EMI Model of an AC/AC Power Converter Jordi Espina 1 , Josep Balcells 1 , Antoni Arias 1 , Carlos Ortega 2 and Nestor Berbel 1 1) Universitat Politècnica de Catalunya, 2) Escola Universitària Salesiana de Sarrià Electronic Engineering Department espina@eel.upc.edu Abstract- This paper deals with the validation of an EMI model to predict electromagnetic interferences (EMI) produced by power converters. A direct AC to AC matrix converter (MC) has been selected as experimental setup for the paper, but the modeling method can be applied to other converter topologies. The method is based on obtaining a high frequency equivalent circuit, using a combined time and frequency domain approach based on: “EMI source identification→ propagation path impedance→ derived disturbance”. The advantages of the proposed procedure are the computational time reduction and the lack of convergence problems, which may arise when using pure time domain procedures. The paper is focused on the prediction of common mode (CM) EMI of a matrix converter. The simulation results of this case will permit the calculation of currents which leak through the ground connections. Two different EMI sources are compared: one based on PWM waveforms obtained from MATLAB ® model and a second one where these data are obtained experimentally from a MC prototype. This procedure allows the comparison of EMI spectra when using simulated waveforms or real source waveforms. I. INTRODUCTION There are different methods to calculate the conducted EMI generated by power converters, which can be broadly classified in two categories: Field computation methods and equivalent circuit methods. Field computation methods are usually based on Maxwell’s equations solved by means of Finite Element Methods (FEM) [1], with different possible approaches: differential methods, such as Finite-Difference Time-Domain (FDTD) or integral methods such as Method of Moments (MoM). On the other hand, equivalent circuit methods may also follow different approaches, namely: Time domain analysis, frequency domain analysis and combined time and frequency domain. Advantages of the time domain methods are high resolution and accuracy. However, their drawbacks are large computational time (large simulation periods and small simulation steps) and possible problems of convergence. Alternatively, frequency domain methods, based on “source-path-victim” method [2-5] introduce a remarkable simplicity in terms of the modeling, which reduces drastically the computational effort required for its simulation. On the other hand, their drawbacks are frequency limitation and that they are valid only for conducted emissions. Combined methods [6], on which this paper is based, integrate time domain and frequency domain methods. Time domain simulation is used for EMI sources identification, combining short time, small steps simulation and long time with wider steps. Frequency domain methods, as in [5] perform the system simulation following the above described procedure ”EMI source identification → propagation path impedance → derived disturbance”. Since the main EMI sources in power electronics come from the converters switching and basically produce conducted emissions, the combined method is very suitable to analyze CM in such applications. Following this approach, in [4] a boost rectifier is analyzed and a complete impedance model is presented. Also, a three phase voltage source inverter (VSI) study is presented in [3], which introduces a simplified model to study the CM of these converters. Another work that studies CM of VSI converter is presented in [5], where not only the VSI itself is modeled but also the cabling, motor and plane ground link. Another frequency method based on “Modular-terminal-method” is presented in [7], which introduces a Norton equivalent circuit as basic noise source. This idea permits a simplification of the noise source model of VSI by changing each phase by a basic Norton circuit. This procedure allows the study of all VSI configurations by only adding/removing Norton circuits depending on the topology. In [8], CM EMI disturbances are addressed when using MC fed wind turbines. The MC model is based on the indirect inverter-rectifier and virtual DC link approach. The present paper presents an EMI model based on circuit of impedances that represents the parasitic impedances and leakage paths which characterize the CM circuit. This model is based on [3] to successfully address the study of CM disturbances in a direct MC configuration [9]. Using a combined time and frequency domain approach will permit a fast prediction of CM produced by matrix converters. II. COMBINED TIME-FREQUENCY METHOD As pointed out above, the time-frequency method is based on the proper identification of the “source, path and victim” for its further modeling process. The modeling of conducted EMI is essentially based on the solution over the whole frequency range of interest of an equivalent circuit for CM and a different circuit for DM, as illustrated in Fig.1. The noise sources to be introduced in the equivalent circuits come from circuit points with large dv/dt, which generate EMI current through parasitic coupling capacitances or from di/dt, which generate voltage sources at parasitic inductances. These represent respectively the CM and DM sources. Propagation paths are modeled by the real circuit impedances plus the parasitic impedances involved in current