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TPS23756: TPS23756 technical questions

Part Number: TPS23756

Hi Team,

Customer has three questions about TPS23756 due to there are some damage in the 2nd side MOSFET.

Would you please help for the questions?

1. how to know the TPS23756 is in POE or POE+? 

2. any TPS23756 pin can inform the POE supply output power?

3. what is the max output current from the transformer that set by TPS23756? how to set the output current?

BR,

Scott

  • Hey Scott,

    1. First, TI designs to the IEEE802.3 standard, not the POE, POE+, POE++ standard. The TPS23756 can identify if the PSE sends 25W of power by detecting if there was a second class finger. I believe the POE+ standard follows this, so the IC should also distinguish between the two. The T2P pin indicates this behavior. 

    2. yes the T2P pin, and we send that signal over the isolation barrier with an optocoupler.

    3. The TPS23756 is rated for 25W at the input. Due to efficiency loss through the DCDC, the output is around 90% of this. Most designs in PoE are flybacks and ACFs. These limit the output current by monitoring the power through the primary side of the transformer. This current goes through the primary MOSFET and sense resistor (usually <1ohm). Changing this resistor affects the output current limit (assuming DCDC can support it). 

    For more debugging guidelines, please review the PoE PD Debug Guidelines user's guide, focusing on page 15: 

    https://www.ti.com/lit/an/slvaf74/slvaf74.pdf?ts=1627485934030&ref_url=https%253A%252F%252Fwww.google.com%252F 

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

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments