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Hot!Center-taps of isolation transformers for use in ENC28J60

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wltse601
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2021/01/21 19:39:26 (permalink)
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Center-taps of isolation transformers for use in ENC28J60

Hi everyone,
I intend to use ENC28J60 in my passive PoE power device. I've searched in market for RJ45 connectors with Ethernet isolation transformers and PoE connection. There are several PoE connectors with center-tap of transformer for data reception and center-tap of transformer for data transmission connecting together (like the one in following link).
https://www.we-online.com/catalog/datasheet/7499211002A.pdf
 
In Figure 2-4 on page 7 of ENC28J60 data sheet, the center-tap of transformer for data reception is kept open. In some other documents like one guideline "Transformerless Applications of Microchip's Ethernet Devices" (AN2190) issued by Microchip, there is a figure (Figure 1 in AN2190) showing both center-taps connecting together to earth via 0.1uF capacitors in typical connection with Ethernet PHY layer.  Is it feasible to use RJ45 connectors with both center-taps of isolation transformers connecting together (e.g. the RJ45 connector in the above link) for connection to ENC28J60?
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    blue_led
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    Re: Center-taps of isolation transformers for use in ENC28J60 2021/01/26 02:30:11 (permalink)
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    I think that magnetics it is not suitable for ENC28J60.
    the problem i see it that magnetics have center tap ( enc side, left side ) conected together TCT pin 3.
    for TX to work CT transformer must be connected to 3.3 V. That have side efect on RX side to be driven also to 3.3 v and i bet that ENC don't like injecting DC into pin and drive pin above 3.3V.
    You have concern about cable side of transformer and that is your least problem. 
     
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    wltse601
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    Re: Center-taps of isolation transformers for use in ENC28J60 2021/01/26 21:22:24 (permalink)
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    Thank you for the information.
     
    LAN-Transformer for connection with ENC28J60 has two isolation transformers with their center-taps not connected together, and has termination impedances at cable side.  For passive 10Mbps PoE powered devices, spared pair 4 & 6 and another spared pair 7 & 8 carry DC+ and DC- voltages respectively, and have DC blocking capacitors in their termination.  All these are necessary requirements to LAN-Transformer for ENC28J60 serving as passive PoE powered device.  If there is any additional requirement, please inform me.
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    oliverb
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    Re: Center-taps of isolation transformers for use in ENC28J60 2021/01/27 01:29:28 (permalink)
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    Since it appears that the only real problem is the centre taps being connected together you might be able to use series capacitors on the ends of the receive transformer to mitigate the effect of the centre tap connection. It seems most likely that there is just a DC offset between the transmitter and reciever, and if that is so then you just need to prevent the 3.3V supply flowing through the transformer and into the reciever.
     
    The differing termination on the cable side should not matter, in the non-POE case the resistors connect to "ground" (actually a 1N capacitor between the termination and chassis) and in the POE case there are blocking capacitors to prevent a DC short and allow power to be injected/extracted. Either way the AC impedance is set.
     
     
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    wltse601
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    Re: Center-taps of isolation transformers for use in ENC28J60 2021/01/27 08:26:23 (permalink)
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    Thank you for the suggestion of connecting capacitors to ends of receive transformer.
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