SNVS124F November 1999 – April 2021 LM2596
Another possible source of increased output ripple voltage or unstable operation is from an open-core inductor. Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to the other end. These magnetic lines of flux will induce a voltage into any wire or PCB copper trace that comes within the inductor's magnetic field. The strength of the magnetic field, the orientation and location of the PC copper trace to the magnetic field, and the distance between the copper trace and the inductor determine the amount of voltage generated in the copper trace. Another way of looking at this inductive coupling is to consider the PCB copper trace as one turn of a transformer (secondary) with the inductor winding as the primary. Many millivolts can be generated in a copper trace located near an open-core inductor, which can cause stability problems or high output ripple voltage problems.
If unstable operation is seen, and an open-core inductor is used, it is possible that the location of the inductor with respect to other PC traces can be the problem. To determine if this is the problem, temporarily raise the inductor away from the board by several inches and then check circuit operation. If the circuit now operates correctly, then the magnetic flux from the open core inductor is causing the problem. Substituting a closed core inductor such as a torroid or E-core will correct the problem, or re-arranging the PC layout can be necessary. Magnetic flux cutting the IC device ground trace, feedback trace, or the positive or negative traces of the output capacitor should be minimized.
Sometimes, placing a trace directly beneath a bobbin inductor will provide good results, provided it is exactly in the center of the inductor (because the induced voltages cancel themselves out). However, problems can arise if the trace is off center one direction or the other. If flux problems are present, even the direction of the inductor winding can make a difference in some circuits.
This discussion on open core inductors is not to frighten users, but to alert users on what kind of problems to watch out for. Open-core bobbin or stick inductors are an inexpensive, simple way of making a compact, efficient inductor, and they are used by the millions in many different applications.