Often in actual operation, the most common damage to the system is low-frequency common mode interference, such as high-power motors, circuit breakers or switches, short circuits, lightning strikes, etc. These types are mostly external common-mode signals, and their pulse width is Between 100 us and s, the longest period is also a few seconds, and such pulses continue to cause high voltage fluctuations to the ground, thereby damaging the system. However, for high-frequency common mode interference, most of the energy is radiated as the energy transmission path from the interference source, and such common mode interference is mostly generated by the system itself.
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1. For grounded products, it is of course desirable to have common mode interference conducted on the cable, through the capacitor or transient suppression device, to the earth or the chassis to prevent it from interfering with sensitive circuits (such as CPU).
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2. For floating products, mainly through the series magnetic ring (or increase the common mode impedance), to prevent the common mode voltage from being converted into differential mode voltage, and to interfere with sensitive circuits; secondly, pay attention to PCB wiring, not only to make PCB board Each circuit of the circuit maintains zero potential at its reference ground (digital ground GND, not the grounded product PG), and is placed in the filter circuit of key signals such as I/O, RST, CS (chip select). In this way, noisy common mode interference will not interfere with the digital circuit.
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3. The first method is to vent (but requires a good grounding or metal casing), and the second method is to block (to avoid common mode disturbances into differential mode interference, affecting the circuit). The former method is mainly used for grounded equipment (such as communication base stations), and the second method is mainly used for vehicle, airborne, and shipborne equipment.
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4. Of course, everyone will say the second method (floating), due to the parasitic capacitance of the PCB board and the ground, the high frequency interference may fail. However, for railway, electric power, and industrial control sites, the main interference is inverters, high-power motors, circuit breakers or switches, and the interference generated is mainly concentrated above 10 MHz. In addition, ground interference (strong electrical short circuit, lightning strike, harmonics, leakage current) is also extremely serious and unstable (usually up to 0.8V), for some key CPU operating voltage of 1.2V, it is simply the devil !
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5. High frequency common mode electromagnetic interference, the energy is generally not very large. For example, mobile phones, high-power radio frequency identification (the maximum power of 3W), because of the high frequency, ferrite magnetic ring or magnetic beads can be absorbed, metal chassis (or sprayed conductive layer inside the plastic case), you can completely Reflect or absorb. ——Now the railway requires 800~1000M, 1.4G~2.1G radiation immunity test (strength up to 20V/M), and 2.1G~2.5G radiation immunity test (strength up to 5V/M), the equipment is almost no There will be problems. Of course, the equipment should pass CS, ESD, EFT and other tests.
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