Ethernet Magnetics / Transformer - Page 1
Forum section: electronics / projects

author: adrifromhh
author_rank: Contributor
timestamp: August 23, 2018, 04:57:10 pm
content: Hello!I am designing a project that's supposed to have Ethernet capabiltiy. I successfully hooked an ENC28J60 china testboard to an AVR and got it working.Now I want to incorporate the ENC28J60 into my own schematic. I put the recommended external circuitry from the datasheet around it.Is this enough?Now it comes to ethernet transformers, which is a completely new subject to me.I would like to know what to look for when using an external transformer vs integrated RJ45 connector with magnetics. Im not sure what will fit my needs better right now.In which ways do different transformers differ? Are they all basically the same?When using an external transformer, do I have to care much about cable length between transformer and RJ45 connector? Can I run a 20cm flat-flex cable between them?Best regards,Adrian

author: adrifromhh
author_rank: Contributor
timestamp: August 23, 2018, 05:02:44 pm
content: My circuit so far:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 24, 2018, 12:04:18 am
content: Trace length between PHY and transformer should be minimal, specifically, an electrical length much less than a bit time.Choose a transformer suitable for your application. If that's a 10/100 PHY, then get a 10/100 transformer. Only needs to be dual, because 10/100 only use two pairs. Connectors with integrated magnetics are great for this. They also include the ESD capacitor and common mode termination resistors.Or if you're doing PoE, get a transformer that supports that, and add the supply/converter as needed. Or so on.Follow the circuit recommended by the manufacturer. Almost always, the PHY outputs are open drain, terminated with 50 ohm resistors, and the transformer CT is tied to +3.3V.Tim

author: bson
author_rank: Unknown
timestamp: August 24, 2018, 02:00:23 am
content: Quote from: adrifromhh on August 23, 2018, 04:57:10 pmThe PHY should always be close to the magnetics, and the only reason really to use external magnetics is if you need to physically separate the PHY from the jack. Otherwise use an integrated magjack and place the PHY close to that; it's simpler, cheaper, smaller, and better. Anything outside the magnetics, including traces, cabling, and any jack is part of the link, so is highly resilient to cabling, jacks, etc. You could easily have a magjack on the PCB, then run a short section of cat5 soldered to a jack that's panel mounted, no worries. 100M-TX isn't going to flinch. In fact, this is commonly done for wall plates.

author: adrifromhh
author_rank: Contributor
timestamp: August 28, 2018, 10:53:20 am
content: Okay, thank you both for your advice!I think I'll use a magjack in this project, but just to make it clear:When the magnetics are close to the PHY, is it ok to run a ~20cm ribbon-cable from the transformer to an RJ45 jack on a front/back panel? (small PCB maybe with jack and shrouded header)If it makes a difference for 10/100/1000 Ethernet, please consider that, as I'm currently only using a 10-BaseT PHY, but will probably upgrade in future projects.Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2018, 11:06:34 am
content: Stuff on the cable side of the transformer is very robust, that's just fine.Tip: use adjacent pairs in the ribbon, ideally spaced by one not-connected line (so, minimum 5 wire ribbon?). This will give a better impedance match, not that it really matters for 10BASE-T whichliterally runs on wet string!Tim

author: adrifromhh
author_rank: Contributor
timestamp: August 28, 2018, 11:15:12 am
content: Okay, thanks for the clarification

author: adrifromhh
author_rank: Contributor
timestamp: December 04, 2018, 10:02:19 am
content: I think it's okay if I dig out this thread of mine..The Project has moved on and I'm actually going to use this PHY now: Microchip KSZ8851-16 (10/100). You can see it's requirements for the transformer below.The problem right now is that I don't seem to find a Magjack that matches all it's requirements. In particular the interwinding capacitance. Most Magjack datasheets don't specify this and the only one I found had 30pf, which is greater than the needed 12pf.I don't have the experience to say if this matters at all.I am searching on Mouser, as I'm probably going to order there. Is basically any 10/100 (symmetrical RX-TX) Magjack fine for the job?https://www.mouser.de/datasheet/2/336/-369765.pdfhttps://www.mouser.de/datasheet/2/643/2250015-1128483.pdfhttps://www.mouser.de/datasheet/2/643/2250056-1368462.pdfhttps://www.mouser.de/datasheet/2/643/dr-MAG-08B0-1D1T-06-F-1309948.pdfToo high interwinding capacitance?https://www.mouser.de/datasheet/2/643/dr-MAG-SI-60062-F-1310538.pdf