Switching power supplies are power supplies that use modern power electronics to control the turn-on and turn-off ratios of switching transistors and maintain a stable output voltage. Switching power supplies typically consist of a pulse width modulation (PWM) control IC and a MOSFET. Compared with the linear power supply, the cost of both of the switching power supply and the linear power supply increases as the output power increases, but the growth rates of the two are different. The linear power supply costs at a certain output power point but is higher than the 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 to low-power output terminals, which provides a broad space for the development of switching power supplies.
The high frequency of the switching power supply is the direction of its development. The high frequency of switching power supply miniaturization and switching power supply into a wider range of applications, especially in high-tech applications, has promoted the miniaturization and lightweight of high-tech products. Change. In addition, the development and application of switching power supply are of great significance in energy conservation, resource conservation and environmental protection.
Switching power supply classification:
In the field of switching power supply technology, people are developing related power electronic devices while developing switching frequency conversion technology. Both promote each other to push the switching power supply towards light, small, thin, low noise, high reliability, and the annual growth rate of more than two digits. The development of anti-jamming direction. Switching power supply can be divided into two major categories: AC/DC and DC/DC. The DC/DC converter is now modularized, and the design technology and production technology are mature and standardized at home and abroad, and have been recognized by users, but AC/DC's modularity, due to its own characteristics, has encountered complex technical and process manufacturing issues in the modularization process. The following describes the structure and characteristics of the two types of switching power supplies.
DC/DC conversion
DC/DC conversion is the conversion of a fixed DC voltage into a variable DC voltage, also known as DC chopping. The chopper works in two ways. First, the pulse width modulation method Ts does not change, and ton (general), the second is the frequency modulation method, ton is constant, and the change Ts (easy to produce interference).
Today's soft-switching technology has led to a qualitative leap in DC/DC. A variety of ECI soft-switching DC/DC converters designed and manufactured by VICOR, USA have maximum output power of 300W, 600W, 800W, etc., and the corresponding power density is (6.2%). , 10, 17) W/cm3, the efficiency is (80 ~ 90)%. Japan's NemicLambda Corporation recently introduced a high-frequency switching power supply module RM series using soft switching technology, its switching frequency is (200 ~ 300) kHz, power density has reached 27W/cm3, the use of synchronous rectifier (MOS-FET instead of SCHOTT The base diodes increase the overall circuit efficiency to 90%.
AC/DC conversion
AC/DC conversion is to convert AC to DC, its power flow can be bi-directional, power flow from the power supply to the load is called "rectification", power flow from the load back to the power supply is called "active inverter." AC/DC converter input is 50/60Hz AC, because it must be rectified and filtered, so relatively large filter capacitor is indispensable, and meets safety standards (such as UL, CCEE, etc.) and EMC directive. The restrictions (eg, IEC, FCC, CSA) must be added on the AC input side with EMC filtering and the use of components that comply with safety standards. This limits the miniaturization of the AC/DC power supply volume. In addition, due to the internal high frequency, high voltage, and large The current switching action makes it more difficult to solve EMC electromagnetic compatibility problems, and it also puts high requirements on internal high-density circuit design. For the same reasons, high voltage and large current switches make the power supply operating losses increase, limiting the Because of the modularization process of AC/DC converters, it is necessary to adopt a power system optimization design method in order to achieve a certain level of work efficiency.
AC/DC conversion can be divided into half-wave circuits and full-wave circuits according to the wiring of the circuit. According to the number of power phases can be divided into single-phase, three-phase, multi-phase. According to the circuit work quadrant can be divided into a quadrant, two quadrants, three quadrants, four quadrants.
Switching power supply selection:
Switching power supply in the input anti-jamming performance, due to its own circuit structure characteristics (multi-stage series), the general input interference such as surge voltage is difficult to pass, in the output voltage stability of the technical indicators compared with linear power supply Larger advantages, the output voltage stability up to (0.5 ~ 1)%.
As a power electronics integrated device, the switching power supply module should pay attention to the following points in the selection:
1 , the choice of output current
Due to the high working efficiency of the switching power supply, it can generally reach more than 80%. Therefore, in the selection of its output current, the maximum absorption current of the power equipment should be accurately measured or calculated so that the selected switching power supply has a high performance-cost ratio.
2 , ground
Switching power supply will generate more interference than linear power supply. Grounding and shielding measures shall be taken for power equipment sensitive to common-mode interference. According to EMC limits such as ICE1000, EN61000 and FCC, the switching power supply adopts EMC electromagnetic compatibility measures, so the switch The power supply should generally have an EMC electromagnetic compatibility filter. For example, a series of HA switching power supplies from Leader's technology can connect its FG terminal to ground or to the user's chassis to meet the EMC requirements.
3 , protection circuit
The switching power supply must have protection functions such as over current, overheating and short circuit in the design. Therefore, the switching power supply module with all protection functions should be preferred in the design, and the technical parameters of the protection circuit should be matched with the working characteristics of the power consumption equipment. Avoid damage to the electrical equipment or switching power supply.
Switching power supply technology trends:
The development direction of switching power supply is high frequency, high reliability, low consumption, low noise, anti-interference and modularization. Because the key technology of switching power supply is light, small, and thin, it is high-frequency, so foreign major switching power supply manufacturers are committed to the simultaneous development of new high-intelligent components, in particular to improve the loss of secondary rectifier devices, and in power iron. Oxygen (Mn  Zn) materials on the increase of scientific and technological innovation, in order to increase high frequency and high magnetic flux density (Bs) to obtain high magnetic properties, and the miniaturization of capacitors is also a key technology. The application of SMT technology has made great strides in switching power supplies, placing components on both sides of the circuit board to ensure that the switching power supply is light, small and thin. The high frequency of the switching power supply will inevitably innovate the traditional PWM switching technology. The soft switching technology of ZVS and ZCS has become the mainstream technology of the switching power supply, and the operating efficiency of the switching power supply has been greatly improved. For high-reliability targets, U.S. manufacturers of switching power supplies have reduced equipment stress by lowering operating currents and lowering junction temperatures, which has greatly improved product reliability.
Modularization is an overall trend of the development of switching power supplies. Modular power supplies can be used to form a distributed power supply system. N+1 redundant power supply systems can be designed and capacity expansion in parallel can be realized. For the shortcomings of high switching noise, the noise of the switching power supply will inevitably increase with increasing frequency. The use of partial resonant conversion circuit technology can theoretically achieve high frequency and reduce noise, but some There are still technical problems in the practical application of resonant conversion technology, so it is still necessary to carry out a large amount of work in this field in order to make this technology practical.
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