Flyback Converter SR Sensing for Accurate Line Feed-Forward
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Solution Overview
Problem
Conventional secondary-side-controlled flyback converters face inefficiencies due to slow detection and inaccurate feed-forward sensing, requiring high-resistance external resistors that increase power loss and fail to meet industry standards, especially under low-load conditions.
Innovation Solution
A secondary-side-controlled flyback converter with a synchronous-rectifier sense architecture that uses an active diode circuit to directly sense the SR_drain voltage, eliminating the need for high-voltage technology and external circuits, and includes a resistor network and line-feed-forward circuit for accurate feed-forward sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a high-resistance external resistor is used to limit voltage on the SR_SEN pin, then the voltage is limited to eliminate high-voltage technology requirements, but the detection speed becomes slow resulting in reduced efficiency due to power loss
Solution Approach 1:
The patent applies dynamics by making the resistance value time-variable through the use of two switches (first switch and second switch) that control the connection of different resistors (first resistor and second resistor) to ground. The resistance transitions from a first value during normal operation to a second, lower value during detection events, enabling both low power loss and fast detection speed at different times.
Solution Approach 2:
The patent prepares the circuit by pre-configuring the resistor network with switches that can rapidly transition between resistance states. The control circuit is ready to activate the low-resistance path when detection events are anticipated, ensuring fast response without requiring permanently high resistance that would cause power loss.
2Speed
If a low-resistance external resistor is used to improve detection speed, then detection becomes faster improving efficiency, but higher DC current flows at low loads failing to meet industry standards
Solution Approach 1:
The patent makes the resistance dynamic rather than static, using control circuits to switch between high and low resistance states based on operating conditions. This allows the system to achieve fast detection speed when needed while maintaining high resistance during low-load conditions to limit DC current and meet industry standards.
Solution Approach 2:
The patent changes the resistance parameter based on operating conditions. The control circuit monitors system state and adjusts the effective resistance value - using high resistance during low-load conditions to limit current and using low resistance during detection events to ensure fast response. This parameter adaptation resolves the contradiction between speed and power consumption.
3Reliability
If a voltage-divider circuit is used to sense SR_drain voltage, then voltage is divided to protect the SSC, but accurate feed-forward sensing becomes difficult requiring complicated and costly line feed-forward circuits
Solution Approach 1:
The patent introduces an intermediary resistor network with switches that acts as a smart mediator between the high-voltage SR_drain node and the low-voltage SSC. Instead of a passive voltage divider, this active network provides controlled impedance matching and signal conditioning, enabling accurate feed-forward sensing while protecting the SSC from high voltage through the switching action.
Solution Approach 2:
The patent replaces the complicated analog voltage-divider-based feed-forward circuit with a digitally-controlled switching network. The control circuit uses digital logic to manage the switches, substituting complex analog signal processing with simpler digital control, thereby reducing overall circuit complexity and cost while maintaining protection and sensing accuracy.
4Measurement precision
If additional external circuits are added to achieve accurate sensing and protection, then sensing accuracy improves, but device complexity and cost increase
Solution Approach 1:
The patent makes the resistor network multi-functional by designing it to simultaneously provide voltage protection, accurate sensing, and feed-forward information extraction. The same switching network that protects the SSC also enables precise measurement of SR_drain voltage and provides the necessary signals for line feed-forward control, eliminating the need for separate dedicated circuits for each function.
Solution Approach 2:
The patent merges multiple functions into a single integrated resistor network with controlled switches. Instead of having separate protection circuits, sensing circuits, and feed-forward circuits, the invention combines these functions into one unified structure that performs all tasks, thereby improving sensing accuracy without proportionally increasing device complexity.
Data Source
AI summary
A flyback-converter with synchronous-rectifier (SR) sense architecture is provided. A secondary side controller includes a SR-sense pin coupled through an external resistor to a drain of an SR on the secondary-side, a negative-sensing-detector, a peak-detector, a zero-crossing-detector, all coupled to the pin, and a resistor network (Rn) coupled between the pin and ground. The Rn includes a first resistor (R1) to couple the pin and to ground through a first switch (S1) during negative-sensing to divide a voltage (VSR_drain) coupled to the pin, and a second, higher resistance resistor (R2) to couple the pin to ground through a second switch (S2) during peak-detection to divide VSR_drain coupled to the pin. S1 and S2 are controlled by register-transfer-level circuit in the SSC. A line-feed-forward (LFF) circuit is coupled to the pin through an active diode to receive an undivided VSR_drain and mirrors diode current to control the converter in LFF mode.