MOSFET Synchronous Rectifier Body Diode Conduction Optimization
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Solution Overview
Problem
Conventional self-synchronized flyback DC-DC converters with synchronous rectification experience inefficient SR turn-off in continuous conduction mode due to fast transients, leading to undesirable reverse currents, and existing solutions are costly.
Innovation Solution
A logic unit with an SR turn-off signal generator, including a body diode conduction detector and optimizer, is used to optimize the body diode conduction time of the SR MOSFET, ensuring efficient turn-off by generating a turn-off signal to terminate SR MOSFET conduction at a selected fixed optimum period, thereby reducing reverse currents and improving power efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional self-synchronized flyback DC-DC converters with synchronous rectification are used, then the converter operates well in discontinuous conduction mode, but it experiences inefficient SR turn-off and significant reverse currents in continuous conduction mode
Solution Approach 1:
The patent applies preliminary action by detecting the body diode conduction status before the SR MOSFET is turned off, and generating a turn-off signal at the optimal moment. The body diode conduction detector monitors the conduction state in advance, and the body diode conduction optimizer determines the precise turn-off timing based on this detection, ensuring the SR MOSFET is turned off before significant reverse currents occur, thus preventing energy losses.
2Reliability
If a delay generator with discrete components is used to delay the turn-on of the primary power switch MOS, then the SR switch MOS can be completely turned off, but the solution becomes costly and complex
Solution Approach 1:
The patent applies self-service by using the body diode's own conduction characteristics to generate the turn-off signal. The body diode conduction detector detects the conduction state of the body diode, and the body diode conduction optimizer uses this information to automatically determine the optimal turn-off timing. This eliminates the need for external delay generators and discrete components, simplifying the circuit while maintaining reliable SR turn-off timing.
Solution Approach 2:
The patent applies feedback by creating a closed-loop control system where the body diode conduction detector continuously monitors the body diode's conduction state and feeds this information back to the body diode conduction optimizer. The optimizer adjusts the SR MOSFET turn-off timing based on this real-time feedback, ensuring optimal performance without requiring complex external delay circuits.
3Loss of energy
If the SR MOSFET is turned off quickly to reduce reverse currents, then power losses are reduced, but the body diode conduction time must be precisely controlled
Solution Approach 1:
The patent replaces mechanical timing methods with electronic detection and optimization. Instead of using physical delay circuits or manual timing adjustments, the system uses the body diode conduction detector to electronically sense the conduction state and the body diode conduction optimizer to electronically generate the precise turn-off signal. This substitution of electronic control for mechanical timing achieves precise body diode conduction time control while reducing power losses.
Data Source
AI summary
To maximize power efficiency, dead time between “on” times of a synchronous rectifier (“SR”) MOSFET switch and a main switch for CCM operation in particular in isolated and non-isolated self-driven synchronous DC-DC converters needs to be optimized. To accomplish that objective, the latest conduction time t of a body diode of the SR MOSFET following conduction thereof is determined, compared with a selected fixed optimum period T1, and incrementally or decrementally adjusted in a subsequent switching cycle while t is unequal to T1, depending on whether t is shorter or longer than T1, so that t eventually is made substantially equal to T1 in length. This process is to be repeated continuously.