How to prevent and solve the electromagnetic interference problem of connectors

2019-04-10

Today, electronic systems have clock frequencies in the hundreds of megahertz (MHZ) range, pulse front and back in the nanosecond range, and high-quality video circuits also use nanosecond pixel rates.These higher processing speeds represent ongoing engineering challenges.So how to prevent and solve the problem of electromagnetic interference of connectors is worth our attention.


interference problem of connectors


The oscillation rate on the circuit becomes faster (up/down time), the voltage/current amplitude becomes larger, and the problem becomes more serious.Today, therefore, solving electromagnetic compatibility (EMC) is harder than ever.


Before the two nodes of the circuit, the rapidly changing pulse current represents what is known as a differential mode noise source, and the electromagnetic field around the circuit can be coupled to other components and intrude into the junction.The noise coupled inductively or capacitively is common mode interference.The rfi currents are identical to each other and the system can be modeled as consisting of a noise source, "victim circuit" or "receiver" and a loop (usually a baseplate).The magnitude of the disturbance is described by several factors: the intensity of the noise source, the area of the interference current, and the rate of change.


Thus, although there is a high probability of unwanted interference in the circuit, the noise is almost always co-model.Once the cable is connected between the input/output (I/O) connector and the housing or ground plane, a few milliamps of RF current can be sufficient to exceed the allowable emission level if certain RF voltages occur.


Coupling and propagation of noise


Common mode noise is caused by unreasonable design.Some typical reasons are different lengths of individual wires in different pairs, or different distances from the power plane or housing.Another cause is defects in components such as magnetic induction coils and transformers, capacitors and active devices (such as special integrated circuits (asics)).


Magnetic elements, particularly the so-called "iron-core choke" type energy storage inductor, are used in power converters and always produce electromagnetic fields.The air gap in the magnetic circuit is equivalent to a large resistance in a series circuit, where more electrical energy is consumed.The core choke, then, is wound around the ferrite rod, generating a strong electromagnetic field around the bar, with the strongest field near the electrode.In the switching power supply with retrace structure, there must be a gap on the transformer with strong magnetic field.The most suitable element to maintain the magnetic field is the spiral tube, which makes the electromagnetic field distributed along the length of the core.This is one of the reasons why helical structures are preferred for magnetic components operating at high frequencies.


Improper decoupling circuits also often become a source of interference.If the circuit requires a large pulsed current, and if local decoupling is not possible to guarantee a small capacitance or a very high internal resistance, the voltage generated by the power supply loop is reduced.This corresponds to a ripple, or a rapid change in voltage between terminals.Because of the stray capacitance of the package, the interference energy is coupled to other circuits, causing the common mode problem.


When common mode current contaminates the I/O interface circuit, the problem must be resolved before passing through the connector.Different applications suggest different ways to solve this problem.In video circuit, where I/O signal is single terminal and common common circuit, to solve it, filter out the noise with small LC filter.In low frequency serial interface networks, some stray capacitance is sufficient to shunt noise to the baseplate.Differential driven interfaces, such as the ether, are usually coupled to the I/O region via a transformer and are provided for coupling by central taps on one or both sides of the transformer.These center taps are connected to the baseplate by high-voltage capacitors to shunt common-mode noise to the baseplate so that the signal is not distorted.


Common mode noise in I/O region


There is no one general solution for all types of I/O interfaces.The main goal of designers is to design circuits well, often ignoring details that are considered simple.Some basic rules minimize noise before it reaches the connector:


1) set the decoupling capacitor close to the load.


2) the size of the loop shall be the minimum for the pulse current of the front and rear edges changing rapidly.


3) keep high-current devices (i.e., drivers and asics) away from I/O ports.


4) test the integrity of the signal to ensure the minimum overshoot and undershoot, especially for the key signals of large current (such as clock, bus).


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


6) provide low impedance lapping on the baseplate or the reference in I/O area on the baseplate.Rf noise and connectors.


Even if the engineer takes many of the precautions listed above to reduce RF noise in the I/O region, there is no guarantee that these precautions will be successful enough to meet the emission requirements.Some of the noise is conducted interference, that is, on the internal circuit board by the common mode current flow.This source of interference is between the baseplate and the circuit, etc.Thus, this RF current must flow through the path of the lowest impedance (between the baseplate and the carrier signal line).If the connector does not exhibit a sufficiently low impedance (at the junction with the baseplate), this RF current flows through the stray capacitance.When this RF current flows through the cable, it inevitably generates emissions.


Another mechanism by which the common mode current is injected into the I/O region is the coupling of a strong disturbance source nearby.Even some "shielded" connectors are useless because the source of the interference is near the connector, such as a PC environment.If there is a gap between the connector and the baseplate, the RF voltage sensed here can degrade the EMC performance.


The shielded connector can be added with finger reed or gasket.The lap joint of the connector is a space filled between the connector and the housing.This method requires a pad.A metal gasket is preferable as long as it is properly handled, 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 sufficient pressure to maintain good, low impedance contact.


Other methods are connector mounting or mounting the connector on the housing.At this point, the maximum contact surface is slightly smaller, and should strictly control the size and flexibility of the joint piece.When installing the shielded connector, open the door on the housing. The side of the opening should be removed with oil stains and made carefully. If the tolerance is not appropriate, the connector will fall into too deep in the housing and make the lap joint interrupted.Every EMC engineer knows that in an "excellent" system, the problem must be met with launch requirements and checked on the production line.Unsecured or bent liner mounted on oil in critical areas will fail.


power connector

ANEN connectors were selected for the following reasons:


1) conductive foam is extremely flexible and can be placed around the entire connector.This eliminates the problems associated with another housing, gasket.


2) mechanical engineers can install connectors within the acceptable tolerance of the system housing.


3) the connector and housing realize low impedance lap joint to ensure good contact.The liner on the inside of the housing wall can be made of a softer material when there is a masking requirement for painting.


4) for designs requiring forced cooling, the gasket should have another feature: the joints between the connector and the housing wall should be sealed to reduce air leakage.In dusty environment, the liner should keep the system clean.


conclusion

There are a variety of connectors on the market today that enable designers to get the best design for a particular interface.


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