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TPS23881: Regarding bob smith circuit

Terry Hu
Expert 3610 points
Part Number: TPS23881

Tool/software:

Hi team,

Could you please advise whether the bob smith circuit is necessary for PoE PSE side? And the DC blocking capacitor 22nF is also needed?

Or only the PoE PD side is needed? And could you please advise why?

Many thanks.

best regards,

Terry

  • +1
    TI__Mastermind 37101 points

    Hi Terry,

    The Bob-Smith (BS) is usually on both PSE and PD sides. It is not mandatory but it is strongly encouraged and most of PoE designs have BS circuits. 

    BS is meant to prevent standing/reflected waves on the data cable in order to reduce CM EMI. If there is an earth/chassis ground then the common BS connection can be connected to earth/chassis through a ~2kV xnF Y-cap.

    On the ethernet side with PoE, each BS branch (center ties of PR12, PR36, PR45, PR78) has a 100-V x0-nF cap adding a 75-Ohm resistor. The cap needs to be >= 100 V considering PoE transient voltage could be >57 V. The caps help to avoid high DC conduction loss on 2x75 Ohm resistors at an up to 57-V DC voltage. The cap value is normally 10 - 47 nF. 

    75-Ohm damping/filtering resistance values are most commonly used in PoE, though some suggest that 40 - 60 ohms is a better value than 75 Ohm in some design cases.

    Best regards,

    Diang