Echelon LONWORKS PLT-22, User Manual

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Switching Power
Supply
Coupling
Circuit
PLT-22
Transceiver
Neuron
Chip
Line
Neutral
Earth
Figure 5.5 Reducing Attenuation Caused by a Switching Power Supply
Including this inductor is desirable because the increase in attenuation for a given
load is much worse when the load, in this case the switching power supply, is
connected directly to the transceiver. In contrast, the loading caused by a switching
supply that is separated from the receiver is reduced by the series inductance of
power line wiring.
The loading effect of a switching power supply almost always dictates the addition of
the inductor for L-to-N coupling circuits. The inductor may or may not be required
for L-to-E coupling circuits depending on the EMC filter topology of the switching
power supply. In either case, the value of the inductor should be chosen such that
the power supply does not impose a low impedance onto the power line in the
communication frequency range of 110kHz-140kHz. The appropriate value of
inductance is a trade-off between required impedance, which is a function of system
topology, and node cost. The selected inductor must have a current rating which is
greater than the peak current drawn by the power supply in order to avoid
impedance reduction due to inductor saturation. Higher impedances require larger
inductance values which are more expensive for a given power supply input current.
For most AC mains distribution systems, where communication frequency
impedances are typically in the range of 1 to 20 Ohms, a power supply input
impedance of 100 Ohms is sufficient to avoid added signal attenuation.
An example of a system that would benefit from an input impedance >100 Ω would be
one in which 100 or more PLT-22 transceiver-based nodes were connected to a 1000m
powered, twisted pair cable. In this case the input impedance of the power supply
could limit either the maximum transmission distance and/or the maximum number
of transceivers that could be connected to the cable. To maximize communication
distance on dedicated twisted pair wiring where the system impedance is
approximately 100 Ohms, a minimum power supply input impedance of 500 Ohms is
recommended. For extreme cases with >100 nodes on dedicated lines longer than
1000m (3279 feet), a power supply input impedance of 2000 Ohms may be needed to
maximize communication distance. Table 5.3 shows the appropriate inductor value
by application.
5-10 Power Supplies for the PLT-22 Transceiver