A power supply system drives a plurality of loads, and includes a power stage circuit, a pulse width modulation controller, a first isolation transformer, at least two second transformers, and at least two rectifier circuits. The power supply system converts external power signals into first alternating current (AC) power signals, and converts the first AC power signals into second AC power signals. The second transformers either boost or buck the second AC power signals, and each has a primary winding connected to a secondary winding of the first isolation transformer in parallel and a center tap grounded. Each of the rectifier circuits is connected to two ends of a secondary winding of a corresponding one of the second transformers, and rectifies the boosted or bucked second AC power signals to output direct current (DC) power signals to a corresponding one of the plurality of loads.