Electronics > Projects, Designs, and Technical Stuff
30W offline isolated flyback and use of shield windings.
dmills:
|O Pretty sure Pace would wind me something with a screen if I asked them for not a lot more then winding one without a screen, same goes for most of the other custom magnetics vendors. One turn of copper tape, terminated at one end is not a big deal in the scheme of things.
Besides to use a screen or not is a DESIGN decision, you make it based on what the best way to get the performance you require is, I don't see a big conspiracy here, interwinding screens are just another tool in the designers toolkit.
Custom magnetics are one of those things that get surprisingly cheap even at only a few hundred pieces, they are well within reach for small volume manufacturing.
73 Dan.
ocset:
Thanks, presumably if you wind an offline, isolated Flyback SMPS with a screen in it, and then you do two conducted EMC scans, one with the secondary fully isolated from the primary, and the other scan with a small shorting wire shorting the primary and secondary together so its non-isolated…….then if the scans are the same, then there was no need to actually use the screen. Would you agree?
(since in a non isolated Flyback SMPS, the capacitive coupling to the secondary doesnt matter too much because the primary and secondary grounds are shorted together, and any noise coupled to the secondary will just get shorted back to the primary anyway)
chris_leyson:
@dmills Thanks Dan, I was considering looking through the Radio Designers Handbook, it's within easy reach but it was getting late. Been going through some of the patents and tracing references back to "prior art" but to be honest I'm putting "shield windings" on the back burner for now as there are a lot of other subtle things that creep in like winding geometry when it comes to common mode currents.
There is an interesting tear down of an Apple iPhone charger on Ken Shirriff's Blog, http://www.righto.com/2012/05/apple-iphone-charger-teardown-quality.html. If you look at the transformer tear down the auxiliary winding is HUGE given that it only needs to supply a few tens of mW. One side of the auxiliary winding is "grounded" to the primary smoothing cap so it's a good example of a primary side shield winding. :-+
dmills:
Yep, transformers a subtle. Lots of fiddly details and they all matter at least some of the time.
Treez,
I would not try to extrapolate from a single design, the parasitics are just too much of a factor, and seriously, the experiments are not that hard to do. A few days with some bobbins, a turns counter, and a collection of suitable wire, tapes and core halves is an education well worth taking the time to acquire if you want to play in this space.
T3sl4co1l:
--- Quote from: treez on April 17, 2019, 06:34:57 pm ---Thanks, presumably if you wind an offline, isolated Flyback SMPS with a screen in it, and then you do two conducted EMC scans, one with the secondary fully isolated from the primary, and the other scan with a small shorting wire shorting the primary and secondary together so its non-isolated…….then if the scans are the same, then there was no need to actually use the screen. Would you agree?
--- End quote ---
I wouldn't. :)
The screen shunts the capacitance to the gradient of voltage along the primary winding. Shorting the common grounds with a wire doesn't do quite the same thing. The gradient gets induced onto the secondary, perhaps increasing the RF ripple (differential) aspect. The ground wire size and length may not be the same, and the capacitances loading them down are not, either.
A non-isolated converter is a much easier thing in general, though. I would not generally expect to need a shield in such a beast. But a LinkSwitch (or equivalent), for example, is an even more compact solution than a flyback.
Tim
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