In this tutorial we will explain about everything you need to know about PC power supplies, including form factors, efficiency, power factor correction (PFC), rails, protections, ripple and much more. You will learn that the power supply power rating should not be the only factor to consider when buying a power supply unit.

But before going further, let’s explain exactly what a power supply does.

As an electrical device, the computer needs power in order for its components to operate properly. The device responsible for supplying power to the computer is the power supply. In a short way, we could say that the main function of the power supply is to convert alternating voltage (a.k.a. AC) which is supplied by the electrical power system into continuous voltage (a.k.a. DC). In other words, the power supply converts the conventional 110V or 220V alternating voltage into continuous voltage used by the PC electronic components, which are +3,3V, +5V, +12V and -12V (alternating voltages vary throughout the world; during this tutorial we will use “110 V” as a catchall label for 110 V, 115 V and 127 V voltages, while we will use “220 V” as a catchall label for 220 V, 230 V and 240 V voltages. Japan is the only country outside this range, using a 100 V power grid). The power supply is also present in the PC cooling process, as we will explain in details later.

There are two basic power supply designs: linear and switching-mode.

Linear power supplies work by getting the 110 V or 220 V from the power grid and lowering it to a lower value (e.g. 12 V) using a transformer. This lower voltage is still AC. Then rectification is done by a set of diodes, transforming this AC voltage into pulsating voltage. The next step is filtering, which is done by an electrolytic capacitor, transforming this pulsating voltage into almost DC. The DC obtained after the capacitor oscillates a little bit (this oscillation is called ripple), so a voltage regulating stage is necessary, done by a zener diode (frequently with the aid of a power transistor) or by a voltage regulator integrated circuit. After this stage the output is true DC voltage.

Although linear power supplies work very well for several low-power applications – cordless phones is an application that come in mind –, when high power is needed, linear power supplies can be literally very big for the task.

The size of the transformer and the capacitance (and thus the size) of the electrolytic capacitor are inversely proportional to the frequency of the input AC voltage: the lower the AC voltage frequency, the bigger the size of those components and vice-versa. Since linear power supplies still use the 60 Hz (or 50 Hz, depending on the country) frequency from the power grid – which is a very low frequency –, the transformer and the capacitor are very big.

Building a linear power supply for the PC would be insane, since it would be very big and very heavy. The solution was to use the high-frequency switching approach.

On high-frequency switching mode power supplies (a.k.a. SMPS, Switching Mode Power Supplies), the input voltage has its frequency increased before going into the transformer (in the range of KHz are typical values). With input voltage frequency increased, the transformer and the electrolytic capacitors can be very small. This is the kind of power supply used on the PC and several other electronic equipments, like DVD players. Keep in mind that “switching” is a short for “high-frequency switching”, having nothing to do whether the power supply has an on/off switch or not…

The power supply is probably the most neglected component on PC. Usually when buying a computer, we just take on account the processor type and clock, the motherboard model, the video card model, the quantity of installed memory, the hard disk storage capacity, and we forget about the power supply, which, in fact, is the one who supplies the "fuel" for the PC parts to operate properly.

A power supply of good quality and with enough capacity can increase the durability of your equipment and reduce your electricity bill (we will explain why when discussing efficiency). Just to have an idea, a high-quality power supply will cost less than 5% of the PC total price. On the other hand, a low-quality power supply can cause several intermittent problems, which are mostly difficult to be solved. A defective or bad-intentioned power supply can lock the PC, can result in hard disk bad blocks, can result in the infamous "blue screen of death" errors and random resets and freezings, added to many other problems.

In this tutorial we will discuss the basics that every user should know. If you want to learn even more about the internals of a PC power supply we recommend you after reading this tutorial to read its sequel, Anatomy of Switching Power Supplies, where we explain in details how the major internal components of a PC power supply work.