Negative Input Resistance and RMS Input Currents

Focus:

According to the author, negative input resistance (NIR) and RMS input currents are mostly overlooked as problems in power supply design. This article explains why a dc-dc converter’s NIR is a source of large oscillations and overstressing of components and why this problem is made worse by input filters. The causes of NIR, its effects, ways to measure it, and examples of it, are explained. Resonant circuit configurations associated with dc-dc converters with input filtering are analyzed to derive the conditions for stability for the converter input. Methods for avoiding or compensating NIR through choice of operating conditions, damping techniques, capacitor selection, etc. are explained. A real-world example of a converter with NIR-induced oscillations and high rms currents and a solution for attenuating these effects is presented. Sources of measurement error and proper measurement techniques concerning NIR effects are discussed.

What you’ll learn:

• How to understand the causes and effects of negative input resistance in dc-dc converters
• How to calculate dc-dc converter susceptibility to oscillations caused by negative input resistance
• How to avoid or compensate for the effects of negative input resistance in dc-dc converters

Notes:

Article appears on pages 56-61 of August 2012 issue in article archive. You must register to access articles in this magazine's archive. However, a link is provided here to the article on powerguru.org.

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Author & Publication:

Dr.-Ing. Artur Seibt, Vienna, Bodo's Power Systems, Aug 01 2012

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