Isolated Power Supply Control for Synchronous Rectifier Timing

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Solution Overview

Problem

Existing isolated power supplies face inefficiencies and safety risks due to simultaneous conduction of primary and secondary circuits, particularly in continuous inductor current mode, which degrades conversion efficiency.

Innovation Solution

A control circuit for an isolated power supply that self-adaptively controls the synchronous rectification transistor based on voltage and current signals of the secondary winding, allowing precise conduction time management in both continuous and discontinuous inductor current modes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of operation

If the synchronous rectification transistor is turned off based on rapid voltage rise and zero-crossing detection in CCM mode, then the control method is simple, but simultaneous conduction of primary and secondary circuits occurs causing safety risks and reduced conversion efficiency

Engineering Contradiction:

Improvecontrol method simplicityVSAvoidsimultaneous conduction prevention

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The patent implements a feedback mechanism where the control circuit detects the voltage across the synchronous rectification transistor in real-time. When the voltage drops below a preset threshold (indicating the transistor is about to turn off), the control circuit extends the conduction time by keeping the transistor on longer. This feedback-based dynamic adjustment prevents simultaneous conduction while maintaining simple control logic.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The control circuit performs preliminary detection of the voltage across the synchronous rectification transistor before it naturally turns off. By detecting when the voltage drops below the threshold and proactively extending the conduction time, the system prevents simultaneous conduction before it can occur, rather than reacting after the problem arises.

Inventive Principle:

Principle #10Preliminary action

2Reliability

If current limitation is implemented to prevent breakdown during simultaneous conduction, then safety is improved, but conversion efficiency deteriorates

Engineering Contradiction:

Improvebreakdown preventionVSAvoidconversion efficiency

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

The patent applies preliminary anti-action by detecting the voltage across the synchronous rectification transistor and extending its conduction time before simultaneous conduction can occur. This preventive measure eliminates the need for current limitation, thereby maintaining high conversion efficiency while ensuring safety.

Inventive Principle:

Principle #9Preliminary anti-action

3Reliability

If the synchronous rectification transistor conduction time is extended to prevent simultaneous conduction, then safety is improved, but conduction time control precision is reduced

Engineering Contradiction:

Improvesimultaneous conduction preventionVSAvoidconduction time control precision

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The control circuit uses feedback-based dynamic adjustment, continuously monitoring the voltage across the synchronous rectification transistor and adjusting the conduction time accordingly. This maintains precise control while preventing simultaneous conduction.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent implements dynamic conduction time adjustment based on real-time voltage detection. The conduction time is not fixed but dynamically adapted to the operating conditions, allowing precise control while preventing simultaneous conduction across different operating modes.

Inventive Principle:

Principle #15Dynamics

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

Enhances safety and efficiency by preventing simultaneous conduction and optimizing conduction time based on current flow, thereby improving overall conversion efficiency.

Implementation Method 1

a transformer comprising a primary winding and a secondary winding

Methodology Applied

Scientific EffectElectromagnetic induction: Electromagnetic Induction

Implementation Method 2

detecting that a voltage at the node, to which the secondary winding and the synchronous rectification transistor are connected, reaches a preset turn-on voltage value

Methodology Applied

Scientific EffectVoltage detection:

Implementation Method 3

detect a voltage/current of the synchronous rectification transistor

Methodology Applied

Scientific EffectCurrent detection:

Data Source

PatentUS12573962B2Isolated power supply control circuit and isolated power supply

Publication Date: 2026.03.10 SHANGHAI BRIGHT POWER SEMICONDUCTOR CO LTD

AI summary

A control circuit for an isolated power supply and an isolated power supply are disclosed. The control circuit includes a secondary control signal generation circuit, which, when detecting a voltage of a secondary winding reaches a first preset voltage value after a primary switching transistor is turned off, provides an instruction to turn on a synchronous rectification transistor. Following elapse of a predetermined period of time after the synchronous rectification transistor is turned on, the secondary control signal generation circuit detects a voltage/current of the synchronous rectification transistor, obtains a first electrical signal and derives a second electrical signal from the first electrical signal. When further detecting that the voltage/current of the synchronous rectification transistor reaches the value of the second electrical signal, the secondary control signal generation circuit provides an instruction to turn off the synchronous rectification transistor.