Technical Articles

Inductor Selection: Design Sensitivities

When you start designing your own circuits, one of the biggest challenges is not obtaining component values, but choosing from many different types of components all having the same value! For example, there are lots of choices for a 14 µH inductor: subminiature iron core, high-current ferrite core solenoid, pot core toroid, standard toroid (both powdered iron and ferrite as described in the article), single-section pi-wound, air-core (of a wide variety of sizes), and the list goes on. What this article illustrates is the idea of design sensitivities—that there are certain aspects of a component that can have a big effect on electrical performance, even if the nominal value is exactly as specified.

Sensitivities can apply to more than just electrical performance, as well. As the author mentions on pages 11 and 12 for L2, L4, and L6, performance of the filter is approximately the same for both powdered iron and ferrite core inductors. However, the higher permeability of the ferrite material allows the inductor to be wound with fewer turns (lower production cost) and larger wire (lower loss due to skin effect). This might result in significant cost savings if many filters are to be made.

Another area where sensitivities may be important is performance far from main frequency of operation. This particular filter might be used anywhere from LF (say the new amateur bands at 2200 and 630 meters) to low VHF (up to 100 MHz) so rejection in the AM band is only part of the story. If the filter’s rejection became lower with increasing frequency due to parasitic capacitance, it might not reject the AM signals when used with a 6 meter receiver, for example. Perhaps the filter’s impedance would drift away from 50 W at low frequencies, causing a drop in signal strength due to increased SWR in the receiving antenna system.

And then there are the concerns about tolerance (variation in value from the specified nominal value) and temperature coefficient (amount of variation in value as temperature changes). These are all considered by the design engineers when specifying particular components for a product. If there are key sensitivities, these must be clearly noted in any design documentation so that if a particular part or manufacturer is no longer available, a substitute will still allow the product to meet its published specifications.

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