With the development of modern electronic technology and power devices, switching power supply with its small size, light weight, high performance and high reliability are widely used in computers and peripheral equipment communications, automatic control, home appliances and other fields, for the people social life and construction of production provides a great help. However, with the rapid development of modern electronic technology, widely used in electrical and electronic equipment, in the same working environment from the various electronic and electrical equipment is getting closer, further deterioration in the external environment of the electronic circuit. Since the switch to work in high-frequency switching power supply, the internal will produce a high current, voltage change rate, resulting in switching power supplies generate strong electromagnetic interference. Electromagnetic interference signals not only cause pollution on the grid, but also directly affect other electrical equipment or the normal operation of the power supply itself, but also as radiation interference into space, causing electromagnetic pollution, which restricts people's production and life.

China in the 1980s and 1990s, in order to strengthen the current domestic electromagnetic pollution control, developed a number of international standards CISPR standard, IEC801 and other corresponding standards. Since August 1, 2003 to enforce China 3C certification (china compulsory certification) work since, setting off a "EMC hot", a close study of electromagnetic interference and control electronics researchers are increasingly causing concern, the current has become a new focus of current research areas. This paper systematically discusses the mechanism related to the suppression of switching power supply for electromagnetic interference.

1. suppression of electromagnetic interference l Switching Power Supply

Three elements form an electromagnetic interference is interference source, transmission and disturbed equipment. Thus, the suppression of electromagnetic interference should these three staff. Sources of interference suppression, eliminate the interference source and destination by coupling and radiation interference between devices, improve immunity disturbed device, thereby improving the electromagnetic compatibility switching power supply energy.

1.1 The use of filter suppresses electromagnetic interference

Filtering is an important method to suppress electromagnetic interference, it can effectively suppress power of electromagnetic interference into the device, it can also suppress electromagnetic interference inside the device into the grid. Mounted on the input and output switching power supply switching power supply filter circuit, not only can solve the problem of conducted interference, but also address important weapon radiated interference. Filtering suppression technology is divided into passive filter and active filter in two ways.

1.1.1 passive filtering techniques

Passive filter circuit is simple, low cost, reliable performance, is an effective way to suppress electromagnetic interference. Passive filter inductor, capacitors, resistors components, its direct effect is to solve the conducted emission. Principle structure of the switching power supply applications, the passive filter shown in Figure 1.

Since the original power circuit large filter capacitance, the rectifier circuit will produce a pulse peak current, this current is very much higher harmonic current composition, on the grid disturbance; another circuit switch is turned on or off, the transformer primary coil will have a ripple current. Because of the high current rate of change, on the surrounding circuit will produce different frequencies induced current, including the differential mode and common mode interference signal, the interference signal can power line conducted by two other lines to the grid and interference from other electronic devices. Figure in differential mode filtering section can reduce the differential mode interference signal switch inside the power supply, but also greatly attenuate electromagnetic interference generated when the work signaling device itself to the grid. And according to the law of electromagnetic induction, was E = Ldi / dt, where: E is the voltage drop across the L; L is inductance; di / dt is the current rate of change. Obviously the smaller the rate of change of the current requirements, the greater the inductance is required.

Interference signal pulse current loop by loop electromagnetic induction and ground or chassis other circuits consisting generated as a common mode signal; generating a strong electric field between the switching power supply circuit switch collector and other circuits, the circuit will produce displacement currents , and this displacement current also belongs to the common-mode interference signals. Figure 1 * mode filter is used to suppress common mode interference, making it subject to decay.

1.1.2 Active Filter

Active filter technology is an effective way to suppress common mode interference. Measures which depart from the noise source taken (Figure 2), the basic idea is to try to remove from the main circuit an electromagnetic interference signal equal in magnitude and opposite in phase compensation signal to balance the original interference signal, in order to achieve The purpose of reducing the interference level. , The current use of the transistor amplification effect shown in Figure 2, by the emitter current equivalent to the base, the base loop for filtering. Filter R1, C2 composition so that the base of the ripple is very small, so that the emitter of the ripple is very small. The capacity of C2 is smaller than the C3, reducing the capacitance volume. This approach is only suitable for small low-voltage power supply situation. In addition, the design and selection should pay attention to the frequency characteristics of the filter, pressure resistance, rated current, characteristic impedance, shielding, and reliability. To properly position the filter is installed, the installation method to be correct, in order to play the expected interference filtering effect.