Variable Turn-Off Threshold Synchronous Rectifier Control
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Solution Overview
Problem
Synchronous rectifiers in switched mode power supplies face challenges in achieving both high efficiency and minimizing inversion currents and drain spiking, particularly due to issues with turn-off threshold settings that affect dead time and system efficiency across varying load conditions.
Innovation Solution
A synchronous rectifier controller implements a multi-step turn-off threshold that increases with load size and switching cycle length, allowing for efficient operation across all load ranges by adjusting the turn-off threshold dynamically during the switching cycle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a positive turn-off threshold is used to reduce dead time and improve efficiency, then system efficiency is improved, but inversion currents and drain spiking occur during transient conditions
Solution Approach 1:
The patent applies dynamics by making the turn-off threshold voltage variable rather than fixed. The controller dynamically adjusts the turn-off threshold based on operating conditions: using a first (higher) turn-off threshold during steady-state operation to minimize dead time and maximize efficiency, and switching to a second (lower or zero) turn-off threshold during transient conditions to prevent inversion currents and drain spiking. This dynamic adaptation resolves the contradiction between efficiency and harmful current suppression.
Solution Approach 2:
The patent changes the parameter of turn-off threshold voltage from a constant value to a variable parameter that adapts to different operating conditions. By modifying this critical parameter based on whether the system is in steady-state or transient mode, the patent achieves both high efficiency during normal operation and protection against harmful currents during transients, thereby resolving the technical contradiction.
2Object-generated harmful factors
If a zero or negative turn-off threshold is used to prevent late turn-off during transients, then inversion currents are prevented, but system efficiency decreases
Solution Approach 1:
The patent uses dynamics by implementing a controller that dynamically selects between different turn-off threshold values based on the operating mode. During transient conditions, a zero or negative turn-off threshold is applied to prevent inversion currents. During steady-state operation, a positive turn-off threshold is used to maximize efficiency. This dynamic switching of threshold values resolves the contradiction between preventing harmful currents and maintaining system efficiency.
Solution Approach 2:
The patent changes the turn-off threshold parameter from a fixed value to a condition-dependent variable. The parameter is set to zero or negative during transients to eliminate inversion currents, and changed to a positive value during steady-state to optimize efficiency. This parameter adaptation strategy resolves the contradiction between harmful current prevention and energy efficiency.
3Device complexity
If a fixed turn-off threshold is used, then control is simple, but performance cannot be optimized across varying load conditions
Solution Approach 1:
The patent applies dynamics by implementing a controller that dynamically adjusts the turn-off threshold based on detected operating conditions such as load changes and transient states. This dynamic control mechanism enables the system to optimize performance across varying load conditions while maintaining relatively simple implementation through automated threshold selection, resolving the contradiction between control simplicity and adaptability.
Solution Approach 2:
The patent changes the turn-off threshold from a fixed parameter to a variable parameter that adapts to different load conditions. The controller automatically modifies the threshold value based on operating mode (steady-state vs. transient) and load level, enabling optimized performance across the full range of operating conditions without significantly increasing control complexity.
Data Source
AI summary
A power converter includes a transformer, a first primary side transistor coupled to the primary winding, a primary side controller that provides a gate signal to a gate of the first primary side transistor, a first synchronous rectifier (SR) transistor, and an SR controller. The first SR transistor has a drain coupled to a secondary winding of the transformer, a gate for receiving a first SR gate signal, and a source coupled to a first output terminal of the power converter. The SR controller is coupled to the gate and drain of the first SR transistor, for activating the first SR gate signal when a voltage on the drain falls below a turn-on threshold, and deactivating the first SR gate signal when a voltage on the drain rises above a variable turn-off threshold, wherein the variable turn-off threshold increases over an expected on-time of the first SR gate signal.