Isolated Power Supply Timing Control for Synchronous Rectifier Shoot-Through
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Solution Overview
Problem
Existing isolated power supply control circuits face challenges in reliably avoiding cross conduction between primary-side transistor switches and synchronous rectification transistors without compromising efficiency or feedback control accuracy, particularly in continuous inductor current mode.
Innovation Solution
A control circuit that generates secondary-side and primary-side control signals based on the turn-off instant of the secondary-side synchronous rectification transistor, determining the actual turn-on instant for the primary-side transistor switch to prevent shoot-through, while allowing feedback control to operate without additional delays in safe conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If signal delaying is used to avoid cross conduction, then reliability of avoiding cross conduction is improved, but feedback control accuracy deteriorates due to extended delay time
Solution Approach 1:
The patent applies dynamics by making the dead time variable rather than fixed. The control circuit dynamically adjusts the dead time duration based on real-time detection of the synchronous rectification transistor's conduction state. When the transistor is detected to be conducting, the dead time is extended to prevent cross conduction; when not conducting, the dead time is reduced or eliminated, thereby maintaining feedback control accuracy while ensuring reliability.
Solution Approach 2:
The patent implements feedback by using the detection circuit to monitor the conduction state of the synchronous rectification transistor and feed this information back to the control circuit. The control circuit then adjusts the dead time accordingly, creating a closed-loop control system that balances cross conduction prevention with feedback control accuracy.
2Reliability
If dead time is extended to prevent cross conduction, then reliability is improved, but power supply performance deteriorates
Solution Approach 1:
The patent makes the dead time dynamic rather than static. By detecting the actual conduction state of the synchronous rectification transistor in real-time, the control circuit adjusts the dead time duration adaptively. This ensures reliable cross conduction prevention when needed while minimizing dead time losses during normal operation, thereby maintaining power supply performance.
3Loss of energy
If synchronous rectification transistor control is implemented in isolated power supply, then efficiency is improved, but control complexity increases due to separate ground references
Solution Approach 1:
The patent introduces a detection circuit as an intermediary between the synchronous rectification transistor and the primary-side control circuit. This detection circuit monitors the conduction state and provides feedback signals that enable coordinated control despite the isolated ground references. The intermediary simplifies the overall control complexity by handling the ground reference isolation issue locally.
Solution Approach 2:
The patent segments the control system into independent primary-side and secondary-side control circuits with separate ground references. The detection circuit on the secondary side independently monitors the synchronous rectification transistor state and provides feedback, allowing each side to operate independently while maintaining coordinated control, thus managing the complexity of isolated ground references.
4Ease of operation
If independent control of synchronous rectification transistor is used, then transportability is improved, but reliability deteriorates due to remaining probability of cross conduction
Solution Approach 1:
The patent implements feedback by having the detection circuit monitor the synchronous rectification transistor's conduction state and provide real-time feedback to the control circuit. This closed-loop control ensures that the dead time is adjusted dynamically based on actual operating conditions, eliminating the remaining probability of cross conduction that exists in open-loop independent control while maintaining transportability.
Data Source
AI summary
Isolated power supply control circuits, isolated power supply and control method thereof are disclosed, the control circuit for controlling an isolated power supply includes a secondary-side control signal generator and a primary-side control signal generator. The secondary-side control signal generator produces a secondary-side transistor switch control signal containing information about a turn-off instant of a secondary-side synchronous rectification transistor, which serves as a second turn-on instant. The primary-side control signal generator derives, from a feedback signal, a supposed turn-on instant for a primary-side transistor switch, which serves as a first turn-on instant. The primary side turn-on signal generator further derives a turn-on instant for the primary-side transistor switch from the second or first turn-on instant whichever is later and responsively generates a primary-side transistor switch control signal. This control circuit can effectively avoid shoot-through of primary- and secondary-side transistor switches at the cost of only insignificantly compromised feedback adjustment accuracy.