Prologue

In our homes, and in general, we use alternating current (AC). When power grids were first built, engineers needed a way to easily change electrical voltage to send power over long distances. This is done using transformers, which is much easier and more cost-effective with AC than with direct current (DC). Doing this with DC at the time would have been prohibitively expensive.

However, AC is not a good choice for powering most modern electronics. Components like microchips require power flowing steadily in one direction (DC) to function correctly, rather than constantly alternating back and forth.

The solution

To solve the problem outlined above, we use power supplies. Power supplies convert alternating current (AC) to direct current (DC) and step down the voltage to a safe level.

Using the formula $\text{Watts} = \text{Volts} \times \text{Amps}$, we can understand how much power a supply can deliver. Because the voltage output is fixed, the total wattage is determined by the current (Amperage). The higher the maximum Amperage a power supply can handle, the more current it can safely deliver, resulting in a higher wattage rating.

The efficiency problem

Converting alternating current (AC) to direct current (DC) is not lossless. Measuring efficiency is simple; we can use the following formula:

$\text{Efficiency} = \left(\frac{\text{DC}}{\text{AC}}\right) \times 100$

The lost power goes into heat.

Most power supplies have around 80% efficiency, meaning to deliver 400W to the components, it must pull 500W from the outlet. However, efficiency is tricky because in normal operating mode (PWM) it is not constant. It is proportionate to the output current. If the output current is low, the efficiency is low. To deliver good efficiency under low loads, power supplies can operate in PFM mode.

Parameter	PWM operation	PFM operation
Light-load efficiency	Low	High
Switching ripple voltage	Small	Large
Transient response	Fast	Slow
EMI base frequency	Fixed	Varying

Noise and audible frequency

Pulse-frequency modulation works by changing the frequency based on the needed current. Pulse-width modulation works by changing the width of the pulse but keeps the frequency constant. The culprit is that, to achieve the efficiency needed under very low loads, the switching frequency in PFM mode can drop down into the human audible range (below 20 kHz).

Why does it go away during high load?

During higher loads, power supplies switch from PFM to PWM mode. In this mode, the frequency stays constant and well above the ceiling of human hearing. That is why you can sometimes hear a slight chirping or buzzing when your power supply is idling, which completely disappears once you start running intensive tasks like gaming or rendering.

It is generally nothing to worry about, though in some cases it may indicate lower-quality components or looser manufacturing tolerances in the internal coils (inductors) that vibrate to create the sound.