Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

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

China in the 20th century, 80 to 90 years, in order to strengthen the domestic electromagnetic pollution on the current governance, developed a number and CISPR standards, IEC801 and other standards corresponding to international standards. Since August 1, 2003 China 3C certification mandatory (china compulsory certification) work has set off a "hot electromagnetic compatibility", close study of electromagnetic interference caused by electronic and control more and more the concern of researchers, the current has become The current research is a new hot spot. This article will focus on switching power supply of electromagnetic interference associated systematically discussed the mechanism of inhibition technology.

1. Electromagnetic interference suppression of switching power supply

    The formation of the three elements of electromagnetic interference is interference source, transmission and disturbed equipment. Thus, the suppression of electromagnetic interference from these three aspects of manpower. Suppress the interference source, eliminate the interference source and the coupling between the perturbed and radiation equipment, improve immunity disturbed equipment, thereby improving the switching power supply electromagnetic compatibility purposes

1.1 The electromagnetic interference suppression using filter

    Filtering is an important method for electromagnetic interference suppression, it can effectively suppress the electromagnetic interference into the power grid equipment, but also can inhibit electromagnetic interference within the equipment into the grid. Input and output in the switching power supply switching power supply filter circuit is installed, not only can solve the problem of conducted interference, but also to resolve the important weapons of radiation interference. Suppression filter is divided into passive filter and active filter 2 ways.

1.1.1 Passive Filtering

    Passive filter circuit is simple, low cost, reliable performance, is an effective way to suppress electromagnetic interference. Passive filter consists of inductor, capacitor, resistor elements, whose role is to address conducted emissions directly.

Since the original power supply filter capacitor circuit capacity, will produce a pulse rectifier circuit peak current, the current is very much higher harmonic current components, interference on the power grid; other circuit in the switch on-or off, the transformer primary coil will have a ripple current. Since the high rate of current change, the surrounding circuitry will generate induced currents at different frequencies, including the differential and common mode interference signals, these interference signals can be conducted by two power lines to the grid lines and other interference with other electronic devices. Differential mode filter graph can reduce the switching power supply part of the internal differential mode interference signal, but also significantly attenuated the work generated by the device itself signals transmitted to the power of electromagnetic interference. Also under the law of electromagnetic induction, was E = Ldi / dt, where: E is the voltage drop across L; L is inductance; di / dt for the current rate of change. Obviously the smaller the current rate of change required, the greater the inductance is required.

    Pulse current loop circuit by electromagnetic induction and ground or chassis of other components of the loop interference signal generated by a common mode signal; switching power supply circuit in the switch between the collector and the other circuits have a very strong electric field, the circuit will produce displacement current , and this displacement current also belongs to common mode interference signals. Figure 1 * mode filter is used to suppress common mode interference, so that by the attenuation.

    1.1.2 Active Filter

    Active Filter suppress common mode interference is an effective method. The method of departure from the noise source measures taken (Figure 2), the basic idea is to try to remove one from the main circuit and the electromagnetic interference signal equal and opposite phase compensation signal to balance the original interference signal, in order to achieve the purpose of reducing the level of interference. Shown in Figure 2, using the current amplification transistor, the emitter current through the equivalent to the base, the base loop filter. R1, C2 to the base composition of the ripple filter is very small, so that the ripple is small emitter. Since the capacity is less than C2 C3, reduce the size of the capacitor. This approach is only suitable for low-voltage low-power power supply situation. In addition, should pay attention to the design and selection of filter frequency characteristics, pressure resistance, current rating, impedance, shielding, and reliability. Filter is installed to appropriate installation method to correct the interference can be expected to play the role of filtering.