Common mode filter

kelven123

What is a Common Mode Choke?
A common mode choke is an electrical filter that blocks highfrequency noise common to two or more data or power lines while allowingthe desired DC or low-frequency signal to pass. Common mode (CM) noisecurrent is typically radiated from sources such as unwanted radiosignals, unshielded electronics, inverters and motors. Left unfiltered,this noise presents interference problems in electronics and electricalcircuits.
How do Common Mode Chokes Work?
In normal or differential mode (single choke), current travels on oneline in one direction from the source to the load, and in the oppositedirection on the return line that completes the circuit. In common mode,the noise current travels on both lines in the same direction.
Common mode chokes have the two or more windings arranged such thatthe common mode current creates a magnetic field that opposes anyincrease in common mode current. This is similar to how single line(differential) inductors function. Inductors create magnetic fields thatoppose changes in current.
In common mode, the current in a group of lines travels in the samedirection so the combined magnetic flux adds to create an opposing fieldto block the noise, as illustrated by the red and green arrows in thetoroid core shown in Figure 1. In differential mode, the current travelsin opposite directions and the flux subtracts or cancels out so thatthe field does not oppose the normal mode signal.
How do I Choose a Common Mode Choke?
The main criteria for selecting a common mode choke are:

[li]Required impedance: How much attenuation of noise is needed?[/li][li]Required frequency range: Over what frequency bandwidth is the noise?[/li][li]Required current-handling: How much differential mode current must it handle?[/li]

High-Speed and Super-Speed Data Line EMI Chokes
USB, high-speed and Super-Speed data line common mode chokeseffectively reduce common mode noise in high-speed interfaces like USB2.0, USB 3.1 Gen 1, HDMI, IEEE 1394, LVDS, HDBaseTTM, MOST® bus, etc.They maintain excellent signal integrity for high-speed communicationswith -3 dB differential mode cutoff frequencies up to 6.5 GHz. Mostprovide greater than 30 dB common mode attenuation at 500 MHz and 25 dBin the GHz band.
Data Line Common Mode EMI Chokes
Surface mount data line common mode chokes are designed to attenuatecommon mode noise up to 100 MHz. The PDLF Series can reduce noise by afactor of 32 from 15 MHz to 300 MHz and are available in 2, 3 and 4 lineversions. The PTRF Series is optimized for FCC and ITU-T (formerlyCCITT) requirements. These parts provide 15 to 25 dB attenuation,greater than 1000 Ohms impedance and 1500 V isolation between windings.M2022 can suppress common mode noise up to 500 MHz in a compact 1812package.
Data/Power Line Common Mode EMI Chokes
LPD, MSD and PFD Families are low profile, miniature footprint commonmode chokes that can be used to attenuate common mode noise ordifferential mode noise in both data and power line applications.
Surface Mount Power Line Common Mode EMI Chokes
Low-cost, high performance surface mount power line common modechokes come in a variety of sizes and packages. They are designed toeliminate AC line-conducted common mode noise across a broad range offrequencies, with up to 1500 Vrms isolation. These common mode chokescan operate over a wide range of current from 0.06 Amps to 15 Amps,providing attenuation where line filtering is needed, such as inswitch-mode power supplies.
Through-Hole Power Line Common Mode EMI Chokes
Low-cost through-hole BU Series high efficiency choke coils aredesigned to eliminate line conducted common mode noise across a broadrange of frequencies. The BU9S and BU9HS are ideal for signal lineapplications; the other BUs can be used in switching power supplies andpower supply circuits. For low profile applications, the BU9 and BU9Sfilters are available in a horizontal configuration that reduces theirheight to under half an inch (12.5 mm).
CMT Common Mode EMI Chokes
CMT toroid style common mode chokes are designed to provide thehighest common mode impedance over the widest frequency range. Theseparts are ideal for any application requiring a high DC current bias andare well suited for use in switch-mode power supplies. These commonmode chokes are most effective in filtering supply and return conductorswith in-phase signals of equal amplitude. Differential mode inductorsare available for filtering out-ofphase or uneven amplitude signals.

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wangyitu

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luchonghui74

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