电磁兼容讲义-3_图文

1、EMC-Chp.3Chp. 3 Interference coupling mechanism and preventive measures4 kinds of interference coupling or transmission modes Electrical, conducted(resistive coupling; Electric field coupling(Capacitive coupling; Magnetic field coupling(inductive coupling; Radiation (HF radiated coupling .3.1 Resist

2、ive couplingWith existing circuit connection Key factor: Common impedance Including two modes: Resistive coupling between operating circuits ; Resistive coupling between operating circuits and grounding circuit. Preventive measures for decoupling such interference Isolating between operating circuit

3、s. Decrease the common conductor impedance. Reducing the distance between the lines, for 2 lines with diameter D, separated in d,L=(/ln(2d/D, stranded lines, match plate lay out in PCB, etc. Absorption capacitance at circuit entrance. With separated supply. But the supply internal impedance cant be

4、eliminated. Separate operating units with significant power assumption. Measures for reducing such coupling (1 Cut-off the grounding circuit. (2 Application of isolating transformer.(3 Application of choke.(4 Bypass techniques.(5 Protective shielding. For DC and LF effective. Other methods Optical c

5、oupler, opto-coupler, OC. Optical fiber. Often used for eliminating common mode interference. operational differentialamplifier, 运算差分放大器,Lmh730154, National Semiconductor.Symmetric systems.3.2 capacitive coupling Notice that the “capacitor” is not a connected component of the circuit, but the stray

6、capacitor between the conductors. Its a kind of near electric field effect. With the increase of the voltage between the conductors, the capacitive coupling effect is increasing significantly. Measures for decreasing U i Shorten the parallel conductors. Coil shape connection, increasing the distance

7、 between the conductors. Decreasing R E , i.e.,application of the of the low impedance line technique. When the receiver is at open circuit status, theinterference is significant. This is used as a characteristic for the judge of such interference. Shielding.Conditions for idea shielding Idea conduc

8、tivity, and inductance-free. i.e., the current flow at the shield will not induce interference voltage at the signal lines. Capacitive leakage should be ignorable. Sylphon bellowss, 波纹管3.3 Magnetic coupling Existing between two or more circuits. Its a kind of near magnetic field induction effect. Ex

9、actly its a mutual inductance effect. The induced noise is a kind of differential mode interference. Measures for reducing U 2i Decreasing the parallel conductors. (decreasingMI/II; Increasing the distance between parallel circuits(alsodecreasing MI/II; orthogonal (normal setting(eliminate MI/II; St

10、randed conductors, decreasing the coupling area.(decreasing A2; Shielding circuit II. (eliminating differential mode noise. ; Application of compensation coil. 3.4 Radiated coupling Its a kind of far field effect. r >>/2 In a EM wave, the magnetic field and electric field is considered simultaneously. The interference to receivers should be analyzed by the traveling wave method. Important concept The traveling wave concept in analysis. If the electric dimension of the receiver is small, it can be considered as uniform field. i.e., the B and E is considered as constant. Distributed