Active Clamp Flyback Dead-Time Control for Short-Circuit Protection
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Solution Overview
Problem
Conventional over-current protection mechanisms in active clamp flyback converters fail to prevent catastrophic failure of switches during short circuits in the secondary windings or rectifiers, due to insufficient dead time allowing rapid voltage spikes that overwhelm the body diodes.
Innovation Solution
Implementing a circuit with a comparator, multiplexer, and monostable multivibrator to extend the dead time between turning off the high-side switch and turning on the low-side switch, based on reverse current sensing signals, to accommodate the reverse recovery time of the body diode, thereby preventing voltage spikes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional over-current protection mechanisms are used with standard dead time, then the converter operates efficiently under normal conditions, but catastrophic failure of switches occurs during short circuits due to rapid voltage spikes overwhelming the body diodes
Solution Approach 1:
The dead time duration is made dynamic rather than fixed. The control circuit automatically adjusts the dead time based on real-time monitoring of reverse current through the body diode. During normal operation, standard dead time is used for efficiency. During short circuit conditions, the dead time is extended automatically when reverse current exceeds a threshold, preventing switch failure while maintaining efficiency under normal conditions.
Solution Approach 2:
A feedback mechanism is implemented where the reverse current through the body diode is continuously monitored during the dead time period. When this reverse current exceeds a predetermined threshold (indicating a short circuit condition), the feedback signal triggers an extension of the dead time duration. This closed-loop control ensures the converter adapts to fault conditions and protects switches without requiring external intervention.
2Reliability
If dead time is extended to accommodate body diode reverse recovery, then switch protection is improved, but converter efficiency decreases due to longer non-conducting periods
Solution Approach 1:
The dead time duration is made dynamic rather than fixed. The control circuit automatically adjusts the dead time based on real-time monitoring of reverse current through the body diode. During normal operation, standard dead time is used for efficiency. During short circuit conditions, the dead time is extended automatically when reverse current exceeds a threshold, preventing switch failure while maintaining efficiency under normal conditions.
Solution Approach 2:
The dead time parameter is changed adaptively based on operating conditions. Instead of using a fixed conservative dead time that would always reduce efficiency, the system monitors reverse current magnitude and adjusts the dead time parameter accordingly. This parameter change ensures minimal dead time (maximum efficiency) during normal operation while providing sufficient protection during fault conditions.
Data Source
AI summary
According to an embodiment, a method for operating an active clamp flyback (ACF) converter is proposed. The method comprises sensing a reverse current sensing signal (RCSS) during a conduction phase of a high-side switch, the RCSS corresponding to a reverse current flowing at the high-side switch during the conduction phase; comparing the RCSS to a threshold; setting a first dead time duration between turning OFF the high-side switch and turning ON a low-side switch in response to the RCSS exceeding the threshold, the first dead time duration being a function of a reverse recovery time of a body diode of the low-side switch; and setting a second dead time duration less than the first dead time duration between turning OFF the high-side switch and turning ON the low-side switch of the ACF converter in response to the RCSS falling below the threshold.