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TPS23861: 54V is detected without PD device issue

Tess Chen
Intellectual 2675 points
Part Number: TPS23861

Hi,

Below is my schematic about TPS23861. At the ethernet port Pin 1 (ETH3_MX1+) and Pin 3 (ETH3_MX2+) there will be about 54V voltage without PD device, please refer to the screenshot for the voltage measured. Could you please guide us whether it is normal behavior? And how to do some improvements? Thanks. 

  • 0
    TI__Mastermind 41760 points

    Hi Tess,

    Yes this is a very unusual behavior and unusual behavior. There should not be high voltage on the port unless it has correctly negotiated with a compliant PoE PD. Also the waveform should be an open circuit waveform like shown in the datasheet. 

    The schematic connections look okay to me; so it makes me wonder if the issue is on the PCB (populated wrong part number or wrong values). Also can you check if FET is damaged? Or has a short across the FET? That is one way for there to be high voltage at the port without negotiation. 

    Thanks!

  • 0 in reply to Darwin Fernandez
    TI__Mastermind 41760 points

    Also, can you send voltage waveforms of VDD and VPWR when you first provide power to the PSE? This is to review that the startup requirement is met. Thanks!

  • 0 in reply to Darwin Fernandez
    TI__Intellectual 2675 points

    Hi Fernandez,

    Thanks for your promptly feedback. Please see my update for the issue above:

    - We may have wrong probe from oscilloscope without ground isolation. The wave that we estimate is just as the Figure 20 in datasheet. For open circuit, there is a periodically 20V Vport last about 300ms each cycle.

    - This Vport voltage seems harmful for our invalid PD device. As our PD side device is just like a normal DC-DC which could not just survive 20V with such duration.

    Questions:

    Does this device able to adjust the Vport for open circuit? Or does there anyway to reduce the Vport duration ? I mean could we make this device give lower power out for open circuit?

  • +1 in reply to Tess Chen
    TI__Mastermind 41760 points

    Hi Tess, the open circuit voltage is not adjustable. however i don't expect it to damage non-PoE equipment. When the non-PoE equipment gets plugged into an ethernet port, the impedance should pull the port voltage down. The idea of the poe standard is to only power compliant PD and not damage anything else that gets connected to it.