Filter capacitor common mode inductance, magnetic beads is a common figure in circuit EMC design, three weapon also eliminate electromagnetic interference. This is the role of these three in the circuit, I believe there are many engineers do not know. From the design, the detailed analysis of the principle of three weapon to destroy EMC.
1. The filter capacitor
Although filter out high frequency noise from the point of view, the resonant capacitor is not desirable, but not always resonant capacitance harmful. When the noise frequency is determined to be filtered out, by adjusting the capacity of the capacitor, the resonance frequency point just fall on harassment. In practical engineering, to filter out the noise frequency electromagnetic often as high as several hundred MHz, even more than 1GHz. For such high-frequency electromagnetic noise feedthrough capacitors can be used effectively filtered. The reason why the common capacitance can not effectively filter out high frequency noise, for two reasons: one reason is the capacitance of the resonant capacitor caused by lead inductance, high-frequency signal exhibits large impedance, the weakening effect of bypassing the high frequency signal; another reason is that a parasitic capacitance between the high-frequency signal wire coupling occurs, reducing the filtering effect.
Feedthrough capacitors is able to effectively filter out high frequency noise, because feedthrough capacitors do not only lead inductance resonant frequency that is too low causes problems and feedthrough capacitors can be directly mounted on the metal panel, the metal panel using a high functions frequency isolation. However, the use feedthrough capacitors, the problem should be noted that installation problems.
Feedthrough capacitors biggest weakness is afraid of high temperature and thermal shock, which caused great difficulties in the feedthrough capacitors welded to the metal panel. Many capacitance damage occurring during welding. Especially when you need a large number of feedthrough capacitors mounted on the panel, as long as there is a damage, it is difficult to repair, because at the time the damaged capacitor removed, can cause damage to other adjacent capacitor.
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2. common mode inductance
Since the EMC problem faced by most of the common-mode interference, and therefore is one of our common mode inductance elements used a strong common mode inductance of a ferrite core common mode interference suppression device, which is the same size by the two , the same number of coil windings wound symmetrically on the same ferrite ring core, forming a four-terminal device having a large inductance to exhibit an inhibitory effect on the common mode signal, and for differential-mode signal exhibits little leakage inductance almost no effect.
When the principle is the common mode current flows in the magnetic flux superimposed on each other, so as to have a considerable inductance, inhibition play a common mode current, and when the two differential mode current flowing through the coil, the magnetic ring through cancel each other, almost no inductance, the differential mode current can pass through unattenuated. Therefore common mode inductance can effectively suppress the common mode interference signals in the balanced lines, no effect on normal differential mode signal transmission line.
In the production of common mode inductor should meet the following requirements: (1) wound on a coil core wire to be insulated from each other, to ensure that the short-circuit breakdown does not occur between the turns of the coil under the action of the transient overvoltage. (2) when an instantaneous large current flows through the coil, the core saturation do not appear. (3) the coil core and the coil to be insulated to prevent breakdown occurs therebetween under the action of a transient overvoltage. (4) a coil winding should be as a single layer, this may reduce the parasitic capacitance of the coil, the coil of a transient overvoltage to enhance the ability of the award.
Typically, bearing in mind the desired band selection filter, the common mode impedance the better, so we need to look at the device selection information is common-mode inductance, the main frequency of an impedance curve selection. Also note that consider differential mode impedance effect on the signal, the main focus differential mode impedance, pay special attention to the choice of high-speed ports.
3. beads
EMC design of products in the digital circuit process, we often use the beads, ferrite material is iron or an iron-nickel alloy, a magnesium alloy, which has a high magnetic permeability material, he can be between coil winding inductance generating high frequency high impedance in the minimum capacitance.
Ferrite material is typically applied at high frequencies, at low frequencies because their main path inductance characteristics, so that the line loss is small. At high frequencies, the reactance primarily in their characteristics vary with frequency and the ratio. In practical applications, ferrite material is used as a high-frequency circuit RF attenuator.
In fact, in the ferrite is preferably parallel equivalent resistance and inductance, low resistance of the inductor is short-circuited, high frequency inductive impedance becomes very high, so that all the current through the resistor. Ferrite is a consumer, in the above high-frequency energy into heat energy, which is determined by his resistance characteristic. Conventional ferrite bead inductor and better characteristics compared to the high-frequency filter. Ferrites exhibit resistive at high frequency, low quality factor corresponding to the inductor, it is possible to maintain a high impedance over a wide frequency range, thereby improving the high-frequency filter performance.
At low frequencies, the impedance constituted by the inductor inductance, R is small, the high permeability of the core at low frequencies, and therefore a large amount of inductance, L plays a major role, electromagnetic interference is reflected suppressed; and at this time a magnetic core loss is small, the entire device is a low loss, high Q inductor, which is likely to cause a resonant inductor in low frequency band and therefore, may sometimes appear after the ferrite bead reinforcing interference phenomenon.
At higher frequencies, the impedance constituted by a resistance component, as the frequency increases, the permeability of the core is reduced, resulting in reduced inductance of the inductor, the inductance component is reduced. However, when the core loss increases, the resistance component results in increase in the total impedance, when the high frequency signal through a ferrite, electromagnetic interference is absorbed and converted into the form of heat dissipated. Ferrite suppression components are widely used in printed circuit boards, power lines and data lines. The end plus ferrite suppression element, can filter out high frequency interference on the PCB, the cable inlets. Ferrite bead or beads dedicated to high-frequency interference suppression and interference peak signal line, power line, it also has the ability to absorb ESD pulse interference. Use of chip beads chip inductor or mainly lies in practical applications. It requires the use of chip inductor in the resonant circuit. When the need to eliminate unwanted EMI noise, using chip beads is the best choice.
Chip beads and chip inductor applications
Chip inductor : a radio frequency (RF) wireless communication and information technology equipment, radar detectors, automotive electronics, cellular telephones, pagers, audio equipment, personal digital assistants (PDAs), wireless remote control system and a low voltage power supply module.
Chip Beads : between the filter circuit, an analog circuit and the digital clock generating circuit, I / O Input / Output internal connector (such as serial, parallel, keyboard, mouse, long-distance telecommunications, local area network), a radio frequency circuit and vulnerable interference between the logic device, the power supply circuit in conductive filter out high frequency interference, computers, printers, video recorders (VCRS), EMI noise television system and a mobile phone suppressed.
Ohms beads, beads because the unit is in accordance with the impedance it produces in a certain frequency to the nominal impedance of the unit is ohm. Typically provide a graph showing a frequency characteristic and impedance on DATASHEET beads, 100MHz generally standard, such as a bead corresponds to the impedance of 1000 ohms when 100MHz frequency. For our band needs to be filtered to select beads impedance bigger the better, usually selected more than 600 ohm impedance. Also note that the choice of bead beads pass flow, typically 80% derating process, to consider the DC impedance of the power supply circuit used in a drop on.