Switching Power Supply Introduction

　　A switching power supply is a power supply that uses modern power electronics technology to control the ratio of the time when the switching transistor is turned on and off to maintain a stable output voltage. A switching power supply is generally composed of a pulse width modulation (PWM) control IC and a MOSFET. Compared with linear power supplies, the cost of both switching power supplies increases with the increase of output power, but the growth rates of the two are different. At a certain output power point, the cost of a linear power supply is higher than that of a switching power supply, which is called the cost reversal point. With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating, and this cost reversal point is increasingly moving towards the low output power end, which provides a broad development space for switching power supplies.

　　The high frequency of switching power supply is its development direction. High frequency makes the switching power supply smaller and allows the switching power supply to enter a wider range of applications, especially in the field of high-tech, which promotes the miniaturization and lightness of high-tech products. In addition, the development and application of switching power supply are of great significance in saving energy, saving resources and protecting the environment.

　　1. Classification of switching power supplies

　　In the field of switching power supply technology, people are developing related power electronic devices while developing switching frequency conversion technology. The two promote each other and drive the switching power supply to develop in the direction of light, small, thin, low noise, high reliability and anti-interference at an annual growth rate of more than double digits. Switching power supplies can be divided into two categories: AC/DC and DC/DC. DC/DC converters have now been modularized, and their design technology and production process have been advanced in China.

Both the internal and external parts are mature and standardized, and have been recognized by users. However, due to its own characteristics, the modularization of AC/DC encounters more complex technical and process manufacturing problems in the process of modularization. The following describes the structure and characteristics of the two types of switching power supplies.

　　1.1 DC/DC conversion

　　DC/DC conversion is to convert a fixed DC voltage into a variable DC voltage, also known as DC chopper. There are two working modes of the chopper: one is the pulse width modulation mode, Ts remains unchanged, ton is changed (universal), and the other is the frequency modulation mode, ton remains unchanged, Ts is changed (prone to interference). Its specific circuits are divided into the following categories:

　　(1) Buck circuit - step-down chopper, whose output average voltage U0 is less than the input voltage Ui and has the same polarity.

　　(2) Boost circuit - step-up chopper, whose output average voltage U0 is greater than the input voltage Ui and has the same polarity.

　　(3) Buck-Boost circuit - a step-down or step-up chopper whose output average voltage U0 is greater or less than the input voltage Ui, with opposite polarity and inductive transmission.

　　(4) Cuk circuit - a buck or boost chopper whose output average voltage U0 is greater or less than the input voltage Ui, with opposite polarity and capacitive transmission.

　　Today's soft switching technology has brought about a qualitative leap in DC/DC. The various ECI soft switching DC/DC converters designed and manufactured by VICOR in the United States have maximum output powers of 300W, 600W, 800W, etc., with corresponding power densities of (6.2, 10, 17) W/cm3 and efficiencies of (80-90)%. The RM series of high-frequency switching power supply modules using soft switching technology recently launched by NemicLambda in Japan has a switching frequency of (200-300) kHz and a power density of 27W/cm3. It uses synchronous rectifiers (MOS-FET instead of Schottky diodes) to increase the efficiency of the entire circuit to 90%.

　　1.2 AC/DC conversion

　　AC/DC conversion is the conversion of AC to DC, and its power flow can be bidirectional. The power flow from the power supply to the load is called "rectification", and the power flow from the load back to the power supply is called "active inversion". The input of AC/DC converter is 50/60Hz AC, which must be rectified and filtered, so a relatively large filter capacitor is indispensable. At the same time, due to the restrictions of safety standards (such as UL, CCEE, etc.) and EMC directives (such as IEC, FCC, CSA), EMC filtering must be added to the AC input side and components that meet safety standards must be used, which limits the miniaturization of AC/DC power supply. In addition, due to the internal high-frequency, high-voltage, and high-current switching action, it is more difficult to solve the EMC electromagnetic compatibility problem, which also puts forward high requirements for the internal high-density installation circuit design. For the same reason, high-voltage and high-current switches increase the working loss of the power supply, which limits the modularization process of AC/DC converters. Therefore, the power system optimization design method must be adopted to achieve a certain degree of satisfaction in its working efficiency.

　　AC/DC conversion can be divided into half-wave circuit and full-wave circuit according to the wiring method of the circuit. It can be divided into single-phase, three-phase and multi-phase according to the number of power phases. It can be divided into one quadrant, two quadrants, three quadrants and four quadrants according to the circuit working quadrant.

　　2. Selection of switching power supply

　　In terms of input anti-interference performance, due to the characteristics of its own circuit structure (multi-stage series connection), general input interference such as surge voltage is difficult to pass. Compared with linear power supplies, it has a greater advantage in output voltage stability, a technical indicator, and its output voltage stability can reach (0.5~1)%. As a power electronic integrated device, the switching power supply module should pay attention to the following points when selecting:

　　2.1 Output current selection

　　Because the working efficiency of the switching power supply is high, generally reaching more than 80%, the maximum absorption current of the electrical equipment should be accurately measured or calculated when selecting its output current, so that the selected switching power supply has a high performance-price ratio. The output calculation formula is usually:

　　Is=KIf

　　Where: Is—rated output current of the switching power supply;

　　If—the maximum absorption current of the electrical equipment;

　　K—margin coefficient, generally 1.5 to 1.8;

       2.2 Grounding

　　Switching power supplies will generate more interference than linear power supplies. For electrical equipment that is sensitive to common-mode interference, grounding and shielding measures should be taken. According to EMC restrictions such as ICE1000, EN61000, and FCC, switching power supplies all take EMC electromagnetic compatibility measures, so switching power supplies should generally be equipped with EMC electromagnetic compatibility filters. For example, the HA series switching power supply of Leader Harvest Technology has its FG terminal connected to the ground or to the user's case to meet the above electromagnetic compatibility requirements.

　　2.3 Protection Circuit

　　The switching power supply must be designed with protection functions such as overcurrent, overheating, and short circuit. Therefore, a switching power supply module with complete protection functions should be preferred during design, and the technical parameters of its protection circuit should match the working characteristics of the electrical equipment to avoid damage to the electrical equipment or the switching power supply.

　　3. Development trend of switching power supply technology

　　The development direction of switching power supplies is high frequency, high reliability, low consumption, low noise, anti-interference and modularization. Since the key technology for making switching power supplies light, small and thin is high frequency, major foreign switching power supply manufacturers are committed to the simultaneous development of new highly intelligent components, especially improving the loss of secondary rectifier devices, and increasing technological innovation in power ferrite (MnZn) materials to improve the high magnetic properties under high frequency and large magnetic flux density (Bs), and the miniaturization of capacitors is also a key technology. The application of SMT technology has made great progress in switching power supplies. Components are arranged on both sides of the circuit board to ensure that the switching power supply is light, small and thin. The high frequency of switching power supplies will inevitably innovate the traditional PWM switching technology to achieve ZVS and Z

CS soft switching technology has become the mainstream technology of switching power supplies and has greatly improved the working efficiency of switching power supplies. For high reliability indicators, American switching power supply manufacturers have reduced the stress of devices by reducing the operating current and junction temperature, which has greatly improved the reliability of products.

　　Modularization is the general trend of the development of switching power supplies. Modular power supplies can be used to form distributed power supply systems, which can be designed into N+1 redundant power supply systems and achieve capacity expansion in parallel. In view of the disadvantage of high noise in the operation of switching power supplies, if high frequency is pursued alone, the noise will also increase. The use of partial resonant conversion circuit technology can theoretically achieve high frequency and reduce noise, but the actual application of partial resonant conversion technology still has technical problems, so a lot of work still needs to be done in this field to make this technology practical.

　　The continuous innovation of power electronics technology has given the switching power supply industry a broad development prospect. To accelerate the development of my country's switching power supply industry, we must take the road of technological innovation and the road of joint development of industry, academia and research with Chinese characteristics, and contribute to the rapid development of my country's national economy.