Interference and suppression of the hottest switch

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Interference and suppression of switching power supply Abstract: the interference generated by switching power supply directly endangers the normal operation of electronic equipment. Suppressing the electromagnetic noise of switching power supply is an important subject in the development and design of switching power supply. This paper briefly introduces the generation and propagation mechanism of electromagnetic interference in switching power supply, and summarizes several main methods to suppress the generation and propagation of electromagnetic interference in switching power supply

key words: switching power supply; Electromagnetic interference; Suppression

0 introduction

switching power supply, as the power supply device of electronic equipment, has the advantages of small size, light weight and high efficiency. It has been widely used in digital circuits. However, it is a strong interference source because it works in the high-frequency switching state, and its interference directly endangers the normal work of electronic equipment. Therefore, suppressing the electromagnetic noise of switching power supply and improving its immunity to electromagnetic interference to ensure the long-term safe and reliable operation of electronic equipment is an important topic in the development and design of switching power supply

1 generation of switching power supply interference

the interference of switching power supply is generally divided into two categories: one is the interference formed by internal components of switching power supply; The second is the interference of switching power supply due to the influence of external factors. Both involve human factors and natural factors

1.1 internal interference of switching power supply

EMI generated by switching power supply is mainly high-order harmonic current interference generated by basic rectifier and peak voltage interference generated by power conversion circuit

1.1.1 basic rectifier

the rectification process of basic rectifier is the most common cause of EMI. This is because the power frequency AC sine wave is no longer a single frequency current after rectification, but becomes a DC component and a series of harmonic components with different frequencies. Harmonics (especially high-order harmonics) will generate conducted interference and radiation interference along the transmission line, which will distort the front-end current. On the one hand, it will distort the current waveform connected to the front-end power line, and on the other hand, it will generate RF interference through the power line

1 1.2 power conversion circuit

power conversion circuit is the core of switching regulated power supply. It has wide production band and rich harmonics. The main components that produce this pulse interference are

1) there is a distributed capacitance between the switch tube and its radiator and the leads inside the shell and the power supply. When the switch tube flows through a large pulse current (generally rectangular wave), the waveform contains many high-frequency components; At the same time, the device parameters used to turn off the power supply, such as the storage time of the switching power tube, the large current at the output stage, and the reverse recovery time of the switching rectifier diode, will cause the circuit to be short circuited instantaneously, resulting in a large short-circuit current. In addition, the load of the switching tube is a high-frequency transformer or energy storage inductor. At the moment when the switching tube is turned on, a large inrush current appears at the primary stage of the transformer, resulting in peak noise

2) the transformer in the switching power supply of high-frequency transformer is used for isolation and voltage transformation, but it will produce electromagnetic induction noise due to leakage inductance; At the same time, under high frequency conditions, the distributed capacitance between transformer layers will transfer the high-order harmonic noise at the primary side to the secondary side, while the distributed capacitance of the transformer to the shell forms another high-frequency path, making it easier for the electromagnetic field generated around the transformer to couple on other leads to form noise

3) when the rectifier diode at the secondary side of the rectifier diode is used as a high-frequency rectifier, due to the reverse recovery time, the charge accumulated in the forward current can not be eliminated immediately when the reverse voltage is applied (due to the existence of carriers, there is current flowing). Once the slope when the reverse current recovers is too large, the inductance flowing through the coil will generate a peak voltage. Under the influence of the transformer leakage inductance and the layout parameters of three production lines expanded by the enterprise, a strong high-frequency interference will be generated, and its frequency can reach tens of MHz

4) the capacitor, inductor and conductor switching power supply will change the characteristics of low-frequency components due to working at a higher frequency, resulting in noise

1.2 external interference of switching power supply the external wall of the building shall be given priority to the external wall internal insulation system

external interference of switching power supply can exist in the form of "common mode" or "differential mode". The type of interference can vary from short duration peak interference to complete power loss. It also includes voltage change, frequency change, waveform distortion, continuous noise or clutter, transient, etc. see Table 1 for the types of power supply interference. It is also necessary to pay more attention to equipment protection, maintenance and ordinary cleaning work

among the several interferences in Table 1, the electrical fast transient pulse group and surge shock wave can be transmitted through the power supply and cause damage to the equipment or affect its work. As long as the power supply does not produce vibration stop, output voltage drop and other phenomena, the displacement stroke is 250mm, There will be no impact on electrical equipment caused by power supply

2 switching power supply interference coupling path

switching power supply interference coupling path has two ways: one is conductive coupling, the other is radiation coupling

2.1 conductive coupling

conductive coupling is one of the main coupling ways between disturbance source and sensitive equipment. Conductive coupling must have a complete circuit connection between the disturbance source and the sensitive equipment. Electromagnetic disturbance transmits electromagnetic disturbance from the disturbance source to the sensitive equipment along this connection circuit to generate electromagnetic interference. According to its coupling mode, it can be divided into circuit coupling, capacitive coupling and inductive coupling. In switching mode power supply, these three coupling modes exist simultaneously and are interrelated

2.1.1 circuit coupling

circuit coupling is the most common and simplest conductive coupling mode. It also has the following types:

1) when the direct conduction coupling wire passes through the environment with disturbance, that is, it picks up the disturbance energy and transmits it to the circuit along the wire to cause interference to the circuit

2) common impedance coupling because more than two circuits have common impedance, when the current of two circuits flows through a common impedance, the voltage formed by the current of one circuit on the common impedance will affect the other circuit, which is common impedance coupling. The common impedance coupling disturbance is caused by the output impedance of the power supply, the common impedance of the grounding wire, etc

21.2 capacitive coupling

capacitive coupling is also called electrical coupling. Because the spike voltage generated by two circuits is a narrow pulse with large amplitude, and there is parasitic capacitance between its frequencies, the charge of one circuit affects the other branch through the parasitic capacitance

2.1.3 inductive coupling

inductive coupling is also called magnetic coupling. When there is mutual inductance between two circuits, when the interference source appears in the form of power supply, the magnetic field generated by this current interferes with adjacent signals through mutual inductance coupling

2.2 radiation coupling

the disturbance coupling caused by radiation is called radiation coupling. Radiation coupling is the transmission of electromagnetic energy from the disturbance source to the receiver in the form of electromagnetic field. There are usually four main coupling ways: antenna coupling, wire induction coupling, closed loop coupling and aperture coupling

2.2.1 radiation coupling between antennas

in practical engineering, there are a lot of antenna electromagnetic coupling. For example, the long signal lines, control lines, input and output leads in the switching power supply have antenna effect, which can receive electromagnetic disturbance and form antenna radiation coupling

2.2.2 inductive coupling of electromagnetic field to conductor

the cable of switching power supply is generally composed of the connecting line of signal circuit, the power supply line of power level circuit and ground wire, in which each conductor is composed of input end impedance, output end impedance and return conductor. Therefore, the cable is the part of the internal circuit exposed outside the chassis, which is most vulnerable to the coupling of the radiation field of the disturbance source to induce the disturbance voltage or disturbance current, and enter the equipment along the wire to form radiation disturbance

2.2.3 coupling of electromagnetic field to closed loop

the coupling of electromagnetic field to closed loop means that the length of the largest part of the loop is less than 1/4 of the wavelength. When the frequency of the radiated disturbance electromagnetic field is relatively low, the radiated disturbance electromagnetic field is coupled with the closed loop

2.2.4 coupling of electromagnetic field through holes and slots

coupling of electromagnetic field through holes and slots refers to the electromagnetic disturbance caused by the radiation disturbance electromagnetic field through the non-metallic equipment shell, the holes and slots on the metal equipment shell, the braided metal shield of the cable, etc

3 some measures to suppress interference

the three elements of electromagnetic interference are disturbance source, propagation path and disturbed equipment. Therefore, appropriate measures should be taken to suppress electromagnetic interference from these three aspects. Firstly, the disturbance source should be suppressed and the cause of disturbance should be eliminated directly; Secondly, eliminate the coupling and radiation between the disturbance source and the disturbed equipment, and cut off the propagation path of electromagnetic interference; The third is to improve the anti-interference ability of the disturbed equipment and reduce its sensitivity to noise. At present, several measures to suppress interference are basically to cut off the coupling channel between the electromagnetic disturbance source and the disturbed equipment. The common methods are shielding, grounding and filtering

1) using shielding technology can effectively suppress the electromagnetic radiation interference of switching power supply, that is, use materials with good conductivity to shield the electric field, and use materials with high permeability to shield the magnetic field. Shielding has two purposes, one is to limit the leakage of internal radiated electromagnetic energy, and the other is to prevent external radiation interference from entering the internal area. Its principle is to use the reflection, absorption and guidance of electromagnetic energy by the shield. In order to suppress the radiation generated by the switching power supply and the impact of electromagnetic disturbance on other electronic equipment, the shielding cover can be processed completely according to the method of shielding the magnetic field, and then the entire shielding cover can be connected with the housing and ground of the system as a whole, so as to effectively shield the electromagnetic field

2) the so-called grounding is to establish a conductive path between two points so as to connect electronic equipment or components to some reference points called "ground". Grounding is an important method for switching power supply equipment to suppress electromagnetic interference. Some parts of the power supply connected with the earth can play a role in suppressing interference. The principle of "one point grounding" shall be followed in the design of circuit system. If multi-point grounding is formed, a closed grounding loop will appear. When the magnetic line of force passes through the loop, magnetic induction noise will be generated. In fact, it is difficult to realize "one point grounding". Therefore, in order to reduce the grounding impedance and eliminate the influence of distributed capacitance, plane or multi-point grounding is adopted. A conductive plane is used as the reference ground, and all parts that need to be grounded are connected to the reference ground nearby. In order to further reduce the voltage drop of the grounding circuit, the bypass capacitor can be used to reduce the amplitude of the return current. In the circuit system where low frequency and high frequency coexist, the ground wires of low frequency circuit, high frequency circuit and power circuit shall be separately connected and then connected to the common reference point

3) filtering is an effective method to suppress conducted interference and plays an extremely important role in the EMC design of equipment or systems. EMI filter, as an important unit to suppress the conducted interference of power line, can suppress the interference from electricity to the power supply itself, and can also suppress the interference generated by switching power supply and fed back to electricity. In the filter circuit, many special filter elements are used, such as through core capacitor, three terminal capacitor and ferrite magnetic ring, which can improve the filter characteristics of the circuit. Properly design or select the filter, and

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