Interference and Suppression of Switching Power Supply

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

1.1 Internal interference

of switching power supply The EMI generated by the switching power supply is mainly caused by the high-order harmonic current interference generated by the basic rectifier and the peak voltage interference generated by the power conversion circuit . 1.1.1 Basic rectifier The rectification process of the basic rectifier is the most common cause of EMI. This is because the industrial 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 radiated interference along the transmission line, causing the front-end current to be distorted. On the one hand, it distorts the current waveform on the front-end power line, and on the other hand, it generates radio frequency interference through the power line. 1. 1.2 Power conversion circuit The power conversion circuit is the core of the switching voltage-stabilized power supply. It has a wide bandwidth and rich harmonics. The main components that produce this pulse interference are 1) Switching tubes There is distributed capacitance between the switching tube and its heat sink and the casing and the leads inside the power supply . When a large pulse current (roughly a rectangular wave) flows through the switching tube, the waveform contains many high-frequency components; at the same time, the device parameters used in the power supply, such as the storage time of the switching power tube , the large current of the output stage, and the reverse recovery time of the switching rectifier diode , will cause the circuit to short-circuit instantly and generate 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 in the primary of the transformer, causing spike noise. 2) High-frequency transformer The transformer in the switching power supply is used for isolation and voltage transformation, but due to leakage inductance, electromagnetic induction noise will be generated; at the same time, under high-frequency conditions, the distributed capacitance between the transformer layers will transfer the high-order harmonic noise on the primary side to the secondary, and the distributed capacitance of the transformer to the casing forms another high-frequency path, making it easier for the electromagnetic field generated around the transformer to couple to other leads to form noise. 3) Rectifier diode When the secondary side rectifier diode is used for high-frequency rectification, due to the reverse recovery time factor, the charge accumulated by the forward current cannot be eliminated immediately when the reverse voltage is applied (due to the existence of carriers, current still flows). Once the slope of the reverse current recovery is too large, the inductance flowing through the coil will produce a spike voltage, which will produce strong high-frequency interference under the influence of the transformer leakage inductance and other distributed parameters, and its frequency can reach tens of MHz. 4) Capacitors, inductors and wires Since the switching power supply operates at a higher frequency, the characteristics of low-frequency components will change, thereby generating noise. 1.2 External interference of switching power supply External interference of switching power supply can exist in "common mode" or "differential mode". The interference type can vary from short-duration spike interference to complete power failure. It also includes voltage changes, frequency changes, waveform distortion, continuous noise or clutter, and transients. The types of power supply interference are shown in Table 1. Among the several types of interference in Table 1, the main ones that can be transmitted through the power supply and cause damage to the equipment or affect its operation are electrical fast transient pulse groups and surge shock waves. As long as the power supply equipment itself does not produce phenomena such as stop vibration and output voltage drop, interference such as electrostatic discharge will not cause the impact on the electrical equipment caused by the power supply. 2 Switching power supply Interference coupling path There are two ways of switching power supply interference coupling: one is conduction coupling and the other is radiation coupling. 2.1 Conductive coupling Conductive coupling is one of the main coupling paths between the interference source and the sensitive equipment. Conductive coupling must have a complete circuit connection between the interference source and the sensitive equipment. Electromagnetic interference is transmitted from the interference source to the sensitive equipment along this connection circuit, generating electromagnetic interference. According to its coupling mode, it can be divided into circuit coupling, capacitive coupling and inductive coupling. In the switching power supply, these three coupling modes exist simultaneously and are interconnected. 2.1.1 Circuit coupling Circuit coupling is the most common and simplest conduction coupling mode. There are several types: 1) Direct conduction coupling When the wire passes through an environment with interference, it picks up interference energy and conducts it along the wire to the circuit, causing interference to the circuit. 2) Common impedance coupling Since two or more circuits have a common impedance, when the current of the 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 interference is formed by the output impedance of the power supply and the common impedance of the ground wire. 2.1.2 Capacitive coupling Capacitive coupling is also called electrical coupling. Since the peak voltage generated between the two circuits is a narrow pulse with a large amplitude, there is parasitic capacitance between the frequencies, so that 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 the two circuits, when the interference source appears in the form of power supply, the magnetic field generated by this current interferes with the adjacent signal through mutual inductance coupling. 2.2 Radiative coupling The interference coupling caused by radiation is called radiative coupling. Radiative coupling is the transmission of electromagnetic energy from the disturbance source to the receiver through space in the form of an electromagnetic field. There are usually four main coupling pathways: antenna coupling, wire inductive coupling, closed loop coupling, and aperture coupling. 2.2.1 Radiative coupling between antennas In actual engineering, there is a large amount of electromagnetic coupling between antennas. For example, the long signal lines, control lines, input and output leads in the switching power supply have antenna effects and can receive electromagnetic disturbances, forming antenna radiation coupling. 2.2.2 Inductive coupling of electromagnetic fields to wires

























































The cable of the switching power supply is generally composed of the connection line of the signal loop, the power supply line of the power level loop and the ground line. Each wire is composed of the input impedance, the output impedance and the return wire to form a loop. Therefore, the cable is the part of the internal circuit exposed outside the chassis. It is most susceptible to the coupling of the radiation field of the interference source and induces interference voltage or interference current, which enters the equipment along the wire to form radiation interference.

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 that is induced is less than 1/4 of the wavelength. When the frequency of the radiated interference electromagnetic field is relatively low, the electromagnetic coupling of the radiated interference electromagnetic field and the closed loop.

2.2.4 Coupling of electromagnetic field through apertures

The coupling of electromagnetic field through apertures refers to the electromagnetic interference of the radiated interference electromagnetic field to the inside of the non-metallic equipment shell, the apertures on the metal equipment shell, the braided metal shielding of the cable, etc.
3 Some measures to suppress interference The three elements that form electromagnetic interference are the interference source, the propagation path and the disturbed equipment. Therefore, the suppression of electromagnetic interference should also start from these three aspects and take appropriate measures. First, the source of interference should be suppressed to eliminate the cause of interference directly; second, the coupling and radiation between the source of interference and the disturbed equipment should be eliminated to cut off the propagation path of electromagnetic interference; third, the anti-interference ability of the disturbed equipment should be improved to reduce its sensitivity to noise. At present, several measures to suppress interference are basically to cut off the coupling channel between the source of electromagnetic interference and the disturbed equipment. Common methods are shielding, grounding and filtering. 1) The use of shielding technology can effectively suppress the electromagnetic radiation interference of the switching power supply , that is, to shield the electric field with materials with good conductivity and to shield the magnetic field with materials with high magnetic permeability. Shielding has two purposes, one is to limit the leakage of electromagnetic energy radiated from the inside, and the other is to prevent external radiation interference from entering the internal area. The principle is to use the reflection, absorption and guidance of electromagnetic energy by the shielding body. In order to suppress the radiation generated by the switching power supply and the influence of electromagnetic interference on other electronic equipment, the shielding cover can be processed completely according to the method of magnetic field shielding, and then the entire shielding cover is connected to the system casing and ground as a whole, which can effectively shield the electromagnetic field. 2) Grounding is to establish a conduction path between two points so that electronic equipment or components can be connected to some reference points called "ground". Grounding is an important method for switching power supply equipment to suppress electromagnetic interference. Connecting some parts of the power supply to the earth can suppress interference. In the design of the circuit system, the principle of "one-point grounding" should be followed. If multiple points are grounded, a closed grounding loop will appear. When the magnetic lines pass through the loop, magnetic induction noise will be generated. In fact, it is difficult to achieve "one-point grounding". Therefore, in order to reduce the grounding impedance and eliminate the influence of distributed capacitance , planar or multi-point grounding is adopted. A conductive plane is used as the reference ground, and the parts that need to be grounded are connected to the reference ground nearby. To further reduce the voltage drop of the ground loop, bypass capacitors can be used to reduce the amplitude of the return current . In a circuit system where low frequency and high frequency coexist, the ground wires of the low frequency circuit, high frequency circuit, and power circuit should be connected separately 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 electromagnetic compatibility design of equipment or systems. As an important unit for suppressing the conducted interference of power lines, EMI filters can suppress the interference from the power grid to the power supply itself, and can also suppress the interference generated by the switching power supply and fed back to the power grid. In the filter circuit, many special filter components are also used, such as through-hole capacitors, three-terminal capacitors, and ferrite magnetic rings, which can improve the filtering characteristics of the circuit. Proper design or selection of filters, and correct installation and use of filters are important components of anti-interference technology. When selecting a filter, pay attention to the following points: (1) Clearly define the operating frequency and the interference frequency to be suppressed. If the two are very close, a filter with a very steep frequency characteristic is required to separate the two frequencies; (2) Ensure that the filter can work reliably under high voltage conditions; (3) When the filter continuously passes the maximum rated current, its temperature rise must be low to ensure that the working performance of the components in the filter is not damaged when it continuously works at the rated current ; (4) In order to make the filter frequency characteristics during operation consistent with the design value, the values ​​of the signal source impedance and load impedance connected to it are required to be equal to the specified values ​​during design: (5) The filter must have a shielding structure, and the shielding box cover and the body must have good electrical contact. The capacitor leads of the filter should be as short as possible. It is best to use a through-hole capacitor with short leads and low inductance; (6) It must have high working reliability, because the faults of filters used to protect against electromagnetic interference are often more difficult to find than those of other components. The following points should be noted when installing the filter: (1) The power line filter should be installed as close to the power inlet of the equipment as possible. Do not allow the power line that has not passed through the filter to meander inside the equipment frame; (2) The capacitor leads in the filter should be as short as possible to avoid resonance at a lower frequency due to the inductive reactance and capacitive reactance of the leads; (3) A large short-circuit current passes through the grounding wire of the filter, which will cause additional electromagnetic radiation, so the filter element itself should be well shielded and grounded; (4) The input and output lines of the filter cannot cross, otherwise crosstalk will be caused by the input and output capacitor coupling path of the filter, thereby reducing the filtering characteristics. The usual method is to add a partition or shielding layer between the input and output ends. 4 Conclusion There are many factors that cause switching power supplies to generate electromagnetic interference, and there is still a lot of work to be done to suppress electromagnetic interference. Comprehensively suppressing various noises of switching power supplies will make switching power supplies operate more safely and reliably.