Research on electromagnetic compatibility issues in high-frequency switching power supply design[Copy link]
Compared with linear regulated power supplies, switching power supplies have many advantages such as low power consumption, high efficiency, small size, light weight, and wide voltage regulation range. They have been widely used in computers and their peripherals, communications, automatic control, household appliances and other fields. However, the prominent disadvantage of switching power supplies is that they can generate strong electromagnetic interference (EMI). EMI signals have a wide frequency range and a certain amplitude. After conduction and radiation, they will pollute the electromagnetic environment and interfere with communication equipment and electronic products. If not handled properly, the switching power supply itself will become a source of interference. At present, the electromagnetic compatibility (EMC) of electronic products has received increasing attention. Suppressing the EMI of switching power supplies, improving the quality of electronic products, and making them meet EMC standards have become issues that electronic product designers are paying more and more attention to. This article discusses the electromagnetic compatibility issues in the design of high-frequency switching power supplies.
1 Composition and working principle of switching power supply 1.1 Composition The block diagram of the switching power supply is shown in Figure 1. It consists of the following parts: 1) The main circuit includes input filter, rectification and filtering, inversion, output rectification and filtering; 2) Control and protection circuit; 3) The detection and display circuit provides various display data in addition to various parameters required by the protection circuit; 4) Auxiliary power supply. Figure 1 Composition block diagram of switching power supply 1.2 Principle of switching voltage-stabilized power supply The circuit of switching voltage-stabilized power supply is shown in Figure 2. The switch K in Figure 2 is repeatedly connected and disconnected at a certain time interval. When K is connected, the input power supply Vin supplies power to the load RL through K and the filtering circuit. When K is disconnected, the input power supply Vin interrupts the supply of energy. It can be seen that the input power supply provides energy to the load intermittently. In order to enable the load to obtain continuous energy supply, the switching voltage-stabilized power supply must have a set of energy storage devices to store part of the energy when the switch is connected and release it to the load when the switch is disconnected. In Figure 2, the circuit composed of energy storage inductor L, filter capacitor C2 and freewheeling diode D has this function. The average voltage VAB between AB can be expressed by formula (1). VAB=Vinton/T=DVin (1) Where: ton is the conduction time of K; T is the working cycle of K; D is the duty cycle, D=ton/T. Figure 2 Schematic diagram of switching voltage regulator circuit It can be seen from formula (1) that changing D can change VAB. Therefore, adjusting D with the change of load and input power supply voltage can keep the output voltage Vo unchanged. This control method is called time ratio control (abbreviated as TRC). According to the TRC principle, there are three modes: 1) Pulse Width Modulation (PWM), in which the switching period is constant and the duty cycle is changed by changing the pulse width; 2) Pulse Frequency Modulation (PFM), in which the on-pulse width is constant and the duty cycle is changed by changing the switching frequency; 3) Hybrid modulation, in which both the on-pulse width and the switching frequency are not fixed and can be changed with each other. It is a combination of the above two methods.
2 Mechanism of electromagnetic interference generated by switching power supply The reason why switching power supply is a strong source of electromagnetic interference is that it comes from high-frequency switching devices and output rectifier diodes, as well as pulse transformers and filter inductors. 2.1 Switching tube and rectifier tube The dv/dt and di/dt generated by the high-frequency switching of the switching tube and the rectifier tube are pulses with large amplitude, wide frequency band and rich harmonics, which is a strong source of interference. 2.2 High-frequency transformer The load of the switching tube is the primary coil of the high-frequency transformer. At the moment when the switching tube is turned on, the primary coil generates a large inrush current and a high surge peak voltage; at the moment when the switching tube is turned off, due to the leakage flux of the primary coil, part of the energy is not transmitted to the secondary coil, but through the capacitor and resistor in the collector circuit to form a peaked attenuation oscillation, which is superimposed on the turn-off voltage to form a turn-off voltage spike, generating the same magnetizing impact current transient as when the primary coil is turned on. This noise will be transmitted to the input and output ends, forming a conducted interference, and in severe cases, it may break down the switching tube. In addition, the high-frequency switching current loop composed of the primary coil of the high-frequency transformer, the switch tube and the filter capacitor may generate large spatial radiation, forming radiation interference. If the capacitor filter capacity is insufficient or the high-frequency characteristics are not good, the high-frequency impedance on the capacitor will cause the high-frequency current to be conducted to the AC power supply in a differential mode to form conducted interference. It should be noted that in the electromagnetic interference generated by the diode rectifier circuit, the |di/dt| of the rectifier diode reverse recovery current is much larger than the |di/dt| of the freewheeling diode reverse recovery current. As an electromagnetic interference source, the interference intensity formed by the rectifier diode reverse recovery current is large and the bandwidth is wide. However, the voltage jump generated by the rectifier diode is much smaller than the voltage jump generated when the power switch tube is turned on and off. Therefore, it is also possible to ignore the influence of |dv/dt| and |di/dt| generated by the rectifier diode and study the rectifier circuit as part of the electromagnetic interference coupling channel. 2.3 Characteristics of the coupling channel affected by stray parameters In the conducted interference frequency band (<30MHz), the coupling channel of most switching power supply interference can be described by circuit networks. However, any actual component in the switching power supply, such as resistors, capacitors, inductors, and even switches and diodes, contains stray parameters, and the wider the frequency band studied, the higher the order of the equivalent circuit. Therefore, the equivalent circuit of the switching power supply, including the stray parameters of each component and the coupling between components, will be much more complicated. At high frequencies, stray parameters have a great influence on the characteristics of the coupling channel, and the existence of distributed capacitance becomes a channel for electromagnetic interference. In addition, when the power of the switch tube is large, the collector generally needs to be equipped with a heat sink. The distributed capacitance between the heat sink and the switch tube cannot be ignored at high frequencies, and it can form radiation interference facing the space and common mode interference conducted by the power line.
3 Electromagnetic compatibility design 3.1 Design of input filter The noise generated by the switching power supply includes common mode noise and differential mode noise. Common mode interference is caused by the potential difference between the current-carrying conductor and the earth. Its characteristic is that the noise voltage on the two lines is at the same potential and in the same direction; while differential mode interference is caused by the potential difference between the current-carrying conductors. Its characteristic is that the noise voltage on the two lines is at the same potential and in the opposite direction. Usually, these two components of interference voltage on the line exist at the same time. For this reason, a filter should be added to the input end of the power supply. The filter impedance should be mismatched with the power supply impedance. The greater the mismatch, the more ideal the attenuation achieved, and the better the insertion loss characteristics obtained. In other words, if the internal resistance of the noise source is low impedance, the input impedance of the EMI filter connected to it should be high impedance (such as a series inductor with a large inductance); if the internal resistance of the noise source is high impedance, the input impedance of the EMI filter should be low impedance (such as a parallel capacitor with a large capacity). Due to the imbalance of line impedance, the two components will transform into each other during transmission, and the situation is very complicated. A typical EMI filter includes two suppression circuits for common mode noise and differential mode noise, as shown in Figure 3. Figure 3 In the power filter diagram: differential mode suppression capacitors Cx1, Cx 20.1～0.47μF; differential mode suppression inductors L1, L2 100～130μH; common mode suppression capacitors Cy1, Cy2 <10000pF; common mode suppression inductors L 15～25mH. The definition of insertion loss is shown in Figure 4. When the filter is not connected, the output voltage of the signal source is V1. When the filter is connected, the voltage of the signal source measured at the output end of the filter is V2. If the output impedance of the signal source is equal to the input impedance of the receiver, both are 50Ω, then the insertion loss of the filter is IL=20log（2） When designing, the resonant frequency of the common mode filter circuit and the differential mode filter circuit must be significantly lower than the operating frequency of the switching power supply, generally lower than 10kHz, that is, f=<10kHz. Figure 4 Definition of insertion loss Figure 5 is a schematic diagram of differential mode and common mode interference. (a) Differential mode interference (b) Common mode interference Figure 5 Schematic diagram of differential mode and common mode interference In actual use, since the common mode and differential mode components generated by the equipment are different, the filter circuit can appropriately increase or decrease the filter components. The adjustment of the specific circuit generally requires EMI testing to obtain satisfactory results. When installing the filter circuit, it is necessary to ensure good grounding and good isolation between the input and output ends, otherwise, the filtering effect will not be achieved. Figure 6 shows two filter circuits, and their filtering effects are shown in the experimental curve of Figure 7. (a) Filter circuit 1 (b) Filter circuit 2 Figure 6 Two filter circuits ① is the filter circuit 1 ② is the filter circuit 2 Figure 7 Experimental curves of the effects of two filter circuits 3.2 Suppression measures for radiated EMI To reduce radiated interference, a voltage buffer circuit can be applied, such as a parallel RCD buffer circuit at both ends of the switch tube, or a current buffer circuit, such as a 20~80μH inductor connected in series to the collector of the switch tube. The collector of the power switch tube is a strong interference source. The heat sink of the switch tube should be connected to the collector to ensure that the current generated by the distributed capacitance between the collector and the heat sink flows into the main circuit. In order to reduce the distributed capacitance between the heat sink and the casing, the heat sink should be as far away from the casing as possible. If conditions permit, a heat sink with shielding measures can be used. The rectifier diode should have a small recovery charge and a short reverse recovery time, such as Schottky diodes. It is best to use a reverse recovery with soft characteristics. In addition, the magnetic beads and parallel RC absorption network at both ends of the Schottky tube can reduce interference. The values of resistance and capacitance can be several Ω and thousands of pF. The capacitor leads should be as short as possible to reduce the lead inductance. The larger the load current, the longer the reverse recovery time of the diode, and the greater the impact of the peak current. Using multiple diodes in parallel to share the load can reduce the impact of short-circuit peak current. The switching power supply must be shielded, using a modular fully sealed structure, generally using galvanized steel plates with a thickness of more than 1mm, and the shielding layer must be well grounded. Adding a shielding layer between the primary and secondary of the high-frequency pulse transformer and grounding it can suppress the electric field coupling of interference. Adding a shielding cover to magnetic components such as high-frequency pulse transformers and output filter inductors can limit the magnetic lines of force within the shielding body with low magnetic resistance. For example, for a switching power supply whose radiated interference exceeds the standard limit of 20dB, the following measures that are easy to implement in the laboratory were adopted to improve it: 1) Connect a 470pF capacitor in parallel across all rectifier diodes; 2) Connect a 50pF capacitor in parallel to the input end of the G pole of the switch tube to form an RC low-pass filter with the original 39Ω resistor; 3) Connect a 0.01μF capacitor in parallel to each output filter capacitor (electrolytic capacitor); 4) Put a small magnetic bead on the rectifier diode pin; 5) Improve the grounding of the shielding body. After the above improvements, the power supply can pass the limit requirements of the radiated interference test. 3.3 Solution to conducted interference The conducted interference of the switching power supply is transmitted outward through the input power line, and there are both differential mode interference and common mode interference. The test frequency range of conducted disturbance is 0.15～30MHz, and the limit requirements are listed in Table 1. Table 1 Conducted disturbance limit table Power port frequency range/MHz Quasi-peak dB/μV Average dB/μV Class A 0.15～0.5 79 66 0.5～30 73 60 Class B 0.15～0.5 66 56 0.5～5 56 46 5～30 60 50 In the frequency range of 0.15～1MHz, the disturbance mainly exists in the form of common mode. In the frequency range of 1～10MHz, the disturbance is in the form of coexistence of differential mode and common mode. Above 10MHz, the disturbance is mainly in the form of common mode. The generation of differential mode disturbance is mainly due to the switch tube working in the switch state. When the switch tube is turned on, the current flowing through the power line rises linearly. When the switch tube is turned off, the current suddenly changes to zero. Therefore, the current flowing through the power line is a high-frequency triangular pulsating current, which contains rich high-frequency harmonic components. As the frequency increases, the amplitude of the harmonic component becomes smaller and smaller. Therefore, the differential mode disturbance decreases with the increase of frequency. The filter circuit of the output loop is shown in Figure 8. The capacitor C5 and the inductor L3 form a low-pass filter. The differential mode conduction disturbance mainly exists in the low-frequency segment. Figure 8 The main reason for the common mode disturbance generated by the filter circuit of the output loop is that there is a distributed capacitance between the power supply and the earth (protective ground). The high-frequency harmonic component of the square wave voltage in the circuit is transmitted to the earth through the distributed capacitance, forming a loop with the power line, and generating common mode disturbance. As shown in Figure 8, L and N are power inputs, C1, C2, C3, C4, C5, L1, and L2 form an input EMI filter, DB1 is a rectifier bridge, and VT2 is a switch tube. When the switch tube is installed on the radiator, the D pole of the switch tube is connected to the radiator, and a coupling capacitor is formed between the radiator and the radiator, as shown in C7 in Figure 8. VT2 works in a switching state, and the voltage of its D pole is a high-frequency square wave. The frequency of the square wave is the switching frequency of the switch tube. The harmonics in the square wave will form a loop through the coupling capacitor, L, and N power lines, generating common-mode interference. The distributed capacitance between the power supply and the earth is relatively dispersed and difficult to estimate, but from Figure 8, the coupling capacitor between the D pole of VT2 and the radiator has the greatest effect. The voltage from DB1 to the inductor L3 is 100Hz, and the voltage from L3 to the connection between VD1 and the D pole of VT2 is a square wave voltage, which contains a large number of high-order harmonics. Secondly, the influence of L3 is also relatively large, but the distance between L3 and the casing is relatively far, and the distributed capacitance is much smaller than the coupling capacitance between the switch tube and the heat sink. Therefore, we mainly consider the coupling capacitance between the switch tube and the heat sink. 3.4 Application of grounding technology "Grounding" includes signal grounding inside the equipment and equipment grounding. The two concepts are different and the purposes are also different. The classic definition of "ground" is "an equipotential point or plane used as a reference for a circuit or system." 3.4.1 Signal grounding of equipment The signal grounding of equipment may be based on a point or a piece of metal in the equipment as the grounding reference point of the signal, which provides a common reference potential for all signals in the equipment. Here we introduce floating ground and mixed grounding. In addition, there are single-point grounding and multi-point grounding. 1) Floating ground The purpose of floating ground is to isolate the circuit or equipment from the public grounding system or the public wire that may cause circulating current. Floating ground can also make it easier to coordinate circuits with different potentials. The methods to achieve floating ground of circuits or equipment include transformer isolation and photoelectric isolation. The biggest advantage of floating ground is good anti-interference performance. The disadvantage of floating ground is that it is easy to cause static electricity accumulation between the two because it is not connected to the public ground. When the charge accumulates to a certain extent, it may cause severe static electricity discharge and become a very destructive interference source. A compromise is to connect a large-resistance discharge resistor between the floating ground and the public ground to release the accumulated charge. Pay attention to controlling the impedance of the discharge resistor. Too low impedance will affect the qualification of the leakage current of the equipment. 2) Mixed grounding Mixed grounding makes the grounding system present different characteristics at low frequency and high frequency, which is necessary in broadband sensitive circuits. Capacitors have higher impedance to low frequency and DC, so they can avoid the formation of ground loops between two modules. When the DC ground and the RF ground are separated, the DC ground of each subsystem is connected to the RF ground through a capacitor of 10~100nF. The two grounds should be connected at a low impedance at one point, and the connection point should be selected at the point where the highest flip speed di/dt signal exists. 3.4.2 Equipment grounding In engineering practice, in addition to carefully considering the signal grounding inside the equipment, the signal ground and the casing of the equipment are usually connected to the earth, and the earth is used as the grounding reference point of the equipment. The purpose of equipment grounding is: 1) to ensure the personal safety of equipment operators; 2) to discharge the charge accumulated on the chassis to avoid the accumulation of charge causing the chassis potential to increase, resulting in unstable circuit operation; 3) to prevent the equipment from changing the potential of the equipment to the earth under the influence of the external electromagnetic environment, resulting in unstable equipment operation. It can be seen that grounding the equipment is not only a consideration for personnel safety and equipment safety, but also an important means to suppress interference. 3.5 Shielding technology Another method to suppress the interference radiation generated by the switching power supply is shielding, the purpose is to cut off the propagation path of electromagnetic waves. Solving the electromagnetic interference problem by electromagnetic shielding will not affect the normal operation of the circuit. It uses materials with good 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 pulse transformer, a closed loop can be used to form a magnetic shield. In addition, the entire switching power supply must be shielded for the electric field. Shielding should take into account heat dissipation and ventilation issues. The ventilation holes on the shielding shell are preferably circular and porous. Under the condition of satisfying ventilation, the number of holes can be large, and the size of each hole should be as small as possible. The joints should be welded to ensure the continuity of the electromagnetic path. If screws are used for fixing, pay attention to the short spacing between the screws. Filtering measures should be taken at the lead-in and lead-out wires of the shielding shell, otherwise, these will become interference transmitting antennas, seriously reducing the shielding effect. If the electric field is shielded, the shielding shell must be grounded, otherwise, it will not have a shielding effect; if the magnetic field is shielded, the shielding shell does not need to be grounded. The outer shell of the non-embedded external switching power supply must be shielded by electric field, otherwise, it is difficult to pass the radiation disturbance test. For the switching power supply, the main thing is to do a good job of housing shielding, high-frequency transformer shielding, switch tube and rectifier diode shielding, and use photoelectric isolation technology. The power switch tube and output diode usually have a large power loss. In order to dissipate heat, it is often necessary to install a heat sink or directly install it on the power supply base. When installing the device, it needs to be insulated with an insulating sheet with good thermal conductivity, which creates distributed capacitance between the device and the base plate and the heat sink. The base plate of the switching power supply is the ground wire of the AC power supply. Therefore, the distributed capacitance between the device and the base plate couples the electromagnetic interference to the AC input end to generate common mode interference. The solution to this problem is to use a shielding sheet sandwiched between two layers of insulating sheets, and connect the shielding sheet to the DC ground to cut off the path for the radio frequency interference to propagate to the input power grid. In order to suppress the influence of the radiated electromagnetic interference generated by the switching power supply 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 can be connected to the system's housing and ground as a whole, which can effectively shield the electromagnetic field. Connecting some parts of the power supply to the ground can play a role in suppressing interference. For example, the grounding of the electrostatic shielding layer can suppress the interference of the changing electric field; the conductor used for electromagnetic shielding can be ungrounded in principle, but the ungrounded shielding conductor often enhances the electrostatic coupling and produces the so-called "negative electrostatic shielding" effect, so it is still better to ground it, so that the electromagnetic shielding can also play the role of electrostatic shielding. The common reference point of the circuit is connected to the earth, which can provide a stable reference potential for the signal loop. Therefore, the safety protection ground wire, shielding ground wire and common reference ground wire in the system are finally connected to the earth after forming a ground bus. 3.6 Component layout and printed circuit board wiring technology The radiated disturbance of the switching power supply is proportional to the current size in the current path, the loop area of the path, and the product of the square of the current frequency, that is, the radiated disturbance E∝IAf2. The premise of using this relationship is that the path size is much smaller than the wavelength of the frequency. The above relationship shows that reducing the path area is the key to reducing radiated disturbance, that is, the components of the switching power supply should be arranged closely to each other. In the primary circuit, the input capacitor, transistor and transformer are required to be close to each other and the wiring is compact; in the secondary circuit, the diode, transformer and output capacitor are required to be close to each other. When designing a printed circuit board, the interrelated components should be placed together as much as possible to avoid interference caused by too long printed lines due to the components being too far away; in addition, the input signal and output signal should be placed as close to the lead port as possible to avoid interference caused by the coupling path. On the printed board, the positive load current conductors are placed close to each other on both sides of the printed board, and try to keep them parallel, because the external magnetic field generated by the parallel and close positive load current conductors tends to cancel each other. Practice has proved that the layout and wiring design of the components of the printed board have a great influence on the EMC performance of the switching power supply. In the high-frequency switching power supply, since there are both small signal control lines with a level of ±5V~±15V and high-voltage power busbars on the printed board, as well as some high-frequency power switches and magnetic components, how to reasonably arrange the position of the components in the limited space of the printed board will directly affect the anti-interference of each component in the circuit and the reliability of the circuit operation. In addition, avoid running two printed signal lines in parallel. If parallel routing cannot be avoided, the following methods can be used to remedy the situation: 1) Add a ground wire between the two signal lines to act as a shield; 2) Try to increase the distance between the two parallel signal lines to reduce the influence of the electromagnetic field between the two lines; 3) Make the currents flowing through the two parallel signal lines in opposite directions. The electromagnetic coupling between the wiring is carried out through the electric field and the magnetic field. Therefore, when wiring, attention should be paid to the suppression of the coupling between the electric field and the magnetic field. The methods for suppressing the electric field are: 1) Increase the distance between the lines as much as possible to minimize the capacitive coupling; 2) Use electrostatic shielding, and the shielding layer should be grounded; 3) Reduce the input impedance of the sensitive line. The methods for suppressing the magnetic field are: 1) Reduce the loop area of the interference source and the sensitive circuit; 2) Increase the distance between the lines to make the mutual inductance between the coupled interference source and the sensitive circuit as small as possible; 3) It is best to wire the interference source and the sensitive circuit at right angles to greatly reduce the coupling between the lines. In addition, by analyzing the characteristic impedance of the printed conductor, the placement, length, width and layout of the printed conductor can be selected. The characteristic impedance of a single conductor consists of DC resistance R and self-inductance L. The shorter the printed line l, the smaller the DC resistance R. At the same time, increasing the width and thickness of the printed line can also reduce the DC resistance R. The shorter the length of the printed line l, the smaller the self-inductance L.Moreover, increasing the width b of the printed line can also reduce the self-inductance L. The characteristic impedance of multiple printed lines is composed of DC resistance R and self-inductance L, and is also affected by mutual inductance M. In addition to being affected by the length and width of the printed line, the distance between the printed lines also plays an important role. Increasing the distance between the two lines can reduce mutual inductance. In view of the above phenomenon, when designing a printed circuit board, the impedance of the power line and the ground line should be reduced as much as possible, because the power line, the ground line and other printed lines have inductance. When the power current changes greatly, a large voltage drop will be generated, and the ground line voltage drop is an important factor in the formation of common impedance interference. Therefore, the ground line should be shortened as much as possible, and the power line and ground line lines can also be thickened as much as possible. In the design of double-sided printed boards, in addition to thickening the power line and ground line lines as much as possible, a decoupling capacitor with good high-frequency characteristics should be installed between the ground line and the power line.
To increase the switching frequency and improve the quality of switching power supply products, electromagnetic compatibility issues must be considered as a priority. This article proposes effective suppression measures based on the analysis of the interference generation mechanism and a large amount of practice. There are still many factors that generate electromagnetic interference in switching power supplies, and there is still a lot of work to be done to suppress electromagnetic interference. When designing, it is also necessary to eliminate the coupling and radiation between the interference source and the disturbed equipment, and cut off the propagation path of electromagnetic interference, so as to reduce the electromagnetic interference of the switching power supply to the lowest point.
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千斤顶
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