This article provides a detailed component level analysis of computer power supply operation. The target audience should be familiar with analog circuit analysis. A more general operational guide can be found at Howstuffworks.

The deconstructed power supply is a 250W unit that outputs +/-5V, +/-12V, and Power Good signal. The operational principles of this switching power supply apply to all present day computer power supplies. Open the companion schematic and begin with the Input Circuit.

Input Circuit. The Forward-Mode Filter Choke, T1, and Capacitors C4, C1, & C12 are used to suppress Radio Frequency Interference (RFI) generated by the power supply. If left unfiltered, RFI escapes onto the AC input line and radiates to other electronic devices. T1 also serves as a choke, reducing line transients, much like a surge protector. Following this filtering network, the AC input is applied to the Bridge Rectifier (BR). BR is a packaged four terminal full-wave rectifying device rated at 600V PRV, 2A. Bridge Rectifier operation depends on Switch SW1, which allows the power supply to run from 110VAC or 220VAC, nominal. The switch is closed for the North American (110V) market and open for most overseas markets (220V). When operated from 220VAC, the bridge acts as a full wave rectifier with C13 & C14 forming a capacitive filter.

When SW1 is closed for 110VAC operation, C13 and C14 are placed in series and alternately charged on each half-cycle, producing a doubled output voltage. In fact, BR-D1 and BR-D3 are used in the configuration; D1 rectifying the positive half cycle and D3 the negative half cycle. Capacitor C13 is charged to the positive RMS value of the 110V AC input, capacitor C14 to the negative RMS value, both with respect to neutral. The Theoretical DC charge of each capacitor is calculated to be 155V, but after diode and other losses, the actual level is closer to 150V. The 300V DC Bus is the potential between C13's positive plate and C14's negative plate. R2 and R3 are bleeder resistors that allow C13 and C14 to discharge when the circuit is turned off.