滤波器选型（Filter selection）

滤波器选型（Filter selection） 滤波器选型(Filter selection) The filter selection needs to consider the following points: 1 voltage This voltage requirement is a range which is the synthesis of the steady-state voltage + ripple voltage. 2 current The current index is critical. It determines the inductance of the filter winding, copper wire and lead wire diameter. If selected, a large current on a thin wire, such as a small cart, causes serious heat and burns. This current is also a range of steady-state current + ripple current maximum. 3 EMC requirements Since it is a filter, in order to filter out some undesired frequency bands, the effect of filtering is usually determined by the visual results of EMC test standards and field applications. In particular, the power filter, it is best to determine the product through which the filter is required to adopt standards, according to the standard requirements of different, in the choice of a specific test band requirements. The main targets of the power filter are transmit emission CE and conduction immunity CS, and the signal filter mainly depends on the requirements of the EMC standard for the undesired input frequency band and the undesired output frequency band. For example, no lamp for rectifier, itself a switch working state, there will be foreign launch, EMC test will focus on examination of the switching frequency and the harmonic components of the conduction interference filter is needed for these specific frequency or frequency has enough filtering effect. 4 safety standards requirements Readers might find it strange to choose filters, and say, what do security standards do? This is because the filter is generally used in the power input and board interface, these parts are the security problems of the hardest hit areas. Is equal to the filter, a body to undertake a number of requirements. The safety regulations related to the filter are three indexes: insulation, withstand voltage, leakage current, residual voltage and residual energy. LN insulation on insulation strength, is the test of the breakdown voltage of Y capacitance, Y capacitance, leakage current can be large, easily lead to safety requirements on leakage current exceed the standard, there are some manufacturers design to take the input of Y capacitor design (Figure). In this way, LN has become LN for G, through L1, Cy1, Cy2, G, and G, while G and G are not connected. If the input terminal is connected to the Y capacitor, the Cy1 and Cy2 will be placed on the left side of the front R, then the test must pay attention to the setting of the insulation voltage and the leakage current size is positively related, and the maximum is not more than 20mA. Had almost returned that filter safety substandard conditions, finally the investigation is the 1500V leakage current is set to 2mA (for 5mA), is the normal alarm test instrument. Another problem is that the choice of R filter, a lot of manufacturer is not installed in the resistance, pull the plug, in a relatively short period of time, to touch the power socket, if there will be a feeling of power, the problem is not installed in this resistor. This is a high resistance, discharge discharge function of the power resistor. 5 filter circuit structure form The parameters of circuit structure and period selection is filter core, but in this part, the choice of application engineers often choose the dark eyes, Although most of the time can be used almost, but neither intimate nor know the design of the way, waste of resources, buried hidden risks will greatly increase the possibility. This is an anachronism in the need for lean design, the manufacture of electronics from China to China, and the growth expectations of junior engineers to senior engineers. The role of the filter is different on the amplification effect by different frequency, the passband frequency is not attenuation to inhibition of the passband is dozens of dB by level of attenuation, so as to achieve the purpose of a sieve. But is the filter in the amplitude of different frequency when taking different magnification, the phase of the electromagnetic wave is also changing, because the phase is frequency dependent, so filter structure form selection, there are still some knowledge. There are three kinds of filter structures: A: the characteristics of Butterworth filter passband amplification flat passband, as frequency increases, the filter magnification remained unchanged; but the drawback is that the pass band cutoff to transition, the transition is gentle. It is said that the boundaries of enemies and friends are not very clear, some friends in doing things there is a part of the enemy, the enemy also help us, this part is to kill or stay in the organization, people very tangled. If the useful frequency and the interference frequency are very close, the function of this filter is very problematic. B Chebyshev filter: it can be a good solution to the shortcomings of the gentle Butterworth transition zone, in the form of filter in the transition zone is very steep, even if the useful frequency and frequency interference is very close, because the transition zone is very steep, so the cutoff frequency of two frequency bands magnification difference is very big, not friends. A good friend is distinguished, not doing the things we are sorry, the enemy wouldn't have done a good thing for us, so friends pulled into the organization of the preferential treatment, the elimination of the enemy is smooth and clean. The mountain side will have the advantages of a deep ravine, inevitably accompanied by a disadvantage, the disadvantage is the Chebyshev filter at the end part of the passband frequency, the gain will have strong fluctuations, namely in the passband, as frequency increases, while the magnification than a lot of big band filter, but the passband the frequency, the amplification factor is not constant, that is, friends of the mood is not stable, not all friends are as in the past give help. (just for example, don't misunderstand your friend's dissatisfaction.) C Bessel filter: this filter is not very general, and used more specifically, because its characteristics are linear phase. The first two concerns are amplification, but if the speech signal, such as the song, has no change in the passband magnification, its melody is no longer melodious. Because the phase change caused the song so hard to listen to the ah oh. At this point, the Bessel filter will take its part. As for which filter circuit structure to choose, circuit engineers may not go into depth research, but must know the characteristics of their own, and to provide filter manufacturers, they help you make the choice. The current power filters are low-pass filters, Through all the frequency 50Hz or 60Hz, it is a useful frequency, all the other is useless, so with the cutoff frequency above 1KHz is more than sufficient, therefore, a blind man on a blind horse like pick filter, there is no problem that a lot of time. Therefore, the selection of the power filter should be paid more attention to in the process and safety regulations. But in the case of specific output or input, the choice of the power filter must be careful. Such as electric knife medical surgery, its working frequency is 500KHz, it will cause interference on the power network, so the foreign interference electric knife to suppress; at the same time, with the electric knife common power supply equipment should be cautious, the 500KHz may also interfere with you. 6 plug curve The filter loss measurement is not true. For example, if we find 100KHz 13dB exceed the standard, the choice of a filter from the insertion loss curve seen in the 100KHz when the insertion loss is 20dB, think this filter up certainly no problem, it is wrong, because the loss curve of manufacturers are inserted under the quasi impedance measured at 50. -50 standard, application site actually, almost certainly not so standard source impedance and load impedance characteristics, so the attenuation effect of the filter will be greatly reduced, therefore, the choice of when the frequency points to be suppressed must set aside at least 20dB margin, as in the example above will need to choose 100KHz when the insertion loss is not less than 33dB filter. In addition the insertion loss insertion loss and differential insertion loss into common mode, more than 30MHz of the general selection of common mode interference, insertion loss filters meet the requirements above, the following selection of differential mode interference 10MHz to meet the requirements of the filter insertion loss, for example 100KHz, select the differential mode filter insertion loss 33dB. 7 installation form of filter This problem is well understood, the general type (a welding pin pin), installation screws, IEC standard (with single-phase 220V three pin input) with switch, IEC, the functional requirements can choose according to the actual structure. 8 installation process specification The installation of the filter is the only technical factor after the structure of the circuit and the component index. Mainly reflected in the filter location, grounding measures. Location close to the input or output, to avoid the high frequency interference input output terminal cable on the radiating effect to other circuit; input output line line not parallel line shall not be close to the line, in order to avoid crosstalk caused by the filter can not clean clean; subject is metal shell, surface grounding and grounding requirements line is not grounded, to ensure that the entire surface with good contact, not only by the grounding wire fixing pin screws or above leads to grounding, grounding wire inductance of large volume, high frequency grounding impedance leads to high frequency grounding is not good, the filtering effect is not good; the grounding cable is connected with twist, must use the welding method. The Q value of the 9 filter The Q value has little influence on the actual filtering effect, but the Q value represents the loss / input power, and the higher the Q value, the greater the loss, which means that some of the energy will be lost on the inductance of the filter. In general low power supply filters and signal filters, this problem will not be too prominent. But on a higher power filter, This is not a loss, can cause fever, fever after the capacitor will cause a greater negative impact, leakage current, voltage and capacitance value will change with the variation of temperature; the two is power consumption will lead to unnecessary loss of power. 10 other The other is the content of the internal filter, as the user is not to see, the thickness of insulating paper between potting potting uniform density, and metal devices, the user can only know through the interrogation, the manufacturer said not to tell the truth, all by fate.