8.3.3 N+1 Power Supply – Typical Connection

The N+1 power supply configuration shown in Figure 9 is used where multiple power supplies are paralleled for either higher capacity, redundancy or both. If it takes N supplies to power the load, adding an extra, identical unit in parallel permits the load to continue operation in the event that any one of the N supplies fails. The supplies are ORed together, rather than directly connected to the bus, to isolate the converter output from the bus when it is plugged-in or fails short. The TPS2412/13 with an external MOSFET emulates the function of the ORing diode.

It is possible for a malfunctioning converter in an ORed topology to create a bus overvoltage if the loading is less than the converter's capacity (for example, N = 1). The ORed topology shown cannot protect the bus from this condition, even if the ORing MOSFET can be turned off. One common solution is to use two MOSFETs in a back-to-back configuration to provide bidirectional blocking. The TPS2412/13 does not have a provision for forcing the gate off when the overvoltage condition occurs, use of the TPS2410/11 is recommended.

ORed supplies are usually designed to share power by various means, although the desired operation could implement an active and standby concept. Sharing approaches include both passive, or voltage droop, and active methods. Not all of the output ORing devices may be ON depending on the sharing control method, bus loading, distribution resistances, and TPS2412/13 settings.