EMI Suppression Filters (EMC and Noise Suppression)Noise Suppression Basic Course Section 1
Chapter 2 Mechanism of Causing Electromagnetic Noise

2-1. Introduction

Chapter 1 described the mechanism of causing interference by electromagnetic noise and the overview of noise suppression. Noise suppression is mainly implemented in the noise transmission path with use of shields and filters as the typical means. In order to use these means efficiently, it is important to have a deep understanding of the mechanism of the occurrence and propagation of electromagnetic noise.
With close reference to the principle of noise occurrence described in Chapter 1 in terms of noise source, there are three factors: noise source origin, transmission path and antenna (assuming that the noise interference is eventually propagated in the form of electromagnetic waves, antenna is included here)  ^{[Reference 1]} as shown in Fig. 2-1-1(a). In case of becoming a noise victim, exactly the same schematic diagram can be used as shown in Fig. 2-1-1(b) by flipping the drawing horizontally and replacing the noise source origin with a noise receiver. That means the mechanism can be considered as the same in both cases of generating and receiving noise.
Therefore, in order to have a deep understanding of the mechanism of noise interference, Chapters 2 to 5 focus on the issues regarding the noise source side and describe the basic theory of the mechanisms of causing, transmitting and emitting noise. In these chapters, shields for shutting out noise and ground connections are also briefly described. Another important factor “filter” will be described in detail in a later chapter.
First of all, Chapter 2 will describe the mechanism of causing noise.

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2-2. Noise source origin

There are various situations to cause an electric current that can be a noise source. For example, a certain signal component is needed for the operation of one circuit while causing a problem for other circuits. In another case, noise may be inevitably generated despite that none of the circuits want it. Sometime the noise may be caused rather as a result of carelessness. Of course, the mind-set towards the noise suppression varies depending on each situation. However, it is easier to handle the situation if you can understand how the particular noise has been caused.
In this chapter, we will take the following three typical cases of noise sources to understand the mechanism of causing noise and the general coping strategies.

1. (i) Signal
2. (ii) Power supply
3. (iii) Surge

2-2-1. In case that signal becomes a noise source or victim

In this section we call the lines that primarily transmit information as signal lines. Generally in order to transmit information through an electric circuit, some amount of electric current is required even if it is very small. Then, the current creates an electromagnetic field around it. When the current changes in accordance with the information, it emits radio waves to the surrounding, which in turn is causing noise.
As the amount of information increases, the frequency of the electric current that goes through the signal line increases, or more lines may be required. Generally the higher the current becomes, or the higher the number of lines becomes, the stronger the radio wave is likely to be emitted. Therefore, the higher the performance of electronic devices becomes, and the more information is handled, the more likely that the signal lines used in the electronic devices can easily cause noise interference.
Electrical circuits that transmit information can be broadly classified into analog circuit and digital circuit, wherein analog signal and digital signal are respectively used. The general characteristics of those are described below from the viewpoint of circuit noise.

(1) Analog circuit

From the viewpoint of noise source origin, analog circuits tend to generate relatively less noise since those only use limited frequencies and are designed to control the flow of electric current.
But still, if some of the energy leaks to the outside, it can be a cause of noise interference. For example, receivers for TV or radio use a signal with a constant frequency called local oscillator frequency in order to selectively amplify the targeted frequency from the radio waves that have been received by the antenna. If this leaks to the outside, it can cause interference to other devices. In order to prevent this from happening, the tuner section is shielded or EMI suppression filter are used for the wiring.

In contrast, from the viewpoint of noise victim, since analog circuits often deal with faint signals and the information is affected by even small fluctuations, the circuit tends to easily become a noise victim. For example, if noise enters to the first stage of a voice amplifier, the speaker will make a loud noise sound by detecting and amplifying it. In order to prevent this from happening, highly sensitive voice amplifiers are shielded or EMI suppression filters are used for the wiring.

(2) Digital circuit

From the viewpoint of noise source origin, digital circuits are likely to become noise sources as the transition between the signal levels of 0 and 1 happens in a very short period of time, which contains an extremely wide range of frequency components. In order to prevent the noise emission, shields and EMI suppression filters are used for digital signals. Noise generated by digital circuits is an important topic and will be described in detail in Section 2-3, since it is not only related to signals but also related to power supply.
However, from the viewpoint of noise victim, the signals are expressed with only two status of 0 and 1 (nothing in between) and have a relatively large amplitude. In addition, the information would not be affected by small induction. So it is unlikely to become a noise victim. However, if it gets high level noise even if it goes for a split second, the data will be completely altered. So it has a vulnerability to pulse noise such as an electrostatic discharge. (Electro-Static Discharge is also called ESD for short)

2-2-2. In case that power supply becomes a noise source

Since power supply is essentially a circuit that provides only direct current or commercial frequency, it should be unlikely to become a cause or pathway of electromagnetic noise. However, in many cases, it actually becomes a cause or pathway of noise. This is considered to be due to reasons as follows:

1. (i) even though the voltage seems stable, its electric current may contain a large amount of high-frequency current flowing to operate the electric circuit
2. (ii) since the power line is a shared wire in the circuit, noise is circulated and affects the entire circuit
3. (iii) since the ground in particular is often shared throughout the equipment and provides a common voltage, it is hard to separate it
4. (iv) since it is the energy source for the equipment, the noise energy also becomes large

Typical examples where the power supply causes noise are contact noise and switching power supply.
The contact noise is a type of noise that occurs at a point of contact when the source current is turned on/off with a switch (it is particularly strong when turned off), which has the same meaning as the switching surge described in (2) of Section 2-2-3. See Section 2-2-3 for details. Since a very high voltage occurs and the flow of transient but high-frequency current spreads radio waves, it can cause a circuit failure or can lead malfunction of the surrounding electronic devices.
Switching power supply is a circuit that changes voltage and frequency by intermittently flowing the electric current with use of semiconductor. Since the section of intermitting the electric current generates a high-frequency energy, it causes noise interference when it leaks out to the outside. For example, the chopper-type DC-DC converter shown in Fig. 2-2-7 generates the output voltage by intermittently flowing the direct current with use of transistor. This intermittent current contains a high-frequency energy. Although most of this energy is usually absorbed by input capacitors and/or output smoothing circuit, even a small amount of leakage can be a noise source for the surrounding circuits. In order to eliminate noise in the switching power supply, a low-pass filter that uses L and C is used in addition to the input capacitors and/or output smoothing circuit (noise can also be suppressed by improving the performance of input capacitors and output smoothing circuit).
Apart from DC-DC converter, an inverter that drives a motor is also a type of switching power supply that can generate noise.

In contrast, from the viewpoint of noise victim, power supply is a circuit that is relatively less likely to get affected. Since the amount of energy used internally is large, it will not be easily affected by interference.
However, power supply can be a conduction path for noise. As shown in Fig. 2-2-8, power line is a conductor to directly join electronic devices to each other and is one of the important conduction paths for noise. For example, when an electronic device is affected by noise, or when an electronic device emits noise, the AC power cable becomes a doorway for noise. Therefore, many electronic devices use EMI suppression filters in the power line. Fig. 2-2-9 shows an example of the configuration of EMI suppression filter for AC power supply.
Since the EMI suppression filters used for power supply generally draw an electric current significantly larger than that of signals, parts that are capable of drawing a large current are required.

2-2-3. Noise occurrence due to surge

An unintended excessive voltage or current due to electrostatic discharge or on-off switching is called surge. Since the level of voltage and/or current is significantly larger than that of normal circuit operation, it can cause malfunction or damage to the circuit. In order to prevent this from happening, surge absorbers are used for the wiring, which surge can enter.
Typical surges are electrostatic surge, switching surge and lightning surge etc. Surge is one of the major categories of EMC measures. It is summarized as follows:

(1) Electrostatic surge

As shown in Fig. 2-2-11, electrostatic surge is a transient noise that happens when an electric charge accumulated in a very small floating electrostatic capacity of about several 100pF sustained in a human body or equipment is discharged to an electronic device or surrounding object. Although its energy is only small, its voltage is as high as several kV or more and a larger current flows instantaneously. Therefore, if it is directly applied to a circuit, it can damage the circuit. Even if it is not directly applied, the circuit may malfunction when the signal line suffers from electromagnetic induction, or when the potential of power supply or ground fluctuates.
One of the tests to simulate electrostatic surge is for example, IEC61000-4-2. Please refer to the test for details.
As shown in Fig. 2-2-12, in order to reduce the interference by static charge,

1. (i) cover up with insulator so as to block discharge, or otherwise, cover up with metal so as to divert it.
2. (ii) release the discharge current through a pathway that does not affect the circuit (release it to the solid earth to avoid flowing into the signal ground: SG).
3. (iii) use an appropriate surge absorber.

(2) Switching surge

When the electric current changes suddenly due to relaying or switching on-off (especially when the circuit is turned off), a transient high voltage is induced at the contact point due to the inherent inductance of the circuit. The phenomenon is called switching surge. The contact noise mentioned in Section 2-2-2 is the noise caused by a switching surge.
Since an extremely high voltage occurs, it can show a spark as shown in Figs. 2-2-13 and 2-2-14 or can spread radio waves due to the strong damped oscillating current by the floating electrostatic capacitance at the contact point resonating with inductance. Therefore, it can damage other electronic devices that are sharing a circuit or cause their malfunction. Since this damped oscillating current contains high-frequency components, it can cause reception interference to radios and TVs.
Since resonance that causes a damped oscillating current is an important topic in noise suppression, it will be further described in a different section.
Other than relays and switches, noise generated by DC motor, for example is often caused by the commutator switching the current. Therefore this can be considered as a type of switching surge.
As shown in Fig. 2-2-15, in order to reduce the interference by switching surge,

1. (i) use a surge absorber such as a capacitor, varistor and snubber at contact point.
2. (ii) provide a shield to shut out any electromagnetic effect.
3. (iii) use EMI suppression filters for the noise-transmitting wires and victimized circuits.

In order to achieve some improvement just by shields and filters, it is important to know where the sections that provide the pathway and antenna for noise are. For example, in Fig. 2-2-15, only shielding the switch section does not achieve any improvement at all in most cases (since the wiring outside the shield works as an antenna and emits most of radio waves).

(3) Lightning surge

Since lightning is a natural phenomenon and has an extremely large energy, it is very difficult to provide protection against a direct hit. In many cases, instead of providing protection against a direct hit, electronic devices are provided with protection against induced lightning.
Induced lightning is a high voltage induced to a relatively long wiring such as a power line or communication line when a lightning stoke occurs near an electronic device. Possible mechanisms of causing an induced lightning are: electric charge is induced to an electric line due to the electric field caused by thunder cloud and then the electric charge is released by a lightning stoke; or a magnetic field due to the lightning current causes an induced electromotive force in an electric line. Not as hard as direct lightning stroke, but induced lightning has a large energy that is enough to damage electric circuits. So protection is needed.
In order to provide protection against induced lightning, a surge absorber such as a varistor is used at the section where power lines and communication lines go in and out the electronic device.

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“2-2. Noise source origin” - Key points