LLC Synchronous Rectifier Control Using Current-Edge DCM Detection
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Solution Overview
Problem
The control of synchronous rectifiers (SRs) in resonant LLC converters is challenging due to misalignment of primary and secondary side transitions and current rises/falls, leading to inaccurate sensing and control, especially in varying operating conditions and modes, with existing methods like voltage sensing being prone to errors and current sensing complicating layout.
Innovation Solution
A secondary-side controller that uses a current sense signal to determine conduction modes (DCM or CCM) based on rising and falling edges, allowing precise control of SRs by switching on rectification branches accordingly, reducing the need for high-speed comparators and improving reaction time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If voltage sensing techniques are used for SR control, then the control implementation is simple, but the sensing accuracy deteriorates due to inductance errors and misalignment
Solution Approach 1:
The patent introduces an intermediary current sensing mechanism that indirectly measures the rectification current through the body diode current. By using a sense resistor to detect the voltage drop across the body diode and comparing it with a reference voltage, the system achieves accurate current detection without directly measuring the high-current rectification path, thus resolving the contradiction between simple implementation and accurate sensing.
2Measurement precision
If direct current sensing is implemented to improve SR control accuracy, then the sensing precision improves, but the circuit complexity and layout difficulty increase
Solution Approach 1:
The patent extracts the current sensing function from the high-current rectification path by utilizing the body diode's inherent current path. The sense resistor is placed in series with the body diode rather than in the main current path, allowing accurate sensing with minimal impact on the overall circuit complexity and layout.
Solution Approach 2:
The body diode serves as an intermediary element that allows indirect current sensing. By measuring the voltage drop across the body diode through a small sense resistor, the system obtains accurate current information without requiring direct measurement of the large rectification current, thus reducing layout complexity while maintaining sensing precision.
3Measurement precision
If adaptive algorithms are used to correct voltage sensing errors, then the sensing accuracy improves, but the control complexity and response time increase
Solution Approach 1:
The patent applies preliminary action by proactively detecting the current mode (DCM or CCM) using the body diode voltage signal before the main rectification switching occurs. The controller pre-determines the appropriate SR switching strategy based on the detected current mode, eliminating the need for complex adaptive correction algorithms and reducing control complexity while maintaining accuracy.
4Ease of operation
If SR turn-off is synchronized to primary side signals, then the control simplicity improves, but the control accuracy deteriorates due to misalignment in varying operating conditions
Solution Approach 1:
The patent implements feedback by continuously monitoring the body diode voltage signal to detect the actual current mode and timing. The controller uses this feedback information to dynamically adjust the SR turn-off timing, ensuring accurate synchronization with the actual rectification current regardless of operating conditions, thus resolving the contradiction between simple control synchronization and accurate SR switching.
Data Source
AI summary
The primary side of an LLC converter includes a primary-side switch network connected to an LLC network having a first winding of an isolation transformer. The secondary side includes a secondary-side switch network having first and second rectification branches coupled to different tap points of a second winding of the isolation transformer. Switching of the secondary-side switch network is controlled based on a drive signal and a current sense signal indicative of current in the rectification branches. For a first part of each switching cycle, discontinuous conduction mode (DCM) is detected based on a falling edge of the current sense signal occurring before a falling edge of the drive signal for the first rectification branch. For a second part of each switching cycle, DCM is detected based on the falling edge of the current sense signal occurring before a falling edge of the drive signal for the second rectification branch.