Защо някои AC адаптери и захранващи устройства правят whining шум?

I have many different AC adapters and power supplies for a variety of devices, ranging from small 5V/1A USB chargers to laptop power adapters and desktop PSUs. However, I often hear a whining noise from some of these power supplies. This happens most often when they are not connected to a device or otherwise in use, and stop making noise when I connect a device that is not fully charged.

Why do some AC adapters and power supplies make this whining noise? Why do some not make this noise? Is there anything that I can do to suppress it?

Most power conversion devices contains coils, such as transformers or inductors. These components use electromagnetism to convert AC main power to low-voltage DC power. The varying magnetic fields generated by these components can cause them to physically vibrate at high frequency, resulting in a high-pitched noise.

Most modern AC adapters are switched-mode power supplies. The internal switching frequency of an SMPS is typically low when unloaded and increases with a load (up to a certain point depending on the design). The no-load frequency is often low enough to be within the human hearing range.

In addition, in low or no-load situations the PWM (Pulse Width Modulation) used to regulate voltage at the inverter stage will be at a low duty cycle and create a “spikey” output profile which is more prone to causing vibration in coils, and the transformer itself will tend to vibrate as well. Together, these can lead to audible noise especially in cheaper units which fail to suppress this noise.

Under a load, a properly functioning SMPS should operate at a frequency well above the human hearing range, typically 50 KHz or higher (although some older designs operate at 33 kHz). However, the same noise can occur under a load with a poorly designed or defective power supply as the coils may vibrate under electrical stress at a sub-harmonic frequency.

This is why you sometimes see a “glue” of sorts on coils inside electronic devices. The glue helps reduce the vibration and noise the coils generate during normal operation. Of course, this means that a user can apply glue onto coils using a glue gun to suppress coil whine-and yes, people have done this successfully with PC motherboards, graphics cards, and power supplies. However, you generally cannot do this easily on small wall chargers of the sort you mentioned without risking damage to the charger or exposure to potentially dangerous voltages.

More information on coil noise can be found in this Wikipedia article.

A coil of copper wire without any “core” metal plates can be made to whine. As the magnetic field increases and decreases (generally about a thousand times a second), the force of the field causes the dimensions of the coil to change slightly, and this vibration leads to a whining sound. Even the wires on a printed circuit board can whine (slightly) under the right circumstances.