I've seen it done for 48 -> 12 power conversion. (something like 20W, I think). It worked pretty good, except for the turns miscount on rev 0 and the itty bitty mod wires snaking under the core that resulted.
It worked because we had a muy multi-layer board in which to pack the windings -- I don't know how it would have flown if we were dealing with higher than 48V.
-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Sure looks like it here as well :-(
But then where is the value added by domestic manufacturers? Then we might as well have it produced in Asia.
-- Regards, Joerg http://www.analogconsultants.com/
It can be done but it's expensive. 8-10 layers, board becomes brittle if skinny, and so on. It does look elegant though.
And probably at a much higher frequency than 100kHz.
-- Regards, Joerg http://www.analogconsultants.com/
Don't most CRTs have flyback xfrmr? Look fer one in an itty bitty tv or o-scope. I've gotta tektonix 213 dmm w/ about a 2" CRT. The flyback hasta be pretty small. (shrug)
nb
-- "Do you recognize me? No! ...cuz I don't work here" Support labelling GMO foods http://www.nongmoproject.org/
They are used more often these days. For example, the Excelsys series of modular supplies use planar transformers with mains volage primaries. A bit pricey, but they have available good "medical" isolation (4kV) high-rel and low noise options.
Here's the output module with the planar transformer:
I have not attempted to determine how many layers are in the board.
Probably. I wasn't advocating that you go do it -- just mentioning that I have seen it done. We had some pretty severe hight constraints and a whole bunch of other conditions (like, we were going to have a bazillion layers on that board anyway, and the lower voltage) that were amenable to it.
When the Linear Tech app engineer suggested it the only thing that kept him from being thrown out of the room was his long history of being right. And dammit -- he was right!
-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
So the 13mmx13mmx6mm, 15-20W flyback transformer also requires reinforced insulation?
RL
It doesn't follow the usual standards, this is a special hi-rel app. We need certain breakdwon limits and creepage paths for everythin line-side. Don't have them in my head right now but more than 0.150" to chassis, for example. This goes into a narrow tube so that presents a real estate issue.
The core itself can be on the isolated (LV) side.
-- Regards, Joerg http://www.analogconsultants.com/
LTC guys are almost always right :-)
-- Regards, Joerg http://www.analogconsultants.com/
Not a flyback, either, I'll bet.
RL
You can see the traces, looks like a megeehoitz switcher. Here, we are talking 100kHz flyback, single-digit Dollars, way different ballgame.
-- Regards, Joerg http://www.analogconsultants.com/
Not this small.
-- Regards, Joerg http://www.analogconsultants.com/
Then the whole core becomes hot which would creat some real issues. But I am not so worried about that, there's the insulating bobbin and we could pot a lot of things.
It's a square. So technically more a "profile" rather than a tube.
RTCA/DO-160.
-- Regards, Joerg http://www.analogconsultants.com/
It would be easier to isolate only the LV winding, considering that most of the guts in the area will be hazardous.
If it's a tube, make use of ALL of the available headroom - unless it's a square tube. (if it were square fitting the board diagonally would also permit increased hight profile ..... and increased board real estate)
Care to reference the standard?
RL
The main trouble is, how many layers can you get and how narrow can you make the traces? Board fab rules really set the limit on voltage here -- easy to wind a transformer with 40AWG wire, much harder to print something equivalent. 180V, at that frequency, on a small core, is probably in the
100t range, which is an awful lot of very fine printing.I've got a design with 10W planar transformers, which works well. The outline is much larger than your requirement, voltage is low, turns count low, and frequency somewhat higher. It's only a two layer board, so the size could come down a lot.
In the same outline, I think I could push 20W, with a high voltage primary, given a 6 or 8 layer board. But the frequency would still be high, and flyback (using only half the B-H curve) would be pushing it (I'd probably do half bridge for such a circuit, at some expense in coupling/bypass caps)...
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
That standard doesn't have physical requirements for isolation barriers. This is usually covered by a different reference in spec or procurement docs for the aircraft company doing the purchasing.
If purchasing told you .15 hazardous to frame, then you'll have to count on completely wrapped/encapped sections in an isolator this small. There is no creepage distance available. Multiply-insulated wire on the secondary might be an option. I don't know how small the diameters get, but the film layers will rapidly dominate their volume.
RTCA also doesn't have 240V sources in any of it's four categories. I don't have access to the newer (2001) section (16) that covers harmonics, so this may have changed too. Were you just going to use the extra headroom to ride through the 180V surges?
Doing a rough calculation for toroids that might limbo under the 6mm headroom, it looks like you'll be wanting peak flux densities at
100KHz, in a complete energy transfer part, that exceed ferrite saturation, just to fit wire in. That's going to cook, no matter what material is used, or core shape.If you went for incomplete energy transfer with a gapped amorphous core ($$$) the gap/turn ratio can come close to 1mm/turn, with peak to average current at ~3:1 at low line, but deltaB will still be a killer. I don't think the PFC chip will go for incomplete energy transfer - looks like a critical conduction job to me and they all want CET.
There might be parts with better volumetric efficiency, but not that much better.
RL
It does specify pressure loss performance and then Paschen's law gets you.
There are many ways to maintain the insulation, for example split bobbin. That's not really my concern because stuff can be potted up, size is.
Yes, of course no 240V but we need headroom for the surges, and at the DC peaks of those.
My preference would be E-core and some newer materials start showing up as E-cores, just not in the small sizes we need. It's like going into a sporting goods store and all running shoes are 11 and up. No problem for me but my wife would not like that.
Cost is not the prime parameter here as long as it isn't outrageous. And that's what I am finding, the millisecond you want a custom size ferrite and bobbin they charge through the nose. But, starting to look overseas now. It could be like with taylored suits, something that is really only affordable in Asia. To say it politely, right now I am a bit non-plussed regarding the willingness of domestic companies to go out of their way and make it happen. This would be "the" chance for one of those to get into a cutting edge market.
If push comes to shove we could use a chip that does flyback-PFC at several hundred kHz. But I haven't found such a beast yet, probably because that usually result in an EMC nightmare.
-- Regards, Joerg http://www.analogconsultants.com/
No worries, I've done things like that before. Sometimes the envelope needs to be pushed. My motivation goes up tremendously when people around me say "It can't be done" :-)
For example, this is how an "impossible" cardiac patient interface came about that was deemed impossible without fiberoptics in the 90's. The sales guys for the super-expensive fiberoptics were miffed, of course. Passed UL and IEC601 right away but went beyond by being defibrillator-proof (I did not consider these med standards safe enough back then and would not design any other way).
They are as stated in the first post. Basically 120VAC in, LV out, up to
20W, about 100kHz, max 0.500" wide and 0.250" high.
Ferrite is ok if someone would make us a "fat" core that's long but no wider than 0.500" and 0.250" high, plus matching bobbin. Without breaking the bank, that is.
Why?
The chip size doesn't matter so much as the transformer takes the lion's share. But ... it needs a modulator for the current control in there in order to do the PFC. Ideally with sim files, that's what I like so much about LTC.
It's mostly a headache because there are no chip that can do more than
150kHz AFAICT but i haven't gone through all yet. First, I'd like to see if we can avoid that.
Susceptibility is not a problem.
I've used those a bucks but they are a stretch at 20W even in flyback configuration. Also, doing PFC with one of these isn't exactly easy.
-- Regards, Joerg http://www.analogconsultants.com/
Well, I sort of admire the relaxed attitude, but it would probably drive me crazy, if I was on the team. I like to see goals that are demonstrably possible, even if just on paper, for some good reasons.
Suggest you nail down actual requirements, if you know you're possibly testing the physical limits in the hardware being designed to do the job.
It doesn't matter what core shape you use, unless the materials exhibut remarkably lower core loss than the current leader
- ie ferrite.
Reports and specs on other materials talk about lower loss, but this is a term used relative to other powder formulations only.
In the dimension limits stated; the use of bobbins over film insulation will just make this worse.
For that matter, any converter chip designed for off-line critical conduction mode could potentially be usable, whether intended for PFC application or not. It won't cut down on real estate, unless you can find an 8-pin part that has also migrated to leadless smd, that requires less external parts to do the job.
About 100KHz operating frequency: In general, the simple CCM control is constant on-time, variable frequency, with a limited upper frequency (maximum power). Controllers likely try to limbo under
150KHz in order to avoid the CISPR/FCC pinch, making controller tolerances and transformer inductance values a critical factor in power transfer. That's going to be a headache in practice, without margins - tough to provide in limited physical/thermal circumstances.RTCA has it's own EMC requirements, with conducted emissions also speced above 150khZ. It also has conducted susceptibility requirements that go down to 10khZ, at current levels that may be significant in lower powered devices.
Although I hesitate to advise actual use of power-integrated parts, you may seem some interestink applications of 'linkswitch' parts, at your targeted power level, at Power Integrations. LNK405-409(EL).
RL
Well, if you were not prepared to explain how it can be done, you'd have run into that a lot and will likely continue to do so. It gets kind of lame after a while.
The kicker is when you're told it's a requirement; no-one has a clue how it's supposed to be achieved; your own paper points to a brick wall.....and you're the one with the job.
I prefer the incremental approach, where simple things you've found out, most likely by accident, or from working in left field, are introduced to save money, or improve performance - and there's a concensus by those expert in their own area of responsibility that it will fly. It's seldom just your own ass on the line.
Must have mis-read the voltage elsewhere. It's the 'up to' that probably needs redefinition, soonest.
Well, it's not OK, because the ideal ferrite part, fitting the space limit, won't do it.
Bobbin wall thicknesses and minimum dimensional tolerances exceed achievable film thickness, due to plastic fab restrictions. Anything that uses volume, unecessarilly, will cut down on power throughput and result in higher temp rises in the same total volume.
Have you got something besides this real estate to convey the losses to the tube walls?
This ain't necessarilly so. No disrespect to LTC.
L6562 FAN7528 MC33260
Never the less, you might find the reading informative. First google search return on LNK405.
RL
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