Power Converter Cross-Conduction Detection for Synchronous Rectifiers
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Solution Overview
Problem
Switch mode power converters face efficiency losses due to cross conduction, which occurs when both primary and secondary switches are conducting, leading to potential damage and reduced duty cycle range, necessitating the minimization of dead time in drive signals while preventing cross conduction.
Innovation Solution
A cross conduction detector circuit is implemented to monitor the forward voltage node, detecting when both switches are conducting and disabling the secondary switch to prevent cross conduction, thereby protecting the switches and maintaining efficiency.
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 a feedback mechanism by monitoring the forward voltage node to detect when both switches are conducting simultaneously. The control circuit uses this feedback information to dynamically adjust the dead time, extending it only when cross-conduction is detected rather than maintaining a fixed extended dead time, thereby preventing efficiency losses while ensuring switch protection.
Solution Approach 2:
The dead time is made dynamic rather than static. The control circuit adjusts the dead time duration based on real-time detection of switch states and forward voltage node conditions. This dynamic adjustment allows the system to minimize dead time during normal operation for maximum efficiency while extending it only when necessary to prevent cross-conduction.
2Loss of energy
If dead time is minimized in drive signals to improve efficiency, then power converter efficiency is improved, but cross conduction risk increases
Solution Approach 1:
The control circuit continuously monitors the forward voltage node and switch states to provide real-time feedback. This feedback enables the system to detect cross-conduction conditions and respond by extending dead time only when necessary, allowing minimal dead time during normal operation for maximum efficiency while maintaining protection against cross-conduction.
Solution Approach 2:
The system takes preliminary action by monitoring switch states and forward voltage node before cross-conduction can occur. The control circuit is prepared to extend dead time immediately upon detecting conditions that precede cross-conduction, preventing the harmful effect while maintaining minimal dead time for efficiency.
3Reliability
If dead time is extended to protect switches from cross conduction, then switch reliability is improved, but available duty cycle range is reduced
Solution Approach 1:
The dead time is made dynamic, adjusting based on actual switch states and forward voltage node conditions. This allows the system to maintain minimal dead time during normal operation, preserving maximum duty cycle range, while extending dead time only temporarily when cross-conduction is detected to ensure switch protection.
Solution Approach 2:
The control circuit uses feedback from switch state monitoring and forward voltage node detection to adjust dead time dynamically. This feedback mechanism ensures dead time is extended only when cross-conduction is detected, rather than maintaining a fixed extended dead time that would unnecessarily reduce duty cycle range.
Data Source
Figure 1A
Figure 1B
Figure 1C
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.