I am designing an application where existing PoE infrastructure should be used to power a device instead of supplying each device with its own power supply. In this application, no data needs to be transferred over the Ethernet connection, in fact the Powered Device is not Ethernet-capable.
In order to reduce BOM costs (PoE Magjacks cost significantly more than standard RJ45 jacks) and the required PCB area (magnetics are usually rather large, and if they aren't, they are expensive again) my idea is to use resistors to create a virtual center tap without transformer-coupling.
Since I am using an isolated flyback topology as a DC-DC converter and since the data lines aren't connected to my device, this method would still leave my device galvanically isolated. Also, the PoE standard allows for 20 Ohms of cabling resistance, and this method introduces only 0.5 Ohms, so it should also allow for correct detection of the PoE device and class and allow for a sufficient power level.
I checked DC-Resistance of PoE Magnetics and they usually were in the 0.5 to 1.5 Ohms max resistance range for the 1-2 or 3-6 pairs, so this should be DC-equivalent to using a transformer center tap.
Is this approach a bad idea, and if it is, why is it a bad idea?
Edit: My reasoning behind the resistors was to limit intra-pair short-circuit current. However, by looking at it again they are not reasonable for this purpose, as they won't adequately limit the high-frequency data current. Since I want a near-short on DC and a near-open on high frequencies, a choke would be the correct choice. But I guess I won't need these too, as Ethernet is already very resilient to shorts since the current is limited by the output transformer. So the resistors are probably not needed. (Thanks @Justme)
I would like to extend the original question to two additional questions:
- Is it a problem if there is a short on all data pairs, i.e. are the PSEs that notice and maybe cut off power to the PD?
- Are there other disadvantages of getting rid of the transformer, such as less decoupling and therefore higher EMI emissions or susceptibility on my ethernet line originating from my or to my PD converter?