1 Introduction
      With the power of electronic technology to tell the development of power electronic equipment and people work and live closer ties between the electronic devices can not do without reliable power into 80 computer power supply of the full realization of the switch, the first complete computer power updating, switching power supply into the 90's have access to a variety of electronic, electrical devices, program-controlled switches, communications, electronic test equipment power supply, power supply and other control devices are widely used in switching power supply, switching power supply also contributed to the rapid development of technology . Switching power supply is the use of modern electronic technology, control switching transistor turn-on and turn-off time ratio of the output voltage to maintain a stable power supply, switching power supply generally by the pulse width modulation (PWM) control IC and MOSFET form. Switching and linear power supply compared to the cost of both the output power increased with the increase of growth, but growth rate varied between the two. Linear power supply costs in a given output power point, rather than switching power supply, the cost of reversal. With the power electronics technology De development and innovation, Shide switching power supply technology is constantly innovating, this cost more to reverse Dian low Shuchudianli Duan Yi Dong, Wei switching power supply that provides a broader space for development.
      High-frequency switching power supply is the direction of its development, high-frequency switching power supply technology to make smaller, making switching power supply into a wider range of applications, especially in the application of high-tech areas, and promote the miniaturization of high-tech products, light of. In addition the development and application switching power supply in the energy, resources and environmental protection are of great significance.

      2 Switching Power Supply Classification
      The people side of switching power supply technology is related to the development of power electronic devices, switching frequency conversion technology development side, the two promote each other to promote the switching power supply to more than two-digit annual growth rate toward the light, small, thin, low noise, high reliability, interference direction. Switching power supply can be divided into AC / DC and DC / DC two categories, DC / DC converter is now modular, and the design and production process maturity and standardization at home and abroad, and has been recognized by the user, but AC / DC modular, because their characteristics make the modular process, the experience is more complex technology and manufacturing process problems. The following are two types of switching power supply for the structure and properties to set.
     2.1 DC / DC transformation
      DC / DC transformation is fixed, variable DC voltage is converted into DC voltage, also known as the DC chopper. Chopper work in two ways, one pulse width modulation Ts constant change ton (GM), and second, frequency modulation, ton constant change Ts (easy to produce interference). The specific circuit consists of the following categories:
      (1) Buck circuit - Buck chopper, the output voltage Uo is less than the average input voltage Ui, the same polarity.
      (2) Boost Circuit - Boost chopper, the output voltage Uo is greater than the average input voltage Ui, the same polarity.
      (3) Buck-Boost Circuit - buck or boost chopper, the output voltage Uo is greater than or less than the average input voltage Ui, polar opposite, inductive transmission.
      (4) Cuk circuit - buck or boost chopper, the output voltage Uo is greater than or less than the average input voltage UI, opposite polarity, capacitance transmission.
      Soft-switching technology makes today's DC / DC undergone a qualitative leap, the United States VICOR company designed and manufactured a variety of ECI soft-switching DC / DC converter, its maximum output power with 300W, 600W, 800W and so on, the corresponding power density (6 , 2,10,17) W/cm3, efficiency (80-90)%. Japan NemicLambda a company's new soft switch technology, high frequency switching power supply module RM Series, its switching frequency (200 ~ 300) kHz, power density has reached 27 W/cm3, synchronous rectifier (MOS-FET to replace Shaw Schottky diode), is the circuit efficiency to 90%.
    2.2 AC / DC transform
      AC / DC transformation is to transform AC to DC, the power flow can be two-way flow of power flow from the power load, known as the "rectification", the power flow from the load back to power as "active inverter." AC / DC converter input AC 50/60Hz, as must be rectified, filtered, the relatively large size of the filter capacitor is necessary, while having to safety standards (such as UL, CCEE, etc.) and EMC Directive restrictions (such as IEC, FCC, CSA), AC input side of the EMC filter must be added to meet safety standards and use of components, so that restrictions AC / DC power supplies small volume, addition, because the internal high-frequency, high voltage, high current switching, making the problem difficult to solve EMC EMC increased, also pairs of the internal circuit design of high-density mounting high demands put forward, for the same reasons, high voltage, high current switching power supply makes the work of consumption increases, limiting AC / DC converter module of the process, it must be used to power system design to optimize the efficiency of a certain level of satisfaction.
      AC / DC transformation circuit according to wiring can be divided into two, half-wave circuit, full-wave circuit. Press the power phases can be divided into several, single, three-phase, multi-phase. Circuit can be divided into quadrants by a quadrant, two quadrant, three quadrant, four quadrants.

     3, the selection of switching power supply
      Interference in the input switching power supply performance, due to the characteristics of its own circuit (Tandem), the average input disturbances, such as surge voltage is difficult to pass, the output voltage stability index of this technology compared with the linear power have greater advantages, its output voltage can reach (0.5 ~ 1)%. Switching power supply module as an integrated power electronic devices, in the selection should note the following:
    3.1 The choice of output current
      Due to the high efficiency switching power supply, generally 80% or more, so the choice of the output current should be accurately measured or calculated maximum absorption current electric equipment, so was chosen for switching power supply has a high cost performance, Usually the output is calculated as: Is = KIf, type in: Is-switching power supply rated output current; If-electric equipment, the maximum absorption current; K-margin coefficient, usually taken from 1.5 to 1.8;
    3.2 Grounding
       Switching Power Supply linear power supply will produce more than the interference, common mode interference on sensitive electrical equipment, grounding and shielding measures should be taken by ICE1000. EN61000. EMC FCC and other restrictions on the shape of switching power supply electromagnetic compatibility EMC measures are taken, so switching power supply should normally be with EMC EMC filters. Such as Leader & Harvest HA series switching power supply technology, to access land or access to FG terminal user case, be able to meet the electromagnetic compatibility requirements.
    3.3 Protection Circuits
      Switching power supply must have been in the design flow, overheating, short circuit protection, it should be the preferred protection in the design of switching power supply modules are available, and the technical parameters of its protection circuit with electrical equipment should match the operating characteristics to avoid damage to electrical equipment or switching power supply.

     4 of switching power supply technology trends
      Switching power supply direction of development is high-frequency, high reliability, low power, low noise, interference and modular. Since switching power supply light, small, thin key technology is the high frequency, thus, leading switching power supply manufacturers abroad are committed to synchronize the development of new components of high intelligence, especially to improve the loss of secondary rectifiers and power rail oxygen (Mn-Zn) material increase scientific and technological innovation to enhance the high frequency and higher magnetic flux density (Bs) obtained a high magnetic properties, while the small capacitor is a key technology. SMT technology allows switching power supply has made considerable progress, both sides of the circuit board layout components to ensure that the switching power supply of light, small, thin. High frequency switching power supply will be bound to the traditional PWM switching technology innovation, realization of ZVS, ZCS soft switching power supply switching technology has become the mainstream technology, and a substantial increase in the switching power supply efficiency. For high reliability, the U.S. manufacturer of switching power supply by reducing operating current to reduce junction temperature and other measures to reduce the stress of the device makes the improved reliability of the product.
      Modular switching power supply is the general trend of development can be composed of modular power distributed power systems can be designed to N +1 redundant power systems, and parallel way to achieve the capacity expansion. Switching power supply noise for the big run this shortcoming, if the exclusive pursuit of their higher frequency noise, which will inevitably increase, Er Zhuanhuan using some resonant circuit technology, in theory, but also can achieve high frequency of Xiangdi noise, but some the practical application of resonant conversion technology, there are still technical problems, it still required much work in this area in order to make the technology to practical use.
Power electronics technology innovation, so that switching power supply industry has a bright future. Switching Power Supply Industry in China to speed up the pace of development must go Technological innovation, out of the Industry and Academia with Chinese characteristics, development road, the rapid development of our national economy to contribute.