Flyback Power Supply On-Time Control via Reset-Time Sensing

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Solution Overview

Problem

Conventional power supply systems for flyback converters require external circuits and suffer from inefficiency due to power dissipation in sense resistors, and are sensitive to input voltage and magnetizing inductance for current regulation.

Innovation Solution

A primary side sensing power control system that uses measured reset-time from previous cycles to determine peak current and off-time for the next cycle, eliminating the need for input voltage and magnetizing inductance knowledge, and employing time measurements instead of voltage for current calculation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If secondary side sensing with external transistor and sense resistor is used for current regulation, then current regulation capability is achieved, but power dissipation increases and system efficiency decreases

Engineering Contradiction:

Improvecurrent regulation capabilityVSAvoidpower dissipation

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

The patent extracts the current sensing function from the secondary side to the primary side by measuring the reset time of the transformer core. This eliminates the need for external sense resistors and transistors on the secondary side, thereby removing the power dissipation associated with these components while maintaining current regulation capability.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces reset time measurement as an intermediary parameter to indirectly sense the output current. Instead of directly measuring current with dissipative resistors, the system uses the reset time (which correlates with current) as a mediator to achieve current regulation without power loss.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If conventional primary side feedback with voltage sensing is used, then current regulation is achieved, but sensitivity to input voltage and magnetizing inductance variations occurs

Engineering Contradiction:

Improvecurrent regulationVSAvoidsensitivity to input voltage and magnetizing inductance

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent changes the sensing parameter from voltage-based measurement to time-based measurement. By measuring the reset time of the transformer core rather than sensing voltages that vary with input voltage and magnetizing inductance, the system achieves current regulation that is insensitive to these parameter variations.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent replaces the voltage sensing mechanism with a time measurement mechanism. Instead of using voltage dividers and analog-to-digital converters to sense output voltage and infer current, the system uses a timer to measure the reset time, which provides a more stable and insensitive measurement to input voltage and inductance variations.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Reliability

If voltage measurement is used for current calculation, then current regulation is achieved, but system complexity increases

Engineering Contradiction:

Improvecurrent regulationVSAvoidsystem complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent extracts only the essential timing information needed for current calculation, removing the need for complex voltage sensing circuits, analog-to-digital converters, and complex calculation algorithms. The reset time measurement provides sufficient information for current regulation with minimal circuitry.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent simplifies the measurement parameter from voltage (which requires complex sensing and conversion circuits) to time (which can be measured with simple digital counters or timers). This parameter change dramatically reduces system complexity while maintaining current regulation accuracy.

Inventive Principle:

Principle #35Parameter changes

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 approach maintains constant current output with varying input voltage and reduces system sensitivity to input voltage and magnetizing inductance, improving efficiency and simplifying current regulation by using time measurements.

Implementation Method 1

The energy is stored in the transformer core gap when the switch is on, and is transferred to the load circuit when the switch is off

Methodology Applied

Scientific EffectElectromagnetic induction: Electromagnetic Induction

Data Source

PatentUS7443700B2On-time control for constant current mode in a flyback power supply

Publication Date: 2008.10.28 DIALOG SEMICONDUCTOR INC

AI summary

A primary side sensing power control system and method for constant current control that utilizes a relationship that involves the measured reset-time from the previous cycle to determine the primary side peak current and off-time for the next cycle. This control mechanism does not need the knowledge of input voltage or magnetizing inductance. Therefore, it removes the sensitivities of input voltage and magnetizing inductance to the output current limit. Furthermore, it uses a time measurement instead of a voltage measurement for the current calculation which in many cases is easier to perform.