Three powerful tools for filtering, anti-interference and eliminating EMC: capacitors/inductors/magnetic beads

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

I believe that many engineers are still confused about the role of these three in the circuit. This article analyzes in detail the principles of the three major tools to eliminate EMC from the perspective of design.

1. Filter capacitor

Although capacitor resonance is undesirable from the perspective of filtering high frequency noise, capacitor resonance is not always harmful.

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

In actual engineering, the frequency of electromagnetic noise to be filtered is often as high as hundreds of MHz, or even more than 1GHz. Such high-frequency electromagnetic noise must be effectively filtered out using a through-hole capacitor.

Ordinary capacitors cannot effectively filter high-frequency noise for two reasons:

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

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

The reason why through-hole capacitors can effectively filter out high-frequency noise is that through-hole capacitors not only do not have the problem of lead inductance causing the capacitor resonant frequency to be too low.

Moreover, the through-hole capacitor can be directly installed on the metal panel, using the metal panel to achieve high-frequency isolation. However, when using the through-hole capacitor, the issue to pay attention to is the installation problem.

The biggest weakness of the through-hole capacitor is that it is afraid of high temperature and temperature shock, which causes great difficulties when welding the through-hole capacitor to the metal panel.

Many capacitors are damaged during the welding process. Especially when a large number of through-hole capacitors need to be installed on a panel, if only one is damaged, it is difficult to repair because when the damaged capacitor is removed, other adjacent capacitors will be damaged.

2. Common mode inductor

Since most of the problems that EMC faces are common-mode interference, common-mode inductors are also one of the powerful components we often use.

The common-mode inductor is a common-mode interference suppression device with ferrite as the core. It consists of two coils of the same size and number of turns symmetrically wound on the same ferrite ring core to form a four-terminal device. It has a large inductance to suppress common-mode signals, but has a very small leakage inductance that has almost no effect on differential-mode signals.

The principle is that when common-mode current flows through the two coils, the magnetic fluxes in the magnetic rings are superimposed on each other, resulting in a considerable inductance, which suppresses the common-mode current. When differential-mode current flows through the two coils, the magnetic fluxes in the magnetic rings cancel each other out, leaving almost no inductance, so the differential-mode current can pass without attenuation.

Therefore, common-mode inductors can effectively suppress common-mode interference signals in balanced lines without affecting the differential-mode signals normally transmitted on the lines.

The common mode inductor should meet the following requirements when it is manufactured:

(1) The wires wound on the coil core must be insulated from each other to ensure that there is no breakdown short circuit between the turns of the coil under the action of transient overvoltage;

(2) When a large instantaneous current flows through the coil, 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 transient overvoltage;

(4) The coil should be wound in a single layer as much as possible. This can reduce the parasitic capacitance of the coil and enhance the coil's ability to withstand transient overvoltage.

Usually, we should also pay attention to selecting the frequency band for filtering. The larger the common-mode impedance, the better. Therefore, we need to look at the device information when selecting common-mode inductors, and mainly choose based on the impedance-frequency curve.

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

3. Magnetic beads

In the EMC design process of digital circuits of products, we often use magnetic beads. Ferrite materials are iron-magnesium alloys or iron-nickel alloys. This material has a very high magnetic permeability. It can minimize the capacitance generated between the coil windings of the inductor under high frequency and high resistance conditions.

Ferrite materials are usually used in high frequency applications because at low frequencies they are mainly inductive, which results in very low losses on the line. At high frequencies, they are mainly reactive and change with frequency. In practical applications, ferrite materials are used as high frequency attenuators in RF circuits.

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

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

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

Do you know the 6 major differences between inductors and magnetic beads?

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

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, the resistance component increases, and the overall impedance increases. When high-frequency signals pass through the ferrite, the electromagnetic interference is absorbed and converted into heat energy and dissipated.

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

Ferrite magnetic rings or beads are specially used to suppress high-frequency interference and spike interference on signal lines and power lines. They also have the ability to absorb electrostatic discharge pulse interference. Whether to use chip magnetic beads or chip inductors depends mainly on the actual application occasion.

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

Applications of Chip Beads and Chip Inductors

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, etc.

Chip ferrite 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), between radio frequency circuits and susceptible logic devices, filtering out high-frequency conducted interference in power supply circuits, EMI noise suppression in computers, printers, video recorders (VCRS), television systems and mobile phones.

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

The DATASHEET of the magnetic bead generally provides a characteristic curve of frequency and impedance, which is generally based on 100MHz. For example, at a frequency of 100MHz, the impedance of the magnetic bead is equivalent to 1000 ohms.

For the frequency band we want to filter, we need to choose a magnetic bead with as large an impedance as possible, usually an impedance of 600 ohms or more.

In addition, when selecting ferrite beads, you need to pay attention to the flow rate of the beads. Generally, they need to be derated by 80%. When used in power supply circuits, the effect of DC impedance on voltage drop must be considered.

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