In EMC conducted emissions, what are differential mode noise and common mode noise?

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In EMC conducted emissions, what are differential mode noise and common mode noise?

Posted Date: 2024-01-26

Conducted noise can be divided into two types: "differential mode noise" and "common mode noise" according to the mode of conduction. This article will introduce these two types of noise.

Differential 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". These two terms are sometimes used differently depending on the conditions, but are treated as the same noun in this article. The other is "common mode noise". Take a look at the picture below. This article focuses on the power supply, so the illustration is an example of a printed circuit board (PCB) with circuits installed in a casing and powered externally.

Differential mode noise is generated between power lines. 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" because the round-trip direction is opposite.

Common mode noise is noise caused by leakage of noise current from stray capacitance, etc., and returns to the power line through the ground. Because the noise current flowing through the (+) and (-) terminals of the power supply flows in the same direction, it is called "Common mode". No noise voltage is generated between power lines. As mentioned earlier, this noise is conducted noise. However, noise is emitted due to noise current flowing in the power line.

Depend onElectric field strength of radiation caused by differential mode noise EdIt can be represented 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 will produce a noise current loop, so the 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.

Depend onElectric field strength Ec of radiation caused by common mode noiseIt can be represented by the formula on the lower right. As shown in the formula below, cable length L is a very important factor.

In order to better understand the radiation characteristics caused by each type of noise, let's calculate the electric field intensity*1 by substituting actual values. Conditions are exactly the same. The observation point of the electric field strength is represented by a blue dot. ●Formula source - Detailed explanation of EMC engineering and practical noise reduction techniques by Henry W.Ott - Tokyo Denki University Press

A very important point in this calculation result is:For the same noise current value, the common mode noise radiation is much greater (about 100 times greater in this case).In any case, if these conducted noise and radiated noise, that is, EMI, exceed the allowable range, noise reduction measures need to be taken. In particular, it is important to remember that when considering radiation noise countermeasures, countermeasures against common mode noise are very important.

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