How to prevent and solve electromagnetic interference problems in connectors?

How to prevent and solve electromagnetic interference problems in connectors?

Nowadays, the clock frequency of electronic systems is hundreds of megahertz, and the front and rear edges of pulses are within the sub nanosecond range. High quality video circuits are also used for sub nanosecond pixel rates. These higher processing speeds indicate that the project is constantly being challenged. Therefore, how to prevent and solve the electromagnetic interference problem of connectors is worth our attention.

The oscillation rate on the circuit increases (rise/fall time), the voltage/current range increases, and there are more problems. Therefore, addressing electromagnetic compatibility issues (EMC) is more difficult than in the past.

Before the two nodes of the circuit, the rapidly changing pulse current represents the so-called differential mode noise source, and the electromagnetic field around the circuit can be coupled to other components and invade the connection part. Inductive or capacitive coupled noise is common mode interference. The RF interference current is the same, and the system can be modeled as a noise source, a "victim circuit" or "receiver", and a circuit (usually a backplane). Describe the magnitude of interference using several factors: the intensity of the noise source, the size of the interference current surrounding area, and the rate of change.

Therefore, although unwanted interference may occur in the circuit, the noise is almost always common to the model. Once a cable is connected between the input/output (I/O) connector and the casing or ground plane, some RF will cause a few milliamperes of RF current to exceed the allowable emission level when voltage occurs.

Coupling and propagation of noise

Common mode noise is caused by unreasonable design. Some typical reasons are that different lines have different lengths of individual wires, or the distance to the power plane or shell is different. Another reason is the defects of components, such as magnetic induction coils and transformers, capacitors, and active devices (such as special integrated circuits (ASICs) for applications).

Magnetic components, especially so-called magnetic components, such as "iron core choke" energy storage inductors used in power converters, always generate electromagnetic fields. The air gap in the magnetic circuit is equivalent to a large resistor in a series circuit, consuming more electrical energy.

Therefore, the iron core choke coil is wound around the ferrite rod, generating a strong electromagnetic field around the rod and having the strongest field strength near the electrode. In a switching power supply using a feedback structure, there must be a gap on the transformer and a strong magnetic field. The most suitable component for maintaining a magnetic field is a spiral tube, which distributes the electromagnetic field along the length of the tube core. This is why spiral structures are the preferred choice for high-frequency magnetic components.

Improper decoupling circuits often become sources of interference. If the circuit requires large pulse currents, local decoupling cannot guarantee small capacitors or very high internal resistance requirements, and the voltage generated by the power circuit will decrease. This is equivalent to a rapid change in voltage between ripple or terminals. Due to the stray capacitance in the packaging, interference can be coupled to other circuits, leading to common mode issues.

When the common mode current contaminates the I/O interface circuit, the problem must be resolved before passing through the connector. It is recommended to use different methods to solve this problem for different applications. In video circuits, where I/O signals are single ended and share the same common circuit, it is necessary to solve this problem by using a small LC filter to filter noise.

In low-frequency serial interface networks, some stray capacitors are sufficient to divert noise onto the substrate. The Ethernet differential drive interface is usually coupled to the I/O area through a transformer, which is coupled by the center tap on one or both sides of the transformer. These center taps are connected to the bottom plate through high-voltage capacitors, diverting common mode noise onto the bottom plate, ensuring that the signal is not distorted.

Common mode noise within the I/O region

There is no one solution to all types of common method I/O interface issues. The main goal of a designer is to design a good circuit, often ignoring some simple details. Some basic rules can minimize noise before reaching the connector

1) Set decoupling capacitors near the load.

2) The loop size of rapidly changing pulse currents should be minimized.

3) Keep high current devices (i.e. drivers and ASICs) away from I/O ports.

4) Measure the integrity of the signal to ensure that overshoot and undershoot are minimized, especially for critical signals with high currents (such as clocks, buses).

5) Using local filtering, such as RF ferrite, can absorb RF interference.

6) Provide low impedance lap I/O to the base plate or base plate, and the reference for this area is on the base plate. RF noise and connectors

Even if engineers take many of the preventive measures mentioned above to reduce RF noise in the I/O area, it cannot be guaranteed that these preventive measures will successfully meet the emission requirements. Some noise is conducted interference, which is the common mode current on the internal circuit board. The interference source is between the backplane and the circuit.

Therefore, this RF current must flow through the path with the lowest impedance (between the base plate and the carrier signal line). If the connector does not exhibit sufficiently low impedance (connection to the backplane), RF current flows through stray capacitors. At this moment. RF inevitably generates emissions when current flows through the cable.

Another mechanism for injecting common mode current into the I/O region is the coupling of nearby strong interference sources. Some people even some people. Shielding the connector is also useless because the interference source is located near the connector, such as in a PC machine environment. If there is a gap between the connector and the backplane, the RF voltage sensed here can degrade EMC performance.

The method of shielding connectors is to add finger springs or gaskets. The overlap of connectors is to fill the space between the connector and the housing. This method requires a cushion. Metal gaskets are better as long as they are handled properly, that is, as long as the surface is not contaminated, as long as hands do not touch or damage the gasket, and as long as there is sufficient pressure to maintain good low impedance contact.

Other methods are to install connector tabs or install the connector on the housing. At this point, the maximum contact surface is slightly smaller, and the size and elasticity of the joint piece should be strictly controlled. When installing shielded connectors, openings should be opened on the casing. The side of the opening should be cleaned of oil and carefully made. If the tolerance is not appropriate, the connector will sink too deep inside the casing, resulting in overlapping interruptions. EMC engineers know that in an "excellent" system, this issue must meet the launch requirements and be promptly inspected on the production line. Untightened or bent gaskets installed on oil stains in critical areas will fail.

The reason for selecting EMI connectors is as follows

1) Conductive foam plastic is very soft and can be placed around the entire circumference of the connector. This eliminates the issue related to another shell and gasket.

2) Mechanical engineers can install connectors within the acceptable tolerance range of the system casing.

3) The connector achieves low impedance overlap with the housing to ensure good contact. When painting is required, softer materials can be used.

4) The design requires forced cooling, and the gasket should have another feature: the joint between the connector and the shell wall should be sealed to reduce leakage. In a dusty environment, the gasket should keep the system clean.