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1、 Modeling, Simulation, and Reduction of Conducted Electromagnetic Interference Due to a PWM Buck Type Switching Power Supply I A. Farhadi Abstract:Undesired generation of radiated or conducted energy in electrical systems is called Electromagnetic Interference （EMI）. High speed switching frequency i

2、n power electronics converters especially in switching power supplies improves efficiency but leads to EMI. Different kind of conducted interference, EMI regulations and conducted EMI measurement are introduced in this paper. Compliancy with national or international regulation is called Electromagn

3、etic Compatibility （EMC）. Power electronic systems producers must regard EMC. Modeling and simulation is the first step of EMC evaluation. EMI simulation results due to a PWM Buck type switching power supply are presented in this paper. To improve EMC, some techniques are introduced and their effect

4、iveness proved by simulation. Index Terms:Conducted, EMC, EMI, LISN, Switching Supply I. INTRODUCTION FAST semiconductors make it possible to have high speed and high frequency switching in power electronics . High speed switching causes weight and volume reduction of equipment, but some unwanted ef

5、fects such as radio frequency interference appeared . Compliance with electromagnetic compatibility （EMC） regulations is necessary for producers to present their products to the markets. It is important to take EMC aspects already in design phase . Modeling and simulation is the most effective tool

6、to analyze EMC consideration before developing the products. A lot of the previous studies concerned the low frequency analysis of power electronics components . Different types of power electronics converters are capable to be considered as source of EMI. They could propagate the EMI in both radiat

7、ed and conducted forms. Line Impedance Stabilization Network （LISN） is required for measurement and calculation of conducted interference level . Interference spectrum at the output of LISN is introduced as the EMC evaluation criterion . National or international regulations are the references for t

8、he evaluation of equipment in point of view of EMC . II. SOURCE, PATH AND VICTIM OF EMI Undesired voltage or current is called interference and their cause is called interference source. In this paper a high-speed switching power supply is the source of interference. Interference propagated by radia

9、tion in area around of an interference source or by conduction through common cabling or wiring connections. In this study conducted emission is considered only. Equipment such as computers, receivers, amplifiers, industrial controllers, etc that are exposed to interference corruption are called vic

10、tims. The common connections of elements, source lines and cabling provide paths for conducted noise or interference. Electromagnetic conducted interference has two components as differential mode and common mode A. Differential mode conducted interference This mode is related to the noise that is i

11、mposed between different lines of a test circuit by a noise source. Related current path is shown in Fig. 1 . The interference source, path impedances, differential mode current and load impedance are also shown in Fig. 1. B. Common mode conducted interference Common mode noise or interference could

12、 appear and impose between the lines, cables or connections and common ground. Any leakage current between load and common ground could be modeled by interference voltage source. Fig. 2 demonstrates the common mode interference source, common mode currents Icm1 and Icm2 and the related current paths

13、. The power electronics converters perform as noise source between lines of the supply network. In this study differential mode of conducted interference is particularly important and discussion will be continued considering this mode only. III. ELECTROMAGNETIC COMPATIBILITY REGULATIONS Application

14、of electrical equipment especially static power electronic converters in different equipment is increasing more and more. As mentioned before, power electronics converters are considered as an important source of electromagnetic interference and have corrupting effects on the electric networks . Hig

15、h level of pollution resulting from various disturbances reduces the quality of power in electric networks. On the other side some residential, commercial and especially medical consumers are so sensitive to power system disturbances including voltage and frequency variations. The best solution to r

16、educe corruption and improve power quality is complying national or international EMC regulations. CISPR, IEC, FCC and VDE are among the most famous organizations from Europe, USA and Germany who are responsible for determining and publishing the most important EMC regulations. IEC and VDE requireme

17、nt and limitations on conducted emission are shown in Fig. 3 and Fig. 4 For different groups of consumers different classes of regulations could be complied. Class A for common consumers and class B with more hard limitations for special consumers are separated in Fig. 3 and Fig. 4. Frequency range

18、of limitation is different for IEC and VDE that are 150 kHz up to 30 MHz and 10 kHz up to 30 MHz respectively. Compliance of regulations is evaluated by comparison of measured or calculated conducted interference level in the mentioned frequency range with the stated requirements in regulations. In

19、united European community compliance of regulation is mandatory and products must have certified label to show covering of requirements IV. ELECTROMAGNETIC CONDUCTED INTERFERENCE MEASUREMENT A. Line Impedance Stabilization Network （LISN）1-Providing a low impedance path to transfer power from source

20、to power electronics converter and load. 2-Providing a low impedance path from interference source, here power electronics converter, to measurement port. Variation of LISN impedance versus frequency with the mentioned topology is presented in Fig. 7. LISN has stabilized impedance in the range of co

21、nducted EMI measurement Variation of level of signal at the output of LISN versus frequency is the spectrum of interference. The electromagnetic compatibility of a system can be evaluated by comparison of its interference spectrum with the standard limitations. The level of signal at the output of L

22、ISN in frequency range 10 kHz up to 30 MHz or 150 kHz up to 30 MHz is criterion of compatibility and should be under the standard limitations. In practical situations, the LISN output is connected to a spectrum analyzer and interference measurement is carried out. But for modeling and simulation pur

23、poses, the LISN output spectrum is calculated using appropriate software.For a simple fixed frequency PWM controller that is applied to a Buck DC/DC converter, it is possible to assume the error voltage （ve） changes slow with respect to the switching frequency, the pulse width and hence the duty cyc

24、le can be approximated by （1）. Vp is the saw tooth waveform amplitude. A. PWM waveform spectral analysis The normalized pulse train m （t） of Fig. 8 represents PWM switch current waveform. The nth pulse of PWM waveform consists of a fixed component D/fs , in which D is the steady state duty cycle, an

25、d a variable component dn/f sthat represents the variation of duty cycle due to variation of source, reference and load. As the PWM switch current waveform contains information concerning EMI due to power supply, it is required to do the spectrum analysis of this waveform in the frequency range of E

26、MI studies. It is assumed that error voltage varies around Ve with amplitude of Ve1 as is shown in （2）. fm represents the frequency of error voltage variation due to the variations of source, reference and load. The interception of the error voltage variation curve and the saw tooth waveform with sw

27、itching frequency, leads to （3） for the computation of duty cycle coefficientsMaximum variation of pulse width around its steady state value of D is limited to D1. In each period of Tm=1/fm , there will be r=fs/fm pulses with duty cycles of dn. Equation （4） presents the Fourier series coefficients C

28、n of the PWM waveform m （t）. Which have the frequency spectrum of Fig.9. B-Equivalent noise circuit and EMI spectral analysis To attain the equivalent circuit of Fig.6 the voltage source Vs is replaced by short circuit and converter is replaced by PWM waveform switch current （Iex） as it has shown in

29、 Fig. 10. The transfer function is defined as the ratio of the LISN output voltage to the EMI current source as in （5）. The coefficients di, ni （i = 1, 2, , 4） correspond to the parameters of the equivalent circuit. Rc and Lc are respectively the effective series resistance （ESR） and inductance （ESL

30、） of the filter capacitor Cf that model the non-ideality of this element. The LISN and filter parameters are as follows: CN = 100 nF, r = 5 , l = 50 uH, RN =50 , LN=250 uH, Lf = 0, Cf =0, Rc= 0, Lc= 0, fs =25 kHz The EMI spectrum is derived by multiplication of the transfer function and the source n

31、oise spectrum. Simulation results are shown in Fig. 11. VI. PARAMETERS AFFECTION ON EMI A. Duty Cycle The pulse width in PWM waveform varies around a steady state D=0.5. The output noise spectrum was simulated with values of D=0.25 and 0.75 that are shown in Fig. 12 and Fig. 13. Even harmonics are i

32、ncreased and odd ones are decreased that is desired in point of view of EMC. On the other hand the noise energy is distributed over a wider range of frequency and the level of EMI decreased .B. Amplitude of duty cycle variation The maximum pulse width variation is determined by D1. The EMI spectrum was simulated with D1=0.05. Simulations are repeated with D1=0.01 and 0.25 and the results are shown in Fig.14 and Fi