A power supply device is utilized for providing operating voltages to circuits of various processes. Said power supply device provides a direct couple path whenever the voltage supply level of a corresponding battery enters into a domain of an operating voltage of a circuit. When the voltage provided by the battery is higher than an ideal operating voltage domain of the circuit, said power supply device utilizes a conventional transformer to supply a required voltage for said circuit. However, when the voltage provided by the battery enters the ideal operating voltage region, the power supply device provides a direct couple path for enabling said battery to directly supply a required voltage for said circuit other than supplying the required voltage by transformation of said transformer. By this mechanism, power consumption generated by the conventional transformer is saved, and related voltage utilization efficiency is enhanced.