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POE Output Load detection Problem

Rajesh Thesiya
Prodigy 90 points
Other Parts Discussed in Thread: TPS23753A

Hi All,

We have selected TPS23753A IC for 12V/1A POE module Design.

We have one third party POE module in which the similar IC used and below are our observation.
During the power ON, Device slowly turn on the internal hot-swap MOSFET and  could not provide load for around 10 Sec.
After this time period, it will provide full load of 13W.
We could not found any detail about in datasheet about this issue.
Please advise, what could be the issue?
  • TI__Expert 6715 points
    Hi Rajesh,

    Could you sent me the schematic and the wavefrom of this issue?
    Do you mean the IC can work normally after 10 sec?

    Regrads,
    Ben
  • in reply to Ben Kuo
    Intellectual 265 points

    Hi BEN,

    We do not have schematic as we have observed this issue in third party module..

    The exact issue is, That POE module get started immediate once input voltage connected but output voltage get reduce toward zero on load condition.

    After around 10S of time it will provide full load at ratted voltage.

    Another thing is,

    Is there any minimum load current requirement to keep signature? We are using adapter oring in secondary side(Output side) with POE output voltage and giving preference to adapter power. For that we have generated 11V from POE circuit and adapter voltage is 12VDC. If some one remove the adapter then the system should not restart. It should continue run from POE power.

  • in reply to kalpesh diyora
    Prodigy 90 points

    Hi Ben,

    Can you please update on below query?

    Thank You

  • in reply to Rajesh Thesiya
    TI__Mastermind 40100 points
    Hi Rajesh,

    Is it possible to send waveforms of VDD-VSS with input current, RTN-VSS, VC-RTN? Also can you show waveform what the output voltage looks like? As Ben said, the PD does not have a 10s timer. My guess is that either there is large cap at the input of the converter, or the VC cap is large as well.

    Yes. there is a minimum requirement of 10mA to keep the PSE power connected to the PD. To keep the PSE ON, I recommend looking at the TPS23753A technical documents section. There is an app note that describes different ORing designs and how to keep the PSE on when you have an adapter powering your system.

    Regards,
    Darwin
  • in reply to Darwin Fernandez
    Prodigy 90 points

    Dear Darwin,

    Thanks for the reply.

    Unfortunately that third party board has damaged during test. We have our own design board by mid of may. If found similar issue in that one then we will share.

    Another is,.. That application gives description about primary side oring. We need to do oring on secondary side with 12V adapter. During adapter present, device should take power from adapter and if adapter get fail then device should smoothly get take over on POE circuit without reset. So, for that 10mA blider resistor is sufficient to keep signature?

    Regards, 

    Kalpesh

  • in reply to Darwin Fernandez
    Prodigy 90 points

    Dear Darwin,

    Thanks for the reply.

    Unfortunately that third party board has damaged during test. We have our own design board by mid of may. If found similar issue in that one then we will share.

    Another is,.. That application gives description about primary side oring. We need to do oring on secondary side with 12V adapter. During adapter present, device should take power from adapter and if adapter get fail then device should smoothly get take over on POE circuit without reset. So, for that 10mA blider resistor is sufficient to keep signature?

  • in reply to Rajesh Thesiya
    TI__Mastermind 40100 points
    HI Rajesh,

    Yes, this is option 3 in the advanced oring app note; there is also a circuit on page 14 of the app note that shows how to implement the circuit to keep the PSE connected (MPS) while an adapter is providing power to the load.
  • in reply to Darwin Fernandez
    Prodigy 90 points

    Hi,

    Thank You.

    There is confusion of 10mA minimum current. We have considered 10mA current on POE Output(12V) but it shows for input.

    In our application, We need to make option3 with adapter priority and clause 3.6 for "Keeping the PSE Powered With Options 2 and 3".

    For 10mA minimum load it would generate around 0.6W power continuously(Consider 42 to 58V input). Is there any other option to hold Signature without dissipation this power?

    If we consider case of low output voltage with only POE(No adapter) like 3.3V with low power requirement design. For 10mA minimum primary current the input power would be 0.010A x 58V = 0.580W. With considering the 75% efficiency the output min load would be 0.580 * 0.75% = 0.435W and minimum output load would be 0.435W / 3.3V = 130mA.
     So, If in some condition like sleep mode this load current may be less then 130mA. Will Device get reboot during this scenario?? Blider resistor to keep minimum load is the only solution to prevent re-booting??

  • in reply to Rajesh Thesiya
    TI__Mastermind 40100 points
    Hi Rajesh,

    The PSE must see the 10mA to keep alive. This means 10mA in the high voltage (48V) rail. If lower (avg) power is needed. The IEEE standard does allow for a pulsed current (60ms ON and 300 OFF) to maintain power. I recommend looking at Table 33-11 in the IEEE802.3at standard for more detailed description.