Analyze several effective switching power supply electromagnetic interference suppression
With regard to the research of the switching power supply EMI (Electro-Magnetic Interference), some of the mechanisms produced from EMI, some of the effects from EMI have proposed a number of practical valuable programs. Here analysis and compare several effective solutions, and new reference recommendations are made for the suppression measures of the switching power supply EMI.
Generation mechanism of switching power supply electromagnetic interference
Interference from the switching power supply, according to the source of noise interference, can be divided into two types of spike interference and harmonic interference; if the coupling path is used, it can be divided into two types of conductive interference and radiation interference. Now, according to the noise interference source, the source is shown separately:
1, interference caused by the reverse recovery time of the diode
When the rectifier diode in the high-frequency rectifier circuit is turned on, there is a large forward current flowing, when it is turned off by the anti-bias voltage, due to the more carriers accumulated in the pn junction, In a period of time before the child disappears, the current will flow in turn, resulting in a sharp decline in the reverse recovery current of the carrier disappearance (Di / DT).
2, harmonic interference generated during switching tube
The power switch is turned through a larger pulse current in turn. For example, an input current waveform of a positive transistor, a push-pull type, and a bridge converter is approximately a rectangular wave in the resistive load, which is rich in high-end harmonic components. This harmonic interference will be small when zero current and zero voltage switches are used. Further, the current mutation caused by the power switch tube during the cutoff, the high-frequency transformer winding leakage, also generates spike interference.
3. Interference from the AC input loop
Switching power supply input tubes in the non-working frequency transformer causes interference from high frequency attenuation oscillation during reverse recovery. Switching power supply generated spike interference energy, the interference formed by propagating out of the input and output line of the switching power source is called a conduction interference; the energy of harmonics and parasitic oscillations, will be in space when propagating by inputting output lines Generate electric fields and magnetic fields. This interference generated by electromagnetic radiation is called radiation interference.
4, other reasons
The parasitic parameters of the components, the schematic design of the switching power supply is not perfect, the printed circuit board (PCB) traces usually use manual arrangement, with great arbitrarily, the near field interference of the PCB is large, and the installation of the device on the printed board, It will cause EMI interference to place, and unreasonable orientation.
Switching power supply EMI characteristics
As the energy conversion device operated in the switching state, the voltage of the switching power supply, the current change rate is high, the generated interference strength is large; the interference source is mainly concentrated during the power switch and the radiator and high plane transformer connected thereto, relative to the number The position of the circuit interference source is clearer; the switching frequency is not high (from tens of kilohertz and Du Zazhertz), the main interference form is conducted interference and near-field interference; and the printed circuit board (PCB) traces usually use manual wiring, With greater arbitrariness, this increases the extraction of PCB distribution parameters and the difficulty of near-field interference estimation.
EMI test technology
Three methods of current diagnosis of differential mold common mode interference: RF current probe, differential mode suppression network, noise separation network. The use of RF current probes is the easiest way to measure differential mold common mode interference, but the measurement results are more complicated with standard limits. Differential mode suppression network structure is relatively simple, and the measurement results can be compared directly to standard limits, but only common mode interference can be measured. The noise separation network is the most ideal method, but its key component transformer has high manufacturing requirements.
Several measures currently inhibiting interference
Three elements that form electromagnetic interference are interference sources, propagation pathways, and interference equipment. Therefore, suppressing electromagnetic interference should also start from these three aspects. First, the source should be suppressed, directly eliminating the cause of interference; second is the coupling and radiation between interference sources and disturbances, cut off electromagnetic interference; the third is to improve the interference resistance of disturbances, reduce noise Sensitivity. Several measures currently inhibiting interference are basically used with coupling channels between electromagnetic interference sources and disturbances, which are independently effective. Commonly used methods are shielding, grounding and filtering.
Electromagnetic radiation interference of the switching power supply can be effectively suppressed by shielding technology. For example, power switching tubes and output diodes typically have a large power loss, and for heat dissipation, the heat sink is often required or mounted directly on the power supply bottom plate. When the device is installed, an insulating sheet having good thermally conductive performance is insulated, which makes a distributed capacitor between the device and the bottom plate and the heat sink, and the bottom plate of the switching power supply is a ground wire of the AC power source, so the distribution capacitance between the device and the bottom plate. The electromagnetic interference is coupled to the AC input to generate common mode interference, and the solution to solve this problem is to use a layer of shielded sheet between the two insulating sheets and connect the shield to DC, and cut the radio frequency interference to input grid. Way. In order to suppress the radiation generated by the switching power supply, the influence of electromagnetic interference on other electronic devices can be processed in accordance with the method of masking the magnetic field shield, and then connect the entire shield with the system's casing and ground to the electromagnetic field. Perform effective shielding. Some parts of the power supply are connected to the earth to suppress the effects of interference. For example, the electrostatic shield layer can inhibit the interference of the electric field; the conductor for electromagnetic shields can be in principle, but the non-ground shield conductor often enhances the electrostatic coupling to generate a so-called negative electrostatic shielding effect, so it is still grounded, so The electromagnetic shield can simultaneously exert the effects of electrostatic shielding. The common reference point of the circuit is connected to the earth, and a stable reference potential can be provided for the signal circuit. Therefore, the security ground wire in the system, the shielding ground line, and the public reference ground line each form the ground bus, and it is ultimately connected to the earth.
In the circuit system design, the principle of grounding should be followed. If a multi-point ground is formed, a closed ground loop will occur. When the magnetic line will generate magnetic induction noise, it is actually a little grounding. Therefore, in order to reduce the ground impedance, the impact of the distribution capacitance is eliminated, the planar or multi-point ground is taken, and the conductive plane (conductive plane layer, etc. of the bottom plate or multilayer printed plate circuit) is used as a reference ground, and the part of the ground is required. Close to the reference ground. To further reduce the pressure drop of the ground loop, the available bypass capacitance is used to reduce the magnitude of the return current. In the low frequency and high frequency coexisting circuitry, the low frequency circuit, high frequency circuit, and power circuitry should be separately connected, and then connected to the public reference point.
Filtering is a good way to suppress conductive interference. For example, on the power input, the filter can suppress the interference of the switching power supply to generate and feedback to the grid, or suppressing the noise of the power supply to the power supply itself. In the filter circuit, many dedicated filter elements, such as wearing an cardiac capacitor, a triple capacitor, a ferrite magnetic ring, which can improve the filtering characteristics of the circuit. Appropriately designed or select a filter and properly install and use a filter, which is an important part of anti-interference techniques.
Original link: https://www.eeboard.com/news/emi/
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