Common-Mode Transformer Aids Noise Reduction In High-Power Supplies

by Dennis Feucht, Innovatia Laboratories, Cayo, Belize, How2Power Today, Jan 15 2015

Focus:
In a typical switched-mode power supply transformer, the capacitance between primary and secondary windings is distributed along the windings. This interwinding capacitance can be represented by an equivalent capacitor, Cseq, across the middle of the primary and secondary windings. This interwinding capacitance offers a path for parasitic currents, which result from voltage differences across the primary and secondary windings. Those parasitic currents, in turn, can become a source of noise, which is particularly troublesome in power supplies with higher power output. However, these parasitic currents can be avoided with the addition of a common-mode transformer. At a glance, this CM transformer might be mistaken for the type used in input EMI filters in power supplies. Although both types of CM transformer are used for noise reduction, they operate differently as explained in this article.

What you’ll learn:

• How to prevent parasitic currents (and the associated noise) caused by interwinding capacitance in SMPS transformers by applying a common-mode transformer

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