Pulse power grid appeared anti-tamper how to deal with

: switching power supply because of its own operating characteristics make electromagnetic interference problem is quite prominent. From the switching power supply electromagnetic interference model to discuss the switching power supply electromagnetic compatibility problems arising from the causes and types, and gives the commonly used measures to inhibit electromagnetic interference switching power supply, filter design and parameter selection. Keywords: switching power supply; electromagnetic interference; analysis and suppression Introduction In recent years, switching power supply has been rapidly developed with the advantages of high efficiency, small size and good output stability. However, due to the high frequency, high di/dt and high dv/dt during the working process of switching power supply makes the electromagnetic interference problem very prominent. The new 3C certification has replaced the CCIB and CCEE certification in China, which makes the requirements for switching power supplies in terms of electromagnetic compatibility more detailed and strict. Nowadays, how to reduce or even eliminate the EMI problem of switching power supplies has become a great concern for switching power supply designers and electromagnetic compatibility (EMC) designers all over the world. This paper discusses the reasons for the formation of switching power supply EMI and commonly used EMI suppression methods.1 Switching Power Supply Interference Sources AnalysisThe most fundamental reason for switching power supply EMI is its high di/dt and high dv/dt generated in the process of its operation, which generates inrush currents and spike voltages to form the source of the interference. Industrial frequency rectifier filtering using large capacitor charging and discharging, switching tube voltage switching during high-frequency operation, the output rectifier diode reverse recovery current are such sources of interference. Most of the voltage and current waveforms in the switching power supply are close to the rectangular periodic waveforms, such as the drive waveform of the switching tube, MOSFET leakage waveforms and so on. For rectangular waveforms, the reciprocal of the period determines the fundamental frequency of the waveform; the reciprocal of the rise time or fall time of the edge of the twofold pulse determines the frequency value of the frequency component caused by these edges, with typical values in the MHz range, and its harmonic frequency is even higher. All of these high-frequency signals interfere with the fundamental signals of the switching power supply, especially those of the control circuits. The electromagnetic noise of switching power supplies can be divided into two main categories in terms of noise sources. One is external noise, for example, transmitted through the power grid **** mode and differential mode noise, external electromagnetic radiation on the switching power supply control circuit interference. Another category is the electromagnetic noise generated by the switching power supply itself, such as switching tube and rectifier current spikes generated by harmonics and electromagnetic radiation interference. As shown in Figure 1, the power grid contains *** mode and differential mode noise on the switching power supply interference, switching power supply in the electromagnetic interference at the same time also on the grid other equipment as well as the load electromagnetic interference (such as return noise, output noise and radiation interference in the figure). Switching power supply EMI/EMC design on the one hand to prevent switching power supply on the grid and nearby electronic equipment interference, on the other hand, to strengthen the switching power supply itself on the electromagnetic harassment environment adaptability. The following specific analysis of switching power supply noise causes and ways.1.1 Electromagnetic Noise Introduced by the Power LinePower line noise is the electromagnetic harassment generated by various power equipment in the power grid along the power line propagation caused by. Power line noise is divided into two categories: *** mode interference, differential mode interference. *** mode interference (Common-modeInterference) is defined as any current-carrying conductor and the reference ground between the unwanted potential difference; Differential-modeInterference (Differential-modeInterference) is defined as any two current-carrying conductors between the unwanted potential difference. The equivalent circuits of the two interferences are shown in Fig. 2 [1]. In the figure, CP1 is the distributed capacitance between the primary and secondary of the transformer, and CP2 is the distributed capacitance between the switching power supply and the heat sink (i.e., between the collector of the switching tube and ground). As shown in Figure 2 (a), switching tube V1 from conduction to cut-off state, its collector voltage suddenly rise to a high voltage, this voltage will cause *** mode current Icm2 to CP2 charging and *** mode current Icm1 to CP1 charging, the charging frequency of the distribution capacitor that is the operating frequency of the switching power supply. Then the total size of the *** mode current in the line is (Icm1 + Icm2). As shown in Fig. 2(b), when V1 conducts, the differential mode current Idm and the signal current IL circulate along the loop composed of the wire, the transformer primary, and the switching tube. From the equivalent model, it can be seen that the *** mode interference current does not pass through the ground line, but is transmitted through the input power line. And the differential mode interference current is transmitted through the ground and input power line loops. Therefore, when we set up the power line filter, we should take into account the difference between the differential mode interference and the *** mode interference, and use the differential mode or *** mode filtering components to suppress their interference on their transmission paths to achieve the best filtering effect.1.2 Noise caused by input current distortionThe input of a switching power supply is commonly used as a bridge rectifier, capacitor filtering type of rectifier power supply. As shown in Fig. 3, in the input stage without PFC function, due to the nonlinearity of the rectifier diode and the energy storage effect of the filter capacitor, which makes the diode's conduction angle smaller, the input current i becomes a periodic spike current with a very short time and high peak value. This distorted current essentially contains rich high harmonic components in addition to the fundamental component. These high harmonic components are injected into the power grid, causing serious harmonic pollution and interference to other appliances on the grid. In order to control the pollution of the switching power supply to the power grid as well as to achieve a high power factor, the PFC circuit is an indispensable part.1.3 Interference generated by switching tubes and transformersThe main switching tube is the core device of the switching power supply, and it is also a source of interference. Its operating frequency is directly related to the intensity of electromagnetic interference. As the operating frequency of the switching tube increases, the switching tube voltage and current switching speed up, and its conduction interference and radiation interference increases. In addition, the reverse recovery characteristics of the clamping diode in anti-parallel on the main switching tube is not good, or improper selection of the parameters of the voltage spike absorption circuit will also cause electromagnetic interference. Switching power supply operating process, by the primary filtering large capacitors, high-frequency transformer primary coil and switching tube constitutes a high-frequency current loop. This loop generates large radiated noise. Switching loop in the switching tube load is a high-frequency transformer primary coil, it is an inductive load, so the switching tube on and off in the high-frequency transformer will appear at the primary ends of the spike noise. The lighter one causes interference, and the heavier one breaks down the switching tube. Distributed capacitance and leakage inductance between the windings of the main transformer are also important factors causing electromagnetic interference.1.4 Interference Generated by Output Rectifier DiodeThe ideal diode cuts off when subjected to reverse voltage, and no reverse current passes through. While the actual diode forward conduction, the charge within the PN junction is accumulated, when the diode is subjected to reverse voltage, the accumulated charge within the PN junction will be released and form a reverse recovery current, which recovers to zero time with the junction capacitance and other factors. The reverse recovery current will produce stronger high-frequency attenuation oscillations under the influence of transformer leakage inductance and other distribution parameters. Therefore, the reverse recovery noise of the output rectifier diode also becomes a major source of interference in switching power supplies. Can be connected in parallel at both ends of the diode RC buffer, in order to suppress its reverse recovery noise.1.5 Distribution and parasitic parameters caused by the switching power supply noise switching power supply distribution parameters is the intrinsic factor of most of the disturbances, switching power supply and heat sink distribution capacitance between the initial stage of the transformer, distribution capacitance, the primary and secondary leakage inductance is a source of noise. *** Mode interference is transmitted through the distributed capacitance between the transformer primary and secondary stages and the distributed capacitance between the switching power supply and the heat sink. Among them, the distributed capacitance of the transformer winding is related to the high-frequency transformer winding structure and manufacturing process. Can be improved through the winding process and structure, increase the insulation between the windings, the use of Faraday shielding and other methods to reduce the distribution capacitance between the windings. The distributed capacitance between the switching power supply and the heat sink is related to the structure of the switching tube and the installation method of the switching tube. The use of insulating pads with shielding can reduce the distributed capacitance between the switching tube and the heat sink. As shown in Fig. 4, components operating at high frequencies have high-frequency parasitic characteristics [2], which have an effect on their operating state. At high frequency operation wires become emitting wires, capacitors become inductors, inductors become capacitors, and resistors become *** vibration circuits. Observation of the frequency characteristic curve in Figure 4 reveals that when the frequency is too high the frequency characteristics of each component produce a considerable change. In order to ensure the stability of the switching power supply in high-frequency operation, the design of the switching power supply should fully consider the characteristics of the components in high-frequency operation, and choose to use a better high-frequency characteristics of the components. In addition, at high frequencies, the inductive reactance of the wire parasitic inductance increases significantly, due to the inductance of the uncontrollability of the inductance, which ultimately turns it into a transmitting wire. It also becomes a source of radiated interference in the switching power supply.2 switching power supply EMI suppression measures electromagnetic compatibility of the three elements are the source of interference, coupling path and sensitive body, inhibit any of the above can reduce electromagnetic interference problems. Switching power supply operating at high voltage and high current high-frequency switching state, the electromagnetic compatibility problems caused by it is more complex. However, still in line with the basic electromagnetic interference model, you can start from the three elements to seek ways to inhibit electromagnetic interference. 2.1 inhibit all kinds of electromagnetic interference sources in the switching power supply in order to solve the input current waveform distortion and reduce the harmonic content of the current, the switching power supply needs to use the power factor correction (PFC) technology. pfc technology makes the current waveform follow the voltage waveform, the current waveform is corrected to an approximate sinusoidal waveform, thus reducing the current harmonic content, and the current waveform is corrected to an approximate sine wave. This reduces the harmonic content of the current, improves the input characteristics of the bridge rectifier-capacitor filter circuit, and also improves the power factor of the switching power supply. Soft switching technology is an important method to reduce the loss of switching devices and improve the electromagnetic compatibility characteristics of switching devices. Switching device turn on and off will produce inrush current and spike voltage, which is the main reason for switching tube electromagnetic interference and switching loss. The use of soft switching technology so that the switching tube at zero voltage, zero current switching conversion can effectively suppress electromagnetic interference. The use of buffer circuits to absorb the spike voltage at the primary coil of the switching tube or high frequency transformer can also effectively improve the electromagnetic compatibility characteristics. The reverse recovery problem of the output rectifier diode can be suppressed by connecting a saturated inductor in series with the output rectifier tube, as shown in Figure 5, where the saturated inductor Ls works in series with the diode. The core of the saturation inductor is made of a magnetic material with a rectangular BH curve. As with the materials used in magnetic amplifiers, the inductor made from this core has a high permeability, and the core has a nearly vertical linear region on the BH curve and easily enters saturation. In practice, in the output rectifier diode conduction, so that the saturated inductor work in saturation, equivalent to a section of wire; when the diode off reverse recovery, so that the saturated inductor work in the inductive characteristics of the state, hindering the reverse recovery current of a large change, thereby suppressing it to the external interference. 2.2 Cut off the transmission path of electromagnetic interference - **** mode, differential mode power line filter design power line Interference can be filtered out using a power line filter, switching power supply EMI filter basic circuit shown in Figure 6. A reasonable and effective switching power supply EMI filter should be on the power line differential mode interference and *** mode interference have a strong suppression effect. In Figure 6 CX1 and CX2 are called differential mode capacitors, L1 is called a ****-mode inductor, and CY1 and CY2 are called ****-mode capacitors. The differential mode filtering element and *** mode filtering element have strong attenuation effect on differential mode and *** mode interference respectively. The ****-mode inductor L1 is made up of two windings with opposite directions and the same number of turns on the same magnetic ring. Usually use ring core, small leakage, high efficiency, but winding difficulties. When the utility network frequency current flows through the two windings for one in and one out, the magnetic field generated by exactly offset, making **** mode inductor on the utility network frequency current does not have any obstruction, can be transmitted without loss. If the utility network contains ***mode noise current through the ***mode inductor, this ***mode noise current is the same direction, flow through the two windings, the magnetic field generated by the superposition of the same phase, so that the ***mode inductor on the interference current shows a larger sense of resistance, which plays a role in suppressing the ***mode interference. the inductance of the L1 and the rated current of the EMI filter I, the specific relationship between the listed in Table 1. Table 1 inductance range and the relationship between the rated current in practical use *** mode inductor two inductor windings due to the winding process there will be a difference in inductance, but this difference is used for differential mode inductance. Therefore, it is not necessary to set up a separate differential mode inductor in the general circuit. The differential inductance of the *** mode inductor and the capacitors CX1 and CX2 form a pi-type filter. This filter provides better attenuation of differential mode interference. In addition to the *** mode inductor, capacitors CY1 and CY2 in Figure 6 are also used to filter out *** mode interference. The attenuation of *** mode filtering is mainly performed by the inductor at low frequencies, while at high frequencies most of it is performed by capacitors CY1 and CY2. Capacitor CY selection should be based on the actual situation, because the capacitor CY is connected to the power line and ground between the voltage is relatively high, so, need to have high withstand voltage, low leakage current characteristics. The formula for calculating the leakage current of capacitor CY is ID=2πfCYVcY where: ID is the leakage current; f is the grid frequency. Generally installed in removable equipment on the filter, its AC leakage current should be <1mA; if installed in a fixed position and grounded equipment on the power supply filter, its AC leakage current should be <3.5mA, the leakage current of the medical device regulations are even smaller. Due to the consideration of the leakage current safety specifications, the size of the capacitor CY is limited, generally 2.2 ~ 33nF. capacitor type is generally porcelain capacitors, the use of the capacitor should be noted in the high-frequency operation of the capacitor CY and the lead inductance of the resonance effect. Differential mode interference suppressor is usually composed of low-pass filtering elements, the simplest is a filter capacitor connected between the two power lines and the formation of the input filter circuit (such as capacitor CX1 in Figure 6), as long as the capacitance selection is appropriate, you can play a role in the suppression of high-frequency interference. The capacitance of high-frequency interference impedance is very low, so the two power lines between the high-frequency interference can pass through it, it is very high impedance of the industrial frequency signals, so the transmission of industrial frequency signals have no effect. The selection of the capacitor is mainly considered the value of the voltage, as long as the power line to meet the voltage rating, and can withstand the predictable voltage impact can be. In order to avoid the impact hazards caused by the discharge current, CX capacitor capacity should not be too large, generally between 0.01 ~ 0.1μF. The capacitor type is ceramic capacitor or polyester film capacitor.2.3 Using shielding to reduce the sensitivity of electromagnetic sensitive equipment An effective way to suppress radiated noise is shielding. You can use materials with good electrical conductivity to shield the electric field and materials with high magnetic permeability to shield the magnetic field. In order to prevent the magnetic field leakage of the transformer, so that the transformer initial stage coupling is good, you can use the closed magnetic ring to form a magnetic shielding, such as the leakage flux of the can-type core is significantly smaller than the E-type. Switching power supply connection line, the power cord should be used with a shielded layer of wire, as far as possible to prevent external interference coupled to the circuit. Or use magnetic beads, magnetic rings and other EMC components to filter out high-frequency interference in the power supply and signal lines, but be careful that the signal frequency can not be interfered with by the EMC components, that is, the signal frequency should be within the passband of the filter. The whole switching power supply shell also needs to have good shielding characteristics, the joints should be in line with the shielding requirements of the EMC regulations. Through the above measures to ensure that the switching power supply is not subject to the external electromagnetic environment and will not interfere with external electronic equipment.3 Conclusion Nowadays, in the switching power supply volume is getting smaller and smaller, the power density is getting bigger and bigger under the trend. EMI / EMC issues become a key factor in the stability of the switching power supply is also one of the most easy to ignore the aspect. The EMI suppression technology of switching power supply occupies a very important position in the design of switching power supply. Practice has proved that the earlier the EMI problem is considered and solved, the smaller the cost and the better the effect. (Information taken from 21IC China Electronics Network) This article is from Shandong Lighting Network | Original link: