since Electronic system noise reduction technology Since its appearance in the mid-1970s Federal Communications Commission In 1990 and european union In 1992, it proposed to Electronics These regulations require companies to ensure that their products comply with strict magnetization and emission guidelines. Products that comply with these regulations are said to have electromagnetic compatibility EMC（Electromagnetic Compatibility）。 electromagnetic interference EMI (Electrokinetic Conference), Yes Conducted interference and Radiated interference Two. Conducted interference refers to the transmission of the Signal coupling (interference) to another Electric network 。 Radiated interference refers to Interference source Coupling (jamming) its signal to another electrical network through space. In high-speed PCB and system design, High frequency signal line 、 Integrated circuit Pins, various connections plug-in unit Are likely to have antenna characteristics Radiated interference source , can emit electromagnetic wave And affect the normal operation of other systems or other subsystems in the system. The so-called "interference" refers to electromagnetic compatibility (EMC), which means that the performance of the equipment is reduced after interference and the interference source that causes interference to the equipment. The first layer means that the thunder and lightning make the radio produce noise, the television picture shows snowflakes after the motorcycle runs nearby, and the radio sound is heard after picking up the phone. These can be referred to as "BC I", "TV I" and "Tel I". These abbreviations all have the same "I" (interference) (BC: radio)
So what part of EMI are EMI standards and EMI detection? Of course, it is the second meaning, that is Interference source , including electromagnetic energy before interference. The second is "electromagnetic". If the charge is static, it is called static electricity 。 When different charges move towards the same direction, electrostatic discharge occurs, generating current, which is generated around the current magnetic field 。 If the direction and size of current continuously change electromagnetic wave 。 Electricity exists in various states, all of which are collectively referred to as electromagnetism. Therefore, EMI standards and EMI detection are to determine the state of the processed electricity, determine how to detect and evaluate it.
There are many ways to classify interference sources.
1.1 Generally speaking Electromagnetic interference source It is divided into two categories: natural interference sources and man-made interference sources. The natural interference sources mainly come from the celestial noise in the atmosphere and the cosmic noise in the outer space of the earth. They are both the earth electromagnetic environment At the same time, it is a source of interference to radio communication and space technology. Natural noise will interfere with the operation of artificial satellites and spacecraft, as well as the launch of ballistic missile carrier rockets. Human interference source is generated by electromechanical or other artificial devices electromagnetism Energy interference, some of which are devices specially used to transmit electromagnetic energy, such as radio equipment such as radio, television, communication, radar and navigation, is called intentional emission interference source. The other part is the emission of electromagnetic energy when completing its own functions, such as traffic vehicles, overhead transmission lines, lighting appliances, electric machinery, household appliances, industrial and medical RF equipment, etc. Therefore, this part becomes an unintentional emission interference source. 1.2 From the electromagnetic interference attribute, it can be divided into functional interference sources and non functional interference sources.
Functional interference source refers to the direct interference to other equipment caused by the equipment in the process of realizing its function; Non functional interference source refers to the side effect generated or added by the electric device while realizing its own function, such as arc discharge interference generated by switch closing or cutting off.
1.3 From electromagnetic interference Signal spectrum Width, which can be divided into broadband interference sources and narrowband interference sources. They are different from the bandwidth of designated receptors.
interference signal If the bandwidth is greater than the specified sensor bandwidth, it is called broadband interference, otherwise it is called narrowband interference source. 1.4 From interference signal frequency range To divide Interference sources can be divided into power frequency and audio frequency interference sources (50Hz and harmonic ）Very low frequency interference source (below 30Hz), carrier frequency interference source (10kHz~300kHz), radio frequency and video interference source (300kHz), microwave interference source (300MHz~100GHz). There are two ways of electromagnetic interference transmission: conduction coupling and radiation coupling.
Any electromagnetic interference must have interference energy transmission and transmission channels (or transmission channels). It is generally believed that there are two ways of electromagnetic interference transmission: one is conductive transmission; The other is radiation transmission. Therefore, from the perspective of disturbed sensors, interference coupling can be divided into conduction coupling and radiation coupling.
Conductive transmission must be complete between the interference source and the sensor Circuit The interference signal is transmitted to the sensor along the connection circuit, and interference occurs. This transmission circuit can include conductors, conductive components of equipment, power supply, public impedance , grounding plate, resistance inductance , capacitance and mutual inductance components, etc. The radiation transmission is transmitted in the form of electromagnetic wave through the medium, and the interference energy is electromagnetic field The law of the launch to the surrounding space. There are three common types of radiation coupling: 1 The electromagnetic wave emitted by antenna A is accidentally received by antenna B, which is called antenna to antenna coupling; 2. Space electromagnetic field passes through wire Induction Coupling is called field to line coupling; 3. High frequency signal induction between two parallel wires is called line to line inductive coupling. In practical engineering, interference between two devices usually involves coupling of many ways. It is precisely because of the existence of multiple ways of coupling, repeated cross coupling, and common interference that electromagnetic interference becomes difficult to control. Sensitive equipment
Sensitive equipment is a general term for interference objects. It can be a very small component or a circuit board component, a separate electrical equipment or even a large system.
Variable frequency drive and electromagnetic interference
Electromagnetic interference is also a major problem of inverter drive system. In many countries, especially in Europe There are strict restrictions on the electromagnetic interference that may be emitted by any system. because digital scroll compressor Loading and uninstall It is a mechanical operation, and the electromagnetic interference generated by the digital vortex system can be ignored. This unique feature not only eliminates the need for expensive electromagnetic suppression electronic devices, but also increases the reliability and simplicity of the digital system. It is more applicable and environmentally friendly to power stations, radio, television, communication, navigation, precision equipment, hospitals, subway control devices and other places. Terms related to electromagnetic interference:
one | EMI filter electromagnetic interference wave filter |
two | electromagnetic interference (EMI) electromagnetic interference |
three | interference, electromagnetic (EMI) electromagnetic interference |
four | Radio Magnetic Interference radio electromagnetic interference |
five | Electromagnetic Interference electromagnetic interference |
six | Conducted Electromagnetic Interference conduction electromagnetic interference |
Electromagnetic interference source
Electromagnetic interference sources include microprocessor 、 micro controller , transmitter, electrostatic discharge and instantaneous power actuator, such as electromechanical relay 、 switch Power Supply , lightning, etc. In the microcontroller system, the clock circuit is the largest broadband noise Generator , and the noise is spread to the whole spectrum. With the development of a large number of high-speed semiconductor devices, the edge jump rate is very fast, and this circuit will generate harmonic interference up to 300 MHz. The noise is coupled to the conductor in the circuit that is easiest to pass through. As shown in the figure, analyze the electromagnetic interference mechanism. If a wireway After passing through a noisy environment, the wire will sense the ambient noise and transmit it to the rest of the circuit. Noise passing power cord When entering the system, the noise carried by the power line is transmitted to the whole circuit, which is a coupling situation. Coupling also occurs in circuits with shared loads (impedances). For example, two circuits share a power supply wire or a ground wire. If one of the circuits needs a sudden large current, and the two circuits share a power line, equivalent access to the same power supply internal resistance, the current imbalance will cause the power supply voltage of the other circuit to drop. The influence of this coupling can be reduced by reducing the common impedance. However, the internal resistance of the power supply and the grounding conductor are fixed. If the grounding is unstable, the return current flowing in one circuit will change the ground potential in the grounding circuit of the other circuit, and the change of the ground potential will seriously reduce the A/D converter 、 Operational amplifier and sensor The performance of analog circuits with equal low level. In addition, the radiation of electromagnetic wave exists in each circuit, which forms the coupling between circuits. When the current changes, electromagnetic waves will be generated. These electromagnetic waves can be coupled to nearby conductors and interfere with other signals in the circuit.
be-all Electronics The circuit may be subject to electromagnetic interference. Although some electromagnetic interference is directly received by means of radio frequency radiation, most electromagnetic interference is received through transient conduction. In digital circuits, critical signals such as reset, interrupt and control signals are most vulnerable to electromagnetic interference. control circuit Low level of simulation amplifier And power supply adjustment circuit are also vulnerable to noise. Emission and anti-interference can be classified according to the coupling of radiation and conduction. Radiative coupling is very common at high frequencies, while conductive coupling is more common at low frequencies.
transmitter And Receiver The radiation coupling between them is caused by the transmission of electromagnetic energy through the radiation path. For example, the electromagnetic energy from nearby equipment is coupled through direct radiation, or similar electricity in nature Magnetic ring Environment coupling enters the receiver. The conductive coupling between the transmitter and the receiver is completed through a direct conductive path connecting the two. For example, when the transmitter and receiver share the same power line for power supply, the interference will be transmitted through the power line; Other ways of transmission are The signal line Or control line, etc. In order to carry out EMC design and meet EMC standards, the purpose is to minimize radiation, that is, reduce the RF energy leaked in the product, and at the same time enhance its anti-interference ability to radiation.
Through the electromagnetic interference model shown in the figure, it is easy to find methods to suppress electromagnetic interference, and the methods are as follows:
·Try to reduce electromagnetic wave radiation source or conduction source;
·Cutting off the coupling path;
·Increase the anti-interference capability of the receiver.
When encountering the problem of electromagnetic interference in practical engineering, it should be discussed with logical analysis. It goes without saying that as long as there is interference, there must be three elements: interference source, coupling path and disturbed object. Therefore, when solving the EMC problem, we should also start with the analysis of these three elements. In general, designing a PCB with good performance to reduce RF energy is the most economical and effective method. The second and third elements tend to adopt shielding technology. This will be described later.
Theoretical and practical research shows that, no matter complex systems or simple devices, any electromagnetic interference must have three basic conditions: first, there should be interference sources; Secondly, there are ways and channels to spread interference energy; Third, there must be a response from the interfered object. In the electromagnetic compatibility theory, the disturbed objects are collectively referred to as Sensitive equipment (or sensor). Therefore, interference sources, interference transmission channels (or transmission channels) and sensitive equipment are called the three elements of electromagnetic interference.
(1) Use shielding technology to reduce electromagnetic interference. In order to effectively suppress the radiation and conduction of electromagnetic waves and the noise current caused by high-order harmonics, the elevator motor cable driven by the frequency converter must be shielded cable. The conductivity of the shielding layer shall be at least 1/10 of the conductivity of each phase conductor core, and the shielding layer shall be reliably grounded. Shielded cable is preferred for control cable; Transmission line of analog signal shall be twisted pair with double shield; Different analog signal lines should be routed independently with their own shielding layers. To reduce the coupling between lines, do not put different analog signals in the same common return line; Low voltage digital signal lines should preferably use double shielded twisted pair, or single shielded twisted pair. The transmission cables for analog signals and digital signals should be shielded and routed in short lengths.
(2) Use grounding technology to eliminate electromagnetic interference. Ensure that all equipment in the elevator control cabinet is well grounded, and the thick grounding wire is connected to the power incoming line grounding point (PE) or grounding bus. It is particularly important that any electronic control equipment connected to the frequency converter should share the ground with it, and short and thick wires should also be used when sharing the ground. At the same time, the ground wire of the motor cable should be directly grounded or connected to the grounding terminal (PE) of the inverter. The above grounding resistance values shall meet the requirements of relevant standards.
(3) Use wiring technology to improve electromagnetic interference. The motor cable shall be routed independently of other cables, and the parallel routing of motor cable and other cables for a long distance shall be avoided to reduce the electromagnetic interference caused by the rapid change of inverter output voltage; When crossing control cable and power cable, they shall be crossed at 90 ° as far as possible, and the shielding layer of motor cable and control cable must be fixed to the mounting plate with appropriate clamps.
(4) Use filtering technology to reduce electromagnetic interference. The incoming reactor is used to reduce the harmonics generated by the frequency converter, increase the power impedance, and help absorb the surge voltage generated when the nearby equipment is put into operation and the peak voltage of the main power supply. The incoming reactor is connected in series between the power supply and the power input terminal of the inverter. When the situation of the main power grid is unknown, it is better to add an incoming line reactor. Low pass filter (the same below for FIR) can also be used in the above circuits. The FIR filter shall be connected in series between the incoming reactor and the frequency converter. For elevator inverter operating in noise sensitive environment, FIR filter can effectively reduce the radiation interference from inverter conduction.
(5) At the site where the interference of lighting line, motor feedback is too large, and the system power line is interfered, the communication interference cannot be eliminated through the above grounding, so magnetic ring can be used to suppress the interference, and the magnetic ring can be added in the following order until the communication returns to normal: 1. If the two power lines of lighting are disconnected at the same time, if the communication returns to normal, Please add a magnetic ring on the two lines of lighting under the control cabinet and wind it for 3 circles (magnetic ring with aperture of 20 to 30, thickness of 10 and length of about 20). If the disconnection of the lighting line has no effect, it means that the lighting line does not interfere with the communication and will not be processed. 2. Add a magnetic ring on the communication line C+, C - from the outgoing line of the main board and wind it for one circle. Note that the winding can only be done once. After more winding, the lift car communication display will become better, but most of the effective signals from the lift car will be filtered out, resulting in the failure of the internal selection registration of the lift car. 3. Add a magnetic ring on the 24V power supply and 0V ground wire output from the mainboard to the lift car and call elevator for 2 to 3 turns. 4. Add a magnetic ring to each three-phase line between the running contactor and the motor and wind it for one circle. After the magnetic ring is added by the above method, the interference of power supply, motor and lighting on site can be handled.
(6) Selection of magnetic ring material: nickel zinc ferrite or manganese zinc ferrite can be selected according to the frequency characteristics of interference signals, and nickel zinc ferrite or manganese zinc ferrite can be selected. The high-frequency characteristics of the former is better than the latter. The magnetic permeability of manganese zinc ferrite is thousands to tens of thousands, while that of nickel zinc ferrite is hundreds to thousands. The higher the permeability of ferrite, the greater the impedance at low frequency and the smaller the impedance at high frequency. Therefore, nickel zinc ferrite should be selected to suppress high-frequency interference; Otherwise, manganese zinc ferrite is used. Or put manganese zinc and nickel zinc ferrite on the same bundle of cables at the same time, so that the interference band can be suppressed is wide. The size selection of the magnetic ring: the larger the difference between the inner diameter and the outer diameter of the magnetic ring, the larger the longitudinal height, the greater the impedance. However, the inner diameter of the magnetic ring must be tightly wrapped around the cable to avoid magnetic leakage. Installation position of the magnetic ring: The installation position of the magnetic ring should be as close to the interference source as possible, that is, close to the inlet and outlet of the cable.
Suppression methods of electromagnetic interference in switching power supply
The electromagnetic interference in switching power supply and is divided into conducted interference and radiated interference. Generally, conducted interference is easy to analyze, and circuit theory and mathematical knowledge can be combined to study the characteristics of various components in electromagnetic interference; However, for the radiated interference, it is difficult to analyze it because of the comprehensive effect of different interference sources in the circuit and the electromagnetic field theory.
Conducted interference can be divided into common mode (CM) interference and normal mode (DM) interference. Due to the existence of parasitic parameters and the high-frequency switching devices on and off in switching power supply, switching power supply produces large common mode interference and normal mode interference at its input (i.e. AC network side).
When the converter works at high frequency, because of the high dvldt, the parasitic capacitance between the transformer windings and between the switch tube and the heat sink is excited, resulting in common mode interference.
According to the principle of common mode interference, the following suppression methods are often used in practical applications:
(1) Optimize the layout of circuit components and try to reduce parasitic and waxy capacitors.
(2) Delay the opening and closing time of the switch, but this is inconsistent with the trend of high-frequency switching power supply.
(3) Buffer circuit is used to slow down the change rate of dv/dt. The current in the converter is switched at high frequency, so that a very high dv/dt is generated on the input and output filter capacitors, that is, the interference voltage is sensed on the equivalent inductance or impedance of the filter capacitor, and then the normal mode interference will be generated. Therefore, selecting high-quality filter capacitors (with very low equivalent inductance or impedance) can reduce the normal mode interference.
Radiated interference can also be divided into near-field interference [distance between measurement point and field source<λ/6 (λ is the electromagnetic wave length of interference)] and far-field interference (distance between measurement point and field source>λ/6). According to Maxwell's electromagnetic field theory, the changing current in a conductor will produce a changing magnetic field in its surrounding space, and the changing magnetic field will produce a changing electric field. Both follow Maxwell's formula. The amplitude and frequency of this changing current determine the magnitude of the generated electromagnetic field and its scope of action. In radiation research, the antenna is the source of electromagnetic radiation. In the switching power supply circuit, the components and wires in the main circuit can be considered as the antenna, which can be analyzed using the electric dipole and magnetic dipole theory. In analysis, diode, switch tube, capacitor, etc. can be regarded as electric dipole; Inductance coil can be regarded as a magnetic dipole, and then it can be comprehensively analyzed with relevant electromagnetic field theory.
It should be noted that the current phase of different branches is not necessarily the same, which is especially important in the calculation of magnetic field. The phase is different, for one thing, there is delay effect (also called hysteresis effect) when the interference propagates from the interference source to the measurement point; Second, the phase difference is caused by the characteristics of the components themselves. For example, the current phase in the inductance lags behind other components. The phase lag caused by the hysteresis effect is the result of the signal frequency, which is only obvious when the frequency is very high (such as GHz or higher); For power electronic devices, the frequency is relatively low, so the hysteresis effect is not significant.
Among the two types of interference generated by switching power supply, conducted interference will cause serious interference to other electronic equipment due to its transmission through the power grid, often causing more serious problems. The commonly used suppression methods include buffer method, reduction of engagement path method, reduction of parasitic components method, etc. In recent years, with the increasingly strict restrictions on electromagnetic interference of electronic equipment, some new suppression methods have emerged, mainly focusing on new control methods and new passive buffer circuit design.
Modulation frequency control
The interference changes according to the switching frequency, and the interference energy is concentrated on these discrete switching frequency points, so it is difficult to meet the requirements of electromagnetic interference (EMI) suppression. By modulating the energy of the switch signal and distributing it in a very wide frequency band to generate a series of discrete sidebands, the interference spectrum can be expanded, and the interference energy is divided into small parts and distributed in these discrete frequency bands, thus it is easier to meet the EMI standard. Modulation frequency control is based on this principle to suppress electromagnetic interference of switching power supply.
At first, people used random frequency control, whose main idea is to add a random disturbance component in the control circuit to make the switching interval change irregularly. Then the spectrum of switching noise changes from discrete spike pulse noise to continuous distribution noise, and its peak value decreases greatly. The specific method is that the pulse generator generates two pulses with different duty ratios, and then samples and selects the error signal generated by the voltage amplifier to generate the final control signal.
However, random frequency control basically adopts PWM control method when it is turned on, and random frequency is only used when it is turned off, so its modulation interference energy is inconvenient to control, and the effect of interference suppression is not very ideal. The latest modulation frequency control solves these problems well. Its principle is to modulate the main switch frequency to generate a series of side bands around the main frequency band, thus distributing the noise energy over a wide band and reducing interference. The key of this control method is to modulate the frequency so that the switching energy is distributed in the side band range, and the amplitude is affected by the modulation coefficient β (modulation coefficient β=△ f/fm, △ f is the interval between adjacent side bands, and fm is the modulation frequency). Generally, the larger the β, the better the modulation effect.
