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Voltage drop when using tps2378 for poe interface, on increasing the load.

Other Parts Discussed in Thread: TPS2378, TPS2378EVM-602, TPS2378EVM-105

I am using TPS2378 for type 2 poe interface in my design. But when I am putting load of approx. 8W across the output, the voltage is dropping from 54V to around 30V and my other circuit stops working.

I have used the same schematic as given in the TPS2378 evm board (SLVU682).

Also there are two similar circuit in the same board for redundancy and when both are POE's are plugged in, the voltage drops further to about 10V in both the section.

Kindly suggest what could the problem be?

  • Hi Abhijit,

    I'm moving your question to the appropriate forum for better support.

    Thanks,
    Siby
  • Hi Abhijit,

    What is your load and input source?
    Because TPS2378 is doing handshake with our PSE. If hanshake is successful, PSE will deliver power to your laod.

    Ben
  • Hello Ben,

              Input source is a UPOE Cisco switch while total load will be around 20W max.

    Regards

    Abhijit

  • Hi Abhijit,

    Because UPoE not a IEEE standard, we don't know are there any risks in your application.
    But base on your description, we guess the negotiation is successful. That is mean your TPS2378 works normal.

    Can you help me to capture the waveform as below?
    1. Vout, Iout wavefrom when the issue happened.
    2.Input current, VDD to RTN, VSS to RTN when start-up with and without load.
    Also if you can attached your PSE and Load detail information, that will be very useful.
    Regards,

    Ben
  • Hi Ben,

                     On looking further we found that the problem was of inrush current, and since our load was of approx. 9W, the output current was stuck at 170mA and RTN was 12V wrt to VSS.

    So we wiil resolve the above problem by managing our load at startup.

     But we are now facing another problem.

    We have two similar circuits for POE for redundancy purpose. When we power up the system using one POE port then it is working fine, but as soon as we insert the another port, output voltage drops and the ciruit starts loading.

    Sometimes it drops to voltage as low as 12V.

    Kindly comment.


    Regards

    Abhijit

  • Hi Ben
    Also the cisco switch we are using supports poe+ which is IEEE 802.3at

    Regards
    Abhijit
  • Hi Abhijit,


    Can you attach the issue waveform for us?
    When you insert anther port is with or without load?
    Please tell to us more test condition.

    Thanks!
    Ben
  • Hi Ben

                     I could not attach the waveform as I am out of office for the remaining week.

         Also we were inserting the second port with the load of around 4W.

       We noticed one more thing when we were using ports of two different switches then it is working fine.

    Regards

  • Hi Abhijit,

    Each switch in PSE is for two pair(VDD and VSS) and connected to our PD.
    If you use the same switch to two PD. The last PD you insert can't do the detection and classification.
    It means the negotiation will be fail.

    -Ben
  • Hi Ben,
    We are still getting the voltage drop problems, many times, even when we are using POE ports of two different switches.
    This happens even when the load is as low as 4W.
    Should I share the schematic with you for reference?
    Also I will attach the waveforms tomorrow. Could you tell me which signals you want the waveforms for?

    Regards
    Abhijit
  • Hi Abhijit,

    Yes, please share your schematic with me.

    Also catch the start-up wavefrom as below.

    1. VDD-RTN, VSS to RTN, inrush current.

    2. Input current and VDD-RTN when this issue happen.

    Thanks!

    -Ben

  • Hi Ben,

                   Please find the attached schematic, for the two TPS2378, along with DC-DC convertor, working in redundancy.

    I will send you the waveforms by the end of the day.

    Regards

    AbhijitTPS2378_SD.pdf

  • Hi Ben,

                      We tried one more thing today, that we isolated the return path for the two ports (rtn_poe), the response was much better but still some glitch occurs when the second port is inserted.

        The voltage falls momentarily as a result of which the load gets reset and and the system is rebooted. And we want to avoid this situation as we want the switchover between the two to be seamless.


        Kindly suggest on how to overcome this problem. Also do we really need to isolate the return path for the two.


    Regards

    Abhijit

  • Hi Ahijit,

    I don't think there are any risk in your schematic.
    Have a waveform of PSE behavior will be very useful for us.
    BTW,Do the voltage droop issue happen in both of your TPS2378? Or just one happened.


    Also we have 51W sulotion which use two TPS2378 - SLVUAG7, hope this document can help you.

    -Ben
  • Hi Ben,

                  I will send the waveform today.

     The voltage drop issue happens in both TPS2378.

    Also we do not require so much power and are using two TPS2378  for redundancy purpose only, so that if one fails the other one takes the load.

    One more thing, when we are shorting the switched rtn of both the modules voltage drops to below 30V and does not recover, but when both are isolated then then the glitch is for few milliseconds and then it recovers.

    Regards

    Abhijit

  • Hi Ben,

                 PFA the attached waveforms.

    Waveforms VDD-RTN and VDD-VSS are taken when only system is powered through one port and the second port is inserted.

    Pink coloured signals is for VDD while Green is for RTN and VSS in respective waveforms.

    Waveform VDD-RTN1 are taken when both the ports are inserted and the master(TPS device which is supplying current to the load) is removed.

    Again VDD is in pink while RTN is in Green.

    Kindly go through the waveforms and comment.


    Regards

    Abhijit

  • Hi Abhijit,

    Have you tried putting ORing FETs on the RTN paths of each PD? Like shown on TPS2378EVM-602? This helps the PD's individually perform as if the other is not there. Or you can replace the FETs with diodes like shown in Figure 1 of SLVA625A.
     
    Is the redundancy to have the other port supply power to the load without smooth transition from one to the other?

  • Hi Darwin,

                                We have taken the schematic from TPS2378EVM-105 board (SLVU682), in which RTN path is through FET's.

    Attached is our schematic, in which we are using two TPS devices for redundancy purpose.

    We will be connecting the system to two switches, and if one fails the switch over to other should be smooth and seamless. But we are facing problem, as shown in the waveforms attached earlier, during insertion and removal of the second LAN port. As a result of which the output voltage drops and then recovers and our whole system is reset.

    In our present design switched RTN of both the devices are finally shorted. Should we avoid this situation?

    Kindly go through our schematic and suggest if any changes are required to acheive the above mentioned requirement.


    Regards

    Abhijit

    6685.TPS2378_SD.pdf