EECS423Lect17

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Transcript EECS423Lect17

Conducted Emissions and Susceptibility
Conducted emissions are simpler to investigate than radiated emissions
because only the product’s power cord needs to be measured.
Measurements of conducted emissions are made using a line impedance
stabilization network (LISN) that is inserted between the product and
the power distribution network.
phase
product neutral
safety
LISN
phase
neutral
safety
a.c. power
distribution
spectrum
analyzer
Figure 1
The purpose of the LISN is twofold:
1) present the same impedance out of the product’s power cord;
2) reduce disturbances due to the a.c. power distribution.
The LISN used for FCC conducted emission tests is:
LISN
phase
50H
IˆP
neutral
50H
1F
1F
1F
50
+
VˆP
receiver
IˆN
-
+
1k
50 VˆN
Dummy
load -
Product
IˆN
1k
IˆP
green
wire
Figure 2
The values of the inductors and the capacitors are chosen so that in the
frequency range of interest the inductors have a large impedance and
the capacitors are almost short circuits.
In this way, the phase current and the neutral current are related to the
voltages by:
VˆP  50 IˆP
VˆN  50 IˆN
(1)
( 2)
Note that for the CISPR 22 conducted emission test, the frequency range
is different (150 KHz-30 MHz) and both capacitors and inductors must
have other values to obtain the desired behavior.
Common Mode and Differential Mode Currents
The simplified representation of a LISN as two 50 resistor is the
following:
IˆP  IˆC  IˆD
50 
IˆD
VˆP
50 
VˆN
phase
neutral
IˆC
IˆC
Green
wire
Figure 3
and, similarly to what we did for the radiated emissions, we introduce
common and differential mode currents as:
1
I D  I P  I N 
(3)
2
1
I C  I P  I N 
(4)
2
In the case of conducted emissions, common mode currents can be of
the order of differential mode currents. If this is the case, then VP and
VN are different since they are given by:
VP  50  I C  I D 
VN  50  I C  I D 
(5)
(6)
Usually one component dominates the other so that the magnitudes of
VP and VN are the same.
Conducted emissions are usually reduced by introducing a power filter.
A power filter contains elements each one of them reducing either
common or differential mode currents
An usual way to block common mode currents is the green wire inductor:
IC
Product
phase
IC
neutral
LGW
Green wire
2IC
LGW
Figure 4
Other ways of reducing emissions are considered in the following.
We want to remind that almost all electronics products contain some
form of power filter where the power cord exists the product.
Sometimes the filter is simply a large transformer or a linear power
supply that provide inherent filtering; for all other cases an “intentional”
power filter is required.
A typical topology of a power supply filter is shown in the following:
Figure 5
LGW :
green wire inductor - blocks common mode currents;
C DL , C DR : divert differential mode currents
- these are line-to-line capacitors or X caps;
CCL , CCR : divert common mode currents
- these are line-to-ground capacitors or Y caps;
L, L: two coupled inductors -- block common mode currents
The purpose of the filter is to reduce IˆD , IˆC so that:
VP  50 ( IˆC'  IˆD' )
VN  50 ( IˆC'  IˆD' )
are below the limits.
(7)
(8)
Let us consider the operation of the power supply filter when only
common mode currents are present:
Figure 6
Common mode currents are represented with current sources. Because
of the symmetry of the circuit, we obtain the equivalent model:
L+M
Figure 7
2 LGW
Note: the common mode choke appears as an inductance L+M, and the
green wire inductor appears doubled because the current through
it is doubled
The operation of the power supply filter under differential mode currents
is understood by considering the following:
Figure 8
Again, in this case, differential mode currents are represented with
current sources; due to the symmetry of the circuit we obtain the
equivalent representation:
L-M
Figure 9
Note: line to line capacitors appear twice as large to differential mode
currents; line to ground capacitors also affect differential mode
currents. In the ideal case, L-M=0 and differential model currents
are completely blocked.
Separation of common mode and differential mode currents
The actual behavior of a power supply filter is not the ideal one and in a
practical situation the conducted emissions may behave as in the
following:
Figure 10
The total current is given by:
IT  IC  I D
(9)
And when one of the components ( I D or I C ) is much larger than the other.
The total current is dominated by the larger component.
Therefore, in order to reduce the total emissions, one should identify (in
a given frequency range) the dominant current component and change
the elements of the power supply filter to reduce its emissions.
The common mode and differential mode components are identified via
a diagnostic tool, such as the one shown in the following:
Figure 11
Additional considerations
Even with the use of power supply filters, only a certain degree of
emission reduction can be accomplished. In general, the best way to
suppress conducted emissions is to reduce them at their source.
However, this is not always possible, as in the case of the noise due to
sharp rise and fall times of clock waveforms
For example, the switched-mode power supplies that are found in
many products are generally the leading cause of conducted emissions.
The operation of these power supplies relies on short rise/ fall times
in order to increase the energy conversion efficiency.
Another simple way to reduce conducted emissions is based on the
appropriate location of the power supply and the power filter.
Let us consider the two following configurations:
Internal fields
Power
Supply
Filter
Internal fields
Filter
Product cabinet
Power
Supply
Product cabinet
(b)
(a)
Figure 12
Case (a) represents a poor choice for the location of the filter and the
power supply because the fields inside the product cabinet may couple
with the wires that lead to the power cord.
Case (b) shows the proper location of both the filter and the power
supply. In this way, the emissions transmitted to the power cord are
reduced to minimum level provided by the filter.
In general, the filter should be placed against the cabinet to minimize
the coupling with the internal fields. For the same reason, the power
supply should go as close as possible to the filter.