More History

Bulky, linear units were the standard until around 30 years ago, when engineers figured out a way to dramatically reduce the weight and heat dissipation of the current crop of supplies. In the race to make electronic products smaller and smaller, physics appeared to be limiting just how tiny any device could shrink. By accepting a huge amount of noise in and frantically trying to filter out the AC (alternating current) ripple at the output, heat dissipation and weight could be reduced by as much as 70-90%.

What's AC ripple? Take the amount of "dirty" voltage, which will be in an AC form and divide that into the absolute value of the DC (direct current) voltage. That result represents a percentage or ratio of the bad stuff (AC) and the good stuff (DC). For example, say the AC hash measures 0.030 volts (30mV) and the DC component is 12 volts. Dividing 0.03 by 12 gives 0.0025 or 0.25% of the output is garbage (dirty). More importantly, this AC garbage has a frequency, sometimes in the worst case scenarios, several frequencies. Ever hear a whine when listening to your car radio that follows the engine rpm? That is usually an indication that your voltage regulators or filter capacitors/inductors are on vacation.

The basics of a power supply

No matter what country where you live, AC comes out of the wall socket. Maybe it's at 100 volts or 120 volts or 240 volts, but its AC. Here's a hand drawn picture of what AC looks like on an oscilloscope.

Actually, the quality or smoothness of your AC depends on your supplier. Some countries are well known for their "dirty" power. A DC curve isn't really a curve, just a flat line.

Some engineer, probably with a mathematics background, figured out that if you chop or slice a very small piece of that AC line, and fiddle with it, a rough approximation of DC begins to take shape. As demand for greater current (amperage) increases, it becomes more difficult to grab a small piece of relatively flat AC, so the snippet nabbed becomes even smaller. Sounds like the frequency of chopping the AC, must be increased, which is exactly what happens.

This means that there exists, inside your power supply, a low frequency generator. These modern switching supplies may not be all that low as their frequencies can top out in the Megahertz range. Back to the radio analogy. Try turning on an AM radio near your box and see what happens. You will probably hear a squeal or squawking that makes listening unbearable. Moving the antenna away from the computer reduces the noise. During high current periods, the obnoxious noises can vary as much as an octave or more.