Analysis Emi Of A Pfc On The Band Pass 150khz30mhz

Analysis EMI of a PFC on the band pass 150kHz30MHz for a reduction of the electromagnetic pollution S. Brehaut, M. Ould El Bechir, J.-C. Le Bunetel, D. Magnon

A. Puzo. SAFT Power Systems Group 37173 Chambray-lès-Tours, FRANCE

Laboratoire de Microélectronique de Puissance LMP 7 avenue Marcel Dassault, 37200 Tours, FRANCE [email protected] Abstract— The boost PFC circuit, widely used to provide a good power factor correction is developed at low and medium power. However, PFC generates electromagnetic interferences in the power converters. In order to have a better comprehension of the causes of the EMI pollution in the circuit, we use a simulation tool. A prototype with a new layout is created to have an experimental validation of the reduction of the EMI. Keywords- Power Resonance, Tracks

I.

factor

correction,

EMI,

Simulation,

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 designed specifications include : output power (Po), input voltage (Vin), line frequency, output voltage (Vout). The controller components are not included in Fig. 1. 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.

INTRODUCTION

TABLE I.

Lots of works have been committed with the aim to understand the high frequency comportment of the power factor corrector (PFC). Many researches have been performed about the simulation of the PFC in high frequency [1][2][3]. These researches have ended on a good modelisation from 150kHz to 1MHz. The influence of the spectrum have been done with the addition of a modulation on the commutation frequency of the switch or with the addition of screen on the source of MOSFET [4][5][6]. The purpose of this paper is to understand the whole spectrum between 150kHz and 30MHz. We have conceived a simulation tool in order to recreate the high frequency comportment of the PFC on the band-pass 150kHz-30MHz. It is easier to understand the causes of pollution’s phenomenon because we can interfere on each electrical parameter (passive, active) and also on parasitic elements. This tool permits to quantify the effects of the layout on the electromagnetic spectrum. A new layout has been studied and has ended in a new prototype. The document is presented as follows. Firstly, we describe the studied converter, the simulation tool and its applications for the PFC. Secondly, we show the propagation of the disturbances in function of the pollution spectrum. Then, the influence of the layout on the electromagnetic spectrum is studied and is validated by experimental results. II.

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 Fig. 2,

SPECIFICATIONS

Input Voltage (VRMS)

230

Output Voltage (VDC)

382

Switching frequency (kHz)

40.5

Input Current (ARMS)

3.2

Inrush limiter circuitry

C C

10 6

Load

102

C105 C Ac 230Vac 50Hz source

Control

Capacitor filter

10 7

Capacitor filter

Figure 1. Boost PFC Stage Schematic

III. BOOST EMI MODELLING Our simulation tool is a resolution of an electrical scheme in the frequential domain. This schema is constituted of a LISN, a PFC and a load. The schematic of PFC is completed by the parasitic inductances and capacitances and the parasitic capacitances between the radiator and the active components, Fig. 2. The switches are modelised by sources of voltage with an impedance to their bounds. The load is a simple winded resistance. In order to simplify the tool, we do not consider the

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effect of saturation of the core or the thermal effects which active components. We admit that the impedance of the network is endless. Each parasitic capacitance and inductance has been measured by a impedance meter on the conducted range frequency under study. Z59

Commutation cell.

Z2

Z5

Z8 Z9

Z12 Z51 Z17 Z21 Z52 Z13 Z18 Z50

Z24

Z28

Z22

Z49

Z27

Z33

Z30

Z31 Z34

Z63

Z37

Z56

Z38

Z58 Z68

Z57

Z41 Z43

Z47

Z62 Z Z 48 72

SiC diode

I36 Z73

Z45

Z25

Z44

Z16 Z46

Si diode

90

Z69

Z64 Z65

Z61

Magnetude (dBµV)

Z4

Load.

Z67

Z66

LISN.

modify the characteristic of the passive and efficiency is greatly improved in radiated mode, from 5dBµV to 20dBµV. In fact, the ringing frequencies are above 30 MHz. We can conclude that the commutation cell pollutes in conducted EMI with the slope of voltage and pollutes in radiated mode with the ringing of voltage.

80

70

Z55 Z3

Z6

Z7

Z10

Z11

Z53 Z14 Z19

Z15

Z54 Z20

Z26

Z32 Z35 Z36

Z74

Z39

Z40

Z42

Z71

Z75

60 10

Z70

Propagation path.

6

Frequency (Hz)

10

7

Figure 4. Pollution spectrum in conducted mode with two various diodes.

Figure 2. Equivalent scheme for EMI modelling, including parasitic components, LISN and converter.

We have ended in a frequency modelisation. The commutation cell represents the source of pollution. The layout, the passive elements and the parasitic ones are the propagation paths. The LISN is the receptor of the pollution. We can notice that the simulation is nearer of the measurement, Fig. 3. The pollution generated by the PFC is above the standard 55022 [9], because we take off a part of the EMI filter. Firstly, the spectrum is constituted by slope between 150 kHz and 7 MHz. Secondly, we can see three peaks of resonance at 8 MHz, 12 MHz and 18 MHz. The robustness of this modelisation has already been demonstrated previously [9].

Simulation

Measurement Standard CEI CISPR 22

Figure 3. Simulation and measurement for the complete HF scheme (150kHz-30MHz)

Si diode

80

SiC diode 60

40

3.107

Frequency (Hz)

Figure 5. Pollution spectrum in radiated mode(30MHz-200MHz)with two various diodes.

The parasitic capacitances to the bounds of switches represent the parasitic capacitance between the active component and the radiator, and the capacitance between the strip and the component. The Fig. 7 shows the effect of parasitic capacitances reduction. This reduction is achieved by the suppression of the capacitance between active component and the radiator. The interference level is reduced to 5 dBµV in low and medium frequency because the pollution is due to slope of current and voltage switching. The use of a effective screen between switch and radiator, connected to the source of the switch should allow to reduce the pollution at low and medium frequency. w ith rin g in g .

IV. STUDY OF THE POLLUTION SPECTRUM. Influence of voltage ripples influence and parasitic capacitances of active components We study the effect of the ripples in EMI . In simulation, Fig. 6, we can see that without the voltage ripples, the spectrum level is lower above 15 MHz. The influence of the voltage ripples is limited for the conducted EMI. Experimentally, this influence can be showed with the change of the diode of commutation cell (Si by SiC diode). The spectrum in conducted mode is the same, Fig. 4, whereas the spectrum in radiated mode is more attenuated, Fig. 5, above 50MHz. The

2.108

A

w it h o u t r i n g i n g .

Figure 6. Simulation of EMI between 150KHz and 30MHz with and without ringing on the switch.

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Module of the boost inductance.

Magnétude

w ith p a ra s itic c a p a c ita n c e .

Phase of the boost inductance.

Phase

w ith o u t p a ra s itic c a p a c ita n c e .

Figure 7. Simulation between 150KHz and 30MHz with and without capacitance MOSFET- radiator

B

Magnitude dBµV

100

2

12 M Hz 18 M Hz

60

106

F re q u e n c y H z

107

Figure 8. The 3 peaks: 8, 12 and 18 MHz

LoopB C 106 C 102

Magnetude

10 6

10 7

Frequency (M Hz)

0 -20 -40

Loop C

-60 -80 Frequency (M Hz)

Loop A

7

10

Figure 11. Module and phase of three loops A,B and C between 1MHz and 10MHz

The three resonant circuits are been recognized by the study of the impedance of all propagation paths. The phase of their impedance pass by zero at the resonant frequency. The MOSFET, the boost inductance and the filter capacitances of the common mode (C106, C107) and of the differential mode (C102, C105) form the whole of the paths with the layout. The Fig. 11 shows an example of the module and the phase of the loops A and C between 1MHz and 10 MHz. The Fig. 11 permits to determinate the loops which intervene at each resonance (Table II). The loop C is responsible for the peak at 8 MHz, the loops A and C are responsible for the peak at 12 MHz. The thirdly peak is created by the loop B. It means that if we remove the capacitance C105, we cut the loop B and we eliminate the peak at 18 MHz.

L BOOST C 105

Loop C

10 6

8 MHz 80

Loop A

Phase

Study of three peaks : 8, 12 and 18MHz The PFC is composed of a whole RLC components which form several resonant circuits. These circuits favour the propagation of the disturbances at particular frequencies. The three peaks identified, on the Fig. 8, correspond to three circuits in parallel, Fig. 9. The impedance, module and phase, of these resonant circuits shows several changes of phase corresponding to the peaks at 8, 12 and 18 MHz.

Figure 10. Module and phase of three loops A,B and C between 1MHz and 10MHz

LoopA

C 107

TABLE II.

RESONANCE LOOPS RESPONSIBLE OF THE PEAKS AT 8, 12 AND 18MHZ

LoopC Peak at 8MHz

Figure 9. Resonant circuits A, B, C

If we study separately the three circuits A, B and C, we have several resonant circuits. At frequencies studies, the impedance of components varies a lot. It is due principally to the impedance of the boost inductance which presents many changes of phase, Fig. 10. Indeed, the inductance becomes capacitive at 170 kHz, and presents three consecutive changes of phase which are increased by the other elements of the circuit.

Peak at 12MHz resonance

Loop A Loop A in parallel Loop B Loop C

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Peak at 18MHz

resonance resonance

resonance

elements. Lm symbolizes the inductive contribution of ground plane. This scheme puts in evidence a low-pass filter of second order with a transfer function following :

Simulation with changes of phase.

H(p)=

1 1+ pRbCb + p 2(Lb − Lm)CT

(1)

The equivalent impedance of the loop is :

Simulation with perfect boost.

Ze= Rb + j((Lb + Lm)ω − 1 ) Cbω

(2)

Figure 12. Simulation with a model boost with and without changes of phase

If we compare the different changes of the boost inductance’s phase, Fig. 10, with the peaks of pollution at 8 and 12MHz, we can see the changes of phase at the same frequency. At 18 MHz, the peak is due to the interaction between the loops A and B. The main element acting on the resonant peak’s, is the boost inductance. We have replaced the boost model with the consideration of phase’s change by a perfect boost in the simulated PFC, Fig. 12. The resonances at 8MHz and 12MHz are eliminated. The comportment of the real boost under 12MHz is inductive like for the perfect boost. So, the peaks at 18MHz remains unchanged. So, the comportment of the boost inductance has an great influence on the pollution in conducted mode. On the one hand, the solution should be to make an inductance without any changing of phase. On the other hand, we can note that the first change of phase at 170KHz is without consequence on the EMI, because the pollution generated by the commutation cell is above the peak created by the change of phase at 170KHz of the Boost inductance. So, the second solution is to bring the change of phase at low frequency or higher than 30MHz. V.

INFLUENCE OF TRACKS ON ELECTROMAGNETIC INTERFERENCES

A

Effect of tracks on resonant loops We use a method of recognition of resonant loop which consists to have a simplified electric view with a RLC circuit [9][10]. This circuit takes the main parasitic elements of passive components and tracks into account. The tracks constitute the propagation paths of electromagnetic interferences which are modelled with the basis elements R, L and C. An equivalent electric scheme of resonant loop is represented by the Fig. 13.

Lb

Rb Vinteference

Cb

Lm

Figure 13. Equivalent electric model of a loop

Rb, Lb and Cb, represent respectively the resistive, inductive and capacitive contribution of tracks and passive

When the phase passes by zero, there is a resonance for the frequency :

f=

1 2π (Lb + Lm)Cb

(3)

The damping factor is :

z=

Rb 2

Cb (L b + L m )

(4)

Then, the frequency and the amplitude of peak are function of Rb, Lb, Cb and Lm values. So, the tracks and passive elements intervene too. To damp the ripple of the resonance, the damping factor must be superior to 0.5. This factor depends to the characteristic impedance of the tracks :

Zc = Lb + Lm Cb

(5)

Optimisation of tracks With the (4), to augment z, we can increase Rb, or Cb or the term (Lb+Lm).To minimize the common mode interferences, the technical is to increase Cb only on area of circuit and to decrease (Lb+Lm). The layout’s modification of circuit allows to act on the parasitic capacitance Cb and on the parasitic inductances (Lb+Lm).

B

The simulation permits to put in evidence the various resonant loops. We have seen before that the peaks are due to the phase’s change of the boost inductance. The effect of track is visible each time that the boost inductance changes the phase. Then, the tracks can modified the time of changes of phase. Indeed, the track’s impedance isn’t negligible, compared to the full impedance in the loop. We know that all the loops determinated by simulation pass by the MOSFET, the boost impedance, and the input capacitances. The tracks which compose this propagation path, are the elements which must be modified in order to control the amplitude and the frequency of pollution peaks. VI. EXPERIMENTAL STUDY The following results are done with a PFC which has the same characteristics that industrial PFC presented previously. Only the layout of circuit is changed. We keep the same

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components to can study the influence of layout on the electromagnetic interferences of PFC. The uncoupling capacitances C105, C106 and C107 are removed. This allows to remove the loop A and B, and the peak at 18 MHz. The reference PFC has the same layout that the industrial PFC.

the impedance of the inductance if we add a capacitance in parallel to shift this frequency to the radiating spectrum. PFC reference.

A

Tracks identification In order to make the tracks less inductive, its must be wide and short. To limit the capacitive comportment of tracks, we removed the ground plane on some areas. This allows to dissociate the capacitive effect and the inductive effect on the studied track. These effects are linked by the characteristic impedance (5). The sensible tracks which intervene anyway in the common and in the differential mode are the tracks of the resonant loop. Experimental results By the reduction of the inductive effect on tracks and by the increase of the capacitive effect, we can see a spectrum attenuation of 3 dBµV between 150kHz and 1MHz, Fig. 14. The decrease of the first peak overtakes 7 dBµV, but the amplitude of second peak doesn’t change much (1 dBµV). There is only a shift of the frequency.

PFC with a new layout without ground plane.

Figure 15. Reduction of EMI by the reduction of inductive and capacitive effects of tracks with out ground plane

B

PFC reference.

C

Limits of influence of the tracks and of the part of passive components We have showed that the effects of tracks aren’t always sufficient to control the frequential comportment of the resonant loops. Indeed, we use the wide of tracks to modify the capacitive and inductive effects. For technological, thermal and electrical reasons, the wide of tracks is limited. To modify the impedance of resonant loops, the comportment of passive components must be adapted to minimize the peaks of resonance. For example, the comportment of the boost inductance and the input filter capacitor can be improved. The choice and the place of the two elements allows to control the equivalent impedance of the resonant loops, and the peaks of pollution.

PFC with a new layout with a ground plane.

Figure 14. Reduction of EMI by the reduction of the inductive effect of tracks with a ground plane

The shift of the second peak is principally due to the decrease of inductance (LP and Lm).This is confirmed by the (3). Indeed, parasitic capacitances between tracks and the ground plane don’t intervene in resonant loops. A second circuit confirms this affirmation. In the second circuit, we have reduced the parasitic capacitances between tracks and the ground plane with a new layout without ground plane. The Fig. 15 shows that the parasitic capacitances don’t modify the frequency of peaks. The suppression of the ground plane on the bottom face of circuit allows to reduce the interference level until 10 MHz. The decrease overtakes 8 dBµV between 150kHz and 4 MHz and 10 dBµV for the first peak. But the second peak increases of 1 dBµV. The impedance of boost inductance explains this phenomenon. Indeed, at 12MHz, the boost inductance becomes progressively inductive. The parasitic inductances of the tracks are negligible compared to the impedance of the boost inductance. Then, the layout of the tracks doesn’t modify the interferences level at this frequency. However, we can modify

VII. CONCLUSION We have analysed the electromagnetic interferences of a PFC on the band pass 150 kHz-30 MHz to reduce the electromagnetic pollution. The EMI modelling takes the real impedance of components integrated in the industrial PFC and the parasitic elements produced by the layout into account. The frequential study of this EMI model allows to determinate the electromagnetic interferences in simulation. So, the consideration of each active, passive and parasitic element in HF, allows to reproduce the full conducted EMI. The simulation tool permits to see and to understand the influence of various parameters like the voltage ripples or the parasitic capacitances of active components. The simulation defines the propagation paths and permits to know the influence of boost inductance in series with filter capacitances. Now, we can identified the cause of the resonant peaks. After to have identified the propagation paths by simulation, we can eliminate one resonant loop by removing one component. The experimental circuit checks this simulation result. We could validate the influence of layout on the resonant peaks. The experimentation and the simulation show the essential incidence of the boost inductance and the limits of the layout modifications on the resonant peaks. Finally, the next studies will concern the passive components, in order to reduce the electromagnetic level without addition of filter components.

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[1]

[2]

[3] [4] [5]

L. Rosseto, S. Buso, G. Spiazzi, «Conducted EMI Issues in a 600-W Single-Phase Boost PFC design». IEEE Transactions on industry application, Vol. 36, NO. 2,pp.578-585, March/April 2000. [7] L. Rosseto, S. Buso, G. Spiazzi, «Conducted EMI Issues in a Boost PFC Design», Proc. of Int. Telecommunications Energy Conf. (INTELEC), San Francisco, October 1998, pp.188-195. [8] CEI CISPR 22, «Radio disturbance characteristics – Limits and methods of measurement (edition 3)», 1997. [9] S. Brehaut, J-C. Le Bunetel, A. Schellmanns, D. Magnon, A. Puzo. «Development of a Conducted EMI Model for an Industrial Power Factor Correction» EPE 2003. [10] A. Pons, «Optimisation de la fonction de filtrage dans les convertisseurs de traction», Alcatel, 1998. [11] A. Puzo, «CEM Chargeur, Méthode de dépollution à la source», Alcatel Astom recherche, 1997. [12] M. Ould El Bechir, «Influence du routage d’un PFC sur la CEM», rapport de stage d’ingénieur de l’Ecole Polytechnique de l’université de Tours, 2003.

[6]

REFERENCES J-C Crebier, « Contribution à l’étude des perturbations conduites dans les redresseurs commandés», Thèse de L’Institut National Polytechnique de Grenoble, 1999. 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. J-C Crebier, M. Brunello, J. P. Ferrieux, «PFC full bridge rectifier EMI forecast analysis», EPE 99. S. Wang, F.C. Lee, W.G. Odendall, «Improving the performance of Boost PFC EMI filters», IEEE 18nd Annual APEC 2003. E. M. Hertz, «Thermal and EMI modelling and analysis of a boost PFC circuit designed using a genetic-based optimisation algorithm», Thèse de l’Institut polytechnique de Virginie, 2001.

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