Powering Your PC: Truths and Misconceptions
A Guide To Power Supplies

So What About My PC Power Supply?

While all PC PSUs are considered "switching power supplies", their design is quite a bit more complicated than the example given a couple pages ago (which had only a single voltage output and everything was greatly simplified). For all practical purposes will we be using the ATX 12V v2.01 specifications as our general guidelines whenever we refer to a "PC power supply" or PSU. It should also be known beforehand that each power supply is designed different, and the details below should only be understood as a "generic" explanation. There are so many combinations of how a power supply can be made that trying to explain every option at every step along the way would make this article way too long.

A PC requires several main voltages that stay within certain tolerances (usually 5% is the spec) to operate properly, specifically 3.3V, 5V, 12V, -12V, and 5Vsb (stand-by). The load (and thus current) on each of these rails varies randomly and independently. By now you are thinking that having a PWM circuit & transformer for each rail would be the way to go, which would yield perfect regulation under any combination of load conditions. However when you look inside a PC power supply, there is only one FET & main transformer (which all the voltage rails are drawn from). Yes, a smaller transformer is present (sometimes two), these are usually for the 5Vsb and -12V, but the low power rating for these rails makes them very easy to implement relatively independent of the rest of the PSU.

To start things out, AC power comes into the PSU and first passes through a thyristor, some filters (to remove noise in the AC line), a fuse (some protection from catastrophic failure), and most importantly the first bridge rectifier. These components are visible in the upper right corner of the picture (the clipped power wires can be seen near the PCB in the upper right). The bridge rectifier is the black rectangular object nearest the heatsink, because of the relatively low-current (but high-voltage) the rectifier doesn't require a heatsink. Optionally, active or passive PFC (power factor correction) circuitry would also be wired in this general stage. (More on the topic of PFC later) Power goes from this stage to the two large capacitors in the bottom right corner of the picture. Their job is kind of like holding tanks whose job is to help smooth out the voltage that is fed to the field-transistors.

Below are some illustrations showing what the power looks like as it passes through this initial stage in the power supply.

Incoming AC Voltage

Through Bridge Rectifier

Smoothed From Capacitors

Next power is switched at a very high frequency (in the kHz range) with varying pulse widths (that whole PWM talk on previous pages) via a high power field transistor mounted on a heatsink. Depending on the power supply two or more transistors could be wired in parallel, so they actually act as one big transistor (this is done for greater load capacity). (I mention this because I'm sure someone would have emailed me mentioning they had more than just one.) The FET feeds power (being switched on-off at a very high speed) to the primary side of the large transformer (yellow object in the center of the picture).

All the various output voltages exit the secondary side of the transformer and get rectified via a set of dual Schottky rectifiers mounted on the second heatsink (as pictured above). The primary advantage using Schottky rectifiers is the very low forward voltage drop and switching speeds that approach zero time (ideal for output stages of switching power supplies). Once rectified the power passes through various DC filter chokes (the rings with wire wrapped around them) which work in tandem with capacitors to filter out any residual AC ripple, and of course store energy.

Because we want a clean DC, chokes are very low frequency cut-off lowpass filters. This can be annoying because, to avoid DC loss in the lumped elements, you sometimes get stuck with big old coils for inductors. In fact, choke often refers to a big inductor, since it performs such a function. Whereas the typical bypass capacitor on an IC serves a similar role (shunting ripple to ground), that function is typically aimed at higher frequencies.³

Feedback from the various rails is monitored by a control circuit that attempts to keep the output voltages within specs. Since there is only one FET & transformer, any load change on one rail will affect the other rails voltage. There are many methods and tricks a PSU manufacturer may choose to implement voltage regulation on their power supply (this factor is primarily driven by cost), this topic alone could go on for several pages and get very technical, so it is probably best if just skipped. Needless to say, the closer actual voltage levels are to their nominal specs under varying load conditions, the better off you will be, and PSU designers strive to achieve this goal.