The filter in the switching power supply circuit
The three elements: the formation of a switching power supply electromagnetic interference source of harassment, transmission and disturbed equipment. Thus, the suppression of electromagnetic interference should also be from the three aspects of manpower, to take appropriate measures. Should inhibit the source of harassment directly to eliminate the cause of the disturbance; followed by the elimination of the coupling between the disturbance source and the disturbed equipment and radiation, cutting electromagnetic interference route of transmission; disturbed equipment immunity to reduce its noise the sensitivity. Interference suppression of several measures are basically use to cut off sources of electromagnetic disturbance and by coupling channel interference between devices. The commonly used method is the shielding, grounding and filtering.
1) shielding technology can effectively suppress electromagnetic radiation interference of switching power supply, good conductivity material to shield the electric field, magnetic field shielding material with high permeability. Shield serves two purposes, restrict internal radiation of electromagnetic energy leaking out of prevent external radiation interference into the internal area. The principle is to use the shield body to electromagnetic energy reflected, absorbed and guiding role. In order to suppress the switching power supply generated by radiation, electromagnetic disturbance to other electronic devices, in full accordance with the method of magnetic shielding to process shield, and then the entire shield system enclosure and connected as one, will be able to electromagnetic fields effective shielding.
2) the so-called ground between two points and create a pathway to the electronic equipment or components connected to certain known as "ground" reference point. The ground is an important method of switching power supply device for electromagnetic interference suppression, connected to the power supply certain parts of the earth can play the role of interference suppression. Circuit system design should follow the principle of "ground", if the formation of multi-point grounding, ground loops closed magnetic induction noise will be generated when the magnetic field lines passing through the loop. Nearest part is actually very difficult to achieve the "ground" Therefore, in order to reduce the ground impedance to eliminate the impact of the distributed capacitance taken by the planar-type multi-point ground, using a conductive plane as the reference ground, to ground connected to the reference earth . To further reduce the pressure drop of ground loops, the the available bypass capacitor to reduce the amplitude of the return current. In the low-frequency and high-frequency circuit system coexist, the ground of the low-frequency circuit, high-frequency circuit, a power circuit should be connected separately, and then connected to a common reference point.
3) filtering is an effective way to suppress conducted interference, electromagnetic compatibility design of the equipment or system has an extremely important role. EMI filter as an important means to suppress power line conducted interference can be suppressed against interference from the power grid to the power supply itself, interference can be suppressed by the switching power supply and feedback to the grid. In many dedicated filter element in the filter circuit, further, such as the feedthrough capacitor, three-terminal capacitors, ferrite magnet, they are able to improve the filtering characteristics of the circuit. Appropriate design or selection filter and properly install and use the filter, is an important part of the anti-jamming technology.
When you select a filter, note the following:
(1) clear operating frequency and to suppress interference frequency, such as the two are very close, you need to apply a very steep filter frequency characteristics, in order to separate the two frequencies;
(2) ensure that the filter can work reliably under high pressure;
(3) continuous pass filter to the maximum rated current, the temperature rise is lower, in order to ensure the continuous operation of the rated current, does not destroy the work performance of the filter element;
(4) frequency characteristics of the filter to make it work with the design values consistent with the requirements specified value with it connected signal source impedance and load impedance value equal to the design:
(5) The filter must have a shielding structure, shielding lid and the body have a good electrical contact, the filter capacitor leads should be as short as possible, the best selection of low-inductance of the short lead feedthrough capacitors;
(6) have a high reliability, protection for electromagnetic interference filter, and its failure more often than the failure of other components are harder to find.
Install a filter should note the following:
(1) Power line filter should be installed as close as possible to the local population from the device power, do not let the power cord through the filter in the device box circuitous;
(2) filter capacitor leads should be as short as possible, so as to avoid lead inductance and capacitance to resonate at lower frequencies;
(3) Filter the grounding conductor through the large short-circuit current, will cause the additional electromagnetic radiation, so the response to the filter element itself is a good shielding and ground handling;
(4) the filter input and output lines can not cross, otherwise it will cause crosstalk due to the filter's input and output capacitive coupling passage, thereby reducing the filtering characteristics, the usual approach is to add a separator between the input and the output terminal or shielding layer .