Development Of A Conducted Emi Model For A Industrial Power

Development of a conducted EMI model for a industrial power factor corrector

BREHAUT STEPHANE

DEVELOPMENT OF A CONDUCTED EMI MODEL FOR AN INDUSTRIAL POWER FACTOR CORRECTOR. Stéphane BREHAUT(1), Jean-Charles LE BUNETEL(1), Ambroise SCHELLMANNS(1) , Didier MAGNON(1) , Antoine PUZO (2). (1) Laboratoire de Microélectronique de Puissance 7 avenue Marcel Dassault BP 407 - 37204 TOURS Cedex 3, (2) SAFT Power Systems Group ZI n°1- 10 rue Jean Perrin 37173 Chambray les Tours Cedex E-Mail : [email protected]

Keywords «EMC/EMI, Power factor correction, Modelling, Simulation».

Abstract The need to provide a good power factor correction is required in a growing number of applications, as in telecommunications. The boost PFC circuit, widely used to fulfill this requirement, is developed at low and medium power. However, PFC generates electromagnetic interferences in the power converters. We must put an expensive and bulky EMI filter. The purpose of this paper is to present a new methodology by calculating the EMI with a developed software. Experiments are performed to verify the consistency with the result of the simulation tool. An industrial PFC for a 600W unit with 230Vac 50Hz input and 380V output has been used for this study.

I. Introduction In power electronics, filters are commonly used for two purposes : to reduce the emission and propagation of electromagnetic interferences (EMI) and to reduce the susceptibility of the converters with respect to external EMI. The presence of filters is dictated by the proper operation of the converters under the worst EMI constraints. In our case, the power supply studied is constituted of both cascaded converters and it is used as a battery charger in telecommunication. The first one is a power factor corrector (PFC) and the second one is a DC-DC insulated converter. Previous studies [1] showed that the worst pollution came from PFC in the power supply. To reduce the EMI pollution, we use expensive and bulky filters. All these facts demonstrate the importance of research in the development of the EMI analysis of power converters working as power factor conversion. There have been recent efforts to provide a more methodical design process by implementing optimisation techniques [2][3]. The method proposed in this paper is based on frequency model for EMI prediction and can thus be easily implemented in a common mathematical tool as MATLAB. Using this software, it is possible to determinate with accuracy the conducted EMI before building an industrial PFC without filter. This information can contribute to acquire a better understanding of the system behaviour in high frequency. This software enables the simulation of the Line Impedance Stabilization Network (LISN) and the EMI receiver. We can determine full EMI spectrum in dBµV in accordance with the CEI CISPR 22 standard [4] in function of all parasitic elements. The paper is presented as follows. In the first part, we present the system under study and its specifications. Then, in the second part, the methodology for modelling of the EMI approach is described and developed in a schematic method. Our purpose is a general application on other converters. At least, the comparison between simulation and measurement results are showed and analysed.

EPE 2003 - Toulouse

ISBN : 90-75815-07-7

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Development of a conducted EMI model for a industrial power factor corrector

BREHAUT STEPHANE

II. System studied, specifications and fixed design variables A. Interest for a follow-up EMI study of a PFC. The power supply studied is a basic structure of a typical two-stages front end converter for DC distributed power systems [5]. The first structure is constituted of a PFC boost converter by the need for a good power factor correction. It is in series with a full-bridge DC/DC converter, a transformer and a rectifier 48VDC 12A (figure 1).

AC line

PFCBoost

High Voltage

Full Bridge DC/DC

Vbus

Fig.1. Two stage front end converter. If we do a spectral analysis, we can say that the spectrum of the global converter has a similar form with the spectrum of the PFC (figure 2). In fact, the two peaks observed for the full power supply at 12MHz and at 18MHz are at the same resonance than the peaks of the PFC. There is a difference of 10 to 15dBµV between the two spectra. The surface of the global converter’s layout is more important than that of the layout of the PFC. So, the parasitic capacitors between the ground board and the layers are more important. These parasitic elements produce common mode pollution [6]. We can conclude that the PFC generates the predominant EMI. We concentrate our research on the modelisation of the PFC.

Frequency global system result.

Frequency PFC result.

Fig.2. Influence of the PFC’s pollution on the global system (150KHz-30MHz). We can notice that the peaks of EMI of the global converter are nearer. Indeed, the switching frequency of the inverter is 100KHz whereas the switching frequency of the PFC is 40.5KHz.

B. Description of the PFC. The system to be designed consists in a boost PFC converter with a part of an EMI filter as showed in Figure 3, which gives an electrical schematic of the circuit. We keep a part of the EMI filter because we need the capacitor above the bridge rectifier for the tripping of the voltage measured by the controller. The capacitors below the bridge will permit to do many tests of robustness. The designed specifications include : output power (Po), input voltage (Vin), line frequency, output voltage (Vout). The controller components are not included in Figure 3. The constant-frequency average-current-mode control for continuous-current-mode operation is the control strategy for the switch. The characteristics of the converter used for this application are given in table I. We use a battery to supply the controller in order to limit pollution.

EPE 2003 - Toulouse

ISBN : 90-75815-07-7

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Development of a conducted EMI model for a industrial power factor corrector

Input Voltage[VAC]

BREHAUT STEPHANE

Inrush limiting

230

limiter circuitry

Output Voltage [VDC] 382 C 106

Switching frequency Fs [KHz]

40.5

Input current [Arms]

3.2

Load

C 102 C 105

Control

Capacitor filter

C 107 Ac 230Vac 50Hz

source

Table I. Specifications

Capacitor filter

Fig.3. Boost PFC Stage Schematic

The snubber composed of a ferrite and a parallel diode with the free wheel diode of the commutation cell allows to reduce the peak of the inverse overlay of the power diode. The diode in parallel allows to discharge the accumulated energy in the ferrite.

III. EMI Tool Description This section is a description of the proposed tool. The main parts of this procedure are presented in the following steps. The principle is to work directly in the frequency domain. The software enables to determine the conducted EMI in dB/µV for the PFC converter, in accordance to the CISPR 16-2 standard.

A. Boost modelling and assumptions. The first step for conducted EMI prediction is to propose a complete electrical equivalent circuit including the converter itself with all the parasites, the measurement equipment (Line Impedance Stabilisation Network-LISN), the cabling impedance between LISN and converter and the commutation cell. We do many hypotheses in order to limit the complexity of the modelisation. In spite of the effect of saturation of core, we assume that the value of the boost inductance requires a unique value in high frequency. The system configuration between the mains and the bridge rectifier is assumed to be symmetrical with respect to ground. We consider that only the switching cell of Boost converter with a snubber generates disturbances. We admit that the impedance of the network is endless. LISN

Commutation cell

ground

Bridge Propagation paths : parasitic rectifier inductances and capacitors

Fig.4. Equivalent scheme for EMI modelling, including parasitic components, LISN and converter.

EPE 2003 - Toulouse

ISBN : 90-75815-07-7

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Development of a conducted EMI model for a industrial power factor corrector

BREHAUT STEPHANE

The propagation paths correspond to electrical connections between interference components and the switching cell. The study of propagation paths requires a previous knowledge of the high frequency behaviour of every components and interference elements. These elements are the parasitic capacitors and inductances, the passive and active components, the circuit layout, the bridge rectifier and the line impedance stabilization network (LISN). The components and layouts were carefully measured. This electrical scheme is presented Figure 4. Each element of the circuit respects the impedance variation along the frequency range (150KHz 30MHz). The switch and the diode are two sources of pollution. These sources produce the pollution in the commutation cell. The layouts are assimilated to inductances and the ground board is represented by capacitors. We suppose that the rectifier does not generate disturbances but the impedances of diode rectifier are included in the simulation. The duty cycle frequency’s of PFC is 10ms, so, the cycle of pollution is 10ms too. During this time equals to the half period of mains, diodes are always in the same position, open or closed, so the position of the diodes is static in the modelisation. For each frequency, electrical elements can be modelled by their impedance. The resolution of the equivalent electrical circuit estimates the EMI level in the LISN resistors. We use a matrix method [7] designed with Kirchoff law. The models presented with the equation 1 and figure 5 are considered.

with

U=Z.I (eq.1) U : sources of pollution produced by the commutation cell. Z : The converter is converted into an impedance matrix. I : Currents of common and differential mode. Z59 I33

I25

Z4

I26

7

Z67

Vd2 Z63 Z8

Z12

I2

I4

I6

Z50

Z2

Z9

Z5

Z51 I3

Z52

0

Z17

Z21

I8

Z13

Z24

I10 18 ZZ

1

Z27

I12 Z22

Z30

Z33

Z37

I16

I18

I20

I14

Z25

Z31

Z28

Z41 I22

Z34 Z38

Z45

Z43

Z16

Z72

Z73

7

Z57 VD Z62 Z68

Z56 I32

Z69 I36

I27

I2 VK

I29 Z60 Z60

I39

I40

Z66 I34 I35 Z4

Z48

Z44 Z64//Z65

4

Z61 I28

Z6

Z3

Z53

Z10

Z14

19 Z23 ZZ 1

I3

I5

I7

I31

I9

Z7

Z11

Z15

Z54

Z20

9 I11

Z26 I13

Z29 I15

Z32 I17

I19

Z35

Z39 Z46 Z55 I23 I21

Z36

Z40

Z7 0

Z71

Z42 I37

Z74

I38 Z75

Fig.5. Impedance model.

B. One sort of Matrix for a frequency range. We develop a simulation, between 150KHz and 30MHz, which takes the impedance evolution of elements in function of frequency into account. The impedance models such as the boost inductance, figure 6, are very complicated. To obtain a more simplified model, we decide to work on many frequency ranges. We use accurate models on some frequency ranges limited.

EPE 2003 - Toulouse

ISBN : 90-75815-07-7

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Development of a conducted EMI model for a industrial power factor corrector

BREHAUT STEPHANE

So, the impedance models of the component are more accessible and the time of calculation is better. An example of an equivalent model of the boost inductance is given in table II.

Module of the boost inductance.

Phase of the boost inductance.

Fig.6. measurement of impedance of boost PFC between 150KHz and 30MHz.

Frequency range

150KHz1MHz

1MHz10MHz

10MHz12MHz

12MHz18MHz

18MHz30MHz

Model used

Table.II. Evolution of impedance model function of frequency.

C. Use of dynamics matrices for the model of semiconductor. To account for the effects of the commutation cell with regard to the rest of the system placed between this cell and the mains, the commutation cell can be replaced by equivalent voltage sources with impedances in series. The first equivalent voltage source VK (t) (figure 7) is located between the collector and the source of the switch. The second voltage source Vd (t) is situated between the anode and the cathode of the diode. The sources of pollution of the semi conductor are represented by a trapezoidal waveform with the ringing. The rise and fall times of the simple waveform have been adjusted in function of real commutations. Impedance VB(t) switch closed

Vk

t Impedance switch open

t

t

VC(t)

Fig.7. Description of the dynamic comportment of the switch. We must use a dynamic matrix because the impedance of both switch and diode changes depending on whether they are open or closed. The source of disturbances of the switch Vk (t) is realised by the

EPE 2003 - Toulouse

ISBN : 90-75815-07-7

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Development of a conducted EMI model for a industrial power factor corrector

BREHAUT STEPHANE

superposition of two sources : the first one for the opening and closing and the second one for the ringing at the conducting and the opening state. Then, we use the theorem of superposition to make the summation of harmonics created by the rise, the fall and the ringing of the switch. First we can characterize these voltage sources in the frequency domain by means of the Laplace transform and then by applying the appropriate conversion to the Fourier representation. The previous voltage waveform is represented by an addition of sinusoids, each of them being a multiple of the fundamental frequency (in our case, the line frequency).

D. Theoretical and experimental EMI results and their comparisons. We make several high frequency (HF) schemes of the PFC with an increasing precision. We show the evolution of the accuracy of the simulation. The first scheme, figure 8, is a simplified model defined by a square matrix (20-20). The second model, figure 4, is defined by a square matrix (40-40). In the simplified model, we keep the source of disturbances of the MOSFET, the parasitic capacitors of the bridge rectifier, of the power switch and of the bus capacitors. We have a simulation similar to the measurement between 150KHz et 500KHz (figure 9).

Fig.8. Equivalent simplified scheme for EMI modelling.

Simula tion

Measurement

Standart CEI CISPR 22

Fig.9. Simulation and measurement for the simplified HF scheme (150kHz-30MHz). For the complete HF scheme, the validity of the EMI model is expected and was experimentally verified in high frequency range (150kHz 30MHz). Figure 10 shows the EMI envelope of the PFC. Firstly, the comparison between disturbance levels and standard limits [6] clearly shows that the

EPE 2003 - Toulouse

ISBN : 90-75815-07-7

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Development of a conducted EMI model for a industrial power factor corrector

BREHAUT STEPHANE

studied converter without the filter does not respect the standard. Secondly, the simulation is nearer to the measurement.

Simulation

Measurement

Standart CEI CISPR 22

Fig.10. Simulation and measurement for the complete HF scheme (150kHz-30MHz). The three peaks at 8MHz, 12MHz and 18MHz are due to some resonance loops of disturbance around the commutation cell. In simulation, we find the three main peaks of resonance, so we can use this tool to determinate the passive elements which produce the resonance. The oscillating circuit is excited by the gradient of voltage of the power switch of the converter. Two solutions can be used to eliminate these peaks of resonance. The first one is to modify the damping factor, with the modification of one of the elements R,L,C of the loop [8][9]. The bigger the coefficient, the less important the peak of resonance is. The second solution is to move the resonance frequency above 30MHz because the filtering is theoretically easier [10]. To reduce the effects of the parasitic loops which may cause EMI noise, the loops area needs to be as small as possible.

IV. Validation of the simulation tool. We keep a part of the PFC filter to know the viability of the model studied. We had changed each value of each capacitor of the filter figure 3 and we have compared the measured and simulated spectrum. We had done the EMI measurements of robustness with a other prototype which is lightly different by her layout. So, the comparaison is qualitative. In the following example, figures 11 and 12,we have change the value of the differential capacitor of 220nF (C105 ) by a value of 440nF.

EMI with the C105 capacitor of 220nF

EMI with the C105 capacitor of 440nF

Fig.11. Measurement of EMI for two different values of the differential capacitor C105 .

EPE 2003 - Toulouse

ISBN : 90-75815-07-7

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Development of a conducted EMI model for a industrial power factor corrector

BREHAUT STEPHANE

EMI with the C105 capacitor of 220nF

EMI with the C105 capacitor of 440nF

Fig.12. Simulation of EMI for two different values of the differential capacitor C105 . We can see that in the two case, measurement and simulation, the EMI disturbance is less important from 12 MHz on. In the second example, figures 13 and 14, we replace two capacitors (C106 , C107 ) , of common filter, of a value of 4.7nF by two capacitors of 1nF.

EMI with the C 106 and C107 capacitors of 4.7nF

EMI with the C 106 and C107 capacitors of 1nF

Fig.13. Measurement of EMI for different values of the common capacitors C106 and C107 .

EMI with the C 106 and C107 capacitors of 4.7nF

EMI with the C 106 and C107 capacitors of 1nF

Fig.14. Simulation of EMI for different values of the common capacitors C106 and C107 . We notice an increase of the EMI pollution between 150KHz and 8MHz as the disappearance of the peak at 18MHz. On the contrary, there is an increase of the spectrum of pollution between 26MHz and 30MHz for the simulation. For the capacitors of filtering, the putting up is sensible. If we change the position of the capacitor, figure 15, we modify the length of the capacitor legs and its parasitic inductance. The parasitic inductance, varying from 1 to 2nH, has not effect at low and medium frequency (150KHz-10MHz) but changes the result of EMI at high frequency (10MHz-30MHz).

Parasitic inductance of the common capacitor.

Fig.15. HF representation of the common capacitor.

EPE 2003 - Toulouse

ISBN : 90-75815-07-7

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Development of a conducted EMI model for a industrial power factor corrector

BREHAUT STEPHANE

V. Conclusion The results obtained by the EMI tool are very closed with the experimental results and tests of robustness which demonstrate the validity of this tool on the range 150kHz-30MHz. With this new model which includes active and passive components, we can make a detailed and quantitative pollution analysis of the PFC. Now, it is possible to change or add new passive or active components easily. We can change the commutation frequency [11] to show the repercussion on the EMI comportment. If we want to add an additional branch, we must develop a new matrix computation. The purpose of this work is to investigate how and under which conditions, EMI constraints can be lowered. We want to achieve a model of the global system of the battery charger with the same methodology. Thus, we will be able to identify the causes of pollution of this circuit and to reduce the EMI before putting in a less expensive filter.

REFERENCES [1]. D. BARILLET-PORTAL, «Rapport de stage», DESS micro-électronique de L’Université de Bordeaux, 2000. [2]. J-C CREBIER, «Contribution à l’étude des perturbations conduites dans les redresseurs commandés, Thèse de L’Institut National Polytechnique de Grenoble, 1999. [3]. S. BUSQUETS-MONGE, J. C. CREBIER, S. RAGON, E. M. HERTZ, J. WEI, J. ZHANG, D. BOROYEVICH, Z. GURDA, P. K. LINDNER, A. ARPILLIERE, «Optimization Techniques Applied to the Design of a Boost Power Factor Correction Converter», PESC 2001 IEEE 32nd Annual, Volume: 2, 2001 pp. 920-925. [4]. CEI CISPR 22, «Radio disturbance characteristics – Limits and methods of measurement (edition 3), 1997. [5] R. WATSON, «New Techniques in The Design of Distributed Power System», Thèse de l’Institut polytechnique de Virginie, 1998. [6]. F.COSTA, «Contribution à l’étude des perturbations conduites par les convertisseurs H.F.» Thèse de doctorat 3ème cycle, Avril 1992, Université d’Orsay. [7]. J-C CREBIER, M. BRUNELLO, J. P. FERRIEUX,«PFC full bridge rectifier EMI forecast analysis», EPE 99. [8]. A. PONS, «Optimisation de la fonction de filtrage dans les convertisseurs de traction», Alcatel, 1998. [9]. A.PUZO, «C.E.M Chargeur, Méthode de dépollution à la source », Alcatel Alsthom recherche, 1997. [10]. E. M. HERTZ, «Thermal and EMI modelling and analysis of a boost PFC circuit designed using a geneticbased optimisation algorithm, Thèse de l’Institut polytechnique de Virginie, 2001. [11]. E. M. HERTZ, S. BUSQUETS-MONGE, D. BOROYEVICH, «Analysis of the Tradeoffs between Thermal Behavior and EMI Noise Levels in a Boost PFC Circuit», Proceeding of the IEEE industry Applications Conference, 2001.

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