【正文】 Protection circuit can be divided over current protection, overvoltage protection, owe voltage protection, the groundclamp protection, overheating protection, automatic restart, soft start, slow startup, etc. Various types. 4) other circuit: if the sawtooth wave generator, offset circuit, optical coupler, etc. 。 In addition, the output will take dummy loads monly (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 pulsewidth 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 don39。t seen practicability of the method appears particularly strong,perhaps with the popularity of digital control, and there are some new control theory into switching power supply. （ 1） The technology: with high frequency switching frequencies increase, switch converter volume also decrease, power density has also been boosted, dynamic response improved. Small power DC DC converter switch frequency will rise to MHz. But as the switch frequency unceasing enhancement, switch ponents and passive ponents loss increases, highfrequency parasitic parameters and highfrequency EMI and so on the new issues will also be caused. （ 2） Soft switching technologies: in order to improve the efficiency of nonlinearity of various soft switch, mutation technical application and hygiene, representative of soft switch technology is passive and active soft switch technology, mainly including zero voltage switch/zero current switch (ZVS/ZCS) resonance, quasi resonant, zero voltage/zero current pulse width modulation technology (ZVS/ZCS PWM) and zero voltage transition/zero current transition pulse width modulation (PWM) ZVT/ZCT technical, etc. By means of soft switch technology can effectively reduce switch loss and switch stress, help converter transformation efficiency （ 3） Power factor correction technology (IC simplifies PFC). At present mainly divided into IC simplifies PFC technology passive and active IC simplifies PFC technology using IC simplifies PFC technology two kinds big, IC simplifies PFC technology can improve AC DC change device input power factor, reduce the harmonic pollution of power grid. （ 4） Modular technology. Modular technology can meet the needs of the distributed power system, enhance the system reliability. （ 5） Low output voltage technology. With the continuous development of semiconductor manufacturing technology, microprocessor and portable electronic devices work more and more low, this requires future DC DC converter can provide low output voltage to adapt microprocessor and power supply requirement of portable electronic devices People in switching power supply technical fields are edge developing related power electronics device, the side of frequency conversion technology, development of switch between mutual promotion push switch power supply with more than two year growth toward light, digital small, thin, low noise and high reliability, antiinterference direction. Switching power supply can be divided into the AC/DC and DC/DC two kinds big, also have AC/AC DC/AC as inverter DC/DC converter is now realize modular, and design technology and production process at home and abroad, are mature and standardization, and has approved by users, but the AC/DC modular, because of its own characteristics in the process of making modular, meet more plex technology and craft manufacture problems. The following two types of switch power supply respectively on the structure and properties o