| For a good electronic product, in addition to the functions of the product itself, the technical level of circuit design and electromagnetic compatibility (EMC) design plays a very critical role in the quality and technical performance indicators of the product. This article introduces the solution to electromagnetic interference in general electronic products by taking the electromagnetic compatibility design of switching power supplies as an example. Modern electronic products are becoming more and more powerful, and electronic circuits are becoming more and more complex. Electromagnetic interference (EMI) and electromagnetic compatibility have become major issues, and circuit design has increasingly higher requirements on the technical level of designers. Advanced computer-aided design (CAD) has greatly broadened the work capabilities of circuit designers in electronic circuit design, but its help in electromagnetic compatibility design is very limited. Electromagnetic compatibility design is actually to optimize the design of electromagnetic interference generated in electronic products so that they can meet the electromagnetic compatibility standards of various countries or regions. The definition of EMC is: in the same electromagnetic environment, the equipment can work normally without being affected by the interference of other equipment, and at the same time, it does not cause interference that affects the work of other equipment. Electromagnetic interference is generally divided into two types, conducted interference and radiated interference. Conducted interference refers to the coupling (interference) of a signal on an electrical network to another electrical network through a conductive medium. Radiated interference refers to the coupling (interference) of an interference source to another electrical network through space. Therefore, the study of EMC issues is the study of the relationship between interference sources, coupling paths, and sensitive equipment.
 The Federal Communications Commission of the United States proposed regulations for commercial digital products in 1990 and the European Union in 1992. These regulations require companies to ensure that their products meet strict susceptibility and emission standards. Products that meet these regulations are called electromagnetically compatible. At present, almost all regions in the world have set up EMC market access certification to protect the electromagnetic environment and competitive advantages of local products in the region. For example, FCC and NEBC certification in North America, CE certification in the European Union, VCCEI certification in Japan, C-tick certification in Australia, BSMI certification in Taiwan, and 3C certification in China are all "passes" to enter these markets. Electromagnetic Induction and Electromagnetic Interference When many people engage in electronic circuit design, they start by understanding electronic components, but engaging in electromagnetic compatibility design should actually start with electromagnetic field theory, that is, from the understanding of electromagnetic induction. Generally, electronic circuits are composed of resistors, capacitors, inductors, transformers, active devices and wires. When there is voltage in the circuit, an electric field is generated around all charged components. When current flows through the circuit, a magnetic field exists around all current carriers. Capacitors are components with the most concentrated electric field. The current flowing through capacitors is displacement current. This displacement current is caused by the two plates of the capacitor being charged and generating an electric field between the two plates. Through the induction of the electric field, the two plates will generate charging and discharging, forming a displacement current. In fact, the current in the capacitor loop does not actually flow through the capacitor, but only charges and discharges the capacitor. When the two plates of the capacitor are opened, the two plates can be regarded as a set of electric field radiation antennas. At this time, the circuit between the two plates will induce the electric field between the plates. Whether the circuit between the two plates is a closed loop or an open circuit, a displacement current will be generated in the conductor in the same direction as the electric field (when the direction of the electric field keeps changing), that is, the current runs forward for a while and runs backward for a while. The definition of electric field strength is potential gradient, that is, the ratio of the potential difference between two points to the distance. When a current of several amperes flows through a wire several meters long, the voltage across its two ends is only a few tenths of a volt at most, that is, an electric field strength of tens of millivolts per meter can generate a current of several amperes in the conductor. This shows how effective the electric field is and how strong its interference ability is.
Inductors and transformers are the components with the most concentrated magnetic fields. The current flowing through the secondary coil of the transformer is an induced current. This induced current is generated when current flows through the primary coil of the transformer, generating magnetic induction. The circuits around the inductor and transformer can be regarded as the induction coil of a transformer. When the magnetic lines of force generated by the leakage inductance of the inductor and transformer pass through a certain circuit, this circuit, as the "secondary coil" of the transformer, will generate an induced current. For a circuit with two adjacent loops, one of the loops can also be regarded as the "primary coil" of the transformer, and the other loop can be regarded as the "secondary coil" of the transformer. Therefore, the two adjacent loops also generate electromagnetic induction, that is, they interfere with each other. 
As long as there is an electric field or magnetic field in an electronic circuit, electromagnetic interference will be generated. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various connectors, etc. may become radiation interference sources with antenna characteristics, which can emit electromagnetic waves and affect the normal operation of other systems or other subsystems within the system. Switching power supply EMC design example At present, most electronic products are powered by switching power supplies to save energy and improve work efficiency; at the same time, more and more products also contain digital circuits to provide more application functions. Switching power supply circuits and clock circuits in digital circuits are the main sources of electromagnetic interference in current electronic products, and they are the main content of electromagnetic compatibility design. Below we analyze the electromagnetic compatibility design process of a switching power supply. Figure 1 is a working principle diagram of a commonly used flyback (or flyback) switching power supply. The 50Hz or 60Hz AC grid voltage is first rectified by the rectifier stack and charged to the energy storage filter capacitor C5, and then supplies power to the load circuit composed of transformer T1 and switch tube V1. Figure 2 is an electrical schematic diagram after electromagnetic compatibility design. 1. Suppression of current harmonics Generally, the capacity of capacitor C5 is large, and the voltage ripple across it is very small, only about 10% of the input voltage. The rectifier diode is turned on only when the input voltage U i is greater than the voltage across capacitor C5. Therefore, within one cycle of the input voltage, the conduction time of the rectifier diode is very short, that is, the conduction angle is very small. In this way, a pulse peak current will appear in the rectifier circuit, as shown in Figure 3. If this pulse peak current is expanded using Fourier series, it will be seen as consisting of a large number of high-order harmonic currents. These harmonic currents will reduce the efficiency of the power supply equipment, that is, the power factor is very low, and will flow back into the power grid, causing pollution to the power grid. In severe cases, it will also cause fluctuations in the power grid frequency, that is, AC power flicker. The pulse current harmonic and AC power flicker test standards are: IEC61000-3-2 and IEC61000-3-3. Generally, the upper limit of the test pulse current harmonic is the 40th harmonic frequency. The solution to the problem of excessive pulse peak current in the rectifier circuit is to connect a power factor correction (PFC) circuit or a differential mode filter inductor in series in the rectifier circuit. The PFC circuit is generally a parallel boost switching power supply, and its output voltage is generally 400V DC. The power factor of the power supply equipment before power factor correction is generally only 0.4~0.6, and can reach up to 0.98 after correction. Although the PFC circuit can solve the problem of excessive pulse peak current in the rectifier circuit, it will bring new high-frequency interference problems, which also require strict EMC design. Using a differential mode filter inductor can effectively suppress the peak value of the pulse current, thereby reducing the current harmonic interference, but it cannot improve the power factor. L1 in Figure 2 is a differential mode filter inductor. The differential mode filter inductor is generally made of silicon steel sheet material to increase the inductance. In order to prevent magnetic saturation when a large current flows through the differential mode filter inductor, the two groups of coils of the differential mode filter inductor generally each have a leakage inductance magnetic loop. 
The L1 differential mode filter inductor can be obtained through experiments or calculated according to the following formula: E=L*di/dt (1) Where E is the difference between the input voltage Ui and the voltage across capacitor C5, that is, the voltage drop across L1, L is the inductance, and di/dt is the current rise rate. Obviously, the smaller the current rise rate is required, the larger the inductance is required. 2. Suppression of ringing voltage Since the primary of the transformer has leakage inductance, when the power switch tube V1 switches from saturation conduction to cut-off, a back electromotive force will be generated, and the back electromotive force will charge and discharge the distributed capacitance of the primary coil of the transformer, thereby generating damped oscillation, that is, ringing, as shown in Figure 4. The voltage amplitude of the back electromotive force generated by the primary leakage inductance of the transformer is generally very high, and its energy is also very large. If no protective measures are taken, the back electromotive force will generally break down the power switch tube. At the same time, the damped oscillation generated by the back electromotive force will also generate strong electromagnetic radiation, which will not only cause serious interference to the machine itself, but also to the surrounding environment of the machine. D1, R2, and C6 in Figure 2 are effective circuits for suppressing the back EMF and the amplitude of the ringing voltage. When the transformer primary leakage inductance generates a back EMF, the back EMF charges the capacitor C6 through the diode D1, which is equivalent to the capacitor absorbing the energy of the back EMF, thereby reducing the amplitude of the back EMF and the ringing voltage. After the capacitor C6 is fully charged, it will discharge through R2. The time constant of RC discharge is correctly selected so that the residual voltage of the capacitor at the next charge is just equal to the amplitude of the square wave voltage. At this time, the working efficiency of the power supply is the highest. 3. Suppression of conducted interference signals In Figure 1, when the power switch tube V1 is turned on or off, a pulsating DC i1 will be generated in the circuit composed of the capacitor C5, the primary of the transformer T1 and the power switch tube V1. If this current loop is regarded as a "primary coil" of a transformer, due to the high rate of change of the current i1, the electromagnetic induction it generates in the "primary coil" will also generate electromagnetic induction on the surrounding circuits. We can regard the surrounding circuits as multiple "secondary coils" of the same transformer. At the same time, the leakage inductance of the transformer T1 also produces an inductive effect on each "secondary coil". Therefore, the current i1 will generate an induced current in each "secondary coil" through electromagnetic induction. We record them as i2, i3, i4, etc. Among them, i2 and i3 are differential mode interference signals, which can be transmitted to other lines of the power grid through two power lines and interfere with other electronic equipment; i4 is a common mode interference signal, which is generated by the current i1 loop through electromagnetic induction of other circuits and the ground or casing, and the other circuits and the ground or casing form a loop through capacitive coupling. The common mode interference signal can be transmitted to other lines of the power grid through the power line and the ground and interfere with other electronic equipment. The circuit connected to the collector of the power switch tube V1 is also the main reason for the generation of common-mode interference signals. This is because in the entire switching power supply circuit, the collector of the power switch tube V1 has the highest potential, which can reach more than 600V, and the potentials of other circuits are lower than it. Therefore, there is a very strong electric field between the collector of the power switch tube V1 and other circuits (including the leads of the power input end). Under the action of the electric field, the circuit will generate a displacement current, which basically belongs to the common-mode interference signal. The capacitors C1, C2 and differential mode inductor L1 in Figure 2 have a strong ability to suppress differential mode interference signals of i1, i2 and i3. Since C1 and C2 are still charged when the power cord is unplugged, which may cause electric shock, a discharge resistor R1 should be connected at both ends of the power input. It is generally difficult to completely suppress the common-mode interference signal i4, especially when there is no metal casing shielding, because in the loop that induces the common-mode interference signal, one of the "components" is the equivalent capacitance between the circuit board and the ground. The value of this "component" is generally unstable, and sufficient margin should be left for the indicators during design. In Figure 2, L2, C3, and C4 are common-mode interference signal suppression circuit components. In circuits with large input power, L2 generally uses two or even three, one of which is usually a toroidal core inductor. 
According to the above analysis, the main reason for electromagnetic interference is the main circuit through which i1 flows. This circuit is mainly composed of capacitor C5, transformer T1 primary and power switch tube V1. According to the principle of electromagnetic induction, the induced electromotive force generated by this circuit is: e=dψ/dt=S*dB/dt (2) Where e is the induced electromotive force, ψ is the magnetic flux, S is the area of the current loop, B is the magnetic induction density, whose value is proportional to the current intensity, and dψ/dt is the rate of change of magnetic flux. It can be seen that the induced electromotive force is proportional to the area of the current loop. Therefore, to reduce electromagnetic interference, the first thing to do is to try to reduce the area of the current loop, especially the area of the loop through which the i1 current flows. In addition, in order to reduce the influence of the transformer leakage inductance on the electromagnetic induction of the surrounding circuit, on the one hand, the transformer leakage inductance is required to be small, and on the other hand, a thin layer of copper must be wrapped around the transformer to form a low-impedance short-circuit coil to dissipate the induced energy generated by the leakage inductance through eddy current. 4. Suppression of radiated interference signals Electromagnetic radiation interference is also generated by electromagnetic induction, from charged bodies or current loops and magnetic induction loops. Any conductor can be regarded as an electromagnetic induction antenna, any current loop can be regarded as a loop antenna, and inductors and transformer leakage inductance are also important components of electromagnetic induction radiation. It is impossible to completely suppress electromagnetic radiation, but the radiation of electromagnetic interference can be greatly reduced by reasonable circuit design or partial shielding measures. For example, shortening the length of circuit leads and reducing the area of current loops as much as possible are effective ways to reduce electromagnetic radiation; using energy storage filter capacitors correctly, installing them as close as possible to both ends of the power leads of active devices, and powering each active device independently, or using a single energy storage filter capacitor (a fully charged capacitor can be regarded as an independent power supply) to prevent active devices (amplifiers) in each circuit from generating crosstalk through the power and ground wires; strictly separating the ground of the power lead from the ground of the signal source, or using a double-line parallel cross-connection method for the signal lead to cancel out interference signals, is also an effective way to reduce electromagnetic radiation; using heat sinks can also be used to partially shield electromagnetic interference, and the signal lead can also be shielded by double ground wires in parallel, with the signal line sandwiched between two parallel ground wires, which is equivalent to a double loop, and interference signals will also cancel each other out, with a very significant shielding effect; using metal casings for machines or sensitive devices is the best way to shield electromagnetic interference, but non-metal casings can also be sprayed with conductive materials (such as graphite) for electromagnetic interference shielding. 5. Elimination of high voltage static electricity In Figure 1, if the output voltage is higher than 1,000V, static elimination must be considered. Although most switching power supplies use transformers to isolate "hot and cold grounds", since the "hot ground", also called "primary ground", can form a loop through the power grid, when the human body touches the "primary ground", it will be "electrically shocked", so people call the "primary ground" "hot ground", which means it cannot be touched. The "cold ground" is also called "secondary ground". Although the voltage is very high, it does not form a loop with the earth. When the human body touches the "secondary ground", it will not be "electrically shocked". Therefore, people call the "secondary ground" "cold ground", which means it can be touched. But no matter it is "cold ground" or "hot ground", its potential difference to the earth cannot be zero, that is, it is still charged. For example, the switching power supply in a color TV has a potential difference of about 400VP-P (peak-to-peak) between the "hot ground" and the earth, and a potential difference of about 1500V P-P (peak-to-peak) between the "cold ground" and the earth. It is easier to understand that the "hot ground" is charged, but it is difficult for most people to understand that the "cold ground" is charged. So how is the voltage of the "cold ground" charged generated? This voltage is generated by the secondary of the transformer. Although one end of the transformer secondary is connected to the "cold ground", the real zero potential is in the center of the transformer secondary coil, or in the middle of the rectifier output filter capacitor medium. This point is called the "floating ground" of the power supply, that is, it is zero potential, but not connected to the ground. From this, it can be seen that the voltage of the "cold ground" is exactly equal to half of the output voltage. For example, the high-voltage anode of the TV picture tube requires a high voltage of about 30,000 volts. The real zero potential is in the middle of the high-voltage filter capacitor (the capacitor between the graphite layers of the picture tube), or at the middle tap of the high-voltage package. From this, it can be calculated that the voltage (static electricity) between the cold ground and the ground in the TV is about 1,5000V. Similarly, the "floating ground" of the "hot ground" circuit is in the middle of the energy storage filter capacitor C5, so the normal voltage of the "hot ground" is half of the rectifier output, about 200 V P (peak value). If the back electromotive force generated when the switch is turned on or off is also superimposed on it, it is about 400V P-P (peak-to-peak value). R3 in Figure 2 is used to reduce the static voltage between the cold ground and the earth, and C8 is used to reduce the dynamic resistance between the hot and cold grounds. Generally, the withstand voltage of digital circuit ICs is very low. If the voltage of the "cold ground" is very high, it is easy to break down the IC through static induction or human touch. The "cold ground" is charged in the category of static electricity, which is equivalent to charging a small capacitor, one end of which is the earth, and the capacitance is equivalent to the equivalent capacitance between the "cold ground" and the earth. In addition, C1, C2, C3, C4, C8, R1, R8, and T1 in Figure 2 are safety devices, and safety requirements must be paid attention to when using them. Common EMC standards:
EMC general series standard: IEC61000-4-X
Industrial environment immunity general standard: EN50082-2
Pulse current harmonic test standard: IEC61000-3-2
AC power flicker test standard: IEC61000-3-3 Author: Tao Xianfang
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