N
Nemo
- Jan 1, 1970
- 0
I designed an isolated switchmode supply using an off-the-shelf surface
mount transformer which is wound on an E shaped core with pins down 2
opposing sides. It works OK, but you can reduce noise by adding a
grounded shield - for my prototype I made this from copper tape,
wrapping around the transformer and looping round the sides which have
no pins. It struck me, on the next PCB iteration I could make this a bit
easier to manufacture by making a pad of copper under it to act as the
length of "copper tape" which goes round the underside of the
transformer. (Production quantities will be very low, maybe a dozen a
year, so getting a custom transformer made wouldn't be cost-effective.
And by making part of the "tape" a "track", I can recover from problems
where a subcontracter delivers an already-mounted transformer he forgot
to customise.)
So, to the question. It is a multilayer PCB. My first thought was, put
this shield "track" on the surface immediately under the transformer,
and run the tracks from the transformer pins - which loop from one to
another under the transformer - on the next layer down. But then I
thought, is it better to put the looping tracks between pins on that top
layer, and bury the shield under them? That way, the shield goes round
the outside of all the "windings":
r-----------------------¬ <-- shield (tape) above xformer
| 888888888 | <-- xformer windings
| | | | | | | | <-- xformer pins
| ~ ~ ~ ~ ~ ~ | <-- tracks linking pins
L-----------------------J <-- buried shield layer
Thing is - my intuition is, this is a good idea; but magnetics are
sometimes counterintuitive. Is running the shield outside all the tracks
& windings equivalent to a shorted turn on a toroid or something?
TIA
mount transformer which is wound on an E shaped core with pins down 2
opposing sides. It works OK, but you can reduce noise by adding a
grounded shield - for my prototype I made this from copper tape,
wrapping around the transformer and looping round the sides which have
no pins. It struck me, on the next PCB iteration I could make this a bit
easier to manufacture by making a pad of copper under it to act as the
length of "copper tape" which goes round the underside of the
transformer. (Production quantities will be very low, maybe a dozen a
year, so getting a custom transformer made wouldn't be cost-effective.
And by making part of the "tape" a "track", I can recover from problems
where a subcontracter delivers an already-mounted transformer he forgot
to customise.)
So, to the question. It is a multilayer PCB. My first thought was, put
this shield "track" on the surface immediately under the transformer,
and run the tracks from the transformer pins - which loop from one to
another under the transformer - on the next layer down. But then I
thought, is it better to put the looping tracks between pins on that top
layer, and bury the shield under them? That way, the shield goes round
the outside of all the "windings":
r-----------------------¬ <-- shield (tape) above xformer
| 888888888 | <-- xformer windings
| | | | | | | | <-- xformer pins
| ~ ~ ~ ~ ~ ~ | <-- tracks linking pins
L-----------------------J <-- buried shield layer
Thing is - my intuition is, this is a good idea; but magnetics are
sometimes counterintuitive. Is running the shield outside all the tracks
& windings equivalent to a shorted turn on a toroid or something?
TIA
