Wednesday, December 29, 2010
Weak current College】 【power EMI filter technical parameters and selection】.
<br> <br> Summary: Describes the power of EMI (ElectroMagneticInterference) interference, power EMI filter technology principle; focused on its technical parameters and selection considerations; typical filter application analysis. <br> Keywords: electromagnetic interference insertion loss testing impedance matching .<br> 1 Introduction <br> In recent years, as the work of electromagnetic compatibility, power filter technology is becoming more and more widely. In order to supply EMI technology have a deeper understanding, in particular its technical principle, selection of technical parameters .when concerns and considerations, as well as filtering effectiveness analysis, etc. Combining work, selection of power supply EMI filter for exploration of deep step. <br> 2 Overview <br> Power supply of the electronic equipment, there are a variety of external .interference signals. Many electronic devices themselves, in the completion of its function, it also produces a variety of EMI signals, as well as man-made and natural EMI signals. These EMI signals, by means of conduction and radiation, affects the environment in .the operation of electronic equipment. <br> <br> 2.1 EMI electromagnetic interference power supply <br> IBM Corporation (USA) study showed that a regular computer equipment monthly to 120 times power interruption and the power problem is the United States more than 45 .per cent of computer devices lose data and the root cause of the failure. One pulse interference 39.5 percent, oscillation transient 49 per cent of the total 88.5 percent, is the power supply is a major component of interference. Power grid load switching, power switch .or other failure may cause transient power occurred resulting impulse noise, it is often also called a transient noise, the wave is a series of single pulse or pulsed beam. <br> <br> For the above power system transient voltage noise, how to .improve equipment (product) on EMI immunity, take effective and reliable measures of EMI filter. We all know, the shield is the control of EMI radiation hazards for the best signal. In deal with EMI interference signal transmission and conduction of certain radiation interference, .power EMI filter is extremely effective devices. Almost all the electronic devices you want to turn to it to control the runtime-generated EMI signals, which is widely used for applications. <br> <br> Power supply EMI filter, also known as electromagnetic .interference filters, power grid filter, electric noise filter and so on, or collectively known as EMI filter. It is a low-pass filters, DC, 50 Hz or 400Hz power without attenuation and transferred to the device, which greatly attenuated by the power .of the incoming signal, EMI protection devices from it; at the same time, and can effectively control the device itself resulting EMI signals, and prevent it into the power grid, pollution, electromagnetic environment against other devices. Power supply EMI filter is to help .the electromagnetic devices and systems meet the standards for electromagnetic compatibility of devices, such as IEC, VDE, FCC, GB9254 and GB6833 MIL-STD-461, etc. Figure 1.2 typical power supply EMI filter filter curve. Turning to the power supply EMI filter .before, let us first to discuss the common mode (also known as asymmetric) disturbing signals and differential mode (also known as symmetric) interference signals. <br> 2.2 common mode interference signal and differential mode <br> <br> About the variety .of EMI signals on the impact of electronic equipment, in which the line (L) and Earth (E) and (N) and middle ground wire (E) the existence of EMI signals between called common mode signal interference, common mode interference signal can .be considered in the L and N line transmission of potentials are equal, the same phase of the signal. The L and N interference signal exists between U3 called differential mode interference signal, you can put it as in the L and N lines have 180 degrees .of phase difference signal interference. For power supply system of conducted noise signal, you can use the common mode signal interference and differential mode. And can also be put on LE and NE and common mode interference. <br> <br> LN on differential .mode EMI interference as an independent source (network port), to analyze the characteristics of EMI signals and signal design inhibiting EMI filtering network. <br> <br> It is composed by the set parameters of the passive network components, the largest network of .dashed line filter metal shielding enclosures. There are two inductors L1 and L2, four capacitor CX and Cy. That is, to filter the power supply side tampering with equipment from the power source, filter load termination device tampering, L1 and L2 and Cy Cy ., respectively constitute LE and NE two independent ports of L-type (Γ) low-pass filter, used to inhibit the power supply system of common mode EMI signals. Of these, L1 and L2 constitute common mode chokes, due to the practical application .of inductive L1 and L2 of the inductance is not necessarily equal, so the difference between L1 and L2 is the differential mode inductance Le, and CX-also constitute LN independent ports a Π type low-pass filter, used to inhibit differential mode EMI signal ., enabling the power supply system EMI signal suppression, protection of electronic devices are not affected by it. <br> Practice, in the power cord is often both common and differential mode interference, generally less than 1MHz frequency interference to differential mode as the interference .than 1MHz frequency in common mode. Therefore power EMI filter is made from common mode filter circuit and differential-mode filter integrated circuits. <br> 3 power supply EMI filter selection <br> Select and use a power supply EMI filter, consider the most .important parameters are rated voltage, rated current, insertion loss, impedance matching, working environment conditions (temperature, etc.), the following respectively, also consider the volume, quality and reliability, and so on. <br> <br> 3.1 rated .voltage <br> Rated voltage is the power supply EMI filter used in specifying power frequency voltage, the work is also the maximum allowed by the filter voltage values. As used in the filter 50Hz single-phase power supply, rated voltage 250V; used in .the filter 50Hz-phase power supply, rated voltage 440V. If the input filterVoltage too high will cause damage to the internal capacitor. <br> <br> 3.2 rated current <br> Rated current (Ir) is specified in the rated voltage and .temperature conditions, to the maximum continuous operating current. <br> <br> As the environment temperature, or because the inductance of the copper wire, magnetic core loss and ambient temperature, and other causes of work temperature is higher than room temperature, then .it will be difficult to ensure the insertion loss of performance. We should be based on actual possible maximum operating current and working temperature to select filters rated current. <br> Except in special instructions, EMI filter specification gives the rated current for room temperature + .25 ° c (nominal temperature) values, the same given typical insertion loss or curve also refer to the value of + 25 ℃. Maximum operating current (Imax), rated current and temperature of the existence of the relationship between as follows: string 3 .<br> <br> -1.0: Imax = the maximum operating current, Ir to room temperature Tmax rated operating current, to the highest working temperature + 85 ° c, the temperature to the actual work of Ta, Tr for room temperature + 25 .° c. According to the type 2.0, Imax / the relationship between Ir and Ta examples: + 25 ° c, Imax = Ir; + 45 ℃, Imax = 0.816Ir; + 85 ℃, Imax = 0. <br> <br .> In addition, some filter companies abroad, + 40 ° c (normal temperature)-operating current value Ir. <br> IEC climate rating is 25/085, specifies the ambient temperature of + 40 ° c (nominal) operating conditions, to .access other ambient temperature allowed by the current curve. <br> <br> <br> Impact of current and environmental relations, one of the main reasons is to filter soft magnetic materials. EMI filter generally use high permeability of MN-Zn ferrite soft .magnetic materials, the initial permeability of μ I = 7000 ~ 10000, but its not the Curie temperature, high-quality only 130 ℃. Magnetic permeability is higher, the lower the Curie temperature, the Curie point decline rapidly after the magnetic permeability, which .causes the filter inductor value decline, severely affect the filter effect. <br> <br> Therefore according to the operating temperature to correctly select the rated current of power line filter, or improve filter heats (environment) to ensure that the filter installation. .<br> 3.3 insertion loss <br> 3.3.1 insertion loss defined <br> Insertion loss is power EMI filter is the most important one of technical parameters, designers and engineering application of staff consider the central issue is to ensure safety, the environment ., filters and reliability to meet the standard requirements, achieving the highest possible insertion loss. <br> <br> Filter insertion loss is the frequency of the function, use dB (decibels). Circuit missed filter, signal source in circuit Terminal voltage ( .power) to U (P), in an interview after the access filter-input voltage (power) to U (P), define the insertion power IL (InsertionLoss) can use the following equation: <br> <br> Assuming that the .actual load impedance in filter insert before and after the stay the same, so the power of 1.1 can be represented by their corresponding load voltage and impedance of expression instead: <br> Equation represents the insertion loss, need any frequencies by taking down and insert .the filter to be measured. <br> <br> 3.3.2 common loss and differential mode losses <br> Power supply EMI filter insertion loss including common mode (expressed as CM) insertion loss and differential mode (expressed as DM) insertion loss .. On their specific test method, no. 17 in CISPR publication has been noted, there is no description. For example, a manufacturer of DNF05-H-6AEMI filter, the relevant standard measured insertion loss. <br> <br> 3.3. .3 factors affecting insertion loss <br> Impact power EMI filter insertion loss factors including impedance matching and installation. Practical applications, EMI filter for input and output impedance is measured figure 2.3 curve of 50 Ω, so its interference signal attenuation, does not mean that .product standards or specifications of the insertion loss. If you choose to use EMI filter network structure and parameters of reasonable, plus installed properly, you may achieve better than the standards set forth in the insertion loss. Conversely, if the network mix and parameter selection ., installation and have problems, you may not get good results, but will have the opposite effect, as in Figure 2.5 insertion loss-gain occurred. <br> <br> Another factor is the filter temperature and operating current. EMI filter insertion loss .measurement standards, CISPR publication No. 17, MIL-STD-220A and GB7343-87 the measurement method, unanimously stressed the need to load the rated current for its insertion loss measurements. Introduction to the foregoing, this is because the filter inductor L used .the ferrite or other magnetic materials, high-current, magnetic saturation caused a deterioration in performance. Figure 2.6 is a problematic EMI filter test, curve ① is normal 50 Ω system under test insertion loss curves, curve ② is 50 Ω system and 30A current .rating of insertion loss under test, comparing the difference between the curve. <br> <br> 3.4 impedance matching <br> 3.4.1 impedance matching of reason <br> Select a filter, the first thing you should choose for your filter circuit .and insertion loss performance. First choose the filter because the and filter to match the conditions of the traditional concept, the so-called match means that the filter you want to keep the input / output signal amplitude is not the same (or a fixed ratio .), with part of the spectrum be expected processing or transformation, and EMI filters, it is a power-on-object of the low-pass filter that is in non-matching conditions, because in practice impossible matches, Such as filter input impedance .RI - power source impedance is with changes of electricity consumption in size, filter the output impedance of the Rl (load impedance) - power supply impedance is with the power load size changes, in order to achieve an ideal suppression effect, should follow the correct .impedance matching. <br> <br> No matter how complex power EMI filter, you can put it in common and differential mode filtering network abstraction. <br> <br> 3.4.2 analysis of impedance mismatch <br> You can analysis out ., usually in the power supply EMI filter circuit network, inductance L as high resistance components, electric capacity c as low resistance components. In order to achieve better results filtering, the filter does not match the principle: If the actual load for the emotional high .resistance, select output load for capacitive low resistance of the filter; if the actual load is capacitive low resistance, then select the output load filter for emotional high resistance. Similarly, for filter input impedance and power source impedance, should follow the principle of impedance .mismatch to select filter. <br> By-1.4, the greater the difference between the Zo and Rl, ρ, port of the reflector. On the control of interference signals, when EMI filter is at both ends of the impedance mismatch, EMI signals .in its input and output ports have a strong reflection. This filter for EMI signal attenuation, equal to the filter's inherent insertion loss and reflection loss. In the actual use of EMI filter, available to the impedance mismatch for EMI Suppression of signal more effective. .This is why the choice of EMI filter, be sure to carefully analyze its port impedance matching of correct so that produce as much reflection, achieve effective control of EMI signals. <br> <br> 3.4.3 the practical application of <br> .In the filter selection, based on our control system DCS product requirements, there are two filter manufacturer offers prototyping and testing, below our actual test application in the analysis and presentations given. <br> <br> EFT output pulse interference of parameters: a .single Pulse rise time is 5ns, single pulse cycle for 50ns, 20MHz, single burst duration to 15ms, pulse train cycle for 300ms, voltage peak value respectively set to 1000V and 2000V. Oscilloscope settings parameters are the same, and the measurement of the connection .. Test results comparing figure 2.10. <br> String 7 <br> <br> (A) peak 1000V, filter effects (left, right, NF-HT type) <br> (B) peak 2000V, filter effects (left, .right, NF-HT type) <br> Figure 2.10 (A) (B) for the two filters in different interference voltage comparison of filtering <br> Test analysis: <br> ⑴ first from two filter circuit analysis on network diagrams, two .circuit networks constitute the fundamental similarity of components, and is double-level filtering network, both on-mode, there is differential mode interference. The only difference is the figure 2.8 (a) NF-network output load capacitance; (b) HT type .filter network output load is emotional. In order to enhance the inhibition, in accordance with the principle of impedance mismatch, NF type should add inductive load, HT-should add capacitive load. <br> <br> <br> ⑵ from test results .graph 2.10 (A), (B) can be seen, oscilloscope channel ① display curve for filtering interference signal channel ② shows curves for interference signals through the filter. Obviously two types of filters for the filtering effect of interference signals vary greatly: model NF's .filter filter effect obvious than HT model filter. Moreover, HT model filter no inhibition to interfere with the signal, but a gain. Analysis of the main reasons is the impedance mismatches, from the test results can also be indirectly draw our DCS system belongs to .the inductive load. <br> <br> According to the actual EMI signal source and load impedances property, in accordance with the greatest mismatch principles to select filter network structure and parameters. However, in actual use, it is necessary to know the filter .output termination load of exact property is not an easy task. <br> <br> The above analysis results only from the filter output load Zo and the actual add-in load Rl analysis, source impedance Rs (grid) and filter input impedance Zi ., the testing process, are the same, so it is no longer expanded analysis. <br> <br> From the above discussion shows that the power supply EMI filter for EMI signal suppression capability not only depends on the filter in 50 Ω System beta .of insertion loss, also depends on the filtering network and EMI signal source and load improperly. Therefore, in the selection of filter, with special attention to the signs of EMI filter, see its content is accurate the filtering network parameters and network infrastructure. Obviously ., that does not provide network parameters, and not a network structure of EMI filter, to correct termination and optimize application brought trouble. <br> In addition, some EMI filter plate indicate power and load termination, this may be to a specific set of .electronic devices, there is no universal significance, only the manufacturer's recommended termination methods. Application of EMI filter, to specific analysis of EMI filter network structure and access circuit equivalent impedance, in accordance with the above principles for terminating impedance matching to properly achieve its desired .objectives. <br> <br> 4 closing <br> <br> By understanding the EMI interference signal, in the selection of power supply EMI filter, in addition to the estimated interference source type - the common mode as the primary or differential mode ., as well as its electrical safety performance-the leakage current and voltage, also note the following points: first, to consider the use of ambient temperature, rated current and rated voltage, it directly affects the filter filtering performance; Secondly, filter insertion loss .is to select the filter must be taken into account; Moreover, to make the power supply EMI filter for EMI Suppression of signal's best, must be based on it to access load impedance, to select power EMI filterThe network structure and parameters, to produce the .maximum possible mismatch loss; Finally, in order to achieve the equipment and system of electromagnetic compatibility, engineering and technical personnel in the undertaking of the device or system design, we should carefully consider the early, right choice, with a good power supply EMI filter .. <br> <br> <br>.