Switching Cell Optimisation Minimising The Common Mode Current For The

SWITCHING CELL OPTIMISATION MINIMISING THE COMMON MODE CURRENT FOR THE POWER FACTOR CORRECTOR

MAGNON Didier

SWITCHING CELL OPTIMISATION MINIMISING THE COMMON MODE CURRENT FOR THE POWER FACTOR CORRECTOR 1

S. Brehaut1, J.C. Le Bunetel1, D. Magnon1, A. Puzo2, D. González3, J. Gago3, J. Balcells3 2 3 LABORATOIRE DE MICROSAFT POWER UNIVERSITAT POLITÈCNICA ELECTRONIQUE DE SYSTEMS GROUP DE CATALUNYA PUISSANCE Chambray, France Electronics Engeenering Department, Rue Pierre et Marie Curie, Terrassa, Spain Tours, France Tel.: +33 / 247367291. Fax: +33 / 247367291. E-Mail: [email protected]

Keywords «EMI», «Power factor correction», «Power supply»

Abstract This paper presents the problem of common mode electromagnetic interference. We study the common mode current phenomena on a single converter like the Boost converter. This converter operates in power factor corrector mode. The advantage of this structure is the presence of one switching commutation cell only. This limits the number of voltage pollution source creating common electromagnetic interference. The elements that intervene in the mechanic of pollution, are switching frequency, the rise and fall time of switches, the parasitic capacitance between electric circuit ant the ground plan, the floating voltage of the converter. In this paper, we are interested in the two last points.

Introduction The power supplies generate a strong electromagnetic pollution [1] [2]. In order to respect the EMI standard some filters are added at input and output converter. The filters’ size reduction is a good way to reduce the cost of power supplies. It involves the reduction of disturbances generated by the commutation cell. These disturbances are conditioned by semi-conductor switching and by impedance of parasitic and passive elements. Several techniques exit to act on the disturbances: on the control [3] or on the layout [4]. We study the mean to decrease the electromagnetic interference by the modifying of the topology converter. This studied power supply is the first stage of battery charger, it‘s a power factor corrector. This converter permits to obtain a input current always sinusoidal in phase with the network voltage. We search what is the interference created by this converter and what elements intervene in this electromagnetic pollution : parasitic capacitance, floating voltage… In the first part, we present the structure studied. After common pollution phenomena are presented, and the topology converter is modified in function studied phenomena. The modified topologies are tried experimentally.

Presentation of structure studied The power supply used in this work is a classical power factor correction. This converter is the input stage of the battery charger used the communication domain. The output stage is an inverter in series with a transformer and a rectifier, figure 1. The specifications of each module is given table I. The controller

EPE 2005 - Dresden

ISBN : 90-75815-08-5

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SWITCHING CELL OPTIMISATION MINIMISING THE COMMON MODE CURRENT FOR THE POWER FACTOR CORRECTOR

MAGNON Didier

components are not included in figure 1. The constant-frequency average-current-mode control for continuous-current-mode operation is the control strategy for the switch. Iin Iout Vin

VBUS

Network 230ac 50Hz

PFC

Vout

Inverter

Load

Fig. 1. Equivalent schematic with the PFC in series with the inverter and with the load

Table I: The title of the Table I Input Voltage, Vin

230 VRMS

Output Voltage, Vout

48 VDC

VBUS

382 VDC

Input Current, Iin

3.2 ARMS

Output Current, Iout

12 ADC

Switching frequency of PFC

50 kHz

Switching frequency of Full Bridge

112 kHz

We are interested only in the first stage: the power factor corrector (PFC). It’s composed of one commutation cell, which simplifies the study of common mode and differential current. The EMI measurement shows the common mode is higher than differential mode (figure 2). The capacitance between switch and heat sink and the floating voltage level on this parasitic capacitance, CP1, justifies this difference (figure 3). Then we study this pollution mode.

Magnitude (dBµV)

Common mode Differential mode

Frequency (Hz) Fig. 2. Common and differential mode comparison

EPE 2005 - Dresden

ISBN : 90-75815-08-5

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SWITCHING CELL OPTIMISATION MINIMISING THE COMMON MODE CURRENT FOR THE POWER FACTOR CORRECTOR

MAGNON Didier

+ C102 CP1

-

ICOMMON=ICP1 (a)

(b)

Fig. 3. creation of the common mode source on the standard PFC If we want reduce this electromagnetic interference, we can act on its floating voltage level or on the switch-heat sink capacitance, CP1. The figure 4 shows the influence of this capacitance, the disturbances level is reduced to 30 dBµV in the low and medium frequency with CP1 equal to zero. But acting on the switch-heat sink capacitance is difficult, because the capacitance value depends on physics parameters. Then we just study how increases the voltage level on this parasitic capacitance. 110

Measure Simulation with Drain-Ground capacitance, CP1 Simulation without Drain-Ground capacitance, CP1

Magnitude (dBµV)

100

90

80

70

60

50 10

6

10

7

Frequency (Hz)

Fig. 4. Simulation with and without Drain-Ground capacitance between 150 kHz et 30 MHz

Study common mode pollution of classical PFC The switching cell of PFC is constituted of two switches : a MOSFET switch and a diode. This permits to connect a current source which is Boost inductance to a voltage source which is a output capacitance. This switching cell is the beginning of the common mode interference. The figure 5 shows in function of the switches state the voltage in the point A is floating in ratio the ground plane. Then this floating voltage is applied to the parasitic capacitance. Also a common mode current flows in this capacity: ICOMMON = ICP1. In order to decrease EMI source, we can act on the rise and fall time of the switch and on the frequency operation. These solutions are limited because its modify the PFC operation by the increase the thermal losses. We can too act on the parasitic capacitance between MOSFET Drain and heat-sink. This capacity

EPE 2005 - Dresden

ISBN : 90-75815-08-5

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SWITCHING CELL OPTIMISATION MINIMISING THE COMMON MODE CURRENT FOR THE POWER FACTOR CORRECTOR

MAGNON Didier

can reduced with the screen inserted between MOSFET Drain and heat-sink and connected to MOSFET Source voltage. This screen has a limited effect that is insufficient to solve the EMI problem. Vin Vin

A VBus

Vin

B VBus

A 0V

Vin-Vbus

-VBus

Vin

0V

0V

C 0V

IP1

0V

ID-M

VBus

VBus

Vin

VBus

0V

0V

B VBus

IP1

C 0V

0V

CP1

CP1

Ground plane

Ground plane ICOMMON=ICP1

ICOMMON=ICP1

(a)

(b)

Fig. 5. Electric schematic in function of switches states We propose to reduce the floating voltage on the MOSFET Drain (point A). The MOSFET Source voltage must become floating to respect the balance voltage. The Drain-ground voltage is divided by two and antisymmetric source is applied on the source-ground capacitance.

Compensation of the common mode interference To symmetry the Boost inductance To symmetry the floating voltage on the drain and on the source MOSFET, the Boost inductance must be placed of symmetric way at input converter (figure 6). The drain-ground voltage is now divided by two, and the source-ground voltage is opposite. The size of two inductances can be minimized if two windings are associated on the same magnetic core.

•

+

C102

CP4

CP1

•

-

IP1 CP2

IP4 IP2

ICOMMON =ICP1-ICP2-ICP4 (b)

(a)

Fig. 6. Creation of the common mode sources on the PFC with the symmetric inductance boost The drawback of the symmetry of Boost inductance is to put the load voltage floating in ratio the ground. Also a common mode current flows in the ground plane: ICOMMON = ICP1 - ICP2 - ICP4. The voltages creating the currents ICP1 and ICP2 are opposite. If the capacitances CP1 and CP2 are equal, then the sum of current ICP1 and ICP2 is zero, the common mode current depends only of CP4. The interference problem isn’t solved, it is shifted toward an other parasitic component. We realized this experimental converter to check the

EPE 2005 - Dresden

ISBN : 90-75815-08-5

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SWITCHING CELL OPTIMISATION MINIMISING THE COMMON MODE CURRENT FOR THE POWER FACTOR CORRECTOR

MAGNON Didier

influence of the new floating voltage points. We compare electromagnetic spectrum between normal PFC and a PFC with the balanced inductance (figure 7). area 2

Magnitude (dBµV)

area 1

attenuation of the peak at 8 MHz

area 3

Classical PFC PFC with symmetric inductance

Frequency (Hz) Fig. 7. EMI spectrum comparison between the normal PFC and the PFC with balanced inductance This topologies changing modifies few the EMI spectrum only about 3 or 5 dBµV in the area 1. With the balanced inductance, the voltage of two load connections changes in function the switching. A new pollution source is then created, because the parasitic capacitances of the output converter aren’t negligible. In the area 2,the peak at 8 MHz is reduced about to 5 dBµV. The peak at 12 MHz keep the same value. In the area 3, the electromagnetic interferences are few modified by this new topology. To reduce the effet of this new floating voltage on the load, we can apply the same principle used on the the floating voltage on the MOSFET Drain. We can create a new floating voltage on the load negative connection.

To symmetry the load We take the classical PFC and we symmetry the diode. Two floating points are created with regard the load (figure 8). The drawback of this solution is increased the number of floating point.

+ C102

CP4

CP1

IP2

IP1

-

IP4

CP2 ICOMMON=ICP1+ICP2-ICP4 (a)

(b)

Fig. 8. Creation of the common mode sources on the PFC with the symmetries diodes Then we add the load common current to the drain-ground common current : ICOMMON=ICP1+ICP2-ICP4 This topology can’t provide some improvement of electromagnetic disturbances level. The figure 9 confirms these arguments. We can see an increase of spectrum level in low and medium frequency range. You conclude this converter version doesn’t present benefit.

EPE 2005 - Dresden

ISBN : 90-75815-08-5

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SWITCHING CELL OPTIMISATION MINIMISING THE COMMON MODE CURRENT FOR THE POWER FACTOR CORRECTOR

Area 2

Magnitude (dBµV)

Area 1

MAGNON Didier

Area 3

amplification of spectrum level due to capacitances CP2 et CP4 Classical PFC PFC with symmetric diode

Frequency (Hz) Fig. 9. Comparison of electromagnetic pollution between classical PFC and PFC with symmetries diodes

To symmetry the Boost inductance and the load If we add a diode between the MOSFET source and the load negative connection, and we symmetry the Boost inductance like the figure 10, the voltages between two load connections and the ground will balance.

•

+

C 102 C P1

-

• I P1

C P2 I P2

I COMM UN =I CP1 -I CP2 (b)

(a)

Fig. 10. Floating voltage with the symmetries diodes and boost inductance on the PFC If the two diodes switch at the same time, the DC bus of the Boost converter is not floating [5]. In this case, there is not common mode current in the DC bus parasitic capacitance. Also a common mode current flows in the ground plane: Icommon = ICP1 - ICP2. The voltages creating the currents ICP1 and ICP2 are opposite. Now, if the capacitances CP1 and CP2 are equal, then the sum of current ICP1 and ICP2 is zero, and the EMI spectrum will be attenuated. We realized this experimental converter to check the influence of the new floating voltage points. We compare electromagnetic spectrum between normal PFC and a PFC with a the balanced inductance and two diodes (figure 11). There is a EMI reduction about 10 to 25 dBµV between 150 kHz-6 MHz. This decrease is limited by the differential mode interference. The pollution peaks are modified too, the first peak is greatly attenuated. The area 3 is few modified.

EPE 2005 - Dresden

ISBN : 90-75815-08-5

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SWITCHING CELL OPTIMISATION MINIMISING THE COMMON MODE CURRENT FOR THE POWER FACTOR CORRECTOR

Area 2

Area 3

Magnitude (dBµV)

Area 1

MAGNON Didier

Classical PFC PFC with symmetric Boost inductance and diode

Elimination of the 8 MHz peak

Frequency (Hz) Fig. 11. EMI comparison between normal PFC and balanced PFC Study at low and medium frequency In low and medium frequency, between 150 kHz and 6 MHz, the result is highly improved but limited. Indeed, the two diodes are considered perfect, and it is not the case in practice. Two diodes from a same batch can have different features. Then, in off state, the voltage across the two diodes are different [6]. In dynamic state, the diode that has stored the least electric charge is the first diode off-state. This diode takes the whole inverse voltage so much as the second diode isn’t off-state. In practice, we notice the inverse voltage of first diode is 250 V when the second diode voltage is 150 V. Then we must take account into the parasitic current from the parasitic capacitance of the load like the figure 12 and the common current flowing in the ground plane is: Icommon = ICP1 - ICP2+ ICP3 - ICP4. Vin

A=Vin/2

B=(Vin+Vbus)/2+100V

Vin

-VBus/2-100V

Vin/2 Vin

0V

A=(Vin+Vbus)/2 B=(Vin+Vbus)/2 0V

(Vin-Vbus)/2

VBus

Vin

VBus

VBus

-VBus/2+100V 0V

C=Vin/2

Vin/2 ICP1 CP1

ICP2

0V

D=(Vin-Vbus)/2-100V ICP3

ICP4 CP3

CP2

0V

(Vin-Vbus)/2 ICP1

CP4

CP1

C=(Vin-Vbus)/2 D=(Vin-Vbus)/2 ICP2

ICP3

CP2

CP3

Plan de masse (a)

ICP4 CP4

Plan de masse ICOMMON=ICP1-ICP2+ICP3-ICP4

(b)

Fig. 12. Floating voltage with balanced actual diode

Result radiated mode between 30 MHz et 200 MHz We took a measure of radiated electromagnetic interference level between 30 MHz and 200 MHz (figure 13). We notice that the balanced PFC creates less electromagnetic interference than the

EPE 2005 - Dresden

ISBN : 90-75815-08-5

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SWITCHING CELL OPTIMISATION MINIMISING THE COMMON MODE CURRENT FOR THE POWER FACTOR CORRECTOR

MAGNON Didier

classical PFC. In spite of a low influence in the area 3 in conducted mode, the electromagnetic pollution reduction in radiated mode isn’t negligible with a reduction from 5 to 10 dBµV on the full range. Classical PFC PFC with symmetric Boost inductance and diode

Fig. 13. Comparison between classical PFC and balanced PFC

Conclusion We studied the electromagnetic interference of the industrial converter. We focused our work on the first stage to reduce the electromagnetic disturbances. We have modified the topologies of the PFC converter in order to compensate the common mode current. We analyzed the balanced PFC in three stage. We conclude the result of balanced PFC is better. In our study, we have considered that the diode like perfect. In practical, the behavior of diode are different and while the conducted state, the voltage of its isn’t the same one. The switching times for two diodes aren’t similar, then one of its will fall before other one. Experimentally there is a one hundred volt difference between the two DC bus and the ground. This difference is equivalent a voltage pollution voltage that creates a common mode current. Moreover, the phenomenon of EMI reduction with the balanced PFC in conduction is reproduced in radiation. In fact, we have a great attenuation from 5 to 10 dBµV on the range 30-300 MHz.

References [1] 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. [2] B.Revol, J. Roudet, EMC modeling of a three phase inverter, EPE 2003, Toulouse, France, 2003. [3] A. Santolaria, J. Barcells, D. Ganzáles, J. Gago, EMI Reduction in Switched Power Converter by Means of Spread Spectrum Modulation Techniques, PESC 2004 IEEE 35th, Aachen, Germany, 2004 [4] S. Brehaut, J-C. Le Bunetel, D. Magnon, A. Puzo, Analysis EMI of a PFC on the band pass 150kHz-30MHz for a reduction of the electromagnetic pollution, APEC 2004 IEEE, Anaheim, USA, feb 2004 [5] Shoyama, M.; Tsumura, T.; Ninomiya, T.; «Mechanism of common –mode noise reduction in balanced boost switching converter» Power Electronics Specialists Conference, 2004. pesc 04. 2004 IEEE 35rd Annual , Volume: 2 , 23-27 June 2004 Pages:1115 – 1120 [6] Rivet, B.; Gauthier, F.; Lanois, F.; «Turbo2 600 V diodes : Optimized solutions for PFC and other applications», International Conference Power Electronics, 2002. PCIM’02. March 2002 Pages:97-102 [7] S. Brehaut, J-C. Le Bunetel, D. Magnon, A. Puzo, A. Santolaria, J. Barcells, D. Ganzáles, J. Gago, Interactions between an input EMI filter and a power supply, EMC Zurich, 16th, feb 2005

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