Unshielded power inductors have an open magnetic circuit where the magnetic flux induced in the core by the current in the winding exits the core and extends through the air to the other side of the core where it completes the flux path. A simple solenoid inductor is a good example of an unshielded inductor. See Fig. 1.
In addition to the evolution of what we ask our inductors to do, the basic nature of inductor operation encourages the use of careful thought to choose correctly. Inductors are passive devices that are far more interesting when something active is happening. Inductor performance is always determined by the operating/excitation conditions.
An example of a shielded inductor is a toroid. See Fig. 2. Conventional inductor designs normally start with a core made out of soft magnetic material, such as ferrite or iron, that is wound with turns of insulated wire to achieve the desired inductance. Some examples of these common power inductors are shown in Fig. 3.
According to Lenz's law, the induced voltage has a polarity (direction) which opposes the change in current that created it. As a result, inductors oppose any changes in current through them. An inductor is characterized by its inductance, which is the ratio of the voltage to the rate of change of current.
Commercial inductors range in value from a fraction of a nanohenry for small surface mount “chip” inductors up to several henries. Some devices exhibit large internal inductances even though they are not specifically used as inductors. One common example is a transformer.
Every inductor winding has some associated distributed capacitance which, along with the inductance forms a parallel resonant tank circuit with a natural self-resonant frequency. For most converters it is best to operate the inductors at frequencies well below the SRF. This is usually shown in the inductor data as a “typical” value.
An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when an electric current flows through it. An inductor typically consists of an insulated wire wound into a coil. When the current flowing through the coil changes, the time-varying magnetic …