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How to design an isolated buck converter

Posted: 27 Dec 2012     Print Version  Bookmark and Share

Keywords:flyback converters  bias supply  asymmetric transformers 

In many applications, one or more low-cost, simple to use, isolated power supplies working from input voltages up to 100V are needed. Traditional solutions use flyback converters to generate this bias supply. Flyback designs typically utilise asymmetric transformers turns ratios for primary and secondary power windings, with an optocoupler and reference, or an auxiliary winding for feedback regulation. Additionally, flyback converters need an elaborate compensation design for stability. This results in a tedious design process, bulky solution, with a higher component count and cost.

An isolated buck converter (flybuck) uses a synchronous buck converter with coupled inductor windings to create isolated outputs. Isolated converters utilising flybuck topology use a smaller transformer for an equivalent power transfer as the transformer primary and secondary turns ratios are better matched. There is no need for an optocoupler or auxiliary winding as the secondary output closely tracks the primary output voltage, resulting in smaller solution size and cost.

This article presents the basic operating principle of an isolated buck converter. The operating current and voltage waveforms are explained and design equations are derived. The design example shows a step-by-step procedure for designing a practical two-output 3W isolated buck converter.

View the PDF document for more information.

Originally published by Texas Instruments at as "Designing an Isolated Buck (Flybuck) Converter".

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