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TPS2372: TPS2372 power on issue.

user4206464
Intellectual 460 points
Part Number: TPS2372
Other Parts Discussed in Thread: PMP20859

Hi

The attached is our customer's design for TPS2372. 

A. We found that some power board will have leakage problem. When plug in POE1, 4.5V voltage can be measured between POE2 ct1 &ct2 Or st1 & st2.

    This problem will cause POE2 detection fail. The fail rate is about 40%.

B. Our customer also find out PQ27 will burn randomly.

Could you help to check this design is ok or not, or do you need any waveform?

And could you also explain PC170 function?

 

2555.TPS2372.pdf

  • 0
    TI__Mastermind 41760 points
    Hi Teresa,

    A. Can you check to see if the "UPoE_PD2 detect circuit" might be causing this leakage (since it is referenced to VSS) by removing PD6 and PR90? This is so can narrow down the issue to a specific circuit.

    B. The PC170 circuit helps with smooth transition (when the voltage changes between the PD inputs). I recommend looking at the training video below that goes into this in detail. Note the PC170 FET must be implemented for smooth transition to work.
    training.ti.com/designing-dual-input-poe-pd-systems-power-redundancy-smooth-transition-and-power-sharing

    For the FET damage, I noticed the gate charge is 2x the FET we used in the TI design (PMP20859). Can they try with the TI FETs?
  • 0 in reply to Darwin Fernandez
    Intellectual 460 points
    Hi Darwin,

    Do you have any concern for this design use full Mosfet bridge?
    Does it cause leakage?

    By the way, when plug in POE1, POE2 Mosfet(PQ30) will turn on.
    Is it normal?
  • 0 in reply to user4206464
    TI__Mastermind 41760 points
    Hi Teresa,

    It should not have very small leakage (meaning it should still pass detection and have tested it with PSE).

    Yes this is how it works, when one pair turns ON, the whole bridge will turn ON.
  • 0 in reply to Darwin Fernandez
    Intellectual 460 points

    Hi Darwin

    The attached are the test waveform.

    Do you have any concern or any suggestion to solve POE2 detection fail?

    CH1: PD1 VDD to VSS; CH2: PD1 input current; CH3: PD2 VDD to VSS; CH4: PD2 input current

    A.     PD1 start up <Handshaking>

      Attached 2 waveform:

             1. with POE2 PSE connected

    2. without POE2 PSE connected

    B.      PD1 is stable and measure PD2 Vdd is 4.5v?

    1. Using passive probe on Ch3 (Remove Ch1 probe to Ch3)

    2. Using differential probe on Ch3

    C.      PD1 is stable, PD2 start up.

    1. Using PDA300 forced on

    2. Using bt PSE, looks no detection & classification action.

    https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/196/POE-waveform.7z

  • 0 in reply to user4206464
    TI__Mastermind 41760 points
    Hi Teresa,

    For a PoE PD to not turn ON, it must fail detection. So any current that pulls on VSS can cause detection failure. The only circuit that's different from the reference design is the UPOE detect circuit. Can we try ruling this circuit out by removing it to see if it is causing the failure?