EEVblog #1122 - Raspberry Pi 3 PoE Hat FAIL Investigation - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: September 13, 2018, 02:42:50 pm
content: The new official Power Over Ethernet (PoE) hat for the Raspberry Pi 3 B+ has been reported to fail when powering USB devices.Dave investigates, until, well, something goes horribly wrong...

author: Zbig
author_rank: Frequent Contributor
timestamp: September 13, 2018, 03:27:34 pm
content: The Register has interviewed the Raspberry Pi guy who has elaborated somewhat on the cause of the issue:https://www.theregister.co.uk/2018/09/11/raspberry_pi_poe_hat_issue/

author: Berni
author_rank: Super Contributor
timestamp: September 13, 2018, 03:59:24 pm
content: This must have been designed by a intern or something because pretty much everythyng is wrong with this design!So the reason that i think this is causing a problem is the sharp rise time in the ripple on the 5V output. That fast risetime will try to quickly charge any capacitance on the other side of the USB hub protection circuitry, once the load is big enough the spikes in current will exceed the trip point and make it go into overcurrent mode. The reason it happens on all ports is that the Raspberry Pi 3 uses a single 5V high side switch with built in protection to switch all 4 ports (Cost saving most likley). The pins on the USB hub are wired together so that a overcurrent signal is sent for all 4 ports, that's why it will report an error for all ports. I know about this because i had to reverse engineer that part of the circuit some time ago.Lets start listing whats wrong with it:- The HV clearance is very important for PoE compliance as you can get massive spikes on long ethernet cables. That slot is doing absolutely nothing due to the copper fill mistake and im sure the clearances are not to spec.- The fact that plugging it in gets in the way of the GPIO header is a major fail. You could have used one of those long pinned troughhole headders that arudinos and so many other Pi Hats use.- That tiny 4 pin connector should have at least had some large copper pads with vias to hold it down (Or even better just use troughhole)- 250mV of ripple is pretty bad, but its made even worse by that sharp rising edge in it- This can NOT run reliably at 130°C on major semiconductor components.- They did not use the opportunity of a copper fill around the diode to cool it (Not even giving it vias to help drain the heat away)- The layout is a bit meh around the secondary side as it has a fair bit of current loop by the time it gets rectified to DC (These are very fast swings in this topology)- The common mode cap over the isolation barier has a really long way around. This will likely make the board radiate radio interference like crazy- That common mode current also has two paths, each one around the fan, not a good design with fast transient currents- Output voltage goes out of control with no load calls for at least some sort of zenner diode to keep it within sane limits (This topology can really run away badly with no load)- It should survive such an output short circuit just fine as im sure a lot of people will eventually short out the 5V rail trough some stupid mistake (Most regulator designs are short circuit tolerant these days)

author: Monkeh
author_rank: Super Contributor
timestamp: September 13, 2018, 04:05:31 pm
content: Quote from: Zbig on September 13, 2018, 03:27:34 pmQuoteI'm not quite sure 'dumb luck' is the term for not performing full tests against all variations of your product.

author: Habropoda
author_rank: Regular Contributor
timestamp: September 13, 2018, 04:20:37 pm
content: The slot is for the camera cable. It has nothing to do with clearance.

author: free_electron
author_rank: Super Contributor
timestamp: September 13, 2018, 06:46:22 pm
content: without even looking at anything else : who the hell suspends a transformer in a hole like that ?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 13, 2018, 07:31:36 pm
content: Quote from: free_electron on September 13, 2018, 06:46:22 pmWatch the video.

author: thm_w
author_rank: Super Contributor
timestamp: September 13, 2018, 09:06:58 pm
content: I don't understand why there was so much hype for POE hat.You can spend less money, and get a properly enclosed product, that can be reused for another project down the road if needed:https://www.aliexpress.com/store/product/Cdycam-IEEE-802-3af-Micro-USB-Active-PoE-Splitter-Power-Over-Ethernet-48V-to-5V-2/1920360_32841790680.htmlhttps://www.aliexpress.com/store/product/Xinfi-DC-12V-POE-Adapter-Injector-Splitter-Connector-IEEE802-3af-Active-10-100Mbps-For-IP-Cameras/1543317_32620368747.htmlThe only advantage I see is its slightly more compact.

author: dardosordi
author_rank: Regular Contributor
timestamp: September 13, 2018, 10:02:37 pm
content: Hey Dave, check this onehttp://www.ivmech.com/magaza/en/development-modules-c-4/poe-hat-for-raspberry-pi-3-model-b-p-111

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 13, 2018, 11:48:41 pm
content: Quote from: thm_w on September 13, 2018, 09:06:58 pmPOE makes it very easy to get power to a distant location. POE cameras are a popular example. You don't have to pay for an electrician. That is big. Cat5 is very cheap compared to power wire.Also POE gives you the ability to remotely control the power.

author: EEVblog
author_rank: Unknown
timestamp: September 14, 2018, 02:43:09 am
content: Quote from: thm_w on September 13, 2018, 09:06:58 pmYeah, plenty of existing robust solutions exist to give you 5V @ 2.5A, but just that it's a separate box.

author: EEVblog
author_rank: Unknown
timestamp: September 14, 2018, 02:45:24 am
content: Quote from: free_electron on September 13, 2018, 06:46:22 pmVery comment technique to get the height profile down. You see it on countless DC-DC converter designs.Bonus is it's also a lower creepage path

author: ogden
author_rank: Super Contributor
timestamp: September 14, 2018, 04:18:37 am
content: Quote from: Berni on September 13, 2018, 03:59:24 pmAs we all saw in the video, Raspberry Pi 3 board fails miserably in this regard.

author: TurboTom
author_rank: Super Contributor
timestamp: September 14, 2018, 05:35:57 am
content: Shame the POE board got damaged. It shouldn't, just from a short at its output rail. What I would have liked to see is an attempt to operate the RPI with the POE unit located separately. The "antenna" effect (induced voltage in the probe ground lead, especially by that magnitude) in the vicinity of the transformer should have made you suspicious. The RPI board is just millimeters away when the POE board is installed and the stray field of the transformer will induce the same kind of voltages there as well. I can very well imagine that this is actually causing all the trouble observed. It might be worth trying to put a short winding (copper foil) externally around the transformer as to be found frequently on bigger switchmode transformers. Anyway, since it's broken now, testing is over...Thanks anyway for demonstrating the problem.Cheers,Thomas

author: Peabody
author_rank: Super Contributor
timestamp: September 14, 2018, 01:57:46 pm
content: So if instead of using the POE hat you provided a nice clean 5V supply into that four-pin connector on the PI that the hat normally uses, there would be no problems with USB? Just a test to eliminate the possiblility that getting 5V from that connector doesn't contribute to the problem in some way.

author: free_electron
author_rank: Super Contributor
timestamp: September 14, 2018, 02:11:59 pm
content: Quote from: EEVblog on September 14, 2018, 02:45:24 amProvided the transformer is mechanically supported .... This one is wafting in the wind. It is pretty big and will cause plenty of stress on the solderjoints and the copper adhesion. i hope they have shot some via's in the pads...

author: free_electron
author_rank: Super Contributor
timestamp: September 14, 2018, 02:18:55 pm
content: a couple of design remarks- why does that output diode run so hot ? That is not normal.- Are we sure that transformer is properly made for that switching frequency ? It wouldn't be the first time they are running at the wrong frequency. some of these chips come in different frequency settings and the transformer needs to be adapted.- primary side regulation is crap. that only works if you use a post-regulator.

author: ogden
author_rank: Super Contributor
timestamp: September 14, 2018, 02:38:51 pm
content: Light (1W) load frequency of that converter was audible frequency, 9.47 KHz. Audio jack output could become unusable when powered from PoE hat. In next video (if any) would be nice to check that as well.

author: Fungus
author_rank: Super Contributor
timestamp: September 14, 2018, 06:17:28 pm
content: That thing needs diagnosing/fixing. It might just be a diode.

author: ricktendo
author_rank: Regular Contributor
timestamp: September 14, 2018, 07:17:03 pm
content: Quote from: thm_w on September 13, 2018, 09:06:58 pmI have one of these and it works fine with my Pi 2B+, but I can report the PoE part works but the LAN cable does not work with the Pi 3B+.Note: the LAN cable on the PoE injector is only 10/100 (Fast Ethernet,) if you can find a 10/100/1000 (Gigabit) version of the same injector it may work.P.S.I have this 3rd party PoE adter from Navo Labs, I will test it see if this too has the same issuehttps://navolabs.com/product/raspberry-pi-3-micro-poe-hat/Edit: I do not have a load tester so I do not know for sure, but as a test I plugged in an external HDD and my Pi did not power down like in Dave's video

author: ogden
author_rank: Super Contributor
timestamp: September 14, 2018, 07:22:25 pm
content: I think there's nothing wrong with diode from design point of view. Diodes usually have to dissipate some power in switching converters (which are not synchronous thus w/o diodes). Most likely they use diode used in MP8007 reference design, SBR8U60P5 or similar. Such diode at 2.5A forward current have ~0.3V drop meaning dissipated power is 0.75W which is more or less what we see in thermal imager. IMHO problem is not diode but specification promising PoE Class0 12.5W with quite lossy dc-dc converter design. They are just trying to jump over their head/hat. Users would be fine with PoE Class2 6.5W (5V @ 1.3A) resulting in much lower heat dissipation.

author: thm_w
author_rank: Super Contributor
timestamp: September 14, 2018, 08:10:12 pm
content: Quote from: JustMeHere on September 13, 2018, 11:48:41 pmI mean the fact that there was a "POE hat" added, was the hype. POE itself is great, didn't think about the remote power control, would be good to remotely reset the unit.Quote from: ricktendo on September 14, 2018, 07:17:03 pmOdd that it wouldn't work on the 3 but does on 2, you'd think the pi would just auto-negotiate to 10 or 100mbps. Maybe forcing 100mbps somehow would help.The second link lets you choose a gigabit option, presumably adds the missing pairs.

author: ogden
author_rank: Super Contributor
timestamp: September 14, 2018, 08:26:31 pm
content: IMHO rPI is screaming for low cost non-isolated "passive" 12V PoE hat which comes with 12V wal-wart supply and "pigtail injector". Usual rPI buyers rarely have 802.3AF-compatible switches around, PoE brick supply Dave used in the video also is not that wildly popular or cheap compared to 12V 1A "plugs with tail".

author: wraper
author_rank: Unknown
timestamp: September 14, 2018, 08:30:08 pm
content: Quote from: free_electron on September 13, 2018, 06:46:22 pmMost of the thin TV PSUs.

author: ricktendo
author_rank: Regular Contributor
timestamp: September 15, 2018, 01:33:14 am
content: Quote from: thm_w on September 14, 2018, 08:10:12 pmHere are detailed PCB pictures of a 12v model, maybe this will provide a cluehttp://www.knutsel.org/2016/05/31/poe-pi-powering-a-raspberry-pi-with-power-over-ethernet/BTW it does appear that they have an updated Gigabit model for the Pi 3B+eBay auction: #123357699802

author: BrianHG
author_rank: Super Contributor
timestamp: September 15, 2018, 07:05:26 am
content: Quote from: free_electron on September 13, 2018, 06:46:22 pmThis super high quality, 10k$, 1kv isolated 4 channel 14 bit scope also has the transformers mounted in the middle of the PCB, and apparently, the high isolation development took over a year and was found to be the best solution:https://www.eevblog.com/forum/blog/eevblog-1119-designing-a-1kv-isolated-oscilloscope/

author: Halcyon
author_rank: Unknown
timestamp: September 15, 2018, 11:54:07 pm
content: Quote from: thm_w on September 13, 2018, 09:06:58 pmBecause POE is a widely used standard. You can plug a non-POE device into a POE port and not have it blow up. Those passive splitters/injectors are fine for crude installations, but when you forget and you plug it into the wrong device, the outcome isn't so good.Also, those splitters use the unused pairs on a 10BASE-T / 100BASE-TX connection. If you used one of those splitters with a Gigabit-capable device, the best you'd get is 100Mbps as it's splitting off those "spare" pairs. A POE/POE+ device can run at 1Gbps while the switch also supplies the power along the same cable.

author: thm_w
author_rank: Super Contributor
timestamp: September 17, 2018, 06:56:40 pm
content: Quote from: Halcyon on September 15, 2018, 11:54:07 pmSecond link has gigabit for $11, as I noted above.

author: Halcyon
author_rank: Unknown
timestamp: September 17, 2018, 11:11:47 pm
content: Quote from: thm_w on September 17, 2018, 06:56:40 pmSure. However that isn't a "dumb" splitter (at least it doesn't purport to be). That device provides phantom power, just like other 802.3af devices do, hence why it's able to be used at Gigabit link speeds. It just has an additional DC barrel jack at the end. The point of the Raspberry Pi hat is so that additional equipment isn't required.

author: viewgsm
author_rank: Newbie
timestamp: September 18, 2018, 01:51:36 am
content: Well, guess what, we did the test for NavoLabs PoE hat for you and it passes with flying colorsthe video is being edited and should be on youtube tonight, you can easy power up 2.5" HDD from the usb, also the ripple is very well under control, 20-40mV Peak-to-peak, also using a DC load we are able to sink 1A easy from the USB port while powered by the NavoLabs PoE hathttps://navolabs.com/product/raspberry-pi-3-micro-poe-hat/

author: Berni
author_rank: Super Contributor
timestamp: September 18, 2018, 05:25:30 am
content: Nice one, very compact design there.Tho by the looks of it that's not a isolated supply so careful plugging anything grounded into that raspberry pi.

author: ogden
author_rank: Super Contributor
timestamp: September 18, 2018, 07:46:58 am
content: Quote from: Berni on September 18, 2018, 05:25:30 amAlso looks that there are no rectifiers nor reverse polarity protection diodeClaiming "IEEE 802.3af POE" is so wrong.

author: LapTop006
author_rank: Unknown
timestamp: September 18, 2018, 11:01:00 am
content: Quote from: ogden on September 18, 2018, 07:46:58 amSome PoE chipsets such as the Si3402 have the diodes integrated, although for a use such as this I'd use external diodes, not sure what chipset they're using on that one.

author: ricktendo
author_rank: Regular Contributor
timestamp: September 18, 2018, 02:42:28 pm
content: Quote from: LapTop006 on September 18, 2018, 11:01:00 amThe chipset the board is using is the:Si3402 B-GMI purchased one while I waited for the official Pi PoE Hat to be released, I added a fan connector to help with cooling.

author: viewgsm
author_rank: Newbie
timestamp: September 18, 2018, 04:36:24 pm
content: It does, take a closer look, plus it has integrated diodes too !

author: viewgsm
author_rank: Newbie
timestamp: September 18, 2018, 04:40:34 pm
content: Quote from: ogden on September 18, 2018, 07:46:58 amDude, do some research before you speak, or at least look good at the photos, it has integrated diodes and it has external too, and its IEEE 802.3af compliant.

author: Monkeh
author_rank: Super Contributor
timestamp: September 18, 2018, 07:02:41 pm
content: Does appear to not be isolated, though. That can be a massive pain.

author: free_electron
author_rank: Super Contributor
timestamp: September 18, 2018, 07:07:01 pm
content: Quote from: wraper on September 14, 2018, 08:30:08 pmyup. through hole ones...or if the transformer is on TOP of the board.in this thing it is hanging BELOW the board causing stress on the copper lamination ! BAD design !

author: ricktendo
author_rank: Regular Contributor
timestamp: September 20, 2018, 01:12:57 am
content: Found another two PoE hat designs... Opinions?eBay auction: #263811907135https://www.tindie.com/products/canerdurmusoglu/power-over-ethernet-hat-for-raspberry-pi3-model-b/

author: ogden
author_rank: Super Contributor
timestamp: September 20, 2018, 05:15:24 am
content: Quote from: viewgsm on September 18, 2018, 04:40:34 pmDude, please be so kind and show (with arrows) where are rectifiers. Seems, you don't even comprehend what rectifiers I am talking about - obviously not about diode at buck converter output QuoteDude, you are The One who needs to do some research - about IEEE 802.3af compliance.Hint: clause "33.4.1 isolation"

author: Berni
author_rank: Super Contributor
timestamp: September 20, 2018, 05:49:00 am
content: Quote from: ricktendo on September 20, 2018, 01:12:57 amThe first one does look better made and it does have real opto feedback so its output should be nice and stable. It has decent sized capacitors on it to smooth things out including some ferrite filters to really clean it up. The parts look reasonably well cooled and it has troughhole connectors. Looks quite good. Tho the isolation doesn't look that great(But it is there so that's a big plus) and the capacitors that bridge the primary and secundary side look kinda small to meet the kilovolt voltage ratings and the way that they route the output back to that capacitor makes a significant loop area in there and probably makes a good antenna to radiate out switching noise.The other one i couldn't reverse engineer very far from the photos but its quite neat in how compact it is, hopefully they did it properly and met the spec.EDIT: And yes the ebay module is the best i seen so far in this whole thread.

author: ogden
author_rank: Super Contributor
timestamp: September 20, 2018, 05:49:49 am
content: Quote from: ricktendo on September 20, 2018, 01:12:57 amBoth isolated. Good. Isolation is "must have" for anything that is more than just hobby toy for living room installation.Ebay module clearly have two rectifiers meaning - will work with any polarity. It also have optoisolated feedback, thus supposedly good regulation at any load. From what I see, eBay module is best I have seen (visually).That IVMTECH module seems to be "OK-ish" but unfortunately no isolated feedback, nor input rectifiers are visible on top of the daughterboard PCB. There are some hints that most likely some components are on the bottom. Hard to tell. Have to see both sides.

author: Halcyon
author_rank: Unknown
timestamp: September 20, 2018, 06:03:56 am
content: Quote from: ogden on September 20, 2018, 05:49:49 amThe feedback on that site doesn't look too good.

author: Monkeh
author_rank: Super Contributor
timestamp: September 20, 2018, 02:57:36 pm
content: Quote from: ogden on September 20, 2018, 05:15:24 amThe rectifier is internal to the IC. This is not hard to understand.

author: ogden
author_rank: Super Contributor
timestamp: September 20, 2018, 07:16:16 pm
content: Quote from: Monkeh on September 20, 2018, 02:57:36 pmI just missed to carefully read DS of Si3402. Two on-chip input diode bridges confirmed, case closed.

author: viewgsm
author_rank: Newbie
timestamp: September 21, 2018, 06:05:44 am
content: intro to the test video for the NavoLabs PoE hat

author: Red Squirrel
author_rank: Super Contributor
timestamp: September 21, 2018, 12:22:31 pm
content: What kind of boggles me is that they would release this as is. That's a pretty major flaw. Didn't they test the design first? "It passes electrical rules check, ship it!"

author: oPossum
author_rank: Super Contributor
timestamp: October 03, 2018, 07:12:28 pm
content: A fix using a LC filter. 4.7 uH + 100 uF x 2

author: chefkoch84
author_rank: Contributor
timestamp: October 22, 2018, 08:10:24 pm
content: FYI:Dave is not the lonley ee nerd anymore - he goes full mainstream ROCKSTAR.This is the german yellow press of IT:https://www.heise.de/make/meldung/Aerger-mit-dem-Raspberry-Pi-Power-over-Ethernet-HAT-4196194.htmlGangnam Style - Stylecheers.

author: darrellg
author_rank: Unknown
timestamp: November 09, 2018, 04:54:58 pm
content: The Raspberry Pi folks have posted a mea culpa discussing what went wrong and what they've done to fix it.https://www.raspberrypi.org/blog/poe-hat-revision/