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TPS23754: Power gap between PSE output and PD input

Jeff Chou78
Prodigy 190 points
Part Number: TPS23754

Hi, Sir,

According to the std of 802.3at, the PSE needs to output 30W and only 25.4W is guaranteed for PD.

4.6W gap was reserved for the loss consumed by cable. We did calculations of the cable loss with different cables.

Could you please comment on the results of cable loss? 

We are looking for the power margin for different cable used in deployement.

Thanks,

Jeff 

  • 0
    TI__Mastermind 34076 points

    Hi Jeff, 

    100m Cat5 and Cat6 cables can have varying cable loss based on the category and materials used for the cable. The power loss set by IEEE 802.3 standards is based on a 100m Cat5 cable and the maximum power loss allowed, this does not reflect power loss of all types of ethernet cables. See excerpt below from EA Overview of IEEE 802.3 bt standards:

    Let us know if further clarification is needed!

    Thanks and Regards,

    Raymond Lin

  • 0
    Prodigy 190 points

    Hi, Raymond,

    Thank you very much for prompt response.

    We tried to access the risk with different cable type vs. cable length. Do you think what need to be included in our table to have better evaluations? Of course, we understood the gap of power budget defined. 

    Many thanks,

    Jeff

  • 0 in reply to Jeff Chou78
    TI__Mastermind 34076 points

    Hi Jeff, 

    One thing I would think about adding is the power loss between 2-pair and 4-pair power. At Class 4/30W the power can be delivered either over 2-pairs or 4-pairs which can effect the cable loss. Over 4-pair the power/cable loss is half compared to if power is delivered over 2-pairs. Do note this is only applicable for Class 4 and below. For Class 5 and above (if follow specifications) 4-pair is always required.  

    For example, if you use 4-pair Class-4 (sending 30W with 50V from PSE side), the cable loss will be (30W/50V)^2*6.25 = 2.25W loss. Over 2-pair Class-4, the loss will be up to 4.5W. 

    Thanks and Regards,

    Raymond Lin

  • +1 in reply to Raymond Lin
    Prodigy 190 points

    Hi, Raymond,

    Thank you so much, this is much clear to me.

    Jeff 

  • 0 in reply to Jeff Chou78
    TI__Mastermind 34076 points

    Hi Jeff, 

    No problem, if you have any further questions or concerns please open a new thread for better tracking! Closing this thread. 

    Thanks and Regards,

    Raymond Lin