Anti-interference Design of Switching Power Supply
The EMC design of switching power supply should consider the following aspects:
1) filter
2) High frequency transformer
3) Soft switching technology 4) Active suppression of common mode interference
5) EMC design of printed circuit board wiring
3EMC Design Measures
3.1 filter
Filtering is a method of suppressing conducted disturbances. For example, connecting a filter at the input end of the power supply can suppress the noise from the power grid from invading the power supply itself, and can also suppress the interference generated by the switching power supply and fed back to the power grid. As an important unit to suppress the conduction interference of the power line, the power filter plays an extremely important role in the electromagnetic compatibility design of the equipment or system. It can not only suppress the conduction interference on the transmission line, but also has a significant suppression effect on the radiated emission on the transmission line. In the filter circuit, the selection of feedthrough capacitors, three-terminal capacitors, and ferrite magnetic rings can improve the filter characteristics of the circuit. Appropriate design or selection of appropriate filters and correct installation of filters are important components of anti-interference technology. The specific measures are as follows: 1) Install a power filter at the AC input terminal, and its circuit is shown in Figure 1. In the figure, Ld and Cd are used to suppress differential mode noise. Generally, Ld is 100-700 μH, and Cd is 1-10 μF. Lc and Cc are used to suppress common mode noise and can be adjusted according to actual conditions.
All power supply filters must be grounded (except those that are not allowed to be grounded according to the manufacturer's special instructions), because the common mode bypass capacitor of the filter must be grounded to work. The general grounding method is not only to connect the filter to the metal casing, but also to connect the filter casing with thicker wires.
Connect to the ground point of the equipment. The lower the ground impedance, the better the filtering effect.
The filter should be installed as close to the power inlet as possible. The input and output ends of the filter should be as far away as possible to avoid interference signals from directly coupling from the input end to the output end.
2) Add an output filter at the output of the power supply. Adding high-frequency capacitors, increasing the inductance of the output filter inductor and the capacity of the filter capacitor can suppress differential-mode noise. If multiple capacitors are connected in parallel, the effect will be better. 3.2 High frequency transformer
Install an RC absorption network on the primary side, secondary side of the high-frequency transformer, between the C and E poles of the switch tube, and on the output rectifier diode.
3.3 Soft switching technology
The application of soft switching technology helps to reduce electromagnetic interference, because the power MOSFET and IGBT are turned on at zero voltage and turned off at zero current, and the fast recovery diode is also soft off, which can reduce the power consumption in the power circuit. The di/dt and dv/dt of the power device can reduce the EMI level. It is proved by experiments that the soft switching technology only has a certain effect on suppressing the high-order harmonics of the ripple.
3.4 Active Suppression Technology of Common Mode Interference
Common-mode interference active suppression technology is a method of taking measures from noise sources to suppress common-mode interference. The idea of this method is to try to extract a compensating EMI noise voltage from the main circuit that is completely opposite to the main switching voltage waveform that causes EMI, and use it to balance the influence of the original switching voltage.
3.5 printed circuit board
Practice has proved that the component layout and wiring design of the printed board have a great influence on the EMC performance of the switching power supply. There are also high-voltage power busbars, as well as some high-frequency power switches and magnetic components. How to reasonably arrange the position of components in the limited space of the printed board will directly affect the anti-interference performance of each component in the circuit. and circuit reliability.
3.5.1 Influence of wire impedance
By analyzing the characteristic impedance of the printed wire, the placement method, length, width and layout method of the printed wire are selected.
The characteristic impedance of a single wire consists of DC resistance R and self-inductance L
Z=R+jωL(1) L=2lln(2)
In the formula: l - the length of the wire;
b - wire width.
Obviously, the shorter the printed line l, the smaller the DC resistance R; at the same time, increasing the width and thickness of the printed line can also reduce the DC resistance R.
It can be seen from formula (2) that the shorter the length l of the printed line, the smaller the self-inductance L, and increasing the width b of the printed line can also reduce the self-inductance L. The characteristic impedance of multiple printed lines is not only composed of DC resistance R and self-inductance L, but also has the influence of mutual inductance M, and the mutual inductance M is not only affected by the length and width of printed lines, but also the distance between printed lines. play an important role. M=2l(3)
In the formula: s——the distance between the two lines, increasing the distance between the two lines can reduce the mutual inductance.
In view of the above phenomenon, when designing the printed circuit board, the impedance of the power line and the ground line should be reduced as much as possible, because the power line, ground line and other printed lines have inductance, when the power supply current changes greatly, it will produce a large Large voltage drop, and the voltage drop of the ground wire is an important factor in the formation of public impedance interference, so the ground wire should be shortened as much as possible, and the power wire and ground wire should be thickened as much as possible.
In the design of double-sided printed boards, in addition to thickening the power line and ground line as much as possible, a decoupling capacitor with good high-frequency characteristics should be installed between the ground line and the power line. In addition, do not run two printed signal lines in parallel. If parallel wiring cannot be avoided, it can be remedied by the following methods: 1) Add a ground wire between the two signal lines for shielding;
2) Keep the distance between the two parallel signal lines as far as possible to reduce the influence of the electromagnetic field between the two lines;
3) The direction of the current flowing through the two parallel signal lines is opposite. (The purpose is to reduce the induced magnetic flux)
3.5.2 Layout of components
When designing a printed circuit board, it is usually difficult to avoid the interference source and the victim due to the limitation of the working conditions. At this time, try to put the interrelated components together to avoid interference caused by too long printed lines because the components are too far away; moreover, place the input signal and output signal near the lead port as much as possible. , to avoid interference due to coupling paths.
4 structural measures
Shielding is an important and effective means to solve electromagnetic compatibility problems
