About EMC, spectrum basics, differential mode noise and common mode noise

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About EMC, spectrum basics, differential mode noise and common mode noise [Copy link]

1. What is EMC?

EMC is the abbreviation of Electromagnetic Compatibility. In Japanese, it is often expressed as "electromagnetic antagonism" or "electromagnetic suitability", and there may be other ways of expression. It means "not to generate electromagnetic interference to other devices, and to maintain the original performance even if electromagnetic interference from other devices is received". It is called "electromagnetic compatibility" because it needs to have both performances.

"Not causing electromagnetic interference to other devices" means that if this performance is not consciously ensured, it will cause electromagnetic interference to other devices. EMI (Electromagnetic Interference) is a term that represents electromagnetic interference (electromagnetic interference, electromagnetic interference). Since the emission of electromagnetic waves will cause interference, it is often used in pairs with the term emission (radiation, emission). In terms of switching power supplies, it refers to the switching noise generated by the on/off operation.

On the contrary, the term related to "even if subjected to electromagnetic interference from other devices" is EMS (Electromagnetic Susceptibility). EMS is often used in pairs with Immunity (tolerance, immunity, and rejection capability). It is required to have the ability to withstand "even if subjected to EMI, it will not cause malfunctions and other problems."

EMI is divided into two types: conducted noise and radiated noise. These two terms are more often expressed in Japanese than in English abbreviations. Conducted noise refers to noise conducted through wires or PCB wiring. Radiated noise refers to noise emitted (radiated) into the environment. For these noises, EMS has immunity requirements for each. Their relationship is as follows.

The above are the explanations and relationships of the relevant terms. In short, EMC is the key to whether EMI and EMS meet the standard specifications. The above explanations are summarized as follows.

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2. Spectrum Basics

"What is a spectrum?" According to the "Encyclopedia Britannica Small Project Edition (Electronic Edition)", "an electromagnetic wave is decomposed into sinusoidal components and arranged in order of wavelength." Expanding this definition means "decomposing something with a complex composition into simple components and arranging these components in order of the size of their characteristic quantities (partially omitted)." Although the quoted explanation is relatively brief, if you think about it again, you will feel that "it is indeed so."

The spectrum here refers to the spectrum of an electrical signal. Specifically, it is based on the data of a spectrum analyzer commonly known as a "spectrum analyzer" (the horizontal axis is frequency and the vertical axis is power or voltage).

Spectrum Basics

As "EMC of switching power supply", the electrical signal is based on the switching signal. First, let's look at the principle diagram below. The pulse waveform representing the switching signal includes tw (pulse width) and ts (rise/fall time).

The middle figure is the theoretical pulse waveform spectrum based on Fourier transform. This is a common spectrum where the amplitude decays as the frequency increases, and the decay slope changes with tw and ts.

The right figure shows the change in the spectrum after the pulse is delayed by ts. It is natural that the 1/πts frequency decreases when the slope becomes -40dB/dec, and the final result is that the amplitude decreases thereafter. In short, "the amplitude of the spectrum decreases when ts is delayed."

Spectrum changes of pulses after ts delay

Next, we will use actual spectrum analyzer data to see how the spectrum changes when frequency and other parameters change. The key point here is "how the spectrum changes with changes in the signal waveform." This is the knowledge necessary to analyze and solve EMC problems through the spectrum related to switching of actual switching power supply circuits.

Waveform changes and spectrum changes

The previous figure is the data under the default conditions for comparison. The conditions in the waveform below are: amplitude 10V, frequency 400kHz, duty (duty cycle) 50%, tr/tf (rise time/fall time) 10ns.

The middle graph shows the relationship between the nth harmonic and the amplitude (V). 1 times the frequency = the fundamental wave, which means that the 400kHz component is the largest, and the spectrum is formed at odd multiples of the frequency.

Harmonics are only odd-numbered, which is a spectral feature of Duty 50% = 1:1. The size of each component is 1/order of the fundamental component, for example, the third harmonic component is 1/3, and the nth harmonic component is 1/n.

The right figure is a logarithmic graph of the amplitude in dBV. By the way, dBV is a dB value based on the voltage ratio with 1V as the reference voltage.

①The spectrum when the frequency is changed to 2MHz. As can be clearly seen from the frequency-amplitude (dBV) relationship graph, the amplitude increases as the frequency increases.

②The spectrum when tr and tf are delayed by 100ns at the same time. The result is as shown in the principle diagram. The frequency decreases when entering -40dB/dec attenuation, and the amplitude of the spectrum decreases.

③The spectrum when the duty is changed from 50% to 20%. Since the duty is not 1:1, even harmonics are generated, but the peak value is basically unchanged. As the pulse width tw becomes narrower, the amplitude of the fundamental spectrum attenuates.

④ Frequency spectrum when only tr (rise time) is delayed. Components related to tr begin to attenuate from lower frequencies due to tr delay.

The results for each case are summarized below. In summary, the spectrum is attenuated when the frequency is lower and the rise/fall is slower. This is more favorable from an EMC point of view when the amplitude of the spectrum is lower.

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3. Differential mode (normal mode) noise and common mode noise

Electromagnetic interference EMI can be roughly divided into two types: "conducted noise" and "radiated noise". Among them, conducted noise can be divided into "differential mode (normal mode) noise" and "common mode noise" according to the conduction method.

Conducted noise can be divided into two types. One is "differential mode noise", also called "normal mode noise". These two terms are sometimes used differently depending on the conditions, but in this article, they are treated as the same term. The other is "common mode noise". Let's look at the figure below. This article is about power supply, so the example is an example of a printed circuit board (PCB) with a circuit installed in a housing and powered from the outside.

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. Since the round trip direction is opposite, it is called "differential mode".

Common mode noise is the noise that leaks through stray capacitance and returns to the power line via the ground. It is called "common mode" because the noise current flowing through the (+) and (-) ends of the power supply has the same direction. No noise voltage is generated between the power lines.

As mentioned earlier, this noise is called conducted noise. However, since noise current flows in the power line, it generates noise.

The radiated electric field strength Ed caused by differential mode noise can be expressed by the formula on the lower left. Id is the noise current in the differential mode, r is the distance to the observation point, and f is the noise frequency. Differential mode noise generates 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.

The electric field strength Ec radiated 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 characteristics of radiation caused by each type of noise, we will substitute actual values to calculate the electric field strength*1. The conditions are exactly the same. The observation points of the electric field strength are represented by blue dots. *1: Formula source - EMC Engineering Detailed Explanation of Practical Noise Reduction Techniques, author Henry W. Ott - Tokyo Denki University Press

The important point in this calculation result is that for the same noise current value, the common mode noise radiation is much larger (about 100 times larger in this case). Regardless, if these conducted noise and radiated noise, or EMI, exceed the allowable range, noise reduction measures must be taken. It is especially important to remember that when considering radiated noise countermeasures, countermeasures for common mode noise are very important.

laker2008

I always couldn't tell the difference between EMC and EMI before, and I couldn't remember the introductions in articles. It was not until I came into contact with the automotive industry last year that I finally understood what EMC is, what EMI is, what EMC tests are involved, what equipment is needed, etc.

led2015

EMC (electromagnetic compatibility) refers to the working state of electronic equipment in an electromagnetic environment. It does not interfere with other electronic equipment and the surrounding electromagnetic environment, and is not affected by the surrounding electromagnetic environment. Electromagnetic compatibility is a very complex issue, which involves many disciplines such as electromagnetic field theory, electrical engineering, and material science. In the process of electronic product development, it is necessary to deal with electromagnetic compatibility issues in an effective way to avoid equipment failure due to interference or interference.

The spectrum basis means that any signal can be decomposed into a set of sine wave signals, each of which has its own frequency, amplitude and phase. A spectrum is composed of many such sine waves, each with a different frequency, and there is a certain relationship between their amplitudes. Spectral analysis refers to the process of decomposing a signal into a set of sine wave signals and determining the frequency, amplitude and phase of each sine wave. Spectral analysis has a wide range of applications in wireless communications, audio processing, image processing and other fields.

Differential mode noise and common mode noise are both forms of noise in the signal transmission process. Differential mode noise refers to the noise that appears on the differential transmission line, which is caused by the asymmetry of the two transmission lines. Common mode noise refers to the noise generated by the voltage difference between the signal pin and the ground. This noise is often caused by the mismatch of the resistance between the signal pin and the ground. Both common mode noise and differential mode noise will interfere with the transmission of signals, so it is necessary to reduce the impact of noise when designing circuits, wiring and selecting cables to ensure the correct transmission of signals.