Implementing Auxiliary Power in PoE

PublishTime：2021-11-17

●Many PoE applications employ auxiliary power sources, typically an AC-powered “wall wart” or a solar panel,connected to the Powered Device (PD). Integrating auxiliary power can be a challenging design task and the PoE designer must understand the various methods, inherent tradeoffs, and pitfalls that exist with each method of implementation. The main issues the designer may need to address are power source priority, smooth transitions between different power sources, and inrush current limiting.
●Depending on the connection point, there are three basic configurations that are commonly used to add auxiliary power to PoE systems:
■Option A: Connection at the front-end of PD (before the pass-FET). It is often called the “Front Aux” technique or “FAUX” option. This is a rarely used option as there is no adapter priority and thus there is wasted power dissipation on the PD chip.
■Option B: This is the most commonly used option. Connection at the input of the switching converter of PD(behind the front-end). It is often called the “Rear Aux” technique or “RAUX” option. There is a sub-option that includes an additional high side or low side OR-ing diode. It offers the ability to introduce adapter priority by turning off the pass-FET. There is no wasted power disipation as the current is not flowing through the PD chip.
■Option C: Connection at the output voltage of the DC-DC converter of PD. It is called “direct OR-ed” or “output OR-ed”. This option has limited use. It requires the adapter voltage to match output and can not be used with sync rectifiers.

Brand of Product：	Microchip
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Product Category：	More
Application：	More
Data Type：	Application note & Design Guide More
Safety & Environment Standards：	More
Operating Temperature：	Please see the document for details
Qualification Grade：	More
Qualification Approval：	More
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Language：	English Chinese Chinese and English Japanese
Date of Creation：	April 2020
Version Number：	Revision A
Data Encoding：	AN3472;DS00003472A
Data Size：	223 KB

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