Filter capacitors, common mode inductors, and magnetic beads are common in EMC design circuits, and they are also the three major tools to eliminate electromagnetic interference.

Filter capacitors, common mode inductors, and magnetic beads are common in EMC design circuits, and they are also the three major tools for eliminating electromagnetic interference.

For the role of these three in the circuit, I believe that there are still many engineers who are not clear. The article analyzes the principle of eliminating the three major EMC weapons in detail from the design.

Who do you know best about the three major tools for eliminating electromagnetic interference?

01 Filter capacitor

Although capacitive resonance is undesirable from the standpoint of filtering out high frequency noise, capacitive resonance is not always detrimental.

When the noise frequency to be filtered is determined, the capacity of the capacitor can be adjusted so that the resonance point just falls on the disturbance frequency.

In practical engineering, the frequency of electromagnetic noise to be filtered is often as high as hundreds of MHz, or even more than 1 GHz. For such high-frequency electromagnetic noise, feedthrough capacitors must be used to effectively filter out.

There are two reasons why ordinary capacitors cannot effectively filter out high-frequency noise:

(1) One reason is that the capacitor lead inductance causes capacitor resonance, which presents a large impedance to high-frequency signals and weakens the bypass effect on high-frequency signals;

(2) Another reason is that the parasitic capacitance between the wires couples high-frequency signals, reducing the filtering effect.

The reason why feed-through capacitors can effectively filter out high-frequency noise is that feed-through capacitors not only do not have the problem of low resonant frequency caused by lead inductance.

Moreover, the feedthrough capacitor can be directly installed on the metal panel, and the metal panel can be used for high-frequency isolation. But when using feedthrough capacitors, the problem to pay attention to is the installation problem.

The biggest weakness of feedthrough capacitors is that they are afraid of high temperature and temperature shock, which causes great difficulties when soldering feedthrough capacitors to metal panels.

Many capacitors are damaged during the soldering process. Especially when a large number of feed-through capacitors need to be installed on the panel, as long as one is damaged, it is difficult to repair, because when the damaged capacitor is removed, it will cause damage to other adjacent capacitors.

02 Common Mode Inductance

Since most of the problems faced by EMC are common mode interference, common mode inductors are also one of the powerful components we commonly use.

The common mode Inductor is a common mode interference suppression device with a ferrite core. It consists of two coils of the same size and the same number of turns wound symmetrically on the same ferrite toroidal core to form a four-terminal The device has a suppressing effect on the common-mode signal with a large inductance, but has little effect on the differential-mode signal with a small leakage inductance.

The principle is that when the common mode current flows through the magnetic flux in the magnetic ring superimposed on each other, it has a considerable inductance, which inhibits the common mode current, and when the differential mode current flows through the two coils, the magnetic flux in the magnetic ring The channels cancel each other out, and there is almost no inductance, so the differential mode current can pass through without attenuation.

Therefore, the common mode inductance can effectively suppress the common mode interference signal in the balanced line, but has no effect on the differential mode signal normally transmitted by the line.

Who do you know best about the three major tools for eliminating electromagnetic interference?

Common mode inductors should meet the following requirements during production:

(1) The wires wound on the magnetic core of the coil should be insulated from each other to ensure that no breakdown short circuit occurs between the turns of the coil under the action of instantaneous overvoltage;

(2) When the coil flows through a large instantaneous current, the magnetic core should not be saturated;

(3) The magnetic core in the coil should be insulated from the coil to prevent breakdown between the two under the action of instantaneous overvoltage;

(4) The coil should be wound in a single layer as much as possible, which can reduce the parasitic capacitance of the coil and enhance the ability of the coil to impart instantaneous overvoltage.

Usually, pay attention to selecting the frequency band to be filtered. The larger the common-mode impedance, the better. Therefore, we need to look at the device data when selecting the common-mode inductor, mainly based on the impedance-frequency curve.

In addition, pay attention to the influence of differential mode impedance on the signal when selecting, mainly focus on differential mode impedance, and pay special attention to high-speed ports.

03 Magnetic beads

In the EMC design process of product digital circuits, we often use magnetic beads. The ferrite material is iron-magnesium alloy or iron-nickel alloy. This material has high magnetic permeability. It can be used between the coil windings of the inductor. The capacitance generated in the case of high frequency and high resistance is the smallest.

Ferrite materials are usually used at high frequencies, because at low frequencies they are mainly inductive, resulting in very little loss on the wire. At high frequencies, they are predominantly reactive and vary with frequency. In practical applications, ferrite materials are used as high-frequency attenuators for radio frequency circuits.

In fact, ferrite is better equivalent to the parallel connection of resistor and inductor. At low frequency, the resistor is short-circuited by the inductor, and at high frequency, the impedance of the inductor becomes so high that all the current passes through the resistor.

Ferrite is a dissipating device on which high frequency energy is converted into heat energy, which is determined by its resistance characteristics. Ferrite beads have better high frequency filtering characteristics than common inductors.

Ferrite is resistive at high frequencies, equivalent to an inductor with a low quality factor, so it can maintain a high impedance over a fairly wide frequency range, thereby improving high-frequency filtering.

In the low frequency band, the impedance is composed of the inductive reactance of the inductor. At low frequencies, R is very small, the magnetic permeability of the magnetic core is high, so the inductance is large, L plays a major role, and electromagnetic interference is reflected and suppressed; and at this time, the magnetic The loss of the core is small, and the whole device is a low-loss, high-Q inductor, which is easy to cause resonance. Therefore, in the low frequency band, sometimes the phenomenon of increased interference after using ferrite beads may occur.

In the high frequency band, the impedance is composed of resistance components. As the frequency increases, the magnetic permeability of the magnetic core decreases, resulting in a decrease in the inductance of the inductor and a decrease in the inductive reactance component.

However, at this time, the loss of the magnetic core increases, and the resistance component increases, resulting in an increase in the total impedance. When the high-frequency signal passes through the ferrite, the electromagnetic interference is absorbed and converted into heat energy and dissipated.

Ferrite suppression components are widely used on printed circuit boards, power and data lines. If a ferrite suppression element is added to the inlet end of the power line of the printed board, high-frequency interference can be filtered out.

Ferrite rings or beads are specially designed to suppress high frequency interference and spike interference on signal lines and power lines, and it also has the ability to absorb electrostatic discharge pulse interference. Whether to use chip beads or chip inductors mainly depends on the actual application.

Chip inductors are required in resonant circuits. When it is necessary to eliminate unwanted EMI noise, the use of chip beads is the best choice.

Applications of Chip Beads and Chip Inductors

Who do you know best about the three major tools for eliminating electromagnetic interference?

Chip inductors: radio frequency (RF) and wireless communications, information technology equipment, radar detectors, automotive electronics, cellular phones, pagers, audio equipment, personal digital assistants (PDAs), wireless remote control systems, and low-voltage power supply modules.

Chip beads: clock generation circuits, filtering between analog circuits and digital circuits, I/O input/output internal connectors (such as serial ports, parallel ports, keyboards, mice, long distance telecommunications, local area networks), RF circuits and susceptible to Interfering logic equipment, filtering high-frequency conducted interference in power supply circuits, EMI noise suppression in computers, printers, video recorders (VCRS), TV systems and mobile phones.

The unit of the magnetic bead is ohm, because the unit of the magnetic bead is nominal according to the impedance it produces at a certain frequency, and the unit of impedance is also ohm.

The characteristic curve of frequency and impedance is generally provided on the DATASHEET of the magnetic beads. Generally, 100MHz is used as the standard. For example, when the frequency of 100MHz is used, the impedance of the magnetic beads is equivalent to 1000 ohms.

For the frequency band we want to filter, it is necessary to select the larger the impedance of the magnetic beads, the better, and the impedance above 600 ohms is usually selected.

In addition, when choosing magnetic beads, you need to pay attention to the flux of the magnetic beads. Generally, it needs to be derated by 80%. When using it in the power supply circuit, the influence of DC impedance on the voltage drop should be considered.

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