A Scientific Approach to EMI Reduction in Switching Power Supplies

The increasingly larger scale of integration in electronic systems has demanded a corresponding increase in the density of the power supplies of these systems. To some extent, this demand has been met by resorting to higher switching frequency power converters (-1 MHz). One of the major problems with high frequency switching converters is the generation of radio frequency signals which may interfere with communication broadcasts. The amount of this electromagnetic interference (EMI) which a product may generate is subject to governmental regulations throughout much of the world. The regulatory standards can generally be met with the addition of an EMI filter on the line side of the power supply. EMI filters may be quite large due to the large storage elements needed to filter the power line. The application of EMI filters is generally quite unscientific and often results in costly iterations and suboptimal results. Power supply and EMI filter designs might be significantly improved by investigation of some of the issues associated with these filters, development of some useful instrumentation procedures, and the application of some methodology and innovation to the design of these filters. This document proposes a strategy for EMI research, outlining the major issues to be investigated. Some of this research has been undertaken in the work of this thesis. A project to characterize and understand the performance of EMI filters led to some understanding of the influence of component parasitics and electromagnetic coupling on filter performance. Work on characterization of power supply EMI sources has also been undertaken. The other major aspect of this work is an investigation into the practical application of active circuitry to EMI filtering at the utility interface of a switching power supply. The major problem addressed is compensation of the feedback loop of the active filter, which includes the variable external impedance of the utility. For the specific application considered herein, that of a conventional switching frequency of 75 kHz, the active filters were not found to be practical. Extending analysis up to higher switching frequencies, however, to which the power supply industry is moving anyway, shows potential for active filtering as a viable means for controlling EMI more effectively with lower cost and volume. Thesis Supervisor: Martin F. Schlecht Title: Associate Professor of Electrical Engineering