Conducted noise from 48 volt DC-DC converters used in telecommunications systems and its mitigation for EMC

Telecommunications switching equipment has been moving toward a distributed DC power concept where 48 volts (or other telephone office voltage) is routed directly onto circuit boards via backplanes. This higher DC voltage is then converted on the circuit board to 5 volts, 3 volts or other logic voltage. One problem with this approach is the generation of a considerable amount of conducted noise current on 48 volt supply leads. Unless some mitigation is used, this noise current, when added to such currents from other boards in the system, can cause failure of electromagnetic compatibility (EMC) tests required by international standards such as EN 300-386-3, or EN 55022, and hence, hurt the ability of the equipment to be marketed. This paper describes in detail the type and frequency range of noise generated by typical 48 volt DC-DC converters as measured in EMC tests on power feeder leads. It provides an analysis of the nature of this noise, a comparison with the requirements of the international standards, and a set of mitigation techniques that not only remove the noise, but satisfy various lightning and grounding requirements, including those of the USA and Europe