• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 62 subscribers
  • Views 347 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS23753A: Limitation of using a non-isolated DC-adapter-supply

Martin Neff
Prodigy 10 points
Part Number: TPS23753A


According datasheet the IEEE standard for ethernet requires adapter supplies to be isolated from ground. The adapter needs to withstand 1500VAC for ORing option 2.

In case we want to use the ethernet connection internally between 2 devices supplied from the same supply, is there still a need to have this supply isolated from ground? Are there limitations concerning cable length? I guess we have to take care to not span a big surface between DC supply and Ethernet cable.

Is such a setup common for industrial applications or a no-go? Thanks for sharing your experience.

Regards

Martin

  • 0
    TI__Mastermind 21320 points

    Hey Martin, 

    Can you confirm my understanding, so you want one PSE that powers two TPS23753A's that are in the same design -- is this two RJ45 ports or one? Maybe a  block diagram would help?

    For the adapter isolation, I believe that 1500VAC requirement is for the actual adapter. Looking at our EVM's and reference designs, we simply have a connector, some EMI filtering and a blocking diode for Option 2 designs. So from my point of view,  you could buy an adapter that meets the 1500VAC requirement and plug it in, or if you are designing it, that is somewhat beyond the PoE application. 

    It is very common to use adapters in industrial applications, we often see 48V adapters. 

    For the cable length, the ethernet cable must be 100meters to be compliant, because we base the detection signature and voltage drop off the impedance of a 100M cable. I am not sure if there is a specified length for adapters. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 10 points

    Hi Michael

    Find below a block diagram of the setup:

    The orange board does not provide PSE capability. Instead of placing an injector in between, we want to take the 24V supply.

    Are there limitations concerning cable length? I guess we have to take care to not span a big surface between DC supply and Ethernet cable. Otherwise voltage differences can be induced, which result in communication problems.

    Is such a setup common for industrial applications or a no-go? Thanks for sharing your experience.

    Regards

    Martin

  • 0 in reply to Martin Neff
    TI__Mastermind 21320 points

    Hey Martin,

    So is there PoE at all in this design? If the Board (without PSE) does not provide PoE capability, where is the PoE option coming from?

    Are you sending 24V across the ethernet cable?

    In general, for PoE, the IEEE802.3. standard is 100m. If you're going non-standard, then what is important to consider is the voltage drop across the cable. As the cable length increases, the impedance will also increase. Therefore, the voltage drop across the cable can become more significant. This is part of the reason why the standard has a specific cable length, so that the  output voltage of the PSE can translate to a specific known range for the input voltage of the PD. 

    The other thing that follows is the increased power loss in the cable. A PoE PSE has a power limit so for a TPS23753A design, it will limit that power to 15W at the PSE port. I am not sure if your power source will do the same, but understand that there will be more power loss in the cable as the length increases. 

    Lastly, if you want to do 24V adapter input, you will either need a wide-Vin design OR you will need to boost the voltage up to 48V. I have some 13W wide-Vin designs I can share in private messenger. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 10 points

    Hi Michael

    The board with the TPS23753A can either be supplied with PoE or with a separate DC-supply. For the application mentioned 2 cables (1 Ethernet, 1 DC-supply) are intended. The main question from my side is, if we have to expect communication issues, in such a case. Is that setup recommended or not for industrial applications.

    Regards

    Martin

  • +1 in reply to Martin Neff
    TI__Mastermind 21320 points

    Hey Martin, 


    Yes it is recommended to have PoE and an Adapter using Option 2 for industrial applications. I do not specialize in the data transmission, but the power and data should be independent. I have not heard of issues with data corruption if PoE or adapter is being used. 

    I sent a 5V/13W wide-Vin design in private messenger for you to review, since a wide-Vin is required for a 24v adapter input. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments