Conducted Interference

7/28/2019 Conducted Interference

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Conducted Interference

Tim Williams

Elmac Services

This presentation discusses the nature of conducted interference with

particular respect to the requirements of present-day commercial test

standards. Equivalent circuits are presented and analysed so that theprinciple modes and routes of coupling can be understood in the general

case. These principles can then be applied to a particular design in order to

diagnose and fix interference coupling problems.


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Conducted coupling

Mains Port Signal Ports

Conducted RFemissions

Mandatory Occasional

Conducted RFimmunity

Usual Usual

Conductedtransient immunit

Mandatory Usual

Conducted interference can be classified according to whether it is incoming

or outgoing, and whether it is coupled via the mains port or via signal ports.

The vast majority of CISPR- and IEC-based EMC test standards requiretesting of all phenomena on the mains port; many require immunity testing

on signal ports and more tests are being proposed. CISPR 22, for instance,

now requires conducted RF emissions testing on telecommunications ports.


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Mains emissions: equivalent circuit

EUT LISN

VCM

VDM

L

N

E

The basic equivalent circuit for conducted emissions testing on the mains

port is shown here.

The mains connection is represented by the AMN/LISN giving a defined RFimpedance between live and earth, and between neutral and earth.

The EUT contains both differential and common mode sources, generalised

here as appearing in one case between live and neutral, and in the other case

between both live and neutral with respect to earth.

The connection to the mains is made via a length of cable which should also

be included in the model for best accuracy, but is not included in this

discussion. If the apparatus is Safety Class II there is no earth wire, but

common mode signals can still return via the stray capacitance to the ground

plane.


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Differential mode sources

EUT

VDM

ZDM

Differential mode sourcevoltage and impedance

Typical source: switching currentdevelops voltage across SMPS DC link

L

N

Differential mode sources appear between live and neutral connections

without reference to the earth connection.

In circuits with switch-mode power supplies or other power switchingcircuits the RF emissions are dominated by interference developed across

the DC link to the switching devices. Although there will normally be a

reservoir capacitor, the high di/dt through this capacitor will generate

voltages at the harmonics of the switching frequency across its equivalent

series impedance.

Diode noise, if it is significant, will also appear in differential mode.


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Common mode sources

EUT

VCM

Common mode

source voltageand impedance

Circuit PS

Typical sources: SMPS

dV/dt and circuit noisevia stray capacitance

Common mode sources are more complex. The common mode voltage

appears between both live and neutral with respect to earth. Since the mains

input is normally isolated from earth, it is usual for common mode couplingto be capacitive.

The coupling is dominated by the interwinding capacitance of the isolating

transformer and the stray capacitances of noise sources, both in the power

supply (e.g. from heatsinks) and the operating circuit. These capacitances

are referred to earth, either directly or via the enclosure if this is conductive.

A well-shielded enclosure will minimise "leakage" of this capacitive

coupling and hence reduced conducted emissions.

Other impedances may appear in the coupling path: for instance the leakage

inductance of the isolating transformer is in series with its interwinding

capacitance and may give a series resonant peak in the MHz range.


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The AMN/LISN

Mains

supply

to EUT

VDM = VN - VL

VCM = VN + VL

VN VL

50

50H

5

The Line Impedance Stabilising Network (LISN) forms part of the overall

equivalent circuit for testing. Its characteristics are defined in CISPR 16-1,

which requires an impedance of 50 in parallel with 50H + 5 betweeneach phase (live and neutral) and the earth reference point. This reference is

connected to the earth reference plane of the test setup.

Voltages are measured across each line with respect to the earth reference

and the higher voltage gives the test result. This test does not measure either

common or differential mode directly, but each can be obtained by summing

or differencing the two lines.


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The mains filter

2 x CYCX

LCM

CX filters differential mode

CY, LCM filter common mode

Application of a mains filter is usual in most cases of mains conducted

interference. The typical mains filter includes components to filter both

differential mode (CX) and common mode (CY, LCM). Although a mainsfilter does not have an "input" or "output", it should be installed in the

correct sense. Normally, the Y-capacitors will face the equipment in order

to provide a capacitive divider with the common mode source capacitance.

The X-capacitor will be more effective on the mains side of the choke, in

order to take advantage of its leakage inductance. In general, capacitors

should face the higher impedance and inductors should face the lower

impedance.


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Mains immunity: equivalent circuit

injectedinterference

Coupling/decoupling

network

EUT

circuit

external

connections

A similar equivalent circuit can be devised for analysing mains-borne

conducted immunity. In all the relevant tests, the interference is injected via

a coupling/decoupling network (CDN) into all three of the mains lines, live,neutral and earth. Testing of the earth line is the major difference between

these tests and the emissions tests.

Common mode coupling is the principle path. The interwinding capacitance

of the isolating transformer passes the interference into the operating circuit.

Here it generates differential voltages across stray impedances at critical

circuit nodes, thereby affecting operation. The return path for the

interference current may include stray capacitance to the test earth reference

plane, and/or it may return via external connections which are coupled to

earth. Filtering all connections to the enclosure to ensure the current

bypasses the circuit will provide the optimum solution.


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Transient coupling (IEC 61000-4-4)

Fast transient bursts

Fast transient burst testing couples the transients into each mains line

capacitively via a CDN. The burst generator is referenced to the earth plane.

The CDN does not have a well-defined source impedance; the burstgenerator is specified to be approximately 50.


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RF coupling (IEC 61000-4-6)

Rtot = 100

Modulated RF

ZS = 50

Conducted RF injection via a CDN on the mains port is very similar to fast

transients, but the CDN is specified differently. All lines are tested

simultaneously. The source impedance at the EUT port of the CDN isspecified as 150 with respect to the earth reference plane.

Modulated RF then appears at all conductors of the mains cable with this

source impedance. The RF current flows through the EUT via both the

power supply and the enclosure (if this is earthed), and returns by stray

capacitance to the earth plane and by any external connections, which in this

test are required to have a stabilised 150 impedance to the earth plane.


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Signal port conducted emissions

PS

Circuit

impedancestabilisation

VN

EUT

mainsconnection

ICM

RF current

measurement

Conducted emissions tests at the signal ports are likely to be increasingly

required in the future.

Various test methods may be applied, but all of these measure the common

mode current or voltage being emitted from each port, into a cable whose

common mode impedance is stabilised. The noise sources may be modelled

as the contribution of all circuit noise referred between the port connection

and the earth reference by stray capacitance, possibly mediated by the

enclosure if this is conductive. The actual desired signal carried by the port

will also contribute if its bandwidth is within the test spectrum and there is

conversion of differential mode signal to common mode, either at the port

or along the cable.


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Signal port conducted immunity

EUT

CircuitPS

Fast transient

bursts

capacitive clamp

RF

CDN

Immunity of signal ports is a frequent requirement of many test standards.

Either continuous RF or transients may be involved. In either case common

mode coupling is the norm.Although the interference is applied in common mode at the port,

differences in internal stray impedances will convert this to differential

interfering voltages at critical circuit nodes. Good PCB layout will minimise

this conversion and hence improve the inherent circuit immunity.

Further protection is provided by common mode filtering at each interface

to divert the interference currents away from the circuit and into the chassis.

The currents then return to the earth reference either directly (if a direct

connection exists) or via stray capacitance from the chassis.


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Summary

interference may be conducted into or

out ofapparatus, in

differential orcommon mode, as

continuous RF ortransients, on

either or both of the mains supply and

signal ports

Conducted interference is dealt with by filtering all appropriate interfaces

and by proper shielding. Understanding the equivalent circuits for each

coupling case allows the correct implementation of these measures.

Tim Williams is with:

Elmac Services, PO Box 111, Chichester, West Sussex, PO19 5ZS

Tel 01243 533361 Fax 01243 790535

http://www.elmac.co.uk

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Conducted Interference