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Well, it's taken me weeks to think of a new absurd question but here I am at last. You're welcome!

I'm looking at using a small, adjustable, switching buck regulator (likely a LM2574) for a very basic supply:

120V AC in --> stepdown transformer --> bridge rectifier --> switching regulator.

This will be used in a commercial product. I've amazingly not designed with one of these before. While I don't think for this product we need to technically be concerned about EMI, I'm curious if a little bitty switching regulator like this generates all that much noise? I'd rather not add cost with upstream filtering if it's not needed.

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    \$\begingroup\$ Your task is to get it to pass CE/FCC/TUV etc qualification safety, noise immunity and emission requirements. This is not a task for noobies. a line filter to suppress emissions and lightning transients is necessary among many functional parameters. \$\endgroup\$ – Tony Stewart EE75 Jun 12 '19 at 23:15
  • \$\begingroup\$ You’re welcome. Next question? \$\endgroup\$ – Tony Stewart EE75 Jun 12 '19 at 23:53
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While I don't think for this product we need to technically be concerned about EMI, I'm curious if a little bitty switching regulator like this generates all that much noise?

Yes, it does, or it can. And if this commercial product is going to be sold, it will most likely need to conform to IEC 61010 and pass the FCC unintentional radiators testing at an ETL. (all of which costs 5k to 10k $ depending on the ETL/Testing).

It doesn't matter what kind of filtering you have, as long as it passes the tests.

Switching regulators can radiate noise and conduct noise into AC mains with an improper design, a good thing to do is find a place or equipment to do precompliance testing. (the less time at an ETL, the less you get billed). Proper EMI design is a must.

For these reasons, if I am designing a commercial product in low-ish quantities, I do a cost analysis and usually I buy and off the shelf power supply to decrease testing costs.

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