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How to prevent and solve the electromagnetic interference problem of the connector?

2021/9/14 13:36:52 点击数:425
Nowadays, the clock frequency of electronic systems is several hundred megahertz, the leading and trailing edges of the pulses used are in the sub-nanosecond range, and high-quality video circuits are also used for sub-nanosecond pixel rates. These higher processing speeds represent constant challenges in engineering. So how to prevent and solve the problem of connector electromagnetic interference is worthy of our attention.

The oscillation rate on the circuit becomes faster (rise/fall time), the voltage/current amplitude becomes larger, and the problem becomes more. Therefore, it is more difficult to solve electromagnetic compatibility (EMC) today than before.
Before the two nodes of the circuit, the rapidly changing pulse current represents the so-called differential mode noise source. The electromagnetic field around the circuit can couple to other components and invade the connection part. Inductively or capacitively coupled noise is common mode interference. The radio frequency interference current is the same as each other, and the system can be modeled as: composed of a noise source, a "victim circuit" or "receiver", and a loop (usually a backplane). Several factors are used to describe the size of the interference: the intensity of the noise source, the size of the area around the interference current, and the rate of change.
Thus, although there is a high possibility of undesired interference in the circuit, the noise is almost always co-model. Once a cable is connected between the input/output (I/O) connector and the chassis or ground plane, when some RF voltage appears, a few milliamps of RF current can be sufficient to exceed the allowable emission level.
Coupling and propagation of noise
Common mode noise is caused by unreasonable design. Some typical reasons are that the lengths of individual wires in different pairs are different, or the distances to the power plane or chassis are different. Another reason is the defects of components, such as magnetic induction coils and transformers, capacitors and active devices (such as the application of special integrated circuits (ASIC)).
Magnetic components, especially the so-called "iron core choke" type energy storage inductors, are used in power converters and always generate electromagnetic fields. The air gap in the magnetic circuit is equivalent to a large resistance in a series circuit, where more power is consumed.
Therefore, the iron core choke coil is wound on the ferrite rod, and a strong electromagnetic field is generated around the rod, and there is a strong field strength near the electrode. In a switching power supply using a retrace structure, there must be a gap on the transformer with a strong magnetic field in between. The most suitable element in which to maintain the magnetic field is the spiral tube, so that the electromagnetic field is distributed along the length of the tube core. This is one of the reasons why the spiral structure is preferred for magnetic elements operating at high frequencies.
Inappropriate decoupling circuits also often become sources of interference. If the circuit requires a large pulse current, and the need for small capacitance or very high internal resistance cannot be ensured during partial decoupling, the voltage generated by the power circuit will drop. This is equivalent to ripple, or equivalent to rapid voltage changes between terminals. Due to the stray capacitance of the package, interference can couple to other circuits, causing common mode problems.
When the common mode current contaminates the I/O interface circuit, the problem must be resolved before passing through the connector. Different applications are suggested to use different methods to solve this problem. In the video circuit, the I/O signals there are single-ended and share the same common loop. To solve it, use a small LC filter to filter out the noise.
In a low-frequency series interface network, some stray capacitance is sufficient to shunt noise to the bottom board. Differentially driven interfaces, such as Ethernet, are usually coupled to the I/O area through a transformer, and the coupling is provided by the center taps on one or both sides of the transformer. These center taps are connected to the bottom plate via a high-voltage capacitor to shunt common mode noise to the bottom plate so that the signal does not distort.
Common mode noise in the I/O area
There is no universal way to solve all types of I/O interface problems. The main goal of designers is to design the circuit well, and they often overlook some details that are considered simple. Some basic rules can minimize noise before it reaches the connector:
1) Set the decoupling capacitor close to the load.
2) The loop size of the rapidly changing pulse current of the front and rear edges should be the smallest.
3) Keep high-current devices (ie, drivers and ASICs) away from the I/O ports.
4) Measure the integrity of the signal to ensure the minimum overshoot and undershoot, especially for critical signals with high currents (such as clocks and buses).
5) Use local filtering, such as RF ferrite, to absorb RF interference.
6) Provide a low-impedance lap connection to the baseboard or a reference in the I/O area on the baseboard. RF noise and connectors
Even if engineers take many of the precautions listed above to reduce the RF noise in the I/O area, they cannot be sure that these precautions will be successful enough to meet the emission requirements. Some noise is conducted interference, that is, common mode current flows on the internal circuit board. The source of this interference is between the backplane and the circuit.
Therefore, this RF current must flow through the path with the lowest impedance (between the bottom plate and the signal-carrying line). If the connector does not exhibit a sufficiently low impedance (at the overlap with the base plate), the RF current flows through the stray capacitance. When this RF current flows through the cable, emission will inevitably occur.
Another mechanism for injecting common-mode current into the I/O area is the coupling of strong interference sources nearby. Even some "shielded" connectors are useless, because the source of interference is near the connector, such as a PC environment. If there is a gap between the connector and the backplane, the RF voltage induced here can degrade EMC performance.
There are methods for shielding connectors, adding finger reeds or gaskets. The overlap of the connector is to fill the gap between the connector and the casing. This method requires a liner. Metal gaskets are better as long as they are handled properly, that is, as long as the surface is not contaminated, as long as the hands do not touch or damage the gasket, and as long as there is enough pressure to maintain good, low-impedance contact.
Another method is to install the connector on the connector or install the connector on the housing. At this time, the maximum contact surface is slightly smaller, and the size and elasticity of the tabs should be strictly controlled. When installing a shielded connector, make an opening on the casing, and remove the oil on the side of the opening. Carefully make it. If the tolerance is not appropriate, the connector will sink too deep in the casing and interrupt the overlap. Every EMC engineer knows that in an "excellent" system, this issue must meet the launch requirements and be checked in time on the production line. Unfastened or bent gaskets, installed on the oil stains in critical areas, will fail.
The EMI connector was selected for the following reasons
1) The conductive foamed plastic is extremely soft and can be placed on the entire circumference of the connector. This eliminates problems related to the other casing and gasket.
2) The mechanical engineer can install the connector within the acceptable tolerance range of the system chassis.
3) The connector and the chassis are connected with low impedance to ensure good contact. The liner on the inner side of the cabinet wall can be made of softer materials when it is required to be painted and has a masking requirement.
4) For designs requiring forced cooling, the gasket should preferably have another feature: the seam between the connector and the casing wall should be sealed to reduce air leakage. In a dusty environment, the gasket should help keep the system clean.
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