Electrical Faults due to EMI
Introduction to EMI
Current is defined as the flow of electrons. It is produced as result of electrons traveling along a wire or trace. Electrons possess a negative charge. We can say that a negative field is associated with a wire or PCB trace when a current flows through it. The greater the number of electrons, the greater the amount of current causing a field of high intensity.
This negative field along the wire/trace is known as the electric field, and is denoted by the letter E. Similarly a magnetic field is also associated with current flowing through conductor/trace. This field is denoted by the letter B. The magnetic field strength varies just like the electric field with a variation in current. Together these two fields are termed the electromagnetic field (H). Both these components move with a speed almost 2/3 to the speed of light.
According to Faraday and the law of electromagnetic induction, when a magnetic field varies, changes in flux lines are produced. This varying magnetic flux produces a current in the adjacent wire. Our TV and other microwave receivers and transmitters work on this principle, although this is not desirable in some cases. The phenomenon is called EMI (electromagnetic interference) or cross talk.
Figure A below (please click to enlarge) demonstrates the magnetic flux along a wire with the current flowing outward. By applying the Right Hand Rule, we can get the direction of the magnetic flux lines, which are anticlockwise. These flux lines cut across the second wire (on the right) and induce an EMF, or electromotive force, in that wire. The magnitude of this induced EMF depends on the coupling coefficient K between the two conductors. The value of K varies from 0 to 1: zero means no induction and one means maximum induction.
According to Lenz’s Law, the induced current will flow inward in conductor B, creating flux lines opposite in direction to conductor A. An observer will see the radiations of one conductor as stronger than the other relative to its distance from conductor. But if we move these conductors close together, then the observer will see the radiation of both conductors as equal in magnitude and opposite in phase, so they cancel each other effect.
From this demonstration we learn that to minimize the effect of EMI in a wire or trace, the return path should be as short as possible. To minimize the effect on return (GND) path, the loop area should be the minimum possible. Loop area is the area followed by current as it flows from the source and returns back to the source. This is easy to say but difficult to implement.