DC-DC for PoE - Page 1
Forum section: electronics / projects

author: sifu0832fjcm2w3fd
author_rank: Newbie
timestamp: April 21, 2015, 10:35:44 pm
content: Recently I dove into the subject of CCTV IP cameras. As a proper nerd the very first thing I want to do with them is to modify themPoE is very neat and saves hustle especially with the external cameras. I'm putting together an injector on my patch panel but as the power supply I have for this task is Earth Ground Referenced - I have a question: Do I have to have a fully isolated DC-DC converter on the Device Under Power side? I do understand the benefits BUT all computers are Earth Ground Referenced so is there any point in doing that? It is worth mentioning that we are talking here about leads up to 40m in length (proper ones - solid copper). Should I go with fully isolated mains power supply and on top of that do the fully isolated DC-DC ?Once I'm done with these I want to make my Raspberry Pi's PoE too. These are hooked to DLP projector and some other equipment.It is my first project with PoE so I lack the experience in this designs. I highly appreciate your help on the subject.

author: moffy
author_rank: Super Contributor
timestamp: April 21, 2015, 11:38:27 pm
content: I would definitely go with isolated supplies to minimise ground loop problems.

author: ve7xen
author_rank: Super Contributor
timestamp: April 22, 2015, 12:43:34 am
content: Definitely isolated. Everything in Ethernet should be isolated from mains. If your main DC supply isn't isolated (or has a ground connection to something that's earth referenced), you should either use a separate isolated AC-DC supply for the PoE, or an isolating DC-DC.If your equipment is "modern" it will probably be using 802.3af / 802.3at which requires/expects some non-trivial negotiation between the PSE (power sourcing equipment) and PD (powered device). It's not as simple as throwing the 48V up the wire. This might work, but isn't guaranteed.This is a big enough market that purpose-built ICs exist to do the necessary negotiation and sometimes power conversion. For example the LTC4263 might suit you pretty well. It's an easy package to work with (SOIC-14), includes the power switch, requires few external components and runs on a single 48V supply. You could add an off-the-shelf mains to 48V brick to power the isolated side of the Ethernet.If you're not using OTS PoE kit that expects standards-compliant implementation, just throw (isolated) power up. Though be wary of voltage drops, it's easy to burn a lot of your voltage margin in wire with 80m of 26awg.

author: LukeW
author_rank: Frequent Contributor
timestamp: April 24, 2015, 03:41:49 am
content: Are you using "real" 802.3af PoE, or "homebrew" PoE without negotiation?If it's the former, the ethernet transformer (either external or integrated magnetics in the jack) should provide galvanic isolation, and the buck converter doesn't have to do it.

author: ve7xen
author_rank: Super Contributor
timestamp: April 24, 2015, 05:07:20 am
content: Quote from: LukeW on April 24, 2015, 03:41:49 amThe magnetics do not isolate the power.

author: AndyC_772
author_rank: Super Contributor
timestamp: April 24, 2015, 05:18:20 am
content: You do, however, need a PoE compatible Ethernet transformer, because you need the centre tap connections on the cable-facing side, and the transformer needs to be able to accept the dc current without saturating.If your product has no other external connections, then isolation isn't strictly required. An ordinary telephone, for example, is not isolated from the line, and a PoE device is no different.

author: Kjelt
author_rank: Super Contributor
timestamp: April 24, 2015, 05:51:03 am
content: It looks like you just want to place a voltage on a few wires of the cat5 cable, this has nothing to do with PoE that requires negotiating protocol, so stop calling it that.My advice: use PoE injectors or a decent PoE switch. DIY PoE is not easy and even pro switches have failures (burnt my 1Gb/s wires in my $2k HP laptop at work)

author: Wilksey
author_rank: Super Contributor
timestamp: April 24, 2015, 11:20:35 am
content: As far as I understand PoE is used with 100Mbps ethernet, so if you have a 1Gbps capable laptop or desktop (who doesn't these days) and you plug (accidently) a PoE enabled cable into your Gbps port, I assume something bad is going to happen as it uses these extra wires that would normally be used for Gbps as PoE lines.I also wonder how much power you can safely put on a standard MAGJACK or even standard RJ45 connector, some people with DIY PoE Injectors do not realise how much power they are pushing through.

author: Monkeh
author_rank: Super Contributor
timestamp: April 24, 2015, 11:37:24 am
content: Quote from: Wilksey on April 24, 2015, 11:20:35 amPoE works fine with gigabit. Ethernet is magnetically coupled, so you use a centretapped transformer and extract DC from it. You can plug a true PoE cable into any normal ethernet device with no ill effects.

author: Howardlong
author_rank: Super Contributor
timestamp: April 24, 2015, 11:46:39 am
content: I looked at putting 802.3af PoE onto a product of mine a year or so ago. After some investigation regrettably I just couldn't make the numbers stack up financially, it is not a cheap option. For the number of punters who'd use the feature compared to the cost of developing and implementing it, and the affect on additional cost of the finished unit, I just said "No". If someone wants PoE, they will have to use a third party adapter, these have become near-commodity items now, at $15 for an 802.3af splitter. It might be a little more messy than an integrated device, but I would have struggled very hard to have made the functionality for that retail price.It's a shame because fundamentally the idea is sound. It's just so expensive to implement if you keep to the IEEE specs.

author: Kjelt
author_rank: Super Contributor
timestamp: April 24, 2015, 12:41:58 pm
content: Quote from: Monkeh on April 24, 2015, 11:37:24 amSo what blew up my 1Gb transformer in my laptop at work?

author: Monkeh
author_rank: Super Contributor
timestamp: April 24, 2015, 12:43:41 pm
content: Quote from: Kjelt on April 24, 2015, 12:41:58 pmIncorrect wiring, somehow placing DC voltage across a winding..

author: Kjelt
author_rank: Super Contributor
timestamp: April 24, 2015, 12:50:22 pm
content: QuoteWell what happened is that I pulled an operating (so power on) PoE cable out of a PoE powered device and put it in the laptop.The switch should have noticed this and should have renegotiated with the laptop finding out that the laptop did not have PoE and keep the socket powered down.But this switch (only costing $600) did not cut the power and smoke escaped from my laptop, and since then I can not connect to 1Gb with that laptop.Just to say this is not something you just build yourself. The receiving side is "easy" in a sense that there are standard chips that will do the negotiation.The switch part is pretty hard since you need to accomodate for many rainy day scenarios that can occur.Another point of warning, some colleagues have blown up their USB ports and debuggerhardware while debugging PoE target devices. So the PoE power is definitely not always isolated in respect to your computer and network. So use USB isolators (as we did from then on) for your debugging tools if you ever going to do this kind of stuff.

author: Monkeh
author_rank: Super Contributor
timestamp: April 24, 2015, 12:55:02 pm
content: Quote from: Kjelt on April 24, 2015, 12:50:22 pmIt may have taken the windings.. or it could just be termination resistors. I forgot they often common those before AC coupling to save parts.

author: madires
author_rank: Super Contributor
timestamp: April 24, 2015, 12:56:46 pm
content: Quote from: Wilksey on April 24, 2015, 11:20:35 amPoE got two modes: A for data pairs and B for spare pairs. Since GigE uses all 4 pairs it's mode A only.

author: madires
author_rank: Super Contributor
timestamp: April 24, 2015, 01:14:01 pm
content: Quote from: Kjelt on April 24, 2015, 12:50:22 pmIt's hard to tell whether the switch's PoE is buggy or if someone configured it that way. During the detection of a PoE device the switch will supply just a few mA after it found a valid signature resistance. But that's not enough to blow up a NIC.

author: Monkeh
author_rank: Super Contributor
timestamp: April 24, 2015, 01:37:55 pm
content: Quote from: madires on April 24, 2015, 01:14:01 pmIf the switch can be configured to explicitly break PoE specs it's buggy!

author: AndyC_772
author_rank: Super Contributor
timestamp: April 24, 2015, 02:19:49 pm
content: Quote from: Kjelt on April 24, 2015, 12:41:58 pmImpatience, probably. Once power has been established on a PoE connection, it's actually quite hard to reliably detect when the powered device has gone away. Often it's done by detecting that the data link has gone down, and this may only be checked every few seconds.If you quickly unplug the cable from a powered device and plug it into a non-powerable device, there's still 48V across the line, and your transformer and/or Bob Smith termination resistors go bang.

author: sifu0832fjcm2w3fd
author_rank: Newbie
timestamp: April 27, 2015, 03:21:22 pm
content: Thank you all for your advice.I actually did an isolated solution. To my huge surprise the AC adapter I had with my IP cameras (Foscam) was doing just fine on 48V. Moreover, it was much more efficient then on 220V ! Imagine that. (Your mileage may very)Power Power Power@ 48V @ 220V drawnsupply supply @ 5V9.792 22.14 7.28.832 18.45 6.1257.68 14.76 4.975.808 10.824 3.7653.888 7.134 2.542.064 3.198 1.2825I used an old patch panel to inject 48V on 4,5 & 7,8 pairs while passing through the other two pairs. Cameras and RaspberryPi are only 100Mbs any way. I have to mention that I did put a 30V Zener and a MOSFET just after Graetz Bridge and before the rest of DC-DC flyback (ex AC adapter). It does work OK for now. I admit, this set-up begs for some current sensing and cut-off on short, on per port basis at the injection point but this will have to do for now. I had no time to design boards for the patch panel. It's a installation at my own place, so unless I screw it up... [ one from time does- Occam's razor ]O! Important thing for anyone who wants to follow suit, in my case each of the 4,5 &7,8 pairs is connected and terminated to the ground with ~75ohm resistor on the camera's board. Of course one has to disconnect both resistorsThis solution, surprising as it is, is just temporary and allowed me to test stuff. I'm planing to make around 10 circuits with TPS23753 (Texas Instruments) especially that the documentation and even eval. circuit are available. Furthermore both the driver & the transformer are available from Mouser.Once I find some spare time, for anyone interested, I may do a write-up of the whole process. I think people who experiment with this may find it interesting. PoE switches are still not the cheapest but having a UPS based power supply on the same cable with data is such an elegant solution. For now I'm planning to create two boards for my patch panel (100M & 1G) and populate them as my needs extend. RJ45 sockets are not as cheap as one would like them to be :/

author: ojousima
author_rank: Newbie
timestamp: May 18, 2015, 09:16:52 am
content: Quote from: sifu0832fjcm2w3fd on April 21, 2015, 10:35:44 pmHi,We're making Raspberry Pi PoE HATs at ELL-i Open Source Co-operative. We're looking for people to assemble and test a few of our boards before we make a production run out of our crowdfunding campaign. You can find more details athttps://www.eevblog.com/forum/crowd-funded-projects/raspberry-pi-power-over-ethernet-hat-looking-for-testers/,contact us if you want to test one of our boards.If you want to roll your own design, please feel free to take a look at our schematic for inspiration. You can find the schematic at forum post linked above.Best regards,Otso