@jjlarkin: "Has anybody made RF transmission-line transformers out of multilayer
PC boards? I've seen switching magnetics integrated into a board, with
traces for the windings and a few strategic holes for mounting an
e-core.
John"
    @OkkimAtnarivik: "<quote>
   I have designed one on an integrated chip, which is more or
less the same thing, just in a smaller scale
   Regards,
            Mikko"
    @rabbitsfriendsandrelations: "<quote>
Philips wrote an app note about using windings in FR4 for a small planar
power transformer. It's probably on the Ferroxcube site now ( they've taken
on the former Philips ferrite lines ). Includes example pcb foil layout.
Graham"
    @my_name: "<quote>
Hi Joerg,
I've built quite a few planar converters, using a combination of clips 
and glue. My last dc-dc converter had a planar xfmr and a planar coupled 
inductor, and we've made about 30,000 so far. Some of these get carted 
about on the back of large trucks )in theory with air riders, in 
practice without), and while there have been problems, the cores have 
been OK for 5 years now. Some of the thermosetting epoxies I tested were 
far stronger than the core, and the operating temperature rise hardens them.
I prefer thermosetting epoxy to glue the cores together, and some form 
of CTE-, tolerance- and vibration-absorbing structure to hold it to the 
PCB. its usually a good idea to space the core slightly away from both 
sides of the PCB, because the permittivity of MnZn ferrite is so high
Of course the production guys then changed to a rather crappy glue that 
softens when it gets hot, because it was easier to apply. It remains to 
be seen if this was a wise idea (they no longer do it), but I suspect not :)
Hi John,
I've seen some papers out of South Africe detailing integrated magnetic 
structures, where the ferrite material is formed around the windings. J 
D Van Wyk springs to mind, but IMBW.
There have also been a bunch from IIRC Korea about coreless transformers 
(spiral windings on FR4) and various uses as gatedrives etc. And some 
spinoff papers looking at the effect of placing Cu and/or ferrite plates 
on the outside. air core gatedrive transformers sound like a great way 
to create mysterious blowups, along with maximising EMI emissions and 
susceptibility.
any transformer you can make with an E-core, you can make with a planar 
core (E-E or E-I). All they have done is reduce the height and increase 
the thickness of the cores. Glued cores can have a lot less permeability 
than a single piece of magnetic material (eg toroid, n-holer), unless 
they are squeezed very close together. several tens of microns can have 
a noticeable effect. very high creepage and clearance can be done, but 
is a pain (buried windings, coatings of gunk, fancy bobbins etc)
Cheers
Terry"
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