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TPS23730: About POE PD application Question TPS23730

KUO WENYUNG
Intellectual 285 points
Part Number: TPS23730


Tool/software:

Dear Tier,

We are looking for a suitable PoE PD solution. Could you help us with some application-related questions?

1. Why is there a distinction between high-side and low-side Hotswap in PD products? What are the advantages and disadvantages?

2. Regarding the PoE handshake protocol issue, if both the external monitor adapter connected to the PD device and the PSE power adapter are grounded together Earth Ground, is there a risk of PSE-PD handshake failure? This consideration involves Hotswap switching between VSS and RTN or VDD and VBUS.

3. If the PD side does not have an enclosure to guide surge energy to the Earth Ground for dissipation, and most PD devices become isolated (islanded) without this path to ground, how will surge energy be suppressed?

www.ti.com/.../slua736.pdf

  • 0
    TI__Mastermind 37111 points

    Hi WENYUNG,

    Thanks for reaching out. We will take a look on your question and get back to you in 3-7 business days. Thanks for your patience.

    Best regards,

    Diang

  • +1 in reply to Diang Xing
    TI__Mastermind 37111 points

    Hi Wenyung,

    Thanks for your patience. 

    1. Low side hotswap could potentially has a better CM impedance on the low side during detection and classification against the noise. Since the current sensor of PSE is on the low side too, low side hotwsap could be better detection/classification accuracy. Low side hotswap could also have a more robust gate driving capability since it refers the low side ground and no need for voltage pump to drive high side. 

    2. There could be a risk during handshake if both adapters are non-isolated (or not well isolated) and if the isolated flyback is not well designed that has serious coupling.

    3. CM surge current amplitude can be reduced by high CM impedance on chokes and isolation. Y-caps could help to reduce the voltage spike due to CM current. The DM current normally can be filtered by dc link cap and exceeded energy can be discharged by TVS diodes.

    Best regards,

    Diang  

  • 0 in reply to Diang Xing
    Intellectual 285 points

    Dear

    Thank you very much for your explanation. During the detection period, the Hotswap FET is disconnected, and before the detection begins. How does this topology provide common-mode interference immunity? I mean before Hotswap, all circuit are same. 

    BR,

    KUO

  • 0 in reply to KUO WENYUNG
    TI__Mastermind 37111 points

    Hi Kuo,

    Thanks for your reply.

    In detection and classification, the hotswap is off but it is unidirectional on due to FET's body diode. This diode is back to back with rectifier, so when hotswap on low side, low side impedance is higher.  

    Best regards,

    Diang