1
\$\begingroup\$

In the Ethernet PHY, we have two types. Voltage mode PHY and current mode PHY.

Can anyone tell me the reason why the center taps of magnetics are shorted in case of connection to a current mode PHY whereas, the center taps of magnetics are individually decoupled in case of voltage-mode PHY?

What is the principle behind this?

\$\endgroup\$

1 Answer 1

1
\$\begingroup\$

Can anyone tell me the reason why the center taps of magnetics are shorted in case of connection to a current mode PHY

They aren't shorted but connected to Vcc. This is because the PHY driver ports are open-collector (current sources) and the centre-tap is a really great place to supply current to the driver.

If you have PHY drivers that are push-pull they will not need the transformer centre connection to enable them to work but, remember, that centre-connection is still needed to be connected to Vcc or GND to deal with surges.

\$\endgroup\$
4
  • \$\begingroup\$ Thank you very much for the answer. Accepted the answer. A Request: Could you just provide a visual image of current flow for both the PHY driver types from the Center tap Magnetics to the PHY \$\endgroup\$
    – user220456
    Jun 7, 2022 at 16:42
  • \$\begingroup\$ I don't know that I have anything that suits what you need. The manufacturers tend to hide what they have inside their chips but they are easily recognizable. The current feeding PHY driver will have termination resistors from each of the two pins to Vcc. Internally they will likely use NPN transistors that receive their collector current through the transformer via Vcc at the centre-tap. A voltage driving PHY will have a termination resistor in series with the two outputs in many cases. \$\endgroup\$
    – Andy aka
    Jun 7, 2022 at 17:24
  • \$\begingroup\$ Page 3 is a bit helpful. \$\endgroup\$
    – Andy aka
    Jun 7, 2022 at 18:18
  • \$\begingroup\$ d3i71xaburhd42.cloudfront.net/… - try this for a current driven output. \$\endgroup\$
    – Andy aka
    Jun 7, 2022 at 18:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.