Common Mode Filter Design Guide - Coilcraft, Inc.

1 Coilcraft, Inc. 2007 Common Mode Filter Design GuideIntroductionThe selection of component values for Common modefilters need not be a difficult and confusing process. Theuse of standard Filter alignments can be utilized to achievea relatively simple and straightforward Design process,though such alignments may readily be modified to utilizepre-defined component filters prevent excessive noise from being conductedbetween electronic equipment and the AC line; generally,the emphasis is on protecting the AC line. Figure 1 showsthe use of a Common mode Filter between the AC line (viaimpedance matching circuitry) and a (noisy) power con-verter. The direction of Common mode noise (noise on bothlines occurring simultaneously referred to earth ground) isfrom the load and into the Filter , where the noise commonto both lines becomes sufficiently attenuated.

2 The result-ing Common mode output of the Filter onto the AC line (viaimpedance matching circuitry) is then 1. Generalized line filteringThe Design of a Common mode Filter is essentially thedesign of two identical differential filters, one for each ofthe two polarity lines with the inductors of each sidecoupled by a single core:L1L2(A)(B)Figure 2. The Common mode inductorFor a differential input current ( (A) to (B) through L1 and(B) to (A) through L2), the net magnetic flux which iscoupled between the two inductors is inductance encountered by the differential signal isthen the result of imperfect coupling of the two chokes;they perform as independent components with their leak-age inductances responding to the differential signal: theleakage inductances attenuate the differential the inductors, L1 and L2, encounter an identicalsignal of the same polarity referred to ground (commonmode signal), they each contribute a net, non-zero flux inthe shared core.

3 The inductors thus perform as indepen-dent components with their mutual inductance respond-ing to the Common signal: the mutual inductance thenattenuates this Common First Order FilterThe simplest and least expensive Filter to Design is a firstorder Filter ; this type of Filter uses a single reactivecomponent to store certain bands of a spectral energywithout passing this energy to the load. In the case of alow pass Common mode Filter , a Common mode choke isthe reactive element value of inductance required of the choke is simply theload in Ohms divided by the radian frequency at and abovewhich the signal is to be attenuated. For example, attenu-ation at and above 4000 Hz into a 50 load would requirea mH (50/(2 x 4000)) inductor.

4 The resulting commonmode Filter configuration would be as follows:50 50 50 VCMout+ VCMoutVCMin+ + VCMin+ mHFigure 3. A first order (single pole) Common mode filterThe attenuation at 4000 Hz would be 3 dB, increasing at6 dB per octave. Because of the predominant inductordependence of a first order Filter , the variations of actualchoke inductance must be considered. For example, a 20% variation of rated inductance means that thenominal 3 dB frequency of 4000 Hz could actually beanywhere in the range from 3332 Hz to 4999 Hz. It istypical for the inductance value of a Common mode chokeDocument 191-1 Revised 11/08/07 Document 191-1 Coilcraft, Inc. 2007to be specified as a minimum requirement, thus insuringthat the crossover frequency not be shifted too , some care should be observed in choosing achoke for a first order low pass Filter because a muchhigher than typical or minimum value of inductance maylimit the choke s useful band of Order FiltersA second order Filter uses two reactive components andhas two advantages over the first order Filter : 1) ideally, asecond order Filter provides 12 dB per octave attenuation(four times that of a first order Filter ) after the cutoff point,and 2) it provides greater attenuation at frequenciesabove inductor self-resonance (See Figure 4).

5 One of the critical factors involved in the operation ofhigher order filters is the attenuating character at thecorner frequency. Assuming tight coupling of the filtercomponents and reasonable coupling of the choke itself(conditions we would expect to achieve), the gain near thecutoff point may be very large (several dB); moreover, thetime response would be slow and oscillatory. On the otherhand, the gain at the crossover point may also be lessthan the presumed -3 dB (3 dB attenuation), providing agood transient response, but frequency response nearand below the corner frequency could be less thanoptimally the Design of a second order Filter , the damping factor(usually signified by the Greek letter zeta ( )) describesboth the gain at the corner frequency and the timeresponse of the Filter .

6 Figure (5) shows normalized plotsof the gain versus frequency for various values of 191-2 Revised 11/08/07 Document 191-2 Figure 4. analysis of a second order (two pole) Common modelow pass filterThe Design of a second order Filter requires more care andanalysis than a first order Filter to obtain a suitableresponse near the cutoff point, but there is less concernneeded at higher frequencies as previously mentioned. A = ; B = ; C = ; D = ; E = 5. Second order frequency response for variousdamping factors ( )As the damping factor becomes smaller, the gain at thecorner frequency becomes larger; the ideal limit for zerodamping would be infinite gain.

7 The inherent parasitics ofreal components reduce the gain expected from idealcomponents, but tailoring the frequency response withinthe few octaves of critical cutoff point is still effectively afunction of ideal Filter parameters ( , frequency, capaci-tance, inductance, resistance).RLRLVCMout+ VCMin+ VCMout+ Frequency, WGain (dB)ABDCEVsVsLRsLCsLCjLRjLCLRLCCMoutCMin LLnnnL()()=++= + =+ 111111212222 radian frequencyR the noise load resistanceLGainjnndB=+ 1122 Coilcraft, Inc. 2007 For some types of filters, the Design and damping char-acteristics may need to be maintained to meet specificperformance requirements. For many actual line filters,however, a damping factor of approximately 1 or greaterand a cutoff frequency within about an octave of thecalculated ideal should provide suitable following is an example of a second order low passfilter Design :1) Identify the required cutoff frequency:For this example, suppose we have a switching powersupply (for use in equipment covered by UL478) thatis actually 24 dB noisier at 60 KHz than permissible forthe intended application.

8 For a second order Filter (12dB/octave roll off) the desired corner frequency wouldbe 15 ) Identify the load resistance at the cutoff frequency:Assume RL = 50 3) Choose the desired damping factor:Choose a minimum of which will provide 3 dBattenuation at the corner frequency while providingfavorable control over Filter ) Calculate required component values:Note:Damping factors much greater than 1 may causeunacceptably high attenuation of lower frequen-cies whereas a damping factor much less may cause undesired ringing and the filtermay itself produce Order FiltersA third order Filter ideally yields an attenuation of 18 dB peroctave above the cutoff point (or cutoff points if the threecorner frequencies are not simultaneous); this is theprominently positive aspect of this higher order Filter .

9 Theprimary disadvantage is cost since three reactive compo-nents are now required. Higher than third order filters aregenerally 191-3 Revised 11/08/07 Document 191-3 Figure 6. analysis of a third order (three pole) low pass filterwhere 1, 2 and 4 occur at the same -3dB frequency of 05) Choose available components:C = F (Largest standard capacitor value thatwill meet leakage current requirements for UL478/CSA No. 1: a 300% decrease from Design )L = mH (Approx. 300% larger than Design tocompensate for reduction or capacitance: Coilcraftstandard part #E3493-A)6) Calculate actual frequency, damping factor, and at-tenuation for components chosen: = (a damping factor of about 1 or more isacceptible)Attenuation = (12 dB/octave) x 2 octaves = 24 dB7) The resulting Filter is that of figure (4) with:L = mH; C = F; RL = 50 RLRLCCVCMout+ VCMout+ VCMin+ L1L1L2L2 DifferentialLoadVCMout sVCMin sRRLsRLssCRLssCRLs LLsLssCLLRsLCsLL CRsLLLLLLL()()()()=+ ++++ + =++++22212121121212311 Butterworth ++ +112212233sssnn n ()()LLRRLLLnnL12111222+==+ ;()LLCn1n2C=2; 2211414=.

10 LLLLnnn12Ln3n2L2n2L2CR=1;RR 33224422=== nnnLfCLLRL=====294248070727502rad / sec= LC=Hz (very nearly 15 KHz) Coilcraft, Inc. 2007 The Design of a generic Filter is readily accomplished byusing standard alignments such as the Butterworth ( maxi-mally flat ) alignments. Figure (6) shows the generalanalysis and component relationships to the Butterworthalignments for a third order low pass Filter . Butterworthalignments provide an inherent of and a -3 dB pointat the crossover frequency. The Butterworth alignmentsfor the first three orders of low pass filters are shown inFigure (7).The Design of a line Filter need not obey the Butterworthalignments precisely (although such alignments do pro-vide a good basis for Design ).