Echelon LONWORKS PLT-22 User Manual
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output loading would result in output voltages >16V. In this application thezener diode does not conduct unless the output load is light and the line voltageis high.In summary, an energy storage linear supply differs from a traditional linearsupply in the following ways:1. Allows the use of a smaller transformer;2. Requires more output capacitance for energy storage;3. May require over-voltage zener diode protection;4. Requires that the node be programmed to enable power management, asdescribed in chapter 3;5. Under typical conditions, exhibits transmit duty cycles which are not limited.Over the full range of line voltage and component tolerance, the PLT-22transceiver’s power management may act to regulate a particular node’stransmit duty cycle (to ≥10% in the above example).Traditional Linear Power SuppliesThis option is usually suitable if the physical size of the power supply is notconstrained in the application, since linear supplies tend to be physically largerthan switching power supplies. Linear power supplies do not load the power linein the range of the PLT-22 transceiver's communication frequencies, nor do theygenerate significant noise. For these reasons, a linear power supply may be usedwithout concern that communication performance will be adversely affected.Switching Power SuppliesWhile generally smaller than linear power supplies, switching power suppliescan be a significant source of noise and power line signal attenuation. As aresult the required design effort is significantly greater than that required forlinear or capacitor-input power supplies.Several design options are available for use if a switching power supply is requireddue to size or application current constraints. These options include a pre-designed switcher, an off-the-shelf switcher, and a custom-designed switcher. Theyare described following the discussion of switching supply design issues.Power Supply-Induced AttenuationThe input stage of a switching power supply typically contains an EMC filterthat includes one or more capacitors connected directly from line to neutral and,in many cases, additional capacitors from line and neutral to ground. When theAC line terminals of the switching power supply are connected to the AC mains(in parallel with the coupling circuit), additional signal attenuation occurs. Thisloss can be avoided by inserting a series inductor between the power supply inputand the power line communication channel as shown in figure 5.5.LON WORKS PLT-22 Transceiver's User Guide 5-9 |
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