Specifications

Chapter 5 – Power Supplies for PL Smart Transceivers
170 PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Boo
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Power Supply Impedance and Noise Requirements
Power Supply-Induced Attenuation
The input stage of a switching power supply typically contains an EMC filter that includes one or more capacitors
connected directly from line to neutral and, in many cases, additional capacitors from line and neutral to ground.
When the AC line terminals of the switching power supply are connected to the AC mains (in parallel with the
coupling circuit), additional signal attenuation occurs. This loss can be avoided by inserting a series inductor
between the power supply input and the power line communication channel as shown in Figure 5.10.
Figure 5.10 Reducing Attenuation Caused by a Switching Power Supply
This inductor is necessary 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 other
switching power supplies on the AC mains that are separated from the receiver is reduced by the series inductance of
power line wiring.
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. 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 the line impedance at the communication frequencies of the PL
Smart Transceiver is typically in the range of 1 to 20 ohms, a power supply input impedance of 100 ohms is
sufficient to avoid added signal attenuation. To maximize communication distance on mains distribution systems
between buildings where the system impedance can be as high as 50 ohms, a minimum power supply input
impedance of 250 ohms is recommended.