“Conducted noise can be divided into two types. One is “differential mode noise”, also known as “normal mode noise”. The two terms are sometimes used differently depending on the condition, but are treated as the same noun in this article. The other is “common mode noise”. Take a look at the image below. This article revolves around power supplies, so the illustration is an example of a printed circuit board (PCB) with circuitry in a housing and powered externally.

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Differential mode (normal mode) noise and common mode noise

Conducted noise can be divided into two types. One is “differential mode noise”, also known as “normal mode noise”. The two terms are sometimes used differently depending on the condition, but are treated as the same noun in this article. The other is “common mode noise”. Take a look at the image below. This article revolves around power supplies, so the illustration is an example of a printed circuit board (PCB) with circuitry in a housing and powered externally.

The differential mode noise is generated between the power lines, and the noise source enters the power line in series, and the noise current is in the same direction as the power current. It is called “Differential mode” due to the reverse direction of the round trip.

Common mode noise is noise returned to the power supply line via the ground by the noise current leaked through stray capacitance or the like. Because the noise current flows in the same direction at the (+) terminal and (-) terminal of the power supply, it is called “common mode”. No noise voltage is generated between power lines.

As mentioned earlier, these noises are conducted noises. However, noise is emitted due to the noise current flowing in the power line.

The electric field strength Ed of radiation caused by differential mode noise can be expressed by the formula on the lower left. Id is the noise current in differential mode, r is the distance to the observation point, and f is the noise frequency. Differential mode noise creates a noise current loop, so loop area S is a very important factor. As shown in the figure and formula, assuming other factors are fixed, the larger the loop area, the higher the electric field strength.

The electric field strength Ec of radiation caused by common mode noise can be expressed by the formula on the lower right. As shown in the figure and formula, the cable length L is a very important factor.

In order to better understand the radiation characteristics caused by each noise, the electric field intensity*1 is calculated by substituting the actual value. The conditions are exactly the same. The observed points of electric field strength are indicated by blue dots. *1: Formula source – EMC Engineering Detailed Explanation of Practical Noise Reduction Techniques by Henry W. Ott – Tokyo Denki University Press

A very important point in this calculation is that the common-mode noise radiation is much larger (about 100 times larger in this example) for the same value of noise current. In any case, if these conducted noise and radiated noise, ie, EMI, exceed the allowable range, noise reduction countermeasures are required. In particular, it is important to remember that countermeasures against common mode noise are very important when considering radiated noise countermeasures.

The specific countermeasures will be introduced gradually in the future. The most principled noise countermeasures are to reduce the loop area S for differential mode noise (for example, using stranded wires for cables), and to minimize the cable length for common mode noise. However, there are bound to be constraints such as configuration and materials. At this time, it is necessary to explore ways to increase filters.

I hope that through this article, you can have a preliminary understanding of the types and properties of noise.