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Measurement and Modeling of the Magnetic Near Field Radiated by a Buck Chopper Ouafae Aouine, C´ ecile Labarre, and Franc ¸ois Costa, Member, IEEE Abstract—Static converters are used with increasingly high switching frequencies. Consequently, they impose increasingly severe electromagnetic interference (EMI) constraints in their environment. In order to study and to quantify the radiated perturbations, we have used a test bench for measuring the magnetic field radiated in the near zone. In this paper, we present measurements of the magnetic field radiated by the switching cell of a Buck chopper in the near field at high and low frequencies. By comparing the mappings of the magnetic field over the switching cell with the ones of a circular loop, we have deduced that the switching cell can be effectively modeled by a loop. The loop’s geometric characteristics and its equivalent current flowing in have been determined from near magnetic field data. The model has been validated at high and low frequencies. Index Terms—Magnetic field measurement, near field, power electronics. I. INTRODUCTION Power electronics are used for energy conversion in many applica- tions such as automotive, aeronautics, or household appliances. The Manuscript received August 28, 2007; revised November 16, 2007. O. Aouine is with the Department of Informatique and Automatique (IA), Ecole des Mines de Douai, 59508 Douai, France, and also with Schneider Electric Company, 38050 Grenoble, France (e-mail: aouine@ensm-douai.fr). C. Labarre is with the Department of Informatique and Automatique (IA), Ecole des Mines de Douai, 59508 Douai, France (e-mail: labarre@ensm- douai.fr). F. Costa is with the SATIE (UMR 8029), ENS de Cachan PRES UniverSud, 94230 Cachan, France, and also with the IUFM de Cr´ eteil, Universit´ e Paris 12, 93000 Saint Denis, France (e-mail: francois.costa@satie.ens-cachan.fr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TEMC.2008.922794 magnetic fields they generate may disturb nearby circuits. Moreover, the variations of currents and voltages in converters are very fast and their amplitudes are great, which causes very high electromagnetic dis- turbances over a large range of frequencies. The aim of this research is to measure the magnetic near field radiated by a typical converter, using a test bench that achieves a mapping of the radiated near field, in order to locate and to model the near-zone radiating sources. Section II will depict the test bench for mapping [1]. The near-field scanner measures the magnitude of the magnetic field over a ground plane at a specific height above the power converter. Thanks to this equipment, it is possible to locate the switching cell in the converter, usually constituted by the circuit delimited by the decoupling input ca- pacitor, a diode and a power switch. Actually, in this circuit, the current and voltage exhibit high-level transients (dV DS /dt, dI DS /dt); outside of it, these quantities are quite constant, showing a low-level ripple. This will be described in Section II (see representation in Fig. 1). In Section III, we will present the measurement results of the near-field scanning. A previous study has shown that the switching cell could be modeled by a circular loop, whose geometrical and electrical charac- teristics can be deduced from the near-field mapping [2]. Section IV will recall these aspects. We have adopted the following methodology. At first, a spectral analysis of the magnetic near field radiated locally over the converter is conducted. The maximal amplitude peak frequencies are identified. Next, a near-field mapping is done for each of these frequencies at a defined height above the converter. Thanks to this, the equivalent sources are identified. II. EXPERIMENTAL SETUP A. Test Bench The test bench consists of a near-field magnetic probe, a ground plane (60 cm × 50 cm), a 2-D displacement table, and an electromagnetic interference (EMI) receiver (9 kHz to 2.9 GHz): the probe is connected to the EMI receiver and is mounted on a two-axis displacement table. A computer monitors the probe displacement (along X and Y ) over the converter and records data provided by the EMI receiver [3]–[5]. The displacements along the X - and Y -axes have been chosen to be close to 50 cm. The heat sink (usually large) cooling a power converter acts as a ground plane for electronics. This is the reason why we have chosen to insert a copper ground plane on the displacement table in order to take into account this characteristic of power electronics. All the displacement devices (motors, electronic drivers), which are likely to induce electromagnetic perturbations, are located under the ground plane. All the structural parts, which stand above this area, are made of unreflecting materials (plexiglas, nylon). We used magnetic probes with a negligible radius (5 mm) as com- pared to the wavelength of the interfering signal. These probes average the magnetic field strength in the loop area of the probe head. Their frequency range is 10 kHz to 50 MHz or 30 MHz to 3 GHz. They have been calibrated with a TEM cell in order to determine their antenna factor. Results are presented in the form of mappings of the magnetic near field radiated, the EMI receiver giving rms values, expressed in decibels per microampere per meter (dB·µA/m). B. Converter Under Test Fig. 1 shows a picture of the studied converter. The buck chopper is fed by a 50-V voltage source E; its switching frequency is f s = 20 kHz. It provides a 2-A current I o in the output resistive load when the duty cycle is 0.5. The control signal is transmitted to the MOSFET, 0018-9375/$25.00 © 2008 IEEE