How Filter Capacitors and Common Mode Inductors and Beads Eliminate Electromagnetic Interference

Filter capacitors, common mode inductors, and magnetic beads are common in EMC design circuits and are the three major tools to eliminate electromagnetic interference. For the role of these three in the circuit, I believe that many engineers are not clear. In this paper, from the design and design, the principle of eliminating the three major weapons of EMC is analyzed in detail.

Three large filter capacitors
Although the resonance of the capacitor is undesirable from the viewpoint of filtering out high frequency noise, the resonance of the capacitor is not always harmful. When the noise frequency to be filtered is determined, the capacitance can be adjusted to make the resonance point just fall on the disturbance frequency.

In actual engineering, the frequency of electromagnetic noise to be filtered is often as high as several hundred MHz or even more than 1 GHz. For such high frequency electromagnetic noise, a feedthrough capacitor must be used to effectively filter out. The reason why ordinary capacitors cannot effectively filter out high-frequency noise is because of two reasons. One reason is that the capacitor lead inductance causes capacitance resonance, which has a large impedance to high-frequency signals and weakens the bypass effect on high-frequency signals. Another reason is that the parasitic capacitance between the wires couples the high frequency signals, reducing the filtering effect.

The reason why the core capacitor can effectively filter out the high frequency noise is because the core capacitor not only has no problem that the capacitor resonance frequency is too low due to the lead inductance, but also the core capacitor can be directly mounted on the metal panel, and the metal panel is used to be high. The role of frequency isolation. However, when using a feedthrough capacitor, the problem to be aware of is the installation problem. The biggest weakness of the feedthrough capacitor is the fear of high temperature and temperature shock, which causes great difficulty in soldering the feedthrough capacitor to the metal panel. Many capacitors are damaged during the soldering process. Especially when a large number of feedthrough capacitors need to be mounted on the panel, as long as there is a damage, it is difficult to repair, because when the damaged capacitor is removed, damage to other capacitors is caused.

Common mode inductance of the three sharp weapons
Since the problem facing EMC is mostly common mode interference, common mode inductor is also one of the commonly used powerful components. The common mode inductor is a common mode interference suppression device with ferrite as the core. It is made up of two identical sizes. The coils with the same number of turns are symmetrically wound on the same ferrite toroidal core to form a four-terminal device, which has a suppressing effect on the common mode signal and a small inductance. Leakage inductance hardly works. The principle is that the magnetic fluxes in the magnetic ring are superimposed on each other when the common mode current flows, so that the inductance is relatively large, and the common mode current is suppressed, and when the two coils flow through the differential mode current, the magnetic in the magnetic ring By canceling each other, there is almost no inductance, so the differential mode current can pass without attenuation. Therefore, the common mode inductor can effectively suppress the common mode interference signal in the balanced line, and has no effect on the differential mode signal that the line normally transmits.

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

1) The wires wound on the coil core should be insulated from each other to ensure that there is no breakdown short circuit between the turns of the coil under the action of instantaneous overvoltage.
2) When the coil flows through a large instantaneous current, the core does not appear to be saturated.
3) The core in the coil should be insulated from the coil to prevent breakdown between the two under transient overvoltage.
4) The coil should be wound as much as possible, which can reduce the parasitic capacitance of the coil and enhance the ability of the coil to instantaneous overvoltage.

Under normal circumstances, at the same time pay attention to select the frequency band required for 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, pay attention to the influence of differential mode impedance on the signal, mainly focusing on differential mode impedance, paying special attention to high-speed ports.

Three sharp weapon beads
In the product digital circuit EMC design process, we often use magnetic beads, the ferrite material is iron-magnesium alloy or iron-nickel alloy, this material has a high magnetic permeability, he can be 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 often used at high frequencies because their main inductance characteristics at low frequencies make the losses on the line small. At high frequencies, they mainly exhibit a reactance characteristic ratio and change with frequency. In practical applications, ferrite materials are used as high frequency attenuators for RF circuits. In fact, the ferrite is preferably equivalent to the parallel connection of the resistor and the inductor. The resistor at the low frequency is short-circuited by the inductor, and the impedance of the inductor at a high frequency becomes so high that the current passes through the resistor. Ferrite is a consumer device in which high-frequency energy is converted into thermal energy, which is determined by its electrical resistance characteristics.

Ferrite beads have better high frequency filtering characteristics than ordinary inductors. Ferrite exhibits electrical resistance at high frequencies, which is equivalent to an inductor with a low quality factor, so it can maintain a high impedance over a relatively wide frequency range, thereby improving high-frequency filtering performance. In the low frequency band, the impedance is composed of the inductive reactance of the inductor. R is small at low frequencies, and the magnetic permeability of the magnetic core is high. Therefore, the inductance is large, L plays a major role, and electromagnetic interference is reflected and suppressed; and magnetic The loss of the core is small, and the whole device is a low-loss, high-Q inductor. This kind of inductance is easy to cause resonance. Therefore, in the low frequency range, the interference enhancement after the use of the ferrite bead may sometimes occur. In the high frequency band, the impedance is composed of a 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 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 to be dissipated.

Ferrite suppression components are widely used in printed circuit boards, power lines, and data lines. High frequency interference can be filtered out by adding a ferrite suppression component to the input end of the power strip of the printed board. Ferrite magnetic rings or magnetic beads are designed 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.

The use of chip beads or chip inductors is mainly in practical applications. A chip inductor is required in the resonant circuit. The use of chip beads is the best choice when eliminating unwanted EMI noise. Chip Beads and Chip Inductors Applications: Chip Inductors: Radio Frequency (RF) and Wireless Communications, Information Technology Equipment, Radar Detectors, Automotive Electronics, Cellular Phones, Pagers, Audio Equipment, PDAs (Personal Digital Assistants), Wireless remote control system and low voltage power supply module. Chip Beads: Clock generation circuit, filtering between analog circuit and digital circuit, I/O input/output internal connector (such as serial port, parallel port, keyboard, mouse, long distance telecommunication, local area network), radio frequency (RF) circuit Between high-frequency conducted interference in the power supply circuit and EMI noise suppression in computers, printers, video recorders (VCRS), television systems, and mobile phones, and interference-prone logic devices.

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 magnetic field DATASHEET generally provides a characteristic curve of frequency and impedance, generally based on 100MHz. For example, the impedance of the magnetic bead is equivalent to 1000 ohms at a frequency of 100MHz. For the frequency band we want to filter, the larger the impedance of the bead is, the better. Generally, the impedance above 600 ohms is selected.

In addition, when selecting the magnetic beads, it is necessary to pay attention to the flux flow of the magnetic beads. Generally, 80% of the derating is required. When using the power supply circuit, the influence of the DC impedance on the voltage drop should be considered.