Power Inductors: Get Higher Performance from New Materials and Fabrication Techniques

Of the three passive components in electronic circuitry, resistors, capacitors, and inductors, the latter is probably the strangest in principle. Inductance was discovered in the 1830s by Michael Faraday, who found that a changing magnetic field could induce an electric current, and by Joseph Henry, who independently studied "self-induction," whereby a conductor induces current in itself.

Until electromagnetics was better understood, it was a mystery how merely forming wire into a coil could change its electrical properties. In the early days of radio, do-it-yourself (DIY) enthusiasts would build crystal radios using a tuning-coil inductor with dozens of turns of wire on a rod or a cardboard tube, just a few inches long.

The schematic symbol for the inductor is based on its physical appearance (Figure 1). Inductor options include air core, iron core, and variable.

Figure 1: Inductors (right) were originally formed from wire wound around an air or iron core; shown are the corresponding schematic symbols (left). (Image source: Hackatronic.com)

Inductance is the property of a conductor that, due to its magnetic field, tends to oppose changes in the current passing through it. For this reason, inductors are sometimes called chokes, in that they “choke” changes in current flow. The relationship between inductance (L), voltage (V), and the rate of change of current (I) is expressed through a simple equation: V = L (dI/dt).

Although still in widespread use, wound-coil inductors are not suitable for many of today’s circuits. They may be too large, unable to provide the required values, exhibit undesirable parasitics, have DC resistance (DCR) that is too high, and show performance degradation at higher frequencies, among other characteristics. In contrast to the days of early DIY radio enthusiasts, it’s now possible to buy off-the-shelf wirewound inductors for radio frequency (RF) applications that are less than 1 millimeter square (mm²).

Modern inductors for power converters

While inductors have come a long way, even enhanced wire-based versions fall short in performance and size for modern circuitry. Modern power inductors are precision components that are carefully modeled and fully specified across primary and secondary parameters, with attributes optimized for different application priorities.

Furthermore, vendors have developed new materials to meet the different needs of switched-mode power topologies, such as the single-ended primary-inductor converter (SEPIC), the Cuk (named after its inventor, Slobodan Ćuk), and various buck-boost configurations.

Most of these use advanced ferrite and powder-based materials with carefully tailored characteristics. These inductors offer extremely low DCR, which significantly increases inductor Q (a standard figure of merit for inductor performance), as well as low inductance roll-off. The latter is a measure of how the actual inductance decreases, or “rolls off,” due to magnetic-core saturation as DC current increases. It is somewhat analogous to a filter’s response-versus-frequency roll-off.

Inductors used in power supplies often must also have relatively high current-handling ratings, usually into the tens of amperes. This parameter is not defined by a single value but by multiple values, such as the root-mean-square (RMS) current (I_rms), peak current (I_peak), and saturation current (I_sat). Vendors offer inductors with different combinations of current ratings and other top-tier parameters to meet the priorities of various topologies.

Vendors have also developed advanced materials and surface-mount technology (SMT) (Figure 2) that can withstand the associated heat without loss of performance or reliability. Shielded versions help minimize RF interference (RFI) issues in sensitive applications.

Figure 2: High-power SMT inductors are now available in a variety of surprisingly small sizes, without performance compromises. (Image source: Eaton)

The range of advances and differentiation among these converter-optimized inductors is seen in the HCM/HPAL molded inductor families from Eaton-Electronics Division. Both families use advanced inductor materials for robustness, high current, and low EMI, while their molded construction provides a soft inductance roll-off across a breadth of current ratings.

Devices in the HCM and HPAL series are available in a range of sizes, while remaining relatively small.

For reliability and robustness, the HCM/HPAL devices’ rated operating temperature is -55 to 125°C (ambient plus self-temperature rise), and they include an anti-rust agent to help prevent surface rust due to humid environments (Moisture Sensitivity Level (MSL) 1).

The HCM family uses an advanced pressed-iron powder for superior I_sat, as seen in two representative parts, the HCM0503V2-R68-R and the HCM0503V2-4R7-R. The HCM0503V2-R68-R is a 680 nanohenry (nH), 8 milliohm (mΩ) DCR unshielded inductor for operation up to 1 megahertz (MHz). It measures just 5.7 × 5.4 × 3.0 mm and features current ratings of 10 amperes (A) (I_rms)/12 A (I_sat). The HCM0503V2-4R7-R comes in the same package size but is suitable if higher inductance is required. It is a 4.7 µH, 47 mΩ unshielded device rated for 4.1 A (I_rms)/6 A (I_sat).

In contrast, HPAL inductors use alloy powder to achieve lower DCR and higher I_rms while maintaining low core losses. Inductors in this family, which span 0.15 μH to 10 μH and 4.5 A to 40 A, include electromagnetic (EMI) shielding, a critical feature in some applications. Example devices include the HPAL1V0630-R47-R, a 470 nH, 4.1 mΩ inductor rated at 18 A (I_rms) and 20 A (I_sat), and the HPAL1V0630-8R2-R, an 8.2 µH, 55 mΩ inductor rated at 5 A (I_rms) and 5.5 A (I_sat).

The graph in Figure 3 shows the roll-off relationship between nominal inductance, DC current, and temperature for the HPAL1V0630-8R2-R inductor.

Figure 3 : Shown is the roll-off and related behavior characteristics for the HPAL1V0630-8R2-R inductor. (Image source: Eaton)

Conclusion

By using advanced materials, fabrication techniques, and packaging, today’s inductors have come a long way from their wound-coil predecessors. They offer high density in small SMT packages, a wide range of inductance and current rating with low resistance, and many other attributes needed for sophisticated, high-performance, efficient, and compact power supplies and converters.