EMI Filter Design Method Incorporating Mix-Mode Conducted Noise For AC Power Line Applications

Transcription

1 EMI Filter Design Method Incorporating Mix-Mode Conducted Noise For AC Power Line Applications Hung-I Hsieh Department of Electrical Engineering National Chiayi University Chiayi, Taiwan, R.O.C. Dan Y. Chen Department of Electrical Engineering National Taiwan University Taipei, Taiwan, R.O.C. Abstract This paper proposed a practical filter design procedure with consideration of the newly discovered mix-mode (MM) conducted EMI noise. Incorporation of the MM EMI considerations in a filter design procedure can lead to significant reductions in filter size. The proposed method can be easily implemented into designing an EMI filter via base-line noise measurements with a balancing capacitor. There is no longer any need for the traditional cut-and-try process in filter design. Design examples are presented in the paper and results are verified by experimental measurements. Based on this theory of MM EMI, the effectiveness of EMI filter X capacitors is then investigated, along with experimental supports. Keywords-Mix-mode EMI; balancing capacitor; filter design procedure; AC power line I. INTRODUCTION Electromagnetic interference (EMI) plays an important role in the use of electronic devices. It is necessary for many subcircuits to operate in close proximity for purposes such as power electronics applications. Thus, EMI filters turns out to be the major concern for practical circuit designers. Recently, progression has been constructed in this regard, and more and more new methodologies have been described for the analysis and design toward a better understanding of the EMI noise suppression techniques and EMI filters [1-4]. Generally speaking, the conducted EMI noise consists of the commonmode (CM) noise and the differential-mode (DM) noise. An EMI filter is rightly separated into two parts, one for CM and the other for DM noise reduction. However, it was discovered recently that there exists a third mode of noise in off-line power converters; the mix-mode (MM) conducted EMI noise [5, 6]. Traditional filter design procedure does not include this important phenomenon and often leads to unnecessarily large filter size design [7]. The main purpose of the paper is to incorporate the newly found MM conducted EMI noise phenomenon into a practical ac power line filter design procedure. Understanding of the MM conducted noise coupling theory gives better insight into filter effectiveness, which may lead to a more compact filter design. Accordingly, the reduction in size of an EMI filter can be achieved. In this paper, a quick and systematic filter design procedure including the MM EMI phenomenon will be presented. Practical filter design examples will be applied to illustrate the proposed design procedure. Experimental results will also be used to support the validity of the proposed design procedure. Further, the effectiveness of X capacitors on EMI filter will also be considered with this theory, along with experimental proofs. II. FILTER DESIGN METHOD INCORPORATING MIX-MODE CONDUCTED EMI NOISE Fig. 1 shows a flow chart for the proposed filter design procedure. To use this design procedure, one needs a noise separator [8], a balancing capacitor, a dual-channel LISN and a spectrum analyzer so that all modes of noise can be clearly identified and a filter designed accordingly. In step 1 of the chart, the base-line (without filter) CM and DM noises of the converter should be measured respectively by using a noise separator and a balancing capacitor. In step 2, the CM and DM attenuation requirements (V req,cm ) db and (V req,dm ) db should be determined. Based on the information obtained in step 2, the filter corner frequencies and component values of an EMI filter will be determined in step 3 and step 4. With the filter design obtained from step 4, one should be able to meet the low frequency specification. However, there is a likelihood of reducing or even eliminating an X capacitor of EMI filter in the design step 5, and emission requirements (EMI specifications) can still be met. In the following, explanation of the fine-tuning mechanisms of the X capacitor will be presented with experimental verifications to demonstrate the analysis. Theoretically speaking, the filter design obtained in step 5 should meet both the low- and high-frequency specifications. If the design also meets the high-frequency specifications, then the design process ends. If not, then other measures are needed to fix the high-frequency problems based on the initial design in step 6. III. STEP-BY-STEP FILTER DESIGN PROCEDURE FOR OFF- LINE SWITCHING POWER SUPPLIES Based on the proposed design procedure, practical design examples are given below to illustrate the design steps described above. EMI filters are designed for a 70 khz 150 W off-line flyback switching power converter using the proposed design procedure incorporating MM conducted EMI noise considerations, as shown in Fig. 2. The input voltage of the converter is 60 Hz/110 RMS V AC and the output voltage is 12 V DC. The limit is FCC Class B conducted emission requirement. The step-by-step filter design procedure with 1617

2 experimental measurements following the flow chart, shown in Fig. 1, will be given below. A. Base-line EMI Measurement using a Balancing Capacitor Place a balancing capacitor C x across the lines, as shown in Fig. 3, and measure both the CM and the DM noise using a noise separator. To provide a balancing impedance path and therefore eliminate MM conducted noise, the impedance of balancing capacitor should be much less than the 50 ohm LISN resistor and the impedance of LISN capacitor, i.e. Z Cx << ( R LISN + Z C-LISN ), at the lowest frequency of interest. However, the impedance of LISN capacitor at noise frequency is much smaller in magnitude than the 50 ohm resistor, hence can be neglected. Therefore, select the balancing capacitor value according to (1). C x >> 1 / (2π f L R LISN ). (1) For the requirement of FCC Class B conducted emission limit, the beginning of frequency f L is 450 khz. From the calculation, a 0.1μF capacitor is selected for the balancing capacitor. Fig. 4 shows the base-line EMI for both the DM and the CM noise measurements. It can be seen that both well exceed the FCC Class B limit. B. Determination of Filter Attenuation Requirements The filter attenuation requirements can be determined by subtracting the magnitude of the limit from the base-line EMI noises. (V CM,measured ) db and (V DM,measured ) db are obtained in Section III-A and (V limit ) db is the conducted EMI limit. The amplitude 3dB is a required correction factor because both CM and DM measurements are 3dB higher than the original value by use of a noise separator [8]. Fig. 5 shows the results. 1) CM attenuation requirement (V req,cm ) (V req,cm ) db = (V CM,measured ) db (V limit ) db + 3dB. (2) 2) DM attenuation requirement (V req,dm ) (V req,dm ) db = (V DM,measured ) db (V limit ) db + 3dB. (3) C. Determination of Corner Frequencies Owing to the resonance frequency of filter components, inductor (L) and capacitor (C), the attenuation is approximated by a 40dB/decade slope going through the derivation of DM and CM section equivalent circuit of the typical EMI filter topology shown in Fig. 2. To obtain the corner frequencies, a dashed line with 40 db/decade slope is therefore drawn tangential to the attenuation requirements. The horizontal intercepts of the lines determine the corner frequencies f R,CM and f R,DM as shown in Fig. 5 (With a comparison to the conventional design procedure, i.e. without using a balancing capacitor to reduce MM EMI, the DM corner frequency is f R,DM = 44 khz). In both figures, Point A indicates the filter attenuation requirement at 450 khz. It is noted that the 40 db/decade slope line, which is tangential to the attenuation requirements, happens to hit the attenuation requirement at the lowest frequency (490 khz) in both modes in this example. But this is not necessarily so in other cases. D. Determination of Filter Component Values 1) CM component values L C and C y To meet safety leakage current requirement, C y is normally limited to 3300 pf for 60 Hz operation. L C value can be calculated from (4). This equation is derived from the fact that the resonant frequency of L C and 2C y determines the CM corner frequency. L C = (1/2π f R,CM ) 2 (1/2C y ) = 2.66 mh. (4) Figure 1. The proposed procedure for filter design including the considerations of mix-mode conducted EMI noise. 1618

3 2) DM component values C x1, C x2, and L D The corner frequency for the DM section of the filter is determined by the resonant frequency of 2L D and C x1 or C x2. In Figure 2. An EMI filter is designed for an off-line flyback power converter using the proposed design procedure incorporating MM EMI considerations. this design approach, C x1 value is first assumed to be equal to that of C x2 in order to obtain a simplified equation for corner frequency. In reality, C x1 and C x2 can be different, and the value of C x2 can be fine-tuned, as will be illustrated later. For a given corner frequency f R,DM of 78 khz obtained in Section III- C, there are many sets of solution for L D and C x1 (or C x2 ) from (5). Two sets of design are given below: a) If C x1 and C x2 are selected to be 0.22 μf, L D is about 9.5 μh. b) If L D is selected to be 30 μh, C x1 and C x2 are both 70 nf. C x1 = C x2 = (1/2π f R,DM ) 2 (1/2L D ). (5) IV. EXPERIMENTAL VERIFICATIONS Figure 3. A balancing capacitor C x is used for MM EMI suppression. A. Summary of the Experimental Results The two EMI filter designs obtained from the proposed procedure are summarized below. For comparison, a design obtained from the conventional design procedure (without using a balancing capacitor) is also shown. All the designs were experimentally verified. 1) Two Filter Designs from the Proposed Procedure a) Design A: C x1 = 0.22 μf, C x2 = 0, L D = 9.5 μh, C y = 3300 pf and L C = 2.66 mh (The experimental result of design A is presented in Fig. 6). b) Design B: C x1 = 70 nf, C x2 = 0, L D = 30 μh, C y = 3300 pf and L C = 2.66 mh (The experimental result of design B is presented in Fig. 6). Figure 4. Base-line EMI measurements with a balancing capacitor: DM noise, CM noise. Figure 5. Filter attenuation requirements and determination of corner frequencies: DM, CM. 1619

4 2) A Filter Design from the Conventional Procedure c) Design C: C x1 = C x2 = 0.22 μf, L D = 30 μh, C y = 3300 pf and L C = 2.66 mh (The experimental result of design C is presented in Fig. 6(c)) B. Comments on Results Fig. 6 shows the results of measured total EMI noise for (c) Figure 6. Total EMI noise measurements: design A, design B, (c) design C. experimental verifications. It can be seen from the proposed designs A, B and the conventional design C that significant reductions in filter size are achieved with the proposed procedure. However, it can be seen that the measured noise level in the 20 MHz range exceeds the limit. This is caused by high-frequency phenomena including: the parasitic capacitance effect of inductors, the parasitic inductance effect of capacitors, the permeability reduction of choke core, and the near-field radiation coupling. These phenomena are often observed in switching power supplies less than 200 W. However, the discussion of high-frequency problems is outside the scope of the present paper. The goal of the present design approach is to quickly obtain a filter design that meets the low-frequency specification. The design obtained will then be the basis for high-frequency modification if necessary. C. Fine-Tuning X Capacitor Value for Space- and Cost- Savings on EMI Filter It will then be mentioned that there is an expectation of reducing or even removing an X capacitor of EMI filter and the designs still meet the low-frequency specification. The reason for this is that the conducted EMI suppression mechanisms of filter components in off-line switching power converters are ascribed to two different concepts: one is the impedance mismatch and the other is the conducted mix-mode EMI noise current balancing. It can be seen from Fig. 2 that the function of C x2 in the DM filter is conventionally thought to provide an impedance mismatching element against the high noise source impedance of off-line power supplies. However, the discovery of the MM EMI phenomenon indicates that the filter capacitor C x1 and the series-connected filter capacitor (1/2)C y also serve as the balancing function. For example, Y capacitors can be a balancing capacitor as long as their impedance meets the MM noise current balancing condition: Z Cy /2 << (50 + jw(l C + L D )). For the reason that the impedance of LISN capacitor at noise frequency is much smaller in magnitude than the R N 50 ohm resistor of LISN, hence can be neglected. Because of the small Y capacitor value commonly used, the chance of meeting the balancing condition is enhanced if there are CM and/or DM filter chokes used in the filter. For this reason, it is easier to meet the balancing condition if Y capacitors are placed on the power supply side. Generally speaking, consideration of the relatively large CM noise source impedance of power supply, it is also better to put the Y capacitors on the power supply side for CM noise reduction. Hence, the balancing condition is indeed satisfied by the designs A and B obtained, and the C x2 can be reduced or even removed from the designs, and the EMI requirement will still be met. Fig. 7 show the comparison of measured EMI for different X capacitor arrangements from design A. Fig. 7 shows the DM noise if both C x1 and C x2 are present. It is interesting to note that if C x1, instead of C x2, is removed; there is a big increase in the noise level measured, as shown in Fig. 7. The reason for this is that the impedance mismatch criterion does not be satisfied in this regard. However, when C x2 is removed, there is very little change in noise level measured, as shown in Fig. 7(c). Similar measurement results were verified by design B shown in Fig

5 V. CONCLUSIONS This work first presented a method for including the mixmode conducted EMI noise considerations in the proposed filter design procedure. Incorporation of the MM phenomenon can lead to significant reductions in filter size. The approach was verified by experimental measurements. This practical design procedure provides a quick and systematic approach to obtain an initial filter design at least meeting low-frequency part of specification. Once designed and built, the filter may need little modification to meet the high-frequency specification. Understanding of the MM EMI coupling theory gives better insight into filter effectiveness, which may lead to design guidelines for more compact filter design. In this paper, the explanation of fine-tuning mechanisms of EMI filter X capacitor was presented with experiments to verify the analysis. The effectiveness of the two separate actions: impedance mismatch and MM EMI noise current balancing were explained with the proposed theory, along with experimental supports. Hence, great savings on both size and weight of EMI filter were achieved. Discussions on the EMI (c) (c) Figure 7. Comparison of DM noise (Design A): DM noise if both C x1 and C x2 are present (C x1 = C x2 = 0.22 μf), DM noise if C x1 is removed (C x1 = 0 and C x2 = 0.22 μf), (c) DM noise if C x2 is removed (C x1 = 0.22 μf and C x2 = 0). Figure 8. Comparison of DM noise (Design B): DM noise if both C x1 and C x2 are present (C x1 = C x2 = 70 nf), DM noise if C x1 is removed (C x1 = 0 and C x2 = 70 nf), (c) DM noise if C x2 is removed (C x1 = 70 nf and C x2 = 0). 1621

6 filter fine-tuning mechanism may exploit a new understanding to achieve a more compact filter were given. Understandings of the issues presented in this paper allow sophisticated designers to have a better grip on this elusive subject. REFERENCES [1] S. Wang, F. C. Lee, and J. van Wyk, A study of integration of parasitic cancellation techniques for EMI filter design with discrete components, IEEE Trans. Power Electron., vol. 23, pp , November [2] T. Neugebauer and D. Perreault, Parasitic capacitance cancellation in filter inductors, IEEE Trans. Power Electron., vol. 21, pp , January [3] S. Wang, F. C. Lee, and W. Odendaal, Characterization and parasitic extraction of EMI filters using scattering parameters, IEEE Trans. Power Electron., vol. 20, pp , March [4] S. Wang, F. C. Lee, W. Odendaal, and J. van Wyk, Improvement of EMI filter performance with parasitic coupling cancellation, IEEE Trans. Power Electron., vol. 20, pp , September, [5] S. Qu and D. Chen, Mixed-mode EMI noise and its implications to filter design, IEEE Trans. Power Electron., vol. 17, pp , July [6] M. Jin and M. Weiming, A new technique for modeling and analysis of mixed-mode conducted EMI noise, IEEE Trans. Power Electron., vol. 19, pp , November [7] Sheng Ye, W. Eberle, and Yan-Fei Liu, A novel EMI filter design method for switching power supplies, IEEE Trans. Power Electron., vol. 19, pp , November [8] Shuo Wang, F. C. Lee, and W. Odendaal, Characterization, evaluation, and design of noise separator for conducted EMI noise diagnosis, IEEE Trans. Power Electron., vol. 20, pp , July