What is Electromagnetic Compatibility?

Electromagnetic compatibility (EMC, Electromagnetic Compatibility) refers to the ability of a device or system to operate in compliance with the requirements in its electromagnetic environment without causing unacceptable electromagnetic disturbance to any device in its environment. Therefore, EMC includes two requirements: on the one hand, it means that the electromagnetic interference (Electromagnetic Disturbance) generated by the device to the environment during normal operation cannot exceed a certain limit; on the other hand, it means that the device has Electromagnetic disturbance has a certain degree of immunity, which is Electromagnetic Susceptibility (EMS).

In the past, outside the military field, research on electromagnetic compatibility was not rigorous, and most equipment manufacturers did not care about electromagnetic compatibility issues. But as the clock frequency of modern digital devices using lower signal voltages increases rapidly, electromagnetic compatibility issues become more and more important. Many countries are aware of this prominent problem and have issued decree on related equipment manufacturers, requiring only equipment that meets the basic conditions to be sold.

Corresponding organizations in various countries have begun to develop standards and maintain government directives. Among the more well-known national organizations are: the United States

Introduction to electromagnetic interference

Electromagnetic interference (Electromagnetic Interference), referred to as EMI for short, has conducted interference and radiated interference. Conducted interference mainly refers to the interference signals generated by electronic devices that interfere with each other through conductive media or public power lines; radiated interference refers to the interference signals generated by electronic devices that pass the interference signals to another electrical network or electronic device through spatial coupling. In order to prevent electromagnetic interference generated by some electronic products from affecting or disrupting the normal operation of other electronic equipment, governments or international organizations of various countries have successively proposed or formulated some relevant regulations or standards for electromagnetic interference to electronic products. Products that meet these regulations or standards It can be called EMC (Electromagnetic Compatibility). The EMC standard is not constant, but it is changing every day. This is also the method often adopted by governments or economic organizations to protect their own interests. ^[1]

Electromagnetic compatibility includes two aspects: EMI (

Rule 1: The relationship between EMC cost-efficiency ratio: The earlier the EMC problem is considered and resolved, the lower the cost and the better the effect. ^[2]

Due to the increasing dependence of microcomputers and the large use of equipment, our electromagnetic environment is complicated. Therefore, external interference such as impulse noise, radiated electromagnetic fields, static electricity, lightning strikes, voltage changes, etc., may cause malfunctions or even damage caused by malfunctions. Situations such as radio communications, radars, big brothers, and television game instruments often interfere with televisions and even cause malfunctions in the use of medical equipment, affecting the safety of flight. ^[3]

Internationally, more and more attention has been paid to the immunity test of electronics, electrical appliances, and industrial equipment products, and it is tending to integrate the IEC (International Electrotechnical Commission) international test standard as the test standard. The European Community took the lead in formulating EMC prevention regulations and began to implement full immunity in 1996. test. ^[3]

In terms of electromagnetic compatibility

A three-phase input power supply is equipped with a noise filter (NFB) between the NFB (no-fuse circuit breaker) and the transformer. The shorter the input line of the filter, the better.
The power supply and high-current wires are close to the bottom of the electrical box and routed along the corners.
The switching power supply is equipped with an isolation cover to prevent radiated emission interference. The filter selector uses or T type to suppress wide-band noise, and the ferrite material (Ferrite) material can suppress radio frequency noise.
Consider using isolated ground at both ends of the power line to prevent the ground loop from forming a common impedance coupling (Common Impedance Coupling) to couple noise to the signal line.
Power or signal wires should be isolated or separated as much as possible.
The power transformer should be shielded and the case must be well grounded.
Single-phase AC control wires are recommended to use twisted wires.
It is recommended to use twisted wires for DC wiring.
Avoid connecting power and signal cables to the same connector.

Electromagnetic compatibility signal line

Signal input and output lines should be avoided to cause interference.
The remaining unused wires of CABLE should be grounded at one end to avoid the induction loop.
Twist is considered for the signal line near the power line.
Signal cables of different categories should not be mixed on one connector, and should be classified according to the category and grounded.
The input signal line and the output line should be avoided on the same connector as much as possible. If it cannot be avoided, the input and output signals should be staggered.
Low-level signal lines with higher sensitivity can be shielded in addition to twisted wires.

In terms of electromagnetic compatibility analog signals

It is recommended to use isolated lines for high-frequency analog signals and pulse wave signal lines.
High-frequency analog signal lines use coaxial isolation lines, and low-frequency analog signal lines use twisted lines. If necessary, additional isolation and shielding can be used.
The installation position of the connector must be cleaned, and the contact resistance of the connector and the metal surface must be less than 2.5m ohm.
Analog circuit interference is mainly waveform distortion, and the suppression method mainly depends on the characteristics selected by the filter, such as bandwidth and frequency response.
The analog signal line and the digital cable must be perpendicular to each other.

Electromagnetic Compatibility Digital Signal

Avoid using unshielded wires to transmit digital signals. It is better to use multiple stranded wires plus isolated wires.
Digital circuit interference is mainly caused by magnetic field interference, and isolation measures should be added.
Digital circuits are susceptible to high-energy electric field interference. Isolation lines must be used for isolation. The best isolation choice is to prevent 200V / m interference from high-energy electric fields in the frequency range of 1-10 MHz.
The digital circuit is mainly used to suppress the interference of pulse waves and spikes from nearby circuits.
Digital circuit transmission avoid the use of excessively long and uninsulated wires.

EMC circuit design

Interfering circuits, such as clocks, drivers, ON and OFF of switching power supplies, and oscillator-type control signals, should be shielded.
For each type of PCB circuit design, low-noise components are selected as much as possible, and the relationship between noise changes and ambient temperature changes must be considered.
Ceramic ferrite core (Ferrite core) is suitable for high frequency filtering, but you must pay attention to the power loss through this coil.
The regulator must consider the problem of suppressing common impedance coupling (Common Impedance Coupling) EMI between lines.
The smaller the output of the oscillator, the better. If a larger output is needed, it should be amplified by an amplifier.
Power amplification should be isolated to prevent radiated emissions.
Electrolyte capacitors are suitable for clearing High Ripple and Transient Voltage changes.
Power line interference includes low voltage (or instantaneous power failure) overvoltage and surges. These interferences usually come from the action of power switches, moments when heavy loads are turned on and off, power semiconductor operations, when fuses blow, lightning induction, etc .
The following must be considered to suppress:

Use a power filter.
Proper power distribution.
The interfered device uses another circuit.
Proper packaging of electronic parts and filters.
Use an isolation transformer.
Device varistor.

The AC electromagnetic contactor coil and solenoid valve must be connected with a spark eliminator.
The thermoelectric relay output side of the electromagnetic switch must be connected to a three-phase spark eliminator.
The DC relay coil is connected to a diode for reverse voltage protection.
The closer the spark eliminator is to the load side, the better.
Putting the surge absorber between the circuit switch and the noise filter, between the line and the line, and between the line and the ground, will effectively absorb the surge.

Design of electromagnetic compatibility distribution box

The distribution box is made of metal. If there is no problem with the welding technology (no deformation), the full-weld method is used to reduce the gap of the joint surface that cannot be fully welded as much as possible. If the distribution box is assembled with screws, the contact surface paint must be scraped off to obtain better conductivity.
The distribution box will inevitably have openings for the entrance and exit of the cable. The radio waves will pass these holes and fail the test. Therefore, the openings should be as small as possible. The unused holes must be covered with metal covers. The contact surface paint must be scraped off and conductive pads for industrial environments must be used.
When the door of the distribution box is closed, the contact surface with the main body of the distribution box must be in close contact with the conductive gasket used in the industrial environment. tight.
The terminals of the distribution box must be grounded, and the ground surface must be paint-proof.