Run Sheng Fung Engineering - switching power supply EMC must master a few concepts

1 generation and transmission of electromagnetic interference
Electromagnetic interference transmission in two ways: one is the conduction transfer mode, the other is radiative transfer approach. Transmission transmission is in interference source and sensitive device between a complete circuit connection, interference signal along the connection circuit is transferred to the receiver and the occurrence of the phenomenon of electromagnetic interference.
Radiation is transmitted through the medium of interference signal in the form of an electromagnetic wave propagates outwards in the form of disturbance. Common coupling to radiation in three ways: 1) an antenna for transmitting the electromagnetic wave is another antenna accidentally receives, known as the antenna on the antenna coupling; 2) electromagnetic field induction and coupled via a wire, called field on line coupling. 3) two equal wire between the high-frequency signal mutual induction and formation of coupling, known as the line on the line by inductively coupled.
2 electromagnetic interference mechanism
From the interference of the sensitive device for angle, interference coupling can be divided into conduction and radiation coupling two class.
Conductive coupling model.
Conductive coupling according to its principle can be divided into the resistive coupling, capacitive coupling, and inductive coupling three basic coupling mode.
The radiation coupling model
Radiation coupling is the coupling of another way, apart from the interference sources to radiation, there are a large number of unintentional radiation. At the same time, the PCB board line either power line, the signal line, the clock line, data line or the line of control, can play the antenna effect, can be radiated interference wave, but also can play receiving function.
3 electromagnetic interference control technology
The transmission channel inhibition
The filter: in design and the selection of filter should be paid attention to when frequency characteristics, pressure resistance, rated current, impedance characteristics, shielding and reliability. The filter is installed correctly or not on the insertion loss characteristic influence, only the mounting position is appropriate, the installation method is correct, in order to reach the expected effect of interference filter. The installation of filters should be considered when the installation position, the input and output side wiring must be shielded isolation, and the high frequency grounding and bonding method.
The shield: according to the principle of electromagnetic shielding can be divided into the electric field shielding, magnetic shielding and electromagnetic field shielding three. The electric field shielding contains electrostatic shielding and alternating electric field shielding; magnetic field shield contains low frequency magnetic field shielding and the high frequency magnetic field shielding. Different types of electromagnetic shield for shielding requirements for different. In the actual shielding, electromagnetic shielding effectiveness is more dependent on the shield body structure, whereby the conductive continuity. The actual shielding body due to manufacturing, assembly, repair, heat dissipation, observation and interface requirements, which are generally provided with different shapes, different sizes of apertures, the apertures on shielding shielding efficiency play an important role, therefore measures must be taken to suppress the aperture electromagnetic leakage.
The ground: grounding safety ground and signal ground two. At the same time, the ground will also introduce the grounding impedance and ground loop interference. Grounding techniques include ground selection, combination circuit, grounding design and inhibition of earthing interference measures such as reasonable application.
The lap: lap is guided between the low impedance connection, only a good lap can cause the circuit to complete its design features, so that the interference of various control measures can play a role in. Lapping method can be divided into permanent and semi-permanent lap lap two, overlapping types are divided into direct and indirect lap lap.
The wiring: wiring printed circuit board electromagnetic compatibility design of key, should choose reasonable conductor width, adopt proper strategies, such as bold ground, ground closed synthesis loop, reduce the conductor discontinuity, using a multilayer board.
The spatial separation
Spatial separation is the inhibition of space radiation harassment and inductive coupling disturbance effectively, through increasing disturbance source and receiver sensitive device and the distance between the harassment, electromagnetic field to sensitive equipment when the intensity attenuation below the receiving equipment sensitivity threshold, so as to achieve the purpose of suppressing electromagnetic interference. By the theory of electromagnetic field, field in near zone induction field to 1 / R3 mode attenuation, far field intensity distribution to 1 / R reduced. Therefore, in order to meet the requirements of electromagnetic compatibility of the system, as far as possible the system each device space distance. In the device, system wiring, restrict parallel cable minimum spacing, to reduce crosstalk. In the PCB design, set the minimum separation between the lead. In addition, the spatial separation is also included in the limited space available, the disturbance source radiation azimuth adjustment, a disturbance source electric field vector and vector space magnetic field orientation control.
The time of separation
When a disturbance source is very strong, not easy to use other methods for reliable inhibition, usually using time division method, so that the useful signals in the disturbance signal to stop firing time transmission, or when strong disturbance signal transmission, are subjected to harassment sensitive device in short time off to avoid damage. Time division control has two kinds of forms, one kind is active time separated, applicable to the useful signal and interference signal in time time is determined successively relationship contexts; another is the passive time division, according to the interference signals and useful signal characteristics make one signal quickly closed, so as to achieve on time not coincident, do not cover the control requirements.
The spectrum management
The spectrum of the planning division of the frequency band division to various radio services, for a particular user set band. The formulation of national standards is to prevent interference and in some cases to ensure the communication system to achieve the required communication performance based. This includes a radio device approval procedures, radio transmitters, receivers and other equipment type approval required minimum performance standards document.
The electrical isolation
Electrical isolation is to avoid circuit of conduction interference in reliable method, but also makes the useful signal normal coupling transmission. Common electrical isolation coupling form of mechanical coupling, electromagnetic coupling, photoelectric coupling. DC / DC converter is widely used for electrical isolation device, it will be a kind of DC voltage into a DC voltage, in order to prevent multiple devices to share a common power source internal resistance interference caused by power supply, the application of DC / DC converter separately on each power supply, to ensure that the circuit is not in a power supply signal interference.
A switching power supply, the origins of interference
Switching power supply the AC rectifier DC inverter for high frequency, then, finally after rectifying and filtering circuit output, to obtain stable DC voltage, thus itself contains a lot of harmonic interference. At the same time, because of the leakage inductance of the transformer and output diode reverse recovery current caused by spikes, are formed in the potential electromagnetic interference. The switching power supply interference source mainly concentrated in the voltage, current changes in components, prominent in the performance of the switch tube, diode, high frequency transformer.
The switching circuit to generate electromagnetic interference
The switch circuit is one of the main sources of interference of switching power supply. The switch circuit is a switching power supply is the core, is mainly composed of a switch tube and a high frequency transformer. It produces Du / dt with the larger amplitude pulse, wide frequency band and harmonic wealth. This pulse interference is the main reason: the switch tube load for high frequency transformer primary coil, is the inductive load. The switch tube is switched on the moment, the primary coil generate large inrush current in the primary coil, and both ends of the high surge peak voltage; the switch tube is disconnected from the moment, due to a primary coil of magnetic leakage flux, which makes a part of energy from a coil to the two coil, stored in the inductor in this part of the energy will and collector circuit capacitance, resistance to form a spike oscillation attenuation, superimposed on the turn-off voltage, forming a turn-off voltage spikes. Supply voltage interrupt will be generated with the primary coil is connected with the same magnetizing inrush current transients, such transients is a kind of electromagnetic interference EMI, not only affects the transformer primary, may also make the conduction interference return distribution system, causing the harmonic electromagnetic interference, thus affecting the safety and economic operation of other equipment.
The rectifier circuit generates electromagnetic interference
The rectifying circuit, the output rectifying diode cut-off when there is a reverse current, it returns to zero time and junction capacitance and other factors. Among them, the reverse current rapidly returned to zero the diode is called a hard recovery characteristics of diode, the diode in the transformer leakage inductance and other parameters distribution will have a strong interference of high frequency, the frequency of up to several tens of MHz. High frequency rectification circuit of the rectifier diode forward breakover had greater positive current flowing in the reverse bias voltage, while the steering stop, because PN junction have more carrier accumulation, and thus in the carrier before a period of time, current will reverse flow, causing carriers lost reverse recovery current sharp decline occurrence of large current changes ( di / dt ).
The high frequency transformer
High frequency transformer primary coil, a switch tube and a filter capacitor consisting of high-frequency switch current loop may have a larger space radiation, the formation of radiation interference. If the capacitor filter low capacity and high frequency characteristic is not good, the capacitor impedance will cause the high frequency current to the differential transmission to AC power formed in the conductive interference. Note that, in the diode rectifier circuit generates electromagnetic interference, rectifier diode reverse recovery current di / dt than the freewheeling diode reverse recovery current of the di / dt much. As the electromagnetic interference source to study, rectifier diode reverse recovery current form interference intensity, frequency bandwidth. However, rectifier diode generates a voltage jump far less than the power switch turn-on and turn-off voltage is generated at the jump. Therefore, can also be produced regardless of rectifier diode / DV / DT / influence, the rectifying circuit for electromagnetic interference coupling path is one part of the.
The interference caused by distributed capacitance
Switching power supply work in high frequency state, so its distribution capacitance can not be ignored. On one hand, the heat sink and the switch tube collector insulating contact area is large, and the insulating film is thin, so the distributed capacitance in high frequency can not be ignored. High frequency current through the distributed capacitance current to the heat sink, and then flows to the chassis ground, produce common mode interference; on the other hand, pulse transformer primary and secondary exists between the distributed capacitance, the primary voltage is coupled directly to the side edge, in the side edge of the DC output of the two power supply lines to produce common mode interference.
The stray parameters affecting the coupling characteristics of the channel
In the conduction interference frequency ( < 30MHz ), a plurality of switching power supply interference coupling channel can be used to describe the circuit network. However, switching power supply in any one of the actual components, such as resistors, capacitors, inductors, diodes and switches contain stray parameter, and the band of the wider, the equivalent circuit of higher order. Therefore, including the components of stray parameters and components of intercellular coupling, equivalent circuit of switching power supply will be more complex. At high frequencies, stray parameters on the coupling characteristics of channel influence, distributed capacitance became the existence of electromagnetic interference channel. In addition, the switch power is larger, the collector general is coupled with the heat sink, the heat sink and the switch tube between the distributed capacitance in high frequency can not be ignored, it can form a space oriented radiation interference and power line transmission of the common mode interference.
Two, the electromagnetic interference of switching power supply control technology
To solve the problem of electromagnetic interference of switching power supply, which can be viewed from 3 aspects: 1) reduce the interference sources produce interference signal; 2) cut off the interference signal transmission; 3) enhance the anti-jamming ability of interference body. Therefore, the switching power supply electromagnetic electromagnetic interference to control techniques mainly include: circuit measures, EMI filtering, component selection, shielding and anti jamming Design of printed circuit board.
Of switch power supply itself interference
The soft switch technology: in the original hard switching circuit of inductance and capacitance elements, using the inductance and capacitance of the resonator, reducing the switching process of Du / dt and di / DT, the switch opens when the voltage drop in the current rise, or turn-off current decline ahead of rising voltage, to elimination of the voltage and current of overlap.
The switching frequency modulation technology: through modulation of the switching frequency fc, the FC concentration in 2FC, 3fc and harmonic ... On the energy dispersed to their surrounding the band, in order to reduce the frequency on the EMI amplitude. The method can reduce the amount of interference, but energy is dispersed to the frequency band, so that for every frequency does not exceed the limits specified in EMI. In order to reduce the noise spectrum peak purposes, there are generally two kinds of treatment methods: random frequency and modulation frequency method.
The common mode interference suppression technology of active: from the main circuit to extract a and cause electromagnetic interference of the main switching voltage waveform completely reversed compensation EMI noise voltage, and uses it to balance the original switching voltage.
The electromagnetic interference reducing buffer circuit: the line impedance stabilization network composition, effect is eliminated in the power supply line of potential disturbances, including power line interference, electrical fast transient, surge, voltage change and power line harmonic etc.. These disturbances on the general power supply, the impact is not great, but the high frequency switching power supply greatly.
The filter: EMI filter is one of the main purposes, is in 150kHz ~ 30MHz frequency range to obtain high insertion loss, but the frequency of 50Hz power frequency signal without attenuation, the rated voltage, current pass smoothly, must also meet certain requirements. Any power line of conduction interference signal, are used in differential mode and common mode signal to said. In the general case, differential mode interference amplitude is small, low frequency, the interference caused by the smaller; common mode interference amplitude, high frequency, but also through wires produce radiation, caused by large interference. Therefore, to weaken the conduction interference, the EMI signal control on the EMC standard limit level, is the most effective method in switch power supply input and output circuit are additionally arranged in the electromagnetic interference filter.
The PCB : PCB design anti-interference design mainly includes the PCB layout, wiring and grounding, the purpose is to reduce the PCB electromagnetic radiation and PCB circuit crosstalk between. Switching power supply layout optimal method and electrical design is similar to. Determining PCB in size and shape, and then determine the special components ( such as a generator, crystal and other ) position. Finally, according to the function of the circuit unit, the circuit layout of all components.
The components of the selection: not easy to produce noise, not easy conduction and radiation noise components. Usually is particularly noteworthy, diode and transformer winding type selection of components. Reverse recovery current small, short recovery time of the fast recovery diode is a high frequency switching power supply rectifier portion of the ideal device.
The cutting interference signal transmission, common mode, differential mode power line filter design
Power line interference can use power line filter. A reasonable and effective switching power supply EMI filter should be on the power line of common-mode and differential-mode interference has strong inhibition effect.
The enhanced sensitivity of anti-jamming ability
It mainly includes the shielding and grounding in two ways.