The Engineer’s Guide To EMI In DC-DC Converters (Part 7): Common-Mode Noise Of A Flyback

Focus:

This article explains the causes of EMI in galvanically isolated dc-dc converter designs focusing on the impact of transformer interwinding capacitance on common-mode (CM) emissions. CM noise is mainly caused by displacement currents within the transformer interwinding parasitic capacitance and the parasitic capacitance between the power switch and chassis/earth ground. This article specifically analyzes CM noise for the dc-dc flyback converter, since it is so widely used as an isolated power supply. At the end of this analysis, a simple, two- capacitor CM noise model of the flyback transformer is developed to model and characterize CM EMI performance. The article begins by reviewing flyback switching behavior and identifying sources of CM noise, then traces the CM noise paths in the flyback. From there, various CM noise models are explained leading to the derivation of the two-capacitor noise model.

What you’ll learn:

• How to reduce EMI in isolated dc-dc converters
• How to model the CM noise paths in a flyback transformer

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Author & Publication:

Timothy Hegarty, Texas Instruments, Phoenix, Ariz., How2Power Today, Dec 18 2018

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