Does power delivery over ethernet always have to use isolated DC/DC converters?

Question

I am examining the possibilities for building a cheap ethernet device that would be powered over ethernet using PoE technology. I came across this article which shows the generic block diagrams:

Looking around, I always find the DC-DC converters used in PoE applications to be isolated. I am trying to understand whether that is always necessary. Assuming that the PoE RJ45 cable would be the only cable connected to my device, is there really any reason for me to isolate from the input voltage? This is also assuming that the data lines are already isolated, so the only potentials without isolation is the power lines.

Edit:

To clarify more: I still want the TX and RX lines to stay isolated via transformers. I just want to skip the DC-DC part isolation (just graetz bridge, bulk capacitor and a DC-DC converter without isolation.) I don't mind being not compliant with the standard here. As for the shielding, I would leave it disconnected.

Answer 1

If you want to call it PoE, you need to stick to IEEE 802.3af-2003 or IEEE 802.3at-2009.

These standards call for isolation. This answer could end here: no, you cannot build a PoE device without isolation.

This is also assuming that the data lines are already isolated, so the only potentials without isolation is the power lines.

You also will have a shield in your ethernet cable. On which end are you going to isolate that? Both? Not much of a sensible shield then.

Also, when doing ethernet, you need the magnetics (==transformers), anyway, so get one with center taps on the cable side – no extra effort, basically every ethernet transformer does, anyway. So, without a DC/DC converter that isolates the regulated voltage from the voltage on the line, you'd tie your device's data logic to a fluctuating, noisy voltage that's not guaranteed to be at any fixed relation to the data signals.

So, really, don't.

Answer 2

I don't have the spec in front of me but I've done a number of PoE designs. As I recall the PoE isolation spec is to meet safety requirements and avoid ground loops. If your device takes the 48 Vdc and uses it within a fully enclosed, non-conductive enclosure, with no other cables, then the 1500 Vac power isolation (typically done with a flyback circuit) isn't needed.

As noted, the RX and TX data pairs will need their own PoE rated pulse transformer since the ethernet transceiver also expects it. Don't forget the PoE power can come in either on the TX / RX pairs or on the other two spare pairs, so to be compliant you'll need two bridges.

Answer 3

Not mentioned elsewhere it is even quite possible that two different devices are powered over longer cables in the same building and then could share grounding through some common signalling cables.

The use of small diameter wires for POE and 48V drive implies that there will be voltage drops across the cables. Such drop is proportional to the current draw of the target device and dissimilar devices would see voltage drop differences that are rather different. The only way that you can mitigate these voltage drop differences is by using isolated DC->DC converters.