Three rules and three elements of EMC
Time:2024-05-27
Views:109
The earlier EMC problems are considered and solved, the smaller the cost and the better the effect.
EMD design in the new product development stage, compared with waiting until the product EMC test failed to improve, the cost can be greatly saved, the efficiency can be greatly higher; On the contrary, efficiency will be reduced and costs will be greatly increased.
Experience tells us that it is the most time-saving and cost-effective to carry out EMC design at the same time as the functional design, to the completion of the prototype, until the EMC test is passed. On the contrary, the product development stage does not consider EMC, after the production found unqualified EMC to improve, not only brings great technical difficulty, and rework will inevitably bring a great waste of cost and time, and even due to the structural design, PCB design defects, can not implement improvement measures, resulting in the product can not be listed
Electromagnetic Compatibility (EMC) is electromagnetic compatibility. It is a technique for studying electromagnetic interference. Electromagnetic interference is the electromagnetic energy around us that causes the operation of electronic devices to produce an undue response. The technical purpose of EMC is to make the electrical device or system under the common electromagnetic environmental conditions, neither affected by the electromagnetic environment, nor to interfere with the environment.
We understand the following three important laws in the EMC field and the three elements of EMC problems:
Three important rules of EMC
Rule 1, the law of EMC cost-effectiveness relationship: the earlier EMC problems are considered and solved, the smaller the cost and the better the effect.
EMC design in the new product research and development stage, compared with waiting until the product EMC test fails to improve, the cost can be greatly saved, the efficiency can be greatly improved; On the contrary, the efficiency will be greatly reduced and the cost will be greatly increased.
Experience tells us that it is the most time-saving and cost-effective to carry out EMC design at the same time as the functional design, and pass the EMC test when the sample and prototype are completed. On the contrary, the product development stage does not consider EMC, after the production is found to be unqualified EMC to improve, not only brings great technical difficulty, and rework will inevitably bring a great waste of cost and time, and even because of the structural design, PCB design defects, can not implement improvement measures, resulting in the product can not be listed.
Rule 2: The larger the high-frequency current loop area S, the more serious the EMI radiation.
The high-frequency signal current flows through the minimum path of the inductor. When the frequency is high, the general line reactance is greater than the resistance, the connection to the high frequency signal is the inductance, and the series inductance causes radiation. Electromagnetic radiation is mostly generated by the high-frequency current loop on the EUT device under test, and the worst case is in the form of an open antenna. The corresponding treatment method is to reduce and shorten the connection, reduce the high-frequency current loop area, and try to eliminate any antenna required for abnormal work, such as discontinuous wiring or excessively long pins of components with antenna effects.
One of the most important tasks to reduce radiation disturbance or improve the anti-interference ability of RF radiation is to find ways to reduce the high-frequency current loop area S.
Rule 3: The higher the loop current frequency f, the more serious the EMI radiation caused, and the electromagnetic radiation field intensity increases in proportion to the square of the current frequency f. The second most important way to reduce radiation disturbance or improve the anti-interference ability of radio frequency radiation is to find ways to reduce the high-frequency current frequency f of the disturbance source, that is, to reduce the frequency f of the disturbance electromagnetic wave.
Two, EMC problems three elements
Because the pulse current and voltage of switching power supply and digital equipment have very rich high-frequency harmonics, they will produce strong radiation. Electromagnetic interference includes radiation-type (high frequency)EMI and conduction type (low frequency)EMI, that is, EMC problems are mainly generated through two ways: one is the form of space electromagnetic wave interference; The other is through the form of conduction, in other words, the three elements of EMC problems are: electromagnetic interference source, coupling path, sensitive equipment. The radiation interference pollutes the space electromagnetic environment in the form of electromagnetic wave mainly through the shell and the connecting line. Conducted interference is the disturbance of the public power grid through the power cord or the influence of other terminals (such as radio frequency terminals, input terminals) on connected devices.
Because the pulse current and voltage of switching power supply and digital equipment have very rich high-frequency harmonics, they will produce strong radiation. Electromagnetic interference includes radiation-type (high frequency)EMI and conduction type (low frequency)EMI, that is, EMC problems are mainly generated through two ways: one is the form of space electromagnetic wave interference; The other is through the form of conduction, in other words, the three elements of EMC problems are: electromagnetic interference source, coupling path, sensitive equipment. The radiation interference pollutes the space electromagnetic environment in the form of electromagnetic wave mainly through the shell and the connecting line. Conducted interference is the disturbance of the public power grid through the power cord or the influence of other terminals (such as radio frequency terminals, input terminals) on connected devices.
Conduction and radiation
Disturbance source--------------------（way）-------------------- Sensitive receptor
Near-field coupling
Possible sources of harassment from IT and AV devices
A) FM receiver, TV receiver local oscillation, the base wave and harmonics are generated by the high-frequency head, the local oscillation circuit;
B) Switching pulse and higher harmonics of switching power supply, synchronization signal square and high frequency harmonics, line and field signals and high frequency harmonics generated by line scan imaging circuit;
C) Various clock signals and high-frequency harmonics required for digital circuit work, as well as their combination, various clocks such as CPU chip working clock, MPEG decoder working clock, video synchronization clock (27MHz, 16.9344MHz,40.5MHz), etc.;
D) digital signal waveform and high-frequency harmonics, high order harmonics generated by crystal vibration, nonlinear circuit phenomena (nonlinear distortion, intermodulation, saturation distortion, cut-off distortion) caused by useless signals, stray signals;
E) Non-sine wave waveform, waveform hairline, overshoot, ringing, parasitic frequency points in circuit design.
F) External disturbances to sensitive receptors through the coupling pathway include surges, rapid pulse clusters, static electricity, voltage drops, voltage changes, and various electromagnetic fields.
Characteristics of electromagnetic disturbance
① The spectrum of the unit pulse is the widest;
(2) The low frequency content in the spectrum depends on the pulse area, and the high frequency component depends on the steepness of the pulse front and back;
(3) The crystal oscillation level must meet a certain amplitude, so that the digital circuit can work according to a certain sequence, so that the disturbance generated by the crystal oscillator presents the characteristics of covering bandwidth and high disturbance level;
(4) The EMI radiation and reception are the most serious when the polarization and direction characteristics of the transmit-receiving antenna are the same; The larger the area of transceiver antenna, the greater the EMI damage;
⑤ Harassment path: radiation, conduction, coupling and combination of radiation, conduction, coupling.
⑥ Power line conduction disturbance is mainly caused by common mode current;
⑦ Radiation disturbance is mainly generated by the loop formed by the differential mode current.
Iii. Concluding Remarks:
Understanding the three laws of EMC and the three elements of EMC problems will make EMC problems have rules to follow, and adhering to the laws of EMC will save time and effort to solve EMC problems, and get twice the result with half the effort.
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