According to the control principle of switch power to classification, we have the following 3 kinds of work mode: 

   1) pulse width adjustment type, abbreviation Modulation PulseWidth pulse width Modulation (PWM) type, abbreviation for。 Its main characteristic is fixed switching frequency, pulse width to adjust by changing voltage 390v, realize the purpose。 Its core is the pulse width modulator。 Switch cycle for designing filter circuit fixed provided convenience。 However, its shortcomings is influenced by the power switch conduction time limit minimum of output voltage cannot be wide range regulation; In addition, the output will take dummy loads commonly (also called pre load), in order to prevent the drag elevated when output voltage。 At present, most of the integrated switch power adopt PWM way。 

    2) pulse frequency Modulation mode pulse frequency Modulation (, referred to PulseFrequency Modulation, abbreviation for PFM) type。 Its characteristic is will pulse width fixed by changing switch frequency to adjust voltage 390v, realize the purpose。 Its core is the pulse frequency modulator。 Circuit design to use fixed pulse-width generator to replace the pulse width omdulatros and use sawtooth wave generator voltage?Frequency converter (for example VCO changes frequency VCO)。 It on voltage stability principle is: when the output voltage Uo rises, the output signal controller pulse width unchanged and cycle longer, make Uo 390v decreases, and reduction。 PFM type of switch power supply output voltage range is very wide, output terminal don't meet dummy loads。 PWM way and way of PFM respectively modulating waveform is shown in figure 1 (a), (b) shows, tp says pulse width (namely power switch tube conduction time tON), T represent cycle。 It can be easy to see the difference between the two。 But they have something in common: (1) all use time ratio control (TRC) on voltage stability principle, whether change tp, finally adjustment or T is pulse 390v。 Although adopted in different ways, but control goals, is all rivers run into the sea。 (2) when load by light weight, or input voltage respectively, from high changed by increasing the pulse width, higher frequency method to make the output voltage remained stable。   

   3) mix modulation mode, it is to point to the pulse width and switching frequency is not fixed, each other can change, it belongs to the way the PWM and PFM blend mode。 It contains a pulse widthomdulatros and pulse frequency modulator。 Because and T all can adjust alone, so occupies emptiescompared to adjust the most wide range, suitable for making the output voltage for laboratories that use a wide range of can adjust switching power supply。 Above 3 work collectively referred to as "Time Ratio Control" (as a Control, from TRC) way。 As noted, pulse width omdulatros either as a independent IC use (for example UC3842 type pulse width omdulatros), can also be integrated in DC/DC converter (for example LM2576 type switching voltage regulators integrated circuit), still can integration in AC/DC converter (for example TOP250 type monolithic integrated circuit switching power supply。 Among them, the switching voltage regulators belong to DC/DC power converter, switching power supply general for AC/DC power converter。

  The typical structure of switch power as figure2shows, its working principle is: the first utility into power rectifier and filtering into high voltage dc and then through the switch circuit and high-frequency switch to high frequency low pressure pulse transformer, and then after rectification and filter circuits, finally output low voltage dc power。 Meanwhile in the output parts have a circuit feedback to control circuit, through the control PWM occupies emptiescompared to achieve output voltage stability。 

  Switching power supply by these four components: 

 1) the main circuit: exchange network input, from the main circuit to dc output。 Mainly includes input filter, rectifier and filtering, inverter, and output rectifier and filtering。