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TPS2378: Fast ramp up and high inrush current

Libor Krejca
Prodigy 180 points
Part Number: TPS2378

Hello

I use TPS2378 on our product, I measured output voltage at TPS2378 (between VDD and RTN pins) and inrush current between PSE (PoE injector) and our device. The ramp up time of the output voltage is very fast, I've measured typically 25 to 30us, see picture below (red line is PoE output voltage, orange is inrush current, ignore blue line). I have a PoE injector with 55V output voltage. There are 4x 2u2 capacitors at the output, so I = 8.8u * 55 / 27u = 18A, I measured about 20A inrush so it corresponds to the capacitors. TPS2378 datasheet says it has "140-mA inrush current limiting", so it is not clear to me why the limiting is not working. I have TPS2378EVM-105 board and I measured same fast ramp and high inrush current on this board too. Do you know why the ramp up and the inrush current is so high? Can I fix it somehow and limit the inrush current?

Thanks a lot

Best regards

Libor

  • 0
    TI__Mastermind 37121 points

    Hi Libor,

    Thanks for reaching out to us!

    Normally both the standard PD and the PSE have the inrush current limitation. And yes, TPS2378 has 140-mA typical current limitation during inrush stage.

    In the PD there is a FET between VSS and RTN to limit the inrush current. I do not think a 20-A current can go through that FET since it will cause the FET saturation (also the PSE side FET cannot go to 20 A).

    - Please check whether the VSS is refereed rather than RTN as the GND.

    - Please check whether the hotswap between VSS and RTN and the PSE's FET are bypassed (like D19's direction) which causes the abnormal high inrush current.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 180 points

    Hi Diang,

    I measured the ramp up again with differential probe, it looks fine with differential probe. It seems the problem is ground connection via oscilloscope, probably PoE injector is not isolated from its grounding point (earth ground pin). So the problem seems to be on system level and not caused by TPS2378. We can close this issue. Thanks

    Best regards

    Libor