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I have a PoE powered design, and it is currently equipped with magnetics not labelled as PoE, HX1188NL. There is also a transformer with a similar topology, HX2019NL.

However, the only difference I can see is the provided isolation.

  • The non PoE one can stand 1650 V for 6 seconds.

  • The PoE one can stand 1500 V for 60 seconds.

Apart from different isolation characteristics, what are the differences between non PoE and PoE transformers ?

shodanex
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2 Answers2

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PoE magnetics are designed to sustain also the DC currents that flow through the magnetics. You should not use magnetics which aren't marketed as PoE magnetics.

For example see coilcraft's

http://www.coilcraft.com/eth1-460.cfm

They're designed to comply with the IEEE PoE standard and are capable of sustaining a certain amount of wattage, in this case 60W but 13-25W magnetics are also common.

Normal magnetics may not be able to handle the DC offset currents during PoE operation.

Using 'normal' magnetics is probably asking for trouble.

Steinar
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Another difference is that PoE transformers must have center taps on some of the network side coils, since that is the power connection into your power supply. There is otherwise no need for center taps on the network side in most cases.

Olin Lathrop
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  • Sometimes you may need access to center taps for setting the common mode voltage on the network side, but not necessary in all case since some Ethernet PHYs have drivers which apply the common mode voltage themselves. – Steinar Oct 02 '13 at 12:40
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    @Steinar: Ethernet is specified with quite high common mode isolation, something around 1000 volts if I remember right. Unless your circuit is prepared to deal with the 1000 volts, it can't touch anything on the network side of the transformer. – Olin Lathrop Oct 02 '13 at 12:44
  • Ah i misunderstood your answer. Thought you were referring to the other side of the transformer, the PHY side. You are correct in that access to the center taps are not neccessary on the network side unless you need to do the terminations yourself. Most RJ45 connector have proper termination (Bob smith termination as an example) built in. But if you have a RJ45 jack without magnetics you may need to place the terminations yourself, which means you require access to the center taps on the network side. – Steinar Oct 02 '13 at 13:31
  • @Steinar: You are still not making sense. If you're not using a transformer, then it's not real ethernet. However, even with a transformer, termination is done on the PHY side, and it doesn't require center taps. Center taps are needed on the network side when using PoE, since you have to access the DC power supplied by the cable. None of this has anything to do with termination. Some PHYs may require center taps on their side, but that is not what we are talking about here. The question was specifically about PoE. – Olin Lathrop Oct 02 '13 at 14:10
  • @OlinLathrop Both transformers have the same topology, so while you are right about the needed center tap for PoE to work at all, you are not really answering to the asked question – shodanex Oct 02 '13 at 15:10
  • @OlinLathrop Regarding termination, I have seen several schematics on ethernet phy datasheets were there is indeed some kind of termination on network side, by connecting center tap to a 75 ohm resistor. So either steinar is right, or a lot of ethernet phy supplier are copy pasting the same old schematics... – shodanex Oct 02 '13 at 15:15
  • @Shod: You seem to think all ethernet transformers have center taps on all windings, which is not the case. They are certainly available without center taps on the network side. Pointing out that these center taps are needed is one answer to your question. – Olin Lathrop Oct 02 '13 at 16:09
  • @Shod: As for termination, that is not done by just connecting a center tap to a 75 Ohm resistor. First, the correct terminating impedance is 100 Ohms. Second, this needs to be accross the pair of wires, not connected to the center tap. Some PHYs specify 50 Ohms between each end and the center tap, which then amounts to 100 Ohms between the ends. – Olin Lathrop Oct 02 '13 at 16:11