Livin’ On The Edge: Switching Converter Slew Rate Is Key To Mitigating EMI In Automotive Environments

Focus:

The state-of-the-art automobile can be viewed as a common chassis that attempts to enclose the RF soup that is radiated and conducted by an increasing plethora of onboard electronics. This article focuses mainly on the brushed dc motors and the choppers or drivers that run them as a noise source. This article will discuss the noise output of these circuits, the applicable radiated EMI standards and how the sensitivity of the AM radio drove those standards. The article describes the limits on radiated EMI for residential equipment established by the FCC’s Part 15 Subpart and then compares these limits with the much stricter automotive standards set by Ford’s EMC specification, EMC-CS-2009.1. Then, to explain why the Ford (and GM) standards are so strict, the article provides a brief review of how the superheterodyne AM receiver works, illustrating its sensitivity to EMI. Various techniques for mitigating noise from switching power circuits are described in brief and then a set of experiments are presented to show how switching-frequency slew rate and PCB layout of switching power converters affect the converters’ radiated EMI. Design tradeoffs associated with applying these techniques for EMI mitigation are discussed.

What you’ll learn:

• How to reduce the EMI produced by brushed dc motors and their drivers in automotive applications
• How to reduce radiated EMI from switched-mode power supplies by controlling switching frequency slew rate and PCB layout
• How to understand radiated EMI requirements in automotive applications

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Author & Publication:

Matt Jenks and Paul L. Schimel, International Rectifier, El Segundo, Calif., How2Power Today, Feb 29 2012

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