EMC

ESD, EMI, EMC design are common problems encountered by electronic engineers in their design. Electromagnetic compatibility (EMC) means that equipment or systems meet the requirements in their electromagnetic environment and do not produce intolerable electromagnetic interference to any equipment in their environment. Ability.

EMC EMI

EMC EMI

Therefore, EMC includes two requirements: on the one hand, the electromagnetic interference generated by the equipment to the environment during normal operation cannot exceed a certain limit; on the other hand, it means that the equipment has a certain degree of electromagnetic interference in the environment.

The degree of immunity, that is electromagnetic susceptibility. The so-called electromagnetic interference refers to any electromagnetic phenomenon that can degrade the performance of a device or system. The so-called electromagnetic interference refers to the performance degradation of equipment or systems caused by electromagnetic interference.

EMC EMI EMS

EMC includes two parts: EMI (electromagnetic interference) and EMS (electromagnetic immunity). The so-called EMI electromagnetic interference refers to the electromagnetic noise generated by the machine itself that is not conducive to other systems in the process of performing its due function, and EMS refers The ability of the machine to not be affected by the surrounding electromagnetic environment in the process of performing its functions.

In the design of electronic products, in order to obtain a good EMC performance and cost ratio, it is important to carry out EMC design on the product; the EMC performance of the electronic product is given by the design. The test only uses a certain quantitative method to characterize the inherent EMC performance of electronic products. For EMC design:

EMC design in the early stage of R&D

If the EMC problem is not considered in the early stage of product design, only hope to solve it in the test stage (indicated by solving the EMC problem of the designed product through rectification, so a lot of manpower and material resources are invested in the later stage of testing/verification and rectification).

Then, even if the product rectification is successful, in most cases, the rectification involves changes in circuit principles, PCB design, and structural molds, which will greatly increase R&D costs and greatly extend the cycle.

Only by considering and predicting EMC issues in the pre-product design process, turning EMC into a controllable design technology, paralleling and synchronizing with the process of producing functional design, can the product be designed at once.

EMC design should be systematically carried out

Improving the EMC performance of electronic products through design is definitely not given by the EMC experts in the enterprise alone, because it is absolutely impossible for EMC to exist without product hardware, structure, and other physical objects. Therefore, to make the designed electronic products achieve good EMC performance at one time, it is necessary to improve the EMC experience and awareness of product design engineers.

Hardware engineers should master the basic knowledge of EMI and EMS anti-interference design in addition to the circuit design knowledge that must be mastered previously;

PCB design engineers need to master the corresponding device layout, stacking design, and high-speed wiring EMC design knowledge;

Structural engineers also need to understand the design knowledge of product structure shielding. Because these engineers who participate in product design together, to realize the opinions put forward by EMC experts in the product design process.

They must understand and comprehend the mystery of the suggestions put forward by EMC experts, and combine them with the design characteristics of their respective fields to combine all the germination of EMC problems is eliminated in the product design stage.

Only all developers involved in product design can improve EMC quality together to design electronic products with high-performance EMC.

In general, the design of electronic products does not consider EMC issues, it will lead to EMC test failure so that it can not pass the certification of relevant laws and regulations.

With the development of electrical and electronic technology, electronic products have become increasingly popular, and electronic, radio and television, post and telecommunications, computers and their networks have become increasingly developed, and the electromagnetic environment has become increasingly complex and deteriorating.

We are gradually paying attention to the working environment of equipment and the electromagnetic environment. As for the impact on electronic equipment, the problems of electromagnetic interference (EMI) and electromagnetic compatibility (EMC) of electrical and electronic products have attracted more and more attention from engineers and manufacturers.

ESD protection

In ESD protection, it can be roughly divided into two categories:

1. Conductive ESD protection

Some protection devices are mainly used to protect the electrostatic current in the circuit, and a protection circuit is formed in the front of the sensitive device to guide or dissipate the current. Such protective devices include ceramic capacitors, varistors, TVS tubes, etc.

2. Radiation ESD protection

For the field generated by static electricity to affect sensitive circuits, the main protection method is to minimize the generation and energy of the field, increase the protection ability through the improvement of the structure, and protect the sensitive circuit.

It is often difficult to protect the field, and a method called alleles has been explored in improved practice. By effectively framing, the shell forms a potential equal body to suppress discharge. Facts have proved that this method is effective and easy to implement.

There are many general methods to protect against static electricity, including reducing the accumulation of static electricity; insulating products to prevent static electricity from occurring; providing branch circuits to shunt static electricity for sensitive lines; shielding the circuits in the discharge area; reducing the loop area to protect the circuit from static electricity The influence of the magnetic field generated by the discharge.

There are direct discharges, and there are couplings for the associated field.

 

What’s the EMI?

Most electrical and electronic equipment will have periodic or intermittent voltage and current changes during operation.

For example, a switching power supply will have a fixed switching frequency. The turn-on and turn-off pulses of the MOS tube will be accompanied by its rising and falling edges. (di/dt), and bring very rich harmonics; in addition to AC-DC, DC-DC switching power supplies used on various occasions, there are also some inverters (photovoltaic, motor control, etc.) PWM signals.

Are interference sources caused by EMI problems? These sources generate some electromagnetic energy concentrated in certain frequencies, which are radiated into the surrounding environment through corresponding circuits, antennas, or equivalent antennas.

  • Two ways of interference conduction and radiation

Conduction test product power port, signal port external interference on the line, and radiation, the test product itself, including cables, interference to space emission.

Three elements of EMC are: interference source, propagation path, and sensitive source

The three major measures for EMC rectification are shielding, filtering, and grounding.

The nature of EMI mentioned above is actually a source of interference. The propagation path is also an extremely important entry point when actually solving problems. In the circuit, many inconspicuous links have actually been playing such an important role.

For example, when PCB wiring, why can’t it be routed at 90°? Only from the perspective of EMI, 90° wiring will cause distortion of the impedance of the line, forming an equivalent transmitting antenna, and high-frequency signals are easily transmitted through this path; in addition, the signal line, the high-level line and the return line of the power line Why is it necessary to route the wires as close as possible to reduce the loop area?

Because of such a loop, from the perspective of EMI, it is still an equivalent transmitting antenna, similar to a loop antenna, and its transmission efficiency is proportional to its area.

Regarding the sensitive source, the antenna simulates the sensitive source during the emission test, and the product itself is the sensitive source during the anti-interference test. As for the anti-interference test, this article will not start. So, when the EMI test fails, how should we rectify it, or what kind of thinking should we take to rectify it?

When the test is unqualified, first eliminate the reasons for the test itself, including the layout of the test harness, product placement, grounding, test equipment such as LISN grounding, measurement switches, 50Ω matching, etc. Secondly, according to the test items, a preliminary judgment can be made.

For example, the conduction test of the power port should start with the investigation of the power circuit, such as disconnecting the power supply of a certain function and then testing, compare the results, and whether the cause can be found. , The second is to check the coupling interference of the product or other lines in the system.

Through targeted and reasonable investigations and comparison tests, after finding the cause of the unqualified, using the three major EMC rectification measures and starting to prescribe the right remedy is the correct rectification idea.

Commonly used EMI rectification devices

EMI interference is generally divided into common-mode interference and differential mode interference, and most devices are also used according to these interference types. (Common mode current: the interference current flows in the loop formed between the wire and the ground at the same potential on the wire; differential mode current: the interference current flows between the signal wires or between the positive and negative lines of the power supply.

1) Capacitance

The basic function of the capacitor is charging and discharging, and the characteristic is to pass and isolate the direct current. Using these characteristics, the flexible use of capacitors, using different capacitance values ​​to suppress interference at different frequencies, can solve many EMI problems; capacitors are also divided into common-mode capacitors (Y Capacitance) and differential mode capacitance (X capacitance).

The X capacitor is connected across the wire and used to absorb differential mode interference, and the Y capacitor is used between the line and the ground. Generally, they appear in pairs to absorb/bleed common-mode interference.

2) Inductance

The basic function of inductance is energy storage, and filter inductance is also divided into common-mode inductance and differential mode inductance; common-mode inductance (common mode choke): the two coils of common-mode inductance are wound on the same magnetic core, and the number of turns same as phase, but the winding direction is opposite.

When the normal current of the product passes, the currents generate reverse magnetic fields in the common-mode inductance coil to cancel each other; and when the common-mode current passes, due to the isotropy of the common-mode current, the same direction will be generated in the common-mode inductance coil.

The magnetic field causes the common-mode inductance to behave as a high impedance attenuated common mode current. Differential mode inductance: In the case of a certain signal frequency, the greater the inductance, the greater the resistance to high-frequency signal current, the smaller the inductance, the smaller the resistance.

3) Magnetic beads

Magnetic beads are specially used to suppress high-frequency noise and spike interference on signal lines and power lines, and also have the ability to absorb electrostatic pulses. The main function of the magnetic beads is above 50MHz. The usage in the circuit is similar to the resistance, which is equivalent to the use of resistance and inductance in series, and higher impedance will be reflected in a specific frequency band;

4) Magnetic ring, magnetic buckle

Pass the wire harness through a magnetic ring or magnetic buckle to form a common mode choke coil. According to the actual situation, the wire harness can be wound several turns on the magnetic ring. The more the number of turns, the greater the inductance, the better the suppression effect on lower frequency interference, and the weaker the suppression effect on higher frequency noise.

According to the material, the magnetic ring is mainly divided into manganese-zinc ferrite, nickel-zinc ferrite, amorphous, nanocrystalline, etc. Manganese-zinc ferrite has better performance at low frequencies, and nickel-zinc ferrite has better performance at high frequencies. Amorphous and nanocrystalline magnetic rings have a higher impedance in a wide frequency range, but the price is very high.

Do a good job in the early EMC design of products

If EMC issues can be taken into consideration in the early stage of product development, it can save the cost of later testing, rectification, board modification, and even re-opening of molds, and it can also greatly shorten the entire product development cycle.

  • Electronic and electrical schematic diagram PCB aspects

1) For the product power supply, communication, and other circuits, it is necessary to design the filter circuit in a targeted manner and reserve the necessary places for the port filter circuit to facilitate the debugging after the product is formed;

2) Unified planning and reasonable layout of devices. For sensitive devices, try to stay away from areas that are susceptible to interference, and cannot be placed near the edge of the board, or near the connectors;

3) The high-speed signal must have a complete ground plane to provide a return path, and if necessary, include the ground for processing;

4) Ensure the continuity of the signal line impedance, reduce the return path, try to control the vias, and do not split across the power supply;

5) The trace should be as short as possible, whether it is a power supply or a signal line, the loop area must be controlled;

6) Keep the input and output away, otherwise, the filter circuit will fail;

7) When the high-frequency signal is grounded, the ground wire should be short, straight, and thick, and the ground wire must not have large distortion or bend;

8) When the working frequency of the product is high, the signal ground, analog ground, and power ground should be separated.

  • Structural layout

1) The layout of high and low voltage modules (such as IGBT and control board) should be well isolated, and the wiring harnesses should not be interspersed with each other or parallel;

2) The input and output wires of each module should be as short as possible, especially high-voltage wires (copper bars), the wires should be short and the loop area should be small, and the wires should be placed together as much as possible;

3) For products with the high power density and strong interference, filter capacitors, magnetic rings, etc. should be reserved at the high and low voltage input and output ports during structural design.

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