Active-Clamp Flyback Circuit Control for Switching Loss Reduction
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Solution Overview
Problem
In flyback switching power supplies, the prior art lacks precise control over the turn-off moment of the switch tube, leading to excessive negative current through the magnetizing inductor, increasing conduction loss, and fails to differentiate between boundary conduction mode (BCM) and discontinuous conduction mode (DCM) requirements.
Innovation Solution
An active-clamp flyback circuit with a primary control circuit that adjusts the turn-off moment of the first switch tube based on the voltages at both ends of the magnetizing inductor, using a switch tube control circuit with a second capacitor to detect voltages and control the turn-off timing, determining the working mode by threshold times and voltage changes to minimize losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the switch tube MA is turned off immediately when the main power switch tube M0 is turned on, then the control is simple, but the negative current through the magnetizing inductor becomes too large which increases conduction loss
Solution Approach 1:
The control circuit performs preliminary action by detecting the voltage across the magnetizing inductor before the main power switch tube turns on, and accordingly controls the turn-off timing of the switch tube MA in advance, so that the negative current through the magnetizing inductor is reduced before it becomes excessively large, thereby reducing conduction loss without significantly increasing control complexity
Solution Approach 2:
The control circuit uses feedback by continuously monitoring the voltage across the magnetizing inductor and adjusting the turn-off moment of the switch tube MA based on this feedback signal, enabling precise control of the negative current magnitude and reducing conduction loss while maintaining manageable control complexity
2Loss of energy
If the turn-off moment of the switch tube MA is precisely controlled based on magnetizing inductor voltage, then the conduction loss is reduced, but the control circuit complexity increases
Solution Approach 1:
The patent introduces an intermediary detection circuit that monitors the voltage across the magnetizing inductor and translates it into control signals for the switch tube MA. This intermediary mechanism enables precise control of the turn-off moment to reduce conduction loss, while the intermediary circuit itself manages the complexity by providing a dedicated, specialized control path rather than requiring complex integration throughout the entire system
3Device complexity
If the switch tube MA is controlled without differentiating between BCM and DCM modes, then the control is simple, but the turn-on loss cannot be optimized for different operating modes
Solution Approach 1:
The control method dynamically adapts to different operating modes (BCM and DCM) by detecting the voltage characteristics of the magnetizing inductor and adjusting the turn-off moment of the switch tube MA accordingly. This dynamic control enables optimization of turn-on loss for each mode while maintaining a unified control framework that does not significantly increase overall system complexity
Data Source
AI summary
The present invention provides an active-clamp flyback circuit and a control method thereof. The active-clamp flyback circuit includes a flyback switching power supply, and further includes a first capacitor and a first switch tube. One end of the first capacitor is connected with a high potential end of an input power supply, the other end is connected with a first end of the first switch tube, and a second end of the first switch tube is connected with a common end of the main power switch tube and the primary winding. A turn-off moment of the first switch tube is adjusted according to time when the main power switch tube is turned on and the first switch tube is turned off and voltages at both ends of the magnetizing inductor. The present invention can reduce the turn-on loss of the main power switch tube.