Generation and transmission of electromagnetic interference
There are two ways of electromagnetic interference transmission: one is conducted transmission and the other is radiated transmission. Conducted transmission is a complete circuit connection between the interference source and the sensitive equipment, and the interference signal is transmitted along the connected circuit to the receiver and electromagnetic interference occurs.
Radiative transmission is a form of interference in which an interfering signal travels outward through a medium in the form of electromagnetic waves. There are three common types of radiative coupling: 1) electromagnetic waves emitted by one antenna are unexpectedly received by another antenna, known as antenna-to-antenna coupling; 2) The space electromagnetic field is coupled by wire induction, which is called field-to-line coupling. 3) The coupling formed by the mutual induction of high-frequency signals between two equal wires is called wire-to-wire inductive coupling.
Mechanism of electromagnetic interference
From the perspective of the sensitive equipment that is being interfered with, interference coupling can be divided into two categories: conductive coupling and radiated coupling.
● Conductive coupling model: conductive coupling can be divided into three basic coupling modes: resistive coupling, capacitive coupling and inductive coupling.
● Radiated coupling modelRadiated coupling is another way of interference coupling, in which there is a large amount of unintentional radiation in addition to the intentional radiation emitted from the interference source. At the same time, the traces on the PCB board, whether it is a power line, a signal line, a clock line, a data line or a control line, can play the role of an antenna, which can radiate interference waves and play a role in reception.
Electromagnetic interference control technology
(1) Transmission channel suppression
● Filtering: Attention should be paid to frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding and reliability when designing and selecting filters. The correct installation of the filter has a great impact on its insertion loss characteristics, and only the correct installation position and installation method can play the expected filtering effect on interference. When installing the filter, the installation location should be considered, the wiring on the input and output sides must be shielded and isolated, and the high-frequency grounding and lapping methods should be considered.
● Shielding: Electromagnetic shielding can be divided into three types: electric field shielding, magnetic field shielding and electromagnetic field shielding according to the principle. Electric field shielding includes electrostatic shielding and alternating electric field shielding; Magnetic field shielding includes low-frequency magnetic field shielding and high-frequency magnetic field shielding. Different types of electromagnetic shielding have different requirements for the shielding body. In actual shielding, the effectiveness of electromagnetic shielding depends more on the structure of the shield, that is, the continuity of conductivity. Due to the manufacturing, assembly, maintenance, heat dissipation, observation and interface connection requirements of the actual shielding body, there are generally holes of different shapes and sizes on it, and these holes and seams play an important role in the shielding effectiveness of the shielding body, so measures must be taken to suppress the electromagnetic leakage of the holes.
● Grounding: There are two kinds of grounding: safety grounding and signal grounding. At the same time, grounding also introduces ground impedance and ground loop interference. Grounding technology includes the selection of grounding points, circuit combinations, the design of grounding and the rational application of grounding interference suppression measures.
● Lap joint: Lap joint is a low-impedance connection between guide bodies, and only a good lap connection can make the circuit complete its design function, so that various suppression measures of interference can play a role. The lap joint method can be divided into two types: permanent lap joint and semi-permanent lap joint, while lap joint types are divided into direct lap joint and indirect lap joint.
● Wiring: Wiring is the key to the electromagnetic compatibility design of printed circuit boards, and a reasonable wire width should be selected, and the correct wiring strategy should be adopted, such as thickening the ground wire, closing the ground wire into a loop, reducing the discontinuity of the wire, and using multi-layer boards.
(2) Spatial separation
By increasing the spatial distance between the disturbance source and the sensitive equipment of the receiver, the intensity of the disturbance electromagnetic field when it reaches the sensitive equipment has been attenuated to lower than the sensitivity threshold of the receiving equipment, so as to achieve the purpose of suppressing electromagnetic interference. According to the electromagnetic field theory, the field strength is attenuated by 1/r3 in the near-region induced field, and the field strength distribution of the far-region radiated field decreases by 1/r. Therefore, in order to meet the electromagnetic compatibility requirements of the system, the spatial distance between the various devices that make up the system should be increased as much as possible. In equipment and system cabling, limit the minimum spacing of parallel cables to reduce crosstalk. In PCB design, the minimum spacing between the leads is specified. In addition, spatial separation also includes the adjustment of the orientation of the radiation direction of the disturbance source and the control of the spatial orientation of the electric field vector and the magnetic field vector of the disturbance source in the case of limited space.
(3) Time separation
When the source of disturbance is very strong and it is not easy to reliably suppress it by other methods, the method of time separation is usually used to make the useful signal be transmitted within the time when the disturbance signal stops transmitting, or when the strong disturbance signal is transmitted, the sensitive equipment that is susceptible to disturbance is turned off for a short time to avoid damage. There are two forms of time separation control, one is active time separation, which is suitable for the situation where there is a definite relationship between the occurrence time of the wanted signal and the occurrence time of the interference signal; The other is passive time separation, which makes one of the signals quickly shut down according to the characteristics of the interference signal and the useful signal, so as to achieve the control requirements of no overlap and no coverage in time.
(4) Spectrum management
The planned allocation of spectrum is to allocate frequency bands to various radio services, and to define frequency bands for specific users. The development of national standards and specifications is the basis for preventing interference and, in some cases, ensuring that the communication system achieves the required communication performance. This includes the approval process for radio equipment and the documentation of minimum performance standards required for the type approval of radio transmitters, receivers and other equipment.
(5) Electrical isolation
Galvanic isolation is a reliable way to avoid conducted interference in a circuit, while also allowing the transmission of desirable signals to be properly coupled. Common forms of galvanic isolation coupling include mechanical coupling, electromagnetic coupling, photoelectric coupling, etc. DC/DC converter is a widely used electrical isolation device, it converts one DC voltage into another DC voltage, in order to prevent multiple devices from sharing a power supply to cause common power supply internal resistance interference, DC/DC converter is used to supply power to each channel separately, so as to ensure that the circuit is not interfered with by the signal in the power supply.
Causes of interference in switching power supplies
The switching power supply first rectifies the power frequency AC to DC, then inverts it into high frequency, and finally outputs it through the rectification filter circuit to obtain a stable DC voltage, so it contains a large number of harmonic interference. At the same time, potential electromagnetic interference is formed due to the leakage inductance of the transformer and the spikes caused by the reverse recovery current of the output diode. The interference sources in the switching power supply are mainly concentrated in the components with large changes in voltage and current, which are prominently manifested in switches, diodes, high-frequency transformers, etc.
(1) Electromagnetic interference generated by switching circuits
The switching circuit is one of the main sources of interference in the switching power supply. The switching circuit is the core of the switching power supply, which is mainly composed of a switch tube and a high-frequency transformer. It produces du/dt with large pulses, wide frequency bands and rich harmonics. The main reason for this pulse interference is that the switch load is the primary coil of the high-frequency transformer, which is an inductive load. At the moment when the switch tube is turned on, the primary coil produces a large inrush current, and a high surge spike voltage occurs at both ends of the primary coil; At the moment when the switch tube is disconnected, due to the leakage flux of the primary coil, a part of the energy is not transmitted from the primary coil to the secondary coil, and this part of the energy stored in the inductor will form an attenuation oscillation with a spike with the capacitance and resistance in the collector circuit, which will be superimposed on the shutdown voltage to form a shutdown voltage spike. The interruption of the power supply voltage will produce the same magnetized impulse current transient as when the primary coil is turned on, which is a kind of conductive electromagnetic interference, which not only affects the primary transformer, but also makes the conductive interference return to the distribution system, causing harmonic electromagnetic interference in the power grid, thereby affecting the safety and economic operation of other equipment.
(2) Electromagnetic interference generated by rectifier circuits
In the rectifier circuit, there is a reverse current at the cut-off of the output rectifier diode, and the time it takes to return to zero is related to factors such as junction capacitance. Among them, the diode that can quickly restore the reverse current to zero is called the hard recovery characteristic diode, which will produce strong high-frequency interference under the influence of transformer leakage inductance and other distribution parameters, and its frequency can reach tens of MHz. When the rectifier diode in the high-frequency rectifier circuit is turned on by the forward conduction, a large forward current flows, and when it is turned off by the reverse bias voltage, the current will flow in the opposite direction for a period of time before the carrier disappears, resulting in a sharp decrease in the reverse recovery current of the carrier disappearance and a large current change (di/dt).
(3) High-frequency transformer
The high-frequency switching current loop composed of the primary coil, switch tube and filter capacitor of the high-frequency transformer may generate large space radiation and form radiated 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 into the AC power supply in a differential-mode manner, resulting in 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 that of the freewheeling diode. As a source of electromagnetic interference, the interference intensity and frequency bandwidth formed by the reverse recovery current of the rectifier diode are large. However, the voltage jump generated by the rectifier diode is much smaller than that generated when the power switch is turned on and off. Therefore, the rectifier circuit can also be studied as part of the electromagnetic interference coupling channel, regardless of the │dv/dt│ effect generated by the rectifier diode.
(4) Interference caused by distributed capacitance
The switching power supply works in a high-frequency state, so its distributed capacitance cannot be ignored. On the one hand, the contact area of the insulating sheet between the heat sink and the collector of the switch tube is large, and the insulating sheet is thin, so the distributed capacitance between the two cannot be ignored at high frequencies. The high-frequency current will flow to the heat sink through the distributed capacitance, and then to the chassis ground, resulting in common-mode interference. On the other hand, there is a distributed capacitance between the primary stage of the pulse transformer, which can directly couple the voltage of the primary side to the secondary side, and generate common-mode interference on the two power lines of the secondary side as the DC output.
(5) Spurious parameters affect the characteristics of the coupling channel
In the conducted interference frequency band ("30MHz"), the coupling channels of most switching power supply interference can be described in terms of circuit networks. However, any actual component in the switching power supply, such as resistors, capacitors, inductors, and even switches and diodes, contains spurious parameters, and the wider the frequency band studied, the higher the order of the equivalent circuit. As a result, the equivalent circuitry of a switching power supply, including the spurious parameters of each component and the coupling between components, will be much more complex. At high frequencies, spurious parameters have a great influence on the characteristics of the coupling channel, and the presence 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 add a heat sink, and the distributed capacitance between the heat sink and the switch tube can not be ignored at high frequency, which can form the radiation interference facing the space and the common mode interference of power line conduction.
Control technology for electromagnetic interference in switching power supplies
To solve the electromagnetic interference problem of switching power supply, we can start from three aspects: 1) reduce the interference signal generated by the interference source; 2) cut off the propagation path of the interference signal; 3) Enhance the anti-interference ability of the victim body. Therefore, the control technology of electromagnetic electromagnetic interference of switching power supply mainly includes: circuit measures, EMI filtering, component selection, shielding and anti-interference design of printed circuit boards.
(1) Reduce the interference of the switching power supply itself
● Soft switching technology: add inductance and capacitance components in the original hard switching circuit, use the resonance of inductance and capacitance to reduce the du/dt and di/dt in the switching process, so that the voltage drop when the switching device is turned on is before the rise of current, or the current drop is before the rise of voltage when the switch is turned off, so as to eliminate the overlap of voltage and current.
● Switching frequency modulation technology: By modulating the switching frequency FC, the FC and its harmonics 2FC, 3FC... to reduce the EMI amplitude at each frequency. This method does not reduce the total amount of interference, but the energy is dispersed to the baseband of the frequency so that the individual frequencies do not exceed the limits specified by the EMI. In order to achieve the purpose of reducing the peak value of the noise spectrum, there are usually two processing methods: random frequency method and modulated frequency method.
● Active suppression technology for common-mode interference: try to take out a compensating EMI noise voltage from the main circuit that is completely inverted to the main switching voltage waveform causing electromagnetic interference, and use it to balance the original switching voltage.
● Buffer circuit to reduce electromagnetic interference: It is composed of a linear impedance stabilization network, which is used to eliminate potential interference in the power supply power line, including power line interference, electrical fast transients, power surges, voltage changes and power line harmonics. These interferences do not have a great impact on the general regulated power supply, but the impact on the high-frequency switching power supply is significant.
● Filtering: One of the main purposes of EMI filter is to obtain high insertion loss in the frequency band range of 150kHz-30MHz, but there is no attenuation of the power frequency signal with a frequency of 50Hz, so that the rated voltage and current can pass smoothly, and it must also meet certain size requirements. Conducted interference signals on any power line can be represented by both differential-mode and common-mode signals. In general, the amplitude of differential mode interference is small, the frequency is low, and the interference caused is small; The common-mode interference amplitude is large, the frequency is high, and the radiation can also be generated through the wire, which causes a large interference. Therefore, the most effective way to reduce conducted interference and control EMI signals below the limit level specified in the relevant EMC standards is to install electromagnetic interference filters in the input and output circuits of switching power supplies.
● PCB design: PCB anti-interference design mainly includes PCB layout, wiring and grounding, the purpose of which is to reduce the electromagnetic radiation of the PCB and the crosstalk between the circuits on the PCB. The best way to lay out a switching power supply is similar to its electrical design. After determining the size and shape of the PCB, the position of special components (such as various generators, crystal oscillators, etc.) is determined. Finally, according to the functional unit of the circuit, all the components of the circuit are laid out.
● Selection of components: select components that are not easy to generate noise, conduction and radiated noise. Of particular note is the selection of winding components such as diodes and transformers. Fast recovery diodes, with small reverse recovery current and short recovery time, are ideal devices for the high-frequency rectification part of switching power supplies.
(2) Cut off the propagation path of interference signals - common mode and differential mode power line filter design
Power line interference can be filtered out using a power line filter. A reasonable and effective switching power supply EMI filter should have a strong suppression effect on both differential mode and common mode interference on the power line.
(3) Enhance the anti-interference ability of sensitive circuits
This mainly includes two ways: shielding and grounding.
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