Power supplies switching have many benefits than other power supplies. and now they substitue them. The most important of benefits than others are listed below. Better effiecency, less fault and smaller in weight, in this type power supply first it converts AC voltage to DC voltage then by one or two switchs in high power supplies four switches are U5ed for transfer energy from primar to secandry of transformer. Frequency of switching are upper than 20 KHZ. At last in seconray, voltage is rectified. and by passing from output filter it would be DC voltage. Output voltage must be stable from changin in the load. for this reason we use negative feedback. Efficency of this type of supplies are about %80 and upper. The switches that used usually are fast bipolar transistor or mospowers. for good switctbing needs to suitable circuits. In transformers usually needs ferrite cores for better trans.fering energy in the secandry of transformer at rectifier the diodes must be shotky or fast type. Usually filter built by inductor and capacitor. Inductor is dc. and in control unit I.C control are used. for example at our project we used TU94 for controling. For isolation between input and output are used transformer and IC optocouplers. And for decreasing output ripple it used snubber circuits on switch transistor and rectifier diodes. and by adding capacitor in suitable places output ripple would be at least values. Load and line regulation at switching power supplies are better than %1 of output voltage at woret conditions