A typical ATX PSU schematic using a TL494. Dianyuan.com forum [Public domain], via Wikimedia Commons. ATX power supplies follow a closely defined standard, so it should come as little surprise ...
At its heart is the ubiquitous TL494 PWM controller that you may be more familiar with in the context of switching power supplies, this one applies the audio drive as PWM to the gate of a MOSFET ...