Weak current College】 【resolves several effective switching power supply for electromagnetic interference suppression】.

<br> <br> At present, many universities and research institutes are switching power supply EMI (ElectromagneticInterference) research, some of them generation mechanism from EMI, some impact from EMI, have raised a number of practical value. Here analysis and comparison of .several effective programmes, and for switching power supply EMI Suppression measures put forward new proposals for reference. <br> <br> 1. switch the power generation mechanism of electromagnetic interference <br> <br> Switching power supply interference, noise source descriptions, .can be divided into peak interference and harmonic interference of two; if by coupling pathway separation, can be divided into conducted interference and radiated interference. Now press the noise interference sources to describe: <br> <br> 1, diode reverse recovery time of .interference caused by <br> <br> The high-frequency rectifier diodes are Wizard-greateropportunities when a forward current flows through, in their turn to reverse bias voltage, because when PN has more carrier accumulation, so the carrier disappears before the period of .time, the current will reverse flow, resulting in disappearance of reverse recovery charge carrier current dramatic reduction in the event of a large current change (di / dt). <br> <br> 2. switch on the work of a harmonic interference < .br> <br> Power switching valve in breakover current flowing through the larger. For example forward, push-pull type and bridge type converter input current waveform in resistance load-time approximation to the square wave, which is rich in higher harmonic components. .When using the zero current, zero voltage switching, the harmonic interference will be very small. In addition, the power switching valve in the closed period, high-frequency transformer leakage caused by current mutation, also produces peak interference. <br> <br .> 3. AC input circuit interference <br> <br> No frequency transformer switching power supply input in reverse recovery rectifiers can cause high frequency during damped oscillatory cause interference. <br> <br> Switching power supply resulting spike interference and harmonic interference energy .by switching power supply's input and output lines spread out of interference call conducted interference; and harmonic and parasitic oscillation of energy, spread through the input and output lines, in the space to produce electric and magnetic fields. This through electromagnetic interference is called radiation interference .. <br> <br> 4. other reasons <br> <br> Components of parasitic parameter, switching power supply's schematic design is not perfect, the printed circuit board (PCB) alignment typically manual layout, with a lot of randomness, PCB's .near-field disturbance, and printing plate of the device's installation and placement and orientation of unreasonable will cause EMI interference. <br> <br> Second, switching power supply EMI characteristics <br> <br> As work on the switch state of energy .conversion devices, switching power supply voltage, current change rate is high, the intensity is large; interference sources mainly focused on the power switch and attached to the heatsink and Gao-transformer, relative to the digital circuit interference source position more clearly; the switching .frequency is not high (from dozens kHz and few MHz), mainly in the form of interference is conducted interference and near-field disturbance; whereas the printed circuit board (PCB) alignment typically manual wiring, with greater randomness, which increases PCB distribution parameter .extraction and the difficulty of near-field disturbance estimation. <br> <br> Third, EMI testing technology <br> <br> Current diagnosis of differential mode EMI in three ways: RF current probe, differential-mode rejection Internet, noise isolation .network. RF current probe is measured in differential mode EMI the easiest method, but measurements and standards for comparison to the more complex conversion. Differential mode suppression network structure simple, the measurement results can be compared directly with the standard limits, but can only measure .the common mode interference. Noise isolation network is the most ideal method, but its key part transformer manufacturing requirements. <br> <br> IV. current suppression disturbance of several measures <br> <br> Formation of the three elements of electromagnetic interference .is interference source, transmission and disturbed. Thus, for electromagnetic interference suppression should also proceed from these three areas. First of all should suppress interference source, direct elimination of interfere with reasons; and second, to eliminate interference and disturbed the coupling between devices and .radiation, cut off the transmission of electromagnetic interference. The third is to improve disturbed equipment of rejection capacity, reduce the sensitivity to noise. Current suppression of interference of some measures are basically used to cut off electromagnetic interference sources and disturbed the coupling channel between devices ., they are an effective way. Commonly used methods is shielding, grounding and filtering. <br> <br> Use of shielding technology can effectively inhibit switching power supply's electromagnetic interference. For example, power switching valve and output diode usually have greater thermal dissipation .power losses, in order to install the heatsink is often required or installed directly on the floor in the power supply. Device installation needs good thermal conductivity of insulating film for insulation, which makes the device and the base plate and heat sink has had distributed capacitance .between, switching power supply's backplane is AC power of Earth, through devices and backplane between distributed capacitance will EMI coupling to produce AC input common mode interference, solution to this problem is to use two layers of insulation between the clip layer shielding film and shielding film .received DC ground, cut off the radio-frequency interference to enter the power grid of transmission. In order to inhibit switching power of radiation, electromagnetic interference to other electronic devices, fully in accordance with the magnetic field shielding method for processing shielding enclosures, then .the entire shielding enclosures and enclosure systems and connection as a whole, can effectively shield the electromagnetic field. Power supply is connected to some parts of the earth can play a role in suppressing interference. For example, static grounding can inhibit change layer electric interference; .electromagnetic shielding for conductors in principle can not Earth, but not a grounded shield conductors often enhanced static coupled to produce the so-called "negative" effects of electrostatic shield, so it is better to ground, so make the electromagnetic shielding can simultaneously play the .role of electrostatic shield. Circuit of public reference points are connected with the Earth, to signal circuit provides stable reference potential. Therefore, system security, ground, screened Earth and public reference ground after their formation ground straps, Ultimately connected with the Earth. < .br> <br> In the circuit system design should follow "grounding" principle, if the form more grounded, there will be a closed ground-loop, magnetosphere pass through the loop when it will produce magnetic noise, actually it is very difficult to .achieve "grounding". Therefore, to reduce the grounding impedance, to eliminate the impact of the distribution capacity to take the plane or multi-point grounding, the use of a conducting plane (floor or multilayer PCB circuit of conducting plane layers, etc. .) as a reference to the need to ground the various parts of the nearest received the reference ground. To further reduce the voltage drop across the ground loop, return the bypass capacitors to reduce available current amplitude. In low frequency and high frequency coexistence of circuit .system, should be low-frequency circuit, high frequency circuit, power circuit of the Earth has a separate connection, and then connect to a common reference point. <br> <br> Filtering is suppressing conduction disturbance of a good idea. For example ., in the power input connection filter, you can suppress switching power generated and interference to the power network feedback, you can suppress noise from the power grid to power itself. In the filter circuit, also takes a lot of dedicated filtering components, such as .through capacitor, three-Terminal capacitor, ferrite magnetic ring, they can improve circuit filtering features. Proper design or select filter, and properly install and use the filter, is an important part of anti-interference technology. <br> <br> EMI .filtering technology is a sharp pulse interference suppression of effective measures that can filter out many reasons of conducted noise. Testing indicates that, as long as the appropriate select components of parameters, they can better suppress switching power supply of conducted noise. <br> < .br> 5. current switching power supply EMI Suppression measures of <br> <br> Existing control measures, mostly from the Elimination of interference and disturbed the coupling between devices and radiation, cut off the transmission of electromagnetic interference, this is indeed a disturbance .of proven approaches, but very few people involved in direct control of sources of interference, to eliminate interference, or increase the interference rejection capabilities of the device, but the latter have a lot of room for development. <br> <br> 6. .improvement measures <br> <br> I think that at present from the transmission of electromagnetic disturbance to suppress interference proceeding, has gradually matured. Our viewpoint to return to the switching power supply to the device itself. From many years of work in practice, .in the circuit, to note the following: <br> <br> (1) of the printed circuit board layout, to analog and digital circuit area reasonably separate, separate power and ground leads, supply Department brings together into one point; PCB wiring ., high frequency digital signal lines to short, the best main signal lines are concentrated in the PCB Board, power supply cable as far as possible, while away from the high frequency digital signal line or separated by the ground. Secondly, according to the coupling .coefficient to wiring to minimize interference coupling. <br> <br> (2) of the printed circuit board of the power and ground wire width of printed pieces as possible in order to reduce the line impedance, thereby reducing public resistance caused by interference noise .. <br> <br> (3) devices more choice of SMD components and minimize component of PIN length, to reduce the impact of inductance components. <br> <br> (4) in the Vdd and Vcc power supply side as close .as possible to the devices access to shorten filter capacitor switching current circulation paths, such as 10 μ F Aluminum Electrolytic Capacitor μ F and 01 and join in power feet. For high-speed digital ICS power-can be used in place of tantalum electrolytic capacitors ., aluminum electrolytic capacitors tantalum electrolytic ground impedance than aluminum electrolysis. <br> <br> Conclusions have switch power electromagnetic interference factors also many, there are a large number of electromagnetic interference suppression. Comprehensive suppression of noise switching power supply makes switching power supply more .widely applied. <br> <br>.