EEVblog #659 - Medical Plugpack Teardown

[Alexei.Polkhanov]

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

[firewalker]

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 wire http://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_tape

The 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

Can you suggest a book (mostly with tips and tricks) for designing smps?

Alexander.

[VK3DRB]

Some half decent tech notes on SMPS's and PFC that are free...

http://www.nxp.com/documents/application_note/APPCHP2.pdf
http://www.onsemi.com/pub_link/Collateral/HBD853-D.PDF

[JVR]

Anyone have any info on the tool used to suck off the transformer?

[fpliuzzi]

It's probably the desoldering tool that Dave reviews in this video...

[wraper]

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=11400
And 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 subsidiary http://jianghai-europe.com/wp-content/uploads/J14_Jianghai_Gesamt.pdf

[wraper]

Optocoupler in cheap PSU is liteon LTV817B http://www.us.liteon.com/downloads/LTV-817-827-847.PDF

[mariush]

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.

Samxon 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.

[wraper]

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.

Samxon 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.

I know that they are fixed, GF are fixed too AFAIK, they are still in production.

[gxti]

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?

[mux]

(...)

Did you even consider the power supply is old?

I.. eh... what? How does this have to do with anything?

[rs20]

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?

[Neilm]

What struck me is how bright and shiney the soldering is.

This is a medical device. You are allowed to use leaded solder if you need reliability.

[thm_w]

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?

Yeah.
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

[batterypowerguy]

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.

[EEVblog]

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?

I 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.

[batterypowerguy]

What struck me is how bright and shiney the soldering is.

This is a medical device. You are allowed to use leaded solder if you need reliability.

not 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.

[batterypowerguy]

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.

[Circlotron]

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.

[jeremybarker]

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.

[daddario]

Can you suggest a book (mostly with tips and tricks) for designing smps?

Bit of an old thread, but one of my favourites is "Switch-Mode Power Supplies: SPICE Simulations and Practical Designs" by Christophe Basso.

[ErikTheNorwegian]

Can you suggest a book (mostly with tips and tricks) for designing smps?

Bit of an old thread, but one of my favourites is "Switch-Mode Power Supplies: SPICE Simulations and Practical Designs" by Christophe Basso.

That'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