What are the three important rules of the EMC inductor and the three elements of the EMC problem?
First, three important rules of EMC.
Law 1. The relationship between EMC cost-effectiveness ratio: The earlier the EMC problem is considered, the earlier it is solved, the smaller the cost and the better the effect.
In the new product development stage carry out the EMC design than when the EMC test is not qualified to improve, the cost can be greatly improved, and 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 EMC design is carried out at the same time as functional design, and EMC testing is completed when the prototype and prototype are completed, which is the most time-saving and most economical. On the contrary, EMC is not considered in the product development stage. After the production is completed, it is found that the EMC is not qualified to be improved. This is not only technically difficult, but also rework will inevitably bring about a great waste of cost and time, even due to structural design and PCB design defects, unable to implement improvement measures, resulting in products not available.
Engineer in the rectification test
In the actual inspection work, the situation often encountered is: through the "surrounding and intercepting" method through the relevant EMC test and certification mandatory requirements, but the actual product availability and product practical applicability of such products is close to zero. This makes the laboratory samples and the actual finished products inconsistent, so the real consideration of the EMC problem is to be taken into account in the product design, and the main countermeasures should not be placed in the product testing phase.
Rule 2: The higher the loop current frequency f, the more severe the EMI radiation caused, and the electromagnetic radiation field strength 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 RF radiation is to find ways to reduce the frequency f of the high-frequency current of the disturbance source, that is, to reduce the frequency f of the disturbance electromagnetic wave. The law of f is caused by shielding and housing design or workmanship, because the higher the frequency, the smaller the wavelength, and the easier it is to leak out of the small gap of the outer casing or shield.
Rule 3: The larger the area S of the high-frequency current loop, the more severe 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 the radiation. Most of the electromagnetic radiation is generated by the high-frequency current loop on the EUT device under test. The worst case is the "antenna form" in the open circuit. The corresponding processing method is to reduce and shorten the connection, reduce the area of the high-frequency current loop, and try to eliminate any "antenna" required for abnormal operation, such as discontinuous wiring or long pins of components with antenna effect. One of the most important tasks to reduce radiated harassment or improve the anti-interference ability of RF radiation is to find ways to reduce the high-frequency current loop area S. Some specific methods in the detection practice are to deal with the grounding problem (power ground and signal ground).
Second, the three elements of the EMC problem
Switching power supplies and digital devices generate strong radiation due to the high frequency harmonics of the pulsed current and voltage. Electromagnetic interference includes radiated (high-frequency) EMI and conductive (low-frequency) EMI, that is, EMC problems occur mainly through two ways: one is in the form of spatial electromagnetic interference; the other is in the form of conduction, in other words, EMC The three elements of the problem are: electromagnetic interference sources, coupling paths, and sensitive equipment. Radiation interference mainly interferes with the space electromagnetic environment through electromagnetic waves in the casing and connecting lines; conducted interference is the equipment that affects the connection through the power line to disturb the public power grid or through other terminals (such as: RF terminals, input terminals).
Possible sources of harassment for T and AV equipment:
A) FM receiver, TV receiver local oscillation, fundamental wave and harmonic generated by high frequency head, local oscillator circuit; B) various clock signals and high frequency harmonics required for digital circuit operation, and their
combinations, Various clocks such as CPU chip operating clock, decoder working clock, video synchronous clock, etc.; C) digital signal square wave and high frequency harmonics, higher harmonics generated by crystal oscillator, nonlinear circuit phenomenon (nonlinear distortion, intermodulation, Unwanted signals and spurious signals caused by saturation distortion, cutoff distortion, etc.; D) switching pulses and higher harmonics of switching power supplies, synchronous signal square waves and high frequency harmonics, line and field signals generated by line scanning imaging circuits and High-frequency harmonics; E) Non-sinusoidal waveforms, waveforms, overshoots, ringing, parasitic frequency points in the circuit design. F) External disturbances received by sensitive receptors via coupling pathways include surges, fast bursts, 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 crystal oscillation level must meet a certain amplitude, and the digital circuit can work according to a certain timing. The disturbance caused by the crystal oscillator is characterized by high coverage bandwidth and high disturbance level.
3.The low frequency content in the spectrum depends on the area of the pulse, and the high frequency component depends on the steepness of the front and back edges of the pulse;
4.When the polarization and direction characteristics of the transmitting and receiving antennas are the same, EMI radiation and reception are the most serious; the larger the area of the transmitting and receiving antennas, the greater the EMI hazard;
5.Harassing path: radiation , conduction, coupling, and a combination of radiation, conduction, and coupling.
6.Power line conducted disturbances are mainly generated by common mode currents;
7.Radiated disturbances are mainly generated by loops formed by differential mode currents.
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