Use Mini Molded Inductors to Save Space, Reduce Losses, and Enhance Power Integrity and Efficiency

Inductors are a crucial component in voltage converter and regulator designs. Due to their energy storage and recovery roles, they are in almost every circuit that regulates power. As applications trend toward smaller and more compact designs that must be increasingly power efficient, designers need to be more discerning about their choice of inductor to accommodate these trends while handling higher currents.

Reducing power losses and improving efficiency depends greatly upon an inductor’s design and core material. For example, the use of mini molded inductors reduces inductor volume, while bringing all the benefits of more conventional inductors, along with greater electromagnetic interference (EMI) shielding, higher power density, and lower core losses.

This article briefly describes inductors and inductance. It then introduces mini molded inductors from Abracon LLC and discusses their selection and application.

Inductors and inductance

Inductors are two-terminal passive components that store and recover energy in the form of a magnetic field. They generally take the form of an insulated wire wound onto a coil. A current applied to the inductor creates a magnetic field proportional to that current within the coil. If the applied current changes, it creates a time-varying magnetic field that induces an electromotive force (EMF) in the conductor. The induced voltage has a polarity that opposes the change in current that created it. Inductors are characterized by their inductance, which is the ratio of the induced voltage to the rate of change of current. The Henry (H) is the unit of inductance, which can be increased by creating a coil with more turns, building a larger cross-section, decreasing the coil length, or using a core with a higher permeability material (Figure 1).

Figure 1: Shown are the factors that determine the inductance of a coil. (Image source: Abracon)

Permeability is a magnetic characteristic, and core materials with higher permeability generate a higher density of magnetic flux, allowing more energy to be stored. Therefore, inductance is also proportional to the permeability of the inductor’s core material. A highly permeable core can reduce the size and weight of the inductor without reducing the inductance value, resulting in a smaller and lighter overall package.

Core materials include air, iron, steel, iron powder, metal powder, ceramic, and ferrite. Ferrites are ceramic materials combined with powdered iron oxide and/or other powdered metals to provide a core material with high permeability. Powder cores use powdered magnetic metals mixed with a binder and a coating. The selection of the metal, binder, and even the inclusion of air bubbles in the mix determines the permeability of the resulting core material.

Inductor specifications

The critical specifications for inductors used in power applications are inductance, DC resistance (DCR), saturation current, temperature rise current, rated current, self-resonant frequency (SRF), and quality factor (Q).

The DCR, sometimes called wire loss, is the measured resistance of an inductor for a DC source. The DCR varies in proportion to the inductance due to the length and cross-sectional area of the wire. Power inductors generally have a DCR in the tens of milliohms (mΩ) to ensure low conduction losses. In most cases, the DCR is specified as a maximum rating.

As the current through an inductor increases, the magnetic field increases proportionally until it reaches saturation; at this point, the permeability begins to decrease. Current increases beyond this point cause the inductance to fall. The saturation current is the current where the resistance drops a specific amount of the nominal inductance. Power inductors usually use a decrease of 10 to 30% as the specification limit.

The temperature rise current is specified as the DC level where an inductor’s case temperature increases by 40°C.

The rated current is specified as the lower value of the saturation current or the temperature rise current, allowing an inductor to operate below the lesser of the two limits.

SRF is the frequency at which the reactance of the parasitic capacitance of an inductor is equal to the reactance. At this point, an inductor operates as a parallel resonant circuit. The net reactance is zero, and the impedance is extremely high and entirely resistive. Inductors are generally operated below their SRF in power applications.

The Q of an inductor is a measure of its efficiency and is the ratio of its inductive reactance to its resistance at a given frequency. A higher Q means lower losses, and the closer an inductor’s behavior reflects that of an ideal inductor.

Molded power inductors

Molded power inductors are surface mount devices (SMDs) that use molding technology to surround and encapsulate the coil of an inductor. Unlike traditional wire wound inductors, a molded inductor’s magnetic powder material is pressed into a mold around a wire coil surrounding the conductors. The molding compound, usually a powdered metal and a binder, sets the permeability of the inductor core. The powdered metal filling offers a softer saturation response than ferrite fillers. It also provides highly effective magnetic shielding, resulting in low magnetic flux leakage. A molded inductor is a solid component suited for harsh environments, protecting against moisture, dust, shock, and vibration. A molded inductor does not emit acoustic noise because it does not have a laminated core. The simple, one-piece construction offers excellent mechanical stability and is compact and lightweight.

Abracon’s mini molded inductors offer all the benefits of molded inductors in a small package measuring under 3 millimeters (mm). Besides their compact size, mini molded inductors include high power density, low core and conduction losses, and excellent EMI shielding.

The AOTA-B1412 and AOTA-B2012 series mini molded inductors are offered with an inductance range from 0.11 to 2.2 micro Henries (µH), and have package dimensions from 1.4 x 1.2 mm (0.055 x 0.047 inches (in.)) to 2.0 x 1.2 mm (0.079 x 0.047 in.) with a maximum height as low as 0.65 mm (0.026 in.). These inductors handle rated currents from 1.9 to 6.4 amperes (A), and are rated to operate over a temperature range of -40°C to +125°C.

An example from the AOTA-B2012 series is the Abracon AOTA-B201208SR11MT, a 0.11 µH SMD mini molded inductor with a rated current of 5.6 A and a saturation current of 10 A (Figure 2). It has a DCR of 13 mΩ and an SRF of 185 megahertz (MHz). It is mounted in a 2.0 mm x 1.2 mm (0.079" x 0.047") package with a seated height of 0.8 mm (0.031").

Figure 2: The AOTA-B201208SR11MT is a typical Abracon mini molded inductor in a sub-3 mm SMD package that protects against environmental factors such as moisture, dust, shock, and vibration. (Image source: Abracon)

At the upper inductance range of the Abracon AOTA-B2012 series is the AOTA-B201208S2R2MT, with an inductance of 2.2 µH, a rated current of 1.8 A, a DCR of 130 mΩ, and an SRF of 42 MHz. The higher inductance requires a greater number of turns, which increases the DCR and decreases the rated current and SRF compared to the AOTA-B201208SR11MT. The package dimensions are the same as the AOTA-B201208SR11MT, 2.00 mm x 1.20 mm (0.079" x 0.047") with a height of 0.8 mm (0.031").

Examples of the Abracon AOTA-B1412 series are the AOTA-B141206SR33MT and the AOTA-B141206SR47MT. These mini molded inductors have the smallest package with dimensions of 1.4 mm x 1.2 mm (0.055" x 0.047") and a package height of only 0.65 mm (0.026"). The AOTA-B141206SR33MT has an inductance of 0.33 µH, a rated current of 3.5 A, a DCR of 32 mΩ, and an SRF of 120 MHz. The AOTA-B141206SR47MT has an inductance of 0.47 µH, a rated current of 2.9 A, a DCR of 41 mΩ, and an SRF of 115 MHz.

Applications for mini molded inductors

Despite their small size, the Abracon mini molded inductors handle substantial power with low core and conductive losses while offering superior EMI shielding. These characteristics make them ideal choices for meeting the unprecedented demand for power converters in ever-smaller form factors.

Typical applications for these components include power decoupling, filtering, and DC/DC converters (Figure 3).

Figure 3: Typical Abracon mini molded inductor applications include power decoupling, filtering, and DC/DC converters. (Image source: Art Pini)

Decoupling integrated circuits from the power bus utilizes the frequency variable impedance of the inductor combined with the complementary impedance characteristics of a capacitor to attenuate high-frequency signals and noise, isolating them from the integrated circuit power inputs. Low DCR and high SRF are the important inductor characteristics.

Filters control the frequency response of the signal path and can be configured as low, high, bandpass, or bandstop. Inductor-capacitor (LC) filters provide passive frequency-selective responses for low-power devices that do not require active devices.

Inductors are the primary energy storage element in DC/DC converters. They store energy while the switch is closed and recover it when it opens.

Conclusion

Abracon mini molded inductors offer the advantages of molded inductors in a compact package smaller than 3 mm. Despite their small size, they can handle significant power levels with low core and conduction losses, ensuring excellent power integrity in small electronic devices.