The following picture shows the 300W ATX power supply schematic diagrams. Click the schematic diagram to enlarge.
the circuit schematic, it includes EMI filter, inrush current limiter,
input select (short for 115VAC), power transformer, input voltage sense,
base drive signal transformer, magamp chokes, coupled output inductor,
combined feedback for 5V and 12V outputs, magamp regulator for 3.3V,
half bridge converter, PWM IC, bias and 5V standby supply based on flyback converter , and opto-isolated feedback for bias supply.