“Filter capacitors, common mode inductors, and magnetic beads are common figures in EMC design circuits, and they are also the three major weapons to eliminate electromagnetic interference.
Filter capacitors, common mode inductors, and magnetic beads are common figures in EMC design circuits, and they are also the three major weapons to eliminate electromagnetic interference.
Regarding 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.
01 filter capacitor
Although the resonance of the capacitor is undesirable from the perspective of filtering high-frequency noise, the resonance of the capacitor is not always harmful.
When the frequency of the noise to be filtered is determined, the capacity of the capacitor 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 hundreds of MHz, or even more than 1 GHz. For such high-frequency electromagnetic noise, through-core capacitors must be used to effectively filter out.
The reason why ordinary capacitors cannot effectively filter out high-frequency noise is due to two reasons:
(1) One reason is that the capacitor lead inductance causes the capacitor resonance, which presents a larger impedance to the high-frequency signal, which weakens the bypass effect of the high-frequency signal;
(2) Another reason is that the parasitic capacitance between the wires causes the high frequency signal to be coupled, which reduces the filtering effect.
The reason why the feedthrough capacitor can effectively filter out high-frequency noise is that the feedthrough capacitor not only has no lead inductance, which causes the capacitor’s resonance frequency to be too low.
Moreover, the through-core capacitor can be directly installed on the metal panel, and the metal Panel is used for high-frequency isolation. However, when using feedthrough capacitors, the problem to be noted is installation.
The biggest weakness of the through-core capacitor is that it is afraid of high temperature and temperature impact, which causes great difficulties when welding the through-core capacitor to the metal panel.
Many capacitors are damaged during the welding process. Especially when a large number of through-core 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, other nearby capacitors will be damaged.
02 Common Mode Inductance
Since most of the problems faced by EMC are common-mode interference, common-mode inductors are also one of our commonly used powerful components.
Common mode Inductor is a common mode interference suppression device with ferrite core. It consists of two coils of the same size and the same number of turns symmetrically wound on the same ferrite toroidal core to form a four-terminal The device should have a suppressive effect on the large inductance of the common mode signal, while the small leakage inductance for the differential mode signal has almost no effect.
The principle is that the magnetic flux in the magnetic ring superimposes each other when the common mode current flows, so that it has a considerable inductance and suppresses the common mode current. When the two coils flow through the differential mode current, the magnetic flux in the magnetic ring Passes cancel each other out, there is almost no inductance, so the differential mode current can pass without attenuation.
Therefore, the common mode inductance can effectively suppress the common mode interference signal in the balanced line, and has no effect on the differential mode signal normally transmitted by the line.
Common mode inductors should meet the following requirements during production:
(1) The wires wound on the coil core 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 the instantaneous large 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 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 ability of the coil to impart transient overvoltage.
Under normal circumstances, pay attention to selecting the frequency band to be filtered at the same time. 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 choosing, mainly pay attention to 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 a high magnetic permeability. It 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 usually used in high frequency situations, because they are mainly inductance characteristics at low frequencies, so that the line loss is very small. In the case of high frequency, they mainly show the reactance characteristic ratio and change 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 resistance and inductance. The resistance is short-circuited by the inductance at low frequencies, and the impedance of the inductance becomes quite high at high frequencies, so that all current flows through the resistance.
Ferrite is a consuming device on which high-frequency energy is converted into heat energy, which is determined by its resistance characteristics. Compared with ordinary inductors, ferrite beads have better high-frequency filtering characteristics.
Ferrite is resistive at high frequency, which is equivalent to an inductor with a very low quality factor, so it can maintain a high impedance in a relatively wide frequency range, thereby improving the efficiency of high-frequency filtering.
In the low frequency band, the impedance is composed of the inductive reactance of the inductance. At low frequencies, R is very small, and the permeability of the magnetic core is high, so the inductance is large. L plays the main role, and the electromagnetic interference is reflected and suppressed; and at this time, the magnetic The core loss is small, and the whole device is a low-loss, high-Q inductor. This inductance is easy to cause resonance. Therefore, in the low frequency range, the interference may increase after ferrite beads are used.
In the high frequency range, the impedance is composed of resistance components. As the frequency increases, the 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 to be dissipated.
Ferrite suppression components are widely used in printed circuit boards, power lines and data lines. For example, adding ferrite suppression components at the entrance of the power cord of the printed board can filter out high-frequency interference.
Ferrite magnetic ring or magnetic beads are specially used to suppress high frequency interference and spike interference on signal lines and power lines. It also has the ability to absorb electrostatic discharge pulse interference. Whether to use chip magnetic beads or chip inductors mainly depends on the actual application.
Chip inductors are required in the resonant circuit. When it is necessary to eliminate unnecessary EMI noise, using chip magnetic beads is the best choice.
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.
Chip beads: Clock generation circuit, filtering between analog circuit and digital circuit, I/O input/output internal connectors (such as serial port, parallel port, keyboard, mouse, long-distance telecommunications, local area network), radio frequency circuit and vulnerable Between interfering logic devices, the power supply circuit filters out high-frequency conduction interference, and suppresses EMI noise 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 generates at a certain frequency, and the unit of impedance is also ohm.
The DATASHEET of the magnetic beads generally provides frequency and impedance characteristic curves, which are generally based on 100MHz. For example, the impedance of the magnetic beads is equivalent to 1000 ohms at a frequency of 100MHz.
For the frequency band we want to filter, we need to select the larger the magnetic bead impedance, the better, usually the impedance above 600 ohms.
In addition, you need to pay attention to the flux of the magnetic beads when choosing the magnetic beads. Generally, you need to derate by 80%. When used in the power circuit, you should consider the impact of DC impedance on the voltage drop.
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