Best way to shield surface mount transformers
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From: Nemo on 15 Jul 2010 17:12 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 -- Nemo |