Switching power supply is a kind of power supply that uses modern power electronics technology to control the ratio of time when the switch is turned on and off, and maintains a stable output voltage. The switching power supply is generally composed of a pulse width modulation (PWM) control IC and a MOSFET. Compared with linear power supplies, switching power supplies increase in cost as output power increases, but the growth rates vary.
The linear power supply cost is higher than the switching power supply at a certain output power point, which is called the cost reversal point. With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating. This cost reversal point is increasingly moving to the low output power end, which provides a broad space for development of switching power supplies.
The high frequency of switching power supply is the direction of its development. The high frequency makes the switching power supply miniaturized, and the switching power supply enters a wider range of applications, especially in the high-tech field, which promotes the miniaturization and lightness of high-tech products. Turn. In addition, the development and application of switching power supplies are of great significance in terms of energy conservation, resource conservation and environmental protection.
The power electronics used in switching power supplies are mainly diodes, IGBTs and MOSFETs.
SCR has a small number of applications in the switching power supply input rectifier circuit and soft start circuit. The GTR drive is difficult, the switching frequency is low, and it is gradually replaced by IGBT and MOSFET.
Three conditions for switching power supplies1, switch: power electronic device works in the switch state instead of the linear state
2, high frequency: power electronic devices work at high frequencies rather than near the low frequency of the power frequency
3, DC: switching power supply output is DC instead of AC
Switching power supply classificationIn the field of switching power supply technology, people develop related power electronic devices and develop switching frequency conversion technology. The two promote each other to promote the switching power supply to a lighter, smaller, thinner, lower noise and high reliability. The direction of anti-interference is developing. Switching power supply can be divided into two major categories: AC/DC and DC/DC. The DC/DC converter has been modularized, and the design technology and production process have been matured and standardized at home and abroad, and have been recognized by users, but The modularity of AC/DC, due to its own characteristics, leads to more complex technical and process manufacturing problems in the process of modularization. The structure and characteristics of the two types of switching power supplies are described below.
2.1 DC / DC conversion
The DC/DC conversion converts a fixed DC voltage into a variable DC voltage, also known as DC chopping. There are two ways to operate the chopper. One is that the pulse width modulation mode Ts is unchanged, the ton is changed, and the other is the frequency modulation mode. The ton is unchanged and the Ts is changed (prone to interference). Its specific circuit consists of the following categories:
(1) Buck circuit - step-down chopper, its output average voltage
U0 is smaller than the input voltage Ui and has the same polarity.
(2) Boost circuit - boost chopper, its output average voltage
U0 is greater than the input voltage Ui and has the same polarity.
(3) Buck-Boost circuit - buck or boost chopper, its
The output average voltage U0 is greater than or less than the input voltage Ui, the polarity is opposite, and the inductance is transmitted.
(4) Cuk circuit - buck or boost chopper, its output average
The voltage U0 is greater than or less than the input voltage Ui, the polarity is reversed, and the capacitance is transmitted.
There are also Sepic and Zeta circuits.
The above is a non-isolated circuit, and the isolated circuit has a forward circuit, a flyback circuit, a half bridge circuit, a full bridge circuit, and a push pull circuit.
Today's soft switching technology makes a qualitative leap in DC/DC. The various ECI soft-switching DC/DC converters designed and manufactured by VICOR in the United States have a 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 company 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, using synchronous rectifier (MOS FET instead of SCHOTT The base diode) increases the overall circuit efficiency to 90%.
2.2AC/DC conversion
AC/DC conversion is to convert AC to DC, and its power flow direction can be bidirectional. The power flow from the power supply to the load is called “rectificationâ€, and the power flow is returned to the power supply by the load called “active inverterâ€. AC/DC converter input is 50/60Hz AC, because it must be rectified and filtered, so the relatively large size of the filter capacitor is essential, and at the same time due to safety standards (such as UL, CCEE, etc.) and EMC directives Restrictions (such as IEC, FCC, CSA), EMC input must be added to the AC input side and components that comply with safety standards are used, which limits the miniaturization of the AC/DC power supply. In addition, due to internal high frequency, high voltage, and large The current switching action makes it more difficult to solve the EMC electromagnetic compatibility problem, which puts high requirements on the internal high-density installation circuit design. For the same reason, the high voltage and high current switch increase the power supply operation loss, which limits the limitation. The modularization process of AC/DC converters must use the power system optimization design method to achieve a certain degree of satisfaction in working efficiency.
AC/DC conversion can be divided into half-wave circuits and full-wave circuits according to the wiring mode of the circuit. According to the number of power phases can be divided into single phase, three phase, multi phase. According to the working quadrant of the circuit, it can be divided into one quadrant, two quadrants, three quadrants and four quadrants.
Switching power supply selectionSwitching power supply has anti-interference performance. Due to its own circuit structure (multi-stage series), general input interference such as surge voltage is difficult to pass. Compared with linear power supply, the output voltage stability is compared with the linear power supply. The larger advantage is that the output voltage stability can reach (0.5~1)%. Switching power supply module as a power electronic integrated device, should pay attention to the following points in the selection:
3.1 Output current selection
Because the switching power supply has high working efficiency, it can generally reach more than 80%. Therefore, in the selection of 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. Usually the output calculation formula is:
Is=KIf
Where: Is—the rated output current of the switching power supply;
If—the maximum absorbed current of the powered device;
K-margin coefficient, generally taking 1.5 to 1.8;
3.2 Grounding
Switching power supply will generate more interference than linear power supply. Grounding and shielding measures should be taken for electrical equipment sensitive to common mode interference. According to EMC limits such as ICE1000, EN61000, FCC, etc., the switching power supply adopts EMC electromagnetic compatibility measures, so the switch The power supply should normally have an EMC EMC filter. For example, Lid Huafu Technology's HA series switching power supply, the FG terminal is connected to the earth or the user's casing to meet the above electromagnetic compatibility requirements.
3.3 protection circuit
The switching power supply must have overcurrent, overheat, short circuit and other protection functions in the design. Therefore, the switching power supply module with complete protection function should be preferred in design, and the technical parameters of the protection circuit should match the working characteristics of the electrical equipment. Avoid damage to electrical equipment or switching power supplies.
Development trend of switching power supply technologyThe development direction of switching power supply is high frequency, high reliability, low consumption, low noise, anti-interference and modularization. Since the key technology of switching power supply is light, small and thin, it is high frequency. Therefore, foreign major switching power supply manufacturers are committed to synchronously develop new high-intelligent components, especially to improve the loss of secondary rectifying devices, and in power iron. The addition of scientific and technological innovations in oxygen (Mn-Zn) materials to improve high magnetic properties at high frequencies and large magnetic flux densities (Bs), and the miniaturization of capacitors is also a key technology. The application of SMT technology has made great progress in switching power supply, and 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 supply will inevitably innovate the traditional PWM switching technology. The soft switching technology of ZVS and ZCS has become the mainstream technology of switching power supply, and greatly improves the working efficiency of switching power supply. For high reliability indicators, US switching power supply manufacturers reduce the stress of the device by reducing the operating current and lowering the junction temperature, which greatly improves the reliability of the product.
Modularity is the general trend of switching power supply development. It can be composed of modular power supply to form a distributed power system. It can be designed as an N+1 redundant power supply system and realize capacity expansion in parallel mode. Aiming at the shortcoming of the noise of the switching power supply, if the high frequency is pursued separately, the noise will increase with the use of the partial resonance conversion circuit technology. In theory, the high frequency can be realized and the noise can be reduced. There are still technical problems in the practical application of the resonant conversion technology, so a lot of work still needs to be done in this field to make the technology practical.
The continuous innovation of power electronics technology has made the switching power supply industry have broad development prospects. To speed up the development of China's switching power supply industry, we must take the road of technological innovation, and walk out the road of joint development of production, education and research with Chinese characteristics, and contribute to the rapid development of China's national economy.
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