Abstract: An intensive widening of the functional capability of radioelectronic equipment results in strengthening of standards of power supplies. So the development engineers are facing one more "headache" - together with the problem of suppression of radiofrequency interferences (RFI) - not to admit RFI, voltage dips, short and long supply interruption and other troubles from the mains to the load. Here an uninterruptible power supplies (UPS) can help, but they are constructed, as a rule, on the basis of switched mode converters, adding the problem connected with electromagnetic compatibility (EMC) too. Therefore the tasks of the paper are: to demonstrate the different UPS contribution of the total RFI level and to point there suppressing qualities. For these tasks solving simplification of the simulation algorithm of RFI formation and propagation in the system "AC Mains-UPS-Load" is suggested.

Keywords: Electromagnetic Compatibility, Radiofrequency Interference, Switched Mode Power Supply, Uninterruptible Power Supply, Simulation.

1. INTRODUCTION

As it's well known, the situations deal with depature of the mains voltage from the norm and even voltage dips (from some mains frequency periods till some seconds or minutes), short and long supply interruptions are possible [1,2]. Nowadays for the protection of the load from such troubles the systems of guaranteed supply are emloyed. To choice of the most suitable UPS it's necessary to solve the complicated task:
1) what type UPS to prefer: "off-line", "on-line" or "line-interactive"?
2) to foresee, what interferences this UPS produces.

The latter problem in connected with UPS inevitably, because of the fact that UPS are constructed on the base of switched mode stages - one of the most widespread sources of RFI.

Below the methodic and results of experimental testing of the system "AC Mains - UPS - Load" to find the levels of RFI, created each of "participants" and applied as to the mains and to the load, are described.

Naturally, to make the best decision about choice of the investigated system structure it's necessary to look into a question beforehand - on the stage of conceptual design.

With this purpose the algorithm of fast simulation of the system "AC Mains - UPS - Load" is suggested.

Such an approach makes it possible to get the preliminary information about RFI levels, omittng the bulky and labour consuming stage of full-scale testing.

2. METHODIC OF THE EXPERIMENT

Here the methodic of EMI experimental investigation in the system "AC Mains - UPS - Load" is described.

The aim of this investigation is to ascertain the interference characteristics of UPS as power supply with switched mode unit. In the experiment the UPS working with the load as the computer is examined. The important feature of the system "AC Mains - UPS - Load" is the switched mode power supply of the computer. So EMI propagation from the mains to the load and the EMI arised on the mains input of the system are investigated.

The block diagram of the testing panel is presented in the fig. 1.

Fig. 1. Block diagram of the testing bed

It consists from the units: line impedance stabilization network (LISN), selective microvolmeter SMV-11 (includes build-in generator of sinusoidal signals with fixed level 0,1V-100dB), generator of sinusoidal signals G3-112 with signal levels up to 10 V (140 dB) in the frequency range 0,02...10 MHz.

Experimental evaluation of the external (from AC Mains) interferences attenuation and their influence on visual characteristics of the computer monitor and computer serviceability are carried out.

The interferences with the levels up to 10 V in frequency range 0,02...30 MHz are examined. The computer is connected to AC Mains directly or through the UPS.

Widely used types of UPS, made by company "Best Power" (Ukraine), are examined: "on-line" (UPS "Sola"), "off-line" (UPS "Patriot") and "line-interactive" (UPS "Fortess") [3].

Measurement of the conductive EMI is realized by the standard methodic [2] at next position of the switches: S1 - 1, S2 - 1, S3 and S4 - 1 or 2.

At appreciation of the input EMI sinusoidal signal with fixed level 0,1 V in the frequency range 0,01...30 MHz (position 1 of the switch S5) or signal with regulated level from 0,1 to 10 V in the frequency range 0,02...10 MHz (position 2 of the switch S5) is applied to the LISN output terminals. EMI voltage levels are measured by the selective microvolmeter on the output terminals of the computer power supply (the switch S6 is in the position 1) at next conditions: 1) the mains voltage isn't applied (switch S1 is in the position 1, switch S2 is in the position 2); 2) the mains voltage is applied (switch S1 is in the position 2, switch S2 is in the position 2).

The similar measurements with the computer connected to the mains through the UPS (switches S3, S4 are in the position 2) are carried out.

The special feature of the experiments deals with the typical usage of the system in the living spaces, where the mains lead-in has no grounding wire and additional protective grounding is absent.

3. EMI CHARACTERISTICS

Results of the EMI levels investigation are illustrated by the fig. 2, 3, 4. Frequency response of AC Mains interference unsymmetrical voltage is presented in the fig.2: curve 1 - AC Mains EMI from the power supply of the computer, curves 2, 3, 4 - AC Mains EMI of the system UPS - computer for different typs of UPS.

Fig. 2. Frequency responses of unsymmetrical voltage of AC Mains EMI of the computer (1) and the system "UPS- Computer Power supply" (2- for the UPS "Patriot", 3 - for the UPS "Fortress", 4 - for the UPS " Sola")

It follows that:
1. The AC Mains EMI from the computer exceed the normative level. It's accounted for low effectiveness of the computer's RFI filters at grounding wire absence.
2. Build - in UPS RFI-filters make it possible to reduce AC Mains EMI levels on 10...15 dB.
3. For "on - line" UPS ("Sola") it's significant that AC Mains EMI levels rise in the frequency range 2...20 MHz because of double transformation of the energy.

Further output EMI of the computer power supply are examined.The results of EMI investigations in fig. 3, 4 are corresponding to fixed EMI level U = 100 dB.

Fig. 3. Frequency responses of unsymmetrical EMI voltage on the output of computer power suppply 230 W (1) and in the system "UPS-Computer Power Supply" (2 - for the UPS "Patriot", 3 - for the UPS "Fortress", 4 - for the UPS "Sola") when supplying from AC Mains

Fig. 4. Frequency responses of unsymmetrical EMI voltage in the System "UPS-Computer Power Supply" (1- for the UPS "Patriot", 2 - for the UPS "Fortress", 3 - for the UPS "Sola")

If UPS and computer are supplied by AC Mains (fig. 3) or when the mains is disconnected (fig. 4), it follows that:
1. All types of UPS let EMI pass differently.
2. At the beginning of the protected frequency range the attenuation is the largest (> 40 dB). Attenuation is calculated as the difference (100 - U), dB.
3. The attenuation reducing (curve 4 fig. 3, curve 3 fig.4) is explained by complication of the system deals with double transformation of the energy in "on-line" UPS.

4. SIMULATION OF THE SYSTEM

Widely used simulators such as Micro CAP, PSPICE, TCAD etc. are based on detailed semiconductor models and provide highly accurate results. For example, PSPICE uses model of bipolar transistor, which is described by 55 parameters, and model of semiconductor diode with 25 parameters. But when begin designing the most part of the model parameters can't be determined and is replaced by default parameters meanings. It decreases the effectiveness of simulation.

Moreover, as an experience shows, at the beginning of the system designing it's not expediently to use the universal simulators, because very high level accuracy isn't needed and faster simulation is preferred. Specialized programs make it possible to cut the simulation time. One of the special feature of the simulator suggested in the paper consists in taking into account the electromagnetic compatibility requirements at the system designing.

Since EMI are generated by switched mode stages in time intervals of active elements switching it's necessary to model switching semiconductor devices with switching time accounting.

To provide fast analysis in the simulator the parametric semiconductor models are used, where the parameter is the internal semiconductor switches resistance [4].

To have possibility to use for analysis highly effective methods of linear circuits theory the simulator applies the linearization of the parametric models in time intervals.

The system investigated is represented as the set of high frequency equivalent circuits where not only regular but parasitic parameters of the system elements are taking into account too. AC Mains is modeling by means of LISN. The processes in time domain are calculated in terms of fast method of fitting and Laplace transformation. To comply with EMC requirements it's necessary to accomplish frequency domain analysis. It's based on the formal correspondence between Laplace and Fourier transfor-mations. As a criterion of EMI requirements accomplishing amplitude-frequency characteristics (AFC) of the solution matrixes elements are accepted. AFC are calculated twice: once as a basic characteristics - when only regular parameters of the system elements are taking into account, and secondly-paying attention to the parasitic parameters of the components. If real AFC differ from basic ones, it's necessary to take measures on EMI suppression.

Fig.5. Flow chart of the algorithm for the System "AC Mains - UPS - Computer" simulation

Flow chart of the algorithm for fast simulation of the system "AC Mains - UPS - Computer Power Supply" is shown in the fig. 5.

Methodic of the computer simulation consists of the stages :
1. choise of the cirquitry of the system "AC Mains-UPS-Load";
2. development of the equivalent circuits in time intervals of linearization;
3. forming of mode admittance matrix [Y₀^{( i )} (p)] and node capacitance matrix [C];
4. calculation of the solution matrix [Y₀^{( i )} (p)] ^-1;
5. calculation of the basic AFC of the solution matrix elements (pRw);
6. operational to time original transformation: calculation of the matrixes of coefficients [D], i.e. the solution matrixes at the end of each interval of linearization;
7. calculation of node voltages in steady-state conditions by fast method of fitting:
   [U^(N)]^k = {[D^(N)][C]:[D⁽¹⁾][I⁽¹⁾]}^k=[b]^k[I⁽¹⁾],
   where N - the number of time intervals of linearization, [b] - the basic matrix, [I⁽¹⁾] - the initial zero conditions, k - the number of switching periods;
8. control of the system output characteristics;
9. development of the equivalent circuits with parasitic parameters (PP) of the components;
10. calculation of the solution matrix subject to PP by modification method, definition of the influencing PP;
11. calculation of real AFC of the solution matrix elements subject to the influencing PP;
12. calculation of critical values of influencing PP;
13. control of the correspondence between basic and real AFC;
14. development of the equivalent circuits where RFI suppressors are used.
15. - control of the system output characteristics taking account of PP; accomplishment of the simulutaion.

5. CONCLUSIONS

6. REFERENCES

[1] IEC 60100-2-2. Electromagnetic compatibility. Environment. Compatibility levels for low-frequency conducted disturbances and signalling in public low-voltage power supply system. First edition, 1990

[2] Electromagnetic compatibility of technical equipment. Power supplies. Test methods of conducted disturbances: State Standard of the Ukraine. Kiev, DSTU 3593, 1997 (Ukrainian)

[3] Obolikshto B. , Shvaichenko V. : Power supply is very important. Networks and Telecommunications, 1998, 1(3), pp. 50-55 (Russian)

[4] Pilinsky V. , Rodionova M. , Rybin A. , Geranin V. , Demura V. : Energy support of the electronics. Kjiv, High School, 1994 (Ukrainian), 258 p.

[5] Rodionova M. : Algorithm of the circuitry design of Switched Mode Power Supplies. Procedeenigs of International Conference "Aerial Navigation and Avionics - 98", Kiev, 1998, pp. 88-89 (Russian)

BIOGRAPHIES

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

Main scientific interests are: power electronics & telecommunication (switched-mode power supplies for electronics, electromagnetic compati-bility of radioelectronics), CAD electronic systems with EMC providing.

In 1963 he graduated from Kyiv Polytechnic Institute.

In 1973 he obtained Ph.D. degree in electrical 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.

Rodionova Mariya V.

She received the degree in electrical engineering from the Faculty of Electroacoustics at National Technical University of Ukraine "Kyiv Polytechnic Institute" in 1972.

She is currently a senior lecturer at NTUU "KPI", Department of Acoustics and Acoustoelectronics.

Her research interests include power electronics, electromagnetic compatibility of electronics with particular attention to analysis and simulation.

She has published more then 70 papers and 2 books.

Shvaichenko Vladimir B. Ph. D. (1989), Accos. Professor (1993), was born in Saint-Petersburg in 1955.

In 1979 he graduated from Kyiv Polytechnic Institute.

From 1979 till now he is the lecturer at Kyiv Polytechnic Institute.

He is the author of more that 60 papers and reports.

The field of his scientific inte-rests includes investigation of conductive interferences in power supplies, design of conductive interference suppressors, providing EMC of electric apparatus.