Introduction to magnetic ring selection and EMC rectification skills

Hello everyone , I'm brother snail . Today, I will share with you the knowledge related to the selection and application of magnetic rings , I hope it helps you .

This paper will elaborate the magnetic ring from the following four aspects .

One 、 Application scenario of magnetic ring

First, let's look at some pictures

chart 1 display VGA Line

chart 2 Adapter cable

chart 3 USB Communication lines

These three lines are common power supply lines or communication lines in our life , They all have one thing , There is a prominent part on the connecting line , What is the outstanding part ？ There is no doubt that this is the added magnetic ring .

Magnetic ring is a commonly used anti-interference component in electronic products , It has a good inhibitory effect on high-frequency noise . Generally, ferrite materials are used （Mn-Zn） Made from .

The magnetic ring has different impedance characteristics at different frequencies , Generally, the impedance is very small at low frequency , When the signal frequency increases , The impedance of the magnetic ring rises sharply , stay EMC In engineering design , The magnetic ring is widely used because of its remarkable function .

Two 、 How the magnetic ring works

chart 4 Equivalent circuit of magnetic ring

Pictured 4, Equivalent circuit of magnetic ring in application .L Is the equivalent inductance ,R Is the equivalent DC impedance of the cable ,C Distributed capacitance generated between windings , Pay special attention to this distributed capacitance , It will reduce the performance of high-frequency filtering .

chart 5 Impedance curve of magnetic ring

Pictured 5, The magnetic ring is unsaturated , The higher the signal frequency , The higher the corresponding impedance , When the frequency exceeds the resonance point , The impedance will show a downward trend .

chart 6 EMC Rectify the commonly used buckle magnetic ring

The biggest difference between the button magnetic ring and ferrite is that it has great loss , Inductance made of this kind of button magnetic ring , Its characteristics are closer to resistance . It is a resistance whose value increases with the increase of frequency , When the high-frequency signal passes through the ferrite magnetic ring , Electromagnetic energy is dissipated in the form of heat .

3、 ... and 、 Classification of magnetic rings

(1) Ferrite magnetic ring

Generally, manganese zinc ring is painted green .

Ferrite magnetic rings mainly include nickel zinc ferrite magnetic rings and manganese zinc ferrite magnetic rings , Classified by permeability ：

The permeability of nickel zinc ferrite is 100-1000 Between , It is called low permeability magnetic ring .

The permeability of Mn Zn ferrite magnetic ring material is generally 1000 above , It is called high permeability magnetic ring .

chart 7 Manganese zinc ferrite high conductivity magnetic ring

Nickel zinc ferrite magnetic rings are generally used for various wire rods , Circuit board end , Anti interference in computer equipment ;

Manganese zinc ferrite magnetic ring , The permeability is very high , This magnetic ring , It is usually used to wind common mode inductors , Suppress low-frequency common mode conducted interference of power interface .

chart 8 Common mode inductance

The common mode inductance suppression frequency band is 500K-30M Between , The filter frequency band is higher than the iron powder core differential mode inductance .

Usually , The lower the permeability of the material , The wider the applicable frequency range ; The higher the permeability of the material , The narrower the applicable frequency range .

(2) Iron powder core magnetic ring

Iron powder core ring uses two colors to distinguish the material , Commonly used -2( red / transparent )、-8( yellow / red )、-18( green / red )、-26( yellow / white ) And -52( green / blue )        

chart 9  Iron powder core magnetic ring

The iron powder core magnetic ring is composed of carbon based iron magnetic powder and resin carbon based iron magnetic powder , The permeability is very low . There is an air gap between the magnetic particle and the insulating material , The general permeability is 20-100 Between . Because the permeability of iron powder core magnetic ring is very low , It is not easy to saturate under the condition of high differential mode current , therefore , Iron powder core magnetic ring winding is often used to make differential mode inductors .

chart 10  Differential mode inductance

Iron powder core differential mode inductance , The filtering frequency band is very low , Several hundred KHz, Suppress the conducted differential mode interference of the power line .

Iron powder core is mainly used in electrical circuits to solve electromagnetic compatibility （EMC） problem . In practice , Various other substances will be added according to different filtering requirements under different bands .

(3) Iron silicon aluminum magnetic ring

Iron, silicon and aluminum are generally all black .

Iron silicon aluminum magnetic ring is one of the magnetic rings with high usage , Simply speaking , Iron silicon aluminum is made of aluminum - silicon - Iron composition , Have a fairly high Bmax(Bmax Is the average maximum magnetic flux density on the cross-sectional area of the magnetic core .), Its core loss is much lower than that of iron powder core and high magnetic flux , There is low magnetostriction （ Low noise ）, It is a low-cost energy storage material , No thermal aging , It can be used to replace iron powder core , The performance is very stable at high temperature .

chart 11 Iron silicon aluminum magnetic ring

The main feature of iron silicon aluminum is that it has lower loss than iron powder core , Have good DC Bias current characteristics . The price is not the highest , Nor is it the lowest , Compared with iron powder core and iron nickel molybdenum .

Iron silicon aluminum magnetic powder core has excellent magnetic properties , Low power loss , High magnetic flux density , stay -55C~+125C When used within the temperature range , With temperature resistance 、 Moisture resistance 、 Vibration resistance and other high reliability ;

meanwhile ,60~160 Wide permeability range available . It is the switching power supply output choke 、PFC The best choice of inductance and resonant inductance , With high cost performance .

(4) Amorphous magnetic ring

Amorphous magnetic ring is a new product , At present, it is gradually popularized .

Amorphous magnetic ring , Generally, most of them are white and black . It has a distinctive feature : The shell is plastic . So it's easy to judge , Because amorphous magnetic rings are wound , It must be wrapped and protected with a plastic shell , Otherwise, it will become debris .

Compared with manganese zinc ferrite magnetic ring , Amorphous magnetic rings have higher permeability , Usually 10 many K Even a few hundred K, The permeability is very high .

chart 12  Amorphous magnetic ring

Amorphous magnetic rings are usually used to wind common mode inductors , Suppress low-frequency conducted interference , Compared with manganese zinc ferrite , Amorphous magnetic rings are expensive , But the permeability is high , The inductance can be made smaller , in addition , The filtering effect is also better than manganese zinc .

It is said that it can be filtered out to dozens MHz, It is close to Mn Zn ferrite magnetic ring . So now in the filter , Amorphous magnetic rings are also used as common mode inductors ！

Four 、EMC Add magnetic ring rectification skills

Hardware notebook

1、 Appearance selection “ As long as possible 、 As thick as possible 、 The inner diameter should be as small as possible ” Magnetic ring . That is, the longer the magnetic ring, the better , The tighter the aperture and the cable it passes through, the better . But in the case of DC or AC bias , There is also the problem of ferrite saturation , The larger the cross-sectional area of the suppression element , The less saturated it is .

chart 13  Magnetic rings with different inner diameters

Hardware notebook

2、 The effect of the magnetic ring on the conditional reflection of electromagnetic waves , Thus reducing the distortion of signal transmission . The position of the magnetic ring sleeve should be as close as possible to the end of the source （ Import and export of cable ）, It will more effectively suppress electromagnetic radiation .

chart 14  The magnetic ring is sleeved on the source end

Hardware notebook

3、 When suppressing high-frequency interference , Nickel zinc ferrite should be selected , Manganese zinc ferrite is used to suppress low-frequency interference . Because the permeability of manganese zinc ferrite ranges from thousands to tens of thousands , Nickel zinc ferrite is hundreds to thousands , The higher the permeability of magnetic ring ferrite , The greater the impedance at low frequency , The impedance at high frequency is smaller .

Hardware notebook

4、 How to avoid magnetic ring saturation ？ When a large current flows through a wire passing through ferrite , Easy to cause saturation , Reduce the performance of components . Avoid this , Connect the two wires of the power supply （ Plus or minus ） Pass through a magnetic ring at the same time .

chart 15  Two wires of the power supply （ Plus or minus ） Pass through a magnetic ring at the same time

Hardware notebook

5、 Low frequency interference , It is recommended to wind the cable 2—3 Turn , On the one hand, it can improve the area through the ring , Increase the equivalent absorption length , On the other hand, make full use of the hysteresis characteristics of the magnetic ring , Improve low frequency characteristics .                                 High frequency interference , No winding （ Because there is parasitic capacitance on the actual magnetic ring , This parasitic capacitance is connected in parallel with the inductance , But when encountering high-frequency interference signal , This parasitic capacitance will short circuit the inductance of the magnetic ring and lose its function .） At this time, you can choose a longer magnetic ring .

chart 16  The magnetic ring is wound

Hardware notebook

6、 Turns winding tips ： Theoretically, the more turns, the better the effect of suppressing low-frequency interference , However, due to the increase of parasitic capacitance , The effect of suppressing high-frequency noise is weak （ Blindly increasing the number of turns to increase the attenuation is a common mistake ）.                                                                                      Practical application , The number of turns needs to be adjusted according to the signal interference frequency . When the frequency band of interference frequency is wide , Two magnetic rings can be sleeved on the cable , Each magnetic ring is wound with a different number of turns , This can suppress both high-frequency interference and low-frequency interference . Nickel zinc and manganese zinc ferrites can also be sleeved at the same time , The interference frequency band suppressed in this way is wide .

Hardware notebook

7、 The magnetic ring is fragile , Therefore, good fixation is required during installation , Avoid the magnetic ring cracking caused by collision during transportation , We usually fix it with ties .

chart 17  Fix the tie （ Then fix the magnetic ring on the equipment ）

Last , The magnetic ring is just EMC Common components in rectification , Used to find the problem , Use only when necessary . Try to add capacitance and inductance in the design , Eliminate interference from the source . It's best not to add anything .

Recommend a classic EMC Information

《 Zheng Junqi EMC( Electromagnetic compatibility ) Design and test case analysis 》,

The background to reply ‘EMC’, Can get PDF writing File download link .

Today's sharing is here , I hope it helps you .

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