How to choose a filter for switching power supply

The three elements of electromagnetic interference of switching power supply are: disturbance source, transmission route and disturbed equipment. Therefore, the suppression of electromagnetic interference should also from these three aspects of the manpower, take appropriate measures. Should first restraining the disturbance source, the interference is eliminated; followed by eliminating interference source and coupling interference between devices and radiation, cut off the route of transmission of electromagnetic interference; the third is improving the anti-jamming ability of disturbance of equipment, reduce its sensitivity to noise. The current suppression of interference is basically the coupling between the electromagnetic disturbance and the disturbed device.. Common methods are shielded, grounded and filtered.

1) using the shielding technology can effectively inhibit the electromagnetic radiation interference of the switching power supply, that is, with good electrical conductivity of the electric field shielding, with high permeability of the magnetic field shielding. Shield has two purposes, one is to limit the internal radiation of electromagnetic energy leak. The second is to prevent external radiation interference in the inner region. The principle is the reflection, absorption and guidance of the electromagnetic energy by the shielding body.. In order to suppress the radiation produced by the switching power supply, electromagnetic disturbance influence on other electronic equipment, fully in accordance with the method of magnetic shield to process a shielding cover, and the shielding cover is connected with the casing of the system and as one can to the electromagnetic field of the effective shielding.

2) the so-called grounding is to establish a conduction passage between two points in order to connect the electronic equipment or components to a reference point called “ground”. Grounding is an important method for the suppression of electromagnetic interference in the switching power supply equipment, and the power supply can play a part in restraining the interference.. In circuit system design should follow the principle of “one point grounding, if the formation of multi point grounding, closed the ground loop, when the magnetic flux through the loop will produce magnetic induction noise. Is actually very difficult to achieve “grounding”. Therefore, to decrease the grounding impedance, eliminate influence of distributed capacitance and adopt planar or multipoint earthing, using a conductive plane as a reference, grounding of the nearest received the reference to needs. To further reduce the pressure drop of the grounding loop, the bypass capacitor can reduce the amplitude of the return current. In the low frequency and high frequency coexistence circuit system, the low frequency circuit, the high frequency circuit, the power circuit of the ground wire are separately connected, and then connected to the public reference point..

3) filtering is an effective method to inhibit the conduction interference, and it has a very important role in the design of electromagnetic compatibility of the equipment or system.. As an important unit of power line conducted interference suppression of EMI filter can be inhibited from interference from the power grid to the power supply itself against, can also inhibit and feed back to the grid in the interference generated by switching power supply. In the filter circuit, the dedicated a lot of filter elements, such as feedthrough capacitor, a three terminal capacitor, ferrite magnet ring bodies, they can improve the filtering characteristics of the circuit. Properly designed or selected filters, and properly installed and used filters, are important parts of the anti-jamming technology.

Select filter when you note the following:

(1) the frequency of the interference frequency and the interference frequency are clearly close to that of the two frequencies, and the frequency characteristic of the filter can be applied to the two frequencies.;

(2) guarantee that the filter can work reliably under high pressure;

(3) the filter is continuously energized at a maximum current rating when its temperature rise is lower, in order to ensure that the working performance of the components in the filter is not destroyed when the rated current is continuously working;

(4) for making the filter frequency characteristic of the work match the design value of the signal source impedance and the load impedance of the signal which is connected with it is equal to the specified value of the design:

(5) the filters must have shielding structure, the shielding box cover and the body to have good electrical contact, filter capacitor lead should be as short as possible, the best selection of short lead low inductance the feedthrough capacitance;

(6) have a high work reliability, because of the protection, electromagnetic interference filter, the fault often fault than the other components of the more difficult to find.

When installing the filter, note the following:

(1) the power supply line filter shall be located in the place where the power source is close to the power source, and do not allow the power cord to be left in the device frame without the filter;

(2) the capacitor lead in the filter should be as short as possible, so as to avoid lead inductance and capacitive reactance at low frequency resonance;

(3) the grounding wire of the filter has a large short-circuit current passing through, will cause additional electromagnetic radiation, so the filter element itself is good shielding and grounding;

Filter (4) input and output should not cross the line, otherwise it will crosstalk caused by the filter input and output capacitance coupling pathway, thereby reducing the filter characteristics, usually is input and output between the partition or shielding layer.

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