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Every plug-in power supply ("wall wart") I have ever used starts off silent, but eventually starts to emit high frequency switching noise. It usually happens only when the charger is on, but not charging anything. I presume it is due to the capacitors failing, or maybe the glue around an inductor.

My question is, why do they all use switching frequencies around 18 kHz which is audible and really annoying? Why not go up to 25 kHz or even higher?

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    \$\begingroup\$ Don't worry. As you get older, you ability to hear these sounds will fade and you will be much less annoyed. \$\endgroup\$ – mkeith Apr 8 '15 at 15:39
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    \$\begingroup\$ @mkeith I think jokes do not really help here. The audio noise from ubiquitous power supplies can lead to sleeping disorder, lack of concentration and can harm the health in long term. We should take Timmmm's question serious. \$\endgroup\$ – Jonas Stein Jan 9 '18 at 17:41
  • \$\begingroup\$ if it is on and not charging anything then it should be switched off or removed from the supply... \$\endgroup\$ – Solar Mike Jan 9 '18 at 20:07
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    \$\begingroup\$ @JonasStein you seem to be in the minority as regards the joke and why "play" with such an old post... \$\endgroup\$ – Solar Mike Jan 9 '18 at 20:09
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In many cases this is a side effect of a converter that uses a variable frequency that shifts down when under light load. Unfortunately, it comes down low enough to be heard. Under normal conditions, the switching frequency should be much higher. This type of converter uses something similar to pulse density modulation to control the output voltage with a fixed pulse width, varying the spacing been the pulses to control the duty cycle.

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Most small offline SMPS use Flyback Fixed frequency peak Current mode. This approach gives a simple cheap circuit that is reliable considering the price. The switching frequencies are well above audio but there are ways that you can still get audible noise. Sub harmonic oscillations can and do occur in these types of supplies. They can be difficult to cure. It is plausible that a say 100 kHz peak current mode switcher could make a noise at 12.5 kHz or 10 kHz etc. Another possibility is an unstable feedback loop which often makes noise around the corner frequency of the output filter which could be say a twentieth of the converter frequency. The part load efficiency of these types of SMPS and some others is poor. Manufacturers do things like frequency fold back, Burst mode, Green mode, Minimum on time etc. All of these standby power saving tricks mean low frequency operation which could mean audible noise.

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The problem is that most switched-mode toplogies will enter "discontinuous mode" under light load. In this mode the duty cycle must be reduced dramatically to keep the output voltage constant.

But there is a minimum feasiable length for the pulses, so at very light load the pulse rate has to drop to maintain output regulation. This brings the switching frequency down into the audible spectrum.

Inductors and transformers aren't totally rigid and they can get less rigid with age. So they can convert the audible switching frequency into actual audio.

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    \$\begingroup\$ This does not add much compared to Alex's answer above... \$\endgroup\$ – Solar Mike Jan 9 '18 at 21:07

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