Common and differential mode noise separation: comparison of two different approaches

Since the EU Directive 336/89 has been published, the manufacturers of electric and electronic equipment have had to face EMC problems. For this reason, the problem of EMI measurements and the design of a proper filter in order to mitigate EMI noise have become very important. In EMI analysis, the separation of common mode and differential mode noises is a procedure well-known in textbooks. However, separated measurements are only sometimes conducted. Therefore, two different approaches are shown for the diagnosis and reduction of conducted noise emission. Two devices have been realized for determining whether the differential mode or common mode component is dominant and in order to determine whether an anticipated change in a value of an element in the power supply filter will be effective. Experimental results are given to show the validity of these approaches.

I. Introduction

Electromagnetic interference (EMI) emission is an important matter for any electric and electronic equipment. When the noise emission of an equipment fails to satisfy the Standard limits, it is usually not easy to find the origin of the noise sources. One of the difficulty in dealing with EMI is the lack of diagnostic tools. Measured emissions are a mixture of common mode (CM) and differential mode (DM) noise. The sources of CM and DM noise in the equipment are of different nature and have to be carefully distinguished [1], [2]. Furthermore, the design procedure for filters is usually divided in common mode and differential mode filters design [3]. Therefore it is very important to discern the two modes in order to design a good line filter. The basic separation of CM and DM is shown in Fig. 1. This scheme represents the equivalent circuit of a Line Impedance Stabilization Network (LISN) in the high frequency behavior and the Equipment under Test (E.u.T.) is shown as an electromagnetic noise source. The differential mode voltage and current , the common mode voltage and current , indicated in Fig.1 are defined as follows: $$\eqalignno{&V_{DM}=(V_{P}-V_{N})=50\cdot{I_{CM} \over 2}=25\cdot I_{CM} &\hbox{(1)} \cr &V_{CM}={(V_{P}+V_{N}) \over 2}=50\cdot 2I_{DM}=100\cdot I_{DM} &\hbox{(2)} \cr &I_{DM}={(I_{P}-I_{N}) \over 2} &\hbox{(3)} \cr &I_{CM}=(I_{P}+I_{N}) &\hbox{(4)} }$$ Fig. 1: Definition of CM and DM.