conducted interference
TRANSCRIPT
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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