Magnetism bead (Ferrite Bead) equivalent circuit is series connection of resistor of a DCR an inductance is shunt-wound a capacitance and a resistor. DCR is constant value, but from the back the function that 3 component are frequency, that is to say their inductance, capacitive reactance and impedance can change as the change of frequency, of course their block value, feeling value and look value are very little.

 

 

Can see from inside equivalent circuit, when frequency under FL (LC syntonic frequency) when, magnetism bead presents inductance character; When frequency is equal to FL, magnetism bead is a pure resistor, right now the impedance of magnetism bead (Impedance) the biggest; When frequency of resonance of frequency prep above nods FL, magnetism bead presents capacitance character. The impedance that the principle that EMI chooses magnetism bead is magnetism bead is in in EMI noise frequency the biggest. For instance if the maximum of EMI noise is in 200MHz, when you choose, that is about to see the characteristic curve of magnetism bead, the maximum of its impedance should be controlled in 200MHz. Next graphs are the actual characteristic graph of a magnetism bead. Everybody can see the peak value of this magnetism bead points out to be controlled in 1GHz now. When the peak is nodded, impedance (Z) curvilinear value and resistor (R) equal. That is to say this magnetism bead is when 1GHz, it is a pure resistor, and impedance value is the greatest.

 

 

 

In front introduced EMI simply the basic and characteristic curve of magnetism bead. From magnetism bead in light of impedance curve, actually its character can be used namely make filter of high frequency signal. Those who need an attention is, the magnetism that the manufacturer that normally everybody sees offers bead impedance curve, it is to be in without slant the curve that the test below circumstance of buy electric current gets. But for the most part the EMI noise that magnetism bead is put on power supply cord to use filter to remove power source normally. And in have slant below the circumstance of buy electric current, the characteristic meeting of magnetism bead produces a few change. A certain 0805 dimension are below the magnetism of 500mA of rated electric current bead slant in what differ the impedance curve below buy electric current. Everybody can see, as voltaic addition, the peak value impedance of magnetism bead is met decrescent, the frequency that at the same time impedance peak value nods also can become tall.

 

 

Before the character that elaborates magnetism bead further, let us see the definition of the major attribute index of magnetism bead first: Z (impedance, impedance Ohm) : ? Fat Lv identifies busy Bian to divide  of  of the sex that promote  to mire M blames  ? the function that it is frequency. Normally everybody uses magnetism bead to regard magnetism bead impedance as the value in the impedance value when 100MHz. DCR (ohm) : Of magnetism bead conductor dc block. Rated electric current: Should magnetism bead is installed at pressworking circuit board is joined constant electric current, oneself temperature rise is worth by the electric current when room temperature ascendant 40C. So EMI magnetism bead have plant by tens of thousands, impedance curve also each are not identical, should we choose appropriate magnetism like the actual application according to us bead?

 

 

Let us see impedance value be 600ohm@100MHz together above all but the magnetism with dimension different measurement bead in different slant the character below electric current of buy electric current and working frequency.

 

 

 

Let us look again below the magnetism of feature of two different curves bead A is mixed the circumstance when magnetism bead B applies at line.

 

 

Magnetism bead A is mixed the impedance peak value of magnetism bead B is between 100MHz and 200MHz, but curve of frequency of impedance of magnetism bead A is flatter, magnetism bead B is abrupter.

 

We put two magnetism bead on the line of the 20MHz that be as follows respectively, look to be able to produce what kind of effect to signal output.

 

Weaveform checks a place

 

 

The undee graph that measures magnetism bead to output end respectively with oscillograph is below

 

 

From the point of output weaveform, the output of magnetism bead B is undee and lack fidelity should be less than magnetism bead A apparently.

 

The impedance frequency weaveform that the reason is magnetism bead B is abrupter, its impedance is taller when 200MHz, only right the attenuation of the signal near 200MHz is bigger, but effect of very broad to spectrum square wave weaveform is lesser. And the impedance frequency character of magnetism bead A is evener, its are wider also to the attenuation spectrum of signal, because this is right the undee impact of square wave is bigger also.

 

The EMI that correspondence of afore-mentioned 3 kinds of circumstances are below checks a result. The result is magnetism bead A is mixed magnetism bead B is met the attenuation with pair of EMI noise very large generation. It is a bit good that magnetism bead A wants in the attenuation inside limits of whole EMI spectrum at magnetism bead B.

 

 

Accordingly, be in specific when choosing magnetism bead, evenness of impedance frequency character magnetism bead A suits to apply at power supply cord quite, and the magnetism with frequency abrupter character bead B suits to apply at line relatively. Magnetism bead B is applying at line, can fall in the condition that carries signal integrality as far as possible, produce the biggest attenuation to the noise near EMI frequency only as far as possible.

 

 

EMC magnetism bead application restrains EMI in electronic circuitry, basically have two kinds of application: 1. The commonnest use at power supply cord. 2. Use at line to resemble frequency, video line. How should be that planted from ten million according to actual application does the EMC of different character choose appropriate magnetism in magnetism bead bead is the system that is used at oneself designed?

 

Forward face place is narrated, if want to choose magnetism bead to be used at power supply cord, should do how to choose?

 

Above all, want to know the working frequency of switch power source. Normally most switch power source works at hundreds of KHz, of a few can work a few MHz. This frequency basically is to be in conduct radiant frequency limits. To initiative will tell at 30MHz radiate frequency, belong to the limits of low frequency. Will tell commonly, the radiation EMI noise that power source produces, be in normally be less than 100MHz-300MHz limits. Accordingly, choose magnetism bead to use choose peak value frequency to be less than 300MHz low frequency model magnetism bead.

 

Next, want to know the working electric current of power source namely. The magnetism that puts park switch or blame dc signal to what bead, want to tell communication signal to change virtual value normally, will choose the rated electric current of magnetism bead with this.

 

To be being used at power supply cord magnetism bead dimension, resemble what tell before us, in contented composing space the design asks the circumstance falls, the magnetism that should choose large size as far as possible bead.

 

The magnetism that is used at power supply cord bead, DCR is very crucial parameter, especially the portable equipment to batteries power supply, resemble a mobile telephone, flat computer. Should choose the magnetism with small DCR as far as possible bead use at power supply cord, in order to improve power source efficiency.

 

Of course, will tell from the angle that checks EMI, the peak value impedance of magnetism bead has been jumped over higher. But normally, the impedance of magnetism bead and DCR concern into inverse ratio. Need oh the applied circumstance with factual basis, one eclectic choice is made between DCR and impedance.

 

Finally, resemble in front what tell, the impedance curve of magnetism bead wants as far as possible evenness, divide the noise of Gao Cixie wave of power source with the filter of utmost.

 

 

If magnetism bead is used at line, how should that choose the sort of magnetism bead? Above all, we should know magnetism bead wants to be used at why be being planted line, it is frequency for instance, video is other still. This should know the working frequency of signal that is to say. In principle, the frequency of impedance peak value of magnetism bead should at least the effective bandwidth of prep above signal, can affect influence signal integrality otherwise, affect regular job of the system thereby. Although to resembling the low frequency signal of frequency and so on, because frequency signal is normally,come by frequency decoder decipher, its EMI noise is the clock frequency harmonic of a few MHz of frequency decoder normally.

 

Accordingly, even if the frequency signal of low frequency, its EMI noise also can be to a few be as high as normally even the high frequency noise of hundreds of MHz. Next, want to know signal electric current. To most signal, resemble video, RS232, it is signal merely just, do not have too big electric current output, because this does not need normally,consider the rated electric current of magnetism bead. But to frequency signal, have power output normally, right now the choice of magnetism bead is about to consider to export electricity. The magnetism that should come to frequency signal convert choose proper and rated electric current into virtual value right now bead.

 

Peak value impedance should choose to be near the frequency dot of problem of likelihood occurrence EMI. The magnetism that is used at high speed signal bead should notice impedance matchs, the magnetism that is used at video line for instance bead impedance wants to be controlled in 50 ohm in 100MHz left and right sides. The magnetism that is used at line bead, do not need to consider magnetism bead DCR normally, it is good that the dimension of magnetism bead wants Yue Xiaoyue. Want with respect to the impedance curve that is magnetism bead finally as far as possible abrupt, lest affect signal integrality. Small magnetism bead, the university asks! I wish article of these a few clumsy can give wide readership some helps.