My first project, a PoE+ power supply

[Ranayna]

Hi everyone,

as already stated in a thread in the beginners section, I will now start a thread here regarding my first bigger electronics project. This is purely a hobby project that i want to do mainly as a learning experience.
The goal of this project is to create a power over ethernet splitter capable of outputting more than 2A at 5 Volts.

This will be my first bigger electronic project, and the first where I will create a custom PCB. I want to reach at least a prototype stage in this project. If it will go any further than prototypes for personal use will have to be seen.

I want to create this thread for a couple of reasons:

- since this is my first project of this kind, i would like to get input at all stages of my project.
- hopefully, a thread like this, where a project is documented from start to (hopefully) finish can help other forum members that want to do something similar
- I also want to create a bit of peer pressure for myself, since I sometimes have massive issues motivating myself to actually do something productive with my free time

I have thougt a lot about this, but have not actually put much to paper yet. A rough outline I see for the project is about as follows:

• set requirements: already done, but additional requirements might surface
• choose the PoE chipset. Since the chipset is the heart of the project, I think it is the first component i have to look at
• determine the rest of the BoM, depending on the chosen chipset, the requirements and component availability. After this point there should be no more changes of requirements
• create schematic
• create board layout. Might result in changes of the BoM
• acquire components for a couple of prototypes
• get the PCB manufactured
• assemble and test

Each of this would be a distinct project step.


Here is my first step in this project, the actual requirements:

• compliant to PoE standards 802.3af (PoE) and 802.3at (PoE+)
• Preserving Gigabit Ethernet
• 5V output with at least 2A (802.3af would be enough for that case)
• overload protection

Optional:

• Auxilliary input to supply power if PoE is not available
• Display current PoE status
• ability to output additional/different voltages for special requirements
• Currently there is no hard size requirement. The board should not be bigger that an Arduino though

Nice to have:

• Some way to communicate with attached devices

I am open regarding some other features that would be nice to have.

The next step is step is selecting the PoE chipset. More on that in the next post.

[Ranayna]

So, before I list the chips that are currently in my list of possible choices, i want to elaborate my thought processes in choosing them.
Due to my work as Network technician, I know about Power over Ethernet and some of the standards and terminology.

So i knew that a google search for PoE chipset would net me some results. And i was not disappointed. I got results fron quite lot of manufacturers, all of which have quite a good list of chips.

The following is just a short list of chips from the manufacturers on the first google page, with their package and price listed. I want to list those, since both will play a big role in my final choice. At this point I have not chosen anything yet.

Analog Devices (former LT)

• LTC4269-1, package: 32pin DFN with centerpad, $2.40
• LTC4269-2, package: 32pin DFN with centerpad, $2.60
• LTC4278, package: 32pin DFN with centerpad, $2.60

prices are on AD page for 1000, so just for reference

Silicon Labs

• Si3406x, 20 or 24pin DFN with center pad, around €1.60 (digikey)

ST

• PM8800, package: HTSSOP16 with centerpad, €2.35 (Digikey)
• PM8801, package: HTSSOP24 with centerpad, €1.72 (Digikey)
• PM8803, package: HTSSOP20 with centerpad, not available: seems to be deprecated?

TI

• TPS23785B, package: HTSSOP24 with centerpad, €2.90 (Digikey)
• TPS23752, package: TSSOP16 with centerpad, not available, deprecated?
• TPS23751, package: TSSOP16 with centerpad, €2.50 (Digikey)
• TPS23756, package: HTSSOP20 with centerpad, not availabe, deprecated?
• TPS23754-1, package: HTSSOP20 with centerpad, can't fint it on digikey
• TPS23754, package: HTSSOP20 with centerpad, €2.31 (Digikey)

That already is a big list, and there are even more I still want to list, but it is getting quite late.
Also one shot question: Looking up prices on Digikey, I saw parts with -ND in their partnumber, those also were not really available. Am i right that "ND" means that part is going out of production? For example the TI parts, on the TI website they were all listed as active...

Anyway, tomorrow I will edit in the rest of the list (Maxim and Microsemi parts), and then I will have to read (and understand )a lot of datasheets...

[westfw]

Am i right that "ND" means that part is going out of production?

No.
https://electronics.stackexchange.com/questions/52545/what-does-nd-mean-on-part-numbers

[Cerebus]

You might find your life easier if you take a slightly roundabout route to picking your central IC. Presumably you'll be fairly closely following a reference design from your chosen chip supplier. That given, look for a manufacturer that has the most comprehensive, easiest to follow, documentation for their reference design. These will all be switched mode DC-DC converters, so you'll need magnetics. Look at the magnetics called out in the reference design and make sure you can easily get hold of them. The availability of magnetics and a well documented reference design will narrow your choice.

[chris_leyson]

Used the TI TPS2378 as a power switch at the device end and LM5017 down converter, max device power was limited to 6W. Pulse HX6096FNL for the Gige transformer and no problems running at Gige speeds.
The only issue I found was that if you are using your own converter design after the power switch you have to use the CDB, converter disable signal connected to UVLO to enable the converter at the right time otherwise it won't start up. Most of it was copied from the TI app note and it worked first time.

[ogden]

Used the TI TPS2378 as a power switch at the device end and LM5017 down converter, max device power was limited to 6W. Pulse HX6096FNL for the Gige transformer and no problems running at Gige speeds.

TI PoE reference design + some buck converter IC with 100V input capability is excellent choice. Mentioned transformer is "Rolls Royce of PoE transformers", but otherwise good for first prototype. You can price-optimize later - when your prototype works.

Any surge protection needed? PoE Gigabit Ethernet is tricky in this regard.

[westfw]

I've always wondered whether you could use one of the 48V DC/DC power modules that show up cheap on the surplus market, to do the actual power supply.  Built-in isolation and reasonable efficiency, no hunting for expensive transformers...
(You'd still need the 802.3af controller.  But those are available by themselves, last I looked.)

[ogden]

I've always wondered whether you could use one of the 48V DC/DC power modules that show up cheap on the surplus market, to do the actual power supply.  Built-in isolation and reasonable efficiency, no hunting for expensive transformers...
(You'd still need the 802.3af controller.  But those are available by themselves, last I looked.)

Yes, indeed you can use 3rd party DC/DC modules. It is even advised in case you don't have SMPS expertise in-house. Match power of the module and test everything at full range of the incoming voltages and output loads, especially power_on/inrush at full load and lowest specified input voltage.

[TimNJ]

Am i right that "ND" means that part is going out of production? For example the TI parts, on the TI website they were all listed as active...

"NRND" might be what you're thinking of. That is, Not Recommend for New Designs. Usually Digikey (et. al.) will list that if the manufacturer has a new series of parts to replace the one you're looking at, but is perhaps not obsolete yet.

[Ranayna]

Thank you for all answers

yes, I have associated ND in my mind with NRND, since I only saw that on parts that were not in stock at Digikey.

Regarding selection of chips: The provided reference designs in the datasheet or on the website are a big factor in my decision. Other factors are solderablility for a hobbyist, price, availablility and also something of a "nose factor". That one is difficult to measure of course

I think that stupidly listing here each and every PoE chipset that I find via google is a bit to much for this thread. That is something for internal spreadsheets and to for a forum, hopefully I did not put people off.
Over the weekend I will start to look at chipsets and will already try to narrow my selections down to a few (5 at most) and will come back here with more concrete questions. I still need to orient myself with this thread and learn when there is too much information.

I also need to focus myself. I already have some project ideas that tend to send me off in other directions, but I really want to get somewhere with this before I get myself distracted again.

This will be big project for me, since in the process I also have to learn using schematics and layout software.