In many electronic designs, we have components that consist of only several turns of a wire, with or without a core. These components are inductors. It is customary to find them in many types of electronic devices, including voltage and power conversion circuits to high-frequency microwave and RF circuits. Typically, inductors resist any change in the current flowing through them, by producing an electromagnetic field.
Available in a variety of package styles depending on their current ratings, inductors are essential components in electronic designs. They function as filters, chokes, and impedance-matching functions. For a practical application, it is essential to understand the important performance parameters of inductors.
Any inductor, whether used for power or RF applications, has the same performance parameters. These include the inductance value, its tolerance, current rating, its DC resistance, SRF or self-resonant frequency, Q or quality factor, and temperature range. However, specific applications may stress more on some of these performance parameters as they have more relevance for that application. For instance, an application involving RF frequencies may give more importance to Q and SRF parameters rather than to the current rating, which is more important for power applications.
The size of an inductor—how big or how small it can be—is usually dependent on the inductance value, its current carrying capacity, and acceptable losses. These are critical parameters when selecting inductors. Selecting an inductor usually begins with the inductance value, typically in nH or in mH, and depends on its function in the circuit. Associated with the nominal inductance value is its tolerance, in %, characterizing the amount of variation of the inductance value, and is determined by the application.
For instance, RF applications typically require inductances closely matched by precise inductance values, and with tight tolerances, such as ±2%. On the other hand, power applications may use inductors with inductance values within a larger band, and with wider tolerances, such as ±20%.
Another important parameter for inductors is their ability to handle current, which can vary greatly by application. This is specifically true for inductors in power circuits such as DC-DC converters, where the current values can change widely, with very high peak-to-average current ratios. Inductors selected on the basis of the application’s highest instantaneous current value may provide an inductor much larger than necessary. On the other hand, selecting an inductor based on the average current value in the circuit may lead to a small inductor resulting in inconsistent performance during peak current deliveries.
The quality of an inductor has more relevance in RF circuits than it has for power applications. Quality or the Q factor is a dimensionless parameter that characterizes the inductor’s bandwidth relative to its center frequency. High Q values are typically matched to narrow bandwidths and low losses, more critical in RF applications.
For power applications, the losses in inductors are more important. Here, the DC losses, characterized by the resistance of the wire, are rather more relevant. Therefore, inductors for power applications tend to be made of wires with larger diameters, so as to increase the area for current travel and thereby reduce the resistance.