PCB News - Solution to the common mode interference leakage to the ground

How to solve common mode interference leakage to the ground? Among them, some netizens suggested one "vent" and two "blocking", then what exactly is this so-called one "venting" and two "blocking", for more details, please see below.

Netizens' views of turning two into one:

1. For grounded products, it is of course hoped that the common mode interference conducted on the cable should be directed to the ground or chassis through capacitors or transient suppression devices to prevent it from interfering with sensitive circuits (such as CPU).

2. But for floating products, it is mainly through the series magnetic ring (or increase the common mode impedance) to prevent the common mode voltage from transforming into the differential mode voltage, which interferes with sensitive circuits; secondly, pay attention to the PCB wiring, not only the PCB board The various circuits of the circuit maintain zero potential for their reference ground (digital ground GND, not the chassis ground PG of grounded products), and are placed in the filter circuit of key signals such as I/O, RST, and CS (chip select). In this way, even harsh common-mode interference will not interfere with digital circuits.

3. The first method is leakage (Teacher Wu talked about it, but it requires a good grounding or metal casing), and the second method is blocking (explained in the second method, to avoid common mode disturbances from transforming into differential mode interference, affecting the circuit ). The first method is mainly used for ground equipment with good grounding (such as communication base stations), and the second method is mainly used for vehicle, airborne, and shipboard equipment.

4. Of course, everyone will say that the second method (floating), due to the parasitic capacitance between the PCB board and the ground, it may fail to high-frequency interference. But for railways, electric power, and industrial control sites, the main interference is frequency converters, high-power motors, circuit breakers or switches, and the interferences they produce are mainly concentrated above 10MHZ. In addition, ground wire interference (strong current short circuit, lightning strike counterattack, harmonics, leakage current) is also extremely serious and unstable (usually as high as 0.8V). For some key CPUs with a working voltage of 1.2V, it is simply a devil. !

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 I have seen is only 3W), because it is high frequency, ferrite magnetic rings or beads can absorb, metal chassis (or sprayed conductive layer in plastic chassis), can be 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), almost no equipment There will be problems. Of course, the equipment must pass CS, ESD, EFT and other tests.

Netizen Evanma's point of view:

For solving external common mode interference, I agree with Brother Hua's point of view. Regarding the influence of parasitic capacitance on high-frequency interference, personal experience, this is not a big problem. The views are as follows;

In actual operation, the most damage to the system is low-frequency common mode interference, such as high-power motors, circuit breakers or switches, short circuits, lightning induction, etc., as mentioned by Brother Hua. Most of these types are external common mode signals. The width is between hundreds of us to s, and the longest period is several seconds. Such pulses continue to cause high voltage fluctuations to the ground, thereby damaging the system. But for high-frequency common-mode interference, starting from the interference source, most of the energy is radiated as the energy transmission path, and such common-mode interference mostly occurs in the system itself.

The above is an introduction to the solution of common mode interference leakage to the ground. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology