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TPS23731: PoE Class 4 without isolation transformer

Jim G
Intellectual 695 points
Part Number: TPS23731
Other Parts Discussed in Thread: TPS2378, LM5116

Tool/software:

I'm developing a passthru power management PCBA based on TPS23731 which will be a Class 4 PoE PD device. 

The downstream system is a single board computer which already has ethernet isolation magnetics and the PHY. 

I'm essentially implementing the reference design of the TPS23731 eval board, but want to omit the isolation magnetics as they are already on the nearby application PCBA and I'm concerned about redundant isolation magnetics causing communication issues.

How can I interface with the upstream PSE using TPS23731 and still adhere to classification etc requirements without isolation magnetics on this power management PCBA?

  • 0
    TI__Mastermind 37121 points

    Hi Jim,

    Thanks for reaching out!

    Standard PoE will require isolation as below pictures. It may not be fully isolated if you have a local load or MCU and using non-isolated DC/DC.

    If you still consider non-isolated solution, it may be easier to use TPS2378 + LM5116 to build a Buck converter. We have a reference design (https://www.ti.com/tool/PMP11257) for non-isolated case. 

    Let me know if you have other concerns or questions.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Intellectual 695 points

    Hi Diang - thanks for your quick reply.

    My total system will have full isolation as the RJ45 connectors on the downstream single board computer have integrated magnetics and terminations, and the TPS23731 flyback provides power isolation for it.

    See the diagram below where I have drawn a pink line around the area which is isolated:

    You can see the system takes in two external ethernet connections.

    One of them at the top is entirely untouched, just a passthru, and the magnetics on the application PCBA provide isolation there.

    The second ethernet connection is PoE+Data. I suspect I will still need the isolation magnetics on the power PCBA to effectively extract Mode A PoE power. I'm a bit concerned as this will effectively place two ethernet isolation magnetics in the data path in a very small space. I'm not sure what impact this may have, but the application will be less than gigabit ethernet so it may not be an issue.

    At the bottom, the TPS23731 drives a flyback which provides power isolation to the downstream application PCBA.

    What I'm unsure about:

    - Is there any recognized approach to extract Mode A PoE and maintain classification handshaking with TPS23731 (or other TI components) which does not require an isolation transformer? Since I already have application level isolation elsewhere.

    - What terminations are recommended on the power PCBA for the passthru data pair, none? Since it is all handled on the magnetics nearby?

  • 0 in reply to Jim G
    TI__Mastermind 37121 points

    Hi Jim,

    Thanks for more details of your question.

    Mode A or Mode B is decided by PSE. Safe way is having both rectifiers on PR12-36, and PR45-78 like below example. Your PD can receive the PoE power from either pairs. And you only need magnetics for the pairs need data isolation.   

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Intellectual 695 points

    Hi Diang - the magnetics for data isolation are already present on a 2nd different and separate PCBA, so I do not have the opportunity to connect to the center tapped transformer connections for PoE there. 

    I am apprehensive to add an additional "upstream" isolation transformer to extract the center tapped PoE connections, as I'm worried about impacting the signal path. 

    To be more specific, I'm trying to eliminate the blue block in my diagram labeled "Isolation Magnetics + Terminations" since the power path will already have isolation via the flyback converter, and the data path will be isolated by the RJ45 integrated magnetics on the 2nd PCBA.

    Is the blue block unavoidable to extract the PoE power without impacting data signals? Isn't there a concern of redundant isolation transformers in the data path so close?

  • +1 in reply to Jim G
    TI__Mastermind 37121 points

    Hi Jim, 

    "Isolation Magnetics + Terminations" is necessary in PoE circuit which can split the PoE power in DC voltage and the data in signal pulse.

    For PoE power aspect, 4 x center taps are needed for PR12, 36, 45, 78. 2 x rectifiers are needed for PR12-PR36, PR45-PR78.   

    For data aspect, magnetic is needed. PoE does not necessarily need the magnetic strictly if without data. 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Intellectual 695 points

    Thank you Diang!

  • 0 in reply to Jim G
    TI__Mastermind 37121 points

    No problem! I will close this thread for now. Please reply or open a new thread if you have further questions.