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Analysis of radiated EMI for power converters switching in MHz frequency range

Analysis of radiated EMI for power converters switching in MHz frequency range
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    ΦΦΦΦ  Abstract -- The higher switching frequency in combination with di/dt loops and dv/dt nodes in the power stages of high frequency power converters generates higher order harmonics which cause Electro Magnetic Interference (EMI). It is commonly perceived that high frequency converters are more vulnerable to radiated EMI, thus, it is very important to analyze the radiated emission of these converters. According to the author’s knowledge, the analysis of the radiated emission of power converters switching in the MHz frequency region has not been presented until now. Therefore, in this paper, the measurements, and analysis of the near field radiated emissions of these emerging power converters is presented. These measurements are beneficial in the early design stage of power converters. Both E-field and H-field and captured and analyzed for a half bridge converter switching at 3 MHz and at the output power level of 25 W. The effects of the magnetic and electric field emissions with the addition of a Y-capacitor and secondary side common mode choke are analyzed.  Index Terms -- Common Mode, Differential Mode, Electro Magnetic Interference, Electro Magnetic Compatibility I. I NTRODUCTION  he switching frequency of power converters is increased in order to increase their power density and improve their dynamic performance. With the utilization of emerging power semiconductor devices such as super- junction, GaN and SiC switching devices, the power converters are designed and developed in the MHz frequency region. The development of high frequency multi-layered PCB and hybrid (POT+I) core power transformers is another revolutionary step for the design and implementation of these converters [1]−[2]. These power converters are compact in size and are highly energy efficient. In the design of these high frequency power converters surface mount devices (SMD) are generally used which are better than through-hole (leaded) devices from the EMI point of view. The SMDs are smaller in size, have reduced parasitic components and a closer placement of components is possible. As a result of this, the length of the return current path is reduced. It is a common belief that high frequency power converters are more vulnerable to EMI. Therefore, it is important to analyze the EMI spectrum of these converters. The analysis of conducted EMI for half bridge converters, switching in frequency range of 1−3 MHz, is presented in the research work [3]. The radiated emissions based on the harmonics components of two and three layers multilayered coreless PCB transformers are estimated in [4]. Since the PCB and hybrid core power transformers are used in high frequency power converters, it is important to analyze their radiated EMI in switching frequency range of 1−4 MHz. These converters are in an early design phase and the analysis of radiated noise emission is extremely important in order to develop them according to the Electromagnetic Compatibility (EMC) regulations, such as the International Special Committee on Radio Interference (CISPR) and the Federal Communication Commission (FCC) etc. It is also very important to suggest measures in order to suppress or eliminate the EMI generated by these converters. In the early design stages of power converters, it is not necessary to characterize the radiated EMI at a special EMC laboratory. Therefore, near field measurements are helpful to pin point the sources of radiated EMI and, based on these results, certain measures should be taken to suppress or eliminate it. The analysis of near field measurements is valuable because it confirms the frequencies and nature of offending emissions and the high frequency response of the circuit. These measurement techniques are of assistance in increasing throughput at a lower cost. The near field probes have finite dimensions, therefore, the orientation of probes or device under test can affect the measurements and thus these measurements only provide relative results. The repeatability of measurements does not provide the same results if the orientation of probe or device is slightly changed. By performing these measurements, the types and strengths of the electric and magnetic fields in different areas of the circuits can be analyzed. Therefore, based on these measurements, the sources of radiated EMI can be detected and remedial measures can be performed in order to solve the problem. In this paper, near field measurements of a half bridge converter, switching at 3 MHz are analyzed. The HAMEG HZ540 EMI probe kit is used for the measurements. The Electric Field (E-field) and Magnetic Field (H-field) strengths are measured. The harmonics amplitudes are plotted and particular measures are taken to suppress the harmonics amplitudes. Analysis of radiated EMI for power converters switching in MHz frequency range A.   Majid 1 , J. Saleem 2 , F. Alam 3 ,   K. Bertilsson 4   1,2,4  Department of Electronics Design, Mid Sweden University,  Holmgatan 10, 851 70 Sundsvall, Sweden 3 SEPS Technologies AB,  Holmgatan 10, 851 70 Sundsvall, Sweden abdul.majid@miun.se T    II. H IGH FREQUENCY HALF BRIDGE CONVERTER USED FOR NEAR FIELD MEASUREMENTS  The near field radiation measurements are performed for an unshielded AC-DC half bridge converter switching at 3 MHz and at an output power level of 25 W. The converter has an input line filter and it is not shielded. The hybrid (POT +I) core center tapped 4:1 power transformer and GaN power MOSFETs are used in the converter. These hybrid transformers have lower values of inter-winding capacitance, i.e, 22.2 pF, and leakage inductance of the secondary side, i.e, 28 nH [2], which are very important from an EMI point of view. The leakage inductance of the power transformer resonates with its inter-winding capacitance and the junction capacitances and secondary side diode. As a result of this, high CM noise peaks occur [5]. Theoretically, the resonance of the leakage inductance with inter-winding capacitance of these power transformers occurs at around 200 MHz. Therefore the near field radiated spectrum of the converter is analyzed in order to investigate this resonance effect on the radiated emission. The analysis of both the H-field and E-field radiated emissions are presented in sections III and IV respectively. III. A NALYSIS AND SUPPRESSION OF RADIATED MAGNETIC FIELD FOR POWER CONVERTER AND CABLES  The radiated signals of a half bridge power converter are measured by using a HAMEG H-Field probe HZ552. These probes provide a better reliability than the conventional field probes. The probe is fixed by means of a nonconductive fixture and connected to a HAMEG HMS300 Spectrum Analyzer. The measurements are performed in the very near field area (at approximately 1-2 cm from the converter and cables). With the assistance of these probes, the effectiveness of the filters can also be judged by measuring the radio frequency interference (RFI), which is conducted along the cables [6]. The measurement results are analyzed by the variation of certain circuit parameters.  A. Power converter H-field measurement The magnetic components such as the inductors and power transformer are the main sources of magnetic field radiations. Because of non-ideal magnetic cores, fringing fluxes exist, which can cause a magnetic field [7]. The major causes of magnetic fields are high currents and low voltages [8], therefore, in order to investigate the sources of radiated magnetic fields, the probe is placed at a distance of 1 cm from the power transformer and secondary side rectifier diodes. The results are plotted in Fig. 2 (a). It is observed that the radiated H-field spectrum contains numerous clusters of higher harmonics. The first cluster contains the highest harmonics peak (approximately 42 dB µV) which occurs at around 200 MHz. The second peak (approximately 35 dB µV) occurs between 250−300 MHz. These harmonics are mainly due to the resonance effect of transformer leakage inductance and inter-winding capacitance. In order to suppress CM EMI, the C  Y   capacitor is connected between the primary and the secondary grounds as shown in Fig. 1. The addition of the C  Y   capacitor has shown a considerable reduction in the amplitude of conducted common mode EMI in these converters [2]. Therefore, the measurements are performed to analyze its effect on radiated emission. From the results of Fig. 2 (b) it can observed that with the addition a 1.5 nF C  Y   capacitor, the radiated emission from the converters is reduced. Almost all the harmonics are suppressed especially the peaks which are prominent in Fig. 2 (a) are significantly reduced. Fig. 1. Half bridge converter with Cy 10 2 10 3 202530354045 Frequency (MHz)    L  e  v  e   l       (      d      B     u      V      )   (a) 10 2 10 3 202530354045 Frequency (MHz)    L  e  v  e   l       (      d      B     u      V      )   (b) Fig. 2. Secondary side H-Field spectrum (a) without C  Y   (b) with C  Y        B. Analysis and Suppression of radiated H-Field from secondary side cables In order to analyze the radiations from cables, the measurements are performed by placing the H-Field probe at a distance of 1 cm from the cables. The EMI generated from these converters may spread while diffusing on the conductor surface of the power transmission lines [9]. The charging and discharging of the load capacitor causes CM EMI, which propagates along the secondary side cables and through the secondary side ground [8], and as a result, radiations from the power lines can also occur. Therefore at higher switching frequencies, the dominant propagation mode of the cables becomes radiation instead of conduction [10]. The H-field measurements of the secondary side cables are shown in Fig. 4 (a). It is observed that the radiations from the cables occur between a frequency range 30−400 MHz. The two peaks, which were prominent in the measurement results of section-II A,  are also present in these measurements. In addition to these two peaks, there are more harmonics in the frequency range of 30−200 MHz. The measurement results of section-II A  reveal that the addition of a 1.5 nF C  Y   capacitor in converter circuit proves to be of assistance in reducing the radiated H-field from the converter. In order to analyze the effect of this C  Y   addition on the radiations from the secondary cables, the measurements are performed and the H-field spectrum is shown in Fig. 4 (b). It is observed that the harmonics peaks around 200 MHz are completely suppressed. There are still some harmonics between 90−160 MHz at a level of 35 dBµV and 250−300 MHz at a level of 32 dBµV. In order to reduce the interference due to the cables, it is desired that these harmonics should be suppressed. In order to further reduce the emission from the secondary side cables, a common mode choke is placed on the secondary side of the power converter as shown in Fig. 3. It is obvious from the analysis of the H-field measurements on the cables (Fig. 4 (a) and (b)) that the harmonic amplitudes are higher between the 30−300 MHz frequency range. Therefore it is desired that the CM choke should have maximum attenuation within this frequency range. The attenuation of CM choke used for the reduction of radiated emission from the cables is shown in Fig. 5. The measurements performed with the addition of the Y-capacitor and the secondary side common mode choke are shown in Figure 6. It is obvious from the results that there is considerable suppression of the harmonic amplitude and the noise spectrum is at the noise floor of the spectrum analyzer. Fig. 3. Half bridge converter with secondary side choke 10 2 10 3 2022242628303234363840 Frequency (MHz)    L  e  v  e   l       (      d      B     u      V      )   (a) 10 2 10 3 2022242628303234363840 Frequency (MHz)    L  e  v  e   l       (      d      B     u      V      )   (b) Fig. 4. Secondary side cables H-Field (a) without C  Y    (b) with C  Y    10 0 10 1 10 2 10 3 -45-40-35-30-25-20-15-10-50510 Frequency (MHz)    A   t   t  e  n  u  a   t   i  o  n       (      d      B     u      V      )   CM attenuationDM attenuation   Fig. 5. Secondary side choke attenuation of    10 2 10 3 2022242628303234363840 Frequency (MHz)    L  e  v  e   l       (      d      B     u      V      )   Figure 6 Secondary side cables H-Field with choke  IV. ANALYSIS AND SUPPRESSION OF RADIATED ELECTRIC FIELD (E-FIELD) In order to analyze the radiated E-field, a highly sensitive HAMEG E-Field probe HZ551 is used. It has the capability to measure the entire radiations from the circuit. This probe can be used to perform relative measurements for pre-compliance tests [6]. The E-field probe is usually placed at a distance of 0.5 to 1.5 meters from the RFI source. The near field measurements are not absolute therefor, in order to know the ambient environment level, the E-Field is measured when the converter is off and the frequency range (30 MHz to 1 GHz) of interest is swept. This is considered as the reference for E-Field measurements. When the converter is on, the measured signals consist of both the ambient and the converter emissions. All the E-field measurements discussed in this paper are performed by placing an E-field probe at a distance of 0.7 meters from the power converter. First of all, the measurements are taken for the half bridge converter without using the C  Y   capacitor and a secondary side CM choke. The comparison of the power converter radiated E-field spectrum with the ambient or reference field is shown in Fig. 7. It is observed that, in the frequency range of 30-600 MHz, the amplitude of the harmonics is higher than the reference level. From the analysis of the H-field in section-III, it was observed that the radiations occur from the converter as well as from the cables. The same phenomenon is observed in E-field measurements. There are higher peaks of harmonics between 30-200 MHz and between 250-300 MHz in the E-field spectrum (Fig. 7). These peaks were also visible in the spectrum of the H-field measurements of the transformer (Fig. 2 (a)) and the secondary side cables (Fig. 4 (a)). However, in the E-field emission spectrum, there are additional peaks at around 400 MHz, which are not present in the H-field measurements. From this analysis, it can be inferred that the transformer, as well as the secondary side cables, have both E and H field radiations. 10 2 10 3 2030405060708090 Frequency (MHz)    L  e  v  e   l       (      d      B     u      V      )   E-field of converterReference   Fig. 7. Comparison of E-Field measurements of the converter (without adding C  Y   and choke) with ambient field  A. Analysis of E-field with addition of C  Y   capacitor The analysis of the H-field measurements presented in section-II B reveal the strength of radiated field with the addition of C  Y  . Therefore, E-field measurements are performed for the half bridge converter after the placement of a 1.5 nF Y-capacitor. The comparison of the E-Field strength with and without C  Y   is shown in Fig. 8. From the comparison results it is observed that the harmonics amplitudes around 200 MHz are suppressed. However some additional harmonics components appear within the 90-150 MHz frequency range. The same phenomenon was observed in the measurements of the H-field radiations from the secondary side cables (Fig. 4(b)). Therefore, it can be concluded that the addition of a Y-capacitor has a similar effect for both the E and H field radiations from the cables. 10 2 10 3 2030405060708090 Frequency (MHz)    L  e  v  e   l       (      d      B     u      V      )   Without CYWith CY   Fig. 8. Comparison of E-Field strength with and without C  Y     B. Analysis of E-field with addition of secondary side common mode choke The common mode choke is used on the secondary side of the power converter and this resulted in a significant reduction of the radiated H-field strength from the output cables. Therefore, in order to analyze the effect of the CM choke placement on the E-field strength, the measurements are performed for the converter having both a 1.5 nF C  Y    capacitor and a secondary side choke. 10 2 10 3 2030405060708090 Frequency (MHz)    L  e  v  e   l       (      d      B     u      V      )   E-field with chokeE-field without choke   Fig. 9. E-field spectrums comparison with and without CM Choke 10 2 10 3 2030405060708090 Frequency (MHz)    L  e  v  e   l       (      d      B     u      V      )   With CY and chokeReference   Fig. 10. Comparison of E-Field strength with reference The E-field spectrum for the converter, with and without CM choke, is shown in Fig. 9. It is observed that with the addition of the CM choke on the secondary side, that the harmonics amplitudes in range of 70-150 MHz are suppressed. The cluster of harmonics around 200 MHz is also suppressed. The E-field measurement results of the converter (having both  C  Y   and CM choke) are compared with the reference spectrum (noise floor of the spectrum analyzer) as shown in Fig. 10. It is obvious that majority of the harmonics are suppressed, however there are still some harmonics between 30-70 MHz and 200-300 MHz frequency range. From the analysis of near field measurements, it is observed that the placement of a 1.5 nF C  Y    capacitor and secondary side common mode choke are necessary for a reduction of the radiated EMI. The RF shielding of the converter will further reduce the emission. V. C ONCLUSION  The focus of this paper is to measure and analyze the near field radiated EMI of the half bridge converter, switching at the 3 MHz frequency range. Both the E-field and H-field measurements are performed by using HAMEG near field probes. From the measurement results, it is observed that these high frequency power converters and, additionally their output cables, generate radiated EMI. It is observed that the addition of a 1.5 nF C  Y   capacitor is helpful for the reduction of both E and H fields due to power converter circuit as well as secondary side cable. In order to eliminate the radiated emission from the output cables, a common mode choke is used on the secondary side of the power converter. It is observed that, with the addition of the CM choke, both E and H-field strengths are significantly reduced. REFERENCES   [1]   Kotte, H. B, Ambatipudi, R, Bertilsson, K, "High-Speed (MHz) Series Resonant Converter (SRC) Using Multilayered Coreless Printed Circuit Board (PCB) Step-Down Power Transformer," Power  Electronics, IEEE Transactions on  , vol.28, no.3, pp.1253,1264, March 2013. [2]   Kotte H, Ambatipudi R, Haller S, Bertilsson K, “A ZVS Half Bridge DC-DC Converter in MHz Frequency Region using Novel Hybrid Power Transformer”, Proceedings of International Exhibition and Conference for Power Electronics, Intelligent Motion, Power Quality (PCIM) 2012, 8-10 May 2012, Nuremberg, Germany.. 2012;:399-406. [3]   Majid. A, Saleem. J, Alam. F, Bertilsson, K, “EMI Suppression in High Frequency (MHz) Half Bridge Converter” Paper submitted for Elektronika ir Elektrotechnika ISSN 1392-1215 (Lithuania) [4]   Ambatipudi, R.; Kotte, H.B.; Bertilsson, K., "Radiated emissions of multilayered coreless printed circuit board step-down power transformers in switch mode power supplies," Power Electronics and  ECCE Asia (ICPE & ECCE), 2011 IEEE 8th International Conference on  , vol., no., pp.960,965, May 30 2011-June 3 2011 [5]   Kong. P, Lee. F.C, "Transformer structure and its effects on common mode EMI noise in isolated power converters,"  Applied Power  Electronics Conference and Exposition (APEC), 2010 Twenty-Fifth  Annual IEEE   , vol., no., pp.1424-1429, 21-25 Feb. 2010. [6]   Hameg Near Field catatog http://www.hameg.com/downloads/flyer/HAMEG_FLYER_E_HZ540_HZ550.pdf  (Last accessed June 4, 2013) [7]   Yu Chen; Xuejun Pei; Songsong Nie; Yong Kang, "Monitoring and Diagnosis for the DC–DC Converter Using the Magnetic Near Field Waveform,"  Industrial Electronics, IEEE Transactions on  , vol.58, no.5, pp.1634,1647, May 2011 [8]   Rashid. M. H, “Electronics Handbook: Devices, Circuits, and Applications” 3 rd   Edition ISBN 978-0-12-382036-5  pp 1103-1104 [9]   Mutoh, N.; Nakashima, J.; Kanesaki, M.; , "Multilayer power printed structures suitable for controlling EMI noises generated in power converters,"  Industrial Electronics, IEEE Transactions on  , vol.50, no.6, pp. 1085- 1094, Dec. 2003 [10]   Tim. W, “EMC for Product Designers” 4th edition ISBN–13: 978-0-75-068170-4.
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