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30W offline isolated flyback and use of shield windings.

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ocset:
We all know that with an offline 30-60W, isolated Flyback SMPS project, for the first prototype, the conducted EMC scan often shows a fail at the 28-30MHz point. As we know, this is down to common mode emissions.
(this fail is often there, even with meticulous PCB layout, and correct use of a Y capacitor across the isolation barrier.)

Often we initially try and get away with just one common mode choke, and no shield windings in the transformer. (also, we don’t like to snub our secondary diode too heavily, as this needs bigger snubber resistors. Also, we don’t like to increase the fet gate resistor too much, because although this conveniently reduces drain voltage dv/dt, it also increases switching losses)

So, in many cases, the transformer shield windings, or the second common mode choke with a higher SRF, is needed to clinch the conducted EMC pass and solve the 28-30MHz conducted problem.
You almost always find that people elect to solve it with shield windings in the transformer rather than with a second, smaller common mode choke. ..
Why is this?
..especially since offline flybacks of 30-60w are almost inevitably sandwich wound, so two shield windings are unfortunately needed in order to fully shield the primary from the secondary.

So why do people always elect for shield windings rather than a second common mode choke with a higher SRF?

(After all, shield windings can often mean having to use a bigger, more expensive transformer, in order to fit the shield windings on to the former.)

T3sl4co1l:
The use of alleged certainty is amusing. :)

Fortunately, we all know why this is; how has it come to be that you need to ask?  Who neglected to tell out about shield windings and common mode chokes?

Tim

MagicSmoker:
Don't assume everyone reading this is an expert on switchers, and the use of first person plural ("as we know," etc) is also annoying.

That said, switchers fail EMC testing at a variety of frequencies and for a variety of reasons and shield windings really shouldn't increase the size of the transformer so much that the next size up is needed. After all, good practice is to oversize the core a bit, anyway, to minimize flux swing and winding layer count, both of which serve to minimize losses (and temperature rise).

In contrast, employing a second common mode choke - which, btw, I have never seen in a commercial switcher - takes up more board space and usually ends up costing more than the shield winding, too.

Controlling the ringing across the secondary diode and switch during turn-off with properly sized RC dampers (rather than just winging it with a generic 10R + 100pF) helps more than anything else, though.

Yansi:

--- Quote from: MagicSmoker on April 08, 2019, 09:45:12 am ---That said, switchers fail EMC testing at a variety of frequencies and for a variety of reasons and shield windings really shouldn't increase the size of the transformer so much that the next size up is needed. After all, good practice is to oversize the core a bit, anyway, to minimize flux swing and winding layer count, both of which serve to minimize losses (and temperature rise).

In contrast, employing a second common mode choke - which, btw, I have never seen in a commercial switcher - takes up more board space and usually ends up costing more than the shield winding, too.

Controlling the ringing across the secondary diode and switch during turn-off with properly sized RC dampers (rather than just winging it with a generic 10R + 100pF) helps more than anything else, though.

--- End quote ---

But the oversized transformer, second common-mode choke and properly engineered big RC dampers cost money. Yo boss wont like that as tell you to cut corners, as is the typical mode of operation of most corpos.

They will probably tell you t otry downsizing the transformer a bit, so it barely works.

MagicSmoker:

--- Quote from: Yansi on April 08, 2019, 09:51:17 am ---But the oversized transformer, second common-mode choke and properly engineered big RC dampers cost money. Yo boss wont like that as tell you to cut corners, as is the typical mode of operation of most corpos.

They will probably tell you t otry downsizing the transformer a bit, so it barely works.

--- End quote ---

I've been self-employed for most of the last 20+ years so if the pointy-haired boss cuts corners I have only myself to blame...  :P

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