Synchronous Rectifier Self-Synchronization via Current Sense Thresholds
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Solution Overview
Problem
Switched mode power supplies using synchronous rectifiers face inefficiencies due to false triggering caused by noise transients, particularly after turn-on events, leading to synchronization errors and improper rectifier control.
Innovation Solution
A self-synchronous rectifier control method that senses a current sense signal and compares it to a RESET threshold, commencing and resetting a minimum off time timer to prevent false triggering, ensuring synchronization with actual conduction phases by using multiple thresholds for on, off, and reset conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If minimum blanking times are used to prevent false triggering, then false triggering is reduced, but synchronization errors occur after turn-on events
Solution Approach 1:
The patent applies preliminary action by resetting the minimum off-time timer when the current sense signal exceeds a reset threshold before the rectifier switch is turned on. This pre-reset action ensures that the timer starts counting from zero at the correct moment, preventing synchronization errors that would occur if the timer had not been reset. The reset threshold is specifically chosen to detect the turn-on event before it occurs, allowing the system to prepare the timer in advance.
2Loss of energy
If synchronous rectification is used to reduce voltage drop, then efficiency is improved, but false triggering from noise transients occurs
Solution Approach 1:
The patent applies local quality by implementing different threshold levels for different detection purposes: an on-threshold for detecting conduction phase start, an off-threshold for detecting conduction phase end, and a reset-threshold for detecting turn-on events. Each threshold is optimized for its specific function, allowing the system to distinguish between legitimate signals and noise transients locally at each detection point, thereby preventing false triggering while maintaining efficient synchronous rectification.
3Measurement precision
If multiple thresholds are used for precise conduction detection, then synchronization accuracy is improved, but device complexity increases
Solution Approach 1:
The patent applies universality by using a single current sense signal for multiple detection functions. The same sense signal is compared against different thresholds (on-threshold, off-threshold, and reset-threshold) to achieve conduction detection, turn-on event detection, and synchronization. This multi-functional approach allows precise conduction detection with multiple thresholds while avoiding the complexity of separate sensing circuits for each function.
Data Source
AI summary
A method and synchronous rectifier controller uses minimum off and on time blanking to avoid switching the switching transistor at incorrect times responsive to transients in the current sense signal. The minimum off time timer is commenced only when the current sense signal is above a reset threshold, and is reset when the current sense voltage falls below the reset threshold. Resetting the minimum off time timer in this manner avoids false starts of the minimum off time timer due to transients and allows the SRC to properly synchronize with the conduction and blocking phases of rectifier operation.