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EMI considerations for isolated AC-DC power supplies – test, troubleshoot & resolve EMI problems

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2019년 5월 3일

EMI is an essential part of every power supply design, but too often gets relegated to latter stages of design flow, where resolving issues can be time consuming, costly, and have a big impact on efficiency. A major objective of this topic is to dispel fears of EMI, allowing you to converse with customers on EMI issues, and be able to help the customer directly in solving them.

This topic will begin with a fundamental review of majors causes and sources of EMI issues. Fundamentally EMI is all about component parasitics that are not even represented in design schematics – parasitics cause EMI – bulk cap esr causes Differential-Mode (DM) interference; transformer input-output parasitic capacitance causes Common-Mode (CM) interference. EMI filter components have parasitic capacitance and inductance that limit their useful frequency range, and can even make EMI worse.

The topic will then go on to show how to design for low EMI at source. Different transformer internal structures are analysed for their relative impacts on CM interference – not all structures are equal, and careful transformer layer structure and arrangement can reduce CM. With additional cancellation windings and clever shielding arrangements, the CM can even be nulled to almost zero. Practical techniques are shown for measuring and assessing the CM performance, separating DM from CM, etc., to show how to debug and root cause power supply EMI issues. Practical examples will be shown to highlight the techniques, solutions, and benefits.

Finally, a summary example is shown of a compact, high-density 65-W USB-PD adaptor, showing how a few basic changes resulted in a 50 dB improvement at the fundamental switching frequency, without needing a major sacrifice in efficiency.

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