EMC rectification six steps to go
Electromagnetic compatibility EMC (Electro Magnetic Compatibility) refers to the ability of a device or system meets the requirements to run does not produce electromagnetic interference can not tolerate any of its equipment environment in its electromagnetic environment. Therefore, EMC requirements include two aspects: on the one hand refers to the environment where electromagnetic interference generated by the device during normal operation should not exceed a certain limit; on the other hand refers to electromagnetic interference present in the environment where the appliance to have a certain degree of immunity that electromagnetic sensitivity.
Interference between the various electronic devices running mainly in electric conductivity, electromagnetic induction and electromagnetic radiation in three ways associated with each other and affect each other, equipment and personnel under certain operating conditions will cause interference effects and hazards. EMC rectification on specific areas of the article is actually a lot.
EMC rectification six steps as follows: Step lookup confirmation radiation, a second step filter, absorbing the third step, the fourth step grounded shield fifth step, a sixth step an energy dispersion method. Specific ideas as shown below:
The first step: Find a radiation source confirmation has exclusion method, frequency search spectrum analyzer method, the natural frequency of the element analysis. With the exclusion pull line method of zoning exclusion, low voltage low current human touch method, the shield exclusion area. Element analysis refers to the natural frequency of the fixed frequency and its harmonic frequency analysis of the typology of some elements, such as the operating frequency of the crystal element and the like DDR are fixed.
Step Two: generally divided into filter capacitor filter, RC filter and LC filter and the like;
The third step: The method of absorbing electromagnetic waves with a series circuit beads method, magnetic method and worn around the absorbing material paste method. Pay special attention to the use of electromagnetic wave absorption methods: excessive radiation frequency electromagnetic waves must be absorbed by the absorber in the frequency range used, or cause the absorbing process will fail.
Step Four: ground method is generally divided into single-point ground method and multi-point ground method.
Step Five: Shielding shield generally applied method of shielding, the shielding housing and PCB trace layout method Shielding.
Step Six: an energy dispersion method refers to the control software of some analyte of energy concentration may show a wide frequency band of the bandwidth to achieve frequency hopping and band energy dispersion and the use of frequency hopping spread spectrum technology to a single point at the additional frequency the energy is reduced, that is, play a single point frequency electromagnetic wave radiation intensity decreases efficacy. Therefore the method of frequency shaped waveform spikes excessive radiation will play a significant effect on the shape of the envelope waveform frequency excessive radiation will not achieve significant effects.
The EMC rectification six-step more suitable for rectification common electronic devices. But six methods above, although in fact Terry Jun think they help to improve the rectification efficiency EMC radiation, saving cycle quickly through the EMC testing, but it is not a program to solve the fundamental problem of EMC, EMC problems or in the best design end to be considered, rather than to emergency afterwards with some "besieged" the program.
EMC (Electromagnetic Compatibility) that we often say EMC technology. It contains two parts EMI and EMS requirements, i.e., in a common electromagnetic environment conditions Electrical devices or systems affected neither by electromagnetic environment, does not give interference to the environment.
EMC rectification method
First, the product diagnose the actual situation, analyze its sources of interference and where the ways and means of mutual interference. According to another analysis, targeted for rectification.
The main rectification general methods as follows
1, on the basis of attenuated interferers find the source of interference, interference can be reduced to allow source range, attenuated method source generally as follows:
a, between GND and Vcc IC decoupling capacitors, the capacitor capacity between the 0.1FF 0.01μF, attention capacitor wire when installing it as short as possible.
b, in the case of adding the attenuator to ensure that the sensitivity and signal to noise ratio. Such as VCD, DVD video disc in the crystal, its electromagnetic compatibility effects are more severe, reducing its amplitude is one feasible method, but it is not the only solution.
c, there is an indirect method of the signal line away from noise sources.
2, wire and cable finishing classification in the electronic device, the coupling between lines is an important way, but also the major cause of interference, because the frequency factor, can be roughly divided into low and high-frequency coupler coupling. Depending coupling, which is different rectification method, are discussed below:
(1) low coupling wire guide is less than or equal to 1/16 of the length of the wavelength, the coupling can be divided into low-frequency electric and magnetic field coupling.
Physical model coupling is a capacitive coupling electric field, therefore the primary object is to reduce the rectification distributed coupling capacitance or coupling amount decreases, the following method may be employed:
a, increasing the pitch of the circuit most effective way to reduce the distributed capacitance.
B, additionally a highly conductive shield, and the shield can effectively single point grounding reject the low frequency electric field disturbance.
c, the additional filter may reduce the amount of coupling between the two circuits.
d, to reduce the input impedance of, for example, high input impedance of the CMOS circuit, the electric field is extremely sensitive to interference, the allowable range may be inherent in the input terminal and connected to a capacitor or low value resistors.
Physical model of the magnetic field coupling is an inductive coupling, coupled distributed mainly through mutual coupling between the lines, so the main method of destruction rectification or reducing the amount of coupling, the following method is generally employed:
adding a filter, the filter is to be noted in the case of adding input and output impedance and frequency response of the filter.
b sensitive circuit and a source circuit is reduced loop area, i.e. as far as possible or the signal line loop current lines near thereto or integrally twisting.
the distance c is increased both circuits, so as to reduce the line to reduce the mutual coupling amount.
d If possible, try to make the circuit sensitive to the source circuit plane orthogonal or nearly orthogonal to reduce the amount of coupling of the two circuits.
e high permeability material wrapped sensitive wire, a magnetic field can effectively solve the interference problem is noteworthy that for forming the closed magnetic circuit and, in an effort to reduce the reluctance of the magnetic circuit will be more effective.
(2) high-frequency coupling means is longer than a 1/4 wavelength of the standing wave due to the trace voltage and current occurring in the circuit, will enhance the amount of coupling, can be employed to solve the following methods:
a, to minimize the ground line, and the housing ground as far as possible by way of surface contact.
B, refresh input and output lines of the filter, to prevent coupling between input and output lines to ensure that the filtering effect of the filter is not deteriorated.
c, shielded cable shielding multi-point grounding.
d, the floating connector pins to ground potential, which prevent the antenna effect.
3, to improve the system ground
Over the ground is a zero impedance, zero potential physical entities, it is not only the reference point signal, the current flows and no voltage drop. In particular in electrical and electronic equipment, which does not exist over the ground, when a current flows through the ground would have a voltage drop. Accordingly base line can interfere with the formation mechanism can be attributed to the following two points. First, to reduce the impedance and low impedance power feed. Second, select the right mode and a barrier to the ground loop, to be divided by grounding suspension, a single point ground, multi-point grounding, the ground mixture. If the line is mainly sensitive to interference from external space or housing system, the suspension can be addressed at this time manner, but the suspension device prone to accumulation of static electricity, when the charge reaches a certain level, will generate an electrostatic discharge, the suspension should not be used to in general electronic equipment. Single-point ground circuit suitable for low frequency, current and frequency in order to prevent other stray current to generate a potential difference between the point of signal ground line, and signal ground and power ground isolation safety, the power cord is located in a large single point connection. Single-point ground is mainly applied to the frequencies below 3MHz. Multi-point grounding the high frequency signal is the only practical way to the ground, when the radio transmission line will exhibit characteristics such that the effectiveness of the multi-point grounding, when the ground conductor is longer than 1/8 wavelength of the highest frequency, require a multi-point grounding equipotential a ground plane. Multi-point grounding applies to 300KHz or more. Since the ground suitable for mixing high-frequency electronic circuits have low
4, the shield
The shield is an important measure to improve the electromagnetic compatibility of electronic systems and the electronic device can, it can effectively inhibit various electromagnetic interference propagating through space. Shielding mechanism can be divided by the electric field shield and the magnetic shield and electromagnetic shield. Electric field shielding should note the following:
A, select material having a high conductivity, and should be grounded.
B, then select the correct location and reasonable shape, preferably directly grounding shield. Magnetic shielding usually refers only to shield the DC or very low frequency magnetic field shielding effectiveness is far better than the electric field shielding and electromagnetic shielding, magnetic shielding is often the key projects, the magnetic shield when:
a, to choose ferromagnetic material.
B, the magnetic shield member away from the magnetic element, to prevent the magnetic short circuit.
c, the shield can be double or even triple-shielded.
d, the upper shield openings to be noted that the direction of opening, so that the long sides parallel to the slit as the magnetic flux flows, a minimum magnetic path length increases. In general, magnetic shielding is not required to ground, but in order to prevent the electric field sensor, or the ground as well. When the electromagnetic field by a metal or attenuation of electromagnetic field barrier, will be a certain degree of attenuation, i.e., the electromagnetic field shielding effect is generated. Depending on the specific needs of the actual rectification process may be selected and which shield shape, size, and the like grounding shield.
5, changing the wiring structure of the circuit board
Some frequencies are distributed down through the circuit board line parameters determined, not by the aforementioned method is useful, such rectification circuit parameters is changed by adding a small structure in the trace inductance, capacitance, magnetic beads, it is moved threshold requirements on higher frequencies. For this type of interference, in order to solve it fundamentally affected, we must re-wiring.
Summary: In short front is good for several ways to improve the electromagnetic compatibility, but the most widely used method is to change the structure and the sort of ground wire and cable, these methods are not only cost savings, but is most effective corrective methods. Although the shield will increase costs, but they sometimes play a shielding effectiveness is not comparable to other methods. Therefore, in the actual rectification should be ground to alter the structure, classification finishing wire and cable, shielding methods mainly supplemented with other methods.
Three important laws in the field of EMC and EMC issues three elements
A, EMC three important laws
A rule, EMC cost-effective law relationship: EMC consider the issue sooner, the sooner resolved, the smaller the cost, the better.
In the new product development stage design for EMC, EMC test failure than to wait until the product was improved, costs can be significant savings, efficiency can be greatly improved; on the contrary, efficiency will be greatly reduced, the cost will be greatly increased. Experience tells us that, in the functional design, while EMC design, to prototype, test the prototype is completed through EMC, is the most save time and most cost-effective. Instead, the product development stage without considering EMC, EMC found that production will only failed to improve, not only to bring technically very difficult, a lot of waste and rework inevitably bring cost and time, even as it relates to the structural design, PCB design defects, unable to implement improvements, resulting in the product not be listed.
When the detected actual Terry detecting operation is frequently encountered: the "dogged" approach by the relevant EMC mandatory testing and certification, but such products can be produced in the actual production and actual product applicability is close to zero . This has resulted in laboratory samples and actual finished product is inconsistent, EMC rectification become "deceiving" the furnishings, so the real question is to consider the EMC product design must be taken into account and should not be the main measures put in the product testing stage.
Law Second, the larger the high-frequency current loop area S, the more severe EMI radiation. A high-frequency signal current flows through the inductor smallest path. When the frequency is higher, generally greater than the resistance wiring reactance, inductance of the connection is high-frequency signal, the series inductance caused by radiation. Most of the high-frequency electromagnetic radiation is measured EUT current loop generated on the device, the worst case is "antenna forms" open circuits. Corresponding to the processing method is to reduce, shorten wiring, reducing high frequency current loop area, try to eliminate any "antenna" non-work required, such as discrete wires or pins of the components of the antenna effect long. Radiation disturbance or reduction of the most important task to improve the anti-interference ability of the RF radiation, is to find ways to reduce the high-frequency current loop area S. In practice a number of days if the detection method is to handle the specific problem of ground (signal ground and power ground).
Law Third, the higher the frequency f of the loop current, the more serious due to EMI radiation, electromagnetic radiation field current increases with the square of frequency f. The most important way to reduce the radiation disturbance or interference rejection capability to improve two RF radiation, is to find ways to reduce the high-frequency current disturbance source frequency F, i.e. the frequency of the electromagnetic wave decreases harassment f. On this law f, Terry empirically is because many of the shield and the housing caused by design or working, because the higher the frequency, the smaller the wavelength, the more readily the small gap from leaking out of the housing or the shield.
Two, EMC issue three elements
Since the switching power supply device and a digital pulse having a current and voltage harmonic very rich, and therefore will have a strong radiation. Including electromagnetic interference emission type (high frequency) the EMI, conduction (low frequency) the EMI, EMC problems which produce mainly via two pathways: a space in the form of electromagnetic interference; the other is in the form of conductive, in other words, generating EMC problems are three elements: a source of electromagnetic interference, coupling path, sensitive equipment. Interference of radiation through the housing and connected to the main line in the form of electromagnetic interference electromagnetic environment space; harassment conducted interference public power grid or by other terminals (such as: a radio terminal, an input terminal) is connected via power supply lines of Effect devices.
T, AV equipment possible source of harassment:
A) FM receiver, the TV receiver local oscillation, harmonic and fundamental wave, the local oscillation circuit generates a high-frequency head;
B) and the switching pulse of the switching power supply higher harmonic, the harmonic sweep synchronization signal side, the row scanning line generated by the imaging circuit, and a higher harmonic field signal;
C) various clock signals and the high frequency harmonics required digital circuit, combinations thereof, and the various clock chip such as CPU operation clock, the clock operation of a decoder, a video synchronization clock and the like;
D) digital signal spread party harmonic, the harmonic generation crystal, nonlinear circuit phenomenon (non-linear distortion, intermodulation, distortion saturation, cutoff distortion) and other unwanted signals, caused by spurious signals;
E) a non-sinusoidal waveform, a waveform burrs, overshoot, ringing, the presence of parasitic circuit design frequency.
F) sensitive receptors for receiving an external coupling path comprises a surge harassment, rapid burst, static electricity, voltage drop, and various changes in the voltage field.
Electromagnetic disturbance characteristics:
① unit pulse spectrum widest;
Area ② spectrum of low-frequency content depends on the pulse, the high frequency component depending on the steepness of the front and rear edge of the pulse;
③ crystal oscillator must meet certain amplitude level, digital circuitry can work according to a certain timing, so that the crystal produced exhibits cover harassment bandwidth harassment high level characteristic;
④ transceiver antenna polarization, the same directional characteristic, EMI radiation and receive the most severe; larger area receiving antenna, EMI harm than large;
⑤ harassment ways: radiation, conduction, and radiation coupling, conductive, coupling combinations.
⑥ power line conducted disturbances produced primarily by the common mode currents;
⑦ radiation disturbance primary loop formed by the differential mode currents.
Zhen Chuan word, fake pass rolls of the book. We know the three laws and three elements of EMC, EMC will make the problem easier and there are rules. Understanding of the law and the spread of elements of EMC, in fact, methods and ideas to solve the EMC problem will clear up, we just interrupt one factor, EMC problems can necessarily be greatly improved.