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30W offline isolated flyback and use of shield windings.
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T3sl4co1l:
The flyback probably isn't meeting its isolation or spacing requirement then.

You can't have both!

Tim
ocset:
Thanks, yes i tend to agree...but i think i subtend from your reply, that in fact, the non isolated flyabck with contiguos ground plane, has much the same emc signature as the buckboost.....(ok a little worse as the flyback has two switching nodes...maybe i should have compared non isolated flyback with uncoupled inductor sepic.)

..Lets suppose I don’t have isolation and spaceing requirement with the  flyback, and just want it because I get LED load referenced to ground, and low side fet drive...isolation not needed.
T3sl4co1l:

--- Quote from: treez on April 19, 2019, 01:32:51 pm ---Thanks, yes i tend to agree...but i think i subtend from your reply, that in fact, the non isolated flyabck with contiguos ground plane, has much the same emc signature as the buckboost.....(ok a little worse as the flyback has two switching nodes...maybe i should have compared non isolated flyback with uncoupled inductor sepic.)
--- End quote ---

Actually, SEPIC with coupled inductors and no capacitor would be almost spot on.  The remaining difference is what ratio if any the flyback has.  Since, if you need a 1:1 ratio, you probably wouldn't choose flyback.

Otherwise, that's not a bad analogy. :)



--- Quote ---..Lets suppose I don’t have isolation and spaceing requirement with the  flyback, and just want it because I get LED load referenced to ground, and low side fet drive...isolation not needed.

--- End quote ---

(Which, speaking of, you also get with SEPIC.  Again, assuming 1:1 ratio in both cases.)

So what?  Now that you've crafted a situation that is different from your premises, should it be surprising that its response also differs?

And this is still ignoring layout, and mechanical constraints, and so on.  The more you transform a circuit, the more those constraints will also change.  What are the chances that any particular embodiment of the isolated flyback topology, will have equivalent layout, mechanics and emissions to a SEPIC or buck-boost of some other particular embodiment?

You could create a series of converters, of each of the major topologies, crafted in such a way that their layouts and emissions are also equivalent.  But this would be a rather verbose way to create supplies for practical use.  Real practical circuits will differ on component choice and mechanical constraints, leading to very different layouts.  (In effect: the ideal layout, for a given set of components, connectivity and electrical and mechanical constraints, is highly nonlinear with respect to the topology, or the input and output ratings.)

Tim
chris_leyson:
I have two flyback transformers, transformer A and transformer B. Transformer B is a clone of transformer A and has the same winding stackup and number of turns but the core and bobbin are slightly larger. Both are wound interleaved with a split primary. Transformer A passes conducted emissions but transformer B fails. Long story short, I needed a sample core and bobbin, transformer B style, so I resorted to taking one of the samples apart. Iransformer A, has single wire primaries wound on a single layer whereas transformer B uses two wires and is wound on two layers. Primary to secondary HF coupling is going to change because of the different winding geometry.
Zero999:
I take it this is for street lighting? Are the EMC requirements more strict than for consumer equipment?

Does the secondary side really have to be SELV? If this is going to be used in a fully insulated luminaire, then I would've thought isolation is unnecessary?

I admit, I've never designed an offline SMPS before, since I always opted for off the shelf, because it normally works out more economical. Nowadays it's generally pointless for one to design their own offline SMPS. Any odd voltages can be generated using DC:DC converters on the secondary side.

I'm curious. I've never heard of a transformer screen, being referred to as a shield winding before. I'm guessing it's simply a piece of copper tape, with insulation at one end to avoid creating a shorted turn?

I've only seen screens on mains transformers before, but it doesn't surprise me they're used on some more high end switched mode supplies.

In an SMPS, is the shield connected to mains earth, or the rectified DC on the primary side?

I imagine, in order to satisfy the safety requirements and pass the hi-pot test, the secondary will need a lot of insulation between it and the secondary, if it's connected to the primary side. Can it be coupled via a suitable class Y capacitor so it doesn't have a direct connection to the mains and therefore less stringent insulation requirements?

Anyway, sorry for derailing the thread with noobie questions. To answer the question. I've had issues with an SMPS failing EMC before. It was a small Traco  DC:DC converter module, used in something to be installed in a military vehicle. From memory (this was about 15 years ago) it emitted in the high HF/low VHF range, similar to the OP's experience and it was resolved by adding a common mode choke, ferrite beads and some capacitors to the input side.
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