EEVblog #659 - Medical Plugpack Teardown - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: September 02, 2014, 04:30:45 am
content: What's inside an IEC60601-1 medical class 5V mains DC-DC plugpack?What is the difference between PCB clearance and creepage?A German FRIWO 5V FW7555M/05 brand medical grade isolation unit vs a cheap Wun Hung Lo consumer brand.Also a comparative look at transformer construction.Datasheets & Links:http://www.farnell.com/datasheets/30889.pdfhttp://en.wikipedia.org/wiki/IEC_60601http://www.rubycon.co.jp/en/catalog/e_pdfs/old/aluminum/e_yxa.pdfhttp://xcnet.de/projekte/xnetcreate.de_v2/pics/kunden/dateimanager/5221/JE8_CD_263_BK.pdfhttp://www.vishay.com/docs/83515/tclt1000.pdf

author: Vgkid
author_rank: Super Contributor
timestamp: September 02, 2014, 04:45:29 am
content: Thanks for the teardown. On a related note it is rather interesting to note build quality practices among various ebay/ oem(real/fake) quality laptop psu's.

author: Dave
author_rank: Super Contributor
timestamp: September 02, 2014, 05:24:34 am
content: I think they didn't route under those optos because they would risk having the board fall apart. Not to mention, there is glue under those optos, so the creepage distance would effectively be increased, as there is probably no direct path under those packages.I didn't like the flimsy heatsinks. The only thing that held them attached to the board were the leads of the components themselves. Mechanical stress on the solder joints should be avoided, especially if you are trying to make a very reliable product. The same goes for pop rivets, the "pop" gives a mechanical shock to the component, which can cause damage to it and significantly reduce its life.

author: coppice
author_rank: Super Contributor
timestamp: September 02, 2014, 05:40:03 am
content: The transformer construction goes a bit farther than most, but the rest of that medical supply is just following standard approvable design - 8mm creepage, air gaps, and so on.I have never understood why you can get away with such ropey transformer construction as most approved supplies have. The opto couplers, capacitors, and board are well controlled by the approvals requirements. They have to not just fail, but fail badly to present a serious risk. Most transformer's, on the other hand, have to be working perfectly to provide safety. The tiniest flaws in the insulation somewhere in those windings, and there's pretty much no isolation at all.

author: Phoenix
author_rank: Frequent Contributor
timestamp: September 02, 2014, 06:40:16 am
content: A few thoughs on some of the components (nothing definitive). Would the signal diode you identified in the DaveCAD be a zener? Then that circuit provides some sort of overvoltage/peaking voltage level feedback.The weird external winding may be providing an electrical connection to the transformer cores since it neatly wraps around both E cores. This might have an EMI/shielding effect. Where does the pin go to on the primary side?The orange wrap looks to be Kapton, rather than Mylar. Though the terms seem to get used interchangibly.

author: firewalker
author_rank: Super Contributor
timestamp: September 02, 2014, 07:26:43 am
content: I believe that the winding above the secondary is not a feedback coil. Just a winding to power the controller.Alexander.

author: mux
author_rank: Regular Contributor
timestamp: September 02, 2014, 07:43:44 am
content: I'm not sure if I said this before on the forum, I remember saying this on the internet once... The reason they are using Samxon (crap garbage bin caps) on the secondary and name brand on the primary is that the secondary caps are much more expensive, simple as that. High voltage aluminum electrolytics are peanuts to build, you can even literally do this yourself and get great stability with some oil paper and aluminum foil. The difference between a name brand and cheap chinese brand cap is less than a cent for these manufacturers, so the choice is easy.The secondary side is a very different story. Low voltage caps that can supply a reasonable amount of ripple current have electrolytes and oxides that have to endure much higher field strengths without breaking down. It's an art to make these caps and they are subsequently usually the most expensive component after the transformer and non-BOM costs. That is why you almost always see crappy secondary caps and good primaries.As for the general construction: Dave, you are giving them way too much credit. This is a crappy power supply. It's not$4 QA reject crappy, certainly, but it's not even near high quality in any respect. It's a phenolic, wave soldered, one-tier-up-from-the-shenzhen-market-brand-components, just-meets-requirements design. I don't necessarily blame them, but I've seen medical supplies from Edac that have proper (4 layer even) FR-4 boards, slot isolated transformers (not this 3M foil garbage), name brand controllers, 125C caps, great output filters, etc..If you ever get a hold of an EEG power supply, I highly recommend tearing those down. They are the absolute best PSUs in the world.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 02, 2014, 07:44:27 am
content: Some comments on the components:The TH20594 DC/DC converter (?) may originate from former company THESYS, a small ASIC maker (design only), as far as I remember.Beneath MELEXIS (mixed signal ASICs fab), it was also a company (VEB) from the former Eastern Germany / Thüringen, nowadays it's XFAB:http://en.wikipedia.org/wiki/X-FabThe two resistors on the 400V cap seems to laid out ok, they have 2M both, (200* 1e4) and are across the cap, intended to discharge it. So they should carry about half of the 400V each.There's also no strange thing about the secondary windings and the MBR1660 Schottky(?)power rectifier diode.Both windings and the diode are in series , therefore you may place the diode everywhere, also "in between" the windings, wouldn't make any difference compared to placing it "outside" the windings, as it's commonly done.The PCB is made from CEM-3, a cheaper, but higher grade composite material:http://en.wikipedia.org/wiki/Composite_epoxy_materialEverything is chosen and designed "on budget", but quality seems to be ok, very similar to Automotive Industry.Nice instructive video about these safety techniques!Frank

author: mux
author_rank: Regular Contributor
timestamp: September 02, 2014, 09:07:34 am
content: Fuses on both line and neutral are only specified for earth-connected (exposed conductor) devices. For 2-pin devices all you need is one fuse for all approval agencies.That's also why the input circuitry is heavily reduced.

author: FrankBuss
author_rank: Unknown
timestamp: September 02, 2014, 09:18:43 am
content: Quote from: mux on September 02, 2014, 07:43:44 amA Rubycon 680 uF / 25 V / 105°C, low ESR capacitor costs EUR 0.58 at Digikey, in single quantities. This power supply costs EUR 64 (at Conrad). Even if you calculate times four the BOM costs to get the retail price, and add a bit for the development costs and profit, it should be possible to use this output capacitor instead of the Samxon. Maybe they think if you can get the medical certification, then you can just double the price, it doesn't matter, the target group will pay it.

author: SNGLinks
author_rank: Regular Contributor
timestamp: September 02, 2014, 10:53:27 am
content: Why is there a green LED on the PCB?The bare wire on the outside of the transformer might be some form of shielding.

author: mux
author_rank: Regular Contributor
timestamp: September 02, 2014, 11:45:47 am
content: Quote from: FrankBuss on September 02, 2014, 09:18:43 amI don't think you appreciate the markets we're talking about here. Power supply manufacturers don't shop at distributors, those prices don't mean anything. Distributor pricing is almost entirely a function of stock, overhead and supplier deals. I'd be surprised if those power supplies go for more than $10-15 a pop in volume.Power supplies, yes even medical grade ones, are an extremely cutthroat business where volumes are in the tens of millions *per model* and margins are sub-10%. A one-cent difference between components is already considered large. And those samxons are definitely more than one cent cheaper than name brands. And yes, certification is definitely a big part of the price, but even if certification is responsible for a $20 increase in price, people are still going to choose for the $34 PSU instead of the $34.50 with slightly better caps.

author: TheRevva
author_rank: Regular Contributor
timestamp: September 02, 2014, 11:56:56 am
content: Some time ago I was looking into making some significant 'mods' to a PC PSU...The most informative article I found on the net was this one:http://www.users.on.net/~endsodds/smps.htmOn that page is a link to his rewind of the main transformer (with pics)http://www.users.on.net/~endsodds/tx.htmIt describes in fairly good detail a lot of the 'safety' aspects of SMPS transformers.You'll even see how ONE of the pictured transformer bobbins 'forces' the presence of a 3mm gap to produce an acceptable 'creepage' distance (when coupled with the teflon insulated lead-in / lead-out of the windings).Anyway, just thought I would 'share'.

author: delmadord
author_rank: Regular Contributor
timestamp: September 02, 2014, 12:13:17 pm
content: I was just wondering, where is the controller on the second unit. It has a one-layer board, as most PSUs have. But there are no SMDs on it, and I could not find any pins or traces that would resemble any THT based SMPS controller. Is it possible to build it without one?

author: VK3DRB
author_rank: Super Contributor
timestamp: September 02, 2014, 02:01:02 pm
content: That odd-ball outer winding of a few turns may have something to do with reducing eddy currents.Rubycon... they were notorious years ago in TV's and audio amplifiers where their dielectric would dry out. The non polarised types were often used in cheapo radios and amplifiers as DC blocking caps between the output of an AF amplifier and the speaker. They may have improved over the years, but I tend to go for prefer Panasonic or EPCOS electrolytics if I want quality.

author: ryanmoore
author_rank: Contributor
timestamp: September 02, 2014, 02:55:05 pm
content: Quote from: SNGLinks on September 02, 2014, 10:53:27 amThere appears to be a plastic window on the front of the device near where the cable exits. You can see it near the start of the video.

author: diyaudio
author_rank: Frequent Contributor
timestamp: September 02, 2014, 07:37:27 pm
content: Quote from: mux on September 02, 2014, 07:43:44 amDid you even consider the power supply is old?

author: diyaudio
author_rank: Frequent Contributor
timestamp: September 02, 2014, 07:42:14 pm
content: Quote from: firewalker on September 02, 2014, 07:26:43 amSwitching controllers are normally powered from a secondary power source, directly from mains and rectified or a smaller transformer (cost more though), I suspect it might be auxiliary tapping or some type of technique used to minimize leakage inductance.

author: Switching Power
author_rank: Contributor
timestamp: September 02, 2014, 09:40:54 pm
content: The IEC60601-1 standard itself will cost you around €200 but that don't include all the other standards its referring to.

author: SNGLinks
author_rank: Regular Contributor
timestamp: September 02, 2014, 09:58:33 pm
content: Quote from: ryanmoore on September 02, 2014, 02:55:05 pmThanks, I missed that.

author: Flump
author_rank: Frequent Contributor
timestamp: September 02, 2014, 11:11:42 pm
content: What struck me is how bright and shiney the soldering is.

author: wigman27
author_rank: Regular Contributor
timestamp: September 02, 2014, 11:23:42 pm
content: Hi Dave!It would be really awesome if you could do a fundamentals Friday on the theory of the power supply design. For example, what purpose the opto couplers have, what the purpose of the y rated cap has and the basics of operation! It would be really helpful :-)ThanksLee

author: patrik
author_rank: Newbie
timestamp: September 02, 2014, 11:26:32 pm
content: Here some Pictures of the same type of Power Supply (FW75550/05, build date October 2003), but not Medical graded.It looks almost the same - the differences seem to be the PCB material, the chosen Optocouplers (PC123) and the Fuse (T630 instead of the T800mA on the medical version).As the Optos are different the routing is altered a bit, but interestingly the landing for the resistors used on the Vishay type are on this PCB as well.The Housing was not welded, is seems to be glued on - nothing what a trusty Swiss army knife couldn't overcome (no need for an Dremel or a sword as Dave is using these days)The other components are similar to the -M type, even the transformer has the same type number (15 1430 OO RH), the caps are JH branded (all of them) and rated for 105°CThe similarity goes so far that the green LED is contained, but you can't see it on the outside

author: electronic_eel
author_rank: Regular Contributor
timestamp: September 02, 2014, 11:44:13 pm
content: Thanks for the teardown video of this power supply. And thanks patrik for the photos of it's non-medical brother.Seems like both units survived the EMC tests without the use of Y-caps. This is something you'll seldom find today and something you need for some applications: the Y-cap floats the output to about half of mains voltage in reference to earth. If you now plug the live power supply into a sensitive device which happens to sit at earth voltage, it could go boom. So if you can't get a psu without Y-caps, always connect the consuming device first and plug the psu into mains afterwards.What I've never seen anywhere else is this dual optocoupler feedback design. Usually you have just a TL431 and a small compensation network with a few caps and resistors on the secondary side, driving one opto. Does anyone have further insight what this dual opto setup is all about and how it works?

author: ddebeer37
author_rank: Newbie
timestamp: September 03, 2014, 12:26:54 am
content: I design power supplies for a living and it keeps surprising me how obscure a subject it seems to be. Very few people seem to understand isolation, creepage distance and clearance distance and the techniques used to achieve compliance.The secondary winding is normally made with triple insulated wire to avoid using a bobbin with a partition in the middle of the bobbin. The secondary in the Chinese supply appears to be wound with triple insulated wire. The medical grade supply uses silicone isolated wire. See here for quality wirehttp://www.rubadue.com/products/reinforced-3-layers-tca3-triple-insulated-wire.The flexible tape between the layers is self vulcanizing tape that forms an isolation barrier between the primary and secondary windings.http://en.wikipedia.org/wiki/Self-amalgamating_tapeThe tape to cover the windings is normally kapton tape for high temperature transformers and cheaper materials for lower specification transformers.The 'winding' that goes around the outside of the transformer is not a secondary or primary winding in the ordinary sense of transformers since the net flux through the winding is zero. To be a normal winding it has to enclose one of the legs of the transformer, either the center leg or one of the outer legs (for half the flux coupling). Since the wire is not isolated with a coating its purpose is to 'connect' to the core pieces. The core pieces are made of ferrite material that has poor conductivity (for low eddy current losses) and you can induce voltages on the surface of the material through capacitive coupling from the windings. That bare wire provides a discharge path for those charges. It is particularly helpful to keeps voltage spikes on the primary from coupling capacitively to the secondary winding.The white spacing tape is to keep the secondary winding well away from the core since it is not a proper isolating material.On low frequency mains transformers the primary and secondary windings are effectively on two separate bobbins and the plastic provide the creepage distance. For high frequency transformers the coupling between the primary and secondary winding is more important and designers prefer to have the secondary on top of the primary. Sometimes the primary winding is split in two with one section wound on the interior of the bobbin followed by the secondary followed by the second half of the primary winding.I hope his helps.Daniel

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: September 03, 2014, 05:41:43 am
content: I was investigating a failure of wallwart that was provided with my cable modem about 2 years ago - it was nothing special, cheap supply but switching transformer also had non-insulated winding with few turns, placed BETWEEN secondary and primary. Now I know WHY! finally!ddebeer37

author: firewalker
author_rank: Super Contributor
timestamp: September 03, 2014, 06:38:15 am
content: Quote from: ddebeer37 on September 03, 2014, 12:26:54 amCan you suggest a book (mostly with tips and tricks) for designing smps?Alexander.

author: VK3DRB
author_rank: Super Contributor
timestamp: September 03, 2014, 12:51:20 pm
content: Some half decent tech notes on SMPS's and PFC that are free...http://www.nxp.com/documents/application_note/APPCHP2.pdfhttp://www.onsemi.com/pub_link/Collateral/HBD853-D.PDF

author: JVR
author_rank: Regular Contributor
timestamp: September 03, 2014, 03:29:55 pm
content: Anyone have any info on the tool used to suck off the transformer?

author: fpliuzzi
author_rank: Regular Contributor
timestamp: September 03, 2014, 03:52:34 pm
content: It's probably the desoldering tool that Dave reviews in this video...

author: wraper
author_rank: Unknown
timestamp: September 04, 2014, 10:59:16 am
content: Those JH capacitors are utter crap indeed. They are plague of APC UPS where they ere used a lot. Didn't yet seen a single low power APC UPS which is older than a 4-5 years which wouldn't have a few dead JH capacitors if they are present there.http://www.badcaps.net/forum/showthread.php?t=11400And Samxons actually are known for good quality unlike Capxons. When I'm repairing some Korean brand equipment, if there is a mix of Samwha, Capxon and Samxon, Samxons are always good unlike others. Have seen like 1-2 failed Samxons ever (except GF series).Edit, completely forgot about Samxon GF which were used in LCD monitors a lot, those was plagued series which failed a lot. Like Japanese Nichicon HM, HN.Edit 2: BTW capacitors are made by Nantong Jianghai Capacitor Factory (China), Jinghai-europe is just their sales subsidiaryhttp://jianghai-europe.com/wp-content/uploads/J14_Jianghai_Gesamt.pdf

author: wraper
author_rank: Unknown
timestamp: September 04, 2014, 11:37:34 am
content: Optocoupler in cheap PSU is liteon LTV817Bhttp://www.us.liteon.com/downloads/LTV-817-827-847.PDF

author: mariush
author_rank: Super Contributor
timestamp: September 04, 2014, 01:38:57 pm
content: Quote from: wraper on September 04, 2014, 10:59:16 amSamxon GF fails everywhere, not only monitors. Nichicon HM and HN were good except for a period of time somewhere until 2004 when there was a manufacturing flaw (I think they were overfilled with electrolyte) but the ones made after 2004 were OK.There were still a lot of failures in Dell small form factor computers but that was simply really bad design, they killed both Nichicon and Rubycon good quality capacitors.

author: wraper
author_rank: Unknown
timestamp: September 04, 2014, 01:57:19 pm
content: Quote from: mariush on September 04, 2014, 01:38:57 pmI know that they are fixed, GF are fixed too AFAIK, they are still in production.

author: gxti
author_rank: Frequent Contributor
timestamp: September 04, 2014, 05:44:23 pm
content: Why does the transformer have an isolation slot in the PCB but the optocoupler does not? (e.g. in the non-medical supply patrik showed). I would think that because the overall width of the optocoupler is shorter than that of the transformer that it would need it more. Is it because the primary windings of the transformer extend closer to the secondary side? And if so what is the risk of creepage over the transformer housing itself?

author: mux
author_rank: Regular Contributor
timestamp: September 04, 2014, 08:27:42 pm
content: Quote from: diyaudio on September 02, 2014, 07:37:27 pmI.. eh... what? How does this have to do with anything?

author: rs20
author_rank: Super Contributor
timestamp: September 05, 2014, 12:38:04 pm
content: I've attached a question in picture form. Basically, the slot seems to be virtually completely useless because the primary-to-secondary creepage is no different -- sure, choose any two points and you may see an increase in creepage -- but the worst case creepage is no different, so I can't understand why the slot's of any use?

author: Neilm
author_rank: Super Contributor
timestamp: September 05, 2014, 03:26:17 pm
content: Quote from: Flump on September 02, 2014, 11:11:42 pmThis is a medical device. You are allowed to use leaded solder if you need reliability.

author: thm_w
author_rank: Super Contributor
timestamp: September 06, 2014, 12:14:58 am
content: Quote from: rs20 on September 05, 2014, 12:38:04 pmYeah.Its common to see holes drilled under a transformer for air circulation, so perhaps the footprint just had it included:https://www.eevblog.com/forum/projects/creepage-vs-isolation-cutouts/msg161397/#msg161397

author: batterypowerguy
author_rank: Newbie
timestamp: September 06, 2014, 12:16:39 am
content: Squib winding is either a piss poor attempt to do some shielding without using a whole piece of foil, or more likely if it is on the primary side it is part of a snubber and core reset circuitry. LT has some good appnotes on doing things like this to do a quasi-resonate snubber that actually is pretty efficient compared to dumping power in a RCD.

author: EEVblog
author_rank: Unknown
timestamp: September 06, 2014, 12:18:59 am
content: Quote from: rs20 on September 05, 2014, 12:38:04 pmI think you've missed the point of creepage. It's about the board getting contaminated and drastically deducing the breakdown voltage. An air gap is always a fixed voltage arcing point, but creepage along a board is not. That is why the slot of there, to make the creepage path as long as possible.

author: batterypowerguy
author_rank: Newbie
timestamp: September 06, 2014, 12:27:15 am
content: Quote from: Neilm on September 05, 2014, 03:26:17 pmnot leaded if its comming out of europe these days, cant even use in medical anymore. Given the older age its possible this is lead though.

author: batterypowerguy
author_rank: Newbie
timestamp: September 06, 2014, 12:29:32 am
content: Also looking at this, i doubt those primary windings are voltage sense, else why use the coupler.... rare to use a current sense transformer these days as well.... though with the flyback you kinda get it for free.

author: Circlotron
author_rank: Super Contributor
timestamp: September 06, 2014, 03:09:42 pm
content: The mosfet on the primary side had the tab go directly to the heatsink. That means that when the psu is running on 240V the mosfet drain tab and therefore the entire heatsink is flapping up and down 680V p/p at the switching frequency with fast rise and fall times. Seeing the psu is enclosed in an unshielded plastic case, one wonders how much the radiated EMI is affected.

author: jeremybarker
author_rank: Newbie
timestamp: September 07, 2014, 08:21:24 pm
content: There's an informative article about the safety requirements for medical equipment here:http://www.internationalcoil.com/Pictures/IEC60601-1%20ARTICLE.pdf(http://tinyurl.com/qbke3yv)The fundamental difference between standard and medical equipment is the maximum leakage current allowed. The isolation standards for medical equipment are now identical to those for IT equipment. Medical equipment demands very low leakage currents (generally 10µA for patient connections) whereas non-medical stuff can have far higher leakage and still meet safety standards. Although the high standard of construction helps the biggest difference is the absence of Y capacitors in medical equipment. As Dave suggests this can make meeting EMC standards more difficult.How on earth the cheap supply Dave took apart managed to get a VDE/TÜV approval I don't know. I feel that it almost certainly would have been binned by the product safety people where I used to work.

author: daddario
author_rank: Regular Contributor
timestamp: October 31, 2014, 03:22:59 pm
content: Quote from: firewalker on September 03, 2014, 06:38:15 amBit of an old thread, but one of my favourites is "Switch-Mode Power Supplies: SPICE Simulations and Practical Designs" by Christophe Basso.

author: ErikTheNorwegian
author_rank: Frequent Contributor
timestamp: April 30, 2022, 04:24:51 pm
content: Quote from: daddario on October 31, 2014, 03:22:59 pmThat's a expensive book!. What's make it SO incredible expensive?Is it some CD , web learning that can be used once that are included that makes it so expensive?2106.99£ ??Bought the used one..https://www.amazon.co.uk/Switch-Mode-Supplies-Simulations-Practical-Designs/dp/0071508589/ref=mp_s_a_1_1?crid=3D3B6C4CGTG3L&keywords=Switch-Mode+Power+Supplies+Spice+Simulations+and+Practical+Designs&qid=1651335793&sprefix=switch-mode+power+supplies+spice+simulations+and+practical+designs%2Caps%2C161&sr=8-1