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EMC design must be mentioned in the three major tools are, many novices are not very clear about this, in order to help you solve this problem, the following editor will explain in detail for you, people with this need can come to learn, I hope you can gain something.

Electromagnetic compatibility (EMC) refers to the ability of equipment or system to meet the requirements in its electromagnetic environment and not cause unbearable electromagnetic interference to any equipment in its environment. Therefore, EMC includes two requirements: on the one hand, it means that the electromagnetic interference caused by the equipment to the environment can not exceed a certain limit; on the other hand, it means that the apparatus has a certain degree of immunity to the electromagnetic interference in the environment, that is, electromagnetic sensitivity. The equivalent circuit of magnetic beads is a DCR resistor in series, an inductor in parallel, a capacitor and a resistor.

DCR is a constant value, but the last three components are all functions of frequency, that is, their reactance, capacitance and impedance vary with frequency, of course, their resistance, inductance and capacity are all very small. Filter capacitors, common-mode inductors and magnetic beads are common figures in EMC design circuits, and they are also the three sharp tools to eliminate electromagnetic interference.

Filter Capacitor of three sharp weapons

Although the resonance of capacitors is undesirable from the point of view of filtering high frequency noise, the resonance of capacitors is not always harmful. When the noise frequency to be filtered is determined, the resonance point just falls on the disturbance frequency by adjusting the capacity of the capacitor.

In practical engineering, the frequency of electromagnetic noise to be filtered is often as high as hundreds of MHz, even more than 1GHz. For such high-frequency electromagnetic noise, heart-piercing capacitance must be used to filter out effectively. The reason why the ordinary capacitor can not effectively filter the high frequency noise is due to two reasons, one is the capacitance resonance caused by the capacitance lead inductance, which presents a large impedance to the high frequency signal and weakens the bypass effect to the high frequency signal; another reason is that the parasitic capacitance between the wires couples the high frequency signal and reduces the filtering effect.

The reason why the core-piercing capacitor can effectively filter out the high-frequency noise is that not only the core-piercing capacitor has no lead inductance, but also the resonant frequency of the capacitor is too low, and the core-piercing capacitor can be installed directly on the metal panel. the metal panel is used to isolate the high frequency.

However, when using the core-piercing capacitor, the problem that should be paid attention to is the installation. The biggest weakness of the core-piercing capacitor is the fear of high temperature and temperature shock, which is very difficult to weld the core-piercing capacitor to the metal panel. Many capacitors are damaged during welding. Especially when a large number of core-piercing capacitors need to be installed on the panel, as long as there is a damage, it is difficult to repair, because when the damaged capacitor is removed, it will cause damage to other adjacent capacitors.

Common mode inductors of three sharp weapons

In the EMC design process of digital circuit, we often use magnetic beads, ferrite material is Fe-mg alloy or Fe-Ni alloy, this material has high magnetic conductivity, it can be inductor coil winding in the case of high frequency and high resistance to produce the smallest capacitance.

Ferrite materials are usually used at high frequency because of their main inductance characteristics at low frequency, which makes the on-line loss very small. In the case of high frequency, they mainly show reactance characteristic ratio 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 the parallel connection of resistors and inductors, the resistance is short-circuited by the inductor at low frequency, and the inductance impedance becomes so high at high frequency that the current passes through the resistor. Ferrite is a consumption device on which high-frequency energy is converted into thermal energy, which is determined by its resistance characteristics.

Ferrite beads have better high-frequency filtering characteristics than ordinary inductors. Ferrite is resistive at high frequency, which is equivalent to the inductor with very low quality factor, so it can maintain high impedance in a wide frequency range, thus improving the efficiency of high frequency filtering.

In the low frequency band, the impedance is composed of the inductance of the inductor, the R is very small at low frequency, and the permeability of the magnetic core is high, so the inductance is larger, L plays the main role, and the electromagnetic interference is reflected and suppressed; and the loss of the magnetic core is small, the whole device is an inductor with low loss and high Q characteristics, this inductance is easy to cause resonance, so in the low frequency band, sometimes the interference may be enhanced after the use of ferrite beads.

In the high frequency band, the impedance is composed of resistance components. With the increase of frequency, the permeability of the core decreases, resulting in a decrease in inductance and inductance components. However, 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 dissipated in the form of thermal energy.

Ferrite suppression elements are widely used in printed circuit boards, power lines and data lines. If a ferrite suppression element is added to the entrance of the power line of the printed board, the high frequency interference can be filtered. Ferrite rings or beads are specially used to suppress high-frequency interference and spike interference on signal lines and power lines, and they also have the ability to absorb electrostatic discharge pulse interference.

The use of chip magnetic beads or chip inductors mainly lies in practical applications. Chip inductors are needed in resonant circuits. When you need to eliminate unwanted EMI noise, the use of chip magnetic beads is the best choice. Applications of chip magnetic beads and chip inductors:

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

Chip beads: filtering between clock generating circuit, analog circuit and digital circuit, I / O input / output internal connectors (such as serial port, parallel port, keyboard, mouse, long distance telecommunication, local local area network), radio frequency (RF) circuit and vulnerable logic device, filtering out high frequency conducted interference in power supply circuit, computer, printer, video recorder (VCRS) EMI noise suppression in TV systems and mobile phones.

The unit of a magnetic bead is an ohm, because the unit of a magnetic bead is defined according to the impedance it produces at a certain frequency, and the unit of impedance is also the ohms. The characteristic curves of frequency and impedance are generally provided on the DATASHEET of magnetic beads, which are generally based on 100MHz. For example, the impedance of magnetic beads is equivalent to 1000 ohms at 100MHz frequency. For the frequency band we want to filter, we need to choose the larger the magnetic bead impedance, the better, usually choose the impedance above 600 ohms.

In addition, when selecting magnetic beads, we need to pay attention to the flux of magnetic beads, which generally needs to be reduced by 80%. When used in the power supply circuit, we should consider the effect of DC impedance on the voltage drop.

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