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Common mode

by Yee Wei Law - Saturday, 17 June 2023, 4:46 PM
 

There are two main types of electromagnetic interference (EMI), namely conducted emission and radiated emission.

Conducted emission can further be classified into two types, namely differential-mode noise and common-mode noise, as shown in Fig. 1.

Fig. 1: Differential-mode noise versus common-mode noise [ROH18].

For differential-mode noise, the noise source appears across power supply lines and is in series with the power supply line.

The noise current flows in the same direction as the power supply current.

Noise is so-called “differential-mode” because the outgoing and return currents are oppositely directed.

For common-mode noise, the noise current that has leaked via a stray capacitance or the like passes through ground and returns to the power supply line.

A noise voltage does not appear across the power supply lines.

Noise is so-called “common-mode” because the direction of the noise currents on the positive (+) and the negative (−) sides of the power supply have the same direction.

References

[ROH18] ROHM, Differential (normal) mode noise and common mode noise - causes and measures, Tech Web Basic Knowledge article, 2018. Available at https://techweb.rohm.com/knowledge/emc/s-emc/01-s-emc/6899.

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Control systems: introduction

by Yee Wei Law - Wednesday, 5 July 2023, 8:38 AM
 
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