Flyback Converter Clamp Timing for Main Switch Soft Turn-On

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Solution Overview

Problem

Conventional flyback power converters experience significant switching loss due to hard switching of the main switch without discharging the parasitic capacitor on the switch terminal.

Innovation Solution

The power converter incorporates a transformer, a clamp capacitor, a main switch, a clamp switch, and a control circuit. The control circuit periodically turns on the clamp switch to generate a reverse current before turning on the main switch, discharging the equivalent capacitor on the common node, and adjusting the duration of the clamp switch's on-time based on the node voltage relative to a threshold voltage.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If hard switching is used for the main switch, then the circuit structure is simple, but the switching loss is serious

Engineering Contradiction:

Improvecircuit structureVSAvoidswitching loss

Core Design Contradiction:

Device complexityVSLoss of energy

Solution Approach 1:

The clamp switch is turned on before the main switch to pre-discharge the parasitic capacitor on the main switch terminal. This preliminary action reduces the voltage across the main switch before it turns on, enabling soft switching and reducing switching loss while maintaining relatively simple circuit structure.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The clamp capacitor and clamp switch are introduced as intermediary components between the main switch and the rest of the circuit. These intermediaries enable the discharge of the parasitic capacitor and facilitate soft switching of the main switch, resolving the contradiction between simple structure and low switching loss.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Ease of operation

If the clamp switch is turned on for a fixed duration, then the control is simple, but the node voltage cannot be precisely controlled to the predetermined range

Engineering Contradiction:

Improvecontrol simplicityVSAvoidnode voltage control precision

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The control circuit detects the node voltage at a predetermined time point after the clamp switch turns off and uses this feedback information to adjust the clamp switch's on-time. This closed-loop feedback mechanism ensures the node voltage is precisely controlled within the predetermined range while maintaining relatively simple control logic.

Inventive Principle:

Principle #23Feedback

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

This solution effectively suppresses the switching loss of the main switch by ensuring it turns on in a soft switching state, with the voltage of the common node being pulled down to a predetermined range across different circuits and environments.

Implementation Method 1

turning on the clamp switch to generate a reverse current into the second terminal of the primary coil before the main switch is turned on

Methodology Applied

Scientific EffectReverse current:

Implementation Method 2

the conventional flyback power converter does not discharge the parasitic capacitor on the terminal of the switch

Methodology Applied

Scientific EffectParasitic capacitor discharge: Capacitance

Implementation Method 3

The transformer includes a first primary coil and a secondary coil, where a first terminal of the first primary coil is used for receiving an input voltage, and the secondary coil is used for generating an output voltage

Methodology Applied

Scientific EffectElectromagnetic induction: Electromagnetic Induction

Data Source

PatentUS12323043B2Method for operating power converter, semiconductor chip and power converter

Publication Date: 2025.06.03 DIODES INC

AI summary

A power converter includes a voltage transformer, a clamp capacitor, a main switch, a clamp switch, and a control circuit. The clamp capacitor is coupled to the primary winding of the voltage transformer, and the main switch is coupled in series between the primary winding and a ground terminal. The clamp switch is coupled in series between the clamp capacitor and the main switch, and the clamp switch, the main switch, and the primary winding are coupled to a common node. The control circuit turns on the main switch periodically, and turns on the clamp switch before turning on the main switch. After turning off clamp switch and before turning on the main switch, the control circuit determines a length of time that the clamp switch should be turned on next time according to a voltage of the common node sensed right before the main switch is turned on and a threshold voltage.