Echelon LONWORKS PLT-22, Руководство пользователя

Страница: 90 / 128

In some instances conducted emissions above 500kHz can be adequately reduced by
the addition of a small value capacitor (e.g., ≤ 470pF) either across the AC mains or
from the line conductor to ground. While nodes using the PLT-22 transceiver have
been demonstrated to pass various limits without an additional capacitor, variations
in node design and layout may require the addition of this small value capacitor. If a
capacitor is added across the line it should be an X2 safety-rated type for maximum
surge reliability. If capacitors are added from either line or neutral to earth, they
should be Y safety rated. Alternately, this capacitor can be added across coupling
circuit inductor L1 (see figures 4.10 and 4.13) or across the secondary winding of
transformer T1 (see figures 4.11 and 4.14). If this option is chosen, either a
metalized polyester capacitor ≥ 250VDC, a ceramic 1000VDC capacitor, or a Y-type
capacitor should be used for surge reliability.
Adding capacitance in any of the above locations reduces the input impedance of the
node and could therefore cause an increase in communication signal attenuation.
The maximum value of capacitance which can be added without significantly
affecting attenuation depends on the application. Table 6.1 shows the maximum
value of added capacitance by application. Under no circumstances should
capacitors >5000pF be used since they will result in excessive signal
attenuation.
Table 6.1 EMC Suppression Capacitor Value versus Application
Application Network Impedance @
130kHz
Capacitor Impedance @
130kHz
Capacitor Value
Single building AC
mains
1-20 Ohms ≥ 250 Ohms ≤ 4700pF
Dedicated cable
≤ 100 nodes
≤ 100m
50 - 100 Ohms ≥ 1000 Ohms ≤ 1200pF
Dedicated cable
>100 nodes
>100m
50 - 100 Ohms ≥ 2500 Ohms ≤ 470pF
Another common method of EMC suppression, the addition of ferrite beads, is
generally unacceptable if they are placed anywhere in the transmit signal path.
Most ferrite beads have several Ohms of impedance at 130kHz. The impedance of
any element placed in series with the transmit signal or return path must be
significantly less than 1 Ohm as described in chapter 4. There is, however, one
means whereby a ferrite bead can be used to reduce common mode high frequency
emissions without affecting the transmit signal. If both the communication signal
and its return conductor (i.e., Line and Neutral for L-to-N coupling or Line and Earth
for L-to-E coupling) pass through the same bead in a common fashion, the bead will
not add any series impedance to the transmitter. This is true since the signal
currents in the two conductors produce opposite polarity (canceling) flux in the
ferrite bead’s core. Common mode noise of equal polarity on both conductors will
produce additive flux in the ferrite bead’s core and will thus be attenuated. Figure
6.3 illustrates both acceptable and unacceptable ferrite bead topologies.
6-8 Design and Test for Electromagnetic Compatibility