminary coordination prevents the lagged turn-off that causes simultaneous conduction, while still ensuring complete current transfer through proper timing design.
3Reliability
If the synchronous rectifier and control circuit are arranged on the secondary side electrically isolated from the primary side, then electrical isolation is achieved, but control coordination becomes challenging
Solution Approach 1:
The control circuit acts as an intermediary that receives feedback signals from the secondary side and generates control signals that are transmitted to both the primary main transistor switch and the secondary synchronous rectifier. This intermediary approach maintains electrical isolation while achieving precise control coordination through a centralized control mechanism.
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 prevents simultaneous conduction, ensuring reliable operation and reducing switching losses, thereby enhancing the efficiency of isolated power supplies by ensuring the secondary synchronous rectifier is turned off at zero current in steady-state.
Implementation Method 1
a secondary voltage signal from a secondary winding in the isolated power supply
Data Source
AI summary
A control circuit of an isolated power supply, an isolated power supply and a control method thereof are disclosed. The control circuit includes a primary original signal generator generating a primary original turn-on signal based on a voltage feedback signal of an output voltage of the isolated power supply, and a secondary control signal generator generating a secondary control signal based on a secondary voltage signal and the primary original turn-on signal. In a current switching period, only when an expected turn-on instant for a primary main transistor switch is earlier than an expected turn-on instant for a secondary synchronous rectifier, will the secondary control signal provide an indication to turn on the secondary synchronous rectifier.