alt Hacker NewsHuh, I figured there had to be AC in there somewhere because otherwise where is the frequency coming from? Or is there a conversion to AC somewhere in the middle of a DC-DC conversion? I had assumed you could achieve it with just resistors. I am ignorant about the physical nature of electronics.
The AC is 50/60Hz. You wouldn't hear that.
What happens is that the AC is rectified to DC and then it is chopped up at many kHz (10-100). The idea being that the higher the frequency, the smaller the inductors, capacitors and transformers can be.
Even if the switching frequency is ultrasonic, it can still excite audible mechanical effects in windings of transformers and other elements to create the canonical coil whine. This effect depends on certain ranges of duty cycle (system load) creating periodic spikes in power.