Flyback Magnetics: Winding-Current Transition Is Key To Efficiency

Focus:

Flyback converters are isolated PWM-switch common-inductor (CL) configured converters, and a common power supply design choice at low power (<100 W). This article presents, with derivations, the design formulas that apply to flyback converter coupled inductors as part of the magnetics design of the converter. After a brief review of flyback circuit operation and the role of the coupled inductors, the design of the coupled inductors or transductor, is discussed at length. First, a primary-side-referred model of the transductor is described and tranductor operation during flyback is explained further. Next, the choice of the clamp voltage is discussed with attention to its impact on the primary-to-secondary current transfer delay time td and that parameter’s effect on converter efficiency. The role of these parameters is further illuminated in the description that follows of current transfer from primary to secondary winding. Finally a transductor model is presented that describes the current transfer during td.

What you’ll learn:

• How to design flyback coupled inductors
• How to understand the impact of clamp voltage and current transfer delay time on flyback converter efficiency

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Author & Publication:

Dennis Feucht, Innovatia Laboratories, Cayo, Belize, How2Power Today, Nov 15 2019

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