In Power over Ethernet (PoE) applications, power is delivered from the Power Sourcing Equipment (PSE) to the Powered Device (PD) via the Ethernet cable infrastructure. Due to the limited current capability of the cable wires, IEEE STD 802.3af specifies that the maximum power deliverable to the PD through one channel (two twisted pairs) is 12.95W. However, some PD devices require more power to function. A typical example is the surveillance camera able to perform the Tilt-Pan-Zoom (TPZ) functions. To support such a high power PD, two PoE channels can be used, which may be connected to the same or different PSEs. Some form of power sharing between the two channels is normally required such that neither of one will exceed the specified maximum power draw.
PD power redundancy is another type of applications that requires two power channels. Typical applications include the support of finance hot lines or security utilities. Two PoE channels serve as redundancy to each other, and power sharing is a desirable feature to share the load between the two PoE channels and to reduce the overall conduction losses on the cable wires when both supplies are present.
If the two channels are from separate PSEs, isolation between the two channels will prohibit power sharing on the primary side of the DC-DC converter stage, and it is necessary to adopt a power sharing scheme on the secondary side of the isolation transformers. This article presents two schemes of power sharing on the secondary side between two isolated PoE channels. One is by series connection of the output ports of two modules, the other by paralleling the module outputs. Experimental results obtained with National's LM5071 and LM5072 Evaluation boards are presented.