Electromagnetic compatibility analysis of electronic circuits

With the rapid development of electronic technology, the world has entered the information age. Electronic and electrical equipment or systems have been used more and more widely. High-power transmitters cause catastrophic interference to high-sensitivity receivers that do not want to receive their information. In large cities with developed industries, the electromagnetic environment is becoming increasingly harsh, often causing electronic and electrical equipment or systems to fail to work properly, causing performance degradation or even damage.

Electromagnetic interference is generated by interference sources. It is an electromagnetic phenomenon that comes from the outside and damages useful signals. The electromagnetic energy generated by the electromagnetic interference source is transmitted to sensitive equipment through a certain transmission path. The sensitive equipment shows a certain form of "response" to this and produces an "effect" of interference. This process and its results are called electromagnetic interference effects. In people's lives, electromagnetic interference effects are ubiquitous and in various forms. If the interference effect is very serious, the equipment or system fails, resulting in serious failures or accidents, which is called electromagnetic compatibility failure. Obviously, electromagnetic interference has become a huge obstacle that must be overcome on the road to the development of modern electronic technology. In order to ensure the normal operation of electronic systems or equipment, it is necessary to study electromagnetic interference, analyze and predict interference, limit the intensity of human interference, study effective technical means to suppress interference, improve anti-interference capabilities, and rationally design the electromagnetic environment.

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 problems. Advanced computer-aided design (CAD) has greatly broadened the ability of circuit design in electronic circuit design, but its help in electromagnetic compatibility design is very limited.

At present, all regions in the world have basically set up EMC market access certification to protect the electromagnetic environment of the region and the competitive advantage of local products. 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. 1 Electromagnetic compatibility

issues

Electromagnetic compatibility design is actually to optimize the design of electromagnetic interference generated in electronic products to make them 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.

Generally, electronic circuits are composed of resistors, capacitors, inductors, transformers, active devices and wires. When there is voltage in the circuit, an electric field will be generated around all charged components. When current flows through the circuit, a magnetic field will exist around all current carriers.

Capacitors are components with the most concentrated electric field. The current flowing through the capacitor is a displacement current. This displacement current is due to the fact that the two plates of the capacitor are charged and an electric field is generated 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 closed or open, when the direction of the electric field changes continuously, a displacement current will be generated in the conductor consistent with the direction of the electric field.

The definition of electric field strength is the 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 at most a few tenths of a volt, that is, an electric field strength of tens of millivolts per meter can generate a current of several amperes in the conductor. It can be seen that the greater the effect of the electric field, the stronger its interference ability.

Inductors and transformers are the components with the most concentrated magnetic fields, and the current flowing through the secondary coil of the transformer is an induced current. This induced current is generated because magnetic induction is generated when a current flows through the primary coil of the transformer. The circuits around the inductor and transformer can be regarded as an 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. The circuit of two adjacent loops can also regard one of the loops as the "primary coil" of the transformer, and the other loop as the "secondary coil" of the transformer, so the two adjacent loops also generate electromagnetic induction, that is, 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 in the system.

2 EMC design of power supply

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. The switching power supply circuit and the clock circuit in the digital circuit are the main sources of electromagnetic interference in electronic products at present, and they are the main contents of electromagnetic compatibility design. The following is an analysis of 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 50 Hz or 60 Hz AC grid voltage is first rectified by the rectifier stack and charged to the energy storage filter capacitor C5, and then the load circuit composed of the transformer T1 and the switch tube V1 is powered. Figure 2 is an electrical schematic 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 Uin 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 by Fourier series, it is considered to be composed 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 to the power grid, causing pollution to the power grid. When it is serious, it will also cause fluctuations in the power grid frequency, that is, AC power flicker. The test standards for pulse current harmonics and AC power flicker are: IEC61000-3-2 and IEC61000-3-3. The upper limit of the general test pulse current harmonic is the 40th harmonic frequency. The solution to the 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 DC 400 V. The power factor of the power supply equipment before power factor correction is generally only 0.4 to 0.6, and after correction, it can reach a maximum of 0.98. 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 and prevent magnetic saturation when a large current flows through the differential mode filter inductor. Generally, the two groups of coils of the differential mode filter inductor each have a leakage inductance magnetic loop. The differential mode filter inductance of L1 can be obtained through experiments or calculated according to the following formula:

E=Ldi/dt

In the formula: E is the difference between the input voltage Uin and the voltage across the 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, a back electromotive force will be generated when the power switch tube V1 switches from saturation conduction to cut-off shutdown. 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 EMF 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 EMF will generally break down the power switch tube. At the same time, the damped oscillation generated by the back EMF 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 ringing voltage amplitude. When the primary leakage inductance of the transformer 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 correct selection of the RC discharge time constant makes the residual voltage of the capacitor just equal to the square wave voltage amplitude when it is charged next time. 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. Since the rate of change of the current i1 is very high, the electromagnetic induction it generates in the "primary coil" will also generate electromagnetic induction on the surrounding circuits. The surrounding circuits can be regarded as multiple "secondary coils" of the same transformer, and the leakage inductance of the transformer T1 also generates an induction effect on each "secondary coil". Therefore, the current i1 will generate an induced current in each "secondary coil" through electromagnetic induction, and they are recorded as i2, i3, ..., in respectively. 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 earth or the housing. The other circuits and the earth or the housing 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 earth 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. 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 600 V, and the potential of other circuits is lower than it. Therefore, there is a strong electric field between the collector of the power switch tube V1 and other circuits (including the leads at the power input end). Under the action of the electric field, the circuit will generate displacement current, which basically belongs to the common-mode interference signal.

The capacitors C1, C2 and the differential-mode inductor L1 in Figure 2 have a strong suppression ability for the differential-mode interference signals of i1, i2 and i3. Since C1 and C2 are still charged when the power cord is unplugged, it is easy to cause electric shock and injury, so 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 circuit that induces the common-mode interference signal, one of the "components" is the equivalent capacitance between the circuit board and the earth. The value of this "component" is generally unstable, and sufficient margin should be left for the indicators when designing. L2, C3, and C4 in Figure 2 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 mostly a toroidal magnetic core inductor.

According to the above analysis, the main reason for the generation of electromagnetic interference is the main circuit through which i1 flows, which 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=dφ／dt=SdB／dt,

where: e is the induced electromotive force; φ is the magnetic flux; S is the area of ​​the current loop; B is the magnetic induction intensity, and its value is proportional to the current intensity; dφ／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, we must first 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 impact of the transformer leakage inductance on the electromagnetic induction of the surrounding circuits, on the one hand, it is required to reduce the leakage inductance of the transformer; on the other hand, a thin layer of copper is wrapped around the outer periphery of the transformer to form a low-impedance short-circuit coil to consume the induction energy generated by the leakage inductance through eddy current.

(4) Suppression of radiated interference signals. Electromagnetic radiation interference is also generated by electromagnetic induction by 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 the inductor coil and transformer leakage inductance are also important components of electromagnetic induction radiation. It is impossible to completely suppress electromagnetic radiation, but by reasonably designing the circuit or taking partial shielding measures, the radiation of electromagnetic interference can be greatly reduced.

For example, shortening the length of the circuit leads and reducing the area of ​​the current loop are effective ways to reduce electromagnetic radiation; using energy storage filter capacitors correctly, installing them as close as possible to the two ends of the power leads of active devices, and each active device is powered independently, or powered by a separate energy storage filter capacitor (a fully charged capacitor can be regarded as an independent power supply) to prevent the active devices (amplifiers) in each circuit from generating crosstalk through the power line and the ground line; strictly separating the ground of the power lead and the ground of the signal source, or using a double-line parallel cross method for the signal lead to cancel out the 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 a double ground line parallel method, so that the signal line is sandwiched between two parallel ground lines, which is equivalent to a double loop, and the interference signals will also cancel each other out, and the shielding effect is very significant; 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 1 000 V, 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 whether it is a "cold ground" or a "hot ground", its potential difference to the earth cannot be zero, that is, it will still be charged. For example, the switching power supply in a color TV has a peak-to-peak potential difference of about 400 V between the "hot ground" and the earth; the peak-to-peak potential difference of the "cold ground" and the earth is about 1 500 V.

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. The "cold ground" charged voltage is generated by the transformer secondary. 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" charged is exactly equal to 50% of the output voltage. For example, the high-voltage anode of the TV picture tube requires a high voltage of about 30,000 V. 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 electrostatic voltage between the cold ground and the ground in the TV is about 15,000 V. Similarly, the "floating ground" of the "hot ground" circuit is in the middle of the energy storage filter capacitor C5, so under normal circumstances, the "hot ground" charged voltage is 50% of the rectifier output, and its peak value is about 200 V. If the back electromotive force generated when the switch is turned on or off is also superimposed on it, its peak-to-peak value is about 400 V.

R3 in Figure 2 is used to reduce the electrostatic voltage between the "cold ground" and the earth, and the function of C8 is to reduce the dynamic resistance between the cold and hot 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 electrostatic induction or human touch. Common EMC standards are as follows:

EMC general series standards: IEC61000-4-X;

General standard for industrial environment immunity: EN50082-2;

Pulse current harmonic test standard: IEC61000-3-2;

AC power flicker test standard: IEC61000-3-3.

R3 in Figure 3 is used to reduce the electrostatic 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 electrostatic induction or human touch.

3 Conclusion

As the switching power supply continues to develop towards high frequency, its anti-interference problem becomes more and more important. In the development and design of switching power supplies, how to effectively suppress the electromagnetic interference of the switching power supply and at the same time improve the anti-interference ability of the switching power supply itself to electromagnetic interference is an important topic. In anti-interference design, several anti-interference measures are both independent and interrelated, and multiple measures must be adopted at the same time to achieve a good anti-interference effect.