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General question about PoE PSE and PD negative rail

Shibuya
Intellectual 2985 points
Other Parts Discussed in Thread: TPS2375, TPS2384, TPS23754, TPS23753, TPS23750, TPS23841, TPS2376-H

Team

Could you tell me reason why protection circuit of both PSE and PD is inserted in negative rail in a general way?  

Best regards,

Tetsuro

  • 0
    TI__Expert 7990 points

    Generally speaking, only one side of the feed is switched (for cost/simplicity), the other passes through from the power supply.  

    Uniformly switching one side at the PSE was done to avoid combinations of PSEs and faults (e.g. shorts on the unswitched leads to earth) from causing "96 V" between wires at places like patch panels where feeds from multiple PSEs comingle.

  • 0 in reply to Martin Patoka
    TI__Intellectual 2985 points

    Martin

     

    Thank you very much for your prompt reply.

    Could you show more tangible example for the combinations of PSEs and faults?   I am not familiar with network topology,

     

    Best regards,

    Tetsuro Shibuya

    TIJ Kyoto FAE

  • 0 in reply to Shibuya
    TI__Expert 7990 points

    There are numberous sources of information about PoE. 

    1)  The PoE standard is available for free download from the IEEE (802.3at the updated clause 33, and the whole IEEE802.3-2008).

    2)  There have been numberous articles published on the web over the last 8 years.  Since you are interested in network topologies, look for general ethernet articles as well.

    3)  Our various datasheets (PSE and PD) have concise sections discussing the technology. (TPS2375, TPS2376-H, TPS23750, TPS23753, TPS23754, TPS2384, TPS23841, TPS23851)

    4)  A number of applications notes on the TI website for PD and PSE.

    As a TIer, you have access to internal materials from various training sessions.  There is one for the TPS23754 plus lots of other materials in ESP.