Abstract: Raising of the requirements to admissible levels of conductive interferences forces designers of EMI filters to develop effective means for suppressing the interference. To solve this problem and to improve the filter's effectiveness the algorithm of mains EMI filters designing is proposed. The main purpose of the report is to bring forward a method for calculation of effective EMI filters (which will have high values of an attenuation) in wide frequency range (of five decades - from 10 kHz to 1 GHz). The proposed method was proved in practice during development of a number of EMI filters of FMPZ-1 type, which have the attenuation higher than 60 dB in the frequency range of five decades. An example of the method application in EMI filter designing which is to provide the pre-set attenuation is given in the final part of the report.

Keywords: EMI filter, electromagnetic compatibility (EMC), conductive interference, filter's attenuation, internal impedance, parasitic parameters of filter's components.

1. INTRODUCTION

The widening of use of switched mode power supplies has increase concerns for controlling conducted emission on the AC power line.

During the last years the relevance to search for solution of problems on rising the effectiveness of the radio frequency interference suppressing devices for switched mode power supplies was brought out by the fact, that in 1992-1994 the countries of European Community in accordance with the Council's Directive on the approximation of the laws of the Member States relating to Electromagnetic Compatibility (EC Directive No. 89/336 EEC) have adopted legislation acts on electromagnetic compatibility, which allowed to provide the unified legal basis for solutions of EMC questions concerning technical devices. This Directive was introduced from the 1^st of January, 1996.

Electromagnetic interference (EMI) filters are the most effective and economical means for reducing the level of conductive EMI.

A correct choice of optimal (for ensuring the highest effectiveness) circuit of EMI filter is often hampered by a number of causes, among which the main ones are the following:
• requirement of high attenuation (up to 50 ... 80 dB) in wide range of frequencies (from tens of kHz to hundreds of MHz or ones of GHz);
• the influence of entire internal impedance of switch mode power supply at high frequencies, with this impedance varying considerably with a frequency;
• parasitic parameters of EMI filter's components which should be taken into account when analyzing the filter's effectiveness.

2. DESIGN OF ELECTROMAGNETIC INTERFERENCE FILTER

When there are no strict limitations to the weight and overall dimensions and cost characteristics of the filter, the problem to ensure its high effectiveness can be solved by increasing the number of the filter's stages, and developing a cascade filter on the basis of L-sections. The number of the filter's stages is determined by a value of the attenuation in the frequency range under protection. Having sequentially (step-by-step) overlapped the whole frequency range under protection with the attenuation of separate section of the filter, the EMI filter which provide the required effectiveness in suppressing the electromagnetic interference can be obtained.

As the result of experimental investigations and calculation of attenuation of EMI filters with different circuit layouts it was found that the attenuation (on the average not lower than 60 dB in the frequency range under protection) in real conditions of the application can be provided by the T-section EMI filters in which, together with the low-frequency components, the high-frequency components such as feedthrough capacitors, ferrite rings, beads and others are used.

The necessity to take into account the entire internal impedance of switch mode power supplies requires to simulate the internal impedance of such devices to provide effective operation of the filter. In a number of practical cases the simulation of internal impedance of switched mode power supplies on the basis of experimental investigations of the frequency characteristics of their internal impedance is suggested to carry out using L-C circuits connected in serial or in parallel.

The experiments and measurements of the internal impedance Z_s of switched mode power supplies have shown that for the most of them the following generalized parameters of the circuit components, modeling Z_s as a source of electromagnetic interference, are typical: for parallel - L_{i par} = 0.1 ... 0.5 mH, C_{i par} = 50 ... 1000 pF, for sequential - L_{i seq} = 1 ... 5 mH, C_{i seq} = 5 ... 15 nF.

When designing the EMI filter, calculating and choice of its components as well as when designing impedance coils with effective operation characteristics in wide frequency range, it is important to take into account the influence of parasitic parameters of the filter's components on the level of it's attenuation. The EMI filter with parasitic parameters of the components on the basis of T-section is shown in Fig. 1.

Fig. 1. T-section EMI filter with parasitic parameters of the filter components.

When calculating the filter's attenuation, the compensation of the parasitic parameters of the filter's components (stray capacitance of the chokes and spurious inductance of the capacitors) allows to reveal troughs in the frequency response at high frequencies. When such troughs are present, the design of the filter has to be changed, as well as type of winding of impedance coils, with an aim to achieve the required parasitic parameters from the standpoint to compensate these troughs.

Designing of EMI filters with high attenuation in wide frequency range is a complicated multi-stage task, solution of which is subordinated to some algorithm, implying step-by-step overlapping of the whole frequency range to be protected by the required value of attenuation. This can be achieved only by meeting the requirements and following the recommendations on choice of regular and parasitic components for the EMI filter and their construction.

Generalized flow chart of designing of the EMI filter with high attenuation in wide frequency range is presented in Fig. 2.

Further the steps of filter designing procedure in accordance with this algorithm are considered:

1) depending on the required value of the attenuation, the circuit of EMI filter is chosen, and the number of the filter's sections is determined which are necessary to provide the required attenuation;

2) according to the required value of the attenuation at low frequencies (from tens of kilohertz to ones of megahertz) preliminary parameters of the EMI filter's components are calculated. The preliminary calculation is advisable to carry on using modern software for analysis of electric circuits such as PSpice, Electronic Workbench, Microcap or others;

3) using the data of the preliminary calculation, components of the EMI filter are chosen. Also, constructive features of chokes to be wound are determined to achieve the required parasitic parameters for required attenuation accomplishing (this includes: determination of type and order of the winding). For toroidal chokes a stray capacitance is calculated using the following formula [1]:

C = ( C₂₁ + ( C₃ ї 2 ) ю ( 1 - W₂ ї W₁ )² + 2 C₂₂ ( W₂ ї W₁ )² ) ю 1 ї N   (1)

where N is the number of sections; W₁ and W₂ are the number of turns in the primary and secondary windings correspondingly; C₃ is a capacity between windings, C₂₁, C₂₂ are interlayer capacities of the primary and secondary sections correspondingly:

C₃=(e_gю e₀ ю (g + g) ю l_W ) ї ( 2 ю b )   (2)

C_2i=( 4 e_g e₀ ю r ю n ю l_W ) ї ( ( 4 a_i - p r ) ю ( m - 1) )   (3)

where

e_g = 3 ... 4 is relative dielectric constant of insulation;
e₀ = 8.85.10-14 F/cm;
g is the width of section at midline of magnetic circuit;
l_W is a mean perimeter of the turns, cm;
r_i is the radius of uninsulated wire, ai is the distance between the axes of the wire turns of the adjacent layers, cm;
i indicates the corresponding winding (primary or secondary);
m is the number of layers;
b is the distance from inner layer of winging to magnetic circuit (core), cm [1].
Parasitic inductance of capacitance components is, as a rule, the reference data;

4) having taken into account the parasitic parameters of the components the attenuation of the EMI filter in the whole protected range of frequencies and frequencies of possible troughs in the frequency response of the attenuation are calculated. The calculation of the attenuation is advisable to carry on using modern software for electric circuit analysis such as PSpice, Electronic Workbench, Microcap or others;

5) using the founded resonance frequencies (frequencies of possible troughs in the frequency response) and the value of the required attenuation at these frequencies, the admissible values of parasitic parameters of the EMI filter's components are calculated: - for parasitic inductance of the capacitors:

L_c=1 ї (4p² ю f_{r C}² ю C)   (4)

where f_{r C} is the resonance frequency, C is the nominal value of the capacitance;

- for stray capacitance of the chokes:

C_l=1 ї (4їp² f_{r L}² L)   (5)

where f_{r L} is a frequency of the resonance, L is the nominal value of the choke's inductance;

6) the EMI filter components with admissible values of parasitic parameters are selected. The type of capacitance components is selected regarding to values of parasitic inductances given in reference data books, and the value of stray (parasitic) capacitance of the chokes is adjusted by using correct selection of the type and order of winding of the choke's windings;

7) having taken into account the admissible parasitic parameters of the components the attenuation of the EMI filter in the whole protected range of frequencies is calculated. Then, the frequencies and level of possible troughs in the frequency response of the attenuation at high frequencies (from hundreds of megahertz to ones of gigahertz) are determined;

8) high frequency interference suppressing components - feedthrough capacitors, ferrite rings, beads etc.- are selected to compensate the frequency troughs of the attenuation in the range of high frequencies;

9) practical realization of its designed EMI filter, and experimental verification of effectiveness in the whole protected range of the frequencies.

An example of the EMI filter calculation according to the algorithm is given below.

Initial data for the calculation are as follows:
• the value of attenuation: A=60 dB in the frequency range from 10 kHz to 1 GHz;
• internal resistance of the interference source and receptor R_s = R_r = 50 Wm;
• admissible leakage current for the apparatus, where the EMI filter is used, is set at the level of I_lc = 3,5 mA. The latter determines the total limit admissible capacitance of asymmetric capacitors according to IEC 380 not more than 0.022 mF.
• the mains voltage is 250 V, (50) 60 Hz;
• the load current is 6 A.

1. To simplify the calculation example, T-circuit is chosen (filter shown in Fig. 1).
2. To obtain the attenuation not less than 60 dB at low frequencies of the protected range (from 10 kHz to 1 MHz), inductance of the bridged T-filter should be not less than 35 mH, having taken into account maximal admissible capacitance of asymmetric capacitors (0.022 mF).

   The inductances are calculated using software for analysis of electronic circuits Electronic Workbench. The values of inductances L₁ and L₂ are set equal each to other to reduce labouriousness of calculation and filter design.

3. Asymmetric capacitor is selected with capacity of 0.022 mF of the K73-18 type. The spurious (parasitic) inductance of this capacitors according to reference data is L_c = 0.01 mH.

   The combined (partly - progressive [2], partly - bulk) type of wire winding is chosen for chokes of the filter, since the number of wire layers should be more than two according to preliminary estimations. When winding several layers of the wire, such type of winding provides low (in comparison with other type of windings) values of parasitic parameters of chokes.

   The order of the wire winding is taken such that ensures absence of the core saturation: two windings are reeled in opposite directions, each along the semi-perimeter of the magnetic circuit (core).

   Using data of choke winding, its stray capacitance is calculated. For chokes with the combined type of winding with inductance of 35 mH the stray capacitance is C_L1 = 90 pF.

4. The calculated frequency response of the attenuation, which takes into account parasitic parameters of the components, is shown in Fig. 3 (curve 1).
5. One can see (Fig. 3, curve 1) that within the frequency range 9 MHz ... 1 GHz the requirement to the filter's attenuation of 60 dB is not met.

   The calculated values of parasitic parameters are:

   • parasitic inductance of the capacitors:

     L_c = 1 ї (4p² f_{r C}² C) > 10.8 mH   (6)

     where
     f_{r C} = 3.95 MHz is the resonant frequency,
     C = 0.022 mF is the nominal value of the capacitance;

   • stray capacitance of the chokes:

     C_L1=1 ї (4p² f_{r L}² L ) > 113 pF   (7)

     where
     f_{r L} = 0.78 MHz is the frequency of the resonant,
     L = 35 mH is the nominal value of the choke's inductance.

   Fig. 3. Frequency responses of mains EMI filter's (250 V, 6 A) attenuation.

   
   
6. The measured values of the parasitic parameters are approximately corresponding to the calculated ones, therefore the type of capacitors as well as the order of the choke winding must not be corrected.
7. - 9. To compensate the attenuation fall down in the range of frequencies 4 MHz ... 1 GHz, the high frequency interference suppressing components - feedthrough capacitors, ferrite rings, beads etc. - are selected. Feedthrough filters of the B 23B type with the attenuation of 60 dB at the frequency of 1 GHz are used. Ferrite rings or beads are installed on the leads of the filter. Then effectiveness of the EMI filter is experimentally verified. If required, the number of feedthrough capacitors and ferrite rings is increased. When the requirement to the attenuation of 60 dB is met in the frequency range up to 1 GHz, the development of the required filter can be considered as accomplished one.

The frequency response of real EMI filter's attenuation, which was developed using the abovementioned procedure, is shown in Fig. 3 (curve 2).

Practical application of the algorithm for designing a number of mains EMI filters rated for different currents and providing the attenuation not less than 60 dB in the range of frequencies from 10 kHz to 1 GHz has proved its effectiveness and high accuracy of the results.

3. CONCLUSION

The obtained data allow to carry on calculation of the attenuation of different EMI filters for specific conditions of their application, to select soundly circuit and parameters of the components of the EMI filter, which will provide the required attenuation in wide frequency range.

REFERENCES:

[1] Gorsky A. N., Rusin Yu. S., Ivanov N. R., Sergeeva L. A.: Calculation of electromagnetic components for the secondary power supplies. Moscow: Radio & Svyaz. 1988, 176 p. (in Russian).

[2] Nave, Mark J.: Power Line Filter Design For Switched-Mode Power Supplies. New York: Van Nostrand Reinhold 1991, 205 p.

BIOGRAPHIES:

Vladimir V. Pilinsky, Ph.D. (1973), professor (1993), was born in Saint-Petersburg in 1941.

Main scientific interests are: power electronics & telecommunication, switch-mode power supplies for electronics, electromagnetic compati-bility of radioelectronic technique, CAD electronic systems with EMC providing.

In 1963 he graduated from Eyiv Polytechnic Institute.

In 1973 he obtained Ph.D. degree in electric engineering.

From 1963 till 1980 he was assistant, lecturer, docent at Kyiv Polytechnic Institute.

From 1980 till 1986 he was the Head of the Technical Universities Department at Ministry of Education of Ukraine. From 1986 till now he is the professor at National Technical University of Ukraine "Kyiv Polytechnic Institute", and the Head of the EMC Center, NTUU "KPI".

He is the author of more than 200 published works, including 7 books.

Vyachslav S. Kotelchuk, engineer, was born in Kyiv in 1968.

In 1994 he graduated from National Technical University of Ukraine "Eyiv Polytechnic Institute".

From 1994 till now he joint the EMC Center.

In 1998 he finished a post-graduate course at NTUU "KPI".

He is the author of seven papers and reports.

The field of his scientific interests includes investigation of conductive interferences in power supplies circuits, design of conductive interference suppressers providing EMC of electric apparatus.