Switching Power SupplySwitching power supply is a voltage conversion circuit, the main work is the step-up and step-down, are widely used in modern electronic products. Always work because the switching transistor in the "on" and "off" state, so called switching power supply. Switching power supply in real terms is an oscillator circuit, the conversion of electrical energy not only used in power circuit, the circuit in other applications are also common, such as LCD backlight circuits, such as fluorescent lamps. Switch the source compared with the transformer high efficiency, good stability, small size and other advantages, disadvantages is the relatively small power, and high-frequency interference on the circuit, circuit complexity, such as easy maintenance.Talking about switching power supply before you familiar with the feedback oscillator circuit transformer, can produce a regular pulse current or voltage of the circuit is called oscillation circuit, transformer feedback oscillator circuit is able to meet these conditions the circuit; it in the basic amplifier circuit with a feedback loop composed of C2, L1 election to form a parallel resonant frequency circuit, the instantaneous power in the circuit turn-VT, this time in the C2, L1, composed of parallel resonant circuits have a very rich harmonic, when the plus parallel resonance frequency and the natural frequency of the same circuit, the circuit to enter a state of oscillation, and VT through L3 feedback to further enlarge the base, and ultimately the formation of a regular pulse current or voltage output to the load RL. Switching power supply is around the transformer and the feedback oscillator circuit design, but the basis of the original increase in the number of protection and control circuits, analysis of our oscillation circuit can be used to analyze the method of switching power supply.Switching Power Supply vibration by way of sub-swing can be dividedinto self-excited and it excited the two, since there is no need for plus-excited self-oscillation signal source, since the excitation can see it as a feedback oscillator circuit transformer, and it is excited is totally dependent on the outside to maintain the oscillation, in the practical application of self-excitation of a comprehensive range of applications. According to the structure of incentives signal classification; can be divided into pulse-width-modulated pulse amplitude modulation and two pulse-width-modulated signal to control the width, that is, frequency, pulse amplitude modulation control signal of the magnitude of the role of the two the same so that oscillation frequency is maintained at within a certain range, to the effect of voltage stability. Winding transformer can be divided into three types in general, a group involved in the primary winding of the oscillation, a group is to maintain the oscillation of the feedback winding, there is a group of the load windings. Household appliances used in switching power supply,after the AC to 220V bridge rectifier, converted into about 300V DC, filter into the transformer is added after the switch to high-frequency oscillations of the collector, feedback winding back to the base to maintain the oscillation circuit, load sensor windings of the electrical signal, by rectification, filtering, the DC voltage regulator has been to provide power to the load. Winding in the provision of electric power load, but also take up the capacity of voltage stability, the theory is then a circuit voltage output voltage sampling devices to monitor the output voltage changes, timely feedback to adjust the oscillation frequency oscillator circuit to achieve the voltage stability The purpose of the circuit in order to avoid interference, the feedback voltage to the oscillator circuit will be isolated optocoupler. Most switches have a standby power circuit, switching power supply in standby mode still oscillating, but the frequency of normal working hours than lower.Some switching power supply are complex, numerous components, many protection and control circuit, in the absence of technical support, maintenance is a headache with the matter. I face this kind of situation is, first of all, I will find the switch and its participation in the external oscillation circuit, it separated from the circuit to see if it met the conditions for oscillation, such as detection bias and whether it is normal, whether positive feedback failure, as well as its own switches, switching power supply has very large protection, after exclusion of the prosecution and load control and protection circuit.the control circuit while output from the sample, with the set standards, then controlled inverter, change its frequency or pulse width output achieve stability, on the other hand, according to test data provided by the circuit by circuit protection identification, Control circuit for the unit for various protection measures.SMPS developments and trends in the United States in 1955 Roje (GH.Roger) invention of the self-excited oscillation push redeem transistor single transformer DC converters, high-frequency conversion is the beginning of control circuit, 1957 United States investigation tournament (Jen Sen) since the invention of the push-pull double Flyback transformers, 1964 American scientists proposes to abolish the frequency transformer series switching power supply scenario, This power to the right size and weight of the decline was a fundamental way. To the 1969 high-power silicon transistor because the pressure increase diode reverse recovery time shortened, and other components improve, and finally turned into a 25 kHz switching power supply.Currently, switching power supply to small, Light volume and the characteristics of high efficiency has been widely used in electronic computer-driven variety of terminal equipment, Communications equipment almost all electronic equipment, the electronic information industryindispensable to the rapid development of a power mode. Currently the market for sale in the switching power supply using bipolar transistors made of 100kHz. use made of 500kHz MOS power, though practical, but its frequency to be further enhanced. To improve the switching frequency, it is necessary to reduce the switching loss, and to reduce the switching loss, you need to have high-speed switching devices. However, the switching speed, will be affected by the circuit inductance and capacitance diode or stored charge arising from the impact of the surge or noise. This will not only affect the surrounding electronic equipment, but also greatly reduce the reliability of the power supply itself. Among them, with the switch to prevent Kai-closed by the voltage surge, it is R-C or L-C Composite bumpers, and the storage charge by the diode current surge caused by the use made of amorphous cores such as magnetic bumper. However, the high frequency of 1 MHz and above, using resonant circuit, Switch to enable the voltage or current through the switch was a sine, which could reduce the switching loss, This can also control the surge occurred. Switches such as resonant mode switching. Currently such SMPS of very active, because this means no significant increase switching speed can theoretically put switching loss fall to zero. and the noise is small, is expected to become high-frequency switching power supply of one of the main ways. At present, many countries in the world are working on several trillion Hz converter practical research.开关电源开关电源是一种电压转换电路，主要的工作内容是升压和降压，广泛应用于现代电子产品。