Resonant Converter Control Circuit for Synchronous Rectification
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Solution Overview
Problem
Resonant converters, such as LLC converters, face misoperation of synchronous rectification switches due to oscillations in drain-source voltage and capacitive current spikes, especially when switching from continuous conduction mode to discontinuous conduction mode or at light loads, leading to inefficiencies and potential negative current feedback.
Innovation Solution
A control circuit with a voltage detection module and delay module dynamically adjusts the delay time for turning on the synchronous rectification switch based on the previous switching cycle's operating state, using timers to filter out oscillations and prevent misoperation by setting strict conditions for switch activation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the synchronous rectification switch is turned on immediately when drain-source voltage reaches conduction threshold, then rectification efficiency is improved, but misoperation occurs due to voltage oscillations and capacitive current spikes
Solution Approach 1:
The control circuit performs preliminary detection of drain-source voltage and capacitive current before triggering the synchronous rectification switch. By detecting whether the voltage has been above the conduction threshold for a predetermined time period and whether the capacitive current has decayed below a threshold, the system ensures proper timing conditions are met before activation, preventing misoperation while maintaining efficiency.
Solution Approach 2:
The control circuit continuously monitors the drain-source voltage and capacitive current of the synchronous rectification switch, using this feedback information to determine the appropriate timing for switch activation. The feedback mechanism allows the system to adapt to different operating conditions (CCM/DCM modes) and prevent misoperation caused by voltage oscillations or capacitive current spikes.
2Reliability
If delay time for switch activation is increased to filter oscillations, then misoperation is reduced, but rectification efficiency deteriorates due to excessive delay
Solution Approach 1:
The control circuit dynamically adjusts the delay time based on real-time detection of voltage and current conditions rather than using a fixed delay. The delay period is determined adaptively by monitoring whether the drain-source voltage remains above the conduction threshold for the predetermined time and whether the capacitive current has decayed, allowing optimal timing under varying load conditions.
Solution Approach 2:
The system changes the timing parameters (delay time, threshold values) based on detected operating conditions. By adjusting these parameters dynamically according to whether the converter is in CCM or DCM mode and the actual voltage/current waveforms, the system achieves both reliable operation and high efficiency without excessive delay.
3Device complexity
If fixed delay time is used for switch activation, then circuit complexity is reduced, but adaptability to different operating conditions (CCM/DCM, light load) deteriorates
Solution Approach 1:
The control circuit uses the inherent voltage and current waveforms from the synchronous rectification switch itself to determine activation timing, without requiring external complex control signals or additional sensing components. The system self-adjusts by detecting its own operating state through the drain-source voltage and capacitive current, achieving adaptability with minimal added complexity.
Solution Approach 2:
The control circuit performs multiple functions using the same detection mechanism: it detects both the voltage threshold crossing and the capacitive current decay, determines operating mode (CCM/DCM), and controls switch activation timing all through a unified detection and control approach. This multi-functional design achieves broad adaptability without proportionally increasing complexity.
Data Source
AI summary
A control circuit for controlling a synchronous rectification switch of a resonant converter, where in a switching cycle, the control circuit is configured to: delay a first time period from a first moment; control the synchronous rectification switch to be turned on when a drain-source voltage of the synchronous rectification switch reaches a first threshold after the first time period; and where the first time period is generated based on an operating state of the synchronous rectification switch in a previous switching cycle.