Everything you need to know about the EMC!

Hi, this is Fiona. Today I would like to share something about the EMC.

What is Electro Magnetic Compatibility(EMC)?

Electromagnetic Compatibility (EMC) refers to the ability of a device or system to operate in its electromagnetic environment without causing unacceptable electromagnetic interference to any device in its environment. Therefore, EMC includes two requirements: on the one hand, the electromagnetic interference generated by the equipment in the normal operation process cannot exceed a certain limit; on the other hand, it means that the appliance has certain electromagnetic interference in the environment. The degree of immunity, ie electromagnetic sensitivity. An electromagnetic environment can be both natural and generated from other products, furthermore to avoid any interference effects.

EMC includes EMI (Electromagnetic Interference) and EMS (Electromagnetic Susceptibility). The so-called EMI electromagnetic interference is electromagnetic noise that is not harmful to other systems in the process of performing the function. The EMS refers to the machine. The ability to be unaffected by the surrounding electromagnetic environment during the performance of the intended function.

Electromagnetic Interference (EMI), including conducted Interference and radiation Interference. Conducted interference is the coupling of signals from one electrical network to another through a conducting medium. Radiated interference is when a source of interference couples its signals through space (interference) to another electrical network. In the design of high-speed PCB and system, high-frequency signal line, pins of the integrated circuit, various connectors and so on may become the radiation interference source with antenna characteristics and can emit electromagnetic waves and affect the normal operation of other systems or other subsystems within the system.

There are many ways to classify interference sources.

1. Normally, the sources of electromagnetic interference can be divided into two categories: natural interference sources and man-made interference sources.

Natural Interference Sources

• > Atmospheric noise interference: such as lightning, wind and snow, heavy rain, hail, sandstorms and other noise generated.
• > Solar noise interference
• > Cosmic noise interference
• > Electrostatic discharge (ESD) : electrostatic voltage that accumulates on a person or device

Man-made Interference Sources

• Any electronic and electrical equipment may produce artificial interference. Such as various signal transmitters, oscillators, motors, switches, relays, neon lamps, fluorescent lamps, engine ignition systems, electric bells, electric heaters, arc welding machines, high-speed logic circuits, gate circuits, silicon controlled inverters, gas rectifiers, corona discharges, various industrial, scientific and medical high-frequency equipment, urban noise, noise from electric railways, and noise from nuclear explosions, etc. .

2. From the electromagnetic interference attribute, can be divided into functional interference source and non-functional interference source.

• Functional interference source refers to the direct interference to other devices in the process of realizing the function of the device, while non-functional interference source refers to the side effect produced or added by the electric device while realizing its function ARC discharge interference, as when a switch is closed or cut off.

3. From the spectrum width of the electromagnetic interference signal, it can be divided into wideband interference source and narrowband interference source.

• They are distinguished by their large or small bandwidth relative to the specified receptor. If the bandwidth of the interference signal is larger than that of the specified receiver, it will become a wideband interference source, otherwise known as a narrowband interference source.

4. From the frequency range of the electromagnetic interference signal, it can be divided into power frequency and audio frequency interference sources (50Hz and its harmonics) , very low frequency interference sources (less than 30Hz) , carrier frequency interference sources (10kHz ~ 300kHz) , radiofrequency and video frequency interference sources (300kHz) , microwave interference sources (300MHz ~ 100Ghz) .

 Propagation routes of electromagnetic interference

There are two ways of EMI Propagation: Conduction coupling and radiation coupling.

The conduction transmission must have a complete circuit connection between the interference source and the sensor, along which the interference signal is transmitted to the sensor, causing interference. The transmission circuit may include a conductor, a conductive member of the device, a power supply, a common impedance, a grounding plate, a resistor, an inductor, a capacitor, a mutual inductor, and so on.

Radiation transmission is in the form of electromagnetic wave propagation through the medium, the interference energy in accordance with the law of electromagnetic field to the surrounding space. There are three common types of radiative coupling: 1. The electromagnetic wave emitted by antenna a is accidentally received by Antenna B, which is called antenna-to-antenna coupling. 2. The coupling of space electromagnetic field is called line-to-line coupling. 3. The high-frequency signal induction between two parallel wires is called line-to-line induction coupling.

In the Electromagnetic compatibility test, EMS is a test item designed to test the anti-interference ability of EUT under various interference conditions. The test items include Electrical Fast Transient (EFT), Electrostatic Discharge (ESD), Voltage Dip (V-DIP), Radiate Susceptibility (RS), Conductive Susceptibility (CS) and so on.

Electromagnetic compatibility design requirements

The requirements of EMC design are as follows:

• 1. Clarify the electromagnetic compatibility index of the system. EMC design includes the electromagnetic interference environment that the system can keep working normally and the allowable index that the system can interfere with other systems.
• 2. On the basis of knowing the source, object and way of interference of the system, these indexes are allocated to each subsystem, subsystem, circuit, component, and device by theoretical analysis.
• 3. According to the actual situations, corresponding measures are taken to suppress the interference source, eliminate the interference way, and improve the anti-interference ability of the circuit.
• 4. Through experiments to verify whether the original target requirements are met. If not, further measures are taken to cycle many times until the original target is reached.

Electromagnetic compatibility design contents

In order for the product to have good electromagnetic compatibility, it is necessary to specifically consider the design content related to electromagnetic compatibility. Electromagnetic compatibility design generally includes the following aspects.

Ground Wire Design

• Many electromagnetic interference problems are caused by ground wire, because ground wire potential is the reference potential of the whole circuit, if the ground wire is not designed properly, the ground wire potential will be unstable, which will lead to circuit failure. The aim of ground wire design is to ensure the ground wire potential to be as stable as possible, so as to eliminate interference phenomenon.

Circuit Board Design

• Whether the equipment emits electromagnetic interference or is affected by external interference, or the circuits interfere with each other, the Circuit Board is at the heart of the problem. So it is very important to design the circuit board to ensure the Electromagnetic compatibility of the equipment. The purpose of circuit board design is to reduce the electromagnetic radiation and the sensitivity to external interference and to reduce the mutual influence between the circuits on the circuit board.

Filtering Design

• For any equipment, filtering is one of the key technologies to solve the problem of electromagnetic interference. Because the wires in the equipment are highly efficient in receiving and radiating antennas. Therefore, most of the radiation emitted by the equipment is realized through various wires, and the external interference is often first received by the wires, and then string into the equipment. The purpose of filtering is to eliminate these interference signals on the wire, to prevent the interference signals in the circuit from being transmitted to the wire, and to prevent the interference signals received by the wire from being transmitted to the circuit by means of the wire radiation.

Shielding and Lap Joint Design

• Shielding is necessary for most devices. Especially with the increasing frequency of circuit work, simply depending on circuit board design often can not meet the requirements of EMC standards. There are many differences between the shielding design of the chassis and the traditional structural design. It is difficult to add the shielding effect to the chassis if the electromagnetic shielding is not considered in the structural design. Therefore, regarding the modern electronic product design, must consider the shield question from the beginning.

The main research object of electromagnetic compatibility

1. All kinds of man-made noise, such as corona noise of power transmission line, automobile noise, the noise of contactor itself, noise caused by discharge when the conductor is turned on, electric locomotive noise, city noise, etc.

2. The mutual influence of various utilities (transmission lines, communications, railways, highways, petroleum and Metal Pipelines, etc.) in the shared corridor.

3.Reflection problems caused by super high-rise buildings, power transmission lines, iron towers, and other large buildings.

4. The effect of the electromagnetic environment on human beings and various living things. These include power frequency fields such as strong wires, the effects of short, medium and microwave electromagnetic radiation.

5. Nuclear electromagnetic pulse. The electromagnetic pulse generated by the high-altitude nuclear explosion can cause widespread damage to command, control, communications, computers and radio systems on the ground.

6. TEMPEST technique. Its essential content is a series of research work from two aspects of information receiving and protection, aiming at electromagnetic radiation and information leakage of information equipment.

7. Misoperation of electronic equipment. In order to prevent misoperation, measures must be taken to improve the anti-interference ability of the equipment.

8. Spectrum allocation and management. Radio waves are limited sources, but not consumptive sources. The resources must be managed scientifically and utilized fully.

9. Electromagnetic compatibility and measurement.

10. Natural influences, etc.

Measures to enhance Electromagnetic Compatibility

1. Using a perfect shield can prevent external radiation into the system, but also prevent the system's interference energy radiation outward. Shielding body should be kept intact, the necessary doors, cracks, vents and cable holes to be properly handled, and the shielding body to have reliable grounding.

2. To design a reasonable grounding system, small-signal, large signal and interference circuit as far as possible separate grounding, grounding resistance as small as possible.

3. Using appropriate filtering technology, the passband of the filter is selected reasonably to reduce the leakage loss as much as possible.

4. Using the limiting technique, the limiting level shall be higher than the working level and shall be bi-directional limiting.

5. To correctly choose the way of connecting cable and wiring, if necessary, using the optical cable instead of a long cable.

6. Balancing the differential circuit, shaping circuit, integral circuit and gating circuit are adopted.

7. System frequency allocation should be appropriate. When there are several main frequency signals in a system, try to avoid each signal frequency, or even avoid each other's resonant frequency.

8. The equipment in the shared corridor shall be kept at a greater distance when conditions permit, in order to mitigate the effects on each other.

The above is something about the EMC, and all related information is from the network. Should there is any need to add, please do not hesitate to contact me.

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