Synchronous Rectifier Controller Voltage Regulation
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Solution Overview
Problem
Flyback converters with synchronous rectification face thermal stress and operational issues due to insufficient power supply voltage during low output voltage periods, leading to inefficient synchronous rectifier switch operation and inaccurate primary-only feedback control.
Innovation Solution
A synchronous rectifier controller that dynamically selects between two charging paths based on switching frequency and duty cycle to maintain a sufficient power supply voltage, using the output voltage when the frequency or duty cycle is low and the drain voltage when high, ensuring continuous operation and reducing thermal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the synchronous rectifier controller uses drain voltage to power the controller during low output voltage operation, then thermal stress is reduced, but the power supply voltage becomes insufficient during low load conditions
Solution Approach 1:
The patent implements dynamic switching between two power supply modes: drain voltage mode during high load operation and output voltage mode during low load operation. The controller monitors load conditions and automatically transitions between power sources, ensuring optimal performance across all operating ranges while preventing thermal stress and maintaining sufficient power supply voltage.
Solution Approach 2:
The patent changes the power supply voltage parameter based on operating conditions. During high load, the drain voltage (which pulses above output voltage during primary switch on-time) is used to power the controller. During low load, the system switches to using output voltage to ensure sufficient average current delivery to the VCC capacitor, maintaining reliable operation across varying load conditions.
2Loss of energy
If the primary switch duty cycle is reduced during low load conditions, then efficiency is improved, but the average current to the VCC capacitor becomes insufficient
Solution Approach 1:
The patent introduces the output voltage as an intermediary power source to supplement the VCC capacitor charging during low duty cycle operation. When the primary switch duty cycle is reduced, the output voltage path becomes the dominant charging source, ensuring the controller maintains sufficient power supply voltage without requiring high switching frequency or duty cycle.
3Reliability
If the synchronous rectifier switch conducts through the body diode due to insufficient power supply voltage, then the controller can continue operating, but accurate voltage sampling for primary-only feedback control is prevented
Solution Approach 1:
The patent takes preliminary action by ensuring sufficient power supply voltage to the controller through dual power path selection before the body diode conduction issue can occur. By proactively switching to output voltage-powered mode during low load conditions, the system prevents the VCC voltage from dropping to levels that would cause body diode conduction, thereby maintaining both controller operation and accurate voltage sampling.
Data Source
AI summary
A flyback converter is provided with a synchronous rectifier (SR) controller including a pulse linear regulator (PLR) charging path and an LDO charging path. The SR controller is configured to monitor the switching period and/or duty cycle of a power switch in the flyback converter to select between the PLR and LDO charging paths.