【正文】 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 of this. Switching power supply is the development direction of high frequency, high reliability, low consumption, low noise, antijamming and modular. Because light switch power, small, thin key techniques are changed, so high overseas each big switch power supply manufacturer are devoted to the development of new high intelligent synchronous rectifier, especially the improvement of secondary devices of the device, and power loss of Zn ferrite (Mn) material? By increasing scientific and technological innovation, to enhance in high frequency and larger magic flux density (Bs) can get high magic under the miniaturization of, and capacitor is a key technology. SMT technology application makes switching power supply has made considerable progress, both sides in the circuit board to ensure that decorate ponents of switch power supply light, small, thin. The high frequency switching power supply of the traditional PWM must innovate switch technology, to realize the ZCS ZVS, soft switch technology has bee the mainstream of switch power supply technical, and greatly improve the efficiency of switch power. For high reliability index, America39。 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。 3） 檢測及保護電路：檢測電路有過電流檢測、過電壓檢測、欠電壓檢測、過熱檢測等；保護電路可分為過電流保護、過電壓保護、欠電壓保護、箝位保護、過熱保護、自動重啟動、軟啟動、緩啟動等多種類型。 （ 3）逆變 :將整流后的直流電變?yōu)楦哳l交流電 ,這是高頻開關(guān)電源的核心部分 ,頻率越高 ,體積、重量與輸出功率之比越小。 開關(guān)電源由以下 4 部分構(gòu)成： 1）主電路：從交流電網(wǎng)輸入，到直流輸出的主要電路。需要指出的是，脈寬調(diào)制器既可作為一片獨立的集成電路使用 (例如 UC3842 型脈寬調(diào)制器 )，亦可被集成在 DC/DC 變換器中 (例如 LM2576型開關(guān)穩(wěn)壓器集成電路 )，還能集成在 AC/DC 變換器中 (例如 TOP250 型單片開關(guān)電 源集成電路 )。 3）混合調(diào)制方式，是指脈沖寬度與開關(guān)頻率均不固定，彼此都能改變的方式，它屬于 PWM 和 PFM 的混合方式。從中可以比較容易的看出兩者的區(qū)別。在電路設(shè)計上要用固定脈寬發(fā)生器來代替脈寬調(diào)制器 中的鋸齒波發(fā)生器，并利用電壓 ?頻率轉(zhuǎn)換器 (例如壓控振蕩器 VCO)改變頻率。目前，大多數(shù)的集成開關(guān)電源采用 PWM 方式。其主要特點是固定開關(guān)頻率，通過改變脈沖寬度來調(diào)節(jié)占 空比，實現(xiàn)穩(wěn)壓目的。針對開關(guān)電源運行噪聲大這一缺點，若單獨追求高頻化其噪聲也必將隨著增大，而采用部分諧振轉(zhuǎn)換電路技術(shù)，在理論上即可實現(xiàn)高頻化又可降低噪聲，但部分諧振轉(zhuǎn)換技術(shù)的實際應(yīng)用仍存在著技術(shù)問題，故仍需在這一領(lǐng)域開展大量的工作，以使得該項技術(shù)得以實用化。由于開關(guān)電源輕、小、薄的關(guān)鍵技術(shù)是高頻化，因此國外各大開關(guān)電源制造商都致力于同步開發(fā)新型高智能化的元器件，特別是改善二次整流器件的損耗，并在功率鐵氧體（ Mn?Zn)材料上加大科技創(chuàng)新，以提高在高頻率