Echelon LONWORKS PLT-22, User Manual

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Off-the-Shelf Switching Supplies
Most commercially available switching power supplies have been designed with some
level of input noise filtering. Frequently this level of filtering is adequate if the supply’s
fundamental switching frequency is within the optimal ranges of Table 5.4 over all
operating conditions and tolerances. Supplies with other fundamental operating
frequencies may be able to meet the required noise masks of the previous sections with
greater effort and/or cost; however, meeting the noise mask requirements with a power
supply fundamental frequency (or second or third harmonic) in the 110kHz to 140kHz
range is very challenging and not recommended .
Most off-the-shelf switching supplies have a very low input impedance and require the
addition of an input inductor as shown in figure 5.5.
Custom Switching Supplies
Design of a custom switching supply, even one which employs a commerically
available controller chip, is very challenging and is only recommended if none of
the other options outlined in this chapter are suitable. If this option is chosen, it
is recommended that it only be pursued by experienced switching supply and
electromagnetic compatibility experts. Even if such personnel are available,
extra time must be allowed for design iterations during development. If this
approach is chosen, it is absolutely essential that the supply is verified to meet
the input and output noise masks of this chapter.
!
Note that many of the popular power supply designs documented by National
Semiconductor in conjuntion with their Simple Switcher™ controller chips do
not meet the required noise masks. Due to their very wide frequency tolerance,
the use of any Simple Switcher controller with a rated operating freqeuncy of
52kHz, 100kHz, or 150kHz is not recommended (e.g., LM2574, LM2575, LM2576,
LM2577, LM2585, LM2587, LM2594, LM2595, LM2596, LM2597, LM2598,
LM2599, LM1575, LM1577). Other Simple Switcher controller chips with
nominal operating frequencies of 200kHz or higher may be considered, providing
the design has been verified to meet the input and output noise mask
requirements of this chapter (over all of the operating conditions).
5-22 Power Supplies for the PLT-22 Transceiver