Inductors: how to choose & buy the right one

There are many different inductors available to buy and there are many different parameters that are needed in the selection.

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Inductors are less widely used than many other forms of electronic component. Coupled to this, there are very many different types of inductor, and this can make choosing and buying the right inductor rather confusing and difficult.

Knowing what to look for when buying an inductor is not as easy as it might seem at first sight, but when simplified out, the process can be made to be far more straightforward.

What type of inductor, leaded, SMD inductor, value, resistance and many other aspects of the performance need to be considered before the final choice is made.

What's the inductor application

The first and most important stage in the inductor selection process is to know the application for the inductor. There are very many different types of inductor and in the main they are designed to have the parameters required for specific applications. Knowing the application is half way to being able to make the choice of what inductor to buy.

Inductors for different applications will have different properties. Those for power supplies will tend to have larger inductance levels, they will have a larger current capability, and will often be physically larger than those that might be used in, for example, RF applications, where much smaller current levels, higher frequencies and smaller sizes are the order of the day.

Inductor manufacturers normally have their inductors sectioned according to their intended application and this makes it very much easier to make the right selection.

Although there are many different types of inductor, some of the main types are those used for RF applications, those used for power, including linear and switch power supplies, and those used for EMC / EMC interference reduction.

Buying RF inductors

One of the main areas for using inductors is for their use in RF designs. Here the inductors can be used in may ways: as chokes to isolate sections of the circuitry from the signals, as inductors in filters, as inductors in oscillators, etc . .

Apart froth inductance of the inductor, there are some other key parameters that need to be considered when selecting and buying an inductor for RF applications.

One of these is the Q of the inductor. The Q factor essential describes the ratio of the reactance of the inductor to its resistance. The lower the Q, the higher the resistive losses and if the inductor is incorporated into a filter or tuned circuit, the selectivity will be reduced. Accordingly high Q inductors are often, but not always needed for RF circuits. Typically data sheets quote a minimum Q level that the inductor will achieve.

Another factor that is often important is the self resonant frequency. This is the frequency where the inductance of the inductor and its self capacitance resonate. Above the self resonant frequency, SRF, the circuit becomes capacitive, which is not what is needed. Accordingly when buying an inductor, it is chosen so that the SRF is above the highest frequency of operation.

Buying power inductors

There is a different set of parameters for power inductors, compared to RF inductors for example. Power inductors tend to chosen dependent upon power related parameters.

The parameters for power inductors often focus on the incremental current and the maximum current.

Incremental current: the incremental current is the current level at which the saturation in the core reduces the inductance by 5%. Different core materials and shapes can affect this.
Maximum current: This is the current at which the temperature rise due tot he heating effects from the current cause the recommended temperature to be exceeded.

In view of the larger current levels, these are two factors apart from the basic inductance that need tone considered when buying an inductor for power applications.

Buying inductors for EMC / EMI

EMI / EMC is an area of great importance on may products. Inductors provide a key part of this, but need to be chosen for their capabilities related to interference and general RF suppression.

Other aspects of buying an inductor

In addition to the specific parameters needing to be considered when buying conductor, a number of other general elements are also important:

Size: In many instances the size of the inductor is critical. With PCB sizes shrinking, ensure that the inductor will fit. It also helps to have some distance between the inductor and other items into which there could be any inductive coupling. Although many inductors are wound to ensure stray linkage is minimised, it is always best to keep them away from other components.
Component format: For large scale production, virtually all the components used are surface mount. Although many leaded inductors are available, there is also a very good selection of SMD inductors. In fact SMD inductors are available for most applications. The choice of a leaded or SMD inductor will be made as a result of the way in which the final circuit will be built.
RoHS : The Restriction of Hazardous Substances. RoHS, Directive 2002/95/EC, issued by the European Union is legislation that requires manufactured items to not use harmful substances. For some products it may be necessary to conform strictly to this legislation to enable items to be sold within he Eu and elsewhere, as other countries have also adopted a similar stance.
Supplier: The supplier of a part may be crucial. If the supplier / distributor can guarantee delivery when it is required,, and on an ongoing basis if the product is to be in long term production, then this can have a major impact upon the choice of which inductor to buy.

These factors, and possibly some others specific to the project, etc can be very important in making the final decision about which inductor to buy. Sometimes aspects like the method of construction will dictate aspects like the use of SMD inductors for example. Although, normally the technical aspects need to take priority, aspects like the supplier, availability and RoHS are equally important. If the supplier has no stock, or the part being proposed does not meet the required RoHS or other standards then it cannot be used.

Although inductors may not be as widely used as resistors and capacitors, their use is no less important. Knowing how to buy the right inductor for any given application can ensure that the best circuit performance can be achieved and the overall system provide the best performance.