How to implement PoE using as little board space as possible? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: October 22, 2016, 06:29:17 pm
content: As the name suggests, I want to implement PoE in one of my projects, but I have to fit the entire project in a switch box in the wall, living along with mains power (!!) So how to implement it without taking up way too much space?Specs:* PoE input voltage = 6V-56V* Isolated* output voltage = 5V (for ATmega328P) and 3.3V (for W5500)* As efficient as possible - the +48V can supply no more than 120mA but W5500 have quite some current draw.* Hand soldered. No pad on the bottom side please.Plan:* Before the isolated power module, use an integrated switch buck converter like LM2597HV to lower the input voltage to 5V.* 5V-5V isolated power module* ATmega328P runs directly off the secondary of the module.* After the power module, use an integrated switch buck converter like TPS562200 to lower the 5V to 3.3V for W5100.

author: kripton2035
author_rank: Super Contributor
timestamp: October 22, 2016, 06:49:14 pm
content: http://www.ebay.com/itm/162230510070will solve all your problems !

author: Monkeh
author_rank: Super Contributor
timestamp: October 22, 2016, 06:55:27 pm
content: If you want to keep the size down, try looking at more modern switchers. LM5018 maybe, much higher frequency and synchronous, in a little DFN package. 300mA should be plenty.

author: technix
author_rank: Super Contributor
timestamp: October 22, 2016, 06:58:33 pm
content: Quote from: Monkeh on October 22, 2016, 06:55:27 pmMaybe something that can be hand soldered?

author: Monkeh
author_rank: Super Contributor
timestamp: October 22, 2016, 07:01:20 pm
content: Quote from: technix on October 22, 2016, 06:58:33 pmWell you didn't ask for hand soldered. You can hand solder a DFN, the pad's a bit tricky. It comes in an SO-8 version, also with pad, and not much bigger. Most of these parts are going to have pads..E: Maxim do the MAX15062 in a padless DFN.

author: technix
author_rank: Super Contributor
timestamp: October 22, 2016, 07:13:35 pm
content: Quote from: Monkeh on October 22, 2016, 07:01:20 pmThat chip is nicer (padless DFN is still pain to solder and inspect though) and it does comes with the feature that can allow me to run the MCU without either BOD or boot delay. But is there anything cheaper and still can cut it?

author: Monkeh
author_rank: Super Contributor
timestamp: October 22, 2016, 07:25:36 pm
content: Quote from: technix on October 22, 2016, 07:13:35 pmWell, you were asking for small.. now it's cheap.That's actually one of the cheapest options for high(ish) voltage synchronous bucks I'm seeing. It's cheaper (from real small quantity suppliers, not your alley vendors) than the LM2597HV..

author: lukier
author_rank: Unknown
timestamp: October 22, 2016, 07:47:09 pm
content: Not exactly something to solder, but I sometimes use PoE modules for IP cameras, like this one:https://www.aliexpress.com/item/PoE-Module-Board-PM3812T-for-Security-CCTV-Network-IP-Cameras-Power-Over-Ethernet-12V-1A-Output/32612107738.html38x38 mm, isolated, 12V 1A (you can step down to 5V with whatever you have in your drawer).I'm not sure though these modules work with Gigabit Ethernet, will have to check. Probably not, as the cameras are usually 100Mbit and there is also the transformer on the board (ethernet comes to one of the JST connectors, signals to the MAC come out on the other connector, third one outputs 12V).

author: technix
author_rank: Super Contributor
timestamp: October 22, 2016, 08:42:43 pm
content: Quote from: lukier on October 22, 2016, 07:47:09 pmI don't want to have things dangling off the board, and that size is probably a but too big for my switch box. And no I am using 100BASE-TX.

author: technix
author_rank: Super Contributor
timestamp: October 22, 2016, 08:46:21 pm
content: Quote from: Monkeh on October 22, 2016, 07:25:36 pmQuality suppliers are simply next to unavailable here so might as well disregard them.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 22, 2016, 08:46:27 pm
content: http://www.innovelec.co.uk/products/power-over-ethernet/

author: nctnico
author_rank: Super Contributor
timestamp: October 22, 2016, 11:30:40 pm
content: Quote from: technix on October 22, 2016, 06:58:33 pmBoth package are easy to solder with some patience. You need vias in the power pad anyway. With hot air you can let solder flow into one via and heat until it comes up in the other(s). The only way this can happen is if there is solder flowing between the pad and the chip.

author: Monkeh
author_rank: Super Contributor
timestamp: October 23, 2016, 02:42:31 am
content: Quote from: technix on October 22, 2016, 08:46:21 pmI can't price up your alleys and boxes off the back of trucks.

author: technix
author_rank: Super Contributor
timestamp: October 23, 2016, 04:21:20 am
content: Quote from: Monkeh on October 23, 2016, 02:42:31 amhttps://www.taobao.com/here is how you price them up.

author: Monkeh
author_rank: Super Contributor
timestamp: October 23, 2016, 04:23:19 am
content: Quote from: technix on October 23, 2016, 04:21:20 amBecause we all speak Chinese.

author: mariush
author_rank: Super Contributor
timestamp: October 23, 2016, 05:16:49 am
content: If you want something really small (but expensive, at $17 each if you buy 100), you could go for a LTM8050 (a micromodule, step-down dc-dc converter with everything inside the package) :http://www.digikey.com/product-detail/en/linear-technology/LTM8050EY-PBF/LTM8050EY-PBF-ND/4356524Looks scary as it's 70 pin bga with lots of pads but really a lot of them have the same role, it's basically 4-5 traces and you only need a couple ceramic resistors and capacitors besides the module to get up to 2A out of it at whatever you want. All in 15mm x 9mm x 4.92mm package. See Figure 3 (page 16) in datasheet for example of use.There's much cheaper modules made by Linear, for something like 7$ each if you buy 100 ... but they only go up to around 36v.As for soldering it, with a bit of liquid flux and a hot air gun, it can be easily soldered to a pcb.

author: technix
author_rank: Super Contributor
timestamp: October 23, 2016, 05:39:47 am
content: Quote from: blueskull on October 23, 2016, 04:27:47 amI am already using the 802.1at/af conforming integrated magnetics Ethernet jack, so I just need to implement my own isolation.