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A switching power supply consists of several stages. A filter for the mains power sits right behind the input, filtering out surges, harmonics, and various other undesirable phenomena found in the mains power. In the second stage, the AC current is rectified and screened. At this point, we’re dealing with about 350 V, which are then transformed through an inverter stage, yielding an alternating current with a frequency between 35 and 50 kHz. This is where our small transformers come in, converting the alternating current.
Eventually, differing voltages of 3.3, 5, and 12 V are required, which means that simple PC switching power supplies have either a single output rail with different taps for each voltage or separate rails for each voltage. Top-end PSUs even have separate transformers for different voltages, which are then corrected and smoothed a second time following transformation. The most important thing is that these voltages have to stay consistent. Regardless of whether the PC is idling or under full load, voltages may not deviate from their spec by more than 5 percent.A regulator circuit ensures this is the case. This is also the reason a switching power supply is always connected to a load. Otherwise, you risk a voltage flashover.
That brings us to our next topic: efficiency. If you’re looking for a new car, you’re going to ask your local dealer, “So, how many miles per gallon does this one get?” Now, PSUs may not burn gasoline, but you still have to look out for their efficiency. Indeed, this is one area where most builders unknowingly waste the most power, increasing the PC's cost over its lifetime. Want to make sure you don’t make that mistake? Take a look at the next page!