Figure Shows the building blocks of a typical high-frequency off-the line switching power-suply. As the name implies, the input rectification in a switching power supply is done directly off-the line, without the use of the low-frequency isolation power transformer, as in the case of linear power supply. The a.c line is directly rectified and filtered to produce a raw high-voltage d.c. which in turn is fed into a switching element.

The switch is operating at the high-frequencies of 20 kHz to 1 MHZ, chopping the dc voltage into a high-frequency square-wave. This square-wave is fed into the power isolation transformer, stopped down to a predetermined value and then rectif‌ied and filtered to produce the required dc output.

A portion of this output is monitored and compared against the fixed reference voltage, and the error signal is used to control the on-off times of the switch, thus regulating the output. Since the switch is either ON or OFF, it is dissipating very little energy, resulting in a very high overall power supply efficiency of about 70 to 80$\%$ .

Another advantage is the power-transformer size which can be quite small due to high operating frequency. Hence the combination of high efficiency and relatively small magnetics, results in compact, light weight power supplies, with power densities up to 30 $\mathrm{W} / \mathrm{in}^{3}$ versus 0.3 $\mathrm{W} / \mathrm{in}^{3}$ for linears.