variety of ac electronic equipment is powered internally by dc. In the early days of power supply design, the common single-phase power supply was linear, more or less. In Fig. 4-1, a schematic of a linear power supply, the output voltage is controlled by means of a transistor (not shown) which operates in its active region. Comparatively speaking, the linear power supply has high power losses (overall efficiency in the 30-60% range). In addition, the 60 Hz transformer represents a significant weight to the power supply and results in a more costly design.