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Integrated magnetics compatible with voltage-mode PHY

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eqc1975

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I am having a REALLY hard time finding multi-port RJ45 jacks with integrated magnetics that are compatible with a PHY using voltage-mode drivers. As I understand it, for the magnetics to be compatible the center-taps for each of the four differential pairs on the PHY side would have to be isolated from each other :


recommended_VM_mag.png


In nearly all of the modular jacks I am finding, the center-taps are connected together :


most_mag.png


Am I missing something here? From what I can tell, voltage-mode TX drivers are not uncommon. Am I misunderstanding the magnetics requirements? Thanks.
 

Not sure what you mean with voltage mode drivers? Are you referring to specific chip? The best correspondence to voltage mode that I can imagine is an old 10Bit-T driver, but it uses only one RX and one RX.
 

Hi,

I wonder how the center taps can be connected.
How can this comply with POE standard?
With POE the center taps are used to transfer (DC, up to 48V) power.

Look for POE compatible magnetics.
If I remember right, then Wurth should have these magnetics.

Klaus
 

I wonder how the center taps can be connected.
How can this comply with POE standard?
With POE the center taps are used to transfer (DC, up to 48V) power.
No problem so far, design is according to standard. PoE hub detects the short and doesn't power the port.
 

Thanks for the replies.

I should have been more specific...I am looking to replace the 2x2 RJ45 modular jack that is currently in our product (BelFuse 0879-2C2R-54) with another 2x2 that is compatible with a 10/100/1000Base-T GbE Switch IC that uses voltage-mode transmit drivers (Microchip KSZ9896C). Single port parts are easy to find, ganged multi-port parts get tougher and stacked multi-port parts are even tougher. I have tried looking for POE compatible magnetics with no luck.
 

Although I believe that the term "voltage mode drive" is missing the operation principle of an Gbit ethernet driver, I see that Microchip specifies transformers with separate center tap as a requirement.

I've shown a suitable 4x1 multi port jack in my previous post, not sure if 2x2 is provided by any manufacturer.
Good luck!
 

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