Conducted EMI Simulation of Switched Mode.pdf

Conducted EMI Simulation of Switched Mode Power SupplyHongyu Li #1, David Pommerenke #2, Weifeng Pan #3, Shuai Xu *4, Huasheng Ren *5, Fantao Meng*6, Xinghai Zhang*7 # EMC Laboratory, Missouri University of Science and Technology 4000 Enterprise Dr. Rolla, MO, USA 1 hlfm4, * 2 davidjp, 3 [email protected] Huawei Technologies CO., LTD. Huawei Industrial Base, Bantian Longgang, Shenzhen, P. R. China 4 xushuai, 5 renhuasheng, 6 mengfantao, 7 [email protected] Abstract—This paper introduces an efficient method to predict the conducted EMI of switched mode power supply (SMPS) through time domain SPICE simulation, which can be used to design or optimize the filter circuit and quantify the suppression degree of the filter. The SMPS modelling method permits modelling of the SMPS as a noise signal source even when its internal structure is unknown. The method is verified by comparing predicted and measured noise signals. Key words: SMPS, DC/DC converter, Conducted EMI, SPICE, Filter efficient method to predict the conducted EMI could permit filters design through simulations rather than experimental procedures and iterations. This paper introduces a method of simulating the conducted EMI of SMPS through the use of time-domain SPICE simulation. Fig. 1 shows the typical conducted EMI test system. output input LISN Filter Circuit SMPS Module ZL I. INTRODUCTION Because of its superior efficiency, switched mode power supply (SMPS) is widely applied in modern electronic systems, [1] and there are many kinds of SMPS modules on the market, including DC/DC converter modules. However, SMPS generates electrical noise due to its high dv/dt and di/dt from switching. Typically, SMPS generates three types of EMI: low frequency conducted noise (<30 MHz), broadband noise (50 – 300 MHz), and high frequency noise (> 300 MHz). [2] This paper focuses on suppression of low frequency conducted noise. Many international agencies specify conducted and radiated emissions limits for electronic products. Included among these are CISPR, FCC, VCCI, and the new CE specifications. Most agency-conducted noise limits apply only to noise currents induced onto the AC power lines in finished products. European Telecommunication Standard Instructions (ETSI) are an exception, applying CE requirements to DC supplies with cables over three meters long. Although not required to do so by agency standards, some system designers apply the conducted emissions requirements to subassemblies within the product to reduce internal interference among subsystems and to reduce the difficulty of meeting overall system requirements. [3] High-density SMPS modules are usually designed to operate at a high switching frequency to reduce the size of the internal filter components. The small EMI filters internal to the modules are often inadequate to meet the stringent international EMI requirements. To meet these requirements, external filtering of the power module is often necessary. [3] Traditional filter design depends heavily on the designers’ experiences and experiments on prototype boards, and the effect of the filter is difficult to quantify. Therefore, an Spectrum Analyzer Fig. 1. Block diagram of a conducted EMI test system To simulate conducted EMI, the entire test system must be modelled including the SMPS module, the filter circuit, and the LISN. The modelling method for each part will be introduced separately in sections II, III, and IV below. Research results show that near-field coupling significantly influences the conducted EMI; these results will be introduced in section V. Finally, all the models will be combined to form one system module to predict the conducted EMI. By comparing predicted and measured signals, the models and the method are fully verified. This simulation method is illustrated with the example of a DC/DC converter module. This is a full-brick package, flyback style, 48V to 28V, 700W output-module, as shown in Fig. 2. II. SMPS MODULE MODEL If the internal components and PCB layout of the SMPS module are known, a common method of modelling the SMPS module is to obtain the main and parasitic parameters of the components from datasheets and measurements, and then extract the parasitic parameters of the PCB using electromagnetic simulation tools and measurements. In many situations, however, the internal structure of the SMPS module is unknown, so this common method is inadequate. Furthermore, if many types of SMPS modules are used, it is not efficient to derive the models of each module using this common method. Therefore, an efficient method is needed to 978-1-4244-4267-6/09/$25.00 ©2009 IEEE 155 and the resistances at the resonant points. the frequency of the excitation signal should not overlap with the noise.. B. 3. 3. Source Current or Voltage According to the Thevenin’s Theorem and Norton’s Theorem. The measurement setup is illustrated in Fig. therefore. and the excitation is input by a current clamp. the measured differential impedance in active mode of the DC/DC converter is shown as a blue line in Fig. Fig. The EQC of the differential impedance of the DC/DC converter module used in this research is shown in Fig. variation in the DC load does not change the internal structure and the coupling path.model the SMPS without knowing the internal structure. an impedance analyser.g. the impedance can be directly measured by a network analyzer or an impedance analyzer. 2. commercial software packages are available for this type of conversion. 2. Several key points of this system require explanation: 1) Differential and common mode: There are differential and common mode noises. 2) Magnitude and frequency of the excitation: The excited signals should not disturb the normal operation of the SMPS. [4] It is better to build the models for the two modes separately. Impedance in active mode was found to be the same as that in passive mode for this particular DC/DC converter module. 2. So that the noise will not be unintentionally detected as the response signal. To address this problem. and then calculate the impedance based on the measured voltages and currents. These measurements were compared with those taken in passive mode using a network analyzer (NWA). 10. The impedance was measured under a driving 200W DC load. This work developed a single automatic measurement system controlled by Matlab code.g. because it is easier to locate the problem when filters are designed using the models. and signal measurement equipment (e. measure the response. 3. this research presented here developed a method to measure the impedance in active mode. However the source impedance in active mode is unknown. differential signal should be injected by putting the two cables in opposite directions in the current clamp. impedance should remain constant regardless of DC load. as illustrated in Fig. either current or voltage source may be used. In this research. 4) Synchronization: The excitation equipment (e. which involves high DC voltage and a very noisy environment where the usual equipment is not suitable. Fig. For differential mode impedance measurement. the signal generator).. Slight differences among measurements in the high frequency range (shown in Fig. This conclusion was verified by comparing impedances measured under various DC load conditions. Since the impedance of the source is already known. the oscilloscope) must run synchronously. therefore. the current clamp injects common mode excitation signals. 3) were caused by variations in the measurements setup. in active mode. Even slight differences between the time sequences of the two instruments will cause errors and reduce the accuracy of the measurement. the equivalent circuit (EQC) can be obtained by calculating the capacitances or inductances in ranges in which the slope is decreasing or increasing by 20 dB/decade. For SMPS modules generally. Source Impedance In passive mode. Measured differential impedance of the DC/DC converter From the measured impedance curve. SMPS module as noise source A. the magnitude of the excitation signal should be as small as possible as long as it can be detected by the measurement equipment. 2. The basic idea of this method is to inject frequency excitations to the SMPS. the source currents or voltages can be calculated from the measured external noise currents and voltages when the SMPS is 156 . One possibility is to take the SMPS module as a noise source and obtain the source impedance and source current or voltage by measurements and calculation. If the curve is complex and manual calculations to obtain the EQC become difficult. In the setup shown in Fig. and the simulated impedance is compared to the measured impedance in Fig. 3) Correction: The measured signals must be corrected by the frequency response characteristic and the time delay of the probes. and the automatic measurement system. 4. Sometimes. It is reasonable. Comparison of voltage signals measured from different loads Fig. the verification method is illustrated by the example of the source current. or the coupling path.1 0 1 2 3 4 5 Time [us] 6 7 8 9 10 Frequency domain Magnitude [dBuA] 100 From filter0 From filter1 Calculate the internal source current at each harmonics in the frequency domain 50 0 0 5 Frequency [MHz] 10 15 Transform the calculated frequency domain source currents into a time domain signal which can be imported in PSpice Fig. the external load does not change the internal structure. 4. the field coupling from the outside to the interior of the module is negligible compared to the conducted coupling. Furthermore. as shown in Fig. indicating that the noise of the SMPS has the primary characteristic of a source. Components For simple components such as capacitors. to measure and derive its Lcm: Common mode inductance. A. Perform FFT on the measured external voltages and currents impedance and source current or voltage as a noise signal source. 5. 7. Common mode choke modelling method B. Here. not only the main parameters. Common mode source comparison Time domain 0. Verification Next. The method to model the common mode choke is shown in Fig. it is reasonable to suppose that the SMPS impedance and the source currents or voltages are essentially constant. it was necessary to verify that an SMPS can be taken as a current or voltage source. 7. Therefore. This research used current source. Rm: Core resistance. The external currents and voltages were measured from filter0 and filter1. Comparison of measured Vip of filter0 and filter2 Time domain Voltage [v] 2 0 -2 From filter0 From filter1 III. Fig. therefore. and the calculation process is shown in Fig. Rw: Winding conductor resistance. the parameters can be taken from the datasheets. the DC bias and temperature characteristics must also be considered. For most SMPS modules. PCB 157 . the high dv/dt and di/dt. some parasitic parameters must be obtained from measurements. but also the parasitic parameters of the components and the PCB are critical to the accuracy of the simulation. [4][5] 0 1 2 3 4 5 Time [us] 6 7 8 9 10 Magnitude [dBuV] Frequency domain 120 100 80 60 40 0 5 Frequency [MHz] 10 From filter0 From filter1 15 Fig.operating. This conclusion was verified by comparing the measured impedance with various filter circuits that are the loads of the AC noise source. 6 demonstrates that different measured signals derived almost the same source current signals. Cp1: Parasitic capacitances in the winding. Cp2: Parasitic capacitances between two windings. Comparison of the calculated current sources Fig. FILTER CIRCUIT MODEL For modelling the conducted noise signals.1 Current [A] From filter0 From filter1 0 Identify the peak value and frequency of the harmonics -0. For the complex components such as the common mode choke. The primary characteristic of a source is that the same source impedance and the current or voltage can be measured and calculated with different loads. 5. Llk: Leakage inductance. Comparison of the measured signals shows the significant differences between these two filters. Source current or voltage calculation process C. 6. 11. and analysis of conducted EMI problems through simulations rather than experimental procedures and iterations. NEAR FIELD COUPLING The near field coupling was found to influence significantly the conducted EMI. VII. The research results indicate that this method of SMPS modelling is reasonable and efficient.. 150 kHz–30 MHz for CISPR22) and shields the measurement against unwanted incoming noises. measured using a 5cmx5cm square loop H field probe [9] Fig. 158 . The cross indicates the position where the common mode choke is mounted. The coupling is apparent from the near magnetic field scanning results shown in Fig. LISN MODEL International standards have defined the reference impedance on which the conducted EMI should be measured. Modelling the coupling effect with SPICE is difficult. and since this effect is not taken into account in the SPICE model. 10. The three subfigures show the magnetic field distribution of directions (x. and the present SPICE model does not yet include this effect. as shown in Fig. This impedance is guaranteed by a Line Impedance Stabilization Network (LISN). 8. Fig. and precisely defined by the CISPR16 document. Some of the coupled signal will bypass the filter and propagate to the LISN directly. the common mode choke was not optimally placed for the research purpose. requires further development. LISN The SPICE model of LISN can be derived from the schematic of this device. The filter circuit model of the DC/DC converter used in this research is shown in Fig.The PCB parameters can be extracted from measurement or using tools such as the Ansoft Q3D Extractor. All the PCB loop and sensitive components can couple the near field. SYSTEM MODEL All of the part models described here were combined to form a system model. Since near field coupling influences the conducted EMI. The LISN offers a 50 :impedance over the frequency of interest (e. therefore it’s near magnetic field coupling is quite strong. including filter design and optimization of external filters for SMPS. however. 10. 8 illustrates the internal structure of the device. the predicted signals closely match the measured signals. and the predicted LISN voltage signals are compared with the measured signals in Fig.g. VI. 9. On the test board used in this research. The method of modelling this effect in SPICE. This simulation method can be helpful to designers in several respects. As shown in Fig. [6]-[8] X Y Z Fig. 11. 9. and both of the two methods were applied in this research to verify the results. the signal was measured after the sensitive components were shielded. Near magnetic field scanning results. and the model used in this experiment is shown in Fig. IV. and z). This method is especially suitable for situations in which many types of SMPS modules are used but the internal structures are unknown. quantification of the filter’s suppression effect. This work also investigated the influence of near-field coupling on the conducted EMI. V. The combined system model accurately predicts the conducted EMI. The system model can be used to predict the conducted EMI. y. CONCLUSION This paper introduces an efficient method of predicting the conducted EMI of an SMPS through SPICE simulation that takes the SMPS as a noise source to model. 10. SMPS LISN Filter Fig. 10. System model lvdfvoltage Diff LISN 100 Magnitude [dBuV] measured shielded simulated 50 0 0 5 10 15 Frequency [MHz] 20 25 30 lvcm voltage Comm LISN 100 Magnitude [dBuV] measured shielded simulated 50 0 0 5 10 15 Frequency [MHz] 20 25 30 Fig. 11. Comparison of predicted and measured LISN voltage signals 159 . 2nd ed. ON Semiconductor.. Pommerenke. Switching Power Supply Design. [3] “FLTR100V20 Filter Module data sheet. [7] C.REFERENCES [1] Abraham I. 2003 IEEE International Symposium. 2005 International Symposium. 2003 Page(s):384 . USA.” Electromagnetic Compatibility.” Electromagnetic Compatibility. Jiang Xanguo. “High frequency model of common mode inductor for EMI analysis based on measurements. on Volume 3. www.465. AND8032/D. D.” Tyco Electronics Power Systems. Inc. Basso. 1997 [8] Christophe P. 3. “EMI specifics of synchronous DC-DC buck converters. Shimoshio. EDN February 3. 1822 Aug.com 160 . McGraw-Hill.. Y. 8-12 Aug. [6] Christophe Basso. “Spice predicts differential conducted EMI from switching power supplies”.. Pressman. TX..” Electromagnetic Compatibility.amberpi.387 vol. on Volume 1. [4] Zhe Li.1. on 21-24 May 2002 Page(s):462 . 2005 Page(s):711 . USA. Pommerenke. 1998.. [5] Liu Dehong. “Common-mode and differential-mode analysis of common-mode chokes.714 Vol. “Conducted EMI Filter Design for the NCP1200”. D. [2] Zhe Li. [9] Measured with the help of Amber Precision Instruments. 002 3rd International Symposium. Basso. Switch-Mode Power Supply SPICE Cookbook. McGraw-Hill.