What are the application examples of electromagnetic compatibility technology?

1, shielding problems

Example 1: A system is a cabinet and a chassis structure, wherein the control part is a chassis structure, a sub-board bus board structure, and a sub-board is installed with a panel. When performing electrostatic test, when the contact discharge is +5.5kv, when the main board panel and the left and right adjacent panels are subjected to electrostatic test, the control board restarts or freezes, and then the finger reed is installed between the panels near the control board, and the system is in contact. The operation is normal when the discharge is ±6.6kv.

Example 2: A system test, using a common cabinet, the system is very sensitive, the group lead line (communication line) is subjected to group pulse test, and the coupling clip coupling method is adopted. When the interference is added, the system is not normal, and the system is added at both ends of the communication line. Magnetic ring, the effect is not obvious, then there is no way, replace the shielded cabinet, carry out the test, there is obvious effect, after a few rounds, the system will appear the idea of the machine, after the communication line into the cabinet to increase the installation of the magnetic ring, the system. The work was normal. After several rounds of tests, there was no downtime and the system worked normally.

Analysis: Many systems are now chassis structure, that is, the control board, acquisition board, and driver board are all installed in the same chassis for data exchange and control. After the installation is completed, each circuit board will have a certain gap. The electrostatic pulse passes through the panel gap, and the distributed capacitance is coupled to the main board, which causes the power supply to be distorted or the control system to restart and crash. The finger-shaped reed is installed between the panels to make the chassis a good shielding body. Due to the "skin effect" of the electric charge, when there is static electricity, the static electricity will be discharged to the earth along the surface, and the influence on the internal circuit is reduced or disappear.

The shielded cabinet handles the gaps and doors of the cabinet. The conductive reed is installed at the gap. The conductive cloth pad is installed at the contact position between the door and the cabinet to improve the shielding effectiveness of the cabinet, improve the overall anti-interference of the cabinet, and the essence of the group pulse interference. It is the accumulation effect of the distributed capacitance energy of the line. When the energy is accumulated to a certain extent, it may cause the circuit (or even the equipment) to work incorrectly. Usually, if the test equipment fails, it will continue to make mistakes, even if the pulse voltage is slightly reduced, the error situation is still explained. Pulse grouping occurs, the pulse repetition frequency is higher, the waveform rise time is short, the energy is small, and generally no equipment failure occurs, which makes the device malfunction.

2, grounding problem

Example 3: A system equipment is doing the RF field induction conduction test of the 422 communication serial port. The twisted pair shielded cable is used, and the single-ended grounding is adopted. The error rate occurring during the test is high, and there is almost no correct data. The terminal is reliably grounded and the communication is normal.

Example 4: When a system equipment is used to test the RF field induction conduction of the video mouse line, the display has ripples in the lower frequency band (below 3M), flashing up and down, and then the display side of the video line is reliably grounded, and the interference is significantly reduced, hardly affects the display.

Analysis: These two phenomena occur when conducting the inductive conduction test of the RF field. The inductive conduction immunity test of the RF field is essentially: the device lead becomes a passive antenna, accepting the induction of the RF field, and becoming a conducted interference intrusion. Inside the device, the near-field electromagnetic field formed by the RF voltage and current affects the operation of the device, mainly based on the low-frequency magnetic field.

Twisted pair can effectively suppress magnetic field interference, not only because there is a small loop area between the two wires of the twisted pair, but also because the current induced on every two adjacent loops of the twisted pair has the opposite. The direction is therefore offset by each other. The denser the twisted pair of twisted pairs, the more obvious the effect.

When the shielding layer is grounded at both ends, the external magnetic field generates an induced current in the loop formed by the original signal and the ground line, and also generates an induced current Is in the loop formed by the shield layer and the ground line, and is also induces a magnetic field, but this the magnetic field is opposite to the original magnetic field and cancels each other, resulting in a reduction in the total magnetic field and a reduction in interference.