Electronic – How to source an Audibly noiseless (< 3dBA) power supply

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This is not a shopping question; this is a "what is the best way to solve this problem" question.

To power a medium-size project, I'm looking for a 15W/12V board mount power supply, taking mains AC in (auto-switching voltage is nice, although US-only would be acceptable) and providing reasonably-regulated 12VDC out.

I first tried the cheapest thing I could find on DigiKey with parametric search: a 35W Delta PSU, cageless (plain board.) Unfortunately, it had quite significant coil noise, which is right out for this application — the device will be in a bedroom and ears are very picky about coil whine. As clarified in the comments, I'm looking for something as low as 3 dBA audible noise at 1 meter.

Second, I tried a more expensive model, with a perforated case: a 25W TDK/Lambda. It, too, has coil noise, although less of it. But, this is still too much.

I understand why the coil noise happens: These switching power supplies regulate the voltage by sending a signal all the way to the mains side, using a flyback regulator.
But, this is not acceptable to me.

Unfortunately, I have not found any parametric search (digi-key, mouser, farnell, jameco) that rates board mount power supplies on noise. And they don't even advertise the topology used — a transformer-based down converter with buck topology on the DC side would probably be fine for my needs, for example.

So, other than buying one of each of everything in stock, which I can't really afford to do, how can I find a power supply that doesn't make any noise? If you had this design requirement, how would you solve it?

Best Answer

Firstly, why <3 dB? This is a ridiculously strict requirement for general use, so do you have a specific need for zero noise?

This aside, the lowest-noise power supplies are linear. This is a basic circuit that pretty much looks like:

transformer -> rectifier -> smoothing capacitors -> regulator -> more smoothing capacitors

See this article for more info.

You can buy these off the shelf. However, they are declining in popularity as switchmode supplies are more compact and efficient. I would just build one yourself, as this way you can find your own specific requirements and parts. For part selection, keep in mind that:

  • Toroidal transformers tend to have the lowest noise
  • The regulator is specific to your voltage, power requirements and features (e.g. variable)
  • These are the only components you really need to choose based on noise, as capacitors and diodes are silent!
  • As @AndrejaKo quite rightly pointed out some capacitors (e.g. ceramics) are indeed not silent, so you should probably steer clear of them. Thanks AndrejaKo!

Look online for example circuits, calculators, and tutorials.

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