7.4.2.10 ORing Challenges

Preference of one power source presents a number of challenges. Combinations of adapter output voltage (nominal and tolerance), power insertion point, and which source is preferred determine solution complexity. Several factors adding to the complexity are the natural high-voltage selection of diode ORing (the simplest method of combining sources), the current limit implicit in the PSE, and PD inrush and protection circuits (necessary for operation and reliability). Creating simple and seamless solutions is difficult, if not impossible, for many of the combinations. However, the TPS23751 and TPS23752 offer several built-in features that simplify some combinations.

Several examples demonstrate the limitations inherent in ORing solutions. Diode ORing a 48 V adapter with PoE (option 1) presents the problem that either source may have the higher voltage. A blocking switch would be required to assure that one source dominates. A second example combines a 12 V adapter with PoE using option 2. The converter draws approximately four times the current at 12 V from the adapter than it does from PoE at 48 V. Transition from adapter power to PoE may demand more current than can be supplied by the PSE. The converter must be turned off while the C_IN capacitance charges, with a subsequent converter restart at the higher voltage and lower input current. A third example involves the loss of the MPS when running from the adapter, causing the PSE to remove power from the PD. If ac power is then lost, the PD stops operating until the PSE detects and powers the PD.