Design Of EMI Filters For DC-DC Converter
Design of EMI Filters for DC-DC converterJ. L. Kotny*, T. Duquesne**, N. Idir**Univ. Lille Nord de France, F-59000 Lille, France* USTL, F-59650 Villeneuve d’Ascq, France** USTL, L2EP, F-59650 Villeneuve d’Ascq, Francenadir.idir@univ-lille1.frAbstract- The subject of this paper is the design of EMI filtersfor the DC-DC converters. It is well known that the staticconverters used in electric traction systems are major sources ofconducted disturbances which are the common mode anddifferential mode. Often, the solution used to reduce conductedemissions consists to use the EMI filters. The design of thesefilters is very difficult because it requires complete mastery ofthe design process. In this paper, we propose a design method ofthe EMI filter based on the simulation of the filter in frequencydomain. Thus, the high frequency models of the filtercomponents are proposed. The obtained models have been testedand give good results on a large frequency range, from 9 kHz to30 MHz.I. INTRODUCTIONThe utilization of the static converter in the electric tractionsystems is very problematic for electromagnetic compatibility(EMC). Indeed, each power converter generates a lot of highfrequency interferences causing a malfunction of the onboardelectronic systems. The main solution to reduce theseconducted emissions is based on the utilization of EMI filters[1] – [3]. It can also be combined with other solutions like:the slowing down the dv/dt during the transitions of thepower semi-conductor components and/or by acting on theconverter controls [4].The EMI filters are made from coupled inductors combinedwith capacitors; the choice of the filter topology depends onnetwork and load impedances. Generally, the Common Mode(CM) and Differential Mode (DM) filters are used for powerconverters. The passive components have a strong impact onfilter efficiency [1]. Parasitic elements of these componentssuch as equivalent series inductor (ESL) of the capacitors andequivalent parallel capacitances (EPC) of the coupledinductors have a negative influence on the EMI filterperformances [5] – [6]. In order to design and optimized filtercharacteristics by simulation, a high frequency models,including parasitic elements of the passive components, mustbe used.In this paper, a high frequency modeling method of themagnetic part of EMI filter is used. Coupled inductors andcapacitor models have been introduced into complete EMIfilters and simulations have been compared to a prototype fortesting its reliability.A procedure for designing EMI filters will be presented inthis study. It is based on the analysis of conducted EMIinduce by the DC-DC converter.978-1-4244-8218-4/10/ 26.00 2010 IEEEII. EMI FILTER DESIGNThe EMI filters are used to reduce the common mode anddifferential mode emissions induced by the power converters.To design of EMC filter, it is necessary to separate conductedemission modes. It is well known that the utilization of theLine Impedance Stabilizing Network (LISN) does not allowto separate the common mode and differential modedisturbances. There are different methods which allow toseparate these modes which are presented in literature [7] –[9]. In this study, we used the current probes (FCC – FC52:10 kHz to 500 MHz) to measure the common mode currentICM and the differential mode current IDM at the output side ofthe LISN as shown in Fig. 1.In this study, in order to reduce the conducted emissionsinduced by the DC-DC converter, common mode anddifferential mode filters are used as shown in Fig. 2. Thecommon mode filter uses a coupled inductors LCM and twocapacitors CY connected to the ground. However, thedifferential mode filter uses two separate inductors LDM andtwo capacitors CX.The measurement method, according to EMC standard, isbased on the utilization of an LISN. Than, the conductedemissions are measured with a spectrum analyzer, infrequency band varying from 0.1 MHz to 30MHz, accordingto CISPR11/ EN55011 standard [10].Fig. 1. Separation of CM and DM currents by using current probesFig. 2. EMI filter structure: common mode and differential mode
A. Common Mode FilterThe equivalent circuit of the common mode filter and LISNis shown in Fig. 3. The common mode emissions, measured(without EMI filter) using current probes, allow to calculatethe voltage under the resistor of the LISN using the followingrelation VCM 25Ω * ICM. The result of the calculation showsthat the level of conducted emissions is over the EMCstandard limit up to 6 MHz (Fig. 4). To meet the EMCstandard, the EMC filter attenuation required is equal to thedifference between the measured level of disturbance and thelimits of the standard (Fig. 5). By drawing a line with slope40dB/dec tangent to the attenuation curve, we obtainsuccessively the cut-off frequencies where the tangent cutsthe frequencies axis. The elements "LCM and CY" aredetermined from the frequency cut-off "fCM" as shown in Fig.5. In the case of common mode and in order to limit theleakage currents to ground, the chosen value of capacitor CYis 4.7nF. Thus, from this value and the cut-off frequency, wecalculate the value of the common mode inductor LCM usingthe following relation:LCM (11)2.2π . f CM2.CY(1)Using the results of Fig. 5, we can determine the first cutoff frequency that is equal to fCM 36 kHz. Thus, we deducethe value of the CM inductor which equal to LCM 2mH. Thesecond cut-off frequency fCMH equal to 10.5 MHz is obtainedfrom the curve shown in Fig. 5. It will allow us thereafter,with fCM, to calculate the stray elements of the common modefilter.Fig. 5. Common mode attenuation requirementB. Differential Mode FilterAs previously, the equivalent circuit of the differentialmode filter and LISN is shown in Fig. 6. The differentialmode emissions, measured (without EMI filter) using currentprobes, allow to calculate the voltage under the resistor of theLISN using the following relation VDM 100Ω * IDM. Theresult of the calculation shows that the level of conductedemissions is over the EMC standard limit up to 3 MHz (Fig.7). The same method is applied to calculate the differentialmode filter. However, in differential mode, the knowledge ofthe cut-off frequency fDM gives a degree of freedom, sinceonly the product LDM * CX is known. For the differentialmode, cut-off frequency of the filter is equal to fDM 115KHz as shown in Fig. 8.Fig. 3. Common mode filter equivalent circuitFig. 6. Differential mode filter noise equivalent circuitFig. 4. Common mode noise without filter (Voltage under 25Ω)Fig. 7. Differential mode noise without filter (Voltage under 100Ω)
frequency equal to 28.9 MHz. On the fig. 5, the second cutoff frequency fCMH is equal to 10,5MHz. In order to haveattenuation allows to meet the EMC standard, it is necessaryto have a coupled inductance with a very low straycapacitance Cf .1) Calculation of the Inductor Stray CapacitanceThe calculation of the stray capacitances of the coupledinductors Cf is based on the geometrical dimensions of theferrite core, the numbers of turns of inductor windings andthe position of windings compared to the core as shown inFig. 9. These calculations are detailed in [11], [12].Dimensions of the ferrite core are following:Fig. 8. Differential mode Attenuation requirementWe can chosen the value of the capacitor CX 1.5nF. Thecalculation, using the relation (1), gives the value of LDM.From Fig. 6, if we taking into account the common modecapacitors, the differential mode equivalent capacitance isequal to: CDM CX CY / 2 3.85nF. Thus, we deduce2LDM 0.5mH. Since, the differential mode filter include twoinductors (Fig. 2), then, the differential mode inductor isequal to LDM 0.25mH. In the next section, we study thehigh-frequency model of the EMI filter.III. HIGH FREQUENCY MODEL OF THE EMI FILTERTo calculate the EMI filter, it is necessary to separate theCM and DM disturbances. The values of the EMI filtercomponents previously calculated are summarized in thefollowing table:de 2.re 26 mm, di 2.ri 14.8 mm,ns 22,dc 2.rc 1 mm,ε0 1/36.π.109.me 2 mm,The calculation of dt and α is given:d t 2.(rt ).sin (α / 2) (rc me / 2) re rc b f α 2. arcsin he 10.6 mm,bf 1 mm,(2)(3)One can then calculate surface as equivalent at to 2 planarplates distant of dt which correspond to 2 winding turns asshown in Fig. 10. The value of as is given by the followingrelation:(d d i )a s 2.d c .(he 2.b f rc e 2.b f rc )(4)2The calculation of the stray capacitances of the coupledinductors is given by:Cf ε0 ns as/dt 4.1pFCommon mode FilterLMC 2mHCY 4.7nF(5)Differential mode filterLDM 0.25mHCX 1.5nFA. Common Mode FilterIn order to realize the common mode inductor LCM 2mH,we used a coupled inductances obtained with two windingand copper wire (diameter 0.8mm) rolled up on a ferrite coreN30 from EPCOS manufacturer. The specific inductance ofthis material is AL 4260nH, since we have AL. ns2 2mH,one deducts the number of turns ns 22. The choice of themagnetic circuit and the number of turns allow to calculatethe value of the stray capacity Cf of the coupled inductors.The cut-off frequency fCMH for the common mode filter willdetermine the choice of the characteristics of the capacitorCY. Indeed, the equivalent circuit (RLC) of this capacitormust have a resonant frequency higher than fCMH. It isinteresting to take a low value of the capacitor because theparasitic inductance Lc shifts the resonance frequency tolower frequencies. In our case, le value of this capacitor isfixed to CY 4.7nF. The stray elements of this capacitor,measured with the impedance analyzer, give a cut-offFig.9. Geometrical dimension of the core and conductorFig.10. Stray capacitance between two turns of the inductor winding
Knowing the stray capacitance of the coupled inductors,and starting from a simple model of this inductance, one canmake the simulation of the EMI filter using the equivalentcircuit shown in Fig. 11. One can see on figure 12 theattenuation variation that corresponds to the common modefilter obtained with the simplified model. This filter uses acoupled inductors LCM 2mH, a stray capacitance Cf 4.1pFand resistance R 10 KΩ. The value of this resistance hasnot the influence on the resonance frequency of the filter. Wehave also added external inductors Lc which correspond tothe connection of the filter.The simulation results of the attenuation, carried out withSPICE, using a simplified model of the filter are shown inFig. 12. The ideal case corresponds to Cf 4pF and Lc 0,the maximum attenuation A 82dB is obtained at 33MHz. Infact, we cannot avoid the filter connection corresponds to theexternal inductance, which is in our case measured using theimpedance bridge, and its value is equal to Lc 39nF. Thesimulation results show that the maximum attenuation valueis equal to A 90dB at 8MHz. Thus, the attenuationdecreases but remains significant and equal to A 45dB at30MHz. The third curve corresponds to Cf 14pF and Lc 39nF. This last curve shows that the attenuation is equal to A 78dB at 8 MHz. These results show that in order to increasethe attenuation of the EMI filter, it's necessary to reduce thestray elements.2) High Frequency Model of the Coupled InductorsHigh frequency model of the coupled inductors, obtained inthe previous study, shown in figure 13 will be used [13]. Thecoupling coefficient of two windings is assumed equal to K 1. The different impedances Z1 and Z2 are used to model thehigh frequency behavior of the coupled inductors. Thus, theimpedances Z1 correspond to the leakage inductances aresymmetrically distributed. However, the impedance Z2 allowsto model the effect of the magnetizing inductance. Thevarious capacitances represent the capacitive effects in thecoupled inductors in high frequencies band.To determine the values of the resistances, inductances andcapacitances, various tests are needed [13]. These tests aredone with impedance analyzer (Agilent 4294A) in afrequency band between 1 kHz to 30 MHz. The necessarytests are: the load test, short circuit test and when thewindings are in parallel with additive and subtractive flux.The test when one winding of the coupled inductors is inopen circuit, with additive and subtractive flux, is used todetermine the elements of the impedances Z1 and Z2. Todetermine the values of stray capacitances, two tests, whentwo winding of coupled inductors are in short circuit, areneeded. These calculations are detailed in [13].We can check the value of the stray capacity which wehave already calculated. According to the high frequencymodel (Fig. 13), the capacitance is equal to Cf 2. (1.4 0.4) 3.6 pF and the calculated value with (5) is equal to 4.1pF.These results validate the method of calculation of the straycapacitances.We can realize now the prototype of the filter by using thecommon mode coupled inductors with the capacitors CY andCX, but in the first time, without differential mode chokes.Fig.11. Simplified diagram of simulation of the Common Mode filterFig.13. High frequency model of the CM coupled inductorsFig.12. Influence of stray elements on the filter attenuation
3) ValidationTo validate the model of the filter, we use the structureshown in Fig. 14. Thus, the measurement of the filterattenuation is carried out with a spectrum analyzer (HP ESAL1500A). It injects (output RF OUT) a signal in thefrequency band between 10 KHz to 30 MHz at the filterinput. The filter is connected to the receiver of the analyzer(Input impedance is 50 Ω). Figure 14 shows the principle ofmeasuring the attenuation of the filter. The resistive divider isused to distribute power on each input of the filter.The common mode and differential mode capacitors havebeen characterized using an impedance analyzer. Theequivalent circuit used for CY is a series circuit: CY 4.9nF,RY 200mΩ and LY 6.2nH. The equivalent circuit used forCX is a series circuit: CX 1.4nF, RX 130mΩ and LX 10nH. Connecting the high frequency models of the commonmode chokes and capacitors CX , CY, the obtained filter modelis simulated with SPICE.Figure 15 gives the results of the comparison of theattenuation measured and simulated with the high frequencymodel. This comparison confirms the validity of the proposedmodel which can be used to study the various structures ofEMI filters. However, these curves show that the reduction ofthe attenuation depends not only of the ferrite characteristicsand capacitors but also of parasitic elements of thesecomponents (the position of L and C elements, positions andconfiguration of the PCB, the length of connections betweenthe elements .). All these parameters reduce the efficiency ofthe filter.In conclusion, the quality of the EMI filters depends on thecharacteristics of the used passive components. However, therealization of the filter requires a rigorous design whichallows to reduce a maximum the parasitic effects.B. Differential Mode FilterDifferential mode filter uses two inductances and of twocapacitors Cx. The characteristics are: 2 coils independent of57 turns for each one with a copper section of 0.6mm. Thus,the DM mode choke value is equal to LDM 0.26mH. Themain characteristic of the magnetic material "ferrite", used torealize the DM chokes, is the saturation current whichappears for the higher values. Thus, inductors saturate from3A.IV. EXPERIMENTAL VALIDATIONTo validate the high frequency model of the EMI filter, weused the experimental setup shown in Fig. 15. It consists of abuck converter which feeds through 4-wire shielded cable anelectrical machine. The choice of this cable and the AC motorwill allow in the future to study a PWM voltage inverter.The coupled inductors have been realized using EPCOSferrite magnetic material: N30. Core shape for CM is a ringR25.3/14.8/10 and windings are made of 2*22 turns of copperwire (diameter 0.8mm). Core shape for DM is a ringR27/14/11.5 and windings are made of 57 turns of copperwire (diameter 0.6mm). The coupled inductors have thenbeen associated with capacitors for achieving a complete EMIfilter. They have been connected together on a PCB circuit asshown in Fig. 16. The high frequency models of the coupledinductors proposed is used to simulate the EMI filter.The comparison of measurement data and simulationresults of the conducted emissions without and with EMIfilter shows a good agreement because the gap is less than10dBμV in all frequency band. The spectra shown in Fig. 17represent the conducted emissions measured with a spectrumanalyzer and an LISN with and without EMI filter.Fig.14. Measurement setup of the filter attenuationFig.15. Experimental setupFig.15. Filter attenuation (measurement and simulation)Fig. 16. Common mode and differential mode filters
These results show the efficiency of the filter, since thelevel of the conducted emissions induced by the powerconverter towards the DC supply is lower than the limit ofEMC standard. To study separately the attenuation of eachfilter (CM, DM), we calculated, from the measured currents(ICM and IDM), the common mode voltage (VCM 25Ω * ICM)and differential mode voltage (VDM 100Ω* IDM) as shown inFig. 18 and Fig. 19.These curves give a comparison of the common mode anddifferential mode emissions with and without EMI filter.These results show that for the DM filter attenuation is higherat 10dBµV up to 5 MHz. However, for the CM filter, theattenuation is importance and reached 40dBµV in allfrequency band.V. CONCLUSIONIn this paper, a design method of EMI filters based on thehigh frequency simulation of the common mode anddifferential mode filters is presented. The high frequencymodel of inductors used in EMI filter made it possible tostudy, by simulation, the influence of the parasitic elementsof the passive components on the efficiency of the EMI filter.The utilization of the EMI filter models is very useful forinvestigating the effects of parasitic couplings on theirperformances, and they therefore offer guides for EMI filterdesign. The objective is to use the high frequency model ofthe filter associated at the high frequency model of powerconverter to optimize the design of the EMI filter. The nextstep is to use these passive components models to study the 3phase EMI filters in the adjustable speed drives.Fig. 17. Conducted emissions measured with LISN (with and without filter)Fig. 18. Differential mode emissions with and without EMI filterFig. 19. Common mode emissions with and without EMI filterREFERENCES[1] C. R. Paul, “A New Method to Characterize EMI Filters,” in proc. IEEEApplied Power Electronics Conference, Anaheim, CA, 15-19 Feb. 1998,pp. 929 -933.[2] F. Y. Shih, D. Y. Chen, Y. P. Wu and Y. T. Chen, "A Procedure forDesigning EMI Filters for AC Line Applications", IEEE Trans. onPower Electronics, vol. 1, N 1, Jan. 1996, pp. 170-181.[3] H. Chen, Z. Qian, Z. Zeng, C. Wolf, “Modeling of Parasitic InductiveCouplings in a Pi-Shaped Common Mode EMI Filter”, IEEETransactions on Electromagnetic Compatibility, Vol. 50 , N 1, Fab.2008, pp. 71 – 79.[4] N. Idir, R. Bausiere, J. J. Franchaud, "Active Gate Voltage Control ofTurn-on di/dt and Turn-off dv/dt in Insulated Gate Transistors", IEEETrans. on Power Electronics, vol.21-4, July 2006, pp.849-855.[5] X. Margueron, J-P. Keradec, "Design of Equivalent Circuits andCharacterization Strategy for n-Input Coupled Inductors", IEEE Trans.on Industry Applications, Vol.43, N 1, Jan./Feb. 2007, pp.14-22.[6] Shuo Wang, F.C. Lee, D.Y. Chen and W.G. Odendaal, “Effects ofParasitic Parameters on the Performance of EMI Filters”, IEEE Trans.on Power Electronics, Vol. 19, May. 2004, pp. 869-877.[7] T.Guo, D.Y.Chen, F.C.Lee “Separation of the Common-Mode andDifferential-Mode Conducted EMI Noisee”, IEEE Trans. on PowerElectronics, vol. 11, N 3, May, 1996, pp.480-487.[8] A.Nagel, R.W. De Doncker “Separating Common Mode and DifferentialMode Noise in EM1 Measurements”, EPE’99, Lausanne, pp.1-8.[9] H. I. Hsieh, “A procedure including mix-mode noise for designing EMIfilters for off-line applications”, Vehicle Power and PropulsionConference (VPPC 2008), September 2008, China, pp. 1 - 6.[10] IEC, "CISPR Publication 11: 2003, Industrial, Scientific, and urbanceCharacteristics Limits and Methods of Measurement", 2004.[11] G. Grandi, M. K. Kazimierczuk, A. Massarini, U. Reggiani, "StrayCapacitances of Single-Layer Solenoid Air-Core Inductors", IEEETrans. on Industry Applications, Vol.35, N 5, Sept./Oct. 1999, pp.11621168.[12] L. Dalessandro, W. G. Hardus Oddendaal, J. W. Kolar, “HFCharacterization and Nonlinear Modeling of a Gapped ToroidalMagnetic Structure, IEEE Transactions on Power Electronics, vol. 21,N 5, Sept. 2006, pp.1167-1175.[13] J. L. Kotny, X. Margueron, N. Idir, "High Frequency Modeling Methodof EMI filters", Energy Conversion Congress and Exposition (ECCE2009), Nov. 2009, San Jose, USA.
A procedure for designing EMI filters will be presented in this study. It is based on the analysis of conducted EMI induce by the DC-DC converter. II. EMI FILTER DESIGN The EMI filters are used to reduce the common mode and differential mode emissions induced by the power converters. To design of EMC filter, it is necessary to separate conducted
EMI FILTER There are several types of filters, for example, passive EMI filter, active EMI filter and hybrid EMI filter. However, the passive EMI filter is simpler than others. A typical procedure used in [4, 5] is considered to design power line EMI filter. A basic network topology is used, C-L-C for DM filter and L-C
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2. Optimal filter design EMI Measurement and Analysis 1. Measure Total Mode Noise 2. CM, DM Noise Analysis 3. Source Impedance Analysis 4. Analysis of each components 5. EMI Filter Design (Basic) 3) Filter Design with EMI Analyzer EMI Analyzer (EA-300)
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