Switching power supply is the use of electronic switching devices (such as transistors, FET, SCR thyristors, etc.), by the control circuit, the electronic switching device constantly "on" and "off" so that the electronic switching devices for input voltage pulse modulation for DC / AC, DC / DC voltage converter, and an output voltage adjustable and automatic voltage regulator. Switching power supply generally has three operating modes: frequency, pulse width fixed mode, fixed-frequency, pulse width variable mode, frequency, pulse width variable mode. The former modes of operation for the DC / AC inverter or DC / DC voltage converter; the latter two modes of operation for the switching power supply. In addition, the switching power supply output voltage also has three operating modes: direct output voltage mode, the average output voltage mode, the amplitude of the output voltage mode. Similarly, before a way of working more for DC / AC inverter or DC / DC voltage converter; the latter two works are used for switching power supply.