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I've a question with the Ethernet PHY connection to the RJ45 connector.

With either discrete or integrated magnetics, either with voltage mode PHY or current mode PHY, what is the purpose of the center tap of the magnetics?

In some places, with voltage mode PHYs, the centre tap is connected to the ground via a capacitor.

In current mode PHYs, the centre taps are pulled to Vcc via a termination resistor and connected to ground via a capacitor.

I want to understand the current/voltage flow across the magnetics and how the capacitor on the centre tap is helpful for this. What happens if I don't have the capacitor on the centre tap?

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  • \$\begingroup\$ Why wouldn't they? -- what would happen with or without a CT connected as such? You might wish to illustrate the effect to yourself by setting up simulations with equivalent output circuits and nonideal transformers. \$\endgroup\$ May 23, 2023 at 8:59
  • \$\begingroup\$ @TimWilliams, could you help with a simple theoretical answer, please? \$\endgroup\$
    – Freshman
    May 23, 2023 at 9:22
  • \$\begingroup\$ You will have to provide more information as far as what you would consider a convincing answer, or where your particular gap of knowledge lies. The reason is obvious enough to me, but I know better than to project my state of knowledge onto others. The most basic answer I would see, as currently worded, is just running the aforementioned simulations for you. Which doesn't seem like it would be very helpful to you, or very interesting. Setting up simulations and doing basic circuit analysis are core EE skills. \$\endgroup\$ May 23, 2023 at 9:44
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    \$\begingroup\$ The center tap is used to inject or tap off PoE power, no? \$\endgroup\$
    – Hearth
    May 23, 2023 at 19:23

2 Answers 2

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Current mode (CM) PHYs' drivers are basically open collectors (current sources) so the centre tap is tied to Vcc. Without Vcc tied to centre tap it won't be possible to induce a voltage at secondary.

Voltage mode (VM) PHYs, on the other hand, have true differential drivers. It's not a crucial requirement to place a capacitor to the centre tap because the transformer is driven with differential drivers. So technically, it's possible to induce voltage at secondary even if the primary is single ended (i.e. not centre-tapped) or in other words, centre tap can be left unconnected.

schematic

simulate this circuit – Schematic created using CircuitLab

NOTE: Ignore the fact that I used op amp symbol and its supply terminals to represent differential output driver.

The capacitors tied to centre taps (for both type drivers) are in fact not critically necessary. But it's a good practice to have them for EMI purposes. The one to the left serves as a decoupling cap, for example.

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  • \$\begingroup\$ Thank you for the answer. Just a couple of clarifications. 1. So, in the current mode PHY, the current from from Vcc -> into centre tap -> out from transformer -> into PHY pins, correct? 2. In the current mode PHY, if the termination resistor is not a single 100ohm resistor but 2 number of 50 ohm resistor connected to each of the lines, can you tell how the current flows in that case? \$\endgroup\$
    – Freshman
    May 23, 2023 at 10:06
  • \$\begingroup\$ Also, the capacitor for the voltage mode PHY, is only for EMI purpose and not anything else, right? \$\endgroup\$
    – Freshman
    May 23, 2023 at 10:07
  • \$\begingroup\$ @Freshman 1) Correct. The current flows from Vcc to the driver pin (whichever is active) through the respective primary winding. 2) but 2 number of 50 ohm resistor connected to each of the lines Clarify this. Do you mean across each line and ground? In that case the drivers will be overloaded. So please clarify. 3) (purpose of VM cap) Basically, yes. \$\endgroup\$ May 23, 2023 at 10:13
  • \$\begingroup\$ Thank you for your comment. 2) I mean, instead of a single 100ohm resistor between the lines, we have one 50 resistor between Vcc and one differential line and another 50 ohm resistor between Vcc and other differential line. \$\endgroup\$
    – Freshman
    May 23, 2023 at 10:22
  • \$\begingroup\$ Could you provide a pictoral respresentation for current flow for easy understanding? \$\endgroup\$
    – Freshman
    May 23, 2023 at 11:41
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For current mode PHYs, the center tap is the only connection for the current to enter the coil. The PHY connections are the only ones from where the current exits the coil. So current mode PHYs can't be used if the transformer has no center tap.

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  • \$\begingroup\$ Thank you for the answer. So, in the current mode PHYs, the current will enter into the magnetics centre tap from the power supply and sink into the PHY pins, correct? So, that's the only purpose of centre tap in current mode PHYs? Any idea on what might be use of centre taps in voltage mode PHYs? \$\endgroup\$
    – Freshman
    May 23, 2023 at 9:56
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    \$\begingroup\$ The transformers have center taps. You can use same transformers with all kinds of PHYs. That's why there is no need to make separate transformers without center taps. You don't use them if you don't need them. \$\endgroup\$
    – Justme
    May 23, 2023 at 10:06

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