I'm trying to grasp the fundamentals of a computer power supply unit. Specifically, how does the PSU take mains AC voltage and reliably output multiple DC rails like 12V, 5V, and 3.3V? What are the main stages involved—rectification, filtering, regulation—and how do they interact to maintain stable voltages under varying loads? Looking for a clear overview.
Understanding PSU: How does a computer power supply convert AC to DC?
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When I first built my own rig back in 2015 I actually opened a cheap PSU just to see what was inside – not that I ever tried to tinker with it, but it helped me understand the flow. The AC from the wall hits a bridge rectifier first, turning the sinusoidal 120/230 V into a pulsating DC. Right after that you get a massive bulk capacitor that smooths the peaks and fills the valleys, giving you a relatively steady high‑voltage DC (around 340 V after the rectifier). From there the PSU’s high‑frequency switch‑mode section takes over: it chops that voltage with a MOSFET and a transformer, stepping it down to the lower levels we need. Each rail (12 V, 5 V, 3.3 V) has its own set of secondary windings, then goes through separate rectifiers and smaller filter caps. Finally, feedback loops with error‑amplifiers and PWM controllers regulate the output – the 12 V rail usually stays in continuous‑conduction mode, while the 5 V and 3.3 V rails use burst‑mode regulation to keep the voltage tight even when the load changes quickly. The whole chain – rectification, bulk filtering, high‑frequency conversion, secondary filtering, and feedback regulation – works together to keep those rails within a few percent of their nominal values, no matter if the GPU spikes or the CPU idles.