Impedance Analysis Of Automotive High Voltage Networks For .
Proc. of the 10th Int. Symposium on Electromagnetic Compatibility (EMC Europe 2011), York, UK, September 26 -30, 2011Impedance Analysis of Automotive High VoltageNetworks for EMC Measurements1M. Reuter1*, S. Tenbohlen1, W. Köhler1, A. Ludwig2Institute of Power Transmission and High Voltage Technology (IEH), University of Stuttgart, Germany,2Research & Technology, Daimler AG, Boeblingen, Germany*Martin.Reuter@ieh.uni-stuttgart.deAbstract — The increasing demand of electric power in electricvehicles cannot be supplied by conventional automotive powernetworks. Therefore novel power bus systems, capable to deliverthe required power are under development. These bus systemshave higher operating voltages, which result in lower currentamplitudes for power transmission. As power efficiency is one ofthe main factors determining the range of an electric car with agiven battery capacity, fast slew rates of the power semiconductors are preferred. But with increasing slew rates alsoemitted radio frequency (RF) disturbances increase. To reduceelectromagnetic interferences (EMI) of power electronics acomplete shielding of the power bus system is necessary. Thus thenetwork configuration is changed from unshielded single coremulti wire harnesses into coaxial conductor layouts, andtherefore also the impedance characteristics of the entire networkare changed. Electromagnetic compatibility (EMC) of automotivecomponents is measured according to CISPR 25, substituting thevehicle environment with line impedance stabilization networks(LISN). Recent research shows that LISNs, developed for lowvoltage networks, are not ideal to measure conducted emissionsof high voltage (HV) components because of changedcharacteristic impedances and additional shielding.This paper deals with a method of determining the highfrequency impedances of automotive HV power networks. AVector Network Analyzer (VNA) is used to measure Scatteringparameters of different HV power cables and an automotive LiIon accumulator battery. Matrix conversions allow calculating animpedance network, which is able to represent an automotive HVnetworks.Filters are a very strong element for suppression of conductedelectromagnetic emissions (EME), but effective filter designrequires knowledge about characteristic impedances of devicesunder test (DUT) and connected HV networks [1].To ensure system’s EMI performance, CISPR 25 defines anartificial network (AN/LISN), which represents a normalizedautomotive LV network with a standardized input impedance[2]. Every automotive component is tested regarding its EMIperformance with this network. Devices connecting LV andHV network, e.g. DC/DC couplers, have to be terminated attheir HV side connectors comparable to their vehicleenvironment. Basically, the LV LISNs can be used as HVtermination, even shielding can be taken into consideration.However, this paper shows that wave impedances of HV cablesdiffer substantially of the LV network characteristicimpedance.HV network impedance knowledge therefore is necessaryfor an effective HV filter design and to enable component EMImeasurements with a realistic line impedance representing avehicle environment.This paper presents a method to characterize automotiveHV network impedances for simulation of an automotive HVenvironment. The simulation of a real vehicle environment is abasic requirement for comparable EMC measurements thatallows to estimate their later performance when mounted in anelectric vehicle.II.Index Terms — EMC, Automotive HV Networks, Charactersistic impedance, HV LISN, RF charactersistics of Li-Ion batteryI.LISNS FOR CONDUCTED EMI TESTINGIn 1983 a method was developed to measure conductedemissions of automotive components [3]. A Line ImpedanceStabilization Network (LISN) was presented that providedmainly three functions:1. Defined input impedance emulating a vehicle LVpower network2. Supply DUT with DC power3. Impedance matching network to measurement systemand decoupling of ambient distortionsINTRODUCTIONThe electrification of the automotive power train requires anovel electrical power bus system. Recent automotive lowvoltage (LV) networks (12 V) are not able to meet powerdemands of electric driving engines because of high currentsrequired for power transmission. Automotive high voltage(HV) power networks, with voltage levels of typically 120 380 V, reduce needed currents to values, which can be handledmore efficiently.One major task in developing electric cars is to control theirelectromagnetic interferences (EMI). Maximum efficiency ofpower electronics requires fast slew rates of switchingsemiconductor devices. But fast slew rates result in busdisturbances consisting of radio frequency (RF) spectralcomponents with high amplitudes.These RF disturbances may propagate along HV powerlines and are able to interfere with other electronic devices.This LISN was developed by impedance measurements atautomotive LV networks having two conductors, battery plus(BP) and battery minus (BM), which usually is grounded toautobody on its LV bus far end. Fig. 1 shows a common twoLISN EMI test setup.Trying to use this LISN for HV component measurementstwo essential barriers occur: HV shielding and changednetwork input impedance.106
Proc. of the 10th Int. Symposium on Electromagnetic Compatibility (EMC Europe 2011), York, UK, September 26 -30, 2011emulating automotive HV networks as close as possible, wouldload HV components as in their vehicle environment andenables correct EMI performance measurements. This networkshould simulate 3-4 m of HV cable with attached accumulatorbattery, as every HV component is connected to this minimumnetwork. More complex networks can be partitioned andreduced to such a minimum HV network.III.HV CABLE MEASUREMENTSFor investigation of their RF behavior, six different automotive HV cables have been examined with a Vector NetworkAnalyzer (VNA). That instrument is used to measureScattering- or S-parameters of electronic circuits and networks.The main high frequency attribute of a cable is itscharacteristic or wave impedance, which is determined by itsline inductivity and capacity to ground. This chapter showshow to calculate these values of measured S-Parameters.Figure 1.A. Devices under Test – HV cablesTable 1 introduces six different HV cables, which havebeen investigated. All cables, except cable 1, are of the samemanufacturer. Cable 6 is not a coaxial type shielded wire (asthe others are); this cable has a common shield for bothconductors.Electric circuit of a two LISN setup for LV automotivecomponent conducted EMI testing.In order to reduce emitted RF radiation of HV powerelectronics a complete shielding of the HV system is used. Asin 1983 there was no need for any shielding of electrical powersystems, the AN is not intended to emulate a shielded system.For security reasons HV systems are assembled floating, thusBM is not grounded. This directly leads to a three conductorsystem, whereas the emulated system of the CISPR 25 is anunshielded two conductor system.By inserting two LISNs into a shielded box, connecting BPto one AN, BM to the second and Ground to the HV cableshields the changed conductor layout can be taken intoconsideration (see fig. 1). This leads to an input impedance of100between BM and BP and of 50between eachconductor to ground. As the characteristic wave impedances ofthe coaxial HV cables is significantly lower than 50 there isa mismatching of these transmission lines and standing wavesand resonances can occur; hence it is not possible to evaluatethe EMI performance of a HV component as in its vehicleenvironment.Recent electric driven cars usually have their electricengine located in the front area and the accumulator battery inthe back, connected by HV cables with the length of 3-4 m.Emitted RF power is mainly generated by fast switching powerelectronics, which are placed near the electric engine in thefront. Conducted EMI can be coupled in cable wires (LV andHV), and is able to interfere with other connected systems. Butthese cable wires also act as antennas and emitted RF radiationmay also disturb galvanically isolated systems such as sensitiveradio antenna preamplifiers. In the investigated frequencyrange the radiated RF power of long cable harnesses is muchhigher than radiated EMI of the device itself (mainly housing).Thus for EMI performance of the complete system itsconducted RF power on these long cables is essential [1].Conducted emissions on automotive LV buses can bemeasured using ANs according to CISPR 25. If these ANs areused to emulate HV networks, disturbing RF sources of HVcomponents are loaded with impedances of LV networks,which are different to the load impedances of the automotiveHV environment. The measurement error cannot be corrected,as it is impossible to determine source impedances of RFsources (switching power semiconductors). A networkTABLE I.INVESTIGATED HV CABLES AND THEIR ASSEMBLYCable 1Cable 2Cable 3Cable 4Cable 5CrossSection25 mm²25 mm²25 mm²35 mm²50 mm²Cable 62x6 mm²Type of shieldingSingle conductor with shield and screening foilSingle conductor with shield and screening foilSingle conductor with meshed shielding, no foilSingle conductor with shield and screening foilSingle conductor with shield and screening foilTwo conductors with common shield and screeningfoilAll cables have a length of 1 m, connected via lowimpedance cable glands to brass adapter boxes for N-Type RFconnections to the measurement system, as shown in fig. 2. Toobtain maximum analogy to vehicle situation two cables of thesame type are arranged parallel to each other in a mutualdistance of d 4.25 cm. As cable 6 has a common shielding ofboth lines, these wires are spaced by the assembly of theirisolation material inside the cable, having a minimum thicknessof 0.28 mm. The cables are situated in a height of h 5 cmabove a conductive, grounded plane as shown in fig. 3.HV-Cable (1-5)N-TypeConnectorCable GlandFigure 2.107Cable 6HV cable assembly with brass adapter boxes and lowimpedances cable glands.
Proc. of the 10th Int. Symposium on Electromagnetic Compatibility (EMC Europe 2011), York, UK, September 26 -30, 2011B. Measurement SetupThe S-parameters of the HV cables are measured with anAgilent ENA E5070B network analyzer in a frequency rangeof f 300 kHz – 200 MHz. A mechanical calibration kit isused.ZW 1 2 Z11 2 Z 31ZW 2 2 Z 22 2 Z 42VNA 1mh 5 cmZX Z 31 Z 42 Z12Z12Z G1 Z122 Z 31 Z 42.Z12 Z 42Z122 Z 31 Z 42Z12 Z 31The measured S-parameters are transformed into Zparameters and inserted in this equation system. An examplefor cable 1 is shown in fig. 5.ZG2 GroundedplaneFigure 3.Measurement setup of HV cable 6 with VNA todetermine RF behavior (S-parameters).1kC. Characterization MethodologyS-parameters are the quotient of reflected or transmittedwave to incident wave amplitudes. They are usually measuredin a 50 system and can easily be converted to Z-, Y-, H- andT-parameters. But they do not lead directly to the characteristicattributes of the measured transmission line. Characteristiccable values like wave impedances, line inductivities, groundcapacitances or dielectric constants need to be calculated infurther steps, which are presented in following chapters. [5] Z / Ω10010ZG1ZG2ZXZL1D. Calculation of Line ImpedancesFor an impedance characterization these S-parameters needto be connected to a given electric network. An electricequivalent circuit (EEC) of these cables can be found in fig. 4.The simplest EEC of a transmission line is a T-network [1]. Asthe DUT consists of two parallel coaxial conductors ahorizontally mirrored T-network is needed, as shown in fig. 4.ZW1ZW210 m300 kHz 1 MHzFigure 5.10 MHz200 MHzValues of HV cable 1 equivalent circuit passiveresistances vs. frequency.Electric equivalent circuit of HV cable, with conductor1 between ports 1-3, conductor 2 between ports 2-4 andcross coupling impedance ZX.Fig. 5 shows that the through resistances ZW1 and ZW2 startat a relatively small value ( 150 m ), increase linearly untilapproximately 50 MHz and begin to oscillate above that. Theresistances of inner conductor to grounded shielding (ZG1 &ZG2) decrease linearly from 1 k with increasing frequency andalso oscillate above 50 MHz. Fig. 5 also shows that bothconductors act very similarly, as the curves of ZW1/ZW2 andZG1/ZG2 are nearly without distinction; they are even equal formost of the investigated frequency range. The cross couplingresistance ZX does not show a clear frequency dependantbehavior, but also oscillates in RF range. There are two clearlydistinguishable frequency ranges of the line impedances: From300 kHz to approximately 30 MHz the cable impedances havecharacteristic frequency dependencies, whereas above anoscillating behavior occurs. This results due to the quotient ofwavelength to cable length: At (f)/ 6 the cable is in quasistationary mode, acting like a network of concentratedelements, and at (f)/6 its transmission line characteristicsoverbalance and lead to this oscillating behavior [1].Calculating the Z-parameters of this 1st order, horizontallymirrored T-network leads to an equation system of fivedifferent equations having five unknowns (ZW1, ZW2, ZX, ZG1,and ZG2). Inverting this equation system to the unknownsdeliversE. Impedance InterpretationThe first resonance of the investigated HV cable 6 iscalculated as first maximum of ZW1 (see fig. 5). The resonanceoccurs at /2 1 m. In this example fR 84.4 MHz,meaning that at this frequency the wavelength is R(f) 2 m. InZG111‘½ ZW1½ ZW13‘½ ZW24‘3ZX22‘½ ZW24ZG2Figure 4.108
Proc. of the 10th Int. Symposium on Electromagnetic Compatibility (EMC Europe 2011), York, UK, September 26 -30, 2011TABLE II.air an electromagnetic wave at f 84.4 MHz has a wavelengthof c0/f 3.55 m. This shortening effect can be traced to theisolation material dielectric constant c0 ε r 2l f R 1 DUT.12πf Z G1 .There are two different ways of calculating wave orcharacteristic impedance of a transmission line [4]:Z L Z 112 Z 312LSerialCGround Z / Ωεr89.82.7819Cable 225117057511.084.43.1521Cable 325116855311.185.83.0465Cable 43514.710246614.884.83.1252Cable 5506.24311026.385.83.0465Cable 62x619.512026821.188.02.8977/// MHzBATTERY MEASUREMENTS1Z G 2πfZL(c)Ls ZW2πf10 mFigure 618.4fRLSCG18.31300 kHzinpF/m363As termination becomes important for the input impedanceof a HV network, the HV batteries also need to becharacterized. For all fast switching power electronics thetermination of extended HV cables is an HV accumulatorbattery. The impedance of this battery is transformed by thetransmission lines into a different input impedance of the entirenetwork.In this paper an automotive Li-Ion battery model ispresented, having an operating voltage of Vop 128.9 VDC at58.3 % state of charge (SOC). For the measurements a Rohde& Schwarz ZVR network analyzer is used. Fig. 7 shows themeasurement setup, consisting of battery, battery controller,DC blocking device and VNA.10010CG25IV.Both formulas lead to an equal result, and are used to verifycalculations. As both ways of determining the wave impedancedo not have anything in common but the raw data, a similarresult indicates correct calculation methodology. Fig. 6 showsan example for a graphical impedance analysis of cable 1.CG LSCompared to usually estimated values of automotive lowvoltage cables (LS 1 H/m, CG 100 pF/m) these measurements show around 10 - 20 times smaller line inductivities and3 - 10 times higher ground capacity values. This leads toconsiderably smaller wave impedances of HV networks.Due to their high line inductivity LV networks haverelatively high line impedance in the frequency region ofinterest for EMC issues. Additionally the transmission linecharacter of HV cables is dominant above 10 MHz, wheretermination is defining its input impedance. Thus terminationof HV cables becomes more important and has to be taken intoaccount, because it is determining the input impedance of a HVnetwork in combination with cable characteristic impedance.and the capacity to ground is calculated via1kZLCable 1/ mm²ZLS W 12πf ,Z L* CrossSectioninnH/m1232In the quasi stationary region of the frequency range, lineand ground impedances show linear frequency dependencies.In this range line impedances are inductive and groundimpedances are capacitive. The line inductivity is determinedbyCG CALCULATED TRANSMISSION LINE ATTRIBUTES OFAUTOMOTIVE HV CABLES1 MHz10 MHz(b)fr(a)(e)200 MHz(d)Graphical evaluations of cable 1 characteristic attributesCG, LS, ZW and fR.F. Measurement ResultsTable 2 lists the measured HV cable characteristics. Itshows, that coaxial type shielded HV cables have a waveimpedance in a range of 6.2 – 18.3 , with direct dependencyto their geometrical properties.Figure 7109Measurement setup with Li-Ion battery (a), ECTASMB adapter (b), battery controller (c), DC blockingdevice (d) and VNA (e).
Proc. of the 10th Int. Symposium on Electromagnetic Compatibility (EMC Europe 2011), York, UK, September 26 -30, 2011A. DC blocking DeviceAs the used network analyzer does not tolerate higher input voltages than 10 VDC, high-pass filtering is required. Toreject DC voltage a series capacitor of size CSeries 100 nF isused. Combined with the input impedance of the VNAthis capacitor forms an RC high-pass filter,Rin,VNA 50having a 3dB cut-off frequency of 31 kHz. The upper cut-offfrequency of this filter is determined by the equivalent seriesinductance (ESL) of the used SMD capacitor components, andmeasurements verified it to be around 500 MHz. This bandpass filtering behavior of the measurement adapter leads to ameasurement range of 40 kHz to 400 MHz. Within thisfrequency range the transmission loss is kept lower than0.5 dB.(a)(b)(c)(e)(d)Figure 8ECTA connector calibration kit for TOM-X calibration, consisting of Open-Open standard (a), MatchOpen (b), Match-Match (c), Through (d) and ECTASMB coaxial adapter (e).The cross coupling attenuation within the DC blockingdevice between channel 1 and channel 2 is beyond 20 dB up to1 GHz, but the measurement is however compromised by it.Therefore a TOM-X calibration was used and showed superiormeasurement accuracy compared to the SOLT method.B. TOM-X Calibration of VNACalibration has a significant impact on the quality of themeasurement results. Valid calibration is needed to correct themeasured S-parameters at the input port of a VNA. The mostimportant source of measurement uncertainty are unavoidablesystematic errors. These include system frequency response, ascable attenuation and phase shift, impedance mismatch or crosscoupling within the test setup. The calibration processmathematically derives the systematic error model and correctsthe compromised raw data. This error model is an array ofvector coefficients used to establish a fixed reference plane ofzero phase shift, zero reflection magnitude, losslesstransmission magnitude and known system impedance. Thearray of coefficients is computed by measuring a set of“known” devices or calibration standards connected at a fixedmeasurement plane [6].Generally the 12 term error correction model is sufficient,using a calibration kit consisting of a known Short-, Open-,Load- and Through-Standard (SOLT). This method is able tocorrect all systematic errors of a single channel. As soon asthere is cross coupling within the measurement path, e.g. inadaptors, this method cannot correct these errors andmeasurement data is compromised. For correction of sucherrors three additional correction coefficients are required,obtaining a 15 term error correction model. Major difference tothe SOLT method is a simultaneous measurement of bothchannel’s standards and therefore setup cross coupling can beconsidered. One 15 term calibration method is the TOM-Xmethod (Through-, Open-, Match-Standard and X-coupling).This process needs a set of four standards, as shown in fig 8.The simultaneous reflection coefficients at both channels aredet
Electromagnetic compatibility (EMC) of automotive components is measured according to CISPR 25, substituting the vehicle environment with line impedance stabilization networks (LISN). Recent research shows that LISNs, developed for low voltage networks, are not ideal to measure conducted emissions of high voltage (HV) components because of changed characteristic impedances and additional .
Odd-Mode Impedance: Z d Impedance seen by wave propagating through the coupled-line system when excitation is anti-symmetric (1, -1). Common-Mode Impedance: Z c 0.5Z e Impedance seen by a pair of line and a common return by a common signal. Differential Impedance: Z diff 2Z d Impedance seen across a pair of lines by differential mode signal .
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Differential Impedance Differential Impedance: the impedance the difference signal sees ( ) ( ) 2 2( ) Z 0 small I V I V diff Z diff one one Differential impedance decreases as coupling increases 1v -1v I one x I two How will the capacitance matrix elements be affected by spacing? C 12 C 11 C 22 Eric Bogatin 2000 Slide -18 www .File Size: 1MBPage Count: 25
DiffZ0 (ohm) - Calculated differential impedance. Like Single Impedance you can change the value for impedance to the needed value. The tool calculates the necessary width. You can change all values of the white boxes to calculate your impedance. Note: If you want to change the material disable “Show Diff Impedance”.
2.2.3. Electrochemical impedance spectroscopy The electrical properties of the bigels were studied using computer controlled impedance analyzer (Phase sensitive multimeter, PSM1735, Numetriq, Japan) The impedance parameters such as impedance, phase angle, capacitance an
2.4. Electrochemical impedance spectroscopy studies Electrochemical impedance spectroscopy (EIS) can provide useful information on the impedance changes of the electrode surface. Lower impedance values indicate higher conductance. Therefore, electrochemical impedance spectroscopy wa
suring acoustic impedance and calibrating impedance heads and propose a general calibration technique for heads with multiple transducers. We consider the effect of transducer errors on impedance measurements and present a technique for distributing any measurement errors over the frequency range. To demonstrate the technique we use an impedance
1.2 Measuring impedance To find the impedance, we need to measure at least two values because impedance is a complex quantity. Many modern impedance measuring instruments measure the real and the imaginary parts
