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Original question:

DP83826I: RBIAS resistor voltage

DP83826I: Discrete magnetics not working

Alberto PAL
Prodigy 60 points

Part Number: DP83826I
Other Parts Discussed in Thread: DP83826EVM

Hello,

I can't make the DP83826I stablish link with this schematic: (If I remove the choke and the transformer and place 4x 33nF capacitors in-line it works perfectly, so the issue is with the magnetics)

The transformer PN is: Wurth 74930000 and the common mode choke PN is: Wurth 744232222.

I've tried removing the choke, adding more capacitance to the CT, trying different discrete transformers... I've also tried it in multiple boards, so it's not a soldering issue...

Can someone help me seeing what's wrong here?

Thanks in advance,

  • +1
    TI__Expert 5425 points

    Hi Alberto,

    Have you used this magnetics in other ethernet applications before?

    The magnetics seem different to the ones we recommend. It seems that pin 3 & 4 is treated as two separate windings instead of a center tap. If you take a look at the magnetics used in the DP83826EVM, the center tap is one pin and has a symmetrical structure.

    Regards,

    Adrian Kam

  • 0
    Prodigy 60 points

    Hi Adrian,

    I haven't used this magnetics before, but because of size restrictions, I need to use these kind of small transformers.

    I have tried looking at similiar sized components but can't find anything similar to the one used in the DP83826EVM.

    I would greatly appreciate any suggestion!

    Thanks,

    Best regards,

  • 0 in reply to Alberto PAL
    TI__Expert 5425 points

    Hi Alberto,

    Since the magnetics cannot be changed, can you provide some scope shots of the MDI lines (TD_P/M, RD_P/M)? I would like to see how this magnetics affected the signal on the lines.

    Regards,

    Adrian Kam

  • 0 in reply to Adrian Kam
    Prodigy 60 points

    Hello Adrian,

    I finally solved the issue by replacing the transformers. The transformer now has a single center tap pin as you suggested, although the windings seem to go to this pin the same way than the other transformers (with separate pins for center tap).

    Thanks for the support,

    Alberto