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Old 22nd December 2005, 10:44 AM   #21
Eva is offline Eva  Spain
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I've found most of the (foregin) academical papers that I have read very useful. However, the fact that I wanted to point out is that in Spanish univestities nobody knows anything about SMPS. For example: One of my friends is studying Telecomunications Engineering (sound and image branch) in Madrid (we don't have such a thing as EE studies here) and his electronics teacher is very interested in seeing (and understanding) a simple push-pull car amplifier SMPS that I helped my friend to build some years ago...

Currently, SMPS is some kind of vodoo for Spanish electronics teachers (and concerning audio amplifiers, they are still teaching 1960's single transistor circuits, my friend is finishing his studies yet he doesn't know what a long tailed pair is...).

Concerning this Spanish paper, I'm almost sure that it was not written by the person that developed that SMPS but by somebody else that had only a vague understanding about it and was provided with some text, graphs and a photo of the prototype...
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Old 22nd December 2005, 11:56 AM   #22
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Quote:
Also, the converter is drawing pulsed current from mains line at high frequencies, but not a single word is said about that and its consequences.
Maybe I'm reading this wrong, but I thought they admitted to a power factor of .346 and a harmonic distortion of 271%.

In any case, someone should write to the editor to point out the defficiencies. (and to find out if it was ever published in print)

That's how the scientific community works, doesn't it? Someone writes a paper, others challenge it. If it remains unchallenged, someone might think it's un-challegeable.

Joseph
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Old 22nd December 2005, 01:18 PM   #23
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Default Re: Push-Pull converter with a gapped transformer ??

Quote:
Originally posted by Joseph Hynes
Problem is, the cores are assembled so sloppy, that glue and varnish got in-between the core halves, creating a gap. I've sacrificed one trafo, by soaking it in paint stripper overnight, to disassemble it.
Most of these transformers can be dissembled by placing in a moderately hot oven for an hour or so -- use your wive's oven mits to dissemble then buy her new ones for Christmas --

National Semiconductor posted a link to their OnLine seminars -- you have to register on their site to download -- Sanjay Maniktala gave an excellent presentation on magnetics about a year ago and there is an accompanying powerpoint or adobe presentation -- http://www.national.com/onlineseminar/
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Old 22nd December 2005, 02:26 PM   #24
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Most of these transformers can be dissembled by placing in a moderately hot oven for an hour or so -- use your wive's oven mits to dissemble then buy her new ones for Christmas --
You have missed a greater conundrum:
getting wife's permission to use oven for non-food purpose.
I'll have to buy her a new oven, not new mitts after that.

In any case, what is moderately hot? --hot enough that it will melt the bobbin as well ?(doesn't take much) some bobbins are made of bakelite , but this one looks like a softie --nylon or polypropylene or something.
Then we're back to the same problem --saving the bobbin.
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Old 22nd December 2005, 02:39 PM   #25
poobah is offline poobah  United States
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Joe,

Don't ask until the deed is done. It is easier to ask for foregivness than permission.

Before you tear those trannies apart... what is your efficiency of these converters as they are now?
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Old 22nd December 2005, 04:05 PM   #26
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Joseph Hynes


You have missed a greater conundrum:
getting wife's permission to use oven for non-food purpose.
I'll have to buy her a new oven, not new mitts after that.

In any case, what is moderately hot? --hot enough that it will melt the bobbin as well ?(doesn't take much) some bobbins are made of bakelite , but this one looks like a softie --nylon or polypropylene or something.
Then we're back to the same problem --saving the bobbin.
I guess that most bobbins can take 150c , better ones even 180c
Try with one. Again
If you have variable temperature controlled soldering iron with scale down to 150c it is quick way to check heat resistance.
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Old 22nd December 2005, 05:18 PM   #27
Eva is offline Eva  Spain
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The cores from the low-cost SMPS transformers that I have dismantled over time were all glued with the own varnish. It appears to start softening above 70șC or so... But I don't even use an oven for that purpose, I just place them over an old electric cooktop (plain heater, non-induction) at minimum power for a few minutes and then the cores are easily splitted with just a knife and a constant mild force (and a lot of care because the thing will be quite hot!). Obviously the core has to be in contact with the (flat) cooktop for proper heat transfer.

There are also (not-so-low-cost) transformers whose cores are joint with some dedicated glue. I haven't been able to split these yet.
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Old 22nd December 2005, 06:41 PM   #28
mzzj is offline mzzj  Finland
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I separated all of my collected cores now..
20-30mins at 175c and most of them were easy to separate. Bobbins are bit soft at this temp but far a way from melting.

Ones made by Coilcraft had better glue, needed 225c to separate them.

Superb easy method.

Edit: Broke one small etd core, but it was probably already damaged before process as it came off with bare hands without leverage help from knife. Still usefull for inductors at least.
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Old 23rd December 2005, 03:01 AM   #29
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I'm not splitting the rest of the trannies.

Now that Eva suggested some of the gap energy may be recovered in the secondary, I have renewed faith in them.

As for efficiency, It seems to be about 90%.

When delivering 50W, all semiconductors get just hot enough to keep my hand on them for about 2-3 seconds, not more. I don't think I need a heat sink at this power.
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Old 24th December 2005, 12:09 PM   #30
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Quote:
Originally posted by Eva
I've found most of the (foregin) academical papers that I have read very useful. However, the fact that I wanted to point out is that in Spanish univestities nobody knows anything about SMPS.
...you are judging quite hard...

Quote:
Originally posted by Eva

For example: One of my friends is studying Telecomunications Engineering (sound and image branch) in Madrid (we don't have such a thing as EE studies here) and his electronics teacher is very interested in seeing (and understanding) a simple push-pull car amplifier SMPS that I helped my friend to build some years ago...
I think it is good if also teachers are interessted to learn more.
...even more brave if somebody who feels, that he should already know this earlier, is trying to learn instead of hiding the lack of knowledge..


Quote:
Originally posted by Eva

Concerning this Spanish paper, I'm almost sure that it was not written by the person that developed that SMPS but by somebody else that had only a vague understanding about it and was provided with some text, graphs and a photo of the prototype...
May be.
I would also not cry "Heureca, that's it !", when seeing that paper.
But I think it could work somehow. From first glance I could imagine that they simply dump the stored magentic energy in the RCD snubber... not nice, but possible without additional demag winding.
Anyway, I feel this circuit will be difficult for universal input range and for partial load conditions...



Quote:
Originally posted by Joseph Hynes

Maybe I'm reading this wrong, but I thought they admitted to a power factor of .346 and a harmonic distortion of 271%.
Your values are the numbers that are stated in the paper to match the class A acc. EN61000-3-2. Class A is the most relaxed class of that standard. Please be aware that even the simple non PFC designs of standard audio amps (transformer, rectifier, fat electrolytic caps) show a power factor around 0.5 and harmonics about 150%.
The circuit in the paper provides better properties.
Their screen shots/diagrams look more like a power factor around 0.8...0.9 and harmonics around 50%.

There are not so many single stage configurations that can provide PFC, voltage regulation and line isolation. Most popular you could use a PFC-flyback (which I would prefer for small power, where the leakage inductance does not cause unsolvable issues).

Other traditional designs like boost PFC followed by a halfbridge forward converter, show good performance, but much higher material costs and component count.

P.S:
NO ! I do not know the author, nor the university of this paper.
I simply feel, that they are not so unbelievable silly as it sounded in this thread here.

BTW:
Merry X-mas 2UAll !
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