How we fight coil whine in the RM Series power supplies

By Jonny Gerow, on September 12, 2013

Over the past few years, Corsair has refined its ZeroRPM fan technology. Our power supply fans remain silent until 20%, 40% and even 60% load. We've developed quieter fans with quieter fan motors and fan blades designed specifically for moving air through the crowded PCB of a power supply. But we haven't forgotten that there are power supply noises that come from other than from the fan, and in this article I will detail some of the additional steps we have taken to limit PSU noise in our new RM Series power supplies.



What is that noise coming from my power supply?

Some power supplies exhibit noises that come from components other than the intake fan. These noises can sound like hissing, buzzing or a high pitched whine. These noises can come and go and can be difficult to troubleshot for a couple of reasons. One reason is that the noise can change, or be completely absent, depending on the load on the power supply. I'll get into more detail about that in a little bit, but what this does mean is that a PSU that makes a noise in one PC during one game or benchmark program may not make the same noise in another... or may make no noise at all. The other reason is that often these noises can be at such a high frequency that only some people are actually able to hear them. Many people, especially those over 25 years of age, cannot hear noises over 15kHz, while others can hear noises all of the way up to 20kHz. So you can have a PC sitting in a room, with a power supply that's making a high pitched squealing noise, and if two people are sitting in the same room; one of them may be able to hear the noise while the other one hears nothing at all.

Is this what people call "coil whine"?

Often, people do call these electrical, non-fan related noises "coil whine". This is because a lot of times this noise is coming from coils. All a coil is comprised of is a core with a copper wire wound around it. In a power supply circuit, coils are used to eliminate left over AC noise from the DC output. But the frequency of this AC current can change with load and at some frequencies, a mechanical resonance may occur that produces a high frequency noise. This noise in coils is often addressed by using better quality coils with tighter windings, and coating the windings with a varnish.

So these noises always come from coils?

"Coil whine" has become a generic term for audible electrical noises. The fact of the matter is, there's a very good chance that these types of noises aren't coming from a coil at all. One component, similar to a coil in construction, is the transformer. A power supply can have several of these and the windings can vibrate under certain frequencies just like they can with any coil. Again, an attempt is made to wind the wire tighter around the transformers core and use varnish and even tape to help isolate any vibration and potential noise.



Above we have a picture of the main transformer and the +5VSB transformer out of our new RM Series power supplies. If we cut all of that tape off...



You can see we have a copper wire wound around a central core (I got a little aggressive with the knife and ended up cutting some of the copper windings, but there was a lot of tape wrapped around there!).

What we've started to do at Corsair is carefully vet our manufacturer of transformers to not only meet performance criteria, but also to meet the criteria we know will give our customers zero noise in their power supply.

To make sure our standard for low noise transformers is maintained, we put in place additional quality control checks in the assembly process. For example: a transformer that doesn't have its windings wound as tight as another transformer is going to show a slightly larger diameter. So using a micrometer, as shown below, we can tell if a transformer meets our requirements.



The transformer on the left is measuring a diameter of 32.8mm, which is good. But the one on the right is measuring 34.08mm despite the same length of copper wire is wound around the core. This means the windings are not as tight as they could be, the transformer could potentially make a noise, and is therefore rejected.

Another source of so-called "coil whine" is actually from capacitors. While many people think that a capacitor that's making noise is a leaking capacitor, the truth is that a capacitor can make a noise similar to "coil whine" for the same reason a coil or transformer will make these noises. The inside of a capacitor is made up of a dielectric material and a metallic film. These two thin layers are rolled up to form the capacitor. Once again, tolerances are important. If there is any non-uniformity in the coil of two layers, a mechanical resonance can occur that produces a high pitched noise.

Here are a handful of capacitors used on the primary side of different RM Series power supplies.



We've found that, under certain circumstances, these types of capacitors can make more noise than any others. Why?

Aluminum electrolytic capacitors, the caps that look like little cans, are rolled up with an electrolyte soaked paper. That paper can actually absorb the mechanical resonance that can create noise. But the capacitors shown above use a metalized polypropylene as a dielectric. This "plastic film" can vibrate with certain frequencies, just like our copper windings on our coils and transformers, and produce a high pitched noise.

Have a look at this capacitor I've opened up and unrolled:



You can probably imagine that if these layers of film aren't rolled up perfectly tight, they can hum like a kazoo. Here too, Corsair has taken extra measures to make sure these capacitors do not make any noise by implementing additional quality control measures when sourcing components to build our power supplies.

As you can see, Corsair's focus on bringing the end user the quietest product possible goes far beyond developing a quieter fan.  And because of Corsair's in house engineering capabilities, we're able to address these issues and provide the end user the quietest power supply product available.