Definition and Acquisition of CM and DM EM1 Noise for General-Purpose

8/11/2019 Definition and Acquisition of CM and DM EM1 Noise for General-Purpose

1/6

2004 3Sth

Annual IEEE

Power Electron ics Specialisrs Conference

Anclten Germany

2 4

Definition and Acquisition of CM and DM EM1 Noise for G eneral-Purpose

Adjustable Speed Moto r Drives

W. Shen,

F.

Wang,

D.

oroyevich, and

Y.

Liu

Center for Power Electronics Systems

The Bradley Department of Electrical and Computer EnFineering

Virginia Polytechnic Institute and State University

Blacksburg, VA 24061 USA

weshen@,vt.edu

Abstract-

Separating conducted

EM1

noise into different

modes, common mode

( C M )

and differential mode

(DM),

is

important to the appro priate application of emission reduction

techniques. While the C M / D M separation is well defined and

understood for the single-phase or D C system, the same canno t

be said for three-phase converter systems, common for

general-purpose adjustable speed drives (ASD). Based on the

study of C M and D M propagation characteristics

of

a three-

phase diode-front converter, this paper

identify

different noise

modes for different front-diode conducting patterns. The

impact on E M 1 filter components by these noise modes is

summarized. Finally, a time-domain based method is proposed

to separate and acquire C M and D M noise components for the

diode front three-phase systems. Simulation and experimental

verifications are presented.

I. INTRODUCTION

The separation of common-mode (CM) and differential-

mode (DM) noise components from total conducted EM1

noise is important to the EM1 filter design and conducted

emission modeling. For DC

or

single-phase AC systems, the

definitions

of

CM and DM conducted EM1 noises are clear

and well understood [I]. Hardware based on signal

transformers or combiner/splitter has been introduced

successfully to acqu ire them [1]-[4].

+

v =-

2

V,, = V, - V ,

V, ,

Vh

:

noise voltages on two lines)

However, when the CM noise is not evenly distributed

between

two

lines, the unbalanced part of C M noise w ould

become DM noise according to the above definition. The

discontinuous conduction

of

the front diode bridge does

cause this CM uneven distribution, the so-called non-

intrinsic differential-mode noise that has been reported in

[5] for diode front-end single-phase power converters. This

affects EM1 filter design and emission modeling, since the

CM and DM noise equivalent circuits cannot be separated

clearly. Specifically, line-to-line capacitors would be chosen

both considering for balancing the unevenly distributed CM

and attenuating DM noises.

For three-phase systems, there is no corresponding CM

and DM definition. However, we still can define the C M

This work was supported primarily by

the

ERC Program of the National

Science Foundation under Award Number EEC-973 1677.

noise for three-phase system as ground loop noise, and

DM noise

as

line-to-line. Once we characterize the CM

and DM in this way, we have already assumed that the

three-phase converters can be decoupled into tw o orthogonal

equivalent modes.

CM

noise is generated by CM noise

source and propagated along ground-included-loop, and DM

noise is from DM noise source and through line-to-line loop.

If the three-phase syr:tem is symmetrical, linear and time-

invariant, CM and DM components can be decoupled and

obtained through y, ti, 0) transformation [6] or some other

orthogonal transformations. Accordingly, several CM/DM

separation methods [7]-[8] are proposed based on this

symmetrical assumption.

However, for three-phase systems widely used in ac

converters (such as motor drives with diode-front), the

circuit is inherently unsymmetrical and time-variant. Similar

to the single-phase c3se [SI, the unevenly distributed CM

noise would appear. Furthermore, the possible diode

commutation would make the three-phase case more

complicated. Under this circumstance, separation of CM and

DM noises meaningfully at the three input lines is a

challenge. In

[9]

a

mapping relationship, scaling the

spectrum by two thirds (3.5dB). is built between DM noise

currents at inputs of liont-end rectifiers and at point of DC

links, where CMIDM separation can be straightforward.

This convolution reflection is valid under the assumption

that the rectifier input current shape is quasi-square-wave of

certain amplitude with a given conduction pattern (each

diode conducts 240).

Following the analysis approach for single-phase

converters [5], this paper tries to clarify the existence of

unevenly distributed CM noise among three phases. Based

on the study of CM and DM propagation characteristics of a

diode-front converter, this paper identifies different noise

modes for different front-end diode rectifier conducting

patterns. The impact on EM1 filter components by these

noise modes is analyzed. Finally, a time-domain based

method is proposed to separate and acquire CM and DM

noise components for the diode-front three-phase systems.

11. CM

AND

DFd

PROPAGATION CHARACTERISTICS

For motor drives with diode-front rectifiers, the DC link

can be treated as three-wire DC system, where the CM and

DM definition and separation are explicit as discussed

above. However, the three-phase EM1 filter at AC input, not

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mailto:weshen@,vt.edumailto:weshen@,vt.edu


2/6

2004 35rh

A n n u a l

IEEE

Po wer

Elecrronics Speciali sts

Con ferace

Aachen Germany 2004

DC EM1 filter,

is

usually chosen for the system. Therefore,

we can find out the mapping relationship between the A

inputs and DC link, to help

us

to understand and define ihe

noise mode. To conduct the app ing analysis, we can

assume that front-end diodes do not contribute

to

EM1

noises emission, as shown by other previous work. DM

noise current circulates between two-conducted phases

under normal operation. There also could be no DM noise

flow under discontinuous-conducted-mode (DCM ), or DM

noise current on all three lines under continuous-conducted-

mode (CCM) because of diode commutations. Meanwhile,

these different diode-conducting conditions also affect the

propagation paths of CM noise currents, which

is

actually

the mechanism of mixed mode noises

[SI.

A simple but representative switching converter is built

and analyzed to understand the mechanism of the mapping

CM and DM noises from DC link to the input of the three-

phase rectifier. The system is shown in the Fig.

I ,

which

consists of three-phase rectifier, DC link capacitor, and

MOSFETllGBT switches. The system represents the

conducted EM1 emission of typical switching power

converter, and the analyzed results can be applied to other

three-phase converter systems such as motor drives.

Fig. I The three-phase diode-front switching

convertersystem

(blue

components representing major parasitics considered during

analysis)

A . D M Noise Propagation

For

converters with DC

link

capacitors, DM noise

propagation mechanism can be described as follows. High

current slew results from each switching instan t, and the AC

source and DC link capacitors would provide the fast

dUdt

together. The portion from AC source is what we concern

for

EM1

noise reduction and standard limits regulate, which

is determined by the equivalent source impedances

of

the

source and capacitor. The presence of

LISN

would cause the

DM current provided by

LlSN

capacitors, if we think the

LlSN

ideally isolate the AC source for high frequency

range. Since the front-end rectifier is nonlinear and time-

variant, we need to examine noise propagation path

specifically for different diode conduction patterns. The

diode conduction

is

determined by the relationship between

line-to-line voltages and the DC link voltage. If we only

focus on the AC inputs, durin g input line current conduction

period, DM noise currents will flo w between the two

conducting phases, as shown in Fig. 2.DM noise current can

still flow to certain extent, when there is no line-frequency

current conducted (DCM), since the

ESL

of DC link

capacitors will lower the DC voltage. Therefore, the

conduction angle of DM noise current is larger than that of

low frequency phase current for each phase. Under this

condition, the three-phase converter is equivalent to three

single-phase converters operating alternatively within one

line cycle. Therefore, we can apply the DM definition by

standard to the circuit.

During the duration of no phase current conducting, there

is no DM noise presenting at the AC input, while the DC

link capacitor would provide the switching current slew, as

illustrated in Fig.

2.

During the short period of diode commutation, there are

all three lines representing low impedance, and the DM

noise current could flow through three LISNs. Although the

noise of one phase is always equal to the

sum

of noise on the

other two phases, the detailed distribution still depends on

the instantaneous circuit parameters. DM is not appropriate

description for the noise mode under this transition duration,

so

we note this mode

as

commutation mode.

- -

- -

1029


3/6

2004 35lh Annual

IEEE

Power Electronics Specia lisls Conference

Aachen,

Germany 2004

Fig. 2. DM

noise propagation

path illustration

during the

period

ofno-

phase, huo-phase,and three-phase

conducting ( b m

op to

bottom

E.

CM NoisePropagation

CM noise current flow is determined by phase voltages

and diode conduction status, which play similar role as

transistor DC voltage and current biasing to small signals.

CM will basically flow through low loop impedance paths,

which are affected by diode conduction and component

high-frequency impedance.

Under two-diode conducting conduction, CM noise

current will be evenly distributed on the two conducted

phases, as in Fig. 3.Since the DC link impedanceis much

smaller than the source

LlSN

impedances, we can treat the

positive and negative D C buses as equal potential points.

For no line current conduction, CM will flow uni-

directinnally through either the most positive phase, when

rear-end switch turns-on (dv/dt>O),

or

the most negative

phase, wh en the switch tu rns-off (dv/dt


4/6

2 4 35th

A n n u l

IEEE Power Electronics Speciolisrs Conference

Aachen Germany, 2004

conducting situation. The DM noise is between the two

conducting phases, while the CM, due to both MOSFET

turn-on and tun-off, also evenly appears on these two

phases. For no-diode conducting duration, Fig.

5

shows the

unidirectional CM noise on each phase, and there is no DM

noise.

... . ~. . . . . . . .. .

.........I

s

simulation (uppa ) and expenmental lower)

IV. SEPARATIONOF CM ANDDM

Since our purpose is to find out the profile of the noise

spectrum, which will be used as bare noises for EM1 filter

design, we can use the spectrum result

of

the diode-

conducted period as worst-case. The effect of noise for

whole line cycle time duration would be the same a s this

worst period, fiom filter design point of view.

After understanding the propagation mechanism of CM

and DM noises under all three possible rectifier diode-

conducting patterns, it is apparent that CM noise can be

obtained at anytime by measuring all three phases together.

For the DM noise, n o m 1 wo-phase conducting would have

bigger noise contribution, for the line current amplitude is

bigger than commutation intervals. Therefore, we can

measure two conducting phase currents during the period of

pair diodes conduction. Accordingly, CM noise can be

obtained by

sum

two-phase currents at this period, and

DM

noise of the phase would be the result of subtracting half of

the CM noise amplitude from each phase current. The

corresponding spectrum of CM and DM noise then can be

obtained through doing Fourier Transform.

50

2

V u

=

F F T [ i , + i , ) * - ] vor instance, pha se

A

and E

conducted) 2)

i i

2

VDU

=

F F T [ e ) * 2 * 5 0 ] or instance, phase A and

B conducted) (3)

To verify the method, we first perform the proposed

method to obtain CM and DM noise components at the

diode rectifier input side. Then we connect

LISN s

to

positive and negative DC outputs of the rectifier, and

directly measure CM and DM spectrum using Common-

mode rejecter (CM R)/Differential-mode rejecter (D MR) and

AgilentB E7402A spectrum analyzer. The comparison is

shown in Fig. 6, and they match well below several mega-

hertz. The high-frequency discrepancy is because of the

coupling

of

noise from the function generator, which is used

to get MOSFET gate signal.

UL j i a m *:m P. .W. I_j

m e

i66' xi

. .. .,.,.. ...

,

..

simulatio n (upper) and experimental lower)

Fig. 5. lhree input line current waveform during no-diode conducting,

103

1


5/6

2004

35Ih Annual IEE E Power Electronics Specialists ConferencE

dBuV

Fig.

6 .

CM

noise

(upper)

and

DM noise (lower) from the proposed

method solid) and Agilat@ E7402A spectrum analyzer dotted)

v. IMPACT

ON

FILTERDESIGN

From the analysis abovementioned, it is clear that CM

and DM noise propagation characteristics are different for

different rectifier diode conduction patterns, and EM1 filter

design needs to consider all of them on time dom ain basis.

For a given filter topology, filter equivalent circuits can be

derived under different periods within one line cycle in time

domain. To guarantee the filter design meeting attenuation

requirements in frequency dom ain, we need to take Laplace

transform of the noises, multiply the corresponding transfer

function of the equivalent filter circuit,

sum

all

s

components, and finally apply Laplace-Fourier transform to

obtain spectrum [IO]

In practice, since there is only one filter for all three

different time d urations within one line cycle, one practical

approach is to identify one duration as the worst case and

design the filter accordingly. The designed filter would

automatically satisfy the frequency domain attenuation

requirements. We can assume the noise amplitude does not

change for different diode condu ction conditions. For DCM

applications, we can design CM and DM parts of EM1 filters

according to the equivalent circuit shown in Fig. 7,

respectively, since X capacitors (usually several pF) make

filter CM equivalent circuit almost the same for different

conditions.

For CCM applications, we need to consider both two and

three diode conducting situations. The filter CM equivalent

circuit can still use the one shown in Fig. 7. After checking

DM equivalent circuits, we can conclude that the two-diode

conducting situation is the worst case. Therefore, we still

can use the equivalent circuit show n in Fig.

7.

AAer

X

capacitors of the filter are ad ded, the path of CM

and DM noise currents will be different on LISN reference

resistors

SOQ)

from input terminals of the rectifier. Since

the value of Cx is usually in the range of 0.5-3pF, which

would short three phases together for the frequency range

of the interest.

Therefore, CM noise current would follow through three

LISNs evenly, to certain degree. In this sense, the Cx also

attenuate CM noise, similar to the reported mixed-mode

phenomena in [SI.Another important point is that the design

~

1032

Aachen.

Gennany

W4

of CM choke would consider one third of total CM noise

curren t for each winding.

Zcm

50R

50R

Fig. 1.

CM

filter

equivalent circuit (upper) and DM filter equivalent

circuit

(lower)

From above discussion, we can conclude that the three-

phase EM1 filter is actually designed as three single-phase

filters, which w ill be effective alternatively along the phase

sequence. The combination of these three identical filters

results in the one three-phase filter for the system.

Furthermore, this conclusion implies that the EM1 filter put

at the DC link could be more optimal from both part count

and size standpoints. Another advantage of the DC link EM1

filter is that the Cy leakage current limitation is not

applicable any more.

It

is true that front-end diode bridge

would generate very limit CM and DM noises, and not

putting of the EM1 filter at the input edge of the system

would d egrade the eflectiveness of the filter. However, the

careful shield and ground design could still make the DC

link filter work w ell.

VI.

CONCLUSIONS

One CM noise and one DM noise are not enough to

characterize the EM1 noise of diode-front type

of

three

phase co nverter systems. Other diode conducting conditions

due to current discontinuous or diode commutation

influences would cause other modes of n oise. However, the

two-diode conducting situation is identified as the worst

case. Therefore, sepaiating CM and DM components from

total noise of each phase can be obtained during two-diode

conducting period, through clearly defined algebraic

calculation and Fourier Transform. This analysis is useful to

the three-phase EM1 filter design, and also provides insights

to the further efforts on EM1 modeling. Both bare noise

spectrum acquisition method and filter design equivalent

circuits are given.


6/6

2004

351h

Annual IEEE Pow er Electronics Specialists Conference Aachen Germany 2 4

ACKNOWLEDGMENl

This work made use of ERC Shared F acilities supported3

by the National Science Foundation under Award Number

EEC -973 1677.

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Documents

Definition and Acquisition of CM and DM EM1 Noise for General-Purpose