Synchronous Rectifier Control for Power Converter Cross-Conduction
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Solution Overview
Problem
Switch mode power converters face efficiency losses due to cross-conduction events, which occur when both the primary and secondary switches are conducting, leading to potential damage and reduced duty cycle range, despite measures like dead time insertion to prevent such events.
Innovation Solution
A cross-conduction detector circuit is integrated into the power converter to monitor the forward node voltage, detecting when the primary switch is conducting and disabling the secondary switch to prevent cross-conduction, using threshold comparisons and control logic to ensure efficient operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dead time is increased in drive signals to prevent cross conduction, then switch protection is improved, but power converter efficiency deteriorates and duty cycle range is reduced
Solution Approach 1:
The patent implements dynamic dead time adjustment where the dead time period is not fixed but varies based on operating conditions. The controller monitors the power converter's state and adjusts the dead time duration dynamically, allowing shorter dead time when cross-conduction risk is low and longer dead time when risk is high, thus optimizing both protection and efficiency across different operating points
Solution Approach 2:
The patent changes the temporal parameter of dead time from a static value to a variable parameter that adapts to operating conditions. By modifying the dead time duration based on real-time monitoring of switch states and power converter performance, the system optimizes the trade-off between protection and efficiency
2Reliability
If dead time is increased in drive signals to prevent cross conduction, then switch protection is improved, but duty cycle range is reduced
Solution Approach 1:
The system dynamically adjusts dead time based on the required duty cycle and operating conditions, allowing the power converter to maintain a wider duty cycle range by reducing dead time when high duty cycle is needed, while still providing adequate protection when operating conditions require it
3Loss of energy
If dead time is minimized in drive signals, then power converter efficiency is improved, but cross conduction risk increases
Solution Approach 1:
The patent employs feedback mechanisms where the controller continuously monitors the states of the first and second switches, along with power converter performance metrics, and uses this feedback to dynamically adjust the dead time period, ensuring efficient operation while preventing cross-conduction
Solution Approach 2:
The controller proactively adjusts the dead time period based on predicted or anticipated operating conditions, preparing the system in advance to prevent cross-conduction before it can occur, rather than reacting after the problem arises
Data Source
AI summary
A controller for a power converter includes a primary controller and a secondary controller. The primary controller is coupled to a primary winding of the energy transfer element and a primary switch of the power converter. The secondary controller is coupled to a secondary winding of the energy transfer element and a secondary switch, e.g., a synchronous rectifier, of the power converter. The secondary controller has a synchronous rectifier control/drive circuit, a control logic circuit, and a cross conduction detector circuit. Cross conduction is when the primary switch turns on when the secondary switch is active. The cross-conduction detector circuit produces a Disable SR signal when cross conduction event detected. The cross-conduction detector circuit detects zero voltage switching cross conduction event and a minimum conduction period cross conduction event. The SR drive circuit disables the synchronous rectifier in response to receiving the Disable SR signal.