A Guide To The Operation And Use Of Input EMI Filters For Switching Power Supplies

Focus:

Conducted noise or electromagnetic interference (EMI) generated by power supply switching can be controlled with an input filter. But many engineers are not clear on how input filters operate, their capabilities and limitations, how to specify them, and how to apply them. This article aims to address these issues, primarily with the non-power supply designer in mind, but in a way that will also inform new power supply designers about key aspects of filter design and application. The article begins with a brief introduction to the FCC DOC 20780, VDE 0806, and MIL STD 461 standards, which set the regulatory requirements for emissions, then explains the role of the input filter in meeting these requirements and discusses filter operation. Factors that influence filter performance or power supply stability including filter cutoff frequency, filter damping, power supply loop crossover frequency, power supply input impedance, and filter output impedance are discussed and design tips are given. The MIL STD 461’s CS01 limits on susceptibility to conducted interference are cited in explaining the role and operation of the filter. The article concludes by explaining how a filter attenuates common-mode and differential-mode noise, first conceptually and then using an example input filter circuit.

What you’ll learn:

• How to understand the role and operation of input EMI filters and how to specify them for use with switched-mode power supplies
• How to understand the factors that influence input filter attenuation and power supply stability when designing or specifying power supply input filters
• How to understand the impact of FCC DOC 20780, VDE 0806, and MIL STD 461 standards on design and selection of input EMI filters (overview)

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Author & Publication:

Anastasios Simopolous, Beta Dyne, Bridgewater, Mass, How2Power Today, Aug 15 2013

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