Switching power supply and its classification and application

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Switching power supply and its classification and application [Copy link]

With the rapid development of power electronics technology, the relationship between power electronics equipment and people's work and life is becoming increasingly close, and electronic equipment cannot do without reliable power supply. In the 1980s, computer power supplies were fully realized as switching power supplies, and the power supply replacement of computers was completed first. In the 1990s, switching power supplies have successively entered various electronic and electrical equipment fields. Programmable switches, communications, power detection equipment power supplies, control equipment power supplies, etc. have widely used switching power supplies, which has promoted the rapid development of switching power supply technology. A switching power supply is a power supply that uses modern power electronics technology to control the time ratio of the switch transistor on and off to maintain a stable output voltage. The switching power supply is generally composed of a pulse width modulation (PWM) control IC and a MOSFET. Compared with a switching power supply and a linear power supply, the cost of both increases with the increase of output power, but the growth rate of the two is 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, the switching power supply technology is also constantly innovating. 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.

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 push 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 the design technology and production process have been mature and standardized at home and abroad, and have been recognized by users. However, the modularization of AC/DC, due to its own characteristics, 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.

2.1 DC/DC conversion
DC/DC conversion is the conversion of 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, in which Ts remains unchanged and ton is changed (universal); the other is the frequency modulation mode, in which ton remains unchanged and 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 - step-down or step-up chopper, whose
output average voltage U0 is greater than or less than the input voltage Ui, with opposite polarity and inductive transmission.
(4) Cuk circuit - step-down or step-up chopper, whose output average voltage
U0 is greater than or less than the input voltage Ui, with opposite polarity and capacitive transmission.
Today's soft switching technology has made 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%.

2.2AC/DC conversion
AC/DC conversion is to convert 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 power. Because it must be rectified and filtered, 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.

3. 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 supply, 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 in selection:

3.1 Selection of output current
Since the working efficiency of the switching power supply is high, generally it can reach more than 80%. Therefore, when selecting its output current, the maximum absorption current of the electrical equipment should be accurately measured or calculated 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—maximum absorption current of the electrical equipment;
K—margin coefficient, generally 1.5~1.8;

3.2 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.

3.3 Protection circuit
The switching power supply must have protection functions such as overcurrent, overheating, and short circuit in the design. Therefore, the 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.

4 Development trends of switching power supply technology

　　The development direction of switching power supply is high frequency, high reliability, low consumption, low noise, anti-interference and modularization. Since the key technology of light, small and thin switching power supply is high frequency, major foreign switching power supply manufacturers are committed to the simultaneous development of new high-intelligent components, especially improving the loss of secondary rectifier devices, and increasing scientific and 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 supply, arranging components 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 supply will inevitably innovate the traditional PWM switching technology. The soft switching technology of realizing ZVS and ZCS has become the mainstream technology of switching power supply and has greatly improved the working efficiency of switching power supply. For high reliability indicators, American switching power supply manufacturers have reduced the stress of devices by reducing the operating current and junction temperature, which greatly improves 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 as N+1 redundant power supply systems, and can 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 inevitably increase. The use of partial resonant conversion circuit technology can theoretically achieve high frequency and reduce noise, but there are still technical problems in the practical application of partial resonant conversion technology. Therefore, 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 China'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 to contribute to the rapid development of China's national economy.