Electromagnetic compatibility of communication switching power supply-EEWORLD

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0 Introduction
Modern communications, the normal operation of electronic and electrical equipment cannot be separated from power supply. Communication power supply has an incomparable important position in communication equipment. With the rapid development of the communication industry, communication tools such as mobile phones, telephones, and computers have become more and more common in people's lives. With the continuous updating of communication equipment, the requirements for communication switching power supply are getting higher and higher. Communication switching power supply has the advantages of small size, light weight, high efficiency, and reliable operation. It is widely used in optical data transmission, program-controlled switching, wireless base stations, cable TV systems and IP networks. It is the "heart" of the normal operation of electronic and electrical equipment.

1 Interference of communication switching power supply
For communication switching power supply to work stably, it must have strong anti-electromagnetic interference ability, and have sufficient anti-interference ability for electric fields, magnetic fields and electromagnetic waves, to ensure that it can work normally and the power supply of communication equipment is stable and uninterrupted, and at the same time, it must not be interfered by the communication system itself due to electromagnetic waves during communication. Generally speaking, the interference sources of switching power supply include interference caused by rapid changes in voltage and current, conducted interference and radiated interference. The interference sources of switching power supplies are: the high-power switch tube of the switching power supply works in the switching state of high voltage and high current, and a surge voltage is formed when switching from the on state to the off state, or a surge current is formed when switching from the off state to the on state. Their high-order harmonic components will be emitted outward through space or conducted through the power line to form an interference source. When switching from the off state to the on state, the rectifier diode on the secondary side of the switching transformer is limited by the reverse recovery characteristics, generating a spike-shaped reverse current, which forms a damped sine oscillation with the diode junction capacitance and lead inductance, and also contains a large amount of harmonic components, which constitutes interference.
Characteristics of electromagnetic interference of communication switching power supplies:
(1) The rectifier or freewheeling diode and the main power transformer work in a high voltage, high current and high frequency switching mode. The rapid change of their current and voltage will cause interference, thereby causing instability in the internal operation of the switching power supply and reducing the performance of the power supply.
(2) The polarization and directional characteristics of the transmitting and receiving antennas EMC radiation. Communication switching power supplies are subject to: 9 kHz to 30 MHz radio frequency magnetic field interference;
30 to 1 000 MHz radio frequency electric field interference.
(3) Part of the electromagnetic field radiates to the surrounding space through the gaps in the switching power supply housing, and together with the radiated electromagnetic field generated by the power line and the DC output line, it propagates through space, causing interference to other high-frequency equipment and equipment that is sensitive to electromagnetic fields, causing other equipment to work abnormally.
Therefore, for communication switching power supplies, it is necessary to limit the amount of radiated electromagnetic field interference generated by the conduction interference generated by the load line and the power line during space propagation, so that they can work normally with other telecommunications equipment in the same environment without interfering with each other.

2 Domestic and foreign electromagnetic compatibility standards
Electromagnetic compatibility refers to the ability of a device or system to work normally in its electromagnetic environment and not cause unbearable electromagnetic interference to anything in the environment. There are a large number of electromagnetic compatibility standards at home and abroad, which set constraints for the mutual electromagnetic compatibility of equipment in the system. Among them, CISPR16, CISPR22 and CISPR24 constitute the electromagnetic compatibility test content and test method requirements for information technology equipment including communication switching power supply equipment, which are the most basic requirements for electromagnetic compatibility design of communication switching power supplies at present.

3 Analysis of electromagnetic compatibility issues of switching power supplies
Because communication switching power supplies work in a high-voltage and high-current switching state, the causes of electromagnetic compatibility issues are quite complex. There are electromagnetic interferences from high-frequency signals of communication systems to switching power supplies; at the same time, switching power supplies will also interfere with communications or other electronic and electrical equipment due to their own circuit design, PCB wiring, and the performance of components. Among them, according to the coupling path, it can be divided into two types: conducted interference and radiated interference; according to the different forms of interference signals on the circuit, the interference in the power supply system can be divided into two types: common-mode interference and differential-mode interference. Usually, any conducted interference signal on the line power line can be expressed in two ways: common-mode interference and differential-mode interference.
In the switching power supply, when the main power switch tube works at high voltage, high current or high-frequency switching mode, the waveforms of the switching voltage and switching current are approximately square waves when the resistive load is applied, and the wave signal contains rich high-order harmonics, the spectrum of which can reach more than 1,000 times the square wave frequency. Since the voltage difference can generate an electric field, the flow of current can generate a magnetic field, and the high-frequency part of the rich harmonic voltage and current generates an electromagnetic field inside the device, it causes instability in the internal operation of the device and reduces the performance of the device. At the same time, due to the leakage inductance and distributed capacitance of the power transformer, as well as the working state of the main power switch device is not ideal, high-frequency and high-voltage peak harmonic oscillations are often generated when the high frequency is turned on or off
. The high-order harmonics generated by the harmonic oscillation are transmitted to the internal circuit through the distributed capacitance between the switch tube and the radiator or radiated to the space through the radiator and the transformer. The communication switching power supply adopts active power factor correction. Although the control is complicated, the effect is independent of the load, which improves the power factor and makes the performance better. At the same time, the switching power supply adopts soft switching technology to reduce the power consumption of the circuit switch, reduce noise, and improve the efficiency and reliability of the circuit. However, the soft switching lossless absorption circuit mostly uses L and C for energy transfer, and uses the unidirectional conductivity of the diode to realize the unidirectional conversion of energy. Therefore, the diode in the resonant circuit becomes a major interference source of electromagnetic interference.
In the communication switching power supply, energy storage inductors and capacitors are generally used to form L and C filter circuits to filter differential and common mode interference signals, and convert AC square wave signals into smooth DC signals. Due to the distributed capacitance of the inductor coil, the self-resonant frequency of the inductor coil is reduced, so that a large amount of high-frequency interference signals pass through the inductor coil and propagate outward along the AC power line or DC output line. As the frequency of the interference signal increases, the capacitance and filtering effect of the filter capacitor continue to decrease due to the effect of the lead inductance, until it reaches above the resonant frequency, completely loses the function of the capacitor and becomes inductive. Improper use of filter capacitors and excessively long leads are also a cause of electromagnetic interference.

[page]4 Electromagnetic compatibility solution
(1) Solve the electromagnetic compatibility inside the switching power supply
Reduce the internal interference when designing the communication switching power supply itself: suppress the noise of the high-frequency switching transformer, absorb and buffer, and reduce leakage inductance; when designing the circuit, the PCB is reasonably wired and loops are avoided as much as possible; put the ones with heavier interference together and keep the ones with low frequency and low voltage interference away; reduce the area of the loop as much as possible; keep the positive and negative wires as close as possible; enhance the design of the input/output filter circuit, improve the performance of the APFC circuit, and eliminate or reduce the rapid change of the diode current. Among them, the commonly used circuits are zero voltage switch ZVS, zero current switch ZCS and quasi-resonant ZVS/ZCS circuits. This technology greatly reduces the electromagnetic interference generated by the switching device. It uses the advantages of lossless absorption technology combined with resonant zero voltage technology and zero current technology in the soft switching technology to solve the resonance loss caused by parallel or series resonant networks in the circuit. It shapes the waveform of the power switch tube; isolates analog and digital, high voltage and low voltage, etc.
(2) Eliminate electromagnetic interference and improve the working performance of the switching power supply

Eliminate conducted interference and radiated interference of communication switching power supplies. Conducted interference is mainly caused by the propagation of signals through the power grid, which will cause serious interference to other electronic equipment, often causing more serious problems. Commonly used suppression methods include: buffer method, reducing coupling path method, reducing parasitic components method, etc. The limit value of conducted interference can refer to the electromagnetic compatibility specifications in the national standard GB9254-1988, (386833.9-87, GB6833.4-1987, GB6833-1987.
In radiation research, the antenna is an electromagnetic radiation source. In the switching power supply circuit, the components and connections in the main circuit can be considered as antennas. At the same time, the data lines and address lines of the MCU and LCD of mobile phones have high operating frequencies, which are also the main sources of radiated interference. The anti-interference ability of small signal circuits that are susceptible to external interference can be improved by adding devices that improve the anti-interference ability; and various grounding measures should be considered comprehensively to improve the overall Electromagnetic compatibility. The switching power supply is susceptible to common-mode/differential-mode interference in the input circuit. At this time, the EMI filter circuit can be used to suppress this interference. The EMI filter circuit is shown in Figure 1(a). Among them, L1 and L2 are common-mode suppression inductors, which form a line low-pass filter with C1~C7: C1, C4, C5 are used to suppress differential-mode noise. Here, 0.33μF polypropylene film capacitors are selected; C2, C3 and C6, C7 are used to suppress common-mode noise. Because they are installed between the casing and the terminals, there will be leakage current flowing to the casing. To prevent electric shock, mica capacitors with small leakage current and not easy to break down and damage are selected here. The capacity is 3.300 pF and 0.1μF; the withstand voltage of C1~C7 is selected to be AC 250 V.
In the internal and external interference and anti-interference of the switching power supply, the common-mode signal has a very complicated relationship with the working mode of the switching device, the installation of the heat sink, and the connection between the PCB board and the casing of the whole machine. The common-mode signal can be converted into a differential-mode signal under certain conditions. In addition to the general EMI filter circuit mentioned above, the circuit can also be improved according to the idea of the following circuit diagram to solve the common-mode interference, so that the switching power supply can be improved in the circuit to improve the performance. Figure 1(b) is a typical circuit of a common-mode/differential-mode interference filter, and Figure 1(c) is a common-mode/differential-mode interference filter circuit that is a variation of Figure 1(b).

(3) Isolate the mutual interference between the power supply and other devices, and enhance the anti-interference ability of the communication switching power supply. In the small signal circuits of the communication port and the control port, select devices with anti-static interference. The frequency spectrum of the unit pulse interference is the widest, and it is easy to be transmitted into the control circuit in a common mode. It can be eliminated by an absorption filter, reducing the distributed capacitance of the common mode inductor, and strengthening the common mode signal filtering of the input circuit to improve the anti-interference performance of the system. Isolate and shield the interference of other interference signals, as well as its own interference to other devices.
Cut off the propagation path of the interference signal: electromagnetic shielding, use a metal shell to enhance the shielding effect, and perform good grounding treatment (note that the earth and the system ground cannot be connected together). The large area grounding between each control unit is shielded by a grounding plate, which can also improve the stability of the internal operation of the switching power supply.

5 Conclusion
This paper analyzes the interference and characteristics that the communication switching power supply is susceptible to during operation. Combined with communication technology and the relevant performance indicators of the switching power supply, and referring to the current domestic and foreign electromagnetic compatibility standards, according to the electromagnetic compatibility problems of the communication switching power supply, a feasible method for solving the electromagnetic compatibility of the communication switch capacitor is proposed to improve the working performance of the communication switching power supply.

Reference address：Electromagnetic compatibility of communication switching power supply

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