Analyze the main components that cause EMC to solve EMC problems

To solve EMC problems, it is necessary to understand the working principles of the main components that affect EMC . This article will introduce the working principles and uses of common-mode inductors, magnetic beads, and filter capacitors.

1. 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 . Common-mode inductors are common-mode interference suppression devices with ferrite as the core. They are composed of two coils of the same size and the same number of turns symmetrically wound on the same ferrite ring core to form a four-terminal device. They have a large inductance to suppress common-mode signals, but have a very small leakage inductance to have almost no effect on differential-mode signals. The principle is that when common-mode current flows through, the magnetic fluxes in the magnetic rings are superimposed on each other, so that there is 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, and there is 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, while having no effect on differential-mode signals transmitted normally 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 or 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 the frequency band of the required filtering. 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, when selecting, we should pay attention to the influence of differential mode impedance on the signal, mainly focusing on differential mode impedance, and pay special attention to high-speed ports .

In the process of EMC design of digital circuits of products , we often use magnetic beads. Ferrite materials are iron-magnesium alloys or iron-nickel alloys. This material has a high magnetic permeability. It can minimize the capacitance between the coil windings of the inductor under high frequency and high resistance conditions. Ferrite materials are usually used in high frequency conditions because at low frequencies they mainly show inductance characteristics, which makes the loss on the line very small. At high frequencies, they mainly show reactance characteristics 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, and 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 is resistive at high frequencies, which is equivalent to an inductor with a very low quality factor, so it can maintain a high impedance in a fairly wide frequency range, thereby improving the high-frequency filtering efficiency. 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 an inductor with low loss and high Q characteristics. 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 the resistance component. 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 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 beads or chip inductors depends mainly on the actual application. Chip inductors are needed in resonant circuits. When it is necessary to eliminate unwanted EMI noise, chip beads are the best choice. Applications of chip beads and chip inductors: Chip inductors: RF and wireless communications, information technology equipment, radar detectors, automotive electronics, cellular phones, pagers, audio equipment, PDAs (personal digital assistants), 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), between radio frequency (RF) circuits and susceptible logic devices, filtering high-frequency conducted interference in power supply circuits, computers, printers, video recorders (VCRS), TV systems, and EMI noise suppression in 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 magnetic beads 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 select a magnetic bead with the larger impedance, the better, usually choose an impedance of 600 ohms or more.

In addition, when selecting magnetic beads , you need to pay attention to the flow rate of the magnetic beads. Generally, it is necessary to reduce the rating by 80%. When used in power circuits, the effect of DC impedance on voltage drop must be considered.

Although the resonance of capacitors is undesirable from the perspective of filtering high-frequency noise, it is not always harmful. When the frequency of the noise to be filtered is determined, the capacitance of the capacitor can be adjusted so that the resonance point falls exactly on the disturbance 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 through-hole capacitors. There are two reasons why ordinary capacitors cannot effectively filter high-frequency noise. One reason is that the inductance of the capacitor leads causes capacitor resonance, presenting a large impedance to the high-frequency signal, weakening the bypass effect on the high-frequency signal; the other reason is that the parasitic capacitance between the wires causes the high-frequency signal 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's resonant frequency to be too low, but also through-hole capacitors can be directly installed on metal panels, using the metal panel to isolate high frequencies. However, when using through-hole capacitors, the problem to pay attention to is the installation problem. The biggest weakness of through-hole capacitors is that they are afraid of high temperatures and temperature shocks, which causes great difficulties when welding through-hole capacitors to metal panels. Many capacitors are damaged during the welding process. Especially when a large number of through-hole 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.