Synchronous Rectifier Control for Resonant Converters
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Solution Overview
Problem
Resonant converter systems face challenges in controlling synchronous rectifier switches to minimize voltage drop and prevent negative currents, especially when switching frequencies vary or exceed the resonant frequency, leading to inefficiencies and potential circuit protection issues.
Innovation Solution
The system employs predictive gate drive signal control techniques that adjust delay times based on previous conduction times to match the zero crossing point of current through the switches, using sample and hold circuitry and delay control mechanisms to ensure minimal body diode conduction time and prevent negative currents, even at varying operational frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If synchronous rectifier switches are controlled to minimize body diode conduction time, then voltage drop across switches is reduced, but negative currents may occur when switching frequency exceeds resonant frequency
Solution Approach 1:
The control system predicts the zero-crossing point of current through the synchronous rectifier switch by using previous conduction time data and iteratively adjusting delay times. This preliminary prediction allows the gate drive signal to be turned off precisely at the zero-crossing point, preventing negative current while minimizing body diode conduction time and voltage drop.
Solution Approach 2:
The system uses feedback from actual conduction times to iteratively adjust the delay time of gate drive signals. By monitoring whether the switch turns off before or after the zero-crossing point, the control system refines the delay time to achieve precise zero-crossing detection, ensuring both minimal voltage drop and prevention of negative currents.
2Reliability
If gate drive signals are adjusted to match current zero crossing points, then negative currents are prevented, but control complexity increases due to iterative delay adjustment
Solution Approach 1:
The control system uses the resonant converter's own operational characteristics (current zero-crossing points and conduction times) to automatically adjust and refine the gate drive signal timing. The system self-calibrates by iteratively modifying delay times based on observed performance, eliminating the need for external complex control mechanisms while achieving precise zero-crossing synchronization.
3Adaptability or versatility
If switching frequency varies above resonant frequency, then power delivery flexibility is improved, but synchronous rectifier control becomes inaccurate leading to negative currents
Solution Approach 1:
The control system dynamically adapts to varying switching frequencies by continuously measuring actual conduction times and iteratively adjusting delay times for gate drive signals. This dynamic adjustment ensures that the gate drive signal remains synchronized with the current zero-crossing point regardless of frequency variations above the resonant frequency, maintaining reliable control across the full operating range.
Data Source
AI summary
A resonant converter system includes a first stage having inverter circuitry and resonant tank circuitry configured to generate an AC signal from a DC input signal, a transformer configured to transform the AC signal, and a second stage. The second stage features synchronous rectifier (SR) circuitry including a plurality of SR switches each having a body diode and SR control circuitry. SR control circuitry is configured to generate gate control signals to control the conduction state of the SR switches so that the body diode conduction time is minimized and a negative current across the SR switches is reduced or eliminated. The method includes controlling the conduction state of SR switches to conduct as the body diode associated with the switch begins to conduct and controlling the SR switch to turn off as the current through the switch approaches a zero crossing.