A power supply device with a high efficiency includes a switch circuit, a transformer, a first capacitor, an output stage circuit, and a PWM (Pulse Width Modulation) IC (Integrated Circuit). The switch circuit generates a switching voltage according to an input voltage, a first PWM voltage, and a second PWM voltage. The transformer includes a main coil, a first secondary coil, and a second secondary coil. A leakage inductor and a magnetizing inductor are built into the transformer. The main coil receives the switching voltage through the leakage inductor. The output stage circuit is coupled to the first secondary coil and the second secondary coil, and is configured to generate an output voltage and an output current. The PWM IC selectively adjusts the switching frequency and duty cycle of the first PWM voltage according to the output current.