反激式开关电源外文翻译.pdf

Measurement of the Source Impedance of Conducted Emission Using Mode Separable LISN Conducted Emission of a Switching Power Supply JUNICHI MIYASHITA 1 MASAYUKI MITSUZAWA 1 TOSHIYUKI KARUBE 1 KIYOHITO YAMASAWA 2 and TOSHIRO SATO2 1Precision Technology Research Institute of Nagano Prefecture Japan 2Shinshu University Japan SUMMARY In the procedure for reducing conducted emissions it is helpful to know the noise source impedance This paper presents a method of measuring noise source complex impedances of common and differential mode separately We propose a line impedance stabilization network LISN to measure common and differential mode noise separately without changing LISN impedances of each mode With this LISN conducted emissions of each mode are measured inserting appropriate impedances at the equipment under test EUT terminal of the LISN Noise source complex impedances of switching power supply are well calculated from measured results 2002 Scripta Technica Electr Eng Jpn 139 2 72 78 2002 DOI 10 1002 eej 1154 Key words Conducted emission noise terminal voltage noise source impedance line impedance stabiliza tion network LISN EMI 1 Introduction Switching power supplies are employed widely in various devices High speed on off operation is accompa nied by harmonic noise that may cause electromagnetic interference EMI with communication devices and other equipment To prevent the interference methods of meas urement and limit values have been set for conducted noise 30 MHz and radiated noise 30 to 1000 MHz Much time and effort are required to contain the noise within the limit values hence the efficiency of noise removal tech niques is an urgent social problem Understanding of the mechanism behind noise generation and propagation is necessary in order to develop efficient measures In particu lar the propagation of conducted noise must be investi gated Modeling and analysis of equivalent circuits have been carried out in order to investigate conducted noise caused by switching 1 2 However the stray capacitance and other circuit parameters of each device must be known in order to develop an equivalent circuit which is not practicable in the field of noise removal On the other hand noise filters and other noise removal devices do not actually provide the expected effect 3 4 which is explained by the difference between the static characteristics measured at an impedance of 50 and the actual impedance Thus it is necessary to know the noise source impedance in order to analyze the conducted noise Regulations on the measurement of noise terminal voltage 5 suggest using LISN in particular the vector sum absolute voltage of two propagation modes namely common mode and differential mode is measured in terms of the frequency spectrum Such a measurement however does not provide phase data and propagation modes cannot be separated therefore the noise source impedance cannot be derived easily There are publications dealing with the calculation of the noise source impedance for example common mode is only considered as the principal mode and the absolute value of the noise source impedance for the common mode is found from the ground wire current and ungrounded voltage 6 or mode separated measure ment is performed by discrimination between grounded and ungrounded devices 7 However measurement of the ground wire current is impossible in the case of domestic single phase two line devices The complex impedance can be found using an impedance analyzer in the nonoperating state but its value may be different for the operating state Thus there is no simple and accurate method of measuring source noise impedance as a complex impedance 2002 Scripta Technica Electrical Engineering in Japan Vol 139 No 2 2002 Translated from Denki Gakkai Ronbunshi Vol 120 D No 11 November 2000 pp 1376 1381 72 The authors assumed that the noise source impedance could be found easily using only a spectrum analyzer provided that the noise could be measured separately for each mode and the LISN impedance could be varied For this purpose a LISN with a balun transformer was devel oped to ensure noise measurement with the common mode and differential mode strictly separated An appropriate known impedance is inserted at the EUT equipment under test terminals and the noise source impedance is found from the variation of the noise level This method was used to measure the conducted noise of a switching power sup ply and it was confirmed that the noise source impedance could be measured as a complex impedance independently for each mode Thus significant information for noise removal and propagation mode analysis was acquired This paper presents a new method of measuring the noise source impedance of conducted emission using mode separable LISN 2 Separate Measurement for Common Mode and Differential Mode The conventional single phase LISN circuit for measurement of the noise terminal voltage is shown in Fig 1 The power supply is provided with high impedance by a 50 H reactor and a meter with an input impedance of 50 is connected between one line and the ground via a high pass capacitor and another line is terminated by 50 Thus the LISN impedance as seen at the EUT is 100 in the differential mode and 25 in the common mode The measured value is the vector sum of both modes and the noise must be found separately in order to find the noise source impedance for each mode There is LISN with Y to delta switching to provide mode separation 8 but its impedance is 150 giving rise to a problem of data compatibility with 50 LISN Thus a new mode separa ble LISN was developed as shown in Fig 2 The circuit is identical to that in Fig 1 from the power supply through the high pass capacitor Switching of the connection pattern ensures measurement with one line of the balun transformer terminated by 50 and another line connected to the meter In Fig 2 the secondary side of the 2 1 balun trans former is terminated by 50 while the primary side has 200 in the differential mode the impedance line to line is 100 since 200 at the high pass capacitor is connected in parallel With the switch set at D the meter is connected to the secondary side of the balun transformer The voltage is one half that of the line to line voltage and measurement is performed in the standard way The common mode current flows from both sides of the balun transformer via the middle tap to the 50 termi nal The currents in the windings are antiphase and no voltage is generated at the secondary side Therefore the impedance of the primary side is the terminal resistance of the tap Since this impedance is connected in parallel to 50 two 100 in parallel at the high pass capacitor the impedance between the common line and ground is 25 With the switch set at C the meter is connected to the middle tap of the balun transformer and the common mode voltage is the line to ground voltage 3 Measurement of Noise Source Impedance 3 1 Measurement circuit and calculation Though the propagation routes are different in the two modes propagation from the noise source to the LISN can be represented in a simplified way as shown in Fig 3 In the initial measurement the load impedance ZL is the LISN impedance ZL can be varied by inserting a known impedance at the EUT terminals Consider three load im Fig 1 Standard 50 50 H LISN Fig 2 Mode separable LISN Fig 3 Schematic circuit of noise propagation 73 pedances namely LISN only and LISN with two different impedances inserted ZL1 R1 jX1 ZL2 R2 jX2 and ZL3 R3 jX3 Using the values I1 I2 I3 scalars measured in the three cases Z0 R0 jX0 is found Since V0 ZL I the following expressions can be derived From the above Here a b and c are as follows Substituting Eq 2 into Eq 1 the following quadratic equation for R0 is obtained Thus R0 and X0 have two solutions each The series of frequency points with positive R0 is taken as the noise source impedance 3 2 Method of measurement An impedance is inserted at the EUT terminals in order to measure the noise source impedance in the LISN as seen at the EUT As shown in Fig 4 the impedance is inserted so as to vary only the impedance in the mode under consideration thus preventing an influence on the imped ance in the other mode In the diagram Vm is the voltage at the meter connected to the LISN while the input impedance of the meter 50 is represented by the parallel resistance Since parameters of both the LISN and the inserted imped ance are known the noise current I can be calculated from Vm Now Z0 is calculated for each mode from the measured data obtained while varying ZL by using Eqs 2 and 3 With the differential mode shown in Fig 4 a CR is inserted between the two lines thus varying the load im pedance ZL In the differential mode Z0 is assumed to be a low impedance and hence the inserted impedance exerts a significant effect on the measured value For this reason 1 0 47 F and 0 0 1 F were inserted which are rather small compared to the LISN impedance The measurement of the common mode shown in Fig 4 b employs common mode chokes that basically have no impedance in the differential mode The common mode chokes are provided with a secondary winding ratio 1 1 so that the impedance at the secondary side can be varied In the common mode Z0 is assumed to have a particularly high impedance in the low frequency band For this reason 5 1 k and 100 pF were used as the secondary load for the common mode choke to obtain a high inserted impedance The measured data for the inserted impedance in the case of resistive and capacitive loads are presented in Fig 5 The impedance of the common mode choke includes its own inductance and the secondary load In the case of a capaci tive load the resonance point is around 200 kHz at higher frequencies the impedance becomes capacitive A single phase two line switching power supply an ac adapter for a PC with an input of ac 100 V a rated power of 45 W and PWM switching at 73 kHz was used as the EUT and the rated load resistance was connected at the dc side Filters were used for both the common and differential 1 2 3 Fig 4 Inserted impedances for each mode 74 modes except for the case in which one common mode choke was removed in order to obtain the high noise level required for analysis Both the EUT and the loads had conventional commercial ratings and were placed 40 cm above a metal ground plate the power cord was fixed 4 Measurement Results and Discussion The results of conventional measurement as well as common mode and differential mode measurement for the LISN without inserted impedance are shown in Fig 6 The measurements were performed in the range of 150 kHz through 30 MHz divided into three bands using a spectrum analyzer with frequency linear sweep Time variable data were measured at their highest levels using the Max Hold function of the spectrum analyzer and only the peak values were employed for calculation of Z0 For this purpose the values measured in every frequency band were subjected to the FFT and all harmonics higher than the fundamental frequency were removed The data were smoothed and about 10 peak points were detected in every frequency band In addition only those peaks that were stronger than the meter s background noise by at least 6 dB were consid ered The results in Figs 6 b and 6 c pertain to the LISN only the level would vary with inserted impedance The noise source impedance for both modes calculated from the measured data using triple measurement is given in Figs 7 and 9 respectively The bold and dashed lines pertain to data acquired with the impedance analyzer at the EUT power plug with the EUT not in operation With the differ ential mode there were no high frequency components as shown in Fig 6 b and hence the impedance is calculated only for significant low frequency peaks The noise source impedance in differential mode can be represented schematically as in Fig 8 The noise source impedance is equal to the impedance between the LISN Fig 5 Inserted impedance in common mode Fig 6 Measured results of standard differential mode and common mode Fig 7 Noise source impedance for differential mode 75 terminals when the noise source is short circuited With switching power supplies filtering is usually performed by a capacitor of 0 1 to 1 F inserted between the lines Since the impedance of the power cord is small in the measured frequency range one may assume that the impedance as seen at the LISN is low and that the phase changes from capacitive toward inductive as with the measured static characteristics However in the case of the given EUT a nonlinear resistor was inserted between the power cord and the filter as shown in Fig 8 and hence the impedance is rather high in the nonoperating state In addition there are rectifying diodes on the propagation route but they do not conduct at the measurement voltage of the impedance ana lyzer The noise levels show considerable variation at 120 Hz which corresponds to the on off frequency of the recti fying diodes however only the peak values are measured and then used for calculation and hence the impedance obtained by the proposed method is considered to pertain to the conductive state For this reason the results do not agree well with static characteristics Thus the impedance in the operating state cannot be measured in the differential mode On the other hand the measured data for Z0 in common mode agree well with the static characteristics as shown in Fig 9 The phase too exhibits a similar variation although the scatter is rather large The resistive part of three load impedances and Z0 may be presented in a simplified way as in Fig 10 From Eq 1 the following is true for R2 R3 and Z0 The distance ratio from Z0 to R3 and R2 on the R X plane that satisfies this equation is I2 I3 which corresponds to a circle with radius r as in Eq 4 with the center lying on the line R3R2 Similar circles for R1 and R2 are also shown in the diagram When Z0 and the load impedances lie on one line the two circles have a common point Equation 4 indicates that if I3 increases slightly the outer circle becomes bigger and the two circles do not adjoin On the other hand when the outer circle becomes smaller the two circles intersect at two points and X0 varies more strongly than R0 In practice the difference in noise level due to the inserted impedance may drop below 1 dB at some frequencies so that the solution for Z0 becomes unavailable because of the scatter or the phase scatters too much The measurement accuracy is governed by the difference in noise level and thus the inserted impedance should have a large enough variation compared to the measurement scatter in addition there should be a phase difference so that the two circles are not aligned as in Fig 10 Figures 7 and 9 pertain to one of the solutions of Eq 3 with larger R0 Here R0 is not necessarily positive and the other solution is not necessarily negative The two solutions may be basically discriminated from the fre quency response and other characteristics but other inser tion data are employed for the sake of accuracy Fig 8 Equivalent circuit of differential mode noise source impedance 4 Fig 9 Noise source impedance for common mode Fig 10 Load impedances and Z0 on R X plane 76 Figure 11 compares the measured data and calculated data for the variation of noise level due to insertion of a commercially available common mode choke with the cal culation based on the results of Fig 9 and the impedance of the common mode choke As is evident the calculation agrees well with the measured values On the other hand a considerable discrepancy was confirmed for the other solu tion The noise source impedance found as explained above is accurate enough to predict the filtering effect The noise source resistance in the common mode can be represented as in Fig 12 Here Z1 is the stray capacitance between the internal circuit and the case and Z2 is the stray capacitance between the case and the ground plate or in the case of the ground wire the impedance of the wire The common mode noise source impedance for a single phase two line EUT is primarily Z2 becoming capacitive at low frequencies Since the EUT is equipped with a filter the influence of the primary rectifying diodes is not related to common mode and hence the data measured by the pro posed method are very close to the static characteristics However this is not necessarily true in the case of a grounded line Z2 short circuited with no filter installed In addition here the full impedance as seen at the LISN is found in practice however a filter or Z1 is employed to suppress noise Therefore the impedance of the power cord is required as well as Z1 and Z2 in order to analyze the filtering effect The impedance of the power cord or grounded wire can be easily determined by measurement or calculation In our experiments without ground the impedance is very close to Z2 on the other hand Z1 might be measured by grounding the case and removing the filter Fig 12 and then used to analyze the filtering effect between the case and the lines However noise propagation in the inner circuit must be further investigated in order to estimate the noise suppressing efficiency of Z1 5 Conclusions A new mode separable LISN is proposed that sup ports noise measurement without changing the impedance depending on the mode The proposed LISN ensures accu rate measurement for each mode thus supporting imped ance analysis With the proposed LISN an appropriate impedance is inserted at the EUT terminals and the noise impedance can be found as a complex impedance just as simply as with conventional measurement of the noise terminal voltage The value of the inserted impedance must be chosen prop erly in order to determine the phase accurately The pro posed method ensures sufficient accuracy not only to investigate noise propagation and design efficient counter measures but also to predict the filtering effect The pro posed technique can supply important data for future analysis of